Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early January 2018. Explosions and pyroclastic flows continued during 2018 and 2019, with several lava flows appearing in late 2019. This report covers continuing activity from January-June 2020 when ongoing explosive events produced ash plumes, pyroclastic flows, and the growth of new dome inside the crater. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Explosions with ash plumes rising up to three kilometers above the summit area were intermittent from late January through early June 2020. Some of the larger explosions produced pyroclastic flows that traveled down multiple flanks. Thermal anomalies within the Nicanor crater were recorded in satellite data several times each month from February through June. A reduction in overall activity led SERNAGEOMIN to lower the Alert Level from Orange to Yellow (on a 4-level, Green-Yellow-Orange-Red scale) during the first week of March, although tens of explosions with ash plumes were still recorded during March and April. Explosive activity diminished in early June and SERNAGEOMIN reported the growth of a new dome inside the Nicanor crater. By the end of June, a new flow had extended about 100 m down the N flank. Thermal activity recorded by the MIROVA project showed a drop in thermal energy in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in thermal and explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June (figure 52).

Figure 52. MIROVA thermal anomaly data for Nevados de Chillan from 8 September 2019 through June 2020 showed a drop in thermal activity in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June. Courtesy of MIROVA.

Weak gas emissions were reported daily during January 2020 until a series of explosions began on the 21st. The first explosion rose 100 m above the active crater; the following day, the highest explosion rose 1.6 km above the crater. The Buenos Aires VAAC reported pulse emissions visible in satellite imagery on 21 and 24 January that rose to 3.9-4.3 km altitude and drifted SE and NE, respectively. Intermittent explosions continued through 26 January. Incandescent ejecta was observed during the night of 28-29 January. The VAAC reported an isolated emission on 29 January that rose to 5.2 km altitude and drifted E. A larger explosion on 30 January produced an ash plume that SERNAGEOMIN reported at 3.4 km above the crater (figure 53). It produced pyroclastic flows that traveled down ravines on the NNE and SE flanks. The Washington VAAC reported on behalf of the Buenos Aires VAAC that an emission was observed in satellite imagery on 30 January that rose to 4.9 km altitude and was moving rapidly E, reaching 15 km from the summit at midday. The altitude of the ash plume was revised two hours later to 7.3 km, drifting NNE and rapidly dissipating. Satellite images identified two areas of thermal anomalies within the Nicanor crater that day. One was the same emission center (CE4) identified in November 2019, and the second was a new emission center (CE5) located 60 m NW.

Figure 53. A significant explosion and ash plume from the Nicanor crater at Nevados de Chillan on 30 January 2020 produced an ash plume reported at 7.3 km altitude. The left image was taken within one minute of the initial explosion. Images posted by Twitter accounts #EmergenciasÑuble (left) and T13 (right); original photographers unknown.

When the weather permitted, low-altitude mostly white degassing was seen during February 2020, often with traces of fine-grained particulate material. Incandescence at the crater was observed overnight during 4-5 February. The Buenos Aires VAAC reported an emission on 14 February visible in the webcam. The next day, an emission was visible in satellite imagery at 3.9 km altitude that drifted E. Episodes of pulsating white and gray plumes were first observed by SERNAGEOMIN beginning on 18 February and continued through 25 February (figure 54). The Buenos Aires VAAC reported pulses of ash emissions moving SE on 18 February at 4.3 km altitude. Ash drifted E the next day at 3.9 km altitude and a faint plume was briefly observed on 20 February drifting N at 3.7 km altitude before dissipating. Sporadic pulses of ash moved SE from the volcano on 22 February at 4.3 km altitude, briefly observed in satellite imagery before dissipating. Thermal anomalies were visible from the Nicanor crater in Sentinel-2 satellite imagery on 23 and 28 February.

Figure 54. An ash emission at Nevados de Chillan on 18 February 2020 was captured in Sentinel-2 satellite imagery drifting SE (left). Thermal anomalies within the Nicanor crater were measured on 23 (right) and 28 February. Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Only low-altitude degassing of mostly steam was reported for the first half of March 2020. When SERNAGEOMIN lowered the Alert Level from Orange to Yellow on 5 March, they reduced the affected area from 5 km NE and 3 km SW of the crater to a radius of 2 km around the active crater. Thermal anomalies were recorded at the Nicanor crater in Sentinel-2 imagery on 4, 9, 11, 16, and 19 March (figure 55). A new series of explosions began on 19 March; 44 events were recorded during the second half of the month (figure 56). Webcams captured multiple explosions with dense ash plumes; on 25 and 30 March the plumes rose more than 2 km above the crater. Fine-grained ashfall occurred in Las Trancas (10 km SW) on 25 March. Pyroclastic flows on 25 and 30 March traveled 300 m NE, SE, and SW from the crater. Incandescence was observed at night multiple times after 20 March. The Buenos Aires VAAC reported several discrete pulses of ash that rose to 4.3 km altitude and drifted SE on 20 and 21 March, SW on 25 March, and SE on 29 and 30 March. Another ash emission rose to 5.5 km altitude later on 30 March and drifted SE.

Figure 55. Sentinel-2 Satellite imagery of Nevados de Chillan during March 2020 showed thermal anomalies on five different dates at the Nicanor crater, including on 9, 11, and 16 March. A second thermal anomaly of unknown origin was also visible on 11 March about 2 km SW of the crater (center). Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Figure 56. Forty-four explosive events were recorded at Nevados de Chillan during the second half of March 2020 including on 19 March. Courtesy of SERNAGEOMIN webcams and chillanonlinenoticia.

In their semi-monthly reports for April 2020, SERNAGEOMIN reported 94 explosive events during the first half of the month and 49 during the second half; many produced dense ash plumes. The Buenos Aires VAAC reported frequent intermittent ash emissions during 1-13 April reaching altitudes of 3.7-4.3 km (figure 57). They reported the plume on 8 April visible in satellite imagery at 7.3 km altitude drifting SE. An emission on 13 April was also visible in satellite imagery at 6.1 km altitude drifting NE.

Figure 57. Sentinel-2 satellite imagery captured a strong thermal anomaly and an ash plume drifting SE from Nevados de Chillan on 10 April 2020. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

During the second half of April 2020, SERNAGEOMIN reported that only one plume exceeded 2 km in height; on 21 April, it rose to 2.4 km above the crater (figure 58). The Buenos Aires VAAC reported isolated pulses of ash on 18, 26, 28, and 30 April. During the second half of April SERNAGEOMIN also reported that a pyroclastic flow traveled about 1,200 m from the crater rim down the SE flank. The ash from the pyroclastic flow drifted SE and S as far as 3.5 km. Satellite images showed continued activity from multiple emission centers around the crater. Pronounced scarps were noted on the internal walls of the crater, attributed to the deepening of the crater from explosive activity.

Figure 58. Tens of explosions were reported at Nevados de Chillan during the second half of April 2020 that produced dense ash plumes. The plume on 21 April rose 2.4 km above the Nicanor crater. Photo by Josefa Carrasco Acuña from San Fabián de Alico; posted by Noticias Valpo Express.

Intermittent explosive activity continued during May 2020. The plumes contained abundant particulate material and were accompanied by periodic pyroclastic flows and incandescent ejecta around the active crater, especially visible at night. The Buenos Aires VAAC reported several sporadic weak ash emissions during the first week of May that rose to 3.7-5.2 km altitude and drifted NE. SERNAGEOMIN reported that only one explosion produced an ash emission that rose more than two km above the crater during the first two weeks of the month; on 6 May it rose to 2.5 km above the crater and drifted NE. They also observed pyroclastic flows on the E and SE flanks that day. Additional pyroclastic flows traveled 450 m down the S flank during the first half of the month, and similar deposits were observed to the N and NE. Satellite observations showed various emission points along the NW-trending lineament at the summit and multiple erosion scarps. Major erosion was noted at the NE rim of the crater along with an increase in degassing around the rim.

During the second half of May 2020 most of the ash plumes rose less than 2 km above the crater; a plume from one explosion on 22 May rose 2.2 km above the crater; the Buenos Aires VAAC reported the plume at 5.5 km altitude drifting NW (figure 59). Continuing pyroclastic emissions deposited material as far as 1.5 km from the crater rim on the NNW flank. There were also multiple pyroclastic deposits up to 500 m from the crater directed N and NE during the period. SERNAGEOMIN reported an increase in steam degassing between Nuevo-Nicanor and Nicanor-Arrau craters.

Figure 59. Explosions produced dense ash plumes and pyroclastic flows at Nevados de Chillan multiple times during May 2020 including on 22 May. Courtesy of SERNAGEOMIN.

Webcam images during the first two weeks of June 2020 indicated multiple incandescent explosions. On 3 and 4 June plumes from explosions reached heights of over 1.25 km above the crater; the Buenos Aires VAAC reported them drifting NW at 3.9 km altitude. Incandescent ejecta on 6 June rose 760 m above the vent and drifted NE. In addition, pyroclastic flows were distributed on the N, NW, E and SE flanks. Significant daytime and nighttime incandescence was reported on 6, 9, and 10 June (figure 60). The VAAC reported emission pulses on 6 and 9 June drifting E and SE at 4.3 km altitude.

Figure 60. Multiple ash plumes with incandescence were reported at Nevados de Chillan during the first ten days of June 2020 including on 6 June, after which explosive activity decreased significantly. Courtesy of SERNAGEOMIIN and Sismo Alerta Mexicana.

SERNAGEOMIN reported that beginning on the afternoon of 9 June 2020 a tremor-type seismic signal was first recorded, associated with continuous emission of gas and dark gray ash that drifted SE (figure 61). A little over an hour later another tremor signal began that lasted for about four hours, followed by smaller discrete explosions. A hybrid-type earthquake in the early morning of 10 June was followed by a series of explosions that ejected gas and particulate matter from the active crater. The vent where the emissions occurred was located within the Nicanor crater close to the Arrau crater; it had been degassing since 30 May.

Figure 61. A tremor-type seismic signal was first recorded on the afternoon of 9 June 2020 at Nevados de Chillan. It was associated with the continuous emission of gas and dark gray ash that drifted SE, and incandescent ejecta visible after dark. View is to the S, courtesy of SERNAGEOMIN webcam, posted by Volcanology Chile.

After the explosions on the afternoon of 9 June, a number of other nearby vents became active. In particular, the vent located between the Nuevo and Nicanor craters began emitting material for the first time during this eruptive cycle. The explosion also generated pyroclastic flows that traveled less than 50 m in multiple directions away from the vent. Abundant incandescent material was reported during the explosion early on 10 June. Deformation measurements showed inflation over the previous 12 days.

SERNAGEOMIN identified a surface feature in satellite imagery on 11 June 2020 that they interpreted as a new effusive lava dome. It was elliptical with dimensions of about 85 x 120 m. In addition to a thermal anomaly attributed to the dome, they noted three other thermal anomalies between the Nuevo, Arrau, and Nicanor craters. They reported that within four days the base of the active crater was filled with effusive material. Seismometers recorded tremor activity after 11 June that was interpreted as associated with lava effusion. Incandescent emissions were visible at night around the active crater. Sentinel-2 satellite imagery recorded a bright thermal anomaly inside the Nicanor crater on 14 June (figure 62).

Figure 62. A bright thermal anomaly was recorded inside the Nicanor crater at Nevados de Chillan on 14 June 2020. SERNAGEOMIN scientists attributed it to the growth of a new lava dome within the crater. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

A special report from SERNAGEOMIN on 24 June 2020 noted that vertical inflation had increased during the previous few weeks. After 20 June the inflation rate reached 2.49 cm/month, which was considered high. The accumulated inflation measured since July 2019 was 22.5 cm. Satellite imagery continued to show the growth of the dome, and SERNAGEOMIN scientists estimated that it reached the E edge of the Nicanor crater on 23 June. Based on these images, they estimated an eruptive rate of 0.1-0.3 m³/s, about two orders of magnitude faster than the Gil-Cruz dome that emerged between December 2018 and early 2019.

