Masaya

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  • Last Known Eruption
  • 11.984°N
  • 86.161°W

  • 635 m
    2083 ft

  • 344100
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Most Recent Weekly Report: 17 December-23 December 2008


Based on analysis of satellite imagery, the Washington VAAC reported that on 17 December a gas plume with possible ash rose to altitudes of 5.3-6.1 km (17,500-20,000 ft) a.s.l.

Source: Washington Volcanic Ash Advisory Center (VAAC)


Most Recent Bulletin Report: June 2012 (BGVN 37:06)


Explosions from Santiago crater began on 30 April 2012

Since our last report covering Masaya’s seismic activity and emissions from November 2011 through March 2012, the Instituto Nicaragüense de Estudios Territoriales (INETER) has maintained monitoring efforts including site visits in April and May 2012. Here we discuss regular gas emissions (SO2 and CO2) and seismic monitoring efforts and highlight events preceding the 30 April 2012 explosion from Santiago crater that ejected ash and incandescent blocks within the bounds of the National Park. That event began a series of explosions; more than 68 explosions occurred between 30 April and 17 May 2012.

On 21 April 2012 INETER conducted routine site visits and made field measurements at Masaya. Maximum temperatures recorded with an infrared sensor found temperatures between 98.7°C and 102°C within Santiago crater. Some jetting sounds were heard from the depths of the crater, cracks were observed on the E wall that emitted abundant gases, and the W interior wall showed signs of rockfalls. INETER field teams also visited Comalito cone, located on the NE flank, and measured maximum temperatures of 72°C to 77°C.

During field investigations on 25 April 2012, INETER volcanologists measured diffuse CO2 emissions from Comalito cone. At night on 26 April, the National Park guards reported incandescence within the crater; the last report of incandescence was in October 2010 (BGVN 36:11). SO2 was measured with Mobile DOAS on 27 April on a traverse between the towns Ticuantepe and La Concha (see map for location in figure 25 from BGVN 36:11).

INETER reported that, on 27 April 2012 at approximately 0500 volcanic tremor appeared in Masaya’s seismic records (figure 34). Tremor slowly increased to 70 RSAM that day, and civil defense authorities released notices to officials that significant seismic unrest was detected at Masaya.

Figure 34. RSAM (averaged seismic amplitude) record from Masaya volcano during April 2012, an interval leading up to and including a 30 April eruption. Tremor drove a notable increase in RSAM on 27 April, diminishing slightly as monochromatic tremor prevailed over the following days. After an abrupt decrease in RSAM, the eruption occurred on 30 April. Courtesy of INETER.

On 28 April 2012, authorities, including the Masaya Volcano National Park, released a public announcement about the unusual seismic activity. Three hours following that announcement, the tremor signal became monochromatic near 15 Hz (figure 35). INETER suggested that this signal arose from magma moving beneath the edifice. RSAM reached 100 units with spectral analysis indicating frequencies oscillating between ~1.26 Hz and ~18.84 Hz. The strongest frequency during one particular time window (figure 35) was centered near 15.8 Hz, with a smaller peak at ~1.5 Hz.

Figure 35. (Upper panel) Seismic signal dominated by ongoing tremor recorded at Masaya on 28 April 2012 on a seismogram (amplitude, y-axis, and time (hours : minutes), x-axis). (Lower panel) A spectral analysis made for the interval shown above (frequency, in Hz, along x-axis). Courtesy of INETER.

INETER noted that before the onset of tremor on 27 April, an average of 35 seismic events per day were recorded. These were low frequency earthquakes that included signals reaching 16 Hz and interpreted as rupture events beneath Masaya. The depths of the earthquakes were determined by the P- and S-wave arrival times indicating a depth range between 3 and 4 km.

On 28 April, tremor continued at 70 RSAM and monochromatic tremor occurred again, reaching 90 RSAM. Up to 40 earthquakes were detected that day.

On 29 April, seismic tremor was slightly lower at 65 RSAM and monochromatic tremor was recorded. A total of 45 earthquakes were recorded. Signals were again monochromatic at peak frequencies of 15.8 Hz.

On 30 April at 0045, the tremor signal dramatically decreased to 30 RSAM. INETER commented that this was abnormal since tremor was often recorded between 40 and 50 RSAM during times of quiescence. Seven hours later, a strong explosion was recorded by seismic instruments and observers within the National Park witnessed a blast of gas and ash from Santiago crater (figure 36).

Figure 36. Ash explosions began on 30 April 2012 from Masaya’s Santiago crater. (A) A large explosion occurred at 0829 on 30 April and was photographed by National Park staff. (B) Later in the day a smaller explosion released a small ash plume. Courtesy of INETER and the Masaya Volcano National Park.

Due to the explosions, the Plaza de Oviedo, an overlook at the edge of Santiago crater, was covered with sand-sized pink and yellow ash and lapilli with some rocks up to 10 cm in diameter. Some of the clasts were incandescent and damaged the roofs of structures near the crater and also burned the asphalt of the plaza (figure 37). Small brush fires were ignited on the N flank of the volcano due to hot blocks falling onto the dry plants. Local firefighters worked with the National Park and Civil Defense for most of the day in order to contain and extinguish the fires. The national park was closed due to the hazardous conditions.

Figure 37. (A) The roofs of several structures near Santiago crater were damaged by volcanic bombs during the 30 April 2012 explosions. (B) Some of the bombs ejected during the primary explosion were incandescent and burned the asphalt of the plaza when they landed. Courtesy of INETER.

INETER reported the explosion ejected a column of ash, gas, and blocks reaching 1,000 m above the summit and the initial explosion was followed by 24 smaller explosions that reached 500 m. Ballistic ejecta covered an area with a 300 m radius to the SSE of the crater and ash fell as far as 3 km to the SE of the crater. Blocks measured from this area had maximum dimensions of 50 x 40 x 30 cm. Ash fell to a thickness of 2 mm in some areas and INETER calculated a total volume of 736 cubic meters of ejecta.

INETER measured temperatures from Santiago crater on 30 April with an infrared thermal camera and detected a maximum of 165°C. During the night of 30 April, 23 explosions were recorded by the seismic network.

Between 30 April and 3 May, a collaborative effort among INETER, Civil Defense, local fire fighters, and the National Park succeeded in maintaining a 24-hour watch of Santiago crater. Over four days, the teams recorded observations and determined that 68 explosions had occurred and the maximum detected crater temperature was 162°C.

On 1 May 2012 at 0223 a small explosion was recorded by the INETER seismic network. This event produced ash and volcanic bombs that fell across the NE-SE sectors including the flanks of Nindirí cone (see figure 30 in BGVN 37:04 for site names). The dimensions of the largest blocks were 60 x 50 x 40 cm.

On 3 May there were two small explosions at 0008 and 0022 with abundant gas and ash emissions. Throughout these events, tremor was constant at 1.5 Hz. On 4 May no earthquakes were recorded but tremor remained between 45 and 50 RSAM; explosions of gas and light ash were observed. On 5 May a total of 19 earthquakes were recorded and RSAM varied between 45 and 58 RSAM; ash and gas explosions were reported by National Park staff. On 6 May between 0700 and 1030 a total of 45 earthquakes were recorded and RSAM increased to 70 units.

Sporadic explosions continued until mid-May (figure 38). INETER noted that in May, RSAM averaged 60 units and a significant increase occurred on 18 May. RSAM reached 120 units and was maintained at that level until 21 May. Low tremor was recorded up to 75 RSAM units after 21 May and two days later reached 85 RSAM units with frequencies in the range 1.5-3.0 Hz. Tremor decreased and remained between 65 and 70 RSAM units until the end of the month. A total of 266 earthquakes were recorded in May.

Figure 38. RSAM record from Masaya volcano during May 2012. Courtesy of INETER.

Long-term gas monitoring. Long-term records of Masaya’s gas emissions (SO2 and CO2) and fumarole temperatures have been developed by INETER. On 2 May, SO2 flux was measured during traverses between Ticuantepe and La Concha (table 5). INETER commented that they observed increasing SO2 flux since December 2011 (648 tons per day) that peaked in March 2012 (1002 tons per day). Flux was decreasing at the time of the explosion on 30 April 2012. INETER noted that overall trends in SO2 flux did not correlate with trends in seismicity, however, they emphasized that difficult-to-constrain variables such as wind speed and direction should be factored into the SO2 data interpretations.

Table 5. SO2 flux detected at Masaya from January 2011 through May 2012 during traverses with a Mobile DOAS. Courtesy of INETER.

Year     Month        SO

2

 flux                     (tons/day)2011     January        642         September      518         October        153         December       6482012     January        801         February       943         March          1002         April          761         May            534

Since 7 December 2008, INETER measured CO2 emissions from Comalito cone, an active fumarolic site on the NE flank of Masaya. Diffuse CO2 was measured from a 9 hectare sector of soil as recently as 1 May 2012 (table 6). INETER reported the highest CO2 emissions were detected in 2008 and decreased between 2010 and 2011. Emissions recorded on 25 April 2012 (before the eruption) were considered low, however, there was a small peak on 1 May that may have been related to the explosive activity.

Table 6. The long-term record of diffuse CO2 analyses from Comalito cone measured from September 2008 through May 2012. Courtesy of INETER.

    Date         Area      CO

2

 emission(dd/mm/yyyy)     (km

2

)      (tons/day) 07/12/2008      0.09         66.4 26/03/2010      0.09         27.4 02/03/2011      0.09         15.1 30/01/2012      0.09         50.8 25/04/2012      0.09         25.2 01/05/2012      0.09         32.2

On 17 May, INETER conducted fieldwork at Santiago crater and determined a maximum temperature of 162°C. While in the field, INETER staff observed two small explosions from the crater. Temperatures were also measured at Comalito cone (figure 39); the maximum recorded temperature was from Fumarole 2, 78.2°C, the highest temperature reading at Comalito cone since February 2012.

Figure 39. Temperatures measured at Comalito cone from January through May 2012. Courtesy of INETER.

New monitoring efforts and installations. Two seismic stations were installed in May 2012. One station, called La Azucena, was installed by INETER on 1 May. This site was located ~4 km N of the active crater and was considered temporary. A second station, called El Comalito, was installed on 15 May; located within the National Park at Comalito cone. INETER recognized potential contributions of background noise from the fumarolic sites close to the station and planned to reevaluate the location after reviewing the results from this station. Both stations transmitted realtime data through radio repeaters.

On 4 May a web camera was installed within the town of La Azucena on a short tower; the camera was programmed to send images through a wireless network every 5 minutes. A second camera was installed in the town of Masaya at the office building of the Center of Disaster Operations (CODE); this camera also captured images every 5 minutes. The camera at CODE suffered malfunctions after installation due to overexposure from direct sunlight. Future fieldwork was planned to fix these problems.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/geofisica/), La Prensa (URL: http://www.laprensa.com.ni/2012/04/30/ambito/99799/imprimir).

Index of Weekly Reports


2008: April | June | August | September | October | November | December
2007: April | June | December
2004: June
2003: October
2001: April | May

Weekly Reports


17 December-23 December 2008

Based on analysis of satellite imagery, the Washington VAAC reported that on 17 December a gas plume with possible ash rose to altitudes of 5.3-6.1 km (17,500-20,000 ft) a.s.l.

Source: Washington Volcanic Ash Advisory Center (VAAC)


26 November-2 December 2008

Based on analysis of satellite imagery, the Washington VAAC reported that on 2 December a plume possibly containing some ash drifted less than 20 km SW.

Source: Washington Volcanic Ash Advisory Center (VAAC)


29 October-4 November 2008

Based on analysis of satellite imagery, the Washington VAAC reported that on 4 and 5 November possible diffuse ash and steam plumes from Masaya drifted SW and S.

Source: Washington Volcanic Ash Advisory Center (VAAC)


8 October-14 October 2008

Based on pilot observations, the Washington VAAC reported that on 9 October an ash plume from Masaya rose to an altitude of 4.6 km (15,000 ft) and drifted NNE.

Source: Washington Volcanic Ash Advisory Center (VAAC)


10 September-16 September 2008

Based on analysis of satellite imagery, the Washington VAAC reported that plumes emitted from Masaya on 10 and 12 September possibly contained ash. Plumes drifted ENE on 10 September.

Source: Washington Volcanic Ash Advisory Center (VAAC)


13 August-19 August 2008

Based on analysis of satellite imagery, the Washington VAAC reported that a diffuse steam plume from Masaya drifted WSW on 12 August and a gas plume was detected on 18 August. Both plumes possibly contained ash.

Source: Washington Volcanic Ash Advisory Center (VAAC)


18 June-24 June 2008

INETER reported that on 18 June, an explosion from Masaya produced an ash-and-gas plume. Local people felt the explosion and reported that the plume was dark in color.

Source: Instituto Nicaragüense de Estudios Territoriales (INETER)


23 April-29 April 2008

Based on observations of satellite imagery and pilot observations, the Washington VAAC reported that an ash plume from Masaya rose to an altitude of 2.1 km (7,000 ft) a.s.l. and drifted SW on 29 April.

Source: Washington Volcanic Ash Advisory Center (VAAC)


19 December-25 December 2007

Based on observations of satellite imagery, the Washington VAAC reported that a small and diffuse plume from Masaya drifted SW on 24 December. Changes in seismic signals correlated with the emission. The plume possibly contained ash.

Source: Washington Volcanic Ash Advisory Center (VAAC)


13 June-19 June 2007

The Washington VAAC reported that a plume from Masaya composed of little to no ash was visible on satellite imagery on 12 June.

Source: Washington Volcanic Ash Advisory Center (VAAC)


6 June-12 June 2007

The Washington VAAC reported a plume from Masaya composed of little to no ash was visible on satellite imagery on 9 June drifting W.

Source: Washington Volcanic Ash Advisory Center (VAAC)


25 April-1 May 2007

The Washington VAAC reported a steam plume from Masaya, visible on satellite imagery and a web camera, drifted WSW on 26 April.

Source: Washington Volcanic Ash Advisory Center (VAAC)


30 June-6 July 2004

On 4 July at 0615, a narrow plume of steam and/or ash from Masaya was visible on satellite imagery extending to the SW. By 0715 the plume extended ~12 km from the summit.

Source: Washington Volcanic Ash Advisory Center (VAAC)


1 October-7 October 2003

A pilot reported seeing an eruption cloud from Masaya on 4 October at 0731 at a height of ~4.6 km a.s.l. Satellite imagery showed a white plume emanating from the volcano, but there were no indications of ash, suggesting that the plume was composed mainly of gas and steam.

Source: Washington Volcanic Ash Advisory Center (VAAC)


23 May-29 May 2001

The Washington VAAC reported that Masaya may have erupted on 23 May at ~1300. Ground observations from the capital city of Managua, 20 km NW of the volcano, indicated that there was a reduction in visibility to the SE of the city due to volcanic "smoke" and steam. The presence of the ash cloud could not be confirmed on satellite imagery due to thunderstorms in the area.

Source: Washington Volcanic Ash Advisory Center (VAAC)


25 April-1 May 2001

INETER personnel reported that volcanic activity at Masaya decreased following the 23 April explosion. Small explosions were observed on 24 and 25 April, but by 27 April only the continuous emission of gas at normal levels was observed with few episodes of strong degassing. Likewise, after the 23 April explosion the level of SO2 emission decreased and normal levels of seismic activity were recorded. INETER warned that further explosions may occur that could affect areas near the crater (within ~500 m).

A tourist at the scene during the 23 April explosion stated that injuries were more serious than was reported either here or in news accounts. Over 100 tourists were near the crater when the explosion occurred, including infants and elderly persons. At least 15 people sustained injuries (bruises and cuts) and one person suffered a broken arm.

Source: Instituto Nicaragüense de Estudios Territoriales (INETER)


18 April-24 April 2001

At 1426 on 23 April a small explosion at Masaya's Santiago crater lasted for ~2 minutes and occurred in three phases. During the first phase volcanic gas under high pressure was explosively released and created a new vent in the bottom of Santiago crater. The eruption sent rock fragments up to 60 cm in diameter as far as 500 m from the crater. Several vehicles parked at a visitors platform near the crater were damaged by the ejecta and one person suffered minor injuries. During the second and third phases a mixture of hot volcanic gas, pieces of lava, and ash ignited dry vegetation near the crater. INETER personnel who monitored the seismic activity before the eruption and scientists from Cambridge University who were working in the crater one hour before the eruption did not notice any unusual activity at the volcano. INETER personnel monitored the volcano after the eruption and found that several small explosions, gas outbreaks, and minor collapses of the crater wall occurred. They warned that further explosions may occur that could affect areas near the crater (within ~500 m).

Sources: Instituto Nicaragüense de Estudios Territoriales (INETER); Associated Press


Index of Bulletin Reports


Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

04/1970 (CSLP 38-70) Minor explosions and lava emission from central vent on old lava lake

04/1978 (SEAN 03:04) Increasing lava lake activity at pit crater

02/1980 (SEAN 05:02) Active lava lake several times larger than in 1977

07/1980 (SEAN 05:07) Large plume; high SO2 output

12/1980 (SEAN 05:12) Continued emission of a large gas plume

01/1981 (SEAN 06:01) Gas emission event continues

03/1981 (SEAN 06:03) Gas emission continues unabated; pit crater enlarges

12/1981 (SEAN 06:12) Large white vapor plume and high SO2 emission rates continue

01/1982 (SEAN 07:01) Small explosion heard followed by ashfall several kilometers south

03/1982 (SEAN 07:03) Bright yellow incandescence seen at night

08/1982 (SEAN 07:08) Strong, continuous gas emission; lava lake glow

10/1982 (SEAN 07:10) Small explosion; strong vapor emission; seismicity

11/1982 (SEAN 07:11) Gas emission continues; incandescence within the inner crater

04/1983 (SEAN 08:04) Gas column and incandescence from lava lake

05/1985 (SEAN 10:05) Small ash eruptions

08/1985 (SEAN 10:08) 400 km plume sighted

11/1985 (SEAN 10:11) Gas column heights in 1985

11/1986 (SEAN 11:11) Rock landslides and wall collapse in Santiago Crater

05/1987 (SEAN 12:05) Collapse and small eruptions from inner crater

01/1988 (SEAN 13:01) Vigorous degassing continues; small tephra eruptions and glow

02/1989 (SEAN 14:02) Lava lake develops in new collapse crater

04/1989 (SEAN 14:04) Lava lake drains; rockslides; gas emission

06/1989 (SEAN 14:06) Lava lake freezes; small explosions

04/1990 (BGVN 15:04) Fumarolic activity

02/1991 (BGVN 16:02) Rockfall activity declines after November 1989 collapse

04/1992 (BGVN 17:04) Weak gas emission; acid gas and rain effects diminish

03/1993 (BGVN 18:03) Crater walls stabilizing

06/1993 (BGVN 18:06) New lava lake appears

07/1993 (BGVN 18:07) Small ash eruption precedes formation of lava lake; fumarole temperatures rise

09/1993 (BGVN 18:09) Incandescence in lava lake

10/1993 (BGVN 18:10) Incandescent hole in lava lake remains active

03/1994 (BGVN 19:03) Incandescence visible in daylight; small eruptions

07/1994 (BGVN 19:07) Sulfur-rich plume and incandescent ejections from opening in lava lake

09/1994 (BGVN 19:09) Temperatures and SO2 flux from incandescent opening continue rising

11/1994 (BGVN 19:11) Red glow from vent on crater floor; gas emission

04/1996 (BGVN 21:04) Incandescent vent in Santiago crater emitting large amounts of gas

03/1997 (BGVN 22:03) Strombolian explosion; incandescent vent in Santiago crater; seismicity increases

06/1997 (BGVN 22:06) Stable and non-eruptive during May-June

07/1997 (BGVN 22:07) Minor morphologic changes and fluctuating incandescence in May

09/1998 (BGVN 23:09) Integrated scientific studies of the caldera area

04/1999 (BGVN 24:04) Continued degassing and marked gravity decreases; previously unreported small explosions

07/2000 (BGVN 25:07) Summary of activity; nearby M 5.4 earthquake at 1 km focal depth on 6 July

09/2000 (BGVN 25:09) Small ash eruptions in March; decreasing levels of degassing

04/2001 (BGVN 26:04) Tourists experience a brief, bomb-charged 23 April 2001 explosion: no fatalities

08/2003 (BGVN 28:08) Fumarolic emissions and low-level seismicity from April 2002 through May 2003

10/2003 (BGVN 28:10) Fumarole temperatures unchanged; landslides, incandescence in Santiago crater

04/2006 (BGVN 31:04) Intermittent ash eruptions November 2003-March 2005; continuing incandescence

03/2009 (BGVN 34:03) Phreatomagmatic explosions in August 2006; intermittent plumes through 2008

11/2011 (BGVN 36:11) Degassing through at least mid-2011; episodic crater wall collapse

04/2012 (BGVN 37:04) Continuous monitoring of emissions and new investigations from collaborators

06/2012 (BGVN 37:06) Explosions from Santiago crater began on 30 April 2012




Bulletin Reports

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


04/1970 (CSLP 38-70) Minor explosions and lava emission from central vent on old lava lake

Card 0924 (30 April 1970)

Jose Viramonte forwarded the following concerning activity at Santiago Crater. "On 4 and 5 April 1970, a little eruption from the central vent on the old lava lake could be observed. The eruption consisted of two of fluidic lava flows. There were many cracks in the surface of the old lava lake through which fumaroles and sublimates showed. Two major explosions were heard on 4 and 5 April. During our visit to the volcano, minor explosions took place, and strong fumarolic activity from the central vent was present as well."

