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 erupt