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).

Anatahan's third historical eruption began on 5 January 2005, and is described in BGVN 29:12. Further details and satellite images were presented in BGVN 30:02, which covered events until mid-February 2005. A 5-6 April 2005 eruption cloud rose to at least 15 km altitude, which was the highest yet seen at the volcano.

Anatahan erupted almost continuously after 5 January 2005, when it started its third eruption in recorded history. An image collected by the Ozone Monitoring Instrument on NASA's Aura satellite shows atmospheric sulfur dioxide (SO₂) concentrations between 31 January and 4 February 2005 (figure 14). A long SO₂ plume extends NE and SW of Anatahan, and the edge of the plume covers Guam (the southernmost island) and the other Mariana Islands immediately to Anatahan's N and S.

Figure 14. Anatahan's atmospheric SO2 as imaged by Aura's OMI instrument on 31 January 2005. The OMI (Ozone Monitoring Instrument) measures SO2 in Dobson Units. Dobson Units, which derive from spectroscopic measurement techniques, can be thought of as the mass of molecules per unit area of Earth's atmospheric column. One Dobson Unit equals 0.0285 grams of SO2 per square meter. The Ozone Monitoring Instrument (OMI) that created this image tracks global ozone change and monitors aerosols like sulfates in the atmosphere. It was added to the Aura satellite as part of a collaboration between the Netherlands' Agency for Aerospace Programs and the Finnish Meteorological Institute. NASA describes the Eos system Aura as "A mission dedicated to the health of the Earth's atmosphere." NASA image courtesy Simon Carn (Joint Center for Earth Systems Technology (JCET), University of Maryland Baltimore County (UMBC)).

Volcanogenic SO₂ combines with water to create a sulfuric acid haze. Called "vog," this haze can cause illness and make breathing difficult. Volcanic haze grew so thick during the first week of February that the National Weather Service issued a volcanic haze advisory for Guam, where several illnesses were reported.

After mid-February 2005, eruptive activity at Anatahan steadily declined to less than 5% of the peak level attained since the eruption started on 5 January. Ash eruptions continued, and the 2003 crater floor was almost entirely covered by fresh lava out to a diameter of ~ 1 km. A MODIS image taken at 0115 on 18 February showed a plume of steam and vog extending about ~ 170 km SW of Anatahan. Seismic and acoustic records during the last week of February 2005 showed very low levels of activity. Seismic amplitudes during 23-28 February were similar to those recorded prior to the 5 January eruption. NASA MODIS (Moderate Resolution Imaging Spectroradiometer) imagery taken on 28 February showed a faint plume of vog and steam trending W of Anatahan.

During the first two weeks of March 2005 volcanic and seismic activity increased relative to the previous weeks. During 14-17 March, seismicity increased and steam rose a few hundred meters above the volcano. The inner E crater had been nearly filled with lava flows and lapilli since early January.

A small eruption began on 18 March at 1544 according to seismic data. On 19 March the Washington VAAC issued an advisory that an ash plume was visible on satellite imagery below 4 km altitude. Small explosions that began late on 20 March lasted for 14 hours. No emissions were visible on satellite imagery, but others were, later in March and April.

A strong outburst apparently began on 21 March, a day when seismicity increased significantly. Seismic amplitudes peaked on the 25th and faded out on the 26th. Near the peak on the 25th, the U.S. Air Force Weather Agency (AFWA) detected a hot spot on the island on satellite imagery and reported an ash plume briefly reaching ~ 5.8 km altitude. The plume height soon dropped to below 3 km altitude, and by near midday on the 27th the plume had changed from ash and steam to steam and vog. On the 27th the plume extended ~ 240 km SW.

On 5 April at about 2200 seismic signals began to increase slowly, and the Washington VAAC began to see increased ash on satellite imagery. On 6 April 2005 around 0300 an explosive eruption began and produced an ash plume to an initial height of ~ 15.2 km altitude, the highest in recorded history from the volcano. Seismicity peaked at the same time.

The AFWA reported an upper level ash plume at ~ 15.2 km altitude blowing E to SE and a lower level ash plume at ~ 4.6 km altitude blowing SW; the upper plume extended more than 465 km. Earth Probe TOMS data on 6 April at 1046 showed a compact sulfur-dioxide cloud drifting E of Anatahan following the eruption.

Chuck Sayon, the superintendent of American Memorial Park noted, "On Saipan at around 10 AM the skies darkened and light ash started falling . . . park operation[s] have been restricted to indoor activities due to irritation to eyes and breathing as ash starts to lightly coat the area. Schools are closed as well as the airport until further notice . . .."

On 6 April during 0400 to 0900 the seismicity at Anatahan decreased to near background. The seismicity surged for about 1 hour, with amplitudes about one-half those reached during the earlier eruption, and subsequently dropped again to near background. Prior to the 6 April eruption, during 31 March to 4 April the amplitudes of harmonic tremor varied, reaching a 2-month high on the 3rd. Small explosions occurred every one minute to several minutes, probably associated with cinder-cone formation. Steam-and-ash plumes drifted ~ 200 km, and vog drifted ~ 400 km at altitudes below ~ 2.4- 4.6 km.

The U.S. Geological Survey (USGS) (in conjunction with the Commonwealth of the Northern Mariana Islands) stated that the "eruption of 6 April 2005 was the largest historical eruption of Anatahan and expelled roughly 50 million cubic meters of ash. The eruption column and the amplitude of harmonic tremor both grew slowly over about 5 hours and both peaked about 0300 on 6 April local time . . .. The peak of the eruption lasted about one hour and then the activity declined rapidly over the following hour."

The 6 April 2005 eruption's plume was captured on satellite images. The image showed a plume that was tan or brown in color and clearly ash laden (figure 15).

Figure 15. A major eruption from Anatahan on 6 April 2005 sent an ash plume to ~ 15 km. The eruption was considered the largest since Anatahan's first recorded eruption on 10 May 2003. This Moderate Resolution Imaging Spectroradiometer (MODIS) image was acquired by NASA's Terra satellite at 0035 UTC, about 8 hours after the eruption began. By this time, the ash plume had spread S to entirely cover Saipan and Tinian, islands immediately to the S. Courtesy of the MODIS Rapid Response Image Gallery, sponsored in part by NASA.

Figure 16 shows SO₂ concentrations in the atmosphere on 7 April 2005, over 30 hours after the large 6 April eruption. SO₂ emissions from the eruption were measured by the Ozone Monitoring Instrument (OMI) on NASA's EOS/Aura satellite. OMI detects the total column amount of SO₂ between the sensor and the Earth's surface and maps this quantity as it orbits the planet. A new perspective on the vertical distribution of the SO₂ is revealed by combining the OMI data with coincident measurements made by the Microwave Limb Sounder (MLS), also part of the Aura mission.

