Frequent activity at Ecuador's Sangay has included pyroclastic flows, lava flows, ash plumes, and lahars reported since 1628. Its remoteness on the east side of the Andean crest make ground observations difficult; remote cameras and satellites provide important information on activity. The current eruption began in March 2019 and continued through December 2019 with activity focused on the Cráter Central and the Ñuñurco (southeast) vent; they produced explosions with ash plumes, lava flows, and pyroclastic flows and block avalanches. In addition, volcanic debris was remobilized in the Volcan river causing significant damming downstream. This report covers ongoing similar activity from January through June 2020. Information is provided by Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), and a number of sources of remote data including the Washington Volcanic Ash Advisory Center (VAAC), the Italian MIROVA Volcano HotSpot Detection System, and Sentinel-2 satellite imagery. Visitors also provided excellent ground and drone-based images and information.

Throughout January-June 2020, multiple daily reports from the Washington Volcanic Ash Advisory Center (VAAC) indicated ash plumes rising from the summit, generally 500-1,100 m. Each month one or more plumes rose over 2,000 m. The plumes usually drifted SW or W, and ashfall was reported in communities 25-90 km away several times during January-March and again in June. In addition to explosions with ash plumes, pyroclastic flows and incandescent blocks frequently descended a large, deep ravine on the SE flank. Ash from the pyroclastic flows rose a few hundred meters and drifted away from the volcano. Incandescence was visible on clear nights at the summit and in the ravine. The MIROVA log radiative power graph showed continued moderate and high levels of thermal energy throughout the period (figure 57). Sangay also had small but persistent daily SO₂ signatures during January-June 2020 with larger pulses one or more days each month (figure 58). IG-EPN published data in June 2020 about the overall activity since May 2019, indicating increases throughout the period in seismic event frequency, SO₂ emissions, ash plume frequency, and thermal energy (figure 59).

Figure 57. This graph of log radiative power at Sangay for 18 Aug 2018 through June 2020 shows the moderate levels of thermal energy through the end of the previous eruption in late 2018 and the beginning of the current one in early 2019. Data is from Sentinel-2, courtesy of MIROVA.

Figure 58. Small but persistent daily SO2 signatures were typical of Sangay during January-June 2020. A few times each month the plume was the same or larger than the plume from Columbia’s Nevado del Ruiz, located over 800 km NE. Image dates are shown in the header over each image. Courtesy of NASA’s Global Sulfur Dioxide Monitoring Page.

Figure 59. A multi-parameter graph of activity at Sangay from May 2019 to 12 June 2020 showed increases in many types of activity. a) seismic activity (number of events per day) detected at the PUYO station (source: IG-EPN). b) SO2 emissions (tons per day) detected by the Sentinel-5P satellite sensor (TROPOMI: red squares; source: MOUNTS) and by the IG-EPN (DOAS: green bars). c) height of the ash plumes (meters above crater) detected by the GOES-16 satellite sensor (source: Washington VAAC). d) thermal emission power (megawatt) detected by the MODIS satellite sensor (source: MODVOLC) and estimate of the accumulated lava volume (million M3, thin lines represent the error range). Courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2020 - N°3, “Actualización de la actividad eruptiva”, Quito, 12 de junio del 2020).

Activity during January-March 2020. IG-EPN and the Washington VAAC reported multiple daily ash emissions throughout January 2020. Gas and ash emissions generally rose 500-1,500 m above the summit, most often drifting W or SW. Ashfall was reported on 8 January in the communities of Sevilla (90 km SSW), Pumallacta and Achupallas (60 km SW) and Cebadas (35 km WNW). On 16 January ash fell in the Chimborazo province in the communities of Atillo, Ichobamba, and Palmira (45 km W). Ash on 28 January drifted NW, with minor ashfall reported in Púngala (25 km NW) and other nearby communities. The town of Alao (20 km NW) reported on 30 January that all of the vegetation in the region was covered with fine white ash; Cebadas and Palmira also noted minor ashfall (figure 60).

Figure 60. Daily ash plumes and repeated ashfall were reported from Sangay during January 2020. Top left: 1 January 2020 (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-2, JUEVES, 2 ENERO 2020). Top right: 20 January 2020 (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-21, MARTES, 21 ENERO 2020). Bottom left: 26 January-1 February 2020 expedition (Martes, 18 Febrero 2020 12:21, EXPEDICIÓN AL VOLCÁN SANGAY). Bottom right: 30 January 2020, minor ashfall was reported in the Province of Chimborazo (#IGAlInstante Informativo VOLCÁN SANGAY No. 006, JUEVES, 30 ENERO 2020). Courtesy of IG-EPN.

A major ravine on the SE flank has been the site of ongoing block avalanches and pyroclastic flows since the latest eruption began in March 2019. The pyroclastic flows down the ravine appeared incandescent at night; during the day they created ash clouds that drifted SW. Satellite imagery recorded incandescence and dense ash from pyroclastic flows in the ravine on 7 January (figure 61). They were also reported by IG on the 9th, 13th, 26th, and 28th. Incandescent blocks were reported in the ravine several times during the month. The webcam captured images on 31 January of large incandescent blocks descending the entire length of the ravine to the base of the mountain (figure 62). Large amounts of ash and debris were remobilized as lahars during heavy rains on the 25th and 28th.

Figure 61. Sentinel-2 satellite imagery of Sangay from 7 January 2020 clearly showed a dense ash plume drifting W and ash and incandescent material from pyroclastic flows descending the SE-flank ravine. Left image uses natural color (bands 4, 3, 2) rendering and right images uses atmospheric penetration (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Figure 62. Pyroclastic flows at Sangay produced large trails of ash down the SE ravine many times during January 2020 that rose and drifted SW. Top left: 9 January (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-9, JUEVES, 9 ENERO 2020). Top right: 13 January (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-14, MARTES, 14 ENERO 2020). On clear nights, incandescent blocks of lava and pyroclastic flows were visible in the ravine. Bottom left: 16 January (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-17, VIERNES, 17 ENERO 2020). Bottom right: 31 January (#IGAlInstante Informativo VOLCÁN SANGAY No. 007, VIERNES, 31 ENERO 2020). Courtesy of IG-EPN.

Observations by visitors to the volcano during 9-17 January 2020 included pyroclastic flows, ash emissions, and incandescent debris descending the SE flank ravine during the brief periods when skies were not completely overcast (figure 63 and 64). More often there was ash-filled rain and explosions heard as far as 16 km from the volcano, along with the sounds of lahars generated from the frequent rainfall mobilizing debris from the pyroclastic flows. The confluence of the Rio Upano and Rio Volcan is 23 km SE of the summit and debris from the lahars has created a natural dam on the Rio Upano that periodically backs up water and inundates the adjacent forest (figure 65). A different expedition to Sangay during 26 January-1 February 2020 by IG personnel to repair and maintain the remote monitoring station and collect samples was successful, after which the station was once again transmitting data to IG-EPN in Quito (figure 66).

Figure 63. Hikers near Sangay during 9-17 January 2020 witnessed pyroclastic flows and incandescent explosions and debris descending the SE ravine. Left: The view from 40 km SE near Macas showed ash rising from pyroclastic flows in the SE ravine. Right: Even though the summit was shrouded with a cap cloud, incandescence from the summit crater and from pyroclastic flows on the SE flank were visible on clear nights. Courtesy of Arnold Binas, used with permission.

Figure 64. The steep ravine on the SE flank of Sangay was hundreds of meters deep in January 2020 when these drone images were taken by members of a hiking trip during 9-17 January 2020 (left). Pyroclastic flows descended the ravine often (right), coating the sides of the ravine with fine, white ash and sending ash billowing up from the surface of the flow which resulted in ashfall in adjacent communities several times. Courtesy of Arnold Binas, used with permission.

Figure 65. Debris from pyroclastic flows that descended the SE Ravine at Sangay was carried down the Volcan River (left) during frequent rains and caused repeated damming at the confluence with the Rio Upano (right), located 23 km SE of the summit. These images show the conditions along the riverbeds during 9-17 January 2020. Courtesy of Arnold Binas, used with permission.

Figure 66. An expedition by scientists from IG-EPN to one of the remote monitoring stations at Sangay during 26 January-1 February 2020 was successful in restoring communication to Quito. The remote location and constant volcanic activity makes access and maintenance a challenge. Courtesy of IG-EPN (Martes, 18 Febrero 2020 12:21, EXPEDICIÓN AL VOLCÁN SANGAY).

During February 2020, multiple daily VAAC reports of ash emissions continued (figure 67). Plumes generally rose 500-1,100 m above the summit and drifted W, although on 26 February emissions were reported to 1,770 m. Ashfall was reported in Macas (40 km SE) on 1 February, and in the communities of Pistishi (65 km SW), Chunchi (70 km SW), Pumallacta (60 k. SW), Alausí (60 km SW), Guamote (40 km WNW) and adjacent areas of the Chimborazo province on 5 February. The Ecuadorian Red Cross reported ash from Sangay in the provinces of Cañar and Azuay (60-100 km SW) on 25 February. Cebadas and Guamote reported moderate ashfall the following day. The communities of Cacha (50 km NW) and Punín (45 km NW) reported trace amounts of ashfall on 29 February. Incandescent blocks were seen on the SE flank multiples times throughout the month. A pyroclastic flow was recorded on the SE flank early on 6 February; additional pyroclastic flows were observed later that day on the SW flank. On 23 February a seismic station on the flank recorded a high-frequency signal typical of lahars.

Figure 67. Steam and ash could be seen drifting SW from the summit of Sangay on 11 February 2020 even though the summit was hidden by a large cap cloud. Ash was also visible in the ravine on the SE flank. Courtesy of Sentinel Hub Playground, natural color (bands 4, 3, 2) rendering.

A significant ash emission on 1 March 2020 was reported about 2 km above the summit, drifting SW. Multiple ash emissions continued daily during the month, generally rising 570-1,170 m high. An emission on 12 March also rose 2 km above the summit. Trace ashfall was reported in Cebadas (35 km WNW) on 12 March. The community of Huamboya, located 40 km ENE of Sangay in the province of Morona-Santiago reported ashfall on 17 March. On 19 and 21 March ashfall was seen on the surface of cars in Macas to the SE. (figure 68). Ash was also reported on the 21st in de Santa María De Tunants (Sinaí) located E of Sangay. Ash fell again in Macas on 23 March and was also reported in General Proaño (40 km SE). The wind changed direction the next day and caused ashfall on 24 March to the SW in Cuenca and Azogues (100 km SW).

Figure 68. Ashfall from Sangay was reported on cars in Huamboya on 17 March 2020 (left) and in Macas on 19 March (right). Courtesy IG-EPN, (#IGAlInstante Informativo VOLCÁN SANGAY No. 024, MARTES, 17 MARZO 2020 and #IGAlInstante Informativo VOLCÁN SANGAY No. 025, JUEVES, 19 MARZO 2020).

Incandescence from the dome at the crater and on the SE flank was noted by IG on 3, 4, and 13 March. Remobilized ash from a pyroclastic flow was reported drifting SW on 13 March. The incandescent path of the flow was still visible that evening. Numerous lahars were recorded seismically during the month, including on days 5, 6, 8, 11, 15, 30 and 31. Images from the Rio Upano on 11 March confirmed an increase from the normal flow rate (figure 69) inferred to be from volcanic debris. Morona-Santiago province officials reported on 14 March that a new dam had formed at the confluence of the Upano and Volcano rivers that decreased the flow downstream; by 16 March it had given way and flow had returned to normal levels.

