The Karangetang andesitic-basaltic stratovolcano (also referred to as Api Siau) at the northern end of the island of Siau, north of Sulawesi, Indonesia, has had more than 50 observed eruptions since 1675. Frequent explosive activity is accompanied by pyroclastic flows and lahars, and lava-dome growth has created two active summit craters (Main to the S and Second Crater to the N). Rock avalanches, observed incandescence, and satellite thermal anomalies at the summit confirmed continuing volcanic activity since the latest eruption started in November 2018 (BGVN 44:05). This report covers activity from December 2019 through May 2020. Activity is monitored by Indonesia's Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), and ash plumes are monitored by the Darwin VAAC (Volcanic Ash Advisory Center). Information is also available from MODIS thermal anomaly satellite data through both the University of Hawaii's MODVOLC system and the Italian MIROVA project.

Increased activity that included daily incandescent avalanche blocks traveling down the W and NW flanks lasted from mid-July 2019 (BGVN 44:12) through mid-January 2020 according to multiple sources. The MIROVA data showed increased number and intensity of thermal anomalies during this period, with a sharp drop during the second half of January (figure 40). The MODVOLC thermal alert data reported 29 alerts in December and ten alerts in January, ending on 14 January, with no further alerts through May 2020. During December and the first half of January incandescent blocks traveled 1,000-1,500 m down multiple drainages on the W and NW flanks (figure 41). After this, thermal anomalies were still present at the summit craters, but no additional activity down the flanks was identified in remote satellite data or direct daily observations from PVMBG.

Figure 40. An episode of increased activity at Karangetang from mid-July 2019 through mid-January 2020 included incandescent avalanche blocks traveling down multiple flanks of the volcano. This was reflected in increased thermal activity seen during that interval in the MIROVA graph covering 5 June 2019 through May 2020. Courtesy of MIROVA.

Figure 41. An episode of increased activity at Karangetang from mid-July 2019 through mid-January 2020 included incandescent avalanche blocks traveling up to 1,500 m down drainages on the W and NW flanks of the volcano. Top left: large thermal anomalies trend NW from Main Crater on 5 December 2019; about 500 m N a thermal anomaly glows from Second Crater. Top center: on 15 December plumes of steam and gas drifted W and SW from both summit craters as seen in Natural Color rendering (bands 4,3,2). Top right: the same image as at top center with Atmospheric penetration rendering (bands 12, 11, 8a) shows hot zones extending WNW from Main Crater and a thermal anomaly at Second Crater. Bottom left: thermal activity seen on 14 January 2020 extended about 800 m WNW from Main Crater along with an anomaly at Second Crater and a hot spot about 1 km W. Bottom center: by 19 January the anomaly from Second Crater appeared slightly stronger than at Main Crater, and only small anomalies appeared on the NW flank. Bottom right: an image from 14 March shows only thermal anomalies at the two summit craters. Courtesy of Sentinel Hub Playground.

A single VAAC report in early April noted a short-lived ash plume that drifted SW. Intermittent low-level activity continued through May 2020. Small SO₂ plumes appeared in satellite data multiple times in December 2019 and January 2020; they decreased in size and frequency after that but were still intermittently recorded into May 2020 (figure 42).

Figure 42. Small plumes of sulfur dioxide were measured at Karangetang with the TROPOMI instrument on the Sentinel-5P satellite multiple times during December 2019 (top row). They were less frequent but still appeared during January-May 2020 (bottom row). Larger plumes were also detected from Dukono, located 300 km ESE at the N end of North Maluku. Courtesy of Global Sulfur Dioxide Monitoring Page.

PVMBG reported in their daily summaries that steam plumes rose 50-150 m above the Main Crater and 25-50 m above Second Crater on most days in December. The incandescent avalanche activity that began in mid-July 2019 also continued throughout December 2019 and January 2020 (figure 43). Incandescent blocks from the Main Crater descended river drainages (Kali) on the W and NW flanks throughout December. They were reported nearly every day in the Nanitu, Sense, and Pangi drainages, traveling 1,000-1,500 m. Incandescence from both craters was visible 10-25 m above the crater rim most nights.

Figure 43. Incandescent block avalanches descended the NW flank of Karangetang as far as 1,500 m frequently during December 2019 and January 2020. Left image taken 13 December 2019, right image taken 6 January 2020 by PVMBG webcam. Courtesy of PVMBG, Oystein Anderson, and Bobyson Lamanepa.

A few blocks were noted traveling 800 m down Kali Beha Barat on 1 December. Incandescence above the Main crater reached 50-75 m during 4-6 December. During 4-7 December incandescent blocks appeared in Kali Sesepe, traveling 1,000-1,500 m down from the summit. They were also reported in Kali Batang and Beha Barat during 4-14 December, usually moving 800-1,000 m downslope. Between 5 and 14 December, gray and white plumes from Second Crater reached 300 m multiple times. During 12-15 December steam plumes rose 300-500 m above the Main crater. Activity decreased during 18-26 December but increased again during the last few days of the month. On 28 December, incandescent blocks were reported 1,500 m down Kali Pangi and Nanitu, and 1,750 m down Kali Sense.

Incandescent blocks were reported in Kali Sesepi during 4-6 January and in Kali Batang and Beha Barat during 4-8 and 12-15 January (figure 44); they often traveled 800-1,200 m downslope. Activity tapered off in those drainages and incandescent blocks were last reported in Kali Beha Barat on 15 January traveling 800 m from the summit. Incandescent blocks were also reported traveling usually 1,000-1,500 m down the Nanitu, Sense, and Pangi drainages during 4-19 January. Blocks continued to occasionally descend up to 1,000 m down Kali Nanitu through 24 January. Pulses of activity occurred at the summit of Second Crater a few times in January. Steam plumes rose 25-50 m during 8-9 January and again during 16-31 January, with plumes rising 300-400 m on 20, 29, and 31 January. Incandescence was noted 10-25 m above the summit of Second Crater during 27-30 January.

Figure 44. Incandescent material descends the Beha Barat, Sense, Nanitu, and Pangi drainages on the NW flank of Karangetang in early January 2020. Courtesy of Bobyson Lamanepa; posted on Twitter on 6 January 2020.

Activity diminished significantly after mid-January 2020. Steam plumes at the Main Crater rose 50-100 m on the few days where the summit was not obscured by fog during February. Faint incandescence occurred at the Main Crater on 7 February, and steam plumes rising 25-50 m from Second Crater that day were the only events reported there in February. During March, steam plumes persisted from the Main Crater, with heights of over 100 m during short periods from 8-16 March and 25-30 March. Weak incandescence was reported from the Main Crater only once, on 25 March. Very little activity occurred at Second Crater during March, with only steam plumes reported rising 25-300 m from the 22nd to the 28th (figure 45).

Figure 45. Steam plumes at Karangetang rose over 100 m above both summit craters multiple times during March, including on 26 March 2020. Courtesy of PVMBG and Oystein Anderson.

The Darwin VAAC reported a continuous ash emission on 4 April 2020 that rose to 2.1 km altitude and drifted SW for a few hours before dissipating. Incandescence visible 25 m above both craters on 13 April was the only April activity reported by PVMBG other than steam plumes from the Main Crater that rose 50-500 m on most days. Steam plumes of 50-100 m were reported from Second Crater during 11-13 April. Activity remained sporadic throughout May 2020. Steam plumes from the Main Crater rose 50-300 m each day. Satellite imagery identified steam plumes and incandescence from both summit craters on 3 May (figure 46). Faint incandescence was observed at the Main Crater on 12 and 27 May. Steam plumes rose 25-50 m from Second Crater on a few days; a 200-m-high plume was reported on 27 May. Bluish emissions were observed on the S and SW flanks on 28 May.

Figure 46. Dense steam plumes and thermal anomalies were present at both summit craters of Karangetang on 3 May 2020. Sentinel 2 satellite image with Natural Color (bands 4, 3, 2) (left) and Atmospheric Penetration rendering (bands 12, 11, 8a) (right); courtesy of Sentinel Hub Playground.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi island. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World: Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

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/); 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); 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/); 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/); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); Bobyson Lamanepa, Yogyakarta, Indonesia, (URL: https://twitter.com/BobyLamanepa/status/1214165637028728832).

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).

Regular plume emissions seen in satellite imagery and by aviators during March-May 2006 (BGVN 31:05) apparently ended in June, with the last reported activity being a pilot report of an ash cloud on 26 June that reached 3 km altitude. A report issued by the U.S. Geological Survey (USGS) on 7 December noted that the Alert Level was being lowered to Green and that seismic activity at Anatahan was very low during late November and early December, although diffuse steam-and-gas plumes were occasionally visible on recent satellite images or by aviators.

According to the USGS, seismometers recorded tremor starting on 24 February (UTC) that continued at high levels through 17 March. During that time, recorded tremor occasionally increased to much higher values. In addition, OMI satellite spectrometer data showed occasionally high amounts of sulfur dioxide over Anatahan. Tremor levels increased significantly starting at 1625 on 9 March (UTC) and continued for over 40 hours. As of 13 March the tremor bursts were infrequent, and some were high amplitude. In addition, a distinct gas plume was visible in Moderate Resolution Imaging Spectroradiometer (MODIS) imagery, suggesting increased emissions. On that day the Alert Level was raised to Advisory.

The MODIS flying onboard the Aqua satellite captured a view of the plume on 18 March 2007 as emissions continued. In the image, the volcanic plume headed SE, then changed direction slightly and trended towards for the islands of Saipan and Tinian. The same day MODIS acquired this image, the U.S. Air Force Weather Agency reported an odor of sulfur, which would also suggest the presence of vog (volcanic smog) on Guam, ~200 km SW of Saipan. USGS and Emergency Management Office air quality instruments on Saipan recorded a maximum 5-minute average of 959 ppb sulfur dioxide and 99 ppb hydrogen sulfide on 18 March.

As of 24 March, the USGS was reporting that tremor levels after 17 March had remained low at pre-24 February levels. The plume visible in MODIS imagery had also remained weak but distinct since 18 March. On 24 March the Alert Level was lowered to Normal, with an aviation color code of Green. No confirmed ash eruptions had occurred after 3 September 2005.

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: Juan Takai Camacho and Ramon Chong, Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO-CNMI), PO Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/); Frank Trusdell, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3591, USA (URL: https://volcanoes.usgs.gov/nmi/activity/); U.S. Air Force Weather Agency (AFWA)/XOGM, Offutt Air Force Base, NE 68113, USA; NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/NaturalHazards/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

The 10-day-long eruption of Etna's Southeast Crater (SEC) in mid-July 2006 (BGVN 31:08 and 31:10) was considered by scientists at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) to represent a distinct phase of 2006 activity for Etna. They identified a very different phase when eruptive activity shifted to SEC's summit vent between 31 August and early 15 September 2006. The latter activity led to lava overflows and repeated collapse on SEC's E side. The seven eruptive activity episodes previously described (BGVN 31:10) have since been renumbered slightly, with Episode 1 taking place between 31 August and 16 September.

The following report was compiled from recent reports by Boris Behncke and Sonia Calvari, based on daily observations by numerous staff members of the INGV Catania (INGV-CT). This issue overlaps with our previous Bulletin reports and then goes on through the end of 2006.

Overview of the 31 August to 14 December eruption. Figure 117 indicates key vents and lava flows during the period 4 September-7 December 2006. It excludes lavas emitted during the short but intense final episode (Episode 20, 11-14 December 2006), but they did not significantly extend beyond flow margins shown here. The longest lava flows of the reporting interval reached ~ 4.7 km SE from their source vent (figure 117).

Figure 117. Map Etna showing lava flows and their corresponding periods of activity: (1) lavas from the summit and flanks of the SEC, 4 September-3 December 2006; (2) lavas from the 2,800-m vent, 13 October-7 December 2006; (3) lavas from the 3,050-m vent, 27 October-27 November 2006; and (4) lavas from 3,180-m vent, 8-27 November 2006. The capital letters indicate the most persistent eruption sources: (A) SEC summit; (B) 2,800?m vent; (C) 3,050-m vent; (D) 3,180-m vent; (E) 3,100-m vent (active between 30 November and 3 December 2006); and (F) the foundation crater of the 23 October 2006 activity (which developed a pit that was also active between 24 November and 7 December 2006). Courtesy of INGV-CT; Behncke, Branca, Neri, and Norini (2006).