Figure 16. Anatahan's 5-6 April 2005 eruption injected significant SO2 high into the atmosphere. This OMI image depicts the concentrations found over 30 hours after the eruption, a time when the SO2 formed two separate zones at distance from the source. Analysis suggests that the westerly zone of SO2 was probably in the lower troposphere and the eastern zone was probably in the upper troposphere or above. Courtesy of Simon Carn.

The MLS data crisscross the OMI image and clearly show that some, but not all, of the SO₂ measured by OMI to the volcano's E was in the upper troposphere or above. At these altitudes, SO₂—and the sulfate aerosols that form from it—can stay in the atmosphere and affect the climate for a longer period of time. A weaker SO₂ signal was also measured in the same region during the nighttime MLS overpass, which crosses the image from upper right to lower left. The daytime data, running from upper left to lower right, coincide with the OMI measurements. The MLS data west of Anatahan show no significant SO₂ signal, indicating that the SO₂ measured by OMI in this region was in the lower troposphere.

MLS measures thermal emissions from the Earth's limb, so unlike the OMI sensor it also collects data at night. It is designed to measure vertical profiles of atmospheric gases that are important for studying the Earth's ozone layer, climate, and air quality, such as SO₂. These images, derived from preliminary, unvalidated OMI and MLS data, show MLS SO₂ columns (filled circles) measured every 165 km along the Aura orbit, plotted over the OMI SO₂ map. The MLS SO₂ columns shown here are derived from profile measurements made from the upper troposphere into the stratosphere (~ 215-0 hPa (hectoPascal, 10² Pa) or ~ 12 km altitude and above), and the circles do not represent the actual size of the MLS footprint, which is roughly 165 x 6 km.

Anatahan's morphological changes were highlighted in before (pre-eruption) versus after (post-eruption) images (figure 17). Seismicity decreased at Anatahan after 6 April and during 7-11 April was at very low levels, near background. On 11 April, a steam-and-ash plume rose ~ 2.7 km altitude and drifted ~ 280 km WSW.

Figure 17. Two satellite images of Anatahan cropped and oriented for direct comparison, one from 20 January 2002 (pre-eruption), the other from 27 April 2005 ("post-eruption" in the sense that it was taken after the May 2003 eruption began). The NASA's Earth Observatory website described the pre-eruption image (bottom) as follows: "In 2002, when the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) captured the lower image, the island was made up of two volcanoes whose conjoined summit calderas formed an elliptical valley at the island's center. Aside from occasional tremors, the island was quiet and no eruption had ever been recorded." Green plants, shown on the false-color (infrared-enhanced) image in red, covered the island and filled the caldera at its center. The same website described the post-eruption image (top) as showing that the volcano was still emitting steam and ash on 27 April 2005. The island's center was completely devoid of plants, covered instead by gray volcanic material. Ash appears to have blanketed the western fringe of the island, where a layer of gray covers the underlying vegetation. The light cloud, ash, and steam that cover the island make it difficult to see changes to the caldera, but it appears that the eruptions may have destroyed its southern wall. On higher resolution images, it also appears that volcanic material may have flowed into the Pacific Ocean on the island's S side. Courtesy of NASA's Earth Observatory.

Occasional data from Anatahan revealed that seismicity appeared to increase during 24-25 April. During 20-25 April, a continuous thin plume of ash-and-steam rose to less than ~ 3 km altitude and drifted more than 185 km from the volcano. Harmonic tremor dropped dramatically on 1 May after being at high levels for several days. During 27 April to 1 May, the main ash-and-steam plume rose to ~ 3 km altitude According to a news article, the volcanic plume from Anatahan reached Philippine airspace on 4 May.

On 5 May an extensive ash-and-steam plume to 4.5 km altitude was visible in all directions. Ash extended 770 km N, 130 km S (to northern Saipan), and 110 km W. Vog extended in a broad swath from 3,000 km W, over the Philippines, to 1,000 km N of Anatahan. By 9 May harmonic tremor amplitude had decreased to near-background levels, with a corresponding drop in eruptive activity. As of 10 May AFWA was reporting ash to about 3 km altitude extending 400 km W and an area of vog less than half that noted on 5 May.

On 11 May AFWA reported thick ash rising to 4.2 km altitude and moving WNW. The thick ash extended in a triangular shape from the summit 444 km to the WSW through 510 km to the NW. A layer of thin ash at 3 km altitude extended another 1,000 km beyond the thick ash. A broad swath of vog extended over 2,200 km W nearly to the Philippines and over 1,400 km NNW of Anatahan. Although the ash plume diminished over the next few days and was not as thick, it remained significant, rising to 2.4 km and extending 370 km WNW on the 13th. Scientific personnel from the Emergency Management Office and the USGS working the next day at a spot 2-3 km W of the active vent heard a continuous roaring sound. They also saw ash and steam rising by pure convection, not explosively, to 3 km altitude.

Reference. Chadwick, W.W., Embley, R.W., Johnson, P.D., Merlea, S.G., Ristaub, S., and Bobbitta, A., 2005, The submarine flanks of Anatahan volcano, Commonwealth of the Northern Mariana Islands: Jour. of Volcanology and Geothermal Res. (In press, June 2005).

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

Information Contacts: Juan Takai Camacho and Ramon Chong, Emergency Management Office of the Commonwealth of the Northern Mariana Islands (CNMI/EMO), PO Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/); Simon Carn, Joint Center for Earth Systems Technology (JCET), University of Maryland Baltimore County (UMBC), 1000 Hilltop Circle, Baltimore, MD 21250, USA; Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/); Charles Holliday, U.S. Air Force Weather Agency (AFWA), Offutt Air Force Base, Nebraska 68113, USA; Randy White and Frank Trusdell, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3591 USA (URL: https://volcanoes.usgs.gov/nmi/activity/); Saipan Tribune, PMB 34, Box 10001, Saipan, MP 96950, USA (URL: http://www.saipantribune.com/); Operational Significant Event Imagery (OSEI) team, World Weather Bldg., 5200 Auth Rd Rm 510 (E/SP 22), NOAA/NESDIS, Camp Springs, MD 20748, USA (URL: https://www.nnvl.noaa.gov/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Chuck Sayon, American Memorial Park, Saipan, MP 96950, USA; NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/).

Awu's eruption on 6 June 2004 and its elevated seismicity in early August 2004 was previously reported (BGVN 29:10). This report covers the last half of August 2004, which had not been reported on previously. Since the 6 June eruption, observation of the summit failed to reveal any significant changes (table 3). The hazard status of Awu during this August report remained at Level 2, having been elevated to 4 (the highest on a scale of 1 to 4) at the time of the 6 June eruption and then lowered on 14 June.

