Japan’s Nishinoshima volcano, located about 1,000 km S of Tokyo in the Ogasawara Arc, erupted above sea level in November 2013 after 40 years of dormancy. Activity lasted for two years followed by two brief eruptions in 2017 and 2018. The next eruption, from early December 2019 through August 2020, included ash plumes, incandescent ejecta, and lava flows; it produced a large pyroclastic cone with a wide summit crater and extensive lava flows that significantly enlarged the island. This report covers the end of the eruption and cooling during September 2020-January 2021. Information is provided primarily from Japan Meteorological Agency (JMA) monthly reports and the Japan Coast Guard (JCG), which makes regular observation overflights.

Ash emissions were last reported on 27 August 2020. The very high levels of thermal energy from numerous lava flows, ash, and incandescent tephra that peaked during early July decreased significantly during August and September. Continued cooling of the fresh lava and the summit crater lasted into early January 2021 (figure 107). Monthly overflights and observations by scientists confirmed areas of steam emissions at the summit and on the flanks and discolored water around the island, but no eruptive activity.

Figure 107. High levels of thermal activity at Nishinoshima during June and July 2020 resulted from extensive lava flows and explosions of incandescent tephra. Although the last ash emission was reported on 27 August 2020, cooling of new material lasted into early January 2021. The MIROVA log radiative power graph of thermal activity covers the year ending on 3 February 2021. Courtesy of MIROVA.

Thermal activity declined significantly at Nishinoshima during August 2020 (BGVN 45:09). Only two days had two MODVOLC alerts (11 and 30), and four other days (18, 20, 21, 29) had single alerts. During JCG overflights on 19 and 23 August there were no ash emissions or lava flows observed, although steam plumes rose over 2 km above the summit crater during both visits. The last ash emission was reported by the Tokyo VAAC on 27 August 2020. No eruptive activity was observed by JMA during an overflight on 5 September, but steam plumes were rising from the summit crater (figure 108). No significant changes were observed in the shape of the pyroclastic cone or the coastline. Yellowish brown discolored water appeared around the western half of the island, and high temperature was still measured on the inner wall of the crater. Faint traces of SO₂ plumes were present in satellite images in early September; the last plume identified was on 18 September. Six days with single MODVOLC alerts were recorded during 3-19 September, and the final thermal alert appeared on 1 October 2020.

Figure 108. No eruptive activity was observed during a JMA overflight of Nishinoshima on 5 September 2020, but steam rose from numerous places within the enlarged summit crater (inset). Courtesy of JMA and JCG (Monthly report of activity at Nishinoshima, September 2020).

Steam plumes and high temperatures were noted at the summit crater on 28 October, and brown discolored water was present around the S coast of the island (figure 109), but there were no other signs of volcanic activity. Observations from the sea conducted on 2 November 2020 by researchers aboard the Maritime Meteorological Observatory marine weather observation ship "Ryofu Maru" confirmed there was no ongoing eruptive activity. In addition to steam plumes at the summit, they also noted steam rising from multiple cracks on the cooling surface of the lava flow area on the N side of the pyroclastic cone (figure 110). Only steam plumes from inside the summit crater were observed during an overflight on 24 November.

Figure 109. On a JCG overflight above Nishinoshima on 28 October 2020 there were no signs of eruptive activity; steam plumes were present in the summit crater and brown discolored water was visible around the S coast of the island. Courtesy of JMA and JCG (Monthly report of activity at Nishinoshima, October 2020).

Figure 110. Observations of Nishinoshima by staff aboard the Maritime Meteorological Observatory ship "Ryofu Maru" on 2 November 2020 showed a steam plume rising from the lava flow area on the N side of the pyroclastic cone (arrow) and minor steam above the cone. Courtesy of JMA (Monthly report of activity at Nishinoshima, November 2020).

JMA reduced the warning area around the crater on 18 December 2020 from 2.5 to 1.5 km due to decreased activity. On 7 December a steam plume rose from the inner wall of the summit crater and thermal imaging indicated the area was still hot. Brown discolored water was observed on the SE and SW coasts. Researchers aboard a ship from the Earthquake Research Institute at the University of Tokyo and the Marine Research and Development Organization reported continued steam plumes in the summit crater, around the lava flows on the N flank, and along the S coast during 15-29 December (figure 111). Steam plumes and elevated temperatures were still measured inside the summit crater during an overflight by the Japan Coast Guard on 25 January 2021, and discolored water persisted on the SE and SW coasts; there was no evidence of eruptive activity.

Figure 111. Observations of Nishinoshima from the sea by researchers from the Earthquake Research Institute (University of Tokyo) and the Marine Research and Development Organization, which took place from 15-29 December 2020, showed fumarolic acitivity not only inside the summit crater, but also in the lava flow area on the N side of the pyroclastic cone (left, 20 December) and in places along the southern coast (right, 23 December). (Monthly report of activity at Nishinoshima, December 2020).

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Another eruption that began offshore in 2013 completely covered the previous exposed surface and enlarged the island again. Water discoloration has been observed on several occasions since. The island is the summit of a massive submarine volcano that has prominent satellitic peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the sea surface 9 km SSE.

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); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: http://www.kaiho.mlit.go.jp/info/kouhou/h29/index.html); Volcano Research Center (VRC-ERI), Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/topics/ASAMA2004/index-e.html); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA 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/).

Nyiragongo is a stratovolcano in the DR Congo with a deep summit crater containing a lava lake and a small active cone. During June 2018-May 2020, the volcano exhibited strong thermal signals primarily due to the lava lake, along with incandescence, seismicity, and gas-and-steam plumes (BGVN 44:05, 44:12, 45:06). The volcano is monitored by the Observatoire Volcanologique de Goma (OVG). This report summarizes activity during June-November 2020, based on satellite data.

Infrared MODIS satellite data showed almost daily strong thermal activity during June-November 2020 from MIROVA (Middle InfraRed Observation of Volcanic Activity), consistent with a large lava lake. Numerous hotspots were also identified every month by MODVOLC. Although clouds frequently obscured the view from space, a clear Sentinel-2 image in early June showed a gas-and-steam plume as well as a strong thermal anomaly (figure 76).

Figure 76. Sentinel-2 satellite imagery of Nyiragongo on 1 June 2020. A gas-and-steam is visible in the natural color image (bands 4, 3, 2) rising from a pit in the center of the crater (left), while the false color image (bands 12, 11, 4) reveals a strong thermal signal from a lava lake (right). Courtesy of Sentinel Hub Playground.

During the first half of June 2020, OVG reported that SO₂ levels had decreased compared to levels in May (7,000 tons/day); during the second half of June the SO₂ flux began to increase again. High levels of sulfur dioxide were recorded almost every day in the region above or near the volcano by the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Copernicus Sentinel-5 Precursor satellite (figure 77). According to OVG, SO₂ flux ranged from 819-5,819 tons/day during June. The number of days with a high SO₂ flux decreased somewhat in July and August, with high levels recorded during about half of the days. The volume of SO₂ emissions slightly increased in early July, based on data from the DOAS station in Rusayo, measuring 6,787 tons/day on 8 July (the highest value reported during this reporting period), and then declined to 509 tons/day by 20 July. The SO₂ flux continued to gradually decline, with high values of 5,153 tons/day in August and 4,468 tons/day in September. The number of days with high SO₂ decreased further in September and October but returned to about half of the days in November.

Figure 77. TROPOMI image of SO2 plume on 27 June 2020 in the Nyiragongo-Nyamulagira area. The plume drifted SSE. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

During 12-13 July a multidisciplinary team of OVG scientists visited the volcano to take measurements of the crater using a TCRM1102 Plus2 laser. They noted that the crater had expanded by 47.3 mm in the SW area, due to the rise in the lava lake level since early 2020. The OVG team took photos of the small cone in the lava lake that has been active since 2014, recently characterized by white gas-and-steam emissions (figure 78). OVG noted that the active lava lake had subsided roughly 20 m (figure78).

Figure 78. Photos (color corrected) of the crater at Nyiragongo showing the small active cone generating gas-and-steam emissions (left) and the active lava lake also characterized by white gas-and-steam emissions on 12 July 2020 (right). Courtesy of OVG (Rapport OVG Juillet 2020).

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); NASA 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/).

Kerinci, located in Sumatra, Indonesia, has had numerous explosive eruptions since 1838, with more recent activity characterized by gas-and-steam and ash plumes. The current eruptive episode began in April 2018 and has recently consisted of intermittent brown ash emissions and white gas-and-steam emissions (BGVN 45:07); similar activity continued from June through November 2020. Information primarily comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data.

Activity has been characterized by dominantly white and brown gas-and-steam emissions and occasional ash plumes, according to PVMBG. Near daily gas-and-steam emissions were observed rising 50-6,400 m above the crater throughout the reporting period: beginning in late July and continuing intermittently though November. Sentinel-2 satellite imagery showed frequent brown emissions rising above the summit crater at varying intensities and drifting in different directions from July to November (figure 21).

Figure 21. Sentinel-2 satellite imagery of brown emissions at Kerinci from July through November 2020 drifting in multiple directions. On 27 July (top left) the brown emissions drifted SW. On 31 August (top right) the brown emissions drifted W. On 2 September (bottom left) slightly weaker brown emissions drifting W. On 4 November (bottom right) weak brown emissions mostly remained within the crater, some of which drifted E. Images using “Natural Color” rendering (bands 4, 3, 2), courtesy of Sentinel Hub Playground.

During June through July the only activity reported by PVMBG consisted of white gas-and-steam emissions and brown emissions. On 4 June white gas-and-steam emissions rose to a maximum height of 6.4 km above the crater. White-and-brown emissions rose to a maximum height of 700 m above the crater on 2 June and 28 July.

Continuous white-and-brown gas-and-steam emissions were reported in August that rose 50-1,000 m above the crater. The number of ash plumes reported during this month increased compared to the previous months. In a Volcano Observatory Notice for Aviation (VONA) issued on 7 August at 1024, PVMBG reported an ash plume that rose 600 m above the crater and drifted E, SE, and NE. In addition, the Darwin VAAC released two notices that described continuous minor ash emissions rising to 4.3 km altitude and drifting E and NE. On 9 August an ash plume rose 600 m above the crater and drifted ENE at 1140. An ash plume was observed rising to a maximum of 1 km above the crater, drifting E, SE, and NE on 12 August at 1602, according to a PVMBG VONA and Darwin VAAC advisory. The following day, brown emissions rose to a maximum of 1 km above the crater and were accompanied by a 600-m-high ash plume that drifted ENE at 1225. Ground observers on 15 August reported an eruption column that rose to 4.6 km altitude; PVMBG described brown ash emissions up to 800 m above the crater drifting NW at 0731 (figure 22). During 20-21 August pilots reported an ash plume rising 150-770 m above the crater drifting NE and SW, respectively.

Figure 22. Webcam image of an ash plume rising above Kerinci on 15 August 2020. Courtesy of MAGMA Indonesia.

Activity in September had decreased slightly compared to the previous month, characterized by only white-and-brown gas-and-steam emissions that rose 50-300 m above the crater; solely brown emissions were observed on 30 September and rose 50-100 m above the crater. This low level of activity persisted into October, with white gas-and-steam emissions to 50-200 m above the crater and brown emissions rising 50-300 m above the crater. On 16 October PVMBG released a VONA at 0340 that reported an ash plume rising 687 m above the crater and drifting NE. On 17 October white, brown, and black ash plumes that rose 100-800 m above the crater drifted NE according to both PVMBG and a Darwin VAAC advisory (figure 23). During 18-19 October white, brown, and black ash emissions rose up to 400 m above the crater and drifted NE and E.

Figure 23. Webcam image of a brown ash emission from Kerinci on 17 October 2020. Courtesy of MAGMA Indonesia.

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

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

Whakaari/White Island, located in the Bay of Plenty 50 km offshore of North Island, has been New Zealand’s most active volcano since 1976. Activity has been previously characterized by phreatic activity, explosions, and ash emissions (BGVN 42:05). The most recent eruption occurred on 9 December 2019, which consisted of an explosion that generated an ash plume and pyroclastic surge that affected the entire crater area, resulting in 21 fatalities and many injuries (BGVN 45:02). This report updates information from February through November 2020, which includes dominantly gas-and-steam emissions along with elevated surface temperatures, using reports from the New Zealand GeoNet Project, the Wellington Volcanic Ash Advisory Centre (VAAC), and satellite data.

Activity at Whakaari/White Island has declined and has been dominated by white gas-and-steam emissions during the reporting period; no explosive eruptive activity has been detected since 9 December 2019. During February through 22 June, the Volcanic Activity Level (VAL) remained at a 2 (moderate to heightened volcanic unrest) and the Aviation Color Code was Yellow. GeoNet reported that satellite data showed some subsidence along the W wall of the Main Crater and near the 1914 landslide scarp, though the rate had reduced compared to previous months. Thermal infrared data indicated that the fumarolic gases and five lobes of lava that were first observed in early January 2020 in the Main Crater were 550-570°C on 4 February and 660°C on 19 February. A small pond of water had begun to form in the vent area and exhibited small-scale gas-and-steam-driven water jetting, similar to the activity during September-December 2019. Gas data showed a steady decline in SO₂ and CO₂ levels, though overall they were still slightly elevated.

Similar activity was reported in March and April; the temperatures of the fumaroles and lava in the Main Crater were 746°C on 10 March, the highest recorded temperature to date. SO₂ and CO₂ gas emissions remained elevated, though had overall decreased since December 2019. Small-scale water jetting continued to be observed in the vent area. During April, public reports mentioned heightened gas-and-steam activity, but no eruptions were detected. A GeoNet report issued on 16 April stated that high temperatures were apparent in the vent area at night.

Whakaari remained at an elevated state of unrest during May, consisting of dominantly gas-and-steam emissions. Monitoring flights noted that SO₂ and CO₂ emissions had increased briefly during 20-27 May. On 20 May, the lava lobes remained hot, with temperatures around 500°C; a nighttime glow from the gas emissions surrounding the lava was visible in webcam images. Tremor levels remained low with occasional slightly elevated episodes, which included some shallow-source volcanic earthquakes. Satellite-based measurements recorded several centimeters of subsidence in the ground around the active vent area since December 2019. During a gas observation flight on 28 May there was a short-lived gas pulse, accompanied by an increase in SO₂ and CO₂ emissions, and minor inflation in the vent area (figure 96).

Figure 96. Photo of a strong gas-and-steam plume rising above Whakaari/White Island on 28 May 2020. Courtesy of GeoNet.

An observation flight made on 3 June reported a decline in gas flux compared to the measurements made on 28 May. Thermal infrared images taken during the flight showed that the lava lobes were still hot, at 450°C, and continued to generate incandescence that was visible at night in webcams. On 16 June the VAL was lowered to 1 (minor volcanic unrest) and on 22 June the Aviation Color Code had decreased to Green.

Minor volcanic unrest continued in July; the level of volcanic tremors has remained generally low, with the exception of two short bursts of moderate volcanic tremors in at the beginning of the month. Temperatures in the active vents remained high (540°C) and volcanic gases persisted at moderate rate, similar to those measured since May, according to an observation flight made during the week of 30 July. Subsidence continued to be observed in the active vent area, as well as along the main crater wall, S and W of the active vents. Recent rainfall has created small ponds of water on the crater floor, though they did not infiltrate the vent areas.

Gas-and-steam emissions persisted during August through October at relatively high rates (figures 97 and 98). A short episode of moderate volcanic tremor was detected in early August, but otherwise seismicity remained low. Updated temperatures of the active vent area were 440°C on 15 September, which had decreased 100°C since July. Rain continued to collect at the crater floor, forming a small lake; minor areas of gas-and-steam emissions can be seen in this lake. Ongoing subsidence was observed on the Main Crater wall and S and W of the 2019 active vents.

Figure 97. Photo of an observation flight over Whakaari/White Island on 8 September 2020 showing white gas-and-steam emissions from the vent area. Photo courtesy of Brad Scott, GeoNet.

Figure 98. Image of Whakaari/White Island from Whakatane in the North Island of New Zealand showing a white gas-and-steam plume on 26 October 2020. Courtesy of GeoNet.

