Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

All times are local (= UTC - 4 hours)

06/1995 (BGVN 20:06) Small phreatic eruptions - the first in recorded history

The following is based on information as of 24 July from the Seismic Research Unit (SRU) team at the University of the West Indies and Volcanic Alert News Releases from the Montserrat Emergency Operations Center. The SRU maintains a seismic network on Montserrat (figure 1), currently composed of seven instruments.

On 18 July, villagers around Soufriere Hills volcano reported unusually loud rumbling noises coming from the fumarolic areas, light ashfall, and a strong sulfur odor. Following confirmation of these reports, an Emergency Operations Center, located in the capital city of Plymouth (on the coast ~4 km W of the summit), was activated and fully operational by 1830 that night. The Emergency Operations Center identified two schools as potential refugee centers, but no evacuation was ordered.

As of the morning of 19 July, based on conversations with Montserrat residents, SRU inferred that the initial explosion was small, phreatic, and only spread minor ashfall around the island. In accord with a small explosion size, the Synoptic Analysis Branch of NOAA saw no evidence of a plume on satellite imagery. Seismicity has been elevated since August 1992, and an earthquake swarm began on 14 July. However, no additional increase in seismicity was associated with the 18 July explosions.

An explosion earthquake at 0924 on 19 July was centered close to the top of Chance's Peak, the summit located on the W side of the crater rim. A field team led by Lloyd Lynch (SRU) trekked in from the N to make an initial inspection just after 1300. They reported minor explosions from an area SW of Tar River Soufriere (a fumarolic area ~1.5 km NE of the summit), explosions discharging from a vent within the summit crater between Chance's Peak and the Tar River area. The explosions took place at intervals of ~20 minutes, sending ash and steam ~40 m high. Based on these observations, no evacuations were recommended. Explosions continued that afternoon (figure 2).

William Ambeh (SRU) led another observation team on the morning of 20 July to the Paradise Estate area (~2 km N of the summit), and additional monitoring equipment was installed in the Long Ground area (~2.5 km NE of the summit). Reconnaissance photographs taken from a Royal Air Force aircraft confirmed the early field reports. Later photographs taken from a Royal Navy helicopter indicated no increased activity in the Long Ground area.

The shallow earthquake swarm that began on 14 July ended on the 21st; depths were 2-4 km, and the largest event was M 3.5. Volcanic earthquakes were concentrated along the ENE and WSW areas of Lang's Soufriere. Phreatic activity continued on 22 July. Early morning ashfall was reported in Plymouth (~4 km W of the summit) and the SW-sector villages of Gages, Parsons, and Amersham. A small steam-and-ash eruption around 0800 lasted ~ 10 minutes. As of 1030 on 23 July, there was no new volcanic activity.

At the request of Montserrat, France sent two scientists (arriving on 25 July) to provide the SRU with technical assistance and additional equipment. They were joined on 26 July by five geologists from the U.S. Geological Survey's Volcanic Crisis Assistance Team.

Information Contacts: R. Robertson, UWI; Montserrat EOC; A. Dennis, Washington DC, USA.

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07/1995 (BGVN 20:07) Steam and ash emissions from two vents in the summit crater

Soufriere Hills volcano (figures 3 & 4) began erupting on 18 July from a fissure vent (Vent 1) within the summit crater (20:6). The initial small phreatic eruption spread minor ash around the island. The next day the airport on Montserrat issued a NOTAM after a reconnaissance flight at 0745 reported flying through volcanic ash. Seismicity and minor phreatic explosions continued in the following days (20:6).

Another NOTAM on 26 July renewed the warning to aircraft and reported sporadic ash emissions. A second vent formed on 28 July (figure 5), and a third on 20 August. Gas samples taken in late July from fumaroles at Tar River and Galway's were unchanged from 1989. Samples of ash showed no juvenile components through at least 30 July. Seismicity in late July remained at about the same level as previously. A distinct odor of hydrogen sulfide (H₂S) was detected in Plymouth . . . during late July, but sulfur dioxide (SO₂) was not detected until 30 July.

Monitoring efforts. Monitoring of the eruptive activity and scientific advice to the Government of Montserrat are being provided by an international team of volcanologists. The first response to the crisis was provided by the Seismic Research Unit (SRU) at the University of the West Indies in Trinidad, which had maintained a seismic network on the island. Their team was later supplemented, at the request of Montserrat, by scientists from the U.S. Geological Survey (USGS), the Guadeloupe Volcano Observatory, and the United Kingdom. French scientists arrived on 25 July to sample gases. The USGS arrived on 26-27 July with additional seismometers, tiltmeters, and a correlation spectrometer (COSPEC).

The USGS team set up a seismic data analysis system to automatically locate earthquakes in near real-time, and made improvements to the existing seismic network. By 30 July, volcanologists were monitoring 10 channels of component signals from eight seismic stations; another station was added soon after. New telemetered tiltmeters at Spring Estate (2.5 km SW of the vents), Amersham (3.7 km WSW), and near Long Ground (2 km NE), were operational by 2 August.

Formation of Vent 2 on 28 July. A volcano-tectonic (VT) earthquake swarm that began at 0854 on 28 July lasted for >2 hours; instruments detected ~ 50 events of M >1. Coincident with this seismicity, a new vent opened SW of Castle Peak (Vent 2), along-strike with the fissure vent that had been intermittently active since 18 July. Vigorous jetting from the new vent at 1814 on 29 July, associated with ~ 22 minutes of tremor and earthquakes, occurred during heavy rainfall and was accompanied by a small mudflow. Following this episode, Vent 2 was estimated to be 1-2 m in diameter and was jetting steam and a small amount of fine ash ~ 100 m high with a loud roaring sound. During an overflight on 30 July Vent 1 was producing only wisps of steam, while Vent 2 continued to jet a large amount of steam and fine ash.

Because of increased steam emissions, on 28 July local authorities ordered an evacuation of the Long Ground area (figure 4). The evacuees returned to their homes the next morning. Two episodes of increased seismicity on 29-30 July caused no observable changes at the vent area. Small (mostly M <1) VT earthquakes continued through 31 July at a relatively steady but low rate. This background activity was punctuated once or twice a day by continuous seismicity lasting several minutes to 2 hours. Some of these episodes may have been associated with vigorous venting of steam and ash. Others were volcanic tremor with coincident earthquakes. One low-amplitude tremor episode on 31 July lasted for several hours.

Low-level activity in early August. During 1-3 August there were fewer high-frequency, VT earthquakes, plus some long-period (LP) earthquakes. Preliminary locations for the LP events were at depths of 5-6 km, slightly deeper than the VT events. Emergent "cigar-shaped" signals, that probably correspond to vigorous steam venting, occurred a few times each day. Heavy rainfall on 3 August triggered a small, non-destructive mudflow during the night in a stream valley that runs through Plymouth. Normal infiltration of rain water may have been reduced by the relatively impermeable layer of fine ash that had accumulated on the upper slopes of the volcano.

Following 12 hours of unusually low seismicity, vigorous steam and ash emission began at 0852 on 4 August. This phreatic eruption lasted ~10 minutes, producing a dark, ash-laden column visible from most of the island. Seismicity associated with the eruption included several LP events. An aerial inspection revealed that the eruption had enlarged Vent 1 to ~10 m across and 10 m deep.

Concern was heightened after the phreatic eruption on 4 August and the increasing seismicity. As a precautionary measure, the elderly and infirm from the villages of Long Ground, Bramble Village, Bethel, Farms, and Trants were resettled on the N part of the island on 6 August. Aged and infirm in areas from Harris to Gages and N and S of the immediate area around Fort Ghaut in Plymouth, were also relocated to the N. Able-bodied residents of Long Ground were advised to move to shelters at night. Further restrictions may have been enforced during the next week.

Vent 2 was full of water on 5 August, apparently ground water forced from the volcano, and muddy water flowed from it through the Hot River drainage. On 6 August a large steam plume with minor ash rose from the vent area. By 7 August Vent 2 had grown ~ 20 m to the NW, in the direction of the 18 July fissure, the muddy water was gone, and jetting of steam and varying amounts of fine ash continued. The abscence of water emissions from the vent area after 7 August suggested that the volcano may have been "drying out," possibly due to increased heatflow.

Eighteen locatable earthquakes (M <1) during 4-5 August were centered <=5 km beneath the vent area or slightly NE of it. There were roughly equal numbers of VT and LP earthquakes. Approximately 200 events during a 24-hour period on 5-6 August were mostly VT events with a few LP events; some were felt in Plymouth. On 7 August, spasmodic tremor and small VT earthquakes occurred throughout the night and into the next day. The seismicity in early August was believed to be related to "boiling" of the hydrothermal system. However, the seismic energy release was dominated by low-amplitude tremor generated by degassing and steam eruptions at Vent 2.

Ground tilt recorded at Long Ground appeared to reverse on 5 August from steady deflation (down toward the vent area) since the station was installed, to apparent inflation (up toward the vent area). Vigorous venting on 6 August caused several microradians of tilt at two tiltmeters. Tiltmeters recorded small tilt events through 7 August, some of which seemed to correlate with periods of strong seismicity. However, there was no consistent pattern, suggesting that any deformation was relatively minor. A small tilt event occurred coincident with the 6-7 August seismic swarm.

Earthquakes declined on 9 August, but tremor caused by steam venting continued. At about 0715-0745 there was a relatively large steam venting episode. Vigorous steaming continued from Vent 2 on 10 August, but tremor intensity decreased and the number of small VT earthquakes increased to 1-2/hour. Most of the earthquakes were centered 2-5 km beneath the vent area. Seismicity was slightly lower during 10-11 August, with seven VT earthquakes, two individual low-frequency events, and three of four periods of continuous tremor lasting ~2 hours. On 10 August tilt appeared stable as measured by titlmeters and along a short leveling line near Broderick's Estate, ~3 km SW of the vent area, that was last measured ~10 years ago.

Reactivation of Vent 1. Vent 1 reactivated on 11 August and seemed to be emitting steam on the 12th. Steady steam emissions continued from both vents through 13 August. Seismicity was slightly lower on 11-12 August, with five small VT earthquakes and two periods of continuous tremor (~2 hours total). Increased gas venting around 1621 on 12 August triggered a swarm of VT events that continued into the next afternoon. The swarm consisted of >134 earthquakes, of which 38 were felt, the largest at around 0221 on 13 August (M ~3.5). Epicenters clustered 2-6 km beneath St. George's Hill (3 km WNW of the summit).

A mudflow from Vent 2 along the Hot River early on 12 August blocked the road for ~1 km between Tar River and Perche's Estates. Steady steam output from Vent 2 continued throughout 14 August. Steam emissions from Vent 1 were intermittent and occasionally changed composition. On 14-15 August there were three VT earthquakes, three B-type events, two low-frequency events, and five degassing episodes followed by tremor.

After three days of consistently lower activity (as of 11 August), daytime occupancy was recommended for able-bodied residents of the evacuated villages. However, the sick and bedridden were to remain at shelters. The reactivation of Vent 1 caused additional evacuations during 12-14 August from the previously mentioned villages, but able-bodied residents returned again on 15 August.

The seismographs recorded sustained low-frequency tremor from noon on 16 August through noon the next day. Degassing from Vent 2 continued at nominal to vigorous rates with occasional increases in acoustic intensity and changes in color of the output. Six of the 13 locatable VT earthquakes on 16-17 August were beneath Soufriere Hills; the others, including one felt strongly in Plymouth at 0143, were scattered within 4 km NE to NW of St. George's Hill. Vent 2 exhibited loud roars and intense venting coincident with heavy rainfall. The morning of 18 August was very overcast and inspection of the vents was not possible, but tremor continued, and there were six locatable VT earthquakes. During 18-19 August there was continuous low-frequency tremor and moderate emissions from Vent 2. Vent 1 was obscured for most of the period, but venting noise was generally low. Several low-amplitude extremely short-duration VT earthquakes on the Gages seismograph were buried within the background tremor signal, locatable events were generally 2-3 km beneath Soufriere Hills.

Tiltmeter observations remained within background noise level between 7 and 19 August. COSPEC measurements of SO₂ flux taken from a helicopter after 30 July averaged 300 metric tons/day (t/d) until the morning of 6 August (table 1). After reaching a high of ~1,200 t/d on 6 August, values decreased and stabilized below an average of 200 t/d through 18 August.

Table 1. Summary of SO₂ measurements at Soufriere Hills determined by COSPEC, 30 July-18 August 1995. Courtesy of the USGS.

    Date     COSPEC SO2 (t/d)   Comments
    30 Jul-    200-600          First flight on 30 July.
    03 Aug    (average 300)     Determined that the only
                                plausible source is a degassing
                                magma body.
    04 Aug     ~550             Measurements spanned an  episode
                                of vigorous venting.
    06 Aug     ~1,200           Morning measurement.
    06 Aug     ~250-300         Later in the day.
    07-09 Aug  ~200
    10 Aug     ~245 ± 25
    11 Aug     ~190 ± 30
    12 Aug         -            Wind not adequate for measurement
    14 Aug     ~150 ± 20
    16 Aug     ~163 ± 25
    17 Aug     ~111 ± 35
    18 Aug     ~180 ± 30

An ash explosion on the morning of 20 August formed a third vent in the summit crater and prompted evacuations of up to 5,000 people. The observatory location was moved as a result, delaying the daily reports. Because this occurred near our deadline, details will be provided next month.

Information Contacts: VDAP, USGS; Seismic Research Unit, UWI; Montserrat EOC.

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08/1995 (BGVN 20:08) Two additional vents open in late August; steam-and-ash emissions

Eruptive activity . . . began with a phreatic explosion on 18 July that caused ashfall around the island (20:6). Formation of a second vent in the summit crater on 28 July and increased steam-and-ash emissions prompted evacuations from communities near the summit (20:7). Activity was variable but generally low in early August, with small mudflows and continued steaming. Vent 1 reactivated on 11 August, and some earthquakes were centered beneath St. George's Hill, 3 km WNW of the summit. Relatively heavy cloud cover and bad weather prevented observations on many days in late August.

Moderate emissions continued from both vents through 19 August (20:7). An ash-and-steam eruption from Vent 1 at 1220 on 19 August was similar in size to the two previous events that caused ashfall in Plymouth. The eruption lasted for ~35 minutes, during which a 10-minute-long phase of more vigorous activity deposited ~1 mm of ash in areas NW of the vent. Ashfall along the road between Lees and Gages has caused reduced traction; police were advising motorists to drive slower on the slippery surface.

Another small phreatic episode at 1657 on 20 August produced minor ashfall. Since the disappearance of continuous tremor on 19 August, seismicity consisted of low-intensity spasmodic tremor and occasional small volcano-tectonic (VT) earthquakes attributed to very shallow activities under the summit area.

Formation of Vent 3 on 22 August. Seismicity generally increased in frequency and amplitude until the largest single phreatic eruption episode to date at 0803 on 21 August produced a column to a height of ~2 km. Ash blown down the W flank engulfed Plymouth within eight minutes, caused darkness for ~25 minutes, and deposited an estimated 2 mm of ash. An analysis of the new ash showed that it consisted only of old altered material. Other reported phenomenon were projectiles in the Long Ground area and a density current in Gages. Over 70 locatable VT earthquakes were recorded between noon on 20 August through noon the next day; most of these events occurred before the 21 August eruption and were very shallow.

Following a contingency plan, the volcano observatory was relocated to the Vue Pointe Hotel in Old Towne (~ 4.5 km N of Plymouth) at approximately 1600 on the 21st; by 1730 all of the seismic instruments were back on-line. Seismicity was less frequent and vigorous through noon on 22 August, with only four eruption signals of smaller intensity and duration than the previous episodes. Over 5,000 residents were evacuated from the capital city of Plymouth to camps on the N part of the island. Day use was permitted, but restricted. Government offices were also relocated.

Six phreatic eruptions occurred between noon on 22 August and noon the next day. The largest, at 1551 on 22 August, produced ashfall ~3-3.5 km to the SW. This event was followed by three long-period (LP) earthquakes (events associated with the movement of pressurized fluids). When the crater area was visited on the afternoon of 22 August it was discovered that a new vent (Vent 3) had opened along the inside of English's Crater rim. Only 24 earthquakes were located during 21-23 August, all very shallow. Seismicity was comparable or slightly lower during the next twenty-four hours. There were six episodes of increased gas venting, some of which were followed by small VT events. Fifteen earthquakes, including two LP events, were located beneath Soufriere Hills at depths ranging from near-surface to ~5 km.

Aerial reconnaissance on the afternoon of 24 August showed a NNW-SSE trending line of several small explosion craters in the summit crater, with Vent 1 at the N end and Vent 3 at the S; Vent 2 was offset to the SE. The rate of steam emission from Vent 2 was very low, while slightly more steam was being emitted from Vent 3. No gas emission was observed from Vent 1. Seismicity continued to be relatively low, with seven earthquakes distributed beneath St. George's Hill and Soufriere Hills at depths of 0-4 km. Three episodes of increased gas venting and 30 minutes of broadband tremor also occurred. A small mudflow originating from Gages Upper Soufriere during heavy rains on the afternoon of 24 August flowed through Fort Ghaut. Seismicity remained low until a swarm of VT earthquakes that lasted from 2157 on 25 August until 0230 the next day. Located earthquakes consisted of 22 VT events at depths of 0-5 km beneath the N flank. Five episodes of gas venting during 25-26 August had repose intervals of ~4 hours.

Formation of Vent 4 on 27 August. On 27 August there was one episode of broadband tremor that lasted ~20 minutes and a small mudflow in Fort Ghaut that began around 0820. From 26 to 28 August, fourteen small VT earthquakes were located beneath Soufriere Hills and St. Georges Hill at depths of <5 km. During this same period there were twelve episodes of increased gas venting. One episode at 1443 on 26 August ejected ash that could be seen from the Vue Pointe Hotel; other emissions caused light ashfall in the Tar River area. Observations on the morning of 28 August confirmed the presence of a fourth vent that had probably opened the day before. Located on the NNE flank of Castle Peak dome, it was vigorously emitting steam and ash through mid-day on 28 August; emissions from the other vents were low. Eight VT earthquakes were located beneath Soufriere Hills at depths of 0-4 km. Five episodes of increased gas venting occurred.

Vent 4 was still emitting mainly steam at a reduced rate on 29-30 August. Another nine episodes of increased gas venting were detected, and five small shallow earthquakes were located beneath and to the N of the Soufriere Hills during 29-30 August. Unusually good visibility allowed Castle Peak dome to be inspected at around 0900 on 30 August. Steam emissions from all vents were low and there was no ash. The main vent system (a linear chain of vents extending from Vent 1 on the NW margin of the dome SE to the S margin of the dome) had enlarged since 24 August. Mud or muddy water was locally present in the bottom of the main vent system. Several pools of standing water were located atop Castle Peak, and the moat pond on the NW side of the dome still existed. A recent mudflow from the W side of the dome southward down the Tar River had buried Vent 2, on the S side of the dome.

In terms of earthquake activity, the 24-hour period beginning at 1400 on 30 August was probably the most active since the 21 August phreatic eruption. Thirty-four shallow earthquakes were located WNW of Soufriere Hills. A few earthquakes were also located beneath Windy Hill (3.5 km NNW) and in the area between Windy Hill and Soufriere Hills. Seismicity decreased the next day, when only ten shallow earthquakes were located WNW of Soufriere Hills; two were also located beneath Windy Hill. In addition, four LP earthquakes occurred at shallow depths beneath the NW edge of Soufriere Hills. During these two days, thirteen episodes of increased gas venting were detected, but steam and ash emissions from all vents remained low.

Deformation and SO₂ measurements. A review of the Brodrick's dry-tilt data completed on 23 August indicated that some deformation of the volcano may have occurred between January and 9 August, confirming that magma may be at a shallow depth (as suggested by the earthquake data). Tiltmeter readings in late August were generally within background noise levels; no tilt related to volcanism was observed. EDM reflectors were deployed on 30 August in Gages Upper Soufriere and on Castle Peak dome.

COSPEC gas measurements taken on the afternoon of 20 August indicated that the rate of SO₂ emission was just above the detection level, ~50 metric tons/day (t/d). Additional measurements taken during favorable conditions on the next afternoon and morning of 22 August did not detect any SO₂. This lack of SO₂ was thought to be a result either of the system running out of gases or a sealing off of the fluid access path to the surface. A COSPEC flight on the afternoon of 23 August detected a slight trace of SO₂ (~ 40 t/d) while a flight the next morning showed none. The flux rate on the morning of 26 August was ~50 t/d, and on 28 August was ~85 t/d. Further COSPEC measurements on 29, 30, and 31 August showed no detectable SO₂.

Information Contacts: VDAP, USGS; Seismic Research Unit, UWI; Montserrat EOC.

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09/1995 (BGVN 20:09) Phreatic eruptions continue; new lava dome in summit crater

Following the formation of Vent 3 and significant ashfall on 22 August (20:8), more than 6,000 residents of southern Montserrat were evacuated to safe areas in the N part of the island. Press sources estimated that by late August ~3,000 people had left for neighboring islands. Vent 4 opened on 27 August and produced mainly steam emissions with some minor ash through 30 August. Although seismicity was high from 30 August through 1 September, steam and ash emissions remained low (20:8).

From 0500 on 1 September through 0500 on the 3rd, only 19 shallow earthquakes occurred beneath the volcano. During that same period, 17 episodes of gas venting were recorded; at least six of those episodes produced some ash, and the two events on 2 September each decayed into a long-period signal of ~10 minutes duration. Venting continued to enlarge vents 2 and 3, but emissions from Vent 4 remained low. A helicopter observation flight on the afternoon of 2 September was in progress when an emission episode began at 1606 with increased steaming that developed rapidly into a small steam-and-ash plume. The emission occurred from a narrow part of the main group of vents that extend SE from Vent 1. Mud on the floor of the vent was expelled during the episode, forming a small mudflow that moved down the S side of the moat and over the area of Vent 2. A gas-and-ash emission at 1912 on 2 September, similar in size and duration to emissions in recent days, was widely observed because of clear conditions. Lightning associated with this activity lasted ~1.5 hours, and an SO₂ odor was detected. Installation of a hardened EDM (electronic distance meter) station in the Tar River area was completed on 2 September.

During 3-4 September there were four gas-venting episodes, twelve volcano-tectonic (VT) earthquakes, and four long-period earthquakes. Aerial observations on the morning of 3 September revealed that the area around the S end of the main group of vents had been enlarged. The moat pond in the NW corner was still present, and fragmental material had collapsed into Vent 1. Afternoon observations showed no new mudflows, and the S moat appeared dry.

On the afternoon of 3 September, scientists at the volcano observatory completed an assessment of the current volcanism since 21 August and prospects for future activity. The rate of eruption signals increased slightly after 21 August, but the size of the eruptions did not. No change in the style of eruptions was anticipated, but areas downwind could be subject to ashfall and temporary darkness. Eruptions were thought likely to be concentrated along the linear vent chain on the W side of Castle Peak dome. The amount of shallow seismicity decreased below that prior to 21 August. SO₂ flux remained near detection limits since 21 August. The rate of long-period seismic events showed no clear pattern, although a slight decrease may have occurred. Initial EDM results indicated no movement of the SE flank of Castle Peak dome or at a site in Upper Gages. Electronic tiltmeters have detected no large-scale deformation since they stabilized on 5 August. Ash samples analyzed through 27 August revealed no juvenile material.

The scientists concluded the following: ". . . eruptions to date have been entirely phreatic, with no direct evidence of magmatic involvement. So long as this behavior pattern persists, it only constitutes a significant hazard to areas within 1.5 km of Castle Peak dome and the areas S of White's Bottom ghaut. All ghauts [ephemeral watercourses] that originate on the flanks of the Soufriere Hills volcano are subject to flooding and should be avoided." Based on this advice, the government approved re-occupation of the areas immediately S of the Belham Valley River from which residents were evacuated on 23 August. All other residents from areas closer to the crater, evacuated since 21 August, were required to stay in the northern third of the island. Controlled entry restrictions were relaxed in most areas to allow residents to prepare for an approaching hurricane. Following passage of the hurricane, on 6 September the remaining evacuation orders were lifted.

Activity during 4-8 September was consistent at a low and generally declining level. At about 1530 on 8 September there was a significant steam explosion. Two hours later, at about 1730, two large ash eruptions produced a vertical plume that formed a mushroom cloud, which drifted to St. Peters (~30 km NNW) and to the N. Soufriere Hills continued to have intermittent swarms of earthquakes from the summit and nearby areas, including three events felt in Woodlands on 11 September. Occasional steam eruptions produced falls of fine ash in communities around the volcano, and morphological changes were continuing in the summit area. These developments suggested to volcanologists that magma was close to the surface under the volcano and that a magmatic eruption was still a possibility.

Two weeks later, on 25 September, a lava dome began growing in the W part of the moat near the linear chain of vents. An explosion between 1100 and 1200 on 27 September caused ashfall on the S part of the island, with minor ashfall also reported in the St. Georges area. Minor explosive activity continued through the end of September.

Information Contacts: Soufriere Hills Volcano Observatory, Plymouth; Seismic Research Unit, UWI; UNDHA; AP; Caribbean News Agency (CANA), Barbados.

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10/1995 (BGVN 20:10) Small ash explosions continue; three new vents form; September dome grows

The observatory was moved on 1 October from the Vue Pointe Hotel to Eifel House on Bishop View Road in Old Towne. A phreatic eruption that day deposited ash across a large area, including the capital city of Plymouth. This eruption was followed by a volcano-tectonic (VT) earthquake swarm, with 70 events located beneath the volcano at depths of 1-6 km. Two of the earthquakes, at 2257 and 2319, had magnitudes of ~2.5 and were felt at the observatory; several were felt in the Long Ground area. After about 0500 on 2 October, the number of located earthquakes dropped to ~5/day. Two episodes of low-amplitude broadband tremor recorded during 1-3 October were related to steam emission. Electronic tiltmeter and EDM observations during that time revealed no significant deformation.

EDM measurements at Tar River completed on 3-4 October continued to show a shortening trend, signaling minor inflation. Shallow VT (12 located events) and long-period (2 events) seismicity continued. Moderate levels of seismicity prevailed during 4-8 October, with 30-40 shallow (< 6 km depth) VT earthquakes each day, rare felt events (M 2-2.5), and a few long-period events. No deformation was detected by electronic tiltmeter.

An explosion around 2355 on 5 October caused heavy ashfall in Plymouth and in the SW part of the island. On 5 October the government announced that over the next two days they would evacuate Plymouth's home for elderly people and the hospital, sending residents to the N part of the island.

Two eruption signals were recorded at 0235 and 0347 on 8 October, and the EDM line at Tar River continued to show minor inflation. Seismicity began decreasing on 8-9 October, when 24 earthquakes were located beneath the volcano, with a few in the Centre Hills area. A small eruption at 1356 on 9 October generated light ashfall in Amersham and Upper Gages. Vent 2 was emitting a small amount of steam again during 7-9 October. Several episodes of broadband tremor may have been caused by increased steam emission. There were only 6 located earthquakes during 9-10 October, but several episodes of broadband tremor. Another minor eruption around 0012 on 10 October caused light ashfall in Plymouth. Visual helicopter inspection of the crater revealed significant steam emission and an increase in the size of the 25 September dome (20:9).

Formation of Vent 5 on 11 October. An ash eruption at 0021 on 11 October came from a new vent on the Tar River side of the Castle Peak dome, and damaged the EDM reflector at Tar River. A small earthquake swarm accompanied this vent formation. There were two more small ash eruptions later that day at 1540 and 1700. Although no significant changes to the dome were noted, steaming continued from its top; Vent 1 was also steaming, and appeared to be larger and deeper. Scientists noted that steam emissions from the crater had generally increased.

Three more ash eruptions occurred on 12 October, at 0901, 0955, and 1114. Continuous steam emission came from several areas in the crater and Vent 5. Two episodes of broadband tremor during 12-13 October were attributed to increased steam emission. Seismicity was low, with only 22 events during 11-13 October. No deformation was detected following this latest series of explosions.

Formation of Vent 6 on 14 October. An eruption at 0708 on 14 October created another vent on the NE flank of Castle Peak dome, generated a significant amount of ash, and ejected blocks as far as the edge of Long Ground, ~1 km E of the vent. A pilot reported that the plume may have reached ~2 km altitude. Another eruption at 1058 caused no reported ashfall. Two gas venting episodes at 2200 and 2345 on the 14th were associated with a small earthquake swarm and broadband tremor episodes. Vent 2 again emitted moderate amounts of steam, accompanied by a loud roaring sound, and Vent 5 continued to emit small amounts of steam. Seismicity decreased from 18 events on 13-14 October to five events accompanied by broadband tremor on 15-16 October.

Seismicity increased again on 16-17 October with 22 events clustered in two areas: one beneath the volcano and the other just E of Windy Hill. Steam-and-ash eruptions were recorded by the seismic network at 1757 and 2245 on 16 October, and at 1150 and 1522 on the 17th. There were also several episodes of broadband tremor and ~30 minutes of low-frequency harmonic tremor starting around 0414 on 17 October. Later that morning an aerial inspection of the crater showed no significant changes and little steaming. During a second flight at 1145, a large mudflow originating within the crater moat beyond Vent 2 was seen running rapidly down the Hot River and reaching the sea. This was probably the largest mudflow (in terms of volume of material) since the current activity began.

During 17-18 October there were 12 scattered earthquakes, several periods of broadband tremor, and some intermediate-frequency tremor. Ash eruptions were recorded at 1739 on the 17th and at 0530 on the 18th. The dome area continued to emit steam, but did not increase in size.

Formation of Vent 7 on 18 October. The 31 earthquakes during 18-19 October were clustered beneath the volcano. Several broadband tremor episodes and one period of low-frequency tremor were also detected. An eruption at 1621 on the 18th was associated with the formation of a new vent within the moat area of English's Crater, just SW of Vent 1. Another eruption was recorded at 2207 on the 18th. An explosive event around 1516 on 19 October generated a mudflow down the Hot River. During 19-20 October there were 28 earthquakes located; the events were scattered throughout S Montserrat, with some clustered beneath Soufriere Hills and St. Georges Hill.

There were 15 VT earthquakes on 20-21 October concentrated around the Long Ground/Soufriere Hills area. Several eruption episodes on 21 October resulted in ashfall that affected villages in the E. Ash fell at the airport for the first time, closing it briefly. No deformation was detected at the Tar River EDM or Long Ground tilt stations. Helicopter observations revealed that Vent 1 had extended E and was responsible for the previous ashfall. There was a small mud flow down the Tar River.

An average of 35 earthquakes/day occurred during 21-23 October. They were scattered throughout S Montserrat with some concentrations in the Long Ground-Tar River area and beneath the volcano. Some broadband tremor was also recorded. Visual observation of English's Crater both from helicopter and Tar River on 22 October revealed light steam emission from vents 2 and 5. When observed on the morning of 23 October, the September dome continued to steam, and was covered with sulfur deposits; it may also have grown since last observed on 20 October. Only one other small area SE of the dome was steaming. An eruption at 1337 on 23 October produced ash deposits within the summit crater and at Tar River. Steam emission increased after this eruption.

Seismicity decreased following this eruption to 10-14 events/day through 29 October, except for 22 events on the 27th. Locations were mainly beneath the volcano, although some were centered in the Windy Hill area and other parts of S Montserrat. An eruption at 1325 on 25 October caused ashfall in the Tar River area. Eruption signals were again recorded at 2314, 2321, and 2347 on 25 October, and at 0447 on the 26th; no ashfall was reported. Several episodes of low-amplitude broadband tremor were recorded during 25-26 October. EDM measurements at Tar River on 26 October indicated a continuation of the minor inflation observed during the past several weeks.

A steam-and-ash eruption at 1317 on 27 October from Vent 1 was followed by more than 30 minutes of low-frequency tremor. Eruption signals were recorded at 0855 and 2018 on 28 October, but no ashfall was reported. Steam emission from Vent 2 was observed that afternoon. Eruptions occurred again at 0326 and 0857 on the 29th, both followed by broadband tremor. An ash-and-steam plume was seen from the observatory following the 0857 event. Steam was seen coming from Vent 1 during a helicopter flight, but no major changes were noted.

Seismicity increased on 29-30 October to 55 events; most were clustered in a region just W of Windy Hill, with some scattered in the Centre Hills and Soufriere Hills areas. Eruption signals were recorded at 2110 on the 29th, and at 0244 and 1310 on the 30th. Two small long-period events were recorded after the first eruption. Ash from the first two of these eruptions was observed in English's Crater by helicopter. The third eruption, witnessed by scientists at the Tar River EDM site, produced a high column that caused ashfall over a wide area. This ashfall was the most significant since 21 August, and was accompanied by a density current of ash in the Gages valley. The morning of 31 October visual observations revealed a significant increase in Vent 1's size, but the 25 September dome appeared unchanged.

Seismicity decreased again the next day to 23 events, but they were located in clusters in the Tar River-Long Ground area and W of Windy Hill. There were also four long-period events and several episodes of broadband tremor. One eruption at 1118 on 31 October had no reported associated ashfall. EDM measurements at Tar River again showed a slight shortening, associated with continued slow inflation of the upper part of the volcanic edifice.

Only 14 seismic events were recorded during 31 October-1 November; most were located beneath the volcano with a few in the Windy Hill and Fox's Bay area. There were three long-period events and several episodes of broadband tremor. A small eruption at 1129 on 1 November caused ashfall within the summit crater.

Information Contacts: Montserrat Volcano Observatory (MVO), Olde Towne.

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12/1995 (BGVN 20:11/12) Dome building, minor ash eruptions

Although there was relative quiet during October (20:10), during the first 10 days of November three large phreatic eruptions occurred. Each of these eruptions blanketed Plymouth, 4.5 km W of the active vent, with ~2 mm of ash (table 2). Dome growth within the crater started on 16 November, the estimated date when juvenile material first reached the surface, and continued through at least December. Estimates of the dome's rate of growth from 16 November to 6 December were on the order of 0.5 m³/sec.

Table 2. Summary of the daily behavior of Soufriere Hills, 1 November through 11 December 1995. The table omits most geophysical and geodedic observations, however, "eruption signal" refers to seismically determined eruptions, and "mudflow signal" refers to seismically determined mudflows. Courtesy of MVO.

    DATE    EVENT & COMMENTS (local time)
    01 Nov  Ashfall (1129).
    02 Nov  Ashfall in Trails, Brodericks, and surrounding areas
            (0118). Explosions accompanied by light ashfall in
            Upper Gages and Chances Peak (1923).
    03 Nov  Mudflow (0254); Steam-and-ash emissions resulting in
            light ashfall in Parson's-Amersham and Plymouth
            (1122). Continued enlargement of Vent 1. Steam-and-
            ash emission (1122). No major changes in Castle Peak.
    04 Nov  Eruption signal (0247), no reported ashfall. Eruption
            signal; one eruption generated an ash plume reaching
            2.5-km high; several millimeters of ash fell in
            Amersham-Plymouth and S of Plymouth (1725).
    05 Nov  Eruption signal (0139), no reported ashfall. Mudflow
            toward Fort Ghaut (0214). Minor eruption without
            visible ash or steam (1307). Eruption signal (2030).
    06 Nov  Minor mudflow (0410). Increase in the size of Vent 1.
            Ashfall, light (0347) in crater area and steam plume,
            1.5-km high. Eruption signals (1044 & 1809), no
            ashfall.
    07 Nov  Eruption signal (0123), no ashfall. Ashfall (0815).
            Eruption signals (2018 & 2358).
    08 Nov  Eruption signal (0935).
    09 Nov  Ashfall, several millimeters accumulated in areas to
            the W and SW of the vent (Kinsale, Amersham,
            Plymouth, and Richmond) (0419).
    10 Nov  Eruption signals (0145, 0420, & 1348). Plume of ash
            and steam (1535), 1.5-km high, blown SW.
    11 Nov  Mudflows in Gages-Fort Ghaut areas (0548 & 0743).
            Eruption signal (0733), no ash emission.
    12 Nov  Eruption signal (0247), no ash emission. Steam
            emission from several new vents SW of main activity
            area. Old vent reopened S of Vent 1.
    13 Nov  Eruption signal (0600). Minor ash and steam (1603),
            blown N.
    14 Nov  Minor ash-and-steam emission (1610). Continued steam
            emissions from vents first observed on 12 November.
            Vent closest to Castle Peak greatly increased in
            size, surrounded by fresh ash.
    15 Nov  Minor ash-and-steam emission (0900-1000). Noise of
            breaking rocks, small landslides, venting heard from
            crater.
    16 Nov  Poor visibility but felt earthquakes, loud venting,
            rock-impact sounds, and light ashfall at Chances Peak
            (1500), with some drifting SW into the Broderick's
            area.
    17 Nov  Episodes of light ashfall in Amersham. Landslides had
            partially filled the Vent 1 crater. The September
            dome grew in height and extended toward Chance's
            Peak. Vigorous steaming at the two vents between
            Castle Peak and the dome.
    18 Nov  Occasional landslides at the edge of Vent 1.
    23 Nov  Noises heard from crater (rock breaking and small
            landslides). CO2 detected in the summit area for the
            first time.
    24 Nov  Noises heard from crater (as above).
    26 Nov  Confirmed emergence of a new spine adjacent to the
            September spine and close to Castle Peak.
    28 Nov  Sound of breaking rocks heard from crater.
    29 Nov  Sound of breaking rocks heard from crater.
    30 Nov  Confirmed lava dome within Vent 1.
    01 Dec  Dome slowly growing in Vent 1 crater; attendant ash
            emission and rock avalanches. A second area of dome
            growth identified NW of September spine. Two small
            ash clouds drifted towards Plymouth.
    05 Dec  Rapid increase in the size of and the number of
            cracks within the new (26 November) spine. Increased
            emission of steam and light ash of reddish color.
    06 Dec  Lava dome glowing, visible from the airport.
    07 Dec  Reddish ashfall (0929) accompanied a small explosion.
            Continued slow growth of lava dome.
    08 Dec  Lava dome had broken along cracks. Deformation
            continued in the area around the September and
            November domes. Ash cloud (1025).
    09 Dec  About 20 minutes of mudflow signal recorded at Gages
            seismic station (0434). Explosion with light ashfall
            (1419, 1520). Dome growth rate slowed.
    10 Dec  Mudflow signal recorded at Gages seismic station
            (2240).
    11 Dec  Rusty brown ash eruptions, ashfall W of crater (0910,
            1455, 1530, & 1604). No major dome growth detected.
            Steam emitted with variable intensity at a vent close
            to Castle Peak.

Small rockfalls from the flanks of the new, locally incandescent dome were witnessed on several occasions. During early December, debris from a larger rock avalanche was seen in the moat of English's Crater. As of early January, neither local avalanches nor material liberated during the failure of spines escaped the crater area. The limited mobility of the rock avalanches suggested they were not propelled by gas explosions with great overpressures. Although floods and dilute mudflows were distinguished seismically, no significant debris avalanches or pyroclastic flows occurred.

Heavy rainfall after 11 December may have triggered several small ash emissions, depositing red-brown ash on the upper W-flanks. The ash presumably consisted of non-juvenile material, from rock avalanches sloughing off the new dome, and some hot juvenile ejecta from small explosions vented in or around the new dome.

Although quantitative SO₂ flux measurements were lacking, as of early December related damage to vegetation extended ~3 km downwind and 1.5 km laterally. Tree damage was severe on the upper W flank. Gases sampled at three of the established fumaroles (soufrieres) around the volcano showed no change in composition. Although gas and acid aerosol production had been at enhanced levels from mid-November to early December, air sampled in Plymouth during early December contained very little SO₂.

Dome growth. Beginning on 30 November, good visibility allowed observers to watch a single dome develop from two smaller bodies (figure 6). One body was NW of the September cryptodome (an intrusion that produces a surficial bulging), and the other at Vent 1. The evolving dome had a rough blocky carapace that initially had some small (<5 m high) protruding spines. Two of these spines became taller in subsequent days; others failed and broke off the dome.

A prominent spine on the new dome's E side grew in height until 7 December when it began to collapse. The spine's maximum vertical growth rate was estimated to be 5-8 m/day. Further dome growth at a slower rate occurred until 9-10 December, and slower growth, or a possible halt, continued as late as 13 December. On 13 December a small, radial crack on the N side of the new dome emitted steam and ash for most of the day. At least two columns reached in excess of 500 m above the crater rim.

A new batch of extruded material reached the surface on 15 December. On the 17th, in addition to widespread incandescence radiating from the new dome, observers saw a new ~ 40-m-tall spine. Between the 17th and 20th the spine grew vertically at 7 m/day, and the adjacent dome also rose, but at a slightly slower rate. The spine's growth rate during some undisclosed intervals reached up to 20 m/day. On 17 December observers also saw a narrow crack in the dome within Vent 1 that emitted glowing ejecta. Many small ash releases sent columns up to ~1.1 km above the summit.

During the week ending 27 December, several spines grew 5-10 m/day then subsequently collapsed. One spine had grown to ~15 m higher than Castle Peak (summit elevation ~910 m) prior to failing late on 25 December.

Explosions on 21 December produced a mildly convecting ash cloud that rose ~1.5 km above the volcano. Ash fell to the N, reaching the N portion of the island. Although apparently phreatic events took place in early- to mid-November, this was the most vigorous explosion since then and it may have been driven magmatically. Steam production remained constant during 21-27 December, feeding a plume that sometimes carried small amounts of ash. From 28 December to 3 January there was relative quiet and slow dome growth. Only 3 m of dome growth took place during the week, and for a least a few days after about 1 January, the dome may have ceased growing.

Deformation. Data from two electronic tiltmeters showed no significant changes during the crisis. Despite their stability, around 10 November deformation in the upper part of the volcanic edifice was recorded by EDM and GPS measurements at Castle Peak Dome and Chances Peak. Four days of significant deformation were followed on 15 November by intense seismic activity (see below). These were followed on 17 and 18 November by an upward extension of the dome that formed in September. The dome also appeared to have extended slightly towards Chance's Peak. Although visibility was poor for the next 10 days, glimpses through steam and cloud cover suggested further doming and rock avalanching. These processes influenced a wide area on the NW side of Castle Peak Dome, including the edge of Vent 1.

From mid-November until about mid-December, the rate of deformation remained very low, with daily shortening on the order of a few millimeters along most lines, even those aimed at the presumably less stable upper flanks.

The EDM data for 10-12 December showed lengthening of the lines to Castle Peak—a deflation of the edifice. Around this time, a longer interval of GPS data also showed their lines had lengthened by >1 cm overall (with some shorter-term variability). This rate was equal to or greater than the average rate during the month of October. Late December deformation measurements using GPS and EDM techniques suggested either a return to slight inflation (14-20 December) or stability (21-27 December).

Seismicity. Montserrat seismic activity falls into four categories: 1) tremor, 2) long-period events, 3) volcano-tectonic earthquakes, and 4) regional earthquakes.

After 15 November, elevated seismicity prevailed with relatively few quiet periods. The pattern appeared very similar to that seen in late September associated with the formation of a cryptodome and possibly associated with the later extrusion of a spine. The elevated seismicity was inferred to be due to a high-level magmatic intrusion.

After 27 November there was a loss of discreet, locatable events. Low-amplitude tremor became intermixed with intervals of intense, low-amplitude, long-period events; these arrived at rates of up to 5/minute but were recorded only on the closest seismic station (MGAT, Upper Gages, figure 7). In early December tremor increased somewhat at other stations farther from the crater (MLGT, Long Ground, and MBCT, Bethel); at this time amplitudes of events at Gages also increased and the RSAM seismic index rose as high as it has been since 15 November.

Until 9 December there were also small, frequent, long-period earthquakes. These were accompanied by low-to-variable amplitude tremor at the Gages station, but tremor disappeared from all other stations by 8 December. The number of locatable earthquakes dropped to 1-2/day, the lowest observed during this crisis. Located earthquakes were mostly volcano-tectonic and at slightly greater depths (0-5 km) than the long-period and hybrid-type earthquakes that had dominated since 24 November. High-amplitude, high-frequency tremor was recorded at station MGAT for several hours during 10-11 December; this was probably due to an increase in steam venting from several areas on Castle Peak.

The dome grew during the week ending on 13 December, with few accompanying earthquakes early on 6 December. In contrast, during 14-20 September there were 2-20 locatable earthquakes/day, many with epicenters along the N flanks at depths of 0-6 km. During the week ending on 20 December all stations registered earthquakes with emergent onsets and a dominant frequency of 2.2 Hz; these took place 5-15 times/day. Some of the earthquakes corresponded to small explosions. Heavy rains on 16-19 December triggered floods and dilute mudflows who's acoustic signals were detected by the seismic network.

Information Contacts: MVO, Plymouth; Seismic Research Unit, UWI.

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01/1996 (BGVN 21:01) Dome growth continues

The dome continued to extrude in the breached summit crater. During January, subtle to dramatic variations occurred in the location, style, and rate of growth (with some areas undergoing up to 1-m vertical rise per day). Numerous spines grew, fell, and shattered. Besides obtaining the first samples of the new dome, fieldworkers established that the emplacement of the old dome (Castle Peak) was accompanied by one or more pyroclastic flows and lahars. The total seismic energy release for the last week of January was the highest since early December. Cumulative deformation measurements suggested inflation of the edifice.

Dome growth and visible observations. On 28-30 December 1995, the dome's E side grew 3 m upwards. Local avalanches accompanied this growth, but by 1 January both the growth and avalanches in this area temporarily slowed to a stop. Adjacent Castle Peak, on the new dome's S side where a small spine had formed on 26 December, volume increased without vertical growth in the week ending on 3 January. During that same week, significant steam escaped near the N part of the dome; the nearly continuous steam plume was sometimes charged with small amounts of ash.

Although dome growth appeared slow during part of the week ending on 10 January, it did not cease. Observers noted local avalanches (off both the dome's N and E sides) coupled with suggested swelling and new lava extrusions in the dome's central region. During this same week, the September spine appeared to move and tilt, and in the subsequent week the spine was pushed S by new dome growth. A new spine was identified on 10 January in the dome's center; this spine grew relatively rapidly until it fell down on 13 or 14 January. Other spines on the new dome also appeared to undergo a growth spurt.

Another spine appeared just after 14 January in the N dome area. Large parts of this spine had fallen by 18 January, possibly contributing to airborne ash seen on two occasions that day; the next day large blocks of the broken spine lay on the talus slope. Yet another spine appeared around 18 January along the S edge of the dome; it grew for two days prior to fall and breakup. Spalling material created a substantial talus pile in the S moat. On 20-21 January another spine grew in roughly the same spot. During the next two days this spine reached 25 m in height and 15 m in basal diameter prior to its partial collapse (an event correlated with significant ash emission on 23 January). Late in the week ending on 24 January, growth of the dome's S edge included growth of spines, spalling debris, slow swelling, and vertical growth.

Steam emissions were generally high during mid-January, and observers first saw new dome material piling up against the crater's W wall at the base of Chances Peak. The other side of the new dome extended a formidable distance up Castle Peak.

The most pronounced dome growth in the final week of January took the form of swelling on the dome's steeply sloping N margin. Although early in the week mass wasting repeatedly sent debris into the adjacent moat, later in the week this took place less frequently. On 26 January a spine was again noted in the dome's S area, but growth there on 28 January was manifested as swelling. Beginning on about 29 January on the N and S ends of the new dome observers saw two elongate ridges trending NE-SW. These appeared as rough mirror images of each other, their forms resembling whale backs.

Lofted ash and mass wasting. Airborne ash seen during January was mainly attributed to mass wasting. For example, a small amount of ash fell in Plymouth early on 4 January; the source was thought to have been a crater-confined rock avalanche off the dome. Minor ash fell in Plymouth four times on 12 January, and one time on both 15 and 16 January. Four ash emission events on 24 January were all associated with major rock-fall events on the lava dome. In some cases very minor ash emissions also issued directly from the dome and some of the material involved in mass wasting may have been dislodged by small explosions.

Visual observations into the crater have enabled good correlation between seismic signals and rock-fall events. During the week ending on 24 January, heat production from the dome appeared somewhat higher than in the past. During January dome incandescence was reported and some material within the rock falls was hot. Rockfalls and avalanches remained confined within the crater area, although the moat continued to gradually fill with debris.

Field studies. Good conditions on 8 January enabled the collection of a sample from the part of the new dome located within the 18 July vent, an area thought to have been extruded in late November. The crystal-rich sample contained dominant plagioclase, subordinate pyroxene and hornblende; parts of the sample were sent to four labs for further analysis.

Around the same time, other fieldwork in flanking drainages (Hot River and Fort Ghaut) found new exposures and established that several charcoal-bearing pyroclastic units (including at least one pyroclastic flow unit) were erupted during the growth of Castle Peak dome. These were also found adjacent to deposits having the character of lahars.

Deformation. EDM lines composed a network consisting of four surveyed triangles around the volcano. The lines continued to be measured routinely. Dry-tilt sites at Amersham and Brodericks on the volcano's W side were re-occupied during the early part of the week ending 10 January; neither showed any change since their last occupation in October. During January, the Spring Hill tiltmeter failed and was moved to a new site, but the Long Ground tiltmeter continued to indicate angular stability.

Looked at in the short term, EDM measurements taken during the first half of January did not show any changes in slant distance (above the error of the method); however, during the week ending 17 January it was reported that a slow shortening had occurred on many of the lines towards the volcano. The shortening indicated swelling of the edifice.

In the week ending on 24 January, it was reported that slant line distance in the NE sector (Tar River to Castle Peak area) underwent a 2.5-cm shortening over the period of a month. During the shorter interval of the final week of January, no changes above the error of the method were detected in slant distance measurements on two deformation triangles in the volcano's S to SW and NE sectors (the Galways-Chance's Peak-O Garra's and Long Ground-White's Yard-Castle Peak triangles).

Seismicity. During January, broadband tremor commonly registered on the Gages station. Tremor was generally absent at the other stations, although the Long Ground station also registered some tremor in mid-January. The number of daily earthquakes typically measured in the thousands (eg. 5,000 to 6,000 events at the closest station on 27 January), too numerous to count on a real-time basis. Instead, MVO often quantified seismicity for rapid dissemination by using located events. These are events for which a hypocenter (the earthquake focus, the point at which the first motion originates) was calculated based on one S-wave and the records from four stations.

An MVO report on 3 January stated that long-period events recorded at most seismic stations had been occurring at a rate of 10-15/day. The hypocenters for these events could not be found but they were thought to be at very shallow depths in the crater area. Later reports in January did not quantify the rate of occurrence for long-period events.

Late on 5 January, broadband tremor picked up slightly in amplitude at Gages station. Then, small long-period events occurred for about the next 12 hours. This was followed by an 8-hour swarm of >300 hybrid events with virtually identical waveforms <3 km beneath the volcano. Lower amplitude, regular hybrid events occurred every 1-2 minutes until 8 January. A smaller series of similar hybrid events took place on 12-15 January. Some initially small hybrid events that first appeared on 23 January grew in amplitude and rate of occurrence (to 6-7/minute) and continued until at least 31 January. During the last week of January these hybrid events formed the dominant seismic activity. repeated shallow hybrid events in early January within the crater preceded new dome growth by a few days. This had happened on at least two previous occassions.

During the first week of January, shallow (0-7 km depth) volcano-tectonic earthquakes with epicenters scattered around the volcano continued at a rate of 2-3/day. An exception was 1 January, when a cluster of 17 volcano-tectonic earthquakes took place just N of the active crater at 1-3 km depths. They occurred in an eight-hour period following an M 5.0 earthquake that struck at a depth of 25 km, centered ~55 km N of Port of Spain, Trinidad. This larger earthquake may have been the trigger for the 1 January seismicity.

Three small, 1-3 km deep, volcano-tectonic earthquakes struck SW of the island during mid-January. Very occasional, small, long-period earthquakes started to appear on the evening of 28 January and a solitary volcano-tectonic earthquake took place early on the 29th. This M 2-2.5 earthquake was located beneath the crater area at a depth of 2.8 km; it was felt by Long Ground area residents.

Crisis management. The eruption driving the current crisis began on 18 July 1995 (BGVN 20:06). According to the Montserrat Government Information Unit (on 13 February 1996), during the crisis there has been no official off-island evacuation. However, a phased relocation of 6,000 residents from the southern half of the island to the northern half immediately followed a large phreatic eruption on the morning of 21 August 1995. Ash from that eruption's cloud, and from a density current that flowed down the flanks of the volcano, caused darkness in the capital (Plymouth) and surrounding areas, and ultimately deposited several millimeters of ash there. The relocation order was partially lifted on 3 September, a day before the passage of Hurricane Luis.

A change in eruptive style in mid-November ultimately lead to the extrusion of lava at the surface. On 1 December 1995 a second relocation of 4,000 residents took place. The relocation lasted a month for residents on the island's SW side and about a month-and-a-half for those on the island's SE side. Some preparatory steps for future emergencies included the continued relocation of Glendon Hospital and newly acquired school buses to move residents.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat.

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02/1996 (BGVN 21:02) Increasingly rapid dome growth

As reported in Montserrat Volcano Observatory (MVO) Scientific Reports, during February the growing dome became higher than Castle Peak and was visible on the volcano's W margins. Based on qualitative estimates, during the third week of February and early March the dome's growth probably reached the highest rates seen since extrusion began around 16 November 1995 (BGVN 20:11/12).

Dome growth and visible observations. As the dome enlarged, the focus of its growth migrated. During 1-7 February the dome's N side grew upward, and the S side grew outward. The dome's N side first became visible from the volcano's W margins beginning on 31 January. Under clear conditions on 2 February it was confirmed that this side of the dome had grown higher than Castle Peak. On 4 February this side of the dome reached a height equal to adjacent parts of the crater wall; talus from the dome's N side filled the adjacent moat and began piling up against the crater wall.

Although low cloud cover generally hampered visibility during the week of 8-14 February, observations around 9 February indicated slowed growth on the N and S coupled with a shift in the focus of activity to the dome's W side. On 11 February a spine was seen in the dome's central sector; its height then was equal to the dome's N side. That day, talus made contact with the crater wall around much of the dome. On 12 February a late morning helicopter flight allowed observers to see a small pyroclastic flow created as debris from the central dome avalanched S. Later in the week, growth took place on the dome's SE side and, in the form of two new protrusions, on the dome's W side.

A second consecutive week of low cloud cover occurred, 15-21 February, and by the end of this interval it was learned that the dome's SE side included a large whaleback-shaped lobe. This new lobe grew to reach the size of the southern whaleback, a lobe emplaced around 19 January (figure 8). The new lobe (not shown on figure 8) was the source of comparatively few rockfalls, and therefore was considered to be relatively massive and coherent. In contrast, frequent rockfalls fell down off the dome's central region and NW side and by the end of the week this area became the focus of growth. The moat's W margin, at the base of Gage's wall (figure 8), received considerable debris. Previously, this area had been the last part of the moat's W margin without appreciable debris.

During the week of 22-28 February, the dome growth rate, which was estimated qualitatively, may have been the highest since extrusion began. High steam and gas fluxes also prevailed. Although the resulting plumes thwarted aerial photo-documentation, the dome grew in both vertical and horizontal directions. Semi-continuous rockfalls from specific zones indicated growth in a pattern similar to the previous week. Specifically, most 22-28 February rockfalls came from the dome's central region, as well as its NW, and to a lesser extent, SE sides. A gas sampling visit on 27 February revealed extensive gas escaping from areas on and surrounding the dome, but the primary vent identified was still the 18 July one (see map, BGVN 20:11/12). At this vent escaping gases were 720°C and red-hot rock was seen ~2 m below the surface.

During the week of 29 February-6 March rockfalls from the new dome were abundant, especially from the SW and NW sides; qualitative estimates suggested the highest rate of growth yet seen. Similar to the previous several months, during February and March ash clouds were produced by rock avalanches. A large avalanche on 1 February detached from the dome's S side resulting in a small convective cloud that deposited fine-grained ash on Chances Peak.

Higher than normal amounts of acidic aerosols were noted in the upper Gages valley and through the first 3 weeks of February. During the last week of February, however, the plume rose higher so there was typically less volcanic fog near the ground. During the first week the largest steam emissions came from the dome's top central region. As a result, brown acid burns on vegetation reached as far as Plymouth and Richmond Hill (~5 km W) and some residents suffered irritations.

Rain water sampled during 1-8 February in the Gages valley had a pH of 2.5 and contained sulfates, <3 mg/l; fluorides, 1.5 mg/l; and chlorides, 106 mg/l. The pH has ranged from 2.5 to 3.5 in weekly rainwater tests made beneath the plume on the volcano's W flanks (Upper Amersham). In contrast, the local source springs used for drinking water, also on the W flank, had consistently shown little or no geochemical perturbation. During February it was reported that gases from both the dome and three fumaroles (soufrieres) around the volcano appeared to have changed little during the course of the increased activity.

Samples of the new dome collected in January were crystal-rich andesites containing hornblende and two pyroxenes. This rock appeared very similar to those forming the old Castle Peak dome. Tephra from small eruptions on 13 and 21 December were of dacitic composition and appeared to be phreatomagmatic in origin, containing accretionary lapilli and a definite juvenile component.

Deformation. Surveys during the first week of February established that the volcano's NW radial direction (the Tar River-Castle Peak EDM line) underwent a shortening of 1.5 cm since its previous measurement on 21 January and a shortening of 2.5 cm since 23 December 1995. A survey on 13 February established that the SW radial direction (the O'Garra-Chances Peak line segment) shortened by 1.4 cm since last measured on 1 February. Surveys during the third week of February established that in the NW radial direction there had been a shortening rate of about 1 mm/day, a rate that appears to have been pretty steady since dome extrusion began. Surveys during the fourth week of February indicated continued deformation. The Long Ground tiltmeter continued to remained stable as it has for several months.

Results obtained on 27-28 February suggested that neither the Castle Peak nor Gages Wall reflectors showed any greater movement than the reflectors farther from the area of dome extrusion. This was taken to indicate a lack of local deformation at these two sites on the edifice.

Seismicity. During the first week of February, tremor was rare. The chief exceptions were a 4-hour interval of low-amplitude tremor and an 18-hour interval of low- to moderate-amplitude tremor. Throughout much of February, and particularly between the 8th and 14th, intermittent episodes of low- to moderate-amplitude tremor were recorded. Increased tremor amplitude was seen on 17, 19, and 20 February; another episode that started on the 25th lasted ~12 hours.

Small (M 0.0-0.5) hybrid events fluctuated in amplitude but occurred often during February. In a particularly intense episode between 23 January and 6 February, they took place 5-6 times/minute. These hybrid events generally took place less frequently, particularly in late February.

Early in February, long-period earthquakes of M <=2.5 were located. During the most intense interval they took place at a rate of 34/day. Late in February, instrumental locations were obtained for many of the larger (M 1.0-1.8) long-period earthquakes. They all occurred <=3 km beneath the volcano. In addition to thedaily seismic events diagnostic of rockfalls, on 7 February a 10-minute-long signal was received exclusively at Gages station. This signal was probably caused by a mudflow down a nearby drainage (Gages Ghaut).

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat.

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03/1996 (BGVN 21:03) Escalating dome growth spawns pyroclastic flows and another evacuation

During March ash plumes continued to blow over the Capital and environs, and the rate of dome extrusion escalated. Later, on 3 April, explosions at the dome and pyroclastic flows down the Tar River prompted an evacuation of the southern part of the island.

Seismicity during March from both rockfalls and deeper sources continued in a manner consistent with dome growth. Tremor was repeatedly recorded at Gages station. Although there were exceptions, deformation mainly continued as a shortening of line lengths equivalent to ~1 mm/day (similar to trends seen since mid-November). The chief exception was on the W flank (Amersham to Chances Steps line), which on March 11 showed a surprising 3 cm lengthening since last measured on February 19. This is a reversal of the shortening that occurred from October to late December on this line.

Numerous rockfalls and avalanches from the dome in early March chiefly appeared on the dome's SW and NW sides. Next, they were repeatedly seen on the NW but some also started in the dome's central area (week 2). Rockfalls then shifted from the dome's NW margin to the E margin (week 3). Later rockfalls descended the NW, W, and E margins (week 4).

Rapidly growing spines continued to be common during much of March. They were noted on the dome's SW (weeks 1 and 2) and NW (week 4). On the NW, one spine achieved the greatest absolute height of any yet seen. It extruded rapidly, rising 10 m over an interval of about one day on 26-27 March. Over a 24-hour interval beginning at 1600 on 21 March, another spine's vertical growth measured ~7 m.

The dome's topography was mapped during week 2 from Farrell's lookout (on the WNW). The resulting map allowed workers to estimate the dome's mid-March volume as ~6.7 x 10⁶ m³, a value comparable to previous, cruder estimates made in the field. It appeared that the dome's growth rate increased 7- to 10-fold in the last few months. Specifically, the late-November and December rate was ~0.2-0.3 m³/second whereas the March rate was closer to 2 m³/sec. On 3 and 12 March the growing dome's summit elevations were 845 and 875 m, a 30 m rise in ~9 days. Later, on 20 March, a visit to Gages Wall revealed that, even though this sector had few rockfalls around the time of the visit, the dome's talus apron had grown to within ~15 m of the wall's top.

During week 2, fine ash carried from some larger rock falls was deposited on the upper W flanks. On 17 March, viewers on Farrell's lookout were enveloped in a warm ash cloud following a rockfall that occurred without a noticeable explosive component. That same day an explosion may have helped drive an ash column to 2,300 m.

Other relatively large ash clouds appeared repeatedly during late March and early April. On 27 March there were ash clouds generated at 0642, 0700, 0848, and 1725. The 0642 event produced an ash column that reached a height of 2,000-2,300 m and blew W blanketing areas in vicinity of the Capital. The 0642 event accompanied a seismic signal comprised of seven pulses in a 14-minute interval; the 0700 event generated a smaller ash column accompanied by three seismic pulses. Except for these intervals of unusual seismicity and frequent signals from large rockfalls, seismicity during the 24-hour interval prior to the 27 March events had been generally quiet. Helicopter observations shortly after the 0700 event disclosed that ash had been channeled to the E down a drainage called the Hot River Ghaut. Hot ash had traveled for ~1 km from the dome, igniting dead trees along its path. Observers witnessed the 0848 event, but it was much smaller and areally restricted.

Several other plumes on 31 March led to a nearly one-hour interval late that day when unusually intense seismicity registered at all the stations. The seismicity was correlated with ash plumes that blew W. On 1 April a helicopter flight confirmed the largest block-and-ash flows yet seen. Although runout distances were similar to those seen on 27 March (on the order of 1 km from the base of the Castle Peak dome), those on 1 April entrained bigger blocks and had a more widely dispersed dilute component that burned a broader swath of trees and foliage around the Tar River Soufriere (~1 km NE of Castle Peak's summit).

Until a small explosive event at 0652 on 3 April, the majority of the airborne ash was thought to have come from rockfalls and avalanches off the dome. This explosion, and several other significant ones the same day, discharged from a fissure on the dome's E flank, a spot that also appeared as the source of recent rockfalls. At various times on 3 April, continuous ash emissions came from the crater area. The activity continued to build during the day, with many small explosive seismic signals and continuous tremor recorded at the closest seismic station on Chances Peak.

At 1518, a pyroclastic flow occurred in the Tar River area. It traveled ~1.9 km down this drainage and burned vegetation and set fire to sulfur at the Tar River Soufriere. It also extended 1.9 km down the Hot River Valley (to where the road crosses the river), stopping ~400 m upslope of the Tar River Estate house. Although no inhabited areas were affected by the pyroclastic flow, the settlement of Long Ground lies ~2 km NE of Castle Peak's summit. The flow generated an ash plume that rose to ~6,700 m. Much of the ash blew N in light and variable winds. Other pyroclastic flows occurred at 1808 and 1818. These events, some of which were captured on NASA GOES satellite images, prompted scientists to note the possibility of further explosive eruptions during the next few days and to urge residents to move to the island's N end. The 3 April evacuation continued through at least 30 April.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/); NOAA/NESDIS Synoptic Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

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04/1996 (BGVN 21:04) Significant explosions and pyroclastic flows; vigorous dome growth

Volcanic activity in the summit crater was very high during early April, but explosions decreased in the second half of the month. Dome growth, most conspicuously in the form of spines, remained vigorous. Activity late in the month was dominated by small to moderate-sized rockfalls with associated ash clouds. Steam production was almost continuous, along with SO₂ emission, throughout the month. Episodes of low-amplitude broadband tremor, usually <1 hour duration, were also recorded, but there were few long-period or shallow volcano-tectonic earthquakes. No major deformation events were detected.

Eruptive activity on 3 April began at 0652 with a small explosion (BGVN 21:03). Near-continuous seismic activity afterwards was a result of more small explosions and ash emission from the dome. After a reassessment of the situation by the Montserrat Volcano Observatory at 1300, the civil authorities began an evacuation of the S part of the island. At 1518, an eruption generated a significant pyroclastic flow in the Tar River valley area and an ash plume that rose to ~6 km altitude. Further pyroclastic flows in the same area were generated at 1808 and 1818. These pyroclastic flows slightly overtopped the N embankment of the Tar River valley but caused no destruction to property in Long Ground, ~2 km NE of the dome. Fires started by the pyroclastic flows continued for several days in the Tar River area.

Several smaller explosions and rockfalls during 4-5 April generated clouds that deposited ash in Plymouth and environs. The most significant of these was a moderately strong explosive eruption at about 1253 on 5 April that produced a column to ~1,500 m altitude and a small pyroclastic flow into the Tar River valley. A series of eruptions starting at 0839 on 6 April generated ash plumes up to ~3 km high and sent at least six small pyroclastic flows into the Tar River area. After 1337 the activity level increased again, with continuous ash emission and several ash plumes. At 1445, a significant explosive eruption began and continued for about an hour. It consisted of two main pulses that sent ash to ~9 km altitude and generated a relatively large pyroclastic flow. Several small-to-moderate eruptions produced ash columns and possibly small pyroclastic flows in the Tar River valley again that afternoon.

A new spine observed close to the center of the dome on the morning of 4 April was ~828 m above sea level at mid-morning the next day. By 6 April it had grown to ~906 m elevation and was visible from many points around the island; by 7 April the spine was taller than Chance's Peak (the highest topographic feature on Montserrat at 915 m). A moderate explosion at 0659 on 7 April was heard at the Bramble Airport ~6 km NE of English's Crater and fed an eruptive column that deposited ash to the NW. During the night of 7 April the top half of the spine broke off but the remnant continued to grow from the base throughout 8 April so that once again it became higher than Chance's Peak; this spine was the largest seen so far. On 8 April there was another series of eruptions, including two large explosions at 1354 and 1357. During this period, near-continuous pyroclastic flows moved into the Tar River valley, and several large ash clouds drifted out to sea. The pyroclastic flows did not reach as far as those on 3 April, but some trees in the Tar River valley were set on fire.

Activity in the crater area during 11-17 April was dominated by rockfalls and explosions creating small ash clouds. The spine that began rising on 5 April collapsed on 12 April towards the SW. A pyroclastic flow from this event was observed at 1559 on 12 April, but remained confined to the upper part of the Tar River valley away from inhabited areas. A twenty-five minute period of explosions and rockfalls began at 2037 on 13 April. On 15 April a new spine was growing to the E of the remnant of the last spine. Break-up of this feature and further break-up of the remnant spine occurred on 17 April.

Seismicity in early April was dominated by rockfalls, but beginning on 7 April hybrid earthquakes centered beneath English's Crater at shallow depths (<2 km) increased in frequency. These events occurred at rates varying from a minimum of 1-2 every 5 minutes (12-24/hour) to a maximum of ~5/minute (300/hour). This intense hybrid type of seismicity, thought to result from dome growth, continued through 17 April. RSAM data showed a steady increase in energy release up to the evening of 15 April when it dropped to low levels. By 17 April the hybrid events were occurring every 2 minutes (30/hour).

A new spine, which had grown over a period of no more than 36 hours, was seen on 18 April. The top of the spine was measured at ~911 m elevation, 30 m above the top of the dome. A smaller spine was observed on the morning of 19 April, with a height of ~20 m. The large spine appeared to fracture on 20 April and the debris fell to the base of the NE part of the old dome. Another small spine was seen in the same location on 24 April. Rockfalls were observed throughout the week, with the largest ones producing significant ash clouds at 1237 on 18 April, 1511 on 21 April, and 0635 on 22 April. The 21 April event generated an ash cloud to 1,700-2,000 m above sea level and sent a small pyroclastic flow ~300 m down the Tar River valley, producing an ash cloud to ~1,300 m altitude.

The number of hybrid earthquakes quadrupled on 18 April, to ~2 events/minute (120/hour). Seismicity then declined gradually back to ~30/hour by 24 April. The longest period of broadband tremor was 8 hours, between 1700 on 23 April and 0100 on 24 April. Volcano-tectonic earthquakes were recorded on 20, 22, and 23 April. The first two were located N of the crater, beneath Farrells Mountain at 0.25 and 4 km depth. During the last week of April, the small repetitive hybrid earthquakes occurred every 2-3 minutes (20-30/hour) but with reduced amplitude. A few volcano-tectonic earthquakes were located, one at a depth of 2 km SE of the South Soufriere Hills. Several very small earthquakes were recorded by the Gages seismic station during this period. Similar swarms have been identified in records from that station, especially during July and August 1995.

Throughout April, measurements to the EDM reflector on the upper flank of Castle Peak dome from both Long Ground and White's Yard continued to show the slow shortening trend of ~1 mm/day observed since late November 1995. The reflector on Gage's Wall was obscured by ash. Occupation of the Dagenham-Amersham-Upper Amersham-Chance's Steps EDM network showed that the very small changes (on the order of 0.3 mm/day) are continuing from December 1995. Two GPS base networks were established in late April. The first is a relatively large-scale network with line lengths of ~7 km. The second is a denser network of 18 stations on the flanks, with an average inter-station spacing of 2 km. This covers most of the volcano, except for the SE sector. No changes have yet been detected above the 1-cm precision of the technique.

Accurate angular measurements of features on the dome have been combined with measurements made from photographs to build a topographic model. This has been compared with a digital terrain model of the old English's Crater and gives a dome volume on 18 April of 9.5 ± 0.5 x 10⁶ m³. This volume gives a mean extrusion rate of ~70,000 m³/day since 30 November 1995.

There has been uncertainty as to whether or not some of the larger ash columns were generated by explosions. The recent ash deposits are uniformly fine-grained, with no clasts above ash-size getting outside the crater. This is inconsistent with an explosive model, where larger ballistic clasts and deposition of lapilli might be expected. A video of one of the smaller pyroclastic flows showed a sizeable thermally convective column being generated when the flow hit the crater wall. Thus the evidence so far indicates that the ash columns are generated from the pyroclastic flows and rockfalls and not from explosions.

Soufriere Hills volcano sits on the N flank of the older South Soufriere Hills volcano, located at the S end of Montserrat Island (13 x 8 km). The summit area consists primarily of a series of ESE-trending lava domes. Block-and-ash flow and surge units associated with dome growth predominate in flank deposits. Pyroclastic-flow deposits associated with the formation of English's Crater have been dated at around 19,000 years BP (before present). A series of eruptions dated at 16,000-24,000 years BP pre-dates the Castle Peak dome in the crater by an unknown period of time. English's Crater is breached to the E. Periods of increased seismicity below Soufriere Hills were reported in 1897-98, 1933-37, and again in 1966-67. There were no reported historical eruptions, but some deposits and features have a young appearance. A radiocarbon date of ~320 ± 54 years BP from a NE-flank pyroclastic-flow deposit is significantly younger than other radiocarbon dates from the volcano, and could have resulted from the latest activity of Castle Peak.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/).

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05/1996 (BGVN 21:05) Dome growth and evacuation continue in May

During May the dome's growth continued, accompanied by small intermittent pyroclastic flows and minor ashfalls that were mostly thought to be generated by rockfalls. Although activity during the first week of May appeared similar to the final week of April, visibility became poor after 5 May. When visible, the dome's new growth was manifested in rapid increases of summit elevation (on 19 April, 865 m; on 30 April, 896 m; on 2 May, 898 m; on 3 May, 909 m). This was followed by an apparent 2-m decrease (i.e. on 4 May, 907 m). Many rockfalls took place on the dome's NE and E flanks. Throughout early May small ash clouds repeatedly blew W depositing very small amounts of ash in the Upper Gages and Amersham areas.

Activity was characterized as slightly less elevated during the second week of May. However, visual observations on 11 May indicated that a small pyroclastic flow had travelled 300 m E of the base of old Castle Peak dome (into the Upper Tar River Valley passing just S of the path of the 3 April pyroclastic flows). Although this flow had set fire to some trees, no significant changes were observed, and small ash clouds again blew W depositing minor ash in the Upper Gages, Amersham, and Fort Barrington areas.

On 12 May the dome area discharged abnormally large ash clouds associated with at least three pyroclastic flows E of the crater down the Tar River. Relatively large ashfalls also took place in the WNW-NW sector at least as far as the coastal area (Fox's Bay). In some places the ashfall reached a maximum thickness of 3 mm. These ashfalls were reported in parts of southern and central Montserrat (including the settlements of Farrell's, Rileys, Windy Hill, Gages, Lees, St. George's Hill, Fox's Bay, Richmond Hill, Garibaldi Hill, Ile Bay, Old Towne, and Salem). Areas affected also included some settlements in the designated safe zone in the N part of the 13-km-long island (including Cork Hill, Weekes', Olveston, and Barzey's) and small amounts of ash fell in the volcano's E sector (Tar River, Long Ground, and Whites).

The 12 May episode began at about 0630 when near-continuous rockfalls took place on the dome's E flank lasting until about 0720. From 0720 to 0945 the rockfalls became intermittent and small but they still produced ash clouds. A further increase in activity produced pyroclastic flows that were seen in the Tar River Valley at around 0945, 0952, 1105 and 1153. The ones at 0945 and 1105 advanced more than 30 m over the sea; the one at 1153 stopped just short of the sea. Activity declined after about 1220 but small-to-moderate rockfalls continued intermittently.

The 12 May pyroclastic flows did not damage any structures but trees were set ablaze in the Tar River Valley area. Excellent views were obtained of the pyroclastic flows.

On 13 May, light ashfalls blew across the volcano's W and SW sectors. On 15 May small ash clouds again blew W; views then suggested that most of the rockfalls producing the ash came from the NE flank of the dome. In addition, on 15 May moderate amounts of steam escaped from the base of the dome's N side; at other times during the second week of May steam mainly escaped from the SW moat.

