Masaya, which is about 20 km NW of the Nicaragua’s capital of Managua, is one of the most active volcanoes in that country and has a caldera that contains a number of craters (BGVN 43:11). The Santiago crater is the one most currently active and it contains a small lava lake that emits weak gas plumes (figure 85). This report summarizes activity during February through May 2020 and is based on Instituto Nicaragüense de Estudios Territoriales (INETER) monthly reports and satellite data. During the reporting period, the volcano was relatively calm, with only weak gas plumes.

Figure 85. Satellite images of Masaya from Sentinel-2 on 18 April 2020, showing and a small gas plume drifting SW (top, natural color bands 4, 3, 2) and the lava lake (bottom, false color bands 12, 11, 4). Courtesy of Sentinel Hub Playground.

According to INETER, thermal images of the lava lake and temperature data in the fumaroles were taken using an Omega infrared gun and a forward-looking infrared (FLIR) SC620 thermal camera. The temperatures above the lava lake have decreased since November 2019, when the temperature was 287°C, dropping to 96°C when measured on 14 May 2020. INETER attributed this decrease to subsidence in the level of the lava lake by 5 m which obstructed part of the lake and concentrated the gas emissions in the weak plume. Convection continued in the lava lake, which in May had decreased to a diameter of 3 m. Many landslides had occurred in the E, NE, and S walls of the crater rim due to rock fracturing caused by the high heat and acidity of the emissions.

During the reporting period, the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system recorded numerous thermal anomalies from the lava lake based on MODIS data (figure 86). Infrared satellite images from Sentinel-2 regularly showed a strong signature from the lava lake through 18 May, after which the volcano was covered by clouds.

Figure 86. Thermal anomalies at Masaya during February through May 2020. The larger anomalies with black lines are more distant and not related to the volcano. Courtesy of MIROVA.

Measurements of sulfur dioxide (SO2) made by INETER in the section of the Ticuantepe - La Concepción highway (just W of the volcano) with a mobile DOAS system varied between a low of just over 1,000 metric tons/day in mid-November 2019 to a high of almost 2,500 tons/day in late May. Temperatures of fumaroles in the Cerro El Comalito area, just ENE of Santiago crater, ranged from 58 to 76°C during February-May 2020, with most values in the 69-72°C range.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras caldera and is itself 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 Nindirí and Masaya cones, 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 6,500 years ago. Historical lava flows cover much of the caldera floor and there is a lake at the far eastern end. 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 have caused 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Shishaldin is located near the center of Unimak Island in Alaska, with the current eruption phase beginning in July 2019 and characterized by ash plumes, lava flows, lava fountaining, pyroclastic flows, and lahars. More recently, in late 2019 and into January 2020, activity consisted of multiple lava flows, pyroclastic flows, lahars, and ashfall events (BGVN 45:02). This report summarizes activity from February through May 2020, including gas-and-steam emissions, brief thermal activity in mid-March, and a possible new cone within the summit crater. The primary source of information comes from the Alaska Volcano Observatory (AVO) reports and various satellite data.

Volcanism during February 2020 was relatively low, consisting of weakly to moderately elevated surface temperatures during 1-4 February and occasional small gas-and-steam plumes (figure 37). By 6 February both seismicity and surface temperatures had decreased. Seismicity and surface temperatures increased slightly again on 8 March and remained elevated through the rest of the reporting period. Intermittent gas-and-steam emissions were also visible from mid-March (figure 38) through May. Minor ash deposits visible on the upper SE flank may have been due to ash resuspension or a small collapse event at the summit, according to AVO.

Figure 37. Photo of a gas-and-steam plume rising from the summit crater at Shishaldin on 22 February 2020. Photo courtesy of Ben David Jacob via AVO.

Figure 38. A Worldview-2 panchromatic satellite image on 11 March 2020 showing a gas-and-steam plume rising from the summit of Shishaldin and minor ash deposits on the SE flank (left). Aerial photo showing minor gas-and-steam emissions rising from the summit crater on 11 March (right). Some erosion of the snow and ice on the upper flanks is a result of the lava flows from the activity in late 2019 and early 2020. Photo courtesy of Matt Loewen (left) and Ed Fischer (right) via AVO.

On 14 March, lava and a possible new cone were visible in the summit crater using satellite imagery, accompanied by small explosion signals. Strong thermal signatures due to the lava were also seen in Sentinel-2 satellite data and continued strongly through the month (figure 39). The lava reported by AVO in the summit crater was also reflected in satellite-based MODIS thermal anomalies recorded by the MIROVA system (figure 40). Seismic and infrasound data identified small explosions signals within the summit crater during 14-19 March.

Figure 39. Sentinel-2 thermal satellite images (bands 12, 11, 8A) show a bright hotspot (yellow-orange) at the summit crater of Shishaldin during mid-March 2020 that decreases in intensity by late March. Courtesy of Sentinel Hub Playground.

Figure 40. MIROVA thermal data showing a brief increase in thermal anomalies during late March 2020 and on two days in late April between periods of little to no activity. Courtesy of MIROVA.

AVO released a Volcano Observatory Notice for Aviation (VONA) stating that seismicity had decreased by 16 April and that satellite data no longer showed lava or additional changes in the crater since the start of April. Sentinel-2 thermal satellite imagery continued to show a weak hotspot in the crater summit through May (figure 41), which was also detected by the MIROVA system on two days. A daily report on 6 May reported a visible ash deposit extending a short distance SE from the summit, which had likely been present since 29 April. AVO noted that the timing of the deposit corresponds to an increase in the summit crater diameter and depth, further supporting a possible small collapse. Small gas-and-steam emissions continued intermittently and were accompanied by weak tremors and occasional low-frequency earthquakes through May (figure 42). Minor amounts of sulfur dioxide were detected in the gas-and-steam emissions during 20 and 29 April, and 2, 16, and 28 May.

Figure 41. Sentinel-2 thermal satellite images (bands 12, 11, 8A) show occasional gas-and-steam emissions rising from Shishaldin on 26 February (top left) and 24 April 2020 (bottom left) and a weak hotspot (yellow-orange) persisting at the summit crater during April and early May 2020. Courtesy of Sentinel Hub Playground.

Figure 42. A Worldview-1 panchromatic satellite image showing gas-and-steam emissions rising from the summit of Shishaldin on 1 May 2020 (local time) (left). Aerial photo of the N flank of Shishaldin with minor gas-and-steam emissions rising from the summit on 8 May (right). Photo courtesy of Matt Loewen (left) and Levi Musselwhite (right) via AVO.

Geologic Background. The beautifully symmetrical Shishaldin is the highest and one of the most active volcanoes of the Aleutian Islands. The glacier-covered volcano is the westernmost of three large stratovolcanoes along an E-W line in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." A steam plume often rises from its small summit crater. Constructed atop an older glacially dissected volcano, it is largely basaltic in composition. Remnants of an older ancestral volcano are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is blanketed by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); 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).

Krakatau, located in the Sunda Strait between Indonesia’s Java and Sumatra Islands, experienced a major caldera collapse around 535 CE, forming a 7-km-wide caldera ringed by three islands. On 22 December 2018, a large explosion and flank collapse destroyed most of the 338-m-high island of Anak Krakatau (Child of Krakatau) and generated a deadly tsunami (BGVN 44:03). The near-sea level crater lake inside the remnant of Anak Krakatau was the site of numerous small steam and tephra explosions. A larger explosion in December 2019 produced the beginnings of a new cone above the surface of crater lake (BGVN 45:02). Recently, volcanism has been characterized by occasional Strombolian explosions, dense ash plumes, and crater incandescence. This report covers activity from February through May 2020 using information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, also known as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), the Darwin Volcanic Ash Advisory Center (VAAC), and various satellite data.

Activity during February 2020 consisted of dominantly white gas-and-steam emissions rising 300 m above the crater, according to PVMBG. According to the Darwin VAAC, a ground observer reported an eruption on 7 and 8 February, but no volcanic ash was observed. During 10-11 February, a short-lived eruption was detected by seismograms which produced an ash plume up to 1 km above the crater drifting E. MAGMA Indonesia reported two eruptions on 18 March, both of which rose to 300 m above the crater. White gas-and-steam emissions were observed for the rest of the month and early April.

On 10 April PVMBG reported two eruptions, at 2158 and 2235, both of which produced dark ash plumes rising 2 km above the crater followed by Strombolian explosions ejecting incandescent material that landed on the crater floor (figures 108 and 109). The Darwin VAAC issued a notice at 0145 on 11 April reporting an ash plume to 14.3 km altitude drifting WNW, however this was noted with low confidence due to the possible mixing of clouds. During the same day, an intense thermal hotspot was detected in the HIMAWARI thermal satellite imagery and the NASA Global Sulfur Dioxide page showed a strong SO₂ plume at 11.3 km altitude drifting W (figure 110). The CCTV Lava93 webcam showed new lava flows and lava fountaining from the 10-11 April eruptions. This activity was evident in the MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data (figure 111).

Figure 108. Webcam (Lava93) images of Krakatau on 10 April 2020 showing Strombolian explosions, strong incandescence, and ash plumes rising from the crater. Courtesy of PVMBG and MAGMA Indonesia.

Figure 109. Webcam image of incandescent Strombolian explosions at Krakatau on 10 April 2020. Courtesy of PVMBG and MAGMA Indonesia.

Figure 110. Strong sulfur dioxide emissions rising from Krakatau and drifting W were detected using the TROPOMI instrument on the Sentinel-5P satellite on 11 April 2020 (top row). Smaller volumes of SO2 were visible in Sentinel-5P/TROPOMI maps on 13 (bottom left) and 19 April (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 111. Thermal activity at Anak Krakatau from 29 June-May 2020 shown on a MIROVA Log Radiative Power graph. The power and frequency of the thermal anomalies sharply increased in mid-April. After the larger eruptive event in mid-April the thermal anomalies declined slightly in strength but continued to be detected intermittently through May. Courtesy of MIROVA.

Strombolian activity rising up to 500 m continued into 12 April and was accompanied by SO₂ emissions that rose 3 km altitude, drifting NW according to a VAAC notice. PVMBG reported an eruption on 13 April at 2054 that resulted in incandescence as high as 25 m above the crater. Volcanic ash, accompanied by white gas-and-steam emissions, continued intermittently through 18 April, many of which were observed by the CCTV webcam. After 18 April only gas-and-steam plumes were reported, rising up to 100 m above the crater; Sentinel-2 satellite imagery showed faint thermal anomalies in the crater (figure 112). SO₂ emissions continued intermittently throughout April, though at lower volumes and altitudes compared to the 11th. MODIS satellite data seen in MIROVA showed intermittent thermal anomalies through May.

Figure 112. Sentinel-2 thermal satellite images showing the cool crater lake on 20 March (top left) followed by minor heating of the crater during April and May 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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: 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/); 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Taal volcano is in a caldera system located in southern Luzon island and is one of the most active volcanoes in the Philippines. It has produced around 35 recorded eruptions since 3,580 BCE, ranging from VEI 1 to 6, with the majority of eruptions being a VEI 2. The caldera contains a lake with an island that also contains a lake within the Main Crater (figure 12). Prior to 2020 the most recent eruption was in 1977, on the south flank near Mt. Tambaro. The United Nations Office for the Coordination of Humanitarian Affairs in the Philippines reports that over 450,000 people live within 40 km of the caldera (figure 13). This report covers activity during January through February 2020 including the 12 to 22 January eruption, and is based on reports by Philippine Institute of Volcanology and Seismology (PHIVOLCS), satellite data, geophysical data, and media reports.

Figure 12. Annotated satellite images showing the Taal caldera, Volcano Island in the caldera lake, and features on the island including Main Crater. Imagery courtesy of Planet Inc.

Figure 13. Map showing population totals within 14 and 17 km of Volcano Island at Taal. Courtesy of the United Nations Office for the Coordination of Humanitarian Affairs (OCHA).

The hazard status at Taal was raised to Alert Level 1 (abnormal, on a scale of 0-5) on 28 March 2019. From that date through to 1 December there were 4,857 earthquakes registered, with some felt nearby. Inflation was detected during 21-29 November and an increase in CO₂ emission within the Main Crater was observed. Seismicity increased beginning at 1100 on 12 January. At 1300 there were phreatic (steam) explosions from several points inside Main Crater and the Alert Level was raised to 2 (increasing unrest). Booming sounds were heard in Talisay, Batangas, at 1400; by 1402 the plume had reached 1 km above the crater, after which the Alert Level was raised to 3 (magmatic unrest).

Phreatic eruption on 12 January 2020. A seismic swarm began at 1100 on 12 January 2020 followed by a phreatic eruption at 1300. The initial activity consisted of steaming from at least five vents in Main Crater and phreatic explosions that generated 100-m-high plumes. PHIVOLCS raised the Alert Level to 2. The Earth Observatory of Singapore reported that the International Data Center (IDC) for the Comprehensive test Ban Treaty (CTBT) in Vienna noted initial infrasound detections at 1450 that day.

Booming sounds were heard at 1400 in Talisay, Batangas (4 km NNE from the Main Crater), and at 1404 volcanic tremor and earthquakes felt locally were accompanied by an eruption plume that rose 1 km; ash fell to the SSW. The Alert Level was raised to 3 and the evacuation of high-risk barangays was recommended. Activity again intensified around 1730, prompting PHIVOLCS to raise the Alert Level to 4 and recommend a total evacuation of the island and high-risk areas within a 14-km radius. The eruption plume of steam, gas, and tephra significantly intensified, rising to 10-15 km altitude and producing frequent lightning (figures 14 and 15). Wet ash fell as far away as Quezon City (75 km N). According to news articles schools and government offices were ordered to close and the Ninoy Aquino International Airport (56 km N) in Manila suspended flights. About 6,000 people had been evacuated. Residents described heavy ashfall, low visibility, and fallen trees.

