Erta Ale is a shield volcano located in Ethiopia and contains multiple active pit craters in the summit and southeastern caldera. Volcanism has been characterized by lava flows and large lava flow fields since 2017. Surficial lava flow activity continued within the southeastern caldera during November 2019 until early April 2020; source information was primarily from various satellite data.

The number of days that thermal anomalies were detected using MODIS data in MODVOLC and NASA VIIRS satellite data was notably higher in November and December 2019 (figure 96); the number of thermal anomalies in the Sentinel-2 thermal imagery was substantially lower due to the presence of cloud cover. Across all satellite data, thermal anomalies were identified for 29 days in November, followed by 30 days in December. After December 2019, the number of days thermal anomalies were detected decreased; hotspots were detected for 17 days in January 2020 and 20 days in February. By March, these thermal anomalies became rare until activity ceased. Thermal anomalies were identified during 1-4 March, with weak anomalies seen again during 26 March-8 April 2020.

Figure 96. Graph comparing the number of thermal alerts using calendar dates using MODVOLC, NASA VIIRS, and Sentinel-2 satellite data for Erta Ale during November 2019-March 2020. Data courtesy of HIGP - MODVOLC Thermal Alerts System, NASA Worldview using the “Fire and Thermal Anomalies” layer, and Sentinel Hub Playground.

MIROVA (Middle Infrared Observation of Volcanic Activity) analysis of MODIS satellite data showed frequent strong thermal anomalies from 18 April through December 2019 (figure 97). Between early August 2019 and March 2020, these thermal signatures were detected at distances less than 5 km from the summit. In late December the thermal intensity dropped slightly before again increasing, while at the same time moving slightly closer to the summit. Thermal anomalies then became more intermittent and steadily decreased in power over the next two months.

Figure 97. Two time-series plots of thermal anomalies from Erta Ale from 18 April 2019 through 18 April 2020 as recorded by the MIROVA system. The top plot (A) shows that the thermal anomalies were consistently strong (measured in log radiative power) and occurred frequently until early January 2020 when both the power and frequency visibly declined. The lower plot (B) shows these anomalies as a function of distance from the summit, including a sudden decrease in distance (measured in kilometers) in early August 2019, reflecting a change in the location of the lava flow outbreak. A smaller distance change can be identified at the end of December 2019. Courtesy of MIROVA.

Unlike the obvious distal breakouts to the NE seen previously (BGVN 44:04 and 44:11), infrared satellite imagery during November-December 2019 showed only a small area with a thermal anomaly near the NE edge of the Southeast Caldera (figure 98). A thermal alert was seen at that location using the MODVOLC system on 28 December, but the next day it had been replaced by an anomaly about 1.5 km WSW near the N edge of the Southeast Caldera where the recent flank eruption episode had been centered between January 2017 and January 2018 (BGVN 43:04). The thermal anomaly that was detected in the summit caldera was no longer visible after 9 January 2020, based on Sentinel-2 imagery. The exact location of lava flows shifted within the same general area during January and February 2020 and was last detected by Sentinel-2 on 4 March. After about two weeks without detectable thermal activity, weak unlocated anomalies were seen in VIIRS data on 26 March and in MODIS data on the MIROVA system four times between 26 March and 8 April. No further anomalies were noted through the rest of April 2020.

Figure 98. Sentinel-2 thermal satellite imagery of Erta Ale volcanism between November 2019 and March 2020 showing small lava flow outbreaks (bright yellow-orange) just NE of the southeastern calderas. A thermal anomaly can be seen in the summit crater on 15 November and very faintly on 20 December 2019. Imagery on 19 January 2020 showed a small thermal anomaly near the N edge of the Southeast Caldera where the recent flank eruption episode had been centered between January 2017 and January 2018. The last weak thermal hotspot was detected on 4 March (bottom right). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. Erta Ale is an isolated basaltic shield that is the most active volcano in Ethiopia. The broad, 50-km-wide edifice rises more than 600 m from below sea level in the barren Danakil depression. Erta Ale is the namesake and most prominent feature of the Erta Ale Range. The volcano contains a 0.7 x 1.6 km, elliptical summit crater housing steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Fresh-looking basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera is renowned for one, or sometimes two long-term lava lakes that have been active since at least 1967, or possibly since 1906. Recent fissure eruptions have occurred on the N flank.