Activity during November was primarily characterized by persistent, moderate-to-large gas-and-steam plumes that drifted downwind for several kilometers but did not reach the mainland. The SO₂ flux was 618 tons/day and the CO₂ flux was 2,390 tons/day. New observations on 11 November noted some occasional ash deposits on the webcams in conjunction with mainland reports of a darker than usual plume (figure 99). Satellite images provided by MetService, courtesy of the Japan Meteorological Agency, confirmed the ash emission, but later images showed little to no apparent ash; GNS confirmed that no eruptive activity had occurred. Initial analyses indicated that the ash originated from loose material around the vent was being entrained into the gas-and-steam plumes. Observations from an overflight on 12 November showed that there was no substantial change in the location and size of the active vents; rainfall continued to collect on the floor of the 1978/90 Crater, reforming the shallow lake. A small sequence of earthquakes was detected close to the volcano with several episodes of slightly increased volcanic tremors.

During 12-14 November the Wellington VAAC issued multiple advisories noting gas, steam, and ash plumes that rose to 1.5-1.8 km altitude and drifted E and SE, based on satellite data, reports from pilots, and reports from GeoNet. As a result, the VAL was increased to 2 and the Aviation Color Code was raised to Yellow. Scientists on another observation flight on 16 November reported that small amounts of ash continued to be present in gas-and-steam emissions, though laboratory analyses showed that this ash was resuspended material and not from new eruptive or magmatic activity. The SO₂ and CO₂ flux remained above background levels but were slightly lower than the previous week’s measurements: 710 tons/day and 1,937 tons/day. Seismicity was similar to the previous week, characterized by a sequence of small earthquakes, a larger than normal volcanic earthquake located near the volcano, and ongoing low-level volcanic tremors. During 16-17 November plumes with resuspended ash were observed rising to 460 m altitude, drifting E and NE, according to a VAAC advisory (figure 99). During 20-24 November gas-and-steam emissions that contained a minor amount of resuspended ash rose to 1.2 km altitude and drifted in multiple directions, based on webcam and satellite images and information from GeoNet.

Figure 99. Left: Photo of a gas observation flight over Whakaari/White Island on 11 November 2020 showing some dark particles in the gas-and-steam plumes, which were deposited on some webcams. Photo has been color corrected and straightened. Courtesy of GeoNet. Right: Photo showing gas, steam, and ash emissions rising above the 2019 Main Crater area on 16 November 2020. Courtesy of GNS Science (17 November 2020 report).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows a total of eleven low-power thermal anomalies during January to late March 2020; a single weak thermal anomaly was detected in early July (figure 100). The elevated surface temperatures during February-May 2020 were detected in Sentinel-2 thermal satellite images in the Main Crater area, occasionally accompanied by gas-and-steam emissions (figure 101). Persistent white gas-and-steam emissions rising above the Main Crater area were observed in satellite imagery on clear weather days and drifting in multiple directions (figure 102). The small lake that had formed due to rainfall was also visible to the E of the active vents.

Figure 100. Low-power, infrequent thermal activity at Whakaari/White Island was detected during January through late March 2020, as reflected in the MIROVA data (Log Radiative Power). A single thermal anomaly was shown in early July. Courtesy of MIROVA.

Figure 101. Sentinel-2 thermal satellite images in the Main Crater area of Whakaari/White Island show residual elevated temperatures from the December 2019 eruption, accompanied by gas-and-steam emissions and drifting in different directions during February-May 2020. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 102. Sentinel-2 images showing persistent white gas-and-steam plumes rising from Main Crater area of Whakaari/White Island during March-November 2020 and drifting in multiple directions. A small pond of water (light blue-green) is visible in the vent area to the E of the plumes. On 11 November (bottom right), the color of the plume is gray and contains a small amount of ash. Images using “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

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

Information Contacts: New Zealand GeoNet Project, a collaboration between the Earthquake Commission and GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.geonet.org.nz/); GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); 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); Brad Scott, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: https://twitter.com/Eruptn).

Suwanosejima, an andesitic stratovolcano in Japan's northern Ryukyu Islands, was intermittently active for much of the 20th century, producing ash plumes, Strombolian explosions, and ashfall. Continuous activity since October 2004 has included intermittent explosions which generate ash plumes that rise hundreds of meters above the summit to altitudes between 1 and 3 km. Incandescence is often observed at night and ejecta periodically reaches over a kilometer from the summit. Ashfall is usually noted several times each month in the nearby community on the SW flank of the island. Ongoing activity for the second half of 2020, which includes significantly increased activity in December, is covered in this report with information provided by the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

A steady increase in activity was reported during July-December 2020. The number of explosions recorded increased each month from only six during July to 460 during December. The energy of the explosions increased as well; ejecta was reported 600 m from the crater during August, but a large bomb reached 1.3 km from the crater at the end of December. After an increased period of explosions late in December, JMA raised the Alert Level from 2 to 3 on a 5-level scale. The MIROVA graph of thermal activity indicated intermittent anomalies from July through December 2020, with a pulse of activity in the second half of December (figure 48).

Figure 48. MIROVA thermal activity for Suwanosejima for the period from 3 February through December 2020 shows pulses of activity in February and April, with intermittent anomalies until another period of frequent stronger activity in December. Courtesy of MIROVA.

Six explosions were recorded during July 2020, compared with only one during June. According to JMA, the tallest plume rose 2,000 m above the crater rim. Incandescent ejecta was occasionally observed at night. The Tokyo VAAC reported a number of ash plumes that rose to 1.2-2.7 km altitude and drifted NW and W during the second half of the month (figure 49). Activity increased during August 2020 when thirteen explosions were reported. The Tokyo VAAC reported a few ash plumes during 1-6 August that rose to 1.8-2.4 km altitude and drifted NW; a larger pulse of activity during 18-22 August produced plumes that rose to altitudes ranging from 1.8 to over 2.7 km. Ashfall was reported on 19 and 20 August in the village located 4 km SSW of the crater; incandescence was visible at the summit and ash plumes drifted SW in satellite imagery on 19 August (figure 50). A MODVOLC thermal alert was issued on 19 August. On 21 August a large bomb was ejected 600 m from the Otake crater in an explosion early in the day; later that afternoon, an ash plume rose to more than 2,000 m above the crater rim. During 19-22 August, SO₂ emissions were recorded each day by the TROPOMI instrument on the Sentinel-5P satellite (figure 51).

Figure 49. An ash emission at Suwanosejima rose to 2.7 km altitude and drifted NW on 27 July 2020. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, July 2020).

Figure 50. Ash drifted SW from the summit crater of Suwanosejima on 19 August 2020 and a bright thermal anomaly was present at the summit. Residents of the village 4 km SW reported ashfall that day and the next. Courtesy of Sentinel Hub Playground.

Figure 51. A period of increased activity at Suwanosejima during 19-22 August 2020 produced SO2 emissions that were measured by the TROPOMI instrument on the Sentinel-5P satellite. Nishinoshima, was also producing significant SO2 at the same time. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Thirteen explosions were recorded during September 2020, with the highest ash plumes reaching 2,000 m above the crater rim, and bombs falling 400 m from the crater. Ashfall was recorded on 20 September in the community located 4 km SSW. The Tokyo VAAC reported intermittent ash plumes during the month that rose to 1.2-2.1 km altitude and drifted in several directions. Incandescence was frequently observed at night (figure 52). Explosive activity increased during October with 22 explosions recorded. Ash plumes rose over 2,000 m above the crater rim, and bombs reached 700 m from the crater. Steam plumes rose 2,300 m above the crater rim. Ashfall and loud noises were confirmed several times between 2 and 14 October in the nearby village. A MODVOLC thermal alert was issued on 6 October. The Tokyo VAAC reported multiple ash plumes throughout the month; they usually rose to 1.5-2.1 km altitude and drifted in many directions. The plume on 28 October rose to over 2.7 km altitude and was stationary.

Figure 52. Incandescence at night and ash emissions were observed multiple times at Suwanosejima during September and October 2020 including on 21 and 26 September (top) and 29 October 2020. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, September and October 2020).

Frequent explosions occurred during November 2020, with a sharp increase in the number of explosions to 105 events compared with October. Ash plumes rose to 1,800 m above the crater rim and bombs were ejected 700 m. Occasional ashfall and loud noises were reported from the nearby community throughout the month. Scientists measured no specific changes to the surface temperature around the volcano during an overflight early on 5 November compared with the previous year. At 0818 on 5 November a small ash explosion at the summit crater was photographed by the crew during an observation flight (figure 53). On 12 and 13 November, incandescent ejecta fell 600 m from the crater and ash emissions rose 1,500 m above the crater rim (figure 54).

Figure 53. A minor explosion produced a small ash plume at Suwanosejima during an overflight by JMA on the morning of 5 November 2020. The thermal activity was concentrated at the base of the explosion (inset). Image taken from off the E coast. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, November 2020).

Figure 54. On 12 and 13 November 2020 incandescent ejecta from Suwanosejima reached 600 m from the crater (top) and ash emissions rose 1,500 m above the crater rim (bottom). Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, November 2020).

During December 2020 there were 460 explosions reported, a significant increase from the previous months. Ash plumes reached 1,800 m above the summit. Three MODVOLC thermal alerts were issued on 25 December and two were issued the next day. The number of explosions increased substantially at the Otake crater between 21 and 29 December, and early on 28 December a large bomb was ejected to 1.3 km SE of the crater (figure 55). A second explosion a few hours later ejected another bomb 1.1 km SE. An overflight later that day confirmed the explosion, and ash emissions were still visible (figure 56), although cloudy weather prevented views of the crater. Ashfall was noted and loud sounds heard in the nearby village. A summary graph of observations throughout 2020 indicated that activity was high from January through May, quieter during June, and then increased again from July through the end of the year (figure 57).

Figure 55. Early on 28 December 2020 a large explosion at Suwanosejima sent a volcanic bomb 1.3 km SE from the summit (bright spot on left flank in large photo). Thermal imaging taken the same day showed the heat at the eruption site and multiple fragments of warm ejecta scattered around the crater area (inset). Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, December 2020).

Figure 56. Ash emissions were still visible midday on 28 December 2020 at Suwanosejima during a helicopter overflight by the 10th Regional Coast Guard. Image taken from the SW flank of the volcano. Two large explosions earlier in the day had sent ejecta more than a kilometer from the crater. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, December 2020).

Figure 57. Activity summary for Suwanosejima for January-December 2020 when 764 explosions were recorded. Black bars represent the height of steam, gas, or ash plumes in meters above the crater rim, gray volcano icons represent explosions, usually accompanied by an ash plume, red icons represent large explosions with ash plumes, orange diamonds indicate incandescence observed in webcams. Courtesy of JMA (Suwanosejima volcanic activity annual report, 2020).

Geologic Background. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed forming a large debris avalanche and creating the horseshoe-shaped Sakuchi caldera, which extends to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

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); 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); NASA 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/).

Karangetang (also known as Api Siau) is located on the island of Siau in the Sitaro Regency, North Sulawesi, Indonesia and consists of two active summit craters: a N crater (Kawah Dua) and a S crater (Kawah Utama, also referred to as the “Main Crater”). More than 50 eruptions have been observed since 1675. The current eruption began in November 2018 and has recently been characterized by frequent incandescent block avalanches, thermal anomalies in the crater, and gas-and-steam plumes (BGVN 45:06). This report covers activity from June through November 2020, which includes dominantly crater anomalies, few ash plumes, and gas-and-steam emissions. Information primarily comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, and various satellite data.

Activity decreased significantly after mid-January 2020 and has been characterized by dominantly gas-and-steam emissions and occasional ash plumes, according to PVMBG. Daily gas-and-steam emissions were observed rising 25-600 m above the Main Crater (S crater) during the reporting period and intermittent emissions rising 25-300 m above Kawah Dua (N crater).

The only activity reported by PVMBG in June, August, and October was daily gas-and-steam emissions above the Main Crater and Kawah Dua (figure 47). MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows intermittent low-power thermal anomalies during June through late July, which includes a slight increase in power during late July (figure 48). During 14-15 July strong rumbling from Kawah Dua was accompanied by white-gray emissions that rose 150-200 m above the crater. Crater incandescence was observed up to 10 m above the crater. According to webcam imagery from MAGMA Indonesia, intermittent incandescence was observed at night from both craters through 25 July. In a Volcano Observatory Notice for Aviation (VONA) issued on 5 September, PVMBG reported an ash plume that rose 800 m above the crater.

Figure 47. Webcam image of gas-and-steam plumes rising above the two summit craters at Karangetang on 16 June 2020. Courtesy of MAGMA Indonesia.

Figure 48. Intermittent low-power thermal anomalies at Karangetang were reported during June through July 2020 with a slight increase in power in late July, according to the MIROVA graph (Log Radiative Power). No thermal activity was detected during August to late October; in mid-November a short episode of increased activity occurred. Courtesy of MIROVA.

Thermal activity increased briefly during mid-November when hot material was reported extending 500-1,000 m NW of the Main Crater, accompanied by gas-and-steam emissions rising 200 m above the crater. Corresponding detection of MODIS thermal anomalies was seen in MIROVA graphs (see figure 48), and the MODVOLC system showed alerts on 13 and 15 November. On 16 November blue emissions were observed above the Main Crater drifting W. Sentinel-2 thermal images showed elevated temperatures in both summit craters throughout the reporting period, accompanied by gas-and-steam emissions and movement of hot material on the NW flank on 19 November (figure 49). White gas-and-steam emissions rose to a maximum height of 300 m above Kawah Dua on 22 November and 600 m above the Main Crater on 28 November.

Figure 49. Persistent thermal anomalies (bright yellow-orange) at Karangetang were detected in both summit craters using Sentinel-2 thermal satellite imagery during June through November 2020. Gas-and-steam emissions were also occasionally detected in both craters as seen on 17 June (top left) and 20 September (bottom left) 2020. On 19 November (bottom right) the Main Crater (S) showed a hot thermal signature extending NW. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). 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/); 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/); 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).

Colombia’s broad, glacier-capped Nevado del Ruiz has an eruption history documented back 8,600 years, including documented observations since 1570. Ruiz remained quiet for 20 years after the deadly September 1985-July 1991 eruption until a period of explosive activity from February 2012 into 2013. Renewed activity beginning in November 2014 included ash and gas-and-steam plumes, ashfall, and the appearance of a slowly growing lava dome inside the Arenas crater in August 2015. Additional information has caused a revision to earlier reporting that eruptive activity ended in May 2017 and began again that December (BGVN 44:12); activity appears to have continued throughout 2017 with intermittent ash emissions and thermal evidence of dome growth. Periods of increased thermal activity alternated with periods of increased explosive activity during 2018-2019 and into 2020; SO₂ emissions persisted at significant levels. The lava dome has continued to grow through 2020. This report covers ongoing activity from July-December 2020 using information from reports by the Servicio Geologico Colombiano (SGC) and the Observatorio Vulcanológico y Sismológico de Manizales, the Washington Volcanic Ash Advisory Center (VAAC) notices, and various sources of satellite data.

Gas and ash emissions continued throughout July-December 2020; they generally rose to 5.8-6.1 km altitude with the highest reported plume at 6.7 km altitude on 7 December. SGC interpreted repeated episodes of “drumbeat seismicity” as an indication of continued dome growth throughout the period. Satellite thermal anomalies also suggested that dome growth continued. The MIROVA graph of thermal activity suggests that the dome was quiet in July and early August, but small pulses of thermal energy were recorded every few weeks for the remainder of 2020 (figure 115). Plots of the cumulative number and magnitude of seismic events at Nevado del Ruiz between January 2010 and November 2020 show a stable trend with periodic sharp increases in activity or magnitude throughout that time. SGC has adjusted the warning levels over time according to changes in the slope of the curves (figure 116).

Figure 115. Thermal energy shown in the MIROVA graph of log radiative power at Nevado del Ruiz from 3 February 2020 through the end of the year indicates that higher levels of thermal energy lasted through April 2020; a quieter period from late May-early August was followed by low-level persistent anomalies through the end of the year. Courtesy of MIROVA.

Figure 116. Changes in seismic frequency and energy at Nevado del Ruiz have been monitored by SGC for many years. Left: the cumulative number of daily VT, LP-VLP, TR, and HB seismic events, recorded between 1 January 2010 and 30 November 2020. The arrows highlight the days with the highest number of seismic events; the number and type of event is shown under the date. Right: The cumulative VT and HB seismic energy recorded between 1 January 2010 and 30 November 2020. The arrows highlight the days with the highest energy; the local magnitude of the event is shown below the date. SGC has adjusted the warning levels over time (bar across the bottom of each graph) according to changes in the slope of the curves. Courtesy of SGC (INFORME TÉCNICO – OPERATIVO DE LA ACTIVIDAD VOLCÁNICA, SEGMENTO VOLCÁNICO NORTE DE COLOMBIA – NOVIEMBRE DE 2020).