Table 9 summarizes the 20 episodes of recent eruptive activity, as currently identified by the INGV staff. Note, however, that episode numbers have changed since discussed in BGVN 31:10. One earlier episode has been added (31 August-15 September). Former Episodes 1-7 as listed in BGVN 31:10 based on earlier INGV reports, have been renumbered to Episodes 2-8. Subsequent episodes (9 through 20) are the main subject of this report.

Table 9. List of eruptive episodes (1-20) at Etna as reported by INGV-CT for the interval 31 August-December 2006. "Former number" refers to the episode numbers stated in BGVN 31:10 but here revised. Geberal morning and afternoon times are indicated by am and pm, respectively. Courtesy of INGV-CT.

Episode 9. Although there were no real paroxysms of Strombolian activity or lava fountaining at the SEC during 26 October-4 November, clear pulses of activity occurred at the effusive vents at 2,800 and 3,050 m elevation, accompanied by ash emission or weak Strombolian explosions at the SEC. These events defined Episode 8, on 27 October, and Episode 29, which took place during 29-30 October. The clear pattern of distinct paroxysms from the SEC finally returned on 5 November and lasted through late that month, before the activity became again more continuous early in December.

Episode 10. Following one week of intermittent ash emissions and weak Strombolian activity on late 4 November, a new strong eruptive episode started at the SEC summit vent at 2004 on 5 November and continued with some fluctuations and intermittent ash emissions for the next 9.5 hours. Light ashfalls occurred over populated areas to the SE. At about 2147 on 5 November, the effusion rate increased at a vent at 3,050 m elevation at the S base of the central summit cone (C on figure 117) which had been continuously active since 27 October. A new lobe of lava traveled S of the summit cone complex across a flat area known as the Cratere del Piano.

An apparent increase in the effusion rate was also noted at the effusive fissure at 2,800 m elevation on the ESE flank (B on figure 117), with active lava lobes extending downslope. Lava effusion from the 3,050-m vent ended during the morning of 6 November, and for the following 48 hours, lava emission continued only at the 2,800-m vent.

Episode 11. Ash emissions from the summit of the SEC occurred on 8 November 2006, followed by vigorous Strombolian activity that continued until about 2200. Around 1600, lava started to flow from a new vent located in the saddle between the SEC cone and the adjacent main summit cone, at an elevation of ~ 3,180 m (D on figure 117). The lava reached the SW base of the SEC cone in a few minutes, where it bifurcated into several short lobes, the largest and westernmost lobe stopping at the E margin of the lava flow field from the 3,050-m vent. Lava from the 3,180-m vent had ceased flowing by about 1845, whereas spattering and lava effusion continued at the 3,050-m vent for some time. Spattering ended at that vent around 1930, but lava continued to flow for another 24 hours.

Episode 12. At 2100 on 10 November 2006, tremor amplitude rapidly increased. Bad weather hampered visual observations until 11 November, when it became evident that this episode was quite similar to its predecessor, with lava emission occurring from both the 3,050-m and 3,180-m vents. Strombolian activity from the SEC summit ceased at 1100 on 11 November. Lava emission from the 3,050-m vent continued until the following night, and the associated lava flow field grew mainly on its W side, with flow fronts descending to ~ 2,800 m. For the next five days, lava emission continued unabated from the 2,800-m-vent, whereas the SEC and all other vents remained inactive.

Episode 13. Following a sharp increase in tremor amplitude at 0500 on 16 November, vigorous ash emissions started at the SEC summit at 0507 and were gradually replaced by intense Strombolian bursts, marking the onset of this eruptive episode.

Very early during the episode, lava issued from the 3,180-m vent, forming a lobe ~ 100 m long before activity at this vent ceased.

Lava effusion from the summit started at 0615 on 16 November and triggered a series of rockfalls down the SE flank of the SEC cone, before the lava descended on the same flank. At 0626, brownish ash was emitted from a spot next to the effusive vent, and major rockfalls and avalanches started shortly thereafter. These originated at the S rim of what remained of the 2004/2005 collapse pit on the E flank of the SEC (see BGVN 30:01 and 30:12). Plumes rising from the descending avalanches contained both brownish ash and white steam. Avalanching was most intense between 0631 and 0640, after which the new lava flow rapidly descended the lower SE flank of the cone and began to extend beyond its base toward the area of the 2,800-m vent. At the same time, strong emissions of black ash marked the opening of another explosive vent next to the summit, and a third explosive vent became active in the same area. For the next several hours, the vents continued to eject ash and occasionally bombs, and to produce vigorous Strombolian activity.

At 0700 on 16 November emissions of white vapor occurred from the SE flank of the SEC cone; a few minutes later large rock avalanches started to descend that flank. Simultaneously a fissure began to open near the summit to downslope on the SSE flank, triggering local rockfalls and dust avalanches. This fissure initially propagated ~ 100 m downslope, then it temporarily stopped; but at 0720, it propagated another 150 m downslope. During the following 15 minutes, another fissure perpendicular to the earlier one cut SE across the flank, generating more rockfalls and dust avalanches. The resulting fissure system had the form of an inverted Y delimiting a block that was actively pushed outward by magma intruding into the cone's flank.

Lava began to issue from the lower end of the W branch of the fissure system at about 0810 on 16 November. At approximately the same time, the 3,050-m vent started to emit lava. By this time, the upper portion of the fissure cutting the SSE flank of the SEC cone had significantly enlarged and became a deep trench. Dense volumes of steam were emitted from this trench at 0831 and were followed a few minutes later by another series of rockfalls and avalanches. Direct observation from ~ 700 m showed that the most energetic of these avalanches resulted from the collapse of low fountains of gas and tephra at the lower end of the large trench. The avalanches and rockfalls lasted about 15 minutes, then a voluminous surge of lava issued from the lower end of the opening trench.

Over the next few hours this sequence of events (vapor emission?rockfalls and avalanches?lava emission) was repeated several times as the trench widened and propagated further downslope. During the few moments when steam and dust clouds cleared and the interior of the trench became visible, a cascade of very fluid lava was seen in the center of the trench. Apparently, the lava issued from a source high in the head wall of the trench, and at times spurted from the vent like a firehose.

At 1100 on 16 November, white steam plumes, rockfalls, and dust avalanches appeared high on the SE flank of the SEC cone, in the area where the summit lava flow was emitted. These phenomena marked a major collapse of the E wall of the trench, which eventually cut into the descending summit lava flow, diverting it into the trench. The original flow, which had descended immediately S of the 2,800-m vent down to ~ 2,600 m elevation, rapidly stopped, although lava continued to drain from the main flow channel and accumulated in a thickening lobe at the cone's base.

At about 1425 on 16 November, several vertical jets of black tephra shot upward from an area at ~ 150 m distance from the cone's base. These emissions were very distinct in color from the brownish dust clouds, which at the same time descended from the trench. The activity at the new site appeared to migrate rapidly both toward the SEC as dark plumes began to rise closer to the cone, while a ground-hugging plume of white vapor shot in the opposite direction. A few ten's of seconds later, very dense clouds of dark brown material began to appear at the base of the surging white cloud and formed a distinct flow that rapidly overtook the front of the white cloud while speeding toward SE. At the slope break along the W rim of the Valle del Bove (~ 2,800 m elevation), both clouds disappeared from view in weather clouds, but at the site where the activity had originated, a huge plume of white vapor soared skyward. White vapor continued to rise from the area and from the path of the white and dark brown clouds for more than 15 minutes.

Another explosive emission of white steam and dark brown plumes occurred at about 1455. Like the 1425 event, it generated ground-hugging clouds of steam and dark brown material, the latter again traveling faster. During the following hours, activity at the SEC gradually decreased, with several spectacular cascades of lava descending through the trench on the cone's SSE side. Steam explosions and rock avalanches occurred at the lower termination of the trench at 1525. Strombolian activity ceased at 1500 on 16 November, but lava emission continued until about midnight. This lava does not seem to have extended far from the base of the SEC cone, since investigation during the following day failed to reveal any fresh lava on top of the debris deposits emplaced during the major explosive events at 1425 and 1455. A minor lava flow was also fed from a new short fissure ~ 80 m E of the 3,050-m vent. During the evening a small lobe of lava was emitted from the accumulation at the SEC cone's base.

Fieldwork and aerial surveys during the two days following 16 November revealed that the 1425 and 1455 explosions and related volcaniclastic density currents (figure 118) had left two main types of deposit. One was of lobate shape and extended a few hundred meters from the source of the explosions to the SE, covering a footpath established by mountain guides to allow tourists to approach the persistently active 2,800-m vent.

Figure 118. One of the peculiar density currents at Etna that occurred during Episode 13, 16 November 2006. The photo was taken from the N side of the large 2002-2003 cone complex, ~ 1.3 km S of the SEC. Seen in the photo are strong emissions of dark gray ash from two vents at the summit (a third caused intense Strombolian activity, but not in the moment shown in the photo). A huge gash carved out of the near right side of the cone emitted a lot of white vapor, with lava flowing from its lower end, and a ground-hugging brownish ash cloud spilling downslope on top of the flowing lava. Photo courtesy of INGV-CT.

On the ground the deposit consisted of very fine grained reddish-brown ash made up almost exclusively of lithic fragments. To the N the deposit gradually thickened and larger clasts were found on its surface, some of which represented fresh magmatic material. Close to the 2,800-m vent, the deposit abruptly graded into a sort of debris flow rich in lithics but with up to 25% of fresh magmatic clasts. These latter showed a peculiar flattened-out morphology. Where this deposit overlay the tourist path near the 2,800-m vent it was 1.52 m thick. In one place the flow had surrounded a plastic-coated sign warning tourists to stay on the path. The plastic lacked evidence of strong heating, indicating that the flow was relatively cool at this point along its path.

Volcanic tremor amplitude began to increase during the late afternoon of 18 November and, during a helicopter flight at 1800, the 2,800-m vent showed vigorous spattering. Active lava from the vent traveled ~ 3 km to Monte Centenari. Bright incandescence was also noted within the 3,180-m vent during this overflight.

Episode 14. At 0400 on 19 November, Strombolian activity at the SEC occurred from 2 vents at the summit while lava flowed through the 16 November trench and divided into numerous braiding lobes on top of the debris deposited 3 days earlier. The longest lobe traveled along the prominent channel in the main debris flow, passing immediately to the S of the 2,800-m vent and extending to an elevation of ~ 2,600 m. This episode was much less violent than its predecessor and lacked the explosions, surges, and flows characteristic of that event. Strombolian activity continued until the late evening, while lava effusion ended early on 20 November. As during previous episodes, lava had also briefly issued from the 3,050-m and 3,180-m vents. In addition, a flow of a few meters in length started from another fissure that opened at ~ 3,200 m, on the saddle between Bocca Nuova and SEC. This upper flow merged with the flow coming out from the 3,180-m vent.

Episode 15. This eruptive episode at the SEC started at 1200 on 21 November 2006, but direct observations were thwarted by inclement weather through nightfall. At about 1500, a black ash plume was seen rising above the cloud cover to ~ 1.5 km above the summit. Light ashfalls occurred along the Ionian coast near Giarre and further N, while at Rifugio Citelli (~ 6 km NE of the SEC), ash deposition was nearly continuous.

After 1900, the cloud cover gradually opened, allowing direct views of the strong Strombolian explosions generating jets sometimes over 300 m high. Lava once more flowed through the 16 November trench on the cone's SSE flank toward the 2,800-m vent. Likewise, the 3,050-m and 3,180-m-vents reactivated, although the latter apparently ceased erupting early during the episode. Lava flowed from the trench until shortly after midnight on 22 November. Bad weather precluded observations until the evening, when all activity was again limited to the 2,800-m vent.

Episode 16. At 0219 on 24 November, there began ash emissions mixed with Strombolian explosions. These were recorded by the INGV-CT thermal camera in Nicolosi (~ 15 km S of the SEC) with a significant anomaly occurring at the SEC summit. Strombolian activity at 0320 was accompanied by voluminous ash emission, which formed a plume that rose ~ 2 km above the summit before being blown to SE.

Two particularly powerful explosions occurred at 0452 and 0455. The latter was followed by lava extruding from a vent presumably located within the 16-November trench. At around 0535, lava began to issue from the 3,050-m vent, forming a small flow on the W side of the lava flow field emplaced since 26 October. A second minor flow issued from another vent located ~ 80 m SE of the 3,050-m vent. Vigorous ash emission from the summit of the SEC caused light ashfalls over populated areas between Zafferana and Acireale (figure 119).