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); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

Rincón de la Vieja is a remote volcanic complex in Costa Rica containing an acid lake that has regularly generated weak phreatic explosions since 2011 (BGVN 44:08). The most recent eruptive period occurred during late March-early June 2019, primarily consisting of small phreatic explosions, minor deposits on the N crater rim, and gas-and-steam emissions. The report period of August 2019-March 2020 was characterized by similar activity, including small phreatic explosions, gas-and-steam plumes, ash and lake sediment ejecta, and volcanic tremors. The most significant activity during this time occurred on 30 January, where a phreatic explosion ejected ash and lake sediment above the crater rim, resulting in a pyroclastic flow which gradually turned into a lahar. Information for this reporting period of August 2019-March 2020 comes from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) using weekly bulletins.

According to OVSICORI-UNA, a small hydrothermal eruption was recorded on 1 August 2019. The seismicity was low with a few long period (LP) earthquakes around 1 August and intermittent background tremor. No explosions or emissions were reported through 11 September; seismicity remained low with an occasional LP earthquake and discontinuous tremor. The summit’s extension that has been recorded since the beginning of June stopped, and no significant deformation was observed in August.

Starting again in September 2019 and continuing intermittently through the reporting period, some deformation was observed at the base of the volcano as well as near the summit, according to OVSICORI-UNA. On 12 September an eruption occurred that was followed by volcanic tremors that continued through 15 September. In addition to these tremors, vigorous sustained gas-and-steam plumes were observed. The 16 September weekly bulletin did not describe any ejecta produced as a result of this event.

During 1-3 October small phreatic eruptions were accompanied by volcanic tremors that had decreased by 5 October. In November, volcanism and seismicity were relatively low and stable; few LP earthquakes were reported. This period of low activity remained through December. At the end of November, horizontal extension was observed at the summit, which continued through the first half of January.

Small phreatic eruptions were recorded on 2, 28, and 29 January 2020, with an increase in seismicity occurring on 27 January. On 30 January at 1213 a phreatic explosion produced a gas column that rose 1,500-2,000 m above the crater, with ash and lake sediment ejected up to 100 m above the crater. A news article posted by the Universidad de Costa Rica (UCR) noted that this explosion generated pyroclastic flows that traveled down the N flank for more than 2 km from the crater. As the pyroclastic flows moved through tributary channels, lahars were generated in the Pénjamo river, Zanjonuda gorge, and Azufrosa, traveling N for 4-10 km and passing through Buenos Aires de Upala (figure 29). Seismicity after this event decreased, though there were still some intermittent tremors.

Figure 29. Photo of a lahar generated from the 30 January 2020 eruption at Rincon de la Vieja. Photo taken by Mauricio Gutiérrez, courtesy of UCR.

On 17, 24, and 25 February and 11, 17, 19, 21, and 23 March, small phreatic eruptions were detected, according to OVSICORI-UNA. Geodetic measurements observed deformation consisting of horizontal extension and inflation near the summit in February-March. By the week of 30 March, the weekly bulletin reported 2-3 small eruptions accompanied by volcanic tremors occurred daily during most days of the week. None of these eruptions produced solid ejecta, pyroclastic flows, or lahars, according to the weekly OVSICORI-UNA bulletins during February-March 2020.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge that was constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of 1916-m-high Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A plinian eruption producing the 0.25 km3 Río Blanca tephra about 3500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); Luis Enrique Brenes Portuguéz, University of Costa Rica, Ciudad Universitaria Rodrigo Facio Brenes, San José, San Pedro, Costa Rica (URL: https://www.ucr.ac.cr/noticias/2020/01/30/actividad-del-volcan-rincon-de-la-vieja-es-normal-segun-experto.html).