Information Contacts: Jose Viramonte, Central American University, Managua.

04/1978 (SEAN 03:04) Increasing lava lake activity at pit crater

The past four months have produced a gradual increase in the intensity of activity at Masaya. Fissures have appeared in the floor of Santiago Crater (figure 1) a collapse feature that formed, along with neighboring San Pedro crater, in 1858. The vent opening about 100 m below the rim of Santiago's pit crater has widened to about three times its size of a few months ago. The persistent lava lake inside the pit crater is usually not visible from Santiago's rim, but splashes of lava can occasionally be seen and minor amounts of lava clots are sometimes thrown from the vent. When the volcano was visited in late March, rare bursts of scoria reached the rim of the pit crater. Gas emission was strong, but has not seriously damaged nearby coffee trees.

Figure 1. Oblique airphoto of the Masaya Complex looking SE, 6 November 1975. The four craters seen in this photo are (clockwise from upper left) Masaya, Santiago, Nindirí, and San Pedro. The Masaya Crater is about 500 m in diameter. Photograph taken by IGN; courtesy of Jaime Incer.

Information Contacts: D. de Jerez, Parque Nacional Volcán Masaya; D. Shackelford, CA.

02/1980 (SEAN 05:02) Active lava lake several times larger than in 1977

In early February, the plume appeared larger than any observed between 1968 and 1977. The diameter of the active lava lake in the pit crater was several times larger than in 1977 and the level of lava has dropped since then.

Information Contacts: R. Stoiber, S. Williams, and M. Bruzga, Dartmouth College.

07/1980 (SEAN 05:07) Large plume; high SO2 output

Emission of a very large plume continued in June. Remote sensing of SO2 gas revealed high output rates. The gas plume allowed only brief glimpses of the small pit crater in which an active lava lake had been observed on many occasions since 1970. The lake was not seen during the brief clear moments, nor did a glow appear in photographs of the pit. The lake's characteristic roaring noise, if present, was masked by the sounds created by gas emission. There were no night observations at Masaya.

Information Contacts: R. Stoiber and S. Williams, Dartmouth College; M. Carr and J. Walker, Rutgers Univ.; A. Creusot, INETER.

12/1980 (SEAN 05:12) Continued emission of a large gas plume

Emission of a very large gas plume has continued without interruption since fall, 1979. Remote sensing of SO2 revealed continued high level flux, with a 1,500-2,000 t/d average for the entire year. The hole through the surface of the lava lake was larger than in previous years and a great deal of sublimation was occurring around its edge. No lava or red glow was visible during daylight. Acid gas and rain continued to cause considerable damage downwind.

Information Contacts: R. Stoiber, S. Williams, H. R. Naslund, L. Malinconico, and M. Conrad, Dartmouth College; A. Aburto Q., D. Fajardo B., Instituto de Investigaciones Sísmicas.

01/1981 (SEAN 06:01) Gas emission event continues

The gas emission event that began in fall 1979 continued with a steady release of very large amounts of SO2 in early 1981. Strong winds carried the gas plume onto populated areas at high elevations. A day of notable rockfall activity in the crater was followed for 1 day by a significantly larger rate of gas release.

Information Contacts: R. Stoiber and S. Williams, Dartmouth College; D. de Jerez, IRENA, Managua; D. Fajardo B., Instituto de Investigaciones Sísmicas.

03/1981 (SEAN 06:03) Gas emission continues unabated; pit crater enlarges

"Scientists from Dartmouth College, IRENA, and the Instituto de Investigaciones Sísmicas report the following based on their continuing cooperative observation of Nicaraguan volcanoes.

"The fourth gas emission crisis of this century continued unabated. Extensive remote measurement of SO2 output (by COSPEC) has revealed a greater variability in emission rates than had previously been recognized (several hundred to several thousand t/d). The pit crater from which the gas is emitted continued to increase slowly in diameter and was strongly elongate in the NW-SE direction. Night observation of the activity was possible and confirmed the complete absence of any incandescence in the pit where lava was visible as recently as November 1978."

Information Contacts: S. Williams, R. Stoiber, Dartmouth College; D. de Jerez, IRENA; D. Fajardo B., Instituto de Investigaciones Sísmicas.

12/1981 (SEAN 06:12) Large white vapor plume and high SO2 emission rates continue

"Emission of a very large white vapor plume continued in late November. SO2 emission rates, measured using COSPEC, were at the same high levels reported since February 1980. Acid rain and gas fumigation continued to cause problems downwind. Incandescence was seen in the bottom of the inner crater through the crust on the surface of Santiago Crater lava lake on 29 November. Park rangers reported that this incandescence has been visible since September 1981, but it was not noted by several observers who specifically looked for it while working around the crater 25-29 November. The roaring sound of gas emission (or possibly lava splashing) may have been louder than in March 1981."

Information Contacts: R. Stoiber, S. Williams, H.R. Naslund, J.B. Gemmell, D. Sussman, Dartmouth College; D. Fajardo B., Instituto de Investigaciones Sísmicas.

01/1982 (SEAN 07:01) Small explosion heard followed by ashfall several kilometers south

"A small eruption occurred from the hole in Santiago Crater lava lake in the early evening of 16 December. No one witnessed the event, but people living S of the caldera reported hearing an explosion that was followed by ashfall several kilometers to the S. Highly vesiculated scoria fragments up to 20 cm in diameter fell as much as 200 m S of Santiago pit crater. As of late January, no subsequent explosive activity had been observed. A very large plume was still being continuously emitted. Incandescence was not readily visible during the day but was evident at night."

Information Contacts: R. Stoiber, S. Williams, Dartmouth College; D. Fajardo B., Instituto de Investigaciones Sísmicas.

03/1982 (SEAN 07:03) Bright yellow incandescence seen at night

"Bright yellow incandescence was plainly visible at night in Santiago Crater in early March. No change had occurred except for a small collapse of the inner crater walls. The huge gas plume still poured out continuously."

Information Contacts: S. Williams and R. Stoiber, Dartmouth College; I. Menyailov and V. Shapar, IVP, Kamchatka; D. Fajardo B., INETER.

08/1982 (SEAN 07:08) Strong, continuous gas emission; lava lake glow

Emission of acid gas from Santiago Crater was strong and continuous in early August. Incandescence from the small pit in Santiago Crater's lava lake was visible at night. The gas and associated acid rain affected vegetation downwind. An explosive gas emission event occurred 6 June at 1622. As of early August, seismographs recorded constant tremor.

Information Contacts: D. Fajardo B., INETER; R. Parnell, Jr., Dartmouth College.

10/1982 (SEAN 07:10) Small explosion; strong vapor emission; seismicity

A small, brief, explosive eruption from the bottom of the lava lake in Santiago Crater occurred at dawn on 7 October. Tephra, including blocks with volumes to 55 cm3, fell 300 m SE and covered an area of 150,000 m2. The eruption killed a few trees and animals near the summit. Heat from the ejecta melted asphalt on a road, which was also slightly damaged by impact from larger tephra. Rumbling and explosion sounds were heard through the day. After the initial explosion, no additional tephra was ejected, but gas emission increased considerably, forming wide vapor columns that reached high altitudes.

The eruption was preceded by a change in the pattern of seismicity and accompanied by a magnitude 2.3 event lasting 3.7 seconds. After the eruption, small earthquakes occurred about every 6 minutes until 1100 on 8 October.

The 7 October eruption was larger than Masaya's previous explosion on 26 December 1981 (7:1). Strong vapor emission has made observations of the bottom of the crater difficult, obscuring any changes that may have occurred to the lava lake.

Information Contacts: G. Hodgson V., INETER.

11/1982 (SEAN 07:11) Gas emission continues; incandescence within the inner crater

"The approximately 3-year gas emission crisis from Santiago Crater continued in late 1982. Total SO2 flux was apparently reduced from the very large levels reported before. Incandescence within the inner crater was dull red-orange, as compared to the brilliant orange observed in February, 1982. The 7 October, 1982 explosion threw out abundant, juvenile, highly vesiculated scoria, which was often flattened and oxidized against the ground surface. Numerous fragments of sublimate minerals torn from the lip of the inner crater were ejected with the juvenile scoria. No new explosions have thrown debris out of Santiago Crater but several gas bursts have been reported by Park guards."

Information Contacts: S. Williams, R. Stoiber, Dartmouth College; G. Hodgson V., D. Fajardo B., INETER.

04/1983 (SEAN 08:04) Gas column and incandescence from lava lake

A strong gas column was observed. Through 9 March, incandescence was noted.

Information Contacts: D. Fajardo B., INETER.

05/1985 (SEAN 10:05) Small ash eruptions

In December 1983, 4,993 microearthquakes were recorded at Masaya. Early that month, a very large gas column was continuing to emerge from Santiago Crater. A series of small ash eruptions occurred in April 1984. There was a small gas explosion on 23 January 1985, and another ash eruption occurred in April 1985.

Information Contacts: D. Fajardo B., INETER.

08/1985 (SEAN 10:08) 400 km plume sighted

An apparent volcanic plume roughly 400 km long was sighted in the Masaya area, extending W over the Pacific Ocean. Its exact source could not be clearly ascertained as Masaya was obscured by clouds. It was broader and appeared to be more dense then the San Cristóbal plume but was still relatively diffuse.

Information Contacts: C. Wood, NASA, Houston.

11/1985 (SEAN 10:11) Gas column heights in 1985

Heights of the gas column above the rim of Santiago Crater (485 m above sea level) were measured on three occasions in 1985: 22 January (48 m), 17 June (78 m), and 21 October (78 m).

On 3 December, during re-entry from mission 61-B, Space Shuttle pilot Brian O'Connor took three 35-mm photographs (nos. 61B-12-020, 021, and 022) of Masaya. These showed a large white plume extending at least 25 km due W toward the Pacific Ocean.

Further Reference. Stoiber, R.E., Williams, S.N., and Huebert, B.J., 1986, Sulfur and Halogen Gases at Masaya Caldera Complex, Nicaragua: Total Flux and Variations with Time; JGR, v. 91, no. B12, p. 12215-12232.

Information Contacts: D. Fajardo B., INETER; C. Wood, M. Helfert, NASA, Houston.

11/1986 (SEAN 11:11) Rock landslides and wall collapse in Santiago Crater

Intensive rock landslides began at midday on 12 November on the S and W sides of Santiago Crater. Part of the SW wall collapsed, extending Santiago into a section of Nindirí crater (figure 2). A floor collapse accompanied the slides. Two seismographs near the crater have recorded only seismicity produced by rockfalls down the 350-m crater wall.

Figure 2. Topographic map of Masaya showing craters and lava flows (from Mooser and others, 1958).

Reactivation of two faults or fissure systems facilitated continued wall collapse. One prominent fault cuts NE/SW through the crater and the other extends NW around the edge of the crater. A substantial decrease in fumarolic activity followed the collapse with only several small fumaroles remaining along the NE/SW fault in the crater bottom.

Guatemalan newspapers reported that the Civil Defense staff closed access to Masaya Volcano National Park on 20 November. A government communique stated that the rock slides closed a fissure (possibly the pit crater) that formerly emitted large plumes and lava.

References. Stoiber, R.E., and Williams, S.N., 1986, Sulfur and halogen gases at Masaya Caldera Complex, Nicaragua: total flux and variations with time: JGR v. 91, no. B12, p. 12, 215-12, 231.

Mooser, F., Meyer-Abich, H., and McBirney, A., 1958, Catalogue of the active volcanoes of the world, Part VI, Central America, 146 p.

Information Contacts: Douglas Fajardo and Petr Hradecky, INETER; Prensa Libre newspaper, Guatemala City, Guatemala.

05/1987 (SEAN 12:05) Collapse and small eruptions from inner crater

Santiago Crater has been in a very active degassing phase, the 4th in this century, since 1979. Such episodes have previously occurred at intervals of 20-25 years and lasted 4-10 years. Volcanic gases, primarily H2O, SO2, HCl, and CO2, have been emitted. The plain of Pacaya (Crucero), in the direction of predominant gas movement, has been most affected by the quantity of SO2, making raising of crops impossible in the area. Degassing phases and the presence of a lava lake are characteristic of Santiago Crater. It is evident that gases emerge along a fissure system, from faults that strike NW-SE.

The landslides from the walls and interior of Santiago Crater that began 12 November 1986 were facilitated by the descent of the magma column or a pressure change of part of the magma chamber below the volcano. The existence of a fracture zone in the crater also facilitated the breakup of its walls. These events were not preceded or accompanied by tremors. The landslide debris partly plugged the gas conduits, although during the next few days a successive resumption of degassing was observed. Thus, sudden transport of gases was expected to lead to a small eruption.

On 18 December 1986 there was a small collapse within the inner crater floor (figures 3 and 4), forming a circular hole and permitting a major flow of gases. No significant changes were observed in January and the beginning of February. On 15 February at 0020 a small eruption occurred in the circular hole. The pressure of lava erupted ash and blocks, the blocks falling back into the bottom of the vent.

Figure 3. Photograph of Santiago's inner crater at Masaya, taken after the collapse events of late 1986 and early 1987. Courtesy of Douglas Fajardo.
Figure 4. Another photograph of Santiago's inner crater at Masaya, taken after the collapse events of late 1986 and early 1987. Courtesy of Douglas Fajardo.

On 20 February, a new hole, also in the bottom of the inner crater, formed by collapse, expelling a larger quantity of gases than from the circular vent. The gas column was persistent, rising above the rim of Santiago Crater. The gas that emerged from the new vent was darker because of its higher content of magmatic gases.

A larger increase in gases has been reported since 22 February. The circular vent continued to produce small eruptions. After each eruption, the vent was completely clear, without gases. At the beginning of April, incandescence began to be noted at night.

The inner crater is 180 m in diameter and 72 m deep.

Information Contacts: Douglas Fajardo B. and Petr Hradecky, INETER.

01/1988 (SEAN 13:01) Vigorous degassing continues; small tephra eruptions and glow

When geologists visited the volcano on 25 December 1987 and 6 and 18 January 1988, two new, growing vents were visible (figure 5). The N vent was the hottest (table 1) and most active, emitting the majority of gases. By 18 January, the inner crater occupied half of Santiago's floor, while Santiago expanded westwards by collapse of the floor of neighboring Nindirí Crater. The NW side of Nindirí also seemed to be sagging along boundary faults. Gas output varied from vigorous (similar to 1985-86) to very little, perhaps because of periodic blockage of the two vents by landslides and rockfalls. One large gas burst was preceded by a loud roar and slightly raised temperatures.

Figure 5. Sketch map of the craters of the Masaya complex as of 18 January 1988, showing the locations of temperature measurements in table 1.

Table 1. Temperatures in Santiago Crater measured by infrared radiometer in late 1987 and early 1988. Surface brightness temperature was measured in the 8-14 µm range with emissivity set at 1. Locations of observation points are shown on figure 5.

    Date                       Location
                        1       2     3(Vn)   4(Vn)   5

    25 December 1987  35-40°  54.6°  66-125°  100.5°  80°
    06 January 1988    40°     --     110°    121°    --
    18 January 1988    36°     --     161°    160°    --

Information Contacts: B. van Wyk de Vries, H. Rymer, and G. Brown, Open Univ; P. Hradecky and H. Taleno, INETER.

02/1989 (SEAN 14:02) Lava lake develops in new collapse crater

Geologists observed increased steam emissions within Santiago's inner crater and incandescent fissures on its SE vent walls 12 February. Fracturing progressed gradually until 19 February at 1900, when collapse occurred beneath the SE vent floor. A lava lake, first observed late 20 February inside the SE vent, was about 5 m in diameter at 2200 and had increased to about 30 m by 0600 the next day (figure 6).

Figure 6. Sketch of the inside of Santiago Crater at Masaya based on a photograph taken on the SW crater rim looking NE, February 1989. Courtesy of D.A. Rothery.

There was some evidence of strong explosive activity, possibly at 2000 on 20 February. Large amounts of Pelé's hair fell on Santiago crater's floor and on the volcano's SW slope. A large hole formed early 21 February in the N wall of the vent. Geologists observed continuous intense lava fountaining (40 m high) in the center of the lava lake that day and the lake level fluctuated by 5-8 m over periods of about 10 minutes. Fluctuations increased to 10-12 m over periods of about 6-8 minutes on 22 February between 2000 and 2400. The lava lake was crescent-shaped and had reached a maximum size of about 40 x 18 m on 22 February. Measurements on 26 February showed the new lava lake to be 175 m below Santiago's crater floor. The lake area varied from hour to hour because of crusting, remelting, and collapse, averaging about 40 m2 28 February-7 March and never exceeding about 100 m2 during that period. Small Strombolian eruptions from the lake surface were frequent. The maximum recorded brightness temperature of the lake was 1,148°C (measured with a Minolta Cyclops 52 infrared thermometer, bandpass 0.8-1.1 µm, 0.33° field of view) during a sustained fountaining event that ejected spatter to ~20 m height. Maximum brightness temperatures for other fountains generally ranged from 1,048° to 1,094°C.

A new vent appeared 23 February in the N corner of the lava lake and exhibited intermittent Strombolian activity. A vent in that vicinity ("glowing vent" in figure 6) had a brightness temperature of 940°C on 28 February, although lava was not visible within it. On 25 and 26 February, small lava flows were slowly extruded from the area between the new vent and the lava lake. Minor amounts of tephra were discharged from the same area on 28 February. Above and west of the lava lake, a third vent that widened by collapse and glowed weakly was the site of considerable degassing but ejected no tephra. Since 20 February, continuous glow and intermittent Strombolian activity inside the 30-m-wide SW vent have suggested the presence of another small lava lake.

Pelé's hair found near the S and SW rims on 1 and 4 March was probably erupted the previous week. Sounds similar to pistol shots were frequently heard from the direction of the active region. A continuously recording seismic station near the summit had registered normal levels of volcanic tremor before the eruptive episodes 19 and 20 February. Beginning 21 February, continuous, high-amplitude, low-frequency tremor was detected.

The last surface lava lake at Masaya was reported in June 1974 and remained continuously visible until January 1978 (3:04). Intermittent glow and deep roaring sounds have been noted since November 1986, when collapse enlarged the crater to 150 m in diameter and 85 m depth (11:11). A small eruption occurred from the inner crater in February 1987 (12:05) and collapse formed 3 small vents in March, June, and December 1987. By January 1988, the inner crater occupied half of Santiago's crater floor (13:01).

Information Contacts: Alain Creusot, Dept of Volcanology, Instituto Nicaraguense de Energía, Managua, Nicaragua; C. Oppenheimer and D. Rothery, Open Univ; B. van Vyk de Vries, O. Castellon, and L. Urbina, INETER.