Table 3. Seismicity at Awu during August 2004 as reported by DVGHM.

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

Information Contacts: Dali Ahmad, Directorate of Volcanology and Geological Hazard Mitigation (DVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

On the morning of 13 May 2005, a new eruption started on uninhabited Fernandina volcano (figure 5). Fernandina last erupted in 1995 (figure 6), and had been quiet and seemingly unchanged when a team from the Ecuadorian Institute of Geophysics (IG) flew over it in late March 2005. On 11 May an M 5.0 earthquake occurred with an epicenter ~ 30 km E of Fernandina's center. Only two other earthquakes have been located by the U.S. Geological Survey (USGS) within 100 km of Fernandina in last 4.5 years (M 4.0 on 23 February 2005 and M 4.6 on 16 April 2005), both having epicenters ~ 70-80 km SE of Fernandina's center. A seismic station, installed by the IG in 1996 on the NE coast of the island, was out of service at the time of eruption.

Figure 5. Sketch map of Fernandina, showing the conspicuous summit caldera, and indicating the flow fields and circumferential vent area from the 13 May 2005 eruption (as mapped on 14 May by airborne reconnaissance and reported by the Charles Darwin Research Station). Key features include the active circumferential fissure vent and two main areas impacted by lava flows. The eastern area contained lava flows still mobile on 14 May; flows to the W had already cooled by 14 May. On the index map of the Galápagos Islands, the largest island, Isabela, is ~ 130 km long and lies to the E of Fernandina island. "La Cumbre"—Spanish for the summit, peak, or top—has been mistakenly applied to the volcano, apparently because the summit was so labeled on an old map. The island has also been called Narborough. The index map is incomplete in its portrayal of both volcanoes and islands of the archipelago. Revised from BGVN 20:01.

Figure 6. A 2002 International Space Station photograph of Fernandina, looking obliquely towards the E (N is towards the left). Labels show key features developed in 1995, 1981, and 1968 eruptions. Note the island's coastline in the lower-right corner and along much of the left margin. Despite the steep walls bounding the 850 m deep, 5 x 6.5 km central caldera, it supports both animal and plant populations. Image ISS05E06997 (Visible Earth v1 ID 18002) with contrast enhanced and labels added by Bulletin editors.

Galápagos National Park workers in western Galápagos were apparently the first to witness the eruption, and IG technicians recognized it on satellite imagery. The University of Hawaii presents hotspot images on their website. Their GOES data lacked hotspots at 0930, but a clear and strong one had developed on the S flank by 0945. Francisco Dousdebes (of Metropolitan Touring) placed the eruption's start time at 0935. S-flank hotspots were comparatively extensive by 1015. The Washington VAAC issued their first full advisory at 1315. Their notices reported that the W-directed plume rose to ~ 5 km altitude, and the S-directed plume went to 9 km; both were visible as late as 1745 on 13 May, depicting the leading portions of Fernandina' s ash plume more than 200 km from the volcano

An overflight of the eruption on the 13th by the National Park resulted in a report by Patricio Ramón and Hugo Yepes, and the eruption was confirmed by Washington Tapia, director of the Galápagos National Park. That evening, Galápagos resident Greg Estes telephoned Dennis Geist to report that the eruptive source was a "circumferential vent near [the] summit, S side . . . 6 km long with an eruptive zone 50 m across." It was uncertain how this fissure was related to the 1981 eruption site (figure 2 and SEAN 09:03). IG also noted that tephra had fallen on neighboring Isabela Island, in the areas of the volcanoes Wolf and Ecuador (~ 40 km from the vent, figure 1).

At 0537 on the second morning, 14 May, the Washington VAAC reported low level ash/steam not visible in infrared imagery, but at 0746, 1½ hours after sunrise, a plume of ash extended ~ 130 km to the W and was moving at 18 km/hour at 1,800 m elevation. The GOES thermal anomaly was greatly diminished by 0930, and remained low to non-existent until resumption around 1415. That afternoon, an overflight by Godfrey Merlen, Wacho Tapia, and Alan Tye (Charles Darwin Research Station) resulted in the fullest report to date.

They said that although the vent area was obscured by clouds, topography suggested a 4.5 km long fissure vent near the S rim, with activity having progressed from SW (near the first and uppermost flows of the 1995 radial fissure eruption) to the E (figure 1). The lava flows "had begun to pond on the gentler outer skirt of the island," and were then 5.5 km from the coast (~ 5 km from the vents). They thought it unlikely that the flows would reach the sea. A follow-up news report in El Comercio (Quito) quoted Tapia as identifying five flows down the S flank. Only one remained incandescent. At 1745 on 14 May, Washington VAAC reported a plume remaining to the NW, but—lacking detectable ash—they discontinued advisories. Thermal anomalies on the GOES satellite remained strong, however, until the next morning.

The report also noted that, "As on previous eruptions, such as that on Cerro Azul in 1998, lava passing through vegetated areas has caused small fires, but these have not spread far from the lava tongues themselves before going out. Most of the new flows have passed over unvegetated older lava, and damage to Fernandina's vegetation is limited."

The team also flew over Alcedo volcano on Isabela, where Project Isabela staff had reported increased fumarole activity. Steam was rising from the "new" fumarole sites (active since the 1990s) and from the area of sulfur deposits and fumaroles in the southwestern area of the rim, but this activity did not appear unusual.

On 15 May, the GOES thermal anomaly was gone before noon, but returned near midnight (about 2330), over a smaller area, and it remained through sunrise (0615) on 16 May. Small anomalies were visible the next several nights (when contrast with adjacent cold flows was strongest), but there was no obvious evidence of continued feeding of the new flows.

The complex thermal anomalies detected in MODIS satellite imagery (provided by the University of Hawaii), were abundant around the time of eruption. They spread over Fernandina's rim, in some cases in the caldera, and broadly over the S flank. They continued through at least the rest of May.

The Washington VAAC reported that a weak hotspot started 29 May 2005 at 1945 (30 May at 0145 UTC) and a very short narrow plume of ash and gases appeared in multi-spectral imagery at 2145 (30 May at 0345 UTC). No ground confirmation of an eruption was available, and there was a layer of low-level weather cloud over the island. At that time, the plume appeared to dissipate as it moved away at ~ 18 km/hour.

Geologic Background. Fernandina, the most active of Galápagos volcanoes and the one closest to the Galápagos mantle plume, is a basaltic shield volcano with a deep 5 x 6.5 km summit caldera. The volcano displays the classic "overturned soup bowl" profile of Galápagos shield volcanoes. Its caldera is elongated in a NW-SE direction and formed during several episodes of collapse. Circumferential fissures surround the caldera and were instrumental in growth of the volcano. Reporting has been poor in this uninhabited western end of the archipelago, and even a 1981 eruption was not witnessed at the time. In 1968 the caldera floor dropped 350 m following a major explosive eruption. Subsequent eruptions, mostly from vents located on or near the caldera boundary faults, have produced lava flows inside the caldera as well as those in 1995 that reached the coast from a SW-flank vent. Collapse of a nearly 1 km3 section of the east caldera wall during an eruption in 1988 produced a debris-avalanche deposit that covered much of the caldera floor and absorbed the caldera lake.