Figure 69. Images from the Rio Upano on 11 March 2020 (left) confirmed an increase from the normal flow rate related to lahars from Sangay descending the Rio Volcan. By 16 March (right), the flow rate had returned to normal, although the large blocks in the river were evidence of substantial activity in the past. Courtesy of IG (#IGAlInstante Informativo VOLCÁN SANGAY No. 018, MIÉRCOLES, 11 MARZO 2020 and #IGAlInstante Informativo VOLCÁN SANGAY No. 023, LUNES, 16 MARZO 2020).

Activity during April-June 2020. Lahar activity continued during April 2020; they were reported seven times on 2, 5, 7, 11, 12, 19, and 30 April. A significant reduction in the flow of the Upano River at the entrance bridge to the city of Macas was reported 9 April, likely due to a new dam on the river upstream from where the Volcan river joins it caused by lahars related to ash emissions and pyroclastic flows (figure 70). The flow rate returned to normal the following day. Ash emissions were reported most days of the month, commonly rising 500-1,100 m above the summit and drifting W. Incandescent blocks or flows were visible on the SE flank on 4, 10, 12, 15-16, and 20-23 April (figure 71).

Figure 70. A significant reduction in the flow of the Upano River at the entrance bridge to the city of Macas was reported on 9 April 2020, likely due to a new dam upstream from lahars related to ash emissions and pyroclastic flows from Sangay. Courtesy of IG-EPN (#IGAlInstante Informativo VOLCÁN SANGAY No. 032, JUEVES, 9 ABRIL 2020).

Figure 71. Incandescent blocks rolled down the SE ravine at Sangay multiple times during April 2020, including on 4 April (left). Pyroclastic flows left two continuous incandescent trails in the ravine on 23 April (right). Courtesy of IG-EPN (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-95, SÁBADO, 4 ABRIL 2020 and INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-114, JUEVES, 23 ABRIL 2020).

Activity during May 2020 included multiple daily ash emissions that drifted W and numerous lahars from plentiful rain carrying ash and debris downstream. Although there were only a few visible observations of ash plumes due to clouds, the Washington VAAC reported plumes visible in satellite imagery throughout the month. Plumes rose 570-1,170 m above the summit most days; the highest reported rose to 2,000 m above the summit on 14 May. Two lahars occurred in the early morning on 1 May and one the next day. A lahar signal lasted for three hours on 4 May. Two lahar signals were recorded on the 7th, and three on the 9th. Lahars were also recorded on 16-17, 20-22, 26-27, and 30 May. Incandescence on the SE flank was only noted three times, but it was cloudy nearly every day.

An increase in thermal and overall eruptive activity was reported during June 2020. On 1 and 2 June the webcam captured lava flows and remobilization of the deposits on the SE flank in the early morning and late at night. Incandescence was visible multiple days each week. Lahars were reported on 4 and 5 June. The frequent daily ash emissions during June generally rose to 570-1,200 m above the summit and drifted usually SW or W. The number of explosions and ash emissions increased during the evening of 7 June. IG interpreted the seismic signals from the explosions as an indication of the rise of a new pulse of magma (figure 72). The infrasound sensor log from 8 June also recorded longer duration tremor signals that were interpreted as resulting from the descent of pyroclastic flows in the SE ravine.

Figure 72. Seismic and infrasound signals indicated increased explosive and pyroclastic flow activity at Sangay on 7-8 June 2020. Left: SAGA station (seismic component) of 7 and 8 June. The signals correspond to explosions without VT or tremor signals, suggesting the rise of a new magma pulse. Right: SAGA station infrasound sensor log from 8 June. The sharp explosion signals are followed a few minutes later (examples highlighted in red) by emergent signals of longer duration, possibly associated with the descent of pyroclastic material in the SE flank ravine. Courtesy if IG-EPN (Informe Especial del Volcán Sangay - 2020 - N°3, “Actualización de la actividad eruptiva”, Quito, 12 de junio del 2020).

On the evening of 8 June ashfall was reported in the parish of Cebadas and in the Alausí Canton to the W and SW of Sangay. There were several reports of gas and ash emissions to 1,770 m above the summit the next morning on 9 June, followed by reports of ashfall in the provinces of Guayas, Santa Elena, Los Ríos, Morona Santiago, and Chimborazo. Ashfall continued in the afternoon and was reported in Alausí, Chunchi, Guamote, and Chillanes. That night, which was clear, the webcam captured images of pyroclastic flows down the SE-flank ravine; IG attributed the increase in activity to the collapse of one or more lava fronts. On the evening of 10 June additional ashfall was reported in the towns of Alausí, Chunchi, and Guamote (figure 73); satellite imagery indicated an ash plume drifting W and incandescence from pyroclastic flows in the SE-flank ravine the same day (figure 74).

Figure 73. Ashfall from Sangay was reported in Alausí (top left), Chunchi (top right) and Guamote (bottom) on 10 June 2020. Courtesy of IG-EPN (#IGAlInstante Informativo VOLCÁN SANGAY No. 049, MIÉRCOLES, 10 JUNIO 2020).

Figure 74. Incandescent pyroclastic flows (left) and ash plumes that drifted W (right) were recorded on 10 June 2020 at Sangay in Sentinel-2 satellite imagery. Courtesy of Sentinel Hub Playground.

Ashfall continued on 11 June and was reported in Guayaquil, Guamote, Chunchi, Riobamba, Guaranda, Chimbo, Echandía, and Chillanes. The highest ash plume of the report period rose to 2,800 m above the summit that day and drifted SW. That evening the SNGRE (Servicio Nacional de Gestion de Riesgos y Emergencias) reported ash fall in the Alausí canton. IG noted the increase in intensity of activity and reported that the ash plume of 11 June drifted more than 600 km W (figure 75). Ash emissions on 12 and 13 June drifted SW and NW and resulted in ashfall in the provinces of Chimborazo, Cotopaxi, Tungurahua, and Bolívar. On 14 June, the accumulation of ash interfered with the transmission of information from the seismic station. Lahars were reported each day during 15-17 and 19-21 June. Trace amounts of ashfall were reported in Macas to the SE on 25 June.

Figure 75. The ash plume at Sangay reported on 11 June 2020 rose 2.8 km above the summit and drifted W according to the Washington VAAC and IG (left). Explosions and high levels of incandescence on the SE flank were captured by the Don Bosco webcam (right). Courtesy of IG-EPN (#IGAlInstante Informativo VOLCÁN SANGAY No. 055, JUEVES, 11 JUNIO 2020 and INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-164, VIERNES, 12 JUNIO 2020).

During an overflight of Sangay on 24 June IG personnel observed that activity was characterized by small explosions from the summit vent and pyroclastic flows down the SE-flank ravine. The explosions produced small gas plumes with a high ash content that did not rise more than 500 m above the summit and drifted W (figure 76). The pyroclastic flows were restricted to the ravine on the SE flank, although the ash from the flows rose rapidly and reached about 200 m above the surface of the ravine and also drifted W (figure 77).

Figure 76. A dense ash plume rose 500 m from the summit of Sangay on 24 June 2020 and drifted W during an overflight by IG-EPN personnel. The aerial photograph is taken from the SE; snow-covered Chimborazo is visible behind and to the right of Sangay. Photo by M Almeida, courtesy of IG EPN (Jueves, 02 Julio 2020 10:29, INFORME DEL SOBREVUELO AL VOLCÁN SANGAY EL 24 DE JUNIO DE 2020).

Figure 77. Pyroclastic flows descended the SE flank ravine at Sangay during an overflight by IG-EPN personnel on 24 June 2020. Ash from the pyroclastic flow rose 200 m and drifted W, and infrared imagery identified the thermal signature of the pyroclastic flow in the ravine. Photo by M Almeida, IR Image by S Vallejo, courtesy of IG EPN (Jueves, 25 Junio 2020 12:24, SOBREVUELO AL VOLCÁN SANGAY).

Infrared imagery taken during the overflight on 24 June identified three significant thermal anomalies in the large ravine on the SE flank (figure 78). Analysis by IG scientists suggested that the upper anomaly 1 (125°C) was associated with explosive activity that was observed during the flight. Anomaly 2 (147°C), a short distance below Anomaly 1, was possibly related to effusive activity of a small flow, and Anomaly 3 (165°C) near the base of the ravine that was associated with pyroclastic flow deposits. The extent of the changes at the summit of Sangay and along the SE flank since the beginning of the eruption that started in March 2019 were clearly visible when images from May 2019 were compared with images from the 24 June 2020 overflight (figure 79). The upper part of the ravine was nearly 400 m wide by the end of June.

Figure 78. A thermal image of the SE flank of Sangay taken on 24 June 2020 indicated three thermal anomalies. Anomaly 1 was associated with explosive activity, Anomaly 2 was associated with effusive activity, and Anomaly 3 was related to pyroclastic-flow deposits. Image prepared by S Vallejo Vargas, courtesy of IG EPN (Jueves, 02 Julio 2020 10:29, INFORME DEL SOBREVUELO AL VOLCÁN SANGAY EL 24 DE JUNIO DE 2020).

Figure 79. Aerial and thermal photographs of the southern flank of the Sangay volcano on 17 May 2019 (left: visible image) and 24 June 2020 (middle: visible image, right: visible-thermal overlay) show the morphological changes on the SE flank, associated with the formation of a deep ravine and the modification of the summit. Photos and thermal image by M Almeida, courtesy of IG EPN (Jueves, 02 Julio 2020 10:29, INFORME DEL SOBREVUELO AL VOLCÁN SANGAY EL 24 DE JUNIO DE 2020).

Geologic Background. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within horseshoe-shaped calderas of two previous edifices, which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been sculpted by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of a historical eruption was in 1628. More or less continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Arnold Binas (URL: https://www.doroadventures.com).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); Bobyson Lamanepa, Yogyakarta, Indonesia, (URL: https://twitter.com/BobyLamanepa/status/1214165637028728832).

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

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

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

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

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

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

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Historical lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

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

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

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

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

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

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

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

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

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

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Disaster Response Operations Monitoring and Information Center (DROMIC) (URL: https://dromic.dswd.gov.ph/); United Nations Office for the Coordination of Humanitarian Affairs, Philippines (URL: https://www.unocha.org/philippines); James Reynolds, Earth Uncut TV (Twitter: @EarthUncutTV, URL: https://www.earthuncut.tv/, YouTube: https://www.youtube.com/user/TyphoonHunter); Chris Vagasky, Vaisala Inc., Louisville, Colorado, USA (URL: https://www.vaisala.com/en?type=1, Twitter: @COweatherman, URL: https://twitter.com/COweatherman); Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue, Singapore (URL: https://www.earthobservatory.sg/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Relief Web, Flash Update No. 1 - Philippines: Taal Volcano eruption (As of 13 January 2020, 2 p.m. local time) (URL: https://reliefweb.int/report/philippines/flash-update-no-1-philippines-taal-volcano-eruption-13-january-2020-2-pm-local); Bloomberg, Philippines Braces for Hazardous Volcano Eruption (URL: https://www.bloomberg.com/news/articles/2020-01-12/philippines-raises-alert-level-in-taal-as-volcano-spews-ash); National Public Radio (NPR), Volcanic Eruption In Philippines Causes Thousands To Flee (URL: npr.org/2020/01/13/795815351/volcanic-eruption-in-philippines-causes-thousands-to-flee); Reuters (http://www.reuters.com/); Agence France-Presse (URL: http://www.afp.com/); Pacific Press (URL: http://www.pacificpress.com/); Shutterstock (URL: https://www.shutterstock.com/); Getty Images (URL: http://www.gettyimages.com/); Google Earth (URL: https://www.google.com/earth/).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

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

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

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

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

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

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

Information Contacts: Information contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/exp.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Kuchinoerabujima encompasses a group of young stratovolcanoes located in the northern Ryukyu Islands. All historical eruptions have originated from the Shindake cone, with the exception of a lava flow that originated from the S flank of the Furudake cone. The most recent previous eruptive period took place during October 2018-February 2019 and primarily consisted of weak explosions, ash plumes, and ashfall. The current eruption began on 11 January 2020 after nearly a year of dominantly gas-and-steam emissions. Volcanism for this reporting period from March 2019 to April 2020 included explosions, ash plumes, SO₂ emissions, and ashfall. The primary source of information for this report comes from monthly and annual reports from the Japan Meteorological Agency (JMA) and advisories from the Tokyo Volcanic Ash Advisory Center (VAAC). Activity has been limited to Kuchinoerabujima's Shindake Crater.