Activity during July-December 2020. Seismic energy increased during July compared to June 2020 with events localized around the Arenas crater. The depth of the seismicity varied from 0.3-7.8 km. Some of these signals were associated with small emissions of gas and ash, which were confirmed through webcams and by reports from officials of the Los Nevados National Natural Park (NNNP). The Washington VAAC reported a possible ash emission on 8 July that rose to 6.1 km altitude and drifted NW. On 21 July a webcam image showed an ash emission that rose to the same altitude and drifted W; it was seen in satellite imagery possibly extending 35 km from the summit but was difficult to confirm due to weather clouds. Short- to moderate-duration (less than 40 minutes) episodes of drumbeat seismicity were recorded on 5, 13, 17, and 21 July. SCG interprets this type of seismic activity as related to the growth of the Arenas crater lava dome. Primarily WNW drifting plumes of steam and SO₂ were observed in the webcams daily. The gas was occasionally incandescent at night. The tallest plume of gas and ash reached 1,000 m above the crater rim on 30 July and was associated with a low-energy tremor pulse; it produced ashfall in parts of Manizales and nearby communities (figure 117).

Figure 117. Images captured by a traditional camera (top) and a thermal camera (bottom) at Nevado del Ruiz showed a small ash emission in the early morning of 30 July 2020. Ashfall was reported in Manizales. The cameras are located 3.7 km W of the Arenas crater. Courtesy of SGC (Emisión de ceniza Volcan Nevado del Ruiz Julio 30 de 2020).

Seismicity increased in August 2020 with respect to July. Some of the LP and TR (tremor) seismicity was associated with small emissions of gas and ash, confirmed by web cameras, park personnel, and the Washington VAAC. The Washington VAAC received a report from the Bogota MWO of an ash emission on 1 August that rose to 6.1 km altitude and drifted NW; it was not visible in satellite imagery. Various episodes of short duration drumbeat seismicity were recorded during the month. The tallest steam and gas plume reached 1,800 m above the rim on 31 August. Despite the fact that in August the meteorological conditions made it difficult to monitor the surface activity of the volcano, three ash emissions were confirmed by SGC.

Seismicity decreased during September 2020 with respect to August. Some of the LP and TR (tremor) seismicity was associated with small emissions of gas and ash, confirmed by web cameras, park personnel and the Washington VAAC. The Washington VAAC reported an ash emission on 16 September that rose to 6.1 km altitude and drifted NW. A minor ash emission on 20 September drifted W from the summit at 5.8 km altitude. A possible emission on 23 September drifted NW at 6.1 km altitude for a brief period before dissipating. Two emissions were reported drifting WNW of the summit on 26 September at 5.8 and 5.5 km altitude. Continuous volcanic tremors were registered throughout September, with the higher energy activity during the second half of the month. One episode of drumbeat seismicity on 15 September lasted for 38 minutes and consisted of 25 very low energy earthquakes. Steam and gas plumes reached 1,800 m above the crater rim during 17-28 September (figure 118). Five emissions of ash were confirmed by the webcams and park officials during the month, in spite of difficult meteorological conditions; three of them occurred between 15 and 20 September.

Figure 118. A dense plume of steam rose from Nevado del Ruiz in the morning of 17 September 2020. Courtesy of Gonzalo.

Seismicity increased during October with respect to September. A few of the LP and tremor seismic events were associated with small emissions of gas and ash, confirmed by web cameras, park personnel, and the Washington VAAC. The Washington VAAC issued advisories of possible ash emissions on 2, 6, 9, 11, 15, 17, 18, and 21 October. The plumes rose to 5.6-6.4 km altitude and drifted primarily W and NW. Steam plumes were visible most days of the month (figure 119). Only a few were visible in satellite data, but most were visible in the webcams. Several episodes of drumbeat seismicity were recorded on 13, 22-25, and 27 October, which were characterized by being of short duration and consisting of very low energy earthquakes. The tallest plume during the month rose about 2 km above the crater rim on 18 October. Ash emissions were recorded eight times during the month by SGC.

Figure 119. A steam plume mixed with possible ash drifted SE from Nevado del Ruiz on 7 October 2020. Courtesy of vlucho666.

During November 2020, the number of seismic events decreased relative to October, but the amount of energy released increased. Some of the seismicity was associated with small emissions of gas and ash, confirmed by webcams around the volcano. The Washington VAAC reported ash emissions on 22 and 30 November; the 22 November event was faintly visible in satellite images and was also associated with an LP seismic event. They rose to 5.8-6.1 km altitude and drifted W. Various episodes of drumbeat seismicity registered during November were short- to moderate-duration, very low energy, and consisted of seismicity associated with rock fracturing (VT). Multiple steam plumes were visible from communities tens of kilometers away (figure 120).

Figure 120. Multiple dense steam plumes were photographed from communities around Nevado del Ruiz during November 2020, including on 18 (top) and 20 (bottom) November. Top image courtesy of Jose Fdo Cuartas, bottom image courtesy of Efigas Oficial.

Seismic activity increased in December 2020 relative to November. It was characterized by continuous volcanic tremor, tremor pulses, long-period (LP) and very long-period (VLP) earthquakes. Some of these signals were associated with gas and ash emissions, one confirmed through the webcams. The Washington VAAC reported ash emissions on 5 and 7 December. The first rose to 5.8 km altitude and drifted NW. The second rose to 6.7 km altitude and drifted W. A single discrete cloud was observed 35 km W of the summit; it dissipated within six hours. Drumbeat seismic activity increased as well in December; the episode on 3 December was the most significant. Steam and gas emissions continued throughout the month; a plume of gas and ash reached 1,700 m above the summit on 20 December, and drifted NW.

Sentinel-2 satellite data showed at least one thermal anomaly inside the Arenas crater each month during August-December 2020, corroborating the seismic evidence that the dome continued to grow throughout the period (figure 121). Sulfur dioxide emissions were persistent, with many days every month recording DU values greater than two with the TROPOMI instrument on the Sentinel 5-P satellite (figure 122).

Figure 121. Thermal anomalies at Nevado del Ruiz were recorded at least once each month during August-December 2020 suggesting continued growth of the dome within the Arenas crater at the summit. Courtesy of Sentinel Hub Playground.

Figure 122. Sulfur dioxide emissions were persistent at Nevado del Ruiz during August-December 2020, with many days every month recording DU values greater than two with the TROPOMI instrument on the Sentinel 5-P satellite. Ecuador’s Sangay had even larger SO2 emissions throughout the period. Dates are at the top of each image. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Additional reports of activity during 2017. Activity appears to have continued during June-December 2017. Ash emissions were reported by the Bogota Meteorological Weather Office (MWO) on 13 May, and by SGC on 28 May. During June, some of the recorded seismic events were associated with minor emissions of ash; these were confirmed by webcams and by field reports from both the staff of SGC and the Los Nevados National Natural Park (PNNN). Ash emissions were confirmed in webcams by park officials on 3, 16, and 17 June. Gas emissions from the Arenas crater during July 2017 averaged 426 m above the crater rim, generally lower than during June. The emissions were mostly steam with small amounts of SO₂. Emissions were similar during August, with most steam and gas plumes drifting NW. No ash emissions were reported during July or August.

SGC reported steam and gas plumes during September that rose as high as 1,650 m above the crater rim and drifted NW. On 21 September the Washington VAAC received a report of an ash plume that rose to 6.4 km altitude and drifted NNW, although it was not visible in satellite imagery. Another ash emission rising to 6.7 km altitude was reported on 7 October; weather clouds prevented satellite observation. An episode of drumbeat seismicity was recorded on 9 October, the first since April 2017. While SGC did not explicitly mention ash emissions during October, several of the webcam images included in their report show plumes described as containing ash and gas (figure 123).

Figure 123. Plumes of steam, gas, and ash rose from Arenas crater at Nevado del Ruiz most days during October 2017. Photographs were captured by the webcams installed in the Azufrado Canyon and Cerro Gualí areas. Courtesy of SGC (INFORME DE ACTIVIDAD VOLCANICA SEGMENTO NORTE DE COLOMBIA, OCTUBRE DE 2017).

The Washington VAAC received a report from the Bogota MWO of an ash emission that rose to 6.1 km altitude and drifted NE on 8 November 2017. A faint plume was visible in satellite imagery extending 15 km NE from the summit. SGC reported that plumes rose as high as 2,150 m above the rim of Arenas crater during November. The plumes were mostly steam, with minor amounts of SO₂. A diffuse plume of ash was photographed in a webcam on 24 November. SGC did not report any ash emissions during December 2017, but the Washington VAAC reported “a thin veil of volcanic ash and gases” visible in satellite imagery and webcams on 18 December that dissipated within a few hours. In addition to the multiple reports of ash emissions between May and December 2017, Sentinel-2 thermal satellite imagery recorded at least one image each month during June-December showing a thermal anomaly at the summit consistent with the slowly growing dome first reported in August 2015 (figure 124).

Figure 124. Thermal anomalies from the growing dome inside Arenas crater at the summit of Nevado del Ruiz appeared at least once each month from June-December 2017. A strong anomaly was slightly obscured by clouds on 3 June (top left). On 2 August, a steam plume obscured most of the crater, but a small thermal anomaly is visible in its SE quadrant (top right). Strong anomalies on 30 November and 20 December (bottom) have a ring-like form suggestive of a growing dome. Atmospheric penetration rendering (bands 12, 11, 8A), courtesy of Sentinel Hub Playground.

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

Information Contacts: 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); 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/); Gonzalo (URL: https://twitter.com/chaloc22/status/1306581929651843076); Jose Fdo Cuartas (URL: https://twitter.com/JoseFCuartas/status/1329212975434096640); Vlucho666 (URL: https://twitter.com/vlucho666/status/1313791959954268161); Efigas Oficial (URL: https://twitter.com/efigas_oficial/status/1329780287920873472).

Mount Ibu is an active stratovolcano located along the NW coast of Halmahera Island in Indonesia. After a two-day eruption in 1911, Ibu was quiet until 1998-1999 when explosions produced ash emissions, a lava flow and dome growth began inside the summit crater. Although possible dome growth occurred in 2001 and 2004, little activity was reported until ash emissions began in April 2008. These were followed by thermal anomalies beginning the next month; ash emissions and dome growth have continued for 12 years and the dome now fills the summit crater (BGVN 45:07). Activity continued throughout 2020, consisting of frequent white-and-gray emissions, ash explosions, ash plumes, and small lava flows. This report updates activity through December 2020, using data from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite instruments.

Activity throughout July-December 2020 was very consistent and similar to activity reported earlier in the year. Tens of daily explosions produced white and gray ash emissions that rose 200-800 m above the summit (figure 25). Occasional larger explosions were reported in VONAs and VAAC notices. The MIROVA graph of log radiative power for the period shows consistent thermal anomalies the entire time (figure 26). Satellite imagery from Sentinel-2 identified thermal anomalies inside the summit crater every month, usually a larger central one and a smaller one to the NW, suggesting continued dome growth and lava flow activity (figure 27).

Figure 25. Between 60 and 90 explosions occurred most days at Ibu during 1 July-31 December 2020. White and gray plumes rose 200-800 m above the summit crater every day. Data courtesy of PVMBG daily reports.

Figure 26. The MIROVA graph of Log Radiative Power at Ibu from 3 February through December 2020 indicated a constant ongoing heat source from the summit of the crater. Courtesy of MIROVA.

Figure 27. Thermal anomalies persisted at the summit of Ibu throughout July-December 2020. One central anomaly was usual accompanied by a smaller one slightly NW of the central spot. Atmospheric penetration rendering (bands 12, 11a, and 8), courtesy of Sentinel Hub Playground.

The Darwin VAAC observed multiple minor ash emissions in satellite imagery drifting W on 6 July 2020 at 1.8 km altitude. A series of discrete puffs of ash were observed on 15 July also at 1.8 km altitude drifting W. Ongoing minor emissions were discernible on visible and RGB imagery at 2.1 km altitude drifting W on 20 July. On 30 July ash plumes rose to 1.8 km altitude drifted NW and a hotspot was present at the summit. A single MODVOLC alert was issued on 8 July. Single MODVOLC alerts were also issued on 11, 18, and 27 August 2020. PVMBG issued a VONA on 5 August, reporting an ash cloud that rose to 1.8 km altitude and drifted N (figure 28). The Darwin VAAC reported an ash emission later that day that rose to 4.3 km altitude and drifted NW for several hours before dissipating. Multiple discrete emissions were identified in satellite imagery drifting N at 2.1 km altitude on 11 August; they dissipated quickly. During 22-25 August intermittent ash emissions rose to 1.5-1.8 km altitude and drifted NW and W. Minor continuous emissions were again reported on 28 August.

Figure 28. Ash plumes rose from the summit of Ibu many days during July and August 2020, including on 8 July (top) and 5 August (bottom). Courtesy of PVMBG.

Many ash emissions during September and October 2020 were not accompanied by VONAs or VAAC advisories (figure 29). PVMBG issued a VONA on 20 September for an ash emission that rose to 1.5 km altitude and drifted N. Continuous discrete ash emissions over several days drifted SW to NW during 25-29 September at 1.8-2.1 km altitude, as reported in multiple VONAs and VAAC advisories. Single MODVOLC alerts were issued on 26 and 30 September. The Darwin VAAC issued an ash advisory on 8 October for intermittent ash emissions rising to 2.1 km altitude and drifting NW. A single MODVOLC alert was issued the next day. On 20 October ash emissions again rose to 2.1 km altitude and drifted NE.

Figure 29. Ash emissions at Ibu were photographed in webcams on 6 September (left) and 12 October (right) 2020. Courtesy of PVMBG.

The Darwin VAAC reported intermittent ash emissions to 1.8 km altitude during 3-5, 12-13, 18-19, and 22 November 2020 that drifted SSW for several hours before dissipating. PVMBG also issued a VONA for an ash cloud on 27 November that rose to 2.1 km altitude and drifted W. They reported faint rumbling at the PGA Ibu station on 10 November and loud rumbling on 16 and 18 November. During December, minor ash emissions rose to 1.8-2.1 km altitude and drifted E on 4 and 6 December, SW on 11 December, and SE on 12-13 December. PVMBG issued a VONA on 19 December for a white to gray ash cloud drifting N at 1.7 km altitude. Single MODVOLC alerts were issued on 10, 13, and 22 December. Numerous ash emissions were captured by the webcams (figure 30).

Figure 30. Ash emissions at Ibu were recorded in webcams on 17 November (top) and 5 December (bottom) 2020. Courtesy of PVMBG.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, contained several small crater lakes through much of historical time. The outer crater, 1.2 km wide, is breached on the north side, creating a steep-walled valley. A large parasitic cone is located ENE of the summit. A smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. Only a few eruptions have been recorded in historical time, the first a small explosive eruption from the summit crater in 1911. An eruption producing a lava dome that eventually covered much of the floor of the inner summit crater began in December 1998.

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/); 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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Etna, on the island of Sicily, Italy, and has had documented eruptions dating back 3,500 years. Its most recent eruptive period began in September 2013 and has continued through November 2020, characterized by frequent Strombolian explosions, effusive activity, and ash plumes. Activity has commonly originated from the summit areas, including the Northeast Crater (NEC), the Voragine-Bocca Nuova (or Central) complex (VOR-BN), the Southeast Crater (SEC, formed in 1978), and the New Southeast Crater (NSEC, formed in 2011). The newest crater, referred to as the "cono della sella" (saddle cone), emerged during early 2017 in the area between SEC and NSEC. This report from August through November 2020 updates activity consisting of frequent Strombolian explosions, ash plumes, summit crater incandescence, degassing, and some ashfall based on information primarily from weekly reports by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV).

Summary of activity during August-November 2020. Intra-crater Strombolian explosions that varied in frequency and intensity throughout the reporting period, and the accompanying ash emissions that rose to a maximum altitude of 4.5 km, primarily originated from the Northeast Crater (NEC), the New Southeast Crater (NSEC), and intermittently from the Voragine Crater (VOR). Degassing of variable intensity typically occurred at the VOR and the Bocca Nuova (BN) Crater. At night, occasional summit crater incandescence was visible in webcam images, accompanied by explosions and gas-and-ash emissions. On 14 August strong Strombolian explosions produced an ash plume that rose to 4.5 km altitude and drifted SE, resulting in ashfall between Pedara, Trecastagni, and Viagrande. INGV reported that the central pit crater at the bottom of BN continued to widen, and on 9 September scientists observed that a new pit crater had formed NW of the central depression and was widening due to crater wall collapses. During late October to 1 November, INGV reported that small lava flows originated from scoria cones in the NEC and were visible from the edge of the crater but did not spill over.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows frequent thermal activity of varying strength throughout the reporting period (figure 308). In late October, the frequency of the thermal anomalies increased, and continued through November. According to the MODVOLC thermal algorithm, a total of 31 alerts were detected in the summit craters during August through November; thermal anomalies were reported for five days in August, four days in September, four days in October, and eight days in November. Frequent Strombolian activity contributed to distinct SO₂ plumes that drifted in multiple directions (figure 309).