Information Contacts: Patricio Ramón and Hugo Yepes, Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Alan Tye, Charles Darwin Research Station, Puerto Ayora, Santa Cruz, Galapagos Islands, Ecuador (URL: http://www.darwinfoundation.org/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Tom Simkin, Dept. of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013-7012, USA; National Earthquake Information Center, U.S. Geological Survey, Box 25046, DFC, MS 966, Denver, CO 80225-0046, USA (URL: https://earthquake.usgs.gov/); MODIS Thermal Alert System; University of Hawaii and Manoa, 168 East-West Road, Post 602, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu).

After a long period of quiescence following the 1991 phreatic eruption, Karthala's seismicity rebounded starting in July 2000 (BGVN 25:10). In October 2000, more than 20 seismic events per day were recorded.

The local observatory and a key source for this report is the Karthala Volcano Observatory (KVO; Netter and Cheminée, 1997). They maintain close ties with the Centre National de Documentation et de Recherche Scientifique des Comores (CNDRS), Reunion Island University, the Institut de Physique du Globe de Paris, Piton de la Fournaise Volcanological Observatory, and various universities in France.

Activity during October 2000-March 2004. Between October 2000 and January 2003, relatively low seismicity was detected beneath Karthala's summit. The seismicity slowly increased. During January instruments recorded 51 earthquakes on the 5th, 58 on the 10th, and 50 on the 11th. During the month of April 2003 instruments registered 732 (i.e. averaging ~ 24 each day).

Seismic instruments detected several short earthquake swarms, each comprised of ~ 150 earthquakes. These swarms took place on 25 March and in April 2003, and each lasted several hours. Moreover, seismologists witnessed another swarm consisting of 183 events on 15 May. Except for that latter swarm, Karthala's seismicity was relatively quiet for 35 days after the 25 April swarm. A photo of the Chahalé crater from the year 2003, well before the April 2005 eruption, appears in figure 6. (For a map of Karthala's summit, see BGVN 16:08.)

Figure 6. Karthala's ~ 300-m-diameter Chahalé crater as seen on 15 August 2003, more than a year prior to the April 2005 eruption. The photo was shot by the automatic camera located at the summit, looking from the NNE towards the SSW. The 2005 eruption dramatically changed this scene, replacing the green lake seen here with a lava lake, and blanketing considerable areas with tephra. Courtesy of Nicolas Villenueve.

During the time interval from early June 2003 to January 2004 instruments registered three periods with elevated seismicity. The first interval spanned 121 days from June until the end of September 2003 and included 6,315 earthquakes. Within that interval there was a major crisis on 6th September, comprised of 345 events, some being felt by local residents (BGVN 28:08).

The second interval began on 11 October 2003, reaching its peak on 4 January 2004 (253 events) and stopped on 31 January 2004. During this interval of 113 days, instruments registered 4,431 earthquakes. The third interval, during the time period of 3 February to 5 March 2004, contained fewer earthquakes. Instruments recorded 832 events in 31 days with a maximum of 143 events per day. After the third interval, KVO recorded only low seismicity until early 2005, when daily events rose to 50-60.

Eruption during April 2005. A seismic crisis began at 0812 on 16 April. Although instruments initially received only short-period events, starting at 0914 they also registered many long-period ones. From 1055 on 16 April a continuous signal was recorded, which was interpreted as tremor marking the beginning of the eruption. At around 1400 that day inhabitants heard a rumbling coming from the volcano. A few minutes later they observed an ash column above the summit. The first ash-fall deposits began to form around 1600, developing on the island's eastern side. According to the firsts reports, ash deposition increased and continued through the night accompanied by a strong smell of sulfur.

On the morning of 17 April ash falls continued on the eastern part of the island and were heavy enough to require inhabitants to use umbrellas to get about. At midday, Jean-Marc Heintz, a pilot for Comores Aviation, flew over the west flank and observed a large plume in the direction of the Chahalé crater. He also clearly observed airborne molten ejecta.

Around 1300, observers saw a very dark plume, spreading into a mushroom shape and accompanied by lightning flashes. Some inhabitants panicked and fled the island's eastern villages. In the afternoon, residents heard rumbling. During the evening, significant rainfall generated small mudflows, and the rumbling became stronger.

At that time, authorities evacuated some eastern villages (according to Agence France Presse (AFP) this affected ~ 10,000 people). Ash there started to fall on the island's western and northern parts, notably, on the country's capital city of Moroni (~ 10 km NW of the summit) and on the Hahaya airport (~ 20 km N of Moroni, ~ 25 km NW of the summit). Figure 7 shows a photo with the base of a vigorous plume over the E flanks on the afternoon of 17 April.

Figure 7. A phreatic eruption as seen from Karthala's eastern slopes on the afternoon of 17 April 2005. The vent lies below the white-colored zone in the center-right portion of the photo. With enlargement, many parts of the image record the descent of large pieces of ejecta. Photo credit to school teacher Daniel Hoffschir.

KVO authorities sometimes witnessed a red color at the plume's base, interpreted as a sign of an ongoing magmatic eruption. At 2105 the KVO seismic network recorded a drastic decrease in the amplitude of the tremor. During the night of 17-18 April, wide variations of the tremor amplitude were recorded with a maximum at 0140 on 18 April and a minimum at 0430 on 18 April. Thereafter, the tremor amplitude did not increase. During the night of 17-18 April the plume and falling ash disappeared.

On an overflight of the Chahalé crater at 0830 on 18 April, KVO personnel observed major modifications at the summit (figures 8-10). A lava lake (figure 8) had replaced the water-bearing lake (figure 6) that had occupied the crater since 1991.

Figure 8. On 18 April 2005 the Karthala eruption generated a lava lake in the Chahalé crater. In this photo, taken the morning of 18 April, considerable portions of the lava lake's surface still remained molten and incandescent.. The lake's surface only remained molten for a few hours. This aerial photo was taken looking from the N. Courtesy of Hamid Soulé.

Figure 9. A 19 April 2005 aerial photograph of Karthala taken from the SE centered on Chahalé crater. The lava lake's surface had chilled and it emitted white vapor. Much of the summit area displays the recently deposited smooth-surfaced tephra blanket. Courtesy of Nicolas Villenueve.