Figure 119. Dark ash plume rising from Etna's SEC during eruptive Episode 16 on the morning of 24 November, photographed from a helicopter provided by the Italian Department of Civil Protection (Dipartimento di Protezione Civile) during that day's particularly explosive episode. A small steam plume at left rises from the area of the 2,800-m vent. More diffuse gas emitted from active lava flows engulfs the photo's extreme left. Etna's other summit craters (Northeast Crater foremost, with Voragine and Bocca Nuova behind) are in the lower right corner of the image, showing normal degassing activity. View is approximately to the S. Courtesy of INGV-CT.

A fracture opened at about 0817 at the SSE base of the SEC cone, producing a violent explosion and a rock avalanche that descended at a speed of several ten's of km/h toward the Valle del Bove, following the path of similar avalanches that had occurred on 16 November. Lava effusion continued from vents at the cone's base, where mild spattering was observed. Upslope from the effusive vent at 2,800 m elevation, a second fracture formed and commenced spattering and lava emission.

During the early afternoon a change in the wind direction drew the plume from its earlier SE-ward course toward Catania and adjacent areas, forcing the closure of the Fontanarossa International airport of Catania. The activity began to diminish, and by 1530 all explosive phenomena ceased. For several more hours lava continued to issue from two vents at the SEC cone's base.

Late in the afternoon of 24 November, weak sporadic Strombolian explosions occurred from a pit located on the E flank of the SEC cone, which had formed during the 23 October eruptive episode (hereafter, '23 October pit' identified as F on figure 117). On 25 November this vent produced pulsating ash emissions that continued intermittently for the next two days.

Episode 17. At around 0410 on 27 November, eruptive activity occurred at the SEC and the thermal monitoring camera at Nicolosi began to record a significant thermal anomaly at the crater and a W-drifting ash plume. Visual observations were hampered by inclement weather. Around 0730, the thermal camera at Nicolosi disclosed lava emission on the W side of the SEC cone, possibly from the vent at 3,180 m elevation in the saddle between the SEC and the Bocca Nuova. About 45 min later, lava emission became evident at the cone's SE base. No further visual observations were available after 0845, but the tremor amplitude remained high until the afternoon, when a sharp drop indicated the end of this eruptive episode.

Bad weather persisted until early on 29 November when observers saw ash emissions from the '23 October pit.' These emissions became more intense after 0545, and the tremor amplitude began to increase rapidly during the late morning. Intermittent, weak Strombolian activity from the '23 October pit' was visible after nightfall; this became notably stronger shortly after 0100 on 30 November and reached its highest intensity around 0130, after which there was a notable decrease. Ash emissions occurred from the same pit at dawn and again from 1240 onward, producing low ash plumes.

Episode 18. At around 1600 on 30 November 2006, lava fountains began to rise from the 2,800-m vent. Two hours later the '23 October pit' emitted a dense ash plume, and Strombolian explosions reached up to 150 m above the vent. At 2045, a fissure opened at ~ 3,100 m elevation, venting spatter several ten's of meters high and releasing a short lava flow towards the 2,800-m vent. After about 10 min the effusion rate at this new fissure diminished, but lava continued to escape at a decreasing rate for ~ 1 hour. The '23 October pit' remained vigorously active for the next 5 hours, producing incandescent jets and a dense tephra plume.

The new fissure at 3,100 m elevation revived around 0115 on 1 December, with vigorous spattering and a new surge of similarly directed lava. At the same time, the '23 October pit' emissions strongly increased. Like on the evening before, the new fissure at 3,100 m elevation remained active only for a short time; lava emission ceased by 0200 on 1 December.

The 2800-m vent produced the largest lava flows during the entire period of activity, in this episode extending lava flows to ~ 1,500 m elevation on the Valle del Bove floor, to a distance of ~ 4.7 km from their source.

Between 1-3 December, the '23 October pit' remained active with nearly continuous emissions of ash interspersed with Strombolian activity. This was accompanied by the 3,100-m fissure emitting low fountaining and lava; lava flows from that fissure were generally short and did not extend far beyond the 2,800-m vent. The last observed activity at the 3,100-m vent occurred during the morning of 3 December. Ash emissions from the '23 October pit' continued for another few days but became progressively weaker; likewise the lava emission at the 2,800-m vent diminished gradually.

Episode 19. Weak Strombolian activity and ash emission occurred at the SEC on the afternoon of 6 December, evidenced by increased tremor, but the amplitude dropped rapidly to very low levels implying that the SEC ceased erupting late on 6 December. Minor lava emissions continued from the 2,800-m vent. On the morning of 8 December, no eruptive activity was visible at any of the numerous vents of the previous weeks. Following several days of very low tremor amplitude, it began to increase again late on 10 December.

Episode 20. Eruptive activity resumed around 0330 on 11 December 2006 from the '23 October pit' on the SEC, with Strombolian explosions documented by INGV-CT's monitoring cameras. Simultaneously, lava emission started from the area of the 2,800-m vent, forming a flow that slowly descended toward the Valle del Bove. Bad weather hampered observations during the following days, but occasional clear views revealed ash emissions from the '23 October pit.' In addition, there were voluminous lava emissions from the 2,800-m vents, feeding a broad lava flow adjacent the N margin of the lava flowfield produced from the same vent between mid-October and early December. The 2,800-m vents generated vigorous Strombolian explosions from two vents that built up a pair of large hornitos, and lava emissions came from a third vent located on the lower E flank of the larger, more easterly of the hornitos. No activity occurred from any other of the numerous vents that had been active during the previous weeks at the summit and in the vicinity of the SEC. Late in the afternoon of 14 December, a sharp drop in tremor amplitude indicated that the end of this final eruptive episode was imminent, and field observations made on the following morning revealed the absence of eruptive activity.

INGV considered Etna's 2006 summit eruptions during 14 July-14 December and made a rough estimate of erupted lava volumes. The total volume produced during those 5 months amounted to ~ 15-20 x 10⁶ m³.

There was a single, relatively small ash emission from Bocca Nuova on 19 March 2007, discharged without an associated seismic signal. This was followed ten days later by a brief episode of violent lava fountaining and tephra emission from the SEC. Details on that and subsequent activity will be reported in a future Bulletin.

References. Behncke, B., and Neri, M., 2006, Mappa delle colate laviche aggiornata al 20 Novembre 2006 (1 page PDF file on the INGV website) and Carta delle colate laviche emesse dall'Etna dal 4 Settembre al 7 Dicembre 2006 (Map of lava flow emissions at Etna from 4 September to 7 December 2006).

Behncke, B., Branca, S., Neri, M., and Norini, G., 2006, Rapporto eruzione Etna: mappatura dei campi lavici aggiornata al 7 Dicembre 2006 (Report of Etna eruption: map of lava flows up to 7 December 2006): INGV report WKRVGALT20061215.pdf.

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: Sonia Calvari and Boris Behncke, Istituto Nazionale di Geofisica e Vulcanologia-Catania (INGV-CT), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

Scientists from Simon Fraser and McGill universities conducted preliminary geophysical and geochemical field studies at Ijen (figure 4) between 13 and 26 August 2006. During this period, volcanic activity was low and restricted to persistent degassing of the solfatara in the SE part of the crater.

Figure 4. Photograph of the acid crater lake and solfatara (bottom left) in the active crater at Ijen, August 2006. View is from the E crater rim. Courtesy of G. Mauri.

Measurements of temperature and pH were made every morning during 14-19 August at four locations: the Banyupuhit River, ~ 5 km from the Banyupuhit River source, the acid lake in the summit crater, and the E shore of the crater lake. Temperatures of the Banyupuhit River were 16-20°C, always above atmospheric temperature by ~ 1-3°C; the pH was ~ 0.4. Lake temperatures varied between 31 and 43°C and the pH was -0.02. The color of the crater lake was generally homogeneous, although large black to brown linear patches, probably sulfur deposits from the solfatara, were observed on the turquoise-green surface. These ephemeral patches were of variable size (e.g. several ten's of meters long and a few meters wide) and moved across the lake during the course of the day, but were not always evident throughout the day. The area near the E shore appeared lighter than the rest of the lake, probably due to a spring at the bottom of the inner E slope.

Pipes driven into the fumaroles are used to extract gases for sulfur mining (figure 5). Temperatures measured 50 cm down into four of those pipes ranged from 224 to 248°C. These measurements almost certainly represent minimum estimates of the true temperatures due to heat loss along the length of the extraction pipes. After the gases had exited less than 50 cm from the pipes, temperatures had dropped below 120°C, the melting point of native sulfur.

Figure 5. Close-up view of the solfatara at Ijen with fumarole temperature of more than 220°C. Note pipes for extracting sulfur gases. Courtesy of G. Mauri.

A survey of sulfur dioxide (SO₂) fluxes made by a portable spectrometer (FLYSPEC) on 21 and 23 August along the SE rim of the crater consisted of seven and twelve walking traverses through the plume, respectively. The gas plume produced directly from the active solfatara near the lake surface rose buoyantly before flowing over the crater rim. During the first survey (conducted over a 2-hour period), the concentration-pathlength of the gas in the plume fluctuated between 1,000 and 2,500 ppm-m. The wind speed (measured by handheld anemometer at plume height) during this time averaged 6.1 m/s and the resultant SO₂ flux was therefore calculated to average 412 metric tons per day (t/d) with a standard deviation of 154 t/d. On 23 August, gas concentrations were somewhat lower, ranging between 500 and 2,000 ppm-m. The average wind speed during the survey period (2 hours) was 3.9 m/s and the resultant SO₂ flux averaged 254 t/d, with a standard deviation of 117 t/d. Based on this very limited survey, the flux of SO₂ was estimated to be 330 t/d.

Gravity surveys (Bouguer and dynamic) were conducted in the active crater and seven gravity stations were selected for future dynamic gravity monitoring. A digital elevation map was prepared (using digital photogrammetric mapping methods) to provide the spatial framework required for interpretation of the geophysical surveys.

The scientists also applied the self-potential (SP) method, also know as spontaneous potential, that measures electrical potentials developed in the Earth by electrochemical action between minerals and solutions with which they are in contact. SP mapping of the active summit crater showed two main hydrologic structures (figure 6). The first is a hydrogeologic zone on the E and NE rim characterized by a negative SP anomaly with a minimum at -100 mV (millivolts), an inverse SP/elevation gradient of -1.6 mV/m, and length of 1,500 m. This almost certainly represents inflow of meteoric water and groundwater.

Figure 6. Self-potential survey results shown on a topographic map of the active crater of Ijen, August 2006. All the SP data were referenced at the Banyupuhit River and at a spring on the inner E slope of the crater. Contour line intervals are 100 m. Courtesy of G. Williams-Jones.

The second structure is the main hydrothermal system located S, W, and N of the crater as well as in the southern inner slope of the crater, places where the surface expressions are solfataras. The SP maxima range between 48 and 60 mV and are located on the slope of the river below a dam on the outer W slope (+52 mV), on the N rim (+48 mV) and in the S part of the solfatara (+ 59 mV). Processing of the SP data along the crater profile by continuous wavelet transform (Mauri and others, 2006) shows that the hydrothermal fluid cells are near the surface (less than 200 m below the topographic surface) suggesting that the hydrothermal system is under high pressure with significant heat flux, as shown by the solfatara.

Reference. Mauri, G., Saracco, G., and Labazuy, P., 2006, Volcanic activity of the Piton de la Fournaise volcano characterized by temporal analysis of hydrothermal fluid movement, 1992 to 2005: AGU, Eos Trans, v. 87, no. 52, Fall Meet. Suppl., Abstract V51A-1653.

Geologic Background. The Ijen volcano complex at the eastern end of Java consists of a group of small stratovolcanoes constructed within the large 20-km-wide Ijen (Kendeng) caldera. The north caldera wall forms a prominent arcuate ridge, but elsewhere the caldera rim is buried by post-caldera volcanoes, including Gunung Merapi, which forms the high point of the complex. Immediately west of the Gunung Merapi stratovolcano is the historically active Kawah Ijen crater, which contains a nearly 1-km-wide, turquoise-colored, acid lake. Picturesque Kawah Ijen is the world's largest highly acidic lake and is the site of a labor-intensive sulfur mining operation in which sulfur-laden baskets are hand-carried from the crater floor. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of cones forms an E-W zone across the southern side of the caldera. Coffee plantations cover much of the caldera floor, and tourists are drawn to its waterfalls, hot springs, and volcanic scenery.

