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 monthly number of recorded explosions declined from a 6-year high of 60 in January, to 16 in February. Seven car wind shields were cracked by lapilli from an explosion at 1009 on 1 February, and two more were cracked at 0630 on 2 February, when the month's highest plume rose 3.5 km. Seismicity was higher than normal, with swarms of volcanic earthquakes recorded on 4, 7-15, 17-19, and 23-29 February.

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

Information Contacts: JMA.

Two blocky lava flows continued to extend down the WSW and W flanks in February (figure 44). The WSW-flank flow, which began in mid-to late November, followed the well-defined levees of the September flow. By the end of February, the active flow had surpassed the older flow's front, advancing several meters daily, burning grass, and reaching 1.8 km length (750 m elevation). The 200-m-wide W-flank lava flow extended ~700 m, to 1,200 m elevation, by the end of February. Gravitational collapse of the W-flank's lava flow front on 24 February produced block-and-ash flows that traveled down valleys to 780 m elevation. Geologists believed that an apparent new amphitheater on the WSW side of crater C had caused lava flows to travel preferentially in that direction during recent months.

Figure 44. Map of late 1991-February 1992 lava flows and the 24 February block-and-ash flow at Arenal. Courtesy of ICE.

Strombolian explosions were low in number and magnitude in February, with 173 recorded during the first 18 days. Many ash emissions, to 1 km height, were observed without obvious explosions. Size analysis of one tephra sample collected on 26 February showed that 85% was coarse-ash and <15% was very coarse ash to fine lapilli. The sample was composed primarily of vesiculated rock fragments, aphanitic and porphyritic in character, and plagioclase crystals.

An average of 10 volcanic earthquakes (a range of 2-24) was recorded daily (at ICE station "Fortuna" 4 km E of the crater) in February. Large increases in tremor period and energy were measured on 6, 7, and 21-25 February, coinciding with increased lava output and strong gas emission. Tremor was recorded up to 24 hours/day.

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

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and R. Van der Laat, OVSCIORI; G. Soto and R. Barquero, ICE.

During 4 March fieldwork, a thin white vapor plume continued to emerge from the crater. The volume of the crater lake seemed unchanged from the previous month at about 600,000 m³, but its pH had dropped to 3, from 5 in February. Lake-water temperature ranged from 31 to 36°C. Solfataras N of the crater had temperatures of 78-101°C, while those S of the crater were at 55-100°C. Deep volcanic earthquakes occurred at a rate of ~1/week.

Geologic Background. The massive Gunung Awu stratovolcano occupies the northern end of Great Sangihe Island, the largest of the Sangihe arc. Deep valleys that form passageways for lahars dissect the flanks of the volcano, which was constructed within a 4.5-km-wide caldera. Powerful explosive eruptions in 1711, 1812, 1856, 1892, and 1966 produced devastating pyroclastic flows and lahars that caused more than 8000 cumulative fatalities. Awu contained a summit crater lake that was 1 km wide and 172 m deep in 1922, but was largely ejected during the 1966 eruption.

Information Contacts: W. Modjo and W. Tjetjep, VSI.

Colima remained quiet from November through January. In mid-January, the top of the cone was snow-covered. The snow later melted and some small landslides were observed.

A team from FIU and Earthwatch visited the summit dome on 28 January. No changes were evident since their previous visit in September 1991. Degassing remained widespread on the dome but was distinctly less vigorous than during active lava extrusion in May. Snow was as much as 2 m deep in some places near the summit, but was absent in fumarolic areas. Four small rockslides occurred on the N flank of the dome during three days of observations, a much lower rate than in May but similar to that of September. Temperatures at four fumaroles were continuously recorded between 1 November and 28 January. Mean temperatures remained between 475 and 535°C. Temperatures were quite steady (except for diurnal variations) and were not affected by unseasonably heavy January precipitation.

Geologists with the CICT reported that six low-magnitude seismic events were recorded during the last three days of February, some only by the Soma station 700 m NW of the cone. No earthquakes were detected 1-3 March, but on 4 March, the Soma station recorded 42 shocks, 17 of which were also recorded by the Yerbabuena station, 7.5 km SW of the summit. No seismicity was evident at more distant stations. Some landslide events were detected at the Soma station, suggesting that they occurred on the NW flank. Seismic activity increased during the first 12 hours of 5 March, when the Soma station registered 39 earthquakes, of higher amplitude than the day before; 24 events were detected at the Yerbabuena station during the same 12-hour period. Geologists observed few morphological changes on the cone's N and NE flanks, although there was some evidence of landslides, probably caused by heavy rain and snow in January. From the W side of the cone, 12 landslides were noted on 5 March between 1145 and 1508; five lasted 3-4 minutes. A gorge near the summit had been recently eroded by the landslides. Although the seismicity and landslides were similar to the activity that preceded the dome extrusion beginning in March 1991, activity had declined to near background by 10 March.

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: Ignacio Galindo, Centro Internacional de Ciencias de la Tierra (with participation of CICT and RESCO staff), Universidad de Colima; S. de la Cruz-Reyna, UNAM; C. Connor and J. West-Thomas, FIU, Miami.

A seismic swarm started on 17 February, with activity peaking by 20 February, and still declining as of 26 February (figure 1). More than 300 small high-frequency earthquakes (eight with M > 3.0) were recorded, the largest (M 4.0) at 0319 on 19 February. Hypocenters show a 3-km-long pattern elongated to the NNW, at 3-5 km depths (figure 2). The focal mechanism for the largest event showed mainly strike-slip motion (right-lateral on a N-S plane, or left-lateral on an E-W plane), with a small normal component. There were no reports of injuries or damages.

Figure 1. Hourly number of earthquakes in the Coso Mountains, 17-26 February 1992. Courtesy of the USGS.

Figure 2. Epicenter map (top) and E-W cross-section showing focal depths (bottom) of >300 high-frequency earthquakes recorded in the Coso Mountains, 17-26 February 1992. Courtesy of the USGS.

The Coso region is an active geothermal area that has had seismic swarms in the past, as in 1982 when thousands of events were recorded, the largest M 4.9. The Volcano Peak cinder cone and lava flow, apparently the youngest features in the Coso Mountains, are believed to have been erupted 0.039 ± 0.033 mybp. (K/Ar age).

Geologic Background. The Coso volcanic field, located east of the Sierra Nevada Range at the western edge of the Basin and Range province consists of Pliocene to Quaternary rhyolitic lava domes and basaltic cinder cones covering a 400 km2 area. Much of the field lies within the China Lake Naval Weapons Center. Active fumaroles and thermal springs are present in an area that is a producing geothermal field. The youngest eruptions were chemically bimodal, forming basaltic lava flows along with 38 rhyolitic lava flows and domes, most with youthful, constructional forms. The latest dated eruption formed the Volcano Peak basaltic cinder cone and lava flow and was Potassium-Argon dated at 39,000 +/- 33,000 years ago. Although most activity ended during the late Pleistocene, the youngest lava dome may be of Holocene age based on geomorphological evidence (Monastero 1998, pers. comm.).