Figure 308. Strong and frequent thermal activity at Etna was detected during August through November 2020, as reflected in the MIROVA data (Log Radiative Power). Beginning in late October, the frequency of the thermal anomalies increased compared to the previous months. Courtesy of MIROVA.

Figure 309. Distinct SO2 plumes from Etna were detected on multiple days during August to November 2020 due to frequent Strombolian explosions, including 29 August (top left), 8 September (top right), 1 October (bottom left), and 11 November (bottom right) 2020. SO2 plumes were observed drifting in multiple directions. Captured by the TROPOMI instrument on the Sentinel 5P satellite, courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during August-September 2020. During August, INGV reported intra-crater Strombolian explosions in the NEC, VOR, and NSEC (including the cono della sella) craters, which produced discontinuous ash emissions rising above each crater (figure 310). Gas-and-steam emissions were the dominant activity in the BN crater. INGV noted that the central pit crater on the floor of BN had been gradually widening since April. On 2 August a slight increase in explosivity resulted in minor ashfall in Trecastagni and Acicastello. Explosive activity occasionally ejected material above the crater rim up to several tens of meters. On the morning of 7 August incandescent Strombolian activity was visible in the NSEC (figure 311). During the evening of 10-11 August surveillance cameras showed the explosions ejecting incandescent material on the surrounding flanks. On 14 August intense Strombolian activity in the saddle cone of the NSEC produced an ash plume that rose to 4-4.5 km altitude and drifted SE, resulting in ashfall between Pedara, Trecastagni, and Viagrande. By the evening activity had sharply declined, according to a VONA (Volcano Observatory Notice for Aviation) report, though sporadic ash emissions continued. A new series of ash emissions associated with explosions of varying intensity began on 15 August in the NSEC. A resulting ash plume rose to 4-4.5 km altitude and drifted ESE. On 17 August gas-and-steam emissions were seen rising above the VOR crater, accompanied by persistent Strombolian explosions. Between the afternoon and early morning of 20-21 August surveillance cameras showed an increased intensity and frequency of ash emissions above the NSEC and NEC that rose to 4-4.5 km altitude and drifted SSE. INGV-OE scientists reported minor ashfall in Trecastagni, Viagrande, and Catania. During 24-30 August ground observers reported that the intra-crater explosions in the NEC originated from two explosive vents; the BN crater exhibited gas-and-steam emissions from the central pit crater, which continued to widen. During 25-26 August explosive activity increased at the NSEC with ash emissions rising to 4.5 km and drifting SSE, which resulted in modest ashfall in Catania, Viagrande, and Trecastagni; by morning, the volume of ash emissions had decreased, though explosions persisted. During 28-29 August discontinuous and modest ash emissions originating from the NSEC rose 4.5 km altitude drifting E and ENE but did not result in ashfall. Emissions had stopped by 1747 on 29 August, though intense gas-and-steam emissions continued, occasionally accompanied by mild explosive activity (figure 312).

Figure 310. An ash plume accompanied Strombolian explosions at Etna on 3 August (top left) and 4 August (top right) and as seen from the Montagnola (EMOV) thermal camera in the NSEC. Continuous Strombolian activity and summit crater incandescence was observed on 7 August (bottom left); an ash plume was visible in the Monte Cagliato surveillance camera during the day on 9 August (bottom right). Courtesy of INGV (Report 33/2020, ETNA, Bollettino Settimanale, 03/08/2020 – 09/08/2020, data emissione 11/08/2020).

Figure 311. Strombolian explosions and summit crater incandescence was observed at Etna’s New Southeast Crater (NSEC “cono della sella”) during the early morning of 7 August 2020 seen from Tremestieri Etneo. Photo by Boris Behncke, INGV.

Figure 312. Photo of the S edge of the Bocca Nuova Crater at Etna on 29 August 2020 showing degassing in the pit crater. The main scoria cone within the Voragine Crater is visible in the background. Courtesy of INGV (Report 36/2020, ETNA, Bollettino Settimanale, 24/08/2020 – 30/08/2020, data emissione 01/09/2020).

Strombolian activity of varying intensity continued in the NSEC and NEC during September, producing sporadic ash emissions (figure 313). The BN and VOR craters were characterized by gas-and-steam emissions. Explosions in the NSEC ejected coarse pyroclastic material above the crater rim several tens of meters, some of which were deposited on the S flank, and accompanied by sporadic ash emissions; these explosions continued to widen the depression in the saddle cone of the NSEC. Intermittent nighttime crater incandescence was observed in the NSEC. Sporadic and weak ash emissions were observed in the VOR. On 9 September INGV scientists reported intense degassing from the center pit crater in the BN. To the NW of this center depression, a new pit crater had formed and began to widen due to the collapse of the crater walls (figure 314). On 26 September explosions in the NSEC produced an ash plume that rose to 4 km altitude and drifted E, though no ashfall was reported.

Figure 313. Webcam image showing explosions in the New Southeast Crater and resulting ash emissions on 1 September 2020. Courtesy of INGV (Report 37/2020, ETNA, Bollettino Settimanale, 31/08/2020 – 06/09/2020, data emissione 08/09/2020).

Figure 314. Photos of the bottom of the W edge of the Bocca Nuova Crater at Etna on 9 September 2020. Gas-and-steam emissions are visible rising above the pit crater in the background. In the foreground a new pit crater had formed to the NW of the central pit crater (yellow dotted line). Photo was taken from the S edge of the BN crater. Courtesy of INGV (Report 38/2020, ETNA, Bollettino Settimanale, 07/09/2020 – 13/09/2020, data emissione 15/09/2020).

Activity during October-November 2020. Similar variable Strombolian activity continued into October in the NSEC (cono della sella) and NEC; isolated and weak ash emissions were visible in the VOR crater and gas-and-steam emissions continued in both the VOR and BN craters. On 1 October an increase in explosive activity in the NSEC occurred around 0800, which produced an ash plume rising to 4.5 km altitude, drifting E. Ash emissions on 3 October were mostly confined to the summit crater, but some drifted toward the Valle del Bove. On 7 October Strombolian explosions in the NSEC generated an ash plume that rose to 4.5 km altitude drifting E and ESE. INGV personnel reported ashfall as a result in the Citelli Refuge. On 9 October drone observations showed at least three active scoria cones on the floor of the NEC with diameters of 30-40 m and heights of 10 m; a fourth vent was later reported in November (figure 315). INGV reported that activity characterized by Strombolian explosions and spatter was fed by these vents, accompanied by intense intra-crater fumarolic activity.

Figure 315. Map of the summit craters of Etna showing the active vents and the area of cooled lava flows (light green) updated on 9 October 2020. The base is modified from a 2014 DEM created by Laboratorio di Aerogeofisica-Sezione Roma 2. The hatch marks indicate the crater rims: BN = Bocca Nuova; VOR = Voragine; NEC = North East Crater; SEC = South East Crater; NSEC = New South East Crater. Red circles indicate areas with ash emissions and/or Strombolian activity, yellow circles indicate steam and/or gas emissions only. Courtesy of INGV (Report 44/2020, ETNA, Bollettino Settimanale, 19/10/2020 – 25/10/2020, data emissione 27/10/2020).

During 12-18 October surveillance cameras captured incandescence in the NEC and pyroclastic material seen during more intense explosions. During the week of 19-25 October several thermal anomalies were detected on the NEC and BN crater floor. Particularly at night, thermal and surveillance cameras observed incandescent ejecta rising above the NSEC (figure 316). On 23 October a helicopter overflight along the W side of Etna showed continued explosions at the NSEC, which produced both ash emissions and incandescent shreds of lava. An associated ash plume rose to 4.5 km altitude and drifted SSE. Sporadic ash emissions were also observed in the BN crater (figure 316). During 26 October to 1 November occasional Strombolian activity resumed in the VOR which ejected material over the crater rim. The BN crater activity was characterized by small intra-crater collapses and consequent ash emissions. In the NEC, similar explosive activity persisted with the addition of small lava flows from the scoria cones, which were visible from the crater edge, though activity remained confined to the crater.

Figure 316. Photos showing Strombolian activity at the New Southeast Crater at Etna on 25 October 2020 (top left); ash emissions were observed during 22 October 2020 (top right). Ash emissions rose above the Bocca Nuova Crater on 22 October (bottom left) and weak ash emissions were seen above the Voragine Crater on 22 October (bottom right). Courtesy of INGV (Report 44/2020, ETNA, Bollettino Settimanale, 19/10/2020 – 25/10/2020, data emissione 27/10/2020).

Activity in November continued with variable Strombolian explosions accompanied by discontinuous ash emissions from the NSEC, NEC, and BN. During more intense explosions, ejecta reached several tens of meters above the crater, sometimes falling just outside the crater rim. Intensive degassing in the BN crater revealed occasional reddish ash in the new W pit crater that formed in September. The central pit crater was primarily characterized by intense gas-and-steam emissions and intra-crater wall collapses. Four vents were observed on the bottom of the NEC during 2-8 November, though only three of them produced Strombolian explosions, the fourth was quiet. On 5 November Strombolian explosions in BN originated from the W pit crater; coarser material was ejected above the pit crater rim. By 12 November Strombolian activity had decreased, explosions in the BN had deposited material on the S flank. Out of the three active NEC scoria cones, only one was continuously exploding, the second had discontinuous explosions, and the third was primarily emitting gas-and-steam. On 15 November faint ash emissions from the E side of the NSEC were observed (figure 317). On 20 November sporadic explosive activity continued from the NSEC and BN, the former of which occasionally ejected material above the crater rim (figure 318).

Figure 317. Webcam images of the New Southeast Crater at Etna on 14 (left) and 15 (right) November 2020 showing Strombolian activity in the cono della sella (left) and the E vent shown by the black arrow (right). Images were taken by the Montagnola webcam. Courtesy of INGV (Report 47/2020, ETNA, Bollettino Settimanale, 09/11/2020 – 15/11/2020, data emissione 17/11/2020).

Figure 318. Drone image of the New Southeast Crater at Etna on 21 November 2020 showing an ash plume rising above the inner crater rim (black line). Fallout is visible within the crater rim (small red circles). Courtesy of INGV (Report 48/2020, ETNA, Bollettino Settimanale, 16/11/2020 – 21/11/2020, data emissione 24/11/2020).

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: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); 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/); 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/); Boris Behncke, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: https://twitter.com/etnaboris).

Copahue is an elongated composite cone located along the Chile-Argentina border. The E summit crater consists of an acidic 300-m-wide crater lake which is characterized by intense fumarolic activity. Previous activity consisted of continuous gas-and-ash emissions during early November 2019, accompanied by nighttime incandescence, minor SO₂ plumes, and the reappearance of the lake in the El Agrio crater during early December 2019 (BGVN 45:03). This report, covering March-November 2020, describes an eruption with gas-and-ash plumes from mid-June through late October, accompanied by thermal anomalies visible in satellite imagery and small SO₂ plumes. Primary information for this report comes from the Servicio Nacional de Geología y Minería (SERNAGEOMIN) Observatorio Volcanológico de Los Andes del Sur (OVDAS), the Buenos Aires Volcanic Ash Advisory Center (VAAC), and various satellite data.

Activity during March-May 2020 was relatively low and consisted primarily of seismicity, sulfur dioxide emissions, and occasional white gas-and-steam emissions rising 300-900 m above the El Agrio crater. On 20 March a series of volcano-tectonic seismic events were detected SSW of the volcano; satellite images showed a decrease in the size of the crater lake. SO₂ emissions had daily averages of 487-636 tons, with the highest value reaching 1,884 tons/day on 16 May. During April slight subsidence was reported in the crater, occurring at a maximum rate of 0.3 cm/month.

Activity during most of June and July consisted of occasional white gas-and-steam emissions rising 350-500 m above the El Agrio crater and SO₂ emissions averaging 592-1,950 tons/day; a high value of 1,897 tons/day was reported on 13 June. However, on 16 June a period of increased seismicity was accompanied by crater incandescence and gas emissions containing some ash. SO₂ plumes increased slightly in July with values of 2,100 and 1,713 tons/day on 2 and 4 July, respectively. Another ash plume was observed by local residents on 16 July, accompanied by elevated seismicity and SO₂ emissions of 4,684 tons/day. On 20 July residents of La Araucanía described an odor that indicated hydrogen sulfide gas emissions. A photo on 23 July showed an ash plume rising above the crater (figure 55).

Figure 55. Photo of a gas-and-ash plume rising from Copahue on 23 July 2020. Courtesy of Valentina Sepulveda, taken from Caviahue, Argentina.

Beginning in early August, and continuing through September 2020, the Sentinel-2 MODIS Thermal Volcanic Activity graph provided by the MIROVA system identified a small cluster of thermal anomalies in the summit area (figure 56). Thermal anomalies during this time were also captured in Sentinel-2 thermal satellite imagery, showing a persistent hotspot of varying strength in the summit crater (figure 57). This thermal activity was accompanied by small sulfur dioxide plumes identified by the TROPOMI instrument on the Sentinel-5P satellite, which exceeded two Dobson Units (DU). Distinct SO₂ emissions greater than two DUs were detected on 6, 11, 21, 22, and 29 August, 1 and 6 September, and 4 and 15 October (figure 58).

Figure 56. A small cluster of thermal anomalies were detected in the summit area of Copahue (red dots) during early August through September 2020 as recorded by the Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A). Courtesy of MIROVA.

Figure 57. Sentinel-2 thermal satellite imagery showed a thermal anomaly (bright yellow-orange) at Copahue during August-October 2020. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 58. Small SO2 plumes were recorded at Copahue during August-October 2020. Top row: 11 August and 1 September 2020. Bottom row: 6 September and 15 October 2020. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

During August, approximately 133 explosive events were detected, in addition to the gas-and-steam and SO₂ emissions (figure 59). On 3 August pulses of ash emissions were reported by SERNAGEOMIN, which resulted in a 2.2-km-long tephra deposit estimated to have a volume of 1 km³. Gray gas-and-ash emissions were observed on 6 August, followed by a thermal anomaly detected in satellite imagery beginning on 8 August. Sulfur dioxide emissions were elevated compared to previous months, measuring an average of 2,641 tons/day with high values of 4,498 tons/day on 12 August that increased to 4,627 tons/day by 27 August. During 16-31 August webcams recorded gas-and-ash plumes rising as high as 1.7 km altitude and were sometimes accompanied by nighttime crater incandescence. Plumes drifted in multiple directions as far as 4.3 km N, 9 km NE, 8 km E, 4 km SE, 4 km SW, 9 km W, and 4.4 km NW.

Figure 59. Photo of a white gas-and-steam plume rising from Copahue on 12 August 2020. Courtesy of Valentina Sepulveda, taken from Caviahue, Argentina.

Elevated activity continued into September with 2-10 explosive events detected during the month; during 1-15 September webcams recorded gas-and-ash plumes rising to 1.1 km altitude, drifting 6-15 km SW and SE, which were sometimes accompanied by nighttime crater incandescence (figure 60). On 7 September a Buenos Aires VAAC advisory reported an ash plume rising to 3.7 km altitude drifting SE. On 11 September a webcam showed a weak gas emission, possibly containing some ash. Three episodes of gas-and-steam plumes were reported, rising 100-1,040 m above the crater, sometimes accompanied by incandescence. SO₂ emissions were in the 1,499-1,714 tons/day range, with a high value of 4,522 tons/day on 28 September. SERNAGEOMIN reported repetitive explosions in the acid lake area alongside fumarolic activity, ejecting some material 1.7 km N, 1.2 km SE, and 4 km E of the crater.

Figure 60. Photos of gas-and-steam plumes rising from Copahue on 6 September (top) and 28 September (bottom) 2020. Courtesy of Valentina Sepulveda, taken from Caviahue, Argentina.