Webcams revealed continued low-level explosive activity and incandescence visible both during the day and at night. By the end of June, webcams recorded a lava flow that extended 94 m down the N flank from the Nicanor crater and continued to advance. Small explosions with abundant pyroclastic debris produced recurring incandescence at night. Satellite infrared imagery indicated thermal radiance from effusive material that covered an area of 37,000 m², largely filling the crater. DEM analysis suggested that the size of the crater had tripled in volume since December 2019 due largely to erosion from explosive activity since May 2020. Sentinel-2 satellite imagery showed a bright thermal anomaly inside the crater on 27 June.

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

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); #EmergenciasÑuble (URL: https://twitter.com/urgenciasnuble/status/1222943399185207296); T13, Channel 13 Press Department (URL: https://twitter.com/T13/status/1222951071443771394); Chillanonlinenoticia (URL: https://twitter.com/ChillanOnline/status/1240754211932995595); Noticias Valpo Express (URL: https://twitter.com/NoticiasValpoEx/status/1252715033131388928); Sismo Alerta Mexicana (URL: https://twitter.com/Sismoalertamex/status/1269351579095691265); Volcanology Chile (URL: https://twitter.com/volcanologiachl/status/1270548008191643651).

Bagana lies in a nearly inaccessible mountainous tropical rainforest area of Bougainville Island in Papua New Guinea and is primarily monitored by satellite imagery of ash plumes and thermal anomalies. After a state of elevated activity that lasted through December 2018 (BGVN 43:05, 44:06, 44:12), the volcano entered a quieter period that persisted through at least May 2020. This report focuses on activity between December 2019 and May 2020.

Atmospheric clouds often obscured satellite views of the volcano during the reporting period. When the volcano could be observed, light-colored gas plumes were often observed (figure 43). Based on satellite and wind model data, the Darwin Volcanic Ash Advisory Centre (VAAC) reported that during 29 February-2 March ash plumes rose to an altitude of 1.8-2.1 km and drifted SW and N. On 1 May an ash plume rose to an altitude of 3 km and drifted NW and W. According to both Darwin VAAC volcanic ash advisories, the Aviation Color Code was Orange (second highest of four hazard levels).

Figure 43. Sentinel-2 image of Bagana, showing a gas plume drifting SE on 13 March 2020, during a period when the Darwin VAAC had not reported any ash explosions (Natural Color rendering, bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

During the reporting period, the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system recorded only intermittent thermal anomalies, all of which were of low radiative power. Sulfur dioxide emissions detected by satellite-based instruments over this reporting period were at low levels.

Geologic Background. Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia's youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue-shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/).

Kerinci is a stratovolcano located in Sumatra, Indonesia that has been characterized by explosive eruptions with ash plumes and gas-and-steam emissions. The most recent eruptive episode began in April 2018 which has included intermittent explosions and ash plumes. The previous report (BGVN 44:12) described more recent activity consisting of intermittent gas-and-steam and ash plumes which occurred during June through early November 2019. This volcanism continued through May 2020, though little to no activity was reported during December 2019. The primary source of information for this report comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC).

Activity during December 2019 consisted of white gas-and-steam emissions rising 100-500 m above the summit. White and brown emissions continued intermittently through May 2020, rising to a maximum altitude of 1 km above the summit on 14 April. During 3-6 and 8-9 January 2020, the Darwin VAAC and PVMBG issued notices reporting brown volcanic ash rising 150-600 m above the summit drifting S and ESE (figure 19). PVMBG published a VONA notice on 24 January at 0828 reporting ash rising 400 m above the summit. Brown emissions continued intermittently throughout the reporting period. On 1 February, volcanic ash was observed rising 300-960 m above the summit and drifting NE; PVMBG reported continuing brown emissions during 1-3 February. During 16-17 February, two VONA notices reported that brown ash plumes rose 150-400 m above the summit and drifted SW accompanied by consistent white gas-and-steam emissions (figure 20).

Figure 19. Brown ash plume rose 500-600 m above Kerinci on 4 January 2020. Courtesy of MAGMA Indonesia via Øystein Lund Andersen.

Figure 20. White gas-and-steam emissions rose 400 m above Kerinci on 19 February 2020. Courtesy of MAGMA Indonesia via Øystein Lund Andersen.

During 1-16 and 25-26 March 2020 brown ash emissions were frequently observed rising 100-500 m above the summit drifting in multiple directions. During 6-8 and 10-15, April brown ash emissions were reported 50-1,000 m above the summit. The most recent Darwin VAAC and VONA notices were published on 14 April, reporting volcanic ash rising 400 and 600 m above the summit, respectively; however, PVMBG reported brown emissions rising up to 1,000 m. By 25-27 April brown ash emissions rose 50-300 m above the summit. Intermittent white gas-and-steam emissions continued through May. The last brown emissions seen in May were reported on the 7th rising 50-100 m above the summit.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); 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/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com, images at https://twitter.com/OysteinLAnderse/status/1213658331564269569/photo/1 and https://twitter.com/OysteinLAnderse/status/1230419965209018369/photo/1).

Tinakula is a remote stratovolcano located 100 km NE of the Solomon Trench at the N end of the Santa Cruz. In 1971, an eruption with lava flows and ash explosions caused the small population to evacuate the island. Volcanism has previously been characterized by an ash explosion in October 2017 and the most recent eruptive period that began in December 2018 with renewed thermal activity. Activity since then has consisted of intermittent thermal activity and dense gas-and-steam plumes (BGVN 45:01), which continues into the current reporting period. This report updates information from January-June 2020 using primary source information from various satellite data, as ground observations are rarely available.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed weak, intermittent, but ongoing thermal activity during January-June 2020 (figure 41). A small cluster of slightly stronger thermal signatures was detected in late February to early March, which is correlated to MODVOLC thermal alert data; four thermal hotspots were recorded on 20, 27, and 29 February and 1 March. However, observations using Sentinel-2 satellite imagery were often obscured by clouds. In addition to the weak thermal signatures, dense gas-and-steam plumes were observed in Sentinel-2 satellite imagery rising from the summit during this reporting period (figure 42).

Figure 41. Weak thermal anomalies at Tinakula from 26 June 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were intermittent and clustered more strongly in late February to early March.

Figure 42. Sentinel-2 satellite imagery shows ongoing gas-and-steam plumes rising from Tinakula during January through May 2020. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

Three distinct thermal anomalies were observed in Sentinel-2 thermal satellite imagery on 22 January, 11 April, and 6 May 2020, accompanied by some gas-and-steam emissions (figure 43). The hotspot on 22 January was slightly weaker than the other two days, and was seen on the W flank, compared to the other two that were observed in the summit crater. According to MODVOLC thermal alerts, a hotspot was recorded on 6 May, which corresponded to a Sentinel-2 thermal satellite image with a notable anomaly in the summit crater (figure 43). On 10 June no thermal anomaly was seen in Sentinel-2 satellite imagery due to the presence of clouds; however, what appeared to be a dense gas-and-steam plume was extending W from the summit.

Figure 43. Sentinel-2 thermal satellite images showing a weak thermal activity (bright yellow-orange) on 22 January 2020 on the W flank of Tinakula (top) and slightly stronger thermal hotspots on 11 April (middle) and 6 May (bottom) in at the summit, which are accompanied by gas-and-steam emissions. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. Similar to Stromboli, it has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The satellitic cone of Mendana is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Frequent historical eruptions have originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

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

Ibu is an active stratovolcano located along the NW coast of Halmahera Island in Indonesia. Volcanism has recently been characterized by frequent ash explosions, ash plumes, and small lava flows within the crater throughout 2019 (BGVN 45:01). Activity continues, consisting of frequent white-and-gray emissions, ash explosions, ash plumes, and lava flows. This report updates activity through June 2020, using data from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Darwin Volcanic Ash Advisory Centre (VAAC), and various satellites.

Volcanism during the entire reporting period dominantly consisted of white-and-gray emissions that rose 200-800 m above the summit drifting in multiple directions. The ash plume with the maximum altitude of 13.7 km altitude occurred on 16 May 2020. Sentinel-2 thermal satellite imagery detected multiple smaller hotspots within the crater throughout the reporting period.

Continuous ash emissions were reported on 6 February rising to 2.1 km altitude drifting E, accompanied by a hotspot visible in infrared satellite imagery. On 16 February, a ground observer reported an eruption that produced an ash plume rising 800 m above the summit drifting W, according to a Darwin VAAC notice. Ash plumes continued through the month, drifting in multiple directions and rising up to 2.1 km altitude. During 8-10 March, video footage captured multiple Strombolian explosions that ejected incandescent material and produced ash plumes from the summit (figures 21 and 22). Occasionally volcanic lightning was observed within the ash column, as recorded in video footage by Martin Rietze. This event was also documented by a Darwin VAAC notice, which stated that multiple ash emissions rose 2.1 km altitude drifting SE. PVMBG published a VONA notice on 10 March at 1044 reporting ash plumes rising 400 m above the summit. PVMBG and Darwin VAAC notices described intermittent eruptions on 26, 28, and 29 March, all of which produced ash plumes rising 300-800 m above the summit.

Figure 21. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and a dense ash plume. Video footage copyright by Martin Rietze, used with permission.

Figure 22. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and ash. Frequent volcanic lightning was also observed. Video footage copyright by Martin Rietze, used with permission.

A majority of days in April included white-and-gray emissions rising up to 800 m above the summit. A ground observer reported an eruption on 9 April, according to a Darwin VAAC report, and a hotspot was observed in HIMAWARI-8 satellite imagery. Minor eruptions were reported intermittently during mid-April and early to mid-May. On 12 May at 1052 a VONA from PVMBG reported an ash plume 800-1,100 m above the summit. A large short-lived eruption on 16 May produced an ash plume that rose to a maximum of 13.7 km altitude and drifted S, according to the Darwin VAAC report. By June, volcanism consisted predominantly of white-and-gray emissions rising 800 m above the summit, with an ash eruption on 15 June. This eruptive event resulted in an ash plume that rose 1.8 km altitude drifting WNW and was accompanied by a hotspot detected in HIMAWARI-8 satellite imagery, according to a Darwin VAAC notice.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected frequent hotspots during July 2019 through June 2020 (figure 23). In comparison, the MODVOLC thermal alerts recorded a total of 24 thermal signatures over the course of 19 different days between January and June. Many thermal signatures were captured as small thermal hotspots in Sentinel-2 thermal satellite imagery within the crater (figure 24).

Figure 23. Thermal anomalies recorded at Ibu from 2 July 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were frequent and consistent in power. Courtesy of MIROVA.

Figure 24. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) showed occasional thermal hotspots (bright orange) in the Ibu summit crater during January through June 2020. Courtesy of Sentinel Hub Playground.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); 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); Martin Rietze, Taubenstr. 1, D-82223 Eichenau, Germany (URL: https://mrietze.com/, https://www.youtube.com/channel/UC5LzAA_nyNWEUfpcUFOCpJw/videos, video at https://www.youtube.com/watch?v=qMkfT1e4HQQ).

Suwanosejima is an active stratovolcano located in the northern Ryukyu Islands. Volcanism has previously been characterized by Strombolian explosions, ash plumes, and summit incandescence (BGVN 45:01), which continues to occur intermittently. A majority of this activity originates from vents within the large Otake summit crater. This report updates information during January through June 2020 using monthly reports from the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and various satellite data.