Figure 10. Tephra deposits left by Karthala's mid-April 2005 eruption altered the landscape and destroyed vegetation. This picture was taken at the entrance to the first caldera on the western trail, viewed looking to the S. Courtesy of Nicolas Villenueve.

On 19 April a new overflight revealed the crater floor containing the lava lake, with its chilled surface emitting steam (figure 13). Lava remained confined to Chahalé crater. Around the caldera area, and particularly on its N, observers saw conspicuous tephra deposits; most of the vegetation had been destroyed (figure 14).

On 20 April a field excursion found that ash deposits varied in thickness from a few millimeters on the coast to ~ 1.5 m at the summit. Near the summit the observers recognized some post-eruptive evaporation and geothermal processes. Specifically, although the lava lake's surface had frozen, there remained sufficient heat under the surface that groundwater migrating towards to the crater's floor evaporated into steam. During another field survey on 8 May, observers noted the renewed presence of lake water inside the crater.

Reference. Netter, C., and Cheminée, J. (eds.), 1997, Directory of Volcano Observatories, 1996-1997: World Organization of Volcano Observatories (WOVO), WOVO/IAVCEI/UNESCO, Paris, 268 p.

Geologic Background. The southernmost and largest of the two shield volcanoes forming Grand Comore Island (also known as Ngazidja Island), Karthala contains a 3 x 4 km summit caldera generated by repeated collapse. Elongated rift zones extend to the NNW and SE from the summit of the Hawaiian-style basaltic shield, which has an asymmetrical profile that is steeper to the S. The lower SE rift zone forms the Massif du Badjini, a peninsula at the SE tip of the island. Historical eruptions have modified the morphology of the compound, irregular summit caldera. More than twenty eruptions have been recorded since the 19th century from the summit caldera and vents on the N and S flanks. Many lava flows have reached the sea on both sides of the island. An 1860 lava flow from the summit caldera traveled ~13 km to the NW, reaching the W coast to the N of the capital city of Moroni.

Information Contacts: Nicolas Villeneuve (CREGUR, Centre de Recherches et d'Etudes en Géographie de l'Université de la Réunion); Hamidou Nassor, and Patrick Bachèlery (LSTUR, Laboratoire des Sciences de la Terre), Université de La Réunion BP 7151, 15 Avenue, René Cassin, 97715 Saint-Denis, Reunion Island; François Sauvestre and Hamid Soulé, CNDRS, BP 169, Moroni, République Fédérale Islamique des Comores (URL: http://volcano.ipgp.jussieu.fr/karthala/stationkar.html).

Lascar, the most active volcano in northern Chile, erupted on 4 May 2005. Although the eruption was substantial, thus far there is an absence of reports from anyone who saw the eruption at close range. Preliminary assessments came mainly from satellite sensors and distant affects witnessed in Argentina. This report is based on one sent to us by Chilean Observatorio Volcanológico de los Andes del Sur (OVDAS) scientists José Antonio Naranjo and Hugo Moreno, discussing events around 4 May, with brief comments on some of Lascar's behavior in the past several years, and suggestions for future monitoring.

Lascar sits ~ 70 km SW of the intersection between Chile, Argentina, and Bolivia, ~ 300 km inland from the Chilean port city of Antofagasta. This part of the coast lies along the Atacama desert, and on flat terrain tens of kilometers W of Lascar resides a large salt pan, the Salar de Atacama (about 50 x 150 km). The settlement of Toconao is ~ 33 km NW of Lascar. Previous reports discussed field observations during 13 October 2002 to 15 January 2003, and fine ash discharged from fumaroles on 9 December 2003 (BGVN 28:03 and 29:01).

Naranjo and Moreno concluded that at roughly 0400 on 4 May an explosive eruption ejected an ash cloud to a tentative altitude on the order of 10 km that dispersed to the SE. About 2 hours later the cloud began dropping ash on Salta, Argentina. Satellite images portrayed the ash cloud's dispersal. An aviation 'red alert' was issued by the Buenos Aires Volcanic Ash Center; they saw the plume over Argentina at altitudes of 3-5 km.

Shortly after atmospheric impacts of the 4 May eruption became apparent, the Buenos Aires VAAC notified OVDAS that NW Argentine cities had reported falling ash. These cities, all SE of Lascar, included Jujuy, Salta, Santiago del Estero, and Santa Fe—locations with respective approximate distances from Lascar of 260, 275, 580, and 1,130 km. The Argentine province of Chaco, along the country's NE margin, was also noted as receiving ash. Buenos Aires (~ 1,530 km SE of Lascar) remained ~ 400 km beyond the point of the farthest detected ashfall.

Patricia Lobera, a professor in Talabre, Argentina, 17 km E of Lascar, said that eruption noises were not heard there on the morning of 4 May. When observers saw the plume from Talabre that morning they reportedly thought the plume looked similar to those on previous days.

Remotely sensed hot spots were detected on a GOES satellite image for 0339 (0639 UTC) on 4 May, showing the first evidence of an eruption. In a later image, at 0409, the thermal anomaly had increased, and the image suggested a growing, ash-bearing cloud then trending ~ 23 km to the SE. The thermal anomaly diminished in intensity by 0439, remaining diminished thereafter, but by that time the plume's leading margin extended over ~ 100 km SE and its tail had detached from the volcano. At 0509 the plume reached 170 km SE. According to a press report, at around 0600 ash fell in Salta (~ 275 km SE of Lascar).

Rosa Marquilla, a geologist at the University of Salta, reported that residents there noticed a mist attributed to the eruption, which hung over the city until at least to 1600, after which, the sky gradually cleared. Preliminary description of the petrography of the ash that fell in Salta came from Ricardo Pereyra (University of Salta) who saw crystal fragments (pyroxenes, feldspars, and magnetite) and fragments of volcanic glass containing plagioclase mircrolites. Lithic fragments were not observed.

The OVDAS authors concluded that, apparently since the year 2000, Lascar underwent constant degassing from an open vent within the ~ 780-m-diameter active central crater. Sporadic explosions as in July 2000 and October 2002, and in this case, 4 May 2005, could be due to diverse causes. For example, there may have been temporarily obstructed conduits at depth, local collapses blocking the vent at the crater floor, or fresh magma injection contacting groundwater. Extrusion of a viscous dome lava also might explain the sudden explosions. That circumstance would presumably lead to visibly increased fumarolic output.

Naranjo and Moreno had several suggestions for ongoing monitoring. First, they suggested developing closer long-term contacts, including people able to visually monitor the volcano directly, as well as continued systematic contact with the Buenos Aires VAAC and their satellite analysts. They recommended ongoing relations with the University of Hawaii (MODVOLC) program to remotely sense hot-spots. They went on to suggest a campaign of stereo aerial photography to detect changes in the active crater. They advocated notifying local inhabitants of the possibility of ash falls before another explosive episode. They pointed out that mountaineers should be made aware of elevated risks within 8 km of the active crater.

