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Bulletin of the Global Volcanism Network

All reports of volcanic activity published by the Smithsonian since 1968 are available through a monthly table of contents or by searching for a specific volcano. Until 1975, reports were issued for individual volcanoes as information became available; these have been organized by month for convenience. Later publications were done in a monthly newsletter format. Links go to the profile page for each volcano with the Bulletin tab open.

Information is preliminary at time of publication and subject to change.


Recently Published Bulletin Reports

Stromboli (Italy) Constant explosions from both crater areas during November 2018-February 2019

Krakatau (Indonesia) Ash plumes, ballistic ejecta, and lava extrusion during October-December; partial collapse and tsunami in late December; Surtseyan activity in December-January 2019

Masaya (Nicaragua) Lava lake persists with decreased thermal output, November 2018-February 2019

Santa Maria (Guatemala) Daily explosions cause steam-and-ash plumes and block avalanches, November 2018-February 2019

Reventador (Ecuador) Multiple daily explosions with ash plumes and incandescent blocks rolling down the flanks, October 2018-January 2019

Kuchinoerabujima (Japan) Weak explosions and ash plumes beginning 21 October 2018

Kerinci (Indonesia) A persistent gas-and-steam plume and intermittent ash plumes occurred from July 2018 through January 2019

Yasur (Vanuatu) Eruption continues with ongoing explosions and multiple active crater vents, August 2018-January 2019

Ambae (Vanuatu) Ash plumes and lahars in July 2018 cause evacuation of the island; intermittent gas-and-steam and ash plumes through January 2019

Agung (Indonesia) Ongoing intermittent ash plumes and frequent gas-and-steam plumes during August 2018-January 2019

Erebus (Antarctica) Lava lakes persist through 2017 and 2018

Villarrica (Chile) Intermittent Strombolian activity ejects incandescent bombs around crater rim, September 2018-February 2019



Stromboli (Italy) — March 2019 Citation iconCite this Report

Stromboli

Italy

38.789°N, 15.213°E; summit elev. 924 m

All times are local (unless otherwise noted)


Constant explosions from both crater areas during November 2018-February 2019

Nearly constant fountains of lava at Stromboli have served as a natural beacon in the Tyrrhenian Sea for at least 2,000 years. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N Area) and a southern crater group (CS Area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the island. Thermal and visual cameras that monitor activity at the vents are located on the nearby Pizzo Sopra La Fossa, above the Terrazza Craterica, and at a location closer to the summit craters.

Eruptive activity from November 2018 to February 2019 was consistent in terms of explosion intensities and rates from both crater areas at the summit, and similar to activity of the past few years (table 5). In the North Crater area, both vents N1 and N2 emitted a mixture of coarse (lapilli and bombs) and fine (ash) ejecta; most explosions rose less than 80 m above the vents, some reached 150 m. Average explosion rates ranged from 4 to 21 per hour. In the CS crater area continuous degassing and occasional intense spattering were typical at vent C, vent S1 was a low-intensity incandescent jet throughout the period. Explosions from vent S2 produced 80-150 m high ejecta of ash, lapilli and bombs at average rates of 3-16 per hour. Thermal activity at Stromboli was actually higher during November 2018-February 2019 than it had been in previous months as recorded in the MIROVA Log Radiative Power data from MODIS infrared satellite information (figure 139).

Table 5. Summary of activity levels at Stromboli, November 2018-February 2019. Low intensity activity indicates ejecta rising less than 80 m and medium intensity is ejecta rising less than 150 m. Data courtesy of INGV.

Month N Area Activity CS Area Activity
Nov 2018 Low- to medium-intensity explosions at both N1 and N2, lapilli and bombs mixed with ash, explosion rates of 6-16 per hour. Continuous degassing at C; intense spattering on 26 Nov. Low- to medium-intensity incandescent jetting at S1. Low- to medium-intensity explosions at S2 with a mix of coarse and fine ejecta and explosion rates of 3-18 per hour.
Dec 2018 Low- to medium-intensity explosions at both N1 and N2, coarse and fine ejecta, explosion rates of 4-21 per hour. Three days of intense spattering at N2. Continuous degassing at C; intense spattering 1-2 Dec. Low- to medium-intensity incandescent jets at S1, low and medium-intensity explosions of coarse and fine material at S2. Average explosion raters were 10-18 per hour at the beginning of the month, 3-4 per hour during last week.
Jan 2019 Low- to medium-intensity explosions at N1, coarse ejecta. Low- to medium-intensity and spattering at N2, coarse and fine ejecta. Explosion rates of 9-16 per hour. Continuous degassing and low-intensity explosions of coarse ejecta at C. Low-intensity incandescent jets at S1. Low- and medium-intensity explosions of coarse and fine ejecta at S2.
Feb 2019 Medium-intensity explosions with coarse ejecta at N1. Low-intensity explosions with fine ash at N2. Explosion rates of 4-11 per hour. Continuous degassing and low-intensity explosions with coarse and fine ejecta at C and S2. Low intensity incandescent jets at S1. Explosion rates of 2-13 per hour.
Figure (see Caption) Figure 139.Thermal activity at Stromboli increased during November 2018-February 2019 compared with the preceding several months as recorded in the MIROVA project log radiative power data taken from MODIS thermal satellite information. Courtesy of MIROVA.

Activity at the N area was very consistent during November 2018 (figure 140). Explosions of low-intensity (less than 80 m high) to medium-intensity (less than 150 m high) occurred at both the N1 and N2 vents and produced coarse material (lapilli and bombs) mixed with ash, at rates averaging 6-16 explosions per hour. In the SC area continuous degassing was reported from vent C with a brief period of intense spattering on 26 November. At vent S1 low- to medium-intensity incandescent jetting was reported. At vent S2, low- and medium-intensity explosive activity produced a mixture of coarse and fine (ash) material at a frequency of 3-18 events per hour.

Figure (see Caption) Figure 140. The Terrazza Craterica at Stromboli on 12 November 2018 as viewed by the thermal camera placed on the Pizzo sopra la Fossa, showing the two main crater areas and the active vents within each area that are discussed in the text. Heights above the crater terrace, as indicators of intensity of the explosions, are shown divided into three intervals of low (basso), medium (media), and high (alta). Courtesy of INGV (Report 46/2018, Stromboli, Bollettino Settimanale 05/11/2018 - 11/11/2018, data emissione 13/11/2018).

Similar activity continued during December at both crater areas, although there were brief periods of more intense activity. Low- to medium-intensity explosions at both N area vents produced a mixture of coarse and fine-grained material at rates averaging 4-21 per hour. During 6-7 December ejecta from the N vents fell onto the upper part of the Sciara del Fuoco and rolled down the gullies to the coast, producing tongues of debris (figure 141). An explosion at N1 on 12 December produced a change in the structure of the crater area. During 10-16 December the ejecta from the N area landed outside the crater on the Sciara del Fuoco. Intense spattering was observed from N2 on 18, 22, and 31 December. In the CS area, continuous degassing took place at vent C, along with a brief period of intense spattering on 1-2 December. Low to medium intensity incandescent jets persisted at S1 along with low-and medium-intensity explosions of coarse and fine-grained material at vent S2. Rates of explosion at the CS area were higher at the beginning of December (10-18 per hour) and lower during the last week of the month (3-4 per hour).

Figure (see Caption) Figure 141. Images from the Q 400 thermal camera at Stromboli taken on 6 December 2018 showed the accumulation of pyroclastic material in several gullies on the upper part of the Sciara del Fuoco following an explosion at vent N2 at 1520 UTC. The images illustrate the rapid cooling of the pyroclastic material in the subsequent two hours. Courtesy of INGV (Report 50/2018, Stromboli, Bollettino Settimanale, 03/12/2018 - 09/12/2018, data emissione 11/12/2018).

Explosive intensity was low (ejecta less than 80 m high) at vent N1 at the beginning of January 2019 and increased to medium (ejecta less than 150 m high) during the second half of the month, producing coarse ejecta of lapilli and bombs. Intensity at vent N2 was low to medium throughout the month with both coarse- and fine-grained material ejected. Explosions from N2 sent large blocks onto the Sciara del Fuoco several times throughout the month and usually was accompanied by intense spattering. Explosion rates varied, with averages of 9 to 16 per hour, throughout the month in the N area. In the CS area continuous degassing occurred at vent C, and low-intensity explosions of coarse-grained material were reported during the second half of the month. Low-intensity incandescent jets at S1 along with low- and medium-intensity explosions of coarse and fine-grained material at S2 persisted throughout the month.

A helicopter overflight of Stromboli on 8 January 2019 allowed for detailed visual and thermal observations of activity and of the morphology of the vents at the summit (figure 142). Vent C had two small hornitos, and a small scoria cone was present in vent S1, while a larger crater was apparent at S2. In the N crater area vent N2 had a large scoria cone that faced the Sciara del Fuoco to the north; three narrow gullies were visible at the base of the cone (figure 143). Vent S1 was a large crater containing three small vents aligned in a NW-SE trend; INGV scientists concluded the vents formed as a result of the 12 December 2018 explosion. Thermal images showed relatively low temperatures at all fumaroles compared with earlier visits.

Figure (see Caption) Figure 142. Thermal images from Stromboli taken during the overflight of 8 January 2019 showed the morphological structure of the individual vents of the N and CS crater areas. Courtesy of INGV (Report 03/2019, Stromboli, Bollettino Settimanale, 07/01/2019 - 13/01/2019, (data emissione 15/01/2019).
Figure (see Caption) Figure 143. An image taken at Stromboli during the overflight of 8 January 2019 shows the morphological structure of the summit Terrazza Craterica with three gullies at the base of the scoria cone of vent N2. The top thermal image (inset a) shows that the fumaroles in the upper part of the Sciara del Fuoco have low temperatures. Courtesy of INGV (Report 03/2019, Stromboli, Bollettino Settimanale, 07/01/2019 - 13/01/2019, data emissione 15/01/2019).

Activity during February 2019 declined slightly from the previous few months. Explosions at vent N1 were of medium-intensity and produced coarse material (lapilli and bombs). At N2, low-intensity explosions produced fine ash. Average explosion rates in the N area ranged from 4-11 per hour. At the CS area, continuous degassing and low-intensity explosions produced coarse and fine-grained material from vents C and S2 while low-intensity incandescent jets were active at S1. The explosion rates at the CS area averaged 2-13 per hour.

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period from about 13,000 to 5000 years ago was followed by formation of the modern edifice. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5000 years ago as a result of the most recent of a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).


Krakatau (Indonesia) — March 2019 Citation iconCite this Report

Krakatau

Indonesia

6.102°S, 105.423°E; summit elev. 813 m

All times are local (unless otherwise noted)


Ash plumes, ballistic ejecta, and lava extrusion during October-December; partial collapse and tsunami in late December; Surtseyan activity in December-January 2019

Krakatau volcano, between Java in Sumatra in the Sunda Straight of Indonesia, is known for its catastrophic collapse in 1883 that produce far-reaching pyroclastic flows, ashfall, and tsunami. The pre-1883 edifice had grown within an even older collapse caldera that formed around 535 CE, resulting in a 7-km-wide caldera and the three surrounding islands of Verlaten, Lang, and Rakata (figure 55). Eruptions that began in late December 1927 (figures 56 and 57) built the Anak Krakatau cone above sea level (Sudradjat, 1982; Simkin and Fiske, 1983). Frequent smaller eruptions since that time, over 40 short episodes consisting of ash plumes, incandescent blocks and bombs, and lava flows, constructed an island reaching 338 m elevation.

Figure (see Caption) Figure 55. The three islands of Verlaten, Lang, and Rakata formed during a collapse event around 535 CE. Another collapse event occurred in 1883, producing widespread ashfall, pyroclastic flows, and triggering a tsunami. Through many smaller eruptions since then, Anak Krakatau has since grown in the center of the caldera. Sentinel-2 natural color (bands 4, 3, 2) satellite image acquired on 16 November 2018, courtesy of Sentinel Hub Playground.
Figure (see Caption) Figure 56. Photo sequence (made from a film) at 6-second intervals from the early phase of activity on 24 January 1928 that built the active Anak Krakatau cone above the ocean surface. Plume height reached about 1 km. View is from about 4.5 km away at a beach on Verlaten Island looking SE towards Rakata Island in the right background. Photos by Charles E. Stehn (Netherlands Indies Volcanological Survey) from the E.G. Zies Collection, Smithsonian Institution.
Figure (see Caption) Figure 57. Submarine explosions in January 1928 built the active Anak Krakatau cone above the ocean surface. View is from about 600 m away looking E towards Lang Island in the background. Photos by Charles E. Stehn (Netherlands Indies Volcanological Survey) from the E.G. Zies Collection, Smithsonian Institution.

Historically there has been a lot of confusion about the name and preferred spelling of this volcano. Some have incorrectly made a distinction between the pre-1883 edifice being called "Krakatoa" and then using "Krakatau" for the current volcano. Anak Krakatau is the name of the active cone, but the overall volcano name is simply Krakatau. Simkin and Fiske (1983) explained as follows: "Krakatau was the accepted spelling for the volcano in 1883 and remains the accepted spelling in modern Indonesia. In the original manuscript copy submitted to the printers of the 1888 Royal Society Report, now in the archives of the Royal Society, this spelling has been systematically changed by a neat red line through the final 'au' and the replacement 'oa' entered above; a late policy change that, from some of the archived correspondence, saddened several contributors to the volume."

After 15 months of quiescence Krakatau began a new eruption phase on 21 June 2018, characterized by ash plumes, ballistic ejecta, Strombolian activity, and lava flows. Ash plumes reached 4.9 km and a lava flow traveled down the SE flank and entered the ocean. This report summarizes the activity from October 2018 to January 2019 based on reports by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Indonesian Center for Volcanology and Geological Hazard Mitigation (CVGHM), MAGMA Indonesia, the National Board for Disaster Management - Badan Nasional Penanggulangan Bencana (BNPB), the Darwin Volcanic Ash Advisory Center (VAAC), satellite data, and eye witness accounts.

Activity during October-21 December 2018. The eruption continued to eject incandescent ballistic ejecta, ash plumes, and lava flows in October through December 2018. On 22 December a partial collapse of Anak Krakatau began, dramatically changing the morphology of the island and triggering a deadly tsunami that impacted coastlines around the Sunda Straight. Following the collapse the vent was located below sea level and Surtseyan activity produced steam plumes, ash plumes, and volcanic lightning.

Sentinel-2 satellite images acquired through October show incandescence in the crater, lava flows on the SW flank, and incandescent material to the S to SE of the crater (figure 58). This correlates with eyewitness accounts of explosions ejecting incandescent ballistic ejecta, and Volcano Observatory Notice for Aviation (VONA) ash plume reports. The Darwin VAAC reported ash plumes to 1.5-2.4 km altitude that drifted in multiple directions during 17-19 October, but throughout most of October visual observations were limited due to fog. A video shared by Sutopo on 24 October shows ash emission and lava fountaining producing a lava flow that entered the ocean, resulting in a white plume. Video by Richard Roscoe of Photovolcanica shows explosions ejecting incandescent blocks onto the flanks and ash plumes accompanied by volcanic lightning on 25 October.

Figure (see Caption) Figure 58. Sentinel-2 thermal satellite images showing lava flows, incandescent avalanche deposits, and incandescence in the crater of Anak Krakatau during October 2018. Courtesy of Sentinel-2 hub playground.

Throughout November frequent ash plumes rose to 0.3-1.3 km altitude, with explosion durations spanning 29-212 seconds (figure 59). Observations by Øystein Lund Andersen describe explosions ejecting incandescent material with ash plumes and some associated lightning on 17 November (figure 60).

Figure (see Caption) Figure 59. Sentinel-2 satellite images showing ash plumes at Krakatau during 6-16 November 2018. Natural color (Bands 4, 3, 2) Sentinel-2 images courtesy of Sentinel Hub Playground.
Figure (see Caption) Figure 60. Krakatau erupting an ash plume and incandescent material on 17 November 2018. Courtesy of Øystein Lund Andersen.

During 1-21 December intermittent explosions lasting 46-776 seconds produced ash plumes that rose up to 1 km altitude. Thermal signatures were sporadically detected by various satellite thermal infrared sensors during this time. On 22 December ash plumes reached 0.3-1.5 km through the day and continuous tremor was recorded.

Activity and events during 22-28 December 2018. The following events during the evening of the 22nd were recorded by Øystein Lund Andersen, who was photographing the eruption from the Anyer-Carita area in Java, approximately 47 km from Anak Krakatau. Starting at 1429 local time, incandescence and ash plumes were observed and the eruption could be heard as intermittent 'cannon-fire' sounds, sometimes shaking walls and windows. An increase in intensity was noted at around 1700, when the ash column increased in height and was accompanied by volcanic lightning, and eruption sounds became more frequent (figure 61). A white steam plume began to rise from the shore of the southern flank. After sunset incandescent ballistic blocks were observed impacting the flanks, with activity intensity peaking around 1830 with louder eruption sounds and a higher steam plume from the ocean (figure 62).

Figure (see Caption) Figure 61. Ash plumes at Krakatau from 1429 to 1739 on 22 December 2018. Courtesy of Øystein Lund Andersen.
Figure (see Caption) Figure 62. Krakatau ejecting incandescent blocks and ash during 1823-1859 on 22 December 2018. The top and middle images show the steam plume at the shore of the southern flank. Courtesy of Øystein Lund Andersen.

PVMBG recorded an eruption at 2103. When viewed at 2105 by Øystein Lund Andersen, a dark plume across the area blocked observations of Anak Krakatau and any incandescence (figure 63). At 2127-2128 the first tsunami wave hit the shore and traveled approximately 15 m inland (matching the BNPB determined time of 2127). At approximately 2131 the sound of the ocean ceased and was soon replaced by a rumbling sound and the second, larger tsunami wave impacted the area and traveled further inland, where it reached significant depths and caused extensive damage (figures 64 and 65). After the tsunami, eruption activity remained high and the eruption was heard again during intervals from 0300 through to early afternoon.

Figure (see Caption) Figure 63. Krakatau is no longer visible at 2116 on 22 December 2018, minutes before the first tsunami wave arrived at west Java. A dark ash plume takes up much of the view. Courtesy of Øystein Lund Andersen.
Figure (see Caption) Figure 64. The second tsunami wave arriving at Anyer-Carita area of Java after the Krakatau collapse. This photo was taken at 2133 on 22 December 2018, courtesy of Øystein Lund Andersen.
Figure (see Caption) Figure 65. Photographs showing damage caused in the Anyer-Carita area of Java by the tsunami that was triggered by the partial collapse of Krakatau. From top to bottom, these images were taken approximately 40 m, 20 m, and 20 m from the shore on 23 December 2018. Courtesy of Øystein Lund Andersen.

Observations on 23 December reveal steam-rich ash plumes and base surge traveling along the water, indicative of the shallow-water Surtseyan eruption (figure 66). Ashfall was reported on the 26th in several regions including Cilegon, Anyer, and Serang. The first radar observations of Krakatau were on 24 December and showed a significant removal of material from the island (figure 67). At 0600 on the 27th the volcanic alert level was increased from II to III (on a scale of I-IV) and a VONA with Aviation Color Code Red reported an ash plume to approximately 7 km altitude that dispersed to the NE. When Anak Krakatau was visible, Surtseyan activity and plumes were observed through the end of December. On 28 December, plumes reached 200-3000 m. At 0418 the eruption paused and the first observation of the post-collapse edifice was made. The estimated removed volume (above sea level) was 150-180 million m3, leaving a remaining volume of 40-70 million m3. The summit of the pre-collapse cone was 338 m, while the highest point post-collapse was reduced to 110 m. Hundreds of thousands of lightning strokes were detected during 22-28 December with varying intensity (figure 68).

Figure (see Caption) Figure 66. Steam-rich plumes and underlying dark ash plumes from Surtseyan activity at Krakatau on 23 December 2018. Photos by Instagram user @didikh017 at Grand Cava Susi Air, via Sutopo.
Figure (see Caption) Figure 67. ALOS-2 satellite radar images showing Krakatau on 20 August 2018 and 24 December 2018. The later image shows that a large part of the cone of Anak Krakatau had collapsed. Courtesy of Geospatial Information Authority of Japan (GSI) via Sutopo.
Figure (see Caption) Figure 68. Lightning strokes during the eruption of Krakatau within a 20 km radius of the volcano for 30 minute intervals on 23, 25, 26, and 28 December 2018. Courtesy of Chris Vagasky.

Damage resulting from the 22 December tsunami. On the 29 December the damage reported by BNPB was 1,527 heavily damaged housing units, 70 with moderate damage, 181 with light damage, 78 damaged lodging and warung units, 434 damaged boats and ships and some damage to public facilities. Damage was recorded in the five regencies of Pandenglang, Serang, South Lampung, Pesawaran and Tanggamus. A BNPB report on 14 January gave the following figures: 437 fatalities, 10 people missing, 31,943 people injured, and 16,198 people evacuated (figure 69). The eruption and tsunami resulted in damage to the surrounding islands, with scouring on the Anak-Krakatau-facing slope of Rakata and damage to vegetation on Kecil island (figure 70 and 71).

Figure (see Caption) Figure 69. The impacts of the tsunami that was triggered by a partial collapse of Anak Krakatau from an update given on 14 January 2019. Translations are as follows. Korban Meninggal: victims; Korban hilang: missing; Korban luka-luka: injured; Mengungsi: evacuated. The color scale from green to red along the coastline indicates the breakdown of the human impacts by area. Courtesy of BNPB.
Figure (see Caption) Figure 70. Damage on Rakata Island from the Krakatau tsunami. This part of the island is facing Anak Krakatau and the scoured area was estimated to be 25 m high. Photographs taken on 10 January 2019 by James Reynolds.
Figure (see Caption) Figure 71. Damage to vegetation on Kecil island to the East of Krakatau, from the Krakatau December 2018 eruption. Photographs taken on 10 January 2019 by James Reynolds.