Manam is a basaltic-andesitic stratovolcano that lies 13 km off the northern coast of mainland Papua New Guinea; it has a 400-year history of recorded evidence for recurring low-level ash plumes, occasional Strombolian activity, lava flows, pyroclastic avalanches, and large ash plumes from Main and South, the two active summit craters. The current eruption, ongoing since June 2014, produced multiple large explosive eruptions during January-September 2019, including two 15-km-high ash plumes in January, repeated SO₂ plumes each month, and another 15.2 km-high ash plume in June that resulted in ashfall and evacuations of several thousand people (BGVN 44:10).

This report covers continued activity during October 2019 through March 2020. Information about Manam is primarily provided by Papua New Guinea's Rabaul Volcano Observatory (RVO), part of the Department of Mineral Policy and Geohazards Management (DMPGM). This information is supplemented with aviation alerts from the Darwin Volcanic Ash Advisory Center (VAAC). MODIS thermal anomaly satellite data is recorded by the University of Hawai'i's MODVOLC thermal alert recording system, and the Italian MIROVA project; sulfur dioxide monitoring is done by instruments on satellites managed by NASA's Goddard Space Flight Center. Satellite imagery provided by the Sentinel Hub Playground is also a valuable resource for information about this remote location.

A few modest explosions with ash emissions were reported in early October and early November 2019, and then not again until late March 2020. Although there was little explosive activity during the period, thermal anomalies were recorded intermittently, with low to moderate activity almost every month, as seen in the MODIS data from MIROVA (figure 71) and also in satellite imagery. Sulfur dioxide emissions persisted throughout the period producing emissions greater than 2.0 Dobson Units that were recorded in satellite data 3-13 days each month.

Figure 71. MIROVA thermal anomaly data for Manam from 17 June 2019 through March 2020 indicate continued low and moderate level thermal activity each month from August 2019 through February 2020, after a period of increased activity in June and early July 2019. Courtesy of MIROVA.

The Darwin VAAC reported an ash plume in visible satellite imagery moving NW at 3.1 km altitude on 2 October 2019. Weak ash emissions were observed drifting N for the next two days along with an IR anomaly at the summit. RVO reported incandescence at night during the first week of October. Visitors to the summit on 18 October 2019 recorded steam and fumarolic activity at both of the summit craters (figure 72) and recent avalanche debris on the steep slopes (figure 73).

Figure 72. Steam and fumarolic activity rose from Main crater at Manam on 18 October 2019 in this view to the south from a ridge north of the crater. Google Earth inset of summit shows location of photograph. Courtesy of Vulkanologische Gesellschaft and Claudio Jung, used with permission.

Figure 73. Volcanic debris covered an avalanche chute on the NE flank of Manam when visited by hikers on 18 October 2019. Courtesy of Vulkanologische Gesellschaft and Claudio Jung, used with permission.

On 2 November, a single large explosion at 1330 local time produced a thick, dark ash plume that rose about 1,000 m above the summit and drifted NW. A shockwave from the explosion was felt at the Bogia Government station located 40 km SE on the mainland about 1 minute later. RVO reported an increase in seismicity on 6 November about 90 minutes before the start of a new eruption from the Main Crater which occurred between 1600 and 1630; it produced light to dark gray ash clouds that rose about 1,000 m above the summit and drifted NW. Incandescent ejecta was visible at the start of the explosion and continued with intermittent strong pulses after dark, reaching peak intensity around 1900. Activity ended by 2200 that evening. The Darwin VAAC reported a discrete emission observed in satellite imagery on 8 November that rose to 4.6 km altitude and drifted WNW, although ground observers confirmed that no eruption took place; emissions were only steam and gas. There were no further reports of explosive activity until the Darwin VAAC reported an ash emission in visible satellite imagery on 20 March 2020 that rose to 3.1 km altitude and drifted E for a few hours before dissipating.