04/1989 (SEAN 14:04) Lava lake drains; rockslides; gas emission

A local newspaper (the Barricada, citing Alain Creusot) reported that on 7 March, the level of the active lava lake in Santiago's crater had dropped considerably (since late February). Spatter was occasionally ejected outside the vent. The lake apparently drained on 9 March. Geologists visited the crater on 14 March and measured a temperature of 76.6°C on the surface of the frozen lake (all reported temperatures were measured by an 8-14 micrometer bandpass infrared thermometer from a distance of about 300 m unless otherwise stated). The two incandescent vents that first appeared on 23 February (14:02) were still present in the lake's N corner. The temperature of the hottest glowing vent was 667°C. On 16 and 18 March, fumes collected in the crater and limited observations. By 28 March, debris from rockslides on the SW inner wall of the crater had covered the site of the former lake, at least 175 m below the floor of Santiago Crater. Gas emission was strong. The two incandescent vents (maximum surface temperature 607°C) remained visible at night. On 12 April, the frequency of rockslides (audible about every 5 minutes) had increased significantly. Most occurred on the SW inner wall of the crater and many lasted for minutes. When geologists drove past Masaya on 18 April the amount of fuming appeared to have dramatically decreased.

Information Contacts: C. Oppenheimer, Open Univ.

06/1989 (SEAN 14:06) Lava lake freezes; small explosions

The February-March lava lake in Santiago Crater (14:02) probably froze over in early March, and degassing from the lake vent had apparently ceased by 12 March. Other vents remained open through April, with occasional strong degassing episodes. Beginning around 11 May, collapses from the W, S, and N sides of the main crater blocked all vents. Little, if any, gas emission was evident until 22 May when park rangers reported more collapses and a plume visible from the Masaya road (6 km from the crater).

On 25 May, geologists found fresh scoria and lithic fragments scattered from Plaza Sapper to the San Pedro crater (figure 7, top). Ten-cm fragments were found to 20 m from the edge of Santiago, 5-cm fragments to 50 m, and fragments <2 cm were found farther away (90% <1 cm). All tephra was highly vesicular, often with smooth surfaces indicating solidification in flight. Many Pelé's tears were found. The fragments were concentrated in small areas, suggesting a number of discrete explosions. Tephra from the explosions rose an estimated 100-300 m above the crater. Most fragments were glassy basalt with occasional small (1-3 mm) fresh plagioclase. Lithic fragments were porphyritic basalts with 10% plagioclase and some were slightly altered hydrothermally.

Figure 7. Sketch of the summit complex at Masaya, May-June 1989 (top) and Santiago Crater, 3 June 1989 (bottom). Courtesy of B. van Wyk de Vries and O. Castellón.

A 3 June visit revealed small amounts of fresh scoria up to 5 cm in diameter as far as 50 m SW of the crater. The tephra was probably erupted on 2 June when inhabitants reported a "brown cloud". Crater geometry was similar to that in February. The lava lake vent and the "cannon" (3rd vent in 14:02) were blocked by collapse debris, but vent No. 2 (glowing vent in 14:02) had enlarged and was thought to be the source of the eruptions. On 25 May the vent was oval and about 4 m across, oriented vertically, rather than horizontally as in February. On the 26th it had enlarged by 1 m, and by 3 June it was 7 x 3 m and rectangular. There appeared to be a considerably larger chamber beneath the vent. The cannon (3rd) deepened slightly between 25 May and 3 June.

Periodic fumarolic activity on the W wall and from a fault on the N side (figure 7, bottom) was also observed. Weak fumaroles along the trend of the fault (on the Nindirí crater floor below La Cruz) had temperatures <45°C. Fumarolic activity decreased from May to June.

Information Contacts: B. van Wyk de Vries and O. Castellón, INETER.

04/1990 (BGVN 15:04) Fumarolic activity

During fieldwork on 17 and 25 April, gas emission in Santiago Crater was limited to a few patches of weakly fuming ground within the inner crater, below the level of the frozen 1965 lava lake. The highest temperature measured on the fuming ground (using an 8-14 µm infrared thermometer from the crater rim) was 50.7°C. Small rockfalls from the inner crater walls were frequently audible. Much of the floor of the innermost crater was covered by debris and the "cannon" vent (first reported in February 1989; 14:02) was no longer visible. However, an opening had formed at the site of a former incandescent vent N of the February-March 1989 lava lake. No incandescence was evident in the crater after dusk on 25 April. Tangential fissures crossing the S rim parking area and nearby had widened in recent weeks.

Information Contacts: C. Oppenheimer, Open Univ; B. van Wyk de Vries, INETER.

02/1991 (BGVN 16:02) Rockfall activity declines after November 1989 collapse

"Growth of Santiago crater has slowed since the November 1989 collapse when 50,000 m3 of rock fell from the S (Plaza Sapper) side (figure 8). The overlook with protective wall and part of the parking lot were lost in this event. Cracks continue to open and widen on Plaza Sapper, but rockfalls decreased to negligible levels by April 1990. Seismic activity has been recorded at a station in the Masaya Volcano Museum, 5 km from the crater, and occasionally at a station in Nindirí crater, but overall, very little activity was detected. The tremor associated with the February 1989 lava lake and subsequent Strombolian activity (May-June 1989) was absent. Three samples of the 1989 ejecta were analysed at the Open Univ (UK); all are typical Masaya tholeiitic basalt, similar to that of 1965 and 1772. Fumarolic activity in Santiago is restricted to a few points surrounded by damp ground. Small areas of yellow sulfur deposits have built up locally. Vegetation has started to colonize the Nindirí and San Pedro craters, and some small grass patches have been established on the 1965 lava lake in Santiago.

Figure 8. Sketch map of Santiago crater, Masaya, 19 December 1990. Courtesy of B. van Wyk de Vries.

"A water well that was drilled 3 years ago, about 5 km N of the caldera (near the village of Veracruz) on the volcanic alignment extending from the volcano, was reported to have started to produce hot (almost boiling) water. Geologists from INETER are investigating the cause of this phenomenon. Two maar craters lie 1 km NE of the well."

Information Contacts: B. van Wyk de Vries, O. Castellón, A. Murales, and V. Tenorio, INETER.

04/1992 (BGVN 17:04) Weak gas emission; acid gas and rain effects diminish

During a 26 April visit to Santiago Crater, extremely weak emissions were observed from two or 3 small, quiet fumaroles at the base of the talus in the inner crater and up the W wall (toward Nindirí Crater). COSPEC measurements indicated an SO2 flux of <10 metric tons/day (t/d), compared to 1500-2000 t/d during lava lake activity in 1980 (SEAN 05:12). Simultaneous use of SO2 and HCl INTERSCAN instruments at the crater indicated HCl concentrations several times greater than SO2. A drive on the WSW (downwind) ridge, the site of extensive acid gas deposition and acid rain during the early 1980's (SEAN 05:12, 06:12, and 07:08), showed that vegetation had recovered somewhat; the same stark deforested appearance was still evident, but low shrubs were healthier and larger.

Information Contacts: S. Williams, Arizona State Univ; Martha Navarro C. and Silvia Arguello G., INETER.

03/1993 (BGVN 18:03) Crater walls stabilizing

"Masaya's Santiago crater, visited on 7 and 13-14 January, contains a few weak fumaroles on the rim of the 1989 vents and on the wall adjoining the Nindirí crater. The crater walls have stabilized since the 1989/90 collapses, and there is now little rockfall activity. Vegetation is beginning to colonize the crater walls."

Information Contacts: Andrea Borgia, Instituto Nazionale di Geofisica, via di Vigna Murata 605, 00143 Roma, Italy; B. van Wyk de Vries, Open Univ; Peter J. Baxter, Dept of Community Medicine, Fenner's, Gresham Road, Cambridge, England.

06/1993 (BGVN 18:06) New lava lake appears

A lava lake reappeared in the bottom of Santiago crater in late June for the first time in 3 years, but seismicity has not increased. The temporary network of seismic stations intermittently installed around the summit area has documented a progressive decline in seismicity since late 1989. Epicenters have mainly been located below the N and NE flanks of the volcano. Since 1989 the number of locatable events decreased to about 1-2/week, with focal depths of 400-800 m. In late 1991 and 1992, maximum fumarole temperatures of 380-400°C were measured during expeditions into the 200-m-deep inner crater. Fumaroles were located just above the previous lava lake, active February-March 1989 (SEAN 14:02, 14:04, and 14:06) and covered by landslides in 1990 (SEAN 15:04). A part of the S crater wall collapsed in November 1989 (SEAN 16:02), dropping 50,000 m3 of rock into the crater. Activity in January 1993 consisted of a few weak fumaroles on the rim of the 1989 vents and on the wall adjoining the Nindirí crater.

An increase in SO2 emission was detected in late May 1993, and 3 seismometers were deployed around the crater in early June. An expedition 8 June installed one seismic station on the crater floor 100 m from the N rim. That same day, a significant increase in the rate of gas release was observed, with temperatures estimated at 400-500°C. Park rangers reported new incandescence in the bottom of the crater the evening of 16 June. Another descent to the crater floor on 20 June revealed a 7-8 m diameter vent with liquid lava splashing at a depth of about 30-40 m; small lava fragments were occasionally ejected. The vent slowly increased in diameter through the end of June, and was elongated NW-SE.

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

07/1993 (BGVN 18:07) Small ash eruption precedes formation of lava lake; fumarole temperatures rise

In the last week of March 1993 a fissure was observed to open in the bottom of Santiago crater, with no associated seismic activity. Enlargement of the fissure to 10 m in diameter was accompanied by high-pressure gas emissions. In the first week of April, an earthquake swarm occurred approximately 3 km E of Masaya City; the largest event was M 2.7. At the same time, minor microearthquake activity was registered beneath Santiago crater (1-2 events/day). On 16 June a small ash eruption in the late afternoon lasted for about 13 minutes. That same night incandescence was observed in the bottom of the Santiago crater (BGVN 18:06).

A 7-8 m diameter vent with liquid lava splashing at a depth of about 30-40 m was present on 20 June according to a report by Alain Creusot (BGVN 18:06). An additional report from Creusot indicates that an incandescent hole that had opened above the lava lake was 1-2 m larger in late July. A significant increase in gas emission has maintained a plume rising from the crater. As of 28 June, the temperature of gas emissions from the crater had increased. INETER reported that fumarole temperatures have generally been around 50°C since the beginning of the year, but have now increased to almost 250°C. Seismicity has generally remained low, with a slight increase on 4 July.

Information Contacts: Oscar Leonel Urbina, Departamento de Volcanes, INETER; Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

09/1993 (BGVN 18:09) Incandescence in lava lake

Bright yellow incandescence was observed on the evening of 31 August through a window in the cooling lava lake at the base of Santiago Crater. Jetting sounds made by escaping gases could be heard from the crater rim. New incandescence in the bottom of the crater, reported on 16 June (BGVN 18:06 and 18:07), was the first since February-March 1989 (14:2, 4, and 6). Fumaroles located on the narrow plateau between Santiago and Masaya craters were passively degassing, and their temperatures ranged from 45-65°C.

Information Contacts: M. Conway and A. Macfarlane, FIU; Charles Connor, CNWA Bldg. 168, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228-0510; Oscar Leonel Urbina and C. Lugo, INETER.

10/1993 (BGVN 18:10) Incandescent hole in lava lake remains active

Scientists approached the incandescent window of the lava lake in Santiago's inner crater on 19 October to sample lava ejected during an episode of increased explosive activity at the beginning of October. The window was 15 m in diameter and 50 m deep with lava splashing every 10-15 seconds. Bright yellow incandescence was reported on 31 August and was first observed on 16 June of this year (BGVN 18:06, 18:07, and 18:09).

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

03/1994 (BGVN 19:03) Incandescence visible in daylight; small eruptions

When visited by a team of scientists from INETER and FIU during 1000-1100 on 1 March 1994, Masaya exhibited two adjacent incandescent openings in the cooling lava lake. The 4- to 7-m-diameter openings appeared at the base of the N wall of a smaller crater within Santiago crater. In September 1993 incandescence was only visible at a single opening, and only at night. According to Canadian Missionaries living in Leon, the second incandescent opening was exposed in mid-February 1994. Several tourists reported seeing ash ejected from the incandescent openings on several occasions, an event documented by a second research team later in the month (see below).

INETER-FIU researchers saw a "diffuse, white, sulfur-rich plume . . . punctuated every several minutes by stronger, short-lived (tens of seconds) pulses of gas. The pulses were accompanied by jetting sounds that were easily heard on the S rim." They also noted a mantle of fresh black ash on the crater floor immediately adjacent to the incandescent openings.

During the period 7-11 March 1994, a research team from Open Univ (OU) revisited a 21 km leveling network established in February 1993. They resurveyed the network using precise leveling to find the vertical deformation. Errors in this portion of their survey were several millimeters. The OU team found that relative to stations 5 km E on the shore of Laguna de Masaya, the summit had shifted 2-3 cm upwards. A zone of uplift trended NE across the summit; the greatest uplift occurred near the caldera wall 2 km SW of the summit.

On 7 March at 1100 the OU team noted that the two incandescent openings remained separate, but by 1800 they had merged as the division between them collapsed. On 11 March the team tied this incandescent opening into their survey net. They used electronic distance measuring (EDM) instrumentation, shooting with double bearings, to determined the elevation of the opening as 233 m (error of 0.2 m). This elevation is equivalent to 294 m below the level of the car parking area on the S rim (150-200 m above sea level). The vent that contained the incandescent openings was elongate N-S, about 12-m long, and at least several meters deep.

Since their previous visit in February 1993, the OU team reported increased summit activity, including "strong smell of SO2" and a "fainter whiff of HCl at times." One team member felt that there were more fumaroles in Santiago crater and also along the uppermost arcuate fracture on the N side of Nindirí crater than in recent years. On 31 August 1993 fumaroles were found between Santiago and Masaya craters (BGVN 18:09), but during March 1994 they were absent. From observations of activity, OU researchers suggested that the top of the magma body is perhaps 30-80 m below the level of the vent.

During the interval 7-22 March the OU team reported that incandescence remained visible, ". . . glowing bright red even in broad daylight." Audible gas exhalations were monitored 16 times during this interval: they averaged 30-40 puffs/minute. Bombs were typically ejected slightly less than once per minute, but each explosion produced 1-10 bombs. They landed at most about 30 m from the vent, to the WSW, W, or NW. Maximum bomb diameter was 50 cm. The blanket of tephra in this quadrant grew noticeably during the observation period.

Even though in September 1993 only one incandescent opening was visible, a short time later, in early October 1993, Masaya underwent an episode of increased explosive activity that included lava splashing every 10-15 seconds (BGVN 18:10). Some previous Masaya reports described fluctuations in the color of incandescent openings (for example in 1982, SEAN 07:11).

In addition to their geological observations, the OU team also remarked that "Hundreds of parrots, which had deserted the crater last year, have returned to nest in holes and crevices in the S walls of Santiago crater now that it is active again." In 1979 Masaya became Nicaragua's first National Park.

Information Contacts: Cristian Lugo, INETER; Michael Conway, Andrew Macfarlane, and Peter LaFemina, Florida International Univ (FIU); J. Murray, B. van Wyk de Vries, and A. Maciejewski, Open Univ.

07/1994 (BGVN 19:07) Sulfur-rich plume and incandescent ejections from opening in lava lake

Scientists from FIU and INETER visited Masaya for about an hour on the afternoon of 26 May 1994 and noted that the two incandescent openings (5-7 m in diameter) in the cooling lava lake observed on 1 March near the N wall of Santiago crater (BGVN 19:03) had coalesced into a single opening 10-12 m long. A sulfur-rich plume was being emitted from the opening at a rate of several pulses/minute; the pulses were accompanied by jetting sounds easily heard from the S rim. Fresh, black ash covered the crater floor immediately SW of the opening. INETER scientists reported that small Strombolian explosions ejected incandescent material from the opening several times during May and June 1994.

Information Contacts: Peter C. La Femina, Michael Conway, and Andrew MacFarlane, FIU; Christian Lugo, INETER.

09/1994 (BGVN 19:09) Temperatures and SO2 flux from incandescent opening continue rising

A red incandescent area that opened in the inner crater during mid-June 1993 remained active at least through June 1994. An unbroken gas plume has often been observed extending several kilometers from the volcano. Average fumarole temperatures, measured with an infrared pyrometer, began increasing in May 1993 from around 50°C to almost 250°C by July 1993 (figure 9 and 18:07). Fumarole temperatures slowly increased to almost 400°C by May 1994, when they suddenly increased again, reaching almost 600°C by the end of July 1994. Measurement of SO2 emissions at the summit were carried out using colorimetric and chemical techniques. An increase from background to ~5 mg/m3 was detected in June 1993 after the incandescent opening first appeared. SO2 increased to ~15 mg/m3 between July and August, and again increased sharply during September-November 1993 to ~30 mg/m3. Steady increases in the SO2 emission rate since then resulted in measurements of ~35 mg/m3 in May-July 1994.

Figure 9. Average fumarole temperatures in the summit crater of Masaya, January 1993-July 1994. Courtesy of INETER.

Information Contacts: H. Taleno, L. Urbina, C. Lugo, and O. Canales, INETER.

11/1994 (BGVN 19:11) Red glow from vent on crater floor; gas emission

When observed during November, the vent in Santiago crater was the same shape as in April 1994. It was possible to see ~20 m down into the hole, which was 10-20 m wide. During daylight a red glow could be seen from the lip of the vent inwards, but no lava or ejecta were observed. Pulses of gas emission occurred every 3-5 seconds.

Information Contacts: B. van Wyk de Vries, Open Univ; Pedro Hernandez, INETER.

04/1996 (BGVN 21:04) Incandescent vent in Santiago crater emitting large amounts of gas

Masaya was visited on 15-16 March by a joint team from the Open University, the Universite de Montreal, Reading University, and INETER. Large amounts of gas exiting a 5-m-wide vent at the bottom of Santiago crater formed a distinct plume clearly visible from the Managua airport. The vent was intensely incandescent, even during mid-day. Eight correlation spectrometer (COSPEC) traverses beneath the gas column on 16 March measured an SO2 flux of 600 ± 290 metric tons/day (t/d). These fluxes are similar to those measured during the degassing crisis of the early to mid-1980's (Stoiber and others, 1986). Microgravity measurements revealed a continued decline of the gravity field in the summit region since re-activation of the volcano in 1993 (BGVN 18:06). Systematic decreases of up to 160 microgals have been recorded during this time near the active crater.

Reference. Stoiber, R.E., Williams, S.N., and Huebert, B.J., 1986, Sulfur and halogen gases at Masaya caldera complex, Nicaragua: Total flux and variations with time: Journal of Geophysical Research, v. 91, p. 12,215-12,231.

Information Contacts: Hazel Rymer and Mark Davies, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom (Email: h.rymer@open.ac.uk); John Stix, Dora Knez, Glyn Williams-Jones, and Alexandre Beaulieu, Departement de Geologie, Universite de Montreal, Montreal, Quebec H3C 3J7, Canada (Email: williamg@ere.umontreal.ca); Nicki Stevens, Department of Geography, University of Reading, Reading RG2 2AB, United Kingdom; Martha Navarro and Pedro Perez, INETER, Apartado Postal 2110, Managua, Nicaragua.

03/1997 (BGVN 22:03) Strombolian explosion; incandescent vent in Santiago crater; seismicity increases

A small Strombolian explosion on 5 December 1996 ejected blocks (<10 cm in diameter), ash, and some Pelee's hair. Some of the inner crater walls collapsed, partly closing the incandescent vent. Prior to this eruption the vent's gas temperature was 1,084°C; afterwards, it dropped to 360°C.

During three consecutive days in 1997, COSPEC SO2 fluxes varied as follows: on 12 February, 159 ± 73 metric tons/day (t/d) (1 sigma, n = 5); on 13 February, 363 ± 182 t/d (1 sigma, n = 6); on 14 February, 290 ± 65 t/d (1 sigma, n = 4). The 363 t/d figure is a minimum estimate since on the first 3 traverses the instrument went off the chosen recording scale indicating still larger values than reported.

A visit in March 1997 yielded COSPEC values of 300-400 t/d; these values were lower than those obtained during March 1996 (BGVN 21:04). Nightime observations of the active Santiago crater revealed that large amounts of incandescent gas were being released frequently through a conduit that had partially collapsed on 5 December 1996. As a result of the collapse, it was not possible to see incandescent magma during the night.