Activity during January 2019. Surtseyan activity continued into January 2019. Øystein Lund Andersen observed the eruption on 4-5 January. Activity on 4 January was near-continuous. The photographs show black cock's-tail jets that rose a few hundred meters before collapsing (figure 72), accompanied by white lateral base surge that spread from the vent across the ocean (figure 73), and white steam plumes that were visible from Anyer-Carita, West Java. In the evening the ash-and-steam plume was much higher (figure 74). It was also noted that older pumice had washed ashore at this location and a coating of sulfur was present along the beach and some of the water surface. Activity decreased again on the 5th (figure 75) with a VONA reporting an ash plume to 1.5 km towards the WSW. SO2 plumes were dispersed to the NE, E, and S during this time (figure 76).

Figure (see Caption) Figure 72. Black ash plumes and white steam plumes from the Surtseyan eruption at Krakatau on 4 January 2019. Courtesy of Øystein Lund Andersen.
Figure (see Caption) Figure 73. An expanding base surge at Krakatau on 4 January 2019 at 0911. Courtesy of Øystein Lund Andersen.
Figure (see Caption) Figure 74. Ash-and-steam plumes at Krakatau at 1702-2250 on 4 January 2018. Lightning is illuminating the plume in the bottom image. Courtesy of Øystein Lund Andersen.
Figure (see Caption) Figure 75. Ash plumes at Krakatau on 5 January 2019 at 0935. Courtesy of Øystein Lund Andersen.
Figure (see Caption) Figure 76. Sulfur dioxide (SO2) emissions produced by Krakatau and drifting to the NE, E, and SE on 3-6 January 2018. Dates and times of the periods represented are listed at the top of each image. Courtesy of the NASA Space Goddard Flight Center.

During 5-9 January intermittent explosions lasting 20 seconds to 13 minutes produced ash plumes rising up to 1.2 km and dispersing E. From 11 to 19 January white plumes were observed up to 500 m. Observations were prevented due to fog during 20-31 January. MIROVA thermal data show elevated thermal anomalies from July through January, with a decrease in energy in November through January (figure 77). The radiative power detected in December-January was the lowest since June 2018.

Figure (see Caption) Figure 77. Log radiative power MIROVA plot of MODIS thermal infrared data for June 2018-January 2019. The peaks in energy correlate with observed lava flows. Courtesy of MIROVA.

Morphological changes to Anak Krakatau. Images taken before and after the collapse event show changes in the shoreline, destruction of vegetation, and removal of the cone (figure 78). A TerraSAR-X image acquired on 29 January shows that in the location where the cone and active vent was, a bay had formed, opening to the W (figure 79). These changes are also visible in Sentinel-2 satellite images, with the open bay visible through light cloud cover on 29 December (figure 80).

By 9 January a rim had formed, closing off the bay to the ocean and forming a circular crater lake. Photos by James Reynolds on 11 January show a new crater rim to the W of the vent, which was filled with water (figure 81). Steam and/or gas emissions were emanating from the surface in that area. The southern lava delta surface was covered with tephra, and part of the lava delta had been removed, leaving a smooth coastline. By the time these images were taken there was already extensive erosion of the fresh deposits around the island. Fresh material extended the coast in places and filled in bays to produce a more even shoreline.

Figure (see Caption) Figure 78. Krakatau on 5 August 2018 (top) and on 11 January 2019 showing the edifice after the collapse event. The two drone photographs show approximately the same area. Courtesy of Øystein Lund Andersen (top) and James Reynolds (bottom).
Figure (see Caption) Figure 79. TerraSAR-X radar images showing the morphological changes to Krakatau with the changes outlined in the bottom right image as follows. Red: 30 August 2018 (upper left image); blue: 29 December 2018 (upper right image); yellow: 9 January 2019 (lower left image). Part of the southern lava delta was removed and material was added to the SE and NE to N shoreline. In the 29 December image the cone has collapsed and in its place is an open bay, which had been closed by a new rim by the 9 January. Courtesy of BNPB, JAXA Japan Aerospace Exploration Agency, and Badan Informasi Geospasial (BIG).
Figure (see Caption) Figure 80. Sentinel-2 satellite images showing the changing morphology of Krakatau. The SW section is where the cone previously sat and collapsed in December 2018. In the upper right image the cone and southern lava delta are gone and there are changes to the coastline of the entire island. Natural color (bands 4, 3, 2) Sentinel-2 satellite images courtesy of Sentinel Hub Playground.
Figure (see Caption) Figure 81. Drone footage of the Krakatau crater and new crater rim taken on 11 January 2019. The island is coated in fresh tephra from the eruption and the orange is discolored water due to the eruption. The land between the crater lake and the ocean built up since the collapse and the hot deposits are still producing steam/gas. Courtesy of James Reynolds.
Figure (see Caption) Figure 82. An aerial view of Krakatau with the new crater on 13 January 2019. Courtesy of BNPB.

References. Simkin, T., and Fiske, R.S., 1983, Krakatau 1883: the volcanic eruption and its effects: Smithsonian Institution Press, Washington DC, 464 p. ISBN 0-87474-841-0.

Sudradjat (Sumartadipura), A., 1982. The morphological development of Anak Krakatau Volcano, Sunda Straight. Geologi Indonesia, 9(1):1-11.

Geologic Background. The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral Krakatau edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of this ancestral volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan, and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption, the 2nd largest in Indonesia during historical time, caused more than 36,000 fatalities, most as a result of devastating tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN ); Geospatial Information Authority of Japan (GSI), 1 Kitasato, Tsukuba, Ibaraki 305-0811, Japan. (URL: http://www.gsi.go.jp/ENGLISH/index.html); Badan Informasi Geospasial (BIG), Jl. Raya Jakarta - Bogor KM. 46 Cibinong 16911, Indonesia. (URL: http://www.big.go.id/atlas-administrasi/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); JAXA | Japan Aerospace Exploration Agency, 7-44-1 Jindaiji Higashi-machi, Chofu-shi, Tokyo 182-8522 (URL: https://global.jaxa.jp/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Øystein Lund Andersen? (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: https://www.oysteinlundandersen.com/krakatau-volcano-witnessing-the-eruption-tsunami-22december2018/); James Reynolds, Earth Uncut TV (Twitter: @EarthUncutTV, URL: https://www.earthuncut.tv/, YouTube: https://www.youtube.com/channel/UCLKYsEXfI0PGXeKYL1KV7qA); Chris Vagasky, Vaisala Inc., Louisville, Colorado (URL: https://www.vaisala.com/en?type=1, Twitter: @COweatherman, URL: https://twitter.com/COweatherman).


Masaya (Nicaragua) — March 2019 Citation iconCite this Report

Masaya

Nicaragua

11.984°N, 86.161°W; summit elev. 635 m

All times are local (unless otherwise noted)


Lava lake persists with decreased thermal output, November 2018-February 2019

Nicaragua's Volcan Masaya has an intermittent lava lake that has attracted visitors since the time of the Spanish Conquistadores; tephrochronology has dated eruptions back several thousand years. The unusual basaltic caldera has had historical explosive eruptions in addition to lava flows and an actively circulating lava lake. An explosion in 2012 ejected ash to several hundred meters above the volcano, bombs as large as 60 cm fell around the crater, and ash fell to a thickness of 2 mm in some areas of the park. The reemergence of the lava lake inside Santiago crater was reported in December 2015. By late March 2016 the lava lake had grown and intensified enough to generate a significant thermal anomaly signature which has varied in strength but continued at a moderate level into early 2019. Information for this report, which covers the period from November 2018 through February 2019, is provided by the Instituto Nicareguense de Estudios Territoriales (INETER) and satellite -based imagery and thermal data.

The lava lake in Santiago Crater remained visible and active throughout November 2018 to February 2019 with little change from the previous few months (figure 70). Seismic amplitude RSAM values remained steady, oscillating between 10 and 40 RSAM units during the period.

Figure (see Caption) Figure 70. A small area of the lava lake inside Santiago Crater at Masaya was visible from the rim on 25 November 2018 (left) and 17 January 2019 (right). Left image courtesy of INETER webcam; right image courtesy of Alun Ebenezer.

Every few months INETER carries out SO2 measurements by making a transect using a mobile DOAS spectrometer that samples for gases downwind of the volcano. Transects were done on 9-10 October 2018, 21-24 January 2019, and 18-21 February 2019 (figure 71). Average values during the October transect were 1,454 tons per day, in January they were 1,007 tons per day, and in February they averaged 1,318 tons per day, all within a typical range of values for the last several months.

Figure (see Caption) Figure 71. INETER carries out periodic transects to measure SO2 from Masaya with a mobile DOAS spectrometer. Transects taken along the Ticuantepe-La Concepcion highway on 9-10 October 2018 (left) and 21-24 January 2019 (right) showed modest levels of SO2 emissions downwind of the summit. Courtesy of INETER (Boletín Sismos y Volcanes de Nicaragua. Octubre 2018 and Enero 2019).

During a visit by INETER technicians in early November 2018, the lens of the Mirador 1 webcam, that had water inside it and had been damaged by gases, was cleaned and repaired. During 21-24 January 2019 INETER made a site visit with scientists from the University of Johannes Gutenberg in Mainz, Germany, to measure halogen species in gas plumes, and to test different sampling techniques for volcanic gases, including through spectroscopic observations with DOAS equipment, in-situ gas sampling (MultiGAS, denuders, alkaline traps), and using a Quadcopter UAV (drone) sampling system.

Periodic measurements of CO2 from the El Comalito crater have been taken by INETER for many years. The most recent observations on 19 February 2019 indicated an emission rate of 46 +/- 3 tons per day of CO2, only slightly higher than the average value over 16 measurements between 2008 and 2019 (figure 72).

Figure (see Caption) Figure 72. CO2 measurements taken at Masaya on 19 February 2019 were very close to the average value measured during 2008-2019. Courtesy of INETER (Boletín Sismos y Volcanes de Nicaragua, Febrero 2019).

Satellite imagery (figure 73) and in-situ thermal measurements during November 2018-February 2019 indicated constant activity at the lava lake and no significant changes during the period. On 14 January 2019 temperatures were measured with the FLIR SC620 thermal camera, along with visual observations of the crater; abundant gas was noted, and no explosions from the lake were heard. The temperature at the lava lake was measured at 107°C, much cooler than the 340°C measured in September 2018 (figure 74).

Figure (see Caption) Figure 73. Sentinel-2 satellite imagery (geology, bands 12, 4, and 2) clearly indicated the presence of the active lava lake inside Santiago crater at Masaya during November 2018-February 2019. North is to the top, and the Santigo crater is just under 1 km in diameter for scale. Courtesy of Sentinel Hub Playground.
Figure (see Caption) Figure 74. Thermal measurements were made at Masaya on 14 January 2019 with a FLIR SC620 thermal camera that indicated temperatures over 200°C cooler than similar measurements made in September 2018.

Thermal anomaly data from satellite instruments also confirmed moderate levels of ongoing thermal activity. The MIROVA project plot indicated activity throughout the period (figure 75), and a plot of the number of MODVOLC thermal alerts by month since the lava lake first appeared in December 2015 suggests constant activity at a reduced thermal output level from the higher values in early 2017 (figure 76).

Figure (see Caption) Figure 75. Thermal anomalies remained constant at Masaya during November 2018-February 2019 as recorded by the MIROVA project. Courtesy of MIROVA.
Figure (see Caption) Figure 76. The number of MODVOLC thermal alerts each month at Masaya since the lava lake first reappeared in late 2015 reached its peak in early 2017 and declined to low but persistent levels by early 2018 where they have remained for a year. Data courtesy of MODVOLC.

Geologic Background. 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.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Alun Ebenezer (Twitter: @AlunEbenezer, URL: https://twitter.com/AlunEbenezer).


Santa Maria (Guatemala) — March 2019 Citation iconCite this Report

Santa Maria

Guatemala

14.757°N, 91.552°W; summit elev. 3745 m

All times are local (unless otherwise noted)


Daily explosions cause steam-and-ash plumes and block avalanches, November 2018-February 2019

The dacitic Santiaguito lava-dome complex on the W flank of Guatemala's Santa María volcano has been growing and actively erupting since 1922. The youngest of the four vents in the complex, Caliente, has been erupting with ash explosions, pyroclastic, and lava flows for more than 40 years. A lava dome that appeared within the summit crater of Caliente in October 2016 has continued to grow, producing frequent block avalanches down the flanks. Daily explosions of steam and ash also continued during November 2018-February 2019, the period covered in this report, with information primarily from Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia) and the Washington VAAC (Volcanic Ash Advisory Center).

Activity at Santa Maria continued with little variation from previous months during November 2018-February 2019. Plumes of steam with minor magmatic gases rose continuously from the Caliente crater 100-500 m above the summit, generally drifting SW or SE before dissipating. In addition, daily explosions with varying amounts of ash rose to altitudes of around 2.8-3.5 km and usually extended 20-30 km before dissipating. Most of the plumes drifted SW or SE; minor ashfall occurred in the adjacent hills almost daily and was reported at the fincas located within 15 km in those directions several times each month. Continued growth of the Caliente lava dome resulted in daily block avalanches descending its flanks. The MIROVA plot of thermal energy during this time shows a consistent level of heat flow with minor variations throughout the period (figure 89).

Figure (see Caption) Figure 89. Persistent thermal activity was recorded at Santa Maria from 6 June 2018 through February 2019 as seen in the MIROVA plot of thermal energy derived from satellite thermal data. Daily explosions produced ash plumes and block avalanches that were responsible for the continued heat flow at the volcano. Courtesy of MIROVA.

During November 2018 steam plumes rose to altitudes of 2.8-3.2 km from Caliente summit, usually drifting SW, sometimes SE. Several ash-bearing explosions were reported daily, rising to 3-3.2 km altitude and also drifting SW or SE. The highest plume reported by INSIVUMEH rose to 3.4 km on 25 November and drifted SW. The Washington VAAC reported an ash emission on 9 November that rose to 4.3 km altitude and drifted W; it dissipated within a few hours about 35 km from the summit. On 11 November another plume rose to 4.9 km altitude and drifted NW. INSIVUMEH issued a special report on 2 November noting an increase in block avalanches on the S and SE flanks, many of which traveled from the crater dome to the base of the volcano. Nearly constant avalanche blocks descended the SE flank of the dome and occasionally traveled down the other flanks as well throughout the month. They reached the bottom of the cone again on 29 November. Ashfall was reported around the flanks more than once every week and at Finca Florida on 12 November. Finca San Jose reported ashfall on 11, 13, and 23 November, and Parcelamiento Monte Claro reported ashfall on 15, 24, 25, and 27 November.

Constant degassing from the Caliente dome during December 2018 formed white plumes of mostly steam that rose to 2.6-3.0 km altitude during the month. Weak explosions averaging 9-13 per day produced gray ash plumes that rose to 2.8-3.4 km altitude. The Washington VAAC reported an ash emission on 4 December that extended 25 km SW of the summit at 3.0 km altitude and dissipated quickly. Small ash plumes were visible in satellite imagery a few kilometers WNW on 8, 12, 30, and 31 December at 4.3 km altitude; they each dissipated within a few hours. Ashfall was reported in Finca Monte Claro on 1 and 4 December, and in San Marcos Palajunoj on 26 and 30 December along with Loma Linda. On 28 December ashfall on the E flank affected the communities of Las Marías, Calahuache, and El Nuevo Palmar. Block avalanches occurred daily, sending large blocks to the base of the volcano that often stirred up small plumes of ash in the vicinity (figure 90).

Figure (see Caption) Figure 90. Activity during December 2018 at Santa Maria included constant degassing of steam plumes, weak explosions with ash plumes, and block avalanches rolling down the flanks to the base of the cone. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Diciembre 2018).

Multiple explosions daily during January 2019 produced steam-and-ash plumes (figure 91). Constant degassing rising 10-500 m emerged from the SSE part of the Caliente dome, and ashfall, mainly on the W and SW rim of the cone, was a daily feature. Seismic station STG-3 detected 10-18 explosions per day that produced ash plumes, which rose to between 2.7 and 3.5 km altitude. The Washington VAAC noted a faint ash emission in satellite imagery on 1 January that was about 25 km W of the summit at 4.3 km altitude. A new emission appeared at the same altitude on 4 January about 15 km NW of the summit. A low-density emission around midday on 5 January produced an ash plume that drifted NNE at 4.6 km altitude. Ash plumes drifted W at 4.3 km altitude on 11 and 14 January for short periods of time before dissipating.

Figure (see Caption) Figure 91. Explosions during January produced numerous steam-and-ash plumes at the Santiaguito complex of Santa Maria. A moderate explosion on 31 January 2019 produced an ash plume that rose to about 3.1 km altitude (top). A thermal image and seismograph show another moderate explosion on 18 January 2019 that also rose nearly vertically from the summit of Caliente. Courtesy of INSIVUMEH (Informe mensual de actividad Volcanica enero 2019, Volcan Santiaguito).

Ash drifted mainly towards the W, SW, and S, causing ashfall in the villages of San Marcos Palajunoj, Loma Linda, Monte Bello, El Patrocinio, La Florida, El Faro, Patzulín and a few others several times during the month. The main places where daily ashfall was reported were near the complex, in the hilly crop areas of the El Faro and San José Patzulín farms (figure 92). Blocks up to 3 m in diameter reached the base of the complex, stirring up ash plumes that settled on the immediate flanks. Juvenile material continued to appear at the summit of the dome during January; the dome had risen above the edge of the crater created by the explosions of 2016. Changes in the size and shape of the dome between 23 November 2018 and 13 January 2019 showed the addition of material on the E and SE side of the dome, as well as a new effusive flow that travelled 200-300 m down the E flank (figure 93).

Figure (see Caption) Figure 92. Near-daily ashfall affected the coffee plants at the El Faro and San José Patzulín farms (left) at Santiaguito during January 2019. Large avalanche blocks descending the flanks, seen here on 23 January 2018, often stirred up smaller ash plumes that settled out next to the cone. Courtesy of INSIVUMEH (Informe mensual de actividad Volcanica enero 2019, Volcan Santiaguito).
Figure (see Caption) Figure 93. A comparison of the growth at the Caliente dome of the Santiaguito complex at Santa Maria between 23 November 2018 (top) and 13 January 2019 (bottom) shows the emergence of juvenile material and a 200-300 m long effusive flow that has moved slowly down the E flank. Courtesy of INSIVUMEH (Informe mensual de actividad Volcanica enero 2019, Volcan Santiaguito).

Persistent steam rising 50-150 m above the crater was typical during February 2019 and accompanied weak and moderate explosions that averaged 12 per day throughout the month. White and gray ash plumes from the explosions rose to 2.8-3.3 km altitude; daily block avalanches usually reached the base of the dome (figure 94). Ashfall occurred around the complex, mainly on the W, SW, and NE flanks on a daily basis, but communities farther away were affected as well. The Washington VAAC reported an ash plume on 7 February in visible satellite imagery moving SW from the summit at 4.9 km altitude. The next day a new ash plume was located about 20 km W of the summit, dissipating rapidly, at 4.3 km altitude. Ashfall drifting SW affected Palajuno Monte Claro on 5, 9, 15, and 16 February. Ash drifting E and SE affected Calaguache, Las Marías and surrounding farms on 14 and 17 February, and fine-grained ash drifting SE was reported at finca San José on 21 February.

Figure (see Caption) Figure 94. Activity at the Caliente dome of the Santiaguito complex at Santa Maria included daily ash-and-steam explosions and block avalanches descending the sides of the dome in February 2019. A typical explosion on 2 February 2019 produced an ash plume that rose to about 3 km altitude and drifted SW (left). A block avalanche on 14 February descended the SE flank and stirred up small plumes of ash in the vicinity (right, top); the avalanche lasted for 88 seconds and registered with seismic frequencies between 3.46 and 7.64 Hz (right bottom). Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 01 al 08 de febrero de 2019).

Geologic Background. Symmetrical, forest-covered Santa María volcano is one of the most prominent of a chain of large stratovolcanoes that rises dramatically above the Pacific coastal plain of Guatemala. The stratovolcano has a sharp-topped, conical profile that is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four westward-younging vents, the most recent of which is Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).


Reventador (Ecuador) — March 2019 Citation iconCite this Report

Reventador

Ecuador

0.077°S, 77.656°W; summit elev. 3562 m

All times are local (unless otherwise noted)


Multiple daily explosions with ash plumes and incandescent blocks rolling down the flanks, October 2018-January 2019

The andesitic Volcán El Reventador lies well east of the main volcanic axis of the Cordillera Real in Ecuador and has historical eruptions with numerous lava flows and explosive events going back to the 16th century. The eruption in November 2002 generated a 17-km-high eruption cloud, pyroclastic flows that traveled 8 km, and several lava flows. Eruptive activity has been continuous since 2008. Daily explosions with ash emissions and ejecta of incandescent blocks rolling hundreds of meters down the flanks have been typical for many years. Activity continued during October 2018-January 2019, the period covered in this report, with information provided by Ecuador's Instituto Geofisico (IG-EPN), the Washington Volcano Ash Advisory Center (VAAC), and infrared satellite data.

Multiple daily reports were issued from the Washington VAAC throughout the entire October 2018-January 2019 period. Plumes of ash and gas usually rose to altitudes of 4.3-6.1 km and drifted about 20 km in prevailing wind directions before either dissipating or being obscured by meteoric clouds. The average number of daily explosions reported by IG-EPN for the second half of 2018 was more than 20 per day (figure 104). The many explosions during the period originated from multiple vents within a large scarp that formed on the W flank in mid-April (BGVN 43:11, figure 95) (figure 105). Incandescent blocks were observed often in the IG webcams; they traveled 400-1,000 m down the flanks.