Information Contacts: J. Mori and W. Duffield, USGS.

The following is from a report by the Gruppo Nazionale per la Vulcanologia (GNV) summarizing Etna's 1991-92 eruption.

1. Introduction and Civil Protection problems. After 23 months of quiet, and heralded by ground deformation and a short seismic swarm, effusive activity resumed at Etna early 14 December. The eruptive vent opened at 2,200 m elevation on the W wall of the Valle del Bove, along a SE-flank fracture that formed during the 1989 eruption.

Since the eruption's onset, the GNV, in cooperation with Civil Protection authorities, has reinforced the scientific monitoring of Etna. Attention was focused on both the advance of the lava flow and on the possibility of downslope migration of the eruptive vent along the 1989 fracture system. The progress of the lava flow has been carefully followed by daily field inspections and helicopter overflights.

Because of its slow rate of advance, the lava did not threaten lives, but had the potential for severe property destruction. The water supply system for Zafferana (in Val Calanna; figure 43) was destroyed in the first two weeks of the eruption ($2.5 million damage). On 1 January, when the lava front was only 2 km from Zafferana, the Minister for Civil Protection, at the suggestion of the volcanologists, ordered the building of an earthen barrier to protect the village. The barrier was erected at the E end of Val Calanna, where the valley narrows into a deeply eroded canyon. The barrier was conceived to prevent or delay the flow's advance, not to divert it, by creating a morphological obstacle that would favor flow overlapping and lateral expansion of the lava in the large Val Calanna basin.

Figure 43. Topographic sketch map showing Etna's 1989 and 1990 lava flows, with preliminary locations of the 1991-92 lava, eruptive fissures, and the barrier constructed in Val Calanna. The area covered by lava since 14 January is shown in a separate pattern. The GNV report, received near press time, included a map that differed somewhat in detail from this map, which was prepared by R. Romano, T. Caltabiano, P. Carveni, M.F. Grasso, and C. Monaco. See pg.4 of Barberi et al., 1990 for a map of the 1989 lava flows, fissures, and monitoring network.

The barrier, erected by specialized Army and Fire Brigade personnel in 10 days of non-stop work, is ~ 250 m long and ~ 20 m higher than the adjacent Val Calanna floor. It was built by diking the valley bottom in front of the advancing lava and accumulating loose material (earth, scoria, and lava fragments) on a small natural scarp. On 7 January, the lava front approached to a few tens of meters from the barrier, then stopped because of a sudden drop in feeding caused by a huge lava overflow from the main channel several kilometers upslope.

A decrease in the effusion rate has been observed since mid-January. There is therefore little chance of further advance of the front, as the flow seems to have reached its natural maximum length. The eruptive fracture is being carefully monitored (seismicity, ground deformation, geoelectrics, gravimetry, and gas geochemistry) to detect early symptoms of a possible dangerous downslope migration of the vent along the 1989 fracture, which continues along the present fracture's SE trend. Preparedness plans were implemented in case of lava emission from the fracture's lower end.

Many scientists and technicians, the majority of whom are from IIV and the Istituto per la Geochimica dei Fluidi, Palermo (IGF) and are coordinated by GNV, are collecting information on the geological, petrological, geochemical, and geophysical aspects of the eruption.

2. Eruption chronology. On 14 December at about 0200, a seismic swarm (see Seismicity section below) indicated the opening of two radial fractures trending NE and SSE from Southeast Crater. Very soon, ash and bombs formed small scoria ramparts along the NE fracture, where brief activity was confined to the base of Southeast Crater. Meanwhile, a SSE-trending fracture extended ~ 1.3 km from the base of the crater (at ~3,000 m asl) to 2,700 m altitude.

Lava fountaining up to 300 m high from the uppermost section of the SSE fracture continued until about 0600, producing scoria ramparts 10 m high. Two thin (~ 1 m thick) lava flows from the fracture moved E. The N flow, from the highest part of the fracture, stopped at 2,750 m altitude, while the other, starting at 2,850 m elevation, reached the rim of the Valle del Bove (in the Belvedere area), pouring downvalley to ~ 2,500 m asl. At noon, the lava flows stopped, while the W vent of the central crater (Bocca Nuova) was the source of intense Strombolian activity.

The SSE fracture system continued to propagate downslope, crossing the rim of the Valle del Bove in the late evening. During the night of 14-15 December, lava emerged from the lowest segment of the fracture cutting the W flank of the Valle del Bove, reaching 2,400 m altitude (E of Cisternazza). Degassing and Strombolian activity built small scoria cones. Two lava flows advanced downslope from the base of the lower scoria cone at an estimated initial velocity of 15 m/s, which dramatically decreased when they reached the floor of the Valle del Bove.

The SSE fractures formed a system 3 km long and 350-500 m wide that has not propagated since 15 December. Between Southeast Crater and Cisternazza, the fracture field includes the 1989 fractures, which were reactivated with 30-50-cm offsets. The most evident offsets were down to the E, with right-lateral extensional movements. Numerous pit craters, <1 m in diameter, formed along the fractures.

Lava flows have been spreading down the Valle del Bove into the Piano del Trifoglietto, advancing a few hundred meters/day since 15 December. The high initial outflow rates peaked during the last week of 1991 and the first few days of 1992, and decreased after the second week in January. Strombolian activity at the vent in the upper part of the fracture has gradually diminished.

Lava flows were confined to the Valle del Bove until 24 December, when the most advanced front extended beyond the steep slope of the Salto della Giumenta (1,300-1,400 m altitude), accumulating on the floor of Val Calanna. Since then, many ephemeral vents and lava tubes have formed in the area N of Monte Zoccolaro, probably because of variations in the eruption rate. These widened the lava field in the area, and decreased feeding for flows moving into Val Calanna. However, by the end of December, lava flows expanded further in Val Calanna, moving E and threatening the village of Zafferana Etnea, ~2 km E of the most advanced flow front. This front stopped on 3 January, on the same day that a flow from the Valle del Bove moved N of Monte Calanna, later turning back southward and rejoining lava that had already stopped in Val Calanna. Since 9 January, lava flows in Val Calanna have not extended farther downslope, but have piled up a thick sequence of lobes.

Lava outflow from the vent continued at a more or less constant rate, producing a lava field in the Valle del Bove that consisted of a complex network of tubes and braiding, superposing flows, with a continuously changing system of overflows and ephemeral vents.

3. Lava flow measurements. An estimate of lava channel dimensions, flow velocity, and related rheological parameters was carried out where the flow enters the Valle del Bove. Flow velocities ranging from 0.4-1 m/s were observed 3-7 January in a single flow channel (10 m wide, ~ 2.5 m deep) at 1,800 m altitude, ~ 600 m from the vent. From these values, a flow rate of 8-25 m³/s and viscosities ranging from 70-180 Pas were calculated. Direct temperature measurements at several points on the flow surface with an Al/Ni thermocouple and a 2-color pyrometer (HOTSHOT) yielded values of 850-1,080°C.