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

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

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

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

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

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

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

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

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Table 15 lists the ground-based 48-inch lidar measurements at 0.69 µm taken with a ruby laser in Hampton, Virginia (37.1°N, 76.3°W) during 1998. The lowest levels of aerosol loading ever reported in the 24-year lidar record at Hampton were measured during the summer of 1998.

Table 15. Lidar data from Virginia, USA, for April-December 1998 showing altitudes of aerosol layers. Backscattering ratios are for the ruby wavelength of 0.69 µm. The integrated values show total backscatter, expressed in steradians^-1, integrated over 300-m intervals from the tropopause to 30 km. Courtesy of Mary Osborne.

Geologic Background. The enormous aerosol cloud from the March-April 1982 eruption of Mexico''s El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin thorugh 1989. Lidar data and other atmospheric observations were again published intermittently between 1995 and 2001; those reports are included here.

Information Contacts: Mary Osborn, NASA Langley Research Center (LaRC), Hampton, VA 23681 USA.

A distinct change in seismic activity began on 3 December. About 120 shallow events of very low magnitude were recorded during 3-6 December. The only days during the episode when observation was not obscured by cloud were 1 and 3-6 December, but no plumes were seen those days.

Geologic Background. Avachinsky, one of Kamchatka's most active volcanoes, rises above Petropavlovsk, Kamchatka's largest city. It began to form during the middle or late Pleistocene, and is flanked to the SE by the parasitic volcano Kozelsky, which has a large crater breached to the NE. A large horseshoe-shaped caldera, breached to the SW, was created when a major debris avalanche about 30,000-40,000 years ago buried an area of about 500 km2 to the south underlying the city of Petropavlovsk. Reconstruction of the volcano took place in two stages, the first of which began about 18,000 years before present (BP), and the second 7000 years BP. Most eruptive products have been explosive, with pyroclastic flows and hot lahars being directed primarily to the SW by the breached caldera, although relatively short lava flows have been emitted. The frequent historical eruptions have been similar in style and magnitude to previous Holocene eruptions.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

The eruption at Colima began by 20 November 1998 following 17 days of continuous seismic unrest and deformation of the summit cone. Gabriel Reyes, Juan José Ramírez, and Yuri Taran noted that fumarolic gases monitored during previous years may have also shown precursory variations in chemical composition and temperature. New fractures in the summit region were observed on repeated occasions by Abel Cortes and J.C. Gavilanes during ascents on 27 November 1997, 18 March 1998, and 5 May 1998. Between 1614 and 1800 on 17 November, Carlos Navarro and Cortes visited La Yerbabuena, a town on the SW flank 9 km from the summit crater, where they heard more than 10 episodes of rumbling noise coming from the volcano. Cloudy weather did not allow direct observation of the volcano, but based on previous experience they interpreted the noises to be the result of small rockslides.

During the morning of 18 November the settlement of La Yerbabuena (~180 inhabitants) was evacuated voluntarily and in orderly fashion with the assistance of the Colima Observatory Information Group and the local civil protection and military authorities. During that day residents also evacuated the settlement of Juan Barragan (120 people) 10 km SE of the summit.

The first helicopter overflight took place between 0800 and 1000 on 19 November but cloudy weather obscured large parts of the summit area. Observers did note a vigorous fumarolic plume blowing W. That night the Red Sismológica Telemétrica del Estado de Colima (RESCO) reported strong seismic activity and harmonic tremor over periods lasting for 6 minutes. They also registered increased rockfall signals.

At 0700 on 21 November the new lava dome had almost entirely filled the 1994 crater (BGVN 23:10; figure 25). At 1130 that morning, lava started spilling out of the summit crater area producing block-and-ash flows to rush down the S slopes at 3- to 5-minute intervals. The block-and-ash flows were mostly emplaced within the eastern branch of Barranca El Cordobán. The most voluminous flows reached the 2,400 m contour, a distance of more than 4 km from the crater.

Figure 25. Photograph of Colima taken from the SW at 0930 on 21 November 1998. By this time the new dome has entirely filled the 1994 crater and was about to spill out onto the S face (on the right-central portion of the photo). Courtesy of J.C. Gavilanes, Universidad de Colima.

Figure 26. An oblique, wide-angle aerial photo taken with the camera somewhat tilted (note horizon in upper corner) showing a pyroclastic event at 0830 on 22 November 1998. The event left a series of block-and-ash flows reaching a maximum distance of 4.5 km W of the summit. Courtesy of Abel Cortés, Colima Volcano Observatory, Universidad de Colima.

Another flight on 21 November revealed that the lava flow had advanced ~150 m downslope, and had a width of 100 m and a thickness of ~20 m. The lava flow continued advancing such that on 22 November it was 170 m long; on 23 November, 270 m; and on 24 November, 370 m. Block-and-ash flows emplaced during the morning of 25 November in the central branch of Barranca El Cordobán reached 1,900 m elevation. Observers and photographs revealed two additional lava flows as seen from both Rancho El Jabalí (10 km SW of the summit) during the night of 25 November and from Cofradia de Suchitlan (15 km SW) during the night of 26 November; these flows also descended the SW flank and headed towards two drainages (the W branch of Barranca El Cordobán and the S branch of Barranca La Lumbre).

Figure 27. Time-lapse photograph (30 minutes) of Colima taken from a point near Suchitlan (15 km SW) around 2300 on 26 November 1998. Courtesy of Claus Siebe, UNAM.

On 28 November, S. Rodríguez, J.M. Espíndola, and C. Siebe observed the advance of the westernmost lava flow from a 3,100-m-elevation vantage point. Most of the time the flow's front and margins relinquished large blocks (up to 10 m across) producing strong rumbling noises. Small block-and-ash flows moved quietly compared with the rockfalls from the lava flow. Still, it was possible to collect samples of the lava flow.

Viewed in thin section the new lava contained, in decreasing abundance, phenocrysts of zoned plagioclase, hypersthene, pleochroic resorbed brown hornblende, and subordinate magnetite in a microcrystalline to glassy matrix. Chemical analysis indicated that the new lava is very similar in composition to previous eruptions (table 5).

Table 5. Chemistry of freshly erupted Colima lava sampled on 28 November 1998. Courtesy of S. Rodríguez, J.M. Espíndola, and C. Siebe; analysis made by Rufino Lozano, Laboratorio de Fluorescencia de Rayos X at Instituto de Geología, UNAM.

On 2 December the three lava flows on the SSW flanks had reached these estimated lengths: 1,000 m (more westerly flow), 1,200 m (central flow), and 900 m (SE flow). Good views of these flows were obtained during an overflight the next day (figure 28). Around this time, it seemed most probable that the ongoing eruption would remain mostly effusive and not exceed the magnitude of eruptions witnessed here during past decades. Accordingly, inhabitants of La Yerbabuena were allowed to return to their homes on 1 December.

Figure 28. The state of lava flow advance on Colima at 0850 on 3 December 1998. Photograph taken from the SW by Juan Carlos Gavilanes. Courtesy of J.C. Gavilanes, Universidad de Colima.

Fine ash produced thus far during the eruption consisted mostly of non-juvenile material related to rockfalls and small block-and-ash flows. Two stations, one located at Rancho El Jabalí (10 km SW of the summit) and the other at La Becerrera (12 km SW of the summit), registered maximum ashfalls on 23 and 26 November, respectively; both with daily loads of around 50 g/m². The wind mostly dispersed this ash towards the SW and W.

COSPEC measurements carried out by Gavilanes and Cortes since 30 October 1998 showed a marked increase in SO₂ flux (table 6). The highest discharge, measured on 26 November, yielded an estimate of more than 16,000 metric tons/day.

Table 6. COSPEC measurements for SO₂ fluxes at Colima volcano at stated dates in 1998. Fluxes are in metric tons/day and were rounded to three significant figures. Measurements on 11 February, 14 April, 2 May, and 25 May were below the detection limit. Extrusion began on 20 November. Courtesy of Juan Carlos Gavilanes and Abel Cortes, Universidad de Colima and Colima Volcano Observatory.

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

Information Contacts: Juan Carlos Gavilanes, Carlos Navarro, Abel Cortés, Alicia Cuevas, and Esther Ceballos, Universidad de Colima; Claus Siebe, Juan Manuel Espíndola, Instituto de Geofísica, UNAM; Sergio Rodríguez-Elizarrarás, Instituto de Geología, UNAM.

The following report summarizes activity observed at each of the four summit craters of Etna (figure 70) from June through September 1998. In early June, Northeast Crater was quiet while Bocca Nuova, Southeast Crater and Voragine were displaying the highest level of activity seen in many months. Generally high levels of activity continued until a major explosive eruption from Voragine on 22 July. Strong Southeast Crater explosions on 15 September destroyed the intracrater cone, which was soon replaced.

Figure 70. Map of the summit craters of Mount Etna, 10 July 1998. Courtesy of J.C. Tanguy and G. Patanè.

During early July all four craters were erupting simultaneously, a fact never recorded since their birth; only the Central Voragine has degree of permanence; Northeast Crater (NEC) appeared in 1911, the Bocca Nuova (BN) in 1968 and the Southeast Crater (SEC) in 1971.

Most of the information for this report was compiled by Boris Behncke at the Istituto di Geologia e Geofisica, University of Catania (IGGUC), and published on his internet web site. The compilation was based on personal visits to the summit, telescopic observations from Catania, and other sources. Additional separate reports were provided by Tanguy and Patanè (10-14 July observations) and Murray, Stevens, and Craggs (15 September observations). Aviation notices were issued by the Toulouse (France) Volcanic Ash Advisory Center.

Activity at Southeast Crater (SEC). There were at least three explosive vents on the intracrater cone during 4-5 June. Activity usually alternated between the N and S vents. When both exploded simultaneously, a third NW vent produced weak incandescent projections. Vigorous growth around the vents elevated the summit to 20 m above the SEC rim. Lava flowed towards the NE flank where it spilled down to the base of the SEC cone. During a visit on 11 June, SEC had the usual two vents active, and fresh bombs scattered over the crater floor. Recent flows had built a high mound on the E side of the cone; an active flow issued from the vent area. By the morning of 15 June the lava flow at SEC had reached its southern base and was advancing slowly.