Figure (see Caption) Figure 104. The number of daily seismic events at El Reventador for 2018 indicated high activity during the first and last thirds of the year; more than 20 explosions per day were recorded many times during October-December 2018, the period covered in this report. LP seismic events are shown in orange, seismic tremor in pink, and seismic explosions with ash are shown in green. Courtesy of IG-EPN (Informe Anual del Volcán El Reventador – 2018, Quito, 29 de marzo del 2019).
Figure (see Caption) Figure 105. Images from IG's REBECA thermal camera showed the thermal activity from multiple different vents at different times during the year (see BGVN 43:11, figure 95 for vent locations). Courtesy if IG (Informe Anual del Volcán El Reventador – 2018, Quito, 29 de marzo del 2019).

Activity during October 2018-January 2019. During most days of October 2018 plumes of gas, steam, and ash rose over 1,000 m above the summit of Reventador, and most commonly drifted W or NW. Incandescence was observed on all nights that were not cloudy; incandescent blocks rolled 400-800 m down the flanks during half of the nights. During episodes of increased activity, ash plumes rose over 1,200 m (8, 10-11, 18-19 October) and incandescent blocks rolled down multiple flanks (figure 106).

Figure (see Caption) Figure 106. Ash emissions rose over 1,000 m above the summit of Reventador numerous times during October 2018, and large incandescent blocks traveled hundreds of meters down multiple flanks. The IG-EPN COPETE webcam that captured these images is located on the S caldera rim. Courtesy of IG Daily Reports (Informe diario del estado del Volcan Reventador, numbers 2018-282, 292, 295, 297).

Similar activity continued during November. IG reported 17 days of the month with steam, gas, and ash emissions rising more than 1,000 m above the summit. The other days were either cloudy or had emissions rising between 500 and 1,000 m. Incandescent blocks were usually observed on the S or SE flanks, generally travelling 400-600 m down the flanks. The Washington VAAC reported a discrete ash plume at 6.1 km altitude drifting WNW about 35 km from the summit on 15 November. The next day, intermittent puffs were noted moving W, and a bright hotspot at the summit was visible in satellite imagery. During the most intense activity of the month, incandescent blocks traveled 800 m down all the flanks (17-19 November) and ash plumes rose over 1,200 m (23 November) (figure 107).

Figure (see Caption) Figure 107. Ash plumes rose over 1,000 m above the summit on 17 days during November 2018 at Reventador, and incandescent blocks traveled 400-800 m down the flanks on many nights. Courtesy of IG Daily Reports (Informe diario del estado del Volcan Reventador, numbers 2018-306, 314, 318, 324).

Steam, gas, and ash plumes rose over 1,200 m above the summit on 1 December. The next day, there were reports of ashfall in San Rafael and Hosteria El Hotelito, where they reported an ash layer about 1 mm thick was deposited on vehicles during the night. Ash emissions exceeded 1,200 m above the summit on 5 and 6 December as well. Incandescent blocks traveled 800 m down all the flanks on 11, 22, 24, and 26 December, and reached 900 m on 21 December. Ash emissions rising 500 to over 1,000 m above the summit were a daily occurrence, and incandescent blocks descended 500 m or more down the flanks most days during the second half of the month (figure 108).

Figure (see Caption) Figure 108. Ash plumes that rose 500 to over 1,000 m were a daily occurrence at Reventador during December 2018. Incandescent blocks traveled as far as 900 m down the flanks as well. Courtesy of IG Daily Reports (Informe diario del estado del Volcan Reventador, numbers 2018-340, 351, 353, 354, 358, 359).

During the first few days of January 2019 the ash and steam plumes did not rise over 800 m, and incandescent blocks were noted 300-500 m down the S flank. An increase in activity on 6 January sent ash-and-gas plumes over 1,000 m, drifting W, and incandescent blocks 1,000 m down many flanks. For multiple days in the middle of the month the volcano was completely obscured by clouds; only occasional observations of plumes of ash and steam were made, incandescence seen at night through the clouds confirmed ongoing activity. The Washington VAAC reported continuous ash emissions moving SE extending more than 100 km on 12 January. A significant explosion late on 20 January sent incandescent blocks 800 m down the S flank; although it was mostly cloudy for much of the second half of January, brief glimpses of ash plumes rising over 1,000 m and incandescent blocks traveling up to 800 m down numerous flanks were made almost daily (figure 109).

Figure (see Caption) Figure 109. Even during the numerous cloudy days of January 2019, evidence of ash emissions and significant explosions at Reventador was captured in the Copete webcam located on the S rim of the caldera. Courtesy of IG Daily Reports (Informe diario del estado del Volcan Reventador, number 2019-6, 21, 26, 27).

Visual evidence from the webcams supports significant thermal activity at Reventador. Atmospheric conditions are often cloudy and thus the thermal signature recorded by satellite instruments is frequently diminished. In spite of this, the MODVOLC thermal alert system recorded seven thermal alerts on three days in October, four alerts on two days in November, six alerts on two days in December and three alerts on three days in January 2019. In addition, the MIROVA system measured moderate levels of radiative power intermittently throughout the period; the most intense anomalies of 2018 were recorded on 15 October and 6 December (figure 110).

Figure (see Caption) Figure 110. Persistent thermal activity at Reventador was recorded by satellite instruments for the MIROVA system from 5 April 2018 through January 2019 in spite of frequent cloud cover over the volcano. The most intense anomalies of 2018 were recorded on 15 October and 6 December. Courtesy of MIROVA.

Geologic Background. Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic Volcán El Reventador stratovolcano rises to 3562 m above the jungles of the western Amazon basin. A 4-km-wide caldera widely breached to the east was formed by edifice collapse and is partially filled by a young, unvegetated stratovolcano that rises about 1300 m above the caldera floor to a height comparable to the caldera rim. It has been the source of numerous lava flows as well as explosive eruptions that were visible from Quito in historical time. Frequent lahars in this region of heavy rainfall have constructed a debris plain on the eastern floor of the caldera. The largest historical eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Instituto Geofísico (IG-EPN), Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).


Kuchinoerabujima (Japan) — March 2019 Citation iconCite this Report

Kuchinoerabujima

Japan

30.443°N, 130.217°E; summit elev. 657 m

All times are local (unless otherwise noted)


Weak explosions and ash plumes beginning 21 October 2018

Activity at Kuchinoerabujima is exemplified by interim explosions and periods of high seismicity. A weak explosion occurred on 3 August 2014, the first since 1980, and was followed by several others during 29 May-19 June 2015 (BGVN 42:03). This report describes events through February 2019. Information is based on monthly and annual reports from the Japan Meteorological Agency (JMA) and advisories from the Tokyo Volcanic Ash Advisory Center (VAAC). Activity has been limited to Kuchinoerabujima's Shindake Crater.

Activity during 2016-2018. According to JMA, between July 2016 and August 2018, the volcano was relatively quiet. Deflation had occurred since January 2016. On 18 April 2018 the Alert Level was lowered from 3 to 2 (on a scale of 1-5). A low-temperature thermal anomaly persisted near the W fracture in Shindake crater. During January-March 2018, both the number of volcanic earthquakes (generally numerous and typically shallow) and sulfur dioxide flux remained slightly above baselines levels in August 2014 (60-500 tons/day compared tp generally less than 100 tons/day in August 2014).

JMA reported that on 15 August 2018 a swarm of deep volcanic earthquakes was recorded, prompting an increase in the Alert Level to 4. The earthquake hypocenters were about 5 km deep, below the SW flanks of Shindake, and the maximum magnitude was 1.9. They occurred at about the same place as the swarm that occurred just before the May 2015 eruption. Sulfur dioxide emissions had increased since the beginning of August; they were 1,600, 1,000, and 1,200 tons/day on 11, 13, and 17 August, respectively. No surficial changes in gas emissions or thermal areas were observed during 16-20 August. On 29 August, JMA downgraded the Alert Level to 3, after no further SO2 flux increase had occurred in recent days and GNSS measurements had not changed.

A very weak explosion was recorded at 1831 on 21 October, with additional activity between 2110 on 21 October and 1350 on 22 October; plumes rose 200 m above the crater rim. During an overflight on 22 October, observers noted ash in the emissions, though no morphological changes to the crater nor ash deposits were seen. Based on satellite images and information from JMA, the Tokyo VAAC reported that during 24-28 October ash plumes rose to altitudes of 0.9-1.5 km and drifted in multiple directions. During a field observation on 28 October, JMA scientists did not observe any changes in the thermal anomalies at the crater.

JMA reported that during 31 October-5 November 2018, very small events released plumes that rose 500-1,200 m above the crater rim. On 6 November, crater incandescence began to be periodically visible. During 12-19 November, ash plumes rose as high as 1.2 km above the crater rim and, according to the Tokyo VAAC, drifted in multiple directions. Observers doing fieldwork on 14 and 15 November noted that thermal measurements in the crater had not changed. Intermittent explosions during 22-26 November generated plumes that rose as high as 2.1 km above the crater rim. During 28 November-3 December the plumes rose as high as 1.5 km above the rim.

JMA reported that at 1637 on 18 December an explosion produced an ash plume that rose 2 km and then disappeared into a weather cloud. The event ejected material that fell in the crater area, and generated a pyroclastic flow that traveled 1 km W and 500 m E of the crater. Another weak explosion occurred on 28 December, scattering large cinders up to 500 m from the crater.

The Tokyo VAAC did not issue any ash advisories for aviation until 21 October 2018, when it issued at least one report every day through 13 December. It also issued advisories on 18-20 and 28 December.

Activity during January-early February 2019. JMA reported that at 0919 local time on 17 January 2019 an explosion generated a pyroclastic flow that reached about 1.9 km NW and 1 km E of the crater. It was the strongest explosion since October 2018. In addition, "large cinders" fell about 1-1.8 km from the crater.

Tokyo VAAC ash advisories were issued on 1, 17, 20, and 29 January 2018. An explosion at 1713-1915 on 29 January produced an ash plume that rose 4 km above the crater rim and drifted E, along with a pyroclastic flow. Ash fell in parts of Yakushima. During 30 January-1 February and 3-5 February, white plumes rose as high as 600 m. On 2 February, an explosion at 1141-1300 generated a plume that rose 600 m. No additional activity during February was reported by JMA. The Alert Level remained at 3.

Geologic Background. A group of young stratovolcanoes forms the eastern end of the irregularly shaped island of Kuchinoerabujima in the northern Ryukyu Islands, 15 km west of Yakushima. The Furudake, Shindake, and Noikeyama cones were erupted from south to north, respectively, forming a composite cone with multiple craters. The youngest cone, centrally-located Shintake, formed after the NW side of Furutake was breached by an explosion. All historical eruptions have occurred from Shintake, although a lava flow from the S flank of Furutake that reached the coast has a very fresh morphology. Frequent explosive eruptions have taken place from Shintake since 1840; the largest of these was in December 1933. Several villages on the 4 x 12 km island are located within a few kilometers of the active crater and have suffered damage from eruptions.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).


Kerinci (Indonesia) — February 2019 Citation iconCite this Report

Kerinci

Indonesia

1.697°S, 101.264°E; summit elev. 3800 m

All times are local (unless otherwise noted)


A persistent gas-and-steam plume and intermittent ash plumes occurred from July 2018 through January 2019

Kerinci is a frequently active volcano in Sumatra, Indonesia. Recent activity has consisted of intermittent explosions, ash, and gas-and-steam plumes. The volcano alert has been at Level II since 9 September 2007. This report summarizes activity during July 2018-January 2019 based on reports by The Indonesia volcano monitoring agency, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, notices from the Darwin Volcano Ash Advisory Center (Darwin VAAC), and satellite data.

Throughout this period dilute gas-and-steam plumes rising about 300 m above the summit were frequently observed and seismicity continued (figure 6). During July through January ash plumes were observed by the Darwin VAAC up to 4.3 km altitude and dispersed in multiple directions (table 7 and figure 7).

Figure (see Caption) Figure 6. Graph showing seismic activity at Kerinci from November 2018 through February 2019. Courtesy of MAGMA Indonesia.

Table 7. Summary of ash plumes (altitude and drift direction) for Kerinci during July 2018 through January 2019. The summit is at 3.5 km altitude. Data courtesy of the Darwin Volcanic Ash Advisory Center (VAAC) and MAGMA Indonesia.

Date Ash plume altitude (km) Ash plume drift direction
22 Jul 2018 4.3 SW
28-30 Sep 2018 4.3 SW, W
02 Oct 2018 4.3 SW, W
18-22 Oct 2018 4.3 N, W, WSW, SW
19 Jan 2019 4 E to SE
Figure (see Caption) Figure 7. Dilute ash plumes at Kerinci during July 2018-January 2019. Sentinel-2 natural color (bands 4, 3, 2) satellite images courtesy of Sentinel Hub Playground.

Based on satellite data, a Darwin VAAC advisory reported an ash plume to 4.3 km altitude on 22 July that drifted to the SW and S. Only one day with elevated thermal emission was noted in Sentinel-2 satellite data for the entire reporting period, on 13 September 2018 (figure 8). No thermal signatures were detected by MODVOLC. On 28-29 September there was an ash plume observed to 500-600 m above the peak that dispersed to the W. Several VAAC reports on 2 and 18-22 October detected ash plumes that rose to 4.3 km altitude and drifted in different directions. On 19 January from 0734 to 1000 an ash plume rose to 200 m above the crater and dispersed to the E and SE (figure 9).

Figure (see Caption) Figure 8. Small thermal anomaly at Kerinci volcano on 13 September 2018. False color (urban) image (band 12, 11, 4) courtesy of Sentinel Hub Playground.
Figure (see Caption) Figure 9. Small ash plume at Kerinci on 19 January 2018 that reached 200 m above the crater and traveled west. Courtesy of MAGMA Indonesia.

Geologic Background. Gunung Kerinci in central Sumatra forms Indonesia's highest volcano and is one of the most active in Sumatra. It is capped by an unvegetated young summit cone that was constructed NE of an older crater remnant. There is a deep 600-m-wide summit crater often partially filled by a small crater lake that lies on the NE crater floor, opposite the SW-rim summit. The massive 13 x 25 km wide volcano towers 2400-3300 m above surrounding plains and is elongated in a N-S direction. Frequently active, Kerinci has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).


Yasur (Vanuatu) — February 2019 Citation iconCite this Report

Yasur

Vanuatu

19.532°S, 169.447°E; summit elev. 361 m

All times are local (unless otherwise noted)


Eruption continues with ongoing explosions and multiple active crater vents, August 2018-January 2019

According to the Vanuatu Meteorology and Geo-Hazards Department (VMGD), which monitors Yasur, the volcano has been in essentially continuous Strombolian activity since Captain Cook observed ash eruptions in 1774, and undoubtedly before that time. VMGD reported that, based on visual observations and seismic data, activity continued through January 2019, with ongoing, sometimes strong, explosions. The Alert Level remained at 2 (on a scale of 0-4). VMGD reminded residents and tourists to remain outside the 395-m-radius permanent exclusion zone and warned that volcanic ash and gas could reach areas influenced by trade winds.

Thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were recorded 6-15 days per month during the reporting period, sometimes with multiple pixels. The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system, also based on analysis of MODIS data, detected numerous hotspots every month. Active crater vents were also frequently visible in Sentinel-2 satellite imagery (figure 50).

Figure (see Caption) Figure 50. Sentinel-2 satellite color infrared image (bands 8, 4, 3) of Yasur on 17 November 2018 showing at least three distinct heat sources in the crater. Courtesy of Sentinel Hub Playground.

Geologic Background. Yasur, the best-known and most frequently visited of the Vanuatu volcanoes, has been in more-or-less continuous Strombolian and Vulcanian activity since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island, this mostly unvegetated pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide, horseshoe-shaped caldera associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department, Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).


Ambae (Vanuatu) — February 2019 Citation iconCite this Report

Ambae

Vanuatu

15.389°S, 167.835°E; summit elev. 1496 m

All times are local (unless otherwise noted)


Ash plumes and lahars in July 2018 cause evacuation of the island; intermittent gas-and-steam and ash plumes through January 2019

Ambae is one of the active volcanoes of Vanuatu in the New Hebrides archipelago. Recent eruptions have resulted in multiple evacuations of the local population due to ashfall. The current eruption began in September 2017, with the initial episode ending in November that year. The second episode was from late December 2017 to early February 2018, and the third was during February-April 2018. The Alert Level was raised to 3 in March, then lowered to Level 2 again on 2 June 2018. Eruptive activity began again on 1 July and produced thick ash deposits that significantly impacted the population, resulting in the full evacuation of the Island of Ambae. This report summarizes activity from July 2018 through January 2019 and is based on reports by the Vanuatu Meteorology and Geo-hazards Department (VMGD), The Vanuatu Red Cross, posts on social media, and various satellite data.

On 1 July Ambae entered a new eruption phase, marked by an ash plume that resulted in ashfall on communities in the W to NW parts of Ambae Island and the NE part of Santo Island (figure 78). On 9-10 July VMGD reported that a small eruption continued with activity consisting of ongoing gas-and-steam emissions. An observation flight on 13 July confirmed that the eruption was centered at Lake Voui and consisted of explosions that ejected hot blocks with ongoing gas-and-steam and ash emissions. Populations on Ambae and a neighboring island could hear the eruption, smell the volcanic gases, and see incandescence at night.

Figure (see Caption) Figure 78. Ash plume at Ambae on 1 July 2018 that resulted in ashfall on the W to NW parts of the island, and on the NE part of Santo Island. Courtesy of VMGD.

On 16 July the Darwin VAAC reported an ash plume to 9.1 km that drifted to the NE. During 16-24 July daily ash plumes from the Lake Voui vent rose to altitudes of 2.3-9.1 km and drifted N, NE, E, and SE (figure 79 and 80). Radio New Zealand reported that on the 16th significant ash emission blocked out sunlight, making the underlying area dark at around 1600 local time. Much of E and N Ambae Island experienced heavy ashfall and the eruption could be heard over 30 km away. The Vanuatu Red Cross Society reported worsening conditions in the south on 24 July with ashfall resulting in trees falling and very poor visibility of less than 2 m (figures 81, 82, and 83). The Daily Post reported that by 19 July lahars had washed away two roads and other roads were blocked to western Ambae. Volcanologists who made their way to the area reported widespread damage (figure 84). The Alert Level was raised from level 2 to 3 (on a scale of 0-5) on 21 July due to an increase in ash emission and more sustained plumes, similar to March 2018 activity.

Figure (see Caption) Figure 79. Ash plumes produced by the Ambae eruption in July 2018 as seen in Terra/MODIS visible satellite images. Images courtesy of NASA Worldview.
Figure (see Caption) Figure 80. Sentinel-2 satellite image of an ash plume from Ambae in Vanuatu on 23 July 2018 with the inset showing the ash plume at the vent. Courtesy of Sentinel Hub Playground.
Figure (see Caption) Figure 81. Ashfall at Ambae, posted on 25 July 2018. Courtesy of the Vanuatu Red Cross Society.
Figure (see Caption) Figure 82. An ash plume at Ambae in July during a day and a half of constant ashfall, looking towards the volcano. Courtesy of Michael Rowe.
Figure (see Caption) Figure 83. Ashfall from the eruption at Ambae blocked out the sun near the volcano on 24 July 2018. Courtesy of the Vanuatu Red Cross Society.
Figure (see Caption) Figure 84. Impacts of ashfall near Ambae in July 2018. Photos by Nicholson Naki, courtesy of the Vanuatu Red Cross (posted on 22 July 2018).

At 2100 on 26 July the ongoing explosions produced an ash plume that rose to 12 km and spread NE, E, SE. A state of emergency was announced by the Government of Vanuatu with a call for mandatory evacuations of the island. Ash emissions continued through the next day (figure 85 and 86) with two episodes producing volcanic lightning at 1100-1237 and 1522-2029 on 27 July (figure 87). The Darwin VAAC reported ash plumes up to 2.4-6.4 km, drifting SE and NW, and pilots reported heavy ashfall in Fiji. Large SO2 plumes were detected accompanying the eruptions and moving towards the E (figure 88).

Figure (see Caption) Figure 85. Ash plumes at Ambae at 0830 and 1129 local time on 27 July 2018. The ash plume is significantly larger in the later image. Webcam images from Saratamata courtesy of VMGD.
Figure (see Caption) Figure 86. Two ash plumes from Ambae at 1200 on 27 July 2018 as seen in a Himawari-8 satellite image. Courtesy of Himawari-8 Real-time Web.
Figure (see Caption) Figure 87. Lightning strokes detected at Ambae on 27 July 2018. There were two eruption pulses, 1100-1237 (blue) and 1522-2029 local time (red) that produced 185 and 87 lightning strokes, respectively. Courtesy of William A. Brook, Ronald L. Holle, and Chris Vagasky, Vaisala Inc.
Figure (see Caption) Figure 88. Aura/OMI data showing the large SO2 plumes produced by Ambae in Vanuatu during 22-31 July 2018. Courtesy of NASA Goddard Space Flight Center.

Video footage showed a lahar blocking a road around 2 August. The government of Vanuatu told reporters that the island had been completely evacuated by 14 August. A VMGD bulletin on 22 August reported that activity continued with ongoing gas-and-steam and sometimes ash emissions; residents on neighboring islands could hear the eruption, smell volcanic gases, and see the plumes.

On 1 September at 2015 an explosion sent an ash plume to 4-11 km altitude, drifting E. Later observations in September showed a decrease in activity with no further explosions and plumes limited to white gas-and-steam plumes. On 21 September VMGD reported that the Lake Voui eruption had ceased and the Alert Level was lowered to 2.

Observed activity through October and November dominantly consisted of white gas-and-steam plumes. An explosion on 30 October at 1832 produced an ash plume that rose to 4-5 km and drifted E and SE. Satellite images acquired during July-November show the changing crater area and crater lake water color (figure 89). VMGD volcano alert bulletins on 6, 7, and 21 January 2019 reported that activity continued with gas-and-steam emissions (figure 90). Thermal energy continued to be detected by the MIROVA system through January (figure 91).