4. Petrography and chemistry. Systematic lava sampling was carried out at the flow fronts and near the vents. All of the samples were porphyritic (P.I.»25-35%) and of hawaiitic composition, differing from the 1989 lavas, which fall within the alkali basalt field. Paragenesis is typical of Etna's lavas, with phenocrysts (maximum dimension, 3 mm) of plagioclase, clinopyroxene, and olivine, with Ti-magnitite microphenocrysts. The interstitial to hyalopitic groundmass showed microlites of the same minerals.

5. Seismicity. On 14 December at 0245, a seismic swarm occurred in the summit area (figure 44), related to the opening of upper SE-flank eruptive fractures. About 270 earthquakes were recorded, with a maximum local magnitude of 3. A drastic reduction in the seismic rate was observed from 0046 on 15 December, with only four events recorded until the main shock (Md 3.6) of a new sequence occurred at 2100. The seismic rate remained quite high until 0029 on 17 December, declining gradually thereafter.

Figure 44. Daily number of recorded earthquakes and cumulative strain release (top), with amplitude (middle) and dominant frequency peaks of volcanic tremor (bottom) at Etna, 1 December 1991-mid-January 1992. Arrows mark the eruption's onset. Courtesy of the Gruppo Nazionale per la Vulcanologia.

At least three different focal zones were recognized. On 14 December, one was located NE of the summit and a second in the Valle del Bove. The third, SW of the summit, was active on 15 December. All three focal zones were confined to <3 km depth. Three waveform types were recognized, ranging from low-to-high frequency.

As the seismic swarm began on 14 December, volcanic tremor amplitude increased sharply. Maximum amplitude was reached on 21 December, followed by a gradually decreasing trend. As the tremor amplitude increased, the frequency pattern of its dominant spectral peaks changed, increasing within a less-consistent frequency trend. Seismicity rapidly declined and remained at low levels despite the ongoing eruption.

6. Ground deformation. EDM measurements and continuously recording shallow-borehole tiltmeters have been used for several years to monitor ground deformation at Etna. The tilt network has recently grown to 9 flank stations. A new tilt station (CDV) established on the NE side of the fracture in early 1990 showed a steady radial-component increase in early March 1991 after a sharp deformation event at the end of 1990 (figure 45), suggesting that pressure was building into the main central conduit. Maximum inflation was reached by October 1991, followed by a partial decrease in radial tilt, tentatively related to magma intrusion into the already opened S branch of the 1989 fracture system, perhaps releasing pressure in the central conduit.

Figure 45. Radial and tangential components measured by the CDV borehole tilt station on the NE side of Etna's 1989 fracture, 1 July—mid-January 1992. The signal has been filtered for daily and seasonal thermoelastic noise. Arrows mark the eruption's onset. Courtesy of the Gruppo Nazionale per la Vulcanologia.

The eruption's onset was clearly detected by all flank tilt stations, despite their distance from the eruption site. The signals clearly record deformation events closely associated in time with seismic swarms on the W flank (before the eruption began) and on the summit and SW sector (after eruption onset). The second swarm heralded the opening of the most active vent on the W wall of the Valle del Bove.

S-flank EDM measurements detected only minor deformation, in the zone affected by the 1989 fracture. Lines crossing the fracture trend showed brief extensions in January 1992.

The levelling route established in 1989 across the SE fracture was reoccupied 18-19 December 1991. A minor general decline had occurred since the previous survey (October 1990), with a maximum (-10 mm) at a benchmark near the fracture.

7. Gravity changes. Microgravity measurements have been carried out on Etna since 1986, using a network covering a wide area between 1,000 and 1,900 m asl. A reference station is located ~ 20 km NE of the central crater. Five new surveys were made across the 1989 fissure zone during the eruption (15 & 18 December 1991, and 9, 13, and 18 January 1992). Between 21 November and 15 December, the minimum value of gravity variations was about -20 mGal, E of the fracture zone. On 9 January, the gravity variations inverted to a maximum of about +15 mGal. Amplitude increased and anomaly extension was reduced on 13 January, and on 18 January gravity variations were similar to those 9 days earlier. Assuming that height changes were negligible, a change in mass of ~2 x 10⁶ tons (~2 x 10⁷ m³ volume), for a density contrast of 0.1 g/cm³ was postulated. However, if gravity changes were attributed to magma movement, a density contrast of 0.6 g/cm³ between magma and country rock could be assumed and magma displacement would be ~ 3 x 10⁶ m³.

8. Magnetic observations. A 447-point magnetic surveillance array was spaced at 5-m intervals near the fracture that cut route SP92 in 1989. Measurements of total magnetic field intensity (B) have been carried out at least every 3 months since October 1989. Significant long-term magnetic variations were not observed between February 1991 and January 1992, although the amplitude of variations seems to have increased since the beginning of the eruption.

9. Self-potential. A program of self-potential measurements along an 1.32-km E-W profile crossing the SE fracture system (along route SP92 at ~ 1,600 m altitude) began on 25 October 1989. Two large positive anomalies were consistently present during measurements on 5 and 17 January, and 9, 18, and 19 February 1992. The strongest was centered above the fracture system, the second was displaced to the W. Only the 5 January profile hints at the presence of a third positive anomaly, on its extreme E end. The persistent post-1989 SP anomalies could be related to a magmatic intrusion, causing electrical charge polarizations inside the overlying water-saturated rocks. A recent additional intrusion was very likely to have caused the large increase in amplitude and width of the SP anomaly centered above the fracture system, detected on the E side of the profile on 5 January 1992.

10. COSPEC measurements of SO₂ flux. The SO₂ flux from Etna during the eruption has been characterized by fairly high values, averaging ~ 10,000 t/d, ~ 3 times the mean pre-eruptive rate. Individual measurements varied between ~6,000 and 15,000 t/d.

11. Soil gases. Lines perpendicular to the 1989 fracture, at ~ 1,600 m altitude, have been monitored for CO₂ flux. A sharp increase in CO₂ output was recorded in September 1991, about 3 months before the eruption began (figure 46). Measurements have been more frequent since 17 December, but no significant variation in CO₂ emission has been observed. Samples of soil gases collected at 50 cm depth showed a general decrease in He and CO₂ contents since the beginning of January. Soil degassing at two anomalous exhalation areas, on the lower SW and E flanks at ~ 600 m altitude, dropped just before (SW flank—Paternò) and immediately after (E flank—Zafferana) the beginning of the eruption, and remained at low levels. A significant radon anomaly was recorded 26-28 January along the 1989 fracture, but CO₂ and radon monitoring have been hampered by snow.

Figure 46. CO2 concentrations measured along Etna's 1989 fracture, late 1990-early 1992, showing a strong increase about 3 months before the December 1991 eruption. Courtesy of the Gruppo Nazionale per la Vulcanologia.