Persistent activity in October consisted of gas-and-steam plumes, ash emissions, and SO₂ emissions. The gas-and-steam plumes rose 1.4 km above the crater, occasionally accompanied by nighttime incandescence. On 5 October the SO₂ emissions were at a high value of 3,824 tons/day. During 12-15 October ash emissions resulted in a wide distribution of ashfall that reached 6.8 km NE, 7 km SE, and 6.7 km SW (figure 61). A pilot reported an ash plume rose to 3.7 km altitude drifting SE, according to a VAAC advisory, though the plume was not visible in satellite data. Sentinel-2 satellite imagery recorded strong gas-and-ash plumes during August-October, drifting generally S and E, which resulted in ash deposits on the nearby flanks (figure 62). Continued emissions had covered all of the flanks with ash by late October.

Figure 61. Photos of a gas-and-ash plume rising from Copahue on 13 October (top) and 15 October (bottom) 2020. Courtesy of Valentina Sepulveda, taken from Caviahue, Argentina.

Figure 62. Sentinel-2 images showing ash gas-and-ash plumes rising from Copahue during August-October 2020, resulting in some ashfall in the nearby areas. The ash plume on 31 August (top left) is drifting S with ashfall observed on the N and S flanks. The ash plume on 7 September (top right) is drifting SE with ashfall on the E and S flanks. The ash plume on 27 September (bottom left) is drifting E and N with ashfall on the NE flanks. The ash plume on 20 October (bottom right) is drifting S with ashfall on all the flanks due to continued activity. Images using “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Similar activity during November decreased, primarily characterized by gas-and-steam plumes and SO₂ emissions. White gas-and-steam emissions, possibly with some ash content, were observed with a webcam on 9 and 12 November, accompanied by low but continuous seismicity. During 11-12 November SO₂ emissions were at a high value of 904 tons/day. A white gas-and-steam plume was observed on 15 November rising 760 m above the crater; typical degassing rose 200-300 m above the crater, according to SERNAGEOMIN. The daily average of SO₂ emissions ranged 366-582 tons.

Geologic Background. Volcán Copahue is an elongated composite cone constructed along the Chile-Argentina border within the 6.5 x 8.5 km wide Trapa-Trapa caldera that formed between 0.6 and 0.4 million years ago near the NW margin of the 20 x 15 km Pliocene Caviahue (Del Agrio) caldera. The eastern summit crater, part of a 2-km-long, ENE-WSW line of nine craters, contains a briny, acidic 300-m-wide crater lake (also referred to as El Agrio or Del Agrio) and displays intense fumarolic activity. Acidic hot springs occur below the eastern outlet of the crater lake, contributing to the acidity of the Río Agrio, and another geothermal zone is located within Caviahue caldera about 7 km NE of the summit. Infrequent mild-to-moderate explosive eruptions have been recorded since the 18th century. Twentieth-century eruptions from the crater lake have ejected pyroclastic rocks and chilled liquid sulfur fragments.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); 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); Valentina Sepulveda, Hotel Caviahue, Caviahue, Argentina (URL: https://twitter.com/valecaviahue, Twitter: @valecaviahue).

Masaya, located in Nicaragua, includes the Nindirí, San Pedro, and San Juan craters, as well as the currently active Santiago crater. The Santiago crater has contained an active lava lake since December 2015 (BGVN 41:08), and often produces gas-and-steam emissions. Similar activity is described in this report which updates information from June through November 2020 using reports from the Instituto Nicareguense de Estudios Territoriales (INETER) and various satellite data.

Volcanism at Masaya has been relatively quiet and primarily characterized by an active lava lake and gas-and-steam emissions. From January to November 2020 there were 8,551 seismic events recorded. A majority of these events were described as low-frequency earthquakes, though a few were classified as volcano-tectonic. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed few low-power thermal anomalies during June through November (figure 87). A small cluster of low-power thermal activity was detected in July and consisted of seven thermal anomalies out of a total of thirteen thermal anomalies recorded during the reporting period. Thermal activity was also observed in Sentinel-2 satellite imagery, which showed a constant thermal anomaly in the Santiago crater at the lava lake during July through October, occasionally accompanied by a gas-and-steam plume (figure 88). Small and intermittent sulfur dioxide emissions appeared in satellite data during each month of the reporting period, excluding July, some of which exceeded two Dobson Units (DU) (figure 89). On 6 July, 11 and 13 August, 7 September, during October, and 9 and 13 November, INETER scientists took SO₂ measurements by making several transects using a mobile DOAS spectrometer that sampled for gases downwind of the volcano. Average values during these months were 1,202 tons/day (t/d), 1,383 t/d, 2,089 t/d, 950 t/d, and 819 t/d, respectively, with the highest average reported in September.

Figure 87. Few thermal anomalies were detected at Masaya between June and November 2020 with a small cluster of thermal activity in July. A total of thirteen low-power thermal anomalies were shown on the MIROVA graph (Log Radiative Power) during the reporting period. Courtesy of MIROVA.

Figure 88. Sentinel-2 thermal satellite imagery showed the active lava lake at the summit crater of Masaya during July through October 2020, occasionally accompanied by gas-and-steam emissions, as seen on 27 July (top left) and 30 September (bottom left). Images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 89. Intermittent sulfur dioxide emissions were captured from Masaya during June through November 2020 by the TROPOMI instrument on the Sentinel-5P satellite. These images show SO2 emissions reaching up to 2 Dobson Units (DU). Top left: 9 June 2020. Top right: 23 August 2020. Bottom left: 7 September 2020. Bottom right: 15 November 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

During June and July persistent gas-and-steam emissions were reported rising above the open lava lake in the Santiago crater (figure 90). On 20 June INETER scientists measured the gases on the S side, inside the Nindirí crater (SW side), and La Cruz (NW side). A perceptible gas-and-steam plume was noted rising above the Nindirí crater and drifting W. Crater wall collapses were observed on the E wall of the Santiago crater; the lava lake remained, but the level of the lake had decreased compared to previous months. During July, thermal measurements were taken of the fumaroles and near the lava lake using a FLIR SC620 thermal camera. INETER reported that the temperature measured 576°C, which had significantly increased from 163°C noted in the previous month.

Figure 90. Images of the lava lake at Masaya during June 2020, accompanied by gas-and-steam emissions (left) and a gas-and-steam plume rising above the Santiago crater (right). Courtesy of INETER (Boletín Sismológico, Vulcanológico y Geológico Junio, 2020).

Small crater wall collapses were detected on the NW and E wall of the Santiago crater, accompanied by abundant gas-and-steam emissions during August (figure 91). On 7 August thermal measurements were taken of the fumaroles and near the lava lake, which showed another temperature increase to 771°C. Continuous collapse of the crater walls began to excavate depressions in the crater floor and along the walls. Similar activity was observed in September with abundant gas-and-steam emissions in the Santiago crater, as well as collapses of the E wall (figure 91). Temperature measurements taken during this month had decreased slightly compared to August, to 688°C.

Figure 91. Photos of the Santiago crater at Masaya during August (left) and September (right) 2020 showing a) an internal collapse on the N wall of the crater floor; b) an internal collapse on the S wall of the crater floor, forming a depression; c) newly excavated crater floor due to wall collapses; and d) an internal collapse on the S wall. In September a significant amount of gas-and-steam emissions originating from the N side of the crater were observed compared to the previous months. Courtesy of INETER (Boletín Sismológico, Vulcanológico y Geológico Agosto and Septiembre, 2020).

Activity in October and November remained consistent with continued wall collapses in the Santiago crater, particularly on the S and E wall, due to fractures in the rocks and erosion, accompanied by gas-and-steam emissions. INETER reported that the level of the lava lake had decreased due to continuous internal wall collapses, which had caused some obstruction in the lava lake and allowed for material to accumulate within the crater. On 9 October thermal measurements were taken of the fumaroles and near the lava lake using a FLIR SC620 thermal camera (figure 92). The temperature had increased again compared to September, to 823°C. By 26 November, the temperature had decreased slightly to 800°C, though activity remained similar.

Figure 92. Thermal measurements of the active lava lake and fumaroles taken in the Santiago crater at Masaya on 1 October 2020 with a FLIR SC620 thermal camera. Temperatures reached up to 823°C. Courtesy of INETER (Boletín Sismológico, Vulcanológico y Geológico Octubre, 2020).

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/); NASA 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).

Nevados de Chillán, located in the Chilean Central Andes, is a volcanic complex composed of late-Pleistocene to Holocene stratovolcanoes. On 8 January 2016 an explosion created the Nicanor Crater on the NW flank of Volcán Viejo. Recent activity consists of explosions, ash plumes, pyroclastic flows, and a new lava dome in the Nicanor Crater (BGVN 45:07). This report covers July through October 2020; activity is characterized by frequent explosions, ash plumes, a lava flow on the N flank, and continued lava dome growth. The primary source of information comes from the Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), the Buenos Aires Volcanic Ash Advisory Center (VAAC), and satellite data.

Since 27 June webcams have showed an active lava flow that originated from the Nicanor Crater and descended the N flank. Activity during July consisted of 210-473 volcano-tectonic seismic events and 565-614 explosive events. Ash plumes rising 1.1-1.2 km above the crater and were accompanied by day and nighttime incandescence on the E edge of the Nicanor Crater. Due to these explosions, SERNAGEOMIN reported that tephra and other pyroclastic deposits were deposited within 400 m to the E of the crater. On 1 July a Buenos Aires VAAC advisory reported that a webcam showed ash emissions rising to 4.3 km altitude. Continuous explosions the next day produced ash plumes that rose 500 m above the crater. During 1-2 July the active lava flow had reached 40 m long and descended at a rate of 0.2 m³ per second. On 6 July an explosion at 0837 generated a gas-and-ash plume that rose 1.2 km above the crater and drifted SE; sporadic ash emissions were also observed on 7 July, according to a VAAC advisory. SERNAGEOMIN webcams showed that the lava flow that began on 27 June continued down the N flank, while a new lobe 55-194 m long moved toward the NE flank of Nicanor Crater. Gas plumes were also observed rising above the active crater, as noted on 20 July (figure 63). On 29 July weak ash emissions rose 3.9 km altitude and drifted SE, according to a VAAC report. During that day, the volume of the lava dome measured 400,000 m³ and grew at a rate of 0.1 m³ per second. Throughout the month, the lava flow continued to descend the N flank of the Nicanor Crater, reaching 520 m at a rate of 0.7-0.6 m per hour. Some unconsolidated blocks up to a meter in size detached from the front of the flow and moved up to 240 m. Sulfur dioxide emissions during the month averaged 823 tons/day with a high value of 1,815 tons/day reported on 29 July.

Figure 63. A white gas-and-steam plume was observed at Nevados de Chillán on 20 July 2020. Courtesy of SERNAGEOMIN webcam, posted by Volcanology Chile.

During August SERNAGEOMIN reported 68-75 volcano-tectonic seismic events and 497-578 explosive events, the latter of which ejected material as far as 300 m E and NE from Nicanor Crater. Associated ash plumes rose 800-980 m above the crater and were accompanied by day and nighttime crater incandescence. The lava dome continued to grow during the month, reaching a thickness of 41 m, according to SERNAGEOMIN. SO₂ emissions were an average value of 134-205 tons/day with a high value of 245 tons/day reported on 3 August. On 15 August a VAAC advisory reported weak and sporadic gas-and-ash emissions at the summit; on 20 August a hotspot was detected in satellite imagery, though an ash plume was not observed. The active lava flow on the N flank extended 490-495 m and moved at a rate of 0.07-0.06 m per hour. On 31 August a webcam showed an ash plume rising above the volcano, accompanied by the advancing lava flow on the N flank (figure 64).

Figure 64. An explosion at Nevados de Chillán produced an ash plume on 31 August 2020. A lava flow accompanies the ash plume on the N flank. Courtesy of SERNAGEOMIN.

Similar activity continued into September, with 45-48 volcano-tectonic and 591-621 explosive events. Ash plumes rose to 1.5 km above the crater and were accompanied by day and nighttime incandescence on the E edge of Nicanor Crater. During 1-15 September explosions at the lava dome produced ash plumes that rose to less than 1.5 km altitude, resulting in ashfall within 300 m E and NE of the crater; ejecta from larger explosions was also observed to the ESE. Satellite images showed partial destruction of the lava dome as well as loss of some material due to successive explosions at the beginning of the month. Overall, the dome continued to increase in size, reaching a volume of 180,000 m³ and a thickness of 45 m since August (41 m). The lava dome measured 93 m NW-SE and 104 m SW-NE. By 15 September the 500-m-long lava flow had descended the NNE flank and continued to advance at a rate of 1.7 m per hour. The W levee of the flow channel had ruptured, which caused the toe of the lava flow to thicken. On 20 September ash emissions rose to 3.7 km altitude and drifted NE and ENE, according to a VAAC advisory. On 22 September gas emissions, weak and sporadic ash emissions, and occasional explosions accompanied the lava flow. Through the remainder of the month, the lava flow persisted, measuring 615 m, and advancing at a rate of 0.4 m per hour; its volume was 487,000 m³ (figure 65). SO₂ emissions were an average value of 111-358 tons/day with a high value of 503 tons/day reported on 22 September.

Figure 65. Photo (color corrected) of the incandescent lava flow at night descending the NNE flank of Nevados de Chillán on 21 September 2020. Photo by Jose Fauna, courtesy of Volcanology Chile.

During October there were 34-61 volcano-tectonic seismic events reported, as well as 607-644 explosive events, seven of which generated ash plumes that rose 1-1.5 km above the crater. Day and nighttime incandescence in the E edge of Nicanor Crater remained. Ash deposits associated with the explosive activity were distributed to the E and NE as far as 300 m from the crater; denser pyroclastic deposits from stronger explosions were located to the N and NE. The lava flow on the N slope persisted, extending 614-683 m from the crater rim at a rate of 0.1-0.82 m per hour with a width of 80.2 m near the crater rim and up to 112.8 m near the toe. The lava dome also continued to grow since it was last measured in September; it was 115 m wide at the base by 107 m high. SO₂ emissions were an average value of 167-355 tons/day with a high value of 588 tons/day reported on 26 October. On 29 October an ash plume was detected in satellite imagery and rose to 3.7 km altitude and drifted W, according to a VAAC advisory (figure 66). SERNAGEOMIN reported that a 25-m-diameter subcrater had formed on the E inner edge of Nicanor Crater at the top of the lava dome. On 30 October, intermittent gas-and-ash emissions were visible at the summit in satellite imagery, rising to 3.9 km altitude and drifting SE.

Figure 66. Webcam image of an explosion at Nevados de Chillán on 29 October 2020 that produced an ash plume that rose 360 m above the crater and drifted SW. Courtesy of SERNAGEOMIN.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows frequent low-power thermal activity beginning in early June and continuing through October 2020 due to frequent explosions, the continued lava dome growth in Nicanor Crater, and the lava flow that descended the N flank (figure 67). On clear weather days, two thermal anomalies in the summit craters are observed in Sentinel-2 thermal satellite imagery; one represents the growing lava dome and the other is the lava flow on the N flank (figure 68). On 25 September an ash plume was observed drifting S.

Figure 67. Frequent low-power thermal activity at Nevados de Chillán continued during July through October 2020, according to the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 68. Sentinel-2 satellite imagery showed a persistent thermal anomaly (bright yellow-orange) in the summit crater of Nevados de Chillán during July through October 2020. On 29 July (top left), a third faint thermal anomaly was detected on the N flank, indicating a lava flow. On 25 September (bottom left) an ash plume was visible drifting S. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The largest stratovolcano, dominantly andesitic, Cerro Blanco (Volcán Nevado), is located at the NW end of the group. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The new Volcán Nuevo lava-dome complex formed between 1906 and 1945 between the two volcanoes and grew to exceed Volcán Viejo in elevation. The Volcán Arrau dome complex was constructed SE of Volcán Nuevo between 1973 and 1986 and eventually exceeded its height.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); 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); Volcanology Chile (URL: https://twitter.com/volcanologiachl); Jose Fauna, Caracol sector, San Fabián de Alicom, Chile (URL: https://twitter.com/josefauna).

Volcanic activity has remained low since the last explosive eruption on 20 February. However, a non-explosive eruption generated an ash plume to 1,400 m altitude on 3 April (19:04). The highest ash plume of the month rose to 1,800 m above sea level at 1506 on 1 May . . . . Two explosions on 30 May caused no damage. Explosive activity has increased since then, with frequent explosions in June.

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

Information Contacts: JMA.