During 3-10 January 2020, 13 explosions were detected from the Otake crater rising to 1.4 km altitude; material was ejected as far as 600 m away and ashfall was reported in areas 4 km SSW, according to JMA. Occasional small eruptive events continued during 12-17 January, which resulted in ash plumes that rose 1 km above the crater rim and ashfall was again reported 4 km SSW. Crater incandescence was visible nightly during 17-24 January, while white plumes rose as high as 700 m above the crater rim.

Nightly incandescence during 7-29 February, and 1-6 March, was accompanied by intermittent explosions that produced ash plumes rising up to 1.2 km above the crater rim (figure 44); activity during early February resulted in ashfall 4 km SSW. On 19 February an eruption produced a gray-white ash plume that rose 1.6 km above the crater (figure 45), resulting in ashfall in Toshima village (4 km SSW), according to JMA. Explosive events during 23-24 February ejected blocks onto the flanks. Two explosions were recorded during 1-6 March, which sent ash plumes as high as 900-1,000 m above the crater rim and ejected large blocks 300 m from the crater.

Figure 44. Surveillance camera images of summit incandescence at Suwanosejima on 29 January (top left), 21 (middle left) and 23 (top right) February, and 25 March (bottom left and right) 2020. Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Figure 45. Surveillance camera images of which and white-and-gray gas-and-steam emissions rising from Suwanosejima on 5 January (top), 19 February (middle), and 24 March 2020 (bottom). Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Nightly incandescence continued to be visible during 13-31 March, 1-10 and 17-24 April, 1-8, 15-31 May, 1-5 and 12-30 June 2020; activity during the latter part of March was relatively low and consisted of few explosive events. In contrast, incandescence was frequently accompanied by explosions in April and May. On 28 April at 0432 an eruption produced an ash plume that rose 1.6 km above the crater rim and drifted SE and E, and ejected blocks as far as 800 m from the crater. The MODVOLC thermal alerts algorithm also detected four thermal signatures during this eruption within the summit crater. An explosion at 1214 on 29 April caused glass in windows to vibrate up to 4 km SSW away while ash emissions continued to be observed following the explosion the previous day, according to the Tokyo VAAC.

During 1-8 May explosions occurred twice a day, producing ash plumes that rose as high as 1 km above the crater rim and ejecting material 400 m from the crater. An explosion on 29 May at 0210 produced an off-white plume that rose as high as 500 m above the crater rim and ejected large blocks up to 200 m above the rim. On 5 June an explosion produced gray-white plumes rising 1 km above the crater. Small eruptive events continued in late June, producing ash plumes that rose as high as 900 m above the crater rim.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed relatively stronger thermal anomalies in late February and late April 2020 with an additional six weaker thermal anomalies detected in early January (2), early February (1), mid-April (2), and mid-May (1) (figure 46). Sentinel-2 thermal satellite imagery in late January through mid-April showed two distinct thermal hotspots within the summit crater (figure 47).

Figure 46. Prominent thermal anomalies at Suwanosejima during July-June 2020 as recorded by the MIROVA system (Log Radiative Power) occurred in late February and late April. Courtesy of MIROVA.

Figure 47. Sentinel-2 thermal satellite images showing small thermal anomalies (bright yellow-orange) from two locations within the Otake summit crater at Suwanosejima. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The steeply sloped 1.4-km-diameter Kadovar Island is located in the Bismark Sea offshore from the mainland of Papua New Guinea about 25 km NNE from the mouth of the Sepik River. Its first confirmed observed eruption began in early January 2018, with ash plumes and lava extrusion resulting in the evacuation of around 600 residents from the N side of the island (BGVN 43:03). A dome appeared at the base of the E flank during March-May 2018 (Planka et al., 2019); by November activity had migrated to a new dome growing near the summit on the E flank. Pulsating steam plumes, thermal anomalies, and periodic ash emissions continued throughout 2019 (BGVN 44:05, 45:01), and from January-June 2020, the period covered in this report. Information was provided by the Rabaul Volcano Observatory (RVO), the Darwin Volcanic Ash Advisory Center (VAAC), satellite sources, and photographs from visitors.

Activity during January-June 2020. Intermittent ash plumes, pulsating gas and steam plumes, and thermal anomalies continued at Kadovar during January-June 2020. MIROVA thermal data suggested persistent low-level anomalies throughout the period (figure 45). Sentinel-2 satellite data confirmed thermal anomalies at the summit on 5 and 25 January 2020, and an ash emission on 20 January (figure 46). Persistent pulsating steam plumes were visible whenever the skies were clear enough to see the volcano.

Figure 45. Persistent low-level thermal activity at Kadovar was recorded in the MIROVA graph of radiative power from 2 July 2019 through June 2020. The island location is mislocated in the MIROVA system by about 5.5 km SE due to older mis-registered imagery; the anomalies are all on the island. Courtesy of MIROVA.

Figure 46. Sentinel-2 satellite data confirmed thermal anomalies at the summit of Kadovar on 5 (left) and 25 January 2020, and an ash emission and steam plume that drifted SE on 20 January (center). Pulsating steam-and-gas emissions left a trail in the atmosphere drifting SE for several kilometers on 25 January (right). Left image uses Atmospheric penetration rendering (bands 12, 11, 8a), center and right images use Natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

On 2 February 2020 the Darwin VAAC reported a minor eruption plume that rose to 1.5 km altitude and drifted ESE for a few hours. Another plume was clearly discernible in satellite imagery on 5 February at 2.1 km altitude moving SE. RVO issued an information bulletin on 7 February reporting that, since the beginning of January, the eruption had continued with frequent Vulcanian explosions from the Main Vent with a recurrence interval of hours to days. Rocks and ash were ejected 300-400 m above the vent. Rumbling could be heard from Blupblup (Rubrub) island, 15 km E, and residents there also observed incandescence at night. On clear days the plume was sometimes visible from Wewak, on the mainland 100 km W. Additional vents produced variable amounts of steam. The Darwin VAAC reported continuous volcanic ash rising to 1.5 km on 22 February that extended ESE until it was obscured by a meteoric cloud; it dissipated early the next day. A small double ash plume and two strong thermal anomalies at the summit were visible in satellite imagery on 24 February (figure 47).

Figure 47. Ash emissions and thermal anomalies continued at Kadovar during February 2020. Two small plumes of ash or dense steam rose from the summit on 24 February 2020, seen in this Natural color rendering (bands 4, 3, 2) on the left. The same image rendered in Atmospheric penetration (bands 12, 11, 8a) on the right shows two thermal anomalies in the same locations as the ash plumes. Courtesy of Sentinel Hub Playground.

The Darwin VAAC reported continuous ash emissions beginning on 13 March 2020 that rose to 1.5 km altitude and drifted SE. The plume was visible intermittently in satellite imagery for about 36 hours before dissipating. During April, pulsating steam plumes rose from two vents at the summit, and thermal anomalies appeared at both vents in satellite data (figure 48). Small but distinct SO₂ anomalies were visible in satellite data on 15 and 16 April (figure 49).

Figure 48. Steam plumes and thermal anomalies continued at Kadovar during April 2020. Top: A thermal anomaly at the summit accompanied pulsating steam plumes that drifted several kilometers SE before dissipating on 4 April 2020. Bottom left: Two gas-and-steam plumes drifted E from the summit on 9 April. Bottom right: Two adjacent thermal anomalies were present near the summit on 19 April. Top and bottom right images use Atmospheric penetration rendering (bands 12, 11, 8a), bottom left image uses Natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 49. Small but distinct SO2 anomalies were detected at Kadovar on 15 and 16 April 2020 with the TROPOMI instrument on the Sentinel-5P satellite. Nearby Manam often produces larger SO2 plumes that obscure evidence of activity at Kadovar. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Two summit vents remained active throughout May and June 2020, producing pulsating steam plumes that were visible for tens of kilometers and thermal anomalies visible in satellite data (figure 50). A strong thermal anomaly was visible beneath meteoric clouds on 8 June.

Figure 50. During May and June 2020 thermal and plume activity continued at Kadovar. Top: Gas-and-steam plumes drifted NW from two sources at the summit of Kadovar on 19 May 2020. Bottom left: Two thermal anomalies marked the E rim of the summit crater on 28 June 2020. Bottom right: A zoomed out view of the same 28 June image shows pulsating steam plumes drifting 10 km NW from Kadovar. Top image is Natural color rendering (bands 4, 3, 2). Bottom images are Atmospheric penetration rendering (bands 12, 11, 8a) of Sentinel-2 images. Courtesy of Sentinel Hub Playground.

Visitor observations on 21 October 2019. Claudio Jung visited Kadovar on 21 October 2019. Shortly before arriving on the island an ash plume rose tens of meters above the summit and drifted W (figure 51). From the NW side of the summit crater rim, Jung saw the actively growing dome on the side of a larger dome, and steam and gas issuing from the growing dome (figure 52). The crater rim was covered with dead vegetation, ash, and large bombs from recent explosions (figure 53). The summit dome had minor fumarolic activity around the summit area and dead vegetation halfway up the flank (figure 54) while the fresh blocky lava of the actively growing dome on the E side of the summit produced significant steam and gas emissions. The growing dome produced periodic pulses of dense steam during his visit (figure 55).

Figure 51. Views looking S show the shoreline dome at the base of the E flank of Kadovar that was active during March-May 2018 (left), and an ash plume drifting W from the summit dome located on the E side of the summit crater (right) on 21 October 2019. Copyrighted photos courtesy of Claudio Jung, used with permission.

Figure 52. A panorama looking SE from the crater rim of Kadovar on 21 October 2019 shows the actively growing dome on the far left with a narrow plume of steam and gas being emitted. A large dome fills the summit crater; the crater rim is visible on the right. Copyrighted photo courtesy of Claudio Jung, used with permission.

Figure 53. The crater rim of Kadovar on 21 October 2019 was covered with dead vegetation, ash, and large bombs from recent explosions. Person is sitting on a large bomb; weak fumarolic activity is visible along the rim. Copyrighted photo courtesy of Claudio Jung, used with permission.

Figure 54. The summit dome of Kadovar on 21 October 2019 had minor fumarolic activity around most of its summit and dead vegetation half-way up the flank (left). The dead tree stumps suggest that vegetation covered the lower half of the dome prior to the eruption that began in January 2018. The fresh blocky lava of the actively growing dome on the E side of the summit dome produced significant steam and gas emissions (right). Copyrighted photos courtesy of Claudio Jung, used with permission.

Figure 55. Dense steam from the growing dome on the E side of the summit drifted W from Kadovar on 21 October 2019. Copyrighted photo courtesy of Claudio Jung, used with permission.

Reference: Planka S, Walter T R, Martinis S, Cescab S, 2019, Growth and collapse of a littoral lava dome during the 2018/19 eruption of Kadovar Volcano, Papua New Guinea, analyzed by multi-sensor satellite imagery, Journal of Volcanology and Geothermal Research, v. 388, 15 December 2019, 106704, https://doi.org/10.1016/j.jvolgeores.2019.106704.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Claudio Jung (URL: https://www.instagram.com/jung.claudio/).

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).

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).

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).

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).

We previously reported fumarolic activity from November 1997 to August 1998, but issued no subsequent Bulletin reports for Chiginagak. This report covers the formation of a summit ice cauldron and crater lake and subsequent draining of the lake resulting in the acidification of Mother Goose Lake during 2000-2010. Records of Chiginagak's past activity are poor. It is not seismically monitored and, because of its remote location, much of the information is limited to observations of nearby residents. The primary source of information for this report has been Alaska Volcano Observatory (AVO) annual reports (McGimsey and others, 1999; McGimsey and others, 2004; Neal and others, 2004; and McGimsey and others, 2008).