The following, from R. Romano, describes activity in February and early March.

The SE-flank fissure eruption was continuing in early March, but was less vigorous than in previous months. An area of ~ 7 km² has been covered by around 60 x 10⁶ m³ of lava, with an average effusion rate of 8 m³/s. The size of the lava field (figure 43) has not increased since it reached a maximum width of 1.7 km in mid-February.

Lava from fissure vents at ~ 2,100 m asl flowed in an open channel to 1,850 m altitude, then advanced through tubes. Flowing lava was visible in the upper few kilometers of the tubes through numerous skylights. Lava emerged from the tube system through as many as seven ephemeral vents on the edge of the Salto della Giumenta (at the head of the Val Calanna, ~ 4.5 km from the eruptive fissure). These fed a complex network of flows in the Salto della Giumenta that were generally short and not very vigorous. None extended beyond the eruption's longest flow, which had reached 6.5 km from the eruptive fissure (1,000 m asl) before stopping in early January. Ephemeral vent activity upslope (within the Valle del Bove) ceased by the end of February. Lava production from fissure vents at 2,150 m altitude has gradually declined and explosive activity has stopped. Degassing along the section of the fissure between 2,300 and 2,200 m altitude was also gradually decreasing.

Small vents were active at the bottom of both central craters. Activity at the west crater (Bocca Nuova) was generally limited to gas emission, but significant ash expulsions were observed during the first few days in March. High-temperature gases emerged from the E crater (La Voragine). Collapse within Northeast Crater, probably between 26 and 27 February, was associated with coarse ashfalls on the upper NE flank (at Piano Provenzana and Piano Pernicana). After the collapse, a new pit crater ~ 50 m in diameter occupied the site of Northeast Crater's former vent. Activity from Southeast Crater was limited to gas emission from a modest-sized vent.

Seismic activity was characterized by low-intensity swarms. A few shocks were felt in mid-February ~ 12 km SE of the summit (in the Zafferana area).

Reference. Barberi, F., Bertagnini, F., and Landi, P., eds., 1990, Mt. Etna: the 1989 eruption: CNR-Gruppo Nazionale per la Vulcanologia: Giardini, Pisa, 75 p. (11 papers).

Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: GNV report:F. Barberi, Univ di Pisa; L. Villari, IIV. February-early March activity:R. Romano and T. Caltabiano, IIV; P. Carveni, M. Grasso, and C. Monaco, Univ di Catania.
The following people provided information for the GNV report. Institutional affiliations (abbreviated, in parentheses) and their report sections [numbered, in brackets] follow names.
F. Barberi (UPI) [1, 2], A. Armantia (IIV) [2], P. Armienti (UPI) [2, 4], R. Azzaro (IIV) [2], B. Badalamenti (IGF) [11], S. Bonaccorso (IIV) [6], N. Bruno (IIV) [10], G. Budetta (IIV) [7, 8], A. Buemi (IIV) [4], T. Caltabiano (IIV) [8, 10], S. Calvari (IIV) [2, 3], O. Campisi (IIV) [6], M. Carà (IIV) [10], M. Carapezza (IGF, UPA) [11], C. Cardaci (IIV) [5], O. Cocina (UGG) [5], D. Condarelli (IIV) [5], O. Consoli (IIV) [6], W. D'Alessandro (IGF) [11], M. D'Orazio (UPI) [2, 4], C. Del Negro (IIV) [7, 8], F. DiGangi (IGF) [11], I. Diliberto (IGF) [11], R. Di Maio (DGV) [9], S. DiPrima (IIV) [5], S. Falsaperla (IIV) [5], G. Falzone (IIV) [6], A. Ferro (IIV) [5], F. Ferruci (GNV) [5], G. Frazzetta (UPI) [2], H. Gaonac'h (UMO) [2, 3], S. Giammanco (IGF) [11], M. Grasso (IIV) [10], M. Grimaldi (DGV) [7], S. Gurrieri (IGF) [11], F. Innocenti (UPI) [4], G. Lanzafame (IIV) [2], G. Laudani (IIV) [6], G. Luongo (OV) [6], A. Montalto (IIV, UPI) [5], M. Neri (IIV) [2], P. Nuccio (IGF, UPA) [11], F. Obrizzo (OV) [6], F. Parello (IGF, UPA) [11], D. Patanè (IIV) [5], D. Patella (DGV) [9], A. Pellegrino (IIV) [5], M. Pompilio (IIV) [2, 3, 4], M. Porto (IIV) [10], E. Privitera (IIV) [5], G. Puglisi (IIV) [2, 6], R. Romano (IIV) [10], A. Rosselli (GNV) [5], V. Scribano (UCT) [2], S. Spampinato (IIV) [5], C. Tranne (IIV) [2], A. Tremacere (DGV) [9], M. Valenza (IGF, UPA) [11], R. Velardita (IIV) [6], L. Villari (IIV) [1, 2, 6].
Institutions: DGV: Dipto di Geofisica e Vulcanologia, Univ di Napoli; GNV: Gruppo Nazionale per la Vulcanologia, CNR, Roma; IGF: Istituto per la Geochimica dei Fluidi, CNR, Palermo; IIV: Istituto Internazionale di Vulcanologia, CNR, Catania; OV: Osservatorio Vesuviano, Napoli; UCT: Istituto di Scienze della Terra, Univ di Catania; UGG: Istituto di Geologia e Geofisica, Univ di Catania; UMO: Dept de Géologie, Univ de Montréal; UPA: Istituto di Mineralogia, Petrologia, e Geochimica, Univ di Palermo; UPI: Dipto di Scienze della Terra, Univ di Pisa.

Occasional emissions of fine ash, sometimes associated with long-period earthquakes or variations in tremor, punctuated the continuous emission of gas and vapor in February. Although seismicity oscillated in February, it has remained stable since the increased activity associated with dome growth in October-November. On 11 February, a M 3.1 earthquake occurred roughly 2 km W of the crater, and was felt 9 km away (in Pasto and Consacá). Electronic tiltmeter measurements [at the Crater and Peladitos stations] were essentially stable, with the latter showing a slight tendency toward inflation.

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: J. Romero, INGEOMINAS-Observatorio Vulcanológico del Sur.

A thin white vapor plume rose 50-100 m above the crater rim in early March, accompanied by an average of 26 volcanic earthquakes/day. Deep volcanic earthquakes increased from 91 during the first week in March to 159 the following week, as the weekly number of shallow volcanic earthquakes grew from 18 to 26.

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

Information Contacts: W. Modjo and W. Tjetjep, VSI.

Ash eruptions occurred on 3 and 15 November 1991, ejecting columns to a maximum of ~150 m above the crater rim. Since then, an average of 47 shallow earthquakes have been recorded monthly, and a white vapor column continued to rise to ~ 50 m above the crater.