During May, Crater C continued its continuous emission of gases, lava flows, and sporadic Strombolian-style eruptions. The lava flows that began to exit in late December (1993) and late April (1994) both continued to move, but some of the smaller lobes had stopped. Though not erupting, Crater D maintained fumarolic activity.

During May, Strombolian eruptions remained low in number and magnitude. As in April, erupted ash reached 100-200 m above Crater C, but no explosive noises were evident ("mute" events). In late June, ICE geologists saw an average of one eruption every half hour, ejecting ash plumes up to 1,200 m above the crater.

During May, the OVSICORI seismic station ("VACR," located 2.7 km NE of the main crater) registered 831 events with frequencies of 1.7-2.3 Hz; the majority of these were associated with eruption of gas and pyroclastics (figure 69). The number of hours of harmonic tremor received for the month was relatively low (a total of 29 hours, figure 69c). Several peaks and troughs in seismic activity took place during the course of the month (figure 69c). The greatest duration of tremor took place around the 12th, when seismicity was moderate to low. A comparison with May data collected at the ICE seismic station ("La Fortuna," 3.5 km E of Crater C) shows good agreement in terms of seismic events and tremor near the middle of the month, but less agreement early and late in the month.

Figure 69. Seismicity and duration of tremor at Arenal, as follows: (a and b) monthly summary for January-May 1994, (c and d) daily summary for May 1994. Courtesy of OVSICORI.

During April and May, surveys of both a W-flank trigonometric-leveling line and the distance-measurement network showed no significant changes.

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

Information Contacts: G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, Univ de Costa Rica; E. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI.

Activity at [Crater 1] has been moderate since an explosion on 20 February 1993 ejected scoriae 100 m above the vent. During the daily rim visit on 2 May 1994, mud ejection was observed for the first time since 10 June 1993. However, the crater floor has been covered by water and frequent water ejections have been observed. Continuous tremor was registered at a seismic station 800 m W of the crater. Average amplitude of continuous tremor had been 0.2 µm through May, but on 7-9 June the average amplitude suddenly increased to >6 µm.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

A vigorous steam plume was observed by pilots on 29 April and by AVO observers on 10 May. No ash was observed on 10 May either in the plume or on the flanks of the volcano. A single ash burst on 25 May generated a plume that rose to ~10.5 km altitude according to two pilot reports between 1700 and 1800 in the afternoon. The plume was described as dark gray and moderately dense by one pilot. Weather clouds obscured the view from satellites immediately following the eruption, but NWS satellite imagery later showed a small volcanic cloud drifting NE over the Bering Sea at ~5 km altitude. Apparently the activity consisted of a single burst without a sustained eruption; no additional eruptive activity was reported through mid-June.

Geologic Background. The beautifully symmetrical Mount Cleveland stratovolcano is situated at the western end of the uninhabited Chuginadak Island. It lies SE across Carlisle Pass strait from Carlisle volcano and NE across Chuginadak Pass strait from Herbert volcano. Joined to the rest of Chuginadak Island by a low isthmus, Cleveland is the highest of the Islands of the Four Mountains group and is one of the most active of the Aleutian Islands. The native name, Chuginadak, refers to the Aleut goddess of fire, who was thought to reside on the volcano. Numerous large lava flows descend the steep-sided flanks. It is possible that some 18th-to-19th century eruptions attributed to Carlisle should be ascribed to Cleveland (Miller et al., 1998). In 1944 it produced the only known fatality from an Aleutian eruption. Recent eruptions have been characterized by short-lived explosive ash emissions, at times accompanied by lava fountaining and lava flows down the flanks.

Information Contacts: AVO; J. Lynch, SAB.

Activity remained at low levels through April and May, similar to January-March of this year. Seismicity was characterized by small-magnitude "butterfly-type" events near the active cone, principally shallow earthquakes associated with rock fractures and fluid movement. It is possible that this activity is influenced by the gravitational field associated with tides (lunar-solar attraction) and by external agents such as rain. Sporadic long-period events are associated with fluid movement, and high-frequency events are associated with rock fractures.

Shallow "butterfly-type" earthquakes were frequent until mid-April, then decreased during May to an average of <10 earthquakes/day toward the middle of the month. High-frequency earthquakes reached a maximum of 3/day and were located mainly 3-4 km W and N of the summit at depths of 2-7 km. On 12 May, one of these earthquakes (M 1.9), was felt in Jenoy, 8 km N of the volcano. Five small-magnitude "screw-type" events were registered from 1 to 12 May. A tremor pulse on 27 May that lasted for ~15 minutes was possibly caused by magma-water interaction; it occurred during a time of strong rains in the region.

Electronic tiltmeters installed on the volcanic structure did not register any deformation in April or May. The SO₂ measurements taken from the gas column during April revealed continued low emission levels. COSPEC measurements of SO₂ in May were also low, with a variation of 50-798 t/d. Most fumarolic activity was toward the W side of the main crater.

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

Information Contacts: INGEOMINAS, Pasto.

Following its May 1993 eruption . . . activity remained high. An explosion in January 1994 at the main crater produced a dark ash cloud 750-1,000 m tall. Small gas explosions were common during February 1994, they often rose 200-400 m above the crater. One or more ash eruptions took place 25-27 March, dusting the village of Rua on the volcano's eastern slopes with thin ash.

Tectonic earthquakes were numerous, especially following the Halmahera earthquake of 21 January, 1994. Prior to the earthquake there were typically 10-25 events/day, following it there were 40 events/day. Volcanic earthquakes remained at normal levels, 3-5 events/day.

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

Information Contacts: W. Tjetjep, VSI; BOM Darwin, Australia; S. Matthews, Univ of Bristol; UPI; Antara News Agency.

. . . earthquake-triggered mudflows swept down steep-walled valleys engulfing multiple villages and settlements (figure 1). The M 6.4 earthquake . . . took place at 1547 on 6 June, apparently falling along the Cauca Romeral fault. It disturbed a wide area, causing minor structural damage in Bogota, but more significant damage to 10 buildings in Cali (100 km W of the epicenter; see inset, figure 1). Near the epicenter, located 10-30 km W of the volcano, the earthquake destroyed at least 40 homes. The most catastrophic damage caused by the earthquake took place when Nevado del Huila released gravitationally unstable rock, snow, and ice down the volcano's slopes. These mudflows are the main focus of the rest of this report.

Figure 1. A 500-m contour interval topographic map (map coordinates approximate) of the Paez river basin, the primary drainage from Nevado del Huila. The map shows villages (large dots), roads (heavy lines), and rivers (broken lines). The index map of SW Colombia shows the epicenter, large rivers, and the chain of active volcanoes (solid triangles) along the Andes as far south as the international border (heavy broken line). After Cepeda (1989).

A . . . topographic map from a published hazard study (Cepeda, 1989) shows the rugged local geography (figure 1, note the contour interval, 500 m). The study also contains a second map that outlines areas of likely risk from lava flows and mudflows. To avoid confusion with the actual event we have omitted this second map, however, it shows the mudflows along drainages down the mountain continuing toward the SSE into the channel of the Paez river. The region of mudflow risk extends all the way to the map's margin near Paical (in the SE corner). Available information suggests the mudflows did basically follow the Paez river as anticipated.

According to a 9 June Reuters news report, "Graphic video images shot by a tourist . . . captured the moment when the huge brown-grey mass of mud roared down the valley, sweeping away trees, rocks, and houses in its path." According to witnesses, the mudflow reached 30-m high. In the wake of the mudflow, access to the area was cut off. Roads and bridges were damaged or blocked by mud, necessitating the use of helicopters. News reports repeatedly cited damage and casualties in the villages of Irlanda, Toez, Talaga, and Paez Belalcazar (figure 1).

A 7 June, UPI report quoted the archbishop of Paez Belalcazar, Jorge Garcia. On a flight over the area, he observed that the village of Toez had been "buried in mud," and "only the roof of the school can be seen." The same news report noted "There were no immediate reports of how many Toez residents managed to escape before the village was smothered, although some 500 people were thought to have been buried." The news report also related that in Paez Belalcazar ". . . 12 people were washed away by the rushing waters."

Overall, the number affected by the widely felt earthquake and the more restricted mudflows was estimated at 50,000. In terms of the mudflows alone, fatality estimates ranged from 253 to over 1,200 people. About 250 people, including many severely injured children, were evacuated by helicopter to hospitals in the provincial capital Neiva. Some 2,500 survivors were brought by helicopters to tent camps in La Plata.

A 6 June Reuters news report told of people hearing a "strong explosion" leading to initial confusion about whether the mudflows were triggered by an eruption or seismic loading. It was reported that geologists monitoring the volcano suggested the explosion may have come from an avalanche in the area.

Problems apparently went beyond the damage from the initial mudflows and subsequent limited access. For example, the 6 June news report stated that at one point: ". . . the river burst through a natural dam created by a mud and rock slide caused earlier by the quake." Other reports cited aftershocks and heavy rains contributing to ground instability, conditions that in some cases injured both survivors and rescue workers.

Reference. Cepeda, H., 1989, Catálogo de los volcanes activos de Colombia: Bol. Geol., v. 30, no. 3.

Geologic Background. Nevado del Huila, the highest peak in the Colombian Andes, is an elongated N-S-trending volcanic chain mantled by a glacier icecap. The andesitic-dacitic volcano was constructed within a 10-km-wide caldera. Volcanism at Nevado del Huila has produced six volcanic cones whose ages in general migrated from south to north. The high point of the complex is Pico Central. Two glacier-free lava domes lie at the southern end of the volcanic complex. The first historical activity was an explosive eruption in the mid-16th century. Long-term, persistent steam columns had risen from Pico Central prior to the next eruption in 2007, when explosive activity was accompanied by damaging mudflows.

Information Contacts: T. Casadevall, USGS; UPI; Reuters.

Phreatic eruptions in July 1993 were preceded by increasing seismicity, but caused no damage. The following report, summarized from . . . VSI (1993a and b), provides additional details about this activity.

The number of volcanic earthquakes started to increase at the end of June 1993. Continuous tremor recorded on 21 June had a maximum amplitude of 0.5-2 mm. The next day, 37 shallow volcanic earthquakes were detected. Tremor amplitude gradually increased from 23 to 30 June. On 26 June, 4 deep volcanic earthquakes occurred. The number of volcanic earthquakes increased until 1 July when a gradual decrease began. However, by 1 July the maximum tremor amplitude was 7-10 mm. Because of the seismic activity, a warning was issued to the local population, to tourists, and to workers at the sulfur mine, saying that the area around the crater was closed.

Water temperature in the crater lake on 2 July was normal (36°C). The lake water was a pale green color, and the surface was covered by dense white vapor to a height of 10 m. Yellowish white vapor was being emitted from the solfatara field, and a very strong sulfur odor could be smelled.

A phreatic eruption at 0845 on 3 July from the center of the crater lake was accompanied by loud eruption sounds. The cloud released from the lake was 10-15 m high and 60-80 m in diameter. Lake water became brownish green, and the surface was dark. Two more phreatic eruptions the next morning (at 0835 and 1045) were smaller than the first; the early morning cloud rose 8-10 m, and no sounds were heard during the second of the 4 July eruptions. Rockfalls occurred at 1000 on 5 July from the S inner crater wall. A rumbling noise indicative of another phreatic eruption was heard at 0215 on 7 July at the sulfur weighing station, ~750 m from the crater.

During the period from 8 to 31 July, seismicity was variable, but there were no phreatic eruptions. Maximum tremor amplitude decreased to 0.5-4 mm. The number of deep volcanic earthquakes fluctuated in the 1-13 events/day range while shallow volcanic earthquakes occurred at a rate of 3-22/day. The temperature of water in the crater lake rose from 39 to 40°C.

Two phreatic eruptions occurred on 1 August starting at 1635; the sound could be heard at the sulfur weighing station. These eruptions were preceded by a tectonic earthquake with an amplitude >46 mm. There were no reports of injuries during any of the phreatic eruptions in July or August.

Seismic activity gradually decreased during 2-21 August when 0-2 deep and 5-23 shallow volcanic earthquakes were recorded each day. Crater lake water temperature through most of August was 39-41°C, and the pH was 1. Maximum tremor amplitude was 1-6 mm until 22 August when tremor was no longer continuous and maximum amplitude decreased to 1 mm. Between 22 August and 9 September deep volcanic earthquakes were recorded at a rate of 1-2/day; shallow events varied from 2 to 17/day. By 10 September, seismic data and visual observations indicated that the volcano had returned to a "normal" level of activity.

References. Volcanological Survey of Indonesia, 1993a, Ijen Volcano: Journal of Volcanic Activity in Indonesia, v. 1, no. 1/2, p. 14.

Volcanological Survey of Indonesia, 1993b, Ijen Volcano: Journal of Volcanic Activity in Indonesia, v. 1, no. 3/4, p. 8-12.

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: W. Tjetjep, VSI; BOM Darwin, Australia; S. Matthews, Univ of Bristol; UPI; ANS.

An ICE report for May stated that fumarolic activity continued in the bottom of the main crater. The warm grass-green-colored lake remained at the same level as in January and March. Water temperature was in the range 20-24.5°C (temperature of the inner lake, 21.4°C), and the minimum pH was 5.5. Fumarole temperatures reached as high as 86°C, and subaqueous fumarolic activity, which involved mainly CO₂, maintained the same vigor as seen in January and March. Fumarolic activity on the NW flank was unchanged. In May, the OVSICORI deformation network did not register significant changes.

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

Information Contacts: G. Soto, Guillermo E. Alvarado, and Francisco (Chico) Arias, ICE; Héctor (Chopo) Flores, Escuela Centroamericana de Geologia, Univ de Costa Rica; E. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI.

Low-level steam and ash emissions continued through late May and the first half of June, although poor weather frequently prevented observations. On several occasions in late May a vigorous steam plume was observed rising through scattered clouds above the volcano. Observers in Adak . . . saw a steam plume over the volcano on 31 May and a gray plume rising 1,000-1,200 m on 9 June. Aerial photographs of the summit area taken by U.S. Navy personnel in late January show that the vent system extends beyond the summit onto the upper W flank, corroborating reports by ground observers during the last several months.

Geologic Background. Symmetrical Kanaga stratovolcano is situated within the Kanaton caldera at the northern tip of Kanaga Island. The caldera rim forms a 760-m-high arcuate ridge south and east of Kanaga; a lake occupies part of the SE caldera floor. The volume of subaerial dacitic tuff is smaller than would typically be associated with caldera collapse, and deposits of a massive submarine debris avalanche associated with edifice collapse extend nearly 30 km to the NNW. Several fresh lava flows from historical or late prehistorical time descend the flanks of Kanaga, in some cases to the sea. Historical eruptions, most of which are poorly documented, have been recorded since 1763. Kanaga is also noted petrologically for ultramafic inclusions within an outcrop of alkaline basalt SW of the volcano. Fumarolic activity occurs in a circular, 200-m-wide, 60-m-deep summit crater and produces vapor plumes sometimes seen on clear days from Adak, 50 km to the east.

Information Contacts: AVO.

"The . . . eruption continued this month with lava entering the ocean along a 500-m-long front between the Kamoamoa and Lae Apuki areas in Hawaii Volcanoes National Park. Explosive activity was reported on 8 May, and continued with increased vigor through the end of the month. Some littoral explosions threw incandescent lava as high as 50 m in the air, and detonations could be heard from the highway (>500 m away). Large cracks were observed running parallel to the pre-April shoreline. Surface flows were rare during May. The Pu`u `O`o lava pond was active and its surface was 79-88 m below the crater rim.

"On 3 June a large channelized aa flow broke out of the lava tube at the 125-m elevation and advanced down to the coastal plain. Within a day, all break-outs from this flow were pahoehoe. The flow spread out on the coastal flats and was within 500 m of the shoreline by 6 June. More skylights opened at 150 m elevation."

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

Information Contacts: T. Mattox, HVO.

Both craters at Langila continued at a low activity level in May. Emissions from Crater 2 consisted of weak-to-moderate white-gray vapour and ash clouds. Occasional forceful ejections of thick, dark-grey ash columns accompanied by explosion noises were reported on the 2nd, 7th, 9th, 20th, 29th, and 31st. Fine ashfall was reported on the 2nd and 20th on the NW side of the volcano. A steady weak red glow was visible on the 5th. Crater 3 released thin white vapour with very low ash content accompanied by thin blue vapor. Seismic activity was at a low level at the beginning of the month. No seismic recording was achieved after the 3rd because of equipment failure."

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: I. Itikarai and C. McKee, RVO.