Seismicity increased since September 1996; in January 1997, 41 events (4 high- and 47 low-frequency) were recorded along with constant tremor. During 22 February-20 March, 18 events occurred, 15 of which were low-frequency and three high-frequency. Since November 1994 background levels of RSAM have varied between 12 and 16 RSAM units. Since mid-January, however, RSAM increased, fluctuating between 22 and 32 units.

In the crater area, gravity decreased steadily during 1993-95; it remained stable in 1996 and possibly increased a little in 1997.

A NE-trending fracture at the base of Comalito cone emitted gases reaching 68°C. In this same vicinity soil gas concentrations contained up to 25% CO2.

Information Contacts: Hazel Rymer and Mark Davies, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom (Email: h.rymer@open.ac.uk); John Stix, Dora Knez, Glyn Williams-Jones, and Alexandre Beaulieu, Departement de Geologie, Universite de Montreal, Montreal, Quebec H3C 3J7, Canada (Email: williamg@ere.umontreal.ca); Nicki Stevens, Department of Geography, University of Reading, Reading RG2 2AB, United Kingdom; Martha Navarro and Pedro Perez, INETER, Apartado Postal 2110, Managua, Nicaragua.

06/1997 (BGVN 22:06) Stable and non-eruptive during May-June

Besides the strong degassing and high tremor, which are normal for this volcano, Masaya lacked signs of abnormal activity during May and June 1997.

Information Contacts: Wilfried Strauch, Department of Geophysics, and Marta Navarro C., Department of Volcanoes, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: wil@ibw.com.ni).

07/1997 (BGVN 22:07) Minor morphologic changes and fluctuating incandescence in May

"On 25 May, observers saw that the small active vent had grown by 30 m and had ceased to be incandescent. Large volumes of gas were still escaping and forming plumes that blew to the W. Masaya park guards reported a resumption of incandescence on 3 June. During the previous day, there was little wind and high humidity, conditions which allowed the gas to produce a sustained vertical column above the crater."

Information Contacts: Benjamin van Wyk de Vries, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom (Email: b.van-wyk-de-vries@open.ac.uk, URL: http://exodus.open.ac.uk/world/uk_groups/open_volcano.html).

09/1998 (BGVN 23:09) Integrated scientific studies of the caldera area

Four teams of Canadian, British, and Nicaraguan volcanologists carried out studies of Masaya caldera during January-April and September 1998. The volcano was examined using correlation spectroscopy (COSPEC), microgravity, Open Path Fourier Transform Infrared spectroscopy (OP-FTIR), and soil-gas studies.

Vent degassing appeared to have increased significantly. COSPEC measurements during February-April 1998 showed SO2 flux varying from 680 t/d to a maximum of 5,580 t/d. Measurements made during the previous year (January-March 1997) showed more stable fluxes of approximately 380 t/d. Measurements in September 1998 showed flux levels varying from 320 to 1,420 t/d.

OP-FTIR measured from the Plaza Oviedo overlooking the "Santiago" pit crater showed consistent SO2/HCl and HCl/HF volume ratios of 2 and 7, respectively. Using the COSPEC-derived SO2 flux, scientists inferred HCl fluxes of 340 to 2,790 t/d and HF fluxes of 97 to 797 t/d.

CO2 soil-gas measurements at the foot of the Comalito cinder cone increased from 23 to 31.3% between March 1997 and February 1998. Fumarole temperatures also increased from 70 to 84°C during February 1998.

Microgravity surveys during March 1997-February 1998 showed a slight increase in gravity immediately beneath the Santiago pit crater. They also showed evidence (increased noise recorded on the meter) of significant seismic activity around the Santiago crater. Similar measurements acquired in September 1998 indicated increased seismic activity throughout the caldera.

Temperatures at the active vent, measured using a Cyclops infrared camera, ranged between 170 and 400°C. The higher measurements occurred when incandescence of the vent walls was visible. In March, a small fumarole emitting low levels of gas appeared, ~15 m from the active vent.

Information Contacts: Glyn Williams-Jones, Dave Rothery, Hazel Rymer, Peter Francis, and Lisa Boardman, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom (Email: G.Williams-Jones@open.ac.uk); Alexandre Beaulieu, Dany Harvey, Pierre Delmelle, Katie St-Amand, and John Stix, Département de Géologie, Université de Montréal, Montréal, Québec H3C 3J7, Canada (Email: Stix@ere.umontreal.ca); Mike Burton, Clive Oppenheimer, and Matthew Watson, Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, United Kingdom (URL: http://www.geog.cam.ac.uk/); Hélène Gaonac'h, Département des sciences de la Terre, Université du Québec - Montréal, Montréal, Québec H3C 3P8, Canada; Martha Navarro and Wilfried Strauch, INETER, Apartado Postal 2110, Managua, Nicaragua; Benjamin van Wyk de Vries, Departement des Sciences de la Terre, Universite Blaise Pascal, 63038 Clermont-Ferrand, France (Email: vanwyk@opgc.univ-bpclermont.fr).

04/1999 (BGVN 24:04) Continued degassing and marked gravity decreases; previously unreported small explosions

The present activity began in mid-1993 with the brief formation of a lava pond and gradual increase in degassing (BGVN 18:04 and 18:07). Small explosions in Santiago Crater on 17 November 1997 and 14 September 1998 ejected lava bombs up to 50 cm in diameter onto the western rim. Canadian, British and Nicaraguan scientists returned between February and March 1999 to continue the study of the degassing crisis (BGVN 23:09).

A gas plume was continuously emitted from a vent with a diameter of 15-20 m at the bottom of Santiago Crater. A characteristic sound, like the breaking of waves, was created by gas emission. Incandescence of the vent walls was visible only at night. Temperatures recorded at the vent with an infrared thermometer, 200-380°C, were highly dependent upon the opacity of the gas plume.

COSPEC measurements of SO2 revealed continued high flux, varying from 1,300 to 4,060 metric tons/day. Remote sensing of the gas plume composition using an open-path Fourier transform infrared spectrometer (OP-FTIR) in a variety of modes reveals a SO2/HCl volume ratio of about 2, comparable to that obtained in February-April 1998.

The OP-FTIR was also run simultaneously with direct plume sampling using a filter pack-collection technique at the summit and on the Llano Pacaya ridge, 15 km from Santiago Crater. Acid gases (CO2, SO2, H2S, HCl and HF) were passively collected from the crater rim using concentrated KOH solutions exposed to the atmosphere. These experiments should allow for a comparison between remote and direct sampling techniques and provide information on variations in plume composition as it disperses.

Fumigation of the land downwind from Santiago Crater continues to affect the local communities. SO2 plume dispersion and deposition was monitored with a large network of diffusion tubes and sulfation plates. Preliminary results indicate that dispersion of the plume is strongly influenced by local topography. Near-ground SO2 concentrations above 100 ppb were measured on the Llano Pacaya ridge in February-April 1999. These high values may indicate a serious local health hazard. Acid rain collected at the summit and about 7 km downwind on 15 March 1999 had pH values between 3.5 and 4.

Microgravity surveys between March 1997 and February 1999 appear to show a consistent decrease in gravity (up to 90 microgals) immediately beneath the Santiago pit crater. This decrease is of the same order as that measured between 1993 and 1994 at the start of the degassing crisis.

Information Contacts: Pierre Delmelle and John Stix, Département de Géologie, Université de Montréal, Montréal, Québec H3C 3J7, Canada (Email: delmellp@ere.umontreal.ca); Glyn Williams-Jones, Dave Rothery, Hazel Rymer, Lisa Horrocks and Mike Burton, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom (Email: G.Williams-Jones@open.ac.uk); Peter Baxter, Department of Community Medicine, University of Cambridge, Cambridge CH1 2H8, United Kingdom (Email: pjb21@medschl.cam.ac.uk); José Garcia Alavarez, Martha Navarro, and Wilfried Strauch, INETER, Apartado Postal 2110, Managua, Nicaragua (Email: ineter@ibw.com.ni).

07/2000 (BGVN 25:07) Summary of activity; nearby M 5.4 earthquake at 1 km focal depth on 6 July

Since the last report on Masaya, of continued degassing and marked gravity decreases (BGVN 24:04), there have been sporadic reports about its activity, which are summarized below prior to discussion of a nearby M 5.4 earthquake on 6 July 2000.

Reports of ash-and-steam emissions. Between November 1999 and January 2000 there were several reports from the Washington VAAC of ash-and-steam emissions from Masaya. On 22 November 1999 the VAAC reported that GOES-8 imagery suggested that Masaya may have awakened. Satellite imagery showed activity at or very near Masaya, including a plume of ash or "smoke" moving to the WSW, and a hotspot that was visible for over two hours. At about 1600 the imagery suggested that an explosion may have occurred and by 1615 the resultant plume was at ~800 m (near Masaya's summit), and had been blown WSW.

On 22 December 1999 the Washington VAAC issued an ash advisory stating that a continuous low-level plume was being emitted from Masaya. Volcanic activity was confirmed by INETER who noted that seismic activity was consistent with ash emissions. The cloud was ~2 km in altitude and was blown to the WSW.

On 18 January 2000 the VAAC reported that GOES-8 imagery through 0845 detected a low-level thin ash plume from Masaya's summit. The plume reached an altitude of ~900 m, was blown to the SW, and rapidly dissipated.

Seismic activity during April 1999-March 2000. Seismic activity at the volcano remained low with eight microearthquakes registered for the month. The RSAM (seismic tremor) stayed at ~30 units. During the first two weeks of April the RSAM signal was not obtained due to technical problems in the seismic power station. On 23 April two explosions were detected by RSAM, which were confirmed by observers at the Masaya Volcano National Park. In that case, RSAM began to show a small increase until 0800, and an hour later the two explosions occurred.

May 1999: The number of microearthquakes was 21 for the month. The RSAM stayed at ~24 units. June: The number of microearthquakes was 18 for the month. The RSAM stayed at ~24 units. August: The number of microearthquakes was 47 for the month. The RSAM remained at ~40 units. Constant gas emissions occurred. September: The number of microearthquakes was 87 for the month. The RSAM stayed constant at ~40 units. Constant gas emissions occurred. October: The number of microearthquakes was 22 for the month. The RSAM stayed constant at ~20 units. Constant gas emissions occurred. November: There were 49 microearthquakes for the month. The RSAM stayed constant. Constant gas emissions occurred. December: Twenty one earthquakes were registered for the month. The RSAM stayed constant.

January 2000: Eleven earthquakes were registered for the month. The RSAM stayed constant. At 1145 on 6 January an explosion occurred in Santiago crater. February: Six microearthquakes and the RSAM remained constant. March: There were three microearthquakes for the month. The RSAM was at a similar level as the previous month.

July 2000 seismicity near Masaya and Laguna de Apoyo. During July 2000 there were over 300 earthquakes near Laguna de Apoyo (Apoyo volcano) and Masaya. The earthquakes, determined to be of tectonic rather than volcanic origin, caused surficial damage at both volcanoes.

At 1329 on 6 July a small M 2 earthquake occurred near the N rim of Laguna de Apoyo that was followed at 1330 by a M 5.4 earthquake (figure 10). It was located ~32 km SE of Managua, at 11.96°N, 86.02°E, with a focal depth less than 1 km (figure 11). The earthquake was felt in most of Nicaragua and was most strongly felt in the cities of Managua (Modified Mercalli V-VI) and Masaya (VI), and in the region near Laguna de Apoyo (maximum intensity of VII or VIII). The earthquake caused numerous landslides down the volcano's crater walls and surface faulting was observed. In towns located in the epicentral zone, trees and electric lines fell and many houses were partially or totally destroyed. About 70 people were injured and four children were killed by collapsing walls or roofs of homes. At Masaya volcano, ~8 km from the epicenter, there were minor collapses of Santiago crater's walls. No change in degassing was observed at the volcano.

Figure 10. Seismogram showing the M 2 and M 5.4 earthquakes near the Masaya volcano station on 6 July 2000. Courtesy of INETER.
Figure 11. Epicenters near Masaya for the M 5.4 earthquake on 6 July, and the M 4.8 earthquake on 25 July 2000 (stars). The aftershocks from these earthquakes are also shown (small circles). Courtesy of INETER.

Immediately after the earthquake there were many smaller, shallow earthquakes in a zone that includes the area between Masaya, Laguna de Apoyo, and W of Granada (figure 11). In the epicentral zone property was destroyed, cracks opened in the ground, landslides occurred, and trees fell. Several landslides occurred at the edges and steep walls of Laguna de Apoyo. A large number of earthquakes continued until 10 July (figure 12 and table 2). The number of earthquakes then diminished until 1554 on 25 July when a M 4.8 earthquake took place, initiating a series of smaller earthquakes that lasted until about 27 July.

Figure 12. Graph showing the number of earthquakes in the Masaya region between 4 and 30 July 2000. Courtesy of INETER.

Table 2. A summary of earthquakes in vicinity of Masaya and Laguna de Apoyo in early July 2000. Courtesy of INETER.

    Date     Time  Number of daily earthquakes     Maximum magnitude

    07 July  1330             180                         5.2
    08 July  1100              70                         3.8
    09 July  1200              81                         3.6
    10 July  1800              27                         3.1
    11 July  1800               6                         3.3
    13 July  1800              16                         2.8

The July earthquakes were the most destructive seismic events since the 1972 Managua earthquake. The epicentral zone of the July 2000 earthquakes correlates with the same active zones of past earthquakes, which are caused by fault movement between the Cocos and Caribbean plates.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/, Email: wil.gf@ineter.gob.ni); Washington VAAC, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

09/2000 (BGVN 25:09) Small ash eruptions in March; decreasing levels of degassing

A previous report (BGVN 25:07) reviewed evidence for steam-and-ash emissions between November 1999 and January 2000, seismicity during April 1999-March 2000, and increased seismicity in the vicinity of both Masaya and Laguna de Apoyo in July 2000. Previously unreported observations and information from March-April 2000 regarding an ongoing international degassing study, and fumarole temperature measurements from INETER, are included below.

Degassing studies during March-April 2000. The current degassing crisis at Masaya began in mid-1993 with the brief formation of a lava pond and gradual increase in degassing (BGVN 18:04 and 18:07). Canadian, Belgian, British, and Nicaraguan scientists returned to Masaya caldera between March and April 2000 to continue the study of the ongoing degassing crisis (BGVN 23:09 and 24:04). Significant amounts of Pele's hair around the W and S rims of Santiago crater (first noted by Alvaro Aleman, Masaya Park guard) were likely the result of a gas-rich explosion one night either at the end of February or during the first week of March 2000. Two minor explosions, which produced small ash plumes, were witnessed at Santiago crater on 2 March at about 1545 and 1645.

A large gas plume was still being emitted from a vent (15-20 m in diameter) at the bottom of Santiago crater. Incandescence of the vent walls was visible only at night. Temperatures recorded at the vent with an infrared thermometer ranged between 200 and 380°C, and were highly dependent upon the opacity of the gas plume. COSPEC measurements of SO2 revealed decreasing but nevertheless high emission rates, ranging from 740 ± 200 t/d to 1,850 ± 300 t/d. Remote sensing of the gas plume composition using an open-path Fourier transform infrared spectrometer (OP-FTIR) in a variety of modes revealed an average SO2/HCl molar ratio of 1.7, comparable to that obtained in February-April 1998 and February-March 1999. The acid emissions continued to affect a vast area downwind of the volcano, and the rural population subsisting on soil cultivation has been severely impacted.

Microgravity measurements between March and April 2000 appeared to show a leveling off of the previous (1993-94 and 1997-99) decreasing gravity change immediately beneath the Santiago pit crater. These values are essentially the same (within error, ± 20 microgals) as those measured at Masaya in June 1999. This leveling off of gravity change and apparent decrease in gas flux is similar to a cycle of activity between 1994 and 1997 and may suggest that Masaya is entering the waning period of the current degassing crisis.

Fumarole temperatures during December 1999-April 2000. Fumaroles from the Cerro El Comalito area (table 3) showed uniform variations in their monthly average temperatures between December 1999 and April 2000. The fumaroles are close to one another, so this outcome was expected. Fumaroles in the Filete San Fernando area exhibited more variation, with some increasing in temperature and others decreasing.

Table 3. Average fumarole temperatures from the Cerro El Comalito and Filete San Fernando areas of Masaya during December 1999-April 2000. All the measurements were carried out with a thermocouple. Courtesy of INETER.

    Fumarole   Dec 1999  Jan 2000  Feb 2000  Mar 2000  Apr 2000

    Cerro El Comalito
    1          72.8ºC    67.3ºC    74.6ºC    73.2ºC    74.7ºC
    2          74.0ºC    68.2ºC    72.9ºC    74.8ºC    73.1ºC
    3          77.6ºC    69.0ºC    77.0ºC    76.3ºC    75.3ºC
    4          76.2ºC    69.5ºC    76.5ºC    76.5ºC    76.5ºC
    5          68.6ºC    63.3ºC    69.8ºC    68.2ºC    69.8ºC
    6          61.3ºC    56.5ºC    60.2ºC    59.0ºC    60.8ºC

    Filete San Fernando
    1          61.4ºC    60.7ºC    60.0ºC    59.7ºC    59.1ºC
    2          61.2ºC    57.2ºC    59.2ºC    58.9ºC    58.7ºC
    3          60.2ºC    69.2ºC    59.2ºC    59.3ºC    59.4ºC
    4          58.6ºC    64.7ºC    55.8ºC    55.3ºC    55.4ºC

INETER also noted that there were no reports of landslides or incandescence from the lava lake in Santiago crater during March-April 2000. Seismic tremor was low throughout that period, and there were only six microearthquakes registered in March, followed by 12 in April.

Information Contacts: Glyn Williams-Jones, Dave Rothery, Hazel Rymer, Department of Earth Sciences, The Open University, Milton Keynes, United Kingdom (Email: G.Williams-Jones@open.ac.uk); Pierre Delmelle, Unité des Sciences du Sol, Université Catholique de Louvain, Louvain-la-Neuve, Belgium (Email: delmelle@pedo.ucl.ac.be); Clive Oppenheimer and Hayley Duffell, Dept. of Geography, University of Cambridge, Cambridge, United Kingdom; José Garcia Alavarez and Wilfried Strauch, INETER, Apartado Postal 2110, Managua, Nicaragua (Email: jalvarez.gf@ineter.gob.ni).

04/2001 (BGVN 26:04) Tourists experience a brief, bomb-charged 23 April 2001 explosion: no fatalities

INETER report. The Nicaraguan group INETER (Instituto Nicaragüense de Estudios Territoriales) stated that Masaya's active summit crater, Santiago, produced an explosion at 1426 on 23 April. The explosion continued for ~2 minutes and a new 10-m-diameter vent opened on the crater floor ~30 m S of the previous vent. Fragments up to 60 cm in diameter flew through the air, falling up to 500 m from the crater. Episodic ashfall was reported near the settlement of Tecuantepe, 6 km NW of Masaya volcano, and people there contended with abnormal concentrations of volcanic gases. Scientists from Cambridge University (UK) who carried out gas measurements at Santiago crater left just one hour before the explosion and had not noticed any unusual precursory behavior. Preliminary, post-event scrutiny of the seismicity failed to reveal precursory signs.

After the explosion, the volcano returned to its typical stable state and monitored parameters remained at normal levels. INETER volcanologists who continuously monitored Santiago in the afternoon and during the night reported several smaller explosions, gas outbreaks and minor collapses of the crater wall. Following the explosion, Masaya National Park closed public access to the crater-rim areas (including the Plaza Sapper visitor platform and parking lot) for the next several days.

The 24 April report noted minor ash-bearing explosions (specifically mentioning one at 1526), but these events did not exceed those typically seen nor did they accompany abnormally large seismic signals. On this day, the previously active vent no longer gave off gases. The report noted that in the current circumstances, the area of primary hazard lay within 500 m of the vent. It also said that areas farther out, particularly as far away as local habitations or along the Managua-Masaya highway, should not be affected.

SO2 monitoring at the visitor's plaza was conducted at 1020 on 24 April. It indicated that, with respect to 23 April at 1800, the ambient gas concentration there had decreased more than 72%. Since these were not flux measurements but were only ambient SO2 concentrations, fresh winds may have contributed to the decreased concentrations. The 25 April report on Masaya noted slightly larger output than the day before, including ash deposition, but noted 29% lower SO2 concentration than the day before. In harmony with the SO2 concentration decrease on 25 April, sulfurous gases then measured ~2.0 ppm in local settlements (Comarcas La Borgoña and San José de los Ríos), half the value measured the previous day.