Figure 14. Lightning produced during the eruption of Taal during 1500 on 12 January to 0500 on 13 January 2020 local time (0700-2100 UTC on 12 January). Courtesy of Chris Vagasky, Vaisala.

Figure 15. Lightning strokes produced during the first days of the Taal January 2020 eruption. Courtesy of Domcar C Lagto/SIPA/REX/Shutterstock via The Guardian.

In a statement issued at 0320 on 13 January, PHIVOLCS noted that ashfall had been reported across a broad area to the north in Tanauan (18 km NE), Batangas; Escala (11 km NW), Tagaytay; Sta. Rosa (32 km NNW), Laguna; Dasmariñas (32 km N), Bacoor (44 km N), and Silang (22 km N), Cavite; Malolos (93 km N), San Jose Del Monte (87 km N), and Meycauayan (80 km N), Bulacan; Antipolo (68 km NNE), Rizal; Muntinlupa (43 km N), Las Piñas (47 km N), Marikina (70 km NNE), Parañaque (51 km N), Pasig (62 km NNE), Quezon City, Mandaluyong (62 km N), San Juan (64 km N), Manila; Makati City (59 km N) and Taguig City (55 km N). Lapilli (2-64 mm in diameter) fell in Tanauan and Talisay; Tagaytay City (12 km N); Nuvali (25 km NNE) and Sta (figure 16). Rosa, Laguna. Felt earthquakes (Intensities II-V) continued to be recorded in local areas.

Figure 16. Ashfall from the Taal January 2020 eruption in Lemery (top) and in the Batangas province (bottom). Photos posted on 13 January, courtesy of Ezra Acayan/Getty Images, Aaron Favila/AP, and Ted Aljibe/AFP via Getty Images via The Guardian.

Magmatic eruption on 13 January 2020. A magmatic eruption began during 0249-0428 on 13 January, characterized by weak lava fountaining accompanied by thunder and flashes of lightning. Activity briefly waned then resumed with sporadic weak fountaining and explosions that generated 2-km-high, dark gray, steam-laden ash plumes (figure 17). New lateral vents opened on the N flank, producing 500-m-tall lava fountains. Heavy ashfall impacted areas to the SW, including in Cuenca (15 km SSW), Lemery (16 km SW), Talisay, and Taal (15 km SSW), Batangas (figure 18).

Figure 17. Ash plumes seen from various points around Taal in the initial days of the January 2020 eruption, posted on 13 January. Courtesy of Eloisa Lopez/Reuters, Kester Ragaza/Pacific Press/Shutterstock, Ted Aljibe/AFP via Getty Images, via The Guardian.

Figure 18. Map indicating areas impacted by ashfall from the 12 January eruption through to 0800 on the 13th. Small yellow circles (to the N) are ashfall report locations; blue circles (at the island and to the S) are heavy ashfall; large green circles are lapilli (particles measuring 2-64 mm in diameter). Modified from a map courtesy of Lauriane Chardot, Earth Observatory of Singapore; data taken from PHIVOLCS.

News articles noted that more than 300 domestic and 230 international flights were cancelled as the Manila Ninoy Aquino International Airport was closed during 12-13 January. Some roads from Talisay to Lemery and Agoncillo were impassible and electricity and water services were intermittent. Ashfall in several provinces caused power outages. Authorities continued to evacuate high-risk areas, and by 13 January more than 24,500 people had moved to 75 shelters out of a total number of 460,000 people within 14 km.

A PHIVOLCS report for 0800 on the 13th through 0800 on 14 January noted that lava fountaining had continued, with steam-rich ash plumes reaching around 2 km above the volcano and dispersing ash SE and W of Main Crater. Volcanic lighting continued at the base of the plumes. Fissures on the N flank produced 500-m-tall lava fountains. Heavy ashfall continued in the Lemery, Talisay, Taal, and Cuenca, Batangas Municipalities. By 1300 on the 13th lava fountaining generated 800-m-tall, dark gray, steam-laden ash plumes that drifted SW. Sulfur dioxide emissions averaged 5,299 metric tons/day (t/d) on 13 January and dispersed NNE (figure 19).

Figure 19. Compilation of sulfur dioxide plumes from TROPOMI overlaid in Google Earth for 13 January from 0313-1641 UT. Courtesy of NASA Global Sulfur Dioxide Monitoring Page and Google Earth.

Explosions and ash emission through 22 January 2020. At 0800 on 15 January PHIVOLCS stated that activity was generally weaker; dark gray, steam-laden ash plumes rose about 1 km and drifted SW. Satellite images showed that the Main Crater lake was gone and new craters had formed inside Main Crater and on the N side of Volcano Island.

PHIVOLCS reported that activity during 15-16 January was characterized by dark gray, steam-laden plumes that rose as high as 1 km above the vents in Main Crater and drifted S and SW. Sulfur dioxide emissions were 4,186 t/d on 15 January. Eruptive events at 0617 and 0621 on 16 January generated short-lived, dark gray ash plumes that rose 500 and 800 m, respectively, and drifted SW. Weak steam plumes rose 800 m and drifted SW during 1100-1700, and nine weak explosions were recorded by the seismic network.

Steady steam emissions were visible during 17-21 January. Infrequent weak explosions generated ash plumes that rose as high as 1 km and drifted SW. Sulfur dioxide emissions fluctuated and were as high as 4,353 t/d on 20 January and as low as 344 t/d on 21 January. PHIVOLCS reported that white steam-laden plumes rose as high as 800 m above main vent during 22-28 January and drifted SW and NE; ash emissions ceased around 0500 on 22 January. Remobilized ash drifted SW on 22 January due to strong low winds, affecting the towns of Lemery (16 km SW) and Agoncillo, and rose as high as 5.8 km altitude as reported by pilots. Sulfur dioxide emissions were low at 140 t/d.

Steam plumes through mid-April 2020. The Alert Level was lowered to 3 on 26 January and PHIVOLCS recommended no entry onto Volcano Island and Taal Lake, nor into towns on the western side of the island within a 7-km radius. PHIVOLCS reported that whitish steam plumes rose as high as 800 m during 29 January-4 February and drifted SW (figure 20). The observed steam plumes rose as high as 300 m during 5-11 February and drifted SW.

Sulfur dioxide emissions averaged around 250 t/d during 22-26 January; emissions were 87 t/d on 27 January and below detectable limits the next day. During 29 January-4 February sulfur dioxide emissions ranged to a high of 231 t/d (on 3 February). The following week sulfur dioxide emissions ranged from values below detectable limits to a high of 116 t/d (on 8 February).

Figure 20. Taal Volcano Island producing gas-and-steam plumes on 15-16 January 2020. Courtesy of James Reynolds, Earth Uncut.

On 14 February PHIVOLCS lowered the Alert Level to 2, noting a decline in the number of volcanic earthquakes, stabilizing ground deformation of the caldera and Volcano Island, and diffuse steam-and-gas emission that continued to rise no higher than 300 m above the main vent during the past three weeks. During 14-18 February sulfur dioxide emissions ranged from values below detectable limits to a high of 58 tonnes per day (on 16 February). Sulfur dioxide emissions were below detectable limits during 19-20 February. During 26 February-2 March steam plumes rose 50-300 m above the vent and drifted SW and NE. PHIVOLCS reported that during 4-10 March weak steam plumes rose 50-100 m and drifted SW and NE; moderate steam plumes rose 300-500 m and drifted SW during 8-9 March. During 11-17 March weak steam plumes again rose only 50-100 m and drifted SW and NE.

PHIVOLCS lowered the Alert Level to 1 on 19 March and recommended no entry onto Volcano Island, the area defined as the Permanent Danger Zone. During 8-9 April steam plumes rose 100-300 m and drifted SW. As of 1-2 May 2020 only weak steaming and fumarolic activity from fissure vents along the Daang Kastila trail was observed.

Evacuations. According to the Disaster Response Operations Monitoring and Information Center (DROMIC) there were a total of 53,832 people dispersed to 244 evacuation centers by 1800 on 15 January. By 21 January there were 148,987 people in 493 evacuation. The number of residents in evacuation centers dropped over the next week to 125,178 people in 497 locations on 28 January. However, many residents remained displaced as of 3 February, with DROMIC reporting 23,915 people in 152 evacuation centers, but an additional 224,188 people staying at other locations.

By 10 February there were 17,088 people in 110 evacuation centers, and an additional 211,729 staying at other locations. According to the DROMIC there were a total of 5,321 people in 21 evacuation centers, and an additional 195,987 people were staying at other locations as of 19 February.

The number of displaced residents continued to drop, and by 3 March there were 4,314 people in 12 evacuation centers, and an additional 132,931 people at other locations. As of 11 March there were still 4,131 people in 11 evacuation centers, but only 17,563 staying at other locations.

Deformation and ground cracks. New ground cracks were observed on 13 January in Sinisian (18 km SW), Mahabang Dahilig (14 km SW), Dayapan (15 km SW), Palanas (17 km SW), Sangalang (17 km SW), and Poblacion (19 km SW) Lemery; Pansipit (11 km SW), Agoncillo; Poblacion 1, Poblacion 2, Poblacion 3, Poblacion 5 (all around 17 km SW), Talisay, and Poblacion (11 km SW), San Nicolas (figure 21). A fissure opened across the road connecting Agoncillo to Laurel, Batangas. New ground cracking was reported the next day in Sambal Ibaba (17 km SW), and portions of the Pansipit River (SW) had dried up.

Figure 21. Video screenshots showing ground cracks that formed during the Taal unrest and captured on 15 and 16 January 2020. Courtesy of James Reynolds, Earth Uncut.

Dropping water levels of Taal Lake were first observed in some areas on 16 January but reported to be lake-wide the next day. The known ground cracks in the barangays of Lemery, Agoncillo, Talisay, and San Nicolas in Batangas Province widened a few centimeters by 17 January, and a new steaming fissure was identified on the N flank of the island.

GPS data had recorded a sudden widening of the caldera by ~1 m, uplift of the NW sector by ~20 cm, and subsidence of the SW part of Volcano Island by ~1 m just after the main eruption phase. The rate of deformation was smaller during 15-22 January, and generally corroborated by field observations; Taal Lake had receded about 30 cm by 25 January but about 2.5 m of the change (due to uplift) was observed around the SW portion of the lake, near the Pansipit River Valley where ground cracking had been reported.

Weak steaming (plumes 10-20 m high) from ground cracks was visible during 5-11 February along the Daang Kastila trail which connects the N part of Volcano Island to the N part of the main crater. PHIVOLCS reported that during 19-24 February steam plumes rose 50-100 m above the vent and drifted SW. Weak steaming (plumes up to 20 m high) from ground cracks was visible during 8-14 April along the Daang Kastila trail which connects the N part of Volcano Island to the N part of the main crater.

Seismicity. Between 1300 on 12 January and 0800 on 21 January the Philippine Seismic Network (PSN) had recorded a total of 718 volcanic earthquakes; 176 of those had magnitudes ranging from 1.2-4.1 and were felt with Intensities of I-V. During 20-21 January there were five volcanic earthquakes with magnitudes of 1.6-2.5; the Taal Volcano network (which can detect smaller events not detectable by the PSN) recorded 448 volcanic earthquakes, including 17 low-frequency events. PHIVOLCS stated that by 21 January hybrid earthquakes had ceased and both the number and magnitude of low-frequency events had diminished.

Geologic Background. Taal is one of the most active volcanoes in the Philippines and has produced some of its most powerful historical eruptions. Though not topographically prominent, its prehistorical eruptions have greatly changed the landscape of SW Luzon. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, and several eruptive centers lie submerged beneath the lake. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all historical eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones that have grown about 25% in area during historical time. Powerful pyroclastic flows and surges from historical eruptions have caused many fatalities.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Disaster Response Operations Monitoring and Information Center (DROMIC) (URL: https://dromic.dswd.gov.ph/); United Nations Office for the Coordination of Humanitarian Affairs, Philippines (URL: https://www.unocha.org/philippines); James Reynolds, Earth Uncut TV (Twitter: @EarthUncutTV, URL: https://www.earthuncut.tv/, YouTube: https://www.youtube.com/user/TyphoonHunter); Chris Vagasky, Vaisala Inc., Louisville, Colorado, USA (URL: https://www.vaisala.com/en?type=1, Twitter: @COweatherman, URL: https://twitter.com/COweatherman); Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue, Singapore (URL: https://www.earthobservatory.sg/); 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/); Relief Web, Flash Update No. 1 - Philippines: Taal Volcano eruption (As of 13 January 2020, 2 p.m. local time) (URL: https://reliefweb.int/report/philippines/flash-update-no-1-philippines-taal-volcano-eruption-13-january-2020-2-pm-local); Bloomberg, Philippines Braces for Hazardous Volcano Eruption (URL: https://www.bloomberg.com/news/articles/2020-01-12/philippines-raises-alert-level-in-taal-as-volcano-spews-ash); National Public Radio (NPR), Volcanic Eruption In Philippines Causes Thousands To Flee (URL: npr.org/2020/01/13/795815351/volcanic-eruption-in-philippines-causes-thousands-to-flee); Reuters (http://www.reuters.com/); Agence France-Presse (URL: http://www.afp.com/); Pacific Press (URL: http://www.pacificpress.com/); Shutterstock (URL: https://www.shutterstock.com/); Getty Images (URL: http://www.gettyimages.com/); Google Earth (URL: https://www.google.com/earth/).