Figure (see Caption) Figure 89. The changing lakes of Ambae during volcanic activity in 2018. Courtesy of Sentinel Hub Playground.
Figure (see Caption) Figure 90. A steam plume at Ambae on 21 January 2019. Courtesy of VMGD.
Figure (see Caption) Figure 91. Log radiative power MIROVA plot of MODIS infrared data at Ambae for April 2018 through January 2019 showing the increased thermal energy during the July 2018 eruption and continued activity. Courtesy of MIROVA.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone dotted with scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Himawari-8 Real-time Web, developed by the NICT Science Cloud project in NICT (National Institute of Information and Communications Technology), Japan, in collaboration with JMA (Japan Meteorological Agency) and CEReS (Center of Environmental Remote Sensing, Chiba University) (URL: https://himawari8.nict.go.jp/); Vanuatu Red Cross Society (URL: https://www.facebook.com/VanuatuRedCross); William A. Brooks and Ronald L. Holle, Vaisala Inc., Tucson, Arizona, and Chris Vagasky, Vaisala Inc., Louisville, Colorado (URL: https://www.vaisala.com/); Michael Rowe, The University of Auckland, 23 Symonds Street, Auckland, 1010, New Zealand (URL: https://unidirectory.auckland.ac.nz/profile/michael-rowe); Radio New Zealand, 155 The Terrace, Wellington 6011, New Zealand (URL: https://www.radionz.co.nz/international/pacific-news/359231/vanuatu-provincial-capital-moves-due-to-volcano); Vanuatu Daily Post (URL: http://dailypost.vu/).


Agung (Indonesia) — February 2019 Citation iconCite this Report

Agung

Indonesia

8.343°S, 115.508°E; summit elev. 2997 m

All times are local (unless otherwise noted)


Ongoing intermittent ash plumes and frequent gas-and-steam plumes during August 2018-January 2019

Agung is an active volcano in Bali, Indonesia, that began its current eruptive episode in September 2017. During this time activity has included ash plumes, gas-and-steam plumes, explosions ejecting ballistic blocks onto the flanks, and lava extrusion within the crater.

This report summarizes activity from August 2018 through January 2019 based on information from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Indonesian Center for Volcanology and Geological Hazard Mitigation (CVGHM), MAGMA Indonesia, the National Board for Disaster Management - Badan Nasional Penanggulangan Bencana (BNPB), the Darwin Volcanic Ash Advisory Center (VAAC), and satellite data.

During August 2018 through January 2019 observed activity was largely gas-and steam plumes up to 700 m above the crater (figures 39 and 40). In late December and January there were several explosions that produced ash plumes up to 5.5 km altitude, and ejected ballistic blocks.

Figure (see Caption) Figure 39. Graph showing the observed white gas-and-steam plumes and gray ash plumes at Agung during August 2018 through January 2019. The dates showing no data points coincided with cloudy days where the summit was not visible. Data courtesy of PVMBG.
Figure (see Caption) Figure 40. A white gas-and-steam plume at Agung on 21 December 2018. Courtesy of MAGMA Indonesia.

The Darwin VAAC reported an ash plume on 8-9 August based on satellite data, webcam footage, and ground report information. The ash plume rose to 4.3 km and drifted to the W. They also reported a diffuse ash plume to 3.3 km altitude on 16-17 August based on satellite and webcam data. During September through November there were no ash plumes observed at Agung; activity consisted of white gas-and-steam plumes ranging from 10-500 m above the crater.

Throughout December, when observations could be made, activity mostly consisted of white gas-and-steam plumes up to 400 m above the crater. An explosion occurred at 0409 on 30 December that lasted 3 minutes 8 seconds produced an ash plume rose to an altitude of 5.5 km and moved to the SE and associated incandescence was observed at the crater. Light Ashfall was reported in the Karangasem regency to the NE, including Amlapura City and several villages such as in Seraya Barat Village, Seraya Tengah Village, and Tenggalinggah Village (figure 41).

Figure (see Caption) Figure 41. A webcam image of an explosion at Agung that began at 0409 on 30 December 2018. Light Ashfall was reported in the Karangasem regency. Courtesy of PVMBG.

White gas-and-steam plumes continued through January 2019 rising as much as 600 m above the crater. Several Volcano Observatory Notices for Aviation (VONAs) were issued during 18-22 January. An explosion was recorded at 0245 on 19 January that produced an ash plume to 700 m above the crater and ejected incandescent blocks out to 1 km from the crater. On 21 January another ash plume rose to an estimated plume altitude of 5.1 km. The next morning, at 0342 on the 22nd, an ash plume to an altitude of 2 km that dispersed to the E and SE.

Satellite data shows continued low-level thermal activity in the crater throughout this period. MIROVA thermal data showed activity declining after a peak in July, and a further decline in energy in September (figure 42). Low-level thermal activity continued through December. Sentinel-2 thermal data showed elevated temperatures within the ponded lava in the crater (figure 43).

Figure (see Caption) Figure 42. Log radiative power MIROVA plot of MODIS infrared data for May 2018 through January 2019 showing thermal anomalies at Agung. The black data lines indicate anomalies more than 10 km from the crater, which are likely due to fires. Courtesy of MIROVA.
Figure (see Caption) Figure 43. Sentinel-2 thermal satellite images showing areas of elevated temperatures within the lava ponded in the Agung crater during August 2018 through January 2019. Courtesy of Sentinel Hub Playground.

Geologic Background. Symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The volcano, whose name means "Paramount," rises above the SE caldera rim of neighboring Batur volcano, and the northern and southern flanks extend to the coast. The summit area extends 1.5 km E-W, with the high point on the W and a steep-walled 800-m-wide crater on the E. The Pawon cone is located low on the SE flank. Only a few eruptions dating back to the early 19th century have been recorded in historical time. The 1963-64 eruption, one of the largest in the 20th century, produced voluminous ashfall along with devastating pyroclastic flows and lahars that caused extensive damage and many fatalities.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).


Erebus (Antarctica) — January 2019 Citation iconCite this Report

Erebus

Antarctica

77.53°S, 167.17°E; summit elev. 3794 m

All times are local (unless otherwise noted)


Lava lakes persist through 2017 and 2018

Between the early 1980's through 2016, activity at Erebus was monitored by the Mount Erebus Volcano Observatory (MEVO), using seismometers, infrasonic recordings to measure eruption frequency, and annual scientific site visits. MEVO recorded occasional explosions propelling ash up to 2 km above the summit of this Antarctic volcano and the presence of two, sometimes three, lava lakes (figure 26). However, MEVO closed in 2016 (BGVN 42:06).

Activity at the lava lakes in the summit crater can be detected using MODIS infrared detectors aboard the Aqua and Terra satellites and analyzed using the MODVOLC algorithm. A compilation of thermal alert pixels during 2017-2018 (table 4, a continuation of data in the previous report) shows a wide range of detected activity, with a high of 182 alert pixels in April 2018. Although no MODVOLC anomalies were recorded in January 2017, detectors on the Sentinel-2 satellite imaged two active lava lakes on 25 January.

Table 4. Number of MODVOLC thermal alert pixels recorded per month from 1 January 2017 to 31 December 2018 for Erebus by the University of Hawaii's thermal alert system. Table compiled by GVP from data provided by MODVOLC.

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SUM
2017 0 21 9 0 0 1 11 61 76 52 0 3 234
2018 0 21 58 182 55 17 137 172 103 29 0 0 774
SUM 0 42 67 182 55 18 148 233 179 81 0 3 1008
Figure (see Caption) Figure 26. Sentinel-2 images of the summit crater area of Erebus on 25 January 2017. Top: Natural color filter (bands 4, 3, 2). Bottom: Atmospheric penetration filter (bands 12, 11, 8A) in which two distinct lava lakes can be observed. The main crater is 500 x 600 m wide. Courtesy of Sentinel Hub Playground.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. The 3794-m-high Erebus is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).


Villarrica (Chile) — March 2019 Citation iconCite this Report

Villarrica

Chile

39.42°S, 71.93°W; summit elev. 2847 m

All times are local (unless otherwise noted)


Intermittent Strombolian activity ejects incandescent bombs around crater rim, September 2018-February 2019

Historical eruptions at Chile's Villarrica, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. An intermittently active lava lake at the summit has been the source of explosive activity, incandescence, and thermal anomalies for several decades. Sporadic Strombolian activity at the lava lake and small ash emissions have continued since the last large explosion on 3 March 2015. Similar continuing activity during September 2018-February 2019 is covered in this report, with information provided primarily by the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN), and Projecto Observación Villarrica Internet (POVI), part of the Fundacion Volcanes de Chile, a research group that studies volcanoes across Chile.

After ash emissions during July 2018 and an increase in of thermal activity from late July through early September 2018 (BGVN 43:10), Villarrica was much quieter through February 2019. Steam plumes rose no more than a few hundred meters above the summit and the number of thermal alerts decreased steadily. Intermittent Strombolian activity sent ejecta a few tens of meters above the summit crater, with larger bombs landing outside the crater rim. A small pyroclastic cone appeared at the surface of the lava lake, about 70 m below the rim, in November. The largest lava fountain rose 35 m above the crater rim in late January 2019.

Steam plumes rose no more than 300 m above the crater during September 2018 and were less than 150 m high in October; incandescence at the summit was visible during clear nights, although a gradual decrease in activity suggested a lowering of the lake level to SERNAGEOMIN. SERNAGEOMIN attributed an increase in LP seismic events from 1,503 in September to 5,279 in October to dynamics of the lava lake inside the summit crater; counts decreased gradually in the following months.

POVI reported webcam evidence of Strombolian activity with ejecta around the crater several times during November 2018. On 5 November the webcam captured an image of an incandescent bomb, more than a meter in diameter, that landed on the NW flank. The next day, explosions sent ejecta 50 m above the edge of the crater, and pyroclastic debris landed around the perimeter. Significant Strombolian explosions on 16 November sent incandescent bombs toward the W rim of the crater (figure 71). The POVI webcam in Pucón captured incandescent ejecta landing on the crater rim on 23 November. POVI scientists observed a small pyroclastic cone, about 10-12 m in diameter, at the bottom of the summit crater on 19 November (figure 72); it was still visible on 25 November.

Figure (see Caption) Figure 71. Strombolian activity at the summit of Villarrica was captured several times in the POVI webcam located in Pucón. An explosion on 5 November 2018 ejected a meter-sized bomb onto the NW flank (left). On 16 November, incandescent bombs were thrown outside the W rim of the crater (right). Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).
Figure (see Caption) Figure 72. A small pyroclastic cone was visible at the bottom of the summit crater at Villarrica (about 70 m deep) on 19 November 2018 (left); it was still visible on 25 November (right). Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

During December 2018 webcam images showed steam plumes rising less than 350 m above the crater. Infrasound instruments identified two small explosions related to lava lake surface activity. SERNAGEOMIN noted a minor variation in the baseline of the inclinometers; continued monitoring indicated the variation was seasonal. A compilation by POVI of images of the summit crater during 2018 showed the evolution of the lava lake level during the year. It had dropped out of sight early in the year, rose to its highest level in July, and then lowered slightly, remaining stable for the last several months of the year (figure 73).

Figure (see Caption) Figure 73. Evolution of the lava pit at Villarrica during 2018. During July the lava lake level increased and for November and December no significant changes were observed. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Between 25 December 2018 and 15 January 2019, financed with funds contributed by the Fundación Volcanes de Chile, POVI was able to install new HD webcams with continuous daily image recording, greatly improving the level of detail data available of the activity at the summit. POVI reported that after a five-week break, Strombolian explosions resumed on 3 January 2019; the lava fountains rose 20 m above the crater rim, and pyroclastic ejecta fell to the E. On 24 January the Strombolian explosions ejected ash, lapilli, and bombs up to 15 cm in diameter; the lava fountain was about 35 m high.

An explosion on 7 February reached about 29 m above the crater's edge; on 9 February a lava fountain three meters in diameter rose 17 m above the crater rim. Sporadic explosions were imaged on 12 February as well (figure 74). During a reconnaissance overflight on 24 February 2019, POVI scientists observed part of the lava pit at the bottom of the crater (figure 75). As of 28 February they noted a slight but sustained increase in the energy of the explosions. SERNAGEOMIN noted that steam plumes rose 400 m in January and 150 m during February, and incandescence was visible on clear nights during both months.

Figure (see Caption) Figure 74. Strombolian activity at Villarrica in January and February 2019 was imaged with a new HD webcam on several occasions. On 24 January 2019 explosions ejected ash, lapilli, and bombs up to 15 cm in diameter; the lava fountain was about 35 m high (left); on 12 February 2019 explosions rose about 19 m above the crater rim (right). Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).
Figure (see Caption) Figure 75. During a reconnaissance overflight on 24 February 2019, POVI scientists observed part of the lava pit at the bottom of the crater at Villarrica; gas and steam emissions and incandescence from small explosions were noted. Courtesy of POVI (Volcán Villarrica, Resumen Gráfico del Comportamiento, November 2017 a Febrero 2019).

Geologic Background. Glacier-clad Villarrica, one of Chile's most active volcanoes, rises above the lake and town of the same name. It is the westernmost of three large stratovolcanoes that trend perpendicular to the Andean chain. A 6-km-wide caldera formed during the late Pleistocene. A 2-km-wide caldera that formed about 3500 years ago is located at the base of the presently active, dominantly basaltic to basaltic-andesitic cone at the NW margin of the Pleistocene caldera. More than 30 scoria cones and fissure vents dot the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Historical eruptions, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/).

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Bulletin of the Global Volcanism Network - Volume 16, Number 08 (August 1991)

Managing Editor: Lindsay McClelland

Aira (Japan)

Explosions continue

Arenal (Costa Rica)

Explosions and seismicity decline; lava flows

Arjuno-Welirang (Indonesia)

White plume from Welirang summit

Barren Island (India)

Basaltic andesite lava from flank vent reaches sea

Colima (Mexico)

Fumarole temperatures increase

Dukono (Indonesia)

Explosions and glow; ashfall to coast; small lahars

Galeras (Colombia)

Explosions eject incandescent tephra; increased seismicity and deformation

Gamalama (Indonesia)

Explosions eject white clouds

Hudson, Cerro (Chile)

Basaltic fissure eruption preceded andesitic paroxysmal phase; strong winds rework major ash deposits

Irazu (Costa Rica)

New fumaroles but seismicity declines

Karangetang (Indonesia)

Explosive activity and glow

Karthala (Comoros)

Details of seismicity and deformation associated with the 11 July eruption

Kilauea (United States)

Continued lava flow into sea; lava pond overflows; magma intrusion

Langila (Papua New Guinea)

Ash clouds; incandescent tephra; lava flows in crater

Lengai, Ol Doinyo (Tanzania)

Carbonatite lava production continues on crater floor

Lokon-Empung (Indonesia)

Incandescent tephra cause fires that burn plantations

Malindang (Philippines)

False report of eruption

Manam (Papua New Guinea)

Occasional ash emission; lava dome

Marchena (Ecuador)

First historical eruption

Pinatubo (Philippines)

Secondary pyroclastic flows feed large ash columns; frequent mudflows; fewer explosions

Poas (Costa Rica)

Crater lake rises, covering fumaroles; low-frequency seismicity

Raung (Indonesia)

Dense plumes

Rincon de la Vieja (Costa Rica)

Explosions eject ash and blocks

Ruapehu (New Zealand)

Lake temperature rises; possible minor eruptions

Ruiz, Nevado del (Colombia)

Seismicity, deformation, and gas emission remain unchanged

Santa Maria (Guatemala)

Continued explosions and block lava production

Stromboli (Italy)

Continued moderate explosive activity; gas data

Unzendake (Japan)

Continued lava dome extrusion and pyroclastic flows

Villarrica (Chile)

Weak explosions

White Island (New Zealand)

Tephra emission; shock waves in crater



Aira (Japan) — August 1991 Citation iconCite this Report

Aira

Japan

31.593°N, 130.657°E; summit elev. 1117 m

All times are local (unless otherwise noted)


Explosions continue

Nineteen explosions occurred . . . in August . . . . Ejecta from an explosion on 5 August at 1057 cracked the windshield of an airliner in flight. A car windshield was cracked by tephra from an explosion at 1249 the same day and another was broken on 20 August at 0851, both on Sakura-jima Island, 3 km from the crater. The month's highest ash cloud rose 4,000 m. A total of 583 g/m2 of ash was deposited [at KLMO]; a change in the usual wind direction had carried ash away from this site in July. Typical volcanic earthquake swarms were recorded on 3, 15, 16, and 29 August.

Similar activity continued through mid-September, adding 15 explosions as of the 14th . . . . The highest September ash cloud reached 1,800 m height.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: JMA.


Arenal (Costa Rica) — August 1991 Citation iconCite this Report

Arenal

Costa Rica

10.463°N, 84.703°W; summit elev. 1670 m

All times are local (unless otherwise noted)


Explosions and seismicity decline; lava flows

An average of 3 explosions/day was recorded in August . . . . Seismicity also decreased, to a daily average of 20 earthquakes (figure 40). Fumarolic activity continued from the active crater, and lava flows continued to travel down the W and SW flanks of the volcano.

Figure (see Caption) Figure 40. Daily number of earthquakes at Arenal, August 1991. Courtesy of the Instituto Costarricense de Electricidad.

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: R. Barquero and G. Soto, ICE; Mario Fernández, Hector Flores, and Sergio Paniagua, Sección de Sismología y Vulcanología, Escuela de Geología, Univ de Costa Rica, San José, Costa Rica.


Arjuno-Welirang (Indonesia) — August 1991 Citation iconCite this Report

Arjuno-Welirang

Indonesia

7.733°S, 112.575°E; summit elev. 3339 m

All times are local (unless otherwise noted)


White plume from Welirang summit

A plume from the summit area of Welirang . . . was photographed by Space Shuttle astronauts on 13 September at [1535] (photo no. S48-151-064) (figure 1). The dense portion of the apparently ash-poor plume extended roughly 50 km N and more diffuse material continued for another 65 km. The summit area was white and apparently de-vegetated. A plume was observed again on direct video downlink from the spacecraft on [17] September at [1306]. No ground reports were available at press time.

Figure (see Caption) Figure 1. Space Shuttle photograph showing a steam plume from Welirang (just east of the central cloud mass). Also, the lack of vegetation at the peak indicates volcanic activity. Volcanoes on Java form an E-W line of peaks the length of the island; five are in this image. NASA Photo ID: STS048-151-064, 13 September 1991.

Geologic Background. The twin volcanoes of Arjuno and Welirang anchor the SE and NW ends, respectively, of a 6-km-long line of volcanic cones and craters. The Arjuno-Welirang complex overlies two older volcanoes, Gunung Ringgit to the east and Gunung Linting to the south. The summit areas of both volcanoes are unvegetated. Additional pyroclastic cones are located on the north flank of Gunung Welirang and along an E-W line cutting across the southern side of Gunung Arjuno that extends to the lower SE flank. Fumarolic areas with sulfur deposition occur at several locations on Gunung Welirang.

Information Contacts: C. Evans and D. Helms, NASA-SSEOP.


Barren Island (India) — August 1991 Citation iconCite this Report

Barren Island

India

12.278°N, 93.858°E; summit elev. 354 m

All times are local (unless otherwise noted)


Basaltic andesite lava from flank vent reaches sea

Lava production continued from the subsidiary vent on the NE face of the volcanic cone, 80 m below the main crater, during a visit on 26 June. Incandescent material was ejected in a pulsating fountain, to [80] m height, more intensely than during the previous visit on 16 May. Satellite monitoring had indicated a temperature of 1,100°C around the vent on 6 May. A dark plume rose 300-400 m from the crater of a large spatter cone that had formed at the eruptive vent. The main crater remained quiet. The lava flow observed in May had bifurcated, with one branch extending along the NW and W valleys, and a new branch extending S. By 26 June, lava had reached the sea at the boat landing near the NW corner of the island (~1.2 km from the vent); during the 16 May fieldwork, the lava front was still 200 m from shore. Vigorous boiling and thick jets of steam were observed for 100 m along the shore. Studies of water near the shore indicated a considerable decrease in pH, and visibility dropped to <10 cm (Srinivas, 1991). Nearby coral was destroyed.

The following is from a GSI report on lava chemistry and petrography. "Thirteen chemical analyses on samples of recent lava collected on 16 May indentify the rocks as basaltic andesites (table 1). They are porphyritic with phenocrysts of plagioclase (dominant; some grains show labradorite composition), with minor clinopyroxene (augite) and forsteritic olivine, set in a fluidal [intersertal] groundmass of brown glass, plagioclase microlites, and Fe-Ti oxides. The amount of mafic phenocrysts is relatively low. The average ratio between phenocryst and groundmass components is around 0.44. The volumetric composition of the phenocrysts indicates: 72% plagioclase, 17% clinopyroxene, and 11% olivine; while the groundmass consists of 43% plagioclase microlites, 37% glass, and 20% Fe-Ti oxides. The amount of glass in the groundmass is highly variable, exceeding 70% in some sections. There is a complete lack of amphibole grains, in both the phenocrysts and [in] the groundmass."

Table 1. Range and average compositions from 13 chemical analyses of recent lava erupted from Barren Island, collected 16 May 1991. Courtesy of the GSI.