Geologic Background. Iliboleng stratovolcano was constructed at the SE end of Adonara Island across a narrow strait from Lomblen Island. The volcano is capped by multiple, partially overlapping summit craters. Lava flows modify its profile, and a cone low on the SE flank, Balile, has also produced lava flows. Historical eruptions, first recorded in 1885, have consisted of moderate explosive activity, with lava flows accompanying only the 1888 eruption.

Information Contacts: W. Modjo and W. Tjetjep, VSI.

Fumarolic activity continued in February. Although the water level continued to drop, the crater lake remained larger than it had been in November (figure 5 and table 3). Water temperatures (measured by UNA) on the N side of the lake near the most active subaqueous fumaroles ranged from 37°C to 73°C; bubbling springs near the edge of the lake were

Figure 5. Oblique view of the crater lake at Irazú, 25 February 1992. Courtesy of ICE.

Table 3. Crater lake characteristics at Irazú, November 1991 and February 1992. Courtesy of ICE.

A monthly total of 234 earthquakes was recorded in February (at UNA station IRZ2, 5 km WSW of the crater), with a maximum of 37 on 21 February. Nine high-frequency earthquakes were recorded in February. Measurements of two geodetic lines across the summit on 13 February indicated contractions of 6.4 ppm in an E-W direction and 15.8 ppm in a N-S direction, since 10 October 1991 (UNA).

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

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and R. Van der Laat, OVSICORI; G. Soto and R. Barquero, ICE.

Lava production from a fissure that extended ~150 m uprift from the lower W flank of Pu`u `O`o began during the evening of 17 February (E-50; 17:1). The small lava lake in Pu`u `O`o crater dropped ~40 m as E-50 began, and the lava surface remained ~80 m below the rim until 19 February, when it rose ~15 m. Lava from the E-50 fissure flowed N and S from the axis of the East rift zone (figure 85). By 19 February, only ~30 m of the fissure was active. The next day, the S flow had stagnated, and all of the lava from the fissure was moving N, where it formed a large ponded area fed by a channel 10 m wide. Overflows from the ponded lava built levees that were 7 m high by 21 February. Lava broke out of the N side of the ponded area on 21 and 22 February, as the eruption rate declined and lava in the channel dropped to a few meters below the levees. The channel had narrowed to ~3.5 m by 23 February. A large flow began to advance southward on 25 February. It stagnated within a few days, but new flows continued to move S atop previous lava.

When observed on 28 February, a thick crust had formed over the lava in Pu`u `O`o crater, although occasional spattering was noted on its margins. Gas-piston activity resumed at the beginning of March, and two separate vents were visible when the lava level was low.

An earthquake swarm in the summit area and upper East rift zone began on 3 March at about 0000. An hour later, the summit began to deflate at a rate of ~0.5 µrad/hour as an intrusion . . . roughly 4-6 km from the caldera rim (between Devil's Throat and Pauahi Crater). Small cracks developed in Chain of Craters Road, but no eruption occurred in the area. By 0930, summit tilt had leveled off. Seismic activity declined through the day, although > 3,000 events were recorded by 5 March at 0800. Activity at the E-50 vent had stopped by 0130, and later observations revealed that the level of lava in Pu`u `O`o crater had dropped to > 100 m below the rim. The large northern aa flow continued to advance sluggishly for much of the day, but stagnated by 1600, and the episode-50 eruption site remained quiet until 7 March.

Episode 51 (E-51). Eruption tremor remained near background levels in the middle East rift zone until shortly before noon on 7 March, when a 1-hour burst of increased activity was noted on the seismic station nearest Pu`u `O`o. At 1340, a helicopter pilot saw lava pouring from a new fissure near the E-50 vents, while the level of lava in Pu`u `O`o crater had risen to ~55 m below the rim. Lava production from the E-51 fissure was intermittent through the evening, but was continuous by 9 March, at rates that appeared slightly less than during E-50 and substantially below those of episode 49. The E-51 fissure appeared to overlap the E edge of the E-50 fissure and extended ~30 m to its E, on the steep W flank of Pu`u `O`o. By 9 March, a spatter cone 6 m high had formed, and lava was ponding on the W side of the fissure. Some flows moved N from the ponded area, but most of the lava fed channelized aa and slabby pahoehoe flows that moved S. Intermittent lava production from the E-51 vent continued through mid-March.

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, HVO.

Steam emission . . . continued steadily in February, reaching 200-300 m height. The ground around the fumaroles was covered by a fine dusting of ash during air reconnaissance on 5, 12, and 18 February. Seismicity was low, with continuous volcanic tremor ceasing on 2 February, and a monthly total of 25 recorded earthquakes . . . .

Geologic Background. Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

Information Contacts: JMA.

"During February the activity continued to be focused at Crater 2, at an intensity similar to that observed in January. However, seismicity increased in the second half of February. Emissions at Crater 2 consisted of pale-grey vapour and ash clouds in low-moderate volumes. Occasionally there were ashfalls on the lower flanks of the volcano. Explosions and rumbling sounds associated with the emissions were heard throughout the month. When the summit was free of cloud at night, a steady weak glow was seen above the crater. Activity at Crater 3 was mostly confined to weak emissions of white and blue vapours. However, there was a large explosion on 11 February that produced an emission cloud ~1 km high. Seismicity was steady at a low level in the first half of the month but then began to increase. By the end of the month seismicity had reached the level recorded in January (up to 17 low-frequency earthquakes per day)."

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: C. McKee, RVO.

Although no lava emission was observed during crater visits, the presence of new lava flows indicated continued activity through December. Photographs taken on 9 October by members of the St. Lawrence Univ Kenya Semester Program, guided by D., M., and T. Peterson, showed no significant changes from 13 August. The crater floor was pale brown and light gray, with no sign of fresh dark lava during the visit. Dark stains were visible on the upper part of cone T5/T9, suggestive of recent spatter, and a considerable amount of young lava (pale gray and pale brown) was apparent around the base of cone T8. A large flow (mid-gray, but with large white areas), possibly from a low dome W of the cones (T18), covered much of the W part of the crater floor, reaching the W wall.

On 7 December, John Gardner reported a large "black jagged" lava flow (F32) extending N-S across the crater floor. The lava was still warm to the touch, with steam being emitted from cracks in its surface, suggesting that the flow had formed within a few hours of Gardner's visit. Steam was reportedly emitted from the estimated 15-m-high cone T5/T9, from cracks in the lava on the crater floor, and from the E rim and E crater wall. Gardner also reported a cone . . . that might be a new feature.

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

Information Contacts: C. Nyamweru, St. Lawrence Univ; D. Peterson, M. Peterson, and T. Peterson, Arusha; J. Gardner, Nairobi, Kenya.