In the northern Tonga region, approximately 80 km NW of Vava’u, large areas of floating pumice, termed rafts, were observed starting as early as 7 August 2019. The area of these andesitic pumice rafts was initially 195 km² with the layers measuring 15-30 cm thick and were produced 200 m below sea level (Jutzeler et al. 2020). The previous report (BGVN 44:11) described the morphology of the clasts and the rafts, and their general westward path from 9 August to 9 October 2019, with the first sighting occurring on 9 August NW of Vava’u in Tonga. This report updates details regarding the submarine pumice raft eruption in early August 2019 using new observations and data from Brandl et al. (2019) and Jutzeler et al. (2020).

The NoToVE-2004 (Northern Tonga Vents Expedition) research cruise on the RV Southern Surveyor (SS11/2004) from the Australian CSIRO Marine National Facility traveled to the northern Tonga Arc and discovered several submarine basalt-to-rhyolite volcanic centers (Arculus, 2004). One of these volcanic centers 50 km NW of Vava’u was the unnamed seamount (volcano number 243091) that had erupted in 2001 and again in 2019, unofficially designated “Volcano F” for reference purposes by Arculus (2004) and also used by Brandl et al. (2019). It is a volcanic complex that rises more than 1 km from the seafloor with a central 6 x 8.7 km caldera and a volcanic apron measuring over 50 km in diameter (figures 19 and 20). Arculus (2004) described some of the dredged material as “fresh, black, plagioclase-bearing lava with well-formed, glassy crusts up to 2cm thick” from cones by the eastern wall of the caldera; a number of apparent flows, lava or debris, were observed draping over the northern wall of the caldera.

Figure 19. Visualization of the unnamed submarine Tongan volcano (marked “Volcano F”) using bathymetric data to show the site of the 6-8 August 2020 eruption and the rest of the cone complex. Courtesy of Philipp Brandl via GEOMAR.

Figure 20. Map of the unnamed submarine Tongan volcano using satellite imagery, bathymetric data, with shading from the NW. The yellow circle indicates the location of the August 2019 activity. Young volcanic cones are marked “C” and those with pit craters at the top are marked with “P.” Courtesy of Brandl et al. (2019).

The International Seismological Centre (ISC) Preliminary Bulletin listed a particularly strong (5.7 Mw) earthquake at 2201 local time on 5 August, 15 km SSW of the volcano at a depth of 10 km (Brandl et al. 2019). This event was followed by six slightly lower magnitude earthquakes over the next two days.

Sentinel-2 satellite imagery showed two concentric rings originating from a point source (18.307°S 174.395°W) on 6 August (figure 21), which could be interpreted as small weak submarine plumes or possibly a series of small volcanic cones, according to Brandl et al. (2019). The larger ring is about 1.2 km in diameter and the smaller one measures 250 m. By 8 August volcanic activity had decreased, but the pumice rafts that were produced remained visible through at least early October (BGVN 44:11). Brandl et al. (2019) states that, due to the lack of continued observed activity rising from this location, the eruption was likely a 2-day-long event during 6-8 August.

Figure 21. Sentinel-2 satellite image of possible gas/vapor emissions (streaks) on 6 August 2019 drifting NW, which is the interpreted site for the unnamed Tongan seamount. The larger ring is about 1.2 km in diameter and the smaller one measures 250 m. Image using False Color (urban) rendering (bands 12, 11, 4); courtesy of Sentinel Hub Playground.

The pumice was first observed on 9 August occurred up to 56 km from the point of origin, according to Jutzeler et al. (2020). By calculating the velocity (14 km/day) of the raft using three satellites, Jutzeler et al. (2020) determined the pumice was erupted immediately after the satellite image of the submarine plumes on 6 August (UTC time). Minor activity at the vent may have continued on 8 and 11 August (UTC time) with pale blue-green water discoloration (figure 22) and a small (less than 1 km²) diffuse pumice raft 2-5 km from the vent.

Figure 22. Sentinel-2 satellite image of the last visible activity occurring W of the unnamed submarine Tongan volcano on 8 August 2019, represented by slightly discolored blue-green water. Image using Natural Color rendering (bands 4, 3, 2) and enhanced with color correction; courtesy of Sentinel Hub Playground.

Continuous observations using various satellite data and observations aboard the catamaran ROAM tracked the movement and extent of the pumice raft that was produced during the submarine eruption in early August (figure 23). The first visible pumice raft was observed on 8 August 2019, covering more than 136.7 km² between the volcanic islands of Fonualei and Late and drifting W for 60 km until 9 August (Brandl et al. 2019; Jutzeler 2020). The next day, the raft increased to 167.2-195 km² while drifting SW for 74 km until 14 August. Over the next three days (10-12 August) the size of the raft briefly decreased in size to less than 100 km² before increasing again to 157.4 km² on 14 August; at least nine individual rafts were mapped and identified on satellite imagery (Brandl et al. 2019). On 15 August sailing vessels observed a large pumice raft about 75 km W of Late Island (see details in BGVN 44:11), which was the same one as seen in satellite imagery on 8 August.

Figure 23. Map of the extent of discolored water and the pumice raft from the unnamed submarine Tongan volcano between 8 and 14 August 2019 using imagery from NASA’s MODIS, ESA’s Sentinel-2 satellite, and observations from aboard the catamaran ROAM (BGVN 44:11). Back-tracing the path of the pumice raft points to a source location at the unnamed submarine Tongan volcano. Courtesy of Brandl et al. (2019).

By 17 August high-resolution satellite images showed an area of large and small rafts measuring 222 km² and were found within a field of smaller rafts for a total extent of 1,350 km², which drifted 73 km NNW through 22 August before moving counterclockwise for three days (figure f; Jutzeler et al., 2020). Small pumice ribbons encountered the Oneata Lagoon on 30 August, the first island that the raft came into contact (Jutzeler et al. 2020). By 2 September, the main raft intersected with Lakeba Island (460 km from the source) (figure 24), breaking into smaller ribbons that started to drift W on 8 September. On 19 September the small rafts (less than 100 m x less than 2 km) entered the strait between Viti Levu and Vanua Levu, the two main islands of Fiji, while most of the others were stranded 60 km W in the Yasawa Islands for more than two months (Jutzeler et al., 2020).

Figure 24. Time-series map of the raft dispersal from the unnamed submarine Tongan volcano using multiple satellite images. A) Map showing the first days of the raft dispersal starting on 7 August 2019 and drifting SW from the vent (marked with a red triangle). Precursory seismicity that began on 5 August is marked with a white star. By 15-17 August the raft was entrained in an ocean loop or eddy. The dashed lines represent the path of the sailing vessels. B) Map of the raft dispersal using high-resolution Sentinel-2 and -3 imagery. Two dispersal trails (red and blue dashed lines) show the daily dispersal of two parts of the raft that were separated on 17 August 2019. Courtesy of Jutzeler et al. (2020).

References: Arculus, R J, SS2004/11 shipboard scientists, 2004. SS11/2004 Voyage Summary: NoToVE-2004 (Northern Tonga Vents Expedition): submarine hydrothermal plume activity and petrology of the northern Tofua Arc, Tonga. https://www.cmar.csiro.au/data/reporting/get file.cfm?eovpub id=901.

Brandl P A, Schmid F, Augustin N, Grevemeyer I, Arculus R J, Devey C W, Petersen S, Stewart M , Kopp K, Hannington M D, 2019. The 6-8 Aug 2019 eruption of ‘Volcano F’ in the Tofua Arc, Tonga. Journal of Volcanology and Geothermal Research: https://doi.org/10.1016/j.jvolgeores.2019.106695

Jutzeler M, Marsh R, van Sebille E, Mittal T, Carey R, Fauria K, Manga M, McPhie J, 2020. Ongoing Dispersal of the 7 August 2019 Pumice Raft From the Tonga Arc in the Southwestern Pacific Ocean. AGU Geophysical Research Letters: https://doi.orh/10.1029/2019GL086768.

Geologic Background. A submarine volcano along the Tofua volcanic arc was first observed in September 2001. The newly discovered volcano lies NW of the island of Vava'u about 35 km S of Fonualei and 60 km NE of Late volcano. The site of the eruption is along a NNE-SSW-trending submarine plateau with an approximate bathymetric depth of 300 m. T-phase waves were recorded on 27-28 September 2001, and on the 27th local fishermen observed an ash-rich eruption column that rose above the sea surface. No eruptive activity was reported after the 28th, but water discoloration was documented during the following month. In early November rafts and strandings of dacitic pumice were reported along the coast of Kadavu and Viti Levu in the Fiji Islands. The depth of the summit of the submarine cone following the eruption determined to be 40 m during a 2007 survey; the crater of the 2001 eruption was breached to the E.

Information Contacts: Jan Steffen, Communication and Media, GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany; Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Klyuchevskoy is part of the Klyuchevskaya volcanic group in northern Kamchatka and is one of the most frequently active volcanoes of the region. Eruptions produce lava flows, ashfall, and lahars originating from summit and flank activity. This report summarizes activity during October 2019 through May 2020, and is based on reports by the Kamchatkan Volcanic Eruption Response Team (KVERT) and satellite data.

There were no activity reports from 1 to 22 October, but gas emissions were visible in satellite images. At 1020 on 24 October (2220 on 23 October UTC) KVERT noted that there was a small ash component in the ash plume from erosion of the conduit, with the plume reaching 130 km ENE. The Aviation Colour Code was raised from Green to Yellow, then to Orange the following day. An ash plume continued on the 25th to 5-7 km altitude and extending 15 km SE and 70 km SW and reached 30 km ESE on the 26th. Similar activity continued through to the end of the month.

Moderate gas emissions continued during 1-19 November, but the summit was obscured by clouds. Strong nighttime incandescence was visible at the crater during the 10-11 November and thermal anomalies were detected on 8 and 10-13 November. Explosions produced ash plumes up to 6 km altitude on the 20-21st and Strombolian activity was reported during 20-22 November. Degassing continued from 23 November through 12 December, and a thermal anomaly was visible on the days when the summit was not covered by clouds. An ash plume was reported moving to the NW on the 13th, and degassing with a thermal anomaly and intermittent Strombolian activity then resumed, continuing through to the end of December with an ash plume reported on the 30th.

Gas-and-steam plumes continued into January 2020 with incandescence noted when the summit was clear (figure 33). Strombolian activity was reported again starting on the 3rd. A weak ash plume produced on the 6th extended 55 km E, and on the 21st an ash plume reached 5-5.5 km altitude and extended 190 km NE (figure 34). Another ash plume the next day rose to the same altitude and extended 388 km NE. During 23-29 Strombolian activity continued, and Vulcanian activity produced ash plumes up to 5.5 altitude, extending to 282 km E on the 30th, and 145 km E on the 31st.

Figure 33. Incandescence and degassing were visible at Klyuchevskoy through January 2020, seen here on the 11th. Courtesy of KVERT.

Figure 34. A low ash plume at Klyuchevskoy on 21 January 2020 extended 190 km NE. Courtesy of KVERT.

Strombolian activity continued throughout February with occasional explosions producing ash plumes up to 5.5 km altitude, as well as gas-and-steam plumes and a persistent thermal anomaly with incandescence visible at night. Starting in late February thermal anomalies were detected much more frequently, and with higher energy output compared to the previous year (figure 35). A lava fountain was reported on 1 March with the material falling back into the summit crater. Strombolian activity continued through early March. Lava fountaining was reported again on the 8th with ejecta landing in the crater and down the flanks (figure 36). A strong persistent gas-and-steam plume containing some ash continued along with Strombolian activity through 25 March (figure 37), with Vulcanian activity noted on the 20th and 25th. Strombolian and Vulcanian activity was reported through the end of March.

Figure 35. This MIROVA thermal energy plot for Klyuchevskoy for the year ending 29 April 2020 (log radiative power) shows intermittent thermal anomalies leading up to more sustained energy detected from February through March, then steadily increasing energy through April 2020. Courtesy of MIROVA.

Figure 36. Strombolian explosions at Klyuchevskoy eject incandescent ash and gas, and blocks and bombs onto the upper flanks on 8 and 10 March 2020. Courtesy of IVS FEB RAS, KVERT.

Figure 37. Weak ash emission from the Klyuchevskoy summit crater are dispersed by wind on 19 and 29 March 2020, with ash depositing on the flanks. Courtesy of IVS FEB RAS, KVERT.

Activity was dominantly Strombolian during 1-5 April and included intermittent Vulcanian explosions from the 6th onwards, with ash plumes reaching 6 km altitude. On 18 April a lava flow began moving down the SE flank (figures 38). A report on the 26th reported explosions from lava-water interactions with avalanches from the active lava flow, which continued to move down the SE flank and into the Apakhonchich chute (figures 39 and 40). This continued throughout April and May with sustained Strombolian and intermittent Vulcanian activity at the summit (figures 41 and 42).

Figure 38. Strombolian activity produced ash plumes and a lava flow down the SE flank of Klyuchevskoy on 18 April 2020. Courtesy of IVS FEB RAS, KVERT.

Figure 39. A lava flow descends the SW flank of Klyuchevskoy and a gas plume is dispersed by winds on 21 April 2020. Courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

Figure 40. Sentinel-2 thermal satellite images show the progression of the Klyuchevskoy lava flow from the summit crater down the SE flank from 19-29 April 2020. Associated gas plumes are dispersed in various directions. Courtesy of Sentinel Hub Playground.

Figure 41. Strombolian activity at Klyuchevskoy ejects incandescent ejecta, gas, and ash above the summit on 27 April 2020. Courtesy of D. Bud'kov, IVS FEB RAS, KVERT.