Information Contacts: Guillaume Mauri and Glyn Williams-Jones, Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada (URL: http://www.sfu.ca/earth-sciences.html); Willy (A.E.) Williams-Jones, Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec, Canada (URL: http://www.mcgill.ca/eps/); Deddy Mulyadi, Center of Volcanology and Geological Hazard Mitigation (CVGHM), Diponegoro 57, Bandung, Jawa Barat 40122, Indonesia (URL: http://vsi.esdm.go.id/).

After a year of quiet following ash ejections from Canlaon in May 2005 (BGVN 30:06), the Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that a new period of activity began on 3 June 2006. In total, twenty-three ash ejections occurred between 3 June and 25 July 2006. These outbursts were all water-driven in nature, characterized by emission of ash and steam that rose up to 2 km above the active crater. The prevailing winds dispersed ash in all directions. The seismic network, however, did not detect significant seismic activity before or after the ash emissions, supporting the idea that the explosions were very near-surface hydrothermal events.

Four explosive episodes that occurred over the days 3, 10, and 12 June ejected mainly steam with some ash, and affected only the summit crater and upper SW slopes. The event at 1430 on 3 June sent dirty white to grayish steam 800 m above the summit. The activity was observed until 1445 when thick clouds covered the summit. Another emission started at 2316 on 10 June and lasted until 0030 the next morning. The plume was estimated to attain heights of 700-1,000 m before drifting SW. After the ash emission, moderate to wispy steam plumes escaped, to maximum heights of 600 m above the summit. Another steam-and-ash episode during 0515-0535 on 12 June caused a plume to rise about 600 m before drifting SW. After the ash emission, generally weak to moderate steaming to a height of ~ 400 m returned. Plumes rose 600-1,000 m and drifted SW; ashfall was confined to the upper slopes. This new period of low-level unrest prompted PHIVOLCS to raise the hazard status to Alert Level 1 on 12 June, suspending all visits to within 4 km of the summit.

Three small steam-and-ash emissions without recorded seismicity occurred again between the afternoon of 13 June and the morning of the 14th. The grayish steam clouds rose ~ 900 m above the active crater and drifted NE and NW. Only traces of ash were observed over the N upper slope. An explosion from 0845 to 0924 on 14 June produced an ash and steam cloud, which rose up to 1.5 km above the summit and drifted N, affecting mainly the upper slopes. Voluminous grayish steam plumes were then seen rising up to 1.5 km above the summit crater after 1640 through the next morning. The seismic network detected only two low-frequency volcanic earthquakes. Kanlaon City proper experienced light ashfall starting at 1630 on 15 June after voluminous dirty white steam was observed rising 1.5-2 km above the summit crater a few hours earlier (from 1346 to 1520). As of 1800, ashfall was still wafting through the city.

The character of this episode changed on the afternoon of 19 June when two episodes of steam-and-ash emission sent clouds 600 m above the crater that drifted SW. Weak to moderate steaming was observed after the second explosion and during the morning observation on the 20th. The initial explosion was recorded by the Cabagnaan station's seismograph as low-frequency tremor with a duration of 13 minutes. One minute of tremor was recorded at the time of the second explosion. No precursor seismicity was detected. Traces of ashfall and sulfurous odors were reported at Barangay Cabagnaan proper in La Castellana. During the 24 hours before 0730 on 20 June, the seismic network detected two cases of low-frequency tremor and three small low-frequency volcanic earthquakes.

An additional six short steam-and-ash emissions took place during 21-25 June. The explosions produced grayish columns that rose 800-1,500 m above the crater and drifted NW, SW, and SSW. Volcanic seismicity was not associated with these events except for a single harmonic tremor before the emission on 25 June. Light ashfall was reported at Upper Cabagnaan in La Castellana. Weak to moderate steaming was observed after the explosions.

Steam-and-ash emissions were not reported again until the afternoon of 2 July. The grayish steam clouds then rose to heights of up to 1,000 m above the active crater and generally drifted NW. Another episode on the morning of 3 July produced a column to a height of 500 m above the crater. The seismograph at Cabagnaan recorded ten volcanic earthquakes while the seismograph at Sto. Bama near Guintubdan in La Carlota City recorded eight local seismic events during the 24 hour observation period that included these emissions.

An explosion-type earthquake with a 10 min, 25 sec duration was recorded at 0426 on 23 July, but cloud cover prevented observations. Traces of ash fell up to about 9 km ENE from the crater, affecting Barangays Pula, Malaiba, and Lumapao. When clouds cleared during 0630-0800 on 25 July, ash-laden steam clouds were seen rising up to 300 m above the crater drifting ENE and SE. Light ashfall was experienced at Gabok, Malaiba, and Lumapao of Kanlaon City, about 9 km from the crater. This emission was not reflected on the seismic record as only two small volcanic earthquakes were detected during the preceding 24 hours. Dirty white steam was observed on the morning of the 26th rising to a maximum of 100 m above the crater.

Explosions ceased after 25 July, and other activity, such as weak steaming and minor seismicity, showed a general trend towards quiescence. After three months with no further explosive emissions, on 2 November 2006 PHIVOLCS lowered the hazard status from Alert Level 1 to Alert Level 0, meaning the volcano has returned to normal conditions.

Geologic Background. Kanlaon volcano (also spelled Canlaon), the most active of the central Philippines, forms the highest point on the island of Negros. The massive andesitic stratovolcano is dotted with fissure-controlled pyroclastic cones and craters, many of which are filled by lakes. The largest debris avalanche known in the Philippines traveled 33 km SW from Kanlaon. The summit contains a 2-km-wide, elongated northern caldera with a crater lake and a smaller, but higher, historically active vent, Lugud crater, to the south. Historical eruptions, recorded since 1866, have typically consisted of phreatic explosions of small-to-moderate size that produce minor ashfalls near the volcano.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, Univ. of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

Moderate activity occurred at Langila between January and March 2006 (BGVN 31:05), with eruptive activity accompanied by a continuous ashfall, rumbling, and weak emissions of lava fragments. Since March 2006, activity has continued at Crater 2.

According to the Darwin Volcanic Ash Advisory Center (VAAC), eruptions at Crater 2 occurred in August 2006 and from October 2006 through March 2007, with explosions of incandescent lava fragments, roaring noises at regular intervals, and continuous emissions of gray-to-brown ash plumes. Plumes generally reached 2.3-3.3 km altitude, although on 31 October a small ash plume rose to an altitude of 4.6 km. Ash plumes were occasionally visible on satellite imagery. During October and through the first part of January 2007, plumes generally drifted N, NW, W, WNW, and NE; between the end of January and March, plumes drifted SE and SW.

Thermal anomalies detected by MODIS instruments on the Terra and Aqua satellites were absent after 2 January 2006 until 21 July 2006. The same system (the HIGP Thermal Alerts System) identified anomalies again on 24 and 31 October, 12 and 21 November, 16 and 27 December 2006, 6 January, 8 March, and 18 March 2007.

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: Rabaul Volcano Observatory (RVO), PO Box 386, Rabaul, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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/).

The rarely visited Lastarria has not erupted in historical time, but has displayed strong fumarolic activity (figure 1) for at least 67 years. This is the first Bulletin report ever issued on this volcano; it presents new images of the steaming edifice. On 2 February 2007, a group of scientists from the Servicio Nacional de Geología y Minería (SERNAGEOMIN) and the Corporación Nacional Forestal (CONAF) observed the fumarolic activity from a distance. The scientists were on a field trip to count flamingos and other Andean birds at Ramsar sites. The Ramsar Convention on Wetlands (http://www.ramsar.org/), named after a city in Iran, is an intergovernmental treaty that provides the framework for national action and international cooperation for the conservation and wise use of wetlands and their resources. The group noted steam plumes blowing NE at mid-day from ~ 47 km SW. Fumarolic gases were again seen, from ~ 35 km WSW, slowly moving down the W slope of the cone (figure 2). Steam plumes were seen intermittently throughout the afternoon.

Figure 1. Lastarria imaged by satellite on an unknown date. Fumaroles can be seen on the SW and SE crater rims. Crater width (E-W) is ~600 m. Courtesy of Google Earth and DigitalGlobe.

Figure 2. Photograph showing Lastarria from ~35 km WSW, 2 February 2007. Fumarolic gases can be seen rising above the cone and moving down the W flank. Courtesy of Héctor Cepeda.

Jose Antonio Naranjo, who has worked at Lastarria since 1983, is very familiar with its spectacular fumarolic activity. He confirmed that the observations of February 2007 reflect Lastarria's normal intense fumarolic emissions. Such activity has continued since at least 1940, when observed by Danko Slozilo. Naranjo noted that in 2007 he saw the same fumarole locations as those he observed in 1983 and in October 2002 (figure 3). The temperatures of these fumaroles were unchanged between 1983 and 2002.

Figure 3. Photograph of the Lastarria cone showing the lava dome overlapping the N crater rim and fumaroles along the rim, October 2002. View is from the N. Courtesy of Jose Antonio Naranjo.

References. Naranjo, J.A., 1985, Sulphur flows at Lastarria volcano in the North Chilean Andes: Nature, v. 313, no. 6005, p. 778-780.

Naranjo, J.A., 1986, Geology and evolution of the Lastarria volcanic complex, north Chilean Andes: Unpublished M Phil. Thesis, The Open University, England, 157 p.

Naranjo, J.A., and Francis, P., 1987, High velocity debris avalanche at Lastarria volcano in the north Chilean Andes: Bull. Volcanol., v. 49, p. 509-514.

Naranjo, J.A., 1988, Coladas de azufre de los volcanes Lastarria y Bayo en el norte de Chile: reologia, genesis e importancia en geologia planetaria: Revista Geologica de Chile, v. 15, no. 1, p. 3-12.

Naranjo, J.A., 1992, Chemistry and petrological evolution of Lastarria volcanic complex in the north Chilean Andes: Geol. Magazine, v. 129, p. 723-740.

Geologic Background. The NNW-trending edifice of 5706-m-high Lastarria volcano along the Chile-Argentina border contains five nested summit craters. The youngest feature is a lava dome that overlaps the northern crater rim. The large andesitic-dacitic Negriales lava field on the western flanks was erupted from a single SW-flank vent. A large debris-avalanche deposit is found on the SE flank. Recent pyroclastic-flow deposits form an extensive apron below the northern flanks of the volcano. Although no historical eruptions have been recorded, the youthful morphology of deposits suggests activity during historical time. Persistent fumarolic activity occurs at the summit and NW flank, and sulfur flows have been produced by melting of extensive sulfur deposits in the summit region.

Information Contacts: Héctor Cepeda and Margaret Mercado, Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile; Jorge Carabantes, Cristian Rivera, Eric Díaz, and Juan Soto, Corporación Nacional Forestal (CONAF), Chile; Jose Antonio Naranjo, Volcano Hazards Programme, Servicio Nacional de Geologia y Mineria, Chile.

The previous Bulletin report (BGVN 31:03) discussed an unusually vigorous eruption during late March and early April 2006. This report revisits the March 2006 eruption and continues to the beginning of 2007, thanks in large part to the reports of many observers posted by Frederick Belton on his website.

March-April 2006 eruption. The March 2006 eruption was initially characterized in the Arusha Times as being more massive than the one in 1966. However, Celia Nyamweru noted that subsequent information indicated that the 2006-2007 event was smaller than the 1966-1967 event. During the March-April 2006 event, the volcano was reported to have emitted "red-hot rivers of molten rock and scalding fumes." Ibrahim Ole Sakay, a resident of Ngaresero (-1.3 km from the volcano) reported that the eruption began on the night of 24 March 2006, continuing the following day, and marked by "rumbling and spitting lava for more than a week."

Several news sources, including CNN, reported that on 30 March 2006 the eruption led to the evacuation of up to 3,000 people from several villages, some quite distant from the volcano. As of 5 April, there was a great deal of contradictory information about this eruption. Belton noted that news media and people distant from the volcano reported explosions, but that people living and working nearby reported a "smoke column" followed by a very large lava flow down the W flank, but no explosions or ash. All evidence now indicates that there was no explosive activity and that this was only a very large eruption of lava.

Visitor observations. Belton posted reports from a number of persons who observed the volcano before and shortly after the March 2006 eruption. One observer, Christoph Weber, drew a new map of the crater in February 2006 (figure 91). Belton visited the volcano in August 2006 and provided (figure 92) an update to Weber's February map as well as a photo of the recent changes (figure 93). The following text and table 11 were taken from observations by visitors, as reported by Belton on his website.