Rockfalls were especially abundant on 16 and 22 May. In addition, one on 19 May generated an ash plume that reportedly reached an altitude of about 1.2 km. Another on 20 May was associated with a small pyroclastic flow that travelled ~2 km NE of Chances Peak down the Upper Tar River Valley (as far as Hermitage).

Visibility was generally poor for most of the third week of May allowing only brief views into the crater to establish the dome's main areas of growth on the N and NE flanks. When visibility improved on 20 May, nine days after the previous observation on 11 May, the dome contained several smaller spines and a large broad spine at the top. The large spine rose ~20 m and leaned slightly NE. Observers saw no morphological clues for the source of the 12 May pyroclastic flows, possibly because any topographic signs may have been erased by mass wasting during the intervening week. During brief observations from a helicopter, rockfalls mainly cascaded down the dome's N and NE flanks; fewer came down the vigorously steaming SE flank. Very poor visibility returned on 21 and 22 May.

During the week ending on 29 May, visibility gradually improved allowing remote measurement of 200-250°C dome surface temperatures. Observers on 24 May saw at least three spines on top of the dome (none more than 15 m high) and vigorous steaming from both the NW moat and several areas of the dome. A mudflow that descended the Upper Tar River Valley had apparently formed due to heavy rainfall on the previous night (23-24 May). Also noted was a clear scar on the dome's lower NE flank. About a meter deep and perhaps 5- to 10-m wide, the scar provided a path for ongoing rockfalls.

Observations on 26 May indicated dome growth focused on the dome's E, NE, S, and W parts. Also during the week ending on 29 May, the absence of strong wind allowed the development of near vertical ash plumes, some of which ascended up to 2-km altitude. On 29 May observers saw several small pyroclastic flows that started near the upper dome and flowed E down the Tar River Valley, stopping no farther than the Tar River Soufriere.

Seismicity during May is summarized in table 3. Intense hybrid seismicity took place on 2-3 May; otherwise seismic activity for late April through May was dominated by near-continuous broadband tremor, in some cases lasting up to several days. Tremor duration remained qualitative because it was saved on analog recorders; the gains and filters on these recorders were periodically changed in order to look at other types of seismicity, leaving no consistent record for quantitative analysis. In addition to tremor, rockfall signals were also common.

Table 3. Seismic data from Soufriere Hills, May 1996. Courtesy of MVO.

    Date     Volcano-    Long-     Hybrid    Rock-     Amount of
             tectonic    period              fall      tremor

    02 May      0          32         52       46      Intermediate
    03 May      1           2        345       50      Intermediate to high
    04 May      0           5         11       27      Intermediate
    05 May      0          11          1       67      Intermediate to high
    06 May      0           2          6       55      Intermediate
    07 May      0           7          5       50      Low
    08 May      0          21          5       64      Low
    09 May      0          21          0       73      Low
    10 May      1          16          0       97      Low
    11 May      1           4          0       62      Low
    12 May      0           6          0      109      Low
    13 May      0          15          0      127      None
    14 May      0          18          0      147      None
    15 May      2          50         67      103      None
    16 May      0           2         12       80      Low to intermediate
    17 May      0           4          8       33      Low to intermediate
    18 May      1          12          2       25      Low
    19 May      1           9         13       34      Low to intermediate
    20 May      0           7          8       43      Intermediate
    21 May      0           4          0       32      Intermediate to high
    22 May      0           7          0       60      Intermediate to high
    23 May      0          12          0       64      Intermediate to high
    24 May      0          19          0       50      Low
    25 May      0          17          1      104      Low
    26 May      0          12          8      114      Intermediate
    27 May      1          13          5       85      Intermediate
    28 May      1          13          4       86      Intermediate to high
    29 May      0          12          3       83      Low to intermediate
    30 May      1           5          0       17      Low to intermediate
    31 May      1          14         96       97      Intermediate to high

Some of the deformation measurements made during May were taken on the E and S triangles on 26 May. The line lengths on the southern triangle had shortened by 8 to 9 mm since 21 April, while the eastern triangle had shortened by ~1 cm since 20 May. These data obtained by the EDM technique were consistent with recent GPS measurements conducted by the Alan Smith and colleagues from the University of Puerto Rico.

The bulk of the SO₂ flux measurements were made with a car-mounted COSPEC driven under the plume (between Cork Hill and St. Patrick's) at ~20 km/hr (table 4). Wind speeds were measured with a hand-held annemometer before and after each day's runs at Windy Hill (3.4 km N of Chances Peak), the windiest spot accessible by road. Typical SO₂ fluxes were in the range of 25-205 metric tons/day (t/d). An exception was the 13 May measurement of 357 t/d.

Table 4. Correlation spectrometer (COSPEC) SO₂ flux measurements at Soufriere Hills, 28 April-22 May 1996. Courtesy of MVO.

    Date       Number of       Mean (t/d)    Sigma
               measurements

    28 April        4             26           5
    29 April        3             86          10
    01 May          5             97          29
    02 May          3            177          29
    03 May          5             89          11
    04 May          5             76          17
    05 May          3             54          10
    09 May          4            138          11
    10 May          5            123          46
    11 May          4             96          30
    13 May          3            357         119
    17 May          5            130          29
    18 May          5            129          39
    19 May          5            203          54
    20 May          4            164          31
    21 May          5            205          56
    22 May         n/a           130          n/a

Resettlement. Since 3 April shelters have housed 1,381 residents. About another 3,000 people rented or shared accommodations in the homes of friends and relatives. The W. H. Bramble airport remained open. Pre-fabricated buildings were erected and church and school buildings were converted to temporary shelters; in addition, the government prepared an ancillary hospital and a power station in the safe area; it made road repairs, upgraded fuel storage, relocated livestock on farms, and established programs for sport, culture, counselling, and guidance.

As of 24 April no plan for mass off-island evacuation for the island's 10,000 inhabitants had been implemented; instead the British and CARICOM governments favored voluntary evacuation. Some residents could remain on Montserrat at the N end of the island, in the area considered comparatively safe by Wadge and Isaacs (1988) and by scientists at MVO. Participants who go to the U.K. could be eligible for employment, income support, housing, and the enrollment of children in British schools for two years.

Reference. Wadge, G., and Isaacs, M.C., 1988, Mapping the volcanic hazards from Soufriere Hills Volcano, Montserrat, West Indies using an image processor: Journal of the Geological Society of London, v. 145, no. 4, p. 541-551.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/); Alan L. Smith, Univ. Puerto Rico, Dept. of Geology, Mayaguez, PR 00680 USA.

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06/1996 (BGVN 21:06) Dome growth continues

The current eruption, which began on 18 July 1995 (BGVN 20:06), started extruding a lava dome on about 16 November 1995 (BGVN 20:11/12). What follows condenses Montserrat Volcano Observatory (MVO) Scientific Reports for the weeks ending 5 and 12 June and Daily Reports for the rest of June.

On 31 May, and 1 and 15 June, pyroclastic flows progressed several kilometers E; one to within 300 m of the sea. Associated plumes reached up to 3 km altitude. Persistent dome growth continued in June. Its talus filled the W moat, allowing rockfalls to begin escaping the crater, but none reached farther than 100 m beyond the rim. Summaries are included for visual observations, seismic observations, daily seismic event counts, and SO₂ flux (tables 5, 6, 7, and 8).

Table 5. Chronology of visual observations at Soufriere Hills, Montserrat, late May through early June 1996. Poor weather conditions often prevented observations. Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

    VISUAL OBSERVATIONS
    Date                Observations

    30 May              Rockfalls concentrated on N and NE; vigorous
                        steaming from many parts of the dome.
 
    31 May              Rockfalls mainly concentrated on N and E, but
                        some travelled into the S moat. Two
                        pyroclastic flows down the Tar River; the
                        first progressed to within 300 m of the sea;
                        the second, to well past the Tar River
                        Soufriere. Associated ash clouds rose to 2-3
                        km and were blown NW. A large S-directed
                        rockfall escaped the S crater but progressed
                        less than another 100 m.
 
    1 June              Rockfalls concentrated on the dome's NE, E,
                        and S. Small pyroclastic flow to the E (into
                        the upper Tar River Valley); the associated
                        plume rose to ~2 km.
 
    2 June              Small "whale back" extruded on the dome's E
                        flank, just N of Castle Peak.
 
    4 June              Although the E dome was quiet, growth was
                        indicated on the S and W sides. No spines were
                        observed; instead the dome's top appeared
                        comparatively rounded.
 
    9 June              Intense incandescent spots coincided with the
                        source areas for rockfalls. New spines in the
                        W summit area. Night incandescence and
                        associated rockfalls; rockfalls on the dome's
                        NE flank to its W flank.
 
    10 June             One of the spines seen on 9 June had fallen
                        over. The dome overtopped the W crater rim at
                        Gages Wall and debris began to travel into the
                        uppermost reaches of Fort Ghaut. Dome growth
                        indicated on the NE.
 
    11 June             Dome growth indicated on the NW side.
 
    12 June             Estimated height of the new dome's summit was
                        943 m.
 
    13 June             Having filled the moat to ~25-m depth, talus
                        spilled out of the partly buried crater for
                        ~100 m into the upper Gages Valley.
 
    14 June             During heavy rainfall, ash erupted and fell NW
                        of the volcano (in the St George's Hill, Cork
                        Hill, and Old Towne areas). Rockfalls were
                        abundant; one ash emission during the
                        afternoon was semi-continuous and lasted about
                        an hour.
 
    15 June             A small (~300-m-long) pyroclastic flow S of
                        Castle Peak; fresh deposits from two others in
                        the upper Tar River valley (on the dome's E
                        and NE flanks).
 
    16 June             Rockfalls on the dome's NE, N, SE, and W
                        sides. On the W it was unclear how much new
                        material escaped the crater along the upper
                        Fort Ghaut.
 
    17 June             Very little new material had travelled down
                        the upper W slopes into the upper Fort Ghaut.
                        Talus on the dome's N side had built up to ~15
                        m below the rim of Farells wall. A projection
                        along the new dome's summit measured 942 m
                        elevation.
 
    26 June             Dome appeared wet and issued heavy steam from
                        the SE flank. A stubby spine was on the NE
                        summit.
 
    30 June             Dome rockfalls relatively rare but
                        considerable new material had been deposited
                        on the dome's E sector, in the Tar river's
                        upper S fork. The dome's most rapid growth
                        appeared to be on the S. Little new mass
                        wasting on the W (down Fort Ghaut); however,
                        steam production was concentrated in the W
                        moat area.
 
    4 July              Viewed a large slab of extruded lava at the
                        top of the SE dome. A few blocks of fresh dome
                        lava lay in the dome's new W drainage (the
                        upper Fort Ghaut).
 
    --
    29 July             Editor's note: As we prepared the final drafts
                        of this issue we received word of a plume
                        rising to 7.6 km altitude based on pilot
                        reports.

Table 6. Chronology of seismically derived observations of volcanic activity at Soufriere Hills, Montserrat, 30 May through July 1996. Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

    SEISMIC OBSERVATIONS
    Date                Observation

    30 May-3 June       Swarm of small hybrid earthquakes of
                        variable amplitude, occurring at the
                        rate of 0.5-2 events/minute.
 
    30 May              Volcano-tectonic earthquake 750 m beneath the
                        crater.
 
    31 May-3 June       Somewhat elevated tremor;
                        volcano-tectonic earthquake 1.5 km
                        beneath the crater.
 
    1 June              Volcano-tectonic earthquake 2 km beneath the
                        crater.
 
    3 June              One ~2-hour continuous tremor episode and a
                        second 5-hour episode running into 4 June.
 
    5 June              Interpreted small, local mudflow in upper Fort
                        Ghaut.
 
    6 June              Sediment-laden flood towards the W (Plymouth)
                        down Fort Ghaut.
 
    9 June              Areas of incandescence seen associated with
                        rockfalls.
 
    13 June             Very early in the morning the Gages and
                        Chances Peak seismic stations started to
                        record a few small repetitive hybrid events;
                        these slowly increased in number and were
                        occurring at a rate of ~1 every two minutes by
                        the end of the reporting period.
 
    15 June             During sustained heavy rainfall, small hybrid
                        events appeared prior to abundant rockfalls.
                        These hybrid events grew from ~1/minute to
                        5/minute and their amplitudes doubled before
                        they decreased in number and size. Larger
                        rockfall signals looked similar to signals
                        from small pyroclastic flows.

Table 7. Daily counts of seismic events at Soufriere Hills, Montserrat, 30 May-1 July 1996. The amount of tremor is described qualitatively (high-low) or using analysis from the Daily Reports (particularly after 12 June). Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

    Date    Daily counts of seismic events        Amount of tremor
            Volcano    Long     Hybrid  Rockfall
            -tectonic  -period

    30 May     1          5        0     17    Low to intermediate
                                                 (3 hours of
                                                 continuous tremor)
    31 May     1         14       96     97    Intermediate to high
    01 June    1          0      307    116    Intermediate to high
    02 June    0          0      132     83    Intermediate to high
    03 June    0          1       19     32    Intermediate to high
    04 June    0          5       18     51    Low to intermediate
    05 June    0         17        8     57    Low to intermediate
    06 June    0         13        4     49    Low to intermediate
    07 June    0          0        1     13    Low to intermediate
    08 June    0          0        1     51    Low to intermediate
    09 June    0          3        1     54    Low to intermediate
    10 June    1         15        2     54    Low to intermediate
    11 June    0         12        5     87    Low to intermediate
    12 June    0          2        1     59    Intermediate
    13 June    1          2   (tab 4)    39    5.5 hours
    14 June   --         12       25     34    --
    15 June   --         --   (tab 4)   198    --
    16 June   --         15       21    149    1 hour
    17 June   --         16        2     49    12 hours
    18 June    1         13        0     81    --
    19 June   --          7        8     92    --
    20 June   --          1        7     63    --
    21 June   --          3        5     53    6.6 hours
    22 June   --          5        0     28    Low
    23 June   --          1        4     60    7 hours
    24 June   --         29        3     62    Very low
    25 June   --          7        2     37    Low
    26 June   --          4        6     37    --
    27 June   --          6        7     59    --
    28 June    2         14       11     66    --
    29 June    1          4        4     55    --
    30 June   --         --      ~55     --    --
    01 July    2         11       12     57    4 hours

Table 8. COSPEC measurements of SO₂ flux at Soufriere Hills. Daily means and weekly averages are both shown; the two may not agree in cases where some daily means went unreported. Courtesy of MVO.

    Date     Number of      SO2 flux (metric tons/day)
           measurements     Daily Mean ()    Average of previous week

    29 May                                   127
    30 May      2           224 (58)
    31 May      2            88 (72)
    01 June     3            52 (13)
    02 June     6           193 (37)
    03 June     3           192 (13)
    04 June     4           240 (65)
    05 June     7           194 (55)
    05 June     -                            169
    07 June     3            84 (54)
    08 June     5           269 (103)
    09 June     4           126 (39)
    10 June     5            59 (21)
    12 June     5           168 (39)
    12 June     -                            141
    15 June     -           135
    17 June     6           125
    19 June     -           170
    24 June     -            76
    28 June     -           160

During the week ending 5 June gas measurements using a Fourier Transform Infrared Spectrometer showed substantial errors (50-100%), but did establish that at ground level on the lower slopes of the volcano (excluding Upper Amersham), the ambient concentrations of HCl and SO₂ in the air were well below 100 ppb. The SO₂:HCl ratio was generally well below 1.0. The only exception to this, on 24 May, was when the measurement errors were large. The SO₂:HCl ratios could be used to make an argument about status of the magma chamber. Assuming that this ground-level SO₂:HCl ratio was the same as in the plume, then the low ratios measured would indicate a moderately degassed magma chamber.

During 6-12 June, ash generation was generally low and few ash clouds emerged from the crater area; seismically detected rockfalls decreased with respect to the previous week and although no swarm of hybrid earthquakes occurred as in the previous week, the abundance of long-period earthquakes did appear similar to the previous few weeks.

EDM deformation measurements often detected an overall shortening rate along survey lines of ~1 mm/day during the interval from the beginning of December through the end of April. During 1 May-4 June there was a shortening rate of 2.4 and 2.1 mm/day in the volcano's N region (White's and Long Ground respectively); however, the shortening rate subsequently returned to ~1 mm/day.

On 12 June it was noted that during the previous 7-day interval, a ~12 mm/day shortening rate occurred on the N line (Long Ground to Castle Peak). In contrast, during this interval the W and N triangles continued to show no changes in line length above the error of the method.

On 13 June the E triangle was remeasured; its previous measurement was on 11 June: the Long Ground to Castle Peak line shortened by 9 mm and the Whites to Castle Peak line shortened by 6 mm. On 15 June it was reported that the W triangle's line lengths had recently shortened by ~1 mm/day. On 19 June it was found that NE sector (White's and Long Ground to Castle Peak) line lengths shortened by 2.5 cm over 5 days; this result extended a 3-week trend of 3-5 mm/day shortening here. Despite these larger than typical deformations in the NE sector during June, during the same month it was reported that the tiltmeters at Long Ground had remained stable for the past 10 months.

Scientists also noted that by June considerable new dome lava and talus had piled against the crater's W wall. Still, the June EDM surveys failed to show corresponding movement in this portion of the older edifice.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/).

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07/1996 (BGVN 21:07) Dome growth continues, rockfalls and pyroclastic flows increase

The following condenses daily Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 1 July-1 August. Seismic and other significant events of this month are also summarized in table 9.

Table 9. Chronology of seismicity and other major events at Soufriere Hills, Montserrat, 1 July through 1 August. The observation period is a 24-hour interval beginning at 1600 the previous day. Courtesy of MVO.

    Date                 Seismic Events
            world/  Long    Hybrid  Rockfall    Tremor
            Tectonic  Period                     Intensity

    01 July    2       11       12       57      Low
    02 July    1        1       18       64      Low
    03 July    -        5        -       59      Low
    04 July    -        5       25       52      Low
             One small ash cloud.
    05 July    -        6       22       27      Low
    06 July    -        9        6       11      High
    07 July    2       12        6       22      High
    08 July    1        3?       3?       3?     High
             Hurricane Bertha.
    09 July    1        1        5        9      Low
    10 July    1        4        2       22      Low
             Two light ash clouds drifted W.
    11 July    2        4        2       32      Low and High
             Flash flooding in Upper Fort Ghaut and Tar River Valley. Few
             very small pyroclastic flows. Light ash fall N of Plymouth.
    12 July    4       10       11        9      Low
    13 July    -        9       22       14      Low-to-Moderate
    14 July    4       22        5       18      Low-to-Moderate
    15 July    2       13       11       14      Low-to-Moderate
             One small ash cloud.
    16 July    2       14       13       15      Low-to-Moderate
             One small ashfall in Brodericks.
    17 July    -        8       14       24      Low
             Two small ash clouds drifted W.
    18 July    -       12       17       16      Low
             One small ash cloud.
    19 July    4       12       11       20      Low
             Four small ash clouds drifted W.
    20 July    -        4        4       21      Low-to-Moderate
             One small ash cloud drifted W.
    21 July  560       19       44       58      Low-to-Moderate
             Some small ash clouds. Few small pyroclastic flows.
    22 July   82      105      114       94      Low-to-Moderate
             Some small ash clouds. Seven small pyroclastic flows.
    23 July   15       24      101     >150      Low-to-Moderate
             Continuous ash clouds production.
    24 July   15        1        9     ~160      Low-to-Moderate
             One ash cloud.
    25 July  106        9       35     ~100      Low-to-Moderate
             One ash cloud drifted NW.
    26 July   98        5       15      102      Low-to-Moderate
             Some very small ash clouds.
    27 July   15        5       36     ~100      Low-to-Moderate
             Some small ash clouds produced light ashfall toward W.
    28 July    5      n/a      n/a      n/a      Low-to-Moderate
             Continuous ash cloud production resulted in heavy ashfall
             toward W. Several small pyroclastic flows.
    29 July  n/a      n/a      n/a      n/a      Low-to-Moderate
             Moderate-sized ash cloud caused ashfall toward WNW.
    30 July   20        6   clusters     89      Low-to-Moderate
             One small ash cloud. Few small pyroclastic flows.
    31 July   88        -      178       93      High
             Large number of pyroclastic flows produced continuous ash
             clouds and heavy ashfall.
    1 Aug     20       36      215      117      Low
             Ashfall continued from the day before.

Activity during 1-20 July. During the first 10 days of July activity remained at a low level, similar to the last week of June (BGVN 21:06). The most significant events were small-to-moderate size rockfalls from the growing S flank of the lava dome. The largest rockfalls produced small ash clouds that drifted with the prevailing winds, principally to the W of the volcano, toward Upper Gages, Amersham, and Plymouth.

Most of the time visibility was poor because of bad weather conditions. On 4 July a brief period of excellent viewing conditions confirmed that the dome was growing mainly in the S section of the crater. A huge slab extruded at the top SE part of the lava dome had a vertical crack down the middle; activity was concentrated around its base. Several large loose boulders were seen on the slopes of the dome. A small quantity of fresh dome material, mainly blocks, was observed in the upper reaches of Fort Ghaut. Moderate steaming and gas production were occurring from several areas.

Seismicity remained low, with volcano-tectonic events concentrated under English's Crater at depths of <2 km. Daily episodes of intermittent low-amplitude broadband tremor lasted from a few minutes to several hours. On 6 and 7 July periods of high-amplitude tremor were associated with heavy rainfall and an increase in steam venting at the summit.

On 7 July a brief period of good visibility revealed a second peak on the dome, and the accumulation of material behind Galways Wall. Intense fumarolic activity was occurring in the saddle between the two peaks. That day the elevation at the top of the dome was measured as 939 m. MVO estimated that the rate of dome growth had not changed significantly since early May.

On 10 July the seismic signals became longer and stronger. That same day a brief view of the dome showed that rockfall activity was spreading to other areas within the active SE section. More fresh material had accumulated down the S side of Castle Peak, while vigorous steaming was observed behind it. Fumaroles were active on the summit of the SE peak and in the saddle area between the two peaks inside English's Crater.

On 11 July, heavy rainfall caused flash floods in Fort Ghaut and possibly Tar River. A fine ashfall was reported in areas N of Plymouth and out to sea. Some small pyroclastic flows went into the Upper Tar River area. Helicopter inspections found that a significant amount of material had come down the N and S sides of Castle Peak and the fresh deposits were still steaming. Several erosion scars were observed on the NE flank of the dome, which was probably the source of the flows.

On 12 July the activity level decreased and it remained low throughout 20 July. However, the broadband tremor increased in amplitude, which was interpreted as a sign of increased steam emission; brief glimpses of the dome eventually revealed vigorous steaming, at times tainted with bluish vapor. That same day rockfall deposits were reported on the S and NE sides of the dome. The wet material on the NE side, around the whaleback feature, had dried out in places and two well-formed erosion chutes were present. Dome elevation was measured at 941 m.

On 17 July more new material was seen over Gages Wall and against Galways Wall. Observers on Perche's Mountain noted that most of the rockfall activity was on the SW flank of the dome.They also reported a small block-and-ash flow down the E flank of the dome around noon. On 19 July a field party working at Farrell's heard frequent rockfall activity and observed one rockfall descending the NE flank of the dome.

Activity during 21 July-1 August. On 21 July, the occurrence of 560 volcano-tectonic earthquakes marked a sharp increase in activity that lasted until August. These events originated from a shallow source beneath the crater, or just slightly NNE at <3 km. Long-period earthquakes were of moderate size whereas hybrid events were always small and occurred in a near-repetitive pattern at times so frequently to resemble continuous tremor. This type of activity had previously been associated with increased dome growth.

When weather conditions allowed, views of the dome revealed very vigorous steam emission from behind the old Castle Peak spine. On 25 July a large spine at the summit of the N peak of the dome was seen from Hermitage.

The rockfall activity, mainly on the NE flank of the dome, increased daily. Periods of near-continuous rockfalls were reported after 27 July. Most of the rockfalls were channeled down the NE-flank gully; none reached as far as the Tar River Soufriere. Small pyroclastic flows from the E and NE parts of the dome occurred daily into the Tar River Valley until they filled the entire valley area. Most of the local vegetation was set on fire by these flows. Associated ash clouds caused light to moderate ashfalls on 27, 28, and 29 July. One eyewitness reported on an electronic forum that during the ashfall of 28 July visibility in Plymouth was reduced to the less than one-half of a city block.

The dominating event on 31 July was a sequence of pyroclastic flows in the Tar River valley. It started at 1150 hours with a series of small- to moderate-sized rockfalls, which gradually led to the pyroclastic flow. Four flows occurred within a period of four minutes, with the last three eventually reaching the sea. A helicopter inspection confirmed that the pyroclastic flows were confined to the Tar River Valley. Light steam emission was observed from the area where the pyroclastic flows entered the sea and from the Tar River Valley.

The ash cloud generated by the pyroclastic flows attained a height of 6.4 km above sea level, according to Bramble Airport Control Tower. The ash cloud produced significant ashfalls in most areas of central Montserrat (Lees, Gages, St. George's Hill, Cork hill, Garibaldi Hill and Fox's Bay) and a far N as Woodlands. Lighter ashfalls were reported in Amersham and Plymouth. An eyewitness posted to an electronic forum that during the 31 July ashfall there was ". . . total darkness, the electricity had gone off . . . then it started to rain. The windows . . . facing the mountain became almost solid black. All the rest were covered with some mud . . . . When I got up to [my pickup the] windows, top, and hood were covered with ~1.5 inches [~3.8 cm] of mud."

That same day five episodes of high-amplitude, low-frequency, harmonic tremor were recorded at intervals of ~4 hours. After each period the signal decayed first into smaller hybrids and then to background noise. These signals could be related to movement of magma at shallow depth as the process of dome growth continued.

GPS, EDM, and COSPEC measurements. The poor weather conditions in July prevented most of these measurements. COSPEC data collected during an all-day experiment on 30 June showed no systematic variation in the SO₂ production; on 10 July ~88 tons/day were measured.

A GPS survey carried out on the E side of the volcano on 7 July showed that no significant movement had taken place there since 18 June. Data collected on 10 July from Tar River, Harry's, O'Hara's, and Dagenham showed changes <5 mm in all lines since the survey started on 11 April.

EDM measurements showed an increase in the shortening rate from a few millimeters/day at the beginning of the month up to 1.5 cm/day toward the end of the month for the lines of the E triangle (Whites-Castle Peak-Long Ground) (table 10). Lengthening (1 and 2.3 cm) was measured on 30 July, when the dome elevation was found to be 923 m.

Table 10. EDM data from Soufriere Hills, Montserrat, 1 July through 1 August 1996. Courtesy of MVO.

    Date      Shortening (-) or      Line or triangle
              lengthening (+)/day

    01 July          -0              Amersham-Dagenham-Chances
    12 July        -few mm           Whites-Chances Peak-Long Ground
    18 July     -1.5 cm/2 days       Whites-Chances Peak
                                     Long Ground-Chances Peak
    24 July     -8 cm/4 days         Whites-Chances Peak                                                     
                                     Long Ground-Chances Peak
    26 July     -3 cm/2 days         Whites-Chances Peak                                                     
                                     Long Ground-Chances Peak
    29 July     -3 cm/3 days         Whites-Chances Peak                                                     
                -1.3 cm/3 days       Long Ground-Chances Peak
                -17.5 cm/14 days     Tar River-Chances Peak
    30 July     +1 cm/day            Whites-Chances Peak                                                     
                +2.3 cm/day          Long Ground-Chances Peak
    01 Aug      -4 cm/day            Whites-Chances Peak

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/).

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08/1996 (BGVN 21:08) Dome growth continues, activity level increases

The following condenses daily reports of the Montserrat Volcano Observatory (MVO) for the period 1 August-1 September. Seismicity and other significant events are summarized in tables 11 and 12. In a comment in the journal Nature, Peter Francis (1996) noted how Soufriere Hills' magma appeared depleted in volatiles with a low SO₂/HCl ratio.

Though not discussed in this report, the largest and most destructive eruption of the 1-year-old eruption began at 2345 on 17 September. No casualties were reported but towns and villages on the NW side of the volcano were evacuated. The resulting plume covered a substantial part of the Caribbean.

Table 11. Chronology of seismicity and other major events at Soufriere Hills, Montserrat, 1 August through 1 September. The observation period is a 24-hour interval beginning at 1600 the previous day. Courtesy of MVO.

    Date    Seismic Events
            world/   Long-   Hybrid  Rockfall    Tremor
            Tectonic   Period          Intensity

    1 Aug     20         36       215     117      Low
    2 Aug     24          6       250      49      Low-to-high
    3 Aug     43        340        58              Low-to-high
    4 Aug    n/a        n/a       n/a     n/a      High
         Ash clouds up to 3.3 km; small pyroclastic flows
    5 Aug     71          0       373      64      High
         Small ash cloud
    6 Aug     22          3       556      48      High
         Small ash clouds and minor pyroclastic flows
    7 Aug     36          2       427      32      High
         Small ash cloud
    8 Aug     47          3       310      47      High
         Small ash clouds rose to 2.3 km; small pyroclastic flows
    9 Aug     97          4       147      67      Low-to-high
         Ash clouds at 1.3-3.1 km; small pyroclastic flows
    10 Aug    74          0       137     153      n/a
         Ash clouds at 2.6 km; small pyroclastic flows
    11 Aug   >37          0       >29    >182      n/a
         Ash cloud reported between 10-13 km; significant ashfall;
         intense pyroclastic flow activity
    12 Aug     7          0        56      82      Low
         Ash cloud reported at 10 km; one large pyroclastic flow and
         several minor ones
    13 Aug     0         39        42     117      Low
         Small ash cloud; pyroclastic flows; one flash flood
    14 Aug    37          4        31      82      n/a
         Ash cloud at 3.3 km; pyroclastic flows traveled 100 m to the sea
    15 Aug    14          1        14      61      n/a
         Ash clouds up to 1.6 km; small pyroclastic flows
    16 Aug    28          0        34      67      Low
         Several small ash clouds and pyroclastic flows; flash floods
    17 Aug    13          0        35     118      Low-to-moderate
         Ash cloud rose to 2 km; few pyroclastic flows; flash floods
    18 Aug     5          4        21      70      Low
         Ash cloud up to 2 km; few small flows
    19 Aug    32          1        22     103      Low
         Ash clouds to 3.3 km; some pyroclastic flows
    20 Aug    30          1        33     161      n/a
         Ash clouds at 1.6-4 km; small pyroclastic flows
    21 Aug     1          2        36     131      Low
         Ash clouds at 5 km; significant ashfall; near continuous pyroclastic flows
    22 Aug    20          1        34     118      Low
         Ash cloud at 2.6 km; two large pyroclastic flows reached the new delta
    23 Aug     0          0         7      51      Low
         Small ash clouds and ashfalls
    24 Aug     2          0         1      26      Low-to-moderate
         Ash at 1.6 km
    25 Aug     3          0         0      62      Low-to-Moderate
         One very small ash cloud
    26 Aug    17          0         8      82      Low
         Some very small ash clouds
    27 Aug    30         10         7     115      Low
         Small ash clouds; one small pyroclastic flow
    28 Aug     0          8        12      49      Low
         Small ash cloud
    29 Aug   n/a       several    n/a     n/a
    30 Aug   146          2        24      55      Low
         Ash cloud rose to 1.1 km
    31 Aug   n/a       several    n/a     n/a
    1 Sept   107         21         4      92      Low
         Ash clouds

Overview. Activity in August continued at a higher level than last month (BGVN 21:07) but small- to moderate-sized rockfalls and small pyroclastic flows continued to be restricted to the Tar River Valley on the E flanks of the growing lava dome. Occasionally the flows traveled as far as the sea, building a new delta that extended 400 m offshore (figure 9). Daily emissions resulted in variable ashfalls to the WNW.

Seismicity included larger numbers of hybrid earthquakes compared to July and the occurrence of volcano-tectonic earthquakes in swarms at depths of <2 km beneath English's Crater. Ash-rich steam frequently escaped from a collapse structure on the upper dome. These emissions were vigorous and associated with intermittent low-amplitude tremor, suggesting magma-groundwater interaction.

Activity during 1-10 August. Several periods of tremor were recorded daily in early August. These episodes started with a series of small volcano-tectonic events followed by small, near-repetitive hybrid earthquakes that increased in frequency and amplitude until they formed a continuous tremor. After a period of sustained peak amplitude, the signal decayed first into smaller hybrid events and then to background levels. After the first week the hybrid earthquakes remained distinct events without evolving into tremor.

Small-to-moderate size rockfalls from the E and NE flanks of the lava dome generated small ash clouds that rose up to 3 km above sea level. A few small pyroclastic flows in the Tar River Valley reached as far as Tar Soufriere, ~1.5 km E; steam was visible from parts of the valley and from the area of entry of the pyroclastic flows into the sea (figure 9).

Ash deposits from the clouds of 29-31 July, not yet washed out, were reported in Upper-Gages, Amersham, and Plymouth. In an electronic newsletter an eyewitness described the ashfall on 2 August as ". . . so heavy in Weeks and Cork Hill areas that the trees and limbs were broken."

Visual observations of the dome on 2 August revealed that the pyroclastic flows on 29 and 31 July originated from a collapsed structure on the new dome. The collapsed structure contained considerable amounts of freshly extruded lava that had a lobate, whale-back form, but parts had also broken into several huge blocks.

By 7 August the new dome had almost filled the collapsed structure and some parts of it appeared over-steepened and unstable. On 9 August, the height to the top of the dome was measured at 929 m above sea level, compared with 923 m on 30 July.

Activity during 11-22 August. Several large pyroclastic flows followed the collapse of parts of the dome's E flank and most of them reached the sea: the two largest flows of the month occurred on 11 and 12 August. Ash clouds were produced almost continuously; they reached 9.1 km altitude and were associated with thunder and lightning. Two pilots flying in the region reported the top of the ash cloud above 12 km. According to satellite imagery and aircraft reports, the near-continuous nature of the activity on 11 and 12 August produced a band of very light ash in the atmosphere extending from Montserrat almost to Puerto Rico.

Significant ashfalls occurred in Plymouth, Richmond Hill, and Fox's Bay. Lighter ashfalls were reported from St. George's Hill, Cork Hill, and Garibaldi Hill. The ash fall of 12 August had the following measured thicknesses: 2 cm in Lovers Lane, 3 cm at Plymouth Police Headquarters, 6 mm in Trials, 2 mm in Gingoes, 19 mm on Fort Ghaut bridge, 3 mm in Fox's Bay, and 2 mm in Weekes.

The highest point on the dome was measured from Chance's Peak on 16 August and yielded an elevation of ~963 m. On 17 August, good visibility showed that the E part of the dome was very unstable and the observed rockfalls generally originated from this area. Within the crater, an eroded gully N of Castle Peak channeled debris from the E dome down the upper reaches of the Tar River valley, almost filling it. On 19 August a large pyroclastic flow reached the new delta but did not enter the sea. On 21 August another flow traveled a distance of ~1.5 km and produced an ash cloud that reached a maximum altitude of ~4.6 km. Near-continuous rockfalls resulted in several ash clouds that caused significant ashfalls in Plymouth and surroundings.

Good visibility on 22 August allowed scientists to conclude that the most recent pyroclastic flows were produced by the loss of material from the lower section of the new dome grown in the collapse structure produced by events of 29 and 31 July 1996. At this time the gully N of Castle Peak was almost empty.

Activity during 23 August-1 September. On 23 August the lowest activity of the month occurred, after which activity remained at a level lower than the previous three weeks. Reports during this interval mentioned small- to moderate-sized rockfalls, very few small pyroclastic flows, ash columns below 1.5 km, and an occasional sprinkling of ash on some central and N areas. On 26 August observers noted that the gully just N of Castle Peak was again filled with debris. On 30 August, brief glimpses of the dome from Bramble Airport Control Tower suggested that two extruded features (possibly spines) existed in the central part of the E dome. Light ash clouds associated with some larger rockfalls and/or small pyroclastic flows were occasionally seen drifting SSE at 1.1 km above sea level. On 1 September the runout of rockfalls and small pyroclastic flows along Tar Valley was generally <1 km.

At the end of the month an increase in volcano-tectonic earthquakes was interpreted as the continuous movement of magma from shallow depths to the surface as the process of dome growth continued.

EDM and COSPEC measurements. EDM measurements (table 12) showed a continuous, though somewhat variable shortening trend. This was especially prominent on the E triangle (Whites-Castle Peak-Long Ground). COSPEC measurements of SO₂ in the volcanic plume gave the following results: 8 August, ~326 metric tons/day (t/d); 12 August, 1, 195 t/d; 13 August, 626 t/d; and 24 August, 258 t/d.

Table 12. EDM data from Soufriere Hills, Montserrat, 1 August through 1 September 1996. Courtesy of MVO.

    Date     Shortening (-) or     Line or triangle
            lengthening (+)/day

    1 Aug        -4 mm             Whites-Castle Peak
    2 Aug     -12 mm/3 days        Whites-Castle Peak-Long Ground
    3 Aug      -2 mm/1 day         Whites-Castle Peak
               -9 mm/1 day         Long Ground-Castle Peak
    6 Aug      -3 mm/2 days        Whites-Castle Peak
               -2 mm/3 days        Long Ground-Castle Peak
    7 Aug      -8 mm/1 day         Whites-Castle Peak
                                   Long Ground-Castle Peak
    9 Aug     -25 mm/2 days        Whites-Castle Peak
              -31 mm/2 days        Long Ground-Castle Peak
    10 Aug     -1 mm/1 day         Whites-Castle Peak
              -16 mm/1 day         Long Ground-Castle Peak
    12 Aug    +24 mm/28 days       Amersham-Amersham slope
              -28 mm/28 days       Amersham-Chances Peak
    16 Aug    -11 mm/2 days        Whites-Castle Peak
                    0              Long Ground-Castle Peak
    18 Aug     +4 mm/2 days        Whites-Castle Peak
               +1 mm/2 days        Long Ground-Castle Peak
    19 Aug    -16 mm/55 days       Galways-Chances Peak
              -12 mm/55 days       Ogarras-Chances Peak
    22 Aug    -16 mm/16 days       Windy Hill-Farrell
    23 Aug          0              Amersham-Amersham slope
              +29 mm/11 days       Amersham-Chances Peak
    25 Aug    -14 mm/7 days        Whites-Castle Peak
               -7 mm/7 days        Long Ground-Castle Peak
               -4 mm/6 days        Galways-Chances Peak
               -5 mm/6 days        Ogarras-Chances Peak
    26 Aug     -1 mm/1 day         Whites-Castle Peak
               -1 mm/1 day         Long Ground-Castle Peak
    27 Aug    -14 mm/1 day         Whites-Castle Peak
               -1 mm/1 day         Long Ground-Castle Peak

Reference. Francis, P., 1996, Volcanoes - danger and dependency: News and Views, Nature, v. 383 (5 Sept.), p. 28.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA; Susan and Eddie Edgecombe, and Betty Dix, Tradewinds Real Estate, Box 365, Plymouth, Montserrat (Email: tradewinds@candw.eg); Glenn Lewis, c/o Doug, Deb, and Paul Darby, 6 Satinwood Road, Rocky Point, NY 11778 USA (URL: http://www.netcom.com/~user22/pkd.html).

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09/1996 (BGVN 21:09) Large destructive explosion 17 September

The following condenses the weekly Scientific Reports of the Montserrat Volcano Observatory (MVO) and stated sources for the period 1 September-1 October.

Observations during 1-14 September. The early days of the month were characterized by several periods of intense rockfalls and pyroclastic flows from the E flank of the lava dome. The steepening of the dome's active flank caused a partial gravitational collapse on 2 and 3 September. The resulting pyroclastic flows were generally confined to the S part of the Tar River valley although they came from N of Castle Peak (figure 10). The pyroclastic flows caused significant erosion in the middle part of the valley and deposition in the lower part and at the mouth of the Tar River, on the pyroclastic-flow delta built up since late July. Excavation of a deep (>10 m) channel from the base of the new dome through the upper part of the talus fan confined the flows giving them greater run-out potential. The scar left on the E flank was soon refilled by continuous rockfall activity and new dome growth. Samples of the pyroclastic-flow deposits on the delta contained less vesicular material than other deposits since late July, and were typically ash-rich, very poorly sorted, and contained juvenile lava blocks to at least 50 cm diameter.

The pyroclastic flows of 2 and 3 September produced ash clouds that rose 6 km, but there was no evidence of vertical columns from the summit of the dome. The ash clouds deposited 1-2 cm of ash in the Cork Hill area, and >5 mm farther N in the Old Towne area. MVO estimated the volume of ash deposited on 2 and 3 September to be equivalent to a rock volume of 7 x 10⁴ m³. In addition to this description from MVO, a local newspaper, The Montserrat Reporter, said these events caused ash to fall on nearly every part of the island from St. Patrick's in the SW, to St. John's in the N, and from Plymouth in the W to Long Ground in the NE, including Bramble Airport. For the remainder of the period, rockfall and associated pyroclastic-flow activity was confined almost exclusively to the E flank. After the major ash falls of 2 and 3 September more moderate amounts were deposited W of the volcano.

Signals from rockfalls and pyroclastic flows dominated the seismic records during this observation period. Long-period and hybrid events remained at background levels and tremor was generally low. Volcano-tectonic earthquakes occurred exclusively in short swarms lasting 1-6 hours. The volcano-tectonic earthquakes were all located <2 km below sea level beneath the crater.

The passage of a hurricane caused several days of strong winds and heavy rain making visual observation of the dome difficult, and causing flash floods that deposited ~60 cm of sediment in Fort Ghaut's lower reaches.

Observations during 15-21 September. Several small pyroclastic flows occurred on 15 September, the largest reaching beyond the Tar River Soufriere. Ash clouds from rockfalls and flows were generally blown NW. Intense ash and steam venting during 1250-1320 on 15 September came from the highest part of the dome W of the active area.

Near-continuous rockfalls started late on the morning of 16 September and by mid-afternoon, numerous pyroclastic flows were being produced by gravitational collapse from the lava dome. Many of these pyroclastic flows reached the sea, extending considerably the depositional fan at the mouth of the Tar River valley. Continuous ash production from the flows fed into a convective column that reached heights of 2-3 km and deposited ash on areas W of the volcano. Activity slowed somewhat in the middle of the evening as pyroclastic flow generation stopped.

Activity restarted at 2342 on 17 September with a small explosive eruption. A laterally directed explosion projected ballistic clasts toward the E (over the Hermitage area and into Long Ground village) and an eruption column was briefly sustained. More than half of the houses in Long Ground were damaged by blocks falling through roofs, doors, and windows. Eight buildings, including the Pentecostal Church, were burnt in Long Ground, all from extremely hot rocks falling on them. The Tar River Estate House was partially demolished by a pyroclastic surge. Gravel-sized material of both pumiceous and dense nature was deposited at Cork Hill, Richmond Hill, and Fox's Bay from the eruption column. The Montserrat Reporter noted that many vehicles had lost their windscreens from "falling pebble rocks". On the other hand, MVO data suggested that the number of windscreen breakages was actually quite low and that ash loading contributed substantially to breakages. All ash erupted during the night was blown W over Plymouth and Richmond Hill and both of these areas received heavy ashfall.

In an electronic forum, Douglas Darby, an eyewitness, reported: "From Iles Bay, you could hear something coming from the direction of the volcano, at about [2345 on 17 September]. It sounded like a low roar, the first time ever in Iles Bay that you could hear any noise from the volcano. Immediately after, thunder and lightning began and it was obvious that this was not anything experienced before . . . And then the rain of stones began . . . Visually you could not really see much at that time but we thought we could see a low level of glowing all across the area where we know is Tar River, from the direction of the pyroclastic flows."

Reports from the NOAA Satellite Analysis Branch indicated that the ash column attained a height of at least 12 km and caused the closure of the airport in Guadeloupe on the morning of 18 September. Pilot and NOAA reports and personal communication with Tom Casadevall indicated that an Air Canada flight inadvertently entered the ash plume on 17 September. Dave Schneider of MTU collected and processed two AVHRR scenes of the ash plume from 18 September: at 0544 the plume was 175 km long E-W and 75 km wide N-S, at 1018 the cloud became very diffuse as it extended 550 km E and 85 km N-S (figure 11).

A major collapse scar cut deeply into the new dome's E flank. Some material was eroded from Castle Peak and a large volume was deposited in the Tar River Valley. The delta at the mouth of the Tar River Valley was enlarged and the vegetation was completely destroyed. MVO estimates stated that perhaps 25-30% of the new dome was removed.

Several small rockfalls from the inner steep-sided walls of the scar, particularly on the N and NW, generated small ash clouds and deposited new debris at the base of the valley. On 19 September field workers found pumice clasts of up to 95 g at 3 km and clasts up to 3.5 g at 6 km. On 22 September a sampling expedition to the Tar River area obtained a temperature of 373°C at a depth of 45 cm in the pyroclastic-flow deposits close to the Tar River Estate House.

Seismicity during this period was characterized by brief swarms of volcano-tectonic earthquakes from a shallow source. These swarms occurred immediately before the most intense rockfalls and increased in frequency and duration preceding the 17-18 September explosion. After 18 September the frequency of volcano-tectonic earthquakes decreased from 2-3 swarms/day to single isolated events at the end of the observation period. Long-period and hybrid events remained low, averaging <11 events/day; low-amplitude tremor was recorded on the Gages seismometer.

Observations during 24-30 September. Activity kept decreasing in intensity during the last part of the month. On 24 September visual observations of the scar's interior showed no signs of new material apart from debris derived from rockfalls off the side walls. Abundant steaming and sulfur deposits were observed at the base of the scar. Rockfalls were very small, mainly concentrated within the scar and associated with continued stabilization of the inner walls of the scar. The lack of large rockfalls suggests that any new dome growth was limited to the interior of the dome, probably at the base of the scar feature caused by the 17 September explosion. On 26 September some red-hot rock and high-temperature gases were seen in the bottom of the scar, suggesting that fresh magma was getting close to the surface again; however material falling from the scar walls covered any new dome growth. Light ashfall, possibly associated with small rockfalls into the scar, was observed by a field team near Chances Peak on 28 September.

On 30 September some areas to the SW and along the base of the scar showed light swelling. This may be due to new dome growth beneath the blocky deposits that line the base of the scar. The N part of the scar had a vertical cliff face with a nearly horizontal, bowl-shaped base, grading downward and outward to the Tar River Valley. Several unstable blocks were observed on the top inner parts of the NE sides of the scar.

Small rockfalls were the most dominant type of seismic signal recorded during this period, but hybrid and volcano-tectonic activity became more prominent during the latter part of the week. Volcano-tectonic earthquakes reappeared from 26 September onwards. They were transitional to hybrid events with a short high-frequency onset and low-frequency coda. The levels of long-period and hybrid events remained comparatively low throughout this period, averaging <11 events/day. Hybrid activity increased somewhat during the latter part of the week in tandem with the increase in volcano-tectonic activity. Tremor levels were high during the earlier parts of the week due to heavy rains. In Fort Ghaut, mudflows resulted from remobilization of thick ash deposits from the 17-18 September explosion.

EDM measurements. Measurements taken on 11 September from White's Yard to Castle Peak showed a 1 cm/day shortening trend, slightly higher than the trend established since mid-July. The Galway's to Chances Peak line was measured on 13 September, but it continued to show inconsistent changes, although shortening was predominant.

On 16 September a shortening of 2.8 cm on the St. George Hill-Farrell's line (N triangle) was measured since 22 August, whereas the two other lines in this triangle -- Windy Hill-Farrell's and St. George's Hill-Windy Hill -- did not change. Between 16 and 21 September the lines St. George's Hill-Farrell's and Windy Hill-Farrell's lengthened by 4 and 9 mm, respectively. These changes, however, are not considered to be related to the 17-18 September explosion. On 25 September the N triangle showed shortening on the St. George Hill-Farrell's and Windy Hill-Farrell's lines of 4 and 11 mm, respectively. Although little consistency is found in the changes of this triangle, a slight overall trend of shortening is observed.

Line lengths between Lower-Upper Amersham and Lower Amersham-Chances Peak showed changes of +48 mm and -1 mm, respectively, during 20-26 September. On 30 September the Galloways-Chances Peak line was found to have lengthened 13 mm during the previous 16 days.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/), NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA; Bennette Roach, The Montserrat Reporter, v. XII nos. 33 and 35, Tom Casadevall, U.S. Geological Survey, Menlo Park, CA 90210 USA (Email: tcasadev@usgs.gov); Dave Schneider, Michigan Technological University, Houghton MI 49931, USA (Email: djschnei@mtu.edu), Doug Darby, 6 Satinwood Road, Rocky Point, NY 11778 USA (Email: darbyd@candw.ag).

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10/1996 (BGVN 21:10) The new dome, first observed on 1 October, continues to grow

The following condenses the weekly Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 1 October-2 November.

As this isue was being finalized in late November, observations of cracks on Galway's Wall caused concern that failure of the crater wall could lead to a catastrophic event.On the afternoon of 26 November the area SW of the volcano was upgraded to risk zone A. This zone extends from S of O'Garra's to Gingoes Gaut. The alert level was also raised to Amber. Additional details will be provided next month.

Visual observations. Clear weather from 1 to 4 October allowed excellent views of a new lava dome (figure 12) growing in the scar left by the 17-18 September explosion (BGVN 21:09). This new dome was light gray in color with a blocky texture and without spines. It developed a flat-topped, steep-sided, solid cap, apparently raised up intact by later endogenous growth. As it grew from 782 to 793 m during 1-2 October, it's base expanded southward to fill the bottom of the scar.

On 7 October the dome was ~75% of the height of Castle Peak dome and appeared to be growing as a single cohesive mass. By 8 October the SE side of the new dome was quite steep and several rockfalls were originating from the base of the E face. On 10 October the new dome was higher than the top of Castle Peak. It had expanded laterally, filling ~25% of the large scar. The dome was a chocolate-brown color, had an even surface, and a nearly rounded conical shape. Several small rockfalls from the flanks and vigorous steaming from all around the base were also observed.

Observations on 11 October revealed regular small cascades of incandescent material from both shoulders of the E face of the dome. During the daytime rockfalls were visible from the coast at the mouth of the Tar River. On 14 October peaks were observed on both the dome's SW and E sides, giving it a blocky morphology, more similar in appearance to previous domes. A small stubby spine was noted on 15 October, but it disappeared by the following day.

After mid-October there was an increase in the number of reported ash clouds. Although these ash clouds were small, reaching no more than 650 m above the crater area, their occurrence suggested a less stable dome surface. A bathymetric survey of the delta, carried out by boat on 16 October, revealed a steep termination, reaching depths of 20 m within a short distance of the shore.

On 18 October it was seen that the dome had grown rapidly on its N side and expanded toward the W, covering most of the scar floor. The E face was quite steep, with several unstable-looking blocks at the top of the slope; small rockfalls were observed from it whenever the clouds lifted.

The height of the dome had increased to 829 m by 22 October (from 808 m on 18 October). Rockfalls were occurring from the N side of the scar and vigorous steaming was observed from the S of the dome. On 23 October some moderate-sized rockfalls produced small ash clouds; the collapse of a small spine was observed and new growth was seen on the S side of the E face of the dome (figure 12).

On the morning of 29 October several rockfalls from the E face of the dome occurred during and after a thunderstorm, and produced small ash clouds. The deposits from these rockfalls went farther down the Tar River valley than any previous rockfalls since the 17-18 September explosion. On 1 November, new deposits suggested that some parts of the E face had collapsed. Other changes to the dome were limited to a general increase in the height, and the appearance of a small stubby spine on its top. On 2 November a new area of activity was observed NW of the new dome, on the edge of the scar. This area was steaming and degassing vigorously, and seemed to be deformed, suggesting a new intrusion was taking place.

Seismicity. Seismicity remained low, a pattern established following the explosion of 17-18 September. Rockfall signals were the dominant events for the first half of the month, whereas volcano-tectonic and long-period earthquakes became more frequent after mid-October. The long-period signals were often associated with observed rockfalls, but were possibly due to some internal dome process (gas expansion?) associated with collapse of the dome. These signals were similar to isolated tremor episodes observed during dome growth at Unzen in Japan.

The first volcano-tectonic swarms occurred from 2200 on 17 October to 0430 on 18 October, and from 1300 on 18 October to 0130 on 19 October. Most locations were shallower than 1 km, and the signals showed extended long-period codas at some stations, consistent with very shallow hypocenters. The two deepest earthquakes were located respectively at ~6 km under Roaches Yard, and at ~15 km, 4 km E of the crater. From 21 October the volcano-tectonic earthquakes occurred in swarms of variable duration and return times. Only a few volcano-tectonic earthquakes were recorded outside of the swarm episodes: one event on 22 October was located at a depth of 5 km to the N of the crater and two events were ~2 km beneath St Georges Hill.

The largest volcano-tectonic swarm (in terms of total numbers of events and duration) recorded since the start of dome growth occurred on 1 November and lasted until 2031 on 02 November (413 events) (figure 13). Many of these events were large enough to be located, and mostly occurred at shallow depths (>2 km). This swarm included a set of deeper events at 3-4 km beneath the crater. Similar swarms occurred in the weeks prior to the 17-18 September explosion, although they were of shorter duration, occurred more frequently, and lacked deeper earthquakes.

The level of tremor was low; Gages station continued to record intermittent tremor. Higher tremor levels were often associated with increased steam venting and as a result of heavy rainfall.

EDM, COSPEC, dome volume, and other measurements. EDM measurements on the lines of the E triangle had the following results: Whites-Castle Peak line shortened by 8 mm (7-8 October), lengthened by 6 mm (8-10 October), and shortened by 2.5 cm (10-19 October); Long Ground-Castle Peak shortened by 3.4 cm (10- 19 October). All the E triangle's lines shortened by 3-4 mm (19-22 October) and by 9 cm ( 23 October-2 November). This was consistent with the long-term trend of line shortening at a rate of 1 cm/day, maintained since mid-July 1996.

For the S triangle, the Galways-Castle Peak line lengthened by 1.8 cm (6-19 October) in its second successive lengthening. For the N triangle, the St George's Hill-Farrells and Windy Hill-Farrells lines lengthened by 3 and 5 mm, respectively, on 21 October, and by 12.5 and 3 mm during 21-26 October. These lines have been quite erratic, but show long-term trends of little or no change. Measurements were also made between the Tar River Estate House and Castle Peak on 28 October, the first time this line was measured since 25 August. The original reflector was destroyed during the 17-18 September explosion, and so the line length could not be compared to previous measurements. Radial W-flank lines were measured on 1 November, between Upper and Lower Amersham, and Dagenham and Chances Steps. Most of these lines were stable, except for the line between Upper and Lower Amersham, which showed an irregular ± 5 cm variation, with no consistent change.

COSPEC measurements were carried out on 10 days in October (13, 15, 17, 21, 23, 24, 26, 27, 28, 31) and on 2 November. All the COSPEC measurements were made by running traverses beneath the volcanic plume along the W coast road. The average flux readings were 550, 650, 1110, 580, 449, 1519, 290, 434, 498, 311, and 267 metric tons/day, respectively. The values over 1,000 t/d obtained on 17 and 24 October were the highest recorded since the beginning of the volcanic crisis.

Three GPS surveys revealed that changes in slope length have not been significant at the 95% confidence level, with all variations within two sigma of the measurement error. This indicates the absence of a widespread deformation field associated with the eruption.

Range-finding binoculars and GPS equipment enabled investigators to estimate the dome's recent effusion rate as 1.8 m³/s. The estimated volumes were the following: 2.0 x 10⁵ m³ (2 October); 4.7 x 10⁵ m³ (3 October); 1 x 10⁶ m³ (10 October); 3 x 10⁶ m³ (18 October); and 3.7 x 10⁶ m³ (23 October).

Results of gas analysis of the Galways Soufriere carried out on 4 July were received from the Volcano Observatory in Guadeloupe. There were no significant changes in gas composition compared with earlier samples; the gas was dominated by CO₂ (60%) and H₂S (39%).

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/).

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11/1996 (BGVN 21:11) Cracks and landslides on SW wall cause major concern

The following condenses the weekly Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 2 November-8 December 1996.

Visual observations. At the beginning of November growth concentrated in the central W part of the 1 October dome, which lies in Englishs crater N and adjacent to Castle Peak, an ancestral dome. The new dome's height was measured at 880 m on 5 November; the same day a new spine was noted on its SW side. Rainfall and a lack of vegetation contributed to a 6 November landslide on the S flank of the crater, above Galway's Soufriere (figure 14). On 7 November incandescent material was seen on the top and N edge of the 1 October dome. The spine first seen on 2 November increased in height and partially collapsed by 9 November. On that day, large amounts of steam were observed over the dome complex especially in the S part at the contact with the Castle Peak dome; several small gullies from recent rockfalls were also seen on the N part of the 1 October dome.

Throughout the second week of the month there was intense steaming from the area around Castle Peak. On the ediface's SW side, heavy rains caused more rock slides on the SW face of Galway's Wall and its upper reaches appeared thin and unstable. On 14 November blocky light gray lava extruded on the dome's N flank, and on its S side it bulged.

On 17 November there were two rockfalls from the N side of the dome and the subsequent small pyroclastic flows were channeled down the canyon's E side. For the first time since the start of the 1 October dome growth, these rockfalls extended ~500 m from the dome, beyond the slope's break at the base of Castle Peak.

Late on 21 November, the new growth areas on the 1 October dome were clearly distinguishable as zones of continuous glow and occasional falls of glowing material mainly from the N and NE faces of the dome, and at times from the NW. Many of the rockfalls emanated from close to the top of the dome, just below several new small spines. On 22 November, the 1 October dome was seen clearly from the NW of the volcano for the first time. The dome was pale gray in color and blocky, in contrast with its scoriaceous, smoother, chocolate-brown appearance in October. Steam escaped at both the SW rim of the September 17-18 explosion scar, and to the W of the active dome.

On 23 November, several vertical cracks on Galway's Wall (SW section of the crater wall) appeared longer than before, extending down much of the face. The lava dome and talus had piled up behind this wall to a depth of ~120 m, with only ~30 m of wall remaining above it.

Growth during the last two weeks of November was predominantly in the N and SW sectors of the 1 October dome, resulting in flattening of the top of the dome into a broader plateau with no change in height. The volume of the 1 October dome was estimated as 4.47 x 10⁶ m³ (dense rock equivalent: DRE) on 24 November and 4.65 x 10⁶ m³ (DRE) by 1 December. This is equivalent to a mean extrusion rate of 47,500 m³/day, significantly less than the 86,250 m³/day estimated for the period 7-24 November. The active NE face of the dome fed two erosive chutes. About 180,000 m³ (DRE) of material was added to the developing talus fans associated with the new dome.

On 26 November, Galway's Wall showed signs of considerable weakening with a series of fractures visible on the wall surface, and talus from small rock avalanches on the inner and outer portions of the wall. The wall was largely composed of interbedded lithified talus and block-and-ash deposits from prehistoric eruptions, cross-cut by ~2-m-wide lateral sills and an anastomosing dike (~3 m wide). The base of the wall largely consisted of relatively thinly bedded tuffs dipping toward the center of English's Crater.

On the morning of 27 November, a large rockfall from Galway's Wall dislodged ~150,000 m³ of material. This avalanche traveled as far as the break in slope, ~300 m from the crest of the wall. Some small pyroclastic flows were also generated on 27 November; they reached a maximum runout of 800 m. Overnight on 27 November, heavy rainfall swept up old pyroclastic-flow material in the Tar River valley and landslide material in the White River and generated lahars. In the meantime, collapses from the 1 October dome caused small pyroclastic flows. Extremely limited dome growth took place in late November.

On 29 November a new series of NW-SE fractures dipping steeply E was observed on Galway's Wall. On 1 December two vertical fractures, trending ~60°, were seen E of Chances Peak near the intersection with Galway's Wall. These large fractures were >50 cm wide and extended at least a few meters below the surface of the wall.

Rockfall activity from Galway's Wall (both on the inner and outer walls) accelerated in association with an intense volcano-tectonic earthquake swarm from 30 November to 8 December. As a result, at least two more avalanches similar to that of 27 November occurred, along with many smaller events. Coarser rockfalls were associated with avalanching from the central portion of the wall.

Seismicity. A swarm of volcano-tectonic earthquakes on 1-2 November marked the most intense period of activity since dome growth began in November 1995 (BGVN 20:10); it was also the only swarm during the current activity with hypocenters deeper than 3 km. All the other earthquakes in the November swarms were <2 km beneath the crater. The week of 2-9 November was also dominated by a swarm of volcano-tectonic earthquakes (389 events). Another swarm (212 events) began on 9 November and lasted until 12 November. Shorter swarms of volcano-tectonic earthquakes occurred on 14 November (40 events), and on 19-20 November (53 events). Apart from these swarms, volcano-tectonic activity was limited to occasional single earthquakes during the rest of November.

The number of rockfall signals increased during 20-24 November, but the level of activity was still much lower than that in the months prior to the September 17-18 explosion. Rockfall activity returned to a low level by 26 November.

A strong inverse correlation between dome rockfalls and the shallow volcano-tectonic swarm activity was observed. Dome rockfalls were not completely absent during seismic swarms, but some may have been due to strong shaking of the unstable dome. On 28 November strong, continuous seismic signals were recorded at most stations in the seismic network. The signals were caused by pyroclastic flows in the Tar River Valley and debris flows along Tar River, White River, and Fort Ghaut.

A correlation between Galway's Wall landslides and seismic signals enabled retrospective identification of previous large landslides: the strong shaking caused by volcano tectonic events triggered landslides on Galway's Wall. Using this criterion, rock avalanches from the Galway's Wall were found to have occurred since at least as early as 24 October, but the largest of these by far occurred on 4 November. No landslides were recorded during the intense seismicity on 1 and 2 November.

The largest volcano-tectonic earthquake swarm recorded since [the start of the eruption] began on 30 November and ended abruptly on 8 December (figure 15). In that time, 1,671 earthquakes were recorded. Figure 14 shows data on the volcano-tectonic activity between 23 November and 8 December as well as the amplitudes of individual events at the Gages seismic station, which was closest to the activity. The plot of amplitude logarithm (proportional to the magnitude) shows that the size of the largest events slowly increased during this swarm. Some of the larger events (M ~3) were felt by residents of Weekes, the closest occupied area, on the NW side of St. Georges Hill (figure 14). A bimodal pattern to the magnitudes was observed, with small earthquakes (M ~1) dominating. During the increase of activity up until 5 December, the number of small earthquakes increased with time, while the number of large ones remained almost constant. This meant that the b-value of the earthquake distribution increased, although these earthquakes clearly did not follow a classical magnitude-frequency relationship.

MVO scientists postulated that the volcano-tectonic earthquakes were caused by pressurized magma at shallow depths. When the magma outlet becomes periodically blocked, a slow down of the dome growth and reduction in rockfalls occur. In the meantime the high-pressure build-up causes rock fracturing around the magma body. If the magma behaves as a non-Newtonian fluid it requires a certain pressure to yield and flow. The correlation between Galway's Wall landslides and the volcano-tectonic swarms suggests that magma pressurization is increasing the stress on the base of the wall. The occurrence of deeper earthquakes at the start of this phase of activity (mid-October) and during the increased activity of 2 November, suggests that the current phase is a response to some deeper volcanic activity, possibly injection of fresh magma.

Ground deformation. EDM measurements made on the E triangle (Long Ground-White's-Castle Peak) between 4 and 8 November were consistent with the recently established shortening trend of ~6 mm/day. The shortening trend on the E triangle continued during the second week of the month, although its rate slowed to ~5 mm/day. Between 18 and 30 November, the lines shortened ~6 mm/day. The last measurement on 4 December showed 3.8 cm shortening over a four-day period, a significant rate increase.

The N triangle (Upper Farrell's-St. Georges Hill-Windy Hill) was measured on 5 November. The lines from Windy Hill and St Georges Hill to Upper Farrell's shortened by 3 mm and 8 mm, respectively, since these were last measured on 27 October. These lines are quite erratic, and show long-term trends of little or no change. The Galway's-Castle Peak line (S triangle) was remeasured on 2 December, following replacement of the EDM reflector on Chances Peak. A line shortening of 2.6 cm since 19 October was recorded, a rate higher than the previous trend on this line.

EDM measurements on the W flank of the volcano on 7 November showed that the line lengths between Upper and Lower Amersham (near Plymouth) shortened by 1 mm since November 1, whereas those between Lower Amersham and Chances Peak lengthened by 5 mm during the same period, without following any particular trend.

Deformation data suggested that movements are confined to the upper flanks and are thought to be due to loading of the upper part of the edifice by the new dome, and localized thermal expansion and pressurization of the magma conduit at shallow depths.

GPS measurements on 3, 6, 16, 23, 25, 26, and 27 November and 5 and 7 December indicated that all line lengths and station heights did not undergo major changes and remained within the 95% confidence level of their long-term means. Only radial lines to the Farrell's benchmark high on the N flank of the volcano showed signs of movement, although within formal errors.

A dome-volume survey was made on 7 November using GPS equipment and range-finding binoculars. The volume estimate was, within error, the same as that obtained from the previous survey on 23 October (BGVN 21:10).

Gas, ash, and rainwater measurements. COSPEC measurements were carried out on nine days in November (3, 5, 10, 11, 13, 14, 16, 18, and 19). All COSPEC measurements were made by running traverses beneath the plume along the W coast road. The average flux readings were 371, 155, 240, 227, 178, 243, 176, 363, and 250 metric tons/day, respectively. These values are similar to recent measurements, indicating that only low amounts of sulfur dioxide are emitted from the volcano during periods of dome growth. Daily fluctuations do not suggest any strong link between SO₂ production and earthquake swarms. Analysis of SO₂ diffusion tubes at five sites W of the volcano showed that the averaged concentrations of SO₂ correlated with the COSPEC measurements.

Rainwater samples collected on 4 November from Upper Amersham, the site closest to the volcano, showed the lowest pH (3.3) recorded. Rainwater collected N and W of the volcano on 10 and 14 November showed continuation of the highly acidic rainfall, particularly W of the volcano. A sample from a pond in the upper Amersham area showed very high levels of chloride. Rainwater samples analyzed for the period 14 to 17 November had low pH values (3.0 to 3.6), with the exception of Weekes, which was neutral.

Hazard assessment. The alert level was raised from Amber to Orange early in the morning of 28 November because of the increased instability of Galway's Wall and fears that catastrophic collapse of the wall might cause a lateral blast. The risk map in effect during October (figure 14) was modified on 26 November because of the increased risk to St. Patricks and surrounding areas on the volcano's SW side. On 3 December, complete closure of zones A to D was recommended as a temporary measure, because the scientific team thought that a larger collapse was possible. Such an event would involve more of the S part of the crater wall, potentially causing major dome instability and pyroclastic flows in any direction. This change was formalized on 5 December with a temporary revision of the risk map, which included all of the S of the island, from Foxes Bay across to Spanish Point, within zone A/B.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/); The Montserrat Reporter, Plymouth, Montserrat (URL: http://www.tiac.net/users/wcwdaj/reporter/reporter.htm).

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12/1996 (BGVN 21:12) Dramatic fracturing on SW wall as dome growth continues

The following condenses the daily Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 9 December 1996-10 January 1997.

Visual observations during 9-31 December On 9 December it was noted that a crack on Galway's wall had opened 33 cm in five days. One side of the crack had moved by 7 cm, consistent with the wall being pushed outwards. Helicopter inspections on 10 December detected 20-m-deep cracks along and on top of this wall, making it very unstable. On 11 December a new dome appeared to the S of the 1 October dome, between Castle Peak and Galway's Wall (see map in BGVN 21:11). New fractures 100 m long and 1 m wide were seen at the E end of Galway's Wall.

On 14 December the new dome volume was estimated at 500,000 m³ ; having grown over a period of 2-3 days, its extrusion was comparable to the initial rates for the 1 October dome. On 15 December the new dome's top was at 910 m, higher than the October 1 dome at that time; growth had occurred along a linear structure oriented ESE. By 16 December the top of the dome was estimated at 920 m. Observations that day showed that the dome had nearly filled the scar left by the explosion in September and that a new spine had grown from its top.

On 17 December the new dome started to overflow the September explosion scar; this caused several moderate-sized rockfalls and small pyroclastic flows into the Tar River that traveled ~250 m from the dome. That day the new dome was 909 m high, its surface rubbly with coarse blocks, and its shape conical with a flat top and two spines. Comparison of recent dome surveys with previous results showed that older material near the new dome rose 80 m, a volume change of perhaps 7 x 10⁶ m³ since the beginning of December. On 17 December several large steam clouds ascended above the volcano, probably caused by steam venting from the new dome.

On 19 December the new dome's E face was near-vertical and appeared very unstable. Discrete pulses of rockfalls and small pyroclastic flows from the dome occurred only a few minutes apart; these descended as far as 1 km along the gully on the S side of Castle Peak creating many small ash clouds that rose 300 m above the crater and drifted slowly W. A dome survey carried out using laser-ranging binoculars estimated the new dome's volume at approximately 800,000 m³, yielding an extrusion rate of 0.5 m³/s. That evening both the E side of the new dome and part of the pre-September dome failed, causing moderately large pyroclastic flows. These flows traveled down the Tar River and over its fan reaching to within 40 m of the sea; ash clouds rose 3 km and were carried SW. As the flows were generated observations from the airport suggested that fresh lava emerged into the dome nearly as quickly as it was lost in the flows. The next day, ground and helicopter observations indicated a reactivation of the 1 October dome growth. Many small rockfalls descended the 1 October dome's N side, fewer from its S side. Small pyroclastic flows generated ash clouds characterized by little convection, possibly suggesting that colder material was involved. This was confirmed by observations during the night using an infrared imaging system. This imaging system also showed that the entire 1 October dome was active.

During the following days, rockfalls and pyroclastic flows caused many more ash clouds which deposited ash in Plymouth; associated clouds displayed robust convection, suggesting that hot, fresh material was involved. A helicopter inspection on 22 December confirmed that the activity was restricted to the 1 October dome and that there was no sign of activity on the 11 December dome.

On 23 December heavy rain caused mudflows in Fort Ghaut that carried half-meter diameter boulders into the sea. On 25 December some uplift was observed on the N flank of the October 1 dome, perhaps due to an injection of fresh lava. On 26 December satellite imagery showed ash ~100 km WSW at a height of 1-2 km.

Ground and helicopter observations on 26 December showed a significant amount of new material on the top of the dome, darker in color and smoother than the older material. On 29 December, glowing all over the NE flank and avalanching of incandescent blocks were observed. Incandescence in daylight suggested that this dome lava may have been hotter than previous dome lavas. A considerable amount of material was observed on 31 December falling down the N flank of the pre-September dome towards Farrell's Wall. The new material at the top of the dome had changed texture, and looked more slabby than before.

Visual observations during 1-10 January 1997. The first seven days of January were characterized by numerous rockfalls and small pyroclastic flows from the 1 October dome, mainly down its NE and E sides. Much of this activity was channeled into the Tar River valley by way of either an erosion chute cutting across the top of Castle Peak or one to its N. At times of peak activity the pyroclastic flows occurred every few minutes and the largest traveled ~300 m past the Tar River Soufriere. Many of the rockfalls and flows generated ash clouds that drifted W and SW, forming a semi-continuous ash plume observed at altitudes of 1.3-1.6 km. On 3 January the plume was reported at 2 km altitude, and on 4 January satellite observations detected the plume 360 km W of Montserrat.

Theodolite measurements of the dome on 5 January showed that although the height had remained relatively constant at ~900 m since 1 January, a new lobe of lava at the top of the dome was ~50 m thick. It was calculated that 4.6 x 10⁶ m³ of material was added between 25 December and 5 January, an extrusion rate of 4.4 m³/s. This was the highest sustained extrusion rate yet measured during this eruption. A helicopter inspection on 5 January revealed material slowly accumulating against the N crater wall; only 7 m of ridge remained above the divide to Tuitt's Ghaut. On 6 January the glowing dome appeared less steep in its upper part.

On 8 January several pyroclastic flows originating from behind Castle Peak moved down the Tar River Valley to reach beyond the Tar River Estate House; at least one pyroclastic flow reached the sea. Further growth was observed on the NW side of the 1 October dome, but was still contained inside the 17-18 September scar. Several new glowing channels eroded by the pyroclastic flows on the E side of the dome were visible. On 10 January a new, unstable-looking extrusion was observed in the middle of the heavily eroded chute crossing Castle Peak. This new extrusion was butterfly shaped and composed of slabs of fresh lava.

Seismicity and seismically detected mass wasting. Seismic activity during 9-11 December was characterized by swarms of shallow volcano-tectonic earthquakes, at times large enough to be felt close to the volcano. A few rockfalls from the dome and some landslides from the Galway's Wall were also detected by the seismic network, indicating that the wall became increasingly unstable during intense earthquake activity. However, a lack of seismicity on 12 December was accompanied by more landslides on Galway's Wall. During the following days rockfalls occurred sporadically, but their number slightly increased after 16 December, as the 11 December dome kept growing. Also, a few landslides from Galway's Wall suggested continued slow deformation. Seismicity increased on 20 December with a shallow volcano-tectonic earthquake swarm that reached the level of intensity of the early December swarms, although maximum magnitudes were not as large as before. Several rockfalls were also detected, mostly from the 1 October dome.

On 22 December the volcano-tectonic seismicity died out, rockfall signals continued, and hybrid seismicity reached April levels. This and increases in the quantity of ash and pyroclastic flows were taken as an indication that the dome growth rate had increased, but poor visibility prevented dome observations. By 24 December hybrid events and continuous tremor dominated the records, but by 27 December banded tremor reached a maximum. Banded tremor, which was last seen between late July and mid-September, had taken place associated with large pyroclastic flows and the 17-18 September explosion.

On 28 December large hybrid events and rockfall signals dominated, but regularly spaced bands of continuous seismic tremor returned on 30 December. Lower in amplitude than before, the banded tremor occurred at ~10-hour intervals. By 31 December the activity was again dominated by banded tremor episodes ~5 hours apart, and by hybrid earthquakes and rockfall signals. This pattern of seismicity continued during the first seven days of January. Volcano-tectonic earthquakes returned on 4 January with signals similar to the November and December events, but possibly from slightly greater depths (2-3 km). From 8 to 10 January, in correspondence with increased dome activity, the seismicity became dominated by rockfall and pyroclastic-flow signals.