Figure 42. Sentinel-2 thermal satellite images of Klyuchevskoy show the progression of the SE flank lava flow through May 2020, with associated gas plumes being dispersed in multiple directions. Courtesy of Sentinel Hub Playground.

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); 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).

Nyamuragira (also known as Nyamulagira) is located in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo and consists of a lava lake that reappeared in the summit crater in mid-April 2018. Volcanism has been characterized by lava emissions, thermal anomalies, seismicity, and gas-and-steam emissions. This report summarizes activity during December 2019 through May 2020 using information from monthly reports by the Observatoire Volcanologique de Goma (OVG) and satellite data.

According to OVG, intermittent eruptive activity was detected in the lava lake of the central crater during December 2019 and January-April 2020, which also resulted in few seismic events. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows thermal anomalies within the summit crater that varied in both frequency and power between August 2019 and mid-March 2020, but very few were recorded afterward through late May (figure 88). Thermal hotspots identified by MODVOLC from 15 December 2019 through March 2020 were mainly located in the active central crater, with only three hotspots just outside the SW crater rim (figure 89). Sentinel-2 thermal satellite imagery also showed activity within the summit crater during January-May 2020, but by mid-March the thermal anomaly had visibly decreased in power (figure 90).

Figure 88. The MIROVA graph of thermal activity (log radiative power) at Nyamuragira during 27 July through May 2020 shows variably strong, intermittent thermal anomalies with a variation in power and frequency from August 2019 to mid-March 2020. Courtesy of MIROVA.

Figure 89. Map showing the number of MODVOLC hotspot pixels at Nyamuragira from 1 December 2019 t0 31 May 2020. 37 pixels were registered within the summit crater while 3 were detected just outside the SW crater rim. Courtesy of HIGP-MODVOLC Thermal Alerts System.

Figure 90. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) confirmed ongoing thermal activity (bright yellow-orange) at Nyamuragira from February into April 2020. The strength of the thermal anomaly in the summit crater decreased by late March 2020, but was still visible. Courtesy of Sentinel Hub Playground.

Geologic Background. Africa's most active volcano, Nyamuragira, is a massive high-potassium basaltic shield about 25 km N of Lake Kivu. Also known as Nyamulagira, it has generated extensive lava flows that cover 1500 km2 of the western branch of the East African Rift. The broad low-angle shield volcano contrasts dramatically with the adjacent steep-sided Nyiragongo to the SW. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Historical eruptions have occurred within the summit caldera, as well as from the numerous fissures and cinder cones on the flanks. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Historical lava flows extend down the flanks more than 30 km from the summit, reaching as far as Lake Kivu.

Information Contacts: Information contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; 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/exp.

Nyiragongo is located in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo, part of the western branch of the East African Rift System and contains a 1.2 km-wide summit crater with a lava lake that has been active since at least 1971. Volcanism has been characterized by strong and frequent thermal anomalies, incandescence, gas-and-steam emissions, and seismicity. This report summarizes activity during December 2019 through May 2020 using information from monthly reports by the Observatoire Volcanologique de Goma (OVG) and satellite data.

In the December 2019 monthly report, OVG stated that the level of the lava lake had increased. This level of the lava lake was maintained for the duration of the reporting period, according to later OVG monthly reports. Seismicity increased starting in November 2019 and was detected in the NE part of the crater, but it decreased by mid-April 2020. SO₂ emissions increased in January 2020 to roughly 7,000 tons/day but decreased again near the end of the month. OVG reported that SO₂ emissions rose again in February to roughly 8,500 tons/day before declining to about 6,000 tons/day. Unlike in the previous report (BGVN 44:12), incandescence was visible during the day in the active lava lake and activity at the small eruptive cone within the 1.2-km-wide summit crater has since increased, consisting of incandescence and some lava fountaining (figure 72). A field survey was conducted on 3-4 March where an OVG team observed active lava fountains and ejecta that produced Pele’s hair from the small eruptive cone (figure 73). During this survey, OVG reported that the level of the lava lake had reached the second terrace, which was formed on 17 January 2002 and represents remnants of the lava lake at different eruption stages. There, the open surface lava lake was observed; gas-and-steam emissions accompanied both the active lava lake and the small eruptive cone (figures 72 and 73).

Figure 72. Webcam image of Nyiragongo in February 2020 showing an open lava lake surface and incandescence from the active crater cone within the 1.2 km-wide summit crater visible during the day, accompanied by white gas-and-steam emissions. Courtesy of OVG (Rapport OVG February 2020).

Figure 73. Webcam image of Nyiragongo on 4 March 2020 showing an open lava lake surface and incandescence from the active crater cone within the 1.2 km-wide summit crater visible during the day, accompanied by white gas-and-steam emissions. Courtesy of OVG (Rapport OVG Mars 2020).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data continued to show frequent strong thermal anomalies within 5 km of the summit crater through May 2020 (figure 74). Similarly, the MODVOLC algorithm reported multiple thermal hotspots almost daily within the summit crater between December 2019 and May 2020. These thermal signatures were also observed in Sentinel-2 thermal satellite imagery within the summit crater (figure 75).

Figure 74. Thermal anomalies at Nyiragongo from 27 July through May 2020 as recorded by the MIROVA system (Log Radiative Power) were frequent and strong. Courtesy of MIROVA.

Figure 75. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) showed ongoing thermal activity (bright yellow-orange) in the summit crater at Nyiragongo during January through April 2020. Courtesy of Sentinel Hub Playground.

Geologic Background. One of Africa's most notable volcanoes, Nyiragongo contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes of a stratovolcano contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 parasitic cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; 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).

Kavachi is a submarine volcano located in the Solomon Islands south of Gatokae and Vangunu islands. Volcanism is frequently active, but rarely observed. The most recent eruptions took place during 2014, which consisted of an ash eruption, and during 2016, which included phreatomagmatic explosions (BGVN 42:03). This reporting period covers December 2016-April 2020 primarily using satellite data.

Activity at Kavachi is often only observed through satellite images, and frequently consists of discolored submarine plumes for which the cause is uncertain. On 1 January 2018 a slight yellow discoloration in the water is seen extending to the E from a specific point (figure 20). Similar faint plumes were observed on 16 January, 25 February, 2 March, 26 April, 6 May, and 25 June 2018. No similar water discoloration was noted during 2019, though clouds may have obscured views.

Figure 20. Satellite images from Sentinel-2 revealed intermittent faint water discoloration (yellow) at Kavachi during the first half of 2018, as seen here on 1 January (top left), 25 February (top right), 26 April (bottom left), and 25 June (bottom right). Images with “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

Activity resumed in 2020, showing more discolored water in satellite imagery. The first instance occurred on 16 March, where a distinct plume extended from a specific point to the SE. On 25 April a satellite image showed a larger discolored plume in the water that spread over about 30 km², encompassing the area around Kavachi (figure 21). Another image on 30 April showed a thin ribbon of discolored water extending about 50 km W of the vent.

Figure 21. Sentinel-2 satellite images of a discolored plume (yellow) at Kavachi beginning on 16 March (top left) with a significant large plume on 25 April (right), which remained until 30 April (bottom left). Images with “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

Geologic Background. Named for a sea-god of the Gatokae and Vangunu peoples, Kavachi is one of the most active submarine volcanoes in the SW Pacific, located in the Solomon Islands south of Vangunu Island. Sometimes referred to as Rejo te Kvachi ("Kavachi's Oven"), this shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 1 km long many times since its first recorded eruption during 1939. Residents of the nearby islands of Vanguna and Nggatokae (Gatokae) reported "fire on the water" prior to 1939, a possible reference to earlier eruptions. The roughly conical edifice rises from water depths of 1.1-1.2 km on the north and greater depths to the SE. Frequent shallow submarine and occasional subaerial eruptions produce phreatomagmatic explosions that eject steam, ash, and incandescent bombs. On a number of occasions lava flows were observed on the ephemeral islands.

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Kuchinoerabujima encompasses a group of young stratovolcanoes located in the northern Ryukyu Islands. All historical eruptions have originated from the Shindake cone, with the exception of a lava flow that originated from the S flank of the Furudake cone. The most recent previous eruptive period took place during October 2018-February 2019 and primarily consisted of weak explosions, ash plumes, and ashfall. The current eruption began on 11 January 2020 after nearly a year of dominantly gas-and-steam emissions. Volcanism for this reporting period from March 2019 to April 2020 included explosions, ash plumes, SO₂ emissions, and ashfall. The primary source of information for this report comes from 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.

Volcanism at Kuchinoerabujima was relatively low during March through December 2019, according to JMA. During this time, SO₂ emissions ranged from 100 to 1,000 tons/day. Gas-and-steam emissions were frequently observed throughout the entire reporting period, rising to a maximum height of 1.1 km above the crater on 13 December 2019. Satellite imagery from Sentinel-2 showed gas-and-steam and occasional ash emissions rising from the Shindake crater throughout the reporting period (figure 7). Though JMA reported thermal anomalies occurring on 29 January and continuing through late April 2020, Sentinel-2 imagery shows the first thermal signature appearing on 26 April.

Figure 7. Sentinel-2 thermal satellite images showed gas-and-steam and ash emissions rising from Kuchinoerabujima. Some ash deposits can be seen on 6 February 2020 (top right). A thermal anomaly appeared on 26 April 2020 (bottom right). Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

An eruption on 11 January 2020 at 1505 ejected material 300 m from the crater and produced ash plumes that rose 2 km above the crater rim, extending E, according to JMA. The eruption continued through 12 January until 0730. The resulting ash plumes rose 400 m above the crater, drifting SW while the SO₂ emissions measured 1,300 tons/day. Ashfall was reported on Yakushima Island (15 km E). Minor eruptive activity was reported during 17-20 January which produced gray-white plumes that rose 300-500 m above the crater. On 23 January, seismicity increased, and an eruption produced an ash plume that rose 1.2 km altitude, according to a Tokyo VAAC report, resulting in ashfall 2 km NE of the crater. A small explosion was detected on 24 January, followed by an increase in the number of earthquakes during 25-26 January (65-71 earthquakes per day were registered). Another small eruptive event detected on 27 January at 0148 was accompanied by a volcanic tremor and a change in tilt data. During the month of January, some inflation was detected at the base on the volcano and a total of 347 earthquakes were recorded. The SO₂ emissions ranged from 200-1,600 tons/day.

An eruption on 1 February 2020 produced an eruption column that rose less than 1 km altitude and extended SE and SW (figure 8), according to the Tokyo VAAC report. On 3 February, an eruption from the Shindake crater at 0521 produced an ash plume that rose 7 km above the crater and ejected material as far as 600 m away. As a result, a pyroclastic flow formed, traveling 900-1,500 m SW. The previous pyroclastic flow that was recorded occurred on 29 January 2019. Ashfall was confirmed in the N part of Yakushima Island with a large amount in Miyanoura (32 km ESE) and southern Tanegashima. The SO₂ emissions measured 1,700 tons/day during this event.

Figure 8. Webcam images from the Honmura west surveillance camera of an ash plume rising from Kuchinoerabujima on 1 February 2020. Courtesy of JMA (Weekly bulletin report 509, February 2020).

Intermittent small eruptive events occurred during 5-9 February; field observations showed a large amount of ashfall on the SE flank which included lapilli that measured up to 2 cm in diameter. Additionally, thermal images showed 5-km-long pyroclastic flow deposits on the SW flank. An eruption on 9 February produced an ash plume that rose 1.2 km altitude, drifting SE. On 13 February a small eruption was detected in the Shindake crater at 1211, producing gray-white plumes that rose 300 m above the crater, drifting NE. Small eruptive events also occurred during 20-21 February, resulting in gas-and-steam emissions that rose 200 m above the crater. During the month of February, some horizontal extension was observed since January 2020 using GNSS data. The total number of earthquakes during this month drastically increased to 1225 compared to January. The SO₂ emissions ranged from 300-1,700 tons/day.

By 2 March 2020, seismicity decreased, and activity declined. Gas-and-steam emissions continued infrequently for the duration of the reporting period. The SO₂ emissions during March ranged from 700-2,100 tons/day, the latter of which occurred on 15 March. Seismicity increased again on 27 March. During 5-8 April 2020, small eruptive events were detected, generating ash plumes that rose 900 m above the crater (figure 9). The SO₂ emissions on 6 April reached 3,200 tons/day, the maximum measurement for this reporting period. These small eruptive events continued from 13-20 and 23-25 April within the Shindake crater, producing gray-white plumes that rose 300-800 m above the crater.

Figure 9. Webcam images from the Honmura Nishi (top) and Honmura west (bottom) surveillance cameras of ash plumes rising from Kuchinoerabujima on 6 March and 5 April 2020. Courtesy of JMA (Weekly bulletin report 509, March and April 2020).

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 W of Yakushima. The Furudake, Shindake, and Noikeyama cones were erupted from south to north, respectively, forming a composite cone with multiple craters. All historical eruptions have occurred from Shindake, although a lava flow from the S flank of Furudake that reached the coast has a very fresh morphology. Frequent explosive eruptions have taken place from Shindake 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), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Soputan is a stratovolcano located in the northern arm of Sulawesi Island, Indonesia. Previous eruptive periods were characterized by ash explosions, lava flows, and Strombolian eruptions. The most recent eruption occurred during October-December 2018, which consisted mostly of ash plumes and some summit incandescence (BGVN 44:01). This report updates information for January 2019-April 2020 characterized by two ash plumes and gas-and-steam emissions. The primary source of information come from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) and the Darwin Volcanic Ash Advisory Center (VAAC).