Element Range (%) Average (%)
SiO2 54.96-56.33 55.64
TiO2 1.13-1.18 1.15
Al2O3 17.66-18.62 18.20
Fe2O3 5.39-9.44 6.32
FeO 0.36-4.23 2.36
MnO 0.18-0.19 0.185
MgO 3.29-3.59 3.39
CaO 7.52-7.93 7.79
Na2O 1.75-2.19 2.01
K2O 0.58-0.79 0.69
P2O5 0.14-0.18 0.16
L.O.I. 0.16-0.48 0.31

Geologic Background. Barren Island, a possession of India in the Andaman Sea about 135 km NE of Port Blair in the Andaman Islands, is the only historically active volcano along the N-S volcanic arc extending between Sumatra and Burma (Myanmar). It is the emergent summit of a volcano that rises from a depth of about 2250 m. The small, uninhabited 3-km-wide island contains a roughly 2-km-wide caldera with walls 250-350 m high. The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic-flow and -surge deposits. Historical eruptions have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast.

Information Contacts: Director General, GSI; S. Acharya, SANE.


Colima (Mexico) — August 1991 Citation iconCite this Report

Colima

Mexico

19.514°N, 103.62°W; summit elev. 3850 m

All times are local (unless otherwise noted)


Fumarole temperatures increase

"Five high-temperature fumaroles on the SW rim of the summit lava dome have been monitored continuously since May. These fumaroles are ~75 m W of the site of the March-May lava extrusion and occur along fractures radial to the dome. Temperatures were measured at 20-minute intervals and radio-telemetered to the Science Center in the city of Colima. Temperatures at two of the fumaroles have increased at a steady rate between May and August (figure 16). Mean late-August temperatures were 506 and 386°C, increases of 66 and 43°C, respectively, since May. Mean temperatures in three other fumaroles have changed <10°C during the same period. Throughout the sampling period, all fumaroles exhibited marked diurnal temperature variation, on the order of 30-80°C/day. The rainy season, which began in mid-June and has continued through August, has had little effect on fumarole temperatures other than occasional low readings during rainstorms."

Figure (see Caption) Figure 16. May-August 1991 temperatures at two fumaroles on the SW rim of Colima's summit lava dome, about 75 m W of the site of March-May lava extrusion. Measurements, at 20-minute intervals, were radio-telemetered to the Science Center, city of Colima. Courtesy of C. Connor.

Geologic Background. The Colima volcanic complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the 4320 m high point of the complex) on the north and the 3850-m-high historically active Volcán de Colima at the south. A group of cinder cones of late-Pleistocene age is located on the floor of the Colima graben west and east of the Colima complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide caldera, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, and have produced a thick apron of debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions (most recently in 1913) have destroyed the summit and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: C. Connor, FIU, Miami.


Dukono (Indonesia) — August 1991 Citation iconCite this Report

Dukono

Indonesia

1.693°N, 127.894°E; summit elev. 1229 m

All times are local (unless otherwise noted)


Explosions and glow; ashfall to coast; small lahars

People living near the volcano reported that a new eruption began during the night of 8-9 June, after nine years of relative quiet. At the onset of the eruption, residents were awakened by rumblings and a red glow from the volcano, which has since remained active. Ashfalls have occurred regularly in coastal towns 15 km NNW to 15 km ENE of the summit (Galela, Mamoya, and Tobelo). When visited by a geologist on 23-28 June, small to moderate explosions occurred every 4-5 minutes, sometimes accompanied by noise and night glow. Small lahars occurred in rivers draining the volcano.

Space Shuttle astronauts photographed an apparently ash-rich plume extending ~30 km from the summit to slightly beyond the coast on 15 September at 2156 (photos STS048-110-34 & 35). The entire summit area appeared ash-covered.

Figure (see Caption) Figure 1. Photograph of Dukono taken from the Space Shuttle, 2156 on 15 September 1991. The summit area appears to be covered with ash, and the plume extends ~30 km W from the summit. Courtesy of NASA-SSEOP; photo STS048-110-35.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland; C. Evans, NASA-SSEOP.


Galeras (Colombia) — August 1991 Citation iconCite this Report

Galeras

Colombia

1.22°N, 77.37°W; summit elev. 4276 m

All times are local (unless otherwise noted)


Explosions eject incandescent tephra; increased seismicity and deformation

Seismic activity increased significantly in August, reaching the highest number of events (>150/day), the greatest reduced displacement (>800 cm2), and the highest released energy (~5.0 x 108 ergs; see figure 52) by long-period events since monitoring began in February 1989. Explosions and continuous ash emission from the crater were accompanied by periodic ejections of incandescent blocks up to tens of centimeters in diameter. Incandescence was visible within the crater at dispersed sites. Although weather conditions impeded direct observations, it was possible to confirm that many of the long-period earthquakes and tremor episodes had associated surface activity. SO2 flux was low, ranging from 7 to ~370 t/d.

Substantial deformation changes were measured by the electronic tiltmeter [at Crater Station], with a resultant vector of 231 µrad of inflation (118° azimuth) in the 2 weeks ending 14 August. Lower levels of deformation, 3.7 µrad at 183° azimuth, were measured [at Peladitos Station].

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large horseshoe-shaped caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate historical eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS-OVP.


Gamalama (Indonesia) — August 1991 Citation iconCite this Report

Gamalama

Indonesia

0.8°N, 127.33°E; summit elev. 1715 m

All times are local (unless otherwise noted)


Explosions eject white clouds

Two strong explosions were seen from Ternate, 6 km ESE of the summit, on 15 June, ejecting mainly white clouds. A 20 June climb revealed only white vapor filling the summit crater.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the thorough documentation of Gamalama's historical activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano. Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland.


Cerro Hudson (Chile) — August 1991 Citation iconCite this Report

Cerro Hudson

Chile

45.9°S, 72.97°W; summit elev. 1905 m

All times are local (unless otherwise noted)


Basaltic fissure eruption preceded andesitic paroxysmal phase; strong winds rework major ash deposits

In one of the largest eruptions of the century, Hudson's mid-August paroxysm produced an eruption cloud 18 km high and deposited ash up to 1,000 km SE. Estimates of tephra volume range between 2 and 6 km3; >1 km3 was deposited in Chile, around 2 km3 in Argentina, and 2 km3 may have fallen in the Atlantic Ocean or been lost to the atmosphere. Satellite data showed that the eruption produced a large SO2-rich cloud, estimated to contain 1.5 megatons of SO2 on 16 August, that was transported twice around the globe in 2 weeks.

The following is from a report by Norman Banks. "The eruption produced 1 to 2+ km3 (dense rock equivalent) of magma. The initial 8-9 August eruption (beginning about 1820 on 8 August) was from a basaltic (50% SiO2) dike through a fissure 4 km long, trending through the NW rim of the 10 x 7 km, ice-filled caldera. The basalt erupted both as a lava fountain and phreatomagmatically, producing a tephra column 12 km high, scoria flows that covered 10 km2 of the western caldera floor and an unknown area outside of the caldera, a 4-km-long lava flow over the WNW flank's Huemules glacier, long-lived (12 hours) floods down the Río Sorpresa (WSW flank) and Río Huemules valleys, and a rather low-volume tephra-fall deposit N of the volcano. This ash had a moderate level (100-300 ppm dry weight) of soluble fluorine that was quickly reduced to 2-10 ppm by heavy rains during the next 2 weeks. Grass growing through this deposit has a fluorine content of about 30 ppm.

"The andesitic eruption of 12-15 August may have been due to secondary boiling triggered by intrusion of the 8 August basalt, or other basaltic dikes into the andesitic magma body under the caldera; bombs and lapilli of pumiceous andesite (60% SiO2) mixed with chilled basalt are common in the tephra-fall deposits. The 3-day andesitic eruption produced a strong Plinian column that ejected pyroclastic material into a very strong SE-directed stratospheric wind (185 km/hr) that kept the plume narrow even 700 km from the volcano. Pumiceous ballistic bombs 1 m in diameter were found 10 km from the vent, where tephra-fall deposits were >2.5 m thick. The 10-cm isopach reached just SE of Chile Chico (120 km SE of the vent) and 1-2 cm of ash was deposited at Argentina's coast (figure 5). [As many as 13 distinct layers of ash were deposited in some locations.] Fortunately, pyroclastic flows did not spill onto the outer snow-covered flanks during this episode, and no additional mudflows were reported. Shortly after the 12-15 August eruption, however, secondary water-and-pumice flows formed on the volcano's flanks during daily melting of the snow. Because most of the thick deposits on the steep mountainous terrain SE of the volcano are on and interleaved with snow, downslope movement and associated hydrological problems for the downstream valleys are certain to accelerate as the summer melting and rains begin. The andesitic ash in Chile had low amounts of soluble fluorine (<20 ppm), and grass covered by or growing through the ash deposits has a relatively low fluorine content. Analysis of fine fractions of the Chilean deposits suggest that downwind (Argentinean) fluorine values will not be significantly higher."

Figure (see Caption) Figure 5. Preliminary isopach map of the 12-15 August 1991 tephra-fall deposits from Hudson. Prepared by N. Banks, H. Moreno, H. Corbella, M. Haller, and H. Ostera.

Steam emission, occasionally containing minor quantities of ash, declined rapidly following the eruption's end on 15 August.

Major reworking of ash deposits in Argentina by strong winds led to several false reports of renewed activity at Hudson. Ash was redistributed N to Comodoro Rivadavia (2 mm at 400 km E of Hudson) and was reported S to Río Gallegos (700 km SSE). In early September, GOES satellite images detected ash clouds, probably below 3 km, carried by ground-level winds at 55-65 km/hr; these clouds extended from near the volcano to over the Atlantic ocean. The densest part of the clouds appeared to be ~250 km SE of the volcano, about halfway to the Argentine coast. Poor visibility, down to a few hundred meters, was reported at Puerto Deseado and Puerto San Julián. Argentine officials have expressed concern over the >2 million sheep and 3,000 cattle in the affected region.

Geologic Background. The ice-filled, 10-km-wide caldera of the remote Cerro Hudson volcano was not recognized until its first 20th-century eruption in 1971. It is the southernmost volcano in the Chilean Andes related to subduction of the Nazca plate beneath the South American plate. The massive volcano covers an area of 300 km2. The compound caldera is drained through a breach on its NW rim, which has been the source of mudflows down the Río de Los Huemeles. Two cinder cones occur N of the volcano and others occupy the SW and SE flanks. This volcano has been the source of several major Holocene explosive eruptions. An eruption about 6700 years ago was one of the largest known in the southern Andes during the Holocene; another eruption about 3600 years ago also produced more than 10 km3 of tephra. An eruption in 1991 was Chile's second largest of the 20th century and formed a new 800-m-wide crater in the SW portion of the caldera.

Information Contacts: N. Banks, USGS; H. Moreno, Univ de Chile; J. Naranjo, SERNAGEOMIN; P. Bitschene, Patagonia Volcanism Program, Argentina; P. Maxwell, US Embassy, Buenos Aires; D. Helms, Lockheed, Houston; S. Doiron and G. Bluth, GSFC.


Irazu (Costa Rica) — August 1991 Citation iconCite this Report

Irazu

Costa Rica

9.979°N, 83.852°W; summit elev. 3432 m

All times are local (unless otherwise noted)


New fumaroles but seismicity declines

Fumarolic activity continued in August, mainly in a large zone of sulfur and chloride deposition in the N section of the crater, while a new zone of fumarolic activity appeared in the SSE part. The crater lake grew to cover almost the entire floor, >150 m in diameter. Seismicity, abnormally high since late May, continued to decrease in August (figure 4). During the second week in June, a new group of fumaroles appeared in the crater.

Figure (see Caption) Figure 4. Monthly number of earthquakes at Irazú, January-August 1991. Courtesy of the Instituto Costarricense de Electricidad.

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

Information Contacts: R. Barquero and G. Soto, ICE; Mario Fernández, Hector Flores, and Sergio Paniagua, Sección de Sismología y Vulcanología, Univ de Costa Rica.


Karangetang (Indonesia) — August 1991 Citation iconCite this Report

Karangetang

Indonesia

2.781°N, 125.407°E; summit elev. 1797 m

All times are local (unless otherwise noted)


Explosive activity and glow

Explosions were clearly visible from the coast (at Ulu Siau) during a visit 2-4 July. A diffuse, red, summit-area glow was continuously observed. Some small earthquakes were felt.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, north of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World: Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts has also produced pyroclastic flows.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland.


Karthala (Comoros) — August 1991 Citation iconCite this Report

Karthala

Comoros

11.75°S, 43.38°E; summit elev. 2361 m

All times are local (unless otherwise noted)


Details of seismicity and deformation associated with the 11 July eruption

The bottom of the summit's Choungou-Chahalé crater, obscured by a cloud of white gas and vapor following the 11 July phreatic eruption, became visible in early August. A new explosion crater (~250 m in diameter) was observed in its SE section. Vigorous degassing occurred through the crater lake and from the wall of the new crater. The following, from Patrick Bachélery, supplements the report in 16:6.

Karthala's 11 July explosion followed an increase in seismicity from 3-5 events/month (June 1988 start of monitoring through early April 1991) to 3-10 events/day in May (figure 2). Earthquakes were centered beneath the crater, mostly at 0-2 km below sea level, with a few 10-20 km below sea level. On 4 May, a swarm of 161 earthquakes (M 0.5-2) was recorded during a 1-hour period beginning at 1609. The number of earthquakes increased to 30-50/day by the end of June, and all were at shallow depths. Deformation measurements showed summit inflation of ~20 µrad during this time; only weak changes in deformation had been measured between the network's installation (May 1987) and June 1989.

Figure (see Caption) Figure 2. Daily number of earthquakes at Karthala, March-June (inset) and May-July 1991. Courtesy of P. Bachélery.

A notable change in seismicity occurred on 30 June at 1645. More than 500 earthquakes (long- and short-period) were recorded that day, as many on the next, and >1,500 daily 2-4 July (figure 2). The short-period events (M 0.5-3.1) were centered in a roughly N-S line below the S part of the summit caldera and the S flank of the volcano (figure 3). Felt shocks caused ~1,000 people to leave the lower part of the S flank.

Figure (see Caption) Figure 3. Epicenter map of short-period earthquakes at Karthala during 30 June-4 July (open squares) and 5-10 July 1991 (filled squares). Courtesy of P. Bachélery.

Seismicity continued to increase from 4 July, with 4,000 earthquakes recorded daily by 10 July. A swarm of nearly continuous seismic events was recorded between 0040 and 0110 the next day. The 4-10 July seismicity was characterized by low-magnitude (mostly M <1, sometimes to M 3.4) short-period events located under the summit at 1-4 km depths, and less numerous deeper earthquakes at 4-8 km depth. Some long-period events with cigar-shaped waveform envelopes were also recorded. The center of seismicity shifted N, resulting in fewer felt shocks in the S part of the island, while several M 3 earthquakes were felt in Moroni (13 km NW of the crater).

Deformation measurements (dry-tilt) the morning of 10 July showed >120 µrad of inflation centered on Choungou-Chahalé and Choungou-Chagnoumeni (figure 4) craters since 28 June. That night, the eruption took place, but no eyewitness accounts are available. Seismicity reached its highest intensity during an 11-hour period that night [but see 16:6], dropping abruptly at 0335 on 11 July to ~100 recorded events/hour. About 1.5 hours later, a strong sulfur odor was detected in Moroni for ~2 hours.

Figure (see Caption) Figure 4. Map showing Karthala's summit region and deposits from the 11 July 1991 explosion. Courtesy of P. Bachélery.

Later visits to the summit revealed that a sizeable phreatic explosion had occurred in Choungou-Chahalé crater. The southern 2/3 of the summit caldera were covered by blocks (up to 10 m3) and ash (figure 4), and the summit vegetation was completely removed from within the limits of the caldera. The crater bottom was hidden by gas and vapor clouds, obscuring the source of a "fountaining" sound heard two weeks after the 11 July explosion. Geologists later believed the sound to have been caused by the forceful arrival of water into the new crater, forming the crater lake.

Seismicity rapidly decreased after the explosion, although several earthquakes of M 3.0-3.5 were recorded through the end of July. In August, 20-40 events/day were recorded, the same level as in June.

Geologic Background. The southernmost and largest of the two shield volcanoes forming Grand Comore Island (also known as Ngazidja Island), Karthala contains a 3 x 4 km summit caldera generated by repeated collapse. Elongated rift zones extend to the NNW and SE from the summit of the Hawaiian-style basaltic shield, which has an asymmetrical profile that is steeper to the S. The lower SE rift zone forms the Massif du Badjini, a peninsula at the SE tip of the island. Historical eruptions have modified the morphology of the compound, irregular summit caldera. More than twenty eruptions have been recorded since the 19th century from the summit caldera and vents on the N and S flanks. Many lava flows have reached the sea on both sides of the island. An 1860 lava flow from the summit caldera traveled ~13 km to the NW, reaching the W coast to the N of the capital city of Moroni.

Information Contacts: P. Bachélery, Univ de la Réunion; D. Ben Ali and J-L. Klein, CNDRS, RFI des Comores; F. Desgrolard, Centre de Recherche Volcanologique, Clermont-Ferrand, France; J-L. Cheminée, J-P. Toutain, and J-C. Delmond, IPGP.


Kilauea (United States) — August 1991 Citation iconCite this Report

Kilauea

United States

19.421°N, 155.287°W; summit elev. 1222 m

All times are local (unless otherwise noted)


Continued lava flow into sea; lava pond overflows; magma intrusion

Lava . . . continued to enter the ocean at two main sites through August (figure 79). By the end of the month, numerous breakouts from the tube system had reduced the volume of lava reaching the sea. Flows produced by major breakouts at ~180 and 340 m (600 and 1,100 ft) elevation spread over the W third of the lava field. Most remained on older lava, but a few lobes reached the field's W edge and ignited small brush fires in the remnants of the Royal Gardens subdivision. One flow from the breakout at 180 m reached 20 m elevation in early August.

Since at least January, a small lava pond has been continuously active in the bottom of Pu`u `O`o crater, covering ~20% of the crater floor on its E side. By April, the pond was circular and surrounded by levees. During the evening of 27 August, bright glow was visible over Pu`u `O`o, and a nearby seismometer recorded frequent bursts of higher amplitude tremor lasting 1-3 minutes. Overflights the next morning revealed that the pond had overflowed its levees, covering the entire crater floor with several meters of active lava that had a thin, frequently overturning, crust. Lava periodically drained back to its former level, remaining confined within the original pond until the next overflow. Similar activity continued through the end of the month. Crater depth remained roughly 80 m.

Seismicity in August included the upper East rift zone's third intrusive swarm since December 1990. More than 200 shallow summit microearthquakes were registered between 1100 and 1200 on 21 August. Earthquake counts quickly declined during the next hour, but elevationated levels of seismicity . . . continued through the next day. The largest concentration of events appeared to be centered just SE of the caldera, and very few occurred beyond Hiiaka crater, 4.5 km from the caldera rim. Most of the month's seismicity in the summit/upper east rift area occurred during the swarm.

Earthquake epicenters since December 1990 (figure 80) have been concentrated in several clusters, the largest of which were associated with the period's three intrusive episodes. The three swarms occurred in different portions of what geophysicists infer to be the same shallow (<5 km deep) structure between the summit and the East rift zone, suggesting a significant role for the summit in the current East rift eruption. During the early December swarm earthquakes were located from the summit roughly 6 km downrift (to Pauahi crater). The largest concentration of events was in the SE part of the caldera, perhaps extending a short distance into the rift zone (toward the Chain of Craters). The March activity occurred away from the summit, with the majority of located events between Pauahi and Mauna Ulu, roughly 3 km farther downrift. Following the early December seismicity and a long-period summit swarm late in the month, seismicity increased between the summit and Hiiaka crater. The same segment of the uppermost East rift zone has consistently shown low levels of shallow seismicity throughout Kupaianaha vent's post-1986 eruptive activity. After the March swarm, seismic activity along this rift segment appears to have increased further, and the August swarm was largely confined to this area.

Figure (see Caption) Figure 80. Plot of earthquake epicenters in Kilauea's summit, upper to middle East rift zone, and south flank areas, December 1990-11 September 1991. Some of the larger craters are labeled. The eruption's two currently active vents, Pu`u `O`o and Kupaianaha, are off the map ~3 and 6 km ENE of Napau Crater. Courtesy of HVO.

Geologic Background. Kilauea, which overlaps the E flank of the massive Mauna Loa shield volcano, has been Hawaii's most active volcano during historical time. Eruptions are prominent in Polynesian legends; written documentation extending back to only 1820 records frequent summit and flank lava flow eruptions that were interspersed with periods of long-term lava lake activity that lasted until 1924 at Halemaumau crater, within the summit caldera. The 3 x 5 km caldera was formed in several stages about 1500 years ago and during the 18th century; eruptions have also originated from the lengthy East and SW rift zones, which extend to the sea on both sides of the volcano. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1100 years old; 70% of the volcano's surface is younger than 600 years. A long-term eruption from the East rift zone that began in 1983 has produced lava flows covering more than 100 km2, destroying nearly 200 houses and adding new coastline to the island.

Information Contacts: T. Mattox and P. Okubo, HVO.


Langila (Papua New Guinea) — August 1991 Citation iconCite this Report

Langila

Papua New Guinea

5.525°S, 148.42°E; summit elev. 1330 m

All times are local (unless otherwise noted)


Ash clouds; incandescent tephra; lava flows in crater

"In August, Crater 3 frequently erupted moderate to strong, pale grey-brown ash and vapour clouds accompanied by weak to loud detonations, roaring or rumbling. The eruptions occurred at intervals of several minutes to a few hours. The emission clouds rose as much as 500 m above the crater. Dull to bright red crater glow was observed on the nights of 7-9, 12, and 13 August.

"During an aerial inspection on the 14th, two active vents were observed in a mound of lava filling Crater 3. The vents were ~5-10 m in diameter, 40 m apart and aligned approximately N-S. The N vent was more active and was filled with incandescent lava. The S vent was clogged with dark lava. Both vents released blue vapour. Lava had flowed eastward to form a short (70 m) lobe in the E part of the crater. A longer (~150 m) lobe of lava was present on the NE flank of Cone 3. This lobe was fresh, having a dark surface, and its source appeared to be a tube within the E lobe. The NE-flank flow was first observed on 13 August, and appeared to be inactive then. However, some activity of this flow had been evident the previous night when prolonged incandescence in this area and some movement of incandescent material were observed. Two other very small lava lobes (both inactive) were observed on the NW flank of Cone 3.