Increased fumarolic activity occurred from November 1997-August 1998. AVO reports that the activity during that time was a result of formation of new fumaroles on the N flank of the volcano. In November of 1997 an increase in steam emission led to increased snowmelt (BGVN 22:11). The steam was accompanied by the smell of sulfur. Through January 1998 a robust steam plume was observed by AVO several times. In March 1998 vigorous fumarolic activity continued, characterized by gray clouds and a strong sulfur smell that was reported up to 49 km away. In August 1998 a plume of black ash and greenish-yellow gas rose from the volcano's fumaroles. In late July-early August 2000 Chiginagak again released a larger than normal plume.

Glacial Outburst Flooding. Between November 2004 and May 2005 non-explosive geothermal activity melted the snow and ice filling Chiginagak's summit crater, forming an ice cauldron ~400 m wide and ~105 m deep. The melt waters formed an acidic lake within the cauldron. The water from the lake melted a tunnel through the summit ice, draining the cauldron. The resulting lahar flowed down the SW flank of the volcano probably in May 2005, photographed August 20, 2005 (figure 1).

Figure 1. Lahar deposits on the SW flank of Chiginagak, caused as a result of draining of the lake, which likely occurred in May 2005. Photograph by Game McGimsey, AVO/USGS, August 20, 2005.

The water from the cauldron continued downstream into Mother Goose Lake, ~27 km downstream to the NW of Chiginagak (figure 2) and in August 2005 Mother Goose Lake became acidic, with pH dropping to 2.9. This killed the majority of aquatic life in the lake and damaged flora surrounding both the lake and the rivers (Indecision Creek and Volcano Creek which transport water from Chiginagak to Mother Goose Lake and King Salmon River that flows from the lake). Below a pH of 4.5, essentially no large fish are able to survive (figure 3). It is not just the acidity that kills aquatic fauna but also high levels of metals such as Al and Fe. At a pH of 5, Al³⁺ becomes insoluble and has a toxic effect on fauna. The acidic water was accompanied by sulfurous, clay-rich debris and acidic aerosols. The high acidity of the lake prevented the annual salmon run that typically ascends into Mother Goose Lake.

Figure 2. Acidic water from Mt. Chiginagak escaped the summit cauldron lake and traveled downstream into Mother Goose Lake. Bold lines indicate drainages that were affected by the acidic water, and thin lines indicate unaffected drainages. Modified from Schaefer and others (2011).

Figure 3. This chart shows the varying pH levels at which aquatic life either leave an environment or die. Courtesy of U.S. Environmental Protection Agency (EPA).

The pH at Mother Goose Lake has been monitored since 2005 and the pH has slowly returned to normal. By 2010 the lake water returned towards normal conditions; pH reached 5.2 and a variety of fish have returned to the lake. By August 2011 the pH had reached 6.9.

In 2005, Kassel (2009) studied the slurry pH deposited at Mother Goose Lake. Slurry pH is the standard method for estimating pH of soils; it is similar to pore water measurements. The details of the process used can be found in Kassel (2009, p.27-30). The slurry pH of Mother Goose Lake in 2005 was approximately the same as the pH of the lake at that time. The assumption can be made that slurry pH reflects lake pH at the time of deposition. Based on slurry pH seen in core samples, at least 7 similar events have occurred at Mother Goose Lake in the last ~3,800 years, including the 2005 event. Only one of these events was associated with tephra deposits, therefore the majority of the events were seemingly triggered by non-explosive geothermal activity, similar to the event in 2005.

According to McGimsey and others (2008), "The area is remote, and the active fumaroles frequently produce visible steam plumes, which have been mistaken for eruptive activity. Unverified reports of minor activity are attributed to 1852, 1929, and 1971. An event similar to the outburst flooding in 2005 may have occurred in the early 1970s according to third-person accounts from a cabin owner on Mother Goose Lake, who reported flooding from the volcano, discoloration of the lakeshore, vegetation damage, and interruption of the annual salmon run (Jon Kent, local lodge owner, oral commun., 2004)."

References. US Environmental Protection Agency, 2008, Effects of Acid Rain - Surface Waters and Aquatic Animals, Updated 1 December 2008, Accessed 15 Febuary 2012 (URL: epa.gov/acidrain/effects/surface_water.html).

Kassel, CM, 2009, Lacustrine Evidence from Mother Goose Lake of Holocene Geothermal Activity at Mount Chiginagak, Alaska Peninsula, Northern Arizona University, 276 p.

McGimsey, RG, and Wallace, KL, 1999, 1997 Volcanic Activity in Alaska and Kamchatka: Summary of Events and Response of the Alaska Volcano Observatory, Open-File Report 99-448, U.S. Department of the Interior, U.S. Geological Survey, 42 p.

McGimsey, RG, Neal, CA, and Girina, O, 2004, 1998 Volcanic Activity in Alaska and Kamchatka: Summary of Events and Response of the Alaska Volcano Observatory, Open-File Report 03-423, U.S. Department of the Interior, U.S. Geological Survey.

McGimsey, RG, Neal, CA, Dixon, JP and Ushakov, S, 2008, 2005 Volcanic Activity in Alaska, Kamchatka, and the Kurile Islands: Summary of Events and Response of the Alaska Volcano Observatory, Scientific Investigations Report 2007-5269, U.S. Department of the Interior, U.S. Geological Survey.

Neal, CA, McGimsey, RG, and Chubarova, O, 2004, 2000 Volcanic Activity in Alaska and Kamchatka: Summary of Events and Response of the Alaska Volcano Observatory, Open-File Report 2004-1034, U.S. Department of the Interior, U.S. Geological Survey.

Schaefer, JR, Scott, WE, Evans, WC, Wang, B and McGimsey, RG, 2011, Summit Crater Lake Observations, and the Location, Chemistry, and pH of Water Samples Near Mount Chiginagak Volcano, Alaska: 2004-2011, Report of Investigations 2011-6, State of Alaska Department of Natural Resources, Division of Geological & Geophysical Surveys.

Geologic Background. The symmetrical, calc-alkaline Chiginagak stratovolcano located about 15 km NW of Chiginagak Bay contains a small summit crater, which is breached to the south, and one or more summit lava domes. Satellitic lava domes occur high on the NW and SE flanks of the glacier-mantled volcano. An unglaciated lava flow and an overlying pyroclastic-flow deposit extending east from the summit are the most recent products of Chiginagak. They most likely originated from a lava dome at 1687 m on the SE flank, 1 km from the summit of the volcano, which has variably been estimated to be from 2075 to 2221 m high. Brief ash eruptions were reported in July 1971 and August 1998. Fumarolic activity occurs at 1600 m elevation on the NE flank of the volcano, and two areas of hot-spring travertine deposition are located at the NW base of the volcano near Volcano Creek.

Information Contacts: 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; State of Alaska Department of Natural Resources, Division of Geological & Geophysical Surveys, 3354 College Rd., Fairbanks, Alaska 99709-3707, USA.

Our last report on Cleveland volcano, August 2011 (BGVN 36:08), described lava dome growth in August-September 2011. This report first addresses late 2011 to early 2012 observations, and then presents some amendments to Bulletin reports over the last decade.

Late 2011-early 2012. According to the Alaska Volcano Observatory (AVO), by the first week of October 2011 satellite images showed the lava dome was within 10 m of the crater rim on the SW and ENE sides of the crater. On 23 October, a TerraSAR-X satellite radar image of Cleveland showed no discernable growth in the lava dome over the course of the past several weeks. Instead, the 23 October image showed deflation or collapse of the dome.

On 3 November 2011, citing lack of dome growth evident in satellite images, AVO lowered both the Aviation Color Code to YELLOW and the Alert Level to ADVISORY. Throughout November, weather permitting, AVO continued to observe thermal anomalies and steam plumes in satellite imagery, consistent with cooling of the emplaced hot dome. Observations in early December 2011 showed continued deflation and cooling of the lava dome, which was about 1x10⁶ m³ in volume.

On 29 December 2011, AVO observed in satellite imagery a detached, drifting ash cloud at an altitude of ~4.6 km and ~80 km ESE of Cleveland. Ground-coupled airwaves from an explosion were also detected at the distant Okmok seismic network, placing the time of explosion at 1312 (UTC) on 29 December 29.

Based on the presence of an ash cloud, on 29 December AVO raised the aviation color code to ORANGE and the alert level to WATCH. On 30 December, with no new explosive activity, AVO lowered the aviation color code to YELLOW and the alert level to ADVISORY. Subsequent satellite images showed that the 25 December (recognized in retrospective data analysis) and 29 December explosions had largely removed the dome.

On 30 January 2012, satellite data showed another small dome within the summit crater, which measured ~ 40 m in diameter by 30 January. On 31 January, AVO raised the aviation color code to ORANGE and the volcano alert level to WATCH. No observations of elevated surface temperatures or ash emissions from Cleveland were noted during 15-21 February. On 17 February, AVO reported that partly-cloudy satellite observations over the past week revealed that the current lava dome had grown to about 60 m in diameter and occupied a small portion of the approximately 200-m diameter summit crater. On 19 February an elevated surface temperature was detected in satellite images. As of this date, there is no real-time seismic monitoring network on Mount Cleveland.

Amendments to Bulletin. According to Diefenbach, Guffanti, and Ewert, (2009), "During the past 29 years, 43 volcanoes within the United States have produced 95 eruptions and 32 episodes of unrest. More than half of the 30 eruptive volcanoes have erupted two or more times. The majority (77 percent) of U.S. eruptions has occurred in Alaska. Akutan volcano in Alaska has produced the most eruptions (11) in the past 29 years, followed by Veniaminof (10), Cleveland (9), and Pavlof (8)."

Because of the relative importance of Cleveland in the Aleutian chain as a source of active volcanism along a busy commercial airline route, we revisited the AVO web site recently to compare information available with that which we used to prepare the Bulletin in the past. As a prelude to this section, table 4 lists Cleveland eruptions reported by the AVO during 2001-2012 and the issues of the Bulletin covering a particular event.

Table 4. Dates of significant eruptions as reported by the AVO web site for Cleveland from January 2001 through January 2012, and related BGVN reports covering the respective eruptions. These data were accessed 9 February 2012; as of that date, the latest eruption reported by AVO was the one of 19 July 2011. From the AVO web site.

We amend some of our previous Bulletin reports with the following excerpts from USGS reports of Cleveland eruptions since 2001, ending with the last Bulletin containing a report on Cleveland (BGVN 36:08). The dates for the eruptions are the start and stop dates from the USGS reports.

Item a, Table 4 - BGVN 26:01: On 19 February 2001, Cleveland volcano erupted explosively at ~1430 UTC and AVO established the eruption termination date as 15 April 2001. However, after the eruption, AVO received reports indicating that precursory emission activity had taken place. Most graphic was a photograph taken on 2 February 2001 by a pilot flying by the volcano showing a dark, lobate deposit on the snow-covered SW flank and robust steaming from the summit crater.

Item a, Table 4 - BGVN 26:04: According to AVO, in 2001, ash fall from the February 2001 eruption of Cleveland was observed only at Nikolski over a ~5 hr on 19 February 2001. A sample from Nikolski showed that the ash was composed of glass shards, crystals, and lithics. The glass was dacitic and had a magmatic morphology rather than phreatomagmatic.

Item b, Table 4 - BGVN 30:09: On 27 April 2005, the Federal Aviation Association (FAA) alerted AVO of a pilot report of eruptive activity (ash cloud 4.6-5.5 km altitude) in the vicinity of Cleveland (based on coordinates from the pilots). Although satellite images and nearby seismic stations showed no evidence of activity, a one-time Urgent Pilot Report and a one-time SIGMET were issued.

Item c, Table 4 - BGVN 31:01: AVO noted that by the end of 6 January 2006 there were no further reports or images of ash production at Cleveland.

Item f, Table 4 - BGVN 33:02: Satellite data from February 2007 revealed evidence of recent activity involving ejection of bombs and debris on the upper flanks and generation of water-rich flows that traveled halfway to the coast. No ash emissions or ash fall deposits were observed. This level of activity -accompanied by persistent thermal anomalies - occurred throughout the spring and early summer. On 4 March 2008, a pilot reported minor ash to 1.5 km above sea level in the vicinity of Cleveland, and a weak thermal anomaly was observed the following day.