Although explosive activity was minimal during the period, SO₂ emissions, and evidence for continued thermal activity were recorded by satellite instruments each month. The TROPOMI instrument on the Sentinel-5P satellite captured evidence each month of SO₂ emissions exceeding two Dobson Units (figure 74). The most SO₂ activity occurred during October 2019, with 13 days of signatures over 2.0 DU. There were six days of elevated SO₂ each month in November and December, and five days in January 2020. During February and March, activity was less, with smaller SO₂ plumes recording more than 2.0 DU on three days each month. Sentinel-2 satellite imagery recorded thermal anomalies at least once from one or both of the summit craters each month between October 2019 and March 2020 (figure 75).

Figure 74. SO2 emissions at Manam exceeded 2 Dobson Units multiple days each month between October 2019 and March 2020. On 3 October 2019 (top left) emissions were also measured from Ulawun located 700 km E on New Britain island. On 30 November 2019 (top middle), in addition to a plume drifting N from Manam, a small SO2 plume was detected at Bagana on Bougainville Island, 1150 km E. The plume from Manam on 2 December 2019 drifted ESE (top right). On 26 January 2020 the plume drifted over 300 km E (bottom left). The plumes measured on 29 February and 4 March 2020 (bottom middle and right) only drifted a few tens of kilometers before dissipating. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 75. Sentinel-2 satellite imagery with Atmospheric penetration rendering (bands 12, 11, and 8a) showed thermal anomalies at one or both of Manam’s summit craters each month during October 2019-March 2020. On 17 October 2019 (top left) a bright anomaly and weak gas plume drifted NW from South crater, while a dense steam plume and weak anomaly were present at Main crater. On 25 January 2020 (top right) the gas and steam from the two craters were drifting E; the weaker Main crater thermal anomaly is just visible at the edge of the clouds. A clear image on 5 March 2020 (bottom left) shows weak plumes and distinct thermal anomalies from both craters; on 20 March (bottom right) the anomalies are still visible through dense cloud cover that may include steam from the crater vents as well. Courtesy of Sentinel Hub Playground.

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical 1807-m-high basaltic-andesitic stratovolcano to its lower flanks. These "avalanche valleys" channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most historical eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent historical eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: 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; 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); Vulkanologische Gesellschaft (URL: https://twitter.com/vulkanologen/status/1194228532219727874, https://twitter.com/vulkanologen/status/1193788836679225344); Claudio Jung, (URL: https://www.facebook.com/claudio.jung.1/posts/10220075272173895, https://www.instagram.com/jung.claudio/).

Near-constant fountains of lava at Stromboli have served as a natural beacon in the Tyrrhenian Sea for at least 2,000 years. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N area) and a southern crater group (CS area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island (figure 168). Periodic lava flows emerge from the vents and flow down the scarp, sometimes reaching the sea; occasional large explosions produce ash plumes and pyroclastic flows. Thermal and visual cameras that monitor activity at the vents are located on the nearby Pizzo Sopra La Fossa, above the Terrazza Craterica, and at multiple locations on the flanks of the volcano. Detailed information for Stromboli is provided by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) as well as other satellite sources of data; September-December 2019 is covered in this report.

Figure 168. This shaded relief map of Stromboli’s crater area was created from images acquired by drone on 9 July 2019 (In collaboration with GEOMAR drone group, Helmholtz Center for Ocean Research, Kiel, Germany). Inset shows Stromboli Island, the black rectangle indicates the area of the larger image, the black curved and the red hatched lines indicate, respectively, the morphological escarpment and the crater edges. Courtesy of INGV (Rep. No. 50/2019, Stromboli, Bollettino Settimanale, 02/12/2019 - 08/12/2019, data emissione 10/12/2019).

Activity was very consistent throughout the period of September-December 2019. Explosion rates ranged from 2-36 per hour and were of low to medium-high intensity, producing material that rose from less than 80 to over 150 m above the vents on occasion (table 7). The Strombolian activity in both crater areas often sent ejecta outside the crater rim onto the Terrazza Craterica, and also down the Sciara del Fuoco towards the coast. After the explosions of early July and late August, thermal activity decreased to more moderate levels that persisted throughout the period as seen in the MIROVA Log Radiative Power data (figure 169). Sentinel-2 satellite imagery supported descriptions of the constant glow at the summit, revealing incandescence at both summit areas, each showing repeating bursts of activity throughout the period (figure 170).