References. Gardeweg, M., 1989, Informe preliminar sobre la evolución de la erupción del volcán Láscar (II Región): noviembre 1989: Servicio Nacional de Geología y Minería, Informe Inédito (unpublished report), 27 p.

Gardeweg, M., and Lindsay, J., 2004, Lascar Volcano, La Pacana Caldera, and El Tatio Geothermal Field: IAVCEI General Assembly Pucón 2004, Field Trip Guide-A2, 32 p.

Gardeweg, M., Medina, E., Murillo, M., and Espinoza, A., 1993, La erupción del 19-20 de abril de 1993: VI informe sobre el comportamiento del volcán Láscar (II Región): Servicio Nacional de Geología y Minería, Informe Inédito (unpublished report), 20 p.

Geologic Background. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: José Antonio Naranjo and Hugo Moreno, Programa Riesgo Volcanico, Servicio Nacional de Geologia y Mineria, Avda. Santa Maria 0104, Casilla 1347, Santiago, Chile; Gustavo Alberto Flowers, Buenos Aires Volcanic Ash Advisory Center (Buenos Aires VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php).

Although lava venting at Ol Doinyo Lengai continued intermittently after February 2004 (BGVN 29:02), no significant changes were detected until July 2004, a time when vigorous venting emitted substantial amounts of the low-viscosity carbonatitic lava typical at this volcano ('flash floods' of lava). This summary report covers the time interval from February 2004 through early February 2005 based on observations made by Frederick Belton, Celia Nyamweru, Bernhard Donth, and Christoph Weber. Websites devoted to Ol Doinyo Lengai, including photographs, information on the evolution, recent history, and current status of the volcano are maintained by Belton, Nyamweru, and Weber.

A map, thermal data, and some new elevation estimates. In February 2005 Weber and others collected location data with a global positioning system (GPS) receiver. Weber used this to create a sketch map of the active crater (figure 82).

Figure 82. Sketch map of crater features at Ol Doinyo Lengai surveyed with a global positioning system (GPS) during 3-7 February 2005. During the course of the report interval, new vents developed at T49G and T58C (amid the N-central group of hornitos; T49G sits ~ 10 m E of T49B). These new vents produced comparatively vigorous eruptions. Courtesy of Chris Weber (Volcano Expeditions International, VEI).

In July 2004 Belton completed the third of a series of distance measurements across crater outflow areas at the crater rim (table 7). Due to the unusually strong eruption on 15 July 2004 (figure 83), deposits comprising the E overflow widened by 3 or 4 m (growing from 44 to 47 m, figure 82). Later, in January 2005, observers noticed a fourth area of overflows had become established on the N crater rim, with lavas pouring over the rim at two adjacent points there (figure 82).

Table 7. For Ol Doinyo Lengai, the width of the three extant lava outflows at the points where they spilled from the active crater ('overflows,' figure 15), as measured during 2 August 2003-29 July 2004. Two additional small overflows formed later, by January 2005, on the N crater rim. The 3-m E-overflow increase occurred during the eruption of T58C on 15 July 2004. Courtesy of Frederick Belton.

Figure 83. A time exposure photograph of Ol Doinyo Lengai taken just after sunset on 15 July 2004. At that time the newly formed vent T58C ("Charging Rhino") issued copious lava. This photo was taken looking approximately NW from the SE part of the crater rim. Courtesy of Frederick Belton.

During 3-7 February 2005 Weber and others completed a series of lava and fumarole temperature measurements that appear as tables 8 and 9. The tables indicate the hottest lava and fumarole temperatures at cracks were 588°C (at T49C, February 2004) and 150°C (at T49G, June 2004), respectively. The hornitos T49C and T49G both lie near T49B, a hornito delineated on figure 82.

Table 8. Repeated maximum lava temperatures measured at Ol Doinyo Lengai during 28 August 1999 to 3 February 2005. The measurements were made employing a digital thermometer (TM 914C with a stab feeler of standard K type). The instrument was used in the 0-1200°C mode, and at least four replicate measurements were made at any one spot. Calibration was by the delta-T method; uncertainties were ± 6°C in the 0-750°C range. Courtesy of C. Weber.

Table 9. Maximum fumarole temperatures measured at cracks in Ol Doinyo Lengai's crater floor over a series of visits during 28 August 1999 to 4 February 2005. Collected using the digital thermometer with procedures and parameters noted with the previous table. For locations, see map (figure 15). Courtesy of C. Weber.

Weber's team GPS measurements suggested a summit elevation of 2,960 ± 5 m. This is consistent with GPS measurements taken in October 2000, by a scientific group led by Joerg Keller, of 2,950-2,960 m (BGVN 25:12). In addition, the tallest hornito in the N-central crater rises to nearly this elevation (see discussion of T49/T56B, below).

During observations in February 2004, Weber measured the tallest hornito at the T49 location (part of T56B) in the center area of the active crater. GPS readings on top of T56B yielded an elevation of 2,886 m. This is only [74 m below the elevation of the summit]. The top of T49 is also ~ 33 m above the adjacent crater floor to the N. In addition, when he measured on 3-7 February, Weber found hornito T58C (a then recent feature) had grown to reach an elevation of ~ 2,870 m.

Observations during February 2004 to February 2005. During February 2004 visits, T56B did not erupt, but instead a new vent erupted at the T49 location (~ 10 m E of T49B, see also BGVN 29:02). This new vent was called T49G (figure 15).

A group from Volcano Expeditions International (VEI) spent 24-30 June 2004 on Lengai and found much of the scene at the vents in the crater similar to that noted in February 2004. They noted that half of the upper 10 m of hornito T56B had collapsed on its E side, and an active lava lake had formed inside this hornito with lava escaping several times through the collapsed opening to its E and flowing out ~ 200 m. The lava was rich in gas with a temperature of 560°C. The hornito T58B was also active and spattered lava many times during these days of observation. Some lava flows from T58B reached about 150 m to the S.

During 2-3 July 2004, Belton observed T58B erupt repeatedly, emitting lava and strombolian displays. The escaping lava flowed S, passing near the base of hornito T47. On 4 July, Belton saw some of the most intense activity of the month. A sequence of lavas erupted on that day and over the next few days. However, events in mid-July and later were also unusually vigorous.

The 4 July 2004 activity included strong strombolian eruptions at T58B and several collapses of its vent area, which released large cascades of lava onto the crater floor. Simultaneously, a tube-fed eruption of pahoehoe lava from the new vent T49G flowed across the NW crater rim to spill down that flank. Early on 5 July numerous eruptions of T58B sent lava flowing toward T47 at an estimated velocity of 10 m/sec. On 6 July, lava flowed out of the lake in T56B and onto the crater floor moving E and entering a cave in T45 for a short distance.