The 27 April report noted few episodes of strong degassing during the previous two days, but normal tremor and little seismicity. A second seismic station was installed on the volcano at a spot near the visitor's platform.

23 April eyewitness account and photos. What started out as a routine sightseeing stop escalated into a local crisis as over 120 tourists found themselves on the crater rim during what was one of the more energetic Masaya explosions reported in the Bulletin in the past 30 years. Few, if any, of those earlier events had been witnessed at close range, and in retrospect it seems fortuitous that in this event no one was killed.

The event highlights the difficulty of assessing, preparing for, and conveying the possibility of infrequent, sudden events. The accompanying photos document the ambiguity of assessing the event's magnitude during the explosion's critical early stages. After the event, the majority of eye witnesses with photographs and videos quickly departed from Nicaragua, having shared almost no information with authorities.

Figures 13-19 show selected scenes the tourists captured on film during and just after the explosion. The photos were taken from the 500-m-diameter Santiago crater's N side (for maps of the crater area see SEAN 14:06 and BGVN 16:02). Figures 13-17 are in chronological order; figures 17-19 show selected scenes in the aftermath of the explosion, after the parking area had been largely vacated of vehicles.

The buses were parked in the crater rim area's N parking area. Progressing upslope and along the crater rim, a foot trail leads to the elevated overlook (~200 m W of the parking lot's center). During the explosion this trail became very exposed to ash and ballistics. Although not shown on any of the included photos, a large cross stands at the elevated overlook in the vicinity of where many of the photographers were standing at the time of the eruption (not shown in figures here but labled "La Cruz" on maps in earlier Bulletins).

The photos were furnished to the Bulletin by Joanne Gordon; Mark Headrick also helped explain the significance of some features in the photos. Photos shot by Headrick used an auto-focusing camera with a fixed-focus lens.

Figures 13-15 show the early progression of the ash- and bomb-charged plume. Although in these photos the rising plume can be seen blown towards the W, during the explosion significant numbers of bombs also fell well beyond the plume's margins. For example, some bombs began pelting the N parking lot, forcing people there to take shelter in buses and cars. Fortunately, comparatively few bombs were launched over the local high adjacent the NNW rim where photographers shot figures 13-16. The tens of tourists who had stood at the elevated overlook later retreated in haste cross-country down the hill's more sheltered but trackless back-side (figure 16).

Figure 13. The earliest of several available photos taken of Masaya's 23 April 2001 explosion that vented from Santiago crater. This photo was taken looking SW from the elevated overlook on the NNW rim. The crater floor appears as the dark zone in the lower left-hand corner. Photo credit: Lillian Reyes.
Figure 14. In this view from the elevated overlook on the NNW crater rim of Masaya, the ash- and bomb-laden plume-top had risen slightly above the rim early in the 23 April eruption. Photo credit: Jay Barron.
Figure 15. The steam- and ash-dominated clouds from Masaya's 23 April 2001 explosion rose well above the crater rim before the hazard presented by the explosion was universally recognized. Among the onlookers in the lower-right of the photograph is a small baby wearing a broad-brimmed hat (second from left). Photo credit: Jay Barron.
Figure 16. Masaya's 23 April 2001 explosion taken in a southward-looking direction on the backside of the elevated overlook. The scene clearly shows tourists making a hasty off-trail retreat away from the crater rim. Photo credit: Mark Headrick.

Figures 17 and 18 show portions of the bomb-strewn parking lot. Many bombs of roughly half-liter to several-liters in volume can be easily seen. Parking stalls in the lot can be assumed to be roughly 2 x 3 m in size (~6 m2) and typically contain about 1 to 3 such bombs. This implies that on average, roughly 1 such bomb landed in each 2 to 3 m2 area.

Some bombs landing in the parking lot broke into bits on impact and sprayed local areas of the lot with their light-colored fragments (figure 18). Both figures 17 and 18 document local, sometimes circular grass fires, suggesting that some of the bombs were hotter than the several hundred degree kindling temperature of the dry, brown vegetation. Several bombs significantly damaged vehicles in the lot, causing fires, breaking windows, and puncturing and deforming bus roofs. One bomb landed in a then-unoccupied bus seat, and another plowed deep into the hood and engine compartment of a car (figure 19).

Figure 17. Fresh bombs litter the N parking area (foreground) as a result of the 23 April Masaya eruption. Tour buses had been parked adjacent the tile-roofed shelters but had moved by the time this shot was taken. Hot ejecta started grass fires, which can be seen in this photograph still burning on the slope behind the shelters. Photo credit: Mark Headrick.
Figure 18. Following Masaya's 23 April explosion, tourists who had been at the elevated overlook regrouped at the edge of the bomb-strewn N parking lot. Some bombs shattered into small aggregates that left several light-colored arrays splashed across the pavement. Fires and smoke appear in the background. Photo credit: Mark Headrick.
Figure 19. A ballistic bomb from Masaya's 23 April eruption ended up lodged in a passenger car's hood. The car was occupied at the time of the incident but there were only minor injuries. Photo credit: Pamela Tores.

Joanne Gordon recounted the events of 23 April as follows: "While traveling by cruise ship from Costa Rica through the Panama Canal towards our final destination of Aruba, we made a one-day stop in Nicaragua for a city bus tour. The Nicaragua stop was the second day of our seven-day cruise. Approximately 150 cruisers [in] five buses were scheduled for the city tour and a short visit to Masaya volcano. The first two buses visited the volcano and were scheduled to have ~ 30 minutes to view the crater. Many of the tourists reported that the odor of sulfur greatly increased during their visit . . . . Those two buses left before the following three buses arrived.

"I was on the last bus to arrive at the crater. After getting off the bus my brother and I excitedly ascended to the top lookout point next to the cross overlooking the lip of the crater. The climb to the top of the crater was about 200 steps up a very unstable staircase . . . . Once we arrived at the top I took a few pictures of the crater and of us. Then I heard a deep rumble and the ground began to shake. It sounded as if it was a huge landslide at the opposite side of the crater. Within seconds I could see a massive black mushroom-shaped cloud of smoke filling the inside of the crater. At that point I ran–thinking, 'Is this normal?' but not wanting to stand at the edge of the crater to find out.

" . . . I ran down the [steep] back side of the hill not realizing it was like running down a slope of marbles . . . . Then I heard a second boom followed by more black smoke, and I heard rocks being thrown from the crater—people screaming—children crying . . . . "I heard a little girl's voice . . . [then] I lost my footing and rolled down the hill. After falling for the second time, I stopped to look for her. She had fallen and was caught on some brush. I could see the sky was black but the smoke was moving away from us and the explosion had stopped. I waited with her, trying to calm her . . . . I could see her dad and my brother rushing down the hill trying to keep from falling. When they caught up to us we traversed our way down the hill to the buses, which had been parked about 10-15 feet [3-5 m] from the crater and had now moved out of sight. They had driven away, moving out from under the shower of rocks.

"At this point I had thought it was just the 30-50 people crowded at the top lookout point that were in danger. Little did I know that while I was running to get away from the blast of smoke from the top, the people at the bottom were dodging rocks. It looked like a war zone. Bus windows [had] broken . . . [and (according to Mark Headrick) one bus with its backside facing the crater suffered extensive damage from ejecta, including the loss of its rear window and severe damage to the fiberglass engine cowling. The damage went deeper, and although it drove a short distance away, this bus soon ceased functioning and had to be abandoned]. One lava rock had landed on the top of a bus, and . . . [wedged into the roof where it caught fire to combustible material]. People [were] bleeding, limping, crying, and desperate to get as far away as possible. The hillside next to the parking lot was filled with burned circles. As the lava rocks hit the ground they caught fire to the surrounding brush.

"We all piled on buses and drove a little ways to the park entrance to make sure all were accounted for. I got out of the bus for first aid. I suffered abrasions down both of my arms and legs . . . . We all wanted to get in the buses and get the heck away from the mountain as quickly as possible. We left just before the news cameras and fire department arrived. We drove for about an hour and a half back to the port to board the ship and . . . [departed] Nicaragua.

"At least 15 of the 90 people on the last buses were treated by the ship's infirmary for wounds ranging from a broken arm [wrist], broken foot, abrasions, and cuts and bruises from falling or being struck by rocks. Most of the people that were struck with rocks were injured after the rocks bounced and hit the legs, shoulders, and backs.

" . . . [the cruise line] was very accommodating—they flew a crisis counselor to the ship to comfort the passengers and the 28 crew members that were also at the volcano."

Report from the ship's doctor. Medical doctor Sydney Schneidman practices emergency medicine and was the acting physician on board the cruise ship, which was moored at San Juan del Sur when the accident occurred. As injured people returned to the ship, Schneidman quickly learned that the people suffered from both physical and psychological trauma, and many of the injuries within each of these groups were quite similar.

He recalled that the most serious physical injury was a broken wrist due to a fall. This and many other injuries to 10-15 people occurred when people fleeing the fallout took the steep, off-trail escape route described above. The visitors were forced to move quickly in this direction because flying debris blocked the trail leading back to the parking lot and the buses. Many of the abrasions obtained on this forced evacuation route were leg wounds from sharp-edged volcanic rocks. In addition, Schneidman noted two or three missile injuries, one a head puncture, two others to the flank (side of the stomach), and one to a hip. (Joanne Gordon noted that a broken foot bone sustained by one of the passengers was diagnosed sometime after the trip was over.)

Regarding the psychological aspect of the injuries, Schneidman described these as mental trauma from people who thought they were going to die. He advised the visitors be treated without delay by a psychologist skilled in dealing with "critical-instance stress" (CIS) debriefing, and when in Panama one day later, the cruise ship had arranged to pick up a psychologist flown in from the USA specializing in trauma counseling. Over the next several days the psychologist held group therapy sessions. Studies have shown that rapid treatment for trauma can circumvent or decrease several years of difficulties, including sleeplessness, anxiety, and depression (Goenjian and others, 2000; Schmookler, 1996).

Media coverage. The explosion and its effect on tourists were discussed in news articles in the Los Angeles Times and at least one other Southern-California paper (Reich, 2001; Lee, 2001). The television show Inside Edition aired videos and photos taken of the explosion by visitors returning from the cruise ship (Inside Edition, 2001). At the time, Nicaraguan papers covering the story included few if any details about the experiences of the tourists from the cruise ship because they lacked contact with them.

References. Goenjian, A.K., Steinberg, A.M., Najarian, L.M., Fairbanks, L.A., Tashjian, M., and Pynoos, R.S. , 2000, Prospective study of posttraumatic stress, anxiety, and depressive reactions after earthquake and political violence: American Journal of Psychiatry, v. 157, no. 6, p. 911-916.

Inside Edition, 2001, Volcano survivors: King World Productions, 8 May 2001 telecast (video ordering information at the phone number 212-817-5656 ext. 5583); 515 W 57th St., New York, NY 10019 USA.

Lee, Jasmine, 2001, Tested by fire—Area residents recount terror of volcano blast in Nicaragua: Daily Breeze (A Copley Newspaper, Torrance, CA), 3 May 2001, p. A1 and A9.

Reich, Kenneth, 2001, Volcano's eruption shook up vacation of southland sisters—Nicaragua: The two dodged rocks and ash on a sightseeing stop at a crater during a Latin American cruise; one broke her arm: Los Angeles Times, Metro News, 3 May 2001, p. B2.

Schmookler, Edward L., 1996, Trauma treatment manual (URL: http://amsterdam.park.org/Guests/Stream/ trauma_manual.htm).

Information Contacts: INETER, Apartado Postal 2110, Managua, Nicaragua (Email: jalvarez.gf@ineter.gob.ni); Joanne Gordon, 222 East Carrillo, Ste. 106, c/o PaineWebber, Santa Barbara, CA 93101 USA (Email: jgheadrick@yahoo.com); Sydney Schneidman, M.D., 1757 Holicong Rd., New Hope, PA 18938 USA (Email: sydney@olympus.net).

08/2003 (BGVN 28:08) Fumarolic emissions and low-level seismicity from April 2002 through May 2003

During April 2002-May 2003, monthly visits were made to Masaya for observations and temperature measurements. This report summarizes the recorded activity.

Between April and October the volcano continued to emit large amounts of gas. Tremors stayed consistently above 40 RSAM units. Seismicity was low, with fewer than 50 total earthquakes during the observation period; temperatures generally remained constant.

Fumarole temperature measurements in the Santiago crater on 22 April 2002 showed only a slight variation from October 2001. On 9 May, however, temperatures showed an increase of 20°C since April; again on 4 June a 20°C increase from May was observed. Measurements by Jaime Cardenas of the National Park at El Comalito and San Fernando on 10 and 30 April also showed little change from previous measurements. Similarly, on 5 and 21 May and in June measurements at El Comalito and San Fernando showed no significant changes. The temperatures at El Comalito and San Fernando fumaroles remained constant through the rest of the year.

In July 2002 tremor stayed above 40 RSAM units, and the volcano continued to emit great amounts of gas. Seismic stations registered 20 earthquakes. On 7 July several rumblings were reported. During a visit to the volcano emissions of dark-colored gases were seen. Landslides were observed to have extended to the inner crater, which had a diameter of 20 m; the diameter was 10 m when the crater opened on 23 April 2001. Gas emanations were abundant; a plume rising more than 1,000 m was observed. Fumarole temperatures varied between 106 and 89.3°C.

In August 2002 gas emissions continued. Martha Navarro and Virginia Tenorio visited on 15 August and observed and clearly heard gases emanating from two locations in the inner crater. Gas columns mixed with vapor reached heights of up to 700 m. The emission of gases was lower than during the previous month, possibly due to decreased rainfall. The tremor continued to stay above 40 RSAM units, and 11 earthquakes were registered.

Navarro visited the volcano twice in September. Gas columns were low and there was little vapor on 13 September; on 30 September she observed greater gas emissions and within the inner crater she could hear with greater force the sound of gas emissions. Weeds within an area of 600 m had been affected by acid rain. A small collapse along the N and E walls was observed within the crater.

On 3 October park guards reported a small collapse from the W wall. Observations on 7 and 28 October showed more water vapor than in September, as well as greater gas emissions and louder sounds associated with them. Through September and October tremor remained above 40 RSAM units; no earthquakes were registered. On 6, 16, and 18 December fumarole temperature measurements were taken with an infrared camera at Santiago crater; measurements on those dates were 216°C, 230°C, and 205°C respectively.

Through December 2002 and January and February 2003 fumarole temperatures at El Comalito and San Fernando remained constant. The low level of gas and vapor exhalation continued; columns reached as high as 100 m at the mouth of the crater. RSAM stayed constant at 30 units, with frequency between 1.5 and 2 Hz. In both January and February two earthquakes were registered. During 25 and 26 February there was a small earthquake swarm in Masaya caldera, with earthquakes located under the lake. Six earthquakes registered in March, with 3 Hz frequency, and five registered in April, with 2.8 Hz. RSAM stayed at 20 units, with frequencies between 1.5 and 2 Hz, in March and April.

Gustavo Chigna (INSIVUMEH-Guatemala) reported that the sulfur-dioxide measurements obtained using COSPEC on 28 March yielded a flux of 840 t/d. Measurements by Glyn Williams-Jones (University of Hawaii) with a 2FlySPEC (gas measurement spectrometer) on 28 March showed a value of 849 t/d. On 8 and 22 May measurements at El Comalito and San Fernando showed little variation. The temperatures at the six fumaroles at El Comalito ranged between 59.5°C at fumarole 6 and 76.4°C at fumarole 3. At San Fernando temperatures ranged from 56.4°C at fumarole 4 to 63.6°C at fumarole 2. The seismic tremor stayed constant with 20 units RSAM, with frequencies of 1.5-2 Hz. Only one earthquake registered, with 3 Hz frequency. Pedro Pérez measured the fumarole temperatures in the Santiago crater at 175°C on 15 May.

Gas monitoring. A scientific and technical team from ITER, INETER, and WESTSYSTEMS (Italy) installed a geochemical station, developed by WESTSYSTEMS, for continuous monitoring of diffuse CO2 and H2S degassing at El Comalito. The observation site was selected after a 1999 diffuse degassing survey at Masaya. The station has been in operation since 15 March 2002.

Information Contacts: Virginia Tenorio, Wilfried Strauch, and Martha Navarro, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (Email: ineter@ibw.com.ni, URL: http://www.ineter.gob.ni/geofisica/); Nemesio M. Pérez, Instituto Tecnológico y de Energías Renovables (ITER), 38611 Granadilla, Tenerife, Canary Islands, Spain (Email: nperez@iter.rcanaria.es); Giorgio Virgili, WESTSYSTEMS, Via Molise, 3 56025 Pontedera, PI (Italy) (Email: g.virgili@westsystems.com, URL: http://www.westsystems.com).

10/2003 (BGVN 28:10) Fumarole temperatures unchanged; landslides, incandescence in Santiago crater

This report summarizes the activity at Masaya during June-September 2003. Activity was generally constant, with fumarole temperature measurements similar to those from previous months (BGVN 28:08). In June, July, and August, during visits made every two weeks, Jaime Cárdenas of Masaya Volcano National Park measured the fumarole temperatures at the Comalito and San Fernando craters (table 4). No changes were observed from previous months. During these months, seismic tremor remained constant with 20 units RSAM. No earthquakes were registered, but on both 21 June and 21 July landslides were reported in the Santiago crater. In September, temperatures obtained from the Santiago crater with a Pyrometer were 187°C and 123°C. It was noted during this visit that the lava sounded like ocean waves, and incandescence was observed at night. Temperatures at El Comalito remained moderate.

Table 4. Temperatures recorded at the El Comalito (EC) and San Fernando (SF) fumaroles of Masaya, 10 June-22 September 2003. All temperatures are in degrees Celsius. Courtesy INETER.

     Date    EC ##1   EC ##2   EC ##3   EC ##4   EC ##5   EC ##6   SF ##1   SF ##2   SF ##3   SF ##4
    (2003)

    10 Jun   65.4    74.5    76.8    72.5    73.4    60.2    59.2    54.8    57.2    55.8
    28 Jun   66.4    75.4    78.4    73.6    73.8    60.4    60.2    55.6    58.8    56.7
    12 Jul   55      76      78.2    74      73.6    60      60      60.2    59.5    57
    26 Jul   66.8    78.4    79.4    75.6    74.2    61      61.2    61.4    60.2    58.2
    15 Aug   66.6    78.2    79.5    76      74.5    61.5    59.7    59.7    57.2    56.2
    29 Aug   67.8    75.6    76.6    74.8    76.4    64.2    59.3    57.4    56.9    57
    22 Sep   68.6    72.3    68.3    65.2     --      --      --      --      --      --

Information Contacts: Virginia Tenorio, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (Email: ineter@ibw.com.ni, URL: http://www.ineter.gob.ni/geofisica/).

04/2006 (BGVN 31:04) Intermittent ash eruptions November 2003-March 2005; continuing incandescence

Previously reported behavior at Masaya through 22 September 2003 consisted primarily of incandescence from Santiago crater (BGVN 28:10). Monthly reports prepared by the Instituto Nicarag?ense de Estudios Territoriales (INETER) since that time noted continuing seismicity and incandescence through March 2005. A small explosions was reported on 29 November 2003. Masaya Volcano National Park workers also reported two ash-and-gas explosions at 0121 on 12 December 2003. A collapse event within the crater was noted on 22 June 2004. A report from the Washington Volcanic Ash Advisory Center (VAAC) noted that on 4 July 2004 at 0015 local time, a narrow plume of steam and/or ash from Masaya was visible on satellite imagery extending to the SW. An hour later the plume had extended ~ 12 km from the summit. The report below notes changes induced in Santiago crater after a landslide in early March 2005. A magnitude 1.9 earthquake at a depth of 2.2 km below Masaya on 30 March 2005 was followed by rumbling noises and gas-and-ash emissions.