COSPEC, EDM, and other measurements. COSPEC measurements were made on 27 and 28 December. The data from 27 December averaged 350 metric tons/day (t/d) but reached ~400 t/d shortly after one of the peaks in seismic tremor. The average fluxes on 28 December, and on 1, 4, 9, and 10 January were 325, 300, 400, 1,130, and 390 t/d, respectively. The increase in emission of SO₂ measured on 9 January was probably due to a partial collapse of the dome.

EDM measurements carried out on the E triangle on 10 and 13 December suggested a continuation of the shortening trend started several weeks earlier. On 16 December, the S triangle had been unchanged since 4 December; on 18 December the lines N of the volcano had also not changed significantly since 5 November. The average shortening on the lines to Castle Peak during 20-22 December was 6 cm; such high rates of deformation had occasionally been seen in the past. In contrast, the shortening seen there during 26-28 December was small (2.4 mm). This drop in the deformation rate roughly coincided with the appearance of new material at the surface on the 1 October dome.

Gravity measurements on the E flank on 22 December showed no significant changes since July 1996. Changes at stations on the upper slope were consistent with the mass added to the dome.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/ volcanoes/west.indies/soufriere/govt).

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01/1997 (BGVN 22:01) Ongoing dome growth, pyroclastic flows, and crack dilation

The following condenses the daily Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 12 January-8 February 1997. This interval was marked by noteworthy pyroclastic flows and visible dome destruction and growth. Galway's Wall, on the volcano SW side, underwent sufficient erosion to allow escape of some dome talus. A photo (figure 16) taken on 28 January, and associated sketch (figure 17), shows the dome within the breached English's Crater.

Visual observations. On 13 January (between 1130-1300 and 2215-2305) two very large pyroclastic flows originated from the SE part of the 1 October dome at a spot close to the 11 December dome. The pyroclastic flows traveled down the S side of the Tar River valley to the sea. These flows eroded a chute to the S of Castle Peak, and created a scar in the 1 October dome. This scar became the site of rapid dome growth from the next day (14 January), when incandescence, small rockfalls and steaming and ash venting from the dome were reported. A helicopter flight on 14 January disclosed new talus deposits along the E and W margins of Galway's Wall.

On the morning of 16 January (between 0530 and 0630) the largest single pyroclastic flow in this eruption occurred. Reports from the airport described a rumbling of rocks and ash accompanying a glowing cloud down to the sea. The entire fan was covered by new material that included blocks 1-2 m in diameter (up to 5 m diameter at the head of the fan). The deposit was quite narrow in the upper reaches of the Tar River valley. The S chute cut back deeply into the dome to the S of Castle Peak, creating a large amphitheater into the mass of the 1 October dome ("Santa lobe") and the 11 December dome. Smaller pyroclastic flows occurred later that day; finally a prominent scar estimated to be ~8 m deep and running NE-SW was noted across the dome. The ash cloud from this activity rose to 6.1 km. The low-level ash was carried SW, causing heavy ashfall in Plymouth. At higher levels, the winds blew the ash S and E, and light ashfall was reported from Guadeloupe, 120 km SE.

On 19 and 20 January observers at Whites saw new extrusions growing rapidly within the scars formed by collapses of the SE flank of the composite October-December dome.

Pyroclastic flows began at 1830 on 20 January and lasted for about one hour. They originated from the SE part of the October-December composite dome, traveled down the S Tar River, and ignited some trees on the lower slopes of Perches Mountain before reaching the sea. A scoop-shaped excavation was carved into the SE flank of the composite dome, in the same place as the 16 January excavation, but larger in size.

The ash cloud from the 20 January pyroclastic flow rose to ~10 km. Low-altitude winds blew ash SW. Higher altitude winds blew ash NE, where it mixed with rain and fell as wet ash over most of N Montserrat. It was suggested that the higher amount of ash production on 20 January, compared with 16 January, was due to the higher lava temperature.

On 22 January, about 40 hours after the activity began, observers noted new growth and a large flower-shaped extrusion within the scar. During the first week of growth, this extrusion was contained within the scar, and only a few rockfalls entered the Tar River valley.

The largest rockfall on 26 January reached only to the foot of the dome. On 28 and 29 January, rockfalls were more common to both the N and S of Castle Peak, and occasional small pyroclastic flows were observed. The dome's vertical growth slowed, as it expanded laterally to the E and NW.

Evidence of further landslides from the Galway's Wall, on the S part of the crater rim, was seen on 26, 27, and 28 January. By 29 January, the central part of the wall's top was flush with the talus of the pre-September dome, and a small amount of dome material had fallen over the wall, leaving a minor scoop in the dome.

From late January through 8 February frequent small pyroclastic flows occurred, often during episodes of tremor. On 1 February a pyroclastic flow caused an ash cloud that drifted NW, depositing ash in Corkhill, Salem, and Old Towne. On 2 February, small pyroclastic flows accompanied by fountains of ash were reported from the area above Castle Peak. On 1 and 2 February heavy rain caused destructive flash flooding in Fort Ghaut. Much of the vegetation on the W slopes of the volcano was destroyed by months of heavy ashfall and acidic rain.

By 3 February, the remains of Castle Peak had either been completely buried or removed by pyroclastic flow activity. On 6 February a pyroclastic flow reached the sea; two flows on 7 February reached about half-way across the delta. The second flow was observed from the air and from Whites. During that event a collapse at the dome's SE face caused four pulses of small-scale decompression, resulting in ash jetting and release of small projectiles.

The most significant development at Galway's Wall during this period occurred sometime around 1-2 February, perhaps associated with heavy rains. A notch was cut through the upper wall at the western end, along one of the near-vertical zones of weakness in the wall. Dome material funneled through the wall, and fell on top of the landslide deposits in the upper Galway's area. The notch was first seen on 4 February, and the volume of material involved did not seem to increase significantly during the rest of the week.

Dome volume measurements. Volume measurements were made of the 20 January dome on 26 and 28 January, giving volumes of 1.46 and 1.52 x 10⁶ m³, respectively. It was also estimated that an accumulation of 150,000 m³ of talus blocks developed at the base of the new extrusion. A survey on 6 February indicated that around 2.3 x 10⁶ m³ of material were produced in nine days, giving an extrusion rate between the 28 January and 6 February of 2.9 m³/s. The average extrusion rate for the 20 January dome up to 6 February was 2.7 m³/s.

Helicopter surveys of the Tar River delta on 5 and 7 February showed that the total volume of the deposits was 14.9 x 10⁶ m³, of which 7.4 x 10⁶ m³ lay above sea level. The previous measurement on 27 September 1996 gave a volume of 11.6 x 10⁶ m³ with a sub-aerial volume of 4.9 x 10⁶ m³. The delta was also measured 600 m offshore, where it was 1.3 km across at its widest point. Even though the delta had not extended significantly since late September, it widened by 150 m and gained 10 m in height at the central point.

Seismicity. Rockfalls, pyroclastic flow signals, and volcano-tectonic events dominated the seismicity during this observation period. Swarms of volcano-tectonic earthquakes were as usual located at shallow depths beneath the crater. All of the major pyroclastic flows were preceded by earthquake swarms, suggesting that the earthquakes may be caused by pulses of magma moving to the surface, with these pulses then causing dome collapses a short time later. Out of nine earthquake swarms, three were followed by pyroclastic flows in the subsequent 27 to 66 minutes. Low-amplitude banded tremor was also observed during this reporting period. The typical tremor duration was about two hours, and the tremor episodes were ~8 hours apart. The amplitude of the tremor increased slightly during 15 January, prior to the collapse early on 16 January. Another tremor episode started before midnight on 16 January, and continued at regular intervals until the end of the period.

In general, after 21 January, volcano-tectonic swarms became fairly regular, occurring twice per day at ~12-hour intervals. This pattern was similar to that observed in the two weeks prior to the explosion of 17 September 1996, when the time interval between the swarms gradually decreased to 4 hours immediately prior to the explosion. At the end, almost all the swarms were accompanied by 2-5 long-period earthquakes (dominant frequencies, ~1 Hz or lower). Again, during 26 January-1 February the volcano-tectonic swarms were regularly spaced, with ~12 hours between each swarm, apart from a gap on 28 January. The duration of the swarms was variable, but generally 1-2 hours. The swarms started at or just after the peak in the theoretical solid-earth tide.

From 2 February, there was an increase in amplitude and duration of tremor episodes that lasted for 40-140 minutes; a gradual increase in amplitude was followed by an abrupt decay. At first the tremor built up gradually from background noise levels, but in the latter part of the observation period the tremor started as individual hybrid earthquakes that grew in number until they formed a continuous signal. During about half of these episodes, pyroclastic flows occurred at or shortly after the peak in tremor.

The time between the earthquake swarms gradually increased between 2 and 7 February. Maximum tremor amplitude also increased, with the highest amplitude recorded on 6 February. Thereafter the maximum tremor amplitude decayed, and no tremor was detected following a swarm on 8 February. Tremor was generally non-harmonic, with frequency in the 1-2.5 Hz range. However, harmonic tremor was recorded during a short interval close to the peak intensity of a tremor episode on 6 February, and may have been present for short periods during other episodes. The dominant frequency of this tremor was 1.25 Hz.

COSPEC and other measurements. On 16 January correlation spectrometer measurements from a boat along lines off the SW coast yielded average SO₂ values of 344 metric tons/day (t/d) at 5.5 km from the dome and 233 t/d at 8.5 km. Measurements were made on the same day along the SW road within a few hours of dome collapse, but due to the wind direction, it was only possible to measure for half of the plume, which yielded a value of 1,017 t/d. On 21 January, 1,260 t/d were measured, a value typical of recent measurements immediately after dome collapses. Daily averages on 24, 29, and 31 January, and 4, 6, and 7 February were 590, 796, 495, 323, 446, and 652 t/d, respectively. During the 7 February traverse, the SO₂ flux maxima (up to 871 t/d) was coincident with tremor and pyroclastic flow activity.

Scientists measured net movement along two prominent cracks on Chances Peak, located at the N intersection of Galway's Wall and Chances Peak. From 12 to 22 January, Crack 1, the most southerly of the two cracks, was oriented NE-SW and showed an extension of 0.69 cm/day and shear of 0.31 cm/day . From 22 to 26 January extension increased to 1.6 cm/day and shear to 0.7 cm/day. From 26 to 28 January extension increased to 1.75 cm/day and shear remained at 0.7 cm/day. Measurements at Crack 2 (22-26 January) showed a steady movement of 0.7 cm/day. Between 26 and 28 January shear increased slightly from 0.06 cm/day to 0.25 cm/day.

SO₂ diffusion tubes downwind of the volcano showed a gradual decrease in this gas species throughout the period 1 October to 30 December, a drop similar to the one prior to the September 1996 explosion event. Dust samples collected in Plymouth in early October had high numbers of particles <0.7 µm in size.

Water samples showed high acidity levels (pH 2-3) in the Amersham area, as a result of acid rain forming in the plume; this effect decayed with distance. Rain water samples collected from various locations around the volcano showed that the rainwater directly W of the volcano continues to be highly acidic and has revealed concentrations of sulfites, chloride, and fluorides.

On 1 February, a Nettlton gravity profile across Whites Ghaut yielded a density of 2.1 g/cm³. On 30 January and 4 February, GPS networks showed that all slope lengths were within two standard deviations of the long-term means since June 1996.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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02/1997 (BGVN 22:02) Dome growth and pyroclastic flows continue; alert status downgraded

The following condenses the daily and weekly Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 9 February-8 March 1997. This interval was characterized by continued dome growth on the crater's E side; late in the interval the NE side of the dome became increasingly unstable. Seismic activity was dominated by repetitive swarms of volcano-tectonic and hybrid earthquakes, with a few episodes of tremor. Also, on 17 February the alert stage at the volcano was reduced from Orange to Amber (table 13).

Table 13. Alert status codes used for Soufriere Hills volcano. Courtesy of MVO.

    NUMBER  ALERT STAGE  OBSERVED PHENOMENA

    0       White        Background seismicity, no new surface
                         manifestation of volcanic activity.
    1       Yellow       Enhanced local seismic activity, ground
                         deformation and mild phreatic activity.
    2       Amber        Dome-forming eruption in progress, periodic
                         gravitationally induced collapses including
                         pyroclastic-flow and rockfall generation.
    3       Orange       Change in style of activity anticipated
                         within a few days. Increasing rockfall and
                         minor pyroclastic-flow activity with
                         associated light ashfall.
    4       Red          Dome collapse under way, pyroclastic flows
                         in valleys adjacent to area of collapse and
                         ash to the W and NW. Explosive event
                         distinct possibility if activity continues.
    5       Purple       Ongoing explosive eruption with heavy
                         ashfall.

The results from a dome survey, undertaken between 18 February and 1 March, show that there was ~42 m of vertical dome growth. The highest point on the dome was measured at 942 m elevation.

Visual and satellite observations. On 10 February, near-continuous rockfalls traveled down the steep face of the dome that began extruding on 20 January (hereafter, "20 January dome"; see BGVN 20:01). Lava extruded from the E side of the dome and moved downhill as fast as it was emitted. Also, two clearly defined chutes cut into the dome's E face at rate that appeared to be equal to the extrusion rate.

Galway's Wall eroded further as dome material continued to fall over the crater wall's top and into the upper reaches of the White River. The dome material generally followed two clearly defined gullies cut into the steep face of the wall. Fresh rockfalls extended to ~50 m above the road at Galway's Soufriere (<1 km S of the dome).

A GOES-8 satellite image, acquired at 1215 on 10 February, revealed an eruption plume that extended 65 km WSW. The plume's maximum width was 10 km. Upper-air analysis suggested the plume rose to ~3,000 m. Another GOES-8 image, acquired at 1415 the same day, showed the plume extending ~120 km WSW. Analysis indicated that the plume remained below 3,000 m.

On the morning of 12 February, a large pyroclastic flow down the Tar River reached about halfway across the delta. Another flow on the evening of 14 February reached as far as the Tar River Estate House (~2 km NE of the dome).

A rockfall deposit in the White River Valley was also observed on 13 February. The deposit did not extend as far as that of 10 February, however it was more voluminous close to the base of Galway's Wall. The deposit had a lobate morphology with well-defined, gravelly levees. Several small, slow-moving, dust-generating rockfalls originated from three well-defined gullies in the top of the wall. By 15 February, the eastern side of the dome had steepened and had a flower-shaped structure with radiating pinnacles at the top of the face. On 15 February a rockfall over Galway's Wall generated a small ash cloud.

During the week of 17 February large blocks fell off the dome's face and moved slowly outwards following deep gullies. At night, incandescence emanating from the dome's face was strongest within these gullies. Galway's wall continued to degrade slowly with the three gullies in the top of the wall becoming more pronounced.

A fresh rockfall deposit observed on 17 February extended as far as the previous longest flows (~1 km from the dome). Several small pyroclastic flows were also observed during the week; most originated from the SE part of the dome and flowed into the Tar River valley. The longest pyroclastic flows reached 1.5 km on the night of 17 February.

On 18 February, the NE part of the dome showed increased activity, with several small pyroclastic flows traveling N towards Farrell's Wall. During 18-27 February the dome mainly grew vertically, rather than the outward growth of the E face that had characterized the previous weeks. On 27 February, observations revealed a small spine at the summit of the January dome. The SE flank of the dome was modified by avalanche; two large blocks, observed on the talus slope on 23 February, were no longer visible, and the deep gullies in the E face were less pronounced. Small rockfalls and pyroclastic flows continued from the N and SE flanks of the dome. One of the pyroclastic flows from the N flank sent ash over Plymouth at 1515.

During a period of increased seismicity on 28 February, several rockfalls occurred on the SE, E, and N faces of the dome. Many of the rockfalls were composed of hot material and resulted in small convective ash clouds. Slow crumbling of Galway's Wall continued, and by 1 March a fourth gully, E of the previous gullies, developed in the top of the wall. Small rockfalls continued to fall over the wall.

Between 1 and 7 March, slow upward growth of the 20 January dome continued, but at a slower rate than the previous few weeks. The NE face of the dome began to show signs of activity as rockfall gullies developed and the entire face began to crumble. Small rockfalls from the active faces of the January and October domes were nearly continuous. Some of these falls generated small pyroclastic flows with accompaning ash clouds that drifted W. Incandescence was observed at night from the active areas of the 20 January dome and gullies in the October dome. Areas of incandescence shifted throughout the week moving from the E to the N and NE faces of the dome.

Dome volume measurements. Between 20 January and 22 February dome growth was confined to the SE area of the dome within the 20 January scar. Extrusion rates varied from 2.7 to 3.2 m³/s, slightly above rates seen since June 1996.

On 1 March a survey concentrating on the 20 January dome indicated that growth since 18 February continued to be confined within the 20 January scar; extrusion was primarily vertical and the dome's height increased 38 m. Talus lying E of the dome's high point had thickened 40 m. Dome profiles from White's (figure 18) showed an increased progradation of the talus slope to the E and also indicated that lateral spreading of the dome, established in the last few weeks, continued. A wedge of the October dome, adjacent to the contact of the October and January domes, appeared to have been thrust up ~ 10 m by the recent extrusion from the 20 January dome.

The volume increase between 18 February and 1 March totalled 3.42 x 10⁶ m³. Added to the previous total amount of dome extrusion, the new total volume was 43.7 x 10⁶ m³. In terms of dense rock equivalence, the total volume of erupted magma, including pyroclastic-flow deposits, was estimated at 72 x 10⁶ m³. The average extrusion rate for the 11-day study period was 4.37 m³/s.

Seismicity. Although seismicity was generally low, signals were predominantly attributed to rockfalls, small pyroclastic flows, volcano-tectonic and hybrid earthquakes, and occasional episodes of continuous tremor.

Earthquake swarms often began as volcano-tectonic events and led into hybrid events during the swarm' second half. Particularly towards the end of the swarms, hybrid earthquakes generally occurred every 1-2 minutes. These swarms did not culminate in periods of continuous tremor, unlike some of the swarms observed in previous weeks (BGVN 22:01). However, occasionally the hybrids joined to form short periods of high-amplitude tremor. Between 26 and 28 February three short episodes of continuous tremor were recorded, each lasting 1-2 hours.

The seismometer at Gages continued to record high-frequency tremor, previously referred to as broadband tremor (frequencies >3 Hz). This type of event has been observed throughout the eruption but has yet to be correlated to any visual phenomena.

COSPEC and other measurements. SO₂ flux measurements were made on 25 January, 1- 3 March, and 5 March. The results were 217, 165, 106, 174, and 159 metric tons/day (t/d), respectively. These values are lower than the last values recorded at the beginning of February (300-700 t/d; BGVN 20:01).

SO₂ diffusion-tube samples, representing the period between 29 December 1996 and 9 February, were analyzed and showed that levels of the gas remain low. This follows a trend that began during the period 1 October-29 December 1996.

Water samples showed that the rainwater directly W of the volcano continued to be acidic and contained high concentrations of certain anions (table 14). Throughout the period, pH levels at individual sample sites fluctuated only slightly. The sample collected from the overflow of the water storage tank at Fairfield (Trials Reservoir) contained anion concentrations and pH levels well within World Health Organization guidelines for drinking water.

Table 14. Rain and surface water geochemistry at Soufriere Hills, 2 March 1997. Courtesy of MVO.

    LOCATION             pH      CONDUCTIVITY   TOTAL DISSOLVED
                                   (mS/cm)        SOLIDS (g/l)

    Upper Amersham       2.60       0.448             0.224
    Lower Amersham       2.85       0.911             0.456
    Police HQ, Plymouth  3.31       0.237             0.118
    Weekes               6.20       0.402             0.201
    Trials Reservoir     7.68       0.618             0.309

    LOCATION             SULFATES    CHLORIDES   FLUORIDES
                          (mg/l)      (mg/l)       (mg/l)

    Upper Amersham          24         118         >1.5
    Lower Amersham           3          52          1.4
    Police HQ, Plymouth      3          53          0.8
    Weekes                   3          44          0.65
    Trials Reservoir        38          90          0.35

Scientists continued to monitor the two widening cracks on Chances Peak. On 11 February, Crack 1, the more easterly of the two cracks, had widened by 44 cm (2.75 cm/day average) and exhibited a dextral shear displacement of 23 cm since the last measurement on 28 January. The original crack had also developed several bifurcations and smaller subparallel cracks. Due to safety precautions, no further measurements will be made on this crack.

Crack 2, ~100 m from the hut on top of Chances Peak, had widened by 2.6 cm with a shear displacement of 6 cm. Before failing on 24 February due to ashfall, an extensometer across the crack transmitted real-time data. Much of the time the crack had opened at an average rate of 0.5 mm/day. Five periods of increased deformation also occurred, coincident with volcano-tectonic swarms (figure 19). The maximum deformation rate was 8 mm in less than 4 hours. During 28 January to 24 February, the total shear displacement along the crack was 18 cm.

EDM measurements were made to the Chances Peak reflector on 17 February. The line from Galway's Plantation to Chances Peak had shortened by 18 mm since the last measurement on 2 December, a reduction in the rate of shortening. Measurements were also made on 19 February on the line from Amersham to Chances Steps. Results there continued to show no significant deformation.

GPS surveys of the WESTNET and EASTNET networks were carried out on 5 and 6 March, respectively. There were no significant changes in baseline length, station position, or station height, indicating an absence of a widespread strain field associated with the eruption.

On the other hand, GPS measurements at one site (FT3), 80 m NW of the dome have shown large movements. The baseline to Harris (M18) shortened by 6.5 cm between 18 January and 3 March. The station had moved radially away from the dome (to the NW) by 12.9 cm (on 3 March) and 14.5 cm (on 8 March). Since 18 January, the station height has remained stable with all occupations yielding values within 1.6 cm of one another.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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03/1997 (BGVN 22:03) Pyroclastic flows advance over Galway's Wall on 29 March

The following summarizes the weekly Scientific Reports of the Montserrat Volcano Observatory for the period 9 March-5 April 1997.

Visual observations. During the first 20 days of March several ash clouds drifted W on the prevailing wind, small pyroclastic flows issued from the E and S areas of the dome, and small-scale rockfalls were confined to the area SE to NE of the dome complex.

Small, relatively cool, pyroclastic flows with a maximum run-out distance of ~1 km were almost continuous from the pre-17 September dome to the N of Galway's Wall (see map in BGVN 22:02). An erosional chute was formed in the pyroclastic-flow deposit leading out from the crater wall. Small landslides occurred from areas E and W of the point on the wall over which the flows traveled. On 18 March fresh deposits with well-developed levee structures reached beyond 1 km from the crater wall to the SW. On 20 March new fractures 100-150 m long and trending SSE were observed running through the S buttress of Galway's Wall, in the area adjacent to Perches Mountain.

Growth continued in the uppermost areas of the 20 January dome: during 13-21 March new spines appeared on the summit area and afterwards moved up toward the E side of the dome. Eventually extrusion in the summit region overgrew the original January scar and overall the shape of the dome changed from flat topped to a more conical geometry.

During the week of 22-29 March a large block tilting to the SSW appeared in the SW region of the dome complex. Two distinct peaks began to develop in the summit area of the dome, the highest being on the S side of the dome overlooking the Galway's Wall. A cleft formed between the two peaks, with material being extruded upwards and to the SW. The dome grew so much above Galway's Wall that there was no barrier left between the new material and the wall itself. Fresh cracks were observed running through the E shoulder of the Galway's Wall on 25 March. Fresh landslide scars and cracks were observed in the Gages Wall on 26 and 28 March, but no fresh activity was noted in the dome behind and above it.

At 1630 on 29 March a large pyroclastic flow occurred over the Galway's Wall into the White River valley. The flow traveled ~400 m farther than previous flows in this region and produced a dark ash cloud that rapidly convected to ~1,500 m. Activity increased at around 1330 on 30 March when another pyroclastic flow occurred over Galway's Wall from the SE summit. Observations from the helicopter of the Galway's Soufriere region revealed that the pyroclastic material was cascading over the Galway's Wall almost contiguously, with the higher velocity flows surging through the smaller, slower-moving flows. Vigorously convecting, co-ignimbrite-type ash clouds rose to heights of 3.5 km. Pyroclastic-flow activity waned at around 1630 after flows traveled 3.6 km down the White River and caused some burning of vegetation. Trees in the distal portion of the flows remained standing, suggesting sluggish movement of flows in the lower part of the valley.

The intense pyroclastic flow activity on 31 March sent material only ~50 m farther down the White River than the previous day. Pyroclastic flows observed from a helicopter on 31 March were valley-confined; on the W side of Galway's Soufriere there were fine-grained deposits and tree flattening associated with pyroclastic surges. Considerable ponding of pyroclastic-flow deposits had occurred, and the Great Alps Falls in the White river were reduced to only ~10 m high from the original 50 m. Finally it was observed that the pyroclastic flows had cut a gully 80 m deep and 50 m wide into Galway's Wall.

The "Easter scar"— the collapse scar formed in the dome complex — was composed of two scallops, one in the 20 January dome with a near vertical head wall, and the second cut into the pre-September dome. Growth of the dome since collapse, and rockfall debris, have rapidly begun to fill the scar.

At around 1500 and 1515 on 31 March two major pyroclastic flows from the NE summit of the dome occurred in the Tar River to the E. The first flow reached ~200 m past the first break in slope, the second flow reached to within ~50 m of the fan. Temperature patches positioned down the track leading into the Tar River valley were engulfed by the surge clouds of the latter flow, and indicated temperatures of 99-149°C for lateral distances of 60 m inside the pyroclastic surge.

Several relatively large pyroclastic flows occurred over the Galway's Wall starting from 1230 on 1 April, but none of them reached as far as those on the 30 and 31 March although considerably more tree flattening occurred in the area directly W of the Galway's Soufriere. This indicated that these flows had a large surge component, probably due to the earlier valley filling.

Large ash clouds associated with the flows rose to 4.3 km and drifted NW producing ash fall over large part of the island as far N as St Peters and St Johns. On 2 April more pyroclastic flows over Galway's Wall generated ash clouds to ~3.3 km of altitude.

Dome growth since the collapse was confined to the Easter scar with upward and southward growth of the steep head wall. The actively growing area had a smooth, scabby, arcuate upper surface with local fractures and N-S running striations indicative of extrusion. Vigorous brown gas jets were seen emerging from cracks in the upper surface. Rockfall debris began to fill the chute carved by the pyroclastic flows into the Galway's wall toward the end of the reporting period. Mudflows during the nights of 3 and 4 April in Fort Ghaut and Aymer's Ghaut left debris on roads and close to houses.

Seismicity. During 8-21 March there were swarms of mainly hybrid events interspersed with periods of relative quiescence. The foci were located at 1-3 km depth below the crater area. Rockfall activity was mainly concentrated in periods between the earthquake swarms, although some of the larger events during a swarm were followed by rockfall and pyroclastic signals from material cascading over the Galway's Wall.

After 22 March the seismicity decreased. The swarms became shorter and less intense, whereas there was a slight increase in the level of rockfall activity and in the number of long-period earthquakes. Occasionally long- period events were present for a few days to weeks, with a maximum of 40 events/day, mostly very small. About 50% of the long-period earthquakes were immediately followed by rockfall signals, as in October and December 1996. It is possible that the long-period events are caused by some dome process, such as gas venting or a sudden growth spurt that leads to partial collapse.

During 30 March-2 April, the dominant seismicity was related to dome collapse, with many rockfall and pyroclastic-flow signals. The level of rockfall and long-period activity decreased abruptly on 3 April, when a swarm of volcano-tectonic events followed by hybrid earth a very slow trend of shortening, respectively. EDM measurements on the N triangle (Windy Hill-Farrells-St. George's Hill) showed an overall stable trend. On 29 March a very slow shortening trend was recorded for the line Windy Hill-St. George's Hill: the total shortening over the past 15 months was ~15 mm.

GPS occupations on 10-11 March with a base station at Harris showed that Hermitage station had moved by 2.5 cm to the NNE since 18 January and had risen by ~9 cm. A GPS occupation of the Eastnet on 15 March recorded a total movement of 2.5 cm to the NNE for Farrells station, ~700 m from the N edge of the dome, since 13 June 1996. During 22-29 March GPS occupations with a base at Harris showed that Station FT3 (Farrells Crater Wall) , had moved 17.6 cm to the NW since 18 January (2.7 mm/day), Hermitage had no significant movement since 17 March, and Perches recorded a 1.6 cm movement to the N since 18 January.

A new crack on the shoulder of Galway's Mountain was measured for the first time on 25 March, with nails hammered into trees on either side of the crack. One array of nails was placed on the steep flank of the mountain at ~ 50 m from the crater wall; the second array was 90 m to the S in a flatter area. All the lines measured on 28 March showed no significant changes in length.

The GPS occupation of Eastnet on 30-31 March and 4 April revealed that the Farrells site had moved ~4 cm to the N since June 1996 at an increasing rate of movement.

Both the GPS and EDM techniques showed the ongoing slow deformation of the N crater wall in the Farrells area; deformation rate drops rapidly with distance from the dome. Neither technique was able to detect any significant deformation around the volcano.

Dome volume measurements. A survey completed on 14 March using the fixed location photographic method showed 1.34 x 10⁶ m³ added to the dome since 1 March, at an average extrusion rate of 1.08 m³/s.

A GPS survey of the talus at the base of the dome combined with the fixed-location photographic method and the GPS/range-finding binocular method resulted in an estimate of 0.8 x 10⁶ m³ material added to the dome from 14 to 19 March. From the photographic profiles it became apparent that the summit dome had grown by 15 m during the same period.

Evidence was found that the pre-September scar material, surrounding the dome on the NW, W, and SW sides, was pushed outward by the growing dome. The amount of movement was 3.9 m during 23 November to 8 January (80 mm/day), and 9.1 m during 8 January-19 March 1997 (13 mm/day).

A GPS bathymetry survey around the pyroclastic fan at the foot of the Tar River Valley on 21 March, combined with a survey of the fan surface on 12 February resulted in a total fan volume estimate of 15.5 x 10⁶ m³.

The results of a 27 March GPS survey indicated that since 19 March the dome volume had increased by 0.98 x 10⁶ m³, at a rate of 1.26 m³/s. This gave a total dome volume of 49.7 x 10⁶ m³ (44.7 x 10⁶ m³ DRE). Digital elevation models created from this survey indicated that growth was focused on the S peak of the dome and the rest of the dome remained relatively unchanged. A GPS dome survey on 2-3 April indicated that the last collapse removed ~1.6 x 10⁶ m³ of material, of which roughly 40% was preSeptember 1996 scar material and 60% new dome material.

Environmental monitoring. Measurements of sulfur dioxide flux were made using the MiniCOSPEC on 10, 14, 15, 17, 24, 28 March, and 4 April and results were as follows: 700, 213, 341, 317, 198, 160, and 573 t/d respectively. The high values on 10 March and 4 April were associated with the recurrence of earthquake swarms and an increase in activity, respectively.

Results for SO₂ diffusion tubes collected during the period 9-23 February showed values similar to those measured over the last few months and are presented in table 15.

Table 15. Sulfur dioxide diffusion tube results at Soufriere Hills for the period between 9 February and 23 February 1997. Courtesy of MVO.

    LOCATION             SO2 (ppb)
		
    Upper Amersham        47.70
    Lower Amersham        17.30
    Airport                0.80
    Police HQ, Plymouth    9.00
    Weekes                 9.00
    Control                0.00

Results from rain water samples collected at 4 locations around the volcano on 9, 16, 23, and 31 March, showed that the rainwater directly W of the volcano was still highly acidic and had high concentrations of certain anions (table 16). One sample collected from the overflow of Trials reservoir in Fairfield was within World Health Organization levels for all measured components.

Table 16. Rain and surface water geochemistry at Montserrat. Courtesy of MVO.

    DATE        LOCATION               pH      CONDUCTIVITY     TOTAL
                                                              DISSOLVED
                                                 (Ms/cm)     SOLIDS (g/l)

    09 March    Upper Amersham        2.39        2.120         1.050
                Lower Amersham        2.55        1.162         0.582
                Police HQ, Plymouth   2.57        0.926         0.464
                Weekes                6.49        0.172         0.086
    16 March    Upper Amersham        2.39        1.883         0.942
                Lower Amersham        2.70        0.731         0.366
                Police HQ, Plymouth   2.81        0.571         0.285
                Weekes                6.11        0.070         0.035
                Trials Reservoir      7.55        0.659         0.330
    23 March    Upper Amersham        2.28        2.41          1.20
                Trials Reservoir      7.63        0.675         0.388

    DATE        LOCATION            SULFATES    CHLORIDES     FLUORIDES
                                     (mg/l)      (mg/l)         (mg/l)

    09 March    Upper Amersham         25          250           1.5
                Lower Amersham         16          115           1.5
                Police HQ, Plymouth    nd           97           1.5
                Weekes                 nd           27           0.3
    16 March    Upper Amersham         34          232           1.35
                Lower Amersham          8          100           1.5
                Police HQ, Plymouth     3           68           1.4
                Weekes                 nd           14.4         0.1
                Trials Reservoir       40           83           0.55
    23 March    Upper Amersham         36          211           1.45
                Trials Reservoir       39           76           0.40

The maximum thickness of ash collected on 31 March in Plymouth was 16 mm, at the American University of the Caribbean, and the total erupted airborne ash volume on 30-31 March was calculated to be 0.1 x 10⁶ m³, dense rock equivalent (DRE).

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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04/1997 (BGVN 22:04) Pyroclastic flows over Galway's Wall reach 500 m from the shore

The following summarizes the visual observations of Stephen O'Meara on 2 April, NOAA/NESDIS satellite observation reports for April, and the weekly Scientific Reports of the Montserrat Volcano Observatory for the period 6 April-10 May 1997. For a map showing the locations of the places mentioned in this report see this BGVN 22:02. An article on the ongoing activity in Montserrat was published in April on Science magazine.

Visual observations. On 2 April O'Meara flew over Montserrat in a small chartered aircraft at an altitude of ~ 3 km. As he approached from the NE around 1050, with good views of the new delta (figures 20 and 21), the volcano was sending light-gray ash-and-steam clouds up to 4 km. Light-brown material covered much of the pyroclastic deposits in the Tar River Valley, while the delta itself retained a weak mantle of grayish ash. Light veils of ash originating from near Farrell's Wall fell to the N, discoloring a kilometer-long channel. Despite ash and clouds covering most of the summit (except for the E section of the dome), boulders could be seen falling down the NE, E, and SE sides, leaving trails of grayish dust.

A reddish-gray ash that obscured about two-thirds of SW Montserrat was slowly blown toward Plymouth. By the time the aircraft started circling the island again, activity had picked up substantially. A vent on the E side of the dome shot tall columns of ash and steam at a 45° angle to the N, and another vent in the direction of Galway's Wall sent a similarly angled plume of ash and steam to the SW. Within 10 minutes a pyroclastic flow originating in the breach at Galway's Wall went down the White River valley. The comparatively slow moving flow had a curved front trailed by tall convecting ash clouds.

On 6 April MVO reported that coarse rockfall debris had completely filled the chute carved by pyroclastic flows during the Easter collapse. The debris resembled a talus slope ~100 m wide and dipping at 50°. In the center of the scarp between the two peaks, observers saw vigorous degassing and milky, ash-laden steam jets.

A survey of the dome on the same day using both GPS and laser range finding binocular showed that the steep headwall representing the active face of the new lobe was then 100 m high, 150 wide, and dipping 60° (i.e. 10 m higher and 30 m wider, but 20° less steep than on 3 April). In a 3-day period the dome's elevation decreased from 968 m to 950 m and at the headwall the dome had also advanced S by about 20 m toward the top of the chute. In contrast, the N peak of the January dome remained at ~965 m elevation, but it appeared more fractured.

A dome collapse on 11 April sent pyroclastic flows ~500 m farther than any previous flows down the White River valley on the volcano's S flank. The flows stopped ~500 m before reaching the seashore at O'Garra's. The deposits in the valley completely buried the Great Alp Falls. Two distinct flow paths developed in the area around the Galway's Soufriere: 1) a main channel in the White River valley itself, and 2) a path over flat ground W of the main valley and then back into the main valley at a point half way along it. In the upper half of the valley surge clouds covered the topography with deposits.

Observations after 11 April revealed that collapses had eaten away another 100 m of pre-September material on the dome's SW side. The lobe built up again, rapidly replacing the material lost.

Satellite imagery on 12 April indicated a low-level (3 km) ash plume reaching as far as 75 km W of the volcano. Maximum width of the visible plume was 15 km at 40 km from the summit. The day after the plume was still extended 95 km WNW from the volcano. A maximum width of 25 km was measured at 75 km from the summit.

A GPS/laser binocular survey conducted on 15 April showed that the S dome's summit, then at 947 m, continued to slowly decrease in elevation. The maximum size of the lobe was calculated at ~ 0.9 x 10⁶ m³, but no change was detected for the N summit. The most noticeable change in the S region was an increase in the width of the lobe to ~180 m. Most of this expansion had occurred on its W side, and fresh material was encroaching on the remains of the September scar. To the E, the edge of the lobe was now against the Easter collapse scar. Shortly after 15 April the smooth surface of the lobe became increasingly fractured, allowing gas to be emitted from it. Gas and ashy steam was also emitted from the saddle between the dome's summits. Until the end of April the chute over the Galway's Wall continued to widen and fill slowly. The material eroded from the base of the chute by the 11 April collapse was being replaced by short-runout rockfalls and small pyroclastic flows. The soufriere area also continued to be filled by pyroclastic-flow deposits that didn't progress much farther.

Volume estimates in the White River area showed that that the rockfall and pyroclastic-flow activity had deposited 2.3 x 10⁶ m³ of material on 2 April and 2.1 x 10⁶ m³ more by 15 April (values given as Dense Rock Equivalents). These values have helped to constrain the magma extrusion rate at 4.6 m³ /s after the 11 April collapse, in agreement with other field observations.

Ash clouds from pyroclastic flows and rockfalls were detected both in visible and infrared imagery from 24 to 27 April, drifting NW and WSW at ~1 km altitude.

During brief breaks in the weather on 2, 3 and 5 May there was evidence of continued pyroclastic flows and dome growth above Galway's Wall, S of the dome. Most of the dome was visible from the Tar River Estate House on 8 May. Rockfalls were heard from the S side of the Tar River valley, and several large blocks bounced toward Perches Mountain.

No major changes were seen on the E face of the dome during the entire observation period. Several fumaroles were present but only very small rockfalls were noted from the N and E flanks of the dome

Seismicity. Seismic activity was low until the dome collapse on 11 April. The collapse began with sustained low-amplitude signals. Two pulses of high-intensity activity were recorded at 1107 and 1155, respectively; the second one lasted 15 minutes. A short, high-amplitude signal was recorded as a pyroclastic flow traveled down the lower reaches of the White River valley. Fifteen hours after the end of the pyroclastic-flow activity a short-lived hybrid earthquake swarm took place. During 23-26 April, rockfall activity reached the highest event counts recorded since the beginning of the year. The largest signals were associated with small pyroclastic flows over the Galway's Wall. For the remainder of the month most of the seismicity dropped to a low level, dominated by rockfalls and long-period earthquakes. The number of long-period earthquakes remained high, though, and on a daily basis 40 to 100% of them triggered rockfalls.

A swarm of 28 shallow (2.4-3.6 km below the crater) volcano-tectonic earthquakes occurred on 7-8 May. During the swarm, the level of rockfall and long-period earthquake activity dropped, only to rise to previous levels when the swarm ended. This anti-correlation pattern between hybrid or volcano-tectonic swarms and rockfalls was observed many times before at this volcano, but had been absent since the explosion in September 1996.

Ground deformation. Survey measurements of the W triangle (Lower Amersham Upper Amersham Chance's Peak) continued to show a very slow shortening of the slant distances. There was indication of very slow subsidence of the two target sites relative to the instrument site at Lower Amersham. This trend was also detected during earlier occupations and seemed to reflect the removal of magma at depth below the volcano.

A long GPS occupation of the sites at Harris Lookout (M18) and the site on the crater rim above Farrell's Yard (FT3) was performed on 6 April. The recent outward movement of the FT3 site (BGVN 22:03) away from the dome appeared to have stopped. However an occupation of the Long Ground-Whites-M18 and Harris Lookout-Windy Hill-Farrell's triangles on 16 April using both the GPS and EDM techniques detected slow movement of the Farrell's site away from the crater toward the NNW. The baseline to Harris had shortened by 4 cm since last June. Further occupations at sites on the crater wall close to the dome with a fixed point at M18 Harris Lookout recorded significant movement of the crater rim close to the lava dome complex. These movements, radially outward from the dome, diminished with distance.

An EDM occupation of the N triangle (Windy Hill Farrell's St. George's Hill) on 15 April showed a 40-mm shortening of the Windy Hill Farrell's slant distance since 2 April. This apparent major change, however, was likely affected by atmospheric conditions. A subsequent occupation on 18 April indicated that the Windy Hill Farrell's slant distance had increased by 22 mm in three days. Later occupations during this report period confirmed slow continuous outward and downward movement of the Farrell's target.

Measurements of the cracks on Chance's Peak and on the E side of the Galway's Wall were carried out on 28 April and 3 May, respectively. The Chance's Peak crack recorded 6 mm of extension and 7 mm of dextral shear since 6 April. The crack on the E side of Galway's Wall opened a few millimeters and underwent a total 25 mm of sinistral shear since late March. These measurements suggested that the area between the cracks, which contains the remains of Galways, wall was moving slowly to the SSW, away from the dome.

COSPEC and other measurements. SO₂ values recorded on 7 and 11 April were 223 and 1524 t/d, respectively. The latter value, measured immediately after the collapse over the Galway's Wall, was one of the highest ever measured at the Soufriere Hills Volcano. Results from SO₂ diffusion tubes collected on 23 March and 6 April (table 17) showed a return to the levels of the past few months, within the recommended limits for occupation or habitation in these areas. The Whites Landfill site on the NE side of the volcano had no detectable sulfur dioxide.

Table 17. Sulfur dioxide diffusion tube results at Soufriere Hills for 23 March and 6 April 1997. Concentrations are in ppb.

    LOCATION           23 March 1997    6 April 1997

    Upper Amersham        45.10            31.05
    Lower Amersham        17.70            11.90
    White Landfill         0.00             1.4
    Police HQ, Plymouth    8.05             5.1
    Weekes                 0.00             0.00
    Control                0.00             0.00

Some analyses of rain water samples collected at various locations from 31 March to 11 May appear in table 18. The rainwater continues to be highly acidic and certain anions are present in high concentrations, but well within World Health Organization guidelines.

Table 18. Rain and surface water geochemistry at Soufriere Hills for 31 March, 12 and 29 April, and 11 May 1997. Courtesy of MVO.

    DATE        LOCATION             pH  CONDUCTIVITY  TOTAL DISS
                                           (mS/cm)     SOLIDS (g/l)

    31 March 97 Upper Amersham       -      1.566         0.784
                Lower Amersham       -      1.394         0.698
                Police HQ, Plymouth  -      1.553         0.727
                Weekes               -      0.058         0.028
    12 April 97 Upper Amersham       -      0.235         0.117
                Lower Amersham       -      0.300         0.150
                Police HQ, Plymouth  -      0.243         0.121
                Weekes               -      0.112         0.055
    29 April 97 Upper Amersham       2.60   1.902         0.953
                Lower Amersham       2.90   1.166         0.584
                Police HQ, Plymouth  3.26   0.622         0.311
                Weekes               5.21   0.223         0.111
    11 May 97   Upper Amersham       2.69   1.525         0.764
                Police HQ, Plymouth  2.98   -             -
                Weekes               5.56   0.051         0.025
                Trials overflow      7.78   0.776         0.388

    DATE        LOCATION          SULFATES  CHLORIDES  FLUORIDES
                                   (mg/l)     (mg/l)     (mg/l)

    31 March 97 Upper Amersham        39       16.2       1.5
                Lower Amersham        50      158        1.4
                Police HQ, Plymouth   20       16.6      >1.5
                Weekes                -        10.3      0.2
    12 April 97 Upper Amersham        20      135.5       1.4
                Lower Amersham        32       64.0      >1.5
                Police HQ, Plymouth   -        28.5      >1.5
                Weekes                3        20.0       0.55
    29 April 97 Upper Amersham        0.60     -          -
                Lower Amersham       >1.5      -          -
                Police HQ, Plymouth   1.25     -          -
                Weekes                1.20     54         8
    11 May 97   Upper Amersham        0.85     166        37
                Police HQ, Plymouth   -         -         -
                Weekes                0.25     11.8       3
                Trials overflow       0.4      103        38

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt); Stephen and Donna O'Meara Nature Stock, PO Box 218, Volcano, HI 96785, USA (Email: someara@interpac.net).

Further Reference. Montserrat Volcano Observatory Team, 1997, The ongoing eruption in Montserrat: Science, v. 276 (5311), p. 371.

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05/1997 (BGVN 22:05) Pyroclastic flows no longer confined by the crater's N wall

On 25 June, unusually large pyroclastic flows swept down drainages on the volcano's NNE side reaching almost as far as the airport. Settlements along their path sustained serious damage. Amid rescue efforts on 27 June, MVO reported at least nine people dead, six injured, and 14 missing. Additional information will be provided in next month's Bulletin.

The following summarizes weekly Scientific Reports of the Montserrat Volcano Observatory for the period 11 May-7 June 1997 and NOAA/NESDIS satellite observations during 12 May-6 June. Many of the places mentioned in this report appear on available maps (e.g. BGVN 22:02; Williams, 1997).

A new risk map was released on 6 June (figure 22). Zone A was expanded from the crater to the N as far as Harris, Bramble, and Bethel villages. Areas designated as Zone B included Tuitt's and Spanish Point on the E and Streatham and Farrell's on the W. Bramble Airport, ~5 km NE of the volcano, was moved into zone C.

At the beginning of this reporting period, dome growth (estimated at 2 m³/s) was concentrated on the crater's S side, above the Galway's area. Rockfalls and a few small pyroclastic flows were shed into both the White River and down the S side of the Tar River valley. After 12 May loud roaring sounds caused by vigorous venting of ash and gas from the dome were heard at Whites, Harris, and Farrell's. These were taken to indicate increased gas pressures within the dome. Furthermore, on 12 May an airplane pilot reported ash between 1,830 and 2,440 m altitude.

On 13 May at 0755 a moderate-size pyroclastic flow from the summit region eroded a narrow channel on the E flank of the dome, a spot underlain by the ancestral Castle Peak. The flow went down the Tar River valley, splitting into two branches that traveled down either side of the upper break in slope, and eventually reached the delta at the coast. The ash cloud from this flow reached 3.3 km altitude and later formed a plume conspicuous in visible satellite imagery for 220 km WNW of the summit.

On 14 and 15 May, small, nearly continuous rockfalls and some small pyroclastic flows occurred on the NE and SE flanks of the dome; these traveled either towards the E (Tar River valley) or S (down Galway's side). Beginning at about 2040 on 15 May a 70-minute-long outburst generated moderate-size pyroclastic flows down the E side, creating a small scar ~40 m N of the one formed on 13 May.

Events on 16 May included small-to-moderate pyroclastic flows from the dome's summit. These traveled down the dome's N and NW sides, towards Farrell's wall, which deflected them E toward the Tar River valley. In addition erosion occurred on the dome's N face: talus continued piling up against the N and NNE crater rim.

During the following days activity was concentrated on the N and E flanks of the dome, with three major rockfall chutes developed on the dome's E, NE, and N sides. At the base of one of these chutes rockfall material piled up against the crater's N wall (Farrell's). Several small rockfalls were also heard on the crater's S side (Galway's wall), where new, relatively fine-grained rockfall deposits had blanketed the entire talus slopes.

On 18 May at 0820 the largest pyroclastic flow of this reporting period occurred. First, a large long-period earthquake took place; observers at Whites reported that the entire dome was being shaken just before the flow started. This pyroclastic flow traveled from the summit down both the 15 May chute and the NE chute. Then it passed down the N side of the Tar River valley to stop a few hundred meters from the delta. During that night intense glows were observed over the dome's entire NW face. There were also some incandescent rockfalls and small pyroclastic flows.

Clear visibility on 19 May revealed a new dark extrusion at the top of the NW dome. This area discharged ash and steam and ejected 5- to 20-cm diameter fragments up to ~60 m above the dome. The source of continuous rockfalls with small pyroclastic flows extended from the 15 May chute on the E to the margin of the September 1996 scar on the NW. The remnant wall of the scar prevented material from reaching the rim of Gages wall. The N side of the crater had filled up, with small amounts of dome material falling into the top of Tuitt's Ghaut, the N- flank drainage.

On 21 and 22 May a few small spines toppled, sending rockfalls down the E, NE, and NW flanks. On 22 May at 1300, after a rockfall on the N flank, some blocks reached ~100 m down the N flank (Tuitt's Ghaut). At 1430 a pyroclastic flow on the same flank produced an almost continuous ash plume; lapilli up to 4 mm in diameter were collected at Dyers and ash fragments ~1 mm in size were reported at Farrell's and from Salem up to St. John's.

Observations on 23 May from Chances Peak revealed several small spines and large blocks in the summit area with vigorous venting and gas emissions in the growth area; there was also a cleft in the middle separating the S lobe from the new extrusion in the N.

On 27 May, a large pyroclastic flow generated high on the E side of the dome traveled down the Tar River valley at a speed of 230 km/h , the fastest flow yet documented during the eruption. In the lower part of the valley the flow slowed considerably, and it stopped ~150 m from the sea. That same day, for the first time, moderate-size pyroclastic flows reached Tuitt's Ghaut; later on 29 May discernible material was deposited 400 m down this drainage.

By 31 May, talus slopes over the dome's E and NE flanks had covered the chutes formed by mid-May pyroclastic flows. The upper part of the dome's E face looked more blocky and relatively inactive. When visibility was good, the presence of ash below ~1,600 m was reported almost daily in satellite imagery.

Small pyroclastic flows down Tuitt's Ghaut on 2 June left fresh deposits ~1 km from the crater rim. By 3 June they reached 1.4 km, and by 4 June, 1.8 km. At 1207 on 5 June a pyroclastic flow extended ~2.9 km from the crater rim; a shorter flow followed to ~2 km. All of these pyroclastic flows were confined to the narrow valley and comparatively slow moving, taking about three minutes to descend it. In the first 500 m of the upland portion of the valley all vegetation was stripped from the valley walls. Farther down, some trees were left standing within the deposits. In the upper 1 km of the deposits there was evidence of several small, lobate flows. In general the thermal effects remained confined to 10 m from the deposit's edge, but on bends it rode up the banks of the ghaut (the so- called "bobsled" effect). The front of the flow was marked by a pile of burned logs and coarse debris, and a finer-grained surge had traveled ~100-200 m farther down the ghaut. A pyroclastic flow at 1845 on 6 June traveled ~2 km down from the crater rim; its front carried particularly large boulders. The flow significantly widened the notch in the crater wall through which it traveled; by this time the domes talus created a smooth slope down the ghaut.

NOAA reported ash clouds on 3, 4, and 5 June in visible satellite imagery up to 2,150 m altitude and crossing over the Virgin Islands, 400 km NW.

Seismicity. The shifting focus of dome growth and rising vigor of emission were reflected in a general decline in the number of long-period earthquakes and an increase in the number of hybrid earthquake swarms. Each swarm lasted for a few hours; some intense swarms during 19-21 May reached up to 35 events/hour. Rockfalls remained common and were concentrated during periods of minor dome collapse. The ratio of maximum rockfall amplitudes measured at Galway's Estate Station and Long Ground station served to differentiate between Tar River and White River pyroclastic flows.

Toward the end of May there was a significant reduction in the number of hybrid and long-period earthquakes, and rockfalls. The hybrid earthquake swarms continued until 27 May; although less frequent, they lasted longer.

The number of long-period earthquakes dropped to the normal background (0-4 events/day), the lowest levels since mid-March. The number of rockfalls increased from 1 June, and for the rest of the period were concentrated on the N and E sides of the dome. Periods of enhanced rockfall and pyroclastic-flow activity occurred every 16-20 hours and lasted ~4 hours. In the lulls, rockfalls continued at greatly reduced levels.

After 4 June the number of both long-period and hybrid earthquakes increased again. Over 50% of these shocks triggered rockfalls.

Ground deformation. GPS measurements at station FT3 (Farrell's wall) on 12 May showed continued movement to the NW, consistent with the total 20 cm of displacement noted since January 1997. Data were somewhat equivocal on 17 and 21 May. A GPS occupation at Chances Peak on 23 May suggested that it had moved 3.5 cm WNW since 28 April. Prior to that date, the movement was toward the NW. The change in direction was thought to reflect the dome's northward shift in activity.

Telemetered electronic tiltmeters installed at Chances Peak on 18 and 21 May (stations CP2 and CP3, W and E of the summit, respectively) registered cycles of inflation and deflation, each lasting ~12-18 hours. Progressive intervals and magnitudes of inflation were greater than those of deflation. Inflation occurred with hybrid earthquake swarms, and deflation correlated with peak rockfall/pyroclastic-flow events. RSAM patterns showed a strong correlation with tilt, with the higher spikes reflecting rockfalls, and the lower intensity patterns reflecting the sum of hybrid events and lesser rockfall activity. Thus tilt and RSAM combined provided a predictive capability. Accordingly, when it was possible, missions to close-in areas were scheduled during early inflation, when the likelihood of pyroclastic flows was considered minimal.

Crack 2, which developed into a zone of broad fracture on Chances Peak, was measured on 23 May, and on 4 and 8 June. The shear along the crack was dextral (E block moving S relative to W block) and reached 6 cm. The shear during 23 May-4 June was 2.5 cm. On 23 May a telemetered extensometer installed across part of Crack 2 that day showed almost 5 mm of diurnal change.

Dome volume, COSPEC, and other measurements. Using a combination of theodolite, GPS, and ranging binoculars, scientists on 19 May estimated the summit at 991 m elevation. One major change since the previous survey (15 April) was the inflation of the highest part of the dome above Galway's wall. Another change was the growth of the new extrusion in the N summit area and the talus accumulation in a 300-m-wide zone against the back of Farrell's wall, due to the activity on the N and NE faces. The volume of the dome from this survey was estimated at 60.1 x 10⁶ m³; this established an average extrusion rate during 15 April-19 May of 2.7 x 10⁵ m³/day (3.1 m³/s).

Later dome-volume surveys were severely hampered by poor visibility; however, brief clear windows allowed photos to be taken for both 31 May and 1 June, documenting continued growth of the dome's N side and summit. On the basis of these photos, the dome's volume was 64.6 x 10⁶ m³, a mean growth rate of 3.5 m³/s during 19 May-1 June. As with the last survey this represented a rate considerably above the mean extrusion rate of 2.1 m³/s.

Mini-COSPEC runs that were completed daily, often both in the morning and afternoon, gave results substantially higher than the usual background flux of 200-300 t/d. May and June SO₂ fluxes were as follows: 24 May, 950 metric tons per day (t/d); 26 May, 940 t/d; 27 May, 971 t/d; 28 May, 616 t/d; 29 May, 770 t/d; 30 May, 510 t/d; 2 and 3 June, 475 t/d; 4 June, 2,129 t/d; 5 June, 2,242 t/d; 6 June, 642 t/d; and 7 June, 505 t/d. The high values on 4-5 June correlated with increased pyroclastic flow activity during 4-6 June. Sulfur diffusion tubes collected on 20 April and 4 May mainly showed values similar to those of previous weeks (table 19). The results from Upper Amersham on 17 May presumably increased because of the increase in the level of eruptive activity.

Table 19. SO₂ concentrations in part per billion (ppb) from diffusion tubes at sites around the volcano. Recommended action level is 100 ppb. Courtesy of MVO.

    LOCATION            20 April     4 May     17 May

    Plymouth Police HQ    7.3          7.8      17.1
    Upper Amersham       45.0         53.2      81.1
    Lower Amersham       12.1         16.9      32.0
    Weekes                0.0          0.0       4.3
    Whites landfill       0.8          1.2       1.2

Rainwater collected W and NW of the volcano on 17 May was more acidic than samples from the previous week and chlorides and sulfates were present at substantially higher levels (table 20). After heavy rainfall and continued winds from the S and SE, a rainwater sample collected on 28 May from Lawyers, 2 km north of Salem, had a pH of 3.3. On those same days, new sites to the N of the volcano were also monitored and showed very low pH values. During this period the fluoride content of the rainwater was also elevated. The pH and fluoride returned to normal values when the wind direction changed to WNW at the end of May. Piped ground water had remained unaffected by the low pH of the rainwater.

Table 20. Rainwater geochemistry from 17 May to 1 June. For comparison, WHO guideline values are as follows: pH, 6.5- 8.5; TDS, 1.0 g/l; fluorides, 1.5 mg/l; chlorides, 250 mg/l; sulfates, 250 mg/l. Courtesy of MVO.

    DATE      LOCATION               pH   CONDUCTIVITY     TOTAL
                                                         DISSOLVED
                                            (mS/cm)     SOLIDS (g/l)

    17 May 1997  Weekes              3.8      0.272        0.136
                 Plymouth Police HQ  2.7      3.51         1.75
                 Upper Amersham      2.4      2.45         1.22
                 Lower Amersham      2.8      4.26         2.13
    25 May 1997  Weekes              2.6      1.286        0.644
                 Upper Amersham      2.0      7.24         3.62
                 Am. cattle trough   7.72     0.335        0.168
                 Trial's reservoir   7.9      0.827        0.414
    27 May 1997  Hope                2.8      0.789        0.37
    28 May 1997  Weekes              2.5      1.557        0.778
                 Molyneux            2.6      1.312        0.657
                 Dyer's              2.8      0.702        0.351
                 Lawyer's            3.0      0.46         0.23
                 M.V.O               2.8      0.863        0.432
    31 Jun 1997  Weekes              3.4      0.257        0.128
                 M.V.O               5.3      0.066        0.033
                 Dyer's              6.7      0.092        0.046
                 Upper Amersham      2.8      0.914        0.458
                 Lower Amersham      3.1      0.533        0.267
                 Am. cattle trough   8.89     0.32         0.16
                 Trial's res.        7.8      0.845        0.423
                  Overflow(from the tap)

    DATE         LOCATION         SULFATES  CHLORIDES  FLUORIDES
                                   (mg/l)    (mg/l)     (mg/l)

    17 May 1997  Weekes               37        50       0.65
                 Plymouth Police HQ  560       710        --
                 Upper Amersham      107       315        --
                 Lower Amersham       97       760        --
    25 May 1997  Weekes                5       133       1.50
                 Upper Amersham       93      1000       0.20
                 Am. cattle trough    --        56       0.55
                 Trial's reservoir    42       112       0.35
    27 May 1997  Hope                 --        70       1.50
    28 May 1997  Weekes               --       126       1.50
                 Molyneux              7        94       1.50
                 Dyer's                3        80       1.40
                 Lawyer's             --        52       1.25
                 M.V.O                --        80       1.50
    31 Jun 1997  Weekes                3        --       1.20
                 M.V.O                --        --       0.35
                 Dyer's                3        --       0.20
                 Upper Amersham       12        --       1.50
                 Lower Amersham       18        --       1.15
                 Am. cattle trough    --        --       0.35
                 Trial's res.       38        --       0.30
                   Overflow(from the tap)

Ash was collected on 17 May following several days of increased volcanic activity. The ash was at least 6 mm thick at Upper Amersham, and 4 mm at Lower Amersham, the Plymouth Police Headquarters, and Dagenham. Ash collected on 1 June was noticeably fine and widely distributed from Brodericks to Dyers with the thickest ash fall (2 mm) at Upper Amersham, Dagenham, and Plymouth Police HQ.

Reference. Williams, A.R., 1997, Montserrat, under the Volcano: National Geographic, v. 192, no. 1 (July 1997), p. 58-77.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Spring, MD 20746, USA.

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06/1997 (BGVN 22:06) Deadly N-directed pyroclastic flows on 25 June; cyclical eruptive behavoir

The following condenses reports from the Montserrat Volcano Observatory (MVO) and stated sources for the period ending 30 June. Although N-flank pyroclastic flows became increasingly common during late May-June (figure 23), the most lethal and destructive eruption in the volcano's historical record traveled N on 25 June (figure 24). That eruption, discussed in MVO Special Report 03 (29 June 1997 draft), sent a plume to ~10-km altitude and produced pyroclastic flows that overran both vacated and partly inhabited NE-flank settlements in the officially evacuated zone. Some flows stopped near the edge of Bramble airport; it has remained closed since that time. Risk associated with repeated pyroclastic flows to the N and W led to a new risk map (figure 25). Early August pyroclastic flows destroyed structures in central Plymouth; details will be provided next month.

During June NOAA's Satellite Analysis Branch repeatedly noted light plumes from the volcano. The plumes were typically "cigar-shaped" and attached to the source; in GOES-8 satellite imagery they frequently remained discernible 50-100 km W to WNW. Two days after the [25] June outburst, an ash cloud to over 10 km moved N-NW to ~200 km from the volcano.

Key events. Table 21 summarizes events during 14 May-25 June 1997. The 14 May rockfalls followed about two-and-a-half months of relative stability; by 19 May the intensity of rockfalls increased and they spilled N into Tuitt's Ghaut. On 29 May, Tuitt's Ghaut was also the scene of a minor pyroclastic flow. Subsequent pyroclastic flows increased during early June; during mid-June they reached into Mosquito Ghaut and Gages Valley (figure 23).

Table 21. Time line summary for Soufriere Hills leading up to the destructive 25 June 1997 outburst. Where unspecified, pyroclastic flow runout distances are measured from the crater. Modified from MVO Special Report 03 (29 June 1997).

    Date        Description of Activity

    14 May      Beginning of rock falls on dome's N face.
    19 May      First rockfall spills N into Tuitt's Ghaut.
    29 May      First pyroclastic flows enter northern ghauts.
    02 Jun      Pyroclastic flow down Tuitt's Ghaut travels 1 km
                  from the crater.
    03 Jun      Pyroclastic flow in Tuitt's Ghaut travels 1.4 km.
    04 Jun      Pyroclastic flow in Tuitt's Ghaut travels 1.8 km.
    05 Jun      Pyroclastic flow in Tuitt's Ghaut travels 2.9 km,
                  250 m from intersection with the Paradise
                  River.
    07-14 Jun   Rockfalls and pyroclastic flows concentrated in
                  Tuitt's Ghaut.
    15 Jun      Pyroclastic flow material advanced 500 m down
                  Mosquito Ghaut. Gage's Valley was the scene of
                  a small rockfall.
    16 Jun      Pyroclastic flows in Gage's Valley travel 1.6 km
                  from the crater rim. Smaller pyroclastic flows
                  in Mosquito and Tuitt's Ghaut.
    17 Jun      Strong deflation on tiltmeters preceded a dome
                  collapse at 2330; pyroclastic flows traveled 2
                  km down Gages Valley (200 m further than
                  previous ones) and 3.5 km down Mosquito Ghaut.
                  Many of the rock samples collected from the
                  pyroclastic flow into Mosquito Ghaut were
                  moderately vesicular and were therefore
                  interpreted as juvenile (not dome material).
    22 Jun      Inflation and subsequent deflation were
                  pronounced and rapid; the latter coincided with
                  pyroclastic flows that traveled ~1 km E down
                  the Tar River Valley. After this event,
                  inflation-deflation cycles shortened and their
                  amplitudes increased. The pyroclastic flows
                  were also followed by a short volcano-tectonic
                  earthquake swarm and the return of hybrid
                  swarms.
    24 Jun      For the first time since 17 June, small
                  pyroclastic flows moved down Mosquito Ghaut;
                  they reached 1 km from the crater rim. Dome
                  growth seen at the top of Mosquito Ghaut.

Limited visibility during June led to the poorly defined, but relatively high extrusion rate of ~3.5 m³/s. The dome's additional bulk furnished less-impeded access to the volcano's N slopes.

Eruption of 25 June. During 1255 to 1320 on 25 June pyroclastic flows sweeping over the volcano's N flanks followed paths down Mosquito Ghaut and the Paradise River almost to the sea (figure 24). Pyroclastic flows and associated surge clouds damaged or destroyed 100-150 houses (severely affecting the villages of Streatham, Dyers, Harris, Bethel, Bramble, Trants, Farm, and Spanish Point). A mid-July official statement confirmed ten people dead and another nine missing and presumed dead. An earlier report mentioned five people who suffered serious burns.

The pyroclastic flows were the largest since the eruption began in 1995; the eruption's intensity exceeded that of the explosion of 17 September 1996. An estimated 4-7 million cubic meters of the lava dome was unloaded during the event, and the resulting flow and surge deposits covered 4 km² (figure 24). The ash fell over W and NW Montserrat. Maximum accumulations reached 2 mm. The event left a steeply-dipping, circular scar ~200 m across in the dome's NNW face.

Table 22 and figures 26, 27, and 28 summarize events on a variety of time scales, the latter two covering intervals just before and during the 25 June outburst. The previously mentioned hybrid earthquake swarm at 0300 had up to 4-5 events/minute, similar to swarms seen during the previous four days. Earthquakes were of moderate amplitude; they caused saturations on the Gages and Windy Hill drum records.

Table 22. Timeline for the destructive 25 June 1997 outburst at Soufriere Hills. Modified from MVO Special Report 03 (29 June 1997).

    Time on     Event
    25 June

    0300        Start of hybrid earthquake swarm
    0600-0800   Deflation accompanied by small pyroclastic flows
                  in Mosquito Ghaut
    1050        Start of hybrid earthquake swarm
    1200        Crater inflation peaked
    1245        Volcanic tremor; steam and ash production
    1255        Start of pyroclastic flow activity
    1257        First seismic pulse
    1300        Second seismic pulse
    1300        First pyroclastic flow observed in Mosquito Ghaut
                  from MVO
    1302        First flow seen from airport
    1303        Loss of seismic signals from eastern stations
    1308        Third seismic pulse
    1315        Second flow seen from airport
    1320        End of seismic activity

Tilt peaked at 0520 and the volcano started to deflate at about 0610 (figure 28). The hybrid earthquake swarm diminished gradually after about 0615. At 0705 the earthquakes gave way to low tremor. Rock falls and minor pyroclastic flows commenced, fitting the established pattern. Between 0600 and 0800 semi-continuous pyroclastic flows ran down Mosquito Ghaut to ~1 km. There were also simultaneous rockfalls and small pyroclastic flows from the dome's SE and E faces. Re-inflation of the dome area began at approximately 0900 and a second hybrid swarm started at 1050 and escalated rapidly, reaching ~6 events/minute between 1130 and 1230. The earthquake amplitudes were uniform, and similar to those in the earlier swarm. At 1200 the inflation trend peaked. By 1245 the seismic record was dominated by tremor, and hybrid earthquakes were barely discernible. A dilute steam and ash cloud blew W at the altitude of ~1.5 km.

Between 1240 and 1250 the tiltmeter registered the start of a sharp deflation. At 1255 a strong seismic signal began and at 1257 and 1300 intense pulses occurred. The latter pulse was roughly coincident with eruption of a dense, dark ash cloud that rose vertically from the N flank of dome above Mosquito Ghaut. This was considered the main event, and sent an ash cloud to 10 km in minutes.

At 1303 the eastern stations of the seismic network stopped transmitting data due to the destruction of either the telephone exchange or the line across the central corridor by a pyroclastic flow down Mosquito Ghaut. Available stations registered a third seismic pulse at 1308.

MVO staff positioned N of the airport witnessed the front of the flow coming around the bend at Pea Ghaut, just up-slope of Trant's village (figure 24). At 1315 MVO observers flying over the airport found that the initial pulse had overrun the lower parts of local settlements (Harris, Farm, and Trant's), and came to within 50 m of the sea. They also reported a final pulse coming down Paradise Ghaut and surges continuing to spread slowly westward in the Spanish Point area. The final pulse advanced at ~30 m/s across flat land near Trants; this was captured on film by a time-lapse video recorder at the airport control tower.

Deposits and destruction. In Mosquito Ghaut, the main part of the flow caused intense scouring to the top (but not over) the steep valley walls; scouring was particularly intense on the outside of bends. The deposits, not extensive in the upper part, generally thickened towards the lower end where Mosquito meets Paradise Ghaut.

Flow deposits completely filled Pea Ghaut and formed a thick, broad fan emerging NW from Paradise Ghaut just N of Bethel (figure 24). Houses 200 m from the edge of the fan were completely buried. A separate lobe of coarse material ran over the lip of Paradise Ghaut immediately W of Bethel. Blocks within this lobe were up to 5 m in size and caused widespread destruction to houses in Bethel village. This was the only area where a high concentration of coarse material spilled from the main ghauts.

As the pyroclastic flows emerged from between peaks B and C (figure 23) and progressed into Mosquito Ghaut, fine-grained pyroclastic surges spread laterally onto the ridges on either side. These surges extended as far E as Paradise Estate, went northward to within 250 m of Windy Hill, inundated the entire village of Streatham, and spread W as far as Gun Hill. They broke and flattened trees on the ridges in the Farrell's and Paradise area. The surges did not spill into Tuitt's Ghaut to the E, but at one or two points they drained into the unnamed ghaut to the W. In Streatham, charring of trees and telegraph poles was limited to the E-SE sides. The orientation of charring, shadow zones behind a few of the houses, and the transport of a water tank indicated that surge movement in this area was WNW.

In the Farrell's area, blocks above 1 m across were rare; occasional blocks ~0.5 m across were present on Farrell's road. The deposits indicated that drainage of flow material into the Dyers river occurred largely in the narrow area S of Gun Hill and W of Riley's Yard. Samples collected in the Farm River area and Spanish Point included both dense and moderately vesicular lithologies.

Pyroclastic flows extended into the Belham valley as far as the last of the tight bends before Cork Hill. The flow-front was marked by a pile of logs aligned cross-valley; still, most trees remained standing, even near the base of the valley. Deposits along the whole length of the Belham valley were fine-grained with a near absence of coarse blocks. In addition, two small concrete bridges were left intact at the base of the valley. The fine grained deposits were interpreted as originating from pyroclastic surges that diverged NW from the main flow in Mosquito Ghaut.

Elevated seismic signals persisted until 1318, and the large deflation recorded by the tiltmeter bottomed out at 1430. Low amplitude tremor with hybrid earthquakes continued until 1500, at which time the seismicity dropped to background levels. The RSAM peak for the event, which lasted for 30 minutes, indicated shorter but more intense activity relative to the explosion of 17 September 1996.

Seismicity overview. Hybrid earthquake swarms occurred during 13 to 27 May (~100 earthquakes/day, figure 26). Rockfalls immediately followed each swarm of earthquakes, and in addition, after the earthquake swarms ended, the rockfall events continued (figure 26).

On the morning of 22 June, after a moderate pyroclastic flow and associated deformation, hybrid earthquakes suddenly restarted (table 23, figure 26). A small swarm of volcano-tectonic earthquakes also appeared; such earthquakes had been rare in recent months, usually occurring in single swarms. Between 22 and 25 June MVO noted seven hybrid swarms; both the duration and number of component events in these swarms increased (figure 26). Within a given swarm, the earthquakes generally had similar magnitudes and the few larger earthquakes were of relatively small magnitude; much larger ones had been recorded previously. Nevertheless, the swarms on 24 and 25 June increased in intensity, reaching a state where repetitive events merged into continuous tremor that was difficult to distinguish from rockfall signals on the drum records.

Long-period earthquakes became more numerous following the 5 June pyroclastic flows (table 23). The number of these earthquakes remained low, not exceeding 40/day, and returned to normal levels after 13 June.

Deformation studies. Figure 27 shows examples of monitored deformation, which includes both Total Station (combined electronic distance measurement (EDM) and theodolite) and global positioning system (GPS) techniques. Cracks in the crater walls were monitored by frequent measurements between fixed points across them. Telemetry links to two tiltmeters and one extensometer at Chances Peak and one tiltmeter at Long Ground.

In early March 1997 GPS surveys detected deformation of the northern crater walls (figure 27a). GPS station FT3 was installed on the crater wall adjacent to Peak C (figure 23); during 13 January-3 March it had moved ~15 cm NW; it continued moving NW with a total displacement by 12 May of 21.5 cm (after which the site was considered too dangerous to visit). Since July 1996, GPS on Chances Peak showed sustained motion away from the dome. By 29 June this site had a total displacement of 16 cm.

In the eruption's early stages, an EDM/GPS station on the N-flank (at Farrells) moved slowly N, away from the dome complex. Thus, by 30 November 1995 a shortening of 9 cm occurred. Although two cycles of lengthening and shortening occurred during 1996, since December 1996 only sustained shortening occurred along certain baselines (Windy Hill and Harris). This shortening continued at an increasing rate until the last measurement on 10 June.

Prior to 16 June, the Chances Peak tiltmeter showed a cyclical pattern of inflation and deflation, centered at the dome, with 12- to 16-hour periods and 16- to 18-µrad amplitudes. During 16-17 June the inflation-deflation cycle flattened to 5- to 10-µrad amplitudes.

At approximately 1600 on 17 June inflation increased steeply, peaking at 2100; rapid deflation followed. This deflation preceded a dome collapse at 2330 that sent pyroclastic flows down Gages and Mosquito drainages. For the next day and a half there was a return of the pronounced inflation-deflation pattern seen prior to 16 June.

In contrast, during 19 June until the early morning of 22 June there prevailed a flattened inflation-deflation pattern. Then, at 0530 on 22 June, a rapid inflation occurred; subsequent sharp deflation at 0630 was coincident with sustained pyroclastic flows.

This event marked the beginning of inflation-deflation cycles with periods shortened to 8 hours and amplitudes increased to ~40 µrad. As previously mentioned, the change was accompanied by a short volcano-tectonic earthquake swarm that preceded the hybrid earthquakes. The number of hybrid earthquakes varied in-phase with the inflation-deflation cycle (i.e. the maximum number of hybrid earthquakes occurred at peak inflation, figure 28).

Following the 25 June pyroclastic flows, the inflation-deflation cycle continued with the same period and amplitude that began 22 June. Prior to 25 June, tiltmeters indicated inflation on the N flank (or deflation on the S); after 25 June, tiltmeters indicated inflation at the dome's center.