Table 7. Monthly summary of activity levels at Stromboli, September-December 2019. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

Figure 169. Thermal activity at Stromboli was high during July-August 2019, when two major explosions occurred. Activity continued at more moderate levels through December 2019 as seen in the MIROVA graph of Log Radiative Power from 8 June through December 2019. Courtesy of MIROVA.

Figure 170. Stromboli reliably produced strong thermal signals from both of the summit vents throughout September-December 2019 and has done so since long before Sentinel-2 satellite imagery was able to detect it. Image dates are (top, l to r) 5 September, 15 October, 20 October, (bottom l to r) 14 November, 14 December 2019, and 3 January 2020. Sentinel-2 imagery uses Atmospheric penetration rendering with bands 12, 11, and 8A, courtesy of Sentinel Hub Playground.

After a major explosion with a pyroclastic flow on 28 August 2019, followed by lava flows that reached the ocean in the following days (BGVN 44:09), activity diminished in early September to levels more typically seen in recent times. This included Strombolian activity from vents in both the N and CS areas that sent ejecta typically 80-150 m high. Ejecta from the N area generally consisted of lapilli and bombs, while the material from the CS area was often finer grained with significant amounts of lapilli and ash. The number of explosive events remained high in September, frequently reaching 25-30 events per hour. The ejecta periodically landed outside the craters on the Terrazza Craterica and even traveled partway down the Sciara del Fuoco. An inspection on 7 September by INGV revealed four eruptive vents in the N crater area and five in the S crater area (figure 171). The most active vents in the N area were N1 with mostly ash emissions and N2 with Strombolian explosions rich in incandescent coarse material that sometimes rose well above 150 m in height. In the S area, S1 and S2 produced jets of lava that often reached 100 m high. A small cone was observed around N2, having grown after the 28 August explosion. Between 11 and 13 September aerial surveys with drones produced detailed visual and thermal imagery of the summit (figure 172).

Figure 171. Video of the Stromboli summit taken with a thermal camera on 7 September 2019 from the Pizzo sopra la Fossa revealed four active vents in the N area and five active vents in the S area. Images prepared by Piergiorgio Scarlato, courtesy of INGV (Rep. No. 37.2/2019, Stromboli, Bollettino Giornaliero del 10/09/2019).

Figure 172. An aerial drone survey on 11 September 2019 at Stromboli produced a detailed view of the N and CS vent areas (left) and thermal images taken by a drone survey on 13 September (right) showed elevated temperatures down the Sciara del Fuoco in addition to the vents in the N and CS areas. Images by E. De Beni and M. Cantarero, courtesy of INGV (Rep. No. 37.5/2019, Stromboli, Bollettino Giornaliero del 13/09/2019).

Strombolian activity from the N crater on 28 September and 1 October 2019 produced blocks and debris that rolled down the Sciara del Fuoco and reached the ocean (figure 173). Explosive activity from the CS crater area sometimes produced ejecta over 150 m high (figure 174). A survey on 26 November revealed that a layer of ash 5-10 cm thick had covered the bombs and blocks that were deposited on the Pizzo Sopra la Fossa during the explosions of 3 July and 28 August (figure 175). On the morning of 27 December a lava flow emerged from the CS area and traveled a few hundred meters down the Sciara del Fuoco. The frequency of explosive events remained relatively constant from September through December 2019 after decreasing from higher levels during July and August (figure 176).

Figure 173. Strombolian activity from vents in the N crater area of Stromboli produced ejecta that traveled all the way to the bottom of the Sciara del Fuoco and entered the ocean. Top images taken 28 September 2019 from the 290 m elevation viewpoint by Rosanna Corsaro. Bottom images captured on 1 October from the webcam at 400 m elevation. Courtesy of INGV (Rep. No. 39.0/2019 and Rep. No. 40.3, Stromboli, Bollettino Giornaliero del 29/09/2019 and 02/10/2019).