After very low activity during 7-10 July 2004, renewed flows and spatter came out on 11 July from T58B, and frequent but short (usually ~ 2 minute) episodes of loud degassing and spattering issued from the lava lake in T56B. At night, this vent emitted incandescent gas. This pattern continued until the morning of 14 July, when eruptions at T58B became more explosive and it expelled small ash clouds. On the morning of 15 July a collapse in the vent area of T58B released large rapid lava flows to the E. The episodes of degassing and spattering from T56B increased in frequency until 1500 on 15 July, when a small hole formed in the crater floor just E of T58B.

Called T58C, the hole became a newly opened vent. It began emitting visible gas puffs mixed with spatter. At this time the degassing episodes from T56B ceased. T58C then began strong degassing and squirted up intermittent lava fountains. The fountains soon fed a large lava stream moving toward the S crater wall.

By 1600 on 15 July 2004 a paroxysm at T58C was in progress, with lava forming 10- to 12-m-tall fountains and 'flash floods' that completely inundated the central-eastern crater floor (in the area between T56B, T58B, T37, T37B, T45, and T57). T58C also ejected strong jets of ash and gas. Turbulent rivers of lava flowing at more than 10 m/sec swept toward the crater's S wall and its E overflow and completely surrounded T37B and T45. Flow rate from the vent was estimated to peak at 10 m³/s.

The momentum of the rapidly outflowing lava carried it nearly 3 m up the W (upstream) side of T45 and obliterated the large cave within that cone. The associated surge of lava poured over the E crater rim and down the flank. It flooded over a 3 m wide swath of vegetation. This triggered a huge cloud of steam and smoke that resembled a small pyroclastic flow. The smoke cloud was accompanied by a loud sizzling sound. A brush fire burned along the crater rim overflow as additional floods of lava arrived. These larger-than-normal flows lasted for little more than 30 sec and were separated by periods of repose of 5 to 6 min. After sunset, incandescent gas flared from the vent during the repose periods. Weak strombolian activity was seen in T56B.

Early on 16 July 2004 the newly formed T58C was a circular pit ~ 2 m in diameter with lava sloshing violently at a depth of ~ 2 m. Two small sub-vents on the N and S edges of the pit interconnected with the main vent. Activity continued sporadically at T58B and T56B with strombolian activity and lava flows. On 21 July there was an exceptionally strong eruption of T58B with loud explosions, jetting of ash-poor clouds, and spatter thrown to above-average heights. Explosions blasted a new vent in the upper E side of T58B. At least four oval bombs 9-12 cm in length flew through the air, along with a great deal of lapilli and ash. Later examination of the bomb's interiors revealed that they all had an outer zone ~ 1.5- to 2-cm thick and a distinctive inner core.

On 23 July 2004, a sloping ~ 4 m² oval section of the crater floor immediately SW of the new spatter cone T58C began to steam and vibrate. Tremor increased and ground movement was visible, manifested as a small section of crater floor rapidly pushed outward and then inward several centimeters, like a membrane vibrating in time to the degassing sounds of lava in T58C just behind it. Abruptly this portion of the crater floor broke outward, and a flood of lava ensued. T58C was observed to grow in height through the time when Belton left the crater on 29 July 2004.

Observations during January and February 2005. Donth reported that during his visit on 10 January 2005, hornito T49B erupted to form many effusive lava flows. For the first time, lava escaped over the northern edge of the crater (see figure 15).

During Weber's crater visit, 3-7 February 2005, the hornito T49B actively emitted lava flows that traveled to the N. Pahoehoe lava flows in motion within small levees on flat terrain were measured from 520°C up to a maximum of 561°C (table 3). The fumaroles at F1 had a maximum temperature of 84°C, and at hornito T46, a maximum of 91°C (table 4). No change in distance was measured across the CR1, CR2, and CR3 cracks cutting the upper crater walls. Adding to visitor safety concerns, which include altitude sickness, burns, falls, and impact from ejecta, Weber's team saw a spitting cobra close to the summit. An overflight by plane on 14 February showed no subsequent change, but did give an excellent view of the crater and its central hornitos (figure 16).

A flight on 14 February failed to reveal subsequent changes. But the effort provided an excellent view of the crater and its central hornitos (figure 84).

Figure 84. An aerial photograph taken looking towards the WSW at the summit crater of Ol Doinyo Lengai on 14 February 2005. The summit, which lies in the upper left corner has a revised elevation based on GPS (see text). In addition, GPS elevations and uncertainties suggest that in 2005 the summit was only marginally higher than the top of the tallest hornito (T56B). Copyrighted photo provided courtesy of T. Schulmeister and C. Weber.

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: Christoph Weber, Volcano Expeditions International, Muehlweg 11, 74199 Untergruppenbach, Germany (URL: http://www.v-e-i.de/); Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617, USA (URL: http://blogs.stlawu.edu/lengai/); Frederick Belton, Developmental Studies Department, PO Box 16, Middle Tennessee State University, Murfreesboro, TN 37132, USA (URL: http://oldoinyolengai.pbworks.com/); Bernard Donth, Waldwiese 5, 66123 Saarbruecken, Germany.

According to Hubert Staudigel (Scripps Institution of Oceanography) and Stanley Hart (Woods Hole Oceanographic Institute), Vailulu'u seamount, the most active Samoan submarine volcano, erupted between April 2001 and April 2005. It formed a 293-m-tall lava cone, which was named Nafanua after the Samoan Goddess of War. This new cone had been growing inside the 2-km-wide caldera of Vailulu'u at a minimum rate of about 20 cm/day since April 2001. Nafanua was discovered during a 2005 diving expedition with the National Oceanic and Atmospheric Agency (NOAA) research submersible Pisces V, launched from the University of Hawaii research vessel Kaimikai O Kanaloa (KOK). It is located in the originally 1,000-m-deep W crater of Vailulu'u (figures 5 to 8).

Figure 5. The route of the 2005 cruise of the research vessel Kilo Moana. Vailulu'u, towards the E end of the Samoan hotspot trail was visited on cruise days 1-4, 4-8 April 2005. Courtesy of H. Staudigel and S. Hart.

Figure 6. Bathymetry of Vailulu'u and nearby Ta'u Island, based on a SeaBeam bathymetric survey performed during R/V Melville's AVON 2 and 3 cruises, augmented with satellite-derived bathymetry from Smith and Sandwell (1996). The inset shows the general location of Vailulu'u with respect to the Samoan Archipelago; two other newly mapped and dredged seamounts (Malumalu and Muli, AVON 3 cruise) are shown as well. Scale: 10' = 18 km. From Hart and others (2000).