Post-event activity and interpretations. After the episode of pyroclastic flows, seismicity remained low for several hours. However, starting at 2000 more inflation was accompanied by a small swarm of hybrid earthquakes. In subsequent days, the inflation and deflation pattern continued, earthquake swarms became more intense, and there were pyroclastic flows in Mosquito Ghaut and Gages valley.

Two small explosions on 27 June caused concern that the activity was still escalating, and the chance of significant explosive activity was judged to have increased. Brief views of the dome on 28 June indicated that a large part had been removed during the pyroclastic flows and rapid growth was occurring within the scar.

The large event was not a surprise because in the weeks prior to 25 June repetitive hybrid earthquake swarms and inflation-deflation cycles suggested that the rate of dome growth and conduit pressure were elevated. The effects of the pyroclastic flow were largely anticipated by the hazard zonation and warnings issued in MVO reports throughout June. The surge into Dyer's Ghaut and the Belham River valley was remarkable in that a relatively fine-grained flow traveled a significant distance off the main flow path.

In the days after 25 June, high activity levels and inflation at the Chances Peak tiltmeter prevailed. Earlier in the eruption significant events were normally followed by a respite in activity and a change in the eruption pattern. This was taken as a sign of further intense activity.

Risk map. During June 1997 MVO published four successive risk maps. With the advent of each map, the A-B zone (with no access) gradually increased in size to cover most of the S part of the island. Seven zones and six possible alert levels produced 42 different options; a new map could simplify the previous system.

The new map in early July (figure 25) contained three zones: the northern, central, and exclusion zones, and only one alert level, "volcanic alert." To decide where the boundaries between risk zones should lie, the distal margins of pyroclastic flows and surges through June 1997 were indicated. There was potential for flows to reach much of the S of Montserrat; thus MVO decided that an exclusion zone should include these areas. The line across the island's center was controlled primarily by topography.

North of the exclusion zone MVO considered that the risk of pyroclastic flows and surges was low enough to allow people to live and work as normal; however, in the case of increased activity it was thought that people in the area directly N of the exclusion should be ready to move at short notice. Therefore, a central zone was designated in which people should be on increased alert. The further that people moved away from the exclusion zone, the safer. Thus, the northern boundary of the central zone was marked as a dotted line. During an increase in alert level, citizens were advised to move uphill and away from the Belham River Valley. To announce an evacuation of the central zone the plan included deployment of wailing sirens and maroons (explosive fireworks).

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

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07/1997 (BGVN 22:07) Activity increased to high levels on 31 July

The following condenses reports from the Montserrat Volcano Observatory (MVO) for July 1997. Activity decreased during the month and the dome appeared to be growing at a lower rate than immediately after the energetic and destructive 25 June pyroclastic flow. Starting on 31 July, however, activity increased.

Visual observations. During 1-5 July several pyroclastic flows traveled down Mosquito, Gages, and Fort Ghauts, the largest ones reaching 3 km downstream. Many of these flows started with resounding explosions and ash columns that rose as high as 11 km at measured rates of 9-17 m/s. Plumes were visible from the Space Shuttle (figure 29).

The two weeks following 5 July were relatively quiet. During this interval rockfalls traveled as far as 500 m down the W and N faces of the dome. A brief glimpse of the dome on the night of 6 July revealed incandescent rockfalls above Mosquito Ghaut and Gages Valley. A partial view during the morning of 7 July showed a new steep-sided post-25 June dome above Mosquito Ghaut and Gages Valley with a broad, relatively flat summit area.

From 8 to 13 July there were fairly frequent emissions of diluted ash, often coinciding with the peak of the tilt cycle, and at times preceding small pyroclastic flows. The ash columns, reaching heights of ~ 3 km before dissipating, appeared to emanate from the W side of the post-25 June dome above Gages Valley. Theodolite measurements on 13 July gave an altitude of 950 m for the old dome and 941 m for the new growth in the 25 June scar. There was a steep 50-m-high protrusion on the new dome above Gages Valley. On 17 July the high point on the old dome (NE) measured 946 m, and the high point on the post-25 June dome 957 m. The spine above Gages valley observed on 13 July was no longer present.

On 21 July a field party at Trant's probing to a depth of 2 m inside the deposits at the end of the 25 June flow found a temperature of 640°C. A helicopter survey on 24 July showed fresh deposits in all of the ghauts around the volcano except Tuitt's. Another surveillance flight on 26 July indicated that most the rockfall activity was confined to Mosquito Ghaut and Gages Valley on the NE, and to the Galways area to the S. Vigorous steaming was coming from the flank of the dome in the Tar River area.

On 29 July between 0600 and 0830 there was more intense activity with several pulses of pyroclastic flows moving down Gages Valley as far as Gages Lower Soufriere. This activity was not preceded by earthquakes or a perceptible increase in rockfall activity. Other small pyroclastic flows occurred throughout the day.

Despite overcast conditions on 30 July, dilute ash plumes were visible from the Observatory during periods of heightened rockfall activity. A late-evening observation flight revealed that pyroclastic-flow deposits from 29 July extended just below the lower soufriere in Gages Valley. Several small pyroclastic-flow deposits from earlier that day (30 July) were noted on the N flank (top of Tuitts Ghaut) and NE flank (Tar River Valley and Galways area).

After 0300 on 31 July there were several periods of intense volcanic activity. A helicopter inspection showed very few new deposits in Gages valley (as far as Gages village) and some small flow lobes in Tuitt's Ghaut (to ~ 2 km from the dome). Many ash plumes were produced throughout the day and the most vigorously convecting clouds reached altitudes above 5 km. It appeared that most of the ash originated from near the top of Gages wall and was not necessarily associated with pyroclastic flows. The ash clouds drifted to the N and NW in light winds, but later in the day they traveled mostly to the W.

Seismicity. After 25 June swarms of hybrid earthquakes typically changed to tremor before the emission of pyroclastic flows. After 8 July hybrid swarms ceased, leaving seismicity dominated by rockfall signals of steady amplitude. A few long-period and hybrid events were recorded, but such activity remained at a very low level.

The number of rockfalls in the upper parts of Mosquito Ghaut and the Gages valley started increasing after 25 July. However, until 30 July the only other seismic signals recorded were a few long-period events. Starting at about 0300 on 31 July the activity became once again very elevated, peaking between 1230 and 1430, when the new Lees Yard seismometer recorded ~2 hours of nearly maximum amplitude signal. During this interval only one moderate- size pyroclastic flow was observed. Still the seismometers registered a significant increase of long-period earthquakes in addition to high-amplitude tremor that continued for much of the day, associated with ash clouds convecting to 6 km.

During the month several periods of low- to moderate-amplitude tremors appeared on both the St. George's Hill and St. Patrick's seismometer (e.g. 28-30 July); they were caused by heavy rains moving recent deposits. The largest volcano-tectonic events of the month occurred at shallow depths beneath English's crater on 24 July.

Ground deformation and volume measurements. EDM measurements showed that in general the inflation-deflation cycle that began on 22 June continued until 5 July with the same period (8 hours) and amplitude. However, after 25 June the trend showed deflation toward the center of the dome. Prior to 25 June inflation occurred to the N and deflation to the S. A survey of EASTNET stations at Harris, Windy Hill, Whites, and Long Ground on 16 July showed that the line to Whites had shortened by 16 mm since last measured on 24 June and by 31 mm from its long term mean. The line to Long Ground showed continued shortening and the line between Long Ground and Windy Hill showed slight lengthening. All the changes were consistent with their current trends although at slightly higher rates.

During 5-19 July the tilt cycles were characterized by lower amplitudes and longer (30-hour) periods; Chances Peak tiltmeter showed a gradual decrease in the rate of subsidence of the x-axis oriented SW. Superimposed on this trend were periods of cyclical inflation and deflation, often associated with hybrid swarms.

Measurements on the EDM line from Waterworks to Lees Yard on 20 and 27 July showed no major changes, although it had consistently shortened since first measured on 12 July 1997. No significant changes were observed on 26 and 27 July on either the new NW triangle (MVO-Garibaldi Hill-Lees Yard) or on the Waterworks-Lees Yard radial line. Finally, 30 July EDM measurements on the NW triangle confirmed the absence of a consistent trend.

A GPS survey on 5 July allowed an estimate of the total volume of deposits in several areas. The 25- June pyroclastic flow area was estimated at 4.61 x 10⁶ m³ and the volume of the flow that propagated into the Belham Valley was 90 x 10³ m³. The combined volume of Mosquito, Paradise, Farms, and Farrell's deposits totalled 9.24 x 10⁶ m³, and the Gages Valley deposit was 3 x 10⁶ m³.

A dome volume of 77 x 10⁶ m³ was calculated based on photographs from 17 July. Cumulative pyroclastic flow deposits were estimated to be 55.05 x 10⁶ m³ (DRE). The previous dome volume estimate on 31 May was 64.6 x 10⁶ m³, and the pyroclastic-flow deposit volume was 43.0 x 10⁶ m³. The average growth rate between 31 May and 17 July was 5.2 m³/s (DRE); visual observations suggested that after 25 June the growth rate was significantly higher.

Environmental monitoring. Rain water and trough water samples were collected from sites around the volcano on 10 and 22 June and 9 July. These values were nearly all within World Health Organization standards for drinking water, but the samples from Upper and Lower Amersham were extremely acidic and had high concentrations of total dissolved solids. All samples collected on 9 July to the N of the volcano had very low pH, probably because of the northerly wind direction on 8 July during heavy rain. Residents in the N of the island reported unusual sulfurous smells and light ashfall at this time.

A miniCOSPEC was used to measure SO₂ flux from the volcano (table 23). Fluxes increased before 25 June and remained comparatively high through 24 June. Since 25 June no measurements were possible along the roads of the central corridor or through Plymouth because of the extreme risk in these areas, thus the value for 17 July were measured by static scanning of the plume from Garibaldi Hill an average of 10 scans.

Table 23. Daily average SO₂ flux at Soufriere Hills using miniCOSPEC (metric tons/day). Courtesy of MVO.

     Date         SO2 flux
     1997     (metric tons/day)
     10 June       842
     11 June       839
     12 June       363
     14 June       442
     15 June       634
     16 June       409
     17 June       450
     19 June       618
     20 June      1171
     21 June       921
     22 June       438
     23 June      1157
     24 June      1933
     17 July       200

Workers collecting ash on 9 June found that small accretionary lapilli were common at the Plymouth sites. The same ash fell over a region including Brodericks and Dyers and it was thickest (2.5 mm) at Upper Amersham. On 17-18 June workers found a similar amount of ash had accumulated although in this deposit they recognized a significantly coarse grained component: it reached up to 5 mm in diameter close to the volcano. After a small explosive event on 27 June, coarse lapilli (up to 10 mm in diameter) were collected from Dagenham and Richmond Hill.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Spring, MD 20746, USA; Cindy Evans, Space Shuttle Earth Observations Office, Mail Code C102, Lockheed Engineering & Sciences, P.O. Box 58561, Houston, TX 77258 USA (Email: cindy.evans1@jsc.nasa.gov).

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08/1997 (BGVN 22:08) Vigorous dome growth continues in August

Most of the following condenses reports from the Montserrat Volcano Observatory (MVO) for August 1997. Vigorous dome growth continued during August and exhibited cyclical patterns of tilt and seismicity, rockfalls, the growing dome's extruding and falling spines, and abundant pyroclastic flows. In addition, NOAA's Satellite Analysis Branch (SAB) reported numerous ash clouds commonly blowing W to NW for tens to hundreds of kilometers. The clouds appeared in GOES satellite images, which are increasingly accessible on the world-wide web. Sketches of a few of these plume images during mid-1997 appear in figure 30. Given the persisting crisis, it is worth noting that the SAB also broadcasts forecasts of the plume's predicted trajectory after energetic eruptions for the benefit of aviators.

Tilt and seismicity. The period 31 July to 20 August was characterized by elevated levels of seismic activity. During 1-6 August tiltmeter readings revealed a cyclic pattern with a 10- to 14-hour periodicity. During this time, regular deflation and inflation of the volcano corresponded with earthquake swarms and continuous pyroclastic flows, respectively. Continuous and near-continuous tremor of varying frequency was noted during several swarms in this period. This pattern is similar to those reported in BGVN 22:06. In addition, on 2 August several very large hybrid events occurred during a hybrid earthquake swarm; at 0900 the hybrid earthquakes peaked at ~3 events/minute with continuous moderate tremor between events.

The 10- to 14-hour cycle continued until 9 August when the pattern was broken with an explosion at 2051. An eruption comprised of several intense bursts caused heavy ash and pumice fall in Old Towne (Plymouth). A new pattern then emerged during 10 to 16 August in which cyclic activity was dominated by hybrid earthquakes and dome growth. Hybrid earthquakes appeared in swarms that were thought to be associated with extrusion of new dome material; the high level of seismic activity suggested a rapid extrusion rate. By 17 August this pattern had become less regular with longer intervals and higher intensity tremors at the end of swarms. After a 17 August swarm, tremor lasting 90 minutes was followed by 6 minutes of well-defined monochromatic seismicity. During 18- 20 August the cycle of alternating hybrid swarms and dome growth developed an 8-hour period.

Seismicity diminished during 21 to 29 August, an interval when rockfall and pyroclastic flow signals were dominant. The chief cyclical pattern noted during this interval took place during 23-26 August: enhanced seismicity at intervals of 12-15 hours. The cycle's peak coincided with modest ash emissions and rockfalls. On 30 and 31 August, activity was reported to be increasing and a 10-hour cycle had become evident.

Pyroclastic flows and ash plumes. During 1-4 August several new pyroclastic flows were reported on the NW flank in the Gages Valley. At 1800 on 3 August a major N-flank flow reached Gages Village and Port Plymouth, causing fires and damage to buildings. The flow was associated with strongly convecting ash plumes that rose up to ~4.6 km altitude and heavy W-flank ashfalls.

On the afternoon of 4 August a dark gray jet was seen projecting from the N flank at 60 degrees to the horizontal, and extending to a height of 600 m. This was followed by pyroclastic flows into the Gages, Mosquito, and Tar River valleys. A vertical ash cloud rose to over ~7.6 km and fragments of rock and pumice up to 11.5 cm diameter fell at the observatory.

On 5 August two periods of intense activity were followed by violent explosions and energetic pyroclastic flows. MVO field teams saw pyroclastic flows enter the sea at the mouth of the Tar River; they covered ~80% of the delta. Researchers investigated submerged portions of the Tar River fan on 25 August. Many of the 5 August pyroclastic flows were subsequently described as rich in pumice, and in addition to the Tar, they also followed the Tuitts, Mosquito, and Gages valleys. Falling pumice reached 8 cm in length; meter-sized impact craters formed up to 1.5 km S of the dome. After the pyroclastic flows there followed a succession of explosive pulses emitting ash at 20- to 30-second intervals; these pulses coincided with elevated 1-Hz harmonic tremor lasting up to 40 minutes.

From 6 to 9 August there were several explosions followed by ash clouds to altitudes of ~5-7 km. Pumice fall and pyroclastic flows were also reported after the explosions. On 7 August field teams saw an ash plume ascend to 1 km at a maximum rate of 47 m/s. From 11 to 31 August, numerous smaller explosions and flows were reported. Throughout the month ash plumes rose up to ~2.4 km, enhanced steaming was coincident with elevated seismicity, and there were small diffuse pyroclastic flows.

Crater observations and measurement. Although high activity occurred during the first week of August, it was not until 7 August that the summit was visible for the first time since July. On 7 August the dome's top contained a bowl-shaped crater oriented with its lower edge facing Gages Mountain. Also, the Gages Valley had become deeply incised. A deep, oval-shaped scar formed from the several small explosions during 6-9 August; on 10 August observers saw new dome material refilling the scar.

On 11 August, the MVO reported theodolite measurements of the new crater taken during clear conditions around 9 August. The crater's volume was 5-7 million cubic meters; the highest point on the dome was measured at 995 m elevation. EDM measurements were also taken on the line from Lee's Yard to Waterworks on several occasions in August; these revealed slow shortening.

A GPS survey conducted on 12 August indicated that a deformation event has occurred in the last 3 months, shifting sites at Whites and Long Ground by about 3 cm N and NE. The results of the survey also suggested that by the end of the interval this movement had slowed or ceased.

On 12 August, a very large spine was observed in the crater, but by the next day it had broken into thirds. A view on 14 August revealed a new cluster of spines above a steep chute at the head of Gages Valley. By 19 August, an area 150 m in diameter perched above Gages wall. It was composed of a series of tall spines; the top of one measured at 969 m. Six days later, on 25 August, it was noted that the mass of spines above Gages wall was breaking up and pyroclastic flows funneled down the Gages valley.

On 27 August fumaroles, rockfalls, and incandescence were noted on the dome's SE flank, an area that had been lacking in recent activity. A secondary phreatic explosion occurred on 30 August after pyroclastic flows had buried a small pond. By 31 August a deep gully had eroded into the deposits below the dome.

Cronan and others (1997) studied Montserrat's hydrothermal discharges in 1995 at spots a few kilometers N of Plymouth near the coast, both on and offshore. The chemical composition of offshore discharges varied the most; for iron, the extreme example, the offshore compositional variation was 300-fold. Temperature also varied at the offshore springs. The authors made a case for monitoring submarine hydrothermal discharges as a means to help predict future eruptions.

Many news reports in the month of August focused on either the ongoing destruction of Plymouth or the lives of people dislocated by the eruption. Press coverage was further heightened when residents made mid-month protests appealing for more governmental support.

For those readers interested in the history of Soufriere Hills, work by Jaggar (1937a, 1937b) and Perret (1939) dealt with the 1933-37 seismic crisis centered at Gages Soufriere. The crisis included abnormal fumarolic activity, earthquakes centered at Gages, and earthquake counts reaching thousands of events per month. Yet, the crisis did not lead to eruptions. Both reports contained an interesting array of experiments and then-new technologies.

References. Cronan, D.S., Johnson, A.G., and Hodkinson, R.A., 1997, Hydrothermal fluids may offer clues about impending volcanic eruptions: Eos, Transactions, American Geophysical Union, p. 341 and 345.

Perret, F.A., 1939, The volcano-seismic crisis at Montserrat, 1933-1937: Washington, D.C., Carnegie Institution of Washington, 76 p.

Jaggar, T.A., 1937A, Adventures and methods in studying West Indian volcanoes, in The Volcano Letter, no. 437 (July 1936, p. 6-7); compiled and reprinted in 1987; edited by Fiske, R.S. and others: Washington, D.C. [ISSN 0890-1996], Smithsonian Institution Press.

Jaggar, T.A., 1937b, Work of F.A. Perret on Montserrat, in The Volcano Letter, no. 449 (July 1937, p. 1-7); compiled and reprinted in 1987; edited by Fiske, R.S. and others: Washington, D.C. [ISSN 0890- 1996], Smithsonian Institution Press.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Spring, MD 20746, USA.

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09/1997 (BGVN 22:09) Repeated pyroclastic flows during 31 August-13 September

The following condenses both Daily Reports and Scientific Report 73 of the Montserrat Volcano Observatory (MVO) for the interval 31 August-13 September 1997. Throughout this two-week period repeated pyroclastic flows left the upper part of Mosquito Ghaut (the drainage directly N of Chances Peak) completely filled. Electronic surveys (EDM) indicated sites in the volcano's NW quadrant (Lee's Yard quadrangle) moved very slowly. Very preliminary analysis of the dome and pyroclastic flow deposits suggested that around early September the extrusion rate was ~6-7 m³/s. Although provisional, this was the highest sustained extrusion rate since the eruption began.

Revisions to the previous hazard map (BGVN 22:06) yielded a new map on the MVO website in September (figure 31). The new map contains two boundaries that had shifted northward compared to the previous one; these boundaries identify three hazard zones described in the figure caption. Chances Peak, the old summit, lies on the W-central side of the volcano's summit area.

31 August-13 September. On 31 August activity remained high and cyclic patterns were evident in the seismic data. After 31 August, pyroclastic flows decreased and generally remained low until 4 September when the level again increased slightly. On 4 September, several hours of pronounced rockfalls were noted; then, at 1540, two detonation sounds were heard associated with a rapidly rising ash cloud and a pyroclastic flow. In the evening, the volcano's upper flanks were unobscured allowing viewers to see a concentration of activity on the dome's N side above Mosquito Ghaut. As had been the case during the last two days of August, rockfall and pyroclastic flow activity continued to show a ~12-hour cyclicity during these first few days of September. This did not continue after 4 September and activity for the rest of the period showed no clear long-lasting pattern although there were a number of earthquake swarms tabulated in MVO's reports.

On 5 and 6 September volcanic activity was generally relatively low. Views of the dome were very clear during the night of 5 September disclosing semi-continuous rockfalls down Mosquito Ghaut and more restricted activity on the upper W flank. Activity increased for about an hour on the morning of 8 September, when a series of moderate pyroclastic flows descended the N flank. Wholly unconstrained by Mosquito Ghaut, these flows spread W (over Farrell's plain) moving ~2 km NNW of Chances Peak (around Riley's Estate) and then progressing towards the W following the Belham drainage system. These flows all moved relatively slowly but reached ~3 km NW of Chances Peak (Dyer's River immediately south of Molyneux). They left the first substantial block-and-ashflows yet deposited in the Belham Valley during the current crisis. In contrast, the pyroclastic flow in the Belham Valley on 25 June deposited a dense ashflow and explosions during August deposited thin pumice flows.

The report for 7 September noted two episodes with intense pyroclastic flow activity due to collapse of hot dome lava. Both of these episodes lasted about an hour-and-a-half and sent material onto the N flank (Tuitt's Ghaut and Farrell's plain). As measured from Chances Peak the pyroclastic flows progressed to distances ~3 km NNE and ~2.6 km NW (to below Harris Lookout on the E and to the to the upper parts of the Belham Valley as far as Dyer's Bridge on the W). After these pyroclastic flows, Farrell's plain was left covered with big boulders. On 7 September, lofted ash blew in an unusual direction, NE. As a consequence ash fell on the neighboring island of Antigua affecting the V.C. Bird International Airport there.

Heightened activity was again noted on 8-9 September. On the latter day there were two main episodes that produced N-directed pyroclastic flows due to dome collapse: the first episode lasted between 0230 and 0430 and was preceded by a hybrid earthquake swarm, the second episode was more intense and lasted from 1005 until 1407 with several discrete pulses. The second episode peaked at 1300 when a series of vigorously convecting pyroclastic flows were observed from MVO's observatory site in Old Towne (7 km NW of Chances Peak). At least two of the flows deposited material on the volcano's northwestern flanks (into an unnamed ghaut in the headwaters of the Belham drainage system). Seismic signals during the venting of the pyroclastic flows and dome collapses had long- period precursors, signals previously associated with gas venting and explosions.

On 9-10 September MVO moved its observatory to a spot along the island's main axis lying ~8 km N and ~1.25 km W of Chances Peak. This spot is called "Mongo Hill" (labeled as Mango Hill on some maps).

The daily report discussing 9-10 September described two hybrid earthquake swarms, each lasting about 2 hours. The first was clearly associated with increased volcanic activity. On 11 September the volcano remained shrouded in cloud for much of the day.

The daily report for the 24-hour period ending at 1600 on 13 September 1997 noted especially vigorous pyroclastic flow activity. The summit of the volcano was shrouded in cloud for much of the day; however, good visual observations were possible because of ash plumes rising above the clouds and of pyroclastic flows descending the N flank. The most vigorous activity in the middle of the afternoon produced pyroclastic flows funneling NW into the uppermost part of the Belham valley in the Dyer's area and also at least one audible explosion from the dome. Ash clouds rose typically to 1.5 km, being generated both from pyroclastic flows and from continuous summit venting. New pyroclastic flows were also noted on the volcano's NNE and NE-E (in Tuitt's and the Tar River drainages), and provisionally to its SW (in the White River).

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Spring, MD 20746, USA.

Corrections: Simon Young provided a thoughtful review of the previous issue (BGVN 22:08), but his comments arrived after press time. Corrections for the opening paragraphs were as follows: 1) Besides the repeated pyroclastic flows, August included a 9-day interval with Vulcanian eruptions, and 2) A major pyroclastic flow entered central Plymouth on 3 August.

In the discussion of tilt and seismicity, Young noted that the hybrid swarms recorded during the first 4 days of August included individual events as large as any recorded on the broadband network (i.e. since October 1996). The volcano went into a sequence of explosive eruptions between 4 and 12 August.

As noted in the discussion of pyroclastic flows and ash plumes, projectiles seen on the afternoon of 4 August rose 600 m; Young noted that these ultimately fell on Farrell's Plain. That same day a vertical ash cloud dropped "fragments of rock and pumice up to 11.5 cm diameter . . ." at the observatory 7 km NW of the volcano. On 7 August field teams saw an ash plume ascend at a maximum rate of 47 m/s; it attained a height of 8 km (not 1 km as stated).

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10/1997 (BGVN 22:10) Dome collapse and explosions

The following condenses Scientific Reports of the Montserrat Volcano Observatory (MVO) for 14 September-12 October. The volcano maintained an extremely high level of activity during this period. During the first week, rockfall and pyroclastic flow activity was concentrated in Tuitt's Ghaut. The flows typically traveled ~3 km and filled the upper reaches of the ghaut. Seismic activity was dominated by pyroclastic-flow and rockfall signals; however, a hybrid swarm preceded a 14 September flow, and there were a large number of long-period events. A major dome collapse on the morning of 21 September generated pyroclastic flows and surges over the NE flank. A phase of explosive activity began on 22 September during which Vulcanian explosions occurred every several hours. The explosions produced vertical eruption columns and collapsed fountains of material that fed pyroclastic flows. The pumice-rich flows traveled down all the ghauts around the volcano, reaching the sea at Tar River valley (figure 32), White River valley, and Spanish Point. After a large explosion on 11 October, a pyroclastic flow in White River valley destroyed a bridge and the Radio Antilles installation. Ash and pumice fall affected N and W Montserrat; Antigua also received occasional light ashfall.

Visual observations. Activity was very high during 14-21 September with moderate-sized pyroclastic flows moving down Tuitt's Ghaut and over Farrell's plain. In the upper reaches of Mosquito Ghaut, material overflowed into Tuitt's Ghaut and created a small debris fan . The largest individual pyroclastic flow in Tuitt's Ghaut occurred on the morning of 17 September, depositing material 200 m beyond the confluence of Tuitt's and Mosquito Ghaut. The dome's SE side continued to degrade, generating rockfall debris and small pyroclastic flows in Tar River.

At 0354 on 21 September a large collapse occurred on the NE flank of the dome. Pyroclastic flows down Tuitt's Ghaut devastated the area from Trant's Yard to White's Yard. The airport terminal building was destroyed as well as most properties in Spanish Point, Bramble, and Tuitt's; in addition, Bethel village was buried. Small fans developed at the mouths of Farm River and White's Ghaut.

Most deposition from the 21 September flows occurred around Trant's Yard and Farm Estate and greatly extended the fan formed on 25 June (BGVN 22:06). A lobe detached from the main Tuitt's Ghaut flow (500 m S of the Mosquito Ghaut confluence) and spread ENE across Bramble village. The deposit was similar to that of the Bethel lobe emplaced on 25 June, consisting of a thin, ashy unit with abundant large blocks sized from one to several meters. Material also spilled from Tuitt's Ghaut into White's Ghaut at a low point in the valley wall. Pyroclastic flows that reached the sea were generated in White's Ghaut; extensive surges traveled over Tuitt's Estate, Bethel Estate, and White's Yard. The collapse generated ~8 x 10⁶ m³ of deposits, a considerably larger volume than on 25 June.

Following the 21 September collapse, a series of Vulcanian explosions began at 0055 on 22 September. During 22-28 September, 15 explosions occurred with an average periodicity of 9.7 hours. The explosions were similar to those of early August (BGVN 22:08); they began with dark-gray explosion clouds comprised of radiating spears. The clouds quickly rose 600-1,000 m above the dome and developed into convecting columns. A rapidly building roar was heard and ballistics were projected up to 1.5 km from the dome during the explosions. Pyroclastic flows were generated during most explosions and traveled down all the six major ghauts (Tar River, Tuitt's, Mosquito, Tyre's, Gages, and White River). The explosion columns rose up to 7,600 m; after each explosion, vigorous pulses of ash venting were observed for up to 1 hour and low rumbling sounds were heard for up to 30 minutes. At night the explosions resembled a large fireball above the volcano; showers of incandescent ballistics occurred over Farrell's plain, Gages Mountain, and Chances Peak. Incandescent surges were seen on several occasions traveling over Farrell's plain and down Tuitt's Ghaut. At the onset of one explosion, observers at Jack Boy Hill saw high levels of gas venting from fumaroles on the dome's E flank.

Thin, pumiceous pyroclastic-flow deposits were generated by the explosions during 22-28 September. They were generated by fountain collapse; a veneer of pumice was left over the area between Gages and Peak 'C', over Chances Peak, over the Galways area, and over much of the dome's surface. The flows were generated soon after the onset of the explosions. Maximum runout distances were ~4.5 km in Tuitt's Ghaut, 2.25 km in Tyre's Ghaut, 4 km in White River, 2.6 km down Gages Valley, and 2.5 km down Farrell's Plain; flows reached the sea at Tar River. In unconfined areas such as the Tar River Fan, the Farrell's Plain, and the Trant's Farm Fan, the flows left long, thin, tongue-like deposits and rarely spread into thin sheets. Flows confined to narrow ghauts were thicker, narrower, and had steeper termini.

Fallout from the 22-28 September explosions varied. Most of N and W Montserrat had moderate ash falls including pumice fragments. Pumice fragments were recorded at the Observatory on Mango Hill and at Little Bay (2 cm), at Cudjoehead and Olveston (3 cm), and on Davy Hill (5 cm). A 6-cm pumice was reported in Woodlands.

During 14-28 September, observations of the dome showed that the scar above Tuitt's Ghaut extended 300 m into the dome; it opened into an explosion crater ~300 m diameter. A bank of pumice and talus separated the crater and the scar; depth of the crater below the bank was ~150 m. Much of the dome's surface and the upper area of Chances Peak were covered by fresh pumice deposits. Two of the three peaks on the northern crater wall (Peaks 'B' and 'C') had severely eroded; they were nearly covered by the dome talus and pumice deposits.

A total of 32 explosions occurred during 28 September-12 October with an average periodicity of ~10 hours, although the intervals varied in duration from 4.9 to 33.7 hours. Explosions after shorter intervals were weaker, produced paler, less vigorous plumes, and were accompanied by smaller pyroclastic flows.

Pyroclastic flows during 28 September-12 October were concentrated in Tuitt's Ghaut and the Tar River Valley, although regular activity also occurred in the White River, Fort Ghaut, and over the Farrell's Plain. Many of the Tuitt's Ghaut flows traveled 4-4.5 km, building a 300-m-wide fan around the Farms and Trants area. The flows over-spilled the ghaut walls in several locations and spread over a wide area, traveling ENE and passing through Spanish Point where they reached the sea. Many pyroclastic flows in the Tar River area reached the sea by traveling down the S side of the valley and down two chutes on the dome's E face. The surface of the fan was almost entirely covered by new pyroclastic-flow deposits; a pronounced hump along the fan's central axis developed. Pyroclastic flow activity in White's River was more limited with only a few flows reaching the area where Great Alps Falls had been. However, pyroclastic flows from an explosion at 0105 on 2 October covered the bridge at O'Garra's, reaching the sea at the mouth of the White River.

A large explosion at 1757 on 11 October covered the field at O'Garra's with pyroclastic-flow deposits, destroyed the Radio Antilles installation, and completely buried a bridge. Incandescent blocks and a glowing cloud were seen from Antigua during the event. Flows in Fort Ghaut spread NW around the Gages fan, W around Upper Amersham and into Plymouth. The Plymouth flows spread through Dagenham and reached within 300 m of the sea. Occasional pyroclastic flows have occurred in Tyer's Ghaut, although the longest runouts were only ~2 km.

Plume heights from the 28 September-12 October explosions varied from 3.6 to 7.6 km. Pumice fallout in inhabited areas only occurred on 1 and 2 October, although heavy ashfall occurred many times on the rest of the island. Antigua received light ashfall on three occasions.

Very good views of the dome were obtained during 28 September-12 October. At the base of the scar above Tuitt's Ghaut, the bank of pumice and talus was steep and appeared consolidated. The lowest point on the bank was 860 m above sea level; behind it was the circular explosion crater, 300 m in diameter. The crater rim had a fairly constant elevation of ~950 m. The highest point on the dome was a spine on the rim above Galways with an elevation of 975 m. Glimpses down into the explosion crater suggested its base was 100-150 m below the level of the pumice and talus bank, probably close to the level of the original English's Crater basement. The crater and scar have both been enlarged slightly by rockfalls from the inner walls, by erosion, and by shaking during the explosions. Chutes were developing along both sides of a large consolidated area of the dome above Galways and the SE flank in the Tar River Valley was degrading slowly.

Much erosion occurred around the margins of the dome on the old crater wall. Peak B on the N crater wall was lowered by 30 m due to the passage of pyroclastic flows this summer. A 90-m-wide chute was cut down the Gages Valley face of the dome immediately S of the original Gages Wall against Chances Peak. This chute was up to 50 m deep and cut at least 30 m into the original basement in places.

Seismicity. During 14-20 September, seismicity was dominated by rockfall and pyroclastic-flow signals. There were few earthquakes, although the pyroclastic flow on 14 September was preceded by a hybrid swarm that included some large events. The hybrids had amplitudes as high as any recorded since the installation of the broadband network in October 1996 at the Windy Hill station. An unusually high number of long-period earthquakes occurred before, rather than after, pyroclastic flows. On 16 September, a "bang" related to a long-period earthquake was heard.

The night of 20-21 September showed a marked change in seismicity. A hybrid swarm occurred before a big pyroclastic flow at 0354, but the hybrids did not decrease very much after the event. Then at 0055 on 22 September, the first in a sequence of explosive events took place that occurred at intervals of 6-10 hours during 22-28 September. As in August, each explosion was recorded on seismometers as a pyroclastic-flow signal preceded by a ~1 Hz signal whose amplitude varied with the associated pyroclastic-flow amplitude. The long- period energy continued throughout the pyroclastic-flow signal and afterwards as low-amplitude tremor. A few explosions were preceded by hybrid swarms, but most had very little precursory seismicity. When swarms occurred, they continued for a short time after the explosion. All of the explosions were followed by between 20 minutes and 3 hours of tremor. The tremor was less harmonic than in August but had two or three well-defined spectral peaks. As in August there was good visual confirmation that the tremor correlated with ash venting. Volcano-tectonic earthquakes during 22-28 September were centered 2-4 km below the dome.

Seismicity was low during 28 September-12 October, except during explosions and subsequent tremor, which on occasion lasted several hours. The explosions, as in August (BGVN 22:08), had a distinctive seismic signal, with an initial low-frequency phase followed by a high-frequency phase and low- frequency tremor. The high-frequency phases are assumed to be caused by pyroclastic flows observed after each explosion. Low-frequency tremor at the start of the signals preceded observed activity in the crater by several seconds. It is assumed to continue throughout the pyroclastic flow signal and become the post-explosion tremor. Both the low frequency phase and the tremor have the same peaked spectrum with the main peaks at 1.2 and 1.7 Hz.

Ground deformation. On 20 September, GPS observations taken at Harris, White's, Long Ground, and Hermitage confirmed the shortening noted in recent reports on the Harris-White's and Harris-Long Ground lines. The length of shortening was ~3.5 cm on these lines due to N movement of White's and Long Ground. The movement did not appear to be accelerating. The Hermitage site was not occupied due to the activity on the NE side since 21 May. The line to Harris showed a further 1-cm shortening (NE movement of Hermitage) consistent with its movement in May. The White's site was affected the following morning by pyroclastic surges. The activity also damaged the permanent GPS site at White's.

No EDM measurements were made during 14-28 September. Ash from pyroclastic flows and explosions obscured the target at Lee's Yard and it was unsafe to enter the area to clean the reflector. No GPS observations were made during 28 September-12 October. The amount of ash on the N, E, and W flanks prevented the helicopter from landing at all but two sites. The O'Garra's GPS site (M46) was destroyed by pyroclastic flows on 11 October. EDM measurements to the Lees reflector were not possible due to ash cover and airborne ash.

Volume measurements. No dome or deposit volume measurements were made during 14- 28 September; however, photographs were obtained from the ground at Whites and from a hover position close to Windy Hill.

A series of accuracy tests were carried out with GPS-laser binoculars from a helicopter to assess their suitability for dome mapping. The working range for this instrument has typically been <200 m, which is considered too dangerous. Although the binoculars had 1-m precision in distance there was too much vibration in the helicopter for them to be useful at distances >400 m.

Several clear days during early October allowed a detailed survey and map of the dome to be completed. Theodolite measurements were made from Jack Boy Hill, Flemings, Garibaldi Hill, and the old observatory in Old Towne. Photographs were taken from White's, Harris, and Jack Boy Hill. A series of photographs at different angles around the dome were taken from the helicopter; the position of the camera was determined with the GPS.

The dome volume was 68 x 10⁶ m³ during early October. The volume has decreased since the last measurement on 28 August when it was 78.1 x 10⁶ m³, at which time the extrusion rate was 8.7 m³/s (average 17 July-28 August). The difference in these volumes represents the volume of the 21 September collapse as well as a substantial amount of pyroclastic flow activity over Farrell's plain and in Tuitt's Ghaut prior to the collapse.

Environmental monitoring. Dust Trak sampling carried out at several sites around the island to evaluate the atmospheric particulate load showed comparatively high values in the N and E area and high values in the Salem area. The central area also showed elevated values. The high levels were the result of fallout from explosive activity.

On 12 September, sulfur dioxide diffusion tubes were collected from four sites to the N of the volcano, at MVO (south), Lawyers, Fogarthy, and Geralds. The diffusion tubes measured the average background level of SO₂ during the exposure period. As in previous sampling periods, SO₂ gas was not present in measurable quantities. On 4 October, sulfur diffusion tubes were left at Weekes and at St George's Hill in the evacuated zone to be collected after two weeks. Under normal prevailing wind conditions (to the W or NW) the sites lie under the plume. Sulfur diffusion tubes at four sites in the inhabited area of the island were being left for four weeks. Until early October there was no detectable SO₂ in the inhabited area.

A mini-COSPEC was deployed on 20 September from a police launch. A series of traverses were made under the plume at different distances from the volcano. The average SO₂ flux was 600 metric tons/day. The launch broke down the next day and was out of action for over a month.

Rainwater collected at three sites on 21 September showed low pH. One site also showed substantially elevated chloride content. High acidity levels persisted during late September.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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11/1997 (BGVN 22:11) Explosions and dome growth

The following summarizes Scientific Reports of the Montserrat Volcano Observatory (MVO) during 12 October-23 November 1997.

General. During September, activity was dominated by collapses with simultaneous pyroclastic flows down ghauts (BGVN 22:10). Particular events differed in magnitude, column height, or pyroclastic runout, possibly due to time elapsed between events. During 12-21 October, 29 explosions were recorded for a total of 61 since the latest episode began on 28 September. After the last explosion on 21 October, a new dome was seen in the crater, extruding at a rate of up to 8 m³/s. The new dome grew during the following week on the S side, weakening the crater wall on the Galway's side (figure 33) and creating two large vertical cracks on the outside of the wall by 2 November. Further growth in the weakened area led to a 4 November collapse, which removed much of the pre-explosion dome complex material. A subsequent collapse on the 6th removed a significant portion of the new dome and old material. Pyroclastic flows from these collapses reached the sea and a fan deposit at the mouth of White River was significantly extended. Dome growth coincided with large swarms of hybrid earthquakes. After the 6 November collapse, the swarms subsided yet seismicity remained relatively high. Low levels of eruptive activity prevailed for the rest of November. Although bad weather limited observation of the dome, the lobe in the Galway's area was seen as the focus of growth during 9-23 November but at a slower rate. Seismicity included rockfall signals and small-amplitude hybrid earthquakes.

Visual observations. Vulcanian explosions up to 21 October resulted in pyroclastic flows into surrounding ghauts. Intervals between explosions averaged 8.5 hours with a range of 2.75 to 20.5 hours. During 14-16 October, 12 explosions occurred; intervals between single events lengthened towards the end of the period. Three vigorous explosions on 20-21 October sent plumes to 9,100 m, pumice to Salem and Olveston, and ash to the N. Pumice from Cork's Hill measured up to 10 cm in diameter and ballistics fell 2 km N from the vent. Pyroclastic flows were generally radial for larger explosions; however, the N ghauts were preferred routes because the crater is open to the N. Some flows had relatively small runouts (<1 km) in only one or two ghauts. Pyroclastic flows over the past month have left thin (0.3-1 m) deposits on all flanks, accumulating and infilling the topography. Fort Ghaut in Plymouth and Mosquito Ghaut were completely filled, and Tuitt's and White's Ghauts were partially filled, resulting in fans advancing into towns. Gage's Soufriere was significantly filled with material stacked in front of St. George's hill.

A new dome was first recorded as an incandescence inside the scar during the evening of 22 October. The next day, fresh lava overspilled the tephra rampart between the scar and crater and, by 25 October, occupied a substantial portion of the scar. The lava appeared to be blocky, coarse material, which, due to oxidation at the top of the conduit, is darker than normal (similar to last October; BGVN 21:10). By 25 October the dome's peak had risen to 910 m, 40 m below the crater rim. Growth to the N and vertical infilling of the scar caused rockfalls that traveled a few hundred meters down Tuitt's Ghaut; however, rockfalls were few in number considering the rate and blockiness of the extrusion as well as the steepness of the ghaut. Dome growth continued over the next few weeks with vigorous ash-and-steam venting. Rockfalls from the new dome and old crater coincided with hybrid earthquake swarms.

An overflight on 2 November revealed two large vertical cracks on the Galway's side of the crater; by the next day, these had evolved to deep gullies. Rockfalls on the dome's S side occurred on the morning of 4 November. At 1206 on 4 November, a wide section of the crater in the Galway's area collapsed and caused an hour of pyroclastic flows. Some of the flows reached the sea at O'Garra's and formed a delta. Ash clouds rose to 3000 m. The collapse removed a large part of the old dome but left the 22 October dome mostly intact. Observations on 6 November included two distinct lobes of the new dome separated by a small crater venting ash; the N lobe remained at its 2 November height of 937 m while the S lobe grew. Following 18 hours of high- amplitude tremor a second collapse in the Galway's area began at 1430 on 6 November and lasted 35 minutes. More material was removed than in the previous collapse, rockfalls occurred in Tar River valley and Gage's areas, and an ash plume reaching 4,500 m drifted W.

After a few days of poor visibility, growth of the new dome in the collapse area was revealed. A fin-shaped lobe had grown almost vertically in the old crater wall position; it had a coarse, blocky outer face but a smooth appearance on the inner surface where it extruded out of a cleft in the dome center that exhibited vigorous degassing and venting of ash. The distinct N and S lobes divided by a central cleft or vent were similar to earlier structures (BGVN 21:08 and 22:05), although in this case the N lobe extruded first to reach a certain size then relaxed while growth shifted to the S lobe; this in turn lead to a catastrophic collapse of the old crater wall. Overflight observation on 11 November showed that the S lobe had doubled in size in 3 days to fill the collapse scar of 4-6 November; however, it was extruding at a slower rate. Ash and steam continued to vent from the central cleft. Ash clouds rose to 1800 m drifting W and fell out over Plymouth. Rockfall spalling off the S lobe eroded chutes S of the dome and accumulated in thick deposits in Galway's Soufriere.

Seismicity. Figures 34, 35, and 36 show seismicity during 12 October-23 November. The sequence that began on 22 September (BGVN 22:10) continued until 21 October. Seventy-six explosions at intervals of 3-34 hours were recorded. The explosions appeared as 1-Hz signals of varying relative amplitude and were followed by pyroclastic-flow signals; long-period energy continued through the flow duration and persisted as lower-amplitude tremor of 0.5 to 3 hours duration. Signals coincided with ash venting but there was little or no precursor activity.

The second explosion of 20 October and the first of 21 October were accompanied by swarms of hybrid and volcano-tectonic earthquakes. The second explosion of 21 October initiated 24 hours of hybrid and volcano- tectonic earthquakes and rockfalls down Tuitt's ghaut before ending in a long, sparse swarm on 23 October, although a high level of long-period earthquakes lingered thereafter. Volcano-tectonic earthquakes typically occurred 2-4 km from the top of the dome.

During late October and early November, intense swarms sometimes merged with tremor having frequencies similar to individual hybrids. Hybrid swarms during 1-2 November produced the highest amplitudes since 24 June, reported from stations in Antigua, Dominica, and Nevis. Large pyroclastic-flow signals were recorded on 4 and 6 November. During 6-8 November, particularly high levels of tremor occurred. Individual hybrids were detected on paper but not on the networks due to high background noise; thus low numbers of events did not reflect low activity. Tremor and hybrids were associated with ash venting at the dome. Small pyroclastic flows were recorded on 9 November, but otherwise hybrid earthquakes did not generate external activity. Amplitudes became progressively smaller later in November; from 14 November to the end of the month, rockfall signals dominated, although a significant number of low-amplitude hybrids not grouped in swarms occurred but were not detected by the network.

Ground deformation. On 20 October, a GPS survey was taken; however, the only sites accessible were White's, Harris, and Windy Hill due to thick ash cover. Measurement from Harris to White's showed a 2-cm increase since 20 September, closer to the pre-June 1997 level. Although less than two standard deviations below the mean, this single measurement did not indicate an acceleration in deformation. The line from Harris to Windy Hill showed slight shortening since 12 August. EDM measurements to Lee's Yard from MVO on 14 October revealed an increase of 1 cm since July.

Volume measurements. Gross morphology of the pre-21 September dome was unchanged since the collapse on that day (BGVN 22:10) until 22 October with some exceptions (see Visual observations). The volume of the 22 October dome was measured by geometric calculation until a survey was taken. Assuming the dome completely filled the explosion crater by 23 October (when overspilling was observed), the volume was approximately 1.7 x 10⁶ m³ resulting in an extrusion rate of 8-10 m³/s, depending on the time of first appearance. A detailed survey was made on 6 November, before the collapse, from several points; theodolite points from Jackboy Hill, Center Hills, and Flemings, a GPS point at Center Hills (to be used in future surveys as an additional static photo point), and helicopter survey photographs of most areas around the dome except the Galway's side. Good coverage of the N lobe of the 22 October dome was obtained. Since this area had not changed since 3 November, the volume was calculated at 5 x 10⁶ m³. Collapse volumes were calculated separately for an average extrusion rate of 5 m³/s over the first 11 days of the "22 October" dome growth. Visual observation revealed that the 4 November collapse involved less material than the 6 November collapse. The latest estimates of collapse volumes were 1.8 x 10⁶ m³ from 4 November and 3.4 x 10⁶ m³ from 6 November. The bulk of the collapse material was deposited in fans at the end of valleys that will be surveyed when the ash subsides. A 17 November survey of the White River valley fan revealed total deposits of 13.6 x 10⁶ m³, an increase of 5.5 x 10⁶ m³ since 15 May, resulting mostly from the 4 and 6 November collapses. The survey did not include recent deposits in the upper valley still covered in ash.

Environmental monitoring. Dust Trak sampling to measure airborne particulates was carried out at four fixed sites. The values at the fixed sites were low (<0.05 mg/m³) during 12 October-23 November, except for the Catholic school site, which sometimes recorded elevated levels (0.05-0.1 mg/m³). This effect is caused by the large amount of human activity at this site and its location near a main road. Towards the end of this reporting period the three sites (not including the school) all had remarkably similar average concentrations each day. A new Dust Trak site was established at Mango Drive in Woodlands on 16 November to replace the Runaway site.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/ volcanoes/west.indies/soufriere/govt).

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12/1997 (BGVN 22:12) Collapse of dome and Galway's wall on 26 December

The following condenses Scientific Reports of the Montserrat Volcano Observatory (MVO) from 23 November 1997 to 4 January 1998.

Overview. Activity during 23 November-21 December was at a relatively low level, although the 22 October dome (BGVN 22:11) continued to grow on the S side in the Galway's area while the N flanks remained quiet. Seismicity was dominated by rockfall and long-period events, with two periods of pyroclastic flow on 27 November and 1 December. During 21 December-4 January, volcanic activity increased to a high level with a large hybrid earthquake swarm on 25 December followed by a debris avalanche and dome collapse down the White River early on 26 December. A series of very large pyroclastic flows destroyed a wide area SW of the volcano and created a surge cloud that may have been associated with a lateral blast.

Visual observations. Good observations of the dome were possible during 23 November-21 December. A small dome collapse occurred on 27 November at 1445 and continued until after 1800. Although some of the flows reached the sea, most of the collapse was comprised of continuous but relatively small flows that traveled less than 2 km. Further pyroclastic flow activity occurred on 1 December starting around 2000 preceded by 2 hours of elevated rockfall activity. Only the largest of these pulses reached as far as the delta. While the flows occurred, glow was observed in the sky above the Galway's area. Neither period of activity produced ash clouds above 3 km or involved significant volumes of material.

On 4 December observers noticed that the dome's talus apron in the Galway's area had significantly increased in height and width. Continued growth of the 22 October dome had begun to encroach on Chances Peak and the Tar River to the E. The height of the active growth center was estimated at ~980 m, well above the height of the explosion crater rim. Around this time, theodolite surveys confirmed that the N lobe of the 22 October dome had not grown recently. Occasional minor rockfalls occurred on the N and E faces in the Tar River valley as old surfaces of the dome gradually degraded.

Near-continuous rockfalls started to cut into the S margin of Chance's Peak during 7-21 December. The talus slope above the Galway's Soufriere area extended significantly SE and the summit area of Galways Mountain degraded slightly. The growing talus apron extended over the remnants of the Galways wall, depositing a large volume of material outside the old crater margins. The remnants of the pre-September 1997 explosion crater above Gages and Tar River eroded, spilling a small amount of material down the Tar River. Theodolite measurements from Jack Boy Hill indicated no movement on the N flanks. A few small rockfalls occurred down Tuitts Ghaut on 10 December.

The low-level activity that had prevailed for 6 weeks was broken at about 0300 on 26 December. Reports from the police checkpoint in Salem indicated explosions at 0315 and 0325. At 0400, observers on Garibaldi Hill reported light ashfall and burning buildings S of Plymouth. A pilot saw an ash plume heading S at ~11 km altitude. Helicopter flights later in the morning revealed extensive damage in the area of Aymer's Ghaut, S of Kinsale, to the N flanks of South Soufriere Hills. Deposits from a large debris avalanche traveled down the White River to the sea and added considerable material to the pyroclastic fan.

Pyroclastic flows and surges covered from Kinsale almost to the summit of South Soufriere Hills. The area closest to the White River, including St. Patrick's and Morris's, was completely destroyed. A small pyroclastic flow traveled down Dry Ghaut towards Sweeney's Well to within 300 m of the SE coast. Brief views of a void at the top of White River indicated the loss of a large amount of material from the area around the Galway's Soufriere and of a portion of Galway's wall. The new talus apron and S lobe above Galways wall had been excavated. Later flights and visits allowed measurement of the deposit's thickness and temperature (table 24).

Table 24. Thicknesses and temperatures of deposits from the 26 December explosion at Soufriere Hills. Courtesy of MVO.

    Date       Site                        Type of deposit

    30 Dec 97  Dry Ghaut (end of flow)     Pyroclastic flow
    30 Dec 97  Dry Ghaut (3 m from end)    Pyroclastic flow
    30 Dec 97  Dry Ghaut (10 m from end)   Pyroclastic flow
    01 Jan 98  S White River Delta         Surge
    01 Jan 98  O'Garra's Quarry            Surge
    01 Jan 98  O'Garra's Quarry            Co-ignimbrite ash

    Date       Site                   Thickness  Temperature (°C)
    30 Dec 97  Dry Ghaut (end of flow)   30 cm    48.7 at 20 cm depth
    30 Dec 97  Dry Ghaut (3 m from end)  40 cm   138.0 at 25 cm depth
    30 Dec 97  Dry Ghaut (10 m from end) 40 cm   122.4 at 35 cm depth
    01 Jan 98  S White River Delta       70 cm   not measured
    01 Jan 98  O'Garra's Quarry          10 cm   not measured
    01 Jan 98  O'Garra's Quarry           7 cm   not measured

Observations from Old Road Bay indicated that a large wave came ashore there immediately after the eruption. At about 0300, an observer in the vicinity of the bay's N end reported that the sea was "sucked backwards" before coming onto land near the jetty. Measurements of detritus on the shore revealed that the wave must have been ~1 m higher than the road, but there was little evidence of substantial waves elsewhere along the coast. This wave probably resulted from a debris avalanche entering the sea; the wave was most likely focused in the Old Road Bay area because of the shape of the bay.

On 1 January increased activity included at least one large pyroclastic flow that traveled down the White River to within 1 km of the sea and much rockfall activity from above Galway's Wall. A new talus apron started to accumulate at the base of the remains of Galway's Wall and some large wall-parallel cracks were observed in the Chance's Peak side. Until 4 January, rockfalls created diffuse ash clouds that generally drifted W over Plymouth.

Seismicity. During 23 November-7 December, the decrease in seismic activity that began around 14 November continued (figure 37). However, the lack of hybrid earthquakes in the second week despite a constant number of rockfalls was unusual. The frequency content of the rockfall signals changed slightly towards longer periods probably due to the attenuation of higher frequencies. This indicated a possible change of ground coupling between rockfalls or pyroclastic flows and unconsolidated deposits.

During 7-21 December seismicity was dominated by rockfall signals and long-period earthquakes. In many cases it was difficult to distinguish between the two event types. A number of the events classified as long-period earthquakes resembled short bursts of harmonic tremor that lasted up to a few tens of seconds and were nearly monochromatic at ~2 Hz, although there were often a few wavelengths at 1 Hz present at the start of an event. Almost all the signals classified as rockfall had a dominant frequency of ~2 Hz, even those clearly correlated visually with rockfalls at the dome. Events were thus classified by the relative importance of the dominant peak, creating a gray area between the types. The rockfall signals may also have included ash venting or degassing; this was supported by a signal recorded during ash venting after an explosion with a corresponding monochromatic seismic signal near 2 Hz.

During 21-24 December seismicity remained relatively quiet. Slightly more hybrids and noticeably fewer rockfall signals occurred than in preceding weeks but it was not indicative of any great changes. The quiet period ended on 24 December when a hybrid swarm began, leading to the large collapse on 26 December (figure 37).

The swarm, which began at 1420 on 24 December, started out sparsely with events every 20 minutes and slowly increased in intensity until about 2000 on 25 December. Individual events also generally increased in amplitude as the swarm progressed, but even the largest were relatively small in amplitude, an order of magnitude smaller than those recorded in early November, for example. At 2000 the hybrids occurred too frequently to trigger the networks and the signal was effectively a tremor signal. The tremor's amplitude increased until about 2300 and then declined until the collapse at 0300 the next day.

At about 0300 a continuous high-amplitude signal lasted ~16 minutes and included several pulses. The signal then continued at reduced amplitude for 9 minutes, during which a roaring sound similar to but louder than that heard after explosions and associated with ash venting. Intervals of monochromatic seismicity at 1.9 Hz were recorded during this time, as was also the case after explosions. The combination of the roaring noise and monochromatic seismicity indicated vigorous degassing after the main pyroclastic flows had finished, suggesting that the conduit was exposed.

After the collapse, seismicity settled into a cyclical pattern with peaks in RSAM every 6-8 hours. Hybrid earthquakes occurred during the periods of raised background amplitude but in most cases there were not enough in any given cycle to constitute a swarm. Two days after the collapse the number of hybrids decreased to previous levels but the cycles in RSAM continued until 4 January.

Ground deformation. GPS surveys of the BIGNET (Harris, Whites, Windy Hill and Broderick's) and LEESNET (Old Towne, WaterWorks, St George's Hill and Lees Yard) networks were taken during 23 November-28 December. During 23 November-7 December the Harris-Whites baseline had recovered half of the shortening that occurred over the last five months; the line was ~2 cm shorter than its pre-June mean. The data suggested a slow rise of the Broderick's site. No clear trends were identified in the data collected on LEESNET.

An 18 December survey of EASTNET (Harris, Whites, Long Ground, Windy Hill, and Hermitage) indicated a 4.5 cm shortening of the Harris-Whites baseline during April-September 1997. Since September the line has returned to within 2 cm of its pre-April mean. It appeared to be stable and the last three baseline measurements were within 4 mm of one another. Hermitage continued to move NE. It has moved 10.7 cm since mid-January 1997; 6.2 cm of this in the last three months, a marked rate increase.

Volume measurements. On 28 November surveys between Mosquito Ghaut in the N and White's Ghaut in the E found that the total volume of material had increased by 24 x 10⁶ m³ since 13 August 1997. The increase included materials from the 21 September collapse, column-collapse flows during explosive activity, and pyroclastic flow activity on the N flank from mid-August to 21 September.

As of 7 December, the volume of erupted material was then estimated at 203 x 10⁶ m³ including the dome, pyroclastic flow material, and estimates of the eruption column material. A previous dome survey measured an extrusion rate of 5 m³/s, down from summer 1997, but the overall trend for the average extrusion rate was still increasing.

A volume survey of the dome on 8 December revealed that the W, N, and E flanks remained mostly unchanged. The S lobe continued its active growth over the whole face. Growth appeared to be caused by the extrusion of large slabs and blocks from the central cleft between the N and S lobes. The talus apron at the base of the S lobe had extended significantly since the last survey, increasing by 26 x 10⁶ m³. Most of the new material accumulated in the Galway's Soufriere region on the S side of the remnants of Galway's wall. Deposits in this area were over 140 m thick. The dome volume was 102 x 10⁶ m³ with the total erupted volume for the entire eruption was estimated at 232 x 10⁶ m³.

A volume survey of the White River valley fan taken on 4 January showed that it had not yet extended into the sea significantly. On the other hand, the steep underwater shelf in this area caused most material that reached the sea to slump off the edge of the fan into deep water where it could not be surveyed. A survey of the material in White River valley was hampered due to unsafe, ashy conditions; however, rough estimates of the amount of material ponded in the valley were 20-30 x 10⁶ m³. Due to poor visibility, the size of the scars in the dome were hard to determine. Estimates for the amount of material lost during the 26 December event were 30-60 x 10⁶ m³, including old Galway's Wall material.

Environmental monitoring. Ambient dust sampling was conducted using a Dust Trak (PM10) instrument at four fixed sites. Each value is an average of the concentration measured over approximately 24 hours. The values at the fixed sites were low from 23 November 1997 to 4 January 1998 except for values at the Catholic school, which sometimes recorded raised levels. The dust concentration was judged to be higher due to proximity of the main road and the large amount of human activity there.

Sulfur dioxide diffusion tubes were re-sited as a result of volcanic activity but showed that in the N of the island there were no significant concentrations of SO₂. On 17 December, scorching of leaves and grass in the Woodlands area resulted from light ashfall.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, P. O. Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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01/1998 (BGVN 23:01) Continued dome growth; low volcanic and seismic activity

The following condenses a scientific report of the Montserrat Volcano Observatory (MVO) for 4-18 January when low volcanic and seismic activity prevailed but dome growth continued.

Visual observations. Activity was dominated by small rockfalls and pyroclastic flows from the new dome growing in the scar left after the 26 December 1997 collapse (BGVN 22:12). Rockfalls were generally located in the upper part of the White River, but some were seen in the Tar River valley; talus accumulated in a depression near the remains of Galway's Wall. The rockfalls produced dilute ash clouds that generally drifted W.

Heavy rain on 11 January generated hot mudflows on the NW flank (Belham River valley). Logs up to 6 m long and rocks up to 60 cm in diameter were transported in the muddy water. The water temperature ~1 hour after the peak in activity was 27.4°C, only about 9°C above air temperature; however, rocks from the main flow on the golf course were still steaming and hot to the touch. Small ash clouds were produced near Molyneaux as recent deposits on the sides of the river valley collapsed.

A helicopter flight around the SW flank on 12 January allowed observation of the 26 December collapse scar, where a new dome with a steep front face and extensive talus slope had grown. Clear weather on 12 and 13 January revealed continuous ash and steam venting; the ash columns rose 2.4-3.0 km.

Seismicity. During 4-18 January seismicity was relatively low and dominated by rockfall signals. Occasional isolated hybrid swarms and one small volcano-tectonic earthquake swarm occurred, but they were not followed by any noticeable increase in surface activity. Short intervals of increased seismic amplitude 6-24 hours apart were recorded on all stations even when no events were being recorded; however, most rockfall signals were recorded during these intervals.

Ground deformation. The GPS network BIGNET (Harris, Whites, Long Ground, Windy Hill, and Broderick's) continued to show slow movements at Long Ground and Whites to the NE and N, respectively. The Harris-Windy Hill baseline has had two lengthening-shortening cycles since measurements began in June 1996. The first cycle, which ended in mid-May 1997, involved a lengthening and shortening of 2 cm. The second cycle involved lengthening and shortening by almost 4 cm; the line was close to its May 1997 length during 4-18 January. Long occupations of the stations at Hermitage and Tar River were made while running a base station at Harris. Hermitage showed continued movement NNE at ~0.3 cm/week. Since 6 March 1997 the Tar River station had moved 5 cm NNE. No clear trends were found in the data for LEESNET (Old Towne, Waterworks, St Georges Hill, and Lees Yard).

26 December 1997 deposits. Inspection of the 26 December deposits were reported in the MVO Special Scientific Report 6. The 26 December dome collapse severely damaged the settlements of Trials and Fairfield (~2 km SW of the summit). In Trials, most buildings had collapsed roofs or fire damage, but remained standing. In Fairfield, some houses had collapsed roofs due to heavy ashfall, but there was little fire damage, indicating that the pyroclastic surge probably did not reach this area. The villages of St. Patrick's and Morris' were almost completely destroyed with only a few foundations remaining.

The 26 December deposits were of three main types: debris avalanche, pyroclastic flow, and ash cloud including co-ignimbrite ash, and blast deposits. There was also considerable erosion of some surfaces, particularly due to the surge. The relationships between these deposits, emplaced in ~15 minutes, were not simple, but it appeared that a sector collapse occurred first.

The edifice that supported the dome complex was fractured, weak, and hydrothermally altered in places. The sector collapse involved slippage of material from around Galway's Soufriere and part of Galway's Wall, and incorporated both new talus and dome rock. This triggered a comparatively large dome collapse with associated pyroclastic flows and ash-cloud surges, and culminated in an energetic lateral blast.

Survey of the 26 December deposits in the upper reaches of the White River Valley on 4 and 17 January revealed a total volume of 46 x 10⁶ m³. Included in this total is the surge component which covered an area of 9.1 km² and consisted of an estimated 1.8-3.2 x 10⁶ m³. The DRE equivalent volume was 44.5 x 10⁶ m³.

Two scars were formed during the Boxing Day collapse. Scar volumes were estimated using data generated from cross-sections, assuming relatively simple geometries. Material lost during the collapse of 26 December comprised 20 x 10⁶ m³ of hydrothermally altered Galway's Soufriere rock, 5 x 10⁶ m³ of Galways wall material, 26 x 10⁶ m³ of the November lava dome, and 26 x 10⁶ m³ of dome talus. Thus, the estimated scar volumes total about 77 x 10⁶ m³. The DRE equivalent volume for collapsed material was 64 x 10⁶ m³. This suggests that about 20 x 10⁶ m³of material came to rest in the sea, a volume consistent with the size of the tsunami that was generated as a result of the collapse.

Thus, the conservative volume of the 26 December collapse deposits (a DRE of 44 x 10⁶ m³ of dome material) is 4-5x larger than previous events. The largest single prior event, on 21 September 1997 (BGVN 22:10), contained of 9 x 10⁶ m³ of material. In an earlier overview Young and others (1997) summarized the extrusive history from 1 November 1995 through early 1997; they provided an annotated plot of volume versus time. Because they show both total extruded volume and dome volume their plot clearly illustrates the pattern of ongoing extrusion and the effect of dome collapses.

Reference. Young, S., Sparks, S., Robertson, R., Lynch, L., and Aspinal, W., 1997, Eruption of Soufriere Hills volcano in Montserrat continues: Eos, Transactions, American Geophysical Union, v. 78, no. 38 (23 September 1997), p. 401.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, P. O. Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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02/1998 (BGVN 23:02) Dome growth continues; discussion of the 26 December dome collapse

The following summarizes a scientific report of the Montserrat Volcano Observatory (MVO) for 18 January-1 February, a time period when seismic and volcanic activity were low but dome growth continued. In addition, this report condenses MVO's Special Report 6 on the 26 December 1997 dome collapse, perhaps the most intense outburst yet recorded during the current crisis.

Visual observations. Few views of the dome complex were obtained due to poor visibility until the end of January, when observers saw active growth in the crater left by the 26 December 1997 dome collapse in the volcano's SW sector (BGVN 22:12). Also reported were occasional rockfalls, ash venting, steaming, and a dilute steam-and-ash plume that drifted WNW. Ash venting and rockfall activity became slightly more vigorous at the end of January, when a shift in prevailing winds sent light ashfall to the N part of the island.

Seismicity. Rockfall signals dominated seismicity; most coincided with a seismic-amplitude cycle with a periodicity of ~12 hours. This regular, slight increase in seismicity despite any major events has continued since the 26 December collapse and has been interpreted to indicate cyclical degassing as the dome grew.

Ground deformation. Displacement vectors for the interval April/May 1997 to January 1998 for sites around the volcano (table 25) revealed that areas NE, E, and SE of the volcano had been significantly displaced. The sector between Whites, Hermitage, and Roches Yard had moved ~6 cm NNE. Similar measurements at Long Ground, Tar River, and Perches suggested that these sites were displaced as a homogenous unit with little deformation. The Hermitage site showed considerably more movement than the others. Because of its proximity to the dome, it may have been more strongly influenced by local pressure or loading effects. Distant sites on the volcano's W and N flanks (Dagenham, Old Towne and Windy Hill) showed less displacement.

Table 25. Displacement vectors during April 1997-January 1998 for sites around Soufriere Hills. The site at Harris is the baseline. The Tar River vector reflects readings beginning in March 1997; the Roches Yard vector, beginning in October 1996. Courtesy of MVO.

    Site           Vector (mm displacement
                  @degrees from grid north)

    Whites               25 @ 353
    Long Ground          66 @ 033
    Hermitage           100 @ 026
    Tar River            57 @ 030
    Perches              59 @ 049
    Roches Yard          66 @ 342
    Windy Hill           15 @ 283
    Dagenham             16 @ 077
    Old Towne (M27)      19 @ 084

New GPS sites were established on the summit of Gages Mountain and in the N part of the island at Drummond's and Blakes. A triple-prism EDM reflector was installed on the remnant of Peak B, a piece of the crater wall between Tuitt's and Mosquito Ghauts. The reflector was installed less than 100 m from the dome's N limit and, along with the new GPS sites, will monitor the N flanks.

Environmental monitoring. Results from diffusion tubes revealed slightly elevated SO₂ levels (11.5 ppb) at St. George's Hill. On 24 January new tubes were placed at various sites on the W side of island. Geochemical sampling showed that all samples had <0.01 mg of cristobalite present. Dust Trak monitoring to measure airborne particulates at four fixed sites showed raised average levels (0.05-0.1 mg/m³) at the CPS site (~7 km NNW of the volcano), presumably due to human activity in this area.

Report on the 26 December dome collapse. The collapse occurred early on 26 December 1997 after the very rapid dome growth that followed the explosive phase of September-22 October 1997 (BGVN 22:09-22:11). Dome growth within the explosion crater and large lobes extruding N and S formed a large dome over the Galway's Wall attaining a summit elevation of 1,020 m (figure 38), the greatest dome height since the eruption began. Seismic activity was generally low but a hybrid swarm beginning at 1430 on 24 December merged to continuous tremor a few hours before the collapse.

The slope failure and dome collapse occurred at about 0300 and lasted ~15 minutes. Seismic evidence provided information on the duration of the event and the timing of specific phenomena, but reconstruction of the event has been done chiefly by evaluating deposits, changes in dome and flank morphology, and changes due to material transportation processes.

The event included a debris avalanche from the Galway's Wall and Galway's Soufriere areas and the consequent collapse of a destabilized portion of the lava dome (figures 38 and 39). The debris avalanche moved down the SW flank following the White River, leaving deposits through much of the valley; these deposits were later blanketed by pyroclastic-flow deposits. A portion of the material may have reached the ocean, generating a small tsunami (BGVN 22:12). The dome collapse produced pyroclastic flows and ash-cloud surges within the White River valley; a considerable volume of this material may have also reached the sea.

Very intense pyroclastic surges occurred during the collapse, causing widespread devastation in the area S of Gingoes Ghaut. Some surges were associated with the main flows, but others may have been caused by explosions in the collapsing dome. A convective ash cloud generated by the pyroclastic flows and surges rose ~14.3 km and deposited fine ash over SW Montserrat.

Deposits. Five main depositional units from the 26 December event were identified (figure 40): debris-avalanche deposits, block-and-ash flow deposits, pyroclastic-surge deposits, co- ignimbrite fallout, and a possible blast deposit.

A ~500 m wide, 25-70 m thick debris-avalanche deposit covered the central delta and lower reaches of the White River valley. The hummocky, orange-brown debris was poorly sorted, coarse, and blocky with an irregular bulbous ~25 m-high front. The deposit resulted from a slope failure of hydrothermally altered rocks in the Galway's Soufriere area, the lower outward flank of the Galway's Wall, and the overlying apron of fresh dome talus. Much of the material had a smoothed, heavily scoured upper surface with discontinuous remnants of pre- existing hydrothermally altered stratigraphy preserved within the deposit.

Block-and-ash deposits left by pyroclastic flows were similar to previous dome collapse flows at Soufriere Hills. They comprised dense to slightly vesicular (friable-textured) blocks in a poorly sorted, ash-rich matrix with little internal organization. The pyroclastic flows were largely confined to the White River valley, although some material spilled out at the river bend (~1.7 km from the coast) and traveled towards Morris'. The flows produced erosion features over the area between the White River valley and Morris' village. The block-and- ash deposits ponded behind and on top of the debris-avalanche deposits, filling the remainder of the White River valley to a maximum depth of 50-70 m. Block-and-ash deposits on the river delta were relatively thin (50-70 cm), broad, and flat-lying. They were poorly sorted with blocks reaching a maximum size of about 1 m (blocks >0.1 m formed ~10% of the surface).

Surge deposits associated with the collapse covered 9.1 km² around the volcano's S flanks. Quite variable, some deposits differed markedly from previous surge deposits associated with pyroclastic-flow emplacement at Soufriere Hills. Conventional ash-cloud-surge deposits were found E of the White River valley on the delta and in the Trials area. These deposits were composed of a fine grained, ash-rich, and sandy layer (6-10 cm thick) with an underlying thin (0.5-2 cm) fines-depleted coarse sand layer. The surge deposits between the White River valley and German's Ghaut varied but the dominant facies was a 15-40 cm-thick, coarse sand/gravel fines- depleted unit. In some areas this deposit was overlain by a second fine-grained surge deposit. The coarse surge deposits largely comprised sub-angular dense dome rock and crystals with little pumiceous or friable component.

Small secondary pyroclastic-flow deposits with abundant charcoal occurred in the deep ghauts that drain the area covered by the surge deposits. One of these flows drained towards the E side of Soufriere Hills down Dry Ghaut. The thin, highly mobile flow was confined to the bottom of the ghaut (average width of 2-4 m) and extended to within 300 m of the sea. The deposit was poorly sorted and 50-70 cm thick, consisting predominantly of fine ash-rich sand.

A possible blast deposit was found on the volcano's SW flank between Gingoes Ghaut and the White River. The deposit comprised angular to sub-angular lithic clasts scattered on the surface, some up to 70 cm in diameter. The surface of the deposit was very subtly corrugated in the flow direction, suggesting a highly energetic emplacement mechanism.. This deposit was distinctly different from thinly spread 'normal' facies block- and-ash flows as it was locally only one clast thick and was completely fines depleted. Dense, fresh, angular dome rock made up most of the deposit, with small amounts of altered dome rock and sub-rounded, semi-vesicular, steely blue-gray dome rock. There was a marked lack of impact craters, bread crust-textured clast, or any ballistic blocks.

Co-ignimbrite ash covered most of the SW part of Montserrat and draped all the 26 December deposits, although heavy rains in early January altered the deposit. Near the coast in the Trials area the co- ignimbrite ash fell as accretionary lapilli, caused by incorporation of steam generated by hot material entering the ocean. The accretionary lapilli were up to 8 mm in diameter and formed a layer up to 4 cm thick. The fine-grained, crystal- rich ash was typical of ash generated from pyroclastic flows sourced from dome collapse. The co-ignimbrite ash plume reached an altitude of ~14 km and light ash fall was reported from Guadeloupe (60 km SSW), as well as St. Vincent and Bequia (both ~400 km SSW).

Temperatures determined from the various deposits several days after the eruption had values up to 293°C (table 26). The debris-avalanche deposit was mainly emplaced cold, although parts of the Galway's Soufriere and dome talus debris would have been warm at the time of incorporation into the avalanche.

Table 26. Temperature measurements for deposits from the 26 December collapse. 'PF' refers to pyroclastic flow; 'DAD', to the debris-avalanche deposit. Courtesy of MVO.

    Deposit       Location       Measurement    Days     Temp
    type                         depth (cm)     after   (deg C)
                                                event

    Secondary PF  Dry Ghaut          20           4        48
    Secondary PF  Dry Ghaut          25           4       138
    Secondary PF  Dry Ghaut          35           4       122
    Surge         White River delta  30           9       155
    Surge         White River delta  60           9       216
    Surge         White River delta  30           9       228
    Surge         White River delta  30           9        83
    Surge         White River delta  50           9        93
    Fumarole      White River delta  30           9        68
    Surge/PF      over DAD           20          13       157
    Surge/PF      over DAD           25          13       103
    Surge/PF      over DAD           60          13       293

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, P. O. Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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03/1998 (BGVN 23:03) Heavy ashfalls and rapid dome growth in February

This report condenses Scientific Reports of the Montserrat Volcano Observatory (MVO) covering February. During 1-14 February, seismic activity increased, heavy ashfalls reached the N part of the island, and dome growth continued. Activity during 15-28 February was dominated by rapid dome growth and elevated seismicity.