Seismicity was recorded during fieldwork on 13-16 January, using a MEQ-800 portable seismograph, at 1,600 m elev. . . . During the observations, the daily number of microearthquakes decreased from 700 on 13 January, and averaged 418 (figure 2). Tremor frequency oscillated between 1 and 1.6 Hz, with a maximum episode-duration of 70 seconds and a maximum daily total of 11.5 hours (13 January). Seismicity was record<->ed at the same site on 25-30 January 1991, when 650 microearthquakes were recorded, with a daily average of 120 events and a maximum of 140 events (27 January). Tremor frequency oscillated between 1 and 1.8 Hz, with a maximum duration of 55 seconds.

Figure 2. Daily hours of tremor (top) and number of earthquakes (bottom) at Llaima, 13-16 January 1992. Courtesy of Gustavo Fuentealba.

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: G. Fuentealba and M. Murillo, Univ de La Frontera; J. Cayupi and M. Petit-Breuilh, Fundación Andes, Temuco.

"Activity at Manam's Southern Crater was at a low-moderate level during February with a slight increase at the end of the month. Southern Crater emissions consisted of weak pale-grey or pale-brown vapour and ash clouds. On a few days the ash content of the emissions was markedly higher, leading to ashfalls in coastal areas (4-5 km from the summit). In general, the emissions occurred without significant sound effects, although rumbling was heard on 29 February in association with thick, dark ash clouds, night glow, and incandescent lava ejections. No activity was observed from Main Crater. Seismicity fluctuated a little but remained at a low level with daily counts of low-frequency events ranging from 100 to 350."

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: C. McKee, RVO.

The following supersedes [16:12 and 17:1].

Increased seismicity preceded the start of summit-area lava extrusion that was first observed on 20 January. Deep (A type, 3.1-3.7 km depth) and shallow (B type,

Glowing rockfalls were first seen on 20 January between 1800 and 2000, emerging from a narrow opening between the NW crater rim (formed by the 1957 lava dome) and the 1984 dome. The rockfalls initially traveled an estimated 125 m from the summit, but they extended farther with time, to ~1,500 m on 31 January (figures 3 and 4). A new lava dome was covering the NW part of the 1984 dome when geologists from the MVO climbed the volcano on 31 January. The 1992 lava was ~50 m higher than the 1984 dome.

Figure 3. Sketch map of Merapi's 1992 lava dome, and the distribution of avalanche-generated, pyroclastic-flow deposits as of 18 February. Courtesy of MVO.

Figure 4. View of Merapi at 0630 on 3 March 1992, drawn by Sadjiman from Jurangjero, ~ 8 km WSW of the summit. Courtesy of MVO.

The first avalanche-generated pyroclastic flow occurred on 31 January at 1535, and three more were detected the next day (table 5).

Table 5. Number of avalanche-generated pyroclastic flows at Merapi, 31 January-2 March 1992. Courtesy of MVO.

The most vigorous pyroclastic-flow activity was on 2 February, when 33 were observed between 1220 and 2221, extending a maximum of 4 km from the summit. These were accompanied by small explosions that were heard 4 km NW of the summit (at Babadan Observatory). Ash rose to 2,600 m above the summit. Sulfur odors were also noted. Volcanic earthquakes were very rare during the eruption.

Pyroclastic-flow intensity then decreased; none have occurred since 2 March, but the lava dome continued to grow as of mid-March. Glowing rockfalls were nearly continuous (>1,000/day since 2 March), but relatively small, extending

Four alert levels have been established by VSI at Merapi: 1) Notifies residents of increased activity and the need for awareness and caution: 2) More serious precursors require increased awareness; local authorities are requested to prepare for hazard prevention and evacuation: 3) All persons living in the danger zone must pack valuables and items that would supply basic needs during an evacuation: 4) Evacuation required because of explosive eruption and the approach of pyroclastic flows toward inhabited areas.

During the 1992 eruption, Alert Level 1 was announced on 24 January, increasing to Level 2 on 1 February at 2215, and to Level 3 the next day at 1430. As the eruption intensity decreased, the alert level was lowered to 2 on 12 February and to 1 on 2 March.

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: S. Bronto, MVO.

An area of green discolored water, 3-5 km long, was observed over the volcano during an overflight on 12 February. Subsequent overflights revealed additional water discolorations on 28 February, and 2, 3, and 4 March, although no discoloration was seen on 21 February. The 4 March discoloration appeared to have a source area 100 m across. Overflights have been conducted almost every month in the Izu and Volcano Islands by the JMSA. This was the first observed incidence of water discoloration since the mid-to-late 1970's, when bubbling, spouting, and discolored water were occasionally sighted.

Geologic Background. Periodic water discoloration and water-spouting have been reported over this submarine volcano since 1975, when detonations and an explosion were also reported. It lies near the SE end of a coalescing chain of youthful seamounts, and is the only historically active vent. The reported depth of the summit of the trachyandesitic volcano has varied between 274 and 30 m. The morphologically youthful seamounts Kita-Hiyoshi and Naka-Hiyoshi lie to the NW, and Ko-Hiyoshi to the SE.

Information Contacts: JMSA.

Two small lahars took place as a result of light rain showers in the Sacobia River drainage in late February, and steam emission continued through early March from a linear trend of fumaroles along the S edge of the 1991 caldera floor. Discrete larger emission episodes were occasionally observed, but there have been no confirmed ash emissions. Weak seismicity has continued at the volcano, including low-amplitude, low-frequency events, at least one of which corresponded with an observed steam emission.

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

Information Contacts: R. Punongbayan, PHIVOLCS.

Gas emission continued in February and occasional small phreatic eruptions were observed. The level of the crater lake decreased for the second consecutive month, and water temperature was 67°C, similar to January. A total of 5,027 low-frequency earthquakes was recorded in February (at station POA3, 2.5 km SW of the crater), with a daily average of 219. No tremor or high-frequency earthquakes were recorded. Long-base dry-tilt measurements 1 km S of the crater on 26 February showed changes of <5 µrad, similar to measurements in 1991.

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

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and R. Van der Laat, OVSICORI.

"There was a slight increase in seismicity in February. The total number of caldera earthquakes was 212 . . . with daily totals ranging from 0 to 35. The highest daily earthquake totals were due to a swarm on 22 February and a series of small discrete events on 29 February. The swarm included several events that were felt in Rabaul, the largest [M_L 3.2]. Earthquakes of this swarm were located in the W part of the caldera seismic zone at a depth of ~3 km. All of the other caldera earthquakes recorded in February were of small magnitude (M_L <0.5). Levelling measurements carried out on 12 February indicated slight subsidence (8 mm) at the S part of Matupit Island since January's measurements. No significant tilt changes were recorded."

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: C. McKee, RVO.

Gas emission has continued over the last several months, punctuated by sporadic phreatic eruptions. Fumarolic activity was concentrated on the active crater's E wall, producing a plume that occasionally reached 500 m height, smelling of sulfur, and irritating eyes and skin. The crater lake was gray, with yellow areas over bubbling points. Concentric and radial fissures, to 1 m wide and to >4 m deep, were found on the upper E, N, and NW flanks. The fissures were probably formed by partial collapse of the crater walls, especially on the E and NW flanks. Seven low-frequency earthquakes were recorded during February, down from a peak of 30 recorded 8 May 1991, associated with a large phreatic eruption. Abnormal seismicity was reported for several months after 8 May.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and R. Van der Laat, OVSICORI.