Figure 91. Sketch map showing features in Ol Doinyo Lengai's active crater as of February 2006 (i.e., before the March-April 2006 eruption). Courtesy of Christoph Weber.

Figure 92. Sketch map showing features in Ol Doinyo Lengai's active crater as of August 2006 (i.e., after the March-April 2006 eruption). Courtesy of Frederick Belton, based on update of the map by Christoph Weber.

Figure 93. Photo of Ol Doinyo Lengai's active crater as seen 7 August 2006, looking N from the S rim. To elucidate recent changes in the crater, see maps in figures 91 and 92 [and earlier maps and photos from BGVN 31:03 (March 2006), 30:10 (October 2005), and 30:04 (April 2005)]. The tall cone is T49B. Slightly to its front and to the right, note the large collapse zone that grew in the spot where cones T56B, T58B, and T58C once stood. The dark lava on the right (E side of crater) was believed to have erupted around 20 June 2006 from T37B. The dark lava to the lower left probably dates from early April 2006. Although it appears dark and fresh here, it had already been highly weathered and easily crumbled into powder if touched. Courtesy of Frederick Belton.

Table 11. Summary of visitors to Ol Doinyo Lengai and their brief observations (from a climb, crater overflight photos, or from the flank) from January 2006 to February 2007 (see figures 91 and 92 for crater features). Detailed observations prior to March 2006 were reported in BGVN 31:03; most of the later observations were detailed in the text. Courtesy of Frederick Belton.

When Rick and Heidi Rosen flew over on 13 March 2006, there appeared to be no activity and many lava flows had turned white. Several flows still contained dark areas, their surface color indicating that they were then only a few days old. Narrow flows extended in all directions from the central cone mound, and a small flow originating on the upper part of T49B extended across the NW crater rim overflow and a short distance down that flank. Lava also appeared to have reached the E crater rim overflow. Most of the flows appeared to have been subject to the same amount of weathering, except for the flow down the NW flank, which looked more recent.

After a 1 April 2006 climb, Matt Jones reported that there was a fairly large lava flow down the W flank. Residents in nearby Ngaresero village and the Ngorongoro District Commissioner said that activity started on 27 March 2006. At the summit in the dark, Jones noted no glowing from lava emissions. The new eruption left a big hole to the left of the climbing path to the crater that emitted a plume of steam. On the following day, abundant steam came from the hornitos and from fissures all around the rim. Two central hornito's had been blown open relatively recently.

According to people interviewed by Amos Bupunga, who visited later, lava had flowed out on 29 (30?) March 2006 and extended to ~ 2 km from a Maasai family village (boma) at Ol Doinyo Lengai's foot. Bupunga heard that residents did not vacate their village. In the crater, lava of unstated ages covered almost all of the NW to SE regions of the crater to a depth of 2 m. At its outlet over the crater's W rim, one or more lava flows was 2.5 m deep and 3 m wide.

On 4 April 2006, Michael Dalton-Smith flew over and observed a very large lava flow that traveled over 1 km down the mountain and into a gorge. He reported that a bush pilot observed a 30 March eruption consisting of a fountain and lava flow, without an ash cloud. Local pilots also noted that on 4 April the eruption stopped. No steam was seen, nor any evidence that the large lava flow was still hot or moving.

On 5 April, Dalton-Smith drove to the foot of the volcano and saw a huge lava fountain coming from one of the summit hornitos. The fountain stopped before he could photograph it, but from the previous overall structure of the hornitos, it appeared that a new one had been building. All hornitos emitted black plumes, and there appeared to be a lake at the summit about the size of the large hornito.

Amos Bupunga visited the crater on 7 or 8 April 2006, and, in addition to the above-mentioned information he gathered relevant to 29 or 30 March, he saw that the fresh lava coming to the surface remained inside the new lava lake.

Table 12 summarizes annual measurements from 2000 to 2006 of widths of lava flows leaving the crater at various rim overflows. The number and size of the overflows have generally grown, although the width of the NW overflow has remained 135 m since 2002.

Table 12. Annual crater rim overflow measurements taken during 2000 to 2006. Stated values are the width of the crater outflow area at the crater rim. Courtesy of Frederick Belton.

Aerial photos made on 1 April by Dean Polley showed that there had been a huge collapse of the upper parts of hornitos T56B and T58B, which merged together and probably contained a lava lake (figure 94); as noted earlier, photos by Rick Rosen showed that the collapse had not occurred by 13 March 2006.

Figure 94. Aerial photo of Ol Doinyo Lengai, taken 1 April 2006, viewing the central crater looking toward the S. The very recent collapse of hornitos T56B and T58B, which appear to have merged together, is evidenced by the depressions sharp edges. Courtesy of Dean Polley.

Polley's 1 April photos show that at the SE base of T58C (just behind the collapse pit) there appeared to be a new vent with prominent lava channels leading away to the SE. Lava from this vent seemingly filled up the low lying areas in the S crater, spilled across the W overflow and down the flank. A similar eruption probably occurred again on 3 April. It was likely that a large amount of the lava was flowing through buried tubes, typical during an eruption of long duration.

From 6-11 May 2006, Jean Perrin and four others from Reunion Island visited ol Doinyo Lengai and reported an absence of active lava flows but small gaseous emissions at some hornitos and plausible rare explosions (which may have also been the sound of rocks collapsing). Due to the very large collapse mentioned above, hornitos T56B, T58B, T58C, and T57B no longer existed. No lava lake activity was seen or heard in the collapsed area. The crater floor was covered with a thick ash layer and looked considerably different than before.

On 12-13 May 2006, Tobias Fischer reported seeing no activity, but the crater was filled with old lava much higher than what was seen the previous year. A very large collapsed cone with sharp rugged edges was noticed in the T58B area. Sulfur dioxide (SO₂) flux was measured using a differential optical absorption spectrometer (mini-DOAS), but the fluxes measured were low, the same as in 2005. Sampled lava were later analyzed and their carbonatite compositions were identical to 2005 lavas. Some possible carbonatite tephra was also sampled. Coming from deep inside the volcano there were discrete rumblings lasting for several seconds and up to 10 seconds; these repeated up to 15 times per hour.

Matthieu Kervyn reported that during his visit to the volcano, 21-28 May 2006, he noted no eruptive activity at all except for fumaroles from cracks in the rim and from most of the hornitos (especially in the afternoons). The collapse pit in the middle was enlarging through rim collapse. Visual inspection showed that the collapse pit might soon cause instability of the very high T49B cone. Maasai guides were also expecting T49B to collapse soon. There were some tremors felt several times per hour within the N crater, as if rocks were collapsing beneath the crater.

During 13-15 July 2006, Steve Beresford, Michelle Carey, and Mark and Rene Tait visited the active crater. Activity at that time was limited to abundant fumarolic degassing from the crater rim and central hornitos. They noted a recent (several days old) major lava flow in the SE part of the N crater, its path emanating from the S end of the lava lake at the crater dominating the central N crater. The pre-March 2006 morphology of the N crater had been the scene of a prominent central hornito cluster (figure 91). During 13-15 July the group found much of that cluster destroyed, with the dominant feature on 13 July being a wide (120 x 120 m) crater hosting a recently active lava lake. The hosting crater's S margin was very unstable and periodic collapse of the crater walls was common over the two days of observation. The crater's N margin was marked by a steep collapse scarp in the T49B hornito. Talus breccia from this scarp partially infilled the N part of the lava lake. Numerous scarp collapses (associated with abundant seismic activity) highlighted the ephemeral nature of the current crater/lava lake outline. Marks around the lava lake recorded former high-stands of lava during recent months. SE- and S-draining tubes were present, both testifying to the lateral draining of lava.

The above group saw the S tubes that emanated from the central lava lake appeared to connect to the T37B hornito. The majority of the lava flow of the March-April eruption appeared to have come from this hornito. The reduction in lava lake level and southerly flow direction suggested that the lava lake dramatically drained to the S and may have provided the lava that escaped in the T37B eruption. Pyroclastics surrounding T37B suggested that early mild Strombolian/Hawaiian style activity preceded or accompanied effusion, as was typical of recent N crater volcanism. The lava flow itself was dominantly slabby to spiny pahoehoe with many aa and frothy pahoehoe breakouts along the E margin. This flow appeared similar to an inflated slabby pahoehoe flow field. Very small toothpaste pahoehoe flows emanated from the slabby pahoehoe flow front.

August 2006 map and its interpretation. During 4-8 August 2006, Fred Belton and Peter and Jennifer Elliston camped on the volcano. The visitors found degassing cones and fumaroles; no lava erupted. Occasional rockfalls occurred in the collapse zone.

To explain the August map and field relationships (figure 21), Belton and the visitors provided the following synopsis of the most recent activity and collateral observations. Some of the following revisits observations already discussed, but other points are new to this report and convey the significance of this stage where substantial lava flows descend out of the summit crater.

Prior to their arrival, lava had flowed from T37B and CP2 and spread over the SE part of the crater floor. Thermal anomaly satellite sensing data from MODIS, analyzed by Matthieu Kervyn, indicated that the eruption probably occurred on 20 June (UTC). An Aster image from June 29 shows new dark lava in the SE part of the crater. During the eruption, lava lakes existed in CP1 and CP2 and lava flowed from CP2 and T37B and covered most of the crater floor lying between T45, T37B, T37, and the crater rim. Lava also flowed across the E overflow and down the flank. The flow was composed of at least two distinct, differently weathered lavas that may have occurred within days or hours of one another. The first eruption phase produced a fine-textured aa no more than 40 cm thick and was the more extensive of the two flows, covering a large area of the crater floor and crossing the E rim overflow. The second phase produced a less extensive but much thicker flow, nearly 2 m deep in places, that stopped before reaching the crater rim or the E overflow. It consisted of broken, ropy pahoehoe slabs. Lava from this eruption and possibly from prior activity completely covered cone T24, which was no longer visible. The collapse of the E half of T46 has revealed an interior cave containing long thin stalactites.

Since March 2006, ~ 8,000 m² of the central crater floor had collapsed. Photographs by several observers indicated that the collapse began just prior to or during the eruption of late March through early April 2006 and continued as an ongoing process. The current collapse zone consisted of two collapse pits, designated CP1 and CP2 in figure 92, plus a fractured area between the two pits and S of CP1 where large sections of terrain had broken away from the crater floor proper and subsided by 1-3 m. The displaced sections had tilted at various angles and were separated from one another and the crater floor by 1- to 2-m-wide fissures. The fissures contain numerous large boulders composed of lavas that were altered by weathering and then lithified.

Cones T58C, T56B, and T58B had collapsed into CP1 and were completely gone. Further enlargement of CP1 claimed the SW half of T57B, the SE base of T49B, and the E half of T46. The SW half of T37B had collapsed into CP2. Tall cone T49B, visible from the Rift Valley floor, appeared likely to collapse in the near future. Failure of its SE base resulted in a talus slope that spilled out onto the floor of CP1. CP1 and CP2 were each ~ 10 m deep with respect to the lowest point on their rims. CP2's floor and E side were talus-covered, but CP1 had a bi-level floor of slabby pahoehoe lava, the surface of a frozen lava lake. A wide lava channel exited CP2 to the SE, near the base of T37B, indicating that it contained a lava lake, which had overflowed onto the crater floor during the March-April eruption. From the lowest point of CP2, a tunnel sloped upward to CP1, connecting the pits. The floor of the tunnel was covered by talus from its unstable walls and roof.

A prominent open lava channel, with a smaller channel diverging from it, led SSE from CP1 past T37 and then wound W and NW to the W overflow, recording the route of the lava that flowed from T58C to Ol Doinyo Lengai's base during the exceptionally strong discharges of roughly 25 March-5 April 2006. Near CP1 the channel's path had thermally eroded to a depth of ~ 3 m, and remained nearly closed at the top. An overhanging ledge contained stalactites. The channel became indistinct in the S part of the crater, but regained prominence near the W overflow, where in places it attained a width of ~ 5 m and depth of ~ 2.5 m. A large chasm just below the W overflow carved by thermal erosion extended ~ 20 m down the flank, with a depth of 5 m and a width of ~ 12 m. Its sides appeared unstable and prone to collapse. Immediately downslope of the chasm, the lava entered an existing gully and could not be easily seen again until the slope moderated near the base of the volcano, at which place the lava chilled only a few meters from the climbing track. From there its path continued into an aa field at its terminus, ~3 km from the summit.