Visual observations. Low clouds during the first two weeks of February often hampered dome observations. However, on 6 February observers on a police boat reported continued growth in the 26 December collapse scar above the White River. By 10 February the growing dome almost completely filled the 26 December scar, approaching the volume prior to the collapse. In addition, two spines were observed on the dome's S side, and the talus slope below the growth area had grown considerably. Steam-and-ash venting continued and was vigorous during periods of elevated seismicity and rockfall.

Rockfalls and small pyroclastic flows occurred mainly on the Galways side of the dome, but a few small rockfalls were observed in the upper part of Tuitt's Ghaut. Fresh pyroclastic-flow deposits in the upper part of the White River were probably emplaced during the elevated activity of 5-6 February.

On 15 February several rockfalls and small pyroclastic flows traveled down the White River valley. Visibility was poor until 25 February when vigorous ash venting, rockfalls in the White River valley, and several stubby spines atop the dome were observed.

Seismicity. Earthquake activity during 1-14 February mainly consisted of rockfalls and hybrid earthquakes with some tremor. Most swarm events, including 21 locatable volcano-tectonic earthquakes, were concentrated below the dome complex's N sector and had shallow focal depths (2-4 km below the summit). During 15-28 February fewer rockfalls but comparatively more earthquakes and seismic swarms (table 27) occurred than in preceding weeks. The swarms were not followed by surface activity.

Table 27. Number of hybrid, long-period (LP), and volcano-tectonic (VT) events detected during earthquake swarms at Soufriere Hills during February 1998. Courtesy of MVO.

    Date    Start time  Duration(hours)  Hybrid  LP  VT

    10 Feb    1154           2.40          21     3  12
    11 Feb    1402           2.93          15     3  13
    11 Feb    2319           0.40           1     -   7
    17 Feb    0452           2.42          10     0   4
    21 Feb    1853           6.48          31     3   8
    23 Feb    0823           3.90          11     5   9
    23 Feb    1350           1.78          14     1   1
    24 Feb    2138           1.87          13     2   1
    25 Feb    1059           2.95          17     3   0
    26 Feb    0536           5.36          82     2  33
    27 Feb    1312          13.12          24     0   0
    28 Feb    1033          10.33          28     0   1
    28 Feb    1457          14.57          48     0   4

At the beginning of February, seismicity displayed a cyclic pattern with peak amplitudes occurring every 6-8 hours; by 14 February, the cycle had lengthened to 8-12 hours. By 22 February, the cycle was ~14 hours long. Peak amplitudes increased during 1-14 February; these peaks generally coincided with elevated rockfall activity. Towards the end of February, the peaks were dominated by hybrid earthquakes and tremor.

Ground deformation. Two GPS occupations of LEESNET (includes sites at Old Towne, Waterworks, St. Georges Hill, and Lees Yard) were made during 1-14 February. No movement within this network was detected. Meanwhile, GPS surveys at Harris, Hermitage, Lees Yard, Perches, St. Georges Hill, Old Towne, Blakes, and Lookout Yard North confirmed that the Hermitage and Perches sites continued to move NNE. Sites on the volcano's N and NW flanks remained relatively stable.

Electronic tiltmeters were installed at Hermitage and on Gages Mountain to provide data on deformation of the volcano's NE flank. The EDM reflector on the N crater wall (Peak B) was shot from Windy Hill during 15-28 February. During 25 January-late February a 5-cm shortening occurred on this line. Lines between the Lees Yard reflector and sites at MVO south and the Waterworks Estate did not show any movement.

Volume measurements. A 10 February theodolite survey of the dome from Garibaldi Hill and the Delta petrol station revealed that the dome's highest point was 970 m. On 27 February, theodolite measurements from Garibaldi Hill and the old observatory in Old Towne showed that the highest point on the dome had reached 997 m. More theodolite measurements on 1 March from South Soufriere Hills and Perches Mountain gave a height of 1011 m, revealing 14 m of vertical growth in only 2 days.

Environmental monitoring. Sulfur dioxide diffusion tube measurements during 1-14 February showed raised (10-12 ppb) SO₂ levels in Plymouth and at St. Georges Hill and low (0-0.6 ppb) levels at Weekes, MVO south, and Lawyers. During 15-28 February SO₂ levels at Plymouth, MVO south, and Lawyers were higher than earlier in the month, but levels at St. Georges Hill were reduced by half. The site in Plymouth showed very high values (30.2 ppb) because it was surrounded by ~30-cm-thick tephra deposits and redeposited debris from nearby pyroclastic-flow deposits.

The mass of fine ash deposited in N Montserrat during several 28 January-7 February ashfalls was calculated using an array of ash collection trays. The mass totaled more than 1 kg/m²; most of this ash was produced during episodes of ash venting and rockfall activity. At most locations the ash collected during 3-5 February accounted for more than 50% of the local monthly ash accumulation.

Dust Trak monitoring at four fixed sites to measure airborne particles revealed elevated values (0.05-0.38 mg/m³) during ashfalls on 4-5 February. Levels were even higher (0.11-0.43 mg/m³) on 7 February due to resuspension of the ash. Sites in the S part of the island showed higher concentrations than in the N. During 15-28 February, no major ash fall occurred and levels were low (<0.05 mg/m³) at all sites; however, a diffuse volcanic plume was occasionally blown N, causing light ash fall and hazy conditions.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, P. O. Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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04/1998 (BGVN 23:04) Low seismic and volcanic activity during March-early April

The following report condenses scientific reports of the Montserrat Volcano Observatory (MVO) for 1 March-12 April 1998. In February there was rapid dome growth and elevated seismicity (BGVN 23:03). However, during 1-15 March volcanic and seismic activity declined. Although spines grew in the summit area early in the month, there was no sign of dome growth after 9 March. During 15 March-12 April activity remained low.

Visual observations. The dome's summit area on 1 March was blocky with a small number of stumpy spines. The summit's highest point was measured at 1,011 m. On 8 March two prominent spines were seen; one broad-based spine's peak was 1,027 m in elevation when measured the next day. No dome or spine growth was observed during 9 March-12 April. During 15-28 March a slight degradation of the upper flanks was the only change in the dome.

Low-level rockfall activity occurred during early March-12 April. During March new rockfall chutes developed on the dome's SW and E sides above the Tar River valley; some large rockfalls reached the base of the talus slope on both sides of the dome. During 29 March-12 April very small rockfalls occurred down the upper flanks over Gages wall and on the upper flanks of the new dome in the SW sector.

During 15-28 March the disintegration of a steep, rocky buttress in the dome complex above the Tar River valley produced several small pyroclastic flows. A field team at Windy Hill observed the largest of these flows traveling down a narrow ravine and reaching the Tar River Estate house. The team also observed small rockfall deposits below the fumarolic area on the dome's E flanks. During 29 March-12 April continued degradation of the buttress sent rockfalls down gullies between the E face and the N flanks or down an incised central chute on the E flanks.

During 1-15 March fumarolic activity was mainly confined to a V-shaped cleft in the pre-explosion dome complex on the dome's E side. No fumarolic activity or ash venting was reported during 15-28 March. During 29 March-12 April moderate fumarolic activity was concentrated within an incised central chute on the E flanks and around a trench between the 26 December collapse scar and fresh dome material on the dome's SW sector.

Pyroclastic-flow deposits in the upper part of Tuitt's Ghaut seen on 28 February were studied in March; the deposits were composed of older material originating from the base and sides of the 22 October 1997 dome (BGVN 22:11). The remnants of this dome formed a large dark mass on the N side of the dome complex. Temperature measurements of pyroclastic-flow deposits produced in a 21 September 1997 dome collapse (BGVN 22:10) were made on 15 March. The maximum temperature found was 590°C at a depth of 1.5 m. During 29 March-12 April, a field team again visited these deposits. Using a thermocouple probe, they measured a maximum temperature of 357°C at a depth of 2 m.

Seismicity. During 1 March-12 April seismicity was low, with small numbers of earthquakes, no swarms, and low rockfall activity. Epicenter locations for all events were in the dome area. During 29 March-12 April, activity consisted principally of volcano-tectonic earthquakes that occurred at irregular intervals. The cycle of seismic amplitudes observed in February (BGVN 23:03) ceased during early March, when the period lengthened and the amplitude decayed so that discrete peaks were not apparent.

During 29 March-12 April, upgrades were completed on the broadband seismic network. Two new stations were installed at South Soufriere Hills and at Mongo Hill, providing increased azimuthal coverage. Both stations use single vertical component seismometers with corner frequencies of 1 Hz. The network now consists of seven stations.

Ground deformation. During 1-15 March GPS occupations at Whites, Long Ground, Windy Hill, Perches, Old Towne, Lees Yard, and Blakes revealed continued NE movement of the Hermitage site and slow movement of the Whites, Long Ground, and Perches sites. The line from the EDM reflector on the remains of the N crater wall (Peak B) to Windy Hill continued to shorten; shortening of 8 cm has occurred on this line since 25 January. Measurements suggested that the shortening rate may have slowed slightly.

During 15-28 March GPS occupations of Blakes, Drummonds, Old Towne, and Dagenham showed that the sites were stable with respect to Harris. Data from the station at Hermitage showed that the site was still moving NE.

During 29 March-12 April GPS occupations at Whites, Gages, Old Towne, Dagenham, Blakes, and Drummonds indicated >3 cm of WNW movement of Gages Mountain's summit had occurred since January. This radial movement away from the dome was almost identical to the movement direction of the pole to the tilt plane on the Gages Mountain summit tiltmeter. The station at Hermitage showed continued NE movement at the highest rate since September 1997. Since March 1997, this site has moved 15 cm.

Volume measurements. A kinematic dome survey consisting of photos and laser range finding binocular measurements was carried out on 10 March. Heights correlated well with previous theodolite measurements. The volume of the dome complex on 10 March was 113 x 10⁶ m³. This figure included 29 x 10⁶ m³ for the talus slope and 84 x 10⁶ m³ for the dome. The volume of the dome just before the 26 December 1997 events was estimated at 115 x 10⁶ m³.

On 30 March, a survey of the dome talus and of deposits in the top of the White River Valley was undertaken. This returned a talus volume of 36 x 10⁶ m³, thus increasing the total dome volume to 120 x 10⁶ m³. The deposits had accrued 8.99 x 10⁶ m³ since a previous survey on 17 January 1997. The volume of erupted material since November 1995, including the dome and deposits, totaled 300 x 10⁶ m³.

Theodolite measurements on 5 April revealed that the dome's highest point was the top of a large, 50-m-tall spine perched near the top of fresh material in the SW sector. The elevation of the spine's top was 1,031 m.

Environmental monitoring. No ashfall was reported on the inhabited sections of the island during 1-15 March. During 15 March-12 April, aerosol levels were low due to low volcanic activity and occasional rains. Comparatively higher aerosol concentrations on 24 March coincided with a small increase in volcanic seismicity during 2200 on 23 March to 0400 on 24 March. Slightly higher aerosol levels recorded on 7 and 8 April and may have been due to Saharan dust in the atmosphere. The Davy Hill area, affected by traffic jams at the time, showed the highest levels.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, P. O. Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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05/1998 (BGVN 23:05) Low activity; deformation and volume measurements

The following condenses scientific reports from Montserrat Volcano Observatory (MVO) for 12 April-10 May 1998.

Summary. Activity during the reporting period continued at low levels: there were no changes in dome morphology and only a few pyroclastic flows occurred. Seismicity was generally low, with occasional volcano-tectonic (VT) earthquakes being the predominant signals recorded by the seismic network. Rockfall activity was particularly low but showed an increase after heavy rains. Several mudflows were generated during the reporting period, most of them moving down Dyers River into the Belham River valley.

Visual observations. Fresh pyroclastic-flow deposits were seen along the N side of the Tar River Valley on 14 April. A small pyroclastic flow occurred during the morning of 19 April and was seen by the remote video camera at MVO as it traveled down the Tar River as far as the site of the Tar River Estate House. Another small pyroclastic flow coursed down the Tar River on the morning of 25 April, but could not be seen from MVO due to low clouds. Three more small pyroclastic flows traveled halfway down the Tar River Valley during 26 April and the morning of 27 April. All of these flows were believed to have originated on the steep upper flanks to the E of the old dome. Each event lasted 2 minutes and generated small ash clouds.

Rockfall activity was minor with small rockfalls occurring down the E and SW flanks. Some of these events are gradually carving deeper chutes on the Tar River side and S flank of the new Galway's dome. Minor rockfall activity also began near the top of the dome over Galway's wall and traveled down a chute on the S flank of Chances Peak.

During a brief clearing in the weather around the dome early on 6 May observers on a reconnaissance flight saw no evidence of new growth, suggesting a lack of significant extrusion since the growth of the summit spine around 10 March (BGVN 23:04). They did see moderate fumarolic activity coming from a point in the main chute on the upper E flank, and in several areas within the trench located between the scar of 26 December (BGVN 22:12) and the fresh growth within it. They also noted that the summit area appeared blanketed by over 5 m of tephra including both fine ash and blocks of glassy dome rock (up to 1 m diameter).

The temperatures of the pyroclastic flows deposited at Trant's during the 21 September collapse (BGVN 22:10) were measured on 28 April. A maximum temperature of 348°C was obtained at a depth of 2 m. They showed only very minor changes since they were last measured 2 weeks ago.

Seismicity. Over the reporting period, seismicity remained low. Volcano-tectonic (VT) earthquake activity continued to be dominate (table 28). VT earthquakes mainly occurred in groups too few in number to constitute swarms, but exceptions to this are shown in table 29, including a swarm of hybrids on 6 May. These were the first hybrids of high amplitude seen for many weeks, but were not followed by others of similar type.

Table 28. Earthquake counts at Soufriere Hills listed by type (based on signal character), 12 April-9 May 1998. These counts were of events that triggered the broadband network's event-recording system between 0000 and 0000 each day. The type Dome RF denotes a dome rockfall. The type LPRF signifies a Long-period (LP) earthquake followed by rockfall signal; and HYRF, a hybrid earthquake followed by rockfall signal. Courtesy of MVO.

    Date        VT   Hybrid   LP   Dome RF   LPRF   HYRF

    12 Apr 98   15      1     --     --       10     --
    13 Apr 98   15     --     --     --        6     --
    14 Apr 98    1     --     --     --       --     --
    15 Apr 98    2      1     --     --       --     --
    16 Apr 98    4     --     --     --       --     --
    17 Apr 98    4     --     --     --       --     --
    18 Apr 98    1     --     --     --       --     --
    19 Apr 98    5     --     --      1        4      1
    20 Apr 98   14     --     --      2        5     --
    21 Apr 98    8      1     --     --        1     --
    22 Apr 98   --     --     --     --        1     --
    23 Apr 98   --     --     --     --        4     --
    24 Apr 98    1     --     --      1        4     --
    25 Apr 98    3      1     --     --        2     --
    26 Apr 98   17     --     --      1        2     --
    27 Apr 98    7     --     --     --        2     --
    28 Apr 98    8     --     --     --        6     --
    29 Apr 98    2     --     --     --       --     --
    30 Apr 98    3     --     --     --       --     --
    01 May 98    7     --     --     --       --     --
    02 May 98   10     --     --     --        3     --
    03 May 98    2      2     --     --        7     --
    04 May 98    4     --     --     --        4     --
    05 May 98    6      2     --     --        3     --
    06 May 98    5     12     --     --       --     --
    07 May 98    6     --     --     --       --     --
    08 May 98   25     --     --     --        2     --

Table 29. Swarms registered at Soufriere Hills during 12 April - 10 May 1998. Courtesy of MVO.

    Date      Local   Duration   Hybrid    Long-    Volcano-
              Time    (minutes)            period   tectonic

    26 April  2247     1.45         0        0         9
    06 May    0445     1.40        11        0        --
    27 May    2257     5.75         0        0         1
    18 May    0841     8.32         0        0        15

Epicenters were located on the E of the volcano at focal depths tightly clustered from 2.5 to 3.5 km below the summit. Fault-plane solutions were calculated using P-wave first-motions detected by the 7 broadband stations along with first motions from the Lee's Yard and Jack Boy Hill stations of the short-period network. The calculated fault-plane solutions are consistent with a strike-slip fault mechanism. The number of recorded rockfall signals was very low. However, in many cases there was a correlation between occurrence of the rockfalls and periods of heavy rainfall.

Ground deformation. With respect to the Harris GPS measuring station, the stations at Dagenham, Old Towne, Lookout Yard, and Windy Hill showed height increases of 5, 5.5, 6, and 4 cm respectively since December 1996. These values are preliminary, as the height component is the least well constrained by GPS. It was judged more likely that the reference at Harris was actually sinking. Height differences between Harris and sites on the E (Long Ground, Tar River and Perches) all showed continued slow movement to the NE of around 7 cm in the last year; Whites and Roches have moved slightly less and in different directions.

A survey from Windy Hill measured the distance to the N crater wall reflector and found it had shortened by only 1 cm since the middle of March. The line to Windy Hill from Harris is stable, as confirmed by repeated measurements since December 1997 that gave site positions lying within a box 3 mm by 7 mm. In contrast, the survey point at Brodericks had shown accelerated movement: 3 cm to the N between November 1997 and January 1998. This coincided with the period of rapid extrusion in the S area of the dome during December, 1997. Subsequently Brodericks appeared to stabilize in its new position.

A new permanent GPS site was installed in the South Soufriere Hills. Telemetry equipment used by the station was installed by the University of Puerto Rico on Antigua and in the Centre Hills.

Volume measurements. A new theodolite site known as Fergus Ridge was set up on the high ridge of the W flank of South Soufriere Hills, to the N of Fergus Mountain, overlooking the White River Valley. Measurements from this site triangulated with measurements from Perches Mountain were obtained on 16 April. In conjunction with the combined photo and GPS data collected on 6 April, a revised total dome volume was calculated to be 113 x 10⁶ m³. This figure differed from the initial estimate of 120 x 10⁶ m³; however, the revised figure incorporated a greater number of theodolite, photo, and GPS points that improved constraints on both the summit area and the new dome on the SW sector of the complex.

Environmental monitoring. Generally, low volcanic activity and the number of rain showers kept aerosol levels low through the reporting period. Extremely wet weather, 14-15 April, produced the lowest aerosol levels since the heavy ash fall at the beginning of February. Rain also prevented the ash produced by the small pyroclastic flow of 19 April from being transported N by wind to any of the sites that were being monitored.

The volcano's small ash output left inhabited N island areas comparatively ash free. Each disturbance of ash by moving vehicles seemed to help the wind and rain remove more ash.

The three pyroclastic flows that occurred on 26-27 April had no effect on the measured levels of airborne ash and dust. On 1 May observers saw a very small venting of ash escape at the top of the Tar River Valley. Scientists working in the SW of the island over the next few days noticed a strong smell of rotten eggs (hydrogen sulfide). Following the hybrid swarm on 6 May dust levels remained low, but aerosol levels doubled. Heavy rain two days later once more reduced levels. Aerosol levels continued low later in this period despite drier weather, except in Salem, an area likely affected by ash blown W from the pyroclastic flows.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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06/1998 (BGVN 23:06) Very low levels of activity in late May

The following condenses the Monserrat Volcano Observatory's (MVO) Scientific Report for 24 May-1 June 1998. Volcanic activity remained at very low levels during the reporting period. No significant changes in dome morphology occurred and seismic activity was limited to small numbers of volcano-tectonic earthquakes.

Visual observations. There were no pyroclastic flows and only occasional small rockfalls down the upper flanks on the E side of the dome complex. Some small rockfalls originated from the summit area of Galway's dome; they traveled down the SW flanks of the complex.

The temperatures of pyroclastic flow deposits that formed during the 21 September collapse were re-measured on 31 May. At a 2-m depth, maximum temperatures reached 355°C, suggesting that the deposits had not cooled significantly since previously measured on 13 May.

Poor weather conditions hindered visual observations; however, no significant changes are believed to have occurred around the dome complex.

Seismicity. Seismicity was generally low. Intervals of scattered volcano-tectonic earthquakes alternated with intervals of almost complete quiet. One exception was a swarm on 25 May. This swarm consisted of many small signals, most of which did not trigger the networks. The signals were originally considered volcano-tectonic earthquakes because of their frequency content, but they were generally emergent and often had 2-3 velocity maxima. In this sense they differed from the simple rise and decay of most volcano-tectonic earthquakes.

The most striking feature of these signals was the difference in arrival times at different stations (often over 10 seconds). Also the order of arrival at different stations changed from event to event. These observations indicated that the signals were propagated as air-waves and that the source was different each time. Crude time-distance calculations, assuming a velocity of 330 m/s, showed that many of the sources were in the Farms and Upper Gages areas. Thus, it was later concluded that these signals were not volcano-tectonic but presumably caused by small phreatic explosions as water reacted with hot pyroclastic deposits. This conclusion was borne out by comparison with data from July 1995, a time when phreatic activity prevailed and the record showed many seismic events with similar emergent starts and large differences in inter-station arrival times.

Ground deformation. Some of the GPS equipment was on loan to the Seismic Research Unit (Trinidad) who conducted surveys on St. Vincent during this period; thus surveys at Montserrat were reduced. Available data indicated that the motion of the Hermitage site was clearly slowing.

Environmental monitoring. Low volcanic activity kept aerosol levels low, with heavy, typically early morning downpours maintaining some of the lowest airborne dust and ash levels over the last month. A sulfurous haze was visible from, and was occasionally smelled at, the MVO-south station. The haze descended to the W over Fort Ghaut, Gages fan, and Plymouth.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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07/1998 (BGVN 23:07) Relatively large pyroclastic flows on 3 July; ash venting

On 3 July, after several months of reduced activity, a 2.5-hour sequence of large pyroclastic flows took place at Soufriere Hills. Ash clouds reached heights of 9-12 km before drifting N and depositing fine ash over the inhabited part of the island. New deposits extended the Tar River delta significantly, adding ~0.25 km² to its area, but not extending it seaward. Winds carried the cloud N, disturbing previously unaffected houses in the Long Ground area. The new materials were typical block-and-ash deposits, associated with fine surge deposits and ash-cloud deposits.

The 3 July pyroclastic flows marked the first major event at Soufriere Hills since dome growth ended in mid-March (BGVN 23:04). The dome had reached a volume of 120 x 10⁶ m³ (the maximum observed throughout the eruption) and a summit height of 1,031 m. The minor pyroclastic flows of previous months (BGVN 23:04 and 23:05) were consistent with mechanical collapse of the very large and unstable dome and were therefore of no great significance. Given the duration of low levels of activity, it was conjectured that the current volcanic crisis might be waning, with important implications for the residents of the island and its administration. Because the 3 July pyroclastic flows apparently represented a re-escalation of activity, the Montserrat Volcano Observatory (MVO) has examined the events and deposits to investigate their significance.

Precursor events. There were no obvious precursors to the eruption. However, possibly related factors included a felt earthquake at 1704 on 25 June centered under Barbuda, and a series of small-to-moderate pyroclastic flows commencing at 1229 on 30 June and lasting 30-40 minutes. It may be significant that the events of 3 July coincided with the passage of a tropical wave, a period of bad weather and heavy rain.

Observations. Abrupt onset of activity was detected by the MVO seismic network at 0302 on 3 July, so opportunities for visual observations were limited. During the first half hour, there were reports from several localities in the W and N of the island of 'stones' and ash falling. At 0400 NOAA satellites detected a large ash column reaching altitudes of 9-12 km moving ENE. As a result of wind shear, the lower part of the plume was blown NW, which resulted in significant ashfall over the N of the island. Light ashfall was also recorded on Nevis and St. Kitts.

At 0422 an observer at Jack Boy Hill reported pyroclastic flows descending the Tar River valley, with abundant lightning and with an ash cloud drifting and rising over the center of the island. Reporters on the W of the island recorded heavy ashfall in the Belham Valley (figure 41), at Salem, and at Woodlands. Thunder and lightning were observed at the front of the ash plume as it headed W over the sea. At 0450 pyroclastic flows were observed at the Tar River delta; the deposits were steaming strongly at 0518. Winds from the S deposited 0.5-1 mm of ash over the entire N end of the island.

Although residents reported falling 'stones,' examination of the deposits revealed no pumice clasts. A maximum 2-mm-thick layer of fine ash was recorded in Salem with lithics and crystals up to 3 mm in size from MVO to the S. Most of the ash fell as accretionary lapilli all over the island, reaching diameters of up to 4 mm. These large lapilli may account for the reports of 'stones'. Heavy rain following the event precluded detailed studies of the deposits.

A helicopter flight at 0700 confirmed that pyroclastic flows had reached the Tar River delta and entered the sea. In the upper Tar River valley, flows were channeled down its S side; lower down they spread out over both sides of the valley. The N slopes of Roche's Mountain and Perche's Estate were eroded and there were indications that small amounts of material, including some ballistics, had overtopped the ridge causing impact scars on steep slopes and starting small fires. Surges associated with the passage of the pyroclastic flow left significant deposits of fine ash in the area of the Tar River estate house, and for the first time since the onset of the crisis the southern parts of Long Ground village were affected. Trees and shrubs were scorched, but no houses caught fire, nor was there any evidence of high velocities associated with the surges. Surge deposits extended to about 400 m N of the Tar River delta.

Small rockfall deposits were observed on the White River flanks of the dome, but there was no activity in Tuitt's, Mosquito, or Tyer's ghauts. Ash continued to drift W during the morning. A smaller event at about 1407 produced a dark cloud about 3 km high, depositing further ash on the Woodlands, Salem, Old Towne, and Olveston areas, but not farther N. This may have been a weak explosion or a small pyroclastic flow. A further 1 mm of fine ash accumulated in the Woodlands and Salem areas.

An observation flight at 1500 on 3 July revealed a large scar on the SE flanks of the dome, with chutes leading down against the N scarp of Perche's mountain and the S edge of the Tar River valley. A large volume had been lost from the dome (later estimated visually to be 15% of total dome volume). The prominent 50-m-high spine at Galway's dome was not visible due to steam and ash in the summit region, but craggy peaks were observed on the S and NE rims of the scar left by the collapse event. There was no evidence of changes on the other flanks of the dome. Strong fumarolic activity was observed along a clearly defined NE-trending, linear fracture 50-100 m in length within the new scar. Several distinct vents along this fracture gave off white steam; one appeared to be tinged with yellow elemental sulfur. To the W of this a dark mass was intermittently visible through the steam: its dark color may have resulted from steam condensation.

An observation flight on 10 July in clear conditions showed that the collapse scar had the shape of an extremely steep sided, long canyon extending deep back into the dome: it was not possible to determine its westernmost limit, but it must have cut through most of the dome. There was no evidence of renewed dome growth.

Associated seismicity. Beginning at 1229 on 30 June signals indicating a moderate-sized pyroclastic flow lasted ~40 minutes. After this event daily rockfall signals increased from an average of 2/day for June (including 11 on 30 June) continuing with 13 and 8 signal on 1 and 2 July.

On 3 July another, much larger pyroclastic-flow signal started at 0302 and lasted ~2.5 hours. The maximum amplitude of this signal was attained immediately after the onset and lasted for 30 seconds. It was greater than that for the flow on 30 June and several times greater than the last big flows down the Tar River valley in May 1997 (BGVN 22:05).

Spectral analysis of the high amplitude signal at the onset suggests that it was not generated by an explosive event similar to those seen during August and September 1997 (BGVN 22:08). It has been suggested, however, that a phreatic explosion may have been involved.

Deposits. New block-and-ash deposits from the 3 July event covered the entire Tar River delta, and new surge deposits extended over the N of the valley into Long Ground (figure 42). Comparisons between aerial photos of the deposit taken on 24 June, 3, 4, and 6 July show a significant increase in area on the N and S of the delta, but no significant increase in the E. The N side of the delta may have extended by up to 250 m; the S side, by only 30-50 m. The maximum width of the delta (along its base) is now ~1.9 km, tapering to 1 km seaward, extending 700 m off shore. This indicates a total area for the delta of 1 km², an increase of about 0.25 km².

An observation flight on 3 July showed intense white and brownish steaming on the N delta and much weaker white steaming on the S. A number of different lobes of various tones and textures were interpreted as successive pulses from different parts of the dome. Overall, the S area appeared light gray and the N appeared brownish.

Deposits along the beach lines of the delta were later visited. Dry material was extremely hot, steaming from small vents locally. The upper layers consisted of very fine grained, dusty surge deposits reaching up to ~0.5 m in thickness, lacking large clasts. This material was very mobile and small jets fountained when disturbed for sampling. Bubbling mud/ash vents up to 0.4 m in diameter were also distributed irregularly over the front of the delta. Underlying the fine surge deposits was a typical block-and-ash deposit of indeterminate thickness, with clasts typically of a few centimeters diameter, though some meter-sized boulders were visible on the surface of the deposit nearby.

Sampling carried out on the delta showed a prevalence of blocks from the dome, associated with a small proportion of pumices of varying density and vessiculation. A few blocks exhibiting bread-crust structure were found in the S delta. The differences in color seen from the air were conspicuous on the ground. The N part appears much finer than the S.

Hydrothermal alteration was observed in both areas. The top of the surge deposit showed some evidence of alteration, forming a slightly more resistant crust, perhaps as a result of steam from below. In the S this alteration crust is much harder. Part of this alteration is the result of a post-depositional process (as evidenced by yellow, white, and red staining on the surface); part may be due to pre-depositional processes, evidenced by individual discolored blocks of different size located on non-altered areas.

Exploratory sections made on 9 July through the delta deposit showed that the S area consists of a new block-and-ash flow deposit ~10 cm thick on top of deposits from an older block-and-ash flow. Close to the sea a significant layer of alteration (about 6 of 13 cm) is also evident in the deposit. The N of the delta shows a surge layer ~10 cm thick on top of the new block-and-ash flow deposit of up to 50 cm. Sections 1 and 2 (close to the original coast line) also hosted a layer of accretionary ash, probably due to secondary explosions when the flow reached the sea. This is consistent with the brownish steam seen to the N of the delta a few hours after the collapse. Section 2, the most complete section observed, shows the presence of a 15 cm thick layer of very fine ash at the base. The new deposits are probably thickest in the central part of the delta, but it was not possible to obtain thickness data there. Temperatures of the deposits were taken at several places (table 30).

Table 30. Temperature measurements in 3 July 1998 deposits at Soufriere Hills. Courtesy of MVO.

    Date,  Time,                Depth    Temperature
    and Location              (cm)        (°C)

    Northern area of delta
    6 July, 1445
    5 m from deposit edge        4        100
                                50        300
    6 m from edge, inside
      inside fluidized area     80        298
    8 m from deposit edge        4        100
                                50        255
    7 July, 1400         
    5 m from deposit edge        4         65
                                50        195
                               100        319
    20 m from deposit edge       4         76
                                50        193
                                70        238

    Southern area of delta
    6 July, 1445
    5 m from deposit edge        4        132
                                50        360
    10 m from edge               4        120
                                50        375
    7 July, 1400
    5 m from deposit edge        4        117
                                50        337
                                70        391
    10 m from deposit edge       4        115
                                50        360
                                70        238

Subsequent events. The pyroclastic flows of 3 July were followed by heightened rock-fall and volcano-tectonic earthquakes until 1407, when there was further high-amplitude seismic signal. This time the signal lasted only 10 minutes and the maximum amplitude was similar to that of the flow on 30 June. Analysis of the most recent event shows that the first 30 seconds were dominated by a 2.4-Hz harmonic signal. This, combined with observations of the color and ascent rate of the ash cloud, suggested an explosive component, which produced a low, dark ash cloud that drifted NW depositing ash on Salem and Woodlands.

On 5 July there were three episodes of ash venting, which produced weak ash plumes to about 3 km that drifted W over Plymouth. The first two occurred at about 0330 and 0500 lasting about 30 minutes each. The third event at 1030 was observed from Salem and lasted 1.75 hours with new, dense pulses of dark gray ash every 5 minutes.

The following week, there continued to be elevated numbers of rock-fall signals, volcano-tectonic earthquakes, and intervals of tremor associated with ash venting from the scar left by the large collapse.

COSPEC observations. COSPEC observations resumed on 5 July. Although the long interval since the last observations (BGVN 22:10) makes comparisons difficult, SO₂ emission rates were clearly elevated, measuring 1,500-3,000 metric tons/day between 5 and 11 July. Although the high flux after the event may have been due to the effects of scattering by fine dust and aerosols in the plume (increasing the effective optical-path length), fluxes one week after the collapse were still significantly higher than during comparable periods earlier in the eruption. This may indicate a change in the magmatic source of the gas, or a change in the degassing regime caused by the depressurizing of a large part of the dome and associated changes in the underlying hydrothermal system. Reports of strong H₂S odors from the volcano over previous months may also be related to a cooler, wetter hydrothermal system.

Interpretations and conclusions. Because the dome had stopped growing in mid-March, and in the absence of any clear seismic or other precursors, the 3 July event was initially interpreted as a large mechanical dome collapse- not triggered by fresh dome growth. Given the continued low level of activity, this may still be the correct interpretation. Seismic records suggested a sudden initial collapse followed by continued erosion of the scar. This inference is supported by the very long, deep collapse scar, which extends across much of the dome. Although there are no close parallels from Montserrat itself, it is possible that the high- amplitude seismic signal at the onset of the event was due to a phreatic explosion.

There is little evidence to indicate renewed dome growth. The high SO₂ fluxes are problematical in the absence of fluxes taken immediately prior to the collapse. There may have been a change in the hydrothermal system, which brought about the conditions leading to collapse.

The difference in temperature, texture, and color between the new deposits in N and S areas of the delta suggests that they have been affected by different processes: the N area was affected by block-and-ash flows and surges; the S area, only by block-and-ash flows. It is likely that the large area affected by surges on the N flanks of the valley, including parts of Long Ground village, was the result of S winds during the emplacement of the pyroclastic flows.

Acknowledgments. The following scientists contributed to these studies: Costanza Bonadonna and Rob Watts, Department of Geology, University of Bristol; Peter Francis, Department of Earth Sciences, Open University; Richard Luckett and Colin Walker, Montserrat Volcano Observatory; Gill Norton and K. Rowley, British Geological Survey; Richard Robertson, Seismic Research Unit, University of the West Indies.

Information Contact: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat, West Indies (Email: mvo@candw.ag; URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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09/1998 (BGVN 23:09) Continuing decrease in activity; hazards reassessed

The following summarizes the Montserrat Volcano Observatory's (MVO) scientific reports for July and August, except information concerning the 3 July pyroclastic flows, which was reported in BGVN 23:07.

Summary. In the weeks following the 3 July pyroclastic flows, no fresh magma reached the surface; however, vesicular ballistic blocks were recovered from craters on Perches Mountain suggesting that there may have been a small Vulcanian explosion. SO₂-flux levels declined steadily throughout July to an average of 1,000 metric tons/day (t/d). Vigorous steam-and-ash venting continued from the dome-collapse scar until the end of July. Activity in August was dominated by several small dome-collapse events and a period of enhanced steam-and-ash venting in the middle of the month. The dome-collapse events were caused by the gravitational collapse of weakened dome rock. The ash venting was intense one day but waned over following days to normal levels. MiniCOSPEC results showed a peak that coincided with the enhanced venting, but there was an overall decline from ~1,000 t/d at the beginning of the month to ~500 t/d at the end of the month.

Visual observations. Ash-and-steam venting immediately after the 3 July event was vigorous. Significant pulses of steam-and-ash continued for 2-3 weeks and fumarolic activity was evident on the S and N flanks of the dome.

A steep buttress overhanging the 3 July scar collapsed on 16 August generating pyroclastic flows that reached the Tar River delta. Large fragments of the buttress were left in the area of the scar's mouth. On 19 August fumarolic activity in the scar increased in intensity: fumaroles on the back wall and at the base of the scar discharged copious quantities of steam and ash in jets. The next day activity decreased in intensity and the fumaroles were generally issuing steam only. Some of the fumaroles were temporarily buried following a rockfall within the scar on 20 August. The fumarolic activity declined steadily, and by 22 August activity had declined to levels observed in the first week of August.

Mudflows continued to be a problem in July. Mudflow deposits built up beneath the Belham Bridge until there was a clearance of only about 30 cm.

Seismicity. After 5 July, seismicity returned to levels similar to the previous month, with the exception of a swarm of volcano-tectonic earthquakes on 25 July (figure 43). This swarm had no outward manifestation at the volcano and activity returned to low levels by the next day.

Seismicity during August was generally low. Activity was dominated by small volcano-tectonic earthquakes located ~3 km below the dome, with occasional rockfalls and pyroclastic-flow signals. On 13 August there were two episodes (at 0519 and 1455) of pyroclastic flow in the White River valley. These flows traveled 1.8 km from the dome and were caused by the collapse of weakened dome rock. Active fumaroles on the Galways side of the dome near Chances Peak undermined part of the dome. A scar immediately above the fumarolic area is believed to be the source of the pyroclastic flows. Each episode was followed by about an hour of continuous rockfall activity. On 19 August a rockfall signal was followed by tremor, which corresponded to vigorous ash venting. The signal lasted two days and varied in amplitude. At times of highest amplitude the tremor was nearly monochromatic at 4 Hz.

Ground deformation. Measurements from GPS survey sites on the flanks of the volcano and in the N of the island indicated widespread major reductions in movement during July. The Hermitage site indicated continued slow movement NE at rate of ~0.5 cm/month. The GPS site at Perches was destroyed in the 3 July event; ballistics were scattered over Perches Mountain and the GPS site was later found at the edge of a 3.4 m diameter impact crater. The rates of movement of sites in August were within the instrumental error. The GPS kit was used for one week by volcanologists from the University of Rhode Island who were conducting a bathymetric survey of the fans at the mouths of the Tar River and White Rivers valleys.

The EDM reflector on Peak B was measured from Windy Hill. The increase in distance of 5 cm during the period May-July may have been caused partially by release associated with the 3 July collapse. The line had shortened by 9 cm between 25 January and 13 May, but between May and August the distance lengthened by a total 8 cm (within 1 cm of its original length) possibly indicating a relaxation in the confining pressure.

Volume measurements. A kinematic GPS survey of the Tar River fan was completed in July. The total volume of the fan was estimated to be 22.1 x 10⁶ m³. A previous survey in August 1997 gave a volume of 15.7 x 10⁶ m³. Much of the increase resulted from the 3 July collapse, which extended the fan 350 m N, although a small part of the increase was due to the accumulation of pyroclastic-flow deposits during the September-October 1997 explosion sequence (BGVN 22:10 and 22:11). The E limit of the fan, defined by a steep shelf extending into the sea, was unchanged. A small deposit was left on the S side of the fan, although above the established shoreline there was only a thin layer of pyroclastic-flow deposits.

No volume measurements were made in August. Attempts to survey the 3 July collapse scar were foiled by deteriorating weather conditions and a lack of helicopter fuel.

Environmental monitoring. MiniCOSPEC observations recommenced on 5 July. In early July SO₂ flux was generally between 1,000 and 2,500 metric tons/day (t/d). On 13 July SO₂ flux measured 4,150 t/d, the highest ever recorded at Montserrat. Throughout the remainder of July there was a gradual decline in SO₂ flux to an average of 1,000 t/d at the end of the month. The cause of the relatively high gas flux in the apparent absence of magmatic activity was being investigated, but may relate to perturbations in the hydrothermal system caused by the dome collapse on 3 July 1998.

MiniCOSPEC measurements in early August showed a consistent SO₂ flux of ~500-1000 t/d. On 19 August levels rose to 1,400 t/d as a result of enhanced venting. Towards the end of the month poor weather limited the number of COSPEC measurements, but there appeared to be a slight decrease to an average of ~500 t/d. Throughout late August the wind direction was variable due to tropical storms in the area. On occasions when the wind blew to the N or NW a strong smell of sulfurous gases was detected in the inhabited area of Montserrat.

Sulfur dioxide diffusion tubes exposed between 29 June and 13 July clearly reflect the high emissions in early July (table 31). The Plymouth area in particular was subjected to very high concentrations of gas. In the second half of July SO₂ concentrations in Plymouth were reduced by half. Populated areas N of the Belham River valley were, as usual, only subjected to very low SO₂ levels in July. In August there was a general decline of SO₂ in the atmosphere. An additional monitoring site in the N of the island was installed to assess SO₂ during shifts in wind direction.

Table 31. Sulfur dioxide diffusion-tube results, 29 June-11 August 1998. Levels are in parts per billion (ppb). Courtesy of MVO.

    Station              29 June-    13 July-    27 July-
                          13 July     27 July     11 August

    Police HQ, Plymouth   207.9       116.5       131.5
    St. George's Hill      22.05        8.55        9.55
    Weekes                  5.75        4.1         2.85
    MVO south               4.3         3.85        --
    Lawyers                 2.2         0           3.8
    Vue Pointe Hotel        --          --          3.25

Hazard assessment. A meeting was held 14-16 July at McChesney's Estate to assess the current hazards and risks associated with Soufriere Hills Volcano. The meeting brought together many of the senior scientists who have worked at MVO during the three-year volcanic crisis. Those who took part were Richie Robertson, Lloyd Lynch and John Shepherd from the Seismic Research Unit in Trinidad; Simon Young, Sue Loughlin, Tony Reedman, and Gill Norton from the British Geological Survey; and many other senior scientists from around the world including Steve Sparks from Bristol University, Peter Baxter from Cambridge University, Barry Voight from Penn State University, Joe Devine from Brown University, Peter Francis from the Open University, Keith Rowley, and Willy Aspinall. Richard Luckett and Richard Herd from MVO provided up-to-date information about the current status of Soufriere Hills volcano.

Discussion was held on various aspects of the activity over the previous six months, including the event on 3 July. Related issues, including the safety of Bramble airport, were also addressed. An assessment of the level of risk associated with the volcano was undertaken. A report was presented to the government of Montserrat and the U.K. on 29 July after which the findings were made public.

According to the report, MVO judged it likely that the volcano has entered a period of repose, with the probability of no further magmatic eruptions in the next 6 months set at about 95%. MVO was confident that renewed magma ascent and escalation to dangerous levels of activity could be identified, although they cautioned that escalation might take place in a very short period of time (e.g. a matter of hours). Most of the island was perceived to be under reduced risk, but areas S of the Belham River Valley remain vulnerable to serious volcanic hazards including pyroclastic flows related to the collapse of the dome, mud flows, and exposure to fine ash. Further dome collapses were deemed likely and could affect all flanks of the volcano, especially the Tar River, Gages Valley, Plymouth area, Galways, and the NE slopes. There is potential for a variety of events to take place, including steam explosions, mud flows, and ash falls, for many years to come but the risks will decline with time. Health risk analysis showed that if magmatic activity does not resume, the potential for harmful exposure to ash will be limited and the risk of developing silicosis will be low in Zones 1 to 3. The same would apply to Population Zone 4 north of the Belham Valley after a clean-up operation has been safely completed. A public education program on the health risks of ash was recommended, including guidance on protection measures during the clean up. Certain groups could be at risk from much higher exposure (e.g. outdoor workers and asthma sufferers) and there may be unknown long-term health risks to young children.

The Volcanic Executive Group (VEG), chaired by Governor Tony Abbott, met to consider the Scientific Review. A statement from the Governor's Office following the meeting rescinded the recommendation that residents leave the Central Zone. Also, there was no longer any objection to commercial organizations operating within the Central Zone. The clean up of Friths, Salem, and Old Towne, which commenced some weeks ago, was intensified. The VEG sought advice on how to ensure that the Zone will be cleaned so that children and those with respiratory problems will not be affected on reoccupation.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat, West Indies (URL: http://www.geo.mtu.edu/ volcanoes/west.indies/soufriere/govt); Richard Aspin, Information & Education Unit, Emergency Dept., St Johns Village, Montserrat, Leeward Islands, West Indies (Email: monmedia@candw.ag).

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11/1998 (BGVN 23:11) Small dome collapses, pyroclastic flows, and ash venting

There was a slight increase in activity in October according to reports from the Montserrat Volcano Observatory (MVO). Five small collapse events occurred on the dome, each producing significant deposits of ash up to 3 km away. Pyroclastic flows occurred along most of the volcano's main drainage. Ash fell predominantly W and NW of the volcano, light ash fell in the N of the island. Dome collapses were commonly followed by periods of volcanic tremor and ash venting, and sometimes swarms of volcano-tectonic earthquakes occurred shortly after the collapse events. The dome gradually eroded, leaving some large fractures in the carapace that could lead to larger collapses in the future.

Visual observations. Intermittent small pyroclastic flows originated from all flanks of the dome. The first significant event, at 0801 on 13 October, produced pyroclastic flows in Tuitt's Ghaut and Tyer's Ghaut. Volcanic tremor after the collapse correlated with ash venting from high on the dome's N flank, the ash cloud rapidly reached 7,500 m. The cloud drifted NW, depositing ash on parts of the island.

At 0916 on 18 October, there was another collapse, the ash cloud rose to around 2,000 m and moved W, although the exact direction was uncertain because a low cloud hampered observation. Subsequent volcanic tremor lasted for several hours.

Another small dome collapse occurred at 2241 on 20 October. The ash cloud from this event rose to an estimated 2,500 m, drifting slowly to the W and NW. Observations the following morning revealed that the pyroclastic flows from this event had traveled towards Plymouth as far as Upper Parsons (2.5 km W of the summit). Fallout included some coarse lithic fragments 4 to 5 mm in diameter.

At 0051 on 26 October, a fourth small collapse occurred. The seismic signal lasted for about 12 minutes followed by an extended period of tremor. Reports were received of thunder from the resultant ash cloud, and there was subsequent wet ashfall as far as 7 km N. Information received from NOAA satellite images indicated that the ash cloud reached to between 6,000 and 7,500 m. Observations during the early hours of the morning suggested that there were two ash cloud lobes, one S of Belham Valley and one over the Salem-Old Towne area. The deepest measured ashfall was 25 mm; 4 mm or more fell in other areas. The ash was fine grained, with common accretionary lapilli. During an observation flight on the 27th, steaming could be seen at the edge of the delta, indicating that the pyroclastic flows had traveled into the sea. The flows also reached NE as the Tar River Estate House (3 km from the summit). On the SW side, down the White River, a thin deposit of ash from the pyroclastic flows could be seen as far as about 700 m from the old coastline at O'Garras; when these deposits were emplaced is unknown.

A fifth small dome collapse occurred at 0418 on 31 October; an ash plume first drifted W, and thenN and NE depositing some ash in occupied areas at the island's N end. An observation flight later that day revealed new deposits: a pyroclastic-flow deposit in the White River reaching Galways Soufriere, and another in the Gages valley that did not extend beyond the top of the Gages fan. The White River deposit had numerous large angular blocks resting on its surface.

A large fissure within the dome extended from its base, where it rests against Chances Peak, to its top in the Galways area (S). At the foot of this crack a triangular-shaped opening had developed and appeared to have been the source of the White River pyroclastic-flow.

Unusual wind directions during the latter part of October directed the plume to the N. As a result, residents in N Montserrat smelled strong sulfurous odors.

On 27 October, probing into the pyroclastic deposits in the area of the Farm River in Trant's yielded these depth-temperature relations: 1.0 m and 86°C; 1.4 m and 146°C; and 2.25 m and 239°C. Unusually clear conditions in the early evening of 27 October enabled observers in Old Towne and Salem to see three small glowing areas on the dome; these areas were thought to reveal the dome's incandescent interior exposed during the recent collapse events.

Seismicity, deformation, and environmental monitoring. Over the reporting period seismicity was generally low; however, small dome collapses triggered volcanic tremor and swarms of volcano-tectonic earthquakes. As in the previous month, tremor correlated with intensified ash-and-steam venting from the N flanks of the dome.

Five small collapses occurred between 13 and 31 October. These were marked by pyroclastic-flow signals that lasted several minutes. The collapse on the 13th was preceded by a swarm of small volcano-tectonic earthquakes. Several much larger volcano-tectonic earthquakes occurred during the collapse, the first approximately 30 seconds after the start of the collapse; hypocenters for these events were tightly clustered directly under the lava dome.

The collapse on the 18th was accompanied by a more intense swarm of earthquakes (table 32). The first earthquake occurred about 40 seconds after the beginning of the collapse and was one of the largest earthquakes recorded since the installation of the broadband network; it was felt in the Woodlands area. This earthquake was much richer in low frequencies than typical volcano-tectonic earthquakes on Montserrat, possibly suggesting a larger source dimension. Hypocenters for the largest earthquakes were located S of the volcano. At the start of the swarm, hypocenters were directly under Roaches Mountain; as the swarm progressed, hypocenters migrated to S of Chances Peak. Preliminary calculations showed that the largest events were consistent with oblique-normal faulting in a NE-SW direction.

Table 32. October 1998 earthquake swarms at Soufriere Hills. Courtesy of MVO.

    Date         Start   Duration (hours)   Hybrid   LP   VT

    13 Oct 98    0249         5.10             0      0   11
    18 Oct 98    0916         6.73             0      0   51
    25 Oct 98    0614        11.32             0      0   24

All GPS sites on the volcano and in the N of the island appear stable and there were no significant changes since last month. The EDM reflector on the northern flank was shot from Windy Hill. The line continues to shorten slowly. The site was later destroyed by a pyroclastic flow.

SO₂ flux, measured using the miniCOSPEC instrument, was (in metric tons/day) 1,300 on 9 October, 340 on 21 October, and 280 on 30 October. These results are similar to those measured in recent months, although an apparent decrease occurred late in the month. Sulfur dioxide was also measured at ground level using diffusion tubes around the island. SO₂ in Plymouth (at Police Headquarters) remained high; elsewhere the average levels were very low.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat, West Indies (URL: http://www.geo.mtu.edu/ volcanoes/west.indies/soufriere/govt).

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12/1998 (BGVN 23:12) Continuing dome collapses and ash deposition in November

Activity during November was dominated by small-volume pyroclastic flows down the Gages, White River, and Tar River valleys. The pyroclastic flows reached the sea and left a narrow, deep cleft in the dome. Ash was deposited over the whole island, but heavy rains cleared the dust from inhabited areas. Seismicity was dominated by rockfalls and volcano-tectonic earthquakes, the latter occasionally occurring in swarms. Some of the larger seismic events were felt throughout the island.

Visual observations. As in October (BGVN 23:10), volcanic activity during November was dominated by intermittent, small pyroclastic flows from all of the dome flanks. On 2 November several small rockfall events were recorded, some followed by low-amplitude tremor.

At 0821 on 3 November a larger dome collapse sent pyroclastic flows down the Tar River as far as the sea and down the White River valley as far as Galway's Soufriere. The ash cloud from this event reached >3,100 m and drifted W. Most of the ash fell S of the Belham valley.

A major dome collapse occurred at 2117 on 5 November. The pyroclastic flows from this collapse traveled down the White River valley to the sea, depositing two blocky lobes on the White River delta. The surge cloud climbed halfway up the N slope of Fergus Mountain. A small, fresh, and predominantly fine-grained pyroclastic-flow deposit was also observed in Ginkgoes Ghaut near Reids Estate. The ash cloud from this event drifted W and reached a height of ~6,200 m. The pyroclastic flows originated from a deep gully between Chances Peak and the dome above Galway's.

Two small pyroclastic flows occurred at 0920 on 8 November and at 0847 on 9 November. These traveled down the White River and the associated ash clouds reached heights of ~1,800 and 3,100 m.

At 0607 on 12 November, the largest dome collapse in the current series occurred, followed by vigorous ash venting. Pyroclastic flows traveled down Gages, Tar River, and White River valleys. The ash cloud reached a height of ~7,700 m; ashfall covered the island but mainly affected the Richmond Hill area. The pyroclastic flows that traveled down Gages valley almost reached the sea at Plymouth; some burning was observed near the port buildings. For the first time, pyroclastic flows reached the War Memorial and the Post Office. Lobes of material reached into the Amersham area and a large water tower was transported into the upper parts of Parsons. Pyroclastic flows also reached the sea at the Tar River delta and the old coastline at the bottom of the White River valley. In the weeks following this collapse there were a few small pyroclastic flows and periods of low-amplitude seismic tremor coupled with ash venting.

Activity during November cut a deep channel into the dome. The channel is ~150 m deep and 30 m wide and bisects the dome between the head of the Tar River and the top of Gages valley. The channel sides are extremely steep and overhanging in places. Several large cracks formed in various sectors of the dome, including in the area above White River and Tyer's Ghaut.

On 16 November, deposits near the War Memorial showed a temperature of 386°C at a depth of 1 m. During 28-29 November, heavy rain caused mudflows down all flanks. New material was deposited on the Belham Bridge (1 m depth), in Plymouth, and on the airport runway.

Seismicity, deformation, and environmental monitoring. A swarm of volcano-tectonic (VT) earthquakes occurred on 1 November (42 events within about 3 minutes); the largest was felt throughout the island. The hypocenters were located SW of the volcano under Chances Peak. Rockfall signals and pyroclastic flows dominated seismicity (70% of recorded events). VT earthquakes (28% of recorded events) beneath the dome often followed rapidly after the larger collapse events. There was a second swarm of VT earthquakes on 25 November with 42 events within about 5 minutes; a pyroclastic flow occurred shortly after the swarm started.

GPS measurements made during the latter part of the month in collaboration with University of Puerto Rico staff determined that Long Ground has moved ~4 cm E since March 1998.

The miniCOSPEC measured an SO₂ flux of 740 metric tons per day on 2 November, similar to the flux measured the previous 2 months. Sulfur dioxide also was measured at ground level using diffusion tubes around the island. SO₂ levels varied depending on the prevailing winds, but overall were lower during November than in previous months.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat, West Indies (URL: http://www. geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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02/1999 (BGVN 24:02) Ash venting and numerous pyroclastic flows in December 1998 and January 1999

Several small dome collapses, some that were initially explosive, generated pyroclastic flows in December. Episodes of ash venting occurred almost daily and seismicity was dominated by volcano-tectonic earthquakes and rockfalls. The number of volcano-tectonic earthquakes declined toward the end of December but the number of long-period signals, corresponding to ash venting, increased slightly. Some explosive eruptions during early- to mid-January generated substantial ash clouds. Brief episodes of ash venting, correlating with seismic tremor, became shorter and weaker toward the end of January. Small-volume pyroclastic flows were generated by dome collapse, but some flows may have been generated by fountain collapse during small explosive eruptions. The average SO₂ flux was elevated throughout December and January. Eastward movement of the Long Ground and Tar River GPS sites continued.

Visual observations.Daily periods of volcanic tremor during December coincided with steam-and-ash venting. On 8 December mudflows occurred all around the volcano.

A pyroclastic flow generated by dome collapse on 14 December reached the sea at the Tar River delta. Deposits were fluidized, fine-grained material with very few blocks. A large ash cloud was generated that rose rapidly to ~6,100 m. Ash fell W and NW of the volcano, attaining a thickness of 2 mm in Salem and containing accretionary lapilli up to 2 mm in diameter. On 19 December a pyroclastic flow reached the Tar River delta in less than five minutes. Powerful black jets of ash and rock burst from the dome at the onset of the event but it is unclear if this explosive activity preceded or followed the dome collapse. The small deposit was almost entirely confined to the incised channel in the Tar River valley on top of the 14 December deposits. On 21 December, at the onset of a sudden large seismic signal, dense black jets of ash and vigorously convecting ash clouds escaped from the main vent in the 3 July scar. Ballistic blocks rose 80 m above the vent. Very vigorous ash venting continued for more than 30 minutes after the initial explosion. A minor dome collapse on 27 December resulted in a small-volume pyroclastic flow reaching the Tar River delta. Poor visibility hampered observations, but a significant ash cloud was generated.

Minor ash venting took place on 1 and 5 January. At 0358 on 7 January, a large long-period seismic signal immediately preceded a 30-minute episode of tremor (usually associated with vigorous ash venting). Later the same day, a small dome collapse generated a pyroclastic flow that traveled half-way down the Tar River valley and a low-level ash cloud that moved W over Plymouth. On 13 January an explosive event generated an ash cloud to 6,100 m and a pyroclastic flow. The onset of the seismic signal had a long-period component, and a pressure wave was recorded at Long Ground. A booming sound was reported by many. The pyroclastic-flow deposit in the Tar River valley was small in volume but its extent suggested that the flow had been very mobile. Narrow small-volume pyroclastic-flow deposits were observed S of the dome as far as the former position of Galway's Soufriere. Two small dome-collapse pyroclastic flows occurred on 14 January. At 0827 on 15 January a small explosive event generated an ash cloud that rose to 4,600 m. The cloud moved NW and light ashfall affected Salem and Old Towne. Ash venting continued in pulses for 15 minutes. Another small explosion on 16 January generated an ash cloud to 3,000 m. Rockfalls were triggered on the inner walls of the 3 July scar and on the outer SE and NE flanks of the dome. A minor dome-collapse pyroclastic flow on 20 January almost reached the sea at the Tar River delta. The resulting steam-rich plume dissipated rapidly. Several brief (20 minute) episodes of tremor preceded by a rockfall corresponded to weak ash venting on 24 January. Further short episodes of ash venting occurred on 25 and 27 January.

Clear conditions on 26 and 27 January enabled MVO staff to survey the dome (figure 44). The canyon, which had been incised through the dome, was clearly visible. It bisected the dome in a NW-SE direction from the top of Tar River Valley to the top of Gages Valley. The inner walls of the canyon were vertical and surfaces looked fresh because of repeated small rockfalls.

Seismicity. Seismicity in December consisted chiefly of volcano-tectonic earthquakes and rockfall signals. Many of the latter were associated with small pyroclastic flows or venting. Small clusters of earthquakes were located under George's Hill to the NW of the dome, under Roaches Yard to the SE, and under Hermitage Estate to the NE.

Overall, January was quiet seismically. Pyroclastic-flow signals had low-frequency precursors. These events were associated with booming noises and were followed by periods of vigorous ash venting, suggesting the collapses were caused by violent degassing of the dome.

Ground deformation. The only area where significant deformation took place in December was on the E flank. The vectors for Long Ground showed eastward movement of these two sites amounting to 5 cm since lava stopped erupting. Most of this movement occurred during the last three months (a time of increased surface activity). The differential movement between Whites and Long Ground since June 1996 is more than 10 cm. The two sites are 733 m apart and the movement between them cannot be fit elastically. A ground inspection on 30 December revealed a possible fault between the two sites. The only surface expression is a linear break in the road and it is not currently known whether this is related to volcanic deformation or to surficial movements. The Tar River GPS pin has followed a similar movement to Long Ground throughout the eruption. The Perches site, until it was destroyed in July, followed a similar path. One possible interpretation is that a sector of the volcano including Long Ground, Perches, and Tar River is moving as a block along faults in a NE direction.

Eastward movement of Long Ground and Tar River continued in January but at a reduced rate. A local EDM network of five pins was set up on 27 January to learn whether the surface feature is a fault.

Environmental monitoring. The miniCOSPEC was used several times in December. The SO₂ flux was elevated and on 22 December and reached a peak average flux of 1,700 metric tons per day (figure 45). Sulfur-dioxide flux decreased throughout January, but generally remained elevated. Concentrations were also measured at ground level by using diffusion tubes around the island.

Ash and rainwater collection continued throughout January. Ash samples from the small explosive events tended to very coarse, with lithic and crystal fragments up to 6 mm in size in the Richmond Hill-St. Georges area. In contrast, ash generated by dome-collapse pyroclastic flows was very fine-grained.

Volume measurements. A detailed photographic and theodolite survey was conducted from twelve sites around the volcano at the end of January. A photographic survey was also conducted from the helicopter with the GPS onboard. The information has been processed to produce a detailed dome map and volume measurement. The dome had a volume of 76.8 x 10⁶ m³ and its highest point was 977 m at the top of the White River Valley. The dome was split deeply by the collapse on 3 July 1998 and by subsequent events. The N part of the dome, which comprises three main buttresses above Gages, the N flank, and Tar River, contains two-thirds of the total dome volume. The scar cuts up to 100 m into the pre-1995 crater floor and has removed a minimum of 5.4 x 10⁶ m³ of old rock from this area.

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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05/1999 (BGVN 24:05) Sporadic explosive eruptions and pyroclastic flows during January-March

January, February, and March were characterized by sporadic low-intensity events and little seismic activity. In early- to mid-January (7, 13, 14, 15, and 16) there was a series of small explosive eruptions, some generating substantial ash clouds. These events were followed by episodes of ash venting and correlated with seismic tremors. There were occasional small-volume pyroclastic flows (mostly generated by dome collapse, but some perhaps due to fountain collapse during earlier eruptions). Several pyroclastic-flow signals (for example, 7 January) had low-frequency precursors and observers heard associated booming noises in the S of the island. Subsequent vigorous ash venting suggested that the collapses came from violent degassing of the dome. SO₂ levels generally decreased during January, although they remained elevated. Ash samples were coarse with lithic and crystal fragments up to 6 mm in size in the Richmond Hills and St. Georges area; those from the dome-collapse pyroclastic flows were very fine-grained.

At the end of January the observatory conducted an extensive photographic and theodolite survey at 12 sites; they also used a GPS-equipped helicopter. The information was used to produce a detailed dome map. The researchers gauged the dome's volume at 76.8 x 10⁶ m³ and measured its highest point (at the top of the White River Valley) at 977 m. Also noted was a deep split in the dome from the 3 July 1998 collapse and subsequent events. The N part of the split comprised two-thirds of the total dome volume (including the three main buttresses above Gages, the N flank, and the Tar River). The scar cuts 100 m into the older English's Crater and has removed a minimum of 5.4 x 10⁶ m³ of old rock.

February activity consisted of a short period of ash venting and pyroclastic flows, and two small mudflows. There was reduced deformation, and the SO₂ flux continued to decline.

Activity in March was dominated by 23 small explosive and ash-venting episodes on 1, 7, 12, 26, and 30 March. The largest produced a 7-km-tall ash cloud, ashfall as far as Salem and Runaway Ghaut, lightning, and pyroclastic flows reaching the Tar River delta. Deformation rates decreased, gas emissions were moderate, and SO₂ fluxes dropped. Seismicity was dominated by signals from ash venting. Events had impulsive origins with gradual declines in amplitude toward the signal's end.

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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07/1999 (BGVN 24:07) Dome collapses, pyroclastic flows, and ash eruptions in April-June

Small explosive blasts with ash venting occurred with regularity during April to July, sending some plumes as high as 5 km, sometimes accompanied by thunder and small pyroclastic flows down the Tar River valley. Vigorous fumarolic activity was observed frequently in the dome above the Tar River as was steaming on the N flanks of the dome. Measurements of SO2 flux typically ranged from ~200 to 500 tons/day, all substantially lower than the peak levels of ~2,000 tons/day.

During April-July there were frequent volcano-tectonic (VT) earthquakes and occasional long period (LP) events. The VT events typically ranged from ~10 to 70/ week; the LP events took place up to 9/week.

Activity increased in May and several events of greater intensity occurred in June and July, the latter with large pyroclastic flows and ash plumes rising to heights of 10 km. Explosions lasting as long as 10 minutes occurred on 10 May and again on 20 May. These were accompanied by ash plumes that rose to altitudes of several kilometers above the summit. Pyroclastic flows were seen traveling down the Tar and Gages rivers and in Tyers Ghaut after the event on 10 May. Observations of the dome on 13 May indicated that a large slab of rock had collapsed from the Galways lobe into the gully between the top of Gages and Tar river valleys.

A distinct increase of activity occurred on 22 May when a VT earthquake swarm was recorded, the first since December 1998. The swarm comprised 141 VT events and lasted ~10 hours. A large pyroclastic flow occurred the next morning, mainly traveling down the E flanks of the volcano before reaching the sea at the Tar River delta. Activity increased on the evening of 5 June when a portion of the dome on the NE flank above Tuitt Ghaut collapsed. About 1.5 x 10⁶ m³ of material were removed from the dome, forming pyroclastic flows that traveled down the Tuitt Ghaut to about 1.5 km from the dome; down the White Ghaut to about 2 km from the dome; and down the Tar River to the sea. A large, dark ash cloud that reached an altitude of ~4 km moved W and NW from the volcano, traveling as far N as Salem with associated thunder and lightning. There was complete darkness in Salem for a short period as ash started to fall. Up to 1 cm of ash was deposited in Salem, Old Towne, and areas S of Belham. Heavy rains on 9 June produced mud flows down all flanks of the volcano depositing new material in the Belham valley.

During the rest of June and most of July, a continuing sequence of small collapse events occurred each week, some with pyroclastic flows that traveled down the Tar River and other valleys. Ash clouds reached 3 km above the summit, causing additional ash deposition in Salem and Old Towne. A 17 June collapse produced a pyroclastic flow that reached the sea at the Tar river delta.

On 20 July a major collapse occurred without any precursory signals; the resulting large pyroclastic flow traveled down the Tar River valley and covered the entire fan. On the N face of Roches Mountain the pyroclastic surge stripped off the remaining vegetation, and the event generated a mobile surge cloud that swept over the mountain and collapsed on its S side forming a secondary flow in the valley leading into Dry Ghaut. Ballistic rocks were thrown over Roches Mountain setting fire to vegetation on its E side. Subsequent visual observations confirmed the large magnitude of the collapse.

On both 25 and 26 July there occurred earthquake swarms, each of 2-hour duration and composed of 22 and 26 events, respectively. Most of the earthquakes were of small magnitude, located at depths of ~1 to 3 km directly beneath the volcano. The first and largest of three explosions that week occurred ~2.5 hours after the second swarm. Heavy meteorological cloud cover precluded visual observation of the effects of these explosions throughout the remainder of the month. However, prolonged steam venting and ash emission, roaring sounds, lightning, and thunder were associated with all three explosions.

Measurements of the volcano deformation rate during the period indicated that it remained low, fluctuating in response to changes in internal pressure. The most recent period of rapid deformation occurred during April and May, coincident with the increase in activity during that time. Electronic distance meter (EDM) measurements reported in late June indicated that the line between Chances Peak Steps and Lower Amersham showed a shortening of 18 cm over the 2-year period since the last measurement, an average rate of about 2 mm/week.

Montserrat Volcano Observatory (MVO) reported that events of the type and size recently observed can occur at any time without any build-up in activity beforehand. MVO warned that the flanks of the volcano remain extremely dangerous and advised residents to keep listening to local radio for updates on the state of the volcano.

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

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01/2000 (BGVN 25:01) Still-vigorous, potentially destructive eruptions during July-November 1999

This report covers a six-and-one-half month period, starting from the major dome collapse on 20 July 1999 (BGVN 24:07) and extending to 4 February 2000. Tremor, explosions, ash venting, and pyroclastic flows occurred frequently during the period from the 20 July dome collapse through late November. Dome growth continued through the end of the interval. Ash clouds were frequent during this period, some rising to above 3 km and several with maximum heights exceeding 6 km. The volcano continued seismic and volcanic activity including rockfalls and abundant earthquakes of several types. The number and intensity of these have varied, but events of all types have been reasonably contiguous throughout the ensuing period. Visibility of the dome area was obscured until early November when the crater was visible for the first time during the early reporting period. Subsequent observations in late November revealed new dome material within the 3 July 1998 dome scarp, the first growth observed since March 1998. This new lava dome continued to grow. Episodes of tremor also began in November (see below).

Figure 46 shows a photo of the dome in December 1999. Early in that month it was estimated to be ~200 m long by 80 m wide and ~70 m high as its widest point. The two small spines at the top were ~8 m high.

Dome growth continued through February; in the last week of January clear weather permitted sufficient measurements and photographs to estimate the dome's volume. Growth rates were above ~3 m³/s for December and early January. This appears consistent with increases in seismicity witnessed during 28 January to 4 February 2000. The first pyroclastic flow from the dome during the reporting interval occurred on 2 February; resulting deposits were seen on 3 February in the S part of the Tar River valley and most of the way across the delta.

During the more than six-month period since the collapse seen on 20 July 1999, rockfalls took place at rates of over 100/week (326 during the last week of January) to as few as 16/week. Weekly earthquake events also varied as shown in table 33. Small earthquake swarms (e.g., 20 events) occurred at numerous times, but were usually of low intensity. A swarm of 213 recorded events occurred during the period 3-8 November; those that could be located occurred at estimated depths between 1.5 and 2 km.

Table 33. A summary of Soufriere Hill's weekly earthquakes during 20 July 1999 through 4 February 2000 emphasizing the maximum and minimum number of events each week. Courtesy of MVO.

    Earthquake type    Maximum    Minimum

    Volcano-tectonic     129        16
    Hybrid               142         6
    Long-period           23         2

On 23 November 1999, cyclic bands of tremor began. They were continuing as of the end of this reporting period. Overall, more than 100 tremor episodes occurred during the reporting period. On average, a cycle included 5 hours and 40 minutes of seismic quiescence followed by 3 hours of tremor. These tremor bands were associated with periods of dome growth, with an increasing amplitude of the seismic peak within each cycle. The slow increase in background long-period seismicity suggested that the dome growth rate was increasing.

Average daily SO₂ flux estimates were made when weather conditions permitted measurement. The results typically varied from about 200 to 600 tons/day, with some peak values exceeding 1,500 tons/day.

Background. A recent reprint volume has been compiled to provide a single source of scientific information on the Montserrat eruption up to late 1997 (Young and others, 1998). In addition, Rozdilsky (1998) discusses some social impacts of the ongoing crisis. The destructive events that began in July 1995 led to the evacuation of ~70% of the population of 11,000 people under emergency conditions. In other words, 7,243 people from 22 settlement areas were evacuated. The settlements were subsequently damaged or destroyed; however, due to the evacuations less than 25 deaths were attributed to the dome collapse.

During 1999, extensive planning for redevelopment of the safer, northern portion of the island was undertaken by the governments of Montserrat (a British Overseas Territory) and the United Kingdom. As a future target, 10,000 persons (approximately the island's population prior to the volcanic crisis) could be relocated at seven new activity centers in N Montserrat.

References. Rozdilsky, J.L., 1999, Disaster recovery in an on-going hazard situation on Montserrat: the July 20, 1999 volcanic dome collapse (preliminary abstract): Ph.D. dissertation, Dept. of Resource Development and Interdepartmental Urban Affairs, Michigan State University.

Young, S. R., Voight, B., Sparks, R.S.J., Rowley, K., Robertson, R.E.A., Lynch, L.L., and W. P. Aspinal (conveners), 1998, Selected papers on the eruption of the Soufriere Hills Volcano, Montserrat: reprints from the Geophysical Research Letters, v. 25, nos. 218 and 219, published by the American Geophysical Union (1998), 3387-3700 p. (ISBN-0-87590-919-1).

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).

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04/2000 (BGVN 25:04) Dome growth continues through May; Vulcanian eruption 20 March

Volcanism at Soufriere Hills volcano between 5 February and 26 May 2000 consisted of tremor, continuous dome growth, dome collapses, several pyroclastic flows, and a Vulcanian eruption.

Dome growth that began in November 1999 (BGVN 25:01) continued this report period with growth concentrated in the NE and E portion of the dome through February 2000. On 7 March a switch in the focus of dome growth was marked by a small swarm of earthquakes recorded from 1530 to 1730. The swarm consisted of a mixture of volcano-tectonic, long-period, and hybrid earthquakes with a total of 25 recorded events. Between 2342 on 7 March and 0349 on 8 March a second swarm of different types of earthquakes occurred. The appearance of spiny material in the summit area on 7 March suggested that some growth occurred to the W of the new dome towards Gages wall. Further observations on 9 and 10 March showed a large spine, ~30 m high in the S part of the new dome.

A magmatic explosion occurred at 1530 on 20 March, which was the largest event of this report period. According to MVO, a series of pyroclastic-flow signals started shortly after 1530. These were probably triggered by heavy rainfall and gradually built up in size. Between 1800 and 1900, there were several large pulses of activity with each pulse successively larger. At about 1915, a very large pyroclastic flow traveled out across the sea. This was followed by at least one, and probably more, vulcanian explosions from the volcano's summit. Lightning was seen, and near-continuous thunder was heard during this eruption. Satellite imagery suggested that the ash cloud reached over 9 km and mostly traveled E and SE. However, all of Montserrat received some light ashfall as part of the ash cloud spread to the N. Reports of ashfall in Guadeloupe and Antigua were also received. The following night, heavy rain washed away most of the ash in Montserrat. Observations of the lava dome after the explosion showed that virtually all of the new dome, growing since November 1999, had collapsed and some of the interior of the old dome had been eroded.

On 24 March observations of the scar confirmed that a new spine of lava had been extruded over the vent area. In addition, excellent views of the lava dome on 31 March showed blocky growth in the base of the scar with abundant steaming from around the dome. The new dome was estimated to be 150 m wide and about 100 m high; three small spiny lobes could be seen on top of the new growth. By 20 April the dome's volume was estimated to be 12 to 15 million m³. The average growth rate was 5-6 m³/s, which was higher than the rate in December and July (above ~3 m³/s: BGVN 25:01). Two areas of active growth were noted; one on the dome's S side, and another on its E side. The highest point on the new lava dome was measured at 848 m above sea level, which means the new lava dome had a total height of ~120 m.

Between 28 April and 5 May there was a significant increase in both hybrid and long-period earthquakes. About half of the long-period earthquakes recorded immediately preceded rockfalls, and visual observations confirmed that these events comprised powerful, vertical ash emissions immediately prior to the start of the rockfall. The nature of the seismicity indicated increased levels of pressure within the upper conduit and lava dome. The main dome growth area was on the dome's NE flank. The increased activity culminated on 6 May when there was a moderate collapse of several million m³ of rock from the dome's NE flank. The pyroclastic flows generated from the collapse traveled down Tar River Valley and the longest flows just reached the sea. Observations on 12 May revealed refilling of the small scar created by the 6 May collapse. As of 26 May growth was concentrated in the dome's NE side, with indications that it may move towards the SE. Also, between 19 and 26 May a continuous, low-level ash plume visible in GOES 8 imagery emanated from the volcano.

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).

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06/2000 (BGVN 25:06) Dome growth continues, reaches 950 m high during May-July 2000

This report covers activity from 26 May to 21 July 2000. During this interval, the lava dome continued to grow; however, between 26 May and 2 June, the direction of the dome's growth changed. Although it continued to grow vertically, the majority of growth appears to have redirected from the E and NE to the S and possibly the W.