Explosive activity on 22 June occurred from three vents on the intracrater cone, and lava issued from a vent halfway up the S flank. Explosive Strombolian activity occurred in distinct cycles separated by quiet periods of up to one hour, although lava effusion persisted. The beginning of each cycle was marked by a flame of burning gas at the summit. More vigorous bursts would then follow at a larger vent. Explosions would become increasingly frequent and rise higher (up to 150 m above the vent), showering the southern part of SEC with bombs. Activity would then shift back to the SW vent where each Strombolian burst was accompanied by a gas flame. Intermittent explosive and effusive activity continued on 24 and 28 June.

During a summit visit by Behncke and members of L'Association Volcanologique Europeenne (LAVE) of Paris on 4 July, explosive activity at SEC was intense, with bombs falling outside the crater. Activity from the top of the intracrater cone sent jets of bombs and scoria up to 150 m. Lava issued from three vents, one feeding a flow over the SW crater rim. On the evening of 7 July LAVE members reported that the active lava flow on the SW flank of SEC was ~200 m long. The summit visit on 13 July was made by Giovanni Sturiale, Sandro Privitera, and Behncke (IGGUC), and Jürg Alean. At SEC, Strombolian activity was vigorous, bombs fell frequently outside the crater, and lava emission was continuing. Recent lava had filled the SW part of the crater to within 1-2 m of the rim. In all other areas the pre-1997 rim of SEC has been buried by overflowing lava. Jürg Alean visited on 14 July and reported that SEC continued to produce Strombolian activity. From the Torre del Filosofo hut a small lava flow could be seen descending the SE flank of SEC; incandescent blocks frequently detached from the flow front.

Vigorous activity occurred at SEC during the 22 July Voragine episode, and during the days after activity was limited to SEC where vigorous lava fountaining and effusion occurred. On 24 July, Sturiale and Privitera observed vigorous Strombolian activity, with many bombs falling outside the crater. However, SEC activity declined and virtually ceased by the end of July.

As of the night of 17-18 August, there had been no resumption of the SEC eruption. The crater was seen erupting later on 18 August by Privitera. When Monaco and Behncke visited on 20 August, virtually no activity was observed. As of 26 August SEC appeared quiet, although the activity in July had built the intracrater cone to 40-50 m higher than the crater rim. Bombs were scattered all over the crater area and beyond. Two post-22 July lava flows had spilled onto the S and NE flanks. No activity occurred during a visit by Behncke and Sturiale on 9 September.

Explosive activity from SEC was observed by scientists from Open University (Murray and Stevens) and the University of London (Craggs) on the morning of 15 September. Several ash clouds erupting from the summit between 0745 and 0800 were seen from 10 km S. At 0815 bomb-laden ash clouds were observed from near the Piccolo Rifugio (4.5 km S of the summit craters). At 0822 an exceptionally large explosion sent meter-sized bombs ~300-400 m above the crater rim. One more minor explosion was observed before the summit was obscured at 0826. Observation recommenced at the Pizzi Denieri volcano observatory. The summit was usually obscured by clouds, but five explosions during 0928-0936 were audible above gale-force winds and engine noise. Ash clouds were seen from Mt. Nero on the NE rift (6 km from the summit craters) at 1003, and at 1006 an explosion was heard.

Explosions continued all afternoon, causing ashfall in inhabited areas on the E flank. During the afternoon, while conducting fieldwork 50 km S of Etna, Behncke and Sturiale saw black ash fountains piercing weather clouds above the summit. These pyroclastic jets rose several hundred meters above the summit before drifting E. Observations by Behncke on 19 September revealed that the explosions ejected lithics and fresh bombs, which were abundant in the saddle between SEC and the main summit cone. Some of the bombs were up to 5 m across and had flattened upon impact. Bombs tens of centimeters in diameter formed a continuous deposit on the NW side of the crater. Most of the intracrater cone was destroyed, and a crater ~80 m across formed in its place (figure 71).

Figure 71. Sketch drawing showing Southeast Crater of Etna as seen from the NW on 19 September 1998. The former intracrater cone has been replaced by an explosion crater that contains a small new cone with two erupting vents, and a small non-eruptive vent that lies below the major breach in the crater wall in the left center. Lava that had overflowed the SW crater rim until July 1998 is shown in a dark pattern at right. Courtesy of Boris Behncke.

Vigorous activity on 17 and 18 September ejected bombs described as having been "several meters across" by a group of British geologists led by J.B. Murray working in the area. The beginning of a lava flow down the NE flank of SEC is not known, but it was reported by mountain guides to have been moving on 17-18 September. During a 19 September visit by Behncke, Strombolian bursts occurred from two vents in the explosion crater, around which a small cone had begun to grow. Lava emission from a vent high on the SEC cone was feeding a flow that advanced towards Valle del Leone.

Activity at SEC was continuing on 21 and 25 September with intense Strombolian activity; incandescent bombs jetted 150-200 m high. Continued vigorous activity during the last week of September caused rapid growth of the intracrater cone until ti was higher than ever before, having almost entirely covered the remains of its predecessor.

Activity at Bocca Nuova (BN). Eruptive activity during 4-5 June was occurring at both previously active areas. Night incandescence and bomb ejections were seen in a deep pit within the SE eruptive area. Noisy activity occurred at the NW eruptive area, at the bottom of the collapsed cone that had grown in 1997. At least five vents were producing explosions and lava fountains accompanied by bursts of burning gas. Several lava flows extended over the crater floor.

Observations were made for 30 minutes on 11 June from the crater rim. The SE vents had fountains of ash and bombs rising ~50 m. At the NW eruptive area, three vents were active, and the collapse pit was filled with pyroclastics and recent lava flows. Two large (30 and 50 m diameter) vents were in the central part of the filled pit while a smaller vent (~5 m in diameter) lay 50-70 m S; this latter one produced weak lava sputterings, building a low hornito. The two larger vents showed a repetitive eruptive behavior for the first 15 minutes of observation, then erupted simultaneously in a series of ash-free lava fountains. For about ten minutes there were bomb ejections from both major vents. Centimeter-sized scoria and Pele's hair were deposited all over the SE sector and on SEC.

Activity was less intense on 15 June; during a 1-hour stay in the summit area, strong explosions from the large cone ejected ash-rich jets of bombs up to 100 m above the crater rim. Visits to BN are dangerous due to frequent blasts of large quantities of meter-sized bombs. Most blasts observed on 22 June lasted up to 10 minutes. The source vent lay in the partially collapsed 1997 cone at the N eruptive area; it produced almost continuous minor explosions between the large detonations, ejecting large clots of fluid lava. A small vent to the south ejected minor sprays of meter-sized bombs. Continuous lava fountaining occurred from a SE vent. During the 22 June visit the central vent was the site of pulsating gas jets, and vigorous lava fountaining occurred at the larger SW vent. A large asymmetrical cone leaning against the thin wall between the Voragine and BN had grown around the vent. Vigorous activity was continuing on 24 and 28 June.

During a summit visit by Behncke and members of LAVE on 4 July, all four summit craters were active. The summit visit on 13 July by Sturiale, Privitera, Behncke, and Alean showed low levels of activity; a small cone had grown around the main vent. The N eruptive area was the site of Strombolian bursts every 5-10 minutes. A fairly large cone had grown at this vent, the first time that significant cone growth had occurred in BN since late 1997. Lava had covered the S crater floor.

A visit by J.C. Tanguy and G. Patanè during 10-14 July revealed that, with respect to the preceding year, the bottom of BN had raised considerably owing to the tephra deposition, so that the strongest explosions from the NW vent (figure 72) sometimes showered the external slope with bombs. By 12 July the explosions were reduced in strength and frequency. Jürg Alean visited on 14 July and reported that the N cone produced fountains heavily charged with bombs; many fell on the crater rims and in the Voragine.

Figure 72. Sketch of the Bocca Nuova and Voragine craters at Etna, 10 July 1998. Courtesy of J.C. Tanguy and G. Patanè.

Vigorous activity occurred at BN during the 22 July Voragine episode. On the afternoon of the 23rd, Carmelo Monaco (IGGUC) saw bright incandescence in BN even in bright daylight from an airplane approaching Catania. Activity was noted on 25 July and increased the following day according to Claude Grandpey (LAVE); activity at the NW area occurred from a small vent while the SE area had three vents emitting gas and bombs. In late July and early August, numerous vents erupted explosively at the NW area; subsidence of the central crater floor by a few meters occurred on 1 August. The SE vents displayed spectacular lava cascades from one vent into the other, the lower vent filling until an explosion cleared it. Kloster (LAVE) reported a lava lake in this area on 7 and 10 August, but during the following days there was only Strombolian activity.

On 20 August Monaco and Behncke observed moderate eruptions at the N vent area. Besides the summit vent, there were at least four smaller flank vents which had erupted recently. On 26 August frequent ash emissions were occurring. Growth of a small cone above the diaframma (septum between the craters) culminated in the fracturing of this cone and a cascade of lava into BN in late August.

A visit to the summit by Behncke and Sturiale on 9 September revealed that one vent at the summit of the NW cone was the site of Strombolian bursts alternating with bomb and ash emissions. Four smaller vents on the flanks on the cone were weakly degassing. Weak Strombolian activity occurred from two SE vents where a small cone was growing in a collapse depression. Behncke saw activity at similar levels on 19 September. As of 25 September there was low-level activity.

Activity at Voragine. On 3 June, near-continuous cannon-shot like detonations were heard kilometers away, and Marco Fulle (Osservatorio Astronomico, Trieste) observed magma bubbles within the vent burst at the onset of fire-fountaining episodes. When observed during 4-5 June, the vent in the SW crater floor had enlarged notably since 6 April and shifted away from the diaframma, and a low pyroclastic cone had grown around it. On the evening of 4 June, activity at the Voragine was observed for about 4 hours. A sustained fountain jetted from the vent, showering the SW part of the crater floor with bombs; many also fell into BN. This fountain lasted about 75 minutes, followed by pyroclastic material sliding from the inner walls of the vent into its throat. After a few minutes, a small vent opened below the inner SE rim of the vent and emitted jets of incandescent lava. Ejections soon resumed at the main vent, and a flame of burning gas persisted at the subsidiary vent accompanied by weak pyroclastic sprays. A new period of fountaining at the main vent resulted in the continuous fall of bombs into BN. The subsidiary vent was soon buried. At times, portions of the inner walls collapsed, causing ash-rich fountains.

The Voragine was not visited on 11 June, but very strong explosive activity was heard more than 10 km S, and high fountains contained meter-sized bombs. On 15 June the focus of activity had shifted to the central vent, previously active between July and December 1997. This vent ejected continuous lava fountains while a lava flow covered the E half of the crater floor. Fountains played up to 200 m above the vent, with all bombs falling back into the crater. At times, the magma level dropped, and the character of the activity changed to discrete explosions. The SW vent exhibited noisy gas emissions alternating with ash emission and lava fountaining. Vigorous activity was continuing on the evening of 24 June. During a summit visit on 28 June, Monaco observed fountaining from the central vent; the SW vent was less active and mostly ejected ash.

A scoria deposit extending SE, produced by a Voragine lava fountaining episode on 1 July, was examined by Behncke and members of LAVE on 4 July. Both vents in the Voragine were in vigorous, alternating activity. Eruptive cycles at the SW vent produced jets of fragmented pyroclastics. As activity waned at this vent, projections of large bombs would initiate at the central vent, increasing in frequency and height into a pulsating fountain at least 100 m above the crater floor.