Low activity and low water temperatures (14-17°C) persisted at Crater Lake through October-December, and seismicity was at background levels. There was no apparent eruptive activity during this time, although moderately strong upwelling continued over the lake's N vents, producing a yellow slick on 11 October. Upwelling was also occasionally observed above the lake's central vents.

A sharp increase in Crater Lake water temperature began in early January. Temperatures paused at ~20°C from 7 to 21 January, then rose at an even higher rate (1.1°/day), reaching 36°C by 8 February (figure 12). Strong sulfur odors were noted at the lake on 3 January, and 9 km N (in Whakapapa Village) during still air and clear weather on 5 February.

During a midday 8 February overflight, January Clayton-Green (Dept of Conservation) reported a gray slick surrounded by blue-green water in the center of Crater Lake, but no anomalous upwelling. Later that day (1500-1600), shortly after the start of a sequence of 30-40 volcanic earthquakes (at 1458; figure 13), Rob McCallum (DOC) observed upwelling 45-60 cm high that produced a surge over the lake's outlet. Agitation of the water was reported as "lasting some time." The next day, McCallum noted that the lake was entirely gray (at 0900), and that a strong sulfur odor was present. Bruce Williams (a Mt. Cook Airlines pilot), reported that Crater Lake, viewed from the air, was a typical blue-green on 8-9 February, but became more active on 10 February, and further increased in activity on 11 February.

Figure 13. Daily number of volcanic (top) and tectonic (bottom) earthquakes at Ruapehu, December 91-9 February 92. Courtesy of DSIR.

Vigorous seismicity continued on 9 February, although earthquake magnitudes dropped from just above M 2 on 8 February (maximum M 2.3), to just below M 2. One episode of low-amplitude, 1-Hz tremor was recorded at 0800-0930 on 9 February. Higher frequency (2 Hz) tremor remained at background levels during this part of February.

A team of scientists from DSIR and DOC visited the crater on 11 February from 1000 to 1450. Four small eruptions were observed (at 1023, 1133, 1257, and 1410), each consisting of a sudden updoming of dark gray water over the central vent, possibly rising several meters and affecting an area 10-20 m across, but rapidly obscured by steam. There was little sound except for a "whooshing" from the agitated water. Small waves (<20 cm high at the shoreline) radiated out from the center, and steam rose approximately 100 m before dissipating.

Water temperature reached 39°C, and outflow was 120 l/s on 11 February (compared to <10 l/s on 17 October and 20 November, and 70 l/s on 3 January). Mg/Cl ratios remained stable, ranging from 0.046 to 0.048 since 3 May 1991, although there did appear to be a slight dilution (from 312 to 295 ppm magnesium, and from 6,526 to 6,245 ppm chloride).

Deformation measurements on 11 February indicated a reversal from apparent deflation to inflation. Fieldwork on 17 October and 3 January had indicated slow deflation since 29 August. Similar deformation reversals were recorded during the 8 other discrete heating episodes since 1985.

A small phreatic eruption was observed on 18 February at about 1100, by airplane pilot Darren Kirkland. The event produced a column of steam, and generated waves estimated at 60-90 cm height. Geologists considered the January-February activity to be typical of the volcano's post-1985 periods of minor phreatic activity. . . .

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake (Te Wai a-moe), is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3,000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.

Information Contacts: P. Otway, DSIR Wairakei.

[A 25 February 1992 overflight during clear weather by a U.S. Coast Guard helicopter revealed no evidence of activity at Mt. Siple. No ash was visible on the surface, and no active fumaroles or fumarolic ice towers could be seen.]

Geologic Background. Mount Siple is a youthful-looking shield volcano that forms an island along the Pacific Ocean coast of Antarctica's Marie Byrd Land. The massive 1,800 km3 volcano is truncated by a 4-5 km summit caldera and is ringed by tuff cones at sea level. Its lack of dissection in a coastal area more susceptible to erosion than inland volcanoes, and the existence of a satellite cone too young to date by the Potassium-Argon method, suggest a possible Holocene age (LeMasurier and Thomson 1990). Its location on published maps is 26 km NE of the actual location. A possible eruption cloud observed on satellite images on 18 September and 4 October 1988 was considered to result from atmospheric effects, after low-level aerial observations revealed no evidence of recent eruptions.

Information Contacts: P. Kyle, New Mexico Institute of Mining & Technology.

After a brief episode of increased seismicity, deformation, and increased crater lake temperatures on 14-15 February, activity returned to more normal levels. Fieldwork by Univ of Savoie personnel indicated that temperatures of the main crater lake were gradually declining, and that seismicity was near background levels. All measurable deformation seemed to have occurred on 14 February. The Alert Level 3 status, announced on 15 February, was lowered to Level 2, and then to Level 1 in early March. Most residents of Taal island have returned home.

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: C. Newhall, USGS.

Fumarolic activity continued in February, with temperatures of 90°C. Similar temperatures have been measured since 1982. A monthly total of 37 low-frequency earthquakes, a maximum of 4/day (4 February), was recorded (at station VTU, 0.7 km from the crater).

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and R. Van der Laat, OVSICORI.

Summit lava dome growth continued through early March, with frequent pyroclastic flows generated by partial dome collapse. Geologists estimated that by late January, the volume of the dome complex was 40 x 10⁶ m³, and that ~ 75 x 10⁶ m³ of lava had been extruded since 20 May 1991. The rate of extrusion was around 3 x 10⁵ m³/day during December-January, a rate that has remained nearly constant since June 1991.

Most of the growth of dome 6 . . . had been endogenous in mid-February through early March, then became dominantly exogenous. The area around the dome swelled upwards, and complicated "petal" structures formed on its surface. Continued thickening of dome 6 forced dome 5 . . . to the NE. The surface of dome 5 was very reddish, implying that it was composed of older, oxidized lavas, and was dominantly a cryptodome. Rockfalls from the E and N faces of dome 5 produced reddish block-and-ash flow deposits and left behind numerous small cliffs (figure 39). Dome 5 in turn pushed dome 4 (split into N and S parts), especially its N part, which moved more than 50 m to the E during mid-February-early March. Much of dome 4 was eroded or buried by material from other domes, bringing the talus slope flush with its top. Incandescence and strong gas emissions were observed along cracks and pit craters in and near dome 3. Emission of ash-laden plumes became continuous from Jigoku-ato Crater in early March.

Figure 39. Sketch of the lava dome complex at Unzen, 27 February 1992. Courtesy of S. Nakada.