Visual observations were severely limited due to clouds throughout the early part of this period. However, during the week of 23-30 June a "rough, spiny area" appeared high on the E face of the dome at the top of the Tar River Valley. The week of 9-16 June, the dome grew to about 914 m. By 25 June, the dome had surpassed the height attained prior to the 20 March 2000 collapse. During this event, instruments for measuring dome volume were damaged. Observations from 30 June through 7 July showed that the area of dome growth had changed to a more slab-like appearance. A new area of spiny growth was first seen on 10 July. This growth appeared on the NE flank at 940 m elevation, which was thought to be the highest point on the dome. On 17 July, a large area of new growth was reported on the S and W sectors of the dome, attaining a height of 950 m.

Pyroclastic flows were reported to the ENE in the Tar River, between 9 and 16 June. The following week, pyroclastic flows were reported in the Gages valley to the W. Additional pyroclastic flows during the week of 7 July went NE into the upper Tar valley; some, if not all, of the flow material originated from the remains of the 1995-98 dome. On 21 July at 0620, there was a small pyroclastic flow with an explosive start. During an observation flight later that day, evidence of pyroclastic flows was observed to the SW in the upper region of the White River valley.

Rockfalls occurred throughout the reporting period (table 34). However, the week of 23 to 30 June was characterized by nearly constant rockfalls and small pyroclastic flows. These rockfalls were concentrated on the E side of the dome and talus accumulated much more slowly to the S above the White River. Prior to this, during the week of 9 to 16 June, the rockfalls occurred almost exclusively in the Tar River valley. During 30 June to 14 July, rockfalls occurring to the E of the dome were infrequent despite the presence of large blocks at the top of the steep E slope. The majority of the rockfall events at this point were occurring to the S and to the W of the dome.

Table 34. Seismic data for Soufriere Hills during 26 May - 21 July 2000. Courtesy of MVO.

    Time Period                 Rockfall  Hybrid  Volcano-   Long    Total
                                Signals           Tectonic  Period

    Week of 26 May-02 Jun 2000     131      54       3         2      190
    Week of 02 Jun-09 Jun 2000     243     172       1        78      494
    Week of 09 Jun-16 Jun 2000     326      49       1        76      452
    Week of 16 Jun-23 Jun 2000     147      11       1        77      236
    Week of 23 Jun-30 Jun 2000     315       4       4       157      480
    Week of 30 Jun-07 Jul 2000     264      47       1       114      426
    Week of 07 Jul-14 Jul 2000     131     103       5        68      307
    Week of 14 Jul-21 Jul 2000     189      24       4        15      232

Seismic records (table 1) revealed a sharp increase in the number of long-period (LP) earthquakes after 2 June. The frequency of LP events continued to increase until its peak during 23-30 June. This same week marked the low point in the number of hybrid earthquakes. The number of volcano-tectonic earthquakes increased towards the end of the reporting period.

A steady production of ash during the week of 9-16 June maintained a dilute ash plume that moved W towards Plymouth and off the coast. Neither this ash plume nor the smaller ash clouds produced by rockfalls during the preceding weeks affected the inhabited parts of the island. During the week of 30 June to 7 July, abundant steaming was observed on the W flanks of the dome. The following week, steaming occurred on the N side between the main masses of the old dome. During this same week, ash venting was also observed from the S side of the dome.

The sharp increase in the number of LP and hybrid earthquakes after 2 June was taken to indicate increasing pressure in the dome. In addition, the dome's filling in of the crater on all sides suggests that rockfalls and pyroclastic flows will increase in the future. These events are expected to affect not only the Tar River valley, but also several other surrounding valleys, particularly Tuitt's Ghaut, White River valley, and Gages valley. These observations also lead to increased concern over the possibility of a substantial dome collapse in the near future.

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).

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09/2000 (BGVN 25:09) Rockfalls and pyroclastic flows, dome growth rate increases

Dome growth continued throughout 21 July-6 October 2000 largely on the S and E flanks of the volcano. Poor weather hampered observations in late July, but during the week of 4-11 August a large ~30-m-high spine was visible on top of a conical mound of new lava. The top of the spine was at ~980 m elevation, substantially higher than the remnants of the 1995-98 dome. By 19 August, the top attained a peak elevation of 1,043 m. When observed again on 20 September, the spine was no longer steeply inclined but was gently inclined to the E. On 24 September a large new spine with near-vertical inclination was seen. A smaller spine on 27 September had an elevation of 1,032 m, and on 28 September a very large, near-vertical spine was seen on the E side of the summit. The latter dominated the E part of the summit during the following week, changing its size and shape throughout that period. By 30 September the top had an elevation of 1,054 m, the highest measurement taken on the dome to date.

The level of seismicity increased substantially after 4 August (table 35) with rockfalls and long-period earthquakes being dominant. Rockfalls were concentrated on the E and S sides of the dome and were almost continuous at times. Subsequent to the increased seismicity, rockfalls caused small ash clouds, reaching up to 3,000 m in height and drifting W. Following the passage of a tropical storm on 21-22 August, unusual wind directions blew some ash to the N of the island. A mudflow down the Belham valley during the early afternoon of 22 August followed two main paths in the lower reaches of the valley, N and S of the golf course. Debris was deposited on the Belham bridge, and the beach at Old Rod Bay was extended further out to sea.

Table 35. Seismic and gas data for Soufriere Hills during 21 July to 6 October 2000. The HCL/SO₂ ratio was determined from FTIR data; SO₂ flux (metric tons/day) is from COSPEC. Courtesy of MVO.

    Time Period        Rockfall  Hybrid  Volcano    Long
                                         -Tectonic  -Period

    21 Jul-28 Jul 2000    217      45        5        17
    28 Jul-04 Aug 2000    220      44        2        14
    04 Aug-11 Aug 2000    296      24        2        42
    11 Aug-18 Aug 2000    257      41       19       119
    18 Aug-25 Aug 2000    277      39        4        63
    25 Aug-01 Sep 2000    390      71        1        55
    01 Sep-08 Sep 2000    872     106       43       110
    08 Sep-15 Sep 2000    411      21        7        44
    15 Sep-22 Sep 2000    386      50        5        50
    22 Sep-29 Sep 2000    665      28       --        66
    29 Sep-06 Oct 2000    169      29       16        25


    Time Period           Total       Ratio    Avg. Daily SO2
                       Earthquakes   HCl/SO2     (tons/day)

    21 Jul-28 Jul 2000     284         --           --
    28 Jul-04 Aug 2000     280         --           --
    04 Aug-11 Aug 2000     364       1.5-2.5      ~ 400
    11 Aug-18 Aug 2000     436         --           --
    18 Aug-25 Aug 2000     383         --         ~ 280
    25 Aug-01 Sep 2000     517         --        1460-2240*
    01 Sep-08 Sep 2000    1131         --        1390-2570*
    08 Sep-15 Sep 2000     483         --         541-890*
    15 Sep-22 Sep 2000     491        2.37        446-630*
    22 Sep-29 Sep 2000     759         --           369*
    29 Sep-06 Oct 2000     239         --         790-948*
    * Data are for specific days during the indicated week.

Small pyroclastic flows were reported on 27 July, 6-7 August, and during the weeks of 15-22 September and 29 September-6 October. The resulting deposits were mostly confined to the Tar River Valley on the E flank, although minor new deposits were seen in the upper reaches of the White River valley. Several small explosions also occurred during the week of 15-22 September. On 19 August a small burst of incandescent gas was observed near the summit of the dome followed by glowing rocks that tumbled down the E face. On 8 and 14 September, a near-continuous rockfall of incandescent material was observed going down the E face of the dome above the Tar River valley; this activity continued to be observed through early October.

Gas monitoring resumed during the week of 4 August using the Cambridge FTIR instrument to measure the ratios of gases in the volcanic plume (table 35). The measured ratio of HCl to SO₂, between 1.5 and 2.5, was about twice the values measured earlier in the year. This is indicative of an increase in extrusion rate since January 2000 and corroborates evidence from visual observations suggesting an increase in the dome growth rate. Gas monitoring also resumed on 24 August with the COSPEC on loan from the Geological Survey of Canada (~).

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).

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02/2001 (BGVN 26:02) Dome growth, rockfalls, and pyroclastic flow continue through March 2001

Dome growth continued from 7 October 2000 through 9 March 2001. Until the end of February 2001, the growth occurred predominantly on the E side of the volcano. However, on 25 February, the direction of the growth and the character of the seismicity changed markedly; the dome growth shifted towards the S, and the weekly number of hybrid earthquakes increased by more than an order of magnitude. During this entire period, residents were advised of the potential dangers associated with pyroclastic flows and advised to avoid the Belham valley during periods of heavy rain. Access to Plymouth, Bramble airport, and beyond was prohibited, and a maritime exclusion zone was declared around the S part of the island extending 3.3 km beyond the coastline. Since November 1999, the dome has grown at an average rate of nearly 3 m³/sec and is now at its largest size since the eruption began in 1995, with a total volume of over 120 x 10⁶ m³ and an elevation greater than 1,000 m.

Recent MVO assessment. A summary assessment of the volcano's activity, status, and related risks covering the period from April 2000 through January 2001 was published recently by MVO. A significant finding from this assessment was: "While one prognosis is for at least a few more years of such eruptive activity, an eruption duration measured in decades has to be contemplated." Other extracts from the report are presented below:

"The period July 1995 to March 1998 was the first phase of the present eruption. A lava dome grew . . . accompanied by several hazardous phenomena . . . . There was then a second phase . . . from March 1998 to November 1999. In this phase, no significant dome growth was detected, but hazardous activity continued . . . . Dome growth resumed in November 1999 . . . [and] represents a third phase of the eruption."

"The seismic monitoring detected relatively intense periods of rockfall and long-period (LP) activity . . . from August to October [2000] . . . and from mid-November 2000 [to January 2001] . . .. There appears to be an underlying 14-week cycle to this type of seismic activity . . .. However, hybrid swarms have been rare and weak by comparison with 1997 activity. Although their generation mechanisms are still poorly understood, hybrid earthquakes may be related to fracturing of plugs that form in the conduit, and their absence suggests that conduit conditions may now be subtly different from previous stages . . .."

"Based on the seismic evidence, the growth of the latest . . . dome seems to have been different in some respects . . .. The only current prognostic feature in the observable seismicity is the weak 14-week cycle."

"Volcanic gases continue to be routinely monitored. The flux of sulphur dioxide over the last ten months has maintained quite high levels . . . . Occasional measurements of chlorine flux indicate that the flux of chlorine relative to sulphur dioxide increases . . . when the dome is growing."

"Two key changes have affected the morphology of the volcano since 1998 . . . which have important implications for hazards. . . . Two remnants of the 1995-1998 dome remain adjacent to the northern wall of English's Crater. Over the last year, a narrow gap between the two remnants has gradually widened and deepened . . .. This deep gully is expected to provide a path for potential collapses on that side of the dome. Since 1997, Mosquito Ghaut has been in-filled by pyroclastic-flow and rockfall deposits and no longer exists as a pathway to channel future pyroclastic flows away to the [NE]. Thus, collapses moving down the outlet gully will run in a generally northerly direction, and these topographic changes mean that pyroclastic flows towards the north are now much more likely to be channeled into the Belham Valley . . . ."

"The group considered the possibility of tsunami hazards that might affect other Caribbean islands. Based on modeling studies carried out by French colleagues a collapse of the current dome . . . into the sea is not expected to generate a tsunami sufficiently large to affect other islands. However, new data . . . show that there have been several very large edifice failures of the Soufriere Hills Volcano in its geological past. Another collapse of such size would cause a significant regional tsunami hazard. Thus, any precursory signs of a major edifice failure should continue to be watched for in the monitoring [program]."

"The duration of the eruption of the Soufriere Hills Volcano now exceeds about 85% of all documented dome eruptions worldwide. Most dome eruptions last only a few years, but some have durations measured in decades. Examples of the latter kind of dome eruption include the Bezymianny volcano in Kamchatka, Russia, which started to erupt in 1956 and is still active, and the Santiaguito dome in Guatemala, which started to erupt in 1923 and is also still active."

"There is currently a lack of clarity about the legal responsibilities and obligations of the MVO . . . [and other official organizations] in relation to providing warnings of detected ash injections to civil aviation authorities. The group recommends that HMG [Her Majesty's Government] investigate as a matter of urgency where responsibility lies and what [organization] should issue such warnings."

Activity since 6 October 2000. Until the end of 2000 the summit was dominated by a broad lava spine inclined at a steep angle towards the E. On 25 October the spine had a peak elevation of 1,030 m and by 13 November had grown to 1,077 m, the greatest height measured throughout the eruption. On 5 December the top of the spine was at 1,060 m, while the flat top of the main dome was between 1,020 and 1,030 m. By the end of December the spine had grown back to 1,071 m. Two large near-vertical spines were observed on 4 February 2001, but both had collapsed by the following week. A large stubby spine visible in the S part of the summit area on 22 February rose to 1,068 m.

Rockfalls took place throughout the period. Until 25 February 2001, these traveled predominantly down the E or NE side of the dome, and occurred typically in numbers of hundreds per week (table 36). Some of these glowed and occasionally produced small ash clouds, but none reached altitudes greater than 3,000 m. These rockfalls contributed to an accumulating talus fan in the upper reaches of the Tar River valley. The talus began to bury the remnant buttress of older dome material on the NE flank that formed during the 1995-98 growth phase. Rockfalls that occurred after 25 February traveled predominantly S of the dome, mainly in the upper reaches of the White River valley. (N.B. The White River is sometimes confused with White's Ghaut, which lies to the N of the dome.)

Table 36. Seismic and gas data from the Soufriere Hills during 20 October 2000 to 9 March 2001. Courtesy of MVO.

    Time Period         Rockfall  Hybrid  Volcano-  Long-   Range of Average
                                          Tectonic  Period  Daily SO2 (t/d)

    20 Oct-27 Oct 2000    214        9        4       35    235-2252
    27 Oct-03 Nov 2000    146       20        3       19

    10 Nov-17 Nov 2000    207       33        7      144    no data
    24 Nov-01 Dec 2000    491       13        -       69    1020 (28 Nov)
    01 Dec-08 Dec 2000    547       15        1       72    no data

    15 Dec-22 Dec 2000    423       12        1       74    400 (20 Dec)
    22 Dec-29 Dec 2000    708       10        2       53    745-1100

    12 Jan-19 Jan 2001    943        -        -       54    345 (18 Jan)
    19 Jan-26 Jan 2001    417        1        -       55    330-350
    26 Jan-02 Feb 2001    313        8       21       45    105-360
    02 Feb-09 Feb 2001    409        5        1       40    180-500
    09 Feb-16 Feb 2001    500        2        1       15    80-670
    16 Feb-23 Feb 2001    486       18        6       53    210-720
    23 Feb-02 Mar 2001    729      388        3       58    180-1400
    02 Mar-09 Mar 2001    629      280        4       45    100-1230

Pyroclastic flows were also produced throughout the period. A small one on 15 November 2000 traveled N from the summit, entered the upper reaches of Tyre's Ghaut, and reached ~1 km away from the dome. On 17 November pyroclastic-flow deposits were noted in the upper reaches of Tuitt's Ghaut and White's Ghaut on the volcano's NE side; this was the first new dome material to have traveled down the notch between the N and NE lobes from the 1995-98 dome. By 8 December 2000 the notch between the central and NE buttresses of the 1995-98 dome was 60 m wide. Another small flow occurred down the White River valley on 1 February 2001 and traveled about 1 km from the dome. On 8 February 2001 new pyroclastic-flow deposits had formed in the upper portion of Tuitt's Ghaut up to ~300 m from the dome. By mid-February new pyroclastic-flow deposits had also formed down the Tar River on the E flank, and, by 23 February, had reached as far as the old coastline. New deposits were also seen by 23 February in the S White River valley just 50 m short of the coastline.

On 25 February 2001 a pyroclastic flow spread over the N and central parts of the White River fan. A hybrid earthquake swarm occurred after this collapse (table 36). Subsequently, small pyroclastic flows traveled into the upper portion of the White River valley and were accompanied by banded tremor and weak hybrid earthquakes. By 9 March steady dome growth appeared to have resumed.

Seismicity and COSPEC measurements of SO₂ are presented in table 36. The SO₂ data are in the range of average daily values (in metric tons/day) measured during the report week and include the range of data obtained from both helicopter and static mounted sensors.

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).

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07/2001 (BGVN 26:07) 29 July dome collapse and rockfalls

This report covers the interval from 9 March to 17 August 2001 and chronicles ongoing dome growth, including a vigorous episode of dome collapse and mass wasting on 28-29 July. As reported in BGVN 26:02, on 25 February 2001, the direction of the continuing dome growth changed markedly, shifting its predominant growth from the volcano's E side towards the S side. Then, as also reported in the Bulletin, the character of the seismicity changed dramatically in early March with the number of hybrid earthquakes exceeding 300/week (table 37). However, by mid-March, seismic activity had decreased significantly. Dome growth with attendant rockfalls, pyroclastic flows, and ash clouds continued at low levels until early-May. A small pyroclastic flow occurred on 9 May and traveled ~2.5 km down the White River to the S of the dome. The number of rockfalls increased substantially in the following week and remained at higher levels until early August. Observations during the week of 11-18 May indicated that the main dome growth was still concentrated in the S sector of the dome, and a lobe of new lava was observed over Galway. Reports from the week of 8-15 June noted that the summit over Galway appeared to contain the highest point on the dome.

Table 37. Seismic and SO₂ data from Soufriere Hills during 16 February to 17 August 2001. Courtesy of MVO.

    Time period   Rockfall  Hybrid  Volcano-  Long-   Range of avg daily
      (2001)                        tectonic  period   SO2 fluxes (t/d)

    16 Feb-23 Feb   486       18        6       53         210-720
    23 Feb-02 Mar   729      388        3       58         180-1400
    02 Mar-09 Mar   629      280        4       45         100-1230
    09 Mar-16 Mar   294        4        0       23         360-460
    16 Mar-23 Mar    84        5        2        8         120-190
    23 Mar-30 Mar    33        5        3        1         200-275
    30 Mar-06 Apr    62       18        1        1         200-370
    06 Apr-13 Apr    52        9        6        3          40-520
    13 Apr-20 Apr    54       48        1        9          20-70
    20 Apr-27 Apr    31       10        1        2         100-250
    27 Apr-04 May    98       10        3        7         130-220
    04 May-11 May   104       34        6       22          80-180
    11 May-18 May   240       17        1       31           170
    18 May-25 May   237       26        0      109           700
    25 May-01 Jun   266       36        3      383          90-370
    01 Jun-08 Jun   224       25        6      164         130-320
    08 Jun-15 Jun   373       71        0      169         770-1410
    15 Jun-22 Jun   462       11        1       77         460-630
    22 Jun-29 Jun   299        1        0       26           860
    29 Jun-06 Jul   295        4        1       28           120
    06 Jul-13 Jul   297        7        0       38           347
    13 Jul-20 Jul   719        5        2       57         709-943
    20 Jul-27 Jul   706        8        1       30         339-854
    27 Jul-03 Aug   453       15        0       67    
    03 Aug-10 Aug   258       13        2       13         680-950
    10 Aug-17 Aug   186        6        3        3

Two notable events occurred during the week of 29 June-6 July. First, on the morning of 30 June, there were prolonged rockfalls that involved ~0.5 x 10⁶ m³ of material transported down the N side of the talus apron in the Tar River valley. Second, on the evening of 4 July, two small pyroclastic flows passed down the W flank of the volcano in the Amersham area, stopping ~1 km short of the sea. Following the pyroclastic flows in the Amersham area, the daytime entry zone (DETZ) was closed until further notice and has remained that way through at least 17 August.

Lava dome collapse. Shortly after 1700 on 29 July, a large pyroclastic flow passed down the Tar River valley on the volcano's E flank and a continuous, dense plume of ash developed and blew W. Pyroclastic-flow output increased gradually over the next three hours, with many of the flows reaching the sea. The downwind plume deposited substantial amounts of wet ash with accretionary lapilli over the residential areas of Salem, Isles Bay, and Olveston.

Pyroclastic-flow activity peaked at ~1950, when surge clouds associated with the largest flow moved out over the sea, followed by rock fragments falling over a wide area in the NW of the island in the sector between Salem and St. Peters. Some fragments were pumiceous, although the majority consisted of angular, dense lithic fragments generally less than a few centimeters in size, but with maximum dimensions of 6 cm. A second peak in pyroclastic-flow output took place shortly after 2200, when another large flow entered the sea and extended out from the shore for 0.5 km or more and rock fragments fell in the Salem area again. After about 0200 on 30 July seismic signals indicated that this dome collapse had largely finished, and the activity level declined rapidly. The ash plume from the collapse dispersed for considerable distances to the NW. Ash was deposited as far away as Puerto Rico and the Virgin Islands.

Observation flights indicated that a large portion of the dome had collapsed. The general summit region dropped ~150 m and there was a complex, amphitheater-shaped scar several hundred meters deep incised into the core of the dome at the head of the Tar River valley. Within this scar, a new dome began extruding. Observations indicated that minor pyroclastic flows also occurred in the upper reaches of White's, Tuitt's, and Gages ghauts, and also on the southern flanks of the dome in the upper reaches of White River. The main pyroclastic flows in the Tar River were highly erosive; they incised a deep canyon extending across the delta region to the shore and split the delta into two distinct lobes. Analysis of seismic data indicated that the two most intensive periods of pyroclastic-flow activity were associated with explosive events related to the collapse of the largest fragments of the dome.

Reports after 3 August noted that activity at Soufriere Hills was at a low level, and it continued that way to the end of the reporting period (17 August). Small-scale rockfalls and minor pyroclastic flows occurred, but clear views of the upper parts of the volcano were hampered by clouds. Occasional views of the dome noted that it was continuing to grow in the scar produced by the 29 July collapse.

Information Contact: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/).

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01/2002 (BGVN 27:01) Small-scale dome collapses and pyroclastic flows through February 2002

Introduction. The Montserrat Volcano Observatory (MVO) reported that during 17 August 2001 through at least 1 February 2002 at Soufriere Hills, a new lava dome continued to grow within the scar produced from the 29 July 2001 partial dome collapse (BGVN 26:07). Activity generally increased at Soufriere Hills during mid-September through November 2001, and remained at a high level through at least 1 February 2002 (table 38).

Table 38. Seismic and SO₂-flux data from Soufriere Hills during 17 August 2001 to 1 February 2002. Courtesy of MVO.

    Date (2001-02)  Rockfall  Long-period  Long-period  Hybrid  Volcano-tect
                    signals   rockfalls    earthquakes  events  earthquakes
        SO2 flux (metric tons/day)

    17 Aug-24 Aug     189          1            36        149        0
        Not Reported

    24 Aug-31 Aug     200          1             6         19       11
        25 Aug: 68; 28 Aug: 151

    31 Aug-07 Sep     218          2            31          8        4
        31 Aug: 242; 01 Sep: 86

    07 Sep-14 Sep     228          0            28         65        1
        13 Sept: 543

    14 Sep-21 Sep     211          4            36        522        3
        avg 200-2000

    21 Sep-28 Sep     297          7            16        326       12
        100-600; avg 250

    28 Sep-05 Oct     202          2            26        451        0
        01 Oct: 418

    05 Oct-12 Oct     285          7            34         20        1
        10 Oct: 388

    12 Oct-19 Oct     207          2             6          9        1
        18 Oct: 320

    19 Oct-26 Oct     208          2             3         46        0
        22 Oct: 574; 23 Oct: 48424 Oct: 292; 25 Oct: 200

    26 Oct-02 Nov     284         --             8         46        2
        77-385; avg 233; 26 Oct: 611

    02 Nov-09 Nov     314          8             5        174        4
        05 Nov: 134

    09 Nov-16 Nov     149          4            20        116        2
        13 Nov: 521; 15 Nov: 450

    16 Nov-23 Nov     251         45           115        413       --
        19 Nov: 140; 20 Nov: 119

    23 Nov-30 Nov     435         82           145        193       --
        <100 avg

    30 Nov-07 Dec     363         37            58        128       --
        Not Reported

    07 Dec-14 Dec     551         97            95         80       --
        11 Dec: 158

    14 Dec-21 Dec     858         42            57         25       --
        19 Dec: 181

    21 Dec-28 Dec    1012         45            75         75       --
        27 Dec: 851
    28 Dec-04 Jan     911         69           103         21       --
        250-1000, avg 457

    04 Jan-11 Jan     939         81            87         24       --
        08 Jan: 898; 10 Jan: 1122

    11 Jan-18 Jan     741         29            52          7       --
        Not Reported

    18 Jan-25 Jan     471         68            70          9       --
        22 Jan: 700

    25 Jan-01 Feb     610         67           140          8       --
        Not Reported

Throughout the report period, the new dome produced pyroclastic flows and rockfalls that traveled E to the upper and middle reaches of the Tar River Valley. Small-scale lava dome collapses generated pyroclastic flows almost continuously, with flows entering the sea on 4, 5, and 14 October, 2 and 28 December 2001, and 5 and 12 January 2002. Dense ash plumes associated with sea entry and ash venting from the summit generally drifted W and reached up to 3.0 km altitude (table 39). During mid-October ash clouds drifted to the W and NW and occasionally deposited small amounts of ash on inhabited areas to the N of the island. A new event began on 28 December at 1330 that produced a large area of dense ash observed on satellite imagery below ~3 km a.s.l. Incandescence was observed at the dome on 3 September, during 2-9 and 16-23 November, and on the E and W sides of dome on 26 and 27 December. Mudflows occurred in the Belham Valley on several days during periods of torrential rainfall.

Table 39. Summary of ash emissions at Soufriere Hills seen on satellite imagery during 26 August 2001- 5 February 2002. Courtesy of Washington VAAC.

    Date (2001-2002)    Altitude (km)   Direction    Size

    26 August 2001         ~2.1            SW        28 km long, 9 km wide
    05 September           ~1               W        160 km long, 28 km wide
    07 September        ~summit level       S
    16 September        ~summit level
    21 September           <1              WNW
    22 September           <1.2            WNW       115 km long
    24 September           ~1.5             W
    25 September           ~1.5             W
    26 September           ~1.5            WSW
    30 September           <3.0             W
    03 October          ~summit level      WSW
    04 October             <1.5             W        36 km long, 23 km wide
    04 October             <2.4            WNW       28 km wide
    05 October             <1.5
    06 October             <1.8             W        168 km long, 17 km wide
    07 October             <1.8
    10 October             ~1.8         vertically, possibly E
    11 October             <1.8             W
    11 October             >2.1            NW
    12 October             <1.8             W
    14 October             ~1.8
    26 October             <2.1             W
    07 November            <1.8            NW       32 km long, 7 km wide
    07 November            <6.0            ENE
    17 November            <5.2            NE
    18 November            <3.0            NE       42 km long, 11 km wide
    03 December            ~2.4             W
    08 December            ~1.8             W       139 km long
    13 December            ~4.0            WSW      60 km long, 13 km wide
    14 December                            WSW
    21 December            <2.4             W       28 km long, 7 km wide
    27 December           2.1-3.0          SSE      22 km wide
    27 December            <3.0            SW
    28 December            <3.0            WNW      47 km long, 11 km wide
    29 December            ~3.0            WNW      70 km wide
    29 December            <3.0             W       129 km long, 16 km wide
    01 January 2002        <1.5             W       133 km long, 10-24 km wide
    02 January             <1.5            WNW      125 km long, 10 km wide
    05 January             <2.4             W
    08 January             ~1.5,            W        140 km long
                       bursts to 2.4
    11 January                              W       41 km long, 9 km wide
    12 January             <3.0            WNW
    13 January             <2.4             W       149 km long
    29 January             ~2.4             W
    05 February           2.4-3.0           W
    05 February             1.5            NW       23 km wide
    05 February             3.0             W       17 km wide

The daytime entry zone (DTEZ), closed after 4 July when two small pyroclastic flows passed down the W flank of the volcano in the Amersham area, reopened on 29 August. However, the Montserrat Volcano Observatory (MVO) warned that activity could still increase quite suddenly, with a dangerous situation developing very quickly. Ash masks were to be worn in ashy conditions, and the Belham Valley was to be avoided during and after heavy rainfall due to the possibility of mudflows. The DTEZ was closed again during 4-11 October due to increased activity.

Morphology of the new lava dome. Observations during August 2001 revealed that the new dome appeared to be growing rapidly and had steep sides and a rugged summit area. During mid-September, MVO reported that the volume of the dome was estimated to be approximately 12 x 10⁶ m³, indicating an average growth rate of ~2.6 m³ per second since the partial dome collapse on 29 July.

On 31 October and 1 November observations revealed that the active lava dome had grown substantially and appeared to switch growth direction from the NE to the E, where a massive, near-vertical headwall had developed. Observations from a helicopter on 8 November revealed that a shallow, circular depression was located over the summit area of the dome, with ash vigorously venting from it. The lava dome's highest point during mid-November was measured on 9 November at 876 m elevation.

During mid-November, lava-dome growth shifted from the E to the W, and the summit area was crowned by spines with an average elevation of 940 m. An elevation of 968 meters was measured on one spine, although one other stood higher. By the end of November, MVO reported these elevations: the dome complex consisting of the stagnant E lobe (870 m), an inactive central lobe (930 m), and the active W lobe (960 m on 27 November). The W lobe had produced several small spines, which collapsed and were replaced by new spines.

Observations of the lava dome on 16 December revealed that although it had not increased noticeably in height, it had increased in volume since November. The top of the dome had developed a broadly rounded and blocky appearance. Most of the growth appeared to occur on the W side of the dome, but rockfalls and small pyroclastic flows also occurred on the E and S flanks.

Observations on 10 January revealed that the summit dome had increased in volume considerably during the previous several weeks and that it was broad with several spines projecting upward. The highest spine reached 1,015 m elevation on 12 January. A large lobe was again active on the upper E flank of the dome, just below the summit level. The W side of the dome appeared to have been inactive for some time, judging from the general weathered appearance and deposits of sulphur. Survey measurements also indicated that the saddle area between the NE and central buttresses lowered by about 20 m during the previous weeks due to rockfall and pyroclastic-flow activity.

On 21 January the dome was crowned by a large 40- to 50-m tall spine inclined steeply upwards towards the E. Although the number of rockfalls gradually decreased over the previous 3 weeks, their size and duration significantly increased during 18-25 January. Rockfalls during that interval yielded seismic signals whose total energy rates exceeded those seen during the previous few months.

Activity of the new lava dome. Lava-dome collapses consisting of 10-15% of the dome's volume occurred on the N side of the dome on 4 and 5 October. On 14 October, after a day of torrential rainfall, several million cubic meters of unconsolidated talus was destabilized on the SE flank of the pre-July 29 dome. Seismic data suggested that the event began at about 1715, peaked at 2245, and ended at about 2300. Ash from the event fell in residential areas on Montserrat to the NW.

On the morning of 16 October a collapse occurred on the S flank of the dome complex, producing numerous pyroclastic flows that traveled W down the White River and reached about two-thirds of the distance to the sea. This collapse involved a substantial amount of unconsolidated talus flanking the pre-July 29 dome; but the actual volume was unknown because clouds prevented observation of the summit region. Small pyroclastic flows also occurred on 2, 4, and 6 December in the upper reaches of White River, originating from the old dome material closest to Chances Peak.

On 31 October and 1 November several small pyroclastic flows were generated by material avalanching off the E flank of the dome. By mid-November, activity had shifted to be mainly concentrated on the W side of the active area. On 2 December pyroclastic flows again originated in several places along the E face of the new lava dome.

A large pyroclastic flow occurred on the night of 14 November; it traveled E and reached the lower parts of the Tar River Valley, stopping a few hundred meters short of the delta. During 1330-1500 on 28 December, several million cubic meters of volcanic material collapsed down the volcano's NE flank, generating a dense W-drifting ash plume that deposited up to a centimeter of ash in the vicinity of Plymouth (~4 km W of the summit).

Seismicity. Weak banded tremor, which indicates rapid magma ascent, began in the early hours of 14 August and continued to strengthen through 22 August. Bands of tremor continued at irregular intervals through mid-November, appearing with periodicities generally ranging between 10 and 27 hours. During these banded-tremor events, rockfall activity and ash venting increased. On 26 August, a particularly vigorous period of ash venting lasted for ~1 hour and sent W-drifting ash up to ~2 km above the volcano. A weak swarm of volcano-tectonic earthquakes (less than M 1) occurred during 29-31 August. During mid-September the bands of tremor occurred about every 13 hours and were slightly more intense when compared with those of the previous week. In addition, the number and strength of hybrid events associated with these tremor episodes increased, which is a pattern consistent with the moderate rate of dome-growth and periods of vigorous degassing.

Continuous low-amplitude tremor was accompanied by increased rockfall activity during 12-14 September. Ash clouds produced from rockfalls rose slightly above the summit and were visible in satellite imagery. Rockfall signals were intense on 9 and 10 November, but then declined significantly and remained low after 12 November. A swarm of hybrid and long-period earthquakes began on 14 November and reached a peak on 21 November, before declining slightly, although the swarm continued to be moderately energetic through the end of the month. An M 3.6 earthquake located just off the NW coast of Montserrat occurred on 29 November at 1248 and was felt throughout the island.

Rockfalls continued through December, and many were preceded by a few seconds of long-period earthquakes. Continuous, weak tremor recorded on 13 December was associated with ash venting, and produced columns that rose to at least 4 km. Periods of intense cyclical rockfalls occurred on 27 December and coincided with weak swarms of hybrid earthquakes. These hybrids were too small to trigger the seismic-event-detection system, and are therefore not included in the count of hybrid earthquakes given in table 39.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

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04/2002 (BGVN 27:04) Rockfalls and pyroclastic flows originate from growing lava dome

During mid-August 2001 through February 2002 at a new lava dome continued to grow at Soufriere Hills. Small-scale dome collapses generated pyroclastic flows almost continuously, with some reaching and entering the sea on several occasions. Dense ash plumes associated with sea entry and ash venting from the summit generally drifted W and reached up to 3 km altitude. Mudflows occurred in the Belham Valley on several days during periods of torrential rainfall (BGVN 27:01). The lava dome continued to grow during February through at least mid-May 2002. Minor episodes of ash venting occurred from the summit of the dome, and at times incandescence was visible. The dome produced numerous rockfalls and small pyroclastic flows in the upper reaches of the Tar River Valley. SO₂ flux rates reached up to 1,200 metric tons per day (table 40).

Table 40. Seismic and SO2-flux data from Soufriere Hills during 1 February-10 May 2002. Courtesy of MVO.

    Date (2002)    Rockfall     LP      LP    Hybrid  VT    SO2 flux
                   signals   rockfalls  EQ's  events  EQ's   (t/d)

    01 Feb-08 Feb     897       64       85     16     --   06 Feb: 160-380
                                                            07 Feb: 665-790
    08 Feb-15 Feb     734       69       83     17      1   09-12 Feb: 150-420
                                                            14 Feb: 350-650
    15 Feb-22 Feb     786       75       74     17     --   16 Feb: 600-780
                                                            19 Feb: 90-130
    22 Feb-01 Mar    1013      124      101      5     --   --
    01 Mar-08 Mar     415       49       56     10     --   60-130
    08 Mar-15 Mar     779       67       92      6     --   40-860
    15 Mar-22 Mar    1002      108      162      3      2   395-1035
    22 Mar-29 Mar     935       80      123      3     --   1100-1200
    12 Apr-19 Apr     841       52       65      6     --   ~1200
    19 Apr-26 Apr     990       66      114     31      1   ~1200
    26 Apr-03 May     741       33       76     42      2   ~600
    03 May-10 May     557       40       82     13     --   --

During flights on 4, 5, and 6 February new pyroclastic-flow deposits were observed in the Tar River to the E (with some flows reaching the sea) and in the White River to the S, derived from the collapse of remnant talus material from the pre-29 July 2001 dome (BGVN 26:07). An observation flight on 14 February revealed minor rockfalls of old, inactive dome material in the upper part of the Gages region. Near-continuous rockfalls and minor pyroclastic flows occurred on the E flank. Minor rockfalls on the N flank of the active dome cascaded between the NE and central buttresses of the older inactive dome.

Activity increased beginning on the evening of 8 March. Small ash clouds (reaching ~2.1 km) arising from small collapses drifted to the W over the Plymouth and Richmond Hill area, although most of the ash fallout occurred over the sea. For a couple days during late March weak winds dispersed the ash towards the NW and N, depositing it over the main populated areas. Large spines on the dome during mid-March periodically collapsed, producing pyroclastic flows down the E flank, some of which reached the Tar River Fan. By late March minor amounts of rockfall debris from the NE flank of the dome had begun to spill into the head of Tuit's Ghaut. Ash venting appeared to have been from a pit-like depression on the summit of the dome.

Increased rockfall and pyroclastic-flow activity over the E flank of the dome coincided with periods of tremor during late April. Small, low-level ash clouds were occasionally visible on satellite imagery. Rockfalls traveled down the SE flank of the dome almost continuously. By early May rockfall talus had begun to spill over the rim of the 29 July 2001 collapse-scar in the extreme SE at the foot of Roches Mountain. Pyroclastic flows on the mornings of 1 and 2 May were the most energetic seismic events recorded for over a month. Activity increased beginning on 8 May, and rockfalls and pyroclastic flows were concentrated on the dome's NE flank.

MVO reported that weather permitting, the daytime entry zone (DTEZ) would remain open. The observatory warned that activity could increase quite suddenly, with a dangerous situation developing in the DTEZ very quickly, and that ash masks should be worn in ashy conditions. The Belham Valley was to be avoided during and after heavy rainfall due to the possibility of mudflows. Access to Plymouth, Bramble airport, and beyond was prohibited. In addition, a maritime exclusion zone around the S part of the island extends two miles beyond the coastline from Trant's Bay in the E to Garibaldi Hill on the W coast.

Seismicity and SO₂ flux. Since 4 February SO₂ measurements were carried out using a remote, telemetered Differential Optical Absorption Spectrometer (DOAS) that scans through the plume, yielding over 600 measurements of SO₂ emission rates per day. The highest SO₂ fluxes were measured after pyroclastic flows. SO₂ emission rates decreased dramatically during early March (table 40).

A swarm of hybrid earthquakes on 22 April was followed by increased numbers of long-period events and a surge in the number of rockfalls over the next four days. Banded tremor also followed the swarm. Weak periods of tremor occurred approximately every 20 hours during 26 April-3 May, and each lasted a few hours. Fluctuations in SO₂ emission rates in late April appeared to reflect variations in the intensity of rockfall activity.

Morphology of the lava dome. During early February the lava dome continued to grow primarily on the E and NE sides, and by late February growth was focused on the E side. The summit of the dome was blocky and massive, in contrast to the spines of previous weeks. On 19 February the dome was crowned by a large spine inclined steeply up towards the SE. The spine changed in size and shape, as it periodically collapsed or disintegrated and grew again as fresh material was extruded. On 26 February the spine had a height of 90 m above the general level of the summit area. At this stage the top of the spine had an elevation of 1,080 m, the highest point measured during the eruption to date.

Observations in early March revealed that the summit of the dome had a generally spiny appearance and on several occasions was crowned by a large spine directed upwards at a high angle towards the E. During mid-March the summit of the dome was dominated by fast-growing large spines (50-70 m high). Theodolite measurements of the dome taken on 20 March yielded a dome height of 1,039 m.

During mid-April, dome growth shifted to the SE area of the dome complex, although small rockfalls occurred in other areas. The summit area had evolved from a large striated lobe to a series of small spines. By late April the lobe on the SE portion of the dome had reached 1,041 m elevation and the NE lobe, which had been highly active during the previous two weeks, stagnated at a height of 1,020 m elevation. Lava dome growth continued on the E side of the dome complex during early May.

The closest GPS station to the dome showed sustained outward movement of ~0.5 cm per month. During periods of dome building, slow subsidence took place at the closest sites at Hermitage, Whites, and Harris. Since January, the EDM reflector on the N flank showed a 5-cm movement away from the lava dome.

Hazard assessment. On 11 March 2002 the Montserrat Volcano Observatory (MVO) issued the following preliminary statement concerning the history and hazard assessment of the current eruption: "The Soufrière Hills Volcano continues its second phase of sustained dome growth, which began in November 1999. Since September 2001, the dome has grown at an average rate of about 2 m³/s (or 400,000 metric tons per day). The summit region of the dome has now reached an altitude of ~990 m, having filled most of the depression formed by the large dome collapse of 29 July 2001. The dome has mainly grown towards the E, although there was a period during late November and early December 2001 when growth was directed W.

"During [September 2001 to March 2002] there have been fluctuations in activity as recorded in seismicity and gas emissions. Pyroclastic flows and almost continuous rockfalls have occurred, mostly directed down the Tar River Valley. For prolonged periods in the last six months, there have been cyclical patterns of enhanced seismicity lasting for a few hours to about a day, during which rockfall and pyroclastic-flow activity has been more intense.

"Continued growth of the dome over this period has meant that hazard levels close to the volcano have increased slightly compared with . . . September 2001. Risk levels will fluctuate as the configuration of the dome changes. In an extreme scenario, a switch in the direction of growth to the N or NW could result in more hazardous conditions along the margins of the Exclusion Zone. Consequently, increased levels of risk might develop in the populated areas bordering the Belham River. Across the remainder of the island, however, it is considered that the general level of risk to the population from volcanic activity is unchanged.

"The main hazards remain pyroclastic flows, explosions, falls of ash and small stones, and volcanic mudflows. The increasing knowledge of the volcano acquired by the experienced observatory staff allows patterns of eruption behavior to be recognized and some forms of activity to be anticipated. During a large dome collapse or explosion, heavy ashfall and the fall of small rock fragments can be expected in the populated areas if the wind is in an unfavorable direction. However, a detailed study of the hazard due to fall of rock fragments has recently been completed, and this indicates that outside the Exclusion Zone significant falls of rock fragments large enough to cause serious injury are unlikely.

"At the moment there is no sign of the volcanic activity diminishing. It is most likely that the eruption will continue for a number of years, although the volcano may be evolving into a persistently active state with the eruption continuing for even longer periods, either continuously or intermittently."

General References. Baker, P.E., 1985, Volcanic hazards on St. Kitts and Montserrat, West Indies: Journal of the Geological Society, London, v. 142, p. 279-295.

Shepherd, J.B, Tomblin, J.F., and Woo, D.A., 1971, Volcano-seismic crisis in Montserrat, West Indies, 1966-67: Bulletin of Volcanology, v. 35, p. 143-163.

Wadge, G., and Isaacs, M.C., 1988, Mapping the volcanic hazards from Soufriere Hills volcano, Montserrat, West Indies using an image processor: Journal of the Geological Society, London, v. 145, p. 541-551.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

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06/2002 (BGVN 27:06) During 19-29 February large spines and plumes occurred at tidal maxima

Stephen O'Meara and four Volcano Watch International (VWI) team members (Robert Benward, Tippy D'Auria, Scott Ireland, and Larry Mitchell) visually monitored Soufrière Hills for 10 days beginning on 19 February 2002. The observations took place on Jack Boy Hill, a spot at ~180 m elevation 6 km N of the volcano. In addition, for 3 hours on the night of 25 February, the group joined Montserrat Volcano Observatory (MVO) scientists Peter Dunkley and Richard Herd on the runway at Bramble Airport. Except for a storm on 20 February, the weather facilitated exceptionally clear views of the dome during both day and night. The team employed a variety of telescopes and other optical equipment and had an interest in astronomy as well as the volcano (O'Meara, 2002).

Benward brought along a homemade night-vision scope (near-infrared image intensifier) that captured images of the dome, even through local atmospheric conditions where visible light was weakened or scattered. The intensifier was coupled to camera lenses. It could be used visually or attached to a video camera (figure 47). The camera's phosphor viewing screen yielded green-colored images of the hot portions of the dome.

One purpose of the VWI team's visit to Montserrat was to chronicle changes in the volcano's visible behavior with approach to the time of the full Moon and its perigee (when the Moon is closest to the Earth). The idea was that the tidal influence associated with the full Moon and its perigee might lead to enhanced activity. With approach of the full Moon, there did seem to be a rise in visible indicators, particularly plume height, a strong pulse of extruded spines, and less-substantial increases in the numbers of rockfalls and pyroclastic flows.

As background on tidal forces, the paths of both the Moon around the Earth, and Earth around the Sun are elliptical throughout the lunar cycle (29.53 days) and solar cycle (the year), meaning that the separations and resulting gravitational forces vary with time. The Earth-Moon separations change by ~50,000 km; when they are smallest (perigee) and largest (apogee) the respective tidal forces are higher or lower than usual. In addition, the gravitational attractions of Moon and Sun on the Earth may act along a common line or at changing angles relative to each other. Particularly large tides affect the Earth's crust and oceans when the Sun and the Moon are lined up with the Earth; this occurs at the new and full phases of the Moon. These orientations lead to what are called spring tides (a name not associated with the season of Spring, but which implies a "welling up"). The amount of tidal enhancement is roughly the same whether the Sun and Moon are lined up on opposite sides of the Earth (full Moon) or on the same side of the Earth (new Moon). In contrast, when the Moon is at first quarter or last quarter (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon produce tidal bulges called neap tides. These are generally weaker than the above-described spring tides.

A two-month record of seismicity and tides at the volcanically active Axial seamount on the Juan de Fuca ridge during 1994 found both bi-weekly and diurnal patterns in earthquakes and volcanic tremor (Tolstoy and others, 2002). The authors concluded that microearthquakes took place at tidal minima.

Montserrat, Moon, and magma. The full Moon occurred at 0518 on 27 February; perigee, ~11 hours later, at 1630. The team's 10-day stay was too short to see more than a partial lunar cycle, but soon after full Moon and perigee, the numbers for the observed visible indicators appeared to drop considerably.

After an initial study of dome activity on 19 February and a storm on 20 February, the group began taking regular visible observations on 21 February. At that time, activity appeared to be on the increase and a high-level of activity was sustained throughout the observation period. According to MVO: "The level of volcanism at Soufrière Hills during 22 February-1 March was higher than it had been in previous weeks." The growing dome was quite active, displaying near continuous rockfall and small pyroclastic flows, most of which traveled E to the Tar River Valley, though some activity was directed to the S and W. During the 10-day observation interval, the dome also rapidly extruded very large spines.

By midnight on 27 February the team had recorded and tabulated 440 observations of notable rockfalls and pyroclastic flows. On the whole during this interval, the number of these events per hour stood well below 10, typically ranging from ~4 to ~8. One low, late on 23 February, only reached 1 event per hour. The average number of these events per hour reached a low of ~5 during 21-23 February rising to ~8 on 27 February. The highest hourly total recorded during the observing period occurred on 27 February with 13 of these events during 0000-0100 and 10 during 1120-1320. These times fall on either side of the full Moon; the second total lies at the midpoint between the full Moon and lunar perigee.

Visible activity decreased sharply on 28 February. The team, which departed on 3 March, made sporadic observations until 1 March. Their observations on and after 28 February suggested dome activity had remained substantially lower than during 21-27 February.

During their interval of observation the team found a direct correlation between the number of large visible events and the size of the dome's emerging and collapsing spines. The mass of each spine also increased during the observation period; the largest spine was observed on 26 February, the day before the full Moon and perigee.

Each of the spines collapsed in less than a day, only to regrow rapidly. The largest (shown on figures 48-53) reached 90 m tall; it enabled the summit to attain 1,080-m elevation, the highest the summit has been during the entire eruption to date (according to the MVO weekly update). It grew rapidly; specifically, it was not present from 1830 to 2100 on the evening of 25 February, but was fully grown by 0600 the following morning. When seen at 0330 on 26 February the new spine appeared as an incandescent obelisk about one-fifth its maximum size. The majority of this massive spine then grew to its record height in 3 hours.

Figure 54 is one of several plots constructed to illustrate the results. It was made by omitting the smaller events, which the team judged from small to medium using a qualitative visual scale that ran from S1 to S₃ and continued upwards from M1 to M₃ (where event sizes are abbreviated as S for "small" and M for "medium" and termed as S-class or M-class, respectively). Thus, the largest events seen were M₃ (i.e., they saw no events in these time periods that they classified as "large"). On their scale, events of size S₃ and M1 were judged to be of very similar magnitude. Figure 54 shows the increase in larger event size seen during 21-26 February, culminating in the highest numbers late on 26 February to early on 27 February.

Figure 54 shows six high tides that occurred at times when observations were conducted (on 21, 22, 24, 26 and 27 February). Five of the six of these tides coincided with observation intervals with the day's highest number of the largest events (the M-class events).

Plume height. As shown on figure 55, an increase in plume height took place around the time of first quarter Moon followed by a decrease, then a gradual rise in plume height, until it reached a maximum at the time of perigee on 27 February. Although atmospheric conditions could clearly affect the extent and height of a plume, the team found the pattern of the plotted data compelling. The plot may disclose tidal effects.

References. O'Meara, S., 2002, Firelight nights: Stargazing from the Caribbean's Emerald Isle; A group of American amateur astronomers helps residents of Montserrat and its neighboring island explore the universe: Sky & Telescope, August 2002, p. 79-83.

Tolstoy, M., Vernon, F.L., Orcutt, J.A., and Wyatt, F.K., 2002, Breathing of the seafloor, tidal correlations of seismicity at Axial Volcano: Geological Society of America (GSA), Geology, v. 30, no. 6, p. 503-506.

Information Contacts: Steve and Donna O'Meara, Robert Benward, Tippy D'Auria, Scott Ireland, and Larry Mitchell, Volcano Watch International, PO Box 218, Volcano, Hawaii 96785, (Email: someara@interpac.net, donna@post.harvard.edu).

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09/2002 (BGVN 27:09) Mid-to-late 2002 dome growth and the start of NE-traveling pyroclastic flows

The Montserrat Volcano Observatory (MVO) reported that during mid-May through mid-September 2002, seismicity at Soufrière Hills was dominated by rockfall signals. Four volcano-tectonic (VT) earthquakes were reported during the first week of June and nine during the week of 9-16 August. SO₂ emission rates were measured using Differential Optical Absorption Spectrometers (DOAS). SO₂ fluxes generally remained at moderate levels. High fluxes occurred at times, such as during rockfall activity on 12 August (up to 690 t/day). On 6 September SO₂ emissions were low at 42-170 t/day, although levels increased to 170-518 t/day through 13 September (table 41).

Table 41. Seismicity at Soufrière Hills during 10 May-13 September 2002. "NR" indicates that the information was not reported. Courtesy MVO.

    Date (2002)    Rockfall    LP       LP      Hybrid  SO2 flux
                   signals   events  rockfalls  events  (tons/day)

    10 May-17 May    553      127        99        5       NR
    17 May-24 May    532       77       111        1       NR
    24 May-31 May    497       57        93        6       NR
    31 May-07 Jun    129       20         4        6       NR
    07 Jun-14 Jun    135       20         3       12     247-955
    14 Jun-21 Jun    226       14        10       17    14-15 Jun: ~170-520;
                                                        16-17 Jun: ~90-350;
                                                        19 Jun: ~600-690;
                                                        20-21 Jun: ~90-350
    21 Jun-28 Jun    102        6         2       19    22-23 Jun: ~170-520;
                                                        24 Jun: ~90-260;
                                                        25-26 Jun: ~170-350;
                                                        26-28 Jun: ~90-170
    28 Jun-05 Jul     42        6         5       11    NR
    05 Jul-12 Jul    108        6         2       17    10-12 Jul: ~90-260
    12 Jul-19 Jul    151        3         4        8    13-14 Jul: 90;
                                                        15-19 Jul: ~130-220
    19 Jul-26 Jul    250       92        28       15    22-26 Jul: 175-250
    26 Jul-02 Aug    260      118        32        3    ~ 90-270
    02 Aug-09 Aug    313      138        52       23    Max: 690; avg: 380
    09 Aug-16 Aug    209       87         8        5    86-430; 12 Aug: ~690
                                                          during rockfall
                                                          activity
    16 Aug-23 Aug    231       44         5        1    16-18 Aug: 170-340;
                                                        19-23 Aug: 170-600
    23 Aug-30 Aug    287       31         9        0    170-340
    30 Aug-06 Sep    453       63         9        1    170-432
    06 Sep-13 Sep    308       63         2        0    6 Sep: 42-170;
                                                        7-13 Sep: 170-518

During mid-May, growth of the summit lava dome continued to be concentrated on the E flank, giving rise to numerous rockfalls and small pyroclastic flows in the upper reached of the Tar River Valley. Pyroclastic flows were observed moving NE in the uppermost part of Tuitt's Ghaut during an observation flight on the morning of May 13. This was the first indication that pyroclastic flows generated on the NE flank of the active dome were able to flow into this drainage system. This new direction of flow was possible after the 29 July collapse scar had become largely buried on this side of the dome. The summit region of the active dome was visible briefly on several occasions during late May. It had a broad blocky appearance, and growth seemed to have become concentrated on the SE, giving rise to rockfalls and small pyroclastic flows on the SE flank of the dome. There was little activity on the NE flank of the dome during the last week of May.

Very clear conditions during 31 May-3 June provided the first good views of the summit region for several months, revealing that since early April a large lobe had been extruded on the dome's upper SE side. The lobe was ~150 m across and reached 1,023 m altitude. The upper surface of the lobe had a spiny though slab-like appearance. Since the dome was last seen, it had developed a small lobe-like protrusion on the summit's W side. Minor June rockfalls occurred on the dome's E and W sectors.

During mid-June, although the dome was mostly covered by clouds, photos of the summit area were captured on many days by the remote digital camera at White's Yard. Despite the low level of rockfall and seismic activity, the massive extrusion lobe on the SE side of the dome continued to grow steadily. Most of the upper surface of the active lobe had the smooth form of a whale's back; it also contained a low-angle spine directed upwards towards the SE. The free face at the front of the lobe on the SE side was steep and blocky in appearance. A theodolite survey of the dome taken during a brief period of clear weather on 11 June measured these altitudes: the general summit area of the active lobe stood at 1,025-1,030 m, and the top of the spine, at 1,048 m.

Rockfall activity increased abruptly on the night of 14 June and remained moderately high until the 18th, when it declined once more. Rockfalls and small pyroclastic flows were produced by material collapsing off the E face of the dome. Several small pyroclastic flows were also produced on the NE flank and were observed flowing into the upper part of Tuitt's Ghaut. By late June, growth of the extrusion lobe on the SE side of the dome appeared to have stagnated. Rockfall activity decreased abruptly on the afternoon of 22 June and declined to very low levels during 25-28 June.

No change in dome morphology occurred during early to mid-July. Rockfall activity on the dome increased slightly on the morning of 3 July, and a small, low ash cloud drifted over Plymouth around 1000. This followed several hours of heavy rain during the night, which was associated with substantial mudflows in the center of Plymouth. Rockfalls increased slightly during 6-8 July, before decreasing to very low levels through 12 July.

Observations of the dome on 15 July suggested that dome growth was continuing at a very low rate. Growth was concentrated on the SE part of the dome, at the lobe that was active during mid- to late June. The level of rockfall activity from this active lobe increased slightly on 15 July, with a small pyroclastic flow at 0800 directed down the Tar River Valley.

A swarm of low-amplitude long-period (LP) earthquakes began on 19 July and increased in strength during the following four days. The swarm continued at an elevated level until it began to decrease slightly during 31 July-2 August.

Observations of the dome on 21 July indicated that significant growth had recommenced, with the extrusion of a new lobe on the NE side of the summit region. Growth of the new extrusion lobe gave rise to rockfalls and small pyroclastic flows off the NE flank of the dome. On the morning of 23 July a minor collapse produced small but continuous pyroclastic flows for about an hour. These mainly flowed into the upper parts of Tuitt's Ghaut and down White's Ghaut for about half the distance to the coast. A few also flowed into the upper part of the Tar River Valley. A similar event, lasting for ~20 minutes, occurred in the early hours on the morning of 26 July.

On the morning of 1 August observations revealed that the new extrusion lobe on the N side of the summit had a broad whaleback form. Growth of this lobe was directed N and, around 2-4 August, the lobe crumbled repeatedly, producing rockfalls and small pyroclastic flows in Tuitt's Ghaut. Limited activity occurred on the NW part of the dome, although one small pyroclastic flow descended the notch between the central and NW buttresses. Individual rocks also reached upper Tyre's Ghaut (behind Gage's Mountain). During 6-9 August, rockfall activity declined substantially due to the lobe becoming more coherent and not collapsing. By mid-August, talus had accumulated in the upper reaches of Tuitt's Ghaut and small pyroclastic flows occurred in both Tuitt's and White's Ghauts. The active lobe also shed more talus into the notch in the NW sector of the old dome, which leads towards Tyre's Ghaut.

Rockfall talus continued to accumulate in the upper reaches of Tuitt's Ghaut during 16-23 August, and there were overspills of talus from the N side of the Tar River Valley into the two tributaries of White's Ghaut. The NE buttress, a remnant of the old dome complex from mid-1997, was now completely buried. Erosion of the E edge of the central buttress continued. Talus continued to slowly accumulate in the notch in the NW sector of the old dome, which leads towards Tyre's Ghaut. During intense rainfall early on 21 August, a small collapse occurred in the Tar River Valley of the talus that had accumulated on the SE sector of the dome during April-May 2002.

During late August, small pyroclastic flows were mainly concentrated on the NE flank where they had been channeled into the upper reaches of Tuitt's Ghaut; although some had spilled eastwards along the N side of the Tar River Valley. Talus also continued to accumulate in the notch in the NW sector of the old dome, which leads towards Tyre's Ghaut. Torrential rainfall produced mudflows in the Belham Valley in the early hours of 28 August.

During early September, growth continued to be focused on the N side of the dome complex although it had become more centralized and the summit height now exceeded 1,050 m. Otherwise the focus of activity remained concentrated on the NE flank, with frequent rockfalls and small pyroclastic flows. Most of these were channeled into the upper reaches of Tuitt's Ghaut; although some had spilled eastwards along the N side of the Tar River Valley.

During mid-September, dome growth remained centralized, and the summit height exceeded 1,050 m. Otherwise the focus of activity remained concentrated on the E flank, with frequent rockfalls and small pyroclastic flows. Around 6-8 September most of these spilled eastwards along the N side of the Tar River Valley, although by 12-13 September activity appears to have refocused northwards onto Tuitt's Ghaut, with subordinate amounts continuing to spill eastwards into the Tar River Valley.

During the reporting interval, the daytime entry zone (DTEZ) remained open, weather permitting. MVO warned that activity could increase suddenly, with dangerous situations developing quickly. Protective masks were to be worn in ashy conditions and the Belham Valley was to be avoided during and after heavy rainfall due to the possibility of mudflows. Access was prohibited to Plymouth, Bramble airport, and points closer to the volcano; including a marine exclusion zone around the southern part of the island ~3 km beyond the coastline, extending from Trant's Bay in the E to Garibaldi Hill on the W.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

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02/2003 (BGVN 28:02) Continued dome growth, rockfalls, and pyroclastic flows

During mid-September 2002 through February 2003 at Soufrière Hills, the dome continued to grow, producing numerous rockfalls and small-to-moderate pyroclastic flows. Most of the activity was concentrated on the NE and N flanks, producing numerous pyroclastic flows in White's Ghaut, the Tar River Valley, and Tuitt's Ghaut. Pyroclastic flows and rockfalls also traveled down the W and NW flanks. Ashfall affected surrounding areas, accumulating in thicknesses up to 9 mm. The Washington VAAC issued notices to the aviation community almost daily. Seismicity was dominated by rockfalls (table 42).

Table 42. Seismicity at Soufrière Hills during 13 September 2002-28 February 2003. *During some weeks, the number of seismic events was under-represented because of problems with the seismic stations. Courtesy MVO.

    Date (2002-2003)    Rockfall  Hybrid  Long-period     LP      Volcano
                        signals   events  (LP) events  rockfalls  -tectonic
                                                                  (VT) events

    13 Sep-20 Sep 2002    689       67        162         41          1
    20 Sep-27 Sep 2002    680       36        260         55          0
    27 Sep-04 Oct 2002    811       15        223         51          2
    04 Oct-11 Oct 2002*   468        3         77         42          0
    11 Oct-18 Oct 2002*   650        2         98         80          1
    18 Oct-25 Oct 2002    536        6        120         27          1
    25 Oct-01 Nov 2002    670        9        148         72          0
    01 Nov-08 Nov 2002    694        3         60         38          0
    08 Nov-15 Nov 2002*   409        0         29          8          1
    15 Nov-22 Nov 2002    592        2         88         37          1
    22 Nov-29 Nov 2002    586        0         44         32          0
    29 Nov-06 Dec 2002    354        0         33         43          0
    06 Dec-13 Dec 2002    427        6         47         30          0
    13 Dec-20 Dec 2002    742        2         50         50          0
    20 Dec-27 Dec 2002    760        5         45         30          0
    27 Dec-03 Jan 2003    863        3         86         41          1
    03 Jan-10 Jan 2003    789        0        120         54          0
    10 Jan-17 Jan 2003    606        7         67         42          2
    17 Jan-24 Jan 2003    566        0         58         24          1
    24 Jan-31 Jan 2003    745        2        177         62          1
    31 Jan-07 Feb 2003    882        6        148        114          0
    07 Feb-14 Feb 2003    840        3        117         78          1
    14 Feb-21 Feb 2003    905        8         87         80          1
    21 Feb-28 Feb 2003   1078        1         92         85          0

Activity during September 2002. Lava-dome growth was directed to the NE during 13-20 September, with frequent rockfalls and small pyroclastic flows sending material to a sector extending from the central Tar River Valley on the E flank to the NE flanks above Tuitt's Ghaut. Some material tumbled through a notch onto the N flank. A major change in direction of extrusion followed a hybrid earthquake swarm between 0703 and 1515 on 19 September. Growth of the previously active NE lobe stagnated during 21-22 September. A near-vertical spine was extruded in the central area around the 21st, possibly indicating a switch in growth direction. On 26 September a swarm of 36 hybrid events occurred between 0330 and 1112. The same day observations revealed a large new dome lobe that had extruded towards the W in the area previously known as Gages Wall. Material spalling off of this lobe produced rockfalls and small pyroclastic flows down Gages Valley that reached up to 1 km.

Notable pyroclastic flows occurred on the evening of 25 September and the morning of the 27th. Growth and rockfall activity then changed towards the N flanks, suggesting a possible stagnation of the recently extruded western lobe. Spectacular incandescence and semi-continuous rockfall activity were observed on the NE and N flanks of the dome on the night of 26-27 September.

On 27 September a 4-hour-period of heightened activity occurred in the afternoon and evening, with small semi-continuous pyroclastic flows traveling down the N flanks and eastwards into the upper portions of Tuitt's Ghaut and then into White's Bottom Ghaut. A newly extruded lobe was visible on 28 September almost directly to the NW with a broad headwall over the N, NW, and W flanks. On the evening of 29 September there was another period of heightened activity on the N flanks that lasted 1.5 hours, with pyroclastic flows just reaching the sea along White's Bottom Ghaut. It was estimated that during this event only 2-3 x 10⁶ m³ of the N edge of the active NW lobe was shed.

The Washington VAAC reported that a low-level ash cloud from an emission at 1510 on 29 September was visible over eastern Puerto Rico on satellite imagery through the following day. On 30 September a light dusting of white ash fell in eastern Puerto Rico at Roosevelt Roads Naval Air Station.

Activity during October 2002. Observations on 1 October revealed that re-growth of the collapsed area had occurred. A brief period of heavy rain on 2 October triggered a moderate-sized mudflow down the Belham Valley. Analysis of seismic data suggested that pyroclastic-flow activity on 2 October began at 1310, and sustained dome collapse continued for 6 hours. Low-energy pyroclastic flows were observed reaching the sea on the Tar River's flanks throughout the collapse, and ash clouds were produced that drifted to the NW. Heavy ashfall occurred in the residential areas of Salem, Old Towne, and Olveston, with deposits up to 9 mm thick. Subsequent observations revealed that this collapse was confined to the E flanks, and that this was again a relatively small event (less than 5 x 10⁶m³ of material was shed off of the E side of the dome complex).

According to the Washington VAAC, after daybreak on 3 October there were several reports of ashfall in Puerto Rico, and visible satellite imagery at 1115 confirmed that an ash cloud around 2.4 km altitude covered most of the island. At 1615 the area of very thin ash was not visible on satellite imagery. By the next day, ash from the previous day's emissions had drifted W, and around 0902 it was located over southern Puerto Rico. A thin plume of ash also extended SSW of St. Croix island.

Early in October the NW extrusion lobe of the lava dome grew to the NW, but later growth remained more centralized and there was noticeable bulking up of the lobe's summit area. Talus continued to accumulate behind the NW buttress and in the head of Tyre's Ghaut. Minor mudflow activity occurred on 9 October. The growth of the lava dome towards the NW prompted the evacuation of populated areas along the fringes of the lower part of the Belham Valley (~300 people) on 8 and 9 October, and the area was declared part of the Exclusion Zone. A relatively small pyroclastic flow traveled NNE down the flanks on 13 October.

On the afternoon of 22 October intense rainfall at midday produced large mudflows NW in the Belham Valley. At the peak of flow, the entire width of the valley floor at Belham Bridge was flooded and standing waves up to 2.5 m high were observed. By 1430, pyroclastic-flow activity began. For several hours, pyroclastic flows from the N flank of the dome were channeled NE into the upper parts of Tuitt's Ghaut, from where they crossed over into White's Bottom Ghaut. Flows also occurred on the dome's E flank in the Tar River Valley.

The volcano was observed using a remote camera and during a flight on 31 October. The active extruded lobe in the NW continued to steadily grow, bulking out on the N and W sides. Rockfalls and pyroclastic flows traveled down the E and N flanks, particularly within Tuitt's Ghaut and the Tar River Valley. A considerable amount of debris also spalled off the W flank of the active extruded lobe and accumulated in the upper parts of Fort Ghaut.

Activity during November 2002. During early November lava-dome growth on the N part of the dome was less directed, with rockfalls dispersed over the summit and flanks. The lobe shed rockfall debris predominately down Tuitt's Ghaut and Tar River Valley, although also onto the NW flank and into the top of Gage's Valley. According to the Washington VAAC, on 8 November strong pyroclastic flows produced ash-and-gas clouds to a height of ~1.5 km.

On 8 and 9 November pyroclastic flows traveled 900-1,000 m NW into Tyer's Ghaut at the headwaters of the Belham Valley. During 12-15 November, the size and energy of the pyroclastic flows increased slightly. During 15-19 November, small pyroclastic flows traveled 1-1.5 km from the dome every few hours in Tuitt's Ghaut to the NE and in the Tar River Valley to the E. On 29 November the active lobe had a broad whaleback-shaped upper surface, which was oriented towards the NNE.

During 29 November-6 December a number of small, short-lived spines formed at the base of the active lobe in the N part of the dome complex, shedding material E into White's Ghaut and the Tar River Valley. Lava blocks continued to spall off the front of the lobe, shedding material NE into Tuitt's Ghaut and onto the northern talus slope. An average of one moderate-sized pyroclastic flow occurred per day and traveled no farther than 1-1.5 km from the lava dome into Tuitt's and White's ghauts and into the Tar River Valley. During 5-6 December, rockfalls and small pyroclastic flows occurred more frequently on the northern talus slope and on the NW, at the top of Tyer's Ghaut.

Activity during December 2002. A sustained dome collapse began on 8 December at 2045, producing energetic pyroclastic flows down White's Ghaut to the sea at Spanish Point. Ash clouds rose to ~3 km altitude and drifted WNW. In Plymouth and Richmond Hill 4 mm of ash was deposited. Seismicity returned to background levels on 9 December by 0045, and several days of weak tremor occurred.

The collapse scar on the dome's NNE flank, estimated to have had a volume of 4-5 x 10⁶ m³, was being filled rapidly with freshly extruded lava. Observations on 13 December revealed a large amount of fragmental lava extruded in a northerly direction on the summit. A large spine was also extruded on the NW side of the summit.

During late December spectacular incandescence of the dome was observed on most nights. Activity increased during 18-20 December, and on 19 December mudflows occurred in White River, Tar River Valley, and Fort Ghaut. During 20-27 December extrusion occurred on the N, and occassionally NW, sides of the summit. A large spine was pushed up at the back of the active extruded lobe during the night of 26-27 December, but was not visible by 2 January. The Washington VAAC reported that on 28 December around 1130 a 3-km-high ash cloud generated from pyroclastic flows drifted over the islands of St. Kitts and Nevis.

Activity during January-February 2003. Activity escalated to very high levels on the night of 27 December. During 27 December-10 January continuous rockfalls and numerous pyroclastic flows spalled off the active extruded lobe on the NNE side of the lava dome. Activity decreased on the night of 2 January to moderate levels on the 3rd.

During mid-January, activity generally declined to a moderate level. During 15-17 January almost all pyroclastic flows occurred in the Tar River Valley, with only minor rockfalls traveling down the dome's NE and N sides. Lava extrusion occurred NE of the lava-dome complex that was associated with rockfalls and small pyroclastic flows down Tar River Valley, White's Ghaut, Tuitt's Ghaut, and on the northern talus slopes. On 18, 20, and 24 January small pyroclastic flows traveled ~1 km down Tyer's Ghaut.

Activity increased during late January. Growth of the active extrusion lobe continued on the N side of the lava dome. The direction of growth was generally towards the NNE, although the focus of rockfall and pyroclastic-flow activity varied from day to day. A pulse of activity occurred at midday on 30 January, during which pyroclastic flows simultaneously descended several flanks of the lava dome traveling to the Tar River Valley, White's Ghaut, Tuitt's Ghaut, and W to Fort Ghaut.

During 31 January-14 February activity remained moderate. Growth of the lava dome was focused on a large, steep lobe directed to the NE. A small amount of rockfall material was directed W towards Fort Ghaut. Rockfalls and small pyroclastic flows also occurred off the N flank of the dome onto the area of Riley's Estate.

During 19-25 February pyroclastic flows and rockfalls were concentrated more on the E flank of the lava dome and in the Tar River Valley, although there were several periods of activity on the N flank, with pyroclastic flows in Tuitt's Ghaut and at the top of Farrell's Plain.

Activity increased slightly during 21-28 February. During an observation flight on 27 February lava-dome growth was concentrated towards the NE. Pyroclastic flows and rockfalls traveled down the lava dome's E and NE flanks via the Tar River Valley and Tuitt's Ghaut. There were also several periods of activity on the N flank, with pyroclastic flows at the top of Farrell's Plain.

SO₂ emission rates varied throughout the report period (table 43), and were especially high following the dome-collapse event on 9 December (2,350 tons per day average).

Table 43. SO₂ emission rates at Soufrière Hills during 13 September 2002 through 28 February 2003. Courtesy MVO.

    Date (2002-2003)      SO2 emissions (tons/day)

    13 Sep-20 Sep 2002    85-518
    11 Oct-12 Oct 2002    260-520, average of 302
    13 Oct 2002           430-860, average of 691
    16 Oct 2002           43-173
    17 Oct-18 Oct 2002    346-518
    19 Oct-21 Oct 2002    85-300
    23 Oct-25 Oct 2002    430-500, peak of 1000
    25 Oct-27 Oct 2002    45-260
    27 Oct 2002           520
    27 Oct-01 Nov 2002    25-260
    01 Nov 2002           240
    02 Nov 2002           208
    03 Nov 2002           200
    04 Nov 2002           508
    06 Nov-07 Nov 2002    220
    08 Nov-15 Nov 2002    520-560
    15 Nov 2002           160
    16 Nov 2002           340
    17 Nov 2002           380
    18 Nov 2002           180
    19 Nov 2002           173
    22 Nov-29 Nov 2002    520-1040
    24 Nov 2002           170-350
    29 Nov-06 Dec 2002    Average 400
    29 Nov-01 Dec 2002    Average 280
    06 Dec-08 Dec 2002    280
    09 Dec 2002           Average 2,350
    10 Dec 2002           620
    06 Jan 2003           130
    07 Jan 2003           200
    09 Jan 2003           430
    10-17 Jan 2003        ~86-1209
    10 Jan 2003           ~170-520, average ~260
    11 Jan 2003           Emissions of ~430 were recorded until mid-morning,
                            but then decreased to ~86 for several hours. In
                            the afternoon they reached ~860-1210 before
                            dropping  to ~430-518
    12 Jan 2003           ~345-605, average ~354
    13 Jan 2003           ~430-780, average ~490
    15 Jan 2003           ~430-605, average ~527
    18 Jan 2003           300
    19 Jan 2003           165
    20 Jan 2003           700
    21 Jan-24 Jan 2003    270
    24 Jan 2003           480
    25 Jan-28 Jan 2003    290
    29 Jan 2003           560
    30 Jan 2003           620
    31 Jan-07 Feb 2003    90-170
    14 Feb-21 Feb 2003    170-350
    21 Feb-28 Feb 2003    400-460
    22 Feb 2003           840
    23 Feb 2003           1120

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Associated Press.

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04/2003 (BGVN 28:04) Continued dome growth, rockfalls, and pyroclastic flows

During 1 March through 2 May 2003, the dome continued to grow, producing numerous rockfalls and moderate pyroclastic flows. Most activity was concentrated on the northern flanks, producing numerous pyroclastic flows in White's Ghaut, the Tar River Valley, and Tuitt's Ghaut. Pyroclastic flows and rockfalls traveled down all flanks of the dome at some time during the period. On 20 March, the greatest dome height recorded to date was measured, 1,098 m. A prominent extrusive lobe was established on the E and SE sides of the summit at the beginning of April. On 22 April, a large spine, inclined to the E, was observed on the summit, the top of which was at an elevation of 1,163 m.

The Washington VAAC issued notices daily to the aviation community regarding ash clouds emanating from the summit. Seismicity during the report period was dominated by rockfalls (table 44). Average daily SO₂ emission rates varied throughout the report period (table 45) with a low of 31 tons/day on 25 March to a maximum of 1,550 tons/day on 1 May.

Table 44. Summary of weekly seismicity at Soufrière Hills during 28 February 2003-2 May 2003. Courtesy MVO.

    Date (2003)     Rockfall   Hybrid   Long-period   Long-period   Volcano
                    signals    events   (LP) events    rockfalls    tectonic
                                                                   (VT) events

    28 Feb-07 Mar     997        0           79            71           4
    07 Mar-14 Mar    1050        5           87           108           0
    14 Mar-21 Mar    1050        2           93           152           2
    21 Mar-28 Mar    1097       16           99           138           7
    28 Mar-04 Apr     754        7           74           101           2
    04 Apr-11 Apr     332        1           66            77          --
    11 Apr-18 Apr     393        7           72            56          --
    18 Apr-25 Apr     966        4           83            88           1
    26 Apr-02 May     813        4          168           121           1

Table 45. Average daily SO₂ emission rates at Soufrière Hills during 28 February 2003-2 May 2003. Courtesy MVO.