Activity during January 2019-April 2020 was relatively low; three faint thermal anomalies were observed at the summit at Soputan in satellite imagery for a total of three days on 2 and 4 January, and 1 October 2019 (figure 17). The MIROVA (Middle InfraRed Observation of Volcanic Activity) based on analysis of MODIS data detected 12 distal hotspots and six low-power hotspots within 5 km of the summit during August to early October 2019. A single distal thermal hotspot was detected in early March 2020. In March, activity primarily consisted of white to gray gas-and-steam plumes that rose 20-100 m above the crater, according to PVMBG. The Darwin VAAC issued a notice on 23 March 2020 that reported an ash plume rose to 4.3 km altitude; minor ash emissions had been visible in a webcam image the previous day (figure 18). A second notice was issued on 2 April, where an ash plume was observed rising 2.1 km altitude and drifting W.

Figure 17. Sentinel-2 thermal satellite imagery detected a total of three thermal hotspots (bright yellow-orange) at the summit of Soputan on 2 and 4 January and 1 October 2019. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

Figure 18. Minor ash emissions were seen rising from Soputan on 22 March 2020. Courtesy of MAGMA Indonesia.

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is located SW of Riendengan-Sempu, which some workers have included with Soputan and Manimporok (3.5 km ESE) as a volcanic complex. It was constructed at the southern end of a SSW-NNE trending line of vents. During historical time the locus of eruptions has included both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

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/); 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).

Heard Island is located on the Kerguelen Plateau in the southern Indian Ocean and contains Big Ben, a snow-covered stratovolcano with intermittent volcanism reported since 1910. Due to its remote location, visual observations are rare; therefore, thermal anomalies and hotspots detected by satellite-based instruments are the primary source of information. This report updates activity from October 2019 to April 2020.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed three prominent periods of strong thermal anomaly activity during this reporting period: late October 2019, December 2019, and the end of April 2020 (figure 41). These thermal anomalies were relatively strong and occurred within 5 km of the summit. Similarly, the MODVOLC algorithm reported a total of six thermal hotspots during 28 October, 1 November 2019, and 26 April 2020.

Figure 41. Thermal anomalies at Heard from 29 April 2019 through April 2020 as recorded by the MIROVA system (Log Radiative Power) were strong and frequent in late October, during December 2019, and at the end of April 2020. Courtesy of MIROVA.

Six thermal satellite images ranging from late October 2019 to late March showed evidence of active lava at the summit (figure 42). These images show hot material, possibly a lava flow, extending SW from the summit; a hotspot also remained at the summit. Cloud cover was pervasive during the majority of this reporting period, especially in April 2020, though gas-and-steam emissions were visible on 25 April through the clouds.

Figure 42. Thermal satellite images of Heard Island’s Big Ben showing strong thermal signatures representing a lava flow in the SW direction from 28 October to 17 December 2019. These thermal anomalies are located NE from Mawson Peak. A faint thermal anomaly is also captured on 26 March 2020. Satellite images with atmospheric penetration (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben because of its extensive ice cover. The historically active Mawson Peak forms the island's high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported at this isolated volcano, but observations are infrequent and additional activity may have occurred.

Information Contacts: 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).

The following report, prepared on 17 March, is from volcanologists of the Institut Francais de Recherché Scientifique pour le Developpement en Cooperation, Office de la Recherché Scientifique et Technique Outre-Mer (ORSTOM), in Vanuatu and Ecuador.

Geological setting. Aoba is the largest basaltic shield volcano in the New Hebrides arc, with the base ~3,000 m below sea level, the summit ~1,500 m asl, and a volume of ~2,500 km³ (Eggins, 1993; Gorton, 1977; Robin and others, 1993). This rainforest-covered island lies in front of the d'Entrecasteaux collision zone, between the N and S Aoba Basins along an ~N50°E fracture transverse to the arc (figure 1; see Greene and others, 1994, for more information). Two concentric summit calderas, the largest 5 km in diameter (figure 2), enclose the central crater containing the 2-km-diameter Lake Voui (Vui) (figure 3). Numerous secondary craters and cones lie along the N50°E fracture, out to the extremities of the island, where previous magma-seawater interactions have produced several maars.

Figure 1. Bathymetric map of central Vanuatu showing the trench and direction of relative movement (arrows), Aoba, and other active volcanoes. Bathymetry is in kilometers. Modified from Greene and others, 1994.

Figure 2. Topographic map of Aoba (Ambae) Island, central Vanuatu. Areas of Recent phreatic explosion cones, spatter and scoria cones, and minor lava flows are approximated from a 1979 geologic map by the New Hebrides Geological Survey (!;100,000). Large dashed circles are 5- and 10-km radius lines. Topographic base map courtesy of C. Robin, ORSTOM.

Figure 3. Photograph of the summit of Aoba Island looking approximately NW. Two concentric calderas enclose the main central crater, which contains the 2-km-diameter Lake Voui (white). A black lake in the E part of the caldera, Lake Manaro, is in the foreground. The photograph was taken by a U.S. pilot during World War II, provided courtesy of C. Robin, ORSTOM.

Eruptive history. Lake Voui and the Manaro Ngoro summit explosion craters and cones formed ~420 years ago. The Ndui Ndui lava flows issued from the N50°E fissure ~300 years ago and reached the NW coast (Warden, 1970). Possible eruption-related lahars (or only secondary mudflows following heavy rains?) annihilated villages on the SE flanks of the island ~120 years ago, producing several casualties. An eruption possibly occurred in 1914 with ashfalls (?) and lahars (12 casualties). . . .

Robin and Monzier (1993, 1994) consider Aoba the most potentially dangerous volcano of the Vanuatu archipelago because of the wide distribution of very young deposits related to strong explosive eruptions. They also cite thick lahar deposits, the presence of Lake Voui, long repose periods (~300-400 years , Warden, 1970), strong degassing at the lake in 1991, and a population of ~3,500 within 10 km of the crater.

Activity in December 1994. Unusual seismicity was felt . . . during 1-7 December 1994 (BGVN 20:01). Records from ORSTOM seismic stations on Santo (70 km W) and Efate (260 km SSE) islands showed that peak activity lasted 24 hours with 13 events, the largest M 4.6 (Regnier, 1995). Crustal hypocenters were located under the S submarine base of the volcano. On 7 December, helicopter reconnaissance showed small areas of rising hot gaseous water at Lake Voui, similar to July 1991 and September 1993, but the rainforest appeared completely burned for up to several hundred meters around the crater. Despite the end of the seismic crisis, ORSTOM emphasized to the NDO the need to remain circumspect of the volcano. In mid-December, according to Robin and Monzier (1994), the following advice was given to NDO: "In the case of a resumption of volcanic activity in the summit area, it will be wise to evacuate, in a first phase, the population of coastal villages of the central part of the island (in a 10 km radius area surrounding Lake Voui) towards the less hazardous NE and SW extremities of the island. If the eruption occurs near these extremities, or spreads along fractures from central vents towards these extremities, then it might be necessary to evacuate part of the population to Santo or Maewo-Pentecost."

Activity in March 1995. According to a VANAIR pilot report on 1 March, Lake Voui was calm with gas emissions from numerous locations. The following day, the lake was steaming all over, bubbling up in the center, and its surface was rough; the pilot also reported black sediment ejections. Early on the morning of 3 March, people on Santo Island observed a gas plume rising 2-3 km above Lake Voui. Simultaneously, crustal seismicity similar to that in December 1994 was recorded.

On 4-6 March, ORSTOM geophysicists (M. Lardy and D. Charley) recorded strong continuous tremor at Ndui Ndui, ~9 km NW from the main crater. This tremor had a monochromatic signal with a 1.4 Hz mean frequency, several hours duration, and an amplitude of 3-4x background. Local observers were trained to watch the activity and the collaboration with VANAIR pilots was reinforced. As usual during the tropical summer, the top of the volcano was covered by thick clouds and rarely visible. However, on 5 March a gas plume was still visible above Lake Voui.

An island resident who stayed several days in the summit area during early March described lake levels and reported that soft mud had been blown all over the shores. On 4 and 6 March the surface of Lake Voui was at least 5.4 m higher than normal. However, on 9 March the lake was hot and steaming, and was ~4.8 m below the normal level, a change of ~10 m within 3 days. Tremor activity remained constant between 9 and 13 March, but with significantly less intensity than during 4-6 March. In addition, shallow, local micro-seismicity was noted since 11 March. During an aerial survey on 13 March, the entire lake was steaming and a strong sulfur smell had been reported around the summit area.

If activity increases in the central crater, magma-water interactions could produce falls of ash, dense lapilli, and accretionary lapilli, as well as pyroclastic flows, base surges and lahars. Lava flows may also erupt from flank fissures, N50°E or other orientations. The ORSTOM seismological team in Vanuatu will be reinforced on 17 March by the arrival of a new seismologist, and 5-7 portable seismic stations will be deployed around the island as soon as possible to improve the focal locations and delineate possible areas of attenuation. Also, a new permanent seismic station will be installed on Aoba. Daily contact is maintained between ORSTOM scientists in Vanuatu and Ecuador; the latter are prepared to move to Vanuatu if necessary.

Evacuation preparations. On 8 March, after discussions between ORSTOM geophysicists in Vanuatu and volcanologists now based in Ecuador, the following advice was given to the Vanuatu Government: ". . .The size of the gas plume observed above Lake Voui crater on March 3, 1995 probably means that magma is now rising within the volcano . . . . Thus, Aoba volcano is now dangerous and it seems necessary to envisage the evacuation of the population of coastal villages located in a 10 km radius area surrounding Lake Voui towards the less hazardous NE and SW extremities of the island . . . ."

Following this advice, Aoba Island was placed on alert and preparations for evacuations were begun. On 9 March, aircraft within a 4-km radius of Aoba up to 2.2 km altitude (7,500 feet) were restricted to scheduled flights and those approved by civil aviation or disaster office authorities. Correcting previous statements that evacuations had already started, the UNDHA reported on 17 March that villages within 10 km of the crater had been identified as threatened, and those within a 5-km radius had been placed on stand-by for immediate evacuation. Evacuation centers were identified, and all available government and several private ships were positioned to assist in a possible evacuation.

References. Eggins, S., 1993, Origin and differenciation of picritic arc magmas, Ambae (Aoba), Vanuatu: Contributions to Mineralogy and Petrology, v. 114, p. 79-100.

Gorton, M.P., 1977, The geochemistry and origin of quaternary volcanism in the New Hebrides: Geochimica et Cosmochimica Acta, v. 41, p. 1257-1270.

Greene, H.G., Collot, J.-Y., Stokking, L.B., and others, 1994, Proceedings of the Ocean Drilling Program, Scientific Results, 134: College Station, TX (Ocean Drilling Program).

Regnier, M., 1995, Rapport préliminaire sur la crise sismique d'Aoba de décembre 1994: Rapport ORSTOM, Port-Vila, 4 p.

Robin, C., and Monzier, M., 1993, Volcanic hazards in Vanuatu: Disaster Management Workshop by National Disaster Management Office, Republic of Vanuatu, 24-28 May 1993, Port-Vila, 8 p.

Robin, C., and Monzier, M., 1994, Volcanic hazards in Vanuatu: ORSTOM and Dept. of Geology, Mines and Water Resources of the Vanuatu Government report, 15 p.

Robin, C., Monzier, M., Crawford, A.J., and Eggins, S.M., 1993, The geology, volcanology, petrology-geochemistry, and tectonic evolution of the New Hébrides island arc, Vanuatu: IAVCEI Canberra 1993, Excursion guide, Record 1993 / 59, Australian Geological Survey Organisation, 86 p.

Warden, A.J., 1970, Evolution of Aoba caldera volcano, New Hebrides: BV, v. 34, no. 1, p. 107-140.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2,500 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: C. Robin and M. Monzier (geologists) ORSTOM, Quito, Ecuador; M. Lardy (geophysicist); M. Regnier, J-P. Metaxian, R. Decourt (seismologists), and D. Charley (technical assistant), ORSTOM, Vanuatu; M. Ruiz (seismologist), Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador; J-P. Eissen (geologist), ORSTOM, France; BOM, Australia; UNDHA.

Lidar data from Russia during April through December 1994 (table 1) continued to show a volcanic aerosol layer over Obninsk, generally between 14 and 21 km altitude. Throughout most of 1994 (see Bulletin v. 19, no. 4 for January-March 1994 data), backscattering ratios and integrated backscatter for the Nd-YAG wavelength generally remained stable at 1.2-1.4 and 0.18-0.34 x 10^-3, respectively. However, after 4 November the backscattering ratio was consistently-3.

Table 1. Lidar data from Russia and Germany showing altitudes of aerosol layers; some layers have multiple peaks. Backscattering ratios are for the Nd-YAG wavelength of 0.53 microns, with equivalent ruby values (0.69 microns) in parentheses for data from Germany. The integrated value shows total backscatter, expressed in steradians^-1, integrated over 150-m intervals from 15-30 km at Obninsk, and over 300-m intervals from the tropopause to 30 km at Garmisch-Partenkirchen.

During December through early February 1995, lidar data from Germany revealed the continued presence of an aerosol layer over Garmisch-Partenkirchen. Peak altitude during this period was usually 16-19 km. The backscattering ratio for the Nd-YAG wavelength, 1.2-1.3, has been unchanged since June 1994 (see Bulletin v. 19, nos. 10-11).

In Germany, a secondary peak on 1 December and the above-20-km peaks on 6 and 16 January may have been fresh volcanic aerosols caused by the 19 September eruption of Rabaul or the 1 October eruption of Kliuchevskoi (Bulletin v. 19, nos. 8-9). A secondary peak at ~24 km altitude was also detected on 5 December at Obninsk, Russia.