Item g, Table 4 - BGVN 33:11: The volcano was relatively quiet until 28 October 2008, when an ash cloud rising to ~4.6 km and drifting E was spotted in satellite imagery. On 29 October, another cloud, 160 km long and drifting NE at an altitude of 3.0 km with little or no ash was observed. A strong thermal anomaly over the summit of the volcano was noted on 30 October 2008, but given the low-level nature of the recent activity, AVO did not elevate the Color Code or Alert Level.

Item k, Table 4 - BGVN 36:05: AVO continued to detect thermal anomalies on 14, 15, 25, and 26 September 2010, and 1 October. During the other days, clouds prevented satellite observation of Cleveland. Although the weather usually prevented observations of Cleveland, weak thermal anomalies were also detected on 14, 19, 25, and 29 October 2010. Clouds completely obscured observations for the week of 1-6 November 2010, but thermal anomalies were again detected on 7 November. The weather then remained cloudy until 16,17, 25, 28, and 30 November 2010, when thermal anomalies were again visible. Thermal anomalies were also recorded on 6, 13, 14, 23, and 27 December 2010, and weak thermal anomalies were visible on 1, 11, and 16 January 2011. A weak thermal anomaly was observed on 1 February 2011, and on 9 February a pilot overflew Cleveland and reported minor, repetitive steam emissions rising hundreds of meters above the summit. The snow on the flanks was pristine, with no indication of recent ash emissions. Steam emissions are common at Cleveland and do not indicate an increased level of unrest.

References. Cervelli, P. F., and Cameron, C. E., 2008, Causation or coincidence? The correlations in time and space of the 2008 eruptions of Cleveland, Kasatochi, and Okmok Volcanoes, Alaska, EOS, Transactions of the American Geophysical Union, Fall Meeting 2008, abstract ##A53B-0278.

Diefenbach, A.K., Guffanti, M., and Ewert, J.W., 2009, Chronology and References of Volcanic Eruptions and Selected Unrest in the United States, 1980-2008, U.S. Geological Survey Open-File Report 2009-1118, 85 p (http://pubs.usgs.gov/of/2009/1118/).

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

Information Contacts: 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; USAToday (URL: http://www.usatoday.com).

Following the 17 September 2006 phreatic eruption of Fourpeaked volcano and subsequent non-juvenile ash emissions and debris flows (Cervelli and West, 2007; BGVN 31:09), low level seismicity (up to M 1.8) and emissions (S0₂ fluxes up to almost 3,000 tons/day) continued during late 2006 and the first half of 2007. Small explosions occurred during February-April 2007 amidst declining gas emissions. The Alaska Volcano Observatory (AVO) lowered the Aviation Color Code and Volcano Alert Level from Yellow/Advisory to Green/Normal on 6 June 2007 (on a scale from Green/Normal to Red/Warning).

Seismic monitoring network. Prior to the 2006-2007 eruption and unrest, Fourpeaked lacked a monitoring network (BGVN 31:09). A network of monitoring instruments was deployed in stages following the onset of unrest in 2006 (figure 8). The network consisted of 4 short-period seismometers (3 newly-deployed and 1 pre-existing), 2 co-located pressure sensors, and a web camera. As a result of the stepwise deployment of the instruments, the precision and number of earthquakes successfully located by AVO increased during the active period. Following the network's successful operation through the winter of 2006-2007, Fourpeaked was formally recognized as the 31st seismically monitored Alaskan volcano on 3 May 2007.

Figure 8. The seismic monitoring network of Fourpeaked volcano. Orange stars indicate the 4 short-period seismometers monitoring Fourpeaked; black stars indicate other nearby seismometers; triangles indicate volcanoes. Modified from Gardine and others (2011).

November 2006-June 2007 activity. AVO reported that low level seismicity and persistent steaming (reaching up to several hundred meters above the summit) continued through the end of 2006. McGimsey and others (2011) reported that an airborne gas survey on 6 November 2006 showed continued elevated S0₂ emissions (~1,000 tons/day). The measured S0₂ flux measured soon after the 17 September eruption (figure 9) was more than 2,000 tons/day (McGimsey and others, 2011). In January 2007, AVO reported an earthquake swarm (swarm IV, figure 9), but stated that it was not considered unusual. Until 8 February, activity was typical of the past few months.

Figure 9. Recorded syn- and post-eruptive seismicity and S02 emissions at Fourpeaked volcano. Plots indicate the number of earthquakes per month (gray bars), timing of earthquake swarms (I-IV, vertical dashed black lines), and measured S02 emissions (tons/day, dark gray points and trend). The data extend 13 months following the 17 September 2006 phreatic eruption (swarm I). Courtesy of Gardine and others (2011).

Beginning on 8 February 2007, AVO reported small explosive events that were registered on seismic and acoustic instruments, and a possible large steam plume that was noticed in a partly cloudy satellite view. A swarm of 13 locatable earthquakes occurred on 18 February, the largest of which was an M 1.8 event at ~4 km deep; this was the largest seismic event of the 2006-2007 Fourpeaked activity (McGimsey and others, 2011). A gas overflight on 22 February recorded S0₂ flux values below those measured in November.

Occasional small eruptions continued through March 2007, while seismicity gradually decreased (McGimsey and others 2011). In the last week of March, AVO reported decreased steam emissions from the vents at the summit. Explosive activity and declining gas emissions continued throughout April, and on 18 May, an aerial gas measurement revealed that the S0₂ flux had decreased to less than 90% of the measured values in September 2006 (Cervelli and West, 2007).

On 6 June 2007, citing declining seismicity and gas emissions, AVO lowered the Aviation Color Code from Yellow to Green, and the Volcanic Activity Alert Level from Advisory to Normal. They noted that "local hazards still [existed] near the summit, including jetting steam and/or very small explosions, unstable snow and ice, hot water and rock, and the possibility for high concentrations of dangerous volcanic gas."

Magma intrusion. Gardine and others (2011) analyzed seismic and gas emission data from the 2006-2007 Fourpeaked eruption and unrest (figure 9) in order to constrain the origin of the eruptive activity. Their findings suggested that the high levels of seismicity and gas emissions during the initial unrest indicated the intrusion of new magma into the upper 10 km of crust. They suggested that the intrusion reactivated fractures, allowing gases exsolved from the magma to be released at the surface. They argued that continued exsolution provided the gases released during the period of unrest, while local stress accumulation led to earthquake swarms (figure 9). They also suggested that the activity ceased only after the magma had cooled and degassed to a point where it became trapped and could no longer overcome the overburden pressure.

References. Cervelli, P.F. and West, M., 2007, The 2006 Eruption of Fourpeaked Volcano, Katmai National Park, Alaska, American Geophysical Union, Fall Meeting 2007, abstract ##V31E-0719.

Gardine, M., West, M., Werner, C., and Doukas, M., 2011, Evidence of magma intrusion at Fourpeaked volcano, Alaska in 2006-2007 from a rapid-response seismic network and volcanic gases, Journal of Volcanology and Geothermal Research, v. 200, issues 3-4, p. 192-200 (DOI: 10.1016/j.jvolgeores.2010.11.018).

McGimsey, R.G., Neal, C.A., Dixon, J.P., Malik, N., and Chibisova, M., 2011, 2007 Volcanic Activity in Alaska, Kamchatka, and the Kurile Islands: Summary of Events and Response of the Alaska Volcano Observatory, US Geological Society Scientific Investigations Report 2010-5242, 103 p.

Geologic Background. Poorly known Fourpeaked volcano in NE Katmai National Park consists of isolated outcrops surrounded by the Fourpeaked Glacier, which descends eastward almost to the Shelikof Strait. The orientation of andesitic lava flows and extensive hydrothermal alteration of rocks near the present summit suggest that it probably marks the vent area. Eruptive activity during the Holocene had not been confirmed prior to the first historical eruption in September 2006. A N-trending fissure extending 1 km from the summit produced minor ashfall.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey (USGS), 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 (ADGGS), 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.dggs.alaska.gov/).

This report on Ol Doinyo Lengai (hereafter, Lengai) is a continuation of previous Bulletin reports that were based in part on those found on Frederick Belton's Lengai web site (Belton, 2012). Our last report was in September 2010 (BGVN 35:09). Figures 149 and 150 show aerial photographs of Lengai in late 2010.

Figure 149. Aerial photo of the N crater of Lengai, looking NE, in late November 2010. Courtesy of Ben Wilhelmi; from Belton (2012).

Figure 150. Dave Simpson, a guide working in Kenya, flew over Lengai on 6 December 2010 and took this photo looking at a steep angle downward into the crater. Cloudy conditions prevailed, but Simpson saw no areas of fresh lava or other activity. Several darker areas on the rim of the crater are results of slope failure. Courtesy of Dave Simpson; from Belton (2012).

On 22 June 2011, Hans Schabel took a group of 8 conservation biologists to the Lengai summit up the regular approach along the NNW trail through the Pearly Gates (PG). During the ascent, the weather was cold, worsened by strong, increasingly sulfurous gusts from above. Minor fumaroles produced small clouds just above the PG. At the summit, conditions were relatively clear, making details of the crater rim and the pit visible. The slump on the E crater that Schabel first saw on his previous climb (16 January 2010, BGVN 35:05) had not expanded significantly, but some of the walls of the crater below had obviously slumped into big piles of rubble below. The group heard a 'whoosh' from two boiling, rolling, lava pools that spilled pitch-black lava into a growing lake flowing E in the crater floor (figure 151).

Figure 151. Lava lake seen at the bottom of Lengai's N Crater (photo looking N), 22 June 2011. Courtesy of Hans Schabel; from Belton (2012).

New Reports. Two recent research papers have been published concerning the 2007-2008 explosive eruptions of Lengai (BGVN 32:11, 33:02, 33:06, 33:08, 34:02, and 34:05). Kervyn and others (2010) and Keller and others (2010) summarize the first relatively closely documented 'cycle' from natrocarbonatite to carbonated nephelinite at Lengai. According to Kervyn and others (2010), on 4 September 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Lengai changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007.

Keller and others (2010) noted that, with its paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Lengai dramatically changed its behavior, crater morphology (figure 152), and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembled the explosive phases of 1917, 1940-1941, and 1966-1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Chemical analyses of the erupted products showed that the 2007-2008 explosive eruptions were associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provided constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, variations observed repetitively in the last 100 years at Lengai.

Figure 152. Morphological evolution of the Lengai volcano active crater from June 2007 to June 2008 illustrated by aerial photographs (date, day-month-year, of each photo shown at left bottom): (a) one central pit crater; (b) the massive lava emission at end of August 2007; (c-d) the progressive growth of an ash cone covering the hornitos in the first months of the 2007 explosive phase; (e-f) the rapidly evolving morphology of the new crater within a prominent ash cone and the changing vent location in January to February 2008; (g) the overgrowth of the inner cone slopes grew outward and here extend to the steep flanks by 18 March 2008; and (g-h) the inner cone grew to the point where its slopes reached the outer slopes of the volcano, and it acquired a deep and wide crater formed by the paroxysmal outbursts of February-March 2008. Photos from Kervyn and others (2010).

Table 25 gives the summary of historical activity of Lengai from Keller and others (2010). The table shows the repeated occurrence of explosive paroxysms with documented or inferred natrocarbonatite activity in between the explosive eruptions.

Table 25. Synopis of the historical activity of Lengai, with observations covering about 100 years (since 1904 to 2008). References cited in the table are listed in the 'References to Table 25' section at the end of this Bulletin report. From Keller and others (2010).