    Date (2003)      SO2 emission (tons/day)

    28 Feb                    1020
    28 Feb-07 Mar         500-1020
    07 Mar-14 Mar         220- 355 
    14 Mar-21 Mar         285- 380
    21 Mar-28 Mar          31- 497
    25 March                    31
    28 Mar-04 Apr         230- 770
    04 Apr-11 Apr         151- 780
    06 Apr                     151
    11 Apr-18 Apr         220- 550
    18 Apr-25 Apr         450- 550
    25 Apr-02 May         390-1550
    01 May                    1550

Throughout the period, access to all areas S of the Belham Valley, to Waterworks, Happy Hill, Lower Friths and Old Towne, and to Bramble airport and beyond was prohibited and a maritime exclusion zone around the S part of the island extended 3.7 km beyond the coastline from Trant's Bay in the E to Lime Kiln Bay on the W coast.

Activity during March 2003. Activity remained at levels similar to that of the previous few weeks (BGVN 28:02), with continued dome growth and moderate pyroclastic-flow activity. Lava extrusion was accompanied by rockfall activity and pyroclastic flows that were focused, during 1-7 March, on the NE and N slopes and valleys. Pyroclastic flows occurred most frequently in Tuitt's Ghaut with a few on Farrell's Plain with run-out distances up to 1 km.

During 8-14 March, rockfalls and pyroclastic flows occurred down all flanks. Dome growth continued and lava extruding into the center of the summit dome complex continued to increase the dome height. Dome glow at night was spectacular in the Tar River Valley and on the NW in Tuitt's Ghaut and the N talus slopes. Small rockfalls and pyroclastic flows occurred infrequently on the W flank and at the top of Gage's Valley. Ash venting was continuous in the summit area.

Lava extrusion during 15-21 March formed a series of spines and ridges. Theodolite measurements on 20 March indicated a dome height of 1,098 m, the highest recorded to date. Activity was dominated by rockfalls and pyroclastic flows mainly in the Tar River Valley, with several small pyroclastic flows in White's and Tuitt's Ghaut and one observed in the upper part of Tyre's Ghaut on 20 March. Ash venting continued.

Dome growth continued through the end of the month. Rockfalls and pyroclastic flows spilled off the active summit in a broad arc extending from the S around the E flanks to the NW. Most activity was towards the NE, with pyroclastic flows in the Tar River Valley and small flows on the N flanks of the dome in White's Ghaut, Tuitt's Ghaut, the upper reaches of Tyre's Ghaut and on Farrell's Plain. Most volcano-tectonic earthquakes (see table 44) occurred in a small swarm late in the evening of 25 March. On the same day, following a brief, intense rainstorm, a 4-5 hour period of increased pyroclastic-flow and rockfall activity occurred on the N and NW flanks of the dome. Observation flights on 27-28 March indicated that rockfalls and small pyroclastic flows were spilling onto the S flanks of the dome.

Activity during April 2003. A prominent extrusive lobe was established on the E and SE sides of the summit at the beginning of April and a large vertical spine, extruded at the back of this lobe on the night of 1-2 April, was the highest point on the dome. During 1-12 April, rockfalls and pyroclastic flows occurred mainly on the E side of the dome in the Tar River Valley. Rockfall activity also continued on the S side of the dome and some pyroclastic flows occurred on the NE flanks in White's Ghaut and Tuitt's Ghaut, and on the NW flank; several of the latter flowed into the upper reaches of Tyre's Ghaut. On 10 April torrential rainfall produced mudflows in the Belham River and triggered pyroclastic flows on the E, N, and NW flanks of the dome.

Helicopter observations during 15 April indicated that the lobe extrusion continued on the ESE side of the dome summit above the Tar River Valley. Vigorous gas venting also was observed on the S side of the summit during this flight. Rockfall and pyroclastic-flow activity occurred throughout the week of 12-18 April on the E and SE sides of the dome with some rockfall activity on the N flanks. On 15 April a small pyroclastic flow occurred in the upper part of Tyre's Ghaut.

On 22 April a large spine was observed on the dome summit, positioned slightly S of the center and inclined at a high angle towards the E. The top of the spine was at an elevation of 1,163 m as compared to the ~1,090 m height of the general summit region of the dome. During 19-25 April, most of the rockfall and pyroclastic-flow activity occurred on the E and SE flank of the dome in the Tar River Valley. A few flows occurred to the NE in White's Ghaut and Tuitt's Ghaut, and to the N and NW onto Farrell's Plain and into the top of Tyre's Ghaut. Observations on 22 April indicated that rockfall debris was starting to spill S into the White River area. On 23 April several large rockfalls were observed on the W side of the dome in the Gages area.

During the last week of April, the prominent spine seen on the summit of the dome the previous week had partly disintegrated. Most of the rockfalls and pyroclastic flows into the Tar River Valley began along the face of the well-developed extrusion lobe present on the ESE side of the summit region. Rockfall debris spilled off the S side of the lobe into the upper reaches of White River, and some flows occurred towards the NE in White's Ghaut and Tuitt's Ghaut, and towards the N and NW on the top of Farrell's Plain and in the top of Tyre's Ghaut. Vigorous pulses of ash-venting occurred on the summit throughout this week.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

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06/2003 (BGVN 28:06) Dome growth, pyroclastic flows, and rockfalls through June

Seismic activity at Soufrière Hills during May-June 2003 was moderate to high, especially during May, and dominated by rockfalls. Most activity was focused on the N and NE flanks of the dome, with rockfalls and pyroclastic flows entering the Tar River Valley and occasionally White's Ghaut. During most of June activity remained, but was at substantially decreased levels. Brief views of the summit in June revealed that the extrusive lobe on the E side persisted.

The Washington VAAC issued daily notices to the aviation community regarding ash clouds that rose to low levels above the summit. Seismicity during the report period was dominated by rockfalls (table 46), particularly during May. Average daily SO₂ emission rates varied throughout the report period (table 47) from 240 to 860 metric tons/day.

Table 46. Summary of weekly seismicity at Soufrière Hills during 2 May-4 July 2003. LP = Long-period; VT = Volcano-tectonic. Courtesy MVO.

    Date (2003)    Rockfall  Hybrid    LP       LP        VT
                   signals   events  events  rockfalls  events

    02 May-09 May    767        7     138       88         2
    09 May-16 May    580        7      65       55        --
    16 May-23 May    774        8      81       75         2
    23 May-30 May    404        1      41       45        --
    30 May-06 Jun    445        5      40       34         1
    06 Jun-13 Jun     79        6      16        8         2
    13 Jun-20 Jun     48       55      --       10        --
    20 Jun-27 Jun     54      135       2        4         1
    27 Jun-04 Jul    193       37       7       61        --

Table 47. Range of average daily SO₂ emission rates measured at Soufrière Hills during 2 May-4 July 2003. Courtesy MVO.

    Date (2003)      SO2 emission (tons/day)

    02 May-09 May           440-850
    09 May-16 May           484-820
    16 May-23 May           300-730
    23 May-30 May           480-860
    30 May-06 Jun           390-560
    05 Jun           Fourier transform infrared
                     spectrometer measurements
                     show HCl:SO2 mass ratio =
                     2.80 in the plume.
    06 Jun-13 Jun           350-520
    13 Jun-20 Jun           295-457
    20 Jun-27 Jun           215-505
    27 Jun-04 Jul           240-840

Throughout the report period, authorities prohibited access to all areas S of the Belham Valley, to Waterworks, Happy Hill, Lower Friths, Old Towne, and to Bramble airport and beyond. A maritime exclusion zone around the S part of the island extended 3.7 km beyond the coastline from Trant's Bay in the E to Lime Kiln Bay on the W coast.

Activity during May 2003. Most of the activity in May was focused on the NE flank of the dome, producing rockfalls and pyroclastic flows in the Tar River Valley and occasionally in White's Ghaut. Brief views of the summit dome on 12 May indicated that the direction of growth had switched towards the NE. On 12-13 May several pyroclastic flows were observed on the N and NW flanks of the dome in the area of Farrell's Plain and in the upper reaches of Tyre's Ghaut. During 16-23 May, rockfalls and pyroclastic flows continued along the N side of the Tar River Valley and White's Ghaut with a number of pyroclastic flows reaching the tops of Farrell's Plain, Tyre's Ghaut, and Tuitt's Ghaut. Pulses of vigorous ash-venting were observed on the summit during clear periods, and intense glow was seen on the summit and NE flanks during the nights of 20-21 May. Clear views of the summit region during an observation flight on 29 May showed that the NE lobe, which had developed over the previous few weeks, was broken up and the summit was irregular and blocky. Lava-dome growth was more centralized, building vertically and accumulating debris in the summit region.

Activity during June 2003. The dome's E and NE flanks continued producing rockfalls and pyroclastic flows into the Tar River Valley, and occasionally White's Ghaut or Tuitt's Ghaut. On the morning of 3 June, a period of increased activity on the NW flank of the dome produced many rockfalls; three pyroclastic flows entered Tyre's Ghaut. Clear views of the summit on 5 June revealed that the active lobe had a well-developed whale-back shape inclined gently upwards towards the E from the summit center. Activity decreased to low levels during the week of 6-13 June and remained low until the last week of the month. Brief views of the summit revealed that the well-developed extrusion lobe on the E side persisted. The focus of activity continued to be on the E and NE flanks of the dome, producing sporadic rockfalls and a few pyroclastic flows in the Tar River Valley, White's Ghaut, and Tuitt's Ghaut. Hybrid earthquakes developed into a diffuse swarm on 22-23 June, with some of the larger events at depths of ~3 km beneath the lava dome. During the last week of June pyroclastic flow and rockfall activity was focused on the N flank with most flows entering Tuitt's Ghaut, and to a lesser extent, Tyre's and White's ghauts. Sporadic flows also occurred in the Gages area on the W side of the dome.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www. mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

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07/2003 (BGVN 28:07) Changes in activity style and dome growth since February 2002

Although detailed reports about the activity and monitoring of Soufrière Hills are provided on a regular basis by the Montserrat Volcano Observatory, this report contains observations made by visitors Stephen O'Meara and Robert Benward. They monitored Soufrière Hills visually and, using some novel electronics, collected data and images for 12 days beginning on 7 February 2003. This visit was similar to one in February 2002 (BGVN 27:06).

The visual observations took place primarily on Jack Boy Hill, 6 km N of the volcano. At the new Montserrat Volcano Observatory, Benward set up a black and white CCD video camera that took a frame every eight (8) seconds and relayed it to a digital video recorder. The camera's low-light sensitivity provided round-the-clock surveillance of dome activity. However, orographic and rain clouds caused problems, and much of the volcanic activity was away from the camera view.

Since the visit in 2002, the dome had increased significantly in size (figure 56). The rockfalls and pyroclastic flows that dominated the activity in February 2002 were concentrated in the E portions of the dome and the Tar River Valley. In 2003, activity occurred in a broader arc that extended from Tar River in the E to Farrell's Plain in the N. Several pyroclastic flows traveled into Tuitt's Ghaut and the upper reaches of Tyre's Ghaut, and onto Farrell's Plain. These events were captured on the surveillance camera and in higher-definition color video taken from Jack Boy Hill.

The dome was impressive at night. The summit was often crowned with thick, blocky spines and sharp pinnacles. An array of spiny ridges (speckled with incandescence) that lined the upper portions of the dome helped channelize many of the rockfalls and pyroclastic flows, the flow channels remaining incandescent. The glow was strong throughout the observation period, but especially during 13-19 February, when episodes of prolonged activity made the dome appear to be melting like candle wax. The glowing dome could be seen from the northernmost reaches of the island at night. Its light was so intense that a homemade spectrograph (attached to a 3-inch telescope brought by Benward) revealed a continuous spectrum.

O'Meara visually observed the dome through a 60 power, 60 mm refractor scope and noticed two curious phenomena. At one point, a mass of viscous, but mobile, lava pushed out of the downslope edge of an incandescent ridge. It slumped onto the dome and formed a pad of molten material that quickly cooled and solidified into linear veins. The behavior was similar to that of a budding toe of pahoehoe lava where internal pressure forces fluid lava through its cooling skin. O'Meara also observed what appeared to be a tiny lateral explosion from the downslope edge of an incandescent ridge which shot out glowing gas and rock fragments like buckshot from a gun.

A significant difference in the style of eruption from that reported in 2002 was the periodic mass dumping of dome material. During these episodes, dome material calved off the highest portions of the dome, creating a wide avalanche of incandescent material which flowed down much of the dome's visible face in a matter of seconds. These episodes differed from the classical pyroclastic flows in that they produced comparatively little ash, being comprised principally of extremely massive and widespread rock and block fall.

A dramatic episode of rockfall and pyroclastic-flow activity occurred during 1745-2000 on 13 February. Massive movement of large, house-sized blocks, many of which self-destructed during their descent, preceded the pyroclastic flows. The subsequent pyroclastic flow activity was accompanied by roiling steel-gray ash clouds that drifted N. One particularly strong pyroclastic flow created an incandescent channel in Tuitt's Ghaut that glowed long into the night. Smaller pyroclastic flows followed this channel downslope, while larger ones overflowed the channel's levees or changed course. Often, when one flow slowed, another would push through it. At times pieces of incandescent rocks appeared to be sliding down the dome in the flow with no detectable rolling motion. At other times, linear threads of glowing gases appeared to advance like the treads of a tank. Another series of pyroclastic flows during 0614-0730 on 14 February were directed N, and spread out across Farrell's Plain. As in February 2002, the night activity was most spectacular when viewed and videotaped in the near-IR using Benward's homemade nightscope.

One purpose of the visit was to chronicle changes in visible behavior when the full Moon approached Earth and at perigee. With the approach of the full Moon, the team reported an apparent rise in the number of visible indicators, particularly an increase in the number of large and prolonged rockfalls and pyroclastic flows, and in the average number of events per hour. There was an impressive episode of spine growth in the 24 hours near the time of full Moon, similar to that in 2002. The limited duration of the observations, however, thwart conclusions about the relationships between lunar positions and volcanism. Convincing theories require baseline data over a considerably longer time period.

Information Contacts: Steve and Donna O'Meara, and Robert Benward, Volcano Watch International, PO Box 218, Volcano, HI 96785, USA (Email: someara@interpac.net).

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08/2003 (BGVN 28:08) Major dome collapse and explosive activity during 12-13 July

Activity at Soufriere Hills has been high over recent months, culminating in the collapse of a major dome and explosive activity during 12-13 July 2003. A summary of reports by the Montserrat Volcano Observatory (MVO) from 27 June to 12 September 2003 is provided below, with sulphur dioxide emissions and activity data (table 48).

Table 48. Summary of activity at Soufriere Hills, 2 May-12 September 2003. Activity occurred as summarized above, with the addition of three explosion signals during 11-18 August. Courtesy of the Montserrat Volcano Observatory.

    Date (2003)    Rockfall   Long-period   Long-period     Hybrid      Volcano-tectonic
                   signals    rockfalls     earthquakes   earthquakes      earthquake

    02 May-09 May    767          88            138            7                2
    09 May-16 May    580          65             55            7               --
    16 May-23 May    774          75             81            8                2
    30 May-06 Jun    445          34             40            5                1
    06 Jun-13 Jun     79           8             16            6                2
    13 Jun-20 Jun     48          10             --           55               --
    20 Jun-27 Jun     54           4              2          135                1
    27 Jun-04 Jul    193          61              7           37               --
    04 Jul-11 Jul    156          12             38            9               --
    11 Jul-18 Jul     58           3             24           84                1
    18 Jul-25 Jul     --           6              5           21               --
    25 Jul-01 Aug     34          --              5           30               --
    01 Aug-08 Aug     25          --              5           35               --
    08 Aug-15 Aug     12          --              7           38                2
    15 Aug-22 Aug      5           1              6           39               --
    22 Aug-29 Aug      7          --              2           26               --
    29 Aug-05 Sep      4          --             --           18               --
    05 Sep-12 Sep      2          --              3           27               --

Activity was generally at a moderate level in early May, increasing over 7-9 May and remaining high through 23 May. Activity mainly focused towards the NE, with rockfalls and numerous pyroclastic flows along the N side of the Tar River and in the Tar River Valley. On 12 and 13 May, flows were seen on the N and NW flanks in the area of Farrell's Plain and the upper reaches of Tyre's Ghaut. During 21-23 May there was increased activity on the N flanks, with a number of pyroclastic flows into the top of Farrell's Plain, Tyre's Ghaut, and Tuitt's Ghaut. Pulses of vigorous ash venting were observed at the summit, and intense glow on the summit and NE flanks was seen on the nights of 20 and 21 May. Sulfur emissions varied during May, with a high of 744 metric tons/day (t/d) (8.6 kg/s) on 14 May and a low of 300 t/d (3.4 kg/s) on 18 May. Extreme highs of 850 t/d (9.9 kg/s) and 820 t/d (9.5 kg/s) occurred on 4 and 9 May, respectively.

During the first week of June, activity was variable, generally declining to a moderately low level. Most activity through 6 June was focused on the E and NE flank, producing rockfalls and numerous pyroclastic flows in the Tar River Valley and occasionally in White's Ghaut and Tuitt's Ghaut. Activity during the week ending 13 June decreased to a low level, and remained low through 27 June, increasing over 26-27 June on the N flanks. Hybrid earthquake activity developed into a diffuse swarm on 22-23 June, some events at depths of 3 km below the lava dome. SO₂ emissions were relatively stable in June, varying between 240 t/d (2.8 kg/s) and 540 t/d (6.3 kg/s).

Sulfur emissions varied between 260 t/d (3 kg/s) and 585 t/d (6.8 kg/s) in July, but jumped to 840 t/d (9.7 kg/s) on 2 July. This could be related to increased activity during the first week of July, with pyroclastic flow and rockfall activity focused on the N flanks of the dome. Most flows occurred in Tuitt's Ghaut, with some in Tyre's Ghaut and White's Ghaut. Sporadic flows also occurred on the W side of the dome in the Gages area.

Activity remained high over the week ending 11 July, with a swarm of several thousand small hybrid earthquakes, at a rate of 1-2 per minute, commencing in the early hours of 9 July. While the size of these earthquakes increased slowly, individual events were below the normal recording threshold. The swarm of hybrid earthquakes intensified slightly over the night of 11 July, with events becoming larger and more closely spaced. Glimpses of the N part of the dome complex on 10 and 11 July confirmed that dome growth switched to the N, as was also shown by the northerly focus to the rockfalls and pyroclastic flows. Pyroclastic flows occurred most frequently in White's Ghaut, Tar River Valley, and Tuitt's Ghaut, with several small flows in Tyre's Ghaut earlier in the week.

By the morning of 12 July, events in the earthquake swarm merged into a continuous tremor signal. A period of prolonged and heavy rainfall between 0600 and 0900 caused mudflows in the Belham Valley. Small pyroclastic flows, the first of which were pale and weakly convective, occurred in the Tar River Valley. Flow activity built slowly through the afternoon until it was almost continuous. There were marked increases in the intensity of the activity at 1827 and again at 2007. Some flows traveled more than 2 km over the surface of the sea at the mouth of the Tar River Valley. Pyroclastic flows also reached the sea in White's Ghaut and the Spanish Point area. These flows resulted in the extremely heavy fallout of ash and accretionary lapilli over the island, particularly S of Woodlands.

A number of explosive events took place towards the end of the dome collapse of 12 July, with the largest occurring between 2300 and midnight. Showers of rock fragments fell on the island, with dense rocks up to 60 mm in diameter recorded. The Washington Volcanic Ash Advisory Center (VAAC) provided a column height of around 16 km for this event. The activity persisted at a high level until around 0200 on 13 July. It began subsiding slowly, declining to very low levels by the following morning, when a sudden Vulcanian explosion occurred from the lava dome. Two more explosions occurred in the next two days, producing pumice that reached 15 cm in size at Richmond Hill (~5 km W) and 4 cm in Olveston. Heavy ashfall from the collapse was experienced over all the inhabited parts of Montserrat, with the greatest thickness (over 15 cm) recorded at Vue Pointe Hotel. North of St Peter's the thickness was less than 1 cm.

The bulk of the dome structure was removed in the collapse, and pyroclastic flows impacted the area between Tar River Valley and Spanish Point. The activity destroyed GPS sites at White's Yard and Hermitage, and a camera site at White's Yard. Solar panels were smashed by falling rocks at Spring Estate GPS site and at Garibaldi Hill. After the collapse, sulfur-dioxide emissions jumped to highs between 1,030 t/d (12 kg/s) and 1,720 t/d (20 kg/s), much higher than any other readings over the past several weeks.

Activity was extremely low through 1 August with only a few events triggering the seismic network. The restrictions of the October 2002 exclusion zone were lifted on 1 August. The pattern of earthquakes through the week of 25 July indicated that dome growth within the explosion crater probably restarted, although it was not possible to confirm this visually due to low clouds. Intense activity began at 0608 on 1 August with an episode of powerful ash venting. There were many strong bursts of gas release and jets of ash; the plume rose to over 3.2 km. This activity declined to very low levels about 0730. Another episode of gas venting began at 0834.

Over the next week activity fluctuated, with periods of relative quiet separating episodes of intense degassing and hybrid earthquake activity. At the beginning of the week the volcano was extremely active with intense ash venting from the explosion crater. It was then fairly quiet with occasional rockfalls and hybrid earthquakes. A good view of the new dome was obtained from the air on 5 August, showing a small southerly directed lobe growing extremely slowly, if at all. Earthquake activity increased on the evening of 7 August with eight large hybrid events occurring overnight.

Through 22 August activity was at low levels; the dome remained a small lobe just over 100 m across. Several small slumps from the interior wall of the 12 July collapse scar produced small rockfalls, light ash in the plume, and the formation of some large fumaroles. By 29 August new fumaroles opened SE of the main explosion crater, towards the upper parts of the Tar River Valley. A strong sulphurous smell and blue haze N of the volcano did not reflect increased activity. SO₂ emissions in August were again variable, with a low of 450 t/d (5.4 kg/s) on 6 August and highs approaching 2,500 t/d (29 kg/s) the following week.

Through the latter part of the week ending 5 September, the gas plume was out of reach of the spectrometer network due to winds from Hurricane Fabian. Activity remained low through 12 September, but several episodes of ash venting occurred with a few small earthquakes.

Information Contacts: Richard Herd, Montserrat Volcano Observatory, Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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10/2003 (BGVN 28:10) Low-level seismicity; ash venting 30 September-1 October

Activity at Soufriére Hills remained at a relatively low level from mid-September into early November 2003. Seismicity consisted mostly of hybrid earthquakes and rockfall signals (table 49). Access continues to be prohibited to some areas after the major dome collapse and explosive activity of 12-13 July 2003 (BGVN 28:08), and there is a maritime exclusion zone around the S part of the island extending 3.7 km beyond the coastline from Trant's Bay in the E to Isles Bay on the W coast.

Table 49. Summary of seismic activity at Soufriére Hills, 5 September-7 November 2003. No volcano-tectonic earthquakes were recorded during this period, but one long-period rockfall event occurred 23-31 October. Courtesy of the Montserrat Volcano Observatory.

    Date (2003)     Rockfall   Long-period     Hybrid
                    signals    earthquakes   earthquakes

    05 Sep-12 Sep      2            3             27
    12 Sep-19 Sep      9            4             20
    19 Sep-26 Sep     13            1             20
    26 Sep-03 Oct      4           --            241
    03 Oct-10 Oct      1           --             15
    10 Oct-17 Oct     12           --              9
    17 Oct-24 Oct      8            2             12
    24 Oct-31 Oct     11            2             19
    31 Oct-07 Nov      8           --             16

During the week of 12-19 September, no growth of the new lava dome was observed. Activity was at a slightly higher level during the week of 26 September-3 October, especially hybrid earthquakes, most of which occurred in a swarm between 1100 and 2100 on 27 September. Some of the hybrids could be located at 2-4 km depth. A period of low-amplitude tremor was also recorded between 0800 on 30 September and 0400 on 1 October coincident with vigorous ash venting, which resulted in ash clouds reaching 2,000-2,500 m altitude and drifting W over Plymouth. Observations on 30 September and 3 October suggested that no new dome growth had occurred.

From 3 October to 7 November, activity returned to a low level. A period of low-amplitude tremor was recorded between 3 and 8 October, and some mudflow signals were also recorded during periods of heavy rain. The tremor coincided with light ash venting. Visibility was poor during this period, so no direct observations of the summit area were possible. The dome was observed clearly on 23 October and a volume survey was carried out from Galways and Perches Mountains. The small dome that extruded in July 2003 had not grown further and appeared to be stagnant, with alteration and degradation occurring such that it appears to be breaking up. The pit crater associated with the explosions of July 2003 had widened slightly, although this was thought to be due to passive slumping of material. Sulfur dioxide and hydrogen chloride emission rates were high during several days around 13-15 October and on 22 October (table 50). An observation flight on 28 October yielded clear views of the scar area and the W scar wall. No changes were observed in the morphology of the scar and no new lava was observed in the vent area.

Table 50. Gas emissions at Soufriere Hills, 5 September-7 November 2003. Hydrogen chloride emissions are calculated from hydrogen chloride to sulfur dioxide mass ratios measured in the volcanic plume using Fourier transform infrared. Values are in metric tons/day. Courtesy of the Montserrat Volcano Observatory.

    Date (2003)       SO2 emissions    HCI emissions
                       (tons/day)        (tons/day)

    12 Sep-19 Sep        700-900           230-300
    19 Sep-26 Sep        500-600             --
    26 Sep-28 Sep        400-500             --
    28 Sep-01 Oct        900-1,200           --
    04 Oct                3,100              --
    05 Oct                1,900              --
    06 Oct-08 Oct        800-1,200           --
    04 Oct & 07 Oct        --              600-1,000
    10 Oct-12 Oct        600-800             --
    13 Oct                1,900              --
    16 Oct                  720              --
    17 Oct-24 Oct        950-1,200           --
    22 Oct                1,850             1,500
    24 Oct-27 Oct        800-900             --
    28 Oct-31 Oct        400-600             --
    31 Oct-07 Nov       800-1,350           --

According to the Washington VAAC, on 1 November resuspended ash was seen in satellite imagery. The ash was moving N to NNW at ~ 10 km/hour from Montserrat between Nevis and Antigua, and the resuspended ash was concentrated in a narrow plume.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

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12/2003 (BGVN 28:12) Dome growth ceased after July 2003 and remained absent 6 months later

Weekly summaries of seismic activity at Soufrière Hills for the period 7 November 2003 to 16 January 2004 are given in table 51. During the week of 14 to 21 November a prominent swarm of hybrid earthquakes lasted for three days. Good views and surveys of the dome during this week confirmed that no growth or changes took place. On 9 December and on 31 December 2003 swarms of small hybrid earthquakes were observed on the drum records, but most of the events were too small to be recorded on the network. Visual observations confirmed that no new dome growth has occurred in the crater since July 2003, although there has been some slumping of old dome material from the crater walls, and degradation of the wall rocks by steam activity.

Table 51. Summary of seismicity recorded at Soufrière Hills, 7 November 2003 to 15 January 2004. Courtesy of Montserrat Volcano Observatory.

        Date       Rockfall      LP          LP        Hybrid          VT
    (2003-2004)    signals   rockfalls  earthquakes  earthquakes  earthquakes

    07 Nov-13 Nov     1           3           1           36           --
    14 Nov-20 Nov     7          --          13          287            4
    21 Nov-27 Nov     5          --           1           50            1
    28 Nov-04 Dec     1          --          --           12            0
    05 Dec-11 Dec    --          --           4           13           --
    12 Dec-18 Dec     2          --          --           12           --
    19 Dec-25 Dec     1          --          --            2           --
    26 Dec-01 Jan     2          --          --            9           --
    02 Jan-08 Jan     2          --          --            2           --
    09 Jan-15 Jan     5          --           1           18           --

Table 52 shows a summary of the gas emissions (mainly sulfur dioxide, but one HCl estimate for 18 December). Instrument problems or unfavorable wind directions disrupted measurements for a number of days during the report interval (dashed lines).

Table 52. Summary of gas emissions recorded at Soufrière Hills, 7 November 2003 to 16 January 2004. The HCl data listed were collected on 18 December. Courtesy of Montserrat Volcano Observatory.

       Date            SO2 emissions        HCl emissions
    (2003-2004)      (metric tons/day)    (metric tons/day)

    07 Nov-13 Nov         200-800                 --
    14 Nov-21 Nov         260-450                 --
    21 Nov-27 Nov             500                 --
    28 Nov-04 Dec         300-600                 --
    05 Dec-11 Dec         300-900                 --
    12 Dec-18 Dec         500-3,600       1,260 (HCl:SO2 = 0.35)
    19 Dec-25 Dec              --                 --
    26 Dec-01 Jan             500                 --
    02 Jan-08 Jan             300                 --
    09 Jan-15 Jan         200-590                 --

Information Contact: Gill Norton, Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/).

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02/2004 (BGVN 29:02) Ash to 7 km altitude on 3 March 2004; pyroclastic flows reached the sea

The Soufrière Hills volcano was quiet for the last few months of 2003, following activity in May and July that included significant dome growth (BGVN 28:10 and 28:12). Light ash-venting had last occurred during a period of low-amplitude tremor 3-8 October. A seismic event in mid-January 2004 and a period of tremor and mudflow activity in late February 2004 were followed by renewed eruptive activity on 3 March 2004.

Between 1 October and 18 December 2003 no dome growth was observed, and only a few earthquakes per week were recorded. Beginning 18 December 2003, SO₂ emissions increased markedly from the previous month's average of 500 tons/day (t/d), reaching 3,600 t/d (see table 53). On 18 January 2004, a swarm of low-amplitude long-period (LP) earthquakes began, with ~ 1,000 separate events over an interval of 36 hours. Fewer than 40 of these earthquakes triggered the automatic seismic-detection systems. Another swarm occurred on 30 January, this time lasting about 30 hours. Again, instruments recorded ~ 1,000 separate events; these, however, were much weaker and only four triggered the detection systems.

Table 53. Summary of SO2 emissions recorded at Soufrière Hills, 5 December 2003 to 12 March 2004, using an array of three scanning UV spectrometers. Courtesy of Montserrat Volcano Observatory.

    Date (2003-2004)    SO2 emissions (metric tons/day)

    05 Dec-11 Dec                 300-900
    12 Dec-18 Dec                 500-3,600
    19 Dec-25 Dec                    --
    26 Dec-01 Jan                   500
    02 Jan-08 Jan                   300
    09 Jan-15 Jan                 200-590
    16 Jan-22 Jan       440 on 22 January (equipment servicing on other days)
    23 Jan-29 Jan                 500-700
    30 Jan-05 Feb                 439-1017
    06 Feb-12 Feb                 350-450
    13 Feb-19 Feb                 350-650
    20 Feb-26 Feb                 496-920
    27 Feb-04 Mar                 480-820
    05 Mar-12 Mar                 340-1250

A period of low-level tremor, consisting of many small LP earthquakes, lasted for about 36 hours beginning 21 February. On 24 February heavy rainfall (10 mm in 2.5 hours) resulted in mudflow activity in the Belham valley; signs of mudflows were also observed in Plymouth.

Beginning the week of 27 February, activity increased significantly. On 2 March, a period of low-level tremor included some small hybrid earthquakes. The tremor continued until afternoon on 3 March, when, at around 1444, seismicity greatly increased and an explosion and collapse event occurred. According to reports from the Montserrat Volcano Observatory (MVO) this was the most significant event since the collapse event of 12-13 July 2003.

The event on 3 March 2004 produced ash clouds that reached altitudes of about 7 km above sea level, and pyroclastic flows were observed in the Tar River, with at least two incidents of flows reaching the sea. Seismicity returned to close to background levels by 1525, but vigorous ash venting continued until the following morning. Low-level tremor accompanied by hybrid earthquakes continued for the next 18 hours, including a series of hybrid earthquakes during the evening of 3 March.

Visual observations first suggested that the 3 March explosion removed the small dome that had grown in the collapse scar in late July 2003. Photographs taken on 28 February and 5 March showed the 3 March collapse to have also removed part of the NW dome remnant originally built up during 1995-1998.

After 3 March, activity remained elevated for several days. A period of low-level tremor occurred on 4 March, beginning at around 1300 and lasting three hours. On 5 March a small explosion was recorded at 1009, followed by a period of ash venting. Between 5 and 12 March activity returned to lower levels, with 1 LP and 15 hybrid earthquakes recorded. On 10 March, however, there was a short (10-20 minutes) period of elevated seismicity early in the morning; later in the day fresh pyroclastic-flow deposits were observed in the upper reaches of the Tar River Valley. During the second half of the week, short episodes of ash and steam venting were periodically observed, and ash fallout occurred as far N as St. Georges Hill.

On 15 March, the Washington VAAC reported a plume of ash extending to the W from the summit. The following day MVO reported a plume extending 250 km (135 nautical miles) W of the volcano. SO₂ emissions fluctuated during February and the first two weeks of March, peaking at 1017 t/d on 1 February and 1250 t/d on 9 March (table 2).

A beautifully illustrated look at the eruption from 1995 to present is now available (Kokelaar, 2002; Druitt and Kokellar, 2002).

References. Kokelaar, B.P., 2002, Setting, chronology and consequences of the eruption of Soufrière Hills Volcano, Montserrat (1995-1999), in Druitt, T.H. and Kokelaar, B.P., eds., 2002: The eruption of the Soufrière Hills Volcano, Montserrat from 1995 to 1999. Geological Society London, Memoir No. 21, p. 1-43.

Druitt, T.H. and Kokelaar, B.P., eds., 2002: The eruption of the Soufrière Hills Volcano, Montserrat from 1995 to 1999. Geological Society London.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov).

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05/2004 (BGVN 29:05) Seismicity generally low except for one dome-disrupting explosion

Although seismicity and volcanism were generally low during this reporting interval, mid-January to early June 2004, several episodes of elevated activity occurred. Weekly summaries for the early part of 2004 are presented in tables 54 and 55. The tables include a summary of seismicity, SO₂ emissions, and forward-looking infrared (FLIR) measurements of the HCl/SO₂ ratio.

Table 54. Summary of seismicity recorded at Soufrière Hills, 16 January to 4 June 2004. Courtesy of Montserrat Volcano Observatory.

    Date (2004)     Activity        Rockfall    Long-period       Hybrid       Volcano-tectonic
                     level           events     earthquakes     earthquakes      earthquakes

    16 Jan-23 Jan     Low               1           38               9                1
    23 Jan-30 Jan   Very low            8            1              10                1
    30 Jan-06 Feb     Low              15            7               9                1
    06 Feb-13 Feb     Low              --           --               3                1
    13 Feb-20 Feb     Low               1           --               3               --
    20 Feb-27 Feb     Low               3            2               5               --
    27 Feb-05 Mar   Increased           4            1              38               --
    05 Mar-12 Mar     Low              --            1              15               --
    12 Mar-19 Mar   Increased           1            4               6                6
    19 Mar-26 Mar   Elevated           --            4               7               --
    26 Mar-02 Apr   Moderate            1            1               1               --
    02 Apr-09 Apr   Low to moderate     5            1              --               --
    09 Apr-16 Apr     Low               1            1              --               --
    16 Apr-23 Apr     Low              --            5              14               --
    23 Apr-30 Apr     Low              --            3               5               --
    30 Apr-07 May     Low              --           --               1               --
    07 May-14 May     Low              --           --               1               --
    14 May-21 May     Low              --           --               1               --
    21 May-28 May     Low              --           --               7               --
                                                              (and 44 'mixed')
    28 May-04 Jun     Low              --           --               4               --
                                                              (and 16 'mixed')

On 18 January a low-amplitude swarm of long-period (LP) earthquakes comprised of 1000 separate events began and continued for ~36 hours. A similar swarm occurred on 30 January, lasting for ~30 hours. On 21 February a period of low-level tremor, including many small LP earthquakes, began at ~0600 and continued for ~36 hours.

A period of low-level tremor began on 2 March and continued until 1444 on 3 March when seismic activity increased significantly and an explosion and collapse event occurred. According to the Washington Volcanic Ash Advisory Center (VAAC), the ash clouds associated with the explosion reached an altitude of ~7 km. During 1445-1500 pyroclastic flows were observed in the Tar River, reaching the sea at the Tar River fan on at least two occasions. Seismicity returned to near background levels by 1525, but vigorous ash venting continued until ~0700 on 4 March. Visual observations reported that the explosion removed the small dome that had grown in the collapse scar in late July 2003, as well as a portion of the NW remnant of the 1995-1998 dome.

[A small amount of ash venting from the volcano's summit occurred on 2 May around 1815.] Episodes of tremor . . . continued until 7 May. During this period, tremor amplitude varied from low to moderate, and tremor duration varied from several days (continuous background) to a few seconds. Tremor peak frequencies were in the 1-10 Hz range. Subsequently, the activity level was low (table 54). The SO₂ flux level dropped to 146 metric tons/day on 13 May (table 55), the lowest value recorded since before the collapse event of 12-15 July 2003. For the remainder of the report period, activity remained at a low level. The seismic network recorded several hybrid earthquakes but also a number of 'mixed' events, characterized by emergent onsets and relatively short durations (~30 seconds) with broad frequency spectra (1-10 Hz), peaking at ~10 Hz.

Table 55. Summary of SO₂ emissions and the HCl/SO2 ratio recorded at SoufriPre Hills, 16 January to 4 June 2004. Courtesy of Montserrat Volcano Observatory.

    Date (2004)             SO2            HCl/SO2
                     (metric tons/day)      ratio

    16 Jan-23 Jan           440           0.36-0.41
    23 Jan-30 Jan         500-700         0.33-0.37
    30 Jan-06 Feb         439-726             --
    06 Feb-13 Feb         350-450            0.32
    13 Feb-20 Feb           --                --
    20 Feb-27 Feb         496-920             --
    27 Feb-05 Mar         480-820             --
    05 Mar-12 Mar         330-1250           0.47
    12 Mar-19 Mar         470-755             --
    19 Mar-26 Mar         370-550         0.53-0.66
    26 Mar-02 Apr         440-480             --
    02 Apr-09 Apr         150-720             --
    09 Apr-16 Apr         540-870             --
    16 Apr-23 Apr          1030               --
    23 Apr-30 Apr         155-290            0.49
    30 Apr-07 May         200-672            0.30
    07 May-14 May         146-695             --
    14 May-21 May         182-428             --
    21 May-28 May         255-922            0.60
    28 May-04 Jun         179-496             --

Information Contact: Gill Norton, Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/).

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09/2004 (BGVN 29:09) Generally low activity; small lake forms in summit crater

According to reports issued by the Montserrat Volcano Observatory (MVO), activity at Soufrière Hills volcano remained low during 21 May-10 September 2004, becoming slightly elevated from 10 September-15 October 2004. Minor events during this period included mudflows, rockfalls, and several small, shallow earthquakes originating at upper regions of the lava dome and conduit.

On 21 May 2004, heavy rainfall caused large mudflows for about two hours (1420 to 1636). The mudflows traveled into the Belham Valley, an area within the exclusion zone on the volcano's NW side, flooding the entire width of the valley floor at Belham bridge. Scientists from MVO noted that at the peak of flow, standing waves of mud reached 2 m high. Intense rains on 25 July, 14 September, and 16 September again caused up-slope erosion and mudflows descended into the Belham Valley.

Surface water from the intense rain of 21 May percolated into the subsurface of the hot dome and vent complex, converting into steam. Steam venting may have initiated the 44 mixed earthquakes recorded the week of 21-28 May (table 56). The earthquakes were short in duration (~ 30 seconds each), and their amplitude decay characteristics suggested that they originated at shallow depths within the remnant dome and at the top of the conduit. The term 'remnant dome' refers to the fact that the late May 2004 dome was considerably reduced in size compared to the dome of about a year before, in large part because of major collapse events on 12 July 2003 and 3 March 2004.

Table 56. Seismicity recorded at Soufrière Hills, 21 May to 15 October 2004. Courtesy of Montserrat Volcano Observatory.

    Date (2004)      Activity Level     Hybrid   Mixed   Volcano-   Long-    Rockfalls
                                                         tectonic   period

    21 May-28 May          Low             7       44       --        --         --
    28 May-04 Jun          Low             4       16       --        --         --
    04 Jun-11 Jun          Low             3        9       --        --         --
    11 Jun-18 Jun          Low             5       --       --        --         10
    18 Jun-25 Jun          Low             5       --        6        --         15
    25 Jun-02 Jul          Low             5       --        6         2          8
    02 Jul-09 Jul          Low             8       --        4        --         10
    09 Jul-16 Aug          Low             6       --        1         1          5
    16 Jul-23 Jul          Low             7       --       --        --          7
    23 Jul-30 Jul          Low             2       --       --        --          8
    30 Jul-06 Aug          Low             1       --       --        --          8
    06 Aug-13 Aug          Low             1       --       --        --          3
    13 Aug-20 Aug          Low             1       --        1        --          1
    20 Aug-27 Aug          Low             1       --       --        --          1
    27 Aug-03 Sep          Low             2       --       --        --         --
    03 Sep-10 Sep          Low            --       --        1        --          2
    10 Sep-17 Sep   slightly elevated     14       --       --         1          1
    17 Sep-24 Sep   slightly elevated      8       --       --         2          2
    24 Sep-01 Oct   slightly elevated      8       --       --         1          3

During 18-25 June, winds blew the volcanic plume NE, and the clouds over the volcano's summit lifted, making the tallest remnants of the dome complex visible for the first time since 7 May. Observers noted a loss of material from the upper regions of the dome due to rockfalls.

Later, on 30 August, MVO personnel on an observation flight discovered a small brown pond within the dome complex (figure 5). This pond was the first seen on the volcano since the beginning of its eruption in 1995. The pond lies in a small crater formed by an explosion and dome collapse on 3 March 2004 (figure 57). It probably developed following the cooling of deposits within the crater, and after recent heavy rainfall.

Sulfur dioxide (SO₂) emissions typically remained low throughout the period. Measured fluxes ranged between about 90 and 1,100 metric tons per day (table 57). Moderate fluctuations occurred during 21-24 May (225-922 metric tons/day) and 7-11 June (169-788 metric tons/day). During 2-9 July, SO₂ emissions dropped to the lowest levels since the collapse event of 12-13 July 2003, and fell even lower during the weeks of 30 July and 17 September.

Table 57. SO₂ gas flux estimates made at Soufrière Hills, 21 May to 15 October 2004. Courtesy of Montserrat Volcano Observatory.

    Date (2004)        SO2 emissions
                     (metric tons/day)

    21 May-28 May        225-922
    28 May-04 Jun        179-496
    04 Jun-11 Jun        169-788
    11 Jun-18 Jun        240-477
    18 Jun-25 Jun             --
    25 Jun-02 Jul        177-364
    02 Jul-09 Jul        120-160
    09 Jul-16 Aug        222-243
    16 Jul-23 Jul        170-400
    23 Jul-30 Jul        175-300
    30 Jul-06 Aug         90-280
    06 Aug-13 Aug        126-296
    13 Aug-20 Aug        200-622
    20 Aug-27 Aug        175-311
    27 Aug-03 Sep        240-456
    03 Sep-10 Sep        175-405
    10 Sep-17 Sep        130-250
    17 Sep-24 Sep         87-454
    24 Sep-01 Oct        200-540
    01 Oct-08 Oct        187-1144
    08 Oct-15 Oct        156-553

Measurements of expansion made by dilatometers embedded on either side of the Soufriere Hills edifice suggested that in July, the volcano changed from contraction to slight expansion.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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10/2004 (BGVN 29:10) Heavy rains cause frequent mudflows and increased seismicity

Table 58, taken from reports of the Monserrat Volcano Observatory (MVO), summarizes activity at Soufrière Hills between 1 October and 26 November. The activity level remained elevated during much of this time period due to increases in seismicity, gas emission, rainfall, and mudflows.

Table 58. Activity recorded at Soufrière Hills, 1 October to 26 November 2004. One of the gas-monitoring sites only functioned on 18 November. Courtesy of Montserrat Volcano Observatory (MVO).

    Date (2004)     Activity   Hybrid   Mixed    VT     LP    SO2 emissions   Rockfalls
                    level       EQ's    EQ's    EQ's   EQ's

    01 Oct-08 Oct   elevated      8      --      --      2      187-1144          1
    08 Oct-15 Oct   elevated      9      --      --     --      156- 553          1
    15 Oct-22 Oct   elevated     49      --       1     --      250-1100          4
    22 Oct-29 Oct   elevated     40      --       1     --      320- 370         --
    29 Oct-05 Nov   elevated     33      --      39     --      140- 440          1
    05 Nov-12 Nov      --        21      --      14     --      147- 225          3
    12 Nov-19 Nov      --        12      --      40      5          1111          3
    19 Nov-26 Nov      --        25      --       5      1      125-330           3

Heavy rains during the first six weeks of the reporting period led to steam venting, which triggered an increase in hybrid and volcanic-tectonic earthquakes. A large number of hybrid and volcano-tectonic (VT) earthquakes was recorded during most of October and early November. The most intense seismicity occurred during 2106-2216 on 12 November and 1335-1436 on 14 November.

Following the rains of 5-12 November, several fumaroles developed along the former Tuitt's Bottom and Pea Ghauts, but by 12 November, drier conditions prevailed and fumaroles diminished. Sulfur dioxide emissions remained low throughout most of the reporting period, however two surges in SO₂ flux occurred during the weeks of 1 October and 15 October. Mudflows occurred since May. As heavy rainfall continued during October and November, more mudflows occurred. Nine separate mudflow events were recorded for this reporting period. The flows of 15, 19, 21, 22-29 October and 1, 3, 9, and 11 November were minor, though one of the flows, which traveled down the NW flank, reached the Belham River. A much heavier flow began around 0620 on 19 November, with a pulse occurring at 1138.

One MVO scientist deemed mudflows the "ongoing legacy of this [the 1995] eruption." Montserrat's rainy season typically continues until December, and more mudflows may occur in coming months. Mudflows have proven to be destructive, whether they have arisen from short, intense downpours or from a buildup over several rains. The example was given of mudflows after two hours of heavy rain on the afternoon of 21 May, which led to burial of the gateway to the Radio Antilles' offices.

MVO personnel made two observation flights during the reporting period (on 28 October and 4 November). Both flights confirmed the presence of the pond seen 30 August in the pit formed by the 3 March dome collapse. Looking into the crater, MVO scientists found no evidence of ongoing dome-building.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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03/2005 (BGVN 30:03) Comparative quiet during 26 November 2004 to 4 March 2005

This report covers the period 26 November 2004 to 4 March 2005. Soufrière Hills volcano remained quiet after late November, with seismic signals, gas emissions, and rockfalls all decreasing (table 59 and BGVN 29:10).

Table 59. Geophysical and geochemical data recorded at Soufrière Hills, 26 November 2004 to 4 March 2005. Wind directions or trouble with gas-monitoring equipment prevented measurement of SO₂ fluxes on some days. Courtesy of MVO.

        Date        Seismicity   Hybrid   Mixed   VT     LP         SO2 flux        Rockfalls
    (2004-2005)       level       EQ's    EQ's    EQ's   EQ's   (metric tons/day)

    26 Nov-03 Dec      --           9      --      7     --          130-590            4
    03 Dec-10 Dec      --           7      --     --     --          250-370            1
    10 Dec-17 Dec      --           6      --      7     --          290-450           --
    17 Dec-24 Dec      --           6      --      1     --          200-500           --
    24 Dec-31 Dec      --           6      --      2      1          300-550           --
    31 Dec-07 Jan      --           4      --     --     --          310-400           --
    07 Jan-14 Jan      --           5      --      5     --          180-511           --
    14 Jan-21 Jan      --           2      --     --     --          300-3801           2
    21 Jan-28 Jan      --           5      --     --      1          350-6701          --
    28 Jan-04 Feb      --           2      --      1      7              4101          --
    04 Feb-11 Feb     low          --      --     --     --             --             --
    11 Feb-18 Feb     low          --      --     --     --             --             --
    18 Feb-25 Feb     low          --      --     --     --          280-9801          --
    25 Feb-04 Mar     low           6      --      3     --              6721           2

While volcanic-tectonic and hybrid earthquakes (as many as 40/week) shook SHV from mid-October to late November, few were recorded between late November and early March (table 1). During the week of 25 February, winds shifted and carried the smell of sulfur to northern parts of the island. However, SO₂ emissions remained low and stable throughout December, January, and February. The average for this reporting period was ~ 400 tons/day, which is below the long-term average of 500 tons/day.

Rain and mudflows have also subsided. A mudflow in the Belham Valley on 15 December was the only event recorded for this reporting period.

An exceptionally clear day enabled scientists to obtain unusually clear photos on 1 February 2005 (figures 58, 59, and 60). Flights were made during November, December, and January as well. During the November-February interval, scientists saw relatively few changes in the surface morphology. Chances Pond, the pool of brownish water sitting in the explosion pit formed on 3 March 2004, still remained. MVO scientists noted around 26 November 2004 that the pond had changed from brownish to milky.

On 3 March 2005, scientists took a Fourier Transform Infrared spectrometer (FTIR) reading of the gas escaping from the crater vent at the summit (figure 59). The gas plume contained a ratio of hydrogen chloride to sulfur dioxide by mass of 0.35. This ratio showed no change since February.

In September 2004, the Scientific Advisory Committee on Montserrat Volcanic Activity (SAC) joined MVO staff at the Montserrat Volcano Observatory to discuss recent activity (SAC, 2004). "Few signs of surface activity" was their appraisal of the period since March 2004. There had been little gas venting, ash venting, or tremor in six months, and no lava extrusion in 15 months. This pause in activity, the SAC predicted, will last ~ 26 months, but could last as long as 170 months. SAC estimated risks for a variety of circumstances.

Readers can access full reports issued by the SAC at the MVO website. The main report summarizes conclusions drawn from the meeting, while the technical report includes long-term monitoring data and risk assessments.

Reference. Scientific Advisory Committee on Montserrat Volcanic Activity (SAC), 2004, Assessment of the hazards and risks associted with the Soufriere Hills volcano, Montserrat: Second Report of the Scientific Advisory Committee on Montserrat Volcanic Activity, parts I (Main Report, 24 p.) and II (Technical Report, 26 p.).

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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06/2005 (BGVN 30:06) Abundant ash-laden plumes, pyroclastic flows, and local ashfall

Soufrière Hills was last reported on in BGVN 30:03, covering November 2004 to March 2005, during which time the volcano remained quiet, with seismic signals, gas emissions and rockfalls all decreasing. This report, from Montserrat Volcano Observatory (MVO), covers the period from late March 2005 to July 2005. The volcano continued to be relatively quiet through April and early May, with activity increasing somewhat through June and several explosive events in late June and in July. Table 60 summarizes the seismicity and SO₂ emissions during the period of this report.

Table 60. Geophysical and geochemical data recorded at Soufrière Hills, 25 March 2005 to 15 July 2005. * Only measurement during report period. **12-hour system failure may have caused events to be missed. Courtesy of MVO.

     Report date    Seismicity    Number of earthquakes          SO2 flux (metric tons/day)
       (2005)         level       Hybrid  VT    LP  Rockfalls    Range      Daily average

    25 Mar-01 Apr      low           1     5     1     --        186-369         290
    01 Apr-08 Apr      low           1     7     1     --        280-650         400
    08 Apr-15 Apr      low          --    19    --     --        261-1877        619
    15 Apr-22 Apr       --           7    37    --      1        122-957         365
    22 Apr-29 Apr       --           7    31    --     --        112-330         304
    29 Apr-06 May       --           1     4    --      1        276-644         439
    06 May-13 May       --           1    38    --      1        221-537         398
    13 May-20 May       --           3    18    --     --        222-363         286
    20 May-27 May       --          --    67    --     --        880*             --
    27 May-03 Jun       --          --     8**  --     --        167-392         261
    03 Jun-10 Jun       --          --    17    --      1        142-671         399
    10 Jun-17 Jun    elevated       17    46    20      7        170-750         460
    17 Jun-24 Jun    elevated        8     4     5      3        430-1150        627
    24 Jun-01 Jul    elevated       19    15     5     --        300-700         470
    01 Jul-08 Jul    elevated       15     9    11     11        241-1700        767

Seismic activity at Soufrière Hills remained at low levels throughout March and most of April 2005. Beginning on 15 April, vigorous steam-and-ash venting occurred on the NW side of Soufrière Hills crater and continued throughout the period of this report. Average daily SO₂ emissions were generally lower than the long-term eruption average of 500 tons/day, but increased in July to above the average.

On 13 June at 0600 an ash plume reached a height of ~ 2.4 km altitude and drifted NE, depositing light ash in Lookout, Geralds, and St. Peters.

Starting around 10 June, seismic and volcanic activity were at elevated levels. The ash venting that began on 13 June declined in intensity during the following week. The ash venting was caused by the rapid release of steam and other volcanic gases, possibly triggered by intense rainfall on the night of 12 June. Ash analyses from this episode did not indicate fresh magma.

On 27 June a steam and ash cloud at ~ 3 km altitude was reported to be drifting W. By 28 June satellite imagery showed a plume of ash and steam at ~ 1.8 km altitude extending NW. Periodic episodes of intense ash venting continued, culminating in an explosive event on 28 June at 1306. During the event, ballistics were ejected onto the Farrell's plain (to the NW), and a column collapse produced pyroclastic flows. The pyroclastic flows reached the sea at the Tar River delta (to the NE), and a smaller volume of material flowed into the top of Tyre's Ghaut (to the N). Ash showed no evidence of fresh magma.

Preliminary analysis of recent ground deformation data from the GPS network at the volcano showed that deflation during April to mid June 2005 had later reversed, and the volcano appeared to be inflating. Periodic ash venting continued and an explosion occurred on 3 July at 0130, which was similar to the explosion on 28 June.

An explosive event at 0301 on 18 July caused widespread ash fallout between Fogarty Hill on the island's NW and Brodericks Yard on the island's SW and almost certainly led to pyroclastic flows to the sea in Tar River. This explosion was similar to, but slightly bigger than, the explosion on 3 July, and ash venting and pyroclastic flows combined to cause dramatic ash clouds which reached to at least 6 km. Winds blew the ash plume in a NW direction causing significant ash fall in Old Towne, Iles Bay, Salem, Olveston, Woodlands and St Peters. The maximum depth of ash measured by scientists in inhabited areas was 1.5 to 2.0 mm; the deepest ash was recorded at Weekes. Activity subsequently returned to background levels.

The MVO collected ash samples from the affected areas to determine whether it was new material from depth or older material from the dome. Ash collected after the 28 June and 3 July 2005 events showed no evidence of new magmatic material.

On 28 July 2005, the Moderate Resolution Imaging Spectroradiometer (MODIS) flying onboard the Aqua satellite acquired an image of a plume of volcanic ash drifting westward in a slightly curving shape as it departs Soufriere Hills (in the middle of the image, figure 61).

Information Contact: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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08/2005 (BGVN 30:08) Through at least 5 September 2005, the lava dome continued to grow

Soufrière Hills was relatively quiet through April and early May 2005, with activity increasing somewhat through June and several explosive events in late June and in July (BGVN 30:06). Table 61 summarizes activity during 8 July thorough 26 August 2005. Further text brings this report through 5 September, with the comment that slow dome growth continued.

Table 61. A summary of the weekly number of earthquakes (EQs), rockfalls, and averaged spot measurements of SO2 flux at Soufriere Hills during July and August 2005. Cases of "mixed earthquakes" were unreported during the reporting interval. Date ranges go from noon on the starting day to noon on the end day. Courtesy of MVO.

    Report date    Number of earthquakes                                     SO2 flux
     (2005)        Hybrid   Volcano-tectonic   Long-period   Rockfalls   (metric tons/day)

    08 Jul-15 Jul    10            2               --           --              660
    15 Jul-22 Jul    16           19               13           11              608
    22 Jul-29 Jul     4           29                5           23              510
    29 Jul-05 Aug     4            8                9           33              986
    05 Aug-12 Aug     3            3                5           14              770
    12 Aug-19 Aug     8            5               13           12              570
    19 Aug-26 Aug     6            5               13           15              900

On 6 August a vigorous eruption sent a plume to ~ 1.8 km above the volcano. Evidence of uplift and fracturing were observed on the crater floor, and an area of blocky lava resembling a small lava dome was observed. Due to poor visibility further observations will be necessary to determine if the feature is a new dome or was caused by the collapse, or uplift, of old dome rock.

Volcanic and seismic activity remained at elevated levels at Soufrière Hills during 12-19 August. Periodic ash venting continued, with a vigorous episode occurring on 18 August at 1800. On 16 August, the presence of a small blocky lava dome with talus slopes was confirmed. There was some ash venting from the dome, but no significant rockfalls were seen. Activity at Soufrière Hills remained at elevated levels during 2-9 September, the end of this reporting period. Observations made on 5 September suggested that slow lava-dome growth continued.

Information Contact: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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12/2005 (BGVN 30:12) Slow lava dome growth continued

Activity at Soufrière Hills remained elevated during the latter half of 2005 (table 62). Since 5 September slow lava dome growth continued. During the week of 30 September through 7 October dome growth increased on the western side. On 26 October a pyroclastic flow with a runout of 2 km was reported around 2400. The pyroclastic flow was confined to the Tar River Valley.

Table 62. Soufrière Hills seismicity during 26 August to 30 December 2005. Courtesy of MVO.

    Report Date              Number of earthquakes
      (2005)         Hybrid    Volcano-tectonic    Long-period

    26 Aug-02 Sep      10              3                9
    02 Sep-09 Sep       5              6               --
    09 Sep-16 Sep      13             70                5
    16 Sep-23 Sep       3              6                5
    23 Sep-30 Sep      --             42                3
    30 Sep-07 Oct      --             50                2
    07 Oct-14 Oct      --            183               --
    14 Oct-21 Oct       1            359               15
    21 Oct-28 Oct      --             53                2
    28 Oct-04 Nov      --             19                2
    04 Nov-11 Nov       2             14                6
    11 Nov-18 Nov      --              3                8
    18 Nov-25 Nov       7             21              176
    25 Nov-02 Dec       2              1               93
    09 Dec-16 Dec      --             --               31
    16 Dec-23 Dec       2              1               40
    23 Dec-30 Dec      19              9               63

On 4 November dome growth on the southern flank was observed. The following week, growth was reported on the E, S, and SE flanks. Radar imaging of the dome indicated a dome volume of about 6.5 million cubic meters, suggesting a growth rate over the past two weeks of between 1.3 and 1.8 cubic meters per second with incandescence visible at night. Observations on the morning of 18 November indicated the dome continued to grow and sent rockfalls in all directions. A pyroclastic flow was observed in the Tar River valley on 15 November and reached to within 1km of the sea. The ash cloud associated with this event rose to ~ 2.1 km.

A pyroclastic flow was observed in the upper reaches of the Tar River valley on 22 November (figure 62). Minor venting of ash occurred, with one event taking place on the afternoon of 24 November, which produced an ash cloud that rose ~ 1 km above the summit (figure 63).

During the month of December, dome growth continued on all flanks although more intense on the S and E flanks. More rockfalls and smaller pyroclastic flows have been reported with incandescence observed at night along the SE and E flanks.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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05/2006 (BGVN 31:05) Big dome collapse and tall plume on 20 May 2006 leave a W-leaning crater

Activity at Soufrière Hills remained at elevated levels (table 63), similar to that previously reported (BGVN 30:12), a state that culminated with a dome collapse on 20 May 2006. Although that event took away considerable portions of the dome (and caused a small tsunami), photographs revealed post-collapse dome growth focused over a broad SE sector extending from the SW around to the NE. Numerous rockfalls continued from the S, E, and NE flanks of the lava dome. The NE-side rockfalls added talus to the upper reaches of the Tar River valley and were visible at night.

Table 63. Soufrière Hills seismicity during 28 December 2005 to 12 May 2006. Courtesy of MVO.

    Report date        Hybrid          VT            LP         Rockfall        SO2 flux
    (2005-2006)      earthquakes   earthquakes   earthquakes    signals     (metric tons/day)

    28 Dec-06 Jan         —             —            11            37             522
    06 Jan-13 Jan         —             1            30           116             724
    13 Jan-20 Jan         —             —            17            61             767
    20 Jan-27 Jan         —             —            11            60             470
    27 Jan-03 Feb         1             3            11            92             594
    03 Feb-10 Feb         2            39            61            84             465
    10 Feb-17 Feb         2             9           121            10             568
    17 Feb-24 Feb         1             3            26            30             286
    22 Feb-03 Mar         1             7           157            18             388
    03 Mar-10 Mar         2             2           148           282             454
    10 Mar-17 Mar         —             4           115           319             480
    17 Mar-24 Mar        13             3           231           336           1,034
    24 Mar-31 Mar        12             1           230           316             523
    31 Mar-07 Apr         —             3            38           507             578
    07 Apr-14 Apr         —             3            99           620             540
    14 Apr-21 Apr         3             —            80           100              *
    21 Apr-28 Apr         —             —            30           589             521
    28 Apr-05 May         —             —           109           279             310
    05 May-12 May         —             —            74           571             702
    12 May-19 May         7             1           130           753             674
    19 May-26 May        89            11           229           373             **
    26 May-02 Jun        62             4           172           195             **
    02 Jun-09 Jun        20             —            28           163             **
    09 Jun-16 Jun         3             —            54            46             **

    *  Due to weather conditions, gas measurements were not made.
    ** As a result of the collapse, instrumentation was lost and gas measurements were not able
       to be measured.

A central spine was first observed on 17 January 2006 when clouds briefly cleared from the dome. On 22 January, two new relatively thin, vertical planar spines were seen on the SE flank of the lava dome and collapsed on 29 January. Helicopter and field observations indicated continued dome growth, particularly in the SE (figure 64).

On 10 February, MVO reported increased activity to the Washington VAAC. Satellite imagery showed a prominent hotspot at the volcano and a NW-drifting ash plume at an altitude of ~ 3 km. A small dark lobe of lava was observed on the western side of the lava dome in the crater. Steaming and venting were observed throughout the day. A photo appears as figure 65.

By early 11 February, this lobe had advanced rapidly towards the NE side of the dome and was visible as a steep-sided plateau of lava from inhabited areas around Salem. Photographs from fixed cameras showed continued changes to this lava lobe over the next few days, and the NE margin could be seen glowing at night and shedding rockfalls into the NE part of the crater. Ash-and-gas emissions continued through 15 February, producing plumes to an altitude of ~ 2.7 km. The initial growth rate of this lobe surpassed 5 cubic meters per second, but the rate declined around 17 February. The new lava lobe began to fill the gap between the lava dome and the N and W crater walls, raising the possibility that small rockfalls could spill over those areas in coming weeks. After 22 February, incandescent rockfalls were visible at night, coursing down the N, E, and SW sides of the dome and into the Tar River Valley (figure 66).

On 26 February, rapid vertical growth of the lava dome at Soufrière Hills was visible on camera images, and by 27 February a large spine about 30 m wide and at least 30 m high had developed at the dome's summit. By 28 February this spine had split into two parts and was leaning precariously to the NE. At about 2115 on 28 February the overhanging parts of the spine disintegrated and generated pyroclastic flows that traveled down the Tar River Valley almost as far as the coast. A low-level ash cloud drifted W. Additional changes to the shape of the spines and the upper NE flank of the volcano were noted in the following days as they disintegrated further. Rockfalls were visible on the N, NE, and E flanks of the volcano. Some fumaroles were observed on the upper outside part of Gages Wall (W of the lava dome) on 27 February, suggesting movement of fluids in this area.

During 3-17 March, lava-dome growth continued and the dome reached an altitude of ~ 950 m. The active lava lobe shed rockfalls and small pyroclastic flows to the W, N, and E. A vigorous gas vent was seen on the W side of the lava dome on 8 March, above Gages valley. Small fumaroles were visible at the top of Gages valley and below the lava dome remnant that stands at the top of Gages Valley.

Observations during 17 March-7 April revealed that lava-dome growth was focused in the summit area and towards the E and NE (figure 67). The N side of the lava dome showed little change. Rockfalls and pyroclastic flows were restricted to the Tar River Valley and were numerous on 19-20 March. The largest pyroclastic flows traveled as far as 2 km.

Lava extrusion continued during 7-21 April. Growth occurred to the E and N, and an eastward-facing lobe developed on the NE side of the dome. Numerous small rockfalls continued from the active eastern flanks of the dome, adding to the talus in the upper reaches of the Tar River valley. Rockfalls were accompanied by minor ash venting. Due to unusual wind conditions, plumes were predominately transported N and NW, shifting to the E on 20 April. As a result of this process, light ashfall occurred over much of Montserrat. Thermal images taken on 27 April indicated some very hot areas on the E flank of the dome.

Deposits from a series of pyroclastic flows occurring on 4 May extended as far as the Tar River delta. Northerly directed winds during the reporting period resulted in light ashfall in areas north of the Belham valley. The dome volume was approximately 80 million cubic meters and the average growth rate through April was about 8 cubic meters per second.

On 18 May, a survey conducted on the southern half of the dome was carried out using a terrestrial laser scanner and showed that the summit of the dome had reached a height of 1,006 m, this is 83 m higher than Chance's Peak (figure 68).

20 May collapse. A major lava dome collapse took place on the morning of 20 May (figure 69). A helicopter flight in the afternoon confirmed that most of the lava dome had gone, together with some remnants of the 2003 lava dome, leaving a broad, deep, eastward-sloping crater at the summit of the volcano. The volume of the lava dome was believed to be about 90 million cubic meters and most of this collapsed over a period of less than three hours. Views of the W part of the crater where ash venting is continuing were not possible but it is unlikely that there is significant dome material remaining there.

At 0222 on 20 May there was a single precursor, a long-period seismic event located 3 km below the dome. A brief episode of heightened seismic amplitude corresponding to ash venting occurred during 0300-0330. During heavy rain, another episode of increased seismic amplitude, interpreted as ash venting, began at 0552, and it developed into a high-amplitude seismic signal. The heavy rain caused mudflows in Belham River valley. By 0632 low-level ash clouds were drifting to the NW of the volcano from the crater area and a steam- plume was rising to 6,000 ft (~ 1800 m). Unconfirmed reports suggested that pyroclastic flows first reached the sea at about 0645. Regular pulses of pyroclastic flows were reaching the sea down the Tar River valley by 0720 with major pulses recorded in seismic amplitude at 0736, 0743, and between 0801 and 0804. Also between 0730 and 0810 a number of long-period seismic events were detected. At 0740 an ash cloud was reported at nearly 17 km, altitude the highest reported ash cloud during the ten years of the eruption. At 0743, pyroclastic surges were observed spreading across the NE flanks of the volcano reaching the Spanish Point area. It was also estimated at this time that surges had spread 3 km offshore from Tar River valley, across the surface of the ocean.

By 0750, lithics were falling in areas NW of the volcano; most were less than 3.5 cm across, and the largest found in the inhabited area was 6 cm across. Six car windscreens were reported broken. The deepest ash fall in inhabited areas was about 3 cm. Activity began to reduce in intensity after 0815 and a high-amplitude seismic signal remained until 0900. At this time, residents in the Old Towne and Salem area were subjected to high levels of volcanic gases particularly hydrogen chloride causing some to move N (figure 69). Widespread and noisy mudflows were reported in the Trants area to the NE of the volcano. Ash venting from the W of the crater continued until about 1700 when it began to decline.

A 1-m-high tsunami was reported from Deshaies beach in Guadeloupe and swells were detected in Little Bay, Montserrat, and at Jolly and English Harbour, Antigua. Relatively light but continuous ash-and-steam venting followed the collapse.

The weeks after the 20 May collapse. Wind direction shifted towards the N late on 21 May causing ash fall and raining mud in most parts of the island. Scientists remained alert to the possibility of further explosive activity but seismic activity was at low levels after the event on 20 May.

Since the May collapse, the lava dome continued to grow. As of 9 June it was approximately 20 million cubic meters in size. This is similar to the size of the dome in early January 2006. The average growth rate since the dome collapsed on 20 May was close to 10 cubic meters per second, well above the average growth rate of 6 cubic meters per second noted between January and April 2006.

By the end of the report period the dome was broad and flat-topped with a growing talus slope extending E. The lava on the summit of the dome is blocky, which is typical of lava extruded at a high rate. Vigorous ash and gas emitted by a vent W of the lava dome occurred during the week of 2 June. The venting is accompanied by a roaring sound that is sometimes audible in the Salem area. Prevailing winds have taken most of this ash and gas to the west over Plymouth. Satellite imagery on 4 June showed a thin area of ash out to St. Croix. In addition, there were multiple SFC and pilot reports of ash over the E portion of Puerto Rico and the Virgin Islands. Mudflows were reported on the 11 and 13 June during heavy rainfall.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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09/2006 (BGVN 31:09) Extrusive dome dynamics during May-September 2006

Since the 20 May 2006 dome collapse, the lava dome at Soufrière Hills has continued to grow. Only weeks after the collapse, the alert level was raised to 4 as a result of increased seismic activity. At approximately 1300 on 30 June, the lava dome partially collapsed again, producing pyroclastic flows that traveled E. According to the Washington VAAC, a pilot reported an ash plume that reached ~ 3 km altitude and drifted NW. At 1830 on 30 June, Montserrat Volcano Observatory (MVO) indicated a second dome collapse that also generated ash plumes to an altitude of 3.0-3.5 km (figure 70). According to MVO, on 27 June (prior to the collapse on 30 June) the lava dome had an estimated volume of 27 million cubic meters.

On 7 July, the alert level was lowered from 4 to 3. Increased rockfall activity and dome growth to the NE were observed on 21 July, and the post-collapse dome developed an asymmetric profile owing to a blocky spine on the NE. On 18 July the spine's summit stood at ~ 895 m elevation. As the dome continued to grow during July (figure 71), visual observations revealed that the still intact blocky spine began leaning E.

During August the dome lost spines from its crest, giving it a more symmetrical profile as it continued to grow E. Heightened activity during the last week of August included an increase in seismicity and pyroclastic flows. On 29 August, pyroclastic flows reached the Tar River valley and generated a steam-and-ash cloud that reached an altitude of ~ 9 km. Heavy rainfall produced mudflows around the base of the volcano.

At 0300 on 31 August, two vigorous ash-and-steam vents opened on the W and N flanks of the dome (figure 72). The venting episode was audible at times from the town of Salem and the surrounding areas. MVO noted the continued dome growth and the opening of these vents when on 31 August they raised the alert level to 4.

Heightened activity continued in September. The dome continued to develop substantially with a majority of growth on the W side. The vents that opened on 31 August remained active, with the vent above Gage's wall emitting a plume of hot gases and the N vent on the dome producing mainly ash-and-steam (figure 73). The opening of these vents coincided with high lava extrusion rates and consequent dome growth.

At 0100 on 10 September, the vent above Gage's wall became more vigorous throughout the day, broadening the vent and generating a wide vertical ash column. By 1300 the venting there became violent and explosive with black jets of ash rising ~ 100 m. Pyroclastic flows traveled down the Gages valley for ~ 1 km (figure 74). The vent formed a crater in the Gages wall, reducing its height compared to that of Chances Peak by 30-50 m. By 11 September, pyroclastic flows from vent emissions had ceased, but vigorous ash venting continued. At 0830 an overhanging lava lobe that developed on the NE collapsed sending a pyroclastic flow almost to the sea at the end of the Tar River valley.

Although volcanic tremor ended early on 16 September, an intense episode of volcanic tremor lasting just half an hour started at 1400 on 19 September. It was accompanied by intense rockfall activity giving rise to minor pyroclastic flows down the N and NE flanks of the lava dome. On 21 September the alert level was reduced to 3.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

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04/2007 (BGVN 32:04) Seismic activity continues at a reduced level through 1 June

Activity returned to normal levels following the strong explosive episode of 10 September 2006 (BGVN 31:09). Activity after September included an occasional minor explosions, rockfalls, minor pyroclastic flows, venting of ash and gases and steam with emissions reaching up to 3 km altitude, minor ashfalls, and mudflows during heavy rains. In September and October, the minor pyroclastic flows primarily moved down the N and NE flanks of the dome. In January, pyroclastic flows traveled down the Gages Valley, Tyres Ghaut, Belham Valley, Tuits Ghaut, Farrells Plain, and especially the lower Tar River Valley E of the volcano.

Lava-dome growth slowed in March, and by the end of April it appeared to have ceased. On 1 June Montserrat Volcano Observatory (MVO) (figure 75) warned that, while the lava extrusion had ceased and the dome may not be actively growing, it remains as a large mass of partially molten lava capable of collapsing or exploding. According to MVO, the amount of material above Tyres Ghaut to the NW was sufficient to generate pyroclastic flows and surges capable of affecting the lower Belham Valley and other areas.

Data provided by MVO (table 64) shows the elevated seismicity (hybrid earthquakes and rockfall signals) related to the increased activity in late August and early September (BGVN 31:09). The high number of long-period earthquakes in late June reflects the dome collapse at that time (BGVN 31:05). The dramatic decrease in long-period events and rockfalls in mid-March corresponds to the observed reduction in dome growth.

Table 64. Seismicity at Soufrière Hills between 16 June 2006 and 25 May 2007. * Data for the first 4 days only. VT: volcanic tectonic; LP: long-period. Courtesy of MVO.