"Throughout the month, Crater 2 (roughly 200 m E of Crater 3) almost continuously emitted moderate amounts of pale grey-brown ash and vapour. This activity was accompanied by nearly continuous low roaring sounds. Occasional stronger explosions took place. Dull glow over the crater was observed on the nights of 7-9, 13, 22, 24, and 27 August. A 30-minute period of strong explosive activity on the night of 13 August resulted in a large volume of incandescent lava fragments being ejected onto the NE flank of Cone 2. Incandescent lava-fragment ejections from Crater 2 were also seen on the night of 20 August. A brief aerial view of the interior of Crater 2 on 14 August indicated that it remains funnel-shaped, with several benches. Detailed observation was prevented, however, by emissions of ash and vapour.

"The ash plume from the combined emissions of the craters was usually directed in a sector between NNE and NW. Fine ashfalls were recorded in coastal areas (9 km distant) on 1, 2, 6, and 12 August.

"Seismicity remained at a moderate to high level throughout the month. It appeared that most of the stronger seismicity was associated with events at Crater 3. The daily number of explosion earthquakes recorded by the summit station fluctuated between 20 and 70, with the largest totals of 40-70 events on 16, 25, and 30-31 August. Meanwhile, the remote station (9 km distant) recorded 0-29 events/day. Numerous low-amplitude, short-duration, tremor-like signals were produced by weaker explosions. Several periods of harmonic tremor were recorded but the source was not determined."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower eastern flank of the extinct Talawe volcano. Talawe is the highest volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila volcano was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the north and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit of Langila. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: B. Talai, C. McKee, and P. de Saint-Ours, RVO.


Ol Doinyo Lengai (Tanzania) — August 1991 Citation iconCite this Report

Ol Doinyo Lengai

Tanzania

2.764°S, 35.914°E; summit elev. 2962 m

All times are local (unless otherwise noted)


Carbonatite lava production continues on crater floor

Photographs taken . . . by D., M., and T. Peterson on 25 January showed few changes since late 1990. Lava flows of varying ages were evident on the crater floor, with the youngest (F25) extending N toward the crater wall from a hornito on the N flank of . . . T5/T9 (figure 22). Its dark brown color and clearly defined margins indicated that it may have been active during the Petersons' visit. Light gray-brown lava had spread from a source near vent T11, across the former saddle (M1M2) to the S wall of the crater, covering more than half of the floor of the former southern depression. Lava of similar age also covered much of the N part of the main crater.

M. Peterson returned . . . 29-30 March, and reported 10-15 minutes of lava production during the evening of the 30th from 2 or 3 vents on the N side of T5/T9, very close to the source of the freshest flow photographed on 25 January. A number of flows moved away from the vents, the longest advancing ~50 m. Flow widths averaged 1-2 m and thicknesses varied from 10 to 20 cm. Steam and sulfur fumes were issuing from several sources on the crater rim, walls, and floor. Older flows in the N part of the crater were dominantly pahoehoe but some aa lava was also observed. Flows entering the S depression were blocky and ~2/3 m thick.

Figure (see Caption) Figure 22. View SE across the crater floor of Ol Doinyo Lengai, 25 January 1991. A recent flow from vent T5/T9 is shown in black. Prepared by C. Nyamweru from a photo taken by the Peterson group.

Little fresh lava was evident on the dominantly pale gray to white crater floor during a visit by Benoit Wangermez on 6 May. A slightly darker flow covered most of the southern depression, showing that lava had advanced S since January from a source slightly NW of T11. Small flows around the base of T5/T9 (active in late March) did not look very young. One new light-colored zone (at M2) appeared to be a vent, currently inactive, that had formed since March.

When T. Peterson arrived at the crater rim on 28 June at about 1000, lava was flowing W from a new vent (T18) W of T5/T9. Activity had subsided 30 minutes later, and the level of lava in the vent had fallen 5 m. Heat was rising from older vents (T5/T9 and T14), while T11 had partially collapsed and looked like a "sulfur cave." Lava flows on the crater floor ranged from dark (fresh) to almost white.

A group led by Luigi Cantamessa climbed to the summit on 12 July. No effusive activity was evident, but black to grayish flows [were] perhaps 1-2 days old . . . . Fumarolic activity occurred from some small hornitos. Many fissures were seen; one extended E-W, parallel to the former saddle dividing the main and southern craters, and cut across the W rim, but was not visible on the volcano's outer flank.

Eruptive activity was very minor . . . on 9 August between 1000 and 1400. Hot, fresh, dark gray natrocarbonatite lava was found near the H6 vent complex (figure 23). Water poured on the lava boiled violently. The extent of other fresh lava flows was similar to that observed 4 days later (see below). A small hornito on the S side of H6 ejected 2-3-mm droplets of spatter. A frozen, but still fresh lava pool ~4 m in diameter was found ~2 m below the average elevation of M1's crater floor (a group of tourists and a local guide reported that vents H6 and M1 had been active 2 days earlier). Vent T5/T9 emitted hot colorless gas, while T11 exhaled SO2. Radial fissures on the W flank of the crater produced almost pure (>95%) CO2, with some SO2. Holes ~0.5-1 m across on the crater's W rim released hot, humid air with no detectable SO2 or CO2. These holes contained a variety of water-loving plants such as moss and algae. Gas compositions were measured with Dräger tubes.

Figure (see Caption) Figure 23. Sketch map of the crater floor of Ol Doinyo Lengai, 13 August 1991. Fresh lava is shown in black. Courtesy of Alain Catté.

Lava production from one vent complex was continuing during a summit climb by Alain Catté and others on 13 August. Irregular, weak, but clearly audible explosions occurred from the 4-5-m-high hornito complex H6, ejecting lava fragments horizontally to 10-15 m from two vents (E1 and E2). Weak effusive activity occurred from a site ([E4]) 5 m below the hornito complex. Young, chocolate-brown flows extended from its base in three directions atop older (>48 hours) whitish flows: ~10 m E; ~40 m NE; and > 100 m N, flowing around other small cones. Production of small flows accompanied vent E1's explosions from the initial observations at 0845 until its activity stopped at about 1000.

When clouds cleared at 1030, a very fluid lava flow 40-50 cm wide was emerging from neighboring vent E2. The flow quickly subdivided into many black lobes ~10 cm wide, with a consistency like lubricating oil. Within a few seconds, these formed channeled pahoehoe flows that turned to aa at their distal ends. Lava also formed tubes that carried it >100 m from the source. No lava temperatures were taken, but it was possible to place one's hand a few centimeters from an active flow, and to touch it after ~2 minutes of cooling. A cascade of lava ~10 cm wide began from a third vent (E3) on the hornito complex at about 1145. Vents E2 and E3 erupted simultaneously and showed parallel fluctuations in activity. Later . . . lava outflow from E2 occurred in a jet 2 m long.

At about noon, lava production resumed from the base of the hornito complex (at [E4]) bubbling out in a manner reminiscent of mud pots. It overflowed after ~45 minutes, gradually building a hornito that grew to 1 m height before activity ceased at about 1330. Above [E4], lava effusion from vent E3 stopped at 1230, emerging from a channel 2 m below in a violent, 3-m jet that reached the base of [E4], beginning to fill the area with lava. The outflow rate increased progressively, and lava had advanced 60 m W by the end of observations at about 1400. Lava production from the H6 complex had roughly quadrupled its size since . . . March.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: C. Nyamweru, St. Lawrence Univ; D. Peterson, M. Peterson, and T. Peterson, Arusha, Tanzania; B. Wangermez, Nairobi, Kenya; L. Cantamessa, Geo-decouverte, Switzerland; P. Vetsch, SVG, Switzerland; T. Dunai, R. Ragettli, K. Schenk-Wenger, and U. Ziegler, ETH Zürich, Switzerland; A. Catté, B. DeMarne, and P. Barois, LAVE.


Lokon-Empung (Indonesia) — August 1991 Citation iconCite this Report

Lokon-Empung

Indonesia

1.358°N, 124.792°E; summit elev. 1580 m

All times are local (unless otherwise noted)


Incandescent tephra cause fires that burn plantations

The press reported that renewed activity on 19 September ejected a plume to ~700 m. Incandescent tephra fell 500 m from the crater, starting fires that burned plantations in seven villages. No casualties were reported. As of the next morning, the eruption was continuing and VSI observers were recording accompanying earthquakes. VSI advised local authorities that residents of nearby villages should remain on alert, but an evacuation was not ordered.

Geologic Background. The twin volcanoes Lokon and Empung, rising about 800 m above the plain of Tondano, are among the most active volcanoes of Sulawesi. Lokon, the higher of the two peaks (whose summits are only 2 km apart), has a flat, craterless top. The morphologically younger Empung volcano to the NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century, but all subsequent eruptions have originated from Tompaluan, a 150 x 250 m wide double crater situated in the saddle between the two peaks. Historical eruptions have primarily produced small-to-moderate ash plumes that have occasionally damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred. A ridge extending WNW from Lokon includes Tatawiran and Tetempangan peak, 3 km away.

Information Contacts: VSI; UPI.


Malindang (Philippines) — August 1991 Citation iconCite this Report

Malindang

Philippines

8.22°N, 123.63°E; summit elev. 2404 m

All times are local (unless otherwise noted)


False report of eruption

Widely distributed reports of increased activity and up to 20,000 evacuees in mid-September proved false. Heavy cloud cover over the volcano and coincidental tectonic earthquakes prompted claims of an imminent eruption. PHIVOLCS scientists found no signs of activity, although they did locate a previously unknown geothermal area on a remote section of the volcano.

Geologic Background. The Pleistocene-to-Holocene Malindang stratovolcano, located on the western margin of Iligan Bay in north-central Mindanao, contains a small summit caldera. Legends record a large eruption from the 2404-m-high, dominantly basaltic-to-andesitic volcano in the past, although no historical eruptions are known (Salise et al., 1991). Reports of increased activity in 1991 at the time of tectonic earthquakes prompted widespread evacuations, but an eruption did not occur, although a previously unknown geothermal area was discovered.

Information Contacts: D. Sussman, Philippine Geothermal, Inc., Manila; Philippine Daily Inquirer and Manila Times, Manila; Reuters.


Manam (Papua New Guinea) — August 1991 Citation iconCite this Report

Manam

Papua New Guinea

4.08°S, 145.037°E; summit elev. 1807 m

All times are local (unless otherwise noted)


Occasional ash emission; lava dome

"Main Crater produced weak emissions of white vapour with low ash content on 1, 2, and 3 August. Blue vapour was visible on 8, 11, and 12 August and only white vapour during the last week of the month. There were no audible noises and no night glow was seen.

"The emissions from Southern Crater consisted of tenuous white vapour with occasional grey-brown ash clouds resulting in fine ashfalls on parts of the island. Occasional weak deep roaring and rumbling noises were heard 2-14 August and a weak red glow was observed around the crater mouth on the night of 7 August. An aerial inspection was carried out on 13 August. Southern Crater was partly filled with vapour but Main Crater was clear. The floor of Main Crater was occupied by a solid plug or mound of lava, at a level ~20 m below the lower (NE) part of the crater rim. White mofettes were released by numerous fumaroles around the base and lower walls of the crater. The crater floor was mostly covered by debris from the crater walls, but in the central area, the lava plug was visible over an area ~5 m in diameter, and consisted of steaming lava surrounded by small blocks and scoriae ejected during a stronger degassing phase. During the aerial inspection, emissions from Southern Crater were low-energy, thermally buoyant clouds, released fairly regularly at ~15-minute intervals.

"Seismicity was at a moderate level and tilt measurements showed a deflation of ~1.5 µrad since mid-August."

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical 1807-m-high basaltic-andesitic stratovolcano to its lower flanks. These "avalanche valleys" channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most historical eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent historical eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: B. Talai, C. McKee, and P. de Saint-Ours, RVO.


Marchena (Ecuador) — August 1991 Citation iconCite this Report

Marchena

Ecuador

0.33°N, 90.47°W; summit elev. 343 m

All times are local (unless otherwise noted)


First historical eruption

Marchena . . . started erupting on 25 September. The TOMS instrument aboard the Nimbus-7 satellite passed at about 1100 and sensed no SO2, but the next pass, at the same time on 26 September, mapped a 300-km plume to the SW with an SO2 content estimated to be close to 100 kt. High SO2 values immediately over the volcano indicated that the eruption was still vigorous at that time. On the following day the plume was nearly twice as long, but had almost vanished by the same time on 28 September. Weather satellite images during this period showed low cloud cover, but no conclusive indication of the volcanic plume. . . .

Geologic Background. The low shield volcano forming Marchena Island contains one of the largest calderas of the Galápagos Islands. The 6 x 7 km caldera and its outer flanks have been largely buried by a cluster of pyroclastic cones and associated lava flows. Its first historical eruption occurred in 1991. Other young lava flows, some of which may be only a few thousand, or even a few hundred years old, filled the caldera and flowed down its outer forested flanks, in some cases to the sea.

Information Contacts: A. Carrasco, Charles Darwin Research Station; S. Doiron, GSFC; SAB.


Pinatubo (Philippines) — August 1991 Citation iconCite this Report

Pinatubo

Philippines

15.13°N, 120.35°E; summit elev. 1486 m

All times are local (unless otherwise noted)


Secondary pyroclastic flows feed large ash columns; frequent mudflows; fewer explosions

Activity continued to decline through 15 September, with only three ash/steam emissions since about 25 August. Heavy monsoon rains triggered numerous mudflows and secondary explosions from the 15-16 June pyroclastic-flow deposits. Two large secondary pyroclastic flows occurred, producing associated ash clouds to 15 km height. The press reported continued fatalities from debris/mudflows and disease in evacuation camps, bringing the number of casualties attributed to the eruption to at least 740 by 20 September. Study of the June deposits has resulted in preliminary estimates of 7-11 km3 of material erupted.

5-11 August. Radar at Clark Air Base detected 13 ash/steam emissions rising to 4.5-13.5 km height; plumes were carried NE by the wind. Most RSAM peaks coincided with these emissions. The majority of seismicity was shallow (<=1 km depth), with magnitudes <1. Seven high-frequency earthquakes were felt at Clark Air Base.

12-18 August. Thirteen ash/steam emissions were detected, three with columns >15 km high (maximum 17.5 km). Wind carried the plumes ENE and NE, and ashfall was reported at Clark Air Base on 13 and 16 August. Ejection velocities ranged from about 300-900 m/min, similar to the ejection velocity on 25 June (estimated at about 450 m/min). A large secondary pyroclastic flow occurred sometime on 12-13 August, in the Marunot drainage on the NW flank. The flow was not observed, but satellite imagery was used to identify the deposits and estimate a deposit volume of 31 x 106 m3 (1.25 km2 areal coverage). The flow, ~10 km long, created a headwall scarp about 20 m high along a 240° arc in the primary pyroclastic-flow deposit source region. During aerial observations, the still-steaming secondary deposits could be differentiated from those of earlier pyroclastic flows by the absence of rills and dissected morphology.

Seismic energy release decreased notably from the previous week (figure 19), although the number of earthquakes remained about the same (102 recorded events/day compared to 95/day the week before). Several shocks were felt at Clark Air Base. RSAM peaks reflected high-frequency earthquakes generated by mudflows, and occasional long-period signals associated with ash/steam emissions from the caldera. Geologists suggested that small long-period events may also be related to secondary explosions from pyroclastic-flow deposits.

Figure (see Caption) Figure 19. Accumulated RSAM energy at Pinatubo, 28 July-18 August 1991. Courtesy of PHIVOLCS.

19-25 August. Ash/steam emissions averaging ~9.8 km high (maximum 15 km) were detected eight times during the week. Ash was carried E. Some may have originated from secondary explosions at the E flank (Sacobia valley) pyroclastic-flow deposits. Seismicity consisted mostly of high-frequency earthquakes (M < 1.0) centered below the caldera or ~3 km NW, at 0-18 km depths (figure 20). Four events (M 2-4) were felt at Clark Air Base, with intensities to IV (adapted Rossi-Forel scale). RSAM peaks coincided with the larger high-frequency earthquakes, and long-period events were associated with ash/steam emissions.

Figure (see Caption) Figure 20. Epicenters of 648 earthquakes recorded near Pinatubo, 19-25 August 1991. Courtesy of PHIVOLCS.

26 August-1 September. Only two ash/steam emissions were detected; plume heights ranged from about 10 to 16 km. Light ashfall occurred to 40 km SE (San Fernando) during secondary explosions that produced columns to 16 km. Ash related to these events caused poor visibility (300 m) on the highway between San Fernando and Angeles (25 km E of the volcano). The number of felt shocks (M < 4.2) increased to 17, with intensities to V (adapted Rossi-Forel scale). Multiple peaks in RSAM plots were due to mudflows, while single peaks were caused by long-period events associated with the two ash/steam emissions.

2-8 September. One ash/steam emission was detected (2 September), producing a 9-km plume that was carried W (highest portion) and NE (lower portion). Secondary explosions, three of which were recorded as low-amplitude, low-frequency earthquakes, generated ash clouds 2-4.5 km high. Geologists proposed that the heavy ashfall and 15-km-high ash column observed at 1400 on 4 September (figure 21) were from a secondary pyroclastic flow, whose fresh deposits were discovered two days later. The absence of a long-period earthquake coincident with the ash cloud suggested that it had not been generated by caldera explosions. The secondary pyroclastic-flow deposits about 3 km SSW of the caldera (in the upper Marella drainage) were estimated to be 1-2 km wide, and 4-6 km long, with a headwall scarp 15-25 m high. The deposit appeared very recent and seemed water-saturated. It was not known whether the ash cloud was generated purely by convection, or by phreatic explosions resulting from an encounter with water on the river bed. A helicopter overflight of the caldera on 6 September revealed no evidence of activity during the prior several days. Steaming was observed along the margins of the caldera and a bluish lake was present. No evidence of a lava dome was found.

Figure (see Caption) Figure 21. Visible and infrared image from the NOAA 11 polar-orbiting weather satellite on 4 September at about 1445, showing a large, 15-km-high ash cloud above Pinatubo believed to have been generated by a secondary pyroclastic flow. Courtesy of G. Stephens.

Recorded earthquakes averaged 88/day, similar to 89/day the previous week. The majority were of high-frequency, and geologists believed that they were caused by tectonic readjustments. Most of the few low-frequency signals coincided with observed secondary explosions. Seismicity remained shallow (about 38% at less than 2 km depth), centered beneath or NW of the caldera. Long-duration, high-frequency earthquakes corresponding to mudflows created peaks in RSAM plots. A magnitude 5.1 earthquake at 0627 on 5 September, centered ~17 km NNW (15.53°N, 120.31°E) at 10 km depth, was felt at Clark Air Base (intensity RF V).

On 4 September, due to the continued decrease in caldera activity, the volcanic alert was reduced from Level 5 (eruption in progress) to Level 3 (numerous magma-related earthquakes, fumaroles, and gas emission), and the danger zone radius was reduced from 20 to 10 km. The principal remaining hazards and their probable durations were identified (table 4).

Table 4. Principal hazards associated with Pinatubo following the 15-16 June 1991 eruption (as of 4 September 1991). Courtesy of PHIVOLCS.

Number Hazard Description Duration
1 Heavy rains may remobilize large volumes of loose pyroclastic materials on the upper slopes of Pinatubo, generating mudflows that will affect communities near drainage channels and in low-lying downstream areas. 2-5 years...for as long as large volumes of loose sediments on slopes are subjected to heavy rain.
2 Moderate to heavy rainstorms that do not generate mudflows will still transport extraordinarily large volumes of sediments to lowland areas. This sediment will fill up river-channel storage capacity, resulting in more frequent and severe floods in lowland areas. Years to decades.
3 Occasional phreatic explosions at the summit caldera can cause light to moderately heavy ashfall in downwind areas with possible damage to aircraft. Possible plume heights may vary from 5-20 km altitude. Months and possibly years, but probably lessening in intensity, heights attained, and recurrence with time.
4 Groundwater that percolates into the hot interiors of pyroclastic-flow deposits can cause steam explosions. Ejected ash and ballistic fragments can be hazardous to distances of hundreds of meters. Months to a year or more...until the interiors of pyroclastic flow deposits have sufficiently cooled.
5 As overall volcano-related seismicity decreases, earthquakes can still generate ground motions sufficiently strong to damage or destroy weak and/or unstable objects and structures. The distribution of these earthquakes probably will be broad and could cover the entire volcano and peripheral areas. Several months to a year.
6 Thick pyroclastic deposits may locally remobilize as secondary pyroclastic flows and threaten areas as much as 10km downslope. Earthquakes, heavy rainfall, and secondary explosions may serve as triggering factors. Weeks to months, dependng on the presence of metastable materials and the occurrence of triggering factors.

9-15 September. Although no ash/steam emissions were detected, ash clouds 2-10 km high were produced by secondary explosions. Vigorous steam emission was noted from the S side of the caldera, and the blue crater lake was still present during observations on 10 September. The average number of earthquakes decreased to 54 recorded daily, most centered ~2 km NW or 2 km S of the caldera, at <2 km and 5-10 km depths. The majority of events were M <2. RSAM and accumulated energy both showed decreases corresponding to the drop in seismicity. Multiple RSAM peaks coincided with mudflows, while single peaks were caused by moderate-sized earthquakes.