Stefano Branca (IGGUC) reported that frequent explosions were audible throughout 6 July at Viagrande, a village at the SE flank of Etna; air concussions associated with the explosions shook windows and rattled doors. The explosions probably originated at the Voragine, the site of recent noisy activity. On 7 July explosions were still audible but less intense. Members of LAVE observed activity that evening from the SW vent that dropped bombs as far as the S rim of NEC.

A visit by J.C. Tanguy and G. Patanè during 10-14 July revealed activity at the large SW cone (figure 72) near the diaframma and from a central cone. By 12 July the two vents hurled large lava lumps and bombs in a fountain-like manner, some of which fell outside the crater. On 13 July this activity was stronger. That afternoon activity decreased, but two flows began from a fissure NE of the central cone. Lava rapidly invaded the northern, lowest part of the Voragine. During the peak effusive activity the two lava flows reached a speed estimated at 3-4 m/s. On the morning of 14 July, only the SW vent showed Strombolian explosions. Lava flows had entirely disappeared under a layer of tephra erupted during the night.

During a visit on 13 July by Sturiale, Privitera, Behncke, and Alean, the most vigorous activity occurred at the Voragine. On 12 July, lava fountains roared up to 200 m above the crater rim for three hours from the SW vent. Powerful jets of bombs mixed with ash were also ejected. The cone around the SW vent was higher in places than the diaframma; the vent was 30-50 m across. Activity varied from isolated powerful explosions to long-lasting lava fountains. At times dense ash plumes with large bombs rose from the vent. Explosions from the central cone blasted lava in all directions. Small lava fountains and ash emissions occurred from two fissures. On at least 20 occasions during 90 minutes of observation the magma surface in the vent domed up, forming a huge bubble that exploded. Explosions later ejected meter-sized bombs to 200 m or higher; many fell into BN, outside the Voragine, or on the E slope of the main summit cone not far from SEC. Jürg Alean visited on 14 July and reported that both vents showed intense activity. The SW vent was filled almost to the rim by lava which was fountaining vigorously. The central vent displayed a similar eruptive behavior as on the previous visit, but no lava bubbles were observed. On 20 July lava fountaining from the Voragine was common.

A major eruptive event began from the Voragine at about 1835 on 22 July. The following is based on preliminary information from scientists of the IGGUC (mainly Giovanni Sturiale and Sandro Privitera) and others who visited after the event as late as 20 August. According to eyewitnesses on the SW side of the main summit cone, huge lava fountains rose from the Voragine, and heavy tephra falls began in the summit area. A large mushroom-shaped tephra column rose up to 10 km above the summit. The plume was then driven S and SE, and widespread ashfalls occurred more than 30 km away. Sand-sized tephra fell in Catania, leaving a deposit about 1 mm thick. For the first time since 24 September 1986 (when NEC had a powerful explosive eruption) the Fontanarossa airport of Catania had to be closed (it was reopened after 15 hours). The Toulouse (France) Volcanic Ash Advisory Center issued 17 aviation notices warning pilots about the ash during 22-29 July. The tephra falls caused traffic problems on roads and highways. Close to the summit, a thick scoria deposit buried the dirt roads leading to the Rifugio Torre del Filosofo and around the western base of the main summit cone. Sturiale and Privitera reported that at Torre del Filosofo the thickness of the scoria deposit was about 50 cm.

It appears that both vent areas produced lava fountains and a tall tephra column. Rapid accumulation of ejecta in the saddle on the NW rim led to a lava flow between the NEC and the main summit cone (figure 73). The flow covered the road connecting the N and S flanks of Etna, and eroded a deep scar into the the S flank of the NEC cone. Continuing pyroclastic activity produced a thick scoria and bomb deposit, with bombs up to 5 m in length. A scoria fan extended 1-1.5 km NW. In the area of the diaframma a lava flow covered the crater floor to several meters depth. On the E flank of the main summit cone a thick pyroclastic deposit formed. In towns on the E and SE flank, the tephra deposit was a few millimeters to a few centimeters thick. Morphological changes within the Voragine consisted mainly of a large amount of filling of the crater followed by subsidence. Parts of the SW crater rim also collapsed.

Figure 73. Sketch map of Etna's summit craters showing features frequently mentioned in the updates, and approximate extent of recent lava flows as of 5 October 1998. Active vents are indicated as gray dots in the craters. Courtesy of Boris Behncke.

Vigorous activity occurred simultaneously at BN and SEC on 22 July Voragine event, indicating that the episode affected much of the central conduit system at some depth, possibly due to the rise of a batch of fresh gas-rich magma. Lava fountaining from the Voragine continued intensely through the night of 22-23 July.

According to Grandpey (LAVE), the Voragine appeared "full of materials" on 25 July with no trace of the former intracrater cones. No further activity occurred until 3 August when Kloster (LAVE) saw explosions ejecting bombs. Two days later, three vents erupted in the center of the Voragine. On 7 August small flows on the crater floor were followed by explosive activity. Powerful Strombolian activity with bomb ejections and ash emission caused light ashfall on the SE flank on 18 August, reaching the outskirts of Catania.

On 19 August explosive ash emissions sent small plumes up to several hundred meters above the summit. When Monaco and Behncke visited on 20 August, vigorous activity occurred from two vents. Very light ashfalls on 21 and 24 August reached Catania; ash emissions were also produced on 26 August. A number of reports indicated continued activity through the end of August.

On 6 September bombs fell on the outer W slope of the Voragine and on 7 September ash emission occurred throughout the day. A visit to the summit by Behncke and Sturiale on 9 September revealed continuous moderately strong Strombolian activity from a SW vent; sporadic explosive activity from the vent next to the diaframma sent bombs over the crater rim. At least three other vents were quietly degassing. Similar activity was continuing as of 19 September. On 30 September strong ash and gas emissions rose hundreds of meters.

Activity at Northeast Crater (NEC). Deep-seated Strombolian activity within the central pit resumed in mid-May according to Vittorio Scribano (Istituto di Scienze della Terra, Catania University). Night glow was observed on the evening of 22 June from 3 km NE. During a summit visit by Behncke and members of LAVE on 4 July, all four craters were active; for the first time since 28 March eruptive activity was observed directly at NEC. The eruption site was a 30-m-diameter vent in the NW part of the central pit while a SW vent (~15-20 m in diameter) emitted dense vapor plumes. Small Strombolian bursts from the larger vent occurred every 2-5 minutes, with most ejecta falling back into the pit.

A visit on 13 July by Sturiale, Privitera, Behncke, and Alean revealed mild Strombolian activity from the central pit that ejected bombs. When Monaco and Behncke visited on 20 August, NEC was degassing quietly. Strong fumarolic activity was occurring on 26 August and 9 September.

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: Boris Behncke, Istituto di Geologia e Geofisica, Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania, Italy; J.C. Tanguy and G. Patanè, University of Catania, Istituto di Geologia e Geofisica, 55 Corso Italia, 95129 Catania, Italy; John Murray and Nicki Stevens, Department of Earth Sciences, Open University, Milton Keynes, United Kingdom; Emma Craggs, Geology Dept, Royal Holloway College, University of London, United Kingdom; Volcanic Ash Advisory Center (VAAC) Toulouse, Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/vaac/)

Since a volcanic crisis in February 1989 (SEAN 14:02-14:05), Observatorio Vulcanológico y Sismológico de Pasto (OVSP) has been constantly monitoring Galeras. The following is from their bi-monthly reports for late 1998.

During September and October 1998, low-level seismic activity continued at Galeras (figure 90). Most of the energy released (1.6 x 10¹⁶ ergs) was due to earthquakes associated with a fracture process. Volcano-tectonic earthquakes registered during these two months totaled 79, ranging from 0.5 to 16 km in depth. One remarkable earthquake occurred at 0209 on 21 September: it was located at 1°15.75'N, 77°19.16'W at a depth of 8 km, released 1.21 x 10¹⁶ ergs of energy, and had a coda magnitude of 3.4. This earthquake was felt in Pasto City and neighboring settlements. It was the most energetic event of 1998 to date.

Figure 90. Location of seismic events at Galeras during September-October 1998. Courtesy OVSP.

Seismic processes related to fluid dynamics (i.e. long-period events and tremor episodes) released a total of 5.18 x 10¹⁴ ergs. Of these events, nine had small amplitudes with long coda and quasi-monochromatic frequencies—so-called "screw type" or "Tornillo" characteristics. Coda values spanned 19-65 s and dominant frequencies ranged 1.82-4.0 Hz. An unusual event occurred 23 October, when harmonic tremor lasted approximately one hour. This episode released 7.09 x 10⁹ ergs.

Galeras, a 4,276 m high andesitic stratovolcano, has a cone that rises 150 m above the floor of the summit caldera. The caldera is open to the west. The active crater is located ~9 km W of Pasto, a city of 350,000 persons. More than 400,000 people live within the volcano's zone of influence. At least six major eruptions have been identified during the past 4,500 years, last in 1886. These eruptions were Vulcanian with inferred low-altitude eruption columns (<10 km) that produced small-volume pyroclastic flows. During the last 500 years eruptions have been characterized by gas-and-ash emissions, small lava flows, and pyroclastic flows that have traveled up to 15 km from the 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: Patricia Ponce V., and Pablo Chamorro C., Observatorio Vulcanológico y Sismológico de Pasto (OVSP), Carrera 31, 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

On 18 December an eruption occurred within the caldera of the subglacial Grímsvötn volcano, 10 km S of the 1996 eruption that resulted in a catastrophic flood. Scientists quickly investigated; the information that follows is from the Nordic Volcanological Institute (NVI).

Eruptive activity. The eruption began at 0920 on 18 December. Ten minutes later a plume (figure 4) was observed that eventually rose 10 km above the Vatnajökull glacier and persisted throughout the day. The plume could be seen from Reykjavik, 200 km W. Winds deflected the plume, causing tephra fallout onto the glacier up to 50 km SE. The London Volcanic Ash Advisory Center issued aviation notices later that day and throughout the eruption.

Figure 4. Photo of the eruption plume from Grímsvötn as it appeared from an aircraft on 18 December 1998. Courtesy of NVI; photo by Karl Grönvold.

The eruption was preceded by a mild increase in seismicity for several weeks. A small earthquake swarm began at 2200 on 17 December and a sharp increase in earthquake activity began at 0330 on 18 December. This latter activity was replaced by continuous tremor at 0920, marking the beginning of the eruption. The Icelandic Meteorological Office and the Science Institute monitored seismicity during the eruption.

Vents were located along a 1,300-m-long E-W oriented fissure on the S caldera fault, similar to eruptions in 1934 and 1983, at the foot of Mt. Grímsfjall (which rises ~300 m above the flat ice shelf of the Grímsvötn subglacial lake). The eruption penetrated the caldera lake and its ice shelf, from ice/water depth of ~100 m. Activity was most vigorous at one crater, but several other craters on the short eruptive fissure were also active with less frequent explosions.

The eruption was slightly less vigorous on 19 December. The plume was continuous, but somewhat lower, rising to 7-8 km. Tephra continued to fall SE. A small part of the Grímsvötn ice shelf next to the eruption site had melted without raising the water level of the caldera lake significantly. Activity was mostly limited to one crater.

An overflight on 20 December from 1045 to 1215 revealed variable activity. The eruption plume extended to 7 km altitude. Initially the plume was light-colored, and narrow at its base. Later the ash content of the plume greatly increased, and the plume turned black. It collapsed down to 1-2 km, created a base surge, and Mt. Grímsfjall disappeared into an ash cloud.