    Report Date      Hybrid         VT             LP             Rockfall   Average SO2 Flux
    (2006-2007)      Earthquakes    Earthquakes    Earthquakes    Signals    (metric tons/day)
                    
    16 Jun-23 Jun     --             --              32            51          --
    23 Jun-30 Jun     54              4            1236           100          --
    30 Jun-07 Jul     17              6             448           194         593
    07 Jul-14 Jul      2              1              49            61         468
    14 Jul-21 Jul      9             --             341           293         523
    21 Jul-28 Jul     12             --             190           144          --
    28 Jul-04 Aug     --              2             162           166         120
    04 Aug-11 Aug      5              1             100           165         230
    11 Aug-18 Aug      8              1              69           253         222
    18 Aug-25 Aug    142             --             124           280         150
    25 Aug-01 Sep     30             12              61           588         351
    01 Sep-08 Sep    154              1              39           366         160
    08 Sep-15 Sep    210              5              38           413         405
    15 Sep-22 Sep     17              1              11           279         232
    22 Sep-29 Sep      1             --              21           383         450
    29 Sep-06 Oct     --              3              83           616         144
    06 Oct-13 Oct     --              1             107           585         150
    13 Oct-20 Oct     --              2             107           807          --
    20 Oct-27 Oct      2              2              88           732         356
    27 Oct-03 Nov      1             --             110           487         420
    03 Nov-10 Nov      1             --             162           346         520
    10 Nov-17 Nov     --              1             209           565         332
    17 Nov-24 Nov      1              1             124           452         845
    24 Nov-01 Dec     --              2             101           298         465
    01 Dec-08 Dec     --             --              81           121         524
    08 Dec-15 Dec     --             --               9           100         574
    15 Dec-22 Dec     --             --              29           257          --
    22 Dec-29 Dec      3              6             163           396         200
    29 Dec-05 Jan      3              3              22           231         152
    05 Jan-12 Jan     --              2              24           348         159
    12 Jan-19 Jan      1              1               2            52         156
    19 Jan-26 Jan     --              7              22            53         204
    26 Jan-02 Feb     --              2             101            57         213
    02 Feb-09 Feb     --              3              69           108         153
    09 Feb-16 Feb     --              3             127           370          --
    16 Feb-23 Feb     --              2             219           353         271
    23 Feb-02 Mar      1              1             189           608         157
    02 Mar-09 Mar     --             --             141           594         150
    09 Mar-16 Mar     --              3              61           383         157
    16 Mar-23 Mar      1              3               1           124         135
    23 Mar-30 Mar     --              8               5            16         158
    30 Mar-05 Apr     --             17               1            45        1035
    06 Apr-13 Apr     --             --               1             8        3114
    13 Apr-20 Apr     --             --               3             8         203*
    20 Apr-27 Apr     --             --               1             3         476
    27 Apr-04 May     --             --              --             9         223
    04 May-11 May     --             --              --             4         125
    11 May-18 May     --             --              --             2         143
    18 May-25 May     --              1              --             1         216

Strong activity during mid-September 2006. On 9 and 10 September, vigorous ash venting from the Gages Wall was accompanied by small explosions. Pyroclastic flows from fountain collapse occurred on all sides of the dome and reached 1 km W down Gages valley. On 11 September, the collapse of an overhanging lava lobe produced pyroclastic flows NE down the Tar River valley. One pyroclastic flow in the same area on 13 September reached the sea. On 14 September, vigorous ash venting resumed. Continuous ash and gas emissions during 13-19 September produced plumes that reached altitudes of 2.4-3.7 km. The Gages Wall vent continued to produce ash and gas emissions into mid-October.

Activity during September-December 2006. During 15 September-6 October the lava dome continued to grow at a moderate rate in the summit area and on the S and E sides of the dome. On 22 September the volume of the dome was about 80 million cubic meters. Lava-dome growth was concentrated on the NE part of the edifice from 6 October until 15 December, when growth moved to the SW part of the dome. A new E-facing shear lobe with a smooth, curved back enlarged during 13-20 October.

During 24 November-1 December, the two cracks in the curved back of the shear E-facing lobe on the summit propagated downward and divided the lobe into three blocks. The dome overtopped the NE crater wall and fresh rock and boulder deposits were observed in that region. During 22-29 December, lava-dome growth was focused on the W, where gas-and-ash venting occurred. A high whaleback lobe directed SW was observed on 26 December.

Aviation notices reported continuous ash and gas emissions almost every day from 15 September through 14 November, with plumes rising above 2 km to a maximum of 4.6 km altitude. Plumes extended 140 km W on 2-3 October. During 17-24 November, ash venting originated from the westernmost of two cracks in the curved back of the shear E-facing lobe on the summit. An explosion produced an ash plume that rose to altitudes of 1.5-1.7 km.

Pyroclastic flows occurred regularly as collapses from the dome sent material in all directions. Pyroclastic flows reached both the upper region of Tuitts Ghaut (N) and the sea via the Tar River Valley (E) on 23 November.

Activity during January-March 2007. Rapid lava-dome growth, pyroclastic flows, and ash venting increased during 3-9 January. Dome growth was concentrated in the NW, the highest part of the dome. Pyroclastic flows were observed in Tyres Ghaut (NW), Gages Valley (W), and N, behind Gages Mountain and accompanied by ash venting. On 4 January, simultaneous pyroclastic flows descended Tyres Ghaut and Gages Valley, and a resultant ash cloud reached an altitude of 2.5 km. The maximum distance for the Gages Valley flow was 4 km. During 6-9 January, distances of pyroclastic flows increased in Tyres Ghaut and possibly exceeded 1.5 km.

During 10-16 January, lava-dome growth was focused on the NW quadrant. During 10-11 January, one pyroclastic flow was observed to the W in Gages Valley and one to the NW in Tyres Ghaut. On 15 January, a relatively large pyroclastic flow traveled E down the Tar River Valley. After 15 January, measurable activity was low. Gas and ash venting that originated from the W side of the dome continued. A clear view on 22 January revealed that the collapse scar from the 8 January event was filled in. A small spine was noted on the W side. On 23 January, a large pyroclastic flow traveled down Gages Valley. The Washington VAAC reported that ash plumes were visible during 26-27 January. On 28 January, a large pyroclastic flow traveled down the Tar River Valley and reached the sea. A diffuse plume rose to an altitude of 1.5 km on 31 January.

During 7-13 February, growth of the lava dome continued on the W side, then was concentrated on the E and N sides for the rest of the month. The lava-dome volume in mid-February was estimated at 200 million cubic meters based on LIDAR data. Previous measurements over-estimated the lava-dome volume due to the perceived location of the dome and the lack of data from inside the crater. Small pyroclastic traveled in multiple directions throughout February. Moderate pyroclastic flows traveled down the Tar River Valley during 24-25 and 27 February. Continuous ash emissions were reported during 14 February-6 March, with plumes to altitudes of 2.1-6.1 km.

Lava-dome growth during 2-9 March was concentrated on an E-facing lobe topped with blocky, spine-like protrusions. Rockfalls affected the E and NE flanks. Pyroclastic flows traveled 2 km in the Tar River Valley. Heightened pyroclastic activity on 7 March resulted in an ash plume that rose to an estimated 2.4 km. On 11 March, a pyroclastic flow traveled down the NE flank into White's Ghaut.

During 9-26 March, lava-dome growth was concentrated on the NE side. Intermittent pyroclastic flows traveled E down the Tar River valley and produced ash plumes. One plume on 12 March rose to 3 km altitude. Pyroclastic flows were observed NW in Tyre's Ghaut and ashfall was reported from the Salem /Old Towne areas. During 23 March-3 April, dome growth apparently stopped.

MODIS thermal data indicated hot pixels at the dome and from pyroclastic flows on 24 March. Another thermal anomaly from a pyroclastic flow Tar River was detected on 29 March. No futher anomalies had been recorded by the HIGP Hotspot system through May. However, the Washington VAAC reported that a SW-drifting, diffuse plume and a hotspot were visible on satellite imagery on 2 April.

During 30 March-13 April, small, intermittent pyroclastic flows from the E-facing shear lobe occurred in the Tar River valley (figure 76). Incandescent rockfalls were seen at night during 5-9 April. On 17 April, a small pyroclastic flow was observed to the NW in the upper part of Tyres Ghaut. In mid-April MVO estimated that the lava-dome volume was about 208 million cubic meters.

The sulfur dioxide (SO₂) flux rate during 6-13 April was high, with an average value of 3,114 metric tons per day (t/d), well above the long-term average for the eruption. The previous week averaged 1,035 t/d, from a low of 71 to a high of 3,818 t/d. The three days from 8 to 10 April showed markedly elevated emissions: 3,550, 7,396 peaking at 7,471 t/d, whereas the remaining days' emissions were extremely low, some below 100 t/d.

During 13-20 April, material originating from the lava dome's E-facing shear lobe was shed down the Tar River Valley. A bluish haze containing sulfur dioxide was observed flowing down the N flanks on 18-20 April. Pyroclastic activity was ongoing on the E and NE sides of the dome during 27 April-4 May. After 4 May the overall structure of the dome changed very little. Low-level rockfall and pyroclastic-flow activity continued into late May.

Reference. Wadge, G., Macfarlane, D.G., Robertson, D.A., Hale, A.J., Pinkerton, H., Burrell, R.V., Norton, G.E., and James, M.R., 2005, AVTIS: a novel millimetre-wave ground based instrument for volcano remote sensing: J. Volcanology and Geothermal Research, v. 146, no. 4, p. 307-318.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Hawai'i Institute of Geophysics and Planetology, MODIS Thermal Alert System, School of Ocean and Earth Sciences and Technology (SOEST), University of Hawai'i, 2525 Correa Road, Honolulu, HI, USA (URL: http://modis.higp.hawaii.edu/).

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04/2008 (BGVN 33:04) Halt in dome growth during March 2007-May 2008

Our previous report on Soufrière Hills characterized the eruptive behavior and monitoring between 16 June 2006 and 25 May 2007 (BGVN 32:04). The current report describes activity between the end of May 2007 through May 2008.

Summary report. A report of a 14 and 16 April 2008 meeting by an advisory committee provides a convenient summary of recent behavior (SAC10, 2008). With minor stylistic changes, important paragraphs are quoted below.

The report indicated that by about mid-March 2007 the volcano stopped extruding dome lava. The authors said that since about October 2007 volcanism at the surface of the volcano has been at a very low level. Further, they noted, "Whilst there have been no major collapses of the dome, or explosions, rockfalls and minor pyroclastic flows [traveling E] into the Tar River Valley have occurred that have eroded the eastern side of the dome. However, the main mass of the 2006-2007 lava dome remains intact, and whilst it remains so it is capable of generating major pyroclastic flows for years to come. Also, the flow of gas continues to stream through the dome from the magma deep in the Earth, forming the visible plume.

"A lidar survey of the shape of the dome undertaken in March 2008 gave an estimate of 195 million cubic meters for the volume of the dome. This figure is within the bounds of uncertainty of the volume estimate of 203 million cubic meters derived from photogrammetry in April 2007.

"The three distinct lobes of lava at the summit of the dome, present at the end of lava extrusion in April 2007, remain. There have been a number of rockfalls and a few minor pyroclastic flows from the dome into the Tar River Valley. As a result of these, the uppermost part of the talus has been removed on the eastern side exposing a steep band of core lava below which a chute channels material to lower levels. Similarly, erosion of talus has begun to re-expose the buried northern crater rim.

"Gases escape from the dome in several areas. On the southern and northern talus slopes multiple gas vents release mainly water-rich gas. Sulphur deposits are evident around the southern vents. These locations have been a common feature for much of the eruption. On the western side of the upper dome, just inside the buried Gage's Wall, is a vent releasing a large flux of gas with a pale blue tint, indicative of sulphur dioxide. This vent formed in February 2006 and has been the source of weak ash generation in the past, roaring noises, and the cause of minor erosion of the Gage's Wall (September 2006).

"The low levels of rockfall seismicity seen in 2007 declined even further during 2008. There were two minor swarms of long-period earthquakes on 23 November 2007 and 28 January 2008, the latter being co-incident with roaring from the Gage's Wall vent. Volcano-tectonic earthquakes occurred between the surface and 4 km below the dome. These may be caused by stress changes around the conduit.

"The reference GPS line between the South Soufrière (SOUF) and old MVO (MVO1) receivers continued the same extensional trend that began when extrusion stopped in April 2007. This extension is slower than the equivalent contracting trend seen during lava extrusion, but is comparable to the extension measured during the first year of the last pause in activity. This pattern is confirmed by most of the other GPS stations and the EDM lines on the northern side of the volcano. This extension is consistent with an island-wide pattern of surface inflation due to the magma reservoir re-charging at depth. Any deformation due to the effects of surface loading by the dome dies away over a much shorter distance from the volcano than that being monitored between MVO1 and SOUF.

"The lack of any fresh, degassing andesite magma high in the conduit was confirmed by low measured HCl/SO₂ ratios. Sulphur dioxide was emitted at a rate above the long-term average (about 500 tonnes/day). Because several instruments in the measurement network have failed, there are some doubts about the absolute values, but a gradually increasing long-term trend seems real. This indicates not only that basalt degassing is ongoing, but also that the system may be becoming more permeable to deep gas loss or that gas production has increased. High values of sulphur dioxide measured by ground-based diffusion tubes to the west of the volcano have been recorded, as was also seen during the previous pause in 2005.

"Ongoing retrospective petrological analysis of the lava erupted over the last few years indicates that the amount of the basalt magma incorporated into the andesite lava that appears at the surface may be greater than previously appreciated. Understanding the mass balance of this interchange and being able to monitor it through time would help to understand the dynamics of the magma chamber.

"The current pause is 13 months long. Previous pauses have lasted 20 months (March 1998–November 1999) and 24 months (July 2003–August 2005). Despite the presence of a large dome, the "residual" surface activity now is far less than was the case during the first pause, when there was also a dome, and is much more like the second pause when there was no dome. The main difference between the first year of this pause and the first year of the second pause is the increasing trend of sulphur dioxide output in 2007-8. A few months prior to the ends of both previous pauses, the level of seismicity, and particularly long period seismicity, increased and there was a resumption of steam-rich explosions."

MVO and other reports. In accord with the summary above, the Montserrat Volcano Observatory (MVO) noted very low seismicity since May 2007. However, at the end of this reporting interval (May 2008), monitoring suggested that volcanic activity seemed headed for an upturn.

Despite the lack of dome growth (or dome destruction) during the entire period of this report the Alert Level remained at 4 (on a scale of 0-5). Authorities prohibited access to many areas near the volcano, including some areas ranging from 2 to 4 km offshore.

The Washington Volcanic Ash Advisory Center (W-VAAC) noted several ash plumes during mid-May 2007 through December 2007 (11 June, 22-28 August, 16 September, 12 October, 15-19 November) and 2008 (7 and 10 January, 10 April, 5 May, 13 -19, 23 and 29 May). Some of the plumes resulted from rockfalls (19 November, and 7 January).

Plumes on 11 June and 15-16 November may have reached 3.7 km altitude. Those on 13 and 29 May rose to 3 km altitude.

Pyroclastic flows were indicated in MVO reports for the May-December 2007 part of the reporting interval on at least 16 days. Particularly noteworthy were days with multiple pyroclastic flows, including 11 June (2), 23 August (4), and 29 November (4). The latter sequence of pyroclastic flows followed minutes after a regional M 7.4 earthquake. A 30 July pyroclastic flow traveled N for a 1.5 km runout distance.

Pyroclastic flows during January-May 2008 occurred on at least five days, and on one of those days, two occurred. One on 15 January had a 2 km runout distance. A pyroclastic flow on 29 May 2008 descended a few hundred meters to the W of the dome and was associated with the above-mentioned ash plume rising to 3 km altitude. An overflight the next day suggested that the explosion and pyroclastic flow originated from the Gages vent.

Lahars were indicated in MVO reports, often one or more per month and sometimes one or more per week, during the 2007-8 reporting interval, typically associated with heavy rains and fresh deposition. Lahars were numerous on 23 October 2007 (descending all drainages), vigorous around 25-26 October 2007, abundant the week of 13-19 February 2008, and noteworthy on 5 May 2008.

A photo shows the little-changing dome as it appeared on 7 January 2008 (figure 77). The photo emphasizes the dome's steep sides and craggy summit, as well as wide areas with emerging plumes. SAC (2008) noted that, although seemingly static, the dome is far from stable and large pyroclastic flows are possible from dome disruptions in the future. In the past, many of the pyroclastic flows traveled E. SAC (2008) noted the possibility (and discussed probabilities) for their transit from the dome towards the WNW along various areas just N of Plymouth.

According to MVO, the level of volcano-tectonic (VT) earthquakes at Soufrière Hills increased during the week of 25 April-2 May 2008, and was the highest since February 2006. During this week, degassing from a vent above Gages Wall was audible in the St. George's Hill area to the NW, and steaming from the area above Tyre's Ghaut to the NW was visible. Light ashfall was reported in the Old Town area about 9 km NW, and in other nearby areas.

During 9-19 May 2008, activity increased. On 13 May a single long-period earthquake occurred, accompanied by a blue sulfur-dioxide plume. An ash plume that rose to an altitude of 3 km drifted NW (dropping ash over much of Iles Bay, Belham, Old Town, and Olveston). Ash emissions from two areas in the Gages vent to the W were observed on 15 May, but may have started the previous day. The resultant ash plume rose about 200 m above the lava dome and drifted W. Both a small rockfall and gentle roaring noises were reported. A new fumarolic area was seen on the SE side of Chances Peak. Ash emissions from Gages vent continued on 16 May. During the week of 17-23 May, activity decreased slightly.

A weekly summary of seismicity and SO₂ fluxes between 25 May 2007 and 30 May 2008 is indicated in table 65. In addition to the rockfall data in the table, there was one long-period rockfall event during each of the weeks of 23-30 November, 4-11 January, and 11-18 January. The long-term SO₂ average is 550 tons/day.

Table 65. Soufrière Hills seismicity between 25 May 2007 and 30 May 2008. (Earthquakes; VT: volcanic- tectonic; LP: long-period. In the last two columns, "—" indicates data were not available.) Courtesy of MVO.

    Report Date      Hybrid (HB)        VT             LP         Rockfall      Avg SO2 flux
    2007-2008        earthquakes    earthquakes    earthquakes    signals     (metric tons/day)

    25 May-01 Jun         1              1             --             5             230
    01 Jun-08 Jun        --              1              1             5             175
    08 Jun-15 Jun        --              1             --            10             288
    15 Jun-22 Jun        --              1              1            10             165
    22 Jun-29 Jun        --              1             --             3             203
    29 Jun-06 Jul        --              1              1            10             200
    06 Jul-13 Jul        --              1             --             4              --
    13 Jul-20 Jul        --              1              1             6             300
    20 Jul-27 Jul        --              3             --             5              --
    27 Jul-03 Aug        --              2              2            11             639
    03 Aug-10 Aug        --              2             --             5              --
    10 Aug-17 Aug        --              2             --             4             818
    17 Aug-24 Aug        --              4             --             4             509
    24 Aug-31 Aug        --             13              1            17             740
    31 Aug-07 Sep        --              1              1             7             575
    07 Sep-14 Sep        --              5             --             6             688
    14 Sep-21 Sep        --             12             --             8              --
    21 Sep-28 Sep        --              4             --             9             300
    28 Sep-05 Oct        --              1              2             3             384
    05 Oct-12 Oct        --             --             --            10             508
    12 Oct-19 Oct        --              5             --             3             691
    19 Oct-26 Oct        --             --              1             9             518
    26 Oct-02 Nov        --             --             --             9             618
    02 Nov-09 Nov        --             12             --            16             596
    09 Nov-16 Nov        --              2             --            11             698
    16 Nov-23 Nov        --             --             20             7             685
    23 Nov-30 Nov        --             --             46             4             868
    30 Nov-07 Dec        --             --             --             4             405
    07 Dec-14 Dec        --              1             --             2             811
    14 Dec-21 Dec        --              9             --             2             865
    21 Dec-28 Dec        --              4             --             8             861
    28 Dec-04 Jan        --              1             --             2             615
    04 Jan-11 Jan        --              8              1             2             513
    11 Jan-18 Jan        --             13              2             3             568
    18 Jan-25 Jan        --             --             --             2             734
    25 Jan-01 Feb        --              3             25            --             468
    01 Feb-08 Feb         1              3             --             2             881
    08 Feb-15 Feb         1             --             --             1           1,004
    15 Feb-22 Feb         1             --              1             1             872
    22 Feb-29 Feb         4             --             --            --             972
    29 Feb-07 Mar         1              1             --            --             824
    07 Mar-14 Mar        --              4             --             3             766
    14 Mar-21 Mar         2              2             --             3           1,070
    21 Mar-25 Apr                            No data
    25 Apr-02 May        --             48              3             1             574
    02 May-09 May        --             10              5             5             630
    09 May-16 May        --             25              1            17             506
    16 May-23 May        --              3              2            11             653
    23 May-30 May        --              8              2            10              --

Since 2002, MVO has been monitoring the SO₂ emission rate in real-time, with spectra telemetered back to the observatory from an array of three fixed, scanning UV spectrometers. MVO has also calculated the HCl:SO₂ ratio by measuring the HCl emission rates indirectly using an open-path Fourier Transform Infrared spectrometer (FTIR). These ratios may be used to evaluate changes in the eruption rate and dome growth. Such mass ratios determined since August 2007 ranged from 0.28 to 0.46, with one ratio at 0.67 (during 9-16 November 2007).

MVO's weekly report for the third week of May states "observations show continuing unrest ... with a gradual increase over the last few weeks. The events of this week suggest that fresh magma is rising beneath the dome. There is now a distinct possibility that lava extrusion will start from the Gages vent without any warning. If this happens, it will probably not be long before there are small pyroclastic flows to the W. Even if lava extrusion does not restart, the dome is still a very large mass of very hot material which is capable of collapsing or exploding at any time."

Seismic signals. Five main seismic signal types have been recognized at many volcanoes, including Soufrière Hills. These include volcano-tectonic (VT) earthquakes, long-period (LP) earthquakes, hybrid earthquakes, rockfall or pyroclastic flow signals, and explosion signals. McNutt (2000) presents illustrations of characteristic seismic traces.

MVO defines a VT earthquake as having an impulsive (i.e., large amplitude) start and then rapidly decreasing in amplitude. These earthquakes often appear in swarms and are predominantly high-frequency signals (over 2 Hz). They are interpreted as due to rock fracturing.

An LP earthquake, as defined by MVO, has a more emergent start (i.e., amplitude growing with time) and generally low, narrow-band frequency content (1-2 Hz). These are interpreted as the result of signal resonance due to gas or magma inside the volcanic conduit.

MVO defines a hybrid (HB) earthquake as a mixture between VTs and LPs; hence they tend to have impulsive starts but contain significant amount of low-frequency signal. They are thought to represent magma forcing its way to the surface. These signals are often associated with periods of rapid dome growth, and are sometimes precursors to major dome collapses or switches in the direction of lava extrusion at the surface. These signals often merge into continuous tremor, which sometimes occurs in bands spaced 4-24 hours apart.

According to MVO, rockfall or pyroclastic flow signals have often been a dominant type of seismic signal recorded here (e.g., table 65). They have an emergent start and a gradual tapering towards the end of the signal and a wide frequency range. They are interpreted as being due to material falling off the dome and traveling down the flanks. Pyroclastic flow signals are similar to those of rockfalls but are generally of longer duration and higher amplitude.

Pyroclastic deposits in the ocean. Trofimovs and others (2006) reported that more than 90% of the pyroclastic material erupted at Soufrière Hills has been deposited in the ocean. The authors describe the characteristics of the deposits at different distances from shore. The coarse material forms steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer materials form turbidity currents that flow to distances greater than 30 km from the shore.

MVO management. For almost 10 years the British Geological Survey (BGS) managed MVO. Beginning 1 April 2008, this service shifted to the Eastern Caribbean's two major geo-hazard organizations, the Seismic Research Unit (SRU) of the University of the West Indies, Trinidad and Tobago and the Institut de Physique du Globe de Paris (IPGP), France. The SRU carried out long-term monitoring prior to the 1995 eruption episode. They were assisted by others as the eruption began. A statement on the new situation included the following paragraph.

"The SRU monitors earthquakes and volcanoes for most of the English-speaking Eastern Caribbean countries. The IPGP has volcano observatories on Martinique and Guadeloupe, i.e. the main French-speaking Antilles. Island arcs such as the Lesser Antilles are regions where complex real-life hazards exist, not only the better known volcanic eruptions, but also the generation of a tsunami by a submarine earthquake or a volcanic landslide. The linking of these two research institutions will provide greater opportunities for studying volcanism and earthquake activity at arc-scale rather than the scale of individual islands, a logical and innovative step towards disaster risk reduction regionally and globally."

Reference. McNutt, S.R., 2000, Volcanic seismicity, in H. Sigurdsson (ed), Encyclopedia of Volcanoes, Academic Press, San Diego, p. 1015-1033.

Trofimovs, J., Amy, L., Boudon, G., Deplus, C., Doyle, E., Fournier, N., Hart, M.B., Komorowski, J.C., Le Friant, A., Lock, E.J., Pudsey, C., Ryan, G., Sparks, R.S.J., and Talling, P.J., 2006, Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995-2003): What happens when pyroclastic flows enter the ocean?: Geology, v. 34, no. 7, p. 549-552.

SAC10, 13 May 2008, Assessment of the hazards and risks associated with the Soufriere Hills volcano, Montserrat, Tenth Report of the Scientific Advisory Committee on Montserrat, Volcanic Activity, based on a meeting held between 14 and 16 April 2008 at the Montserrat Volcano Observatory, Montserrat (Part I: Main Report), 23 pp. (URL: http://www.mvo.ms/).

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

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10/2008 (BGVN 33:10) Dome collapse and eruption on 28 July, followed by renewed dome growth

Our previous report on Soufrière Hills (BGVN 33:04), characterized the eruptive behavior and monitoring of dome growth during March 2007-May 2008. The current report describes activity from the end of May 2008 through 4 December 2008.

Through the end of May, the Montserrat Volcano Observatory (MVO) generally reported continued pause in dome growth and low seismicity. An explosion on 29 May produced an ash plume that rose to an altitude of ~ 3 km and drifted SW; a pyroclastic flow descended a few hundred meters to the W. Aerial observation the following day suggested that the activity originated from the Gages vent. The explosion, which had no precursory seismicity, was heard in multiple areas to the NW.

Throughout June and the first three weeks of July, the background activity, while low, indicated continuing unrest. The pause in dome growth continued, but MVO emphasized that despite the lack of substantial lava extrusion, the dome remained hot and hazardous.

Mild ash was ejected from the Gages vent on 19 June. The event lasted for ~ 2 hours, and included several pulses. Due to strong E winds at low altitudes, the ash plume remained below ~ 1,200 m altitude and left Old Towne and Olveston untouched.

During mid- and late June, sporadic heavy rainfalls triggered minor mudflows down the Belham River. Access was accordingly prohibited. Also, a Maritime Exclusion Zone kept boats away from the island's S shore.

On 21 July, four mild eruptions occurred, typically venting ash or pyroclastic flows. The previous day there had been a large swarm of shallow volcano-tectonic earthquakes beneath the volcano and seismicity proceeded continuously throughout each of the first three eruptions. The first three eruptions lasted for about 50, 40, and 75 minutes, respectively. The third one generated the largest seismic signals, the fourth event was much smaller. Volcano-tectonic and hybrid seismic activity continued for the rest of the week without significant reduction. Small pyroclastic flows from collapses in the eroded chute on the dome's SE and E flanks traveled down the Tar River valley, with the largest reaching to within 500 m of the sea.

All four events generated ash columns rising more than 2 km. The first two events also generated ash clouds above the upper Tar River valley, probably caused by small pyroclastic flows. Ash clouds drifted W over Plymouth and St George's Hill; the source of the ash was probably the vent at Gages. Light ashfall occurred in parts of Old Towne. Rumbling, continuous at times, was heard in Salem, Old Towne and Olveston during each of the first three events. Lightning strikes could also be heard and sometimes seen. These events were most probably caused by ash venting from the lava dome, accompanied by small collapses on the E flank of the dome; however, there was no apparent change in the lava dome's shape.

After 20-25 July, seismicity increased significantly. On 26 July, a series of hybrid earthquakes slowly increased in both numbers and magnitude, eventually reaching about 15 events per hour. Seismicity decreased for a few hours, then increased again. Hybrid earthquakes with a few long-period events peaked at a rate of more than one per minute.

On the morning of 27 July, a short series of eruptions occurred. The first eruption generated a non-energetic ash column that rose ~ 2.5 km; the source of ash could not be seen due to cloud cover, but was probably the Gages vent. The ash cloud was blown to the W and NW, and there was ashfall in Plymouth and St George's Hill; pyroclastic flows were absent. Two other eruptions during the next 45 minutes were much smaller, with ash clouds below 1.5 km altitude. Seismicity continued at a slightly reduced level following these eruptions.

28 July dome collapse. On 28 July the seismic signals built up gradually over a few minutes, signals interpreted as consistent with a dome collapse rather than an explosion. Seismicity then displayed a series of sharp peaks consistent with explosive activity, but this activity stopped within about an hour. Next, there was a partial collapse on the dome's W side. A few explosions issued from the dome during the collapse. An infrasound sensor on St Georges Hill, which records low frequency sound waves, recorded a clear explosion signal that coincided with the largest peak recorded in the seismic signals.

The collapse generated three pyroclastic flows that traveled down the flanks. The largest, from the Gages area, split into two and traveled to Lee's Yard and Plymouth. A pyroclastic flow in Plymouth also split into two as it diverted around Round Hill, with a pyroclastic surge traveling over the top of the hill.

The two lobes of this W-traveling pyroclastic flow traveled almost to the sea, with one reaching the old Police headquarters and the other reaching the Pentecostal Church and the old Government House. This pyroclastic flow set fire to trees and vegetation on Gages Mountain, the lower flank of St George's Hill, and some buildings in Plymouth.

Another pyroclastic flow emerged from the channel created by erosion on the dome's SE flank. It descended E into the Tar River valley and traveled as far as the old Montserrat coastline. A much smaller flow followed a gully cut in volcanic material choking the upper White River; this flow only reached ~ 2 km or less from the dome.

Post-eruption examination of the deposits found that the pyroclastic flows at the Tar River appeared to contain significant amounts of old dome material, which would reflect the partial dome collapse. In contrast, the pyroclastic flows at both Plymouth and White River contained mainly juvenile pumice, material thought to have risen some distance in a plume.

The material collapsed from the dome on the 28th occupied a volume of ~ 200,000-300,000 cubic meters. Satellite radar images indicated that the vent above Gages wall was enlarged by the explosion to ~ 150 x 60 m, elongated E-W. MVO interpreted the 28 July eruption as caused by input of new magma, possibly triggered by the partial collapse of existing dome material.

MVO stated that the 28 July eruption generated a large ash column and the fallout of airborne pumice in nearby communities. The ash column reached a maximum altitude of ~ 12 km and drifted primarily NW. While almost no ash fell on inhabited areas near the volcano, there were reports of ashfall from St Croix, Puerto Rico, and Guadeloupe. Satellite sensors indicated the release of at least 2,000-3,000 tons of sulfur dioxide (table 66). Two minor eruptions on 29 July generated small ash clouds. During the period of this activity, the Washington VAAC published numerous advisories for aviation (table 67).

Table 66. Sulfur dioxide emission were almost continuous and appear here as weekly averages and Minimum/ Maximum values. SO2 values in tons/day. Courtesy of MVO.

    Dates (2008)     Average SO2    Minimum    Maximum

    31 May-06 Jun       206            --          --
    07 Jun-13 Jun       228           161        294
    14 Jun-20 Jun       254           201        347
    21 Jun-27 Jun       323           256        472
    28 Jun-04 Jul       329           276        440
    05 Jul-11 Jul       339           242        564
    11 Jul-18 Jul       414           243        561
    18 Jul-24 Jul       378           216        794
    25 Jul-01 Aug        --             --          --
    28 Jul           2-3,000 tons SO2 released during the eruption
    01 Aug-08 Aug     1,121           671      2,069
    09 Aug-15 Aug     1,016           364      1,791
    16 Aug-22 Aug     1,122           274      2,033
    23 Aug-29 Aug       466           239        758
                     (3 days)
    30 Aug-05 Sep       --              --          --
    06 Sep-12 Sep     1,422           562      4,599
    13 Sep-19 Sep       989           657      1,217
    20 Sep-26 Sep     1,239            --          --
                     (2 days)
    26 Sep-03 Oct       840           463      1,523
    03 Oct-10 Oct       522           201        968
    10 Oct-17 Oct        --             --          --
    17 Oct-24 Oct       531           277        668
    25 Oct-30 Oct     1,283           689      2,540

Table 67. Washington VAAC advisories as a result of ash plumes from Soufrière Hills during 21 July 2008-20 October 2008. All reports were based on GOES-12 satellite source information.

    Date      Time     Altitude    Drift    Remarks
              (UTC)

    21 Jul    1145     ~ 1.8 km      W
    21 Jul    1315     ~ 1.8 km      --      Plume to ~ 2 km; 16 km wide.
    21 Jul    1915     ~ 1.8 km    W, S
    22 Jul    0108        --          --      Ongoing emissions
    22 Jul    0708     ~ 1.8 km      W      Intermittent low level ash emissions
    22 Jul    1245     ~ 1.8 km      W      Reduced seismicity
    22 Jul    1845     ~ 1.8 km      W
    27 Jul    1345     ~ 2.4 km      W      Small bursts of venting gases and ash; seismic
                                              signals increased
    27 Jul    1945     ~ 2.4 km      W      Dome collapse event began
    29 Jul    0415     ~ 12 km       W      Partial dome collapse on dome's W flank, accompanied
                                              by several explosions that generated ash plumes.
                                              Highest ash level noted at about 0340 UTC moving
                                              ESE.
    29 Jul    0340     ~ 12 km      SE      Weak hotspot in multi-spectral satellite data
    29 Jul    1215     ~ 7.6 km     NW      Partial dome collapse 0327 UTC
    29 Jul    1815     ~ 7.6 km     NW
    30 Jul    0615     ~ 2.7 km     SW      Low-level emissions
    30 Jul    1215     ~ 2.7 km     NW      Thin low level plume
    30 Jul    2345     ~ 7.6 km      W      Residual ash and ongoing summit emissions
    20 Oct    1415     ~ 5.5 km     NW      Ash associated with a pyroclastic flow

Monitoring dome shape and finding new rockfall material. The 6 August MVO report noted that the only significant change in the past few months occurred in the area of the Gage's Wall vent. That area was the source of ash and mild explosive activity in the last few months. During the first weeks of August seismicity was relatively low.

X-band radar images of the dome (figure 78) taken from different sides allow comparisons between 9 October 2007 and 1 August 2008. Images such as these help MVO interpret changes in topography and other features such as the surface texture of pyroclastic flows. Radar images provide data not available using optical techniques such as aerial photography or satellite imagery. For example, this image illustrates an effect called layover, where topography appears to lean. The images are also quite sensitive to the moisture content (affecting conductivity) and roughness of the ground surface (which scatters the radar energy).

In the radar images color channels represent different points or intervals of time: red, 9 October 2007; green, 1 August 2008; and blue, the difference between those two dates. The result is that yellow areas depict terrain unchanged between those times. Areas of magenta had rougher surfaces in 2007 than 2008; areas that are cyan had rougher surfaces in 2008 than in 2007.

A new lava extrusion started from the W side of the lava dome sometime between the 28 July dome collapse event and 8 August when a new channel of fresh rockfall material was seen below Gages Wall.

On 14 August the dome's W side was visible and observers noted that the explosion crater of 28 July was almost filled with new lava and lava had spilled over the lower and W side of the crater and generated rockfalls.

On 8 August, the Government of Montserrat instituted a new Hazard Level System, which replaces the Alert Level system. The system divides the southern two-thirds of the island into six zones, and includes two Maritime Exclusion Zones. Access into each of the zones is restricted depending on the Hazard Level assigned (1-5); the current level has been set at 3.

During the week of 15-22 August, MVO found evidence of increased growth of the dome's W side. Earthquakes and rockfalls increased. Rockfalls occurred on the dome's W side in a new channel below Gages Wall. Ash plumes occasionally generated by the rockfalls were most noticeable on 16 and 17 August. On 19 August a pyroclastic flow again descended the Tar River Valley. According to news reports, on 25 August a rainfall-induced pyroclastic flow on the W flank split into two parts and caused ashfall to the N. The event enlarged and steepened the rockfall gully below Gages Wall. Lahars likely descended the Tar River Valley on 29 and 31 August.

On 1 September, a lahar descended the Belham River valley to the NW; the event lasted ~ 50 minutes. A new vent was observed on the NW part of the lava dome, a little further N of the Gages vent. Incandescence was also observed at a scar on the lava dome and in an area N of the scar. Rockfalls descended the W side of the dome. MVO reported that seismicity continued at a low level and dome growth continued throughout September.

During October, slow growth on the W side of the dome was accompanied by mudflows. As a result of slow and continuous erosion of the lower part of the dome, occasional rockfalls occurred on both the W side in the gully over Gages Wall and on the E side in the Tar River Valley.

One notable volcano-tectonic event occurred on 5 October in coincidence with the arrival of seismic waves from a M 6.6 earthquake in central Asia. Although the rate of lava extrusion had declined significantly, thermal imagery captured during an overflight on 8 October revealed that a major E-W oriented fracture in the dome, aligned with Gages valley and extending vertically over a few tens of meters, was associated with very elevated temperatures. Several other very hot areas on the dome were visible as points of incandescence that night. Also on 8 October, mobile, hot lahars were observed in Plymouth near the Pentecostal Church. This indicated that the 28 July pumice flows were still very hot.

Toward the middle of October, activity was low and consisted mainly of mudflows spurred by tropical storms that evolved to become hurricane Omar. Strong headward erosion affected the dome's talus slope on the Tar River side. A large gap developed in the talus, exposing the dome's core and forming a large vertical cliff.

Between 10-17 October, instrumentation recorded five long-period, five hybrid, and one volcano-tectonic earthquakes and only two rockfalls. By the third week of October, activity had increased slightly. Seismicity for the week consisted of 22 long-period, eight hybrid, and eight volcano-tectonic earthquakes, and two rockfalls. Incandescence was again observed from MVO on 17 October. On 20 October, three small pyroclastic flows descended to the Tar River Valley, generating small ash clouds that drifted over unpopulated areas to the W and SW. These pyroclastic flows were probably caused by the slightly increased seismic activity and continued interaction of the hot dome with water from the intense rainfall following passage of hurricane Omar. As of 24 October, there was no evidence of ongoing lava extrusion. Through the end of October, activity was at a low level. MVO recorded only four rockfalls, two long-period rockfalls, and one volcano-tectonic event. Several mudflow signals were also recorded during periods of heavy rainfall. Limited observations on 26 October confirmed that a few small pyroclastic flows traveled ~ 1.5 km E on the Tar River side.

Headward erosion continued along several V-shaped chutes at the base of the dome on both the dome's Tar River and SE sides. A small pyroclastic flow descended the Tar River (runout of ~ 1 km) on 27 October; it generated small ash clouds that drifted over unpopulated areas to the W, and to the SW. On the dome's W flanks, the talus pile on the Galways side developed a well-incised network of gullies leading into the White River.

On 2-5 December a series of explosions took place without clear seismic precursors. The first was the largest; MVO reported that incandescent blocks were ejected to 1 km from the dome's Gages vent. Pyroclastic flows began within 15 seconds of the first explosion's start at 0935 local time. They soon set vegetation and a few buildings into flames at Plymouth, and some of the fires continued for hours, one into the next day. The flows appeared devoid of pumice and were thought to be composed mainly of hot dome material. The event was judged smaller than the one on 28 July 2008, although the plume rose to over 10 km. The accompanying ash columns became the path for lightning strikes. Inhabited areas remained free of ash, which blew W. As of early December scientists had not assessed the impact of the 2 December events to the dome.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Caribbean Net News (URL: http://www.caribbeannetnews.com/).

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10/2009 (BGVN 34:10) A lull during late 2008 and intermittent high activity during late 2009

Dome collapse at Soufrière Hills (figure 79) and an eruption on 28 July 2008 was followed by dome regrowth (BGVN 33:10). During October through 4 December 2008, low-level activity included occasional earthquakes and explosion, lahars, and small pyroclastic flows. The current report describes activity from 5 December 2008 through 10 December 2009, primarily based on information provided by the Montserrat Volcano Observatory (MVO). Activity was intermittent during 2009 (table 68), with high-level activity resuming in November and continuing through at least 11 December.

Table 68. Ash plumes or plumes that may have contained ash from Soufrière Hills between 5 December 2008 and 2 December 2009. Courtesy Washington Volcanic Ash Advisory Center (VAAC), based on analysis of satellite imagery, information from MVO, and pilot reports.

    Date              Ash plume height,            Remarks
                      flow direction

    13 Dec 2008       4.6-5.2 km 
    14 Dec 2008       1.8 km, W 
    15 Dec 2008       2.4-3 km, SW
    16 Dec 2008       S                            Thermal anomaly
    19-23 Dec 2008    4.3 km, various              Thermal anomalies on 19 and 21 Dec
    24 Dec 2008       3 km                         Caused by pyroclastic flow
    26-30 Dec 2008    2.1-4.9 km,                  Thermal anomalies on 27 Dec
                        various (28 and 30 Dec)    Thermal anomaly
    03 Jan 2009       2.4-10.7 km, various
    04 Jan 2009       W, WSW
    25 Feb 2009       W                            Caused by pyroclastic flow
    06 Apr 2009       2.7-4.9 km, NW
    24 May 2009       --                           Caused by pyroclastic flow
    04-06 Oct 2009    3.4-5.5 km, W, WNW
    23-25 Oct 2009    W                            Caused by pyroclastic flows
    24 Nov 2009       6.1 km                       Caused by pyroclastic flows
    02 Dec 2009       4.6-6.1 km                   Caused by pyroclastic flows

Activity during December 2008. Subsequent to the four explosions between 2-5 December 2008, the MVO reported that seismicity from the lava dome remained elevated. The volcano continued to inflate and discharge lava and ash during December 2008. Frequent pulses of ash rose from multiple places on the NW face of the lava dome and from a low on the dome behind Gages Mountain (as seen from Salem). A series of pyroclastic flows and rockfalls descended the Gages Valley and other valleys during December 2008, at least two reaching Plymouth (~ 5 km W). Significant lava dome growth on the SW flank was observed. Photographs showed that most of the growth had taken place since 8 December; lava was filling in the area between the lava dome and Chance's Peak. Initial calculations suggested that the dome grew at a rate of 1 m³/s during this time.

During the last two weeks of December 2008, the lava dome was characterized by increased lava extrusion, rockfalls, and pyroclastic flows. Lava extrusion on the N, W, and SW sides of the dome continued and incandescence on the dome was visible at night when weather was favorable. On 22 December, the Hazard Level was increased to 4 (on a scale 1-5) due to the repeated occurrences of pyroclastic flows in the lower part of Tyers Ghaut.

On 24 December, a large pyroclastic flow that reached Plymouth, and possibly the sea, generated an ash plume that rose to an altitude of 3 km. Ashfall was reported in areas 6-7 km NW. Large incandescent blocks, deposited by rockfalls and pyroclastic flows, were visible on multiple occasions at night in the lower parts of Tyers Ghaut. Fires triggered by surges were visible in the neighboring valley.

Activity during January-May 2009. On 2-3 January 2009, activity from the lava dome increased drastically. On 2 January, an energetic pyroclastic flow and associated surge traveled down Tyers Ghaut (NW) and reached the upper part of Belham River. On 3 January, after a period of elevated seismicity, two explosions produced large ash plumes that rose to altitudes greater than 10.7 km. Ashfall affected most of the island at elevations of 1.2 km and above. The explosions had significant "jet components" that rose to at least 500 m above the dome. In-column collapses resulted in pyroclastic flows that traveled W and reached Plymouth (~ 5 km W). According to news articles, about 70 people were evacuated from an area about 6-8 km NW.

According to MVO, the level of seismic activity decreased dramatically after 3 January. It increased slightly in early February, with occasional rockfalls, and several small pyroclastic flows. On 19 February 2009, the Hazard Level was lowered to 3. Seismic activity remained low during March through May. Occasionally, lahars caused by heavy rainfall descended through multiple river valleys. Thermal images of a pyroclastic flow on 25 February 2009, and other videos, can be viewed on the MVO YouTube channel (http://www.youtube.com/user/montserratvolcanoobs).

In mid-May 2009, activity from the Soufrière Hills lava dome increased slightly, but generally remained at a low level. Tectonic earthquakes were noted on 16, 18, 20, and 21 May at depths less than 3 km beneath the lava dome. Two possible explosions were detected on 21 May. The second and larger signal was followed by an ash plume that drifted W over Gages Mountain. During 21-22 May, a strong smell of sulfur dioxide was noted from Salem (6 km NW) to Woodlands (1 km N of Salem). Heavy rainfall caused erosion of the lava dome and hot pyroclastic flow deposits; steam plumes occasionally laden with ash occurred periodically from the base of Tyre's ghaut. Lahars traveled down multiple river valleys on 18 May.

Activity during May-December 2009. Between the latter part of May and 4 October 2009, activity remained low with only periodic rockfalls and small pyroclastic flows. On 4 October 2009, a short volcano-tectonic earthquake swarm from the Soufrière Hills lava dome was detected. A period of tremor and vigorous ash venting followed about an hour later. The resulting ash plume drifted WNW across the island and out to sea, causing ashfall in Old Towne and Olveston. The seismic signals indicated no explosive activity or pyroclastic flows, but only two rockfalls after the ash-venting event. On 5 October, intermittent ash venting continued (figure 80), and ash fell S of inhabited areas. Early on 7 October, the ash-venting events from the lava dome ceased after a total of 13 had occurred. The last three were associated with small pyroclastic flows that traveled about 500 m down Tyers Ghaut to the NNW.

By mid-October 2009, activity from the Soufrière Hills lava dome rose to a high level. A new lava dome, first reported on 9 October, continued to grow. The new lava dome summit was about 60 m above the old dome structure. Seismicity was high and cycles of low-level tremor occurred at regular intervals. Over 1,200 rockfalls were detected and pyroclastic flows traveled down every major drainage valley except the Tar River valley to the E, resulting in ash plumes (figure 81). Heavy rainfall caused a lahar in the Belham Valley to the NW on 14 October. On 16 October, several large pyroclastic flows descended the White River to the S and reached the sea. Moderate-sized pyroclastic flows traveled 3 km NE down Tuitts Ghaut and White Bottom Ghaut, and a few smaller pyroclastic flows descended Tyers Ghaut to the N. Extensive ash clouds rose to an altitude of 6 km and drifted WNW, resulting in multiple minor ashfall in inhabited areas. Venting on 6 October 2009 can be seen on the YouTube channel for the Government Information Unit of Montserrat (http://www.youtube.com/user/GIUGOV). Lahars traveled NW down the Belham valley.

During the last week of October 2009, seismicity decreased slightly. However, numerous pyroclastic flows, some of which produced ash plumes, occurred in most of the major drainage valleys. Rockfalls were concentrated in the S. Heavy rainfall continued to cause lahars in the Belham Valley. On 29 October, a 40-m-high spine was seen protruding from the summit. Changes in lava-dome morphology seen on 30 October, and occurrences of pyroclastic flows traveling NE, indicated that growth was concentrated in the central part of the lava dome.

By 30 October, activity was again at a high level. Hybrid earthquakes were recorded for the first time since the renewal of activity in early October. Numerous pyroclastic flows occurred in most of the major drainage valleys. The frequency of pyroclastic flows increased on 5 November and particularly vigorous flows occurred in Tuitt's Ghaut to the NE. Ash fell in inhabited areas on a few occasions. Lahars descended the Belham Valley several times. Good views of the lava dome on 9 and 10 November revealed that recent lava-dome growth was concentrated on the WSW side, immediately NE of Chances Peak; intense incandescence and rockfalls were noted at night. Ash fell across the Montserrat on 11 November, and about 6-8 km NW in Salem, Old Towne, Olveston, and Woodlands on 12 November. One pyroclastic flow nearly reached the sea at Kinsale village (WSW).

By mid-November, activity from the Soufrière Hills lava dome consisted of ash venting along with semi-continuous rockfalls and pyroclastic flows that were concentrated on the W flank. Ashfall occurred across many areas of the island. On 19 November, heavy ashfall occurred to the NW between Old Towne and Brades. Views of the lava dome on 16 November showed that the dome height had decreased because of collapses and that a deep channel had developed NE of Chances Peak. Pyroclastic flows in the Gages Valley (W) continued down Spring Ghaut and Aymer's Ghaut, and spread onto the alluvial fan below St. Georges Hill.

On 21 November 2009, activity returned to a high level. Periods of tremor were detected on 23 November. Lava extrusion during this period shifted from the W side of the lava dome to the summit region. As a result, abundant pyroclastic flows traveled NE down Tuitt's Ghaut on 23 November for the first time in several weeks. On 24 November there was a period of 120 minutes of continuous pyroclastic flow activity, followed by 90 minutes of semi-continuous activity. The pyroclastic flows traveled W down Gages Valley and into Spring Ghaut, and NE down Tuitt's Ghaut and Whites Bottom Ghaut reaching Tuitt's village. Associated ash plumes rose to an altitude of 6.1 km. On 26 November, a pyroclastic flow that descended the Tar River valley was caused by collapse of part of the old, pre-2009 lava dome. Ashfall occurred in Old Towne and parts of Olveston. Incandescent material seen in a photograph taken at night on 29 November traveled down the flanks of the lava dome in several areas.

High-level activity from the lava dome continued through the first half of December 2009. Dome growth was concentrated on the N side, which has led to approximately 100 m of lateral growth of the lava dome in a northward direction. This growth has increased the available material for the formation of pyroclastic flows. Pyroclastic flows down the N flank became more abundant and their runout distance steadily increased. Pyroclastic flows also occurred to the NE and W, and one reached within 200 m of the sea. Ash vented from the S part of the lava dome.

On 10 December 2009, a large pyroclastic flow traveled down Tyers Ghaut. This pyroclastic flow reached to below the west end of Lees village in the Dyers river, some 3.5 km from the lava dome. This event prompted MVO to raise the Hazard Level from 3 to 4. The higher Hazard Level signifies that larger pyroclastic flows moving down the Belham valley are a more likely possibility. According to MVO, larger pyroclastic flows could be formed by a partial dome collapse which could involve several million cubic meters of material. Helicopter observations have shown that the head of Tuitt's Ghaut down to the junction with Whites Bottom Ghaut is full of pyroclastic flow deposits such that there is now a continuous surface across from Farrell's plain. The head of Tyers Ghaut is also now nearly full. This means that future pyroclastic flows are likely to be less confined by topography and will spread more readily across the N flanks of the volcano.

Thermal anomalies. MODIS satellite imagery recorded many thermal anomalies during December 2008, a smaller number in January 2009, none in February 2009, one in March 2009, and none during April through 10 October 2009. Beginning on 11 October through 11 December 2009, MODIS recorded a large number of thermal anomalies.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/VAAC/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://hotspot.higp.hawaii.edu/); U.S. Air Force Weather Agency (AFWA)/XOGM, Offutt Air Force Base, NE 68113, USA (Email: Charles.Holliday@afwa.af.mil).

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03/2010 (BGVN 35:03) Lava dome growth continuing; pyroclastic flows reached the ocean

Montserrat Volcano Observatory (MVO) reported a strong increase in dome growth at Soufrière Hills (figure 82) and energetic explosive activity, including pyroclastic flows and substantial ash clouds, during the 6 months ending early April 2010 (the end of this reporting interval). Energetic extrusions were particularly noteworthy during January and February 2010 (table 69). From mid-December 2009 through early April 2010 there was continuing seismicity and gas emissions (table 70) as well as weekly ash emissions and pyroclsatic flows (table 71). Partial dome collapse on 11 February 2010 led to a plume that rose to ~ 15 km altitude.

Table 69. Key features of the five Vulcanian explosions that occurred at Soufriere Hills in January and February 2010. Units in valley columns are pyroclastic-f low runout distances in kilometers. From Cole and others (2010) with due credit to Washington Volcanic Ash Advisory Center (VAAC) for satellite and aviation-based plume altitude estimates.

    Date (2010)    Time(local)    Lapilli fallout       Plume

    08 Jan         1449-1500      No: Ash from PFs      7.6 km (25,000 ft)
          Runout Distances
            White's Bottom Ghaut        4.7 km
            Tar River Valley            2 km
            Farrells Plain              2 km
            Tyers Ghaut/Belham Valley   5.8 km
            Gages                       4 km
            Gingoes Ghaut               2.6 km
            White River                 1.5 km

    10 Jan         0128-0135      Not known             6.7 km (22,000 ft)
          Runout Distances
            White's Bottom Ghaut        >2 km
            Tar River Valley            —
            Farrells Plain              1.5 km
            Tyers Ghaut/Belham Valley   2.5 km
            Gages                       3 km
            Gingoes Ghaut               —
            White River                 —

    10 Jan         2027-2031      Yes: pumice           5.5 km (18,000 ft)
          Runout Distances
            White's Bottom Ghaut        1.5 km
            Tar River Valley            2 km
            Farrells Plain              —
            Tyers Ghaut/Belham Valley   —
            Gages                       —
            Gingoes Ghaut               —
            White River                 —

    05 Feb         1349-1356      Yes: non-pumiceous    6.7 km (22,000 ft)
          Runout Distances
            White's Bottom Ghaut        1.5 km
            Tar River Valley            2 km
            Farrells Plain              1.5 km
            Tyers Ghaut/Belham Valley   2 km
            Gages                       4 km
            Gingoes Ghaut               1.5 km
            White River                 1.5 km

    08 Feb         1957-2003      Not known             4.6 km (15,000 ft)
          Runout Distances
            White's Bottom Ghaut        —
            Tar River Valley            —
            Farrells Plain              —
            Tyers Ghaut/Belham Valley   —
            Gages                       3.5 km
            Gingoes Ghaut               —
            White River                 —

Table 70. Soufrière Hills seismicity and gas measurements from weekly reports between 4 December 2009 and 19 March 2010. MVO seismicity terminology as follows: RF = rockfall signals (featureless, high-frequency events, which correlate to large rockfalls from the dome); VT = volcanic- tectonic (high frequencies >5 Hz, often impulsive P-phases and usually clear S-phases); LP = long-period (generally phaseless events with predominant frequency ~ 1 Hz); H = hybrid (repetitive transient events of intermediate frequency, 3-5 Hz, without discernible S-phases; initial high-frequency waveforms at some stations) (MVO, 1996). Numbers refer to the total over the period indicated. Hydrochloric acid/sulfur dioxide ratios (HCl/SO2) are derived from Fourier Transform Infrared (FTIR) gas measurements. Cycles of activity refer to rock falls, ash venting, and pyroclastic flows. "—" indicates that data was not reported. Courtesy of MVO.

    Reporting period      RF     LP    VT    H    Other related brief observations

    04 Dec-11 Dec 2009    957    207    3    6    Activity (pyroclastic flow, ash venting, rock
                                                    falls, etc.) continued in cycles more
                                                    irregular in time in the last few days; 10
                                                    Dec-hazard level raised from 3 to 4.
    11 Dec-18 Dec 2009    977    134    3   58    Cycles of activity continue, varying between 5
                                                    and 6 hours; intensity of cycles decreased
                                                    slightly through the week, however an
                                                    increase in intensity occurred since about
                                                    1600 on 17 Dec.
    18 Dec-24 Dec 2009    594    154    3   25    Cycles of activity with periods between 6 and 7
                                                    hours; heavy ashfall NW Montserrat.
    24 Dec-31 Dec 2009    270     52    —    6    Cycles of activity with periods between 6 and 8
                                                    hours.
    31 Dec-08 Jan 2010    135    73    1    16    Cycles of activity with periods between 8 and
                                                    10 hours; ashfall in Old Towns, Salem,
                                                    Olveston, Woodlands.
    08 Jan-15 Jan 2010     68    25    2    10    Three explosions occurred during the week (1449
                                                    on 8 Jan and 0128 and 2027 on 10 Jan), each
                                                    accompanied by seismic signals that lasted
                                                    11, 7, and 4 minutes, respectively, and ash
                                                    plumes that reached altitudes of 7.6, 6.7,
                                                    and 5.5 km, respectively.
    15 Jan-22 Jan 2010    196    38    —    18    Cycles of activity with 6-8-hour periods;
                                                    several houses buried and set on fire in
                                                    Kinsale; ash clouds associated with
                                                    pyroclastic flows reached 3-km altitude.
                                                    Hybrid swarm of 7 larger quakes on 20 Jan.
    22 Jan-29 Jan 2010    565   113    2    18    Cycles of activity with periods between 5 and 7
                                                    hours; 25 Jan-heavy rain caused vigorous
                                                    steaming of hot pyroclastic flows.
    29 Jan-05 Feb 2010    552    87    6    64    Cycles of activity with periods between 7 and
                                                    12 hours. On 5 Feb, a 30-m-high
                                                    pyramidal-shaped extrusion was first seen;
                                                    although it temporarily put the summit
                                                    elevation at 1,170 m, it was destroyed by an
                                                    explosion at 1349 that day; resulting
                                                    pyroclastic surges moved NW across the sea
                                                    near Plymouth.
    05 Feb-12 Feb 2010    512   141    4    82    Two explosions on 5 and 8 Feb; 11 Feb-partial
                                                    dome collapse, plume rose to altitude of
                                                    ~15.2 km.
    12 Feb-19 Feb 2010     53    34    1     4    17 Feb data consistent with quite slow
                                                    extrusion of lava; MVO not yet able to make
                                                    observations into the deep crater at the dome
                                                    summit. HCl/SO2 = 0.76 (17 Feb).
    19 Feb-26 Feb 2010     11     —    —     6    23 Feb-hazard level lowered from 4 to 3.
                                                    HCl/SO2 = 0.74 (19 Feb); 0.7 (22 Feb).
    26 Feb-05 Mar 2010      7     1    —     9    Swarm of 7 hybrids on 4 Mar.  HCl/SO2 = 0.81
                                                    (1 Mar); 0.71 (2 Mar); 0.98 (4 Mar).
    05 Mar-12 Mar 2010     47     9    2     7    Hybrid swarm of 6 on 11 Mar
    12 Mar-19 Mar 2010     41     3    —     7    17 Mar- SO2 flux 2,315 tons/day. HCl/SO2 = 0.6
    19 Mar-26 Mar 2010     28     3    1     3    avg. SO2 flux 342 tons/day
    26 Mar-02 Apr 2010     17     —    —     1    avg. SO2 flux 194 tons/day
    02 Apr-09 Apr 2010      9     1    3     3    3-day avg. SO2 flux 376 tons/day

Table 71. Brief summary of dome emissions compiled from MVO reports, 4 December 2009-1 April 2010. Date entries indicated with a * are discussed in the text. Courtesy of MVO.

    Reporting Period        Dome activity
        Location of pyroclastic flows (PF) and rockfalls (RF) (runout distance from dome)

    11 Dec-31 Dec 2009      Hottest and most active areas located on NW flank.
        Whites Ghaut to Whites Bottom Ghaut to the sea (4 km); Tyres Ghaut (~ 1-2 km); Gages
        valley (~ 2 km); Tar River valley; Gingoes Ghaut; Farrells plain, Dyers village (~ 2.5
        km), Spring Ghaut.

    31 Dec-08 Jan 2010     Growth on N side; 2 January-40-m high, 150-m wide lobe of lava
                           extruded onto dome.
        Whites Ghaut, Farrells plain, Tyers Ghaut.

    08 Jan-15 Jan 2010 *   NE flank; 2 Jan-40-m high, 150-mwide lobe of lava extruded onto N
                           summit of dome; 11 Jan-dome growth resumed on top, central part of
                           dome.
        8 Jan-collapsing fountain of tephra generated PF down Whites Bottom Ghaut, Tuitts Ghaut
        (within several hundred meters of the sea), Tyers Ghaut, Belham valley, Tar River valley;
        10 Jan-explosion produced PF down Whites Bottom and Tuitts Ghaut, Tyers Ghaut, Gages
        valley.

    15 Jan-22 Jan 2010 *   18 Jan-partial dome collapse on W side of dome.
        18 Jan-PF reached sea down Aymers Ghaut (Gages valley to Spring Ghaut to Aymers Ghaut);
        houses inundated/burned in Kinsale.

    22 Jan-29 Jan 2010     Dome growth on SE side of summit; NE side of summit has steep,
                           vertical walls; NW part more rounded.
        Increase in PF in Tar River valley (several reached sea); Whites Ghaut; heavy rain on 25
        caused vigorous steaming of hot PF in Belham valley; some lahars formed.

    29 Jan-05 Feb 2010     5 Feb-central W part of lava dome grew to altitude of ~1,070 m.
        Gages valley to Spring Ghaut (~ 2-3 km; head of Springs Ghaut neearly full of PF
        deposits), Whites Ghaut.

    05 Feb-12 Feb 2010 *   W side of dome; 9 Feb-activity shifted to N side of dome; 11
                           Feb-partial dome collapse, scar ~ 300 m wide on N flank of volcano
                           (MVO-"largest event for volcano since May 2006").
        5 Feb-volcanian explosion sent PF to Plymouth and into sea ~ 500 m, Tyers Ghaut (~ 2 km),
        Whites Ghaut, plume to ~ 8.4 km altitude; 8 Feb-small vulcanian explosion generated PF
        down Gages valley (over 2 km altitude), plume to ~ 5 km drifted E and ENE to Antigua; 11
        Feb-PF reached on E side of island (coastline extended E ~ 650 meters at airport), Tyers
        Ghaut into Belham valley.

    12 Feb-19 Feb 2010     Low activity, some incandescence on dome.
        PF deposits ~ 15 m thick in Trant's region, PF razed many buildings in Harris and
        Streatham.

    19 Feb-26 Feb 2010     Low activity.

    26 Feb-05 Mar 2010     26 Feb-crater at summit of dome <100 m deep and ~ 200 m wide.
        4 Mar-Tar River valley.

    05 Mar-12 Mar 2010 *   Moderate activity.
        8-9 Mar-rainfall caused degradation of dome; Gages valley (~ 2 km).

    12 Mar-19 Mar 2010 *   Low activity; some incandescence on 14 Mar.

    19 Mar-26 Mar 2010     Low activity.
        25 Mar-Spring Ghaut (~ 2 km).

    26 Mar-02 Apr 2010     Low activity.

    02 Apr-09 Apr 2010     Low activity; some incandescence on dome.
        Lahars in Farm River and Trant's area.

MVO issued a synthesis to the Scientific Advisory Committee (SAC) on volcanism between 15 August 2009 and 28 February 2010 (Cole and others, 2010). That report figures heavily in the following summary, but the included tables and comments also came from MVO reports, and there is a section on satellite thermal monitoring. Two similar earlier reports were published in 2009 (Robertson and others, 2009 and Stewart and others, 2009).

Since the dome remained active and at the same time represented the volcano's highest point, the summit elevation varied. The historical value of 915 m was a high point on the crater rim. Cole and others (2010) noted that the dome's summit was 1,050 m in September 2009, with the elevation being 1,130 m on 29 January 2010. Some taller heights involved blocky spines that did not last.

Extrusive Phase 5 activity. Extrusive Phase 4 finished on 3 January 2009 and was followed by 10 months of comparative inactivity with intermittent small pyroclastic flows and ash venting 5-7 October (BGVN 34:10). Phase 5 occurred from 4 October 2009 to 11 February 2010 (figure 83). Seismic records enabled MVO to subdivide this phase into three episodes of inferred dome growth as follows: 9 October-20 November 2009 (Episode 1); 20 November 2009-8 January 2010 (Episode 2); and 8 January-11 February 2010 (Episode 3). Cole and others (2010) noted that "A characteristic feature of Phase 5 dome growth has been the simultaneous occurrence of PFs in more than one direction, sometimes on the opposite side of the lava dome." Throughout Phase 5, ash often fell on inhabited areas.

Phase 5 began with a swarm of 24 volcano-tectonic (VT) earthquakes and ash venting. Gas fluxes had been low for two days prior to the onset of activity. The dome variously grew to the S, W, and N, and pyroclastic flows traveled in many directions. The eruptive style was described as "ash venting" rather than "explosions" due to the mild character of the associated seismic signals and the absence of ballistic fragments. Fallout deposits included comparatively coarse, well-sorted ash.

October dome growth mostly occurred on the S, with shed material filling the upper part of the SW flank's White River and covering what had stood as a protective wall for material traveling WSW. As a result, for the first time, substantial pyroclastic flows entered the WSW flank's Gingoes and Aymer's Ghauts, reaching the sea there with runout distances of over 4 km in those drainages.

Cyclic episodes of tremor occurred particularly during episode 2. On 23 November tremor occurred all day; it then waned and began to appear in cycles at 4-hour intervals, initially with signals of long-period and hybrid earthquakes. The tremor appeared associated with increased venting lasting 0.5-2 hours with plume heights to 5 km altitude. At 0640 on 10 December 2009, a large pyroclastic flow traveled down Tyers (Tyres) Ghaut and reached ~ 3.5 km from the lava dome.

Vigorous Vulcanian explosions occurred on five occasions during January-February 2010 (table 69), episode 3. All of these involved collapsing ash columns, producing fountain collapse pyroclastic flows that typically descended more than one ghaut. One explosion on 8 January, the largest by volume during January-February, sent a pyroclastic flow ~ 6 km down the Belham Valley. Two more Vulcanian explosions occurred during the night on 10 January.

Dome collapse of 11 February 2010. A large dome collapse took place in the early afternoon of 11 February, one day after a shift in dome-growth direction, and had several pulses. The collapse comprised 40-50 million cubic meters of material, and represented roughly 20% of the dome's total volume. A collapse scar ~ 300 m wide developed on the N flank of the dome. The collapse ended with vertically-directed explosions that created a new crater behind the collapsed part of the dome.

The collapse produced large pyroclastic flows and surges, mainly to the N and NE, that extended the E coastline (between Trants and Spanish Point), adding ~ 1 km² of new land. Two smaller flows also traveled NW and entered the Belham Valley.

A large ash column resulted from the collapse that reached ~ 15 km altitude, causing extensive ashfall on Guadeloupe (~ 60 km SE) and other parts of the eastern Caribbean. After 11 February, both seismicity and surface activity quieted but deep deformation returned. Gas measurements also indicated that the system remained active.

Pyroclastic flows traveled N and NE toward the old airport. The extensive pyroclastic-flow deposits extended the coastline 300-400 m out to sea. The coastal area impacted extended from Whites Bottom Ghaut to Trants Bay, just N of the old Bramble airport (figures 84 and 85). The effects were clearly visible on the NE flanks. Some flows, ~ 15 m thick, reached the sea at Trant's Bay. These flows extended the island's coastline up to 650 m to the E.

Towards the end of the collapse there was an energetic pyroclastic flow directed N over Streatham and Harris. This sent flows over the Harris Ridge into Bugby Hole and down the Farm River (~ 3.5 km from the dome) for the first time. The flows razed many buildings in both Harris and Streatham down to their foundations, and trees were felled by pyroclastic surges in the Gun Hill area and at the head of Farm River in Bugby Hole.

It was unclear whether there was any new dome growth within the crater during the week after the collapse. Night-time views of the dome revealed several small points of incandescence. Observations of the crater at the summit of the dome on 26 February found that it was then 50-100 m deep and ~ 200 m wide (figure 86). There was no newly extruded lava visible inside the crater.

Heavy rain on 8-9 March caused vigorous steaming of the hot 11 February deposits (figure 87). Strong geysering was visible at Trants near the old Bramble airport, with ash and steam fountaining occurring. In addition, lahars traveled down several drainages, including the Belham valley. Small spots of incandescence on the dome were visible again on 14 March. Occasional small pyroclastic flows and rockfalls were still occurring mainly from the western and southern parts of the dome.

MODVOLC Thermal Alerts. According to the Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, no satellite thermal alerts were measured over Soufrière Hills between 29 March 2007 and 3 December 2008. Satellite thermal alerts were measured almost daily during 11 October 2009 through 15 February 2010. An isolated thermal alert was measured on 10 March 2010. Previously shorter periods of thermal alerts were measured during 11-29 March 2007 and 3 December 2008-3 January 2009.

References. Cole, P., Bass, V., Christopher, C., Fergus, M., Gunn, L., Odbert, H., Simpson, R., Stewart, R., Stinton, A., Stone, J., Syers, R., Robertson, R., Watts, R., and Williams, P., 2010, Report to the Scientific Advisory Committee on Montserrat Volcanic Activity, Report on Activity between 15 August 2009 and 28 February 2010, Open File Report OFR 10-01a, Prepared for SAC 14: 22-24 March 2010. Montserrat Volcano Observatory (MVO).

Robertson, R., Babal, L., Bass, V., Christopher, T., Chardot, L., Fergus, M., Fournier, N., Higgins, M., Joseph, E., Komorowski, J.-C., Odbert, H., Simpson, R., Smith, P., Stewart, R., Stone, J., Syers, R., Tsaines, B., and Williams, P., 2009, Report for the Scientific Advisory Committee on Montserrat Volcanic Activity, Prepared for SAC 13: 7-9 September 2009, MVO Open File Report 09/03.

Stewart, R., Bass, V., Chardot, L., Christopher, T., Dondin, F., Finizola, A., Fournier, N., Joseph, E., Komorowski, J.-C., Legendre, Y., Peltier, A., Robertson, R., Syers, R., and Williams, P., 2009, Report for the Scientific Advisory Committee on Montserrat Volcanic Activity, Prepared for SAC12: 9-11 March 2009, MVO Open File Report 09/01.

Wadge, G., Macfarlane, D.G., Robertson, D.A., Hale, A.J., Pinkerton, H., Burrell, R.V., Norton, G.E., and James, M.R., 2005, AVTIS: a novel millimetre-wave ground based instrument for volcano remote sensing: J. Volcanology and Geothermal Research, v. 146, no. 4, p. 307-318.

MVO, 1996, MVO/VSC Open Scientific Meeting, 27 November 1996, Seismicity of Montserrat Soufrière Hills Volcano Eruption, July 1995-November 1996 (URL: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/meetings/nov1996/02.html).

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/VAAC/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://hotspot.higp.hawaii.edu/).

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08/2011 (BGVN 36:08) Extrusive pause follows 11 February 2010 partial dome collapse

Our previous report on Soufrière Hills volcano, Montserrat, (BGVN 35:03), was published in the month following the 11 February 2010 partial dome collapse, which consisted of 40-50 x 10⁶ m³ of material (~ 20% of the dome's pre-collapse volume) and left a collapse scar ~ 300 m wide on the N flank of the dome. The ash column from this event reached ~ 15 km altitude, and large pyroclastic flows extended the island's E shoreline, creating ~ 1 km² of new land surface. This event marked the end of Extrusive Phase 5, the most recent phase of extrusive activity since the volcano's awakening in 1995 (figure 88). Since then, and as recently as 9 September 2011, Soufrière Hills experienced a low level of activity with pyroclastic flows and rockfalls, but no significant extrusion of lava; a behavior typical of pauses in extrusion at this volcano since 1995 (figure 88). The Hazard Level remained at 3 (on a scale from 1 to 5). Most of the data and information in this report is from Cole and others, 2010 and Cole and others, 2011. Information has also been contributed by the Montserrat Volcano Observatory (MVO).

Pyroclastic flows and rockfalls removed 3.65 x 10⁶ m³ (~ 1.9 % volume) of the dome from 12 February 2010 to 30 April 2011. Of these events, 54 were small pyroclastic flows (maximum runout of 1 km) and 31 were moderate-large pyroclastic flows (maximum runout of 2 km), generated mainly from three source regions: (1) above Gages Valley (W side); (2) N of the head of the 11 February 2010 collapse scar; and (3) at the head of the Tar River Valley (ESE side).

Undercutting of the dome face above Gages Valley (W side) by heavy summer rainfall in 2010 compromised dome stability and led to the largest pyroclastic flows. Pyroclastic flows occasionally generated small plumes and ashfall in inhabited areas of the island.

Lahars in the Belham Valley (on the volcano's W side) decreased from 40 measurable events (during late February-October 2010) to 10 measurable events (during November 2010-April 2011). The lahars were well correlated with peak rainfall events, and the 40 lahars occurring from 28 February to 31 October 2010 were attributed to especially heavy rainfall occurring in the months of April, late July, September, and October 2010. Figure 89 shows a house in Belham Valley partially covered by lahar deposits.

As of 9 September 2011 the Washington Volcanic Ash Advisory Center (VAAC) reported eruptive plumes only a handful of times since 28 February 2010. Based on a METAR weather report and analyses of satellite imagery, on 10 August 2010 a narrow plume drifted 100 km WNW. According to the Washington VAAC, on 2 October 2010 the MVO reported that an ash plume rose to an altitude of 2.1 km altitude and drifted W. The next day an ash plume seen in satellite imagery drifted 55 km WNW and NW. A few hours later, an area of ash at an altitude of 2.1 km altitude was observed 140 km to the WNW. On 11 October a diffuse steam-and-gas plume drifted NNW.

Seismicity generally remained low, with low-frequency seismicity, long-period (LP), hybrid, and LP-rockfall events detected between 1 November 2010 and 9 September 2011 (figure 88). Short periods of volcano-tectonic (VT) earthquake activity on 25 June and 2 July 2010 were followed by ash venting; roaring could be heard N of the volcano. Another VT short swarm on 3 December 2010 was associated with an increase in gas emission and additional minor ash venting.

SO₂ flux also remained low, with the daily average from 28 February 2010 to 30 April 2011 measured at 375 tons/day; low values typical of periods of pause. Elevated SO₂ flux occurred in early May 2010 (up to 1,250 tons/day), November 2010, and March 2011 (figure 88).

During the reporting interval, mid-February 2010 to 9 September 2011, steady but weak inflation occurred, with only minor changes to the dome. The changes stemmed from mass wasting such as rockfalls and from pyroclastic flows.

Incandescence from various places on the lava dome was observed and analyzed (figure 90). The largest feature was a fumarolic gas vent on the floor of the 11 February collapse scar (see "Gas vent", figure 90 B, D). From the incandescent color, the temperature is estimated to be 500-600°C. Dome interior temperatures are considered likely nearly magmatic.

References. Cole, P., Bass, V., Christopher, T., Eligon, C., Murrell, C., Odbert, H., Smith, P., Stewart, R., Stinton, A., Syers, R., Robertson, R., and Williams, P., 2010, Report to the Scientific Advisory Committee on Volcanic Activity at Soufrière Hills Volcano Montserrat, Report on Activity between 28 February 2010 and 31 October 2010, Open File Report OFR 10-02a, Prepared for SAC 15: 15 – 17 November 2010. Monsterrat Volcano Observatory (MVO).

Cole, P., Bass, V., Christopher, T., Murrell, C., Odbert, H., Smith, P., Stewart, R., Stinton, A., Syers, R., and Williams, P., 2011, MVO scientific report for volcanic activity between 1 November 2010 and 30 April 2011, Open File Report OFR 11-01. Monsterrat Volcano Observatory (MVO).

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/VAAC/).

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