Lava blocks frequently fell from near the head and front of dome 6, generating pyroclastic flows to the SE and occasionally to the E and NE (figure 40). Clouds of elutriated ash descending to the S sometimes reached the N cliff of Mt. Iwatoko, but the accompanying block-and-ash flows stopped about 300 m short of this point. Thus, trees on the N slope of the cliff were covered by the elutriated ash clouds, but they were neither bent over nor burned. Larger pyroclastic flows occurred on 2 and 12 February. Flows at 2020 and 2028 on 12 February had durations of 290 and 300 seconds, respectively, the longest since 15 September.

Figure 40. Map showing distribution of 1991-92 pyroclastic-flow deposits at Unzen, February 1992. The 1991 pyroclastic-surge deposits are not shown. Courtesy of S. Nakada.

Heavy rainfall triggered a large debris flow at 0130 on 1 March, along the E flank's Mizunashi River, following the route of the previous large debris flow on 30 June 1991. The flow reached a point 100 m from the coast, 8 km E of the summit, crossing Routes 57 and 251, and burying a 200-m section of the Shimabara Railway. No damage occurred in previously untouched areas, and rail service was resumed within 6 days. As of early March, roughly 7,600 people remained evacuated.

February's 6,434 recorded earthquakes represent the largest monthly total since the eruption began, but seismicity started to decline on 4 March. Seismicity has been at very high levels since October.

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: S. Nakada, Kyushu Univ; JMA.

Explosive activity continued through January. A large ash emission event on 17 January deposited ash 50 km S, and was associated with a large high-frequency seismic episode. The 17 January event marked a change from Strombolian ejections of scoriaceous bombs and juvenile ash, to emissions of ash-sized tephra dominated by lithics and altered glass.

Tephra ejection, December to mid-January. R. Fleming (Waimana Helicopters pilot) reported that Wade crater (formed in mid-October 1991) remained very active in late December and early January, emitting scoriae and bombs (to 30 m height) that were scattered over most of the W end of the main crater floor. The largest bombs were ejected after heavy rainfall at the beginning of January, but volcano noise (booming at 1-2-second intervals) heard during earlier visits had diminished after the rainfall. TV1 Crater (formed in October 1990) occasionally emitted ash, but no emissions were observed from May 91 vent.

B.J. Hogg and P. Horn reported observing an eruption from a boat 8 km E of the island shortly after 2000 on 16 January, coinciding with a recorded E-type earthquake. The initial gray-brown plume, ~150-180 m high, was followed by a separate brown ash column that rose ~900-1,500 m. Ashfall quickly obscured the W and S portions of the island. Roughly 15 minutes into the eruption, ash was observed cascading down the outer margins of the eruption column. Vigorous ash emission continued for at least an hour.

Strong explosion, 17 January. At 0932 on 17 January, seismometers registered the largest discrete seismic event ever recorded at the volcano (figure 16). Boats contacted at 1000-1015 reported limited visibility due to deteriorating weather, but that a "change to heavy ashfall had occurred within the last half hour." The New Zealand Herald reported that a yacht sailing close to the S coast of White Island at about 1100 had its sails coated with mud, and was later dismasted. Ashfall was reported 50 km S (in the Whakatane area) between 1115 and 1130. Geologists suggested that the 17 January explosion was probably caused by subterranean collapse of Wade Crater's conduit wall onto the top of the magma column at considerable depth. This resulted in a change from "open-vent" Strombolian eruptions of scoriaceous bombs, to "closed vent" phreatomagmatic eruptions of altered, lithic-dominated, mostly ash-sized ejecta.

Figure 16. Seismogram showing a large high-frequency event at White Island, 0932 on 17 January 1992. Ticks are at 1-minute intervals. Courtesy of DSIR.

Post-17 January fieldwork. Only a thin layer of light gray ash covered the island during fieldwork on 22 January, suggesting that most of the ash erupted on 17 January had been carried offshore by strong winds. About 32 cm of tephra had been deposited on the 1978/90 Crater rim (S of TV1) since 5 December, of which 11 cm were believed to be associated with 17-22 January activity. No surge deposits were recognized. The largest of the ash-covered blocks and bombs (up to 1.3 m long), found ~200 m E of Wade Crater, had been deposited before 17 January.

No significant changes had occurred to visible parts of the three recently active vents since fieldwork on 5 and 6 December. Wade Crater emitted a vigorously convoluting column of very fine dark gray-brown ash and white gas. White blocks (perhaps baked lithic material) were occasionally ejected. Most of the ash fell back into the vent. Noise from the crater was subdued, in comparison with 5 December, and the dull "booms" had no obvious correlation with emissions. TV1 Crater quietly emitted a small continuous plume of light gray ash that fell to ~100 m ENE, onto an area covered by a layer of recent ash and blocks.

During fieldwork on 23 January, Wade Crater erupted fine red ash, which became more predominant through the day. A distinctive gray-white ash deposit was apparent around the NE margin of 1978/90 Crater Complex, above TV1 Crater. Deposits of fine yellow-green ash, not apparent in photos taken on 22 January, mantled the ground elsewhere on Main Crater floor and on the outer SW slopes. Ash emissions from Wade Crater were stronger on 24 January and conspicuously redder. When geologists left the area at 1635, ash was falling at sea, downwind of the island.

On 31 January, a steam column with small quantities of pink ash from Wade Crater and a light gray column from TV1 combined to form a weakly convoluting pink-brown plume 400 m high. Solar panels 600 m SE of Wade had accumulated ~20 mm of ash since 22 January.

Seismicity. Before 9 December, episodic medium-frequency volcanic tremor accompanied open-vent Strombolian activity at variable, but low amplitude. Tremor declined after 12 December, and was replaced by more discrete, medium-frequency (C-type) events (~200/day) that lasted until 22 December. Relatively brief E-type (eruption) events were recorded on 11, 13, 16, and 17 December (at 1802, 1003, 1921, and 0723, respectively), and rare B-type events were recorded after 16 December. No signal was received 23-27 December.

B-type shocks and microearthquakes dominated the seismic records by 1 January, with 5-10/minute occurring in bursts lasting 3.5-8 hours. Microearthquake activity declined about 6 January, while the number of B-type earthquakes increased, peaking at >20/day on 11 January. A-type earthquakes remained constant, around 3-4/day. E-type sequences reappeared on 7 January, and occurred daily until 17 January, as B-type earthquakes decreased in number. A distinctly different, high-frequency, long-duration event (figure 16) occurred at 0932 on 17 January, shortly before reports of heavy ashfall. A sequence of 18 A-type earthquakes followed in the next 10 hours, and medium- to low-frequency volcanic tremor of variable but increasing amplitude commenced. After 18 January, 5-6 B-type and fewer A-type earthquakes were recorded daily. E-type events were recorded on 21 and 25 January (at 0312 and 1438, respectively), the latter accompanying a voluminous ash eruption. Increasing ash emission interrupted the seismic telemetry link on 26 January.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: I. Nairn and B. Scott, DSIR Rotorua.