The terminus of the flow lies within 1 km of a Masai boma on the flank, the only habitation evacuated as a result of the eruption. The lava channel near the climbing track was ~ 3 m high and at one point formed a tumulus ~ 5 m in height (tumulus, an elliptical, domed structure formed on the surface of a pahoehoe flow on flat or gentle slopes, created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward). A video of this segment of the lava flow (made during the eruption viewed from the escarpment to the W) showed a rapid, turbulent flow with blobs of lava becoming airborne. The lava near the base of Ol Doinyo Lengai had a dark gray-black coloration and appeared less weathered than might be expected based on its age of 4 months.

Lava flows from the same eruption also covered much of the S part of the crater floor to a depth of at least 2 m. Based on the indistinctness of the main lava channel in the S part of the crater, it appeared likely that the low areas of the S part of the crater were filled by lava prior to spilling over the W crater rim overflow and down the flank. Hornitos T27 and T30, formed in 1993, were completely covered by this flow.

Satellite IR data for 2006 (MODIS and MODLEN). Remote thermal monitoring by satellite using an algorithm called MODLEN was analyzed by Matthieu Kervyn. The analysis suggested an increase in volcanism around 11-13 March 2006. MODLEN is the name of a semi-automated algorithm using MODIS night-time imagery to record thermal activity and detect abnormally high-intensity eruptive events. It is built upon MODVOLC, an algorithm developed by the University of Hawaii, which provides a fully-automated global-coverage hot-spot-detection system. MODLEN was specifically tailored to Ol Doinyo Lengai's low-temperature and small scale eruptive activity (Kervyn and others, 2006a and 2006b).

Table 13 shows the MODIS/MODVOLC thermal anomalies for the year 2006. MODIS thermal alerts on 25, 27, and 29 March 2006 indicated a small but intense area of activity, possibly in the form of a large lava lake. A thermal alert at about 2255 on 29 March was consistent with eye-witness reports and air photos by Polley (mentioned above). A thermal alert for a large area of the flank on 3 April probably indicated a second lava flow to the base of the volcano.

Table 13. MODIS thermal anomalies detected at Ol Doinyo Lengai during 2006. Courtesy of Hawai'i Institute of Geophysics and Planetology.

Kervyn reported that the MODIS algorithm indicated a strong thermal anomaly in the crater on 20 June 2006 (table 13). He interpreted this anomaly as likely thermal signatures from new lava in the SE part of the crater and the lava lakes that later observers reported. No thermal alerts were detected through the remainder of 2006.

Early 2007 observations. Tom Pfeiffer reported that during a visit from 31 January-2 February 2007, no lava erupted from the summit vents. According to local Masai guides, the form of the central area of the crater with the large collapse pit near the tall hornito T49b appeared unchanged since the summer of 2006. From an open vent in the NE corner at the bottom of the pit at the base of the hornito, continuous sounds of loud sloshing suggested mobile lava in some caverns just beneath that area, an assumption confirmed by the glow of lava visible at night from a second, smaller vent located about 30 m S of the large vent in the base of the collapse pit. One guide confirmed he had seen spattering of lava from this vent some two weeks earlier. In addition to the loud sound of moving lava underground, a constant, deep rumbling could be heard from the ground, resembling the sounds of very distant thundering. It was strongest in the NW area of the crater between the collapse pit and the fissure vents of the March 2006 lava flow.

References. Kervyn, M., Harris, A.J.L., Mbede, E., Jacobs, P., and Ernst, G.G.J., 2006a, MODIS thermal remote sensing monitoring of low-intensity anomalies at volcanoes: Oldoinyo Lengai (Tanzania) and the MODLEN algorithm: Geophysical Research Abstracts, v. 8, p. 03887.

Kervyn, M., Harris, A.J.L., Mbede, E., Jacobs, P., and Ernst, G.G.J., 2006b, MODLEN: A semi-automated algorithm for monitoring small-scale thermal activity at Oldoinyo Lengai Volcano, Tanzania: International Association for Mathematical Geology XIth International Congress, Université de Liège, Belgium, 3-8 September 2006, paper SO9-15.

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: Frederick Belton, Developmental Studies Department, PO Box 16, Middle Tennessee State University, Murfreesboro, TN 37132, USA (URL: http://oldoinyolengai.pbworks.com/); Christoph Weber, Volcano Expeditions International, Muehlweg 11, 74199 Untergruppenbach, Germany (URL: http://www.v-e-i.de/); Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617, USA (URL: http://blogs.stlawu.edu/lengai/); Matthieu Kervyn, University of Ghent, Geology Department, Ghent, Belgium (URL: http://homepages.vub.ac.be/~makervyn/); Arusha Times, Arusha, Tanzania (URL: http://www.arushatimes.co.tz/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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/).

Volcanic activity from Lopevi has continued intermittently since November 1998 (BGVN 24:02). Though there are no permanent residents on the island, which is known as Vanei Vollohulu in the local language, the nearby islands of Epi (~ 17 km SW) and Paama (~ 10 km WNW) are heavily populated. Ambrym, another active volcanic island 18 km NNW, is also at risk of ashfall from Lopevi. Ash plumes during active periods are often reported by aviators, and thermal anomalies are frequently detected by the MODIS instrument on the Terra and Aqua satellites. Ash plumes and lava flows have most recently been reported in January, May, and July 2006.

Activity during 2006. Vertical plumes were observed by aviators reaching altitudes of 2.1-2.4 km on the morning of 24 January, and ~ 2.7 km the next morning. Further advisories issued by the Wellington VAAC reported that "smoke" plumes with a "steady rate of growth" rose to ~ 2.1 km on the morning of 26 January and drifted S. Lava flowing down the S flank was also reported on the 26th.

Based on information from a pilot report, the Wellington VAAC reported that on 7 May 2006 a small ash plume was visible below an altitude of ~ 3 km and an active lava flow was observed. On 10 May, a slow moving plume reached 3 km altitude. The next day a plume rose to 4.6 km and trended SE. During 12-13 May, the plume heights lessened to 3 km as the eruption vigor reportedly decreased. News media also reported heavy ashfall on Ambrym and Paama from an eruption on 15 May. An official spokesperson for Vanuatu's National Disaster Management Office reported no new ashfall during 17-22 May.

A situation report from the UN Office for the Coordination of Humanitarian Affairs (OCHA) noted that the May eruptive episode caused heavy ashfall on Paama and SE Ambrym, affecting water supplies and crops. The total population of Paama is 1,572, comprised of 23 villages and 511 households. On the island of Paama, the two main cash crops of vanilla and pepper were damaged badly. On both islands, staple foods such as wild yams, kumala, taros, bananas, and coconut trees were either damaged or destroyed. Residents experienced health problems caused by the consumption of contaminated food and water as well as the inhalation of ash. Head pain, skin infections, diarrhea, vomiting and respiratory difficulties were reported.

The Wellington VAAC received pilot reports of an eruption plume on 5 July that reached an unknown altitude. Another pilot report indicated that the eruption may have started on 27 June. The eruption continued over the next few days, with dark ash plumes reaching altitudes of 3.7 km and drifting E and SE. No plumes were reported after the morning of 10 July.

MODIS thermal anomalies during 2005-2006. Thermal anomalies were detected by MODIS during 26-31 March 2005, though no corresponding explosive activity was reported. No hot spots were identified at Lopevi again until 27 October 2005, after which anomalies were present on most days through 26 January 2006; ash plumes were not reported until the end of this period, 24-26 January.

Later in 2006, thermal anomalies were detected by MODIS on most days during 25-28 April, 2-16 May, 25-28 May, 26 June-9 July, and 18 July-1 August 2006. The largest number of alert pixels (24) during this time occurred at 2225 on 2 May. These data indicated two significant episodes of activity that included both explosive activity and probably lava emission during 25 April-28 May and 26 June-1 August. Two periods of plumes observations discussed previously, during 7-15 May and 27 June-10 July, fall within these longer episodes defined by the thermal data. No MODIS thermal anomalies were detected between 2 August 2006 and mid-March 2007.

Geologic Background. The small 7-km-wide conical island of Lopevi, known locally as Vanei Vollohulu, is one of Vanuatu's most active volcanoes. A small summit crater containing a cinder cone is breached to the NW and tops an older cone that is rimmed by the remnant of a larger crater. The basaltic-to-andesitic volcano has been active during historical time at both summit and flank vents, primarily along a NW-SE-trending fissure that cuts across the island, producing moderate explosive eruptions and lava flows that reached the coast. Historical eruptions at the 1413-m-high volcano date back to the mid-19th century. The island was evacuated following major eruptions in 1939 and 1960. The latter eruption, from a NW-flank fissure vent, produced a pyroclastic flow that swept to the sea and a lava flow that formed a new peninsula on the western coast.

Information Contacts: Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://vaac.metservice.com/); MODVOLC Alerts Team, Hawai'i Institute of Geophysics and Planetology (HIGP), SOEST, University of Hawaii and Manoa, 168 East-West Road, Post 602, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Department of Geology, Mines, and Water Resources, PMB 01, Port-Vila, Vanuatu (URL: http://www.suds-en-ligne.ird.fr/fr/volcan/vanu_eng/lopevi1.htm); Port Vila Presse, PO Box 637, Port Vila, Efate, Vanuatu (URL: http://www.news.vu/en/); ReliefWeb, Office for the Coordination of Humanitarian Affairs, United Nations, New York, NY 10017, USA (URL: https://reliefweb.int/).

Merapi, one of the most dangerous volcanoes in the world owing to its perched lava dome and location in populous central Java, underwent vigorous dome growth during early to mid-2006, and its increasingly unstable summit dome released numerous pyroclastic flows and incandescent avalanches. Thousands of residents evacuated and the volcano became prominent in international news. The longest pyroclastic flows of mid-2006 took place on 8 and 14 June, with respective run-out distances from the summit area of ~ 5 and 7 km. Merapi's summit lies 32 km N of the large city of Yogyakarta.

This report contains summary notes on activity during 7 March to 1 July 2006. These notes were assembled and reported by scientists from the Merapi Volcano Observatory and the Center of Volcanology and Geological Hazard Mitigation (CVGHM), formerly the Volcanological Survey of Indonesia, and augments material presented previously (BGVN 31:05 and 31:06).

The USGS provided a satellite image with labels showing key drainages and features near the summit (figure 27). The dome's instability leads to pyroclastic flows and various kinds of rockfalls and other mass wasting episodes down the labeled drainages. During the 7 March to 1 July reporting interval, pyroclastic flows followed the headwaters of the Gendol , Krasak, Boyong, and Sat rivers, which trend to the SE, SW, SSW, and W, respectively.

Figure 27. An annotated Ikonos satellite image of Merapi taken 10 May 2006. Image resolution is 2 m; N is to the top, and the scale is such that the entire distance N-S on the image is approximately 1 km. The labeled arrows indicate key rivers into which upslope avalanche shoots drain. Multiple drainage names are separated by a slash, and many western headwaters descend into the Woro river. The "K." stands for Kali, Indonesian for stream. Lava domes and viscous flows ("L") are labeled with the year of extrusion. The Gegerbuaya ridge was formed by 1911 lavas. Garuda, Woro, and Gendol identify headwaters. Letters reference locations used by scientists to facilitate communication. The Kaliurang Observatory lies ~ 4 km to the SE of the summit. The labeled image was a collaborative effort provided here courtesy of John Pallister, USGS. Image copyright 2006, GeoEye.

Tectonic earthquake on 27 May 2006. The tectonics of Java are dominated by the subduction of the Australia plate to the NNE beneath the Sunda plate with a relative velocity of ~ 6 cm/year. The Australia plate dips NNE from the Java trench, attaining depths of 100-200 km beneath the island of Java, and depths of 600 km to the N of the island. The earthquake of 27 May 2006 occurred at shallow depth in the overriding Sunda plate, well above the dipping Australia plate.

The pace of volcanism and the intensity of the regional crisis increased after 27 May 2006. At 0553 that day, a destructive M_w 6.3 earthquake occurred leaving damage across central Java's southern coastal and inland areas (figure 28). The earthquake occurred at 10 km focal depth. The epicenter (at 7.962°S, 110.458°E) was 20 km SSE of Yogyakarta (population, 511,000; 6 million in the larger metro area). Some initial estimates put the earthquake at MR 5.9; this was later revised and even the newer (above-stated) seismic parameters are preliminary.