Geologic Background. 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 thorugh 1989. Lidar data and other atmospheric observations were again published intermittently between 1995 and 2001; those reports are included here.

Information Contacts: Sergey Khmelevtsov, Institute of Experimental Meteorology, Lenin Str. 82, Obninsk, Russia; Horst Jager, Fraunhofer -- Institut fur Atmospharische Umweltforschung, Kreuzeckbahnstrasse 19, D-8100 Garmisch-Partenkirchen, Germany.

A new eruption . . . was first noticed by the Indian Navy on 20 December 1994. A team composed of scientists from the GSI and Zoological Survey of India arrived at the island early on 24 January, and an aerial survey . . . was made on the 31st. As of 22 February, this mainly Strombolian eruption was still "in its initial stage, gradually gaining momentum."

During January and February, thick clouds of pale brownish gas, dark ash particles, and white steam from the crater area were rising ~200 m at intervals of 30 seconds, accompanied by continuous rumbling and intermittent "cracking" sounds. Two new vents were active, the first within the main crater near the SW corner, and the second ~50 m from the summit down the SW flank. The eruption is believed to have started from the flank vent, around which a new 100-m-diameter subsidiary crater had formed.

Incandescent material (cinder and volcanic bombs) rising to heights of 20 m could be seen from 4 km offshore. Particles ranged in size from a few cubic centimeters to ~1 m³, with the average size being slightly less than 10 cm³. Ejecta filled a valley on the S side of the western-most 1991 lava bed. Lava flows traveled ~1.5 km from the active vents into the sea, producing profuse steaming at the ocean entry. The moving lava front was ~50 m wide and 6 m thick by 22 February. Megascopically the lava was basaltic andesite, similar to that erupted during September 1991, with a high percentage of large plagioclase phenocrysts and frequent olivine in a dark-gray glassy groundmass.

On 9 March at around 0530 GMT astronauts on the Space Shuttle noticed a small plume rising from Barren Island. They made a short video recording (~15 seconds) showing a V-shaped plume that extended for ~3 km before dispersing. Visible imagery from the NOAA-14 (at 0730 GMT) and GMS (0430-0830 GMT) satellites failed to reveal a volcanic plume. A photograph taken from the Shuttle on 14 March at 0749 GMT again showed a small plume blowing W towards the Andaman Islands (figure 2). As this issue went to press, an aviation notice to airmen (NOTAM) on 27 March stated that the intensity of the eruption was unpredictable and advised all aircraft to avoid overflying the area.

Figure 2. Oblique photograph of the Barren Island eruption plume taken from the Space Shuttle, 14 Mar 1995 at 0749 GMT, looking NW. Ash plume is blowing generally W towards the Andaman Islands. NASA photograph STS 067-721A-052. Courtesy of Cindy Evans.

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; C. Evans, NASA-SSEOP; J. Lynch, SAB.

Scientists from the geologic group of CUICT (Centro Universitario de Investigaciones en Ciencias de la Tierra), RESCO (Red Sismologica Telemetrica de Colima), and the Colima Volcano Observatory at the University of Colima visited the summit on 4 and 15 February 1995.

During a previous ascent on 20 May 1994, temperature measurements of fumaroles were taken at 21 locations in two areas, E and NE of the summit; values were in the 274-304°C range. A gas sampling experiment (SO₂ and CO₂) used an aspirating pump (Matheson-Kitagawa toxic gas detector system) with 100-ml precision detector tubes and 1-5 minute collection times. SO₂ values of 200 ppm were measured at both sites; CO₂was 0.2 and 0.3%, respectively. Low temperatures (<60°C) at the gas sampling sites were required. A second ascent later in 1994 was not undertaken because of increased seismicity following a phreatic explosion in July.

During February 1995, the group visited the same points as in May 1994, as well as the bottom of the July 1994 crater. On 4 February, fumarole temperatures measured at 17 locations in the E summit area averaged 372°C, with a high value of 504°C. Temperatures in the NE sector averaged 398°C. Gas sampling (HF, HCl, SO₂, and CO₂) was again conducted at almost the same sites. Values in the E and NE sectors, respectively, were as follows for each gas: HF, 17.4 and 78.3 ppm; HCl, 8.0 and 63.3 ppm; SO₂, 180 and 460 ppm; CO₂, 0.25 and 0.85%. On 15 February, temperatures taken inside the E rim of the July 1994 crater averaged 230°C. A survey showed the crater to have a rim diameter of 135 m, a depth of 40 m, a floor diameter of 37 m, and an internal slope of 30° on the E side (figure 21).

Figure 21. Sketch map and topographic profiles of the summit of Colima, February 1995. Courtesy of Andrea Csillag Tirelli, Universidad de Colima.

A flight was made during clear weather on 11 February with a correlation spectrometer (COSPEC) to measure the SO₂ flux. Ten traverses at 3,050 m altitude were made between two navigational benchmarks using the aircraft global positioning system (GPS), assuming that the traverses were perpendicular to the plume axis. Wind speed and direction was computed using GPS at two points beneath the plume as well as before and after the traverses above the summit. Wind direction was 289° with an average velocity of 10.9 m/s. The SO₂ flux was determined to be 386 ± 160 metric tons/day, and was calculated according to instructions provided by S. Williams during a June 1994 workshop at UNAM in México City.

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 high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the 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, producing thick debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Carlos Navarro, Juan-José Ramirez, Abel Cortes, and Juan-Carlos Gavilanes, Colima Volcano Observatory and CUICT, Universidad de Colima; Andrea Csillag Tirelli, RESCO-CICBAS, Universidad de Colima.

A NOTAM issued from the Ujung Pandang aviation control center on 30 January noted the presence of a volcanic ash cloud from Dukono with both altitude and drift direction unknown. Satellite imagery gave no indication of the presence of volcanic ash, although there was evidence of a low-level smoke plume.

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: BOM Darwin, Australia.

The fissure eruption... has continued sending lava flows down the SW flank and into the sea. All of the new flows appeared to be aa lavas (figure 2). Godfrey Merlen compared the eruption intensity in late January to 5 March and concluded that it had decreased significantly... eruptions continued through at least 19 March.

Figure 2. SW Fernandina Island sketch map from an original ~9 February map by Godfrey Merlen with later annotations by Tui De Roy. GPS points A, B, and C were recorded on 7 March. Point A lay at the extreme S end of a new 80-m-wide aa flow that also passed through point B. Point C lay at the foot of the S side of an active cone.

Tui De Roy was on the island during 8-16 February and part of her report follows (the term "kipuka" refers to an area of older rocks surrounded by younger lava flows). She saw two vent areas (figure 2): 1) an early eruptive site (active before she arrived) in the crater of an old cone ("Old Cone"), and 2) a main vent where the sustained activity that she witnessed took place ("Main vent"). She also had a reconnaissance view of some small finger-like lava flows at higher elevation ("inexact" on figure 2 and discussed below under Early Activity).

"All of the activity has taken place along a prominently marked, prehistoric radial fissure running from about half way up the volcano right down to the shore. This fissure is marked by numerous old cones of varying ages, ranging from a very old, elongated (and perfectly aligned) well-vegetated cones covered in ancient ash at the edge of a kipuka ["Old Cone"], to a string of 6-8 very recent looking cones on the lower flats coming right down to the shore [figure 2]. Significantly, a couple of very small new spatter cones had been active briefly early in this eruption within the crater of the old cone.... The entire length of this radial fissure had built up through previous eruptions something of a ridgeline down the flank of the volcano, which served to deflect most of the current lava to its northern watershed, although later in our stay an increasing number of flows were beginning to spill over through a gap to the S, posing an imminent threat to the wildlife oasis of Cape Hammond...."

De Roy also noted that in many cases the paths of lava flows descending the flank "could not be readily followed because of undulations in the land and the fact that many of the flows disappeared into lava tubes at several points." But, she did describe flows that were visible, as follows.

"Both the active flows, as well as some that appeared to have now stopped, meandered and braided down the slope, with arms crisscrossing through irregular-shaped kipukas far to the NW of the main and most direct path to the sea. A new flow (as shown on Godfrey's map) reached the sea S of the main flows at about 0800 on 8 February where it formed a new delta and continued to advance steadily before halting a couple of days later."

Although there were slight variations, the intensity and height of the fountaining remained "remarkably steady" during her stay. The single active main vent displayed continuous fountaining 50-100 m tall. Fountains shot up both vertically and at oblique angles on either side of the vent. During 8-16 February the spatter cone around the vent grew considerably broader, but little taller. She camped near the vent on 9 and 13 February (figure 2) and watched the growth of a very blocky mass of rubble at the E base of the cone.

The migration of flows toward the N is emphasized by comparing De Roy's 16 February annotations of lava extent to the map completed by Merlen about a week earlier (figure 2). Starting about 12 February new flow paths developed high on the slope. Some lava flowed N as small fingers, but beginning at about 1600 on 12 February a large lobe flowed more southward than before. This migration of lava flows to the N and S corresponded with a progressive decrease in lava flow rate at the ocean entry (even though, as previously mentioned, the fountaining at the vent showed no marked decrease). By the time De Roy departed at noon on 16 February ". . . there seemed to be no more flowing of lava into the sea, with only slight wisps of steam still rising along the shore." On the nights of 13-15 February the glow from lava on the flats 1-2 km inland seemed to increase.

Although De Roy's observation of smoke and other airborne material was from upwind positions, she reported the following: "Only a very small amount of solid airborne particles appear to have been emitted during the initial stage of the eruption. A minimal amount of Pele's hair was evident near the shore, barely increasing in density closer to the vent. Within 1-2 km of the vent a thin dusting of light, gassy scoria littered the ground as in all previously observed Fernandina eruptions, but in much lower amount than some of the caldera eruptions of the 1970s and 1980s. Such scoria was still being produced at the time of our visit, with constant fallout in the area of our camp of 9 February whenever the eruption cloud drifted above us. No signs of ash from this eruption were present anywhere; although I did hear comment of 'ash' dusting one of the early boats to visit the site.

"Intense heat was rising from the main vent, with only moderate amounts of bluish-white smoke. It rose vertically into a constantly contorting, billowing, major thermal head, resembling a thunderhead. In addition, a pall of amber-colored fumes surrounded this cloud column and spread westward at all times, regardless of the shifting directions of the wind at lower elevations, which caused the main cloud to waver in various directions at different times of day or night. This pall was particularly evident when traversed by sunshine or moonshine, which took on a brownish hue. This plume should have been evident on satellite images, regardless of the main cloud possibly being mistaken for the normal thunderhead prevalent over the island during this El Niño season. The 'smoke' from the vent did seem to increase very gradually during our stay."

Besides the main vent, the eruption also produced voluminous amounts of gases from two other sources: 1) several areas of the main lava flow ~2 km below the main vent where degassing took place at the mouths of lava tubes, and 2) at the lava's ocean entry where mainly steam was rising. The first source of gases came out of the main lava flow and was thought to be degassing at the mouths of lava tubes.

Weather satellites (and shuttle astronauts)... have thus far been unable to obtain clear views of the eruption plume. The difficulty has been screening from high clouds coupled with inadequate eruptive plume heights. The TOMS instrument that has successfully imaged Galápagos eruptions since 1979 failed in December 1994.

Having seen the eruption in late-January, Godfrey Merlen returned... on the night of 5 March and noted a reduction in the comparative intensity of the eruption. In March the molten lava at the ocean entry was "dripping rather than flowing." Though less intense than in February, lava outflows remained concentrated at the site where lava had initially entered the sea in January; in March this amounted to about 10 separate outpourings over a 90-m lateral distance. Merlen noted that the small delta created there was ~5-m high and already cut back by waves forming an almost vertical cliff face. In contrast to earlier stages of the eruption, floating dead fish and the abundant wildlife feeding on them were largely absent. In March the sea surface temperature was up to 45°C, while it was ~24.5°C at a distance from the new delta. These temperatures were down from those in mid-Feb when at equivalent spots temperatures were >60°C and ~ 27°C (table 5). No new lava flows had moved to the S. Though still very hot, the new flow appeared to have left nearby vegetation nearly green, suggesting it may have been cooler when erupted than some of the earlier lavas. Scoria thickness on the new cone's upwind base averaged 5 cm.

Table 5. A summary of measurements and remarks comparing offshore seawater and nearshore turgid water close to the lava's ocean entry for the vigorous part of the eruption (late January and early February). Courtesy of Godfrey Merlen.

As previously mentioned, the "old cone" (figure 2) contained two or three early vents within its crater. These vents were marked by steep black spatter. The spatter had been flung 20-30 m, coating and charring trees. Those trees closest to the vents (~15 m from them) had their bark steamed off and were deep orange in color. Although these vents were only briefly active, they discharged a very rough aa flow.

Around the old cone many of the larger trees (Palo Santo and Opuntia cacti) had lost limbs or been knocked down (uprooted or snapped off at mid-height). The trees had predominantly fallen in a downhill direction, radiating roughly away from the main vent. An absence of directional scouring from scoria, and the presence of Waltheria bushes repeatedly twisted around their bases, suggested violent multidirectional wind gusts (a "tornado") rather than a well-defined unidirectional blast. Within a kilometer of the vent, however, Jasminocercus cacti consistently showed mild blistering from excess heat on their ventward sides.

Merlen noted that during the eruption lightning and heavy rain were commonly seen. For example, on the night of 28 January (prior to the release of ponded lava into the sea at about 2230) there was considerable sheet lightning coming from high clouds. Merlen also noted that high columns of thick white steam rose on occasion to ~4 km. The ascent of these plumes appeared dependant on the flux of lava into the sea.