Figure 7. Bathymetry of the Vailulu'u crater between the 1999 and 2005 surveys, showing the emergence of Nafanua. Courtesy of H. Staudigel and S. Hart.

Figure 8. Bathymetric map of the Vailulu'u seamount from multibeam data during the April 2005 survey. Note the new inner cone named Nafanua. Contour interval is 20 m. Courtesy of H. Staudigel and S. Hart.

Seismic monitoring during April-June 2000 showed substantial seismicity, ~ 4 earthquakes per day with hypocenters beneath Nafanua (Konter and others, 2004; BGVN 26:06), which can now be interpreted as pre-eruption seismic activity. These observations are consistent with previous reports highlighting the volcanic and hydrothermal activity of Vailulu'u (Hart and others, 2000; Staudigel and others, 2004). The scientists suggested that continued volcanic activity could bring the summit region of Vailulu'u to a water depth of ~ 200 m. At that point, Nafanua would overtop the crater rim and further growth would require a build-up of the lower flanks, areas that rise from the 5,000-m-deep floor of the ocean.

Staudigel and Hart teamed up in April 2005 on the Hawaiian Research Vessel Kilo Moana to study the Samoan hotspot thought to underlie Vailulu'u. They named their expedition ALIA after the ancient twin-hulled canoe that Samoan warriors used to explore the SW Pacific. The Kilo Moana left Pago Pago on 4 April 2005 to study active and extinct underwater volcanoes along the chain of Samoan islands. The expedition investigated previously uncharted seamounts and the submarine portions of some islands, scattered over almost 600 nautical miles, from its most recent and quite active Vailulu'u submarine volcano in the E to Combe Island in the W.

The Nafanua cone was first mapped by using the center beam of the research vessel KOK in several crossings of the W crater. An active hydrothermal system was apparent from evidence such as the murky water that limited visibility during two submersible dives, several microbial biomats covering pillow lavas that were centimeters thick, and a large number of diffuse vents. A dive on 30 March 2005 to examine Nafanua reported "that it must have grown in the last 4 years because CTD (conductivity-temperature-depth) crossings in 2001 still were consistent with the old crater morphology ... the basal portion of the cone displayed relatively large pillows, and higher up pillows look almost like very fluid pahoehoe that collapsed and/or transitioned into aa flows. Nafanua . . . grew very fast with abundant breccia material from collapsing and draining pillows, in particular in the summit region."

On 1 April, another dive along the outer flanks of Vailulu'u found that during the up-slope transit, observers saw a few additional areas of active venting and many sites where there had been venting in the past. Large and perfectly formed pillow lavas were present in most sites, with a few areas being dominated by broken talus fragments and some having completely black glassy pillows with no oxidation, apparent evidence for relatively recent formation. The topography was extremely rough, the slope being punctuated with numerous fissure systems and edifices of pillow lava.

A primary plan for the ALIA expedition was to study the water in and around the seamount for several days using a CTD probe. To sample the inside of the volcano for a full tidal cycle, the scientists varied the depth of the CTD between 40 and 930 m (almost to the crater floor), collecting various data, including visibility. At Vailulu'u, the particulates given off by hydrothermal venting are flushed out of its caldera during each tidal cycle into the surrounding water. In 2005, a dense layer of particulates was found in the water within the crater, but the water was clear outside the crater rim. This contrasts with observations seen from the cruise in 2000, when there was a dense ring of particulates around the whole volcano. It appears that in 2005 the particulates were rising above the crater and then later sinking, instead of forming the widespread ring observed in 2000.

In addition, the expedition crew conducted dredging of the new summit of Nafanua. Staudigel and Hart noted that the rocks from the first dredge haul were quite newly formed, containing pristine olivine-phyric volcanic rocks. Abundant large vesicles in the rocks from Nafanua suggest a volatile-rich magma capable of submarine lava fountaining and explosive outgassing in shallower water. Dredging from a second site, outside of Vailulu'u, recovered rocks that were both much older and far less fragile.

References. Hart, S.R., Staudigel, H., Koppers, A.A.P., Blusztajn, J., Baker, E.T., Workman, R., Jackson, M., Hauri, E., Kurz, M., Sims, K., Fornari, D., Saal, A., and Lyons, S., 2000, Vailulu'u undersea volcano: The new Samoa: Geochemistry, Geophysics, Geosystems (G3), American Geophysical Union, v. 1, no. 12, doi: 10.1029/2000GC000108.

Konter, J.G., Staudigel, H., Hart, S.R., and Shearer, P.M., 2004, Seafloor seismic monitoring of an active submarine volcano: Local seismicity at Vailulu'u Seamount, Samoa: Geochemistry, Geophysics, Geosystems (G3), American Geophysical Union, v. 5, no. 6, QO6007, doi: 10.1029/2004GC000702.

Lippsett, L., 2002, Voyage to Vailulu'u: Searching for Underwater Volcanoes. Woods Hole Oceanographic Institution, Fathom online magazine (URL: http://www.fathom.com/feature/122477/).

Staudigel, H., Hart, S.R., Koppers, A., Constable, C., Workman, R., Kurz, M., and Baker, E.T., 2004, Hydrothermal venting at Vailulu'u Seamount: The smoking end of the Samoan chain: Geochemistry, Geophysics, Geosystems (G3), American Geophysical Union, v. 5, no. 2, QO2003, doi: 10.1029/2003GC000626.

Geologic Background. Vailulu'u, a massive basaltic seamount not discovered until 1975, rises 4,200 m from the sea floor to a depth of 590 m about one-third of the way between Ta'u and Rose islands at the E end of the American Samoas. It is considered to mark the current location of the Samoan hotspot. The summit contains a 2-km-wide, 400-m-deep oval-shaped caldera. Two principal rift zones extend E and W from the summit, parallel to the trend of the hotspot. A third less prominent rift extends SE of the summit. The rift zones and escarpments produced by mass wasting phenomena give the seamount a star-shaped pattern. On 10 July 1973, explosions were recorded by SOFAR (hydrophone records of underwater acoustic signals). An earthquake swarm in 1995 may have been related to an eruption. Turbid water above the summit shows evidence of ongoing hydrothermal plume activity.

Information Contacts: Hubert Staudigel, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, Univ. of California, San Diego, La Jolla, CA 92093-0225, USA (URL: https://earthref.org/whoswho/ER/hstaudigel/, https://igpp.ucsd.edu/); Stanley R. Hart, Woods Holes Oceanographic Institute, Geology and Geophysics Dept., Woods Hole, MA 02543, USA; ALIA Expedition, Samoan Seamounts, R/V Kilo Moana (KM0506), supported by the San Diego Supercomputer Center and the Scripps Institution of Oceanography (URL: https://earthref.org/ERESE/projects/ALIA/).