Figure 28. Epicenter of the 27 May 2006 earthquake in Central Java, including impact on regions around Merapi. The histograms show numbers of people killed (on left bar) and injured (right bar). As mentioned in text, some of the seismic parameters stated were later revised. Modified from a UN OCHA ReliefWeb Map Centre (1 June 2006) map in a 2006 United Nations report (see References).

A US Geological Survey (USGS) summary stated that the earthquake caused 5,749 deaths, 38,568 injuries, and led to as many as 600,000 people displaced in the Bantul-Yogyakarta area. The shaking left more than 127,000 houses destroyed and an additional 451,000 houses damaged in the area, with the total loss estimated at ~3.1 billion US dollars. Modified Mercalli intensities were as follows: at Bantul and Klaten, IX; at Sleman and Yogyakarta, VIII; at Surakarta, V; at Salatiga and Blitar, IV; and at Surabaya, II. The earthquake was felt in much of Java and at Denpasar, Bali. The website of the US Geological Survey's Earthquake Hazards Program features a large number of photos (captioned in English) depicting various aspects of the earthquake.

Events during 7 March-1 July 2006. Tables 17 and 18 summarize some of the details during the reporting interval. Merapi's activity had increased to include volcanic earthquakes and deformation of the summit area a year earlier (in July 2005). Although the number of daily lava avalanches and pyroclastic flows had increased almost a week earlier, a tectonic earthquake, MR 6.3 (Richter scale magnitude), at 0555 (local time, WIB) on 27 May was followed by another significant increase in those events for another week (tables 17 and 18). Pyroclastic flows and lava avalanches between 10 May and 30 June were rare in the W-flank Sat drainage (31 May, 2 June, and 10 June), and did not descend into the Boyong drainage (SSW) after 4 June (table 18). The Krasak river drainage (SW) had material entering it on an almost daily basis after 27 May, except for a brief time during 14-19 June, with maximum run-out distances of 4 km. The Gendol drainage (SE) also experienced daily pyroclastic flows and lava avalanches starting on 28 May. Most of these flows to the SE did not extend more than 5 km, but on 14 June a pyroclastic flow descended 7 km.

Table 17. A compilation of seismic events at Merapi during 7 March to 1 July 2006. In creating this table Bulletin editors merged the category "landslides" with the category "lava avalanches". Similarly, the category "hot cloud reports" was interpreted to be equivalent to "pyroclastic flow" and those were also merged. Those mergers were driven by sudden shifts in terminology found in CVGHM reports. No data was available for 26-27 April, 29 April-5 May, 8 May, 12-21 May, 24-26 May, 9 June, or 16-18 June. * Earthquake, MR 6.3 (Richter scale magnitude) recorded at 0555 (local time, WIB). ** Incomplete data only 0000-0600 (local time). All data courtesy of CVGHM.

Table 18. Record of run out distances (km) of pyroclastic flows and lava avalanches (the latter, in parentheses) toward river drainages on Merapi from 10 May to 30 June 2006. No data was reported for 16-18 June, and weather obscured views on21-22 June. Courtesy of CVGHM.

Because of the vigor of activity, the Alert Level rose in several steps as follows: 19 March (Green to Yellow), 12 April (Yellow to Orange), and 13 May (Orange to Red). The step to Red (which is the highest alert level, and sometimes also referred to as Level 4) followed clear deformation at the dome during elevated seismicity. On 28 April, a new lava dome emerged. By 20 May, pyroclastic flows several kilometers long were regularly seen passing down several key drainages (table 18). Figure 29 shows a 15 May pyroclastic flow (seen two days after the alert status rose to red).

Figure 29. A photo taken on 15 May 2006 (0555 local time) of a pyroclastic flow traveling down the W flank of Merapi (the Krasak headwaters). Photo taken from the Kaliurang Observatory; courtesy of CVGHM.

Volcano enthusiasts and photographers Martin Rietze and Tom Pfeiffer viewed Merapi on the morning of 27 May, during the destructive earthquake, from a high-elevation parking area ~ 4 km S of the summit. Prior to the earthquake, Rietze took several spectacular photos of incandescent avalanches pouring down avalanche shoots (figure 30 A-B). During the earthquake, he described horizontal swinging motion and dull rumbling sounds lasting perhaps 20 seconds. Dust rose from the volcano. Plants rubbing together also produced a rustling noise. Cries and engine noises in the background came from distant residents responding to the earthquake. At ~1-minute intervals, Merapi emitted about six pyroclastic flows and a substantial ash cloud grew overhead, reaching several kilometers in altitude above them. The photo in figure 30 C depicts the scene on Merapi around that time (which Rietze lists as 0555 on 27 May). His companion, Tom Pfeiffer, also took photos just after the large earthquake (e.g., figure 30 D).

Figure 30. (A and B) Pre-dawn shots of incandescent material traveling down S-flank avalanche shoot(s) at Merapi on 27 May 2006 (prior to the M ~ 6 earthquake). (C) A photo of Merapi's response at 0555 on 27 May during or just after the M ~ 6 earthquake, with several pyroclastic flows clearly visible. (D) A second photo of the scene on Merapi during or just after the earthquake. This photo captured the chaotic scene at the summit and upper slopes, including a complex array of billowing ash clouds seemingly from multiple sources, and suspended dust hanging over many parts of the volcano (particularly distinguishable along the photo's lower central and right-hand areas). Copyrighted photos; those labeled A-C, used with permission of Martin Rietze; the one labeled D, with permission of Tom Pfeiffer.

During early June the activity level of Merapi remained at red and on 4 June, the increase in volume of the new lava dome had caused the southern part of the crater wall called Gegerbuaya (1910 lavas) to collapse. Prior to its collapse, Gegerbuaya had functioned as a barrier to prevent pyroclastic flows moving southward from entering the Gendol River, which they did later in June.

On 8 June, multiple pyroclastic flows reached 4 km from the Krasak and Boyong Rivers and up to 4.5 km down the Gendol River. On 9 June, ash drifted W and NW and accumulated as ashfall ~ 1.5 mm thick. Pyroclastic flows traveled as far as 4 km toward the Gendol River. Figures 31 and 32 show pyroclastic flows on 7 and 10 June.

Figure 31. A pyroclastic flow at Merapi at 08:54:37 on 7 June 2006 shown traveling down Merapi's upslope region in a generally SE direction. Photo credit to BPPTK (The Research and Technology Development Agency for Volcanology, Yogyakarta). Provided courtesy of CVGHM.

Figure 32. A Merapi pyroclastic flow in its early stages as seen at 08:50:53 on 10 June 2006. Photo credit to BPPTK; provided courtesy of CVGHM.

In the period after the hazard level was raised to red, the lava dome grew and by 22 May its volume was ~ 2.3 million cubic meters. The M 6.3 earthquake in S-Central Java on 27 May triggered additional activity at Merapi. The dome's growth rate increased from the previous rate of around 100,000 cubic meters/day, leading to a lava dome volume on 8 June 2006 of ~4.3 million cubic meters. That lava dome stood 116 m above the nominal summit elevation of Merapi's peak (Garuda peak).

Dome collapse created the longest pyroclastic flow of the reporting interval, which took place on 14 June 2006. That pyroclastic flow attained a run-out distance of 7.0 km (table 18, figures 33 and 34, and previously reported in BGVN 31:05).

Figure 33. Deserted houses and dislodged lumber amid ash and volcanic rocks from Merapi (left-background) as seen in the village of Kaliadem (E of Kinahrejo near Bebeng, on the SE flank ~ 5 km from the summit) shortly after the 14 June 2006 pyroclastic flows passed through the settlement. Courtesy of Agence France Presse (photo by Tarko Sudiarno).

Figure 34. Night photo of Merapi (unknown date) showing incandescence on the slopes and, in the foreground, the large pyroclastic flow deposited on 14 June 2006. This photo is taken from nearly the same spot as the photos of 27 May (figure 30, above). Copyrighted photo used with permission of Tom Pfeiffer.

At least in part owing to loss of topographic relief at the Gegerbuaya ridge along the S crater wall (figure 27), the 14 June pyroclastic flow took a different path. It crossed the former barrier and descended the Gendol drainage. As previously noted (BGVN 31:05), the 14 June pyroclastic flow took two lives when the underground bunker where the victims sought refuge was buried by the pyroclastic flow.

The bunker overridden on 14 June resides in Kaliadem village (~ 5 km SE of the summit). News stories showed pictures of the rescue attempt with initial digging commencing using picks and shovels, with the excavation by soldiers wearing dust masks and standing on boards or wooden platforms, presumably to reduce the heat flow from the fresh deposit. The article also noted that the soldiers wore heat-retardant clothes. A report from the Taipei Times of 16 June 2006 and credited to the Associated Press said that "The fierce heat melted the troops' shovels and the tires on a mechanical digger brought in to plow through more than 2 m of volcanic debris covering the bunker, built for protection from volcanic eruption . . .." Later news reports noted that authorities unearthed the bunker, which lay beneath more than 2 m of steaming pyroclastic flow deposit. The two bodies had suffered burns and the facility's door was ajar. A BBC report showed deeper portions of the hole being excavated by a large backhoe. They also noted that upon deeper excavation a probe into the deposit with a hand-held digital thermometer apparently indicated temperatures reached ~ 400°C. Several grim photographs circulated in the press showing the excavated entrance of the bunker and a team in the process of removing the victim's bodies. No report has been found discussing the exact reason for the bunker's failure, although several comments in the press suggested it was not designed to withstand burial by a pyroclastic flow.

Prior to that, on 13 June, the alert status dropped to orange, but it rose back to red again the next day after the pyroclastic flow and increases in multi-phased earthquakes. Activity remained stable but high through June 29 but began to decrease after 30 June. During July the intensity and frequency of pyroclastic flows and rock falls decreased. On 10 July, authorities reduced the alert status to orange on all but the S slopes. By the end of July 2006, pyroclastic flows had ceased.

Merapi's long-term dome growth continued at low to modest levels during the rest of 2006 and early 2007. The Darwin Volcanic Ash Advisory Center noted a plume to 6.1 km altitude drifting NE on 19 March 2007. These later incidents will be discussed in more detail in a forthcoming issue of the Bulletin.

MODVOLC Thermal Alerts. The Hawai'i Institute of Geophysics and Planetology MODIS Thermal Alert System web site lacked any thermal alerts for over a year preceding May 2006. Thermal alerts over Merapi began 14 May 2006 and extended through early September 2006 on nearly a daily basis. The alerts continued intermittently into 2007.

Reference. United Nations, 2006, Indonesia Earthquake 2006 Response Plan: United Nations, OCHA Situation Report No. 5, Issued 31 May 2006, GUDE EQ-2006-000064-IDN, 42 p.

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: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); United Nations-Office for the Coordination of Humanitarian Affairs (OCHA), United Nations, New York, NY 10017, USA; National Earthquake Information Center, US Geological Survey, PO Box 25046, Denver Federal Center MS967, Denver, CO 80225, USA (URL: http://earthquake.usgs.gov/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/advisories/); John Pallister, Volcano Disaster Assistance Program, USGS Cascades Volcano Observatory, 1300 SE Cardinal Court, Suite 100, Vancouver, WA 98683-9589, USA (URL: http://volcanoes.usgs.gov/); Tom Pfeiffer and Martin Rietze, Volcano Discovery (URL: http://www.decadevolcano.net/), http://www.tboeckel.de/); Tarko Sudiarno, Agence France Presse (AFP) (URL: http://www.afp.com/english/home/); Taipei Times (URL: http://www.taipeitimes.com/); Associated Press (URL: http://www.ap.org/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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/).

As previously reported, the Rabaul Volcano Observatory noted a large, sustained Vulcanian eruption at Rabaul on 7 October 2006. Since that initial event at the Tavurvur cone, activity has varied in intensity (BGVN 31:10). During 13 December 2006 through the end of March 2007, generally mild eruptive activity continued, often with loud roaring noises and in some cases with ash plumes rising 1.5 to 3.7 km above Tavurvur's summit.

During December 2006, there was only low level seismicity, including high-frequency earthquakes and mild eruptive activity. During 24-29 December, ash clouds rose 1-3.7 km above the summit before being blown variably to the NE and SW. On 25, 27, and 28 December, fine ash fell downwind, including in Rabaul Town, and occasional roaring noises were heard. Seismic activity continued at low levels. No high-frequency earthquakes were recorded. Low seismicity continued during most of January.