Submarine acoustic recordings were also made by Merlen on 27-29 January using a Benthos hydrophone. The recordings detected extremely loud, echoing explosions at least 7 km from the lava's ocean entry. These sounds were not heard during subsequent visits (on 6-7 and 10 February); however, during all visits the hydrophones received a cacophony of hissings, poppings, and low-level thumps.

Some of Merlen's oceanographic observations are summarized in table 1. Within the discolored water Merlen also noted a ~100-m-diameter circular patch of upwelling water that was "glassy-smooth" and encircled by standing waves up to a meter in height on its margins. Located near the shore and not shifting in position, the upwelling water was cool and sufficiently turbulent to make steerage of the dingy difficult. In contrast to the cool (19.6°C) upwelling water, only 2-3 m away from its margin very hot (50°C) water was found. The upwelling water was brought to his attention by seabirds attracted to it. "Around this dramatic phenomenon and spreading out from it were a quantity of dead fish representing a mesopelagic fauna, including hatchet fish (Argyopelecus sp.), what appears to be a scabbard fish (Aphanopus sp.), and others that have yet to be identified." Although a limited amount is known about the vertical ranges of these kinds of fish, their presence at the surface may help determine the sources of this cold upwelling water.

Biological impact. De Roy noted that the wildlife appeared unable to comprehend the dangers from the intense heat of the lava. Marine iguanas were attracted to the warmth of active flows, climbed onto them, and were ignited before being able to escape. On the other hand, sea turtles and adult fur seals cruised through steaming waters within meters of the lava flow edge and showed no immediate signs of discomfort or injury. In other cases, it was unclear if the water temperature or chemistry was more critical in causing death (eg. pelicans, marine invertebrates, moray eels, and fish). In the sea and along the shore, many animals were attracted by the abundance of dead marine life floating on the surface. These opportunistic species included frigate birds, boobies, brown noddies, storm petrels, and many hundreds of pelicans. Merlen mentioned pelicans with pouches scalded from diving into hot seawater. In addition, De Roy saw sharks, sea lions, and flightless cormorants feeding. The eruption also killed some land iguanas. If lava flows were to reach Cape Hammond this would threaten flightless cormorants, penguins, and marine iguanas as well as one of the largest breeding populations of Galápagos fur seals. Merlen closed his 28 February report with the words: "the overall impression was that of biology in confusion."

Geologic Background. Fernandina, the most active of Galápagos volcanoes and the one closest to the Galápagos mantle plume, is a basaltic shield volcano with a deep 5 x 6.5 km summit caldera. The volcano displays the classic "overturned soup bowl" profile of Galápagos shield volcanoes. Its caldera is elongated in a NW-SE direction and formed during several episodes of collapse. Circumferential fissures surround the caldera and were instrumental in growth of the volcano. Reporting has been poor in this uninhabited western end of the archipelago, and even a 1981 eruption was not witnessed at the time. In 1968 the caldera floor dropped 350 m following a major explosive eruption. Subsequent eruptions, mostly from vents located on or near the caldera boundary faults, have produced lava flows inside the caldera as well as those in 1995 that reached the coast from a SW-flank vent. Collapse of a nearly 1 km3 section of the east caldera wall during an eruption in 1988 produced a debris-avalanche deposit that covered much of the caldera floor and absorbed the caldera lake.

Information Contacts: T. De Roy, Golden Bay, New Zealand; G. Merlen and D. Day, Estacion Cientifica Charles Darwin; J. Lynch, SAB; C. Evans, Lockheed.

Activity during January and February consisted of slow gas release, decreasing the chances of an eruption in the near future. Overflights on 6 and 9 January revealed no changes in the fumarolic activity. On 23 January a visual inspection of the active cone from the police station revealed increased fumarolic activity in the W sector. In several visits to the summit, the principal points of gas emission were La Joya, Las Deformes, Las Chavas, and El Paisita fumarolic areas, and low-pressure zones on the interior of the main crater and the inside W crater wall (figure 72); fumarolic columns rose <30 m. Temperature measurements at Las Deformes and La Joya fumaroles (average 130°C) showed a small decrease compared to 21 July 1994.

Figure 72. Sketch map of the Galeras summit crater, 24 January 1995. Courtesy of INGEOMINAS.

SO₂ measurements obtained by COSPEC increased compared to December (2 remained stable during February (~200 t/d), and deformation measurements showed no variations.

A total of 89 screw-type seismic events were recorded between 20 October 1994 and 9 January. These types of signals, associated with pressure in the system, preceded five of the six eruptions between June 1992 and July 1993. Long-period events were recorded after 9 January. A swarm of "butterfly" events (a hybrid long-period, high-frequency event) on 20 January was the first since July 1994; a peak of 210 events was recorded on the 21st. The number of high-frequency events was very low in early 1995, but increased slightly after 23 January. These signals, which have a similar wave form to long-period events, were located principally in the W sector of the active crater at depths of <4 km.

Shallow high-frequency seismicity in February was concentrated near the crater. There was also sporadic fracturing activity from the W part of the crater (small magnitudes with depths <6 km) and from a N source (M <1.9 and depths of 5-7 km). "Butterfly" events were observed through mid-Feb with an average of 50 events/day before decreasing to 15 events/day toward the end of the month. These events were concentrated near the active cone, at depths <1 km. Few long-period events occurred during the month, but after 26 February a new type of high-frequency signal (called "Pseudo-Screw") began with dominant peaks of 8-10 Hz.

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, Pasto.

On 15 February, inhabitants of the Huemules valley 40 km NW of Hudson heard noises coming from the volcano. The next day a sulfurous odor reached the city of Coihaique, 75 km NE of Hudson. The day after that (17 February), inhabitants of the Huemules valley again heard noises and smelled sulfur. Furthermore, the Huemules river rose such that its banks shifted laterally by 30-40 m from its normal course.

Based on an interpretation of a NOAA satellite image, personnel of the Centro de Estudios Espaciales de la Universidad de Chile reported a 10-km-diameter, 37°C thermal anomaly over the E sector of the caldera. Rodrigo Rodrigues (ONEMI) overflew the area on 21 February but saw no fresh ash upon the snow. He only saw minor fumarolic activity, mainly discharging steam. The steam escaped from part of crater 1, an area in the glacial ice cap along the W wall of the 9-km-diameter summit caldera (see BGVN 16:07-16:11).

As on 14 March 1994, this event may have generated phreatic explosions, local subglacial melting, and steam production, all possibly due to heat remaining from the 1991 eruptive cycle. Similar activity was also reported during 10-13 April 1993 and a rainy summer season in 1991-1992 caused extensive reworking of pyroclastic debris, particularly down the Huemules river (BGVN 17:03). Prior to the overflight, on 6 February 1995 a pilot flying near the Chile-Argentina border (close to Balmaceda, 45.52°S, 72.43°W) noted "strong volcanic activity." Since prevailing winds blow from the W, this might have been new ash from Hudson, but it also might have been dust or Hudson ash re-suspended from previous ground deposits.

Preliminary tephrochronology indicates that in the last 7,000 years Hudson has had at least 3 large magnitude eruptions (possibly in the VEI 4-6 range). Minor Plinian eruptions had a recurrence interval of 500 to 1,000 years (Stern and Naranjo, in press).

Hudson produced one of the largest eruptions of the 20th century starting on 8 August 1991 from a fissure cutting the caldera rim. The paroxysmal phase began on 12 August, sending columns up to 16-18 km for 3 days, resulting in ash fall on the Falkland Islands, 1,000 km away. Pyroclastic flows were mostly restricted to the caldera floor, and a lava flow traveled 4 km down the WNW flank following the glacier along the upper reaches of the Huemules valley. The eruption plume of 14-15 August was blown rapidly E by the Roaring Forties winds so that about 5-6 days later a "strange haze" arrived in Australia, 15,000 km E.

Reference. Stern, C.R., and Naranjo, J.A., in press, Summary of the Holocene eruptive history of the Hudson volcano, in Bitschene & Mendia (Eds.). The 1991 eruption of the Hudson volcano: a thousand days after, Naturalia Patagonica: Universidad Nacional de la Patagonia, Comodoro Rivadavia and Publicacion Series of the Argentianian Geological Survey, Buenos Aires, Argentina.

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: Jose Antonio Naranjo, Servicio Nacional de Geologia y Mineria, Avenida Santa Maria 0104, Casilla 1347, Santiago, Chile.

Both the Lae`apuki and Kamoamoa lava flows had many breakouts on the coastal plain during February, and several aa and pahoehoe flows were observed on the Pulama pali flow field (figure 96). Poor weather conditions and thick fume clouds obscured the Pu`u `O`o lava pond during the first half of February, but it was very active and 75 m below the crater rim on 24 February.

On 2 February, lava that broke out of the Kamoamoa tube system at ~600 m elevation fed flows that burned forest and cascaded down Pulama pali. This fast-moving pahoehoe flow reached Paliuli on the 14th, 700 m W of the Lae`apuki flow, and headed for the Chain of Craters Road, burning grasslands and setting off methane explosions. The flow front stagnated within 150 m of the road on 27 February. Lava broke out of the tube again on 10 February at ~615 m elevation and formed a channelized aa flow 1 km W of the main flow field that reached the base of Pulama pali by the 13th. In the second half of February the Lae`apuki flow had several breakouts between Paliuli and the ocean that spread W, covering new land and starting brush fires and methane explosions.

Lava flows were active at four ocean entries during the month (figure 96). Lava continued to enter the ocean across a wide front on the Kamoamoa flow, and built benches into the ocean. Explosions following a small bench collapse at the W Kamoamoa entry spread spatter 30-40 m inland of the sea cliff. A lava flow also advanced to the E edge of the Kamoamoa flow field and on 10 February entered the ocean within a few hundred meters of the Kupaianaha flow (Kamokuna entry). This entry then built a large bench that merged with Kupaianaha flows.

Low-amplitude tremor dominated the east rift zone throughout the first half of February. The number of microearthquakes was low beneath the summit and rift zones except for a slight pickup in LPC-C activity (5-13 km depth, 1-5 Hz) on 10-11 February. A series of three small earthquakes in the lower east rift on 10 February (M 2-2.5) originated from a shallow source near Puulena Crater, E of the Leilani Estates subdivision; a few residents felt the events. Tremor amplitudes in the second half of February were slightly higher at a fairly constant level 3x background, interrupted by a few bursts of higher-amplitude tremor. Activity beneath the summit and rift zones was low except for a steady swarm of LPC-C events. During 24-27 February, intermediate, long-period microearthquake counts were high, averaging nearly 200 events on 26-27 February.

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.

"Activity during February continued to be focused at Crater 2, at the moderately low level observed since December. Emissions consisted mainly of white-to-grey vapour-and-ash clouds in low or moderate volumes. Occasionally, an explosion produced a larger and darker cloud that rose a few hundred meters above the crater and produced fine ashfall SE of the volcano. Rumbling noises accompanying the emissions were heard intermittently throughout the month, and weak glow was seen on most clear nights. Activity at Crater 3 consisted essentially of fumarolic emission of thin white vapour. The seismograph was not in operation during February."

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 E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N 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. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: P. de Saint-Ours, R. Stewart, and B. Talai, RVO.

One of the most active volcanoes in Chile, Llaima's last reported eruptive episode began on 17 May 1994. An overflight made in the late morning of 15 February (in conjunction with Simon Young and John Simmons) disclosed only minor fumarolic activity. The fumarolic activity focused on the N internal wall of the main crater. In accord with the minor fumarolic activity, no new ash was seen. The summer ice melt has exposed the May and August 1994 scoria deposits (BGVN 19:04, 19:05, and 19:08), layers blackening the glaciers and rocks on the volcano's slopes. Along the crater's SSW border, a roughly 200-m-deep notch exposed alternating lava and tephra layers that mantle the edifice. A small scoria cone surrounding the source vent sat in the SE portion of the crater after the 26-30 August 1994 eruption. That feature later collapsed without leaving a visible trace. The crater itself had a depth of ~350 m.

The episode that began on 17 May 1994 generated a Strombolian-to-subplinian eruption with associated lahars and flooding, and produced a column ~4-5 km above the summit. Tephra fell over a cigar-shaped zone trending about ESE. A 500-m-long, SW-trending fissure produced explosions and lava fountains. Lava flowed across the bottom of a glacier on Llaima's W flank, melting snow and ice that caused lahars to descend into the Calbuco and Quepe rivers. Flooding occurred farther from the volcano.

Geologic Background. Llaima, one of Chile's largest and most active volcanoes, contains two main historically active craters, one at the summit and the other, Pichillaima, to the SE. The massive, dominantly basaltic-to-andesitic, stratovolcano has a volume of 400 km3. A Holocene edifice built primarily of accumulated lava flows was constructed over an 8-km-wide caldera that formed about 13,200 years ago, following the eruption of the 24 km3 Curacautín Ignimbrite. More than 40 scoria cones dot the volcano's flanks. Following the end of an explosive stage about 7200 years ago, construction of the present edifice began, characterized by Strombolian, Hawaiian, and infrequent subplinian eruptions. Frequent moderate explosive eruptions with occasional lava flows have been recorded since the 17th century.

Information Contacts: Jose Antonio Naranjo, Servicio Nacional de Geologia y Mineria, Avenida Santa Maria 0104, Casilla 1347, Santiago, Chile.

"Activity during February decreased further from January levels. Both South and Main craters released weak white vapours in low to moderate volumes. One explosion from South Crater on 19 February emitted a grey cloud, and a weak glow was seen on the night of the 24th. Seismicity was low during the first half of February, but increased somewhat during the 2nd and 3rd weeks. No significant change was shown by the water-tube tiltmeter 4 km SW of the summit."