Photographs from 27 December showed intermittent eruptive activity between 1124 and 1240. The plume was discontinuous, fed by intermittent crater activity. It rose to a maximum of 4.5 km and distributed ash near the crater; bombs up to 0.5 m in diameter were ejected onto Grímsfjall. The eruption has resulted in the formation of a tephra ring that lies partly on ice, but its inner part is likely to be made completely of ash overlying bedrock.

The eruption ended on 28 December. Continuous tremor recorded at the Grimsfjall seismograph, 3 km from the eruption site, stopped at 1050 on 28 December. Small tremor bursts were recorded for another 3 hours, but activity stopped completely at 1400.

This eruption was located 10 km S of the 1996 eruption in Vatnajökull (Gudmundsson and others, 1997), which caused a catastrophic outburst flood from the glacier. This time no major flood ensued because only a small amount of the Grímsvötn ice shelf near the eruption site melted, and water did not flow towards the Grímsvötn caldera lake.

Chemical analyses of ash. The ash analyzed fell during 1000-1200 on 20 December in Suðursveit, ~60 km SE of Grímsvötn. The ash was well sorted with an average grain size of 0.05 mm and density of ~2.7 g/cm³. The areal density of ash fall was estimated at 93 g/m². The ash was aphyric; the glass composition (table 1) can be compared with Grímsvötn ash samples from earlier this century. The composition is similar to earlier samples; however, the recent sample is slightly less evolved, with higher MgO/FeO, Al₂O₃, and CaO, but lower TiO₂. The composition was markedly different from more evolved samples from the 1996 eruption or most of the samples available from the neighboring Bárðarbunga volcanic system.

Table 1. Microprobe analyses of the glass phase from the 20 December 1998 Grímsvötn eruptions (standard deviation in parentheses) and two Grímsvötn hyaloclastites. The analyses from the 1983, 1934, 1922, and 1903 eruptions are from Grönvold and Johannesson (1984). The analyses of the hyaloclastites are from Heikki Makipaa (1978). All analyses are in weight percent. Courtesy NVI.

The potential chemical pollution of the fallout ash was tested by leaching a batch of ash with 6.7 times its mass of de-ionized water. The pH of the leachate was 5.12; the water-soluble components were as follows (mg leachate / kg ash): SiO₂, 7.2; Na, 315.3; K, 32.7; SO₄, 557.8; F, 346.5; Cl, 366.2.

References. Grönvold, K., and Jóhannesson, H., 1984, Eruption in Grímsvötn 1983, course of events and chemical studies of the tephra: Jökull, 34:1-11.

Gudmunsson, M., Sigmundsson, F., and Björnsson, H., 1997, Ice-volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland: Nature, v. 389, p. 954-957.

Geologic Background. Grímsvötn, Iceland's most frequently active volcano in historical time, lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by a 200-m-thick ice shelf, and only the southern rim of the 6 x 8 km caldera is exposed. The geothermal area in the caldera causes frequent jökulhlaups (glacier outburst floods) when melting raises the water level high enough to lift its ice dam. Long NE-SW-trending fissure systems extend from the central volcano. The most prominent of these is the noted Laki (Skaftar) fissure, which extends to the SW and produced the world's largest known historical lava flow during an eruption in 1783. The 15-cu-km basaltic Laki lavas were erupted over a 7-month period from a 27-km-long fissure system. Extensive crop damage and livestock losses caused a severe famine that resulted in the loss of one-fifth of the population of Iceland.

Information Contacts: Karl Grönvold and Freysteinn Sigmundsson, Nordic Volcanological Institute (NVI), Grensásvegur 50, 108 Reykjavík, Iceland (URL: http://nordvulk.hi.is/); Pall Einarsson, Science Institute, University of Iceland; Icelandic Meteorological Office, Reykjavík, Iceland (URL: http://en.vedur.is/).

Measurements and sampling were made in the acid lake of Kawah-Ijen during two transits with a rubber rowboat on 7 and 10 December 1998. The 7 December measurements occurred just after heavy rain and the lake's color was pale and nearly white. In the middle of the lake, the surface temperature was 23.5°C with a pH of 0.48 as a result of dilution by the rain. The temperature near the solfatara at the S side of the lake ranged between 24.6 and 24.9°C with a pH of 0.45. Near the hot sublacustrine spring the temperature was as high as 61.7°C and the pH was 0.60. In the Banyupahit River, 3 km from the dam that closes the lake, the water had a temperature of 21.1°C and a pH of 0.47.

Three days later, 10 December, the lake was pale green with localized brown coloration; the temperature of the surface was 24.8-25.2°C and the pH 0.36-0.38. The highest measured temperature of the solfatara was 224°C, while the CO₂ content of the atmosphere near the lake surface was normal, ~300 ppm.

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: Jacques-Marie Bardintzeff, Laboratoire de Petrographie-Volcanologie, bat 504 Universite Paris-Sud, 91405 Orsay, France.

Seismicity remained above background levels during 1 November-7 December. Low-level Strombolian activity, including 100-200 earthquakes and gas explosions each day, continued to characterize activity at the volcano. On 24 November a pilot in the vicinity reported an explosive event that sent an ash column 6 km above the summit. The color-coded hazard status remained at Yellow.

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry; Tom Miller, Alaska Volcano Observatory.

A significant collapse of the lava bench on the coast SE of Kilauea occurred in early December. Lava continued to flow into the sea via a tube from the Pu`u `O`o vent, and a pit at the vent continued to grow.

A large part of the active lava delta on the SE coast collapsed into the sea sometime between 1200 on 10 December and 0930 on 11 December. A comparison between the shoreline as mapped on 11 and 24 November (figure 125), and the shoreline on 11 December, showed that ~5.8 hectares (ha) was lost. The missing shoreline included ~3.4 ha of land built since August and ~2.4 ha built W of the current lava-entry area (indicated by the steam cloud at the top of figure 125) between 1992 and 1997. Judging from observations of earlier bench collapses, the collapsed area most likely slid into the sea in several segments over a period of tens of minutes to several hours.

Figure 125. View of the Kilauea lava entry point area looking S on 24 November 1998. Courtesy HVO; photograph by J. Kauahikaua.

The eruption of Pu`u `O`o continued in November as lava flowed to the sea through a lava tube that developed on the coastal plain after a major pause in magma supply to the vent on 12-14 August (BGVN 23:08). Another brief pause occurred on 7-8 November (pause #21 of the current eruptive episode) leading to several small `a`a and pahoehoe flows on the coastal plain, none of which reached the sea. Scientists measured a slight increase in the discharge of lava from the tube system—from 3/day in late October to just over 400,000 m<sup>3</sup>/day in early December. Dense volcanic fumes continued to obscure various pits within Pu`u `O`o most of the time, but sloshing sounds of lava degassing could be heard from the crater rim.

A new pit that developed high on the S flank of Pu`u `O`o about one year ago enlarged significantly in 1998, and recent measurements of cracks around the edge of the pit showed that its walls were slumping slowly into the pit.

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: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii Volcanoes National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/); Ken Rubin and Mike Garcia, Hawaii Center for Volcanology, University of Hawaii, Dept. of Geology & Geophysics, 2525 Correa Rd., Honolulu, HI 96822 USA (URL: http://www.soest.hawaii.edu/GG/hcv.html).

Following one month of build-up in seismicity and radial tilt (figure 10), intensive eruptive activity resumed on 5 October 1998—the first since its fatal eruption of November-December 1996 (BGVN 21:12).

Figure 10. Seismicity and radial tilt from a water-tube instrument at Manam, August-October 1998. Courtesy of RVO.

Visible increases in activity started on 23-25 September, with intermittent dark ash emissions and night-time incandescent projections to ~200 m above South Crater. In subsequent days of October, the activity decreased to continuous white vapor emissions, first profuse then very weak, and occasional roaring sounds and fluctuating glow. This corresponded to a slight decrease in seismic amplitude levels, but the radial tilt continued to show inflation.

On the morning of 5 October a rapid build-up of activity took place. At 0800 ash emissions became forceful, rising ~2 km above South Crater. By 0815 the first small pyroclastic flows started down SE Valley at 5-10 minute intervals. At 0850, the now-dark ash column rose ~3 km, surrounded by blue vapor. Pyroclastic flows started at 0913, penetrating down SW Valley, and the island's E side underwent heavy ash and scoria fall. After 1020 this crater produced several loud explosions every 10-15 minutes. Loud roaring and banging starting at 1205 heralded a decline in activity. By 1600, thick, dark clouds still rose intermittently, but by 1800 only weak, thin gray emissions were visible. Roaring and banging sounds were heard through the night. Although short-lived, this phase also fed a lava flow into SE Valley that branched into two lobes below 900 m elevation and stopped at ~450 m. A lava flow also started toward SW Valley but stopped at the headwall.

In the following days, the tiltmeter 4 km from the summit (at Tabele Observatory) recorded a drop of 2 µrad while the seismicity decreased to near background levels. Until 15 October, gray ash clouds and occasional deep roaring sounds were observed. Not even red glow remained. By the evening of the 16th, red glow reappeared and incandescent projections rose 100-200 m above South Crater. On 17 October, dark ash clouds rose forcefully with rumbling sounds, and minor ash fell on the island's N side.

On the 18th, Main Crater occasionally emitted gray-brown plumes to 600-700 m, and the seismic amplitude increased. Activity in South Crater became sub-continuous, with incandescent projections to 1,000-1,100 m. On the morning of the 19th, a lava flow issued by South Crater descended into SW Valley. The strength of the eruption declined after 1415 and again after 1600. Yet, by 2225 there was a fountain of incandescent projections 1,400-1,600 m above the crater accompanied by loud roaring all night.

Emissions on the morning of the 20th comprised a thick, dark, ash-laden column. In the afternoon, small pyroclastic flows at 1415 and 1750 reached only to the head of SW Valley. By that time, the lava flow extended to within ~2 km of the coast. A single large explosion at 1715 ejected ballistic blocks 1,500 m above the crater. That night, on-going Strombolian explosions rarely reached 1,100 m above the crater.

On the 21st, Main Crater produced dark columns, rising to ~1,000 m, while the roaring Strombolian eruption persisted in South Crater. That night, small tongues of lava flowed in the upper SE Valley.

Activity began to decrease on the 23rd, when the Strombolian projections gave way to intermittent dark ash clouds to ~800 m above the crater. After 1000 on the 23rd, rumbling diminished. The next day Main Crater forcefully ejected dark columns with ballistic fragments and South Crater continued to issue subdued white emissions, with occasional ones that were gray and forceful. This activity persisted until the end of the month, without sound except for occasional low roaring and a faint glow.

While the seismicity noticeably reflected the variations in eruptive strength, tilt was not affected by the second eruptive phase and it resumed rising steadily thereafter as late as early November (figure 10).

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: Ben Talai and Patrice de Saint-Ours, RVO.

According to reports from local news sources and USAID's Office of Foreign Disaster Assistance (OFDA), earthquake swarms began on the island of Dominica on 11 September and continued intermittently into October. The Seismic Research Unit (SRU) of the University of the West Indies started monitoring the activity on 28 September and determined that [seismicity] was occurring in the S part of the island beneath Morne Patates volcano. By 23 October the [seismicity] had subsided to about two [earthquake] events per hour, with 10% large enough to be felt.