Figure 174. Ejecta from Strombolian activity at the CS crater area of Stromboli rose over 150 m on multiple occasions. The webcam located at the 400 m elevation site captured this view of activity on 8 November 2019. Courtesy of INGV (Rep. No. 45.5/2019, Stromboli, Bollettino Giornaliero del 08/11/2019).

Figure 175. The Pizzo Sopra la Fossa area at Stromboli was covered with large blocks and pyroclastic debris on 6 September 2019, a week after the major explosion of 28 August (top). By 26 November, 5-10 cm of finer ash covered the surface; the restored webcam can be seen at the far right edge of the Pizzo (bottom). Courtesy of INGV (Rep. No. 49/2019, Stromboli, Bollettino Settimanale, 25/11/2019 - 01/12/2019, data emissione 03/12/2019).

Figure 176. The average hourly frequency of explosive events at Stromboli captured by surveillance cameras from 1 June 2019 through 5 January 2020 remained generally constant after the high levels seen during July and August. The Total value (blue) is the sum of the average daily hourly frequency of all explosive events produced by active vents.

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5,000 years ago due to a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); 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).

Semeru is a stratovolcano located in East Java, Indonesia containing an active Jonggring-Seloko vent at the Mahameru summit. Common activity has consisted of ash plumes, pyroclastic flows and avalanches, and lava flows that travel down the SE flank. This report updates volcanism from September 2019 to February 2020 using primary information from the 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).

The dominant activity at Semeru for this reporting period consists of ash plumes, which were frequently reported by the Darwin VAAC. An eruption on 10 September 2019 produced an ash plume rising 4 km altitude drifting WNW, as seen in HIMAWARI-8 satellite imagery. Ash plumes continued to rise during 13-14 September. During the month of October the Darwin VAAC reported at least six ash plumes on 13, 14, 17-18, and 29-30 October rising to a maximum altitude of 4.6 km and moving primarily S and SW. Activity in November and December was relatively low, dominated mostly by strong and frequent thermal anomalies.

Volcanism increased in January 2020 starting with an eruption on 17 and 18 January that sent a gray ash plume up to 4.6 km altitude (figure 38). Eruptions continued from 20 to 26 January, producing ash plumes that rose up to 500 m above the crater that drifted in different directions. For the duration of the month and into February, ash plumes occurred intermittently. On 26 February, incandescent ejecta was ejected up to 50 m and traveled as far as 1000 m. Small sulfur dioxide emissions were detected in the Sentinel 5P/TROPOMI instrument during 25-27 February (figure 39). Lava flows during 27-29 February extended 200-1,000 m down the SE flank; gas-and-steam and SO₂ emissions accompanied the flows. There were 15 shallow volcanic earthquakes detected on 29 February in addition to ash emissions rising 4.3 km altitude drifting ESE.

Figure 38. Ash plumes rising from the summit of Semeru on 17 (left) and 18 (right) January 2020. Courtesy of MAGMA Indonesia and via Ø.L. Andersen's Twitter feed (left).

Figure 39. Small SO2 plumes from Semeru were detected by the Sentinel 5P/TROPOMI instrument during 25 (left) and 26 (right) February 2020. Courtesy of NASA Goddard Space Flight Center.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed relatively weak and intermittent thermal anomalies occurring during May to August 2019 (figure 40). The frequency and power of these thermal anomalies significantly increased during September to mid-December 2019 with a few hotspots occurring at distances greater than 5 km from the summit. These farther thermal anomalies to the N and NE of the volcano do not appear to be caused by volcanic activity. There was a brief break in activity during mid-December to mid-January 2020 before renewed activity was detected in early February 2020.

Figure 40. Thermal anomalies were relatively weak at Semeru during 30 April 2019-August 2019, but significantly increased in power and frequency during September to early December 2019. There was a break in activity from mid-December through mid-January 2020 with renewed thermal anomalies around February 2020. Courtesy of MIROVA.

The MODVOLC algorithm detected 25 thermal hotspots during this reporting period, which took place during 25 September, 18 and 21 October 2019, 29 January, and 11, 14, 16, and 23 February 2020. Sentinel-2 thermal satellite imagery shows intermittent hotspots dominantly in the summit crater throughout this reporting period (figure 41).

Figure 41. Sentinel-2 thermal satellite imagery detected intermittent thermal anomalies (bright yellow-orange) at the summit of Semeru, which included some lava flows in late January to early February 2020. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com).

Frequent historical eruptions have been reported from Mexico's Popocatépetl going back to the 14th century. Activity increased in the mid-1990s after about 50 years of quiescence, and the current eruption, ongoing since January 2005, has included numerous episodes of lava-dome growth and destruction within the 500-m-wide summit caldera. Multiple emissions of steam and gas occur daily, rising generally 1-3 km above the summit at about 5,400 m elevation; many contain small amounts of ash. Larger, more explosive events with ash plumes and incandescent ejecta landing on the flanks occur frequently. Activity through August 2019 was typical of the ongoing eruption with near-constant emissions of water vapor, gas, and minor ash, as well as multiple explosions with ash plumes and incandescent blocks scattered on the flanks (BGVN 44:09). This report covers similar activity from September 2019 through February 2020. Information comes from daily reports provided by México's Centro Nacional de Prevención de Desastres (CENAPRED); ash plumes are reported by the Washington Volcanic Ash Advisory Center (VAAC). Satellite visible and thermal imagery and SO₂ data also provide helpful observations of activity.

Activity summary. Activity at Popocatépetl during September 2019-February 2020 continued at the high levels that have been ongoing for many years, characterized by hundreds of daily low-intensity emissions that included steam, gas, and small amounts of ash, and periods with multiple daily minor and moderate explosions that produce kilometer-plus-high ash plumes (figure 140). The Washington VAAC issued multiple daily volcanic ash advisories with plume altitudes around 6 km for many, although some were reported as high as 8.2 km. Hundreds of minutes of daily tremor activity often produced ash emissions as well. Incandescent ejecta landed 500-1,000 m from the summit frequently. The MIROVA thermal anomaly data showed near-constant moderate to high levels of thermal energy throughout the period (figure 141).

Figure 140. Emissions continued at a high rate from Popocatépetl throughout September 2019-February 2020. Daily low-intensity emissions numbered usually in the hundreds (blue, left axis), while less frequent minor (orange) and moderate (green) explosions, plotted on the right axis, occurred intermittently through November 2019, and increased again during February 2020. Data was compiled from CENAPRED daily reports.

Figure 141. MIROVA log radiative power thermal data for Popocatépetl from 1 May 2019 through February 2020 showed a constant output of moderate energy the entire time. Courtesy of MIROVA.

Sulfur dioxide emissions were measured with satellite instruments many days of each month from September 2019 thru February 2020. The intensity and drift directions varied significantly; some plumes remained detectable hundreds of kilometers from the volcano (figure 142). Plumes were detected almost daily in September, and on most days in October. They were measured at lower levels but often during November, and after pulses in early and late December only small plumes were visible during January 2020. Intermittent larger pulses returned in February. Dome growth and destruction in the summit crater continued throughout the period. A small dome was observed inside the summit crater in late September. Dome 85, 210-m-wide, was observed inside the summit crater in early November. Satellite imagery captured evidence of dome growth and ash emissions throughout the period (figure 143).

Figure 142. Sulfur dioxide emissions from Popocatépetl were frequent from September 2019 through February 2020. Plumes drifted SW on 7 September (top left), 30 October (top middle), and 21 February (bottom right). SO2 drifted N and NW on 26 November (top right). On 2 December (bottom left) a long plume of sulfur dioxide hundreds of kilometers long drifted SW over the Pacific