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 basaltic-andesitic stratovolcano to its lower flanks. These 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 observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent 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: P. de Saint-Ours, R. Stewart, and B. Talai, RVO.

Workers at the GMU Geophysical Laboratory and Martin Beisser of GFZ-Potsdam recorded seismic data during the [summit lava dome] collapse from their station at Klathakan, 1.8 km WNW of the summit. Their broad-band seismic instrument showed the associated disturbance beginning on 22 November at 1007 and 32 seconds (radial-component data shown on figure 14). So far as the GMU and GFZ workers know, the wide dynamic range of their broad-band instrument preserved the event with a minimal amount of high-amplitude signal "clipping." Also, in their interpretation, the collapse and seismic disturbance began simultaneously. In other words, the initial displacement at the beginning of the seismic record is thought to correspond to the arrival of signals from the inception of the collapse.

Figure 14. Seismic record for the Merapi 22 November 1994 dome collapse. The component shown is horizontal, radial to the edifice; amplitude scale is arbitrary. The data were recorded on a data logger connected to a Streckeisen STS2 seismometer (with a 50 Hz sampling rate, a 8.33 mHz to 50 Hz linear response, and a 32-bit analog-to-digital converter). Courtesy of A. Brodscholl and K. Brotopuspito.

The collapse-related seismic event lasted for almost an hour (figure 14). The initial signals were set against a moderately quiet background, and maximum amplitude generally increased with time. Highest-amplitude signals were received ~40 minutes after the event began. These largest signals had amplitudes that reached approximately 30 mm/second, whereas at the beginning of the collapse the maximum amplitudes were only ~0.05 mm/second. Thus, on the seismic records, amplitudes ultimately grew to 600x as large as the initial signals.

The eruption and collapse also appear in a 200-hour time window showing measured seismic amplitude in specified wavelengths (figure 15). The figure was prepared using signal processing techniques, which for the high frequency (0.1-1.0 Hz) data involved significant averaging of the maximum values (to once an hour). These depictions show that one or two noteworthy seismic disturbances took place at ~150 and 180 hours prior to the collapse (cause unknown). Compared to the other seismic disturbances on these records, the collapse and eruption induced larger amplitude and much more sustained signals. The post-collapse signals were also followed by an interval of at least 10 hours of elevated background (most noticeable in the 1-12 Hz range).

Figure 15. Radial component of the Merapi 22 November 1994 dome collapse showing a seismic amplitude (arbitrary scale) versus time for stated wavelength ranges. The inception of the collapse lies at the zero point of the time scale. Courtesy of A. Brodscholl and K. Brotopuspito.

Using the available data, the investigators failed to find any clearly related premonitory seismic signals for the collapse. Sufficient collateral data (for example, teleseismic and meteorological data) might help constrain detected collapse and eruption earthquakes, or shed light on the cause of the pre-collapse seismic disturbances.

Since our last report (19:12), continued dome building occurred at Merapi. On 5 January another collapse brought 1 x 10⁶ m³ of debris downslope. This collapse produced a small pyroclastic flow on the S slope.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: A. Brodscholl and K. Brotopuspito, GMU; M. Beisser, GFZ-Potsdam, Germany; W. Tjetjep, VSI.

On the morning of 21 February at 1105, for the first time since eruptions began on 21 December 1994, Claus Siebe was able to look into the crater from a helicopter without fumes or ash impeding visibility. A small crater surrounded by a tuff cone composed of light-brown to gray silty-sandy ash occupied the site of the former lake. Judging from the color, he interpreted the loose ash to be mostly non-juvenile. A plume was emitted from a depression in the ash cone at 1115 and rose ~3 km above the crater rim. No snow has fallen in recent weeks, and all the snow and ice in the summit area was covered by a thin coat of ash.

Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: Claus Siebe, Instituto de Geofísica, UNAM, Coyoacán.

"Eruptive activity resumed at Tavurvur on 13 February after one and a half months of quiescence; no precursory activity was detected. Following the end of the 1994 eruption on 23 December, Tavurvur had exhibited only fumarolic activity. The amount of vapour released declined during January and emissions became intermittent in the first half of February. Seismicity was low, although some volcanic earthquakes continued to be recorded. The deflation rate of the caldera was also extremely low.

"After about 0100 on 13 February, small explosions started from Tavurvur's 1994 crater. Activity increased during the early hours of the morning, and large explosions occurred at 0328, 0857, 0919, 0937, 1012, 1100, and 1230. Each of these lasted 2-3 minutes and generated ash clouds that rose 1,500-3,000 m above the crater. Some of the ash clouds were laced with lightning. Ballistic blocks were seen falling onto the flanks of the cone and into the sea around Tavurvur. Between the larger explosions, emissions were less energetic or in "puffs" over periods of 5 minutes or more. After the first day, the emissions generally rose 500-1,000 m above the crater and were blown SE, producing a 10-km-long discontinuous, diffuse, pale-grey plume.

"Each of the explosions was accompanied by a distinctive explosive or low-frequency earthquake whose amplitude corresponded to the size of the explosion. Changes in the eruptive activity could therefore be tracked using RSAM data from station KPTH on Matupit Island (figure 23). An analysis of RSAM 1-minute data produced the event counts and mean amplitudes shown in figure 23. These showed that after a few hours of large events, at an average rate of ~10/hour, the activity was dominated by smaller explosions that peaked after about a day and a half on 14 February, at an average rate of 15/hour. The number of explosions and their amplitude then declined over the next 2-3 days. On 17 and 18 February, however, the activity increased again, perhaps associated with heavy rain on the 16th and 17th. The event count stayed fairly constant until the end of the month, although event amplitudes exhibited a slowly increasing trend.

Figure 23. Rabaul tilt and seismicity measured at stations MPT and KPTH on Matupit Island, 1 February-10 March 1995. Positive N and W tilts indicate deflation of the caldera. Note that times are GMT (= local time - 10 hours). Courtesy of RVO.

"Apart from the low-frequency explosive events associated with the Tavurvur eruption, earthquake activity at Rabaul was very low in February. There were only four small high-frequency earthquakes recorded, compared to 28 in January. Two were located at shallow depths near Vulcan and the other two were outside the seismic network to the NE.

"Throughout the first part of February, ground deformation data continued to show the slowing deflationary trend seen since September 1994, with the deflation centered S of Matupit Island. Electronic tilt data from station MPT on Matupit Island showed deflation of ~0.5 µrad/day during this period (figure 23). Seashore survey measurements around Greet Harbour were in good agreement, with subsidence of <1 cm/month. Following the renewal of activity at Tavurvur, ground deformation rates seem to have decreased, with only 3 µrad of tilt at MPT in 3 weeks, and no measurable changes in seashore levelling data. The gap in the tilt data on figure 23 was because the battery at MPT was stolen the day before the explosive activity began.

"There were three aerial inspections of Tavurvur during this period. On the morning of 13 February, before the large explosions took place, there was no marked change in the configuration of the bowl-shaped crater compared to the previous inspection in January. There also was no open vent, although the explosive emissions rose through the central part of the crater floor, which was covered with ash and rubble. On 20 February, emissions were seen rising from an obstructed vent in the SE part of the crater, while a strong fumarole was active on the W side of the crater (at the head of the 1994 lava flow). A small mound of lava seen on the 27th at the base of the crater was 20-30 m wide, only a few meters high, and was partly mantled with ash. Emissions were released through cracks in the lava or from between blocks near the edges.

"Throughout February, Vulcan continued to exhibit only very weak fumarolic activity from diffuse sources around the edge of the floors of both the 1937 and 1994 craters. At some time in late January or February, hot steaming springs appeared along the N shore of the Vulcan headland. Measured temperatures were consistently around 100°C.

"The Gazelle Peninsula has remained under a State of Emergency, with access to Rabaul controlled because of the risk from mudflows and flooding. Although the rainy season has been unusually mild so far, mudflows and flash floods are causing much damage to the roads into Rabaul and are flooding the remaining buildings in the town and in nearby villages."

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the 688-m-high asymmetrical pyroclastic shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: P. de Saint-Ours, R.Stewart, and B. Talai, RVO.

Activity during February-March was characterized by almost no magma supply to the dome. The dike at the top of the endogenous dome had almost stopped moving in late January. No changes at the dome were observed during either helicopter or ground-based inspections. No large rockfalls or pyroclastic flows have occurred since early February. Emissions of SO₂ from the dome declined to below the COSPEC detection limit, according to SEVO (Shimabara Earthquake and Volcano Observatory, Kyushu University).

Dome outlines observed from several fixed points using theodolite by both SEVO and JMA showed no change during February. EDM measurements by the Geological Survey of Japan indicated that mirrors located on the upper NW to SW flanks near the dome moved little, except one 500 m SW of the dome. The distance between the latter mirror and a point ~1.5 km S has been decreasing at a steady rate of ~0.3 mm/day during the last four years (there were no data prior to dome extrusion).

Except for a swarm of 55 events on 4 February, microearthquakes beneath the lava dome occurred at a rate of <5/day. A total of 81 events registered in February at the seismic station 3.6 km SW of the dome. However, there have been isolated tremors, but these were much smaller and scarcer that those that preceded dome extrusion in 1991. Only two pyroclastic flows were detected at a seismic station 1 km WSW of the dome, both of which traveled ~500 m SE.

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: Setsuya Nakada, Volcano Research Center - Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan; Volcanological Affairs Office, Seismological and Volcanological Dept, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

The RSP stations in Tahiti registered acoustic T-waves (tertiary waves traveling through the ocean) beginning on 8 January. This seismic swarm ended after 9 small and 5 stronger events in early February. The total number of recorded events during this swarm was 100 small and 45 larger events. Twelve of the larger events in January (M 4.2-4.8), detected and located by the world-wide seismic network, showed that the swarm was spread ~130 km along a NW-SE trend,~50 km NE of Ta'u Island (see figure 1) in the E Samoa Islands.

Geologic Background. Vailulu'u, a massive basaltic seamount not discovered until 1975, rises 4,200 m from the sea floor to a depth of 590 m about one-third of the way between Ta'u and Rose islands at the E end of the American Samoas. It is considered to mark the current location of the Samoan hotspot. The summit contains a 2-km-wide, 400-m-deep oval-shaped caldera. Two principal rift zones extend E and W from the summit, parallel to the trend of the hotspot. A third less prominent rift extends SE of the summit. The rift zones and escarpments produced by mass wasting phenomena give the seamount a star-shaped pattern. On 10 July 1973, explosions were recorded by SOFAR (hydrophone records of underwater acoustic signals). An earthquake swarm in 1995 may have been related to an eruption. Turbid water above the summit shows evidence of ongoing hydrothermal plume activity.

Information Contacts: F. Schindele, LDG, Tahiti; NEIC.

Geologists who made an overflight of the stratovolcano late on the morning of 15 February (in conjunction with Simon Young and John Simmons) observed increasing fumarolic activity. Villarrica gave off moderate puffs of bluish, sulfurous gases at 1-2 minute intervals that rose 300-400 m above the crater before dispersing to the SE.

Between 1040 and 1245 on 15 February the local seismic station (VVN) registered an average of 3 tremor episodes per minute. This tremor had frequencies of 1.3-1.5 Hz, 0.3 Hz below the frequency customarily received (1.8 Hz), and considered a possible indication of a slightly deeper source than typical for both the tremor and the puffs. This behavior continued until 1900 on 15 February. Afterwards tremor diminished and puffing ceased at the fumaroles. These later conditions prevailed until at least 19 February.

The crater, a little more than 200 m in diameter, contained a nested terrace (figure 4). The inner crater floor sat ~200 m below the crater rim, the bottom 50 m of which was black in color, possibly composed of scoria. At the very bottom center an opening exposed ~20 m of material with a bright red glow.

Figure 4. Sketch of Villarrica's crater as seen on 15 February 1995. Courtesy of J. Naranjo, G. Fuentealba, and P. Peña.

Black ash on the glaciers of the E and S flanks extended 4.6 km in the S20 E direction and 2.5 km in the S direction (figure 5). These ash lobes could correspond to eruptions on 25 and 29 December 1994 (19:12).

Figure 5. Distribution of black ash from Villarrica's crater as seen on 15 February 1995. Courtesy of J. Naranjo, G. Fuentealba, and P. Peña.

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: J. Naranjo, SERNAGEOMIN, Santiago; G. Fuentealba and P. Peña, SAVO.

A hydrothermal explosion around 1430 on 11 February killed four people at a highway construction site, located in a geothermal area along the narrow Azusa-gawa River ~2 km SE of the summit. The Geological Survey of Japan reported that there were at least two explosions from the vent (12 m long and 6 m wide). The first, a large explosion, created a 1,500-m-high plume composed of mud and gas, and caused collapse of the river bank, burying the primary vent. A second explosion scattered mud and gas within 200 m of the vent. JMA staff who surveyed the site on 12 February and 13 March noted that fume rising to a height of 20 m was almost at the boiling point. No explosions have been reported since 12 February.

Geologic Background. Yakedake rises above the popular resort of Kamikochi in the Northern Japan Alps. The small dominantly andesitic stratovolcano, one of several Japanese volcanoes named Yakedake or Yakeyama ("Burning Peak" or "Burning Mountain"), was constructed astride a N-S-trending ridge between the older volcanoes of Warudaniyama and Shirataniyama. Akandanayama, about 4 km SSW, is a stratovolcano with lava domes that was active into the Holocene. A 300-m-wide crater is located at the summit, and explosion craters are found on the SE and N flanks. Frequent small-to-moderate phreatic eruptions have occurred during the 20th century. On 11 February 1995 a hydrothermal explosion in a geothermal area killed two people at a highway construction site.

Information Contacts: JMA.