Field work during February-March 2005. Patricia Nadeau and Glyn Williams-Jones sent us a report of an intensive, multi-component field campaign conducted at Masaya from 16 February 2005 to 12 March 2005. Two FLYSPEC ultraviolet spectrometers were used in tandem with two Microtops sun photometers to constrain passive SO2 and aerosol fluxes and also to evaluate potential downwind loss of SO2 by conversion to aerosols. Additionally, self-potential geophysical measurements were performed at Masaya's summit in a preliminary attempt to delineate the hydrothermal system of the volcano.

On the morning of 3 March, Park workers reported that a landslide had occurred within Santiago crater the previous night. A visibly diminished plume from the crater's active vent suggested that the landslide may have caused a blockage that reduced the escape of SO2 (figures 20 and 21).

Figure 20. A photo taken from the tourist parking lot on 1 March 2005 showing the inner crater at Masaya emitting a large plume prior to the 2-3 March 2005 landslide. The diameter of the crater in this view is estimated to be 150-200 m. Courtesy of Patricia Nadeau and Glyn Williams-Jones.
Figure 21. A view into the Santiago Crater at Masaya and its diminished plume rising from the inner crater, as taken from the tourist parking lot on 3 March 2005. The diameter of the outer crater is approximately 500 m; the inner crater is about 200 m across. Courtesy of Patricia Nadeau and Glyn Williams-Jones.

The visual observations were supported by subsequent SO2-flux measurements, which confirmed a significant drop in SO2 emissions from an average of ~ 300 tons/day prior to the landslide to an average of ~ 80 tons/day following the landslide (figure 22). This decrease in emissions led to concerns over the possibility of a small vent-clearing explosion such as the one that occurred on 23 April 2001 (BGVN 26:04). That explosion was preceded by a similar drop in SO2 emissions for several weeks due to a blockage of the vent that was active at the time. The 2001 explosion resulted in the opening of a new vent, which has since been the site of Masaya's degassing. After the 2001 explosion, the previously active vent no longer degassed and was assumed to be completely inactive.

Figure 22. Graph showing Masaya's daily SO2 fluxes during 25 February 2005-17 April 2005 (normalized to a wind speed of 1 m/s) before and after the landslide during the night of 2-3 March 2005. Courtesy of Patricia Nadeau and Glyn Williams-Jones.

In the days following the 2 March 2005 landslide, gas output was monitored closely, both visually and with the FLYSPEC, for any further decreases, which could have been indicative of further blockage and possible pressurization. Visual observations of the crater on the nights of 4 March and 11 March revealed that while the currently degassing vent was not incandescent, the older vent (believed to be inactive after the April 2001 explosion) was indeed incandescent, though not degassing (figure 23).

Figure 23. A photo taken from the second parking lot overlooking Masaya's Santiago Crater captured the scene at two vents within the inner crater on 10 March 2005. The younger, actively degassing vent and plume are in the foreground; the older, non-degassing vent is in the background. The latter vent was incandescent at night. The diameter of the active vent in this view is estimated to be 30-40 m. Courtesy of Patricia Nadeau and Glyn Williams-Jones.

As of 10 March, the visible gas emissions were the lowest seen, despite the apparent open conduit, as indicated by incandescence in the old vent. Rumbling and sloshing sounds from within the crater had increased from sporadic to nearly constant. However, the days following were marked by a decrease in acoustical noise, as well as the apparent beginning of a climb back to higher SO2 emission rates (~ 120 tons/day on 16 March). These observations were consistent with devlopments in the upper conduit.

References. Williams-Jones, G., Horton, K. A., Elias, T., Garbeil, H., Mouginis-Mark, P. J., Sutton, A. J., and Harris, A. J. L., Accurately measuring volcanic plume velocity with multiple UV spectrometers: Bulletin of Volcanology, in press.

Williams-Jones, G., Delmelle, P., Baxter, P., Beaulieu, A., Burton, M., Garcia-Alvarez, J., Gaonac'h, H., Horrocks, L., Oppenheimer, C., Rymer, H., Rothery, D., St-Amand, K., Stix, J., Strauch, W., and van Wyk de Vries, B., (2001?), Projecto Laboratorio Geofisico-Geoquimico Volcán Masaya, Geochemical, geophysical, and petrological studies at Masaya volcano (1997-2000), on INETER website at <http://www.ineter.gob.ni/geofisica/vol/masaya/doc/gases-glyn2000/gases-glyn2000.html>.

Information Contacts: Patricia Nadeau and Glyn Williams-Jones, Department of Earth Sciences, Simon Fraser University, Burnaby, Canada (Email: panadeau@sfu.ca, glynwj@sfu.ca); Kirstie Simpson, Geological Survey of Canada, Vancouver, Canada (Email: ksimpson@nrcan.gc.ca); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/VAAC/); Wilfried Strauch and Martha Navarro, Instituto Nicarag?ense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (Email: ineter@ibw.com.ni).

03/2009 (BGVN 34:03) Phreatomagmatic explosions in August 2006; intermittent plumes through 2008

Our previous report on Masaya, in April 2006 (BGVN 31:04), summarized intermittent ash eruptions and continuing incandescence through March 2005. At that time, the visible SO2 gas emissions were the lowest seen, a condition attributed to the landslide of 2-3 March 2005 blocking the degassing vent. MODIS/MODVOLC data revealed only one pixel on 24 April 2006.

Activity during 2005-2006. The level of tremor slowly decreased from 20 RSAM units in April 2005 to 10 RSAM units a few months later. A short INETER report noted that there were no micro-earthquakes registered in October 2005. Tremor then stood at 15 RSAM (Real-time Seismic Amplitude Measurement) units, occurring with frequencies of 1.5 Hz. Gas fumes remained steady and strong. No activity was reported from April 2005 until 25-30 April 2006 when there was a small increase in emissions, with columns of gases rising ~ 100 m from the crater; there was also a strong odor of sulfur. During May, increased precipitation resulted in acid rains that burned the vegetation. In June, an observer reported a wall collapse in Santiago crater.

On the evening of 3 August 2006 seismic tremor began to increase, reaching approximately 130 RSAM units. This level was maintained throughout the next day; typically RSAM levels are at about 5 units. INETER volcanologists traveled to the volcano on 4 August and around 1030 observed two small phreatomagmatic explosions from the crater with dark gray ash. From the crater rim incandescence was seen at the bottom of the crater, and jet engine sounds could be heard. Civil Defense also reported that residents of Leon saw ash and gas emissions in the morning. Small amounts of ash fell in Cristo Rey, W of the volcano and in Las Marías to the N. Gas emissions remained strong on 4 August. Small explosions on the morning of 6 August again ejected ash. Activity decreased afterwards, with no further ash emissions and a drop in seismicity to 20 RSAM units. Minor gas emissions continued.

Overall during August 2006 the frequency of tremor shifted slightly from 1.5 to 2.0 Hz, which remained constant through August. Gas emissions increased in August 2006 at a point ~ 800 m from the cone. Gas emissions were released from the old crater as well. Temperatures at the San Fernando and Comalito cones remained unchanged. On 20 August, Martha Navarro (INETER) and Gustavo Chigna (INSIVUMEH, Guatemala), measured SO2 emissions with a COSPEC near El Crucero, (16 km W of the summit) and noted a level of ~ 900 tons of SO2 per day.

On 4 September 2006 tremor remained at 15 RSAM units, with frequencies of 1.5 Hz, a level that continued through October. Gas emissions remained constant, steady and strong. INETER reporting on 25 October 2006 discussed a new vent that opened on the floor of Santiago crater with a small lava lake. It displayed intense degassing. Following heavy rains, landslides spilled down the crater walls. Instability was noted at an overlook parking area.

Activity during 2007-2008. The Washington Volcanic Ash Advisory Center (VAAC) provided occasional reports of plumes from 26 April 2007 to 17 December 2008, predominately from GOES-12 satellite observations. Pilots and local residents also contributed observations through the VAAC and INETER.

A steam plume that drifted WSW on 26 April 2007 was visible on satellite imagery and a web camera. Additional plumes on 9 and 12 June, with little or no ash, were noted. No further plumes were reported until 24 December 2007, when a small diffuse plume, possibly containing ash, moved SW; a change in seismicity corresponded to the emission.

Pilots reported an ash plume on 29 April 2008 that was also seen in satellite imagery moving SW at 2.1 km altitude. An explosion on 18 June 2008 registered on the seismometer E of the volcano. The event discharged moderate quantities of gas and volcanic ash, and the resulting cloud was dark in color. Nearby inhabitants felt the explosion.

Satellite imagery during August 2008 revealed plumes described as steam on 12 August and gas on 18 August, both possibly containing ash. Similar plumes on 10 and 12 September drifted ENE. Pilots reported that on 9 October an ash plume rose to an altitude of 4.6 km and drifted NNE. Analysis of satellite imagery through the rest of 2008 showed possible diffuse ash and steam plumes to the SW and S on 4-5 November, a plume with possible ash on 2 December that moved SW, and a gas plume with possible ash to an altitude of 5.3-6.1 km on 17 December.

Information Contacts: Wilfried Strauch, Virginia Tenorio, and Martha Navarro, Instituto Nicarag?ense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (Email: ineter@ibw.com.ni); Jaime Cardenas Masaya Volcano National Park, Gustavo Chigna (INSIVUMEH, Guatemala); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/VAAC/).

11/2011 (BGVN 36:11) Degassing through at least mid-2011; episodic crater wall collapse

This report on Masaya presents a summary of activity through mid-2011. Our last report was issued in March 2009 (BGVN 34:03) and highlighted the intermittent plumes and explosions of 2006 and 2008.

From 2008-2010 activity generally consisted of degassing with sulfur dioxide (SO2) fluxes typically under 1,200 tons per day. Instability of the S andW crater walls was a concern for the National Park and monitored by the agency INETER (Instituto Nicaragüense de Estudios Territoriales). Mass wasting, frequently triggered by heavy rain, occurred within the crater with debris occasionally blocking the active vents.

Throughout this 3-year period, fumarole temperatures ranged from 58 to 84°C and regular monitoring of the El Comalito cinder cone showed that degassing continued. Tremor sources shallowed during 2008-2010, rising from a 2008 depth of 26 km to a 2010 depth of ~ 1 km.

On 12 October 2010 incandescence occurred in the intra-crater area's largest opening (figure 24). Temperature at the points of incandescence reached 207°C. Differential optical-absorption spectroscopy (DOAS) measurements from vents registered SO2 fluxes of 465 tons per day. SO2 emissions increased throughout October 2010, reaching 586 tons per day. INETER reports contain plots with more detailed SO2 data.

Figure 24. Incandescence seen in Masaya's Santiago crater on 12 October 2010. Note the crater's vertical walls and depth. Courtesy of INETER.

SO2 fluxes in 2011. In January 2011, INETER's team measured SO2 fluxes while in transit along the easternmost route on figure 25 (between the town of Ticuantepe and the community of San Juan). Those SO2 measurements averaged 642 tons per day, an increase over 2010 that was attributed to increased gas and magma output.

Figure 25. Vehicle routes (heavy lines) used while recording Masaya's SO2 fluxes. The scale at bottom shows distance in meters. The topographic margin of Masaya's main caldera sits ~2 km E of the easternmost vehicle route. Courtesy of INETER.

During 7-30 March 2011 collaborators from the University of East Anglia, Heidelberg University, and Oxford University measured Santiago crater's SO2 and other gas emissions. A Mini-DOAS mobile was one of the many instruments used to monitor the atmosphere and SO2 fluxes (figures 26-28).

Figure 26. SO2 measurements underway at Masaya on 20 January 2011. The vehicle passed beneath Masaya's gas plume on the Southern Pan-American Highway. The laptop displays a well-defined red histogram representing SO2 measured along the plume transect. Courtesy of INETER.
Figure 27. Instruments used during the 7-30 March 2011 campaign to measure SO2 on a continuous basis from a viewing platform overlooking Masaya. Courtesy of INETER.
Figure 28. A compact, automatic meteorological station used to measure wind velocity, air humidity, and other parameters that could refine and enable comparisons with the gas measurements. Courtesy of INETER.

In addition to mobile DOAS and fixed gas monitoring systems, a small dirigible (Zeppelin) represented a novel monitoring approach. One potential use for the dirigible was as a platform from which to measure gas concentrations inside the volcanic plume at altitude. Unfortunately, when deployed on its trial launch, heavy winds quickly blew it out of control (figure 29).

Figure 29. The dirigible (Zeppelin) deployed at Masaya during 7-30 March 2011. The dirigible undergoing instrumental work (top), and floating above Santiago crater moments before being blown away by heavy winds (bottom). Courtesy of INETER.

Crater-wall collapse leads to 6 August 2011 Park closure. More than a dozen crater-wall collapses occurred at Santiago crater during June and July 2011. INETER geologist Marisol Echaverry López noted that the SW and W sides of the crater wall had severely eroded. Echaverry recommended that, should the situation worsen, nearby residents be evacuated since debris-covered vents could pressurize the system and lead to explosions. On 14 July, geologist Martha Ibarra found that debris shed from the steep walls was accumulating and the recent collapses had blocked two gas vents. The deep, steep wall of Santiago crater frequently collapsed along fracture zones.

On 6 August 2011, Masaya National Park officials alerted INETER that significant portions of the SW crater rim had collapsed and completely covered the active vent. The park closed for the day during inspections by INETER. The SW rim was the site of frequent failures and field investigators noted that gas emissions were blocked for ~ 10 minutes. No additional failures were observed and activity did not escalate.

During field investigations in September and October 2011, INETER described and measured temperatures from three new fumaroles within Santiago crater. These sites were located at the edges of debris fill within the crater, along the S and E walls and were degassing with temperatures from 48 to 74°C. SO2 measurements from Mini-DOAS indicated decreasing emissions during this time period, from 518 tons per day in September to 153 tons per day in October 2011.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/geofisica/).

04/2012 (BGVN 37:04) Continuous monitoring of emissions and new investigations from collaborators

In our last report on Masaya volcano, we reviewed field investigations and gas measurements from 2008-2011 including the attempt to launch a small Zeppelin as an experiment to measure gas emissions in March 2011 (BGVN 36:11). Here we present results from monitoring efforts focused on the elevated activity that has continued from Masaya’s Santiago crater, one of the nested summit craters in Nindirí cone (figure 30). New gas measurements and field observations have become available from the Instituto Nicaragüense de Estudios Territoriales (INETER) from November 2011 through March 2012. Reports were also available for Masaya’s Comalito cinder cone, a site of continuous gas emissions and elevated temperatures. In February 2012, INETER met with collaborators from both Simon Fraser University (Canada) and The Open University (UK). We highlight some of the results from these collaborators including mapping and modeling of Masaya’s hydrothermal complex, results from long-term SO2 flux monitoring, and a conceptual model that links magma chamber dynamics with intermittent explosive activity.

Figure 30. In this false-color image, Masaya caldera is well-defined. Landsat bands 4,3,2 emphasize vegetation (red) and soil (brown to yellow) and the panchromatic analysis improved the distinction between dark rock (lava) and water (Masaya lake, at the E edge of the caldera) (NASA Landsat Program, 2007). Annotation is based on sketch maps by Mooser and others (1958) and Girard and van Wyk de Vries (2005); image processed by GVP.

The false-color image of Masaya (figure 30) and the surrounding area is a standard composite image (bands 4,3,2) captured by Landsat on 25 March 2001, during Nicaragua’s dry season (November through April). Here, vegetation appears in shades of red (darker in areas with denser vegetation), urban areas are cyan blue, and soils vary from dark to light browns. Located just 500 m E of Santiago crater, Masaya crater is distinguished by older deposits, last active around 150 AD, and contains a ring of vegetation (which appears as a pale pink circle). Masaya’s recent lava flows have been contained within the larger caldera except for those dating from 1670 when lava ponded along the northern caldera rim and spilled over to cover more than 1 km2 outside the caldera.

In November 2011, INETER recorded little activity from Masaya. No field visits were made and no earthquakes were large enough to locate hypocenters. Seismicity that month was low, at 50 RSAM units.

On 12 December 2011, INETER conducted site visits to Masaya’s active crater (Santiago) and Comalito cinder cone. With an infrared thermometer, temperatures were measured from vents within Santiago crater; the highest temperatures measured were 42 and 45°C. The field investigators learned from National Park personnel that, recently during a 2-hour period, booming noises were heard from Santiago crater. INETER suggested that the noise resulted from strong gas release from deep within the crater - no visible material was ejected during the episodes. Areas of gas release could be visually identified within the crater; these were also locations where debris had been shed from the S and SW walls. Rockfalls from these locations were likely affecting gas emissions.

Additional visits to Comalito cone (figure 30), a satellite cone located less than 2 km NE of Santiago crater, allowed in situ measurements of fumarole temperatures. Four sites were measured; the highest temperature was 79°C, the lowest was 75°C (fumaroles 4 and 1 respectively). These temperatures were considered typical compared to others during 2011 (as compiled by INETER; figure 31). The lowest temperatures of the year 2011 were recorded in May and July with some values as low as 60-65°C.

Figure 31. Temperature measurements made by INETER during 2011 at Masaya’s Comalito cone. Four fumaroles were measured consistently throughout the year. Courtesy of INETER.

To quantify SO2 gas emissions, INETER used a mobile Mini DOAS throughout the year transported on two different routes. The road between Ticuantepe and San Juan de la Concepción was the closest route available when the plume trended SW. An additional route, at a greater distance (figure 25 from BGVN 36:11) was available between Las Quatro Esquinas and El Crucero. On 13 December, cloud cover limited the number of successful traverses; however, an average SO2 flux of 648 metric tons per day (t/d) was calculated from three of the six traverses. This was a significant increase compared to values obtained in October 2011 when 13 successful traverses that month yielded an average of 153 t/d. These values (and others in this report) have not been corrected for meteorological conditions and error calculations were not available during this reporting period.

On 23 January 2012 INETER conducted traverses below Masaya’s plume with a Mini DOAS. Measurements along both routes, proximal (Ticuantepe and San Juan de la Concepción) and distal (Las Quatro Esquinas and El Crucero) were attempted. From 10 calculations, SO2 flux from the proximal route yielded 801 t/d. From the distal route, the average flux rate was 543 t/d.

INETER conducted fieldwork during 30-31 January 2012, visiting Santiago crater and Comalito cone. Temperatures from fumarole sites on Comalito had maximum temperatures of 70°C (fumarole 4) and 78°C (fumarole 2) on 30 January. The maximum temperature measured from Santiago crater had increased to 95°C.

On 1 February 2012, INETER visited Comalito cone and reported fumarole temperatures. The highest temperature was 97°C (fumarole 1); on 23 February the highest temperature was 86°C (fumarole 2). Fieldwork also included visits to Santiago crater; temperatures within the crater were relatively low, 75 and 70°C (from 1 February and 23 February, respectively). SO2 flux from Mini DOAS from the closest route (Ticuantepe and San Juan de la Concepción) yielded an average of 943 t/d based on 12 traverses, continuing the trend of increased SO2 emissions since December 2011.

In March 2012, National Park personnel reported that acoustic noise from the crater was less frequent compared to the previous month. Also, visible gas emissions appeared concentrated at the SW and innermost portions of Santiago crater. On 12 March 2012, INETER visited Masaya and measured temperatures from Santiago crater. A wide range of values was recorded: 100°C to 43°C. Relatively stable temperatures were measured from Comalito cone: 73°C to 76°C. The highest temperatures were measured at fumaroles 3 and 4.

On 20 March INETER conducted Mini DOAS traverses beneath Masaya’s SW-trending gas plume. On the proximal route, 12 traverses were successful and determined an average SO2 flux of 1002 t/d suggesting the increasing trend that began in early December 2011 was continuing. Without error calculations and assessing meteorological conditions, however, this trend could not be directly interpreted.

Geohydrology. Long-term interest in diffuse CO2 gas emissions spurred recent investigations into Masaya’s hydrothermal system. Mauri and others (2012) found active hydrothermal anomalies under many of the cinder cones and investigated these conditions with field measurements of soil CO2 concentration, self-potential (SP), soil temperatures, and flow-path modeling (figure 32). Self potential methods make observations “of the static natural voltage existing between sets of points on the ground (Sheriff, 1982)”. From Comalito cone, Nindirí cone, and the lower slopes of Masaya, CO2 gas concentrations ranged from 26 to 43 ppm (mean values). During a 5-year investigation, the authors collected SP geophysical data over extensive transects within the caldera. The datasets yielded significant correlations between high CO2 soil concentrations and SP anomalies. Water depths were determined by processing the SP data with mathematical techniques (wavelets from the Poisson kernel family). They concluded that interconnected structures (ring faults, fissures, and dikes) serve as flow paths for gas, fluids, and heat. These structures also have the potential to block groundwater flow, a conclusion suggested by their models of groundwater contributions to Masaya Lake (Laguna de Masaya) (figure 32).