An earthquake recorded by SRU at 1018 on 24 September had its epicenter at 15.28°N, 61.37°W. It occurred at a depth of 15 km with a body-wave magnitude of 2.9 and a Richter magnitude of 1 to 3. A spasmodic (new events were starting before the previous were finished) sequence of activity started about 1500 on 22 October. These events were less than 6 km deep and had a maximum magnitude of 3.5 Richter and an intensity of MM V. On 23 October, an SRU aerial reconnaissance revealed no surface manifestations of the events (i.e., scarps, vents).

The strong [felt earthquakes] on 22-23 October were described as the longest and most intense in recent times. These [earthquakes] caused landslides and road closures, including the main road from the capital, Roseau, to the communities on the S end of the island. The SRU stated on 22 October that 27 was the maximum number of [events] recorded within a 24-hour period since 28 September, noting that the daily numbers were not as high as during the 1974 sequence.

Morne Patates, at the southern tip of Dominica, is an arcuate structure open to Soufriere Bay on the west. It was constructed within an irregular depression on the SW flank of a larger stratovolcano, Morne Plat Pays, whose summit is only 3 km NE. The latest eruptions occurred at about 450 ± 90 years BP (Roobol and others, 1983) from the Morne Patates lava dome just prior to European settlement. At least ten swarms of small-magnitude earthquakes have occurred since 1765. The most recent swarm, between March and October 1986, consisted of 10-30 recorded A-type volcanic shocks in about two hours. No eruptive activity followed any of these swarms and no systematic shallowing was documented to indicate upward migration of magma.

General References. Roobol, M.J., Wright, J.V., and Smith, A.L., 1983, Calderas or gravity-slide structures in the Lesser Antilles Island Arc?: JVGR, v. 19, p. 121-134.

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 Morne Plat Pays 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: Tina Neal, OFDA/USAID, 1300 Pennsylvania Ave. NW, Washington, DC 20523-8602 (URL: http://www.info.usaid.gov/ofda/ofda.htm); CaKaFete News, 25-12 Street, Canefield, Dominica (URL: http://www.cakafete.com/).

A change in the typical low-level steam-and-gas emission regime in late November and early December suggested that a new lava body was growing inside the crater. The following has been condensed from CENAPRED bulletins.

Low-level activity continued during the first three weeks of November, and included low-intensity, short-duration exhalations of steam and gas with occasional eruptions of ash. Bad weather obstructed observations on many days. Authorities recommended that no one approach within 5 km of the crater because of the danger of sudden explosions. The volcanic alert level remained at yellow, indicating a state of heightened caution. Several A-type earthquakes occurred (on 6, 14, 15, 16, and 17 November; M 2.1-2.9), generally 3-4 km E or SE from the crater, none of which seemed to affect eruptive activity. One exceptional emission occurred at 0109 on 9 November; its intense phase lasted one minute and was followed by 12 minutes of high-frequency tremor.

At 1753 on 19 November a moderately large eruption was followed by five smaller ones. The series lasted seven minutes and produced an ash column that rose 2-3 km above the summit and dissipated NNW. Light ash fall was reported in the neighboring town of Amecameca. At 2019 a slightly smaller exhalation lasted nine minutes.

At 1302 on 22 November the volcano began a substantial increase in activity, starting with a sequence of small ash emissions; light ashfall was reported at Paso de Cortés and Amecameca. This activity continued into the night with about 40 separate emission events by midnight. Exhalations increased, and at about 0430 on 23 November harmonic tremor episodes were recorded. At 0530 incandescence at the crater could be seen, at 0854 high-frequency tremor started, and at 0922 a moderate ash emission generated a column 3 km above the summit. By noon about 100 exhalations had been recorded. Dense fumarolic clouds of gas and steam were blown NW. Beginning at 1245 activity increased again: high-frequency tremor and emissions occurred at a rate of one per minute. Although the summit was obscured by cloud, it was assumed, based on reports from local towns, that ash emissions were continuous. After 1515 seismicity increased to saturation levels on most of the recording instruments. Later, an emission of steam, gas, and ash could be seen. At 1630 seismicity started to decrease.

Small, low-frequency tremor signals began around 0200 on 24 November, and intensified between 0300 and 0600. The tremor was accompanied by continuous emissions of gas, steam, and some ash, blown to the SW. At 1257 another increase of activity began. Low-frequency tremor of variable amplitude was recorded until 1600. Poor visibility prevented direct observation of the summit during most of the day.

A steam plume that rose 2-2.5 km over the summit persisted until 0803 on 25 November when a moderately large explosion lasting one minute produced an ash plume that rose 3-4 km over the summit (figure 28) and threw rock fragments to a distance of 2 km. The top of the plume moved N, while the lower part moved SW; ashfall warnings were issued to towns in those directions. A low-frequency tremor signal followed the explosion and persisted through the day. Other explosions occurred at 1205 and 1658 on 25 November. Although the explosions were heard in nearby towns, there were no reports of large ash emissions, and it is likely that the ejected rock fragments were dispersed around the crater.

Figure 28. Characteristic explosions and associated plumes from Popocatépetl taken by CENAPRED's video monitor during 25-26 November 1998. Scenes 1-3 (top) are from 0803-0805 on 25 November; scenes 4-5 (bottom) are from 1013-1015 on 26 November. Courtesy of CENAPRED.

An increase in tremor was followed by new explosions at 0654 and 0719 on 26 November. Moderate steam-and-ash plumes rose to a height of 1,500 m above the summit. A stronger exhalation at 0931 produced a moderate plume of steam and ash rising 3-3.5 km above the summit. Other explosions at 1013 (figure 28) and 1104 produced higher ash columns. In all cases warnings were issued to air-traffic controllers. A new warning to the general population recommended approaching no closer than 7 km from the crater. Tremor was followed by volcanic earthquakes at 2113 and 2220; both events produced moderately large explosions and ash plumes, and during the later event incandescent lava fragments were thrown to a distance of ~1.5 km.

Moderate explosions were detected in the crater at 1206, 1333, 1749, and 2345 on 27 November, and at 0242 and 1021 on 28 November. All of them, except the third, expelled incandescent fragments of lava around the crater to a distance of 0.5-2 km, and produced moderately large emissions of ash, rising in most cases up to 4 km over the summit. This activity was detected against a background of low-level exhalation and tremor signals of decreasing amplitude. Light ashfall had been reported in Tlacotitlán at 0130 on 28 November. During 28 November activity increased again following several short harmonic tremor signals at 2130. At 2228 a moderate volcano-tectonic event was followed by small tremor episodes.

At 0002 and 0305 on 29 November two explosions were preceded by low-frequency tremor. The second explosion produced a shock wave clearly heard at Paso de Cortes and San Nicolás de los Ranchos. Large quantities of glowing rocks ejected from the crater could be seen falling in a area of ~3 km radius. There was also a large ash emission. At 0654 a moderately large emission, lasting seven minutes, formed an ash plume 4 km above the summit. At 1118 there were several low-frequency harmonic tremors. A moderately large explosion at 1645 ejected incandescent lava blocks around the cone and produced an ash plume up to 7 km above the summit (according to personnel working close to Paso de Cortes).

Tremor episodes and moderate emissions of steam, ash, and gas with occasional explosions persisted over the next week. One explosion at 0929 on 30 November began with a strong shock wave and blast, ejected fragments over its flanks 2-3 km from the crater, and produced an ash column 4 km above the summit. At 1853 on 3 December an explosion ejected incandescent fragments over the SE flanks and produced a moderately large ash cloud, carried by the wind to the SE. The explosion signal lasted one minute, followed by 15 minutes of tremor. At 1255 on 4 December an explosion threw hot debris on the SE flanks and produced an ash plume that rose 4-5 km above the summit. Another explosive eruption at 1511 on 6 December ejected incandescent rocks over the E and N flanks and produced an ash column 5 km above the summit that dispersed to the NW. This event lasted 1.5 minutes and was followed by high-frequency tremor for four minutes. Three explosions were recorded on 7 December at 0241, 0449, and 0623; glowing fragments fell on the E and N flanks and an ash column rose 4 km. The last of these events lasted 1.5 minutes and was followed by high-frequency tremor for 10 more minutes. During 8 December frequent exhalations with durations of 3-10 minutes each produced steam-and-ash columns 2 km above the summit.

Activity became stable at lower levels during the second week of December, persisting until the time of this report (15 December).

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

Information Contacts: Servando De la Cruz-Reyna^1,2, Roberto Quaas^1,2, Carlos Valdés G.², and Alicia Martinez Bringas¹. 1 Centro Nacional de Prevencion de Desastres (CENAPRED) Delfin Madrigal 665, Col. Pedregal de Santo Domingo,Coyoacan, 04360, México D.F. (URL: https://www.gob.mx/cenapred/); 2 Instituto de Geofisica, UNAM, Coyoacán 04510, México D.F., México.

Tilt, leveling, and sea-shore surveys continued to record the slow resurgence of the caldera floor observed since April 1996. During October, continued slow magma supply into Rabaul Caldera kept feeding mild Vulcanian activity at Tavurvur cone. Emissions occurred at irregular intervals, from a few minutes to several hours apart. Longer time intervals usually resulted in more powerful and voluminous explosions.

In the beginning of the month, explosions ejected a grayish ash plume 500-1,000 m above the crater. Following a particularly large explosion at 2138 on October 5 (which littered the cone with incandescent ballistic blocks, and displayed dramatic lightning within the dark rising cloud) emissions were larger for a few days, rising to 1,000-3,000 m, although without sounds. During 10-15 October emissions were again milder, hardly rising over 600 m above the crater. Emissions occurring 16-20 October rose to ~1,000 m and were often accompanied by roaring sounds. After 29 October, emissions were again noiseless, and from the 26th onward they became lower in ash content and energy.

October was the transitional period of wind shift. From the 19th, the NW wind began to dominate and bring welcome relief after seven months of very unpleasant, corrosive, and toxic ashfall to Rabaul and neighboring residents.

The recorded seismicity consisted almost exclusively of low-frequency events accompanying the Vulcanian activity from Tavurvur. However, two types of signals were observed: usual short-duration events, and low-amplitude, long-duration (1-3 minutes) events. Their combined number, with an increase in August and September, averaged 46 per day but increased to 81 and 143 on the last two days of October without any corresponding change in visible eruptive activity. The two types of signals usually occurred in subequal amounts, although on 5-7 October the number of long-lasting events started to dominate, while the shorter events prevailed for a few days after the 8th. The amplitude of both types fluctuated substantially for several multi-day intervals during October. Short-duration harmonic signals were also recorded during 16-18 and 24 October. On 20 October the system registered the month's only significant high-frequency event.

A visit to Rabaul by professional photographer George Casey resulted in several images of Tavurvur during August. Casey appreciated the aid kindly given him by RVO staff and was gracious enough to provide us with photos, including one of a small plume on 4 August (figure 32).

Figure 32. The Tavurvur cone at Rabaul emits a small plume on 4 August in this photograph looking SE from the bay's shore. Courtesy of George Casey.

Figure 33. An E-facing close-up photograph of the Tavurvur cone at Rabaul from the bay's shore; it emphasizes its low conical shape, wide-aperture crater, and ongoing emissions. Courtesy of George Casey.

Figure 34. SE-looking overview photograph taken on 11 August from Rabaul caldera's topographic margin looking out over the city of Rabaul, Simpson harbor, and the cones that bound the harbor's NE side. The now-resurging caldera is breached by the sea on its E side. Tavurvur cone, ~10 km distant, lies in the peninsular lowlands to the right of the highest peak, Mother (Kombiu), and the lower peak, South Daughter. Courtesy of George Casey.

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: Ben Talai and Patrice de Saint-Ours, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Seismicity was generally at background levels during 1 November-7 December. Clouds obscured the volcano throughout much of the reporting period. On 1, 2, and 6 November steam-and-gas plumes were seen to rise 300 m above the summit before dispersing. High-frequency tremor increased over six hours on both 13 and 15 November. Periods of high-frequency tremor lasted 0.7 hours on 17 November and 3.5 hours on 22 November. Two hours of high-frequency tremor and 3 hours of low-frequency spasmodic tremor were recorded on 2 December. On 5-6 December a plume rose 150 m above the summit.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

There was a slight increase in activity in October according to reports from the Montserrat Volcano Observatory (MVO). Five small collapse events occurred on the dome, each producing significant deposits of ash up to 3 km away. Pyroclastic flows occurred along most of the volcano's main drainage. Ash fell predominantly W and NW of the volcano, light ash fell in the N of the island. Dome collapses were commonly followed by periods of volcanic tremor and ash venting, and sometimes swarms of volcano-tectonic earthquakes occurred shortly after the collapse events. The dome gradually eroded, leaving some large fractures in the carapace that could lead to larger collapses in the future.

Visual observations. Intermittent small pyroclastic flows originated from all flanks of the dome. The first significant event, at 0801 on 13 October, produced pyroclastic flows in Tuitt's Ghaut and Tyer's Ghaut. Volcanic tremor after the collapse correlated with ash venting from high on the dome's N flank, the ash cloud rapidly reached 7,500 m. The cloud drifted NW, depositing ash on parts of the island.

At 0916 on 18 October, there was another collapse, the ash cloud rose to around 2,000 m and moved W, although the exact direction was uncertain because a low cloud hampered observation. Subsequent volcanic tremor lasted for several hours.

Another small dome collapse occurred at 2241 on 20 October. The ash cloud from this event rose to an estimated 2,500 m, drifting slowly to the W and NW. Observations the following morning revealed that the pyroclastic flows from this event had traveled towards Plymouth as far as Upper Parsons (2.5 km W of the summit). Fallout included some coarse lithic fragments 4 to 5 mm in diameter.

At 0051 on 26 October, a fourth small collapse occurred. The seismic signal lasted for about 12 minutes followed by an extended period of tremor. Reports were received of thunder from the resultant ash cloud, and there was subsequent wet ashfall as far as 7 km N. Information received from NOAA satellite images indicated that the ash cloud reached to between 6,000 and 7,500 m. Observations during the early hours of the morning suggested that there were two ash cloud lobes, one S of Belham Valley and one over the Salem-Old Towne area. The deepest measured ashfall was 25 mm; 4 mm or more fell in other areas. The ash was fine grained, with common accretionary lapilli. During an observation flight on the 27th, steaming could be seen at the edge of the delta, indicating that the pyroclastic flows had traveled into the sea. The flows also reached NE as the Tar River Estate House (3 km from the summit). On the SW side, down the White River, a thin deposit of ash from the pyroclastic flows could be seen as far as about 700 m from the old coastline at O'Garras; when these deposits were emplaced is unknown.

A fifth small dome collapse occurred at 0418 on 31 October; an ash plume first drifted W, and thenN and NE depositing some ash in occupied areas at the island's N end. An observation flight later that day revealed new deposits: a pyroclastic-flow deposit in the White River reaching Galways Soufriere, and another in the Gages valley that did not extend beyond the top of the Gages fan. The White River deposit had numerous large angular blocks resting on its surface.

A large fissure within the dome extended from its base, where it rests against Chances Peak, to its top in the Galways area (S). At the foot of this crack a triangular-shaped opening had developed and appeared to have been the source of the White River pyroclastic-flow.

Unusual wind directions during the latter part of October directed the plume to the N. As a result, residents in N Montserrat smelled strong sulfurous odors.

On 27 October, probing into the pyroclastic deposits in the area of the Farm River in Trant's yielded these depth-temperature relations: 1.0 m and 86°C; 1.4 m and 146°C; and 2.25 m and 239°C. Unusually clear conditions in the early evening of 27 October enabled observers in Old Towne and Salem to see three small glowing areas on the dome; these areas were thought to reveal the dome's incandescent interior exposed during the recent collapse events.

Seismicity, deformation, and environmental monitoring. Over the reporting period seismicity was generally low; however, small dome collapses triggered volcanic tremor and swarms of volcano-tectonic earthquakes. As in the previous month, tremor correlated with intensified ash-and-steam venting from the N flanks of the dome.

Five small collapses occurred between 13 and 31 October. These were marked by pyroclastic-flow signals that lasted several minutes. The collapse on the 13th was preceded by a swarm of small volcano-tectonic earthquakes. Several much larger volcano-tectonic earthquakes occurred during the collapse, the first approximately 30 seconds after the start of the collapse; hypocenters for these events were tightly clustered directly under the lava dome.

The collapse on the 18th was accompanied by a more intense swarm of earthquakes (table 32). The first earthquake occurred about 40 seconds after the beginning of the collapse and was one of the largest earthquakes recorded since the installation of the broadband network; it was felt in the Woodlands area. This earthquake was much richer in low frequencies than typical volcano-tectonic earthquakes on Montserrat, possibly suggesting a larger source dimension. Hypocenters for the largest earthquakes were located S of the volcano. At the start of the swarm, hypocenters were directly under Roaches Mountain; as the swarm progressed, hypocenters migrated to S of Chances Peak. Preliminary calculations showed that the largest events were consistent with oblique-normal faulting in a NE-SW direction.

Table 32. October 1998 earthquake swarms at Soufriere Hills. Courtesy of MVO.

All GPS sites on the volcano and in the N of the island appear stable and there were no significant changes since last month. The EDM reflector on the northern flank was shot from Windy Hill. The line continues to shorten slowly. The site was later destroyed by a pyroclastic flow.

SO₂ flux, measured using the miniCOSPEC instrument, was (in metric tons/day) 1,300 on 9 October, 340 on 21 October, and 280 on 30 October. These results are similar to those measured in recent months, although an apparent decrease occurred late in the month. Sulfur dioxide was also measured at ground level using diffusion tubes around the island. SO₂ in Plymouth (at Police Headquarters) remained high; elsewhere the average levels were very low.

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat, West Indies (URL: http://www.mvo.ms/).

On the basis of waveform features and locations, earthquakes in the vicinity of the volcano during November were identified as constituting a separate group. Since September 1998 more than 20 events with magnitudes ranging from 0.5 to 1.0 occurred at shallow depths (<5 km).

Geologic Background. Ushkovsky volcano (formerly known as Plosky) is a large compound volcanic massif located at the NW end of the Kliuchevskaya volcano group. It consists of the flat-topped Ushkovsky (Daljny Plosky), which is capped by an ice-filled 4.5 x 5.5 km caldera, and the adjacent slightly higher peak of Krestovsky (Blizhny Plosky) volcano. Two glacier-clad cinder cones with large summit craters form a high point within the Ushkovsky caldera. Linear zones of cinder cones are found on the SW and NE flanks and on lowlands to the west. The younger caldera at the summit of Daljny was formed in association with the eruption of large lava flows and pyroclastic material from the Lavovy Shish cinder cones at the foot of the volcano about 8600 years ago. The only known historical activity was an explosive eruption from the summit cone in 1890.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

This report summarizes daily visual observations by members of the Proyecto de Observación Villarrica (POVI), volcano guides, and other sources during February to November 1998. In late February, after two months of subsidence, the magmatic column reached the crater floor with a weak and irregular degassing. By mid-March the lava pond was clearly visible as an intermittent red glow from 12 km away. In April and May, three convective magmatic pushes, gas-poor, filled half of the funnel-shaped crater with pahoehoe lava. On 13, 25, and 30 June, small phreatic emissions rose up to 200 m above the summit. Since mid-October, the activity level in the lava pond has varied, with the low levels of degassing intensity occurring at irregular intervals. On 8 November, the red glow was seen for the only time that month.

It is inferred that the red glow indicates that a small volume of usually gas-enriched magma has reached the crater floor in phases and at irregular intervals. This causes a sudden occurrence of the glow, sometimes with increasing intensity and lasting from a few hours up to 3 days. Subsequently, a distinct reduction of the glow intensity is interpreted to mean that an insufficient supply of convecting magma and gas allows the lava pond to form a crust. During the report period, 16 such magmatic pulses were observed and 10 additional pulses were inferred for periods of non-observation due to weather conditions.

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

Information Contacts: Werner Keller U., Proyecto de Observacion Villarrica (P.O.V.I.), Wiesenstrasse 8, 86438 Kissing, Germany (URL: https://www.povi.cl/).

Minor eruptive activity continued at White Island through November and early December. The level of activity varied, but observations during visits and instrumental indicators in early December were sufficient to raise the Alert Level from 1 to 2 on 3 December. The current style of activity was expected to continue for some time.

There was evidence that molten magma was the direct cause of eruptive activity, although only weak volcanic tremor accompanied the ash eruptions. A surveillance visit was made on 1 December to assess the ongoing activity, conduct deformation and magnetic surveys, and collect ash and gas samples.

Observations. The active vent at the base of the NW wall of 1978/90 Crater continued to erupt fine-grained volcanic ash during the 1 December visit. The vent size had not changed since the 2 November visit (BGVN 23:10). During the later visit, an ash-charged, tan-brown convecting plume rose to ~800 m before trailing downwind 10-15 km. The volume of ash in the plume was greater than that observed any time during November. The eruptive activity had deposited up to 45 mm of fine, dark gray and brown ash at the crater rim. Samples of ash that fell on 1 December showed a significant change from ash collected on 23 November and earlier. The 1 December ash samples contained fresh, vesiculated glass, suggesting that magma may have risen in the vent and was contributing directly to the eruption. Previously the ash was derived from solidified lava.

A ground-deformation survey showed a consistent trend of minor inflation across the main crater floor, with continued subsidence near the rim of 1978/90 Crater (figure 34). Large-scale post-1990 inflation was evident at the more distal sites (Pegs C and J), with only minor changes over the last 2-3 months. Collapse about the crater rim, which started in July, was continuing but at a lesser rate (Pegs M and W). Provisional results from the magnetic survey indicated heating at depth and shallow cooling about the crater rim area.

Figure 34. Contour plot from White Island showing the height changes (mm) between measurements made 2 November and 1 December 1998. Courtesy IGNS.

Fumarolic discharge pressures from sites 1, 6a (base of Donald Mound), and 13a were not significantly stronger than those observed on 2 and 16 November, and temperatures remained high at these features: site 1, 124°C; site 6a, 107°C; and site 13a, 120°C. Molten sulfur was found in vents at sites 1 and 13a, which is consistent with the temperatures in excess of 119°C. The sulfur mound at site 1 had grown over the vent during November, suggesting that sulfur was being remobilized from depth in response to elevated temperatures. The discharge at site 6a was mildly superheated, but of high pressure, indicating a relatively high gas content. These observations were consistent with general heating of the hydrothermal system.

The lake, which had reformed in the main crater, was the likely result of recent rains. The lake water was cool (~20°C) and had the brown color of the ash falling into it.

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: B.J. Scott, Manager of Volcano Surveillance, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).