. . .The eruption produced Strombolian-fed, partially subglacial lava flows. The resulting meltwater caused lahars and chocolate-colored floods (figure 5). On 17 May Llaima also produced a column composed of ash, gas, and steam that reached ~ 4,000-5,000 m above its summit. Tephra fell over a 300-km-long, cigar-shaped zone trending about ESE (figure 6); it fell mainly on 17 May but limited falls also took place on 18 and 19 May.

Figure 5. Annotated sketch map of the area near Llaima on 21 May 1994. Contour intervals are 50 m (but note that in some snow-and ice-covered areas intermediate contours are missing due to data-transmission problems). The map emphasizes lava and subsequent lahars produced when lava melted glacial ice. The extent and path of the subglacial lava flow are incompletely known. Courtesy of H. Moreno.

Figure 6. Isopach and isopleth map of the Llaima tephra falls of 17-19 May 1994. Values given are in units of millimeters with thicknesses shown first, and grain-size diameters in parentheses. Courtesy of H. Moreno.

Observations prior to eruption. Hugo Moreno compiled the following list of pre-eruptive observations. In July 1993 after a long rainstorm, Conguillío lake, located on the NE foot of Llaima (figure 6), rose ~ 10 m above its typical seasonal height. It stayed at this elevated height until at least late-December. The magnitude and duration of the lake level rise were unprecedented since 1957, the year of the last big eruption that brought lava to the surface.

In November 1993, rangers of Conguillío national park reported underground rumbling on the N foot of the volcano (Captrén). A video taken from a small aircraft on 25 December showed that the crater area lacked many visible fumaroles. Specifically, the main crater, which was covered by ice, only hosted a very weak fumarole on its SW side. Llaima typically exhibits more vigorous fumaroles; their absence was an anomaly.

A seismic survey (14-17 February, ~10 km E of Llaima at Verde lake: figure 6) found seismic events had an average frequency of about 1.0 Hz, a typical result for Llaima (e.g., 1.2-1.4 Hz in September 1992, 17:8). During 16-17 February a 2-fold increase in the number of events took place, from 90 to 180 events. The events were interpreted as due to magma degassing. A later seismic survey from the same area, 8-10 March, recorded 150-160 events/day with average frequencies in the range 1.0-2.4 Hz. On 22 March a portable seismic station on the W slope of the volcano (Los Paraguas) recorded events reaching still higher average frequency (1.6-3.0 Hz). The consistent increase of the average frequency since February was interpreted as due to slow ascent of magma along the volcano's main conduit.

H. Moreno and S. Barrientos conducted precise leveling, dry tilt, and electronic distance meter (EDM) measurements during 24 February-1 March on the volcano's E flank. Again, except for weak fumes on the SW rim, no fumaroles were seen coming from the main crater. The S summit area ("Pichillaima," figure 5) displayed many small fumaroles; these have progressively increased since 1984.

17-19 May eruptions. The first report of an eruption came from the Melipeuco Police Station, located ~20 km S of the volcano (figure 6), where at 0500-0600 on 17 May observers saw explosions above the main crater. At about 0600 they watched a dense column of ash, gas, and steam issue from the crater; a strong wind dispersed these products toward the ESE.

Between 0900 and 1000 three Chilean domestic (LAN) flights reported the ash column rising 4-5 km above the summit. Ultimately, the plume disrupted several other commercial flights, especially in Argentina.

Between 1100 and 1530 a Chilean Air Force helicopter carried observers to the erupting snow- and ice-capped stratovolcano's W and N sides. On the SSW side of the main crater the aerial observers saw at least four lava fountains escaping from a fissure. The areas covered by spatter from these fountains are shown on figure 5. The fissure was ~500-m long, trending N10°E; it vented small explosions at intervals of ~3 seconds. Lava fountains reached up to ~ 200 m high and joined a lava flow that ran under the adjacent glacier to the W. Llaima's western glacier is significant. Prior to the eruption it had an area of ~ 17.2 km² and a liquid-phase volume of ~ 367 x 10⁶ m³. Along the fissure the ice underwent rapid, violent melting and vaporization. Many explosions penetrated through the ice.

Aerial observers noted that downslope of the eruption fissure the W glacier discharged steam and explosions. These exhalations indicated that the lava continued some distance beneath the glacial ice, apparently turning toward the W and entering the alpine reaches of either the Lanlan or Calbuco rivers, or both (figure 5). The lava's subglacial path became more apparent later, on 21 May, when the volcano next became visible from the ground. The main crater rim then contained a small notch on its SSW side. The notch held an "ice channel" with a pronounced westerly bend (figure 5). On 21 May, the channel's width varied from about 50 m above the bend to 150 m below it.

On 17 May the invading lava melted sufficient glacial ice so that at about 1200 a lahar was identified moving down the Calbuco river (figure 5). Downstream at a village off the W edge of figure 5 (El Danubio, ~16 km WSW of the summit), the lahar passed at about 1245-0100 carrying trees, sediments, ice blocks, and boulders up to 9 m in diameter. Within a deep gully the lahar reached 35-m wide, 19-m high, and its volume was estimated as 2.5 x 10⁶ m³.

After the lahar reached the Quipe river (~25 km W of Llaima's summit) it advanced as a chocolate-colored flood. At about this point observers in the helicopter flew to the town of Vilcún (43 km W of Llaima's summit), landed on a small bridge, and alerted residents of the advancing floodwaters. The floodwaters arrived at 1515; subsequently the river rose 4.3 m and widened from 32 to 61 m. Estimated water velocity was 13-14 km/hr. During the interval 1630-1700, observers at El Danubio noted the passage of a second flood. In addition to stranding and killing thousands of fish, the lahar and associated flooding nearly covered a cemetery, cut across roads, and destroyed five bridges across the Rio Calbuco; 59 people were rescued from its path.

Observers near the volcano on 17 May saw the ash column blow toward the ESE in the region below about 5 km elevation. During the interval from 0800-1230 ash affected the area immediately adjacent Llaima's E and SE flanks (the Trufultruful river-Verde lake area). During 1000-1330, peaking at 1300-1330, ash fall increased in the area along the ash-distribution axis near the E border of Chile (the Icalma-Cruzaco area). The ash column contained both ash- and water-rich zones.

At 2000 on 18 May, a new, coarser ash fell for several minutes on Cruzaco (~46 km SSE of Llaima). Cruzaco again received ash for the last time on 19 May at 1200; this time it was very fine. Ash samples collected in Cruzaco contained 0.1-4 mm diameter grains of black and reddish-colored scoria with phenocrysts of plagioclase, olivine, and magnetite. Some samples were also taken of water and Coirón grasses that feed livestock, in order to make sulfide, chloride, and other chemical analyses.

Seismic and satellite data. Abnormally high seismicity occurred after the eruption until at least 14 June when monitoring ceased. During this interval, increased seismicity took place on 31 May-1 June, coincident with loud subterranean noises reported from 20 km S at Melipeuco, and summit incandescence seen from 24 km W at Cherquenco.

During the nights of 13-19 June, subterranean rumblings were heard by Pablo Parra of the Hosteria Hue-Telen (Melipeuco) when he was at Verde Lake (figure 6). The rumblings lacked associated smoke-puffs or incandescence. He also reported that although clouds and rain generally shrouded the summit in mid-June, on either 14 or 15 June clear weather revealed a gray-white plume ("normal" for the volcano) changing to a dark-gray plume (distinctly different from "normal"). Parra also noted that Pichillaima exhibited a recent slump on its SE side. He thought the slump was reminiscent of the one seen prior to the explosive 1957 eruption, and he recalled how he and area residents heard similar rumblings for several years prior to that eruption.

Satellite data of Llaima includes GOES-E images collected between 17 and 23 May, excepting 19 May. Steve Matthews, Kath Walley, and Robin Sharphouse have stored the GOES-E images in PDF and TIFF computer format.

The first GOES-E image, at 0230, shortly before the eruption, shows no eruption plume. Plume-like reflectors were observed on the E side of the Chile-Argentina border as follows: (a) on 18 May at 0926, (b) on 20 May at 0926 and 1430, and (c) on 22 May at 1430. On other days cloud cover obscured the area.

The GOES-E image for 18 May contains a small, compact reflector ~100 km E of the volcano. The two 20 May images depict an elongate, plume-like reflector extending from the border directly east of the volcano for ~ 150 km in a SE direction. On the 22 May image a similar feature extends from the border for ~150 km in a NE direction. In all cases these features were more intense than nearby clouds and may represent the ash plume.

Other remarks. The 17 May eruption was ranked by Hugo Moreno as VEI 2 with a strong phreatic component. The exact extent of the subglacial lava flow remains uncertain. The eruption caused no reported casualties.

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

Information Contacts: H. Moreno¹, G. Fuentealba², M. Murillo², M. Petit-Breuilh², J. Cayupi², and P. Peña², SERNAGEOMIN, Temuco, Chile; A. Rivera, Univ de Chile, Santiago; D. Lescinsky, Arizona State University; S. Mathews, Univ of Bristol, U.K.; K. Walley and R. Sharphouse, Ulverston Victoria High School, U.K.

During May, activity . . . remained low. Crater emissions consisted of thin white vapor released at weak to moderate rates. Throughout the month seismic activity remained at low to moderate inter-eruptive levels. Tilt, measured in the water-tube tiltmeter . . . , remained stable.

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

Information Contacts: I. Itikarai, and C. McKee, RVO.

A fire of unknown origin burned 10 m² of accumulated sulfur deposits in the Soufriere Sulphur Springs area (~700 m SSW of the summit), causing false eruption reports. The alleged eruption was reported by residents to have started on 24 April with the formation of small lava flows. Authorities in the capital of Roseau passed the information to the Seismic Research Unit in Trinidad. A team was sent to investigate the report on 27 April. No local seismic activity was detected at the permanent seismographic station, located 1.5 km away, or by the portable seismometer installed at the site during the visit.

Geologic Background. The Morne Plat Pays volcanic complex occupies the southern tip of the island of Dominica and has been active throughout the Holocene. An arcuate caldera that formed about 39,000 years ago as a result of a major explosive eruption and flank collapse is open to Soufrière Bay on the west. This depression cuts the SW side of Morne Plat Pays stratovolcano and extends to the southern tip of Dominica. At least a dozen small post-caldera lava domes were emplaced within and outside this depression, including one submarine dome south of Scotts Head. The latest dated eruptions occurred from the Morne Patates lava dome about 1270 CE, although younger deposits have not yet been dated. The complex is the site of extensive fumarolic activity, and at least ten swarms of small-magnitude earthquakes, none associated with eruptive activity, have occurred since 1765 at Morne Patates.

Information Contacts: W. Ambeh, L. Lynch, and R. Robertson, UWI.

In May, gases from the shrunken and nearly dry lake, Laguna Caliente, continued to present an environmental problem. Dry weather and persistent eruptive activity led to a decrease in the level of both the lake and surrounding groundwater. The retreat of the lake had reached the point that it appeared nearly dry in March, but fumarolic degassing persisted from a number of locations on the crater floor (figure 48). In the absence of abundant water, volcanic gases vented more directly into the atmosphere, causing fumaroles to degas more vigorously and sometimes even to resemble low-energy explosions.

Figure 48. The active crater at Poás in late May 1994. Original sketch provided by G.J. Soto of ICE.

Volcanic gas concentrations have risen in the area adjacent to the National Park (SE, S, SW, and W of the main crater); residents in its vicinity have reported a "strong sulfur smell." These odors forced the Park to close on 26-27 May and at least once in June. They were particularly strong at dawn, and some emissions had yellow and bluish colors. Acidic rainfall also increased such that economic losses since 1988 were on the order of several million dollars (US). Areas of loss encompassed timber, crops, machinery, grazing land, livestock, habitations, and human health. Health complaints have included nausea and coughing, and irritated throat, eyes, and skin.

In contrast, the fumaroles located on the S part of the crater toward the dome appeared comparatively unchanged. They had stable temperatures (89°C) and continued to emit steam-rich components.

ICE reported that microseismicity at Poás has mainly consisted of low-frequency events located beneath the crater lake. From last January through May 1994 the microseismicity has doubled.

OVSICORI reported that during May, station POA2 (located 2.5 km SW of the active crater) registered a total of 5,228 low-frequency events (figure 49). POA2 registered medium-frequency events (99), and high-frequency events (9). POA2 also registered continuous low-frequency tremor with peak-to-peak amplitude slightly under 3 mm, at times reaching 5 mm. The tremor signal was strong in the frequency range 2.0-3.2 Hz (figure 50). The highest seismicity took place on 25 and 31 May, the lowest, 15 May, a day that still received continuous tremor.

Figure 49. Poás seismicity for January-May 1994. Courtesy of OVSICORI.

Figure 50. Poás tremor beginning at 1343 GMT, 16 May 1994 (top) and spectral analysis of the tremor (bottom). Amplitudes are arbitrary. Courtesy of OVSICORI.

Compared with the month of April, low-frequency seismicity decreased 13%, medium-frequency increased 76%, and the high-frequency remained about the same. In May, the number of hours of tremor increased—coincident with the above mentioned rise in the vigor of fumarolic activity. On 18 May a M 2.5 earthquake took place at a depth of 15 km centered 3.3 km NE of the active crater. During April and May there was no significant deviation in deformation.

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

Information Contacts: G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, UCR; E. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI.

Cordón Caulle began generating a series of small to moderate felt earthquakes and discontinuous subterranean noises during the final week of May. The Univ of Chile and the Univ of the Frontera monitored the activity with two seismometers on 28 and 29 May. They detected harmonic tremor and small earthquakes centered N of Puyehue, generally located on Cordón Caulle. Santiago radio reported that four tremors were felt in the area over a 12-hour period on the night of 29 May. The tremors shook with Mercalli-scale intensity IV and V.

The radio report said that the activity had also drawn a team of professionals from the Geosciences Institute of Valdivia Austral Univ to the area. Meanwhile, the police, army officers, civil authorities, and scientists had formed an emergency action committee and established a "White Alert," which signifies the detection of possibly abnormal volcanic activity and mandates that emergency plans be reviewed and updated.

Geologic Background. The Puyehue-Cordón Caulle volcanic complex (PCCVC) is a large NW-SE-trending late-Pleistocene to Holocene basaltic-to-rhyolitic transverse volcanic chain SE of Lago Ranco. The 1799-m-high Pleistocene Cordillera Nevada caldera lies at the NW end, separated from Puyehue stratovolcano at the SE end by the Cordón Caulle fissure complex. The Pleistocene Mencheca volcano with Holocene flank cones lies NE of Puyehue. The basaltic-to-rhyolitic Puyehue volcano is the most geochemically diverse of the PCCVC. The flat-topped, 2236-m-high volcano was constructed above a 5-km-wide caldera and is capped by a 2.4-km-wide Holocene summit caldera. Lava flows and domes of mostly rhyolitic composition are found on the E flank. Historical eruptions originally attributed to Puyehue, including major eruptions in 1921-22 and 1960, are now known to be from the Cordón Caulle rift zone. The Cordón Caulle geothermal area, occupying a 6 x 13 km wide volcano-tectonic depression, is the largest active geothermal area of the southern Andes volcanic zone.

Information Contacts: N. Banks, US Embassy, Santiago.

"During May, 694 earthquakes were detected, compared to 397 in April and 458 in March. Of these, 51 earthquakes were located, 28 with errors <1 km.

"Seismic activity was low until 25 May; it consisted of small swarms and discrete events. On 25 May, Rabaul was subjected to its strongest seismic activity in about a year. Starting at 1043, earthquakes were felt for ~20 minutes. The maximum felt intensity was in the airport region, IV-V on the modified Mercalli scale. Two spatially separated swarms were involved. The first, including an M_L 3.3 earthquake, was located in a linear zone between the airport region and Vulcan. The second swarm, which included an M_L 3.0 earthquake, started ~15 minutes after the first. The second swarm was located just off the E shore of Vulcan and Vulcan Island, near the site of swarm activity in February and April (19:2-3). Both swarms were shallow (< 2 km), consistent with previous activity in these areas. Seismic activity at both centers continued throughout the rest of the day at a declining rate.

"For the rest of the month, seismic activity consisted of small and discrete events, probably located in the same region as the large swarms on the 25th. On the 26th there were two earthquakes just off the SW shore of Matupit Island, at depths around 2.2 km. These locations are not on the ring fault system.

"At 0212 on 26 May, a low-frequency earthquake was recorded on the harbor network. The signal had dominant frequencies around 1 Hz and probably originated near Matupit Island. There may have been as many as 10 similar events in the 24-hour period following the felt earthquakes.