Figure 32. Groundwater flow model for Masaya volcano taken from Mauri and others (2012). (a) A map indicating key geographic and geologic features including groundwater flow. (b,c) Two vertical profiles with a legend at the bottom. The groundwater was mapped using two geo-electrical prospecting techniques. The self-potential (SP) technique yielded data processed with multi-scale wavelet tomography (MWT). The second technique was the transient electromagnetic method (TEM) (see key and text).

In Figure 32a, we see the spatial localization of uprising fluids associated with hydrothermal activity (green diamonds) and gravitational water flow (blue squares) within Masaya caldera for which depths have been determined. The names of volcanic cones are in blue; crater names and ground structures are in dark red; dark green dashed lines are the fissure vent structures; solid red lines represent the inferred structures (faults, fissures) based on previous work by Crenshaw and others (1982) and Harris (2009). The red dashed lines are the hypothetical structures (faults, fissures) (Crenshaw and others, 1982). The black dashed line is the inferred limit of the caldera.

The three segments traced in Figure 32a correspond to cross-sections along A-D-B (figure 32b) and C-D-B (figure 32c). Cross-section A-D-B represents the water flow direction across the caldera while the cross-section along profile C-D-B represents the water flow direction through the active Santiago crater and across the caldera. The dashed red lines represent underground structures in cases where the dip orientation is unknown and are based on the work of Williams (1983) and Crenshaw and others (1982). Blue lines with a single dot above the center represent water flow having a flow direction different than the cross-section view. Solid arrows represent the flow direction inferred from the self potential/elevation gradient. Elevations of the shallow flow direction (blue and solid green arrows) were estimated from multi-scale wavelet tomography (MWT). MWT is a signal processing method based on waves that allow for location of dipole and monopole sources which correspond to the electrical anomalies generated by water flow through bedrock. The dashed grey line and dashed blue arrows are deep hypothetical flows from the transient electromagnetic method (TEM) results (MacNeil and others, 2007). TEM results were considered in this study because they offered a different level of sensitivity to SP method and, at the time of the study, direct well data was not available to correlate results, making it difficult to determine which model (MWT or TEM) best represented the true water depth.

Long-term SO2 fluxes and windspeed-induced errors. Nadeau and Williams-Jones (2009) consolidated data spanning three decades (figure 33) and assessed current methods for constraining uncertainties in SO2 data collected on traverses with UV correlation spectrometers (COSPEC/DOAS/FLYSPEC). The authors agreed with previous investigators that the following factors contribute to uncertainties: variable local windspeed, emission rate, dry deposition of sulfur from the plume, and conversion of SO2 to sulfate aerosols within the plume. Of these factors, the authors stressed that for low-lying volcanoes such as Masaya, the local wind patterns cause the largest errors. “One must be wary of using one blanket plume speed value for all data collected at different locations, as it can result in misleading variations within the SO2 flux dataset (Nadeau and Williams-Jones, 2009).” At Masaya, this led to as much a 50% apparent decrease in measured SO2 flux between the proximal and distal routes.

Figure 33. Mean SO2 fluxes grouped by month from numerous field campaigns at Masaya. Error bars represent 1 standard deviation of 1 month of measurements. Note the break in the x-axis. Data from Nadeau and Williams-Jones (2009), which expanded on previous work by numerous investigators listed in that publication.

Modeling Masaya’s magma system. Glyn Williams-Jones from Simon Fraser University visited Masaya with student researchers on 21 February 2012. At the National Park facilities, this group presented recent research and results from the 8-year-long collaborative effort between Simon Fraser University, The Open University, and INETER. Williams-Jones reviewed the primary monitoring techniques applied to Masaya and preliminary results regarding the environmental impact of the persistent degassing. In particular, gravity measurements, GPS, and DOAS/FLYSPEC have been used to characterize activity. SO2 flux and air quality measurements have been part of an additional effort to characterize environmental impacts related to resident’s health. The varying trend in the SO2 flux observed since 1976 has been interpreted as being related to varying rates of magma convection in the volcanic plumbing system, as opposed to models invoking intermittent magma supply (Williams-Jones and others, 2003; Stix, 2007).

The model invoking convection within the system links Masaya’s periodic explosive activity with intense, long-term degassing (Williams-Jones and others, 2003; Stix, 2007). The accumulation of a gas-rich magma within a shallow reservoir could develop a buoyant, pressurized foam. This setting would be susceptible to disruptions (by convection cells or structural adjustments, for example) and could be destabilized, leading to explosive activity.

References. Crenshaw, W.B., Williams, S.N., and Stoiber, R.E., 1982, Fault location by radon and mercury detection at an active volcano in Nicaragua, Nature, 300: 345?346.

Harris, A.J.L., 2009, The pit-craters and pit-crater-filling lavas of Masaya volcano, Bulletin of Volcanology, 71(5): 541?558.

MacNeil, R.E., Sanford, W.E., Connor, C.B., Sandberg, S.K., and Diez, M., 2007, Investigation of the groundwater system at Masaya Caldera, Nicaragua, using transient electromagnetics and numerical simulation, Journal of Volcanology and Geothermal Research, 166(3?4): 216?232.

Mauri, G., Williams-Jones, G., Saracco, G., and Zurek, J.M., 2012, A geochemical and geophysical investigation of the hydrothermal complex of Masaya volcano, Nicaragua, Journal of Volcanology and Geothermal Research, 227?228: 15?31.

Nadeau, P.A. and Williams-Jones, G., 2009, Apparent downwind depletion of volcanic SO2 flux-lessons from Masaya Volcano, Nicaragua, Bulletin of Volcanology, 71: 389?400.

NASA Landsat Program, 2007, Landsat ETM+ scene 7dx20010325, Orthorectified, USGS, Sioux Falls, Mar. 25, 2001.

Sheriff, R.E., 1982, Encyclopedic Dictionary of Exploration Geophysics, Eighth Edition, Society of Exploration Geophysicists, Tulsa, OK, 266 pp.

Stix, J., 2007, Stability and instability of quiescently active volcanoes: the case of Masaya, Nicaragua. Geology, 35(6):535?538.

Williams, S.N., 1983, Geology and eruptive mechanisms of Masaya Caldera complex, Nicaragua [PhD Thesis]: Hanover, New Hampshire, Dartmouth College, 169 p.

Williams-Jones, G., Rymer, H., and Rothery, D.A., 2003, Gravity changes and passive SO2 degassing at the Masaya caldera complex, Nicaragua, Journal of Volcanology and Geothermal Research, 123: 137?160.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/geofisica/), Glyn Williams-Jones, Department of Earth Sciences, Simon Fraser University, Burnaby, Canada (URL: http://www.sfu.ca/earth-sciences/), Hazel Rymer, Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, UK (URL: http://www8.open.ac.uk/science/environment-earth-ecosystems/).

06/2012 (BGVN 37:06) Explosions from Santiago crater began on 30 April 2012

Since our last report covering Masaya’s seismic activity and emissions from November 2011 through March 2012, the Instituto Nicaragüense de Estudios Territoriales (INETER) has maintained monitoring efforts including site visits in April and May 2012. Here we discuss regular gas emissions (SO2 and CO2) and seismic monitoring efforts and highlight events preceding the 30 April 2012 explosion from Santiago crater that ejected ash and incandescent blocks within the bounds of the National Park. That event began a series of explosions; more than 68 explosions occurred between 30 April and 17 May 2012.

On 21 April 2012 INETER conducted routine site visits and made field measurements at Masaya. Maximum temperatures recorded with an infrared sensor found temperatures between 98.7°C and 102°C within Santiago crater. Some jetting sounds were heard from the depths of the crater, cracks were observed on the E wall that emitted abundant gases, and the W interior wall showed signs of rockfalls. INETER field teams also visited Comalito cone, located on the NE flank, and measured maximum temperatures of 72°C to 77°C.

During field investigations on 25 April 2012, INETER volcanologists measured diffuse CO2 emissions from Comalito cone. At night on 26 April, the National Park guards reported incandescence within the crater; the last report of incandescence was in October 2010 (BGVN 36:11). SO2 was measured with Mobile DOAS on 27 April on a traverse between the towns Ticuantepe and La Concha (see map for location in figure 25 from BGVN 36:11).

INETER reported that, on 27 April 2012 at approximately 0500 volcanic tremor appeared in Masaya’s seismic records (figure 34). Tremor slowly increased to 70 RSAM that day, and civil defense authorities released notices to officials that significant seismic unrest was detected at Masaya.

Figure 34. RSAM (averaged seismic amplitude) record from Masaya volcano during April 2012, an interval leading up to and including a 30 April eruption. Tremor drove a notable increase in RSAM on 27 April, diminishing slightly as monochromatic tremor prevailed over the following days. After an abrupt decrease in RSAM, the eruption occurred on 30 April. Courtesy of INETER.

On 28 April 2012, authorities, including the Masaya Volcano National Park, released a public announcement about the unusual seismic activity. Three hours following that announcement, the tremor signal became monochromatic near 15 Hz (figure 35). INETER suggested that this signal arose from magma moving beneath the edifice. RSAM reached 100 units with spectral analysis indicating frequencies oscillating between ~1.26 Hz and ~18.84 Hz. The strongest frequency during one particular time window (figure 35) was centered near 15.8 Hz, with a smaller peak at ~1.5 Hz.

Figure 35. (Upper panel) Seismic signal dominated by ongoing tremor recorded at Masaya on 28 April 2012 on a seismogram (amplitude, y-axis, and time (hours : minutes), x-axis). (Lower panel) A spectral analysis made for the interval shown above (frequency, in Hz, along x-axis). Courtesy of INETER.

INETER noted that before the onset of tremor on 27 April, an average of 35 seismic events per day were recorded. These were low frequency earthquakes that included signals reaching 16 Hz and interpreted as rupture events beneath Masaya. The depths of the earthquakes were determined by the P- and S-wave arrival times indicating a depth range between 3 and 4 km.

On 28 April, tremor continued at 70 RSAM and monochromatic tremor occurred again, reaching 90 RSAM. Up to 40 earthquakes were detected that day.

On 29 April, seismic tremor was slightly lower at 65 RSAM and monochromatic tremor was recorded. A total of 45 earthquakes were recorded. Signals were again monochromatic at peak frequencies of 15.8 Hz.

On 30 April at 0045, the tremor signal dramatically decreased to 30 RSAM. INETER commented that this was abnormal since tremor was often recorded between 40 and 50 RSAM during times of quiescence. Seven hours later, a strong explosion was recorded by seismic instruments and observers within the National Park witnessed a blast of gas and ash from Santiago crater (figure 36).

Figure 36. Ash explosions began on 30 April 2012 from Masaya’s Santiago crater. (A) A large explosion occurred at 0829 on 30 April and was photographed by National Park staff. (B) Later in the day a smaller explosion released a small ash plume. Courtesy of INETER and the Masaya Volcano National Park.

Due to the explosions, the Plaza de Oviedo, an overlook at the edge of Santiago crater, was covered with sand-sized pink and yellow ash and lapilli with some rocks up to 10 cm in diameter. Some of the clasts were incandescent and damaged the roofs of structures near the crater and also burned the asphalt of the plaza (figure 37). Small brush fires were ignited on the N flank of the volcano due to hot blocks falling onto the dry plants. Local firefighters worked with the National Park and Civil Defense for most of the day in order to contain and extinguish the fires. The national park was closed due to the hazardous conditions.

Figure 37. (A) The roofs of several structures near Santiago crater were damaged by volcanic bombs during the 30 April 2012 explosions. (B) Some of the bombs ejected during the primary explosion were incandescent and burned the asphalt of the plaza when they landed. Courtesy of INETER.

INETER reported the explosion ejected a column of ash, gas, and blocks reaching 1,000 m above the summit and the initial explosion was followed by 24 smaller explosions that reached 500 m. Ballistic ejecta covered an area with a 300 m radius to the SSE of the crater and ash fell as far as 3 km to the SE of the crater. Blocks measured from this area had maximum dimensions of 50 x 40 x 30 cm. Ash fell to a thickness of 2 mm in some areas and INETER calculated a total volume of 736 cubic meters of ejecta.

INETER measured temperatures from Santiago crater on 30 April with an infrared thermal camera and detected a maximum of 165°C. During the night of 30 April, 23 explosions were recorded by the seismic network.

Between 30 April and 3 May, a collaborative effort among INETER, Civil Defense, local fire fighters, and the National Park succeeded in maintaining a 24-hour watch of Santiago crater. Over four days, the teams recorded observations and determined that 68 explosions had occurred and the maximum detected crater temperature was 162°C.

On 1 May 2012 at 0223 a small explosion was recorded by the INETER seismic network. This event produced ash and volcanic bombs that fell across the NE-SE sectors including the flanks of Nindirí cone (see figure 30 in BGVN 37:04 for site names). The dimensions of the largest blocks were 60 x 50 x 40 cm.

On 3 May there were two small explosions at 0008 and 0022 with abundant gas and ash emissions. Throughout these events, tremor was constant at 1.5 Hz. On 4 May no earthquakes were recorded but tremor remained between 45 and 50 RSAM; explosions of gas and light ash were observed. On 5 May a total of 19 earthquakes were recorded and RSAM varied between 45 and 58 RSAM; ash and gas explosions were reported by National Park staff. On 6 May between 0700 and 1030 a total of 45 earthquakes were recorded and RSAM increased to 70 units.

Sporadic explosions continued until mid-May (figure 38). INETER noted that in May, RSAM averaged 60 units and a significant increase occurred on 18 May. RSAM reached 120 units and was maintained at that level until 21 May. Low tremor was recorded up to 75 RSAM units after 21 May and two days later reached 85 RSAM units with frequencies in the range 1.5-3.0 Hz. Tremor decreased and remained between 65 and 70 RSAM units until the end of the month. A total of 266 earthquakes were recorded in May.

Figure 38. RSAM record from Masaya volcano during May 2012. Courtesy of INETER.

Long-term gas monitoring. Long-term records of Masaya’s gas emissions (SO2 and CO2) and fumarole temperatures have been developed by INETER. On 2 May, SO2 flux was measured during traverses between Ticuantepe and La Concha (table 5). INETER commented that they observed increasing SO2 flux since December 2011 (648 tons per day) that peaked in March 2012 (1002 tons per day). Flux was decreasing at the time of the explosion on 30 April 2012. INETER noted that overall trends in SO2 flux did not correlate with trends in seismicity, however, they emphasized that difficult-to-constrain variables such as wind speed and direction should be factored into the SO2 data interpretations.

Table 5. SO2 flux detected at Masaya from January 2011 through May 2012 during traverses with a Mobile DOAS. Courtesy of INETER.

Year     Month        SO

2

 flux                     (tons/day)2011     January        642         September      518         October        153         December       6482012     January        801         February       943         March          1002         April          761         May            534

Since 7 December 2008, INETER measured CO2 emissions from Comalito cone, an active fumarolic site on the NE flank of Masaya. Diffuse CO2 was measured from a 9 hectare sector of soil as recently as 1 May 2012 (table 6). INETER reported the highest CO2 emissions were detected in 2008 and decreased between 2010 and 2011. Emissions recorded on 25 April 2012 (before the eruption) were considered low, however, there was a small peak on 1 May that may have been related to the explosive activity.

Table 6. The long-term record of diffuse CO2 analyses from Comalito cone measured from September 2008 through May 2012. Courtesy of INETER.

    Date         Area      CO

2

 emission(dd/mm/yyyy)     (km

2

)      (tons/day) 07/12/2008      0.09         66.4 26/03/2010      0.09         27.4 02/03/2011      0.09         15.1 30/01/2012      0.09         50.8 25/04/2012      0.09         25.2 01/05/2012      0.09         32.2

On 17 May, INETER conducted fieldwork at Santiago crater and determined a maximum temperature of 162°C. While in the field, INETER staff observed two small explosions from the crater. Temperatures were also measured at Comalito cone (figure 39); the maximum recorded temperature was from Fumarole 2, 78.2°C, the highest temperature reading at Comalito cone since February 2012.

Figure 39. Temperatures measured at Comalito cone from January through May 2012. Courtesy of INETER.

New monitoring efforts and installations. Two seismic stations were installed in May 2012. One station, called La Azucena, was installed by INETER on 1 May. This site was located ~4 km N of the active crater and was considered temporary. A second station, called El Comalito, was installed on 15 May; located within the National Park at Comalito cone. INETER recognized potential contributions of background noise from the fumarolic sites close to the station and planned to reevaluate the location after reviewing the results from this station. Both stations transmitted realtime data through radio repeaters.

On 4 May a web camera was installed within the town of La Azucena on a short tower; the camera was programmed to send images through a wireless network every 5 minutes. A second camera was installed in the town of Masaya at the office building of the Center of Disaster Operations (CODE); this camera also captured images every 5 minutes. The camera at CODE suffered malfunctions after installation due to overexposure from direct sunlight. Future fieldwork was planned to fix these problems.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/geofisica/), La Prensa (URL: http://www.laprensa.com.ni/2012/04/30/ambito/99799/imprimir).

Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.

Summary of Holocene eruption dates and Volcanic Explosivity Indices (VEI).

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
2008 Apr 29 2008 Dec 17 (?) Confirmed 1 Historical Observations Santiago
2006 Aug 4 2006 Oct 25 (?) Confirmed 1 Historical Observations Santiago
2005 Mar 4 (?) 2005 Mar 30 (?) Confirmed 1 Historical Observations Santiago
[ 2004 Jul 4 ] [ 2004 Jul 4 ] Uncertain 1   Santiago
2003 Sep 22 (in or before) 2003 Dec 12 (?) Confirmed 1 Historical Observations Santiago
2001 Apr 23 2001 Apr 25 (?) Confirmed 1 Historical Observations Santiago
1999 Nov 22 2000 Mar 2 (?) Confirmed 1 Historical Observations Santiago
1998 Sep 14 1998 Sep 14 Confirmed 1 Historical Observations Santiago
1997 Jun 3 (?) 1997 Nov 17 Confirmed 1 Historical Observations Santiago
1996 Dec 5 1996 Dec 5 Confirmed 1 Historical Observations Santiago
1993 Jun 16 1994 Nov (in or after) Confirmed 1 Historical Observations Santiago
1989 Feb 20 1989 Nov Confirmed 1 Historical Observations Santiago
1987 Feb 15 1987 Feb 22 (in or after) Confirmed 1 Historical Observations Santiago
1965 Oct 10 (?) 1985 Apr (?) Confirmed 1 Historical Observations Santiago
1948 Sep 1948 Sep Confirmed 1 Historical Observations Santiago
1946 Jun 1947 Dec (?) Confirmed 1 Historical Observations Santiago
1925 Apr Unknown Confirmed 2 Historical Observations Santiago
1919 1924 Confirmed 2 Historical Observations Santiago
1918 Jan Unknown Confirmed 1 Historical Observations Santiago
1913 Jul 12 Unknown Confirmed 1 Historical Observations Santiago
1906 Jan 2 1906 Jan 9 (in or after) Confirmed 2 Historical Observations Santiago and upper NE flank near El Pelón
1904 May 1904 Jun Confirmed 2 Historical Observations Santiago
1902 Jul 15 1903 Nov Confirmed 2 Historical Observations Santiago
1858 Nov 10 1859 Mar 27 Confirmed 2 Historical Observations Santiago, San Pedro
[ 1858 Apr ] [ Unknown ] Uncertain    
1856 Dec 1857 Jan Confirmed 2 Historical Observations Santiago or San Pedro
1853 Apr 9 (?) 1853 Sep 15 (in or after) Confirmed 1 Historical Observations Santiago
1852 Jun 1852 Jul Confirmed 2 Historical Observations Between Masaya and Nindirí Craters
[ 1775 ] [ Unknown ] Discredited    
1772 Mar 16 1772 Mar 25 (?) Confirmed 2 Historical Observations North side of Old Masaya Crater
1670 Unknown Confirmed 3 Historical Observations Nindirí
[ 1613 ] [ Unknown ] Uncertain 0   Nindirí
[ 1586 ] [ Unknown ] Uncertain 0   Nindirí
1570 Unknown Confirmed 0 Historical Observations Nindirí
1551 Unknown Confirmed 0 Historical Observations Nindirí
1524 1544 (?) Confirmed 0 Historical Observations Nindirí
0150 (?) Unknown Confirmed 5 Tephrochronology Masaya Tuff
0170 BCE ± 100 years Unknown Confirmed 5 Radiocarbon (uncorrected) Masaya Triple Layer
4050 BCE (?) Unknown Confirmed 6 Tephrochronology NW of caldera, San Antonio Tephra

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.