References. Belton, F., 2012, Mountain of God (URL: http://oldoinyolengai.pbworks.com/w/page/33191422/Ol Doinyo Lengai2C The Mountain of God).

Keller, J., Klaudius, J., Kervyn, M., Ernst, G.G.J., and Mattsson, H.B., 2010, Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania: I. New magma composition during the 2007-2008 explosive eruptions, Bulletin of Volcanology, v. 72, no. 8, pp. 893-912. DOI 10.1007/s00445-010-0371-x.

Kervyn, M., Ernst, G.G.J., Keller, J., Vaughan, R.G., Klaudius, J., Pradal, E., Belton, F., Mattsson, H.B., Mbede, E., and Jacobs, P., 2010, Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania: II. Eruptive behaviour during the 2007-2008 explosive eruptions, Bulletin of Volcanology, v. 72, no. 8, pp. 913-931. DOI 10.1007/s00445-010-0360-0.

Klaudius, J., and Keller, J., 2006, Peralkaline silica lavas at Oldoinyo Lengai, Tanzania, Lithos, v. 91, no. 1-4, pp. 173-190.

Mattsson, H.B., and Reusser, E., 2010, Mineralogical and geochemical characterization of ashes from an early phase of the explosive September 2007 eruption of Oldoinyo Lengai (Tanzania), Journal of African Earth Sciences, v. 58, no. 5, pp. 752-763.

Wiedenmann, D., Keller, J., and Zaitsev, A.N., 2010, Melilite-group minerals at Oldoinyo Lengai, Tanzania, Lithos, v. 118, no. 1-2, pp. 112-118.

References to Table 25. Barns, T.A., 1921, The highlands of the Great Craters, Tanganyika Territory, Geographic Journal, v. 58, pp. 401-416.

Dawson, J.B., 1962, The geology of Oldoinyo Lengai, Bulletin of Volcanology, v. 24, pp. 348-387.

Dawson, J.B., Bowden, P., and Clark, G.C., 1968, Activity of the carbonatite volcano Oldoinyo Lengai, 1966, Geol Rundsch, v. 57, pp. 865-879.

Dawson, J.B., Pinkerton, H., Norton, G.E., and Pyle, D., 1990, Physicochemical properties of alkali carbonatite lavas: data from the 1988 eruption of Oldoinyo Lengai, Tanzania, Geology, v. 18, pp. 260-263.

Dawson, J.B., Smith, J.V., and Steele, I.M., 1992, 1966 ash eruption of the carbonatite volcano Oldoinyo Lengai: mineralogy of lapilli and mixing of silicate and carbonate magmas, Mineralogical Magazine, v. 56, pp. 1-16.

Dawson, J.B., Keller, J., and Nyamweru, C., 1995, Historic and recent eruptive activity of Oldoinyo Lengai. In: Bell K, Keller J (eds) Carbonatite volcanism: Oldoinyo Lengai and the petrogenesis of natrocarbonatites, IAVCEI Proceedings on Volcanology, v. 4. Springer, Berlin, pp. 4-22.

Fischer, G.A., 1885, Bericht über die im Auftrage der Geographischen Gesellschaft in Hamburg unternommene Reise in das Masai-Land 1882-1883. II: Begleitworte zur Original-Routenkarte, Mitt Geogr Ges Hamburg 1885, pp. 189-237.

Guest, N.J., 1956, The volcanic activity of Oldoinyo L'Engai, 1954, Rec Geol Surv Tanganyika, v. 4, pp. 56-59.

Hay, R.L., 1983, Natrocarbonatite tephra of Kerimasi volcano, Tanzania, Geology, v. 11, pp. 599-602.

Keller, J., and Krafft, M., 1990, Effusive natrocarbonatite activity of Oldoinyo Lengai, June 1988, Bulletin of Volcanology, v. 52, pp. 629-645.

Keller, J., Zaitsev, A.N., and Klaudius, J., 2007, Geochemistry and petrogenetic significance of natrocarbonatites at Oldoinyo Lengai, Tanzania: composition of lavas from 1988 to 2007, Goldschmidt Conference 2007, Cologne, Abstracts.

Kervyn, M., Klaudius, J., Keller, J., Kervyn, F., Mattsson, H., Belton, F., Mbede, E., Jacobs, P., and Ernst,G.G.J., 2008, Voluminous lava floods at Oldoinyo Lengai in 2006: chronology of events and insights into the shallow magmatic system. Bulletin of Volcanology, v. 70, pp. 1069-1086.

Neumann, O., 1894, In: Matschie, P., Nachrichten aus den deutschen Schutzgebieten. Deutsch-Ostafrika. Von der wissenschaftlichen Expedition Oskar Neumanns, Deutsches Kolonialblatt, v. 21, pp 421-424.

Nyamweru, C. 1990, Observations on changes in the active crater of Oldoinyo Lengai from 1960 to1988, Journal of African Earth Sciences, v. 11, pp. 385-390.

Nyamweru, C., 1997, Changes in the crater of Oldoinyo Lengai, Journal of African Earth Sciences, v. 25, pp. 43-53.

Reck, H., and Schulze, G., 1921, Ein Beitrag zur Kenntnis des Baues und der jüngsten Veränderung des L'Engai Vulkans im nördlichen Deutsch-Ostafrika, Z Vulk, v. 6, pp. 47-71.

Richard, J.J., 1942, Volcanological observations in East Africa. I Oldoinyo Lengai. The 1940-1 eruption, Journal of East Africa Uganda Natural Historical Society, v. 16, pp. 89-108.

Uhlig, C., 1905, Bericht über die Expedition der Otto-Winter-Stiftung nach den Umgebungen des Meru. Zeitschrift der Gesellschaft für Erdkunde zu Berlin, Jg 1905, pp. 120-123.

Zaitsev, A.N., Keller, J., Spratt, J., Perova, E.N., and Kearsley, A., 2008a, Nyerereite-pirssonite-calcite-shortite relationships in altered natrocarbonatites, Oldoinyo Lengai, Tanzania, Canadian Mineralogy, v. 46, pp. 1077-1094.

Zaitsev, A.N., Keller, J., Spratt, J., Jeffries, T.E., and Sharigin, V.V., 2008b, Chemical composition of nyerereite and gregoryite in natrocarbonatites of Oldoinyo Lengai Volcano, Tanzania, Procedings of the Russian Mineralogical Society, v. 137, pp. 101-111.

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

Information Contacts: Frederick Belton, Developmental Studies Department, PO Box 16, Middle Tennessee State University, Murfreesboro, TN 37132, USA (URL: http://oldoinyolengai.pbworks.com/); Laura Carmody, Department of Earth Science, University College London, Gower Street, London, WC1E 6BT, United Kingdom; Michael Dalton-Smith, Digital Crossing Productions (URL: http://digitalcrossing.ca/); Adrian P. Jones, Department of Earth Science, University College London, Gower Street, London, WC1E 6BT, United Kingdom; Sonja Joplin, One Heart Source (URL: http://www.oneheartsource.org); Matthew J. Genge, Department of Earth Science and Engineering, Royal School of Mines, Prince Consort Road, Imperial College London, SW7 2BP United Kingdom; Wendy Nelson, Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd, NW Washington, DC 20015, USA; Hans Schabel, retired forestry professor; Dave Simpson, Dave Simpson, professional guide, Kenya, East Africa (URL: http://www.davesimpsonsafaris.com); Ben Wilhelmi, commercial pilot (URL: http://benwilhelmi.typepad.com/benwilhelmi/).

Activity at Mount Martin volcano since our last report (March 1995, BGVN 20:03) was marked by typical activity (summit fumarolic activity, often generating thick steam plumes reaching up to 1 km above the summit; Neal and others, 2009), occasionally interrupted by increased seismicity. The most notable event was a seismic swarm in January 2006.

Outstanding activity. An increase in seismicity during October 1996 was attributed to an actively degassing intrusion at the neighboring Mount Mageik volcano, ~7 km ENE of Martin (Jolly and McNutt, 1999). Other increases in seismicity occurred in December 1998, May-July 1999, January 2006 (the largest swarm at Martin since it has been monitored, discussed below), and May-June 2007 (figure 1).

Figure 1. Number of earthquakes recorded per month at Mount Martin since 1996. Five episodes of increased seismicity are shown, the most notable of which was the January 2006 seismic swarm at Martin. Note the break in scale on the y-axis, denoted by the horizontal dashed line. Modified from Dixon and Power (2009).

January 2006 seismic swarm. The January 2006 Mount Martin seismic swarm included 860 locatable earthquakes (figures 1 and 2), more than four times the number of earthquakes seen during other periods of increased seismicity or seismic swarms since the region has been monitored. No recorded earthquakes during the swarm were much greater than M 2 (figure 2d), and a significant number of earthquakes were of magnitudes below the magnitude of completeness, M_c (figure 2a-c). M_c is the minimum magnitude needed to reliably locate an earthquake, reported by Dixon and Power (2009) to be M_c = 0.2 for Mount Martin.

Figure 2. Plots highlighting the January 2006 Mount Martin seismic swarm. (A) Number of earthquakes per day; (B) cumulative number of earthquakes; (C) cumulative seismic moment; (D) magnitude of each recorded earthquake. In plots A-C, black symbols indicate all recorded earthquakes, and gray symbols indicate locatable earthquakes (earthquakes with magnitudes equal to or above the magnitude of completeness, M ≥ Mc = 0.2 (explained in text).

Dixon and Power (2009) concluded that the pattern of the seismicity of the January 2006 swarm was characteristic of a volcanic earthquake sequence (as opposed to a tectonic earthquake sequence, which begins with a large mainshock) since the located hypocenters of the swarm occurred in the same space as those during previous background periods (figure 3). However, citing the short duration of the swarm, similar focal mechanisms compared to background periods, and the lack of long-period earthquakes, Dixon and Power (2009) stated that the data was not suggestive of a large intrusion of magma beneath Martin.

Figure 3.Located earthquake hypocenters at Mount Martin during March 2002-December 2005 (map view shown in A, cross section in B) and during the January 2006 seismic swarm (map view shown in C, cross section in D). The graphs indicate that the hypocenters of the seismic swarm earthquakes occurred within the same volume as those that occurred during previous background period, suggesting that the earthquakes were characteristic of a volcanic earthquake sequence. Modified from Dixon and Power (2009).

References. Dixon, J.P., and Power, J.A., 2009, The January 2006 Volcanic-tectonic earthquake swarm at Mount Martin, Alaska, in Haeussler, P.J., and Galloway, J.P., eds, Studies by the U.S. Geological Survey in Alaska, 2007: U.S. Geological Survey Professional Paper 1760-D, 17 p.

Jolly, A.D., McNutt, S.R., 1999, Seismicity at the volcanoes of Katmai National Park, Alaska; July 1995-December 1997, Journal of Volcanology and Geothermal Research, vol. 93, issues 3-4, pg. 173-190 (DOI: 10.1016/S0377-0273(99)00115-8).

Neal, C.A., McGimsey, R.G., Dixon, J.P., Manevich, A., and Rybin, A., 2009, 2006 Volcanic Activity in Alaska, Kamchatka, and the Kurile Islands: Summary of Events and Response of the Alaska Volcano Observatory, U.S. Geological Survey Scientific Investigations Report 2008-5214, 102 p.

Geologic Background. The mostly ice-covered Mount Martin stratovolcano lies at the SW end of the Katmai volcano cluster in Katmai National Park. The volcano was named for George C. Martin, the first person to visit and describe the area after the 1912 eruption. It is capped by a 300-m-wide summit crater, which is ice-free because of an almost-constant steam plume and contains a shallow acidic lake. The edifice overlies glaciated lava flows of the adjacent mid- to late-Pleistocene Alagoshak volcano on the WSW and was constructed entirely during the Holocene. Martin consists of a small fragmental cone that was the source of ten thick overlapping blocky dacitic lava flows, largely uneroded by glaciers, that descend 10 km to the NW, cover 31 km2, and form about 95% of the eruptive volume of the volcano. Two reports of historical eruptions that originated from uncertain sources were attributed by Muller et al. (1954) to Martin.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey (USGS), 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 (ADGGS), 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.dggs.alaska.gov/).