Debris flows. All of Pinatubo's major drainage systems experienced debris flows, ranging from mudflows to hyperconcentrated flows and floods. Numerous flows also occurred in more distant drainages in which significant quantities of tephra were deposited. To help alleviate hazards and to aid in studying debris-flow processes, rain gauges were installed, observation posts were set up at strategic locations along rivers, and cross sections were monitored at bridges. Timely warnings and evacuations considerably reduced the number of injuries and casualties. High rainfall (to > 30 cm/day) and still-hot pyroclastic-flow deposits generated numerous hot mudflows that deposited as much as several meters of material.

On the SE flank's Pasig-Potrero River, pyroclastic-flow deposits had formed a dam behind which a 1,000 x 600 m lake had formed. The lake drained on 7 September, causing muddy flash floods that reached 1.2 m high in about 5-10 minutes at Bacolar (35 km SE of the volcano). Press reports indicated that 800 homes were destroyed and seven people were confirmed dead. By 15 September, continued flooding and mudflows resulted in the deaths of 12 more people at Bacolar, where 45,000 of the 68,000 residents had fled.

News reports placed the death toll from the eruption, mud flows, and disease at more than 740 by 20 September [see also 16:9]. Of the fatalities in evacuation camps, an estimated 95% were Aeta tribesmen and 75% were children. The Aeta reportedly refused most medical assistance such as vaccinations.

Fieldwork on June eruptive products. Preliminary estimates have been calculated for pyroclastic-flow deposits and airfall tephra from the paroxysmal June eruptive activity. The bulk of the material erupted was found in pyroclastic flow deposits (5-7 km3); several drainage systems included more than 1 km3. An estimated 0.48 km3 of airfall tephra was deposited within the 15-cm isopach (table 5); the total volume of tephra-fall material erupted, including that deposited in the South China Sea or lost to the atmosphere, was believed to be between 2 and 4 km3. The total volume, therefore, is estimated as 7-11 km3 (roughly 3-5 km3 dense rock equivalent).

Table 5. Preliminary volume calculations (±10% error) of June 1991 eruptive products from Pinatubo. Total tephra deposit volume: 0.48 km3. Total pyroclastic-flow volume: 7.0 km3. Courtesy of PHIVOLCS.

Deposit Type Location Volume (km3)
Tephra 50 cm isopach 0.07
Tephra 40 cm isopach 0.03
Tephra 30 cm isopach 0.10
Tephra 25 cm isopach 0.01
Tephra 20 cm isopach 0.11
Tephra 15 cm isopach 0.16
 
Pyroclastic-flow O'Donnell drainage 1.0
Pyroclastic-flow Sacobia-Pasig-Abacan drainage 1.6
Pyroclastic-flow Marella drainage 1.3
Pyroclastic-flow Balin-Barquero-Maraunot-Bucao drainage 3.1

Geologic Background. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Information Contacts: R. Punongbayan, PHIVOLCS; K. Rodolfo, Pinatubo Lahar Hazards Taskforce, Univ of Illinois; W. Scott, USGS CVO; G. Stephens, NOAA/NESDIS; NEIC; AP; Reuters; UPI.


Poas (Costa Rica) — August 1991 Citation iconCite this Report

Poas

Costa Rica

10.2°N, 84.233°W; summit elev. 2708 m

All times are local (unless otherwise noted)


Crater lake rises, covering fumaroles; low-frequency seismicity

In August, the crater lake grew to cover all crater fumaroles, while fumarolic activity continued at levels considered "normal" for the volcano. The yearly total of recorded microearthquakes (almost all of low frequency) exceeded 32,500 by the end of the month (figure 40), a decrease from 1990.

Figure (see Caption) Figure 40. Monthly number of earthquakes at Poás, January-August 1991. Courtesy of ICE.

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: R. Barquero and G. Soto, ICE; M. Fernández, H. Flores, and S. Paniagua, UCR.


Raung (Indonesia) — August 1991 Citation iconCite this Report

Raung

Indonesia

8.119°S, 114.056°E; summit elev. 3260 m

All times are local (unless otherwise noted)


Dense plumes

The crew of Qantas flight 41 (Sydney-Jakarta) observed a very dense black plume emerging intermittently from a flank vent on 10 September at 1508. The plume was drifting N at ~6 km altitude, well below the aircraft's altitude of nearly 12 km. A voluminous, dense, mostly white plume with small pulses of ash in its center was observed from a commercial flight two days later.

Geologic Background. Raung, one of Java's most active volcanoes, is a massive stratovolcano in easternmost Java that was constructed SW of the rim of Ijen caldera. The unvegetated summit is truncated by a dramatic steep-walled, 2-km-wide caldera that has been the site of frequent historical eruptions. A prehistoric collapse of Gunung Gadung on the W flank produced a large debris avalanche that traveled 79 km, reaching nearly to the Indian Ocean. Raung contains several centers constructed along a NE-SW line, with Gunung Suket and Gunung Gadung stratovolcanoes being located to the NE and W, respectively.

Information Contacts: ICAO; J. Post, SI.


Rincon de la Vieja (Costa Rica) — August 1991 Citation iconCite this Report

Rincon de la Vieja

Costa Rica

10.83°N, 85.324°W; summit elev. 1916 m

All times are local (unless otherwise noted)


Explosions eject ash and blocks

After reports of strong sulfur odors, geologists visited the summit area on 28-30 August. A sulfurous odor was noted at Copelares on the S flank (1,400 m elevation), during the evening of 28 August. An explosion was heard at 0151 the next morning, followed several seconds later by the sound of falling material. Examination of 29 August records from a seismic station 6 km SW of the crater (RIN3) showed that a small earthquake occurred at 0148:47, then a larger earthquake sequence lasting 7.5 minutes began at 0151:40, coinciding with the first audible explosion. As the ascent continued later that morning, traces of fresh ash were observed beginning at about 1,500 m elevation. Large quantities of ash and blocks, ranging from 15 to 75 cm in diameter, were found deposited in the summit area. Impact craters reached 120 cm in diameter and 35 cm deep.

Bad weather obscured the view of the crater floor, but several explosions were heard, and the largest, at 0930, rained very wet ash on the scientists. Near the crater, the smell of sulfur was very strong, making breathing difficult and stinging the eyes. Nearby vegetation was partially or completely dead. Rain collected at Copelares had a pH of 4.1.

On 30 August, scientists visited Ríos Azul and Pénjamo, which flow down the N flank from the crater area. Both rivers were gray-white with suspended sediment, which was also visible, but in lower concentrations, in the Ríos Colorado and Blanco on the S and SE flanks.

[On 6 September, strong fumarolic activity (jet engine noise) was seen in the active crater. During explosive events of May-August 1991 the ejecta was mainly composed of gray mud (sulfide-rich), lithics, and bread-crust bombs (~10% by volume).]

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge that was constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of 1916-m-high Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A plinian eruption producing the 0.25 km3 Río Blanca tephra about 3500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: J. Barquero and E. Fernández, OVSICORI; R. Barquero and G. Soto, ICE; Mario Fernández, Héctor Flores, and Sergio Paniagua, Univ. de Costa Rica.


Ruapehu (New Zealand) — August 1991 Citation iconCite this Report

Ruapehu

New Zealand

39.28°S, 175.57°E; summit elev. 2797 m

All times are local (unless otherwise noted)


Lake temperature rises; possible minor eruptions

A brief period of strong heating in Crater Lake was accompanied by small volcanic earthquakes and possibly by minor eruptions. Continuously recorded lake temperature data showed a gradual decline to 16°C by mid-June, then little change until a sharp increase began about 1 July. Temperatures reached 24.4°C on the 18th before declining again to 13° by late August. A series of small volcanic earthquakes occurred 5-14 July, none exceeding M 1.8.

Severe winter weather limited observations near the time of the increased activity, although the lake appeared normal on 11 July. When briefly observed on 12 August, evidence of 1-2 m of surging was visible under fresh (about 10 August) snow around the lake margin. More detailed observations during fieldwork 27 and 29 August revealed dirty, ash-covered ice under fresh snow 1-2 m above lake level, and widening of the lake's outlet channel by previous strong outflow or surging. No clear patterns were evident in summit-area deformation data.

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The 110 km3 dominantly andesitic volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake, is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.

Information Contacts: P. Otway, DSIR Wairakei.


Nevado del Ruiz (Colombia) — August 1991 Citation iconCite this Report

Nevado del Ruiz

Colombia

4.892°N, 75.324°W; summit elev. 5279 m

All times are local (unless otherwise noted)


Seismicity, deformation, and gas emission remain unchanged

Seismicity remained at low levels in August, with earthquakes mainly W and N of the crater at 0-5 km depths. Tremor episodes were brief and of low energy. Deformation showed no significant changes. The monthly average SO2 flux was 1,135 t/d, similar to July.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.


Santa Maria (Guatemala) — August 1991 Citation iconCite this Report

Santa Maria

Guatemala

14.757°N, 91.552°W; summit elev. 3745 m

All times are local (unless otherwise noted)


Continued explosions and block lava production

During a brief visit on 11 September, vertical explosions occurred hourly, producing plumes to about 1200 m height. The block lava flow erupting from the E summit of Caliente continued to flow down to the Río Nima II.

Geologic Background. Symmetrical, forest-covered Santa María volcano is one of the most prominent of a chain of large stratovolcanoes that rises dramatically above the Pacific coastal plain of Guatemala. The stratovolcano has a sharp-topped, conical profile that is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four westward-younging vents, the most recent of which is Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: W.I. Rose, Michigan Technological Univ.


Stromboli (Italy) — August 1991 Citation iconCite this Report

Stromboli

Italy

38.789°N, 15.213°E; summit elev. 924 m

All times are local (unless otherwise noted)


Continued moderate explosive activity; gas data

Explosive activity was restricted to crater C1 (NE part of the summit area; figure 17) during 9 August fieldwork (by F. Iacop, Institute of Earth Sciences, Univ of Udine). C1's central cone ejected hot tephra at ~20-minute intervals, and as a result, it had grown more rapidly than the crater's other two active cones. Glow from two small radial fissures in crater C2 was clearly visible at night. Sustained noisy gas emissions occurred about once an hour. Volcano guides had reported that activity was concentrated in crater C3 (SW part of the summit area), but at its cone 1 only hot vapor emission was occurring, from two vents, on 9 August. Rare explosions, mostly ejecting tephra, took place at bocca 4. The average number of recorded earthquakes remained near the normal value of 6/hour in July, declining below that level in the month's last week (figure 18). Average tremor amplitude also remained relatively constant through the end of July, while large shocks nearly disappeared after a peak on 29 June (figure 19). [see 16:09 for 28-29 August observations].

Figure (see Caption) Figure 17. Active craters at Stromboli as seen from the somma, 6 September 1991. Crosses mark small vents active during the 6 September fieldwork. Courtesy of the Société Volcanologique Européenne.
Figure (see Caption) Figure 18. Average number of explosion events/hour at Stromboli, 22 June-31 July 1991. The mean value for the period is shown. Courtesy of M. Riuscetti.
Figure (see Caption) Figure 19. Number of seismometer-saturating events/day (lower curve) and average daily tremor amplitude (upper curve) at Stromboli, 22 June-31 July 1991. Courtesy of M. Riuscetti.

Moderate activity was observed in early September, with explosive episodes about every 15 minutes at crater C3 and roughly hourly at C1. Activity increased in the 3 hours of observations after 2300 on 6 September, with many moderate to strong explosions from the SW part of C3. Ejections of incandescent bombs and scoria sometimes lasted several minutes. Thick white vapor plumes rose from C2 and a small cone in its center, while blue SO2-rich plumes emerged from several other vents. Explosions from C1 were vigorous, ejecting glowing fragments and dark brown columns that rose 200 m above the crater. C3's smaller explosive bursts, consisting of tephra-poor incandescent gas jets, were usually preceded by comparatively brief periods of increasing, noisy gas puffs; larger explosions that ejected a higher proportion of tephra followed longer intervals, with fewer or no precursory gas puffs. Geologists attributed this pattern to intermittent closure (by cooling) of the lava-filled conduits to gas-bubble rise from the underlying magma body, allowing higher pressure to build at depth.

Airborne COSPEC measurements by an Italian-French cooperative program in May-July indicated a total SO2 flux somewhat lower than that measured by the same means in 1980 and 1984 (1,000 ± 200 t/d average; Allard and others, in press), consistent with the current moderate activity. Geologists concluded that combined with microprobe determination of the initial and residual sulfur content of Stromboli's lava, the SO2 flux data require the degassing of 0.1 km3/year (average) of magma, three orders of magnitude more than the co-erupted volume. Thus, gas output is essentially derived from magma stored within the volcano. To assess the amount of diffuse magmatic degassing through the volcanic pile, other than from the craters, infrared mass spectrometric profiling of CO2 concentrations in the ground began on 11 September. High CO2 levels (80-90%), associated with subsurface thermal anomalies, were found to characterize the Pizzo sopra La Fossa crater terrace (at the summit rim, SE of the active craters). Concentrations gradually decreased toward the rim of this former crater, and no CO2 anomaly was detected in outer areas to the S (down to the Vancori rampart).

Reference. Allard, P., Carbonelle, J., Le Bronec, J., Metrich, N., and Zetwoog, P., Volatile flux and magma degassing budget at Stromboli volcano: Geophysical Research Letters, in review.

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period from about 13,000 to 5000 years ago was followed by formation of the modern edifice. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5000 years ago as a result of the most recent of a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: M. Riuscetti, Univ di Udine; Patrick Allard, CNRS-CEA, France; J.C. Baubron, BRGM, France; H. Gaudru and Rolf Haubrichs, SVE, Switzerland; Yvonne Miller, Univ de Genève, Switzerland.


Unzendake (Japan) — August 1991 Citation iconCite this Report

Unzendake

Japan

32.761°N, 130.299°E; summit elev. 1483 m

All times are local (unless otherwise noted)


Continued lava dome extrusion and pyroclastic flows

Lava extrusion continued at Jigoku-ato crater through mid-September, generating destructive pyroclastic flows that advanced down two valleys. More than 12,000 people remained evacuated and no new casualties were reported.

A summit seismic swarm that began 11 August peaked 12-13 August (figure 29), then gradually declined through the 19th. Incandescent block ejection was seen between 0000 and 0200 on 12 August, followed by continuous ash emission through the day. The number of seismically detected pyroclastic flows from the lava dome decreased suddenly to a few events daily on 12 August. A new lava dome, first recognized from the air on 13 August, emerged W of the former dome, and began to produce pyroclastic flows on 25 August. Pyroclastic flows had previously traveled down the Mizunashi River valley but those from the new dome (C dome; see below) moved ENE down the Oshigatani Valley, which extends N of and parallel to the Mizunashi, then joins it several kilometers downstream. Some of the larger pyroclastic flows from the new dome advanced 3 km down the Oshigatani valley from late August through mid-September, and pyroclastic surges burned vegetation. The mayor of Shimabara city ordered the evacuation of about 500 people from an area (Senbongi) 3.5 km NE of the dome on 31 August. Frequent pyroclastic flows during the afternoon of 3 September included one of about 1 x 105 m3 volume that advanced down the Oshigatani Valley at 1611. The accompanying cloud rose about 1,500 m and ash fell to the N part of Shimabara city. Ashfalls from pyroclastic flow elutriation clouds disrupted traffic around Shimabara city throughout the following day; the cloud from a flow at 1311 was 2,500 m high.

Figure (see Caption) Figure 29. Daily numbers of earthquakes (top), tremor episodes (middle), and pyroclastic-flow events (bottom) recorded at Unzen, 1 May-20 September 1991. Courtesy of JMA.

Another seismic swarm began beneath the crater on 6 September, and a pyroclastic flow that evening at 2121 advanced about 3.5 km down the Oshigatani Valley. Hypocenters and seismic wave characteristics were similar to those of mid-August, although the September swarm was more vigorous.

By 12 September, the lava dome had broken into numerous small blocks. Seismic activity declined through 14 September but increased again on the 15th. Seismometers near the summit began to record larger pyroclastic flows, with longer durations than any since 8 June, on 15 September at 1644 (150 seconds) followed by others at 1759 (120 seconds), 1842 (360 seconds), and the largest at 1854 (670 seconds). The latter moved down the Oshigatani valley, entered the Mizunashi valley, and continued to within 500 m of highway 57, a total of 5.5 km. The main body of the pyroclastic flow turned east into the Mizunashi valley, where it damaged 50 houses in Shimabara city, but the pyroclastic surge continued about 800 m southward, destroying 26 houses and 74 other buildings including those of a primary school (in Onokoba district, Fukae town). All of the affected area had previously been evacuated, so there were no casualties. The largest pyroclastic flow was associated with the collapse of a section of the new lava dome about 250 m wide, 300 m long, and 50 m thick, a volume exceeding 3 x 106 m3. This is about 20% of the total volume of lava domes erupted to date, and 3 times the volume of material removed by the 8 June pyroclastic flow. Two days later, a new lobe had grown to 100 x 200 m and 30 m high (0.3 x 106 m3/day), about twice the June-August extrusion rate (see below).

A total of 292 pyroclastic-flow events was recorded in August, down from 326 in July, but the more frequent episodes toward mid-September raised that month's total to 310 as of the 17th. September earthquake counts had reached 2075 through the 17th, up from 559 in August and 133 in July.

The following, from Setsuya Nakada, describes eruption products through early September.

The size and frequency of pyroclastic flows had decreased until July, and travel distances were almost always <2 km. However, collapse episodes from the E lava dome remained frequent and lava blocks had filled the narrow headwaters of the Mizunashi River, along which the 3 and 8 June pyroclastic flows had descended. As a result, cliffs along the valley disappeared, and valley-fill deposits (talus) became thick enough to act as a cushion to soften the shock of falling blocks. The E dome flowed southeastward on the valley-fill deposits. After the end of June, the horseshoe-shaped depression had filled with dome materials, and lava blocks began to fall northeastward onto the floor of Myoken caldera (figure 30). They filled the E end of the floor with talus, which overflowed the caldera rim at the end of July. Lava blocks then fell down the E and NE flanks as pyroclastic flows and their paths widened northeastward. Some reached the N bank of the Mizunashi River. The E margin of the E dome widened; because the NE slope under the dome was steeper than the SE slope, the northern half of the E dome migrated northeastward, while the southern half did not move and solidified. By the middle of August, the caldera rim NE of the dome had been eroded away by the falling lava blocks.

Figure (see Caption) Figure 30. Sketch map of Unzen's lava dome, 8 September 1991. Courtesy of Setsuya Nakada.

At the beginning of August, the ash-laden plume from the small vent at the northern base of the remnant W dome became stronger, and new lava was extruded on the western part of the E dome. On 5 August, many bubbles were observed coming from an old water-filled crater near the W dome. The small explosions that took place from the W dome on 12 August (see above) enlarged the vent to 20 m across and built a tuff cone around it. The E dome temporarily thickened for a few days prior to the new lava extrusion; the western part of the E dome, just above the former Jigoku-ato Crater, had swelled vertically. By the time new lava appeared 13 August, magma supply into the E dome had stopped, since the E dome did not lengthen and the surface of the dome did not move eastward (figure 31). It was difficult to accurately estimate the change in magma supply rate; talus and pyroclastic flows were deposited over an extensive area with irregular topography, which causes difficulties in calculating volumes of talus plus pyroclastic deposits.

Figure (see Caption) Figure 31. Tracings of photographs from a fixed point about 4.4 km from Unzen's E lava dome, illustrating its growth 10 July-20 August 1991. Courtesy of Setsuya Nakada.

At the end of August, the new dome (central, or C dome) was 375 m long, 275 m wide, and 60-80 m high. The C dome grew eastward and northeastward, keeping a constant thickness. It covered the E dome and talus, plus a part of the old volcanic edifice, which was bulldozed by the growing dome from the former crater wall to the caldera rim. Talus also formed on the E dome. At the end of August, the volume of C dome was about 4 x 106 m3 and the total volume of the domes was about 12 x 106 m3. The resulting dome growth rate is about 0.15 x 106 m3/day for 8 June-28 August.

Lava blocks fell down the E and NE margins of C dome into the Oshigatani Valley, forming pyroclastic flows beginning 25 August. The upstream area of the valley was the source area for lahars on 30 June. The pyroclastic flows traveled a maximum distance of 3 km from the dome, and had associated ash-cloud surge and seared zones like those of 3 and 8 June (figure 32). Flows moving down the Oshigatani Valley changed course southeastward when they encountered a high point dividing the valley and a residential area. Ash-cloud surges climbed the barrier, burning or searing trees, but block-and-ash flows did not. The devastated area was widest for pyroclastic flows that took place within the first week. By mid-September, Oshigatani Valley had been almost filled by pyroclastic-flow deposits.

Figure (see Caption) Figure 32. Map showing the distribution of pyroclastic flows from Unzen as of 9 September 1991. Deposits from lahars, which occurred mainly on 30 June, are omitted. Courtesy of Setsuya Nakada.

Average speeds of pyroclastic flows were estimated using travel distances observed by Ground Self-Defense Force radar and durations of tremor signals. The higher the average speed of a pyroclastic flow, the longer its travel distance: about 100 km/hour for flows reaching 3 km distance and 50 km/hour for flows 1 km long. The average speed of a pyroclastic flow at the end of August was estimated at 93 km/hour using the time lag between the start of the tremor signal and the time when the seismometer was broken by the flow.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: JMA; S. Nakada, Kyushu Univ; M. Takahashi, SI; Yomiuri Shinbun, Tokyo.


Villarrica (Chile) — August 1991 Citation iconCite this Report

Villarrica

Chile

39.42°S, 71.93°W; summit elev. 2847 m

All times are local (unless otherwise noted)


Weak explosions

An increase in fumarolic activity and weak explosions were observed in the crater during August-September. On 26 August, water in a nearby river (Río Carmelito) was cloudy and the river level abnormally high. Four days later, on 30 August, small ash emissions and continuous explosions were observed from 1430 to 1500, followed by a strong explosion at 1506. A weak emission of gray ash and a white gas plume 1 km high were observed on 17 September. Seismicity was at normal levels for the volcano.