"Routine leveling on 27 May showed that about 35-40 mm of uplift had taken place at the S end of Matupit Island since . . . 2 May. Additional leveling to Vulcan Point on 30 May showed an uplift of ~30 mm since September 1993."

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: I. Itikarai and C. McKee, RVO.

In May a glow was noticed on the crater floor of Barujari cone, which has undergone no significant activity since August 1966. A portable seismograph (PS-2) and telemetry seismograph (Teledyne) were put into operation on 27 May and 9 June, respectively. One volcanic earthquake event/day was recorded on 27, 28, 30, and 31 May. After 4 June, however, volcanic tremor with a maximum amplitude of 35 mm was recorded, presumably associated with the upward movement of magma.

At 0200 on 3 June, Barujari cone began erupting by sending an ash plume 500 m high. One 8 June press report described emission of "smoldering lava" and "thick smoke," as well as ashfall in nearby villages from an ash cloud rising 1,500 m above the summit. Between 3 and 10 June, up to 172 explosions could be heard each day from the Sembalun Lawang volcano observatory (~15 km NE). During this period, seismic data indicated a dramatic increase in the number of explosions per day, from 68 to 18,720 (figure 2). Eruptions were continuous at least through 19 June, with maximum ash plume heights of 2,000 m on 9-11 June (figure 2).

Figure 2. Daily number of explosion earthquakes and height of the ash plume at Rinjani, 3-19 June 1994. Courtesy of VSI.

The ash plume generally drifted SE, depositing up to 30 mm of ash on the island (figure 3). Strombolian eruptions ejected pyroclastic material <2 m in size as high as 600 m above the vent; this material fell in a restricted proximal area around the cone and in the lake. Lava flows began on 8 June and partially covered previous lava flows from Rombongan (in 1944) and Barujari (in 1966) (figure 4).

Figure 3. Distribution of ash from Rinjani eruptions, 3-19 June 1994. Courtesy of VSI.

Figure 4. Segara Anak caldera lake at Rinjani showing lava flows from the Rombongan dome (1944), Barujari cone (1966), and the recent lava flows of June 1994 (slash pattern). Courtesy of VSI.

A series of aircraft warnings based on pilot reports and weather satellite images indicated much larger plumes than suggested by the ground observations. First, an eruption at about 1200 on 7 June produced a long plume that caused a large number of aviation warnings. The plume, located on satellite imagery, extended 120 km S of Rinjani and was beginning to disperse by 1530. A pilot report at 1645 on 7 June indicated a "smoke" plume to 13.5 km altitude moving ESE, but by 2345 the plume was indistinguishable on satellite imagery. The imagery showed a plume around 0633 on 8 June, which extended at least 83 km SE of the volcano. Aircraft were advised to avoid this area to an altitude of 10.5 km (35,000 feet).

Second, at 1645 on 9 June a cloud with volcanic ash was evident on satellite imagery within 56 km of the volcano rising up to an altitude of 4.5 km (15,000 feet). The plume was apparently not elongated on the image but the report stated: "Expect cloud to drift W."

In apparent conflict with ground observations and satellite imagery observed by Australian meteorologists, a GOES satellite image at 1831 on 9 June obtained by Steve Matthews revealed a N-directed plume. This straight, distinct plume originating from Lombok Island trailed N for 800 km over SE Borneo, where it merged with a dense cloud bank. The plume widened from ~50 km across at a point 100 km N of the island to 100 km across where it met the Borneo coast.

Satellite imagery at 0830 on 10 June indicated a cloud with ash from 74 km SE to 56 km NW of the volcano to an altitude of 9 km (30,000 feet) with upper level drift to the S. Between 1700 and 2330, an ash cloud (bounded by the following corner points: 8°S, 116°E; 8°S, 117°E; 10°S, 117°E; and 12°S, 118.5°E) reached a height of almost 10 km (34,000 feet). The tongue of ash cloud previously detected drifting S was no longer evident on satellite imagery by 0600 on 11 June, but at 1940 the ash cloud was detected within an area slightly smaller than the previous day. The plume, as seen on satellite imagery at 0800 on 13 June through about 0500 on 16 June, remained over the vicinity of the island, but it exhibited some streaming to the N. At that time the plume began streaming E before drifting N. Pilot reports indicated a plume to 7.5 km (25,000 feet), with patches to 10.5 km (35,000 feet) and spreading N and NE. On 17 June, islands could be seen through the thin plume on satellite imagery. Enhanced AVHRR imagery indicated the probable presence of ash within the plume through 1300 on 18 June. Pilot reports at ~1200 on 18 June again confirmed an ash "smoke" cloud SW of the volcano for a distance of 80 km and an altitude of 10 km (34,000 feet). The plume was consistently observed on the imagery during the night of 18-19 June, but remained thin.

Geologic Background. Rinjani volcano on the island of Lombok rises to 3726 m, second in height among Indonesian volcanoes only to Sumatra's Kerinci volcano. Rinjani has a steep-sided conical profile when viewed from the east, but the west side of the compound volcano is truncated by the 6 x 8.5 km, oval-shaped Segara Anak (Samalas) caldera. The caldera formed during one of the largest Holocene eruptions globally in 1257 CE, which truncated Samalas stratovolcano. The western half of the caldera contains a 230-m-deep lake whose crescentic form results from growth of the post-caldera cone Barujari at the east end of the caldera. Historical eruptions dating back to 1847 have been restricted to Barujari cone and consist of moderate explosive activity and occasional lava flows that have entered Segara Anak lake.

Information Contacts: W. Tjetjep, VSI; BOM Darwin, Australia; S. Matthews, Univ of Bristol, UK; UPI; ANS.

Heatflow during April remained low (table 4), but evidence of convection (dark slicks from the central vent) on 6 May indicated some recent increase. Lake temperature at 20 m depth continued to decline from 47°C on 18 February to 23.6°C on 6 May. Two bursts of strong tremor, on 5 and 8 May, corresponded to a renewed steady temperature rise to 24.9°C by 11 May. As with the previous heating phase, this activity occurred several weeks after strong low-frequency acoustic signals were recorded.

Table 4. Temperature, outflow measurements, and water analyses from the crater lake of Ruapehu, 18 January 1994 to 27 August 1994. Discharge of "0" indicates a lake level below overflow stage. A dash (--) signifies no measurement. Courtesy of IGNS.

On 18 April the lake was a uniform battleship gray color with no evidence of upwelling, although the N vents were not fully visible from the observation point. No signs of surging were seen around the shoreline or at Outlet. A dark khaki-green slick emanating from the central vent area on 6 May drifted slowly onto the SE shore, but no upwelling was observed. Broken yellow slicks originating from several weak upwelling cells in the N vent area were also present over the N half of the lake. The general color of the lake was the same as in April, and there was no sign of recent activity. Prior to the heating episode in February, the ratio of Mg to Cl in the lake water decreased slightly from 0.042 in late 1993 to 0.038 in January (table 4), due mainly to a decrease in Mg. This ratio had remained stable at least through 18 April.

Inspection of photographs taken during the reported steam eruption on 1 March revealed an apparently passive steam cloud, a common atmospheric effect at the crater lake. The rising cloud was most intense over an area of discolored water, and may have been caused by vigorous convection or a minor phreatic event shortly beforehand. This incident is a reminder that even reports from reliable eyewitnesses should be treated with caution; reports of possible eruptions in February-April should be regarded as unproven.

The only deformation change of possible volcanic significance detected on 6 May was a reversal of the 9 mm contraction of the crater width indicator line recorded between 12 and 28 March. This suggested a return to the mildly inflated level first recorded in January. It is not yet known if the evidence of minor inflation is significant. A leveling survey on 18 April indicated 21 µrad of tilt towards the crater (deflation) at the Dome location over the past year, the largest tilt since 1981. Because this follows a period of slow apparent deflation (0.7 µrad/year), the measurement may not be reliable. Southern benchmarks may have been lowered by downhill creep of a lava slab. However, large systematic apparent tilts of

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

Information Contacts: P. Otway, IGNS Wairakei.

A high-frequency earthquake swarm in mid-March and early April ended nearly two years of low activity. Significant long-period earthquakes began in mid-April. Several swarms on 19, 22, and 23 April culminated in an explosion at 1554 on the 23rd. Seismic activity gradually declined after the explosion. The Emergency Committee of Caldas declared a yellow alert and suspended visitor and tourist passes until the seismicity had decreased to acceptable levels. [INGEOMINAS stated that there was no emission of ash at the time of the 23 April earthquake swarm.]

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

Information Contacts: INGEOMINAS, Manizales; U.S. Embassy, Bogota.

A . . . small eruption at Suoh hot-spring field that expelled gas-charged hot mud [followed] a major, destructive earthquake in the same region (19:02). The earthquake, Ms 7.2, took place at 1707 GMT on 15 February, or in terms of local time and date, at 0007 on 16 February.

"We sent our team to investigate the area where the phreatic explosion occurred. The team arrived at Suoh on 19 February, three days after the earthquake. Two new mud explosion pits, 5 m in diameter, were found W of the Suoh depression. Liquifaction was consistently found at fractures associated with the earthquake. The two explosion pits contained boiling water."

Tables 1 and 2 present data on water and gas samples taken from two sites in the Suoh area during the investigation.

Table 1. Water chemical analyses for two sites in the Suoh hot-spring field (sampled 19 February 1994). Courtesy of VSI.

Table 2. Gas chemical analyses for two sites in the Suoh hot-spring field (sampled 19 February 1994). Courtesy of VSI.

Geologic Background. The 8 x 16 km Suoh (or Suwoh) depression appears to have a dominantly tectonic origin, but contains a smaller complex of overlapping calderas oriented NNE-SSW. Historically active maars and silicic domes lie along the margins of the depression, which falls along the Great Sumatran Fault that extends the length of the island. Numerous hot springs occur along faults within the depression, which contains the Pematang Bata fumarole field. Large phreatic explosions (0.2 km2 tephra) occurred at the time of a major tectonic earthquake in 1933. Very minor hydrothermal explosions produced two 5-m-wide craters at the time of a February 1994 earthquake.

Information Contacts: R. Sukhyar, VSI.

Annual fieldwork was carried out on 30 March and 29 April 1994. Maximum fumarole temperatures had fallen to 78°C by the end of April. ... There was insufficient fumarole discharge for adequate sampling, and temperatures and pressures were at the lowest levels ever recorded. Except for minor landslide debris, no significant changes were noted in the Ngauruhoe crater.

Tilt leveling surveys were carried out at the Tama Lakes (1.7 km SSW) and Mangatepopo (1.8 km NNW) locations on 30 March. Apparent tilt recorded at Tama Lakes during the previous 11 months represented 4 µrad of inflation, but was within the range of random fluctuations recorded since installation in 1978. At Mangatepopo approximately 14 µrad of tilt towards Ngauruhoe (deflation) was recorded over the same period. This is ~2-3x the past noise level resulting from normal survey errors and seasonal movements. The most likely explanation, based on earlier experiences, is that two benchmarks near a walking trail have settled.

Repairs were made to the three highest crater rim stations on 30 March and two new stations were installed; two old stations are scheduled for removal after the 1995 survey. All six rim sites were surveyed for horizontal deformation on 29 April. Measurements were made by EDM and theodolite from 2 km N on Tongariro volcano. Relative movement vectors for the 1992-94 period at three stations were well within the normal noise range. Instabilities noted at the other sites resulted from various surface movements. Overall, there was no indication of recent volcanic deformation.

Geological mapping of the crater, N flank, and SW flank accomplished during these visits is part of the ongoing mapping project of the Tongariro complex.

Geologic Background. Tongariro is a large volcanic massif, located immediately NE of Ruapehu volcano, that is composed of more than a dozen composite cones constructed over a period of 275,000 years. Vents along a NE-trending zone extending from Saddle Cone (below Ruapehu) to Te Maari crater (including vents at the present-day location of Ngauruhoe) were active during several hundred years around 10,000 years ago, producing the largest known eruptions at the Tongariro complex during the Holocene. North Crater stratovolcano is truncated by a broad, shallow crater filled by a solidified lava lake that is cut on the NW side by a small explosion crater. The youngest cone, Ngauruhoe, is also the highest peak.

Information Contacts: P. Otway, IGNS Wairakei.

The increase in the level of venting activity . . . continued into May. Throughout the month the summit crater emitted moderate to thick white vapor, although there were occasional reports of gray and blue emissions on 17 and 18 May, and towards the end of the month. On 23 May, because of the ash cloud, pilots in the region were notified to "exercise caution and to report any increase in activity including height and movement of the ash cloud." In addition, during most nights in the first three weeks of the month the crater emitted a red glow that remained weak but steady.

May seismic activity underwent a slight progressive decrease: Daily earthquake totals early in the month were in the range 400-600; by month's end they had dropped to 400. Since the end of April earthquake amplitudes also decreased.

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: I. Itikarai, and C. McKee, RVO; BOM, Darwin.

Endogenous dome growth to the W and NW . . . had ceased by the end of April. However, the dome began to grow in a SW direction in mid-May. This SW growth continued through at least mid-June at a rate of 1-2 m/day. EDM measurements taken by the GSJ revealed that a line on the N flank had shortened between January and April, implying that inflation of the entire mountain had ceased by the end of April, but the same line showed elongation in May.

Elevations of lava-dome peaks have steadily increased since the eruption began (figure 71). The highest peak in early June was 250 m above the level of the Jigokuato crater floor. Peaks were commonly formed just above the magma-supply vent during both exogenous and endogenous growth, but no lava extrusion has taken place above 1,420 m elevation.

Figure 71. Elevation of lava-dome peaks at Unzen, 20 May 1991-June 1994. The highest peak as of June 1994 is lobe 12 (L12); base elevation shown (1,250 m) is for the Jigokuato crater. Different lobes are indicated by symbols and lobe numbers. All measurements were made using a theodolite and mirror-less laser distance meter by geologists from the Joint University Research Group. Courtesy of S. Nakada.

A time plot of the eruption rate shows two pulses of magma during the current eruption (figure 72). The first pulse (May 1991-December 1992) was characterized mainly by exogenous growth. The second pulse (December 1992), which started with lobe 9, was dominated by exogenous growth early (first half of the pulse), but then changed to endogenous growth. The volume of magma erupted during the first pulse, 1.3 x 10⁸ m³, is roughly double that erupted during the second pulse (0.6 x 10⁸ m³). Total volume of the lava dome, based on analysis of aerial photos by the GSJ, was 90 x 10⁶ m³ as of 9 April. The lava extrusion rate between 7 February and 9 April was 60,000 m³/day (figure 72). The eruption rate declined in May to3/day as determined by the Joint University Research Group. No fresh lava has been extruded onto the dome surface since February.

Figure 72. Daily eruption rate at Unzen, 20 May 1991-June 1994, showing two distinct pulses of magma-supply. Eruption rates were estimated by the Joint University Research Group (JURG) using photographs from daily helicopter inspections and theodolite surveys. Only aerial photographs were used by the Geographical Survey Institute (GSI), the Public Works Research Institute (PWRI), and the Geological Survey of Japan (GSJ) to calculate the volume change of eruption products. Courtesy of S. Nakada.

Most pyroclastic flows traveled down the SW and SE flanks, only rarely did they descend N of the dome. The longest pyroclastic flow of the month went 2.5 km on 3 May. Pyroclastic flows are detected seismically at a station ~1 km WSW of the dome. Real-time monitoring of both the dome and pyroclastic flows is conducted from the Unzen Weather Station using four visible and thermal infrared video cameras. Microearthquakes beneath the dome averaged >100/day. The total of 3,171 earthquakes in May continues the decline in seismicity . . . .

The Coordination Committee for Prediction of Volcanic Eruption had a meeting on 3 June. A statement issued after the meeting noted that both the lava dome and the entire volcanic edifice were very unstable, and that pyroclastic flows generated by collapse of lava might occur despite the decline in lava extrusion. As of 31 May, 3,307 people remained evacuated.

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

Information Contacts: S. Nakada, Kyushu Univ; JMA.

Residents in Perryville . . . reported a large steam plume rising from Veniaminof on the afternoon of 20 May. Inclement weather prevented observation of any other activity during the second half of May. Residents of Port Heiden . . . who were able to see the volcano on 2 June reported that no plume was present over the summit caldera. However, they did observe a steam plume on 9 June. AVO received no pilot reports of continuing eruptive activity in early June.

Geologic Background. Veniaminof, on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3,700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: AVO.