Cones

Feature Name Feature Type Elevation Latitude Longitude
Arenal, Cerro Pyroclastic cone
Comalito Pyroclastic cone
Coyotepe, Cerro el Pyroclastic cone 11° 59' 6" N 86° 5' 53" W
Errant Cone Pyroclastic cone
Media Luna, La Pyroclastic cone
Montoso, Cerro Pyroclastic cone
Pelón, El Pyroclastic cone
Renón, El Pyroclastic cone
Sastres, Las Pyroclastic cone

Craters

Feature Name Feature Type Elevation Latitude Longitude
Masaya
    San Fernando
Crater
Nindirí Crater
San Juan Crater
San Pedro Crater
Santiago Crater
Sierras, Las
    Nubes, Las
Pleistocene caldera 923 m
Ventarrón, El Caldera
Masaya is one of Nicaragua's most unusual and most active volcanoes. It is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300-m high. The caldera is filled on its NW end by more than a dozen vents erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya are seen here from the east caldera rim above Lake Masaya. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted several attempts to extract the volcano's molten "gold."

Photo by Jaime Incer.
A broad expanse of youthful lava flows extends across the floor of Nicaragua's Masaya caldera, whose wall forms the arcuate rim in the background. The lava flows originated from the post-caldera cones of Masaya and Nindirí and constrain Lake Masaya against the eastern caldera wall. Recent lava flows have flooded much of the caldera and have overflowed its rim in one location on the NE side. This view from the NW shows Mombacho volcano in the distance.

Photo by Jaime Incer.
A tumulus on the surface of a pahoehoe lava flow in the foreground is exposed on the western flank of Masaya volcano, with Nindirí cone rising in the background. Nindirí is the westernmost cone of Masaya's summit complex and is cut by a large summit crater not apparent from this vantage point.

Photo by Jamie Incer.
Masaya Volcano National Park provides visitors with the opportunity to observe activity at a frequently active volcano. This observation point on the NE side of Santiago crater is easily accessible from the capital city of Managua. The hazards of close access were underscored in April 2001, when a sudden explosion without precursors ejected blocks onto the parking lot and the surrounding area during a visit by cruise ship passengers. Fortunately, only minor injuries occurred.

Photo by Jaime Incer.
Steep-walled Santiago crater provides a dramatic perspective into the vent of an active volcano. The crater floor is covered by recent lava flows and fume rises from an inner crater. The walls of the 600-m-wide crater expose stacked lava flows, truncated lava lakes, and pyroclastic material erupted from earlier vents.

Photo by Jaime Incer.
A new lava lake appeared on the floor of Santiago Crater in October 1965, marking the beginning of Masaya's third 20th-century lava-lake cycle. By 1969 activity was restricted to a small spatter cone in the center of the crater. Several lava flows erupted on the crater floor between 1965 and 1972. The spatter cone collapsed in 1972 and lava lake activity was visible until 1979, when activity subsided and was followed by dense gas emissions. Small ash eruptions took place on single days in 1981, 1982, 1984, and 1985.

Photo by Jaime Incer.
Three Masaya summit craters are visible in this view to the NW; Santiago crater appears in the foreground, with the flat, high surface of Nindirí crater in the middle and San Pedro crater behind it. The flat-lying, lighter-colored lavas in the center are a part of a lava lake emplaced in Nindirí crater during an eruption in July 1852. A small lava flow was also emitted at that time, a few years before the formation of Santiago and San Pedro pit craters in 1858-1859. The NW wall of Masaya caldera forms the low ridge in the middle distance.

Copyrighted photo by Dick Stoiber, 1963 (Dartmouth College).
An incandescent vent glows on the floor of Santiago crater in July 1972. A new lava lake formed in Santiago crater in October 1965. By 1969 activity was restricted to a small spatter cone; after it collapsed in 1972 a lava lake was visible until 1979. Periodic lava lake activity has occurred since the time of the Spanish Conquistadors. The partially yellow incandescence of the active lava lake was taken for molten gold, prompting several attempts to mine it.

Copyrighted photo by Dick Stoiber, 1972 (Dartmouth College).
Fresh black lava fills the floor of Santiago crater in December 1976. A new lava lake appeared on the floor of Santiago Crater on October 20, 1965, marking the beginning of Masaya's third 20th-century lava lake cycle. Several lava flows occurred on the crater floor between 1965 and 1972. The spatter cone collapsed in 1972 and lava lake activity was visible until 1979, when the lava lake subsided, producing thick gas emissions.

Copyrighted photo by Dick Stoiber, 1976 (Dartmouth College).
Early monitoring efforts at Masaya volcano included tilt meter stations such as this one, operated by the Instituto Nicaragüense de Estudios Territoriales (INETER). Inflation of the volcano's surface produced by ascent of magma can be detected by sensitive instruments such as the one installed here in 1976.

Copyrighted photo by Dick Stoiber, 1976 (Dartmouth College).
Masaya volcano is noted for long-duration periods of voluminous gas emission. This February 1982 photo from the NE shows a gas plume pouring from Santiago crater, during the 4th gas emission crisis of the 20th century. Emission of a very large gas plume had continued without interruption since the fall of 1979. Remote sensing of SO2 revealed continued high level flux, with a 1500-2000 tons/day average during 1980. Distribution of the gas plumes by prevailing winds caused widespread crop damage.

Copyrighted photo by Dick Stoiber, 1982 (Dartmouth College).
A small eruption occurred from Santiago crater on February 15, 1987 following blockage of the vent by landslides in November and December 1986. The small eruption ejected ash and blocks, which fell back into the bottom of the vent. Additional collapses took place on February 20, and the circular vent continued to produce small eruptions after February 22. This photograph of Santiago's inner crater (180 m in diameter and 72 m deep) was taken after the collapse events of late 1986 and early 1987.

Photo by Douglas Farjado, 1987 (INETER).
An aerial oblique photo from the NW shows the summit crater complex of Masaya volcano. The wall of the 6 x 11 km wide caldera inside which the central cone complex was constructed can be seen at the upper right and the extreme upper left (where it contains Lake Masaya).

Aerial photo by Instituto Geográfico Nacional, 1975 (courtesy of Jaime Incer).
An October 1971 photo from the crater rim shows a fresh black lava flow extruded onto the crater floor. Fumaroles rise from the surface of the flow at the right. Masaya began a long-duration eruptive period in 1965. Several lava flows, such as this one, were erupted between 1965 and 1972. An active lava lake was visible until 1979, and intermittent small explosive eruptions lasted until 1985.

Photo by Bill Rose, 1971 (Michigan Technological University).
The Masaya central cone complex within the Holocene Masaya caldera (foreground) lies within a larger Pleistocene caldera, the Las Sierras (or Las Nubes) caldera, which was formed following the eruption of a major ignimbrite about 30,000 years ago. The ignimbrite deposit from this eruption is found in the Managua area, where it is cut by faults of the Managua Graben, which extends to the north, constraining the SE margins of Lake Managua.

Photo by Jaime Incer, 1990.
Santiago crater, seen here from the NE, formed in 1853. A strong detonation on April 9, 1853 was followed by heavy gas emission with no noticeable ejection of pyroclastic material. Flames were seen at the summit of the volcano on the night of September 15 from the town of Masatepe and later investigation revealed a new 80 x 65 m wide crater (later known as Santiago) with incandescent lava surrounded by volcanic bombs. Santiago pit crater expanded significantly in 1858-1859 and is now 600 m wide.

Photo by Jaime Incer, 1996.
Masaya (left) and Nindirí (right) cones are seen here from the NW across the floor of Masaya caldera. Santiago crater, the source of most of Masaya's historical eruptions, lies in the saddle between the two cones. Historical and prehistorical lava flows blanket the caldera floor.

Photo by Jaime Incer, 1990.
Fume rises in 1978 from a vent on the floor of Santiago crater, seen here from its SE rim. The floor of the crater is covered by fresh lava flows erupted between 1965 and 1972. A lava lake was present on the part of the crater floor until 1979, and intermittent explosive activity continued until 1985.

Photo by Jaime Incer, 1978.
Incandescent magma is visible in 1996 at the bottom of Santiago crater from a vent below the south crater wall. A small strombolian eruption on December 5, 1996 ejected blocks (<10 cm in diameter), ash, and Pele's hair. Some of the inner crater walls collapsed, partly closing the incandescent vent. Prior to this eruption the vent's gas temperature was 1,084°C; afterwards, it dropped to 360°C.

Photo by Jaime Incer, 1996.
Santiago crater was initially formed in 1853, but the present pit crater formed during the 1858-1859 eruption. A column of flames and ash eruptions were reported on January 27 and March 27, 1859 accompanied by collapse of Santiago and San Pedro Craters. After its initial collapse, Santiago pit crater was a vertical cylinder about 150 m deep and 600 m across. By the time of the 1865 visit of Seebach, the craters had evolved to their present configuration.

Photo by Jaime Incer, 1992.
The sparsely vegetated lava flow in the foreground was emplaced during an eruption in 1772. The flow originated from a vent on the north side of Old Masaya crater and traveled to the north. One lobe passed through a notch in the northern caldera rim, while the lobe seen in this photo was deflected by the caldera rim and traveled to the SE into Lake Masaya, which is ponded against the SE caldera rim. The twin-peaked stratovolcano in the distance is Mombacho volcano.

Photo by Jaime Incer.
Nindirí crater is partially truncated by the walls of Santiago crater, which formed in 1858-1859. The crater walls reveal flows from lava lakes erupted between 1524 and 1670. An active lava lake was apparently present in Nindirí crater from 1524 to 1544, as reported by Spanish Friars passing through Nicaragua. The Spanish chronicler Oviedo observed the lava lake when he climbed the volcano in July 1529. In 1534 Friar Bartolomé de las Casas reported that a letter could be read at night in the town of Nindirí (6 km away) by the glow of lava.

Photo by Jaime Incer, 1991.
The fresh-looking lava flow in the foreground was erupted in 1772. On March 16 of that year lava emerged from a fissure on the north side of Old Masaya Crater, accompanied by bomb ejection. Part of the flow was diverted by the caldera wall and flowed into Masaya Lake, but much of the flow overtopped the caldera rim and traveled a total of 16 km to near the shores of Lake Managua, cutting the road between Nindirí and Managua and destroying croplands. The eruption lasted about 9 days.

Photo by Jaime Incer, 1994.
The 1670 lava flow covering much of this photo originated from Nindirí crater on the horizon. A lava lake in the crater eventually overflowed the rim, producing a lava flow that traveled from Nindirí crater for 5 km down the northern flank of Masaya's post-caldera cone. Some accounts confused the 1670 flow from Nindirí with the 1772 flow from Old Masaya crater.

Photo by Jaime Incer, 1994.
The 1670 lava flow from Nindirí crater in the background is seen here spilling over the crater rim. A series of lava lakes emplaced in 1670 eventually overflowed the rim through this narrow notch and produced a large lava flow that traveled 5 km down the north flank of the intracaldera cone.

Photo by Jaime Incer, 1977.
The blocky lava flow in the foreground was erupted in 1772 from a vent on the northern side of Old Masaya cone (extreme left). The dark flow at the upper right spilling over the rim of Nindirí crater was erupted in 1670 and traveled down the northern flank of the cone.

Photo by Paul Kimberly, 1998 (Smithsonian Institution).
A vigorous gas plume is seen exiting from Masaya volcano in November 1998. Masaya's latest episode of degassing activity began in mid 1993. Prevailing winds typically distribute plumes from Masaya to the west, where they come in contact with the higher-elevation Las Sierras highlands (left horizon). The volcanic emissions during 1998-1999 affected a 1250 sq km area downwind, causing health hazards and extensive vegetation damage, resulting in economic losses to coffee plantations.

Photo by Lee Siebert, 1998 (Smithsonian Institution).
A vigorous steam plume pours from Masaya volcano in this November 9, 1984 Space Shuttle image taken near the end of a two-decade-long eruptive episode. North lies to the lower right, with Lake Nicaragua at the lower left and Lake Managua at the lower right. To the left of the plume from Santiago crater is Lake Masaya (ponded against the rim of Masaya caldera) and the circular lake-filled Apoyo caldera. The two caldera lakes at the lower right are Apoyeque (light blue) and Jiloa (dark-colored), across the bay from the city of Managua.

Photo by National Aeronautical and Space Administration (NASA), 1984.

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

Bice D C, 1980. Tephra stratigraphy and physical aspects of recent volcanism near Managua, Nicaragua. Unpublished PhD thesis, Univ Calif Berkeley, 422 p.

Bice D C, 1985. Quaternary volcanic stratigraphy of Managua, Nicaragua; correlation and source assignment for multiple overlapping plinian deposits. Geol Soc Amer Bull, 96: 553-566.

Branan Y K, Harris A, Watson I M, Phillips J C, Horton K, Williams-Jones G, Garbeil H, 2008. Investigation of at-vent dynamics and dilution using thermal infrared radiometers at Masaya volcano, Nicaragua. J Volc Geotherm Res, 169: 34-47.

Burton M R, Oppenheimer C, Horrocks L A, Francis P W, Polet J, Kanamori H, 2000. Remote sensing of CO2 and H2O emission rates from Masaya Volcano, Nicaragua. Geology, 28: 915-918.

Carr M J, 1984. Symmetrical and segmented variation of physical and geochemical characterisitics of the Central American volcanic front. J Volc Geotherm Res, 20: 231-252.

Costantini L, Bonadonna C, Houghton B F, Wehrmann H, 2009. New physical characterization of the Fontana Lapilli basaltic Plinian eruption, Nicaragua. Bull Volc, 71: 337-355.

Delmelle P, Stix J, Baxter P J, Garcia-Alvarez J, Barquero J, 2002. Atmospheric dispersion, environmental effects and potential health hazard associated with the low-altitude gas plume of Masaya volcano, Nicaragua. Bull Volc, 64: 423-434.

Duffell H J, Oppenheimer C, Pyle D M, Galle B, McGonigle A J S, Burton M R, 2003. Changes in gas composition prior to a minor explosive eruption at Masaya volcano, Nicaragua. J Volc Geotherm Res, 126: 327-339.

Freundt A, Kutterolf S, Schmincke H-U, Hansteen T, Wehrmann H, Perez W, Strauch W, Navarro M, 2006. Volcanic hazards in Nicaragua: past, present, and future. In: Rose W I, Bluth G J S, Carr M J, Ewert J W, Patino L C, Vallance J W (eds), Volcanic hazards in Central America, {Geol Soc Amer Spec Pap}, 412: 141-165.

Girard G, van Wyk de Vries B, 2005. The Managua Graben and Las Sierras-Masaya volcanic complex (Nicaragua): pull-apart localization by an intrusive complex: results from analog modeling. J Volc Geotherm Res, 144: 37-57.

Harris A J L, 2009. The pit-craters and pit-crater-filling lavas of Masaya volcano. Bull Volc, 71: 541-588.

Hradecky P, 1997. Estudio geologico para reconocimiento de riesgo natural y vulnerabilidad geologica en el area de Managua. Cesky Geologicky Ustav Praha, Instituto Nicaraguense de Estudios Territoriales, Managua (INETER), 81 p.

IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior..

Incer J, 1970. Nuevo Geografia de Nicaragua. Mangua: Talere S de Editorial Recalde, 582 p.

Incer J, 1987. . (pers. comm.).

McBirney A R, 1956. The Nicaraguan volcano Masaya and its caldera. Eos, Trans Amer Geophys Union, 37: 83-96.

Mooser F, Meyer-Abich H, McBirney A R, 1958. Central America. Catalog of Active Volcanoes of the World and Solfatara Fields, Rome: IAVCEI, 6: 1-146.

Parsons Corporation, 1972. The Geology of Western Nicaragua. Nicaragua Tax Improvement and Natural Resources Inventory Project, Final Technical Rpt, v. IV.

Perez W, Freundt A, 2006. The youngest highly explosive basaltic eruptions from Masaya caldera (Nicaragua): stratigraphy and hazard assessment. In: Rose W I, Bluth G J S, Carr M J, Ewert J W, Patino L C, Vallance J W (eds), Volcanic hazards in Central America, {Geol Soc Amer Spec Pap}, 412: 189-207.

Perez W, Freundt A, Kutterolf S, Schmincke H-U, 2009. The Masaya Triple Layer: a 2100 year old basaltic multi-episodic plinian eruption from the Masaya caldera complex (Nicaragua). J Volc Geotherm Res, 179: 191-205.

Rymer H, van Wyk de Vries B, Stix J, Williams-Jones G, 1998. Pit crater structure and processes governing persistent activity at Masaya volcano, Nicaragua. Bull Volc, 59: 345-355.

Sapper K, 1925. The Volcanoes of Central America. Halle: Verlag Max Niemeyer, 144 p.

Schmincke H-U, Kutterolf S, Perez W, Rausch J, Freundt A , 2009. Walking through volcanic mud: the 2,100-year-old Acahualinca footprints (Nicaragua) I: Stratigraphy, lithology, volcanology and age of the Acahualinca section. Bull Volc, 71: 479-493.

Stix J, 2007. Stability and instability of quiescently active volcanoes: the case of Masaya, Nicaragua. Geology, 35: 535-538.

van Wyk de Vries B, 1993. Tectonics and magma evolution of Nicaraguan volcanic systems. Unpublished PhD thesis, Open Univ, Milton Keynes, 328 p.

Viramonte J G, Incer-Barquero J, 2008. Masaya, the "Mouth of Hell", Nicaragua: volcanological interpretation of the myths, legends and anecdotes. J Volc Geotherm Res, 176: 419-426.

Viramonte J G, Navarro Collado M, Malavasi Rojas E, 1997. Nicaragua-Costa Rica Quaternary volcanic chain. IAVCEI General Assembly, Puerto Vallarta, Mexico, January 19-24, 1997, Fieldtrip Guidebook, 17 p.

Walker J A, Williams S N, Kalamarides R I, Feigenson M D, 1993. Shallow open-system evolution of basaltic magma beneath a subduction zone volcano: the Masaya Caldera Complex, Nicaragua. J Volc Geotherm Res, 56: 379-400.

Wehrmann H, Bonadonna C, Freundt A, Houghton B F, Kutterolf S, 2006. Fontana Tephra: a basaltic plinian eruption in Nicaragua. In: Rose W I, Bluth G J S, Carr M J, Ewert J W, Patino L C, Vallance J W (eds), Volcanic hazards in Central America, {Geol Soc Amer Spec Pap}, 412: 209-223.

Williams S N, 1983. Plinian airfall deposits of basaltic composition. Geology, 11: 211-214.

Williams-Jones G, Rymer H, Rothery D A, 2003. Gravity changes and passive SO2 degassing at the Masaya caldera complex, Nicaragua. J Volc Geotherm Res, 123: 137-160.

Volcano Types

Caldera
Stratovolcano(es)
Pyroclastic cone(s)

Tectonic Setting

Subduction zone
Continental crust (> 25 km)

Rock Types

Major
Basalt / Picro-Basalt
Andesite / Basaltic Andesite

Population

Within 5 km
Within 10 km
Within 30 km
Within 100 km
989,888
989,888
1,914,707
2,926,954

Affiliated Databases

Large Eruptions of Masaya Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).
WOVOdat WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.
EarthChem EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).
Smithsonian Collections Search the Smithsonian's NMNH Department of Mineral Sciences collections database. Go to the "Search Rocks and Ores" tab and use the Volcano Name drop-down to find samples.