Geologic Background. Glacier-clad Villarrica, one of Chile's most active volcanoes, rises above the lake and town of the same name. It is the westernmost of three large stratovolcanoes that trend perpendicular to the Andean chain. A 6-km-wide caldera formed during the late Pleistocene. A 2-km-wide caldera that formed about 3500 years ago is located at the base of the presently active, dominantly basaltic to basaltic-andesitic cone at the NW margin of the Pleistocene caldera. More than 30 scoria cones and fissure vents dot the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Historical eruptions, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: G. Fuentealba and P. Riffo, Univ de la Frontera.


White Island (New Zealand) — August 1991 Citation iconCite this Report

White Island

New Zealand

37.52°S, 177.18°E; summit elev. 321 m

All times are local (unless otherwise noted)


Tephra emission; shock waves in crater

Emission of gas/tephra columns from May 91 vent continued through August. During early-August helicopter overflights, R. Fleming noted flashes and strong low-frequency detonations as a hot, dilute eruption column rose from the vent. Crumbly white lithic blocks and lapilli with rare juvenile scoriae had been deposited nearby. Larger-than-normal plumes were often visible from the North Island coast, roughly 50 km away.

During fieldwork 28-29 August, a convoluting pink-brown column was emitted from May 91 vent. It contained very little ash and no evident incandescent material. Visible shock waves emerged from the vent every few seconds as "flashing arcs," lighting clouds above with a flickering glow like that from a poorly-functioning fluorescent tube. The strongest shock waves were manifested as an instantaneous displacement of the plume at the vent, and could be felt 150 m away. Some could be seen to bounce off the crater walls and travel back through the clouds. The shock waves did not seem to affect the rate of plume emission. The activity was accompanied by dull booming and sloshing noises, and occasional sharp detonations. The sloshing sounds were much like those heard in 1988 at Yasur (Vanuatu), where large gas bubbles were bursting through the surface of an active lava lake. Geologists noted that the activity at May 91 vent was consistent with similar gas-bubble discharge through a liquid magma column.

About 200 mm of coarse and fine ash had been deposited just N of May 91 vent since the previous fieldwork on 27 May. Little new ash was evident elsewhere on the main crater floor, but small (< 0.3 m) lithic blocks and their impact craters were found >200 m SE of the vent and to its W. Scarce, widely scattered scoria bombs, most 0.1-0.2 m across but some reaching 0.3 m, were found on top of the May ash, with only a light ash coating. The bombs seemed most abundant a few hundred meters SE-NE of the vent. They had highly vesiculated interiors of black glass with large pyroxene and plagioclase phenocrysts. Internal vesicles were up to 30 mm across, but decreased rapidly to sub-millimeter size toward the surface.

The pattern of deformation between late May and late August was similar to that of the previous 3 months. Strong subsidence at roughly double the previous rate continued to be centered SE of May 91 vent, while relative inflation persisted ~200 m farther SE. A new zone of inflation was measured E of Noisy Nellie fumarole (NE of May 91 vent). Minor deformation associated with activity at May 91 vent is unlikely to be detected, as the nearest part of the levelling network is 100 m away. Most fumarole temperatures had changed little since May, although values at Noisy Nellie had increased from 240 to 411°C.

The volcano had remained seismically quiet until mid-June, when B-type events became more common, continuing at rates of 2-7/day through the end of the month. Very weak volcanic tremor was sometimes visible on seismic records. A sequence of >45 tectonic earthquakes (to ML 3.7) occurred near White Island 1-2 July. A- and B-type events increased markedly on 7 July, accompanied by a small increase in background volcanic tremor amplitude. E-type eruption earthquakes were recorded on 1, 7, and 11 July. Seismicity had declined by 15 July, but a 3-day swarm of >200 A-type events began on 20 July. Significant volcanic tremor also resumed and continued through mid-August, increasing again 21-28 August. Tremor varied from a nearly pure 1.8 Hz signal to a complex pattern with spectral peaks to 8 Hz. A-type events did not occur daily in August, but often numbered 8-10/day. B-type events were very rare after 24 July. E-type eruption shocks were recorded on 14, 15, 19, 20, 23, 27, 29, and 30 August.

Geologic Background. Uninhabited 2 x 2.4 km White Island, one of New Zealand's most active volcanoes, is the emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes; the summit crater appears to be breached to the SE, because the shoreline corresponds to the level of several notches in the SE crater wall. Volckner Rocks, four sea stacks that are remnants of a lava dome, lie 5 km NNE. Intermittent moderate phreatomagmatic and strombolian eruptions have occurred throughout the short historical period beginning in 1826, but its activity also forms a prominent part of Maori legends. Formation of many new vents during the 19th and 20th centuries has produced rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project.

Information Contacts: B. Houghton, I. Nairn, and B. Scott, DSIR Geology & Geophysics, Rotorua.

Atmospheric Effects

The enormous aerosol cloud from the March-April 1982 eruption of Mexico's El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin. Descriptions of the initial dispersal of major eruption clouds remain with the individual eruption reports, but observations of long-term stratospheric aerosol loading will be found in this section.

View Atmospheric Effects Reports

Special Announcements

Special announcements of various kinds and obituaries.

View Special Announcements Reports

Additional Reports

Reports are sometimes published that are not related to a Holocene volcano. These might include observations of a Pleistocene volcano, earthquake swarms, or floating pumice. Reports are also sometimes published in which the source of the activity is unknown or the report is determined to be false. All of these types of additional reports are listed below by subregion and subject.

Kermadec Islands


Floating Pumice (Kermadec Islands)

1986 Submarine Explosion


Tonga Islands


Floating Pumice (Tonga)


Fiji Islands


Floating Pumice (Fiji)


Andaman Islands


False Report of Andaman Islands Eruptions


Sangihe Islands


1968 Northern Celebes Earthquake


Southeast Asia


Pumice Raft (South China Sea)

Land Subsidence near Ham Rong


Ryukyu Islands and Kyushu


Pumice Rafts (Ryukyu Islands)


Izu, Volcano, and Mariana Islands


Acoustic Signals in 1996 from Unknown Source

Acoustic Signals in 1999-2000 from Unknown Source


Kuril Islands


Possible 1988 Eruption Plume


Aleutian Islands


Possible 1986 Eruption Plume


Mexico


False Report of New Volcano


Nicaragua


Apoyo


Colombia


La Lorenza Mud Volcano


Pacific Ocean (Chilean Islands)


False Report of Submarine Volcanism


Central Chile and Argentina


Estero de Parraguirre


West Indies


Mid-Cayman Spreading Center


Atlantic Ocean (northern)


Northern Reykjanes Ridge


Azores


Azores-Gibraltar Fracture Zone


Antarctica and South Sandwich Islands


Jun Jaegyu

East Scotia Ridge


Additional Reports (database)

08/1997 (BGVN 22:08) False Report of Mount Pinokis Eruption

False report of volcanism intended to exclude would-be gold miners

12/1997 (BGVN 22:12) False Report of Somalia Eruption

Press reports of Somalia's first historical eruption were likely in error

11/1999 (BGVN 24:11) False Report of Sea of Marmara Eruption

UFO adherent claims new volcano in Sea of Marmara

05/2003 (BGVN 28:05) Har-Togoo

Fumaroles and minor seismicity since October 2002

12/2005 (BGVN 30:12) Elgon

False report of activity; confusion caused by burning dung in a lava tube



False Report of Mount Pinokis Eruption (Philippines) — August 1997

False Report of Mount Pinokis Eruption

Philippines

7.975°N, 123.23°E; summit elev. 1510 m

All times are local (unless otherwise noted)


False report of volcanism intended to exclude would-be gold miners

In discussing the week ending on 12 September, "Earthweek" (Newman, 1997) incorrectly claimed that a volcano named "Mount Pinukis" had erupted. Widely read in the US, the dramatic Earthweek report described terrified farmers and a black mushroom cloud that resembled a nuclear explosion. The mountain's location was given as "200 km E of Zamboanga City," a spot well into the sea. The purported eruption had received mention in a Manila Bulletin newspaper report nine days earlier, on 4 September. Their comparatively understated report said that a local police director had disclosed that residents had seen a dormant volcano showing signs of activity.

In response to these news reports Emmanuel Ramos of the Philippine Institute of Volcanology and Seismology (PHIVOLCS) sent a reply on 17 September. PHIVOLCS staff had initially heard that there were some 12 alleged families who fled the mountain and sought shelter in the lowlands. A PHIVOLCS investigation team later found that the reported "families" were actually individuals seeking respite from some politically motivated harassment. The story seems to have stemmed from a local gold rush and an influential politician who wanted to use volcanism as a ploy to exclude residents. PHIVOLCS concluded that no volcanic activity had occurred. They also added that this finding disappointed local politicians but was much welcomed by the residents.

PHIVOLCS spelled the mountain's name as "Pinokis" and from their report it seems that it might be an inactive volcano. There is no known Holocene volcano with a similar name (Simkin and Siebert, 1994). No similar names (Pinokis, Pinukis, Pinakis, etc.) were found listed in the National Imagery and Mapping Agency GEOnet Names Server (http://geonames.nga.mil/gns/html/index.html), a searchable database of 3.3 million non-US geographic-feature names.

The Manila Bulletin report suggested that Pinokis resides on the Zamboanga Peninsula. The Peninsula lies on Mindanao Island's extreme W side where it bounds the Moro Gulf, an arm of the Celebes Sea. The mountainous Peninsula trends NNE-SSW and contains peaks with summit elevations near 1,300 m. Zamboanga City sits at the extreme end of the Peninsula and operates both a major seaport and an international airport.

[Later investigation found that Mt. Pinokis is located in the Lison Valley on the Zamboanga Peninsula, about 170 km NE of Zamboanga City and 30 km NW of Pagadian City. It is adjacent to the two peaks of the Susong Dalaga (Maiden's Breast) and near Mt. Sugarloaf.]

References. Newman, S., 1997, Earthweek, a diary of the planet (week ending 12 September): syndicated newspaper column (URL: http://www.earthweek.com/).

Manila Bulletin, 4 Sept. 1997, Dante's Peak (URL: http://www.mb.com.ph/).

Simkin, T., and Siebert, L., 1994, Volcanoes of the world, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Information Contacts: Emmanuel G. Ramos, Deputy Director, Philippine Institute of Volcanology and Seismology, Department of Science and Technology, PHIVOLCS Building, C. P. Garcia Ave., University of the Philippines, Diliman campus, Quezon City, Philippines.


False Report of Somalia Eruption (Somalia) — December 1997

False Report of Somalia Eruption

Somalia

3.25°N, 41.667°E; summit elev. 500 m

All times are local (unless otherwise noted)


Press reports of Somalia's first historical eruption were likely in error

Xinhua News Agency filed a news report on 27 February under the headline "Volcano erupts in Somalia" but the veracity of the story now appears doubtful. The report disclosed the volcano's location as on the W side of the Gedo region, an area along the Ethiopian border just NE of Kenya. The report had relied on the commissioner of the town of Bohol Garas (a settlement described as 40 km NE of the main Al-Itihad headquarters of Luq town) and some or all of the information was relayed by journalists through VHF radio. The report claimed the disaster "wounded six herdsmen" and "claimed the lives of 290 goats grazing near the mountain when the incident took place." Further descriptions included such statements as "the volcano which erupted two days ago [25 February] has melted down the rocks and sand and spread . . . ."

Giday WoldeGabriel returned from three weeks of geological fieldwork in SW Ethiopia, near the Kenyan border, on 25 August. During his time there he inquired of many people, including geologists, if they had heard of a Somalian eruption in the Gedo area; no one had heard of the event. WoldeGabriel stated that he felt the news report could have described an old mine or bomb exploding. Heavy fighting took place in the Gedo region during the Ethio-Somalian war of 1977. Somalia lacks an embassy in Washington DC; when asked during late August, Ayalaw Yiman, an Ethiopian embassy staff member in Washington DC also lacked any knowledge of a Somalian eruption.

A Somalian eruption would be significant since the closest known Holocene volcanoes occur in the central Ethiopian segment of the East African rift system S of Addis Ababa, ~500 km NW of the Gedo area. These Ethiopian rift volcanoes include volcanic fields, shield volcanoes, cinder cones, and stratovolcanoes.

Information Contacts: Xinhua News Agency, 5 Sharp Street West, Wanchai, Hong Kong; Giday WoldeGabriel, EES-1/MS D462, Geology-Geochemistry Group, Los Alamos National Laboratory, Los Alamos, NM 87545; Ayalaw Yiman, Ethiopian Embassy, 2134 Kalorama Rd. NW, Washington DC 20008.


False Report of Sea of Marmara Eruption (Turkey) — November 1999

False Report of Sea of Marmara Eruption

Turkey

40.683°N, 29.1°E; summit elev. 0 m

All times are local (unless otherwise noted)


UFO adherent claims new volcano in Sea of Marmara

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Information Contacts: Erol Erkmen, Tuvpo Project Alp.


Har-Togoo (Mongolia) — May 2003

Har-Togoo

Mongolia

48.831°N, 101.626°E; summit elev. 1675 m

All times are local (unless otherwise noted)


Fumaroles and minor seismicity since October 2002

In December 2002 information appeared in Mongolian and Russian newspapers and on national TV that a volcano in Central Mongolia, the Har-Togoo volcano, was producing white vapors and constant acoustic noise. Because of the potential hazard posed to two nearby settlements, mainly with regard to potential blocking of rivers, the Director of the Research Center of Astronomy and Geophysics of the Mongolian Academy of Sciences, Dr. Bekhtur, organized a scientific expedition to the volcano on 19-20 March 2003. The scientific team also included M. Ulziibat, seismologist from the same Research Center, M. Ganzorig, the Director of the Institute of Informatics, and A. Ivanov from the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences.

Geological setting. The Miocene Har-Togoo shield volcano is situated on top of a vast volcanic plateau (figure 1). The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Pliocene and Quaternary volcanic rocks are also abundant in the vicinity of the Holocene volcanoes (Devyatkin and Smelov, 1979; Logatchev and others, 1982). Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Figure (see Caption) Figure 1. Photograph of the Har-Togoo volcano viewed from west, March 2003. Courtesy of Alexei Ivanov.

Observations during March 2003. The name of the volcano in the Mongolian language means "black-pot" and through questioning of the local inhabitants, it was learned that there is a local myth that a dragon lived in the volcano. The local inhabitants also mentioned that marmots, previously abundant in the area, began to migrate westwards five years ago; they are now practically absent from the area.

Acoustic noise and venting of colorless warm gas from a small hole near the summit were noticed in October 2002 by local residents. In December 2002, while snow lay on the ground, the hole was clearly visible to local visitors, and a second hole could be seen a few meters away; it is unclear whether or not white vapors were noticed on this occasion. During the inspection in March 2003 a third hole was seen. The second hole is located within a 3 x 3 m outcrop of cinder and pumice (figure 2) whereas the first and the third holes are located within massive basalts. When close to the holes, constant noise resembled a rapid river heard from afar. The second hole was covered with plastic sheeting fixed at the margins, but the plastic was blown off within 2-3 seconds. Gas from the second hole was sampled in a mechanically pumped glass sampler. Analysis by gas chromatography, performed a week later at the Institute of the Earth's Crust, showed that nitrogen and atmospheric air were the major constituents.

Figure (see Caption) Figure 2. Photograph of the second hole sampled at Har-Togoo, with hammer for scale, March 2003. Courtesy of Alexei Ivanov.

The temperature of the gas at the first, second, and third holes was +1.1, +1.4, and +2.7°C, respectively, while air temperature was -4.6 to -4.7°C (measured on 19 March 2003). Repeated measurements of the temperatures on the next day gave values of +1.1, +0.8, and -6.0°C at the first, second, and third holes, respectively. Air temperature was -9.4°C. To avoid bias due to direct heating from sunlight the measurements were performed under shadow. All measurements were done with Chechtemp2 digital thermometer with precision of ± 0.1°C and accuracy ± 0.3°C.

Inside the mouth of the first hole was 4-10-cm-thick ice with suspended gas bubbles (figure 5). The ice and snow were sampled in plastic bottles, melted, and tested for pH and Eh with digital meters. The pH-meter was calibrated by Horiba Ltd (Kyoto, Japan) standard solutions 4 and 7. Water from melted ice appeared to be slightly acidic (pH 6.52) in comparison to water of melted snow (pH 7.04). Both pH values were within neutral solution values. No prominent difference in Eh (108 and 117 for ice and snow, respectively) was revealed.

Two digital short-period three-component stations were installed on top of Har-Togoo, one 50 m from the degassing holes and one in a remote area on basement rocks, for monitoring during 19-20 March 2003. Every hour 1-3 microseismic events with magnitude <2 were recorded. All seismic events were virtually identical and resembled A-type volcano-tectonic earthquakes (figure 6). Arrival difference between S and P waves were around 0.06-0.3 seconds for the Har-Togoo station and 0.1-1.5 seconds for the remote station. Assuming that the Har-Togoo station was located in the epicentral zone, the events were located at ~1-3 km depth. Seismic episodes similar to volcanic tremors were also recorded (figure 3).

Figure (see Caption) Figure 3. Examples of an A-type volcano-tectonic earthquake and volcanic tremor episodes recorded at the Har-Togoo station on 19 March 2003. Courtesy of Alexei Ivanov.

Conclusions. The abnormal thermal and seismic activities could be the result of either hydrothermal or volcanic processes. This activity could have started in the fall of 2002 when they were directly observed for the first time, or possibly up to five years earlier when marmots started migrating from the area. Further studies are planned to investigate the cause of the fumarolic and seismic activities.

At the end of a second visit in early July, gas venting had stopped, but seismicity was continuing. In August there will be a workshop on Russian-Mongolian cooperation between Institutions of the Russian and Mongolian Academies of Sciences (held in Ulan-Bator, Mongolia), where the work being done on this volcano will be presented.

References. Devyatkin, E.V. and Smelov, S.B., 1979, Position of basalts in sequence of Cenozoic sediments of Mongolia: Izvestiya USSR Academy of Sciences, geological series, no. 1, p. 16-29. (In Russian).

Logatchev, N.A., Devyatkin, E.V., Malaeva, E.M., and others, 1982, Cenozoic deposits of Taryat basin and Chulutu river valley (Central Hangai): Izvestiya USSR Academy of Sciences, geological series, no. 8, p. 76-86. (In Russian).

Geologic Background. The Miocene Har-Togoo shield volcano, also known as Togoo Tologoy, is situated on top of a vast volcanic plateau. The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Information Contacts: Alexei V. Ivanov, Institute of the Earth Crust SB, Russian Academy of Sciences, Irkutsk, Russia; Bekhtur andM. Ulziibat, Research Center of Astronomy and Geophysics, Mongolian Academy of Sciences, Ulan-Bator, Mongolia; M. Ganzorig, Institute of Informatics MAS, Ulan-Bator, Mongolia.


Elgon (Uganda) — December 2005

Elgon

Uganda

1.136°N, 34.559°E; summit elev. 3885 m

All times are local (unless otherwise noted)


False report of activity; confusion caused by burning dung in a lava tube

An eruption at Mount Elgon was mistakenly inferred when fumes escaped from this otherwise quiet volcano. The fumes were eventually traced to dung burning in a lava-tube cave. The cave is home to, or visited by, wildlife ranging from bats to elephants. Mt. Elgon (Ol Doinyo Ilgoon) is a stratovolcano on the SW margin of a 13 x 16 km caldera that straddles the Uganda-Kenya border 140 km NE of the N shore of Lake Victoria. No eruptions are known in the historical record or in the Holocene.

On 7 September 2004 the web site of the Kenyan newspaper The Daily Nation reported that villagers sighted and smelled noxious fumes from a cave on the flank of Mt. Elgon during August 2005. The villagers' concerns were taken quite seriously by both nations, to the extent that evacuation of nearby villages was considered.

The Daily Nation article added that shortly after the villagers' reports, Moses Masibo, Kenya's Western Province geology officer visited the cave, confirmed the villagers observations, and added that the temperature in the cave was 170°C. He recommended that nearby villagers move to safer locations. Masibo and Silas Simiyu of KenGens geothermal department collected ashes from the cave for testing.

Gerald Ernst reported on 19 September 2004 that he spoke with two local geologists involved with the Elgon crisis from the Geology Department of the University of Nairobi (Jiromo campus): Professor Nyambok and Zacharia Kuria (the former is a senior scientist who was unable to go in the field; the latter is a junior scientist who visited the site). According to Ernst their interpretation is that somebody set fire to bat guano in one of the caves. The fire was intense and probably explains the vigorous fuming, high temperatures, and suffocated animals. The event was also accompanied by emissions of gases with an ammonia odor. Ernst noted that this was not surprising considering the high nitrogen content of guano—ammonia is highly toxic and can also explain the animal deaths. The intense fumes initially caused substantial panic in the area.

It was Ernst's understanding that the authorities ordered evacuations while awaiting a report from local scientists, but that people returned before the report reached the authorities. The fire presumably prompted the response of local authorities who then urged the University geologists to analyze the situation. By the time geologists arrived, the fuming had ceased, or nearly so. The residue left by the fire and other observations led them to conclude that nothing remotely related to a volcanic eruption had occurred.

However, the incident emphasized the problem due to lack of a seismic station to monitor tectonic activity related to a local triple junction associated with the rift valley or volcanic seismicity. In response, one seismic station was moved from S Kenya to the area of Mt. Elgon so that local seismicity can be monitored in the future.

Information Contacts: Gerald Ernst, Univ. of Ghent, Krijgslaan 281/S8, B-9000, Belgium; Chris Newhall, USGS, Univ. of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; The Daily Nation (URL: http://www.nationmedia.com/dailynation/); Uganda Tourist Board (URL: http://www.visituganda.com/).