During 4-10 January 2007 plumes occasionally bearing ash rose 0.9-3.3 km above the cone and drifted E and NE. Vapor emissions accompanied by pale gray ash clouds occurred on 13, 16, and 24 January. The emissions rose 0.4- 2.5 km above Tavurvur's summit and blew E, NE, and N. During 24-25 January there were nine low-frequency earthquakes recorded. Ground deformation measurements showed no significant movement apart from a slight deflation of about 1 cm during the last few days of January. From 29 January onwards, seismicity increased to a moderate level. Three high-frequency earthquakes were recorded, one on 27 January, and two on 30 January, all originating NE of the caldera. Low-frequency earthquakes began 24 January. A total of 16 events were recorded during 24-28 January, and a further 50-60 small events 29-31 January.

Two small explosions occurred at 0448 and 0548 on 27 January and a large explosion occurred at 0130 on 31 January. The latter explosion showered the cone's flanks. The accompanying ash clouds rose a couple of hundred meters straight above the summit. Fine ashfall occurred at Rabaul Town and surrounding areas.

Mild eruptive activity continued during early February with associated seismicity at very low levels. The small low-frequency earthquakes had declined in number by about half. Ground deformation data indicated a noticeable deflation of the caldera. Mild eruptive activity continued intermittently during the latter half of February, associated with low seismicity. Ash fell on surrounding villages on 20 February. On 16, 19, and 21 February, low-frequency earthquakes and white vapor emissions containing very low ash content rose as high as 3 km above Tavurvur's summit. The emissions were not accompanied by high-frequency signals or significant ground deformation.

Moderate explosions occurred on 21, 26, and 27 February. A larger explosion, at 1150 on 28 February, showered the cone's flanks with lava fragments. Thick ash clouds rose 2 km above the summit and blew NE.

Between 3 and 4 March, multiple explosions occurred; the biggest on 3, 4, and 8 March. The explosion's shockwaves rattled houses in Rabaul Town and surrounding villages. Thick ash and lava fragments showered the flanks of the cone. Other emissions consisted of white gray ash clouds that drifted E and SE. On 4 and 6 March ash plumes rose as high as 2.7 km above the summit. A weak glow was visible only during forceful emissions. During 6 to 21 March, ash plumes intermittently rose as high as 3.7 km. From 16 to 25 March, multiple explosions again produced shockwaves felt in Rabaul Town, and ash fell in surrounding villages. Incandescent material was seen rolling down the cone's flanks. During the period 27-30 March only low level vapor emissions rising to 400 m above the cone were visible. Seismic activity continued to remain at a very low level, with just three or four short (< 30 second) low-frequency events. There were no high-frequency events.

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: Steve Saunders and Herman Patia, Rabaul Volcanological Observatory (RVO), Department of Mining, Private Mail Bag, Port Moresby Post Office, National Capitol District, Papua, New Guinea; Andrew Tupper, Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Darwin, Australia.

A moderate volcanic earthquake struck Ruapehu at 2230 on 4 October 2006. The M 2.8 event falsely triggered the lahar warning system. A visit to the crater lake on 7 October revealed evidence that a small hydrothermal eruption had occurred. Wave action reached up to 4-5 m above the lake surface around the basin, but was insufficient to overflow the tephra dam where it might have formed a lahar on the outer slopes. Since the last measurement (date not specified) the lake's temperature rose ~8°C, and the water level increased ~ 1 m. Both of these effects were expected. Seismic activity remained at typical background levels on 7 October 2006.

At about 1300 on 18 March 2007, Crater Lake partly emptied and its runoff traveled rapidly downstream as a powerful lahar. A subsequent issue will discuss that dramatic event and its impact.

Since the last report in February 2004 (BGVN 29:02), from May 2003 to October 2006, there were eight alerts issued by the Institute of Geological & Nuclear Sciences (IGNS, table 12), indicating appreciable changes in both the level of the lake and its temperature; these alerts can be compared with the temperature data (table 13).

Table 12. Institute of Geological & Nuclear Sciences (IGNS) alerts posted for Ruapehu volcano, May 2003 to October 2006. Compiled from IGNS reports.

Table 13. Lake temperature data recorded at Ruapehu during 2003-2006. Some months have multiple sets of readings. Data were rounded to two significant figures. Compiled from IGNS reports.

Volcanic tremor was recorded during July 2005 and continued at varying levels. Although tremor is not unusual at Ruapehu, this was the strongest recorded since November 2004. Prominent steam plumes rose above Ruapehu on the morning of 13 September 2005. The crater lake temperature had recently risen from 23°C in August 2005 (table 13) to 39°C in early September 2005. By 12 October 2005 it had fallen to 30°C, indicating the end of the heating cycle. Thereafter, another cycle of lake heating took place in middle to late October 2005. During the period when the lake was at its hottest, steam plumes appeared on several days, but no eruptive activity was observed. Seismic activity continued at about normal levels except for a slight increase in the occurrence of volcanic earthquakes over the previous two weeks.

Lahar hazard. The last report on Ruapehu (BGVN 29:02) reviewed the government of New Zealand's efforts to lessen potential damage and loss of life from the possible collapse of the ash dam surrounding the lake that sits directly within the crater. An illustrative model of the most likely potential lahar was presented in the previous Bulletin (BGVN 29:02). Figure 27 provides more details on the regional geography.

Figure 27. Composite maps of the Ruapehu area modified from part of a lahar hazards poster titled "How will the Lahar Affect Me?" The schematic map (at left) shows that the Tongariro river trends N, crosses State Highway 1 two times, and eventually enters Lake Taupo. The shaded relief map (right) of Ruapehu and adjacent flanks along its E-sector. Note the multiple chutes created to divert flood waters and lahars toward the S on the Whangaehu river. These chutes are intended to protect the Tongariro river's headwaters. Courtesy of the NZ Department of Conservation.

According to IGNS and related government websites, the most likely lahar's path starts from a 7-m-thick tephra dam sitting above bedrock along the low point in Ruapehu's crater rim. This path descends along the Whangaehu valley, a drainage that initially travels radially down the cone to the E. Where the Whangaehu reaches beyond ~ 10 km from the rim (figure 27), the channel curves sharply S and then SW, ultimately crossing Ruapehu's S side. In contrast, just upstream of the above-mentioned bend, the intersecting Tongariro river flows N. At that connection between the two drainages (a divide), engineers added a 300-m-long embankment (a levee or bund), to keep substantial material from entering the Tongariro drainage. Engineers also added one or more chutes to direct some of the Whangaehu river S and away from the critical junction. Protecting the Tongariro river from sudden influx of water and debris protects infrastructure along and downstream of that river. For example, the Tongariro river enters Lake Taupo, a 30 x 40 km caldera lake. Lake Taupo drains to the N along the Waikato river and dams along that river generate hydroelectric power.

According to the Institute of Geological & Nuclear Sciences (IGNS), about 60 lahars have swept down the mountain's southern side in the past 150 years. Lahars are not limited to the Whangaehu valley as eruptive and mass wasting processes can result in sudden influx of water and debris in other drainages as well. Lahar episodes since 1945 appear on figure 28.

Figure 28. Lahar episodes occurring at Ruapehu since 1945, as grouped into four categories. The categories are those associated with an extended eruption, a sudden (blue-sky) eruption, rain mobilization, and dam break or failure. From Harry J. R. Keys (date unknown), Department of Conservation (see Reference, below).

Figure 29 contains plots of the crater lake's surface elevation during the past several years. The plot is part of a poster available on the Department of Conservation website. The poster also notes the approximate volume of the crater lake, 107 m³. The tephra dam allows lake water to seep through it, considerably complicating estimates of the late-stage-filling rates, and any predicted date of overflow or related failure. Derek Cheng wrote an 8 January 2007 New Zealand Herald news piece stating that the lake then stood ~2.7 m below the dam's top. According to Chang's news story, the tephra dam allowed lake water to seep through it at a rate of ~10 L per second.

Figure 29. A plot of the surface elevation with time (1996 to mid-2006) of Ruapehu's crater lake. Absolute lake elevations in meters above sea-level apply to the curve labeled "Lake level" and correspond to the y-axis scale at the right. Indices of lake fullness (percent above or below the elevation 2,440 m) apply to the curve describing "Lake volume as percent of fullness." This curve corresponds to the y-axis at left (i.e., 0 % full = 2,440 m a.s.l.; 100% full = 2,529.3 m a.s.l.). The dotted horizontal line indicates the elevation of the base of the tephra dam that lies over the rim's low point. This plot came directly from an informative poster on the lahar available online at the Department of Conservation website (Keys, (date unknown), in reference list below).

Crater Lake observations. Ruapehu's Crater Lake had warmed following periods of volcanic tremor, with heating cycles getting to temperatures ranging from about 15 to 40°C (eg., 39°C during February 2004 and ~36°C during late October 2006; table 13). The IGNS website notes that Ruapehu's heating cycles typically occur every 9-12 months and normally last 1-3 months.

An innovative approach to covering the current lahar hazard status can be found at the Department of Conservation website. As of early February 2007 the reports were "updated every 1-2 weeks depending on weather conditions and [field] site visits."

Reference. Keys, H.J.R., (date unknown), Lahars from Mount Ruapehu—mitigation and management; NZ Dept. of Conservation website (a poster conveyed as a PDF file; creation/publication date unknown) (URL: http://www.doc.govt.nz/templates/summary.aspx?id=42442).

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: Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/, https://www.geonet.org.nz/); New Zealand Department of Conservation, Private Bag, Turangi, New Zealand (URL: http://www.doc.govt.nz/).

A previous report (BGVN 31:02) described small earthquakes on 1-2 March 2006, accompanied by "gray-blue emissions." Subsequent ongoing eruptions continued at Ulawun through 18 January 2007, generating almost daily aviation reports describing plumes blowing W to NW and of generally modest height (table 3). The tallest plume of the reporting interval rose to 4.6 km altitude.

Table 3. A summary of key events at Ulawun observed during the reporting interval 22 March 2006-18 January 2007. Reported plumes did not attain an altitude of over 4 km except on 12 November, when they reached an altitude of 4.6 km. Information based primarily on satellite data and pilot reports from the Darwin VAAC and in a few cases, the US Air Force Weather Agency (AFWA).

No MODIS thermal alerts were identified between March 2006 and January 2007 on the Hawai'i Institute of Geophysics and Planetology MODIS Thermal Alert System web site. The lack of thermal anomalies may indicate explosive eruptions, and not lava emissions. However, such activity has occurred at the summit in the past. One such episode, in November 1985, generated Strombolian activity and pyroclastic flows (figure 11).

Figure 11. Photograph of Ulawun taken from a helicopter on 25 November 1985. The view from the NE shows emission of large clots of molten lava into the air above the vent and pyroclastic flows (right). The other large stratovolcano in the background is 2,248-m-tall Bamus. Photographs were taken and provided by James Mori, Disaster Prevention Research Institute, Kyoto University.

Four Volcanic Ash Advisory Centers (VAAC): Tokyo, Washington, Darwin, and Wellington, have an interest in this volcano, because plumes may enter their areas of responsibility (figure 12). The VAACs came into existence to keep aviators informed of volcanic hazards. A key player in their development was the International Civil Aviation Organization (ICAO), a United Nations Related Agency that is the recognized international authority regarding a large number of aviation isses. Nine VAAC were created, in Anchorage (Alaska), Buenos Aires (Argentina), Darwin (Australia), London (England), Montreal (Canada), Tokyo (Japan), Toulouse (France), Washington (United States), and Wellington (New Zealand). These centers are tasked with monitoring volcanic ash plumes and providing Volcanic Ash Advisories (VAA) whenever those plumes enter their assigned airspace. The VAACs are often integrated with aviation weather centers; many have developed back-up sites. For example, the Washington VAAC is backed-up by the US Air Force Weather Agency; the Tokyo by Japan Meteorological Association Headquarters, and Darwin by the National Meteorological & Oceanographic Centre.

Figure 12. Map of Indonesia and Papua New Guinea showing selected volcanoes, including Ulawun on New Britain (right center), with areas of responsibility for local VAACs. Courtesy of Darwin VAAC.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: Rabaul Volcano Observatory (RVO), P. O. Box 386, Rabaul, Papua New Guinea; 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/); US Air Force Weather Agency (AFWA), Satellite Applications Branch, Offutt AFB, NE 68113-4039, USA; Hawai'i Institute of Geophysics and Planetology (HIGP) 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/); James Mori, Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan (URL: http://eqh.dpri.kyoto-u.ac.jp/~mori/).