Cerro Negro remained non-eruptive from 2003 to 2011; explosive activity was last recorded in December 1999 (BGVN 24:11). Our last report reviewed Cerro Negro's fumarolic field observations, including descriptions of passive degassing and measurements of temperatures from June 2002 through May 2003, provided courtesy of Instituto Nicaragüense de Estudios Territoriales (INETER) and international collaborators (BGVN 28:07). No volcanic ash advisory reports for the area of Cerro Negro were released by the Washington VAAC office during 2003-2011. The following report reviews seismicity from 2003 to 2011, field observations, and emission measurements provided by INETER. The primary physical features of Cerro Negro highlighted in this report include the 1992 and 1995 central craters as well as the three 1999 craters, which continued to steam in 2007 (figure 15).

Figure 15. A composite Landsat 7 ETM+ image of Cerro Negro processed by GVP with geospatial software (NASA Landsat Program, 2003). The image had an original resolution of 30 m and was collected on 15 November 1999. Lava flow ages shown (1923 to 1999) are based on GVP online photo captions ("Photo Gallery") and published literature by McKnight and Williams (1997) and Hill and others (1998).

Figure 15 consists of a false-color image made from visible, near- and mid-infrared bands (3,7,2) to enhance geological features. Cerro Negro appears dark-red in the center of the image. The central cone, which was the source of many lava flows, lies immediately to the left of "1960" (the label dating the eruption associated with one of the lava flows). The tiny, arcuate pink and green zones at the central cone represent the rim of the nested craters there. Those craters are the scene of the highest fumarolic activity.

On the cone's S flanks, the three small cones created during the 1999 eruption appear as bright pink points. In figure 15 these appear immediately right of "1999".

Several volcanoes of the NW trending Marrabios range of Western Nicaragua are labeled on figure 15. Along the range to the SE is the historically active El Hoyo (Las Pilas) volcanic complex, which in figure 15 is partly cloud-covered. The complex includes Las Pilas, Cerro Grande, and Cerro Ojo de Agua eruptive centers. To the N and NW of Cerro Negro lie the volcanic centers Cerro la Mula and Rota.

Post-eruptive seismicity from 1999 to 2003. The INETER December 2003 report discussed seismicity after the small-scale, cone-forming events in 1999. INETER described Cerro Negro as relatively quiet since the 1999 episode; minor ash and gas explosions occurred as late as 25 December 1999. Earthquake counts from August 1999 to December 2003 ranged from 40 to 100 earthquakes per month, typically volcanic-tectonic (VT) events. Low amplitude tremor (frequency ranges of 8-19 Hz) was detected throughout 2003.

Figure 16 depicts multi-year seismicity and illustrates comparitive highs during 2003, particularly in January, September, and December when the the number of monthly earthquakes rose to over 100. These swarms led to counts roughly 10-fold higher than the 18-month interval of quiet from middle to late 2001. The later seismic swarm, occurring from 30 to 31 December, comprised 37 events too small to locate.

Figure 16. Histogram presenting the number of earthquakes recorded at Cerro Negro from January 2000 through December 2003. Two swarms occurred during 2003 (labeled). Courtesy of INETER.

During 2003, INETER visited the volcano and found the scene without visible sign of change, without felt earthquakes, and lacking anomalous gas emissions. Fumarole temperatures from eight sites were in the range ~100-400°C. The only anomalous temperature increase in 2003 appeared at two fumaroles measuring ~550°C on 27 August. That was an increase of more than 200°C since July 2003.

2004 banded tremor and elevated seismicity. Although not ploted on figure 16, elevated seismicity continued through January, February, and March 2004. Banded tremor was recorded until 20 January, when it began to diminish. In January, RSAM was not greater than 50 units, but several cautionary public announcements were made regarding persistent tremor and its typical association with explosive activity.

Although INETER reported decreased tremor toward the end of January 2004, a seismic swarm occurred from 23 to 27 January. On 26 January the highest number of earthquakes registered (203 earthquakes, ~50 more than high of December 2003).

Of the ~1,200 earthquakes registered during January 2004, only three were located. During 3-29 February, ~400 events were registered and 33 were located. In March, 23 earthquakes were located and during the following months, significant events became rare averaging ~3 events located per month for the rest of the year. In March, tremor reached only 5 RSAM units.

Field visits by INETER determined that fumarole temperatures in March, May, June, and July 2004, ~50-350°C, spanned a wider range than those from the previous year. INETER had been measuring temperatures from several fumaroles (three to eight sites) within the crater since 1999 (figure 12 in BGVN 24:06 shows two primary fumarole locations in a map developed after major crater changes in 1995).

Press accounts regarding the seismic swarms. Officials interviewed by the newspaper La Prensa on 17 January 2004 included the mayor of León, who stated that the municipality's Emergency Committee was activated and on standby. The director of INETER's Volcanology program, Martha Navarro, also explained that caution was merited due to experience from Cerro Negro's 1999 escalation. Similar seismic tremor was recorded recently from the volcano, but conditions had clearly changed since 1999 and no explosions had occurred. The director also noted that on 11 January 2004 visiting scientists had looked for substantial sulfur-dioxide emissions but found them absent.

On 22 January 2004, a Civil Defense representative told La Prensa that recent reports of plumes from the crater were false and that no physical changes had occurred at Cerro Negro during the December-January seismic unrest. Passive degassing had been occurring at the summit and from fumaroles since the 1999 events but may have appeared anomalous to local observers. Regular monitoring by INETER had shown elevated temperatures from the fumaroles and steam frequently escaped from the three 1999 cinder cones (figure 15). According to La Prensa, the Civil Defense representative also shared details regarding new installations of seismic stations and gas-monitoring sites. A collaborative effort between Civil Defense and INETER made this possible.

2005-2011 rockfalls and altered crater morphology. Routine monitoring by INETER from 2005 through 2011 has been recorded in monthly reports available online in Spanish with English abstracts, works that chiefly documented passive degassing through this time period. Fumarole temperatures ranged from 13°C to ~400°C. In May 2003, seven fumaroles had elevated temperatures (BGVN 28:07), but in April 2008, six of these sites had ceased discharging measurable emissions. By July 2008, four fumarole sites were emitting gas and elevated temperatures ranging from 96 to 285°C that month and appeared stable through 2011.

INETER began reporting significant rockfalls along Cerro Negro's S and SW interior crater walls in 2009. These rockfalls continued through 2011 and released meter-sized blocks of coherent rock as well as highly altered material that collected within the crater (figure 17). INETER suggested that some of the large rockfalls may have been caused by large rainfalls, particularly those events during July 2009 and May-July 2010.

Figure 17. Photos of the S and SW interior crater walls of Cerro Negro. (Left) A photo (with people for scale) taken in August 2009 shows meter-sized blocks had collected on the crater floor and (in the middle ground) a diffuse plume from fumarolic degassing. (Right) A photo taken in September 2009 depicts massive blocks cropping out along the upper portion of the near-vertical wall and extensive areas of loose scree and blocks that have already fallen free. Courtesy of INETER.

A significant geomorphic change at Cerro Negro was noted by INETER investigators on 11 January 2011. A N-trending fault had appeared since the last field visit (10 November 2010) on the SE interior crater wall (figure 18). Offset along the fault measured ~30 cm. Based on field relations INETER suggested this feature appeared gradually. The fault intersected fumarole ##1, a reliable site for thermal measurements. A major system of normal faults had already been documented to the NW of Cerro Negro, and the new fault on the cone appeared to trend parallel to it.

Figure 18. Observations made on 11 January 2011 by INETER detail the surface expression of a new fault intersecting fumarole ##1 on the SE segment of the interior crater wall (viewed looking S). The terms 'Upthrown' and 'Downthrown' refer to relative motion on the fault. The fault trends N-S and underwent ~30 cm of lateral displacement. Courtesy of INETER.

Seismicity at Cerro Negro remained generally low from 2005 through 2011 although tremor was detected regularly. At times, tremor was as low as 5 RSAM units (July 2009) and as high as 30 (December 2010 and September 2011). Numerous VT events were recorded in 2006 (~347) and in 2011 (~240) and accordingly, the number of significant located events was higher for those years as well, 25 and 32, respectively (table 4).

Table 4. Significant earthquakes located near Cerro Negro from 2003 through 2011. For each year, the table lists the number of located earthquakes, range of local magnitudes (M_L), range of focal depths, and most frequently-occurring focal depth. Courtesy of INETER.

The range of focal depths was relatively large in 2006 and 2011. The deepest earthquake during 2003-2011 struck on 23 December 2008 with local magnitude (M_L) 3.1 and located ~190 km below sea level. The most frequently occurring focal depth during 2005-2011 was very shallow, 2 km below sea level, under M_L 3.5.

During field campaigns on 21-27 February 2011, a collaborative effort between Spain's Instituto Tecnológico y de Energías Renovables (ITER) and INETER mapped the spatial CO₂-flux pattern. The team was able to map CO₂ fluxes from multiple diffuse sources over the cone and within Cerro Negro's 1992 and 1995 craters (figure 19). An overall total CO₂ flux of 43 tons per day was determined; a similar measurement was obtained in 2010 (44 tons per day). Collaborative efforts between ITER and INETER have applied this mapping technique since 1999 in order to locate anomalous areas of emissions from the cone and to calculate total flux (Dionis, S. and others 2010). These investigators noted that the years following the 1999 explosion were marked by decreasing levels of CO₂ however, an increasing trend appeared from December 2008 to March 2009; values ranged from 12 tons per day to 38 tons per day.

Figure 19. Results of a CO2 measuring campaign from 21 to 27 February 2011. Courtesy of ITER and INETER.

References. Dionis, S., Melián, G., Barrancos, J., Padilla, G., Calvo, D., Rodríguez, F., Padrón, E., Nolasco, D., Hernández, Pedro A., Pérez, N. M., Ibarra, M., and Muñoz, A., 2010. Dynamics of diffuse CO₂ emission and eruptive cycle at Cerro Negro volcano, Nicaragua, Cities on Volcanoes 6, Puerto de la Cruz, Tenerife, 31 May-4 June, 2010, Abs, p 103.

Hill, B. E., Connor, C.B, Jarzemba, M.S., La Femina, P.C., Navarro, M., and Strauch, W., 1998, 1995 eruptions of Cerro Negro volcano, Nicaragua, and risk assessment for future eruptions, Geological Society of America Bulletin, 110, no. 10;1231-1241.

NASA Landsat Program, 2003, Landsat ETM+ scene 7dt19991115, SLC-Off, USGS, Sioux Falls, Nov. 15, 1999.

McKnight, S.B. and Williams, S.N., 1997, Old cinder cone or young composite volcano?: The nature of Cerro Negro, Nicaragua, Geology, 25, 339-342.

Geologic Background. Nicaragua's youngest volcano, Cerro Negro, was created following an eruption that began in April 1850 about 2 km NW of the summit of Las Pilas volcano. It is the largest, southernmost, and most recent of a group of four youthful cinder cones constructed along a NNW-SSE-trending line in the central Marrabios Range. Strombolian-to-subplinian eruptions at intervals of a few years to several decades have constructed a roughly 250-m-high basaltic cone and an associated lava field constrained by topography to extend primarily NE and SW. Cone and crater morphology have varied significantly during its short eruptive history. Although it lies in a relatively unpopulated area, occasional heavy ashfalls have damaged crops and buildings.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Global Land Cover Facility ( URL: http:// http://www.glcf.umiacs.umd.edu/); Instituto Tecnológico y de Energías Renovables (ITER), 38611 Granadilla, Tenerife, Canary Islands, Spain (URL: http://www.iter.es/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); La Prensa de Nicaragua, Managua, Nicaragua (URL: http://www.laprensa.com.ni/).

The 2004 unrest at Mount Spurr (BGVN 29:10) continued for nearly two years before the Alaska Volcano Observatory (AVO) lowered the Level of Concern Color Code from Yellow to Green (on a scale from Green to Yellow to Orange to Red) on 21 February 2006. During those two years, hydrothermal and fumarolic activity within the ice-filled summit crater resulted in the formation of a summit ice cauldron and emplacement of debris-flow deposits on the upper slopes of Spurr. The summit crater and cauldron remained active after most other signs of unrest had declined. This report discusses observations of the development of the unrest following November 2004 (activity prior to that time covered in BGVN 29:10).

A depression in the summit ice was observed in its early stages in June 2004 (figures 12 and 13). The subsidence became more pronounced, and was recognized as an ice cauldron on 2 August, following debris flows emplaced in late July (figure 14). The cauldron housed a lake, whose water was described by Neal and others (2005) and McGimsey and others (2008) as "dark battleship gray" and turquoise in color, respectively, likely due to dissolved sulfur compounds (figure 15).

Figure 12. Photograph of Mount Spurr's summit (viewing SSW) on 20 June 2004, showing the initial development of a depression (dashed outline) in the ice and snow covering the summit. Crevassing of the snow and ice downslope is indicated by arrows. This is the earliest image of the 2004 development of the summit ice cauldron. Photograph courtesy of Bruce Hopper, Alaska Volcano Observatory (AVO).

Figure 13. Satellite photography highlighting the 2004 development of Mount Spurr's summit ice cauldron, acquired on (A) 15 June 2002 and (B) 10 August 2004. Modified from Coombs and others (2005).

Figure 14. Debris flow deposits on the upper slopes of Mount Spurr, photographed on 15 July 2004, viewing NNW. The debris flow deposits prompted an observation flight, which resulted in the observation of the summit ice cauldron on 2 August. Courtesy of Christina Neal, Alaska Volcano Observatory (AVO).

Figure 15. Aerial photograph and Forward Looking Infrared Radiometer (FLIR) images of Mount Spurr's summit ice cauldron and lake, taken on 25 April 2005. Hottest parts of the FLIR image correspond to the exposed bedrock on the shore of the lake (see temperature scale at right). The light blue gray color of the lake is likely due to dissolved sulfur compounds (Neal and others, 2005). Courtesy of McGimsey and others (2008).

Ice cauldron widens. Neal and others (2005) reported that measurements made on 10 August and 30 October 2004 revealed enlargement of the ice cauldron from ~65 m x 95 m to ~130 m x 130 m in two and a half months' time. Gas measurements during the same time revealed that CO₂ emissions had more than doubled (figure 16).

Figure 16. Measured gas emissions (tons/day) at Mount Spurr during the 2004-2006 active period. SO2 (in parentheses) and CO2 fluxes plotted on left axis; H2S flux plotted on right axis. Data courtesy of Doukas and McGee (2007).

Forward Looking Infrared Radiometer (FLIR) measurements on 24 September showed that the crater lake was ~0 °C (substantially warmer than the surrounding ice and snow), and the surrounding exposed bedrock (and main fumarolic emission area) was as hot as ~39 °C (figure 15).

By the end of 2004, seismicity remained elevated, and most located earthquakes were within 0-5 km depth below sea level (figure 17; Neal and others 2005).

Figure 17. Earthquakes located beneath Mount Spurr during 2004 showed increased seismicity correlating to increased gas emissions and hydrothermal activity responsible for the formation of the summit ice cauldron. Plots show (A) number of earthquakes per day and (B) hypocenter depths below sea level. Symbol size indicates the relative magnitude of the earthquakes; triangles indicate located hypocenters at depths greater than 20 km. Courtesy of Neal and others (2005).

During 2005, growth of the summit ice cauldron continued (figures 18 and 19), and areas of exposed bedrock increased along the N and NW walls of the crater. According to McGimsey and others (2008), FLIR measurements on 25 April 2005 showed similar temperatures to those measured in September 2004 (figure 15).

Figure 18. Mount Spurr's summit ice cauldron extent as it expanded during 2004-2006. Colored lines indicate the rim of the cauldron as measured on the dates indicated. Courtesy of Coombs and others (2006).

Figure 19. Two plots showing (A) the area of Mount Spurr's ice cauldron and (B) the number of earthquakes per week during March 2004-March 2006. Courtesy of Coombs and others (2006).

May 2005 debris flow. A small debris flow was captured on webcam views of the summit on 2 May 2005 (figure 20). Observations a week later revealed that the cauldron lake level had dropped by ~15 m, and fumaroles on the N shore of the lake had been exposed (McGimsey and others, 2008). The fumaroles were described as vigorous by McGimsey and others (2008). FLIR measurements during an observation flight on 21 June indicated increasing temperatures of exposed bedrock within the crater (up to 60 °C; orange areas within the ice cauldron outline, figure 20) and observers noted strong upwelling within the N half of the cauldron lake.

Figure 20. Map and interpretive cross-section highlighting the locations of debris flows emplaced onto the summit cone of Mount Spurr during 2004-2005. Dates of observation or emplacement of debris flows are provided in the explanation; associated outflow points are indicated by red dots. Orange areas indicate zones of elevated thermal activity and/or exposed bedrock. Schematic cross-section (bottom right) is along the line A-A', and indicates the probable pathway of debris-laden water from the cauldron lake to the outflow points of the debris flows. Base image from QuickBird satellite image, acquired 10 August 2004. Modified from Coombs and others (2006).

The likely (or at least nearly) contemporaneous lake level drop and debris flow on 2 May were not associated with any significant ice collapse into the cauldron lake; Coombs and others (2006) thus concluded that the debris flows were the result of widening of englacial or subglacial pathways by erosion, heating, or glacial flow (cross-section, figure 20). They also interpreted the main source of the debris carried in the debris flows to be melted glacial ice containing layers of tephra and ash. The primary source of the tephra and ash layers was likely the 1992 eruptions of Crater Peak (Spurr's satellite cone and youngest vent) and possibly the 1989-90 eruptions of Mount Redoubt (Coombs and others, 2006). Some component of the debris was also likely sourced from the summit crater floor and wall rocks.

Snow/ice melts from summit crater. By 1 August 2005, the ice cauldron had reached its largest size (i.e. the snow/ice had melted from within the perimeter of the summit crater; figures 18 and 19), and was thus no longer termed the "ice cauldron", but simply the summit crater (McGimsey, personal communication, 2012). In September, the crater lake was observed to be completely ice-free, and most likely remained as such through mid January 2006. McGimsey and others (2008) reported that, as of 3 November 2005, ~5.4 x 10⁶ m³ of ice and snow had been melted and consumed by the summit lake.

Decreasing seismicity prompted the AVO to lower the Level of Concern Color Code from Yellow to Green on 21 February 2006. With the exception of an earthquake swarm during 11-12 April, seismicity continued to decrease, and reached background levels by May 2006. During the earthquake swarm, Neal and others (2009) reported 157 volcano-tectonic earthquakes (reaching M ~2.3) that occurred at less than 5 km depth below sea level and ~1-3 km W of the summit. FLIR measurements two days after the earthquake swarm revealed that fumaroles within the summit crater were as hot as 150 °C (Neal and others, 2009). By mid July, however, snow and ice had started accumulating on the lake's surface, and by 17 November 2006, a rise in the level of the lake was observed. As fumarolic activity continued, yellow, sulfur stained ice and snow, as well as a strong sulfur smell, was often reported by pilots passing the summit.

Since 2006, most of the sides and bottom of the summit crater have been covered by snow, with the exception of the fumarole field in the N part of the crater floor. As of the last observation flight, the fumarole field maintained a small patch of snow/ice free bedrock on the summit crater's floor, an area still active as of 28 August 2011 (figure 21). Later satellite imagery suggested that the fumarole field had been covered as the summit crater filled with snow and ice during the first part of the 2011-2012 winter (figure 22), but there have been no observation flights to confirm this as of 24 February 2012.

Figure 21. Aerial observation photograph of Mount Spurr's summit on 28 August 2011. Although the crater had begun refilling with snow, the fumarole field on the crater floor remained in a clear patch of bedrock. Photograph courtesy of Game McGimsey, Alaska Volcano Observatory (AVO).

Figure 22. WorldView-1 daytime optical panchromatic imagery detailing the apparent filling of Mount Spurr's summit crater with snow and ice during the early 2011-2012 winter. (B) shows Spurr on 11 August 2011, with the fumarole field and area of exposed bedrock in the N of the summit crater (enlarged, inset). Snow appears white, and vegetation at the bottom of the image appears dark. (C) shows the same area on 15 October, with the fumarole field and exposed bedrock areas apparently covered by snow at the bottom of the summit crater. Vegetation is no longer visible at the bottom of the image. (D) shows the same area on 27 October, but the summit crater appears to be completely filled with snow, no longer exhibiting a depression. Topographic map of the same area is shown for reference (A). '*' symbol indicates a prominent topographic ridge to the N of Spurr that is visible in each image. Scale is approximate for the satellite imagery (B-D). Courtesy of Alaska Volcano Observatory (AVO) and Digital Globe, Inc. (B-D).

Coombs and others (2006) stated that the overall effect of the hydrothermal activity (including water/debris flow releases from the summit) on the glacial system of Spurr were likely minimal, pointing out that the volume of water released was relatively small and probably easily accommodated "without significant modification of the icemass."

References. Coombs, M.L., Neal, C.A., Wessels, R.L., and McGimsey, R.G., 2006, Geothermal disruption of summit glaciers at Mount Spurr Volcano, 2004-6: An unusual manifestation of volcanic unrest: U.S. Geological Survey Professional Paper 1732-B, 33 p.

Doukas, M.P., and McGee, K.A., 2007, A compilation of gas emission-rate data from volcanoes of Cook Inlet (Spurr, Crater Peak, Redoubt, Iliamna, and Augustine) and Alaska Peninsula (Douglas, Fourpeaked, Griggs, Mageik, Martin, Peulik, Ukinrek, and Veniaminof), Alaska, from 1995-2006: U.S. Geological Survey Open-File Report 2007-1400, 16 p.

McGimsey, R.G., Neal, C.A., Dixon, J.P., and Ushakov, S., 2008, 2005 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2007-5269, 94 p.

Neal, C.A., McGimsey, R.G., Dixon, J.P., Manevich, A., and Rybin, A., 2009, 2006 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2008-5214, 102 p.

Neal, C.A., McGimsey, R.G., Dixon, J., and Melnikov, D., 2005, 2004 Volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report 2005-1308, 71 p.

Geologic Background. The summit of Mount Spurr, the highest volcano of the Aleutian arc, is a large lava dome constructed at the center of a roughly 5-km-wide horseshoe-shaped caldera open to the south. The volcano lies 130 km W of Anchorage and NE of Chakachamna Lake. The caldera was formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an ancestral edifice. The debris avalanche traveled more than 25 km SE, and the resulting deposit contains blocks as large as 100 m in diameter. Several ice-carved post-caldera cones or lava domes lie in the center of the caldera. The youngest vent, Crater Peak, formed at the breached southern end of the caldera and has been the source of about 40 identified Holocene tephra layers. Eruptions from Crater Peak in 1953 and 1992 deposited ash on the city of Anchorage.

Information Contacts: Bruce Hopper, Game McGimsey, and Christina Neal, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey (USGS), 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 (ADGGS), 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.dggs.alaska.gov/); Digital Globe, Inc. (URL: http://www.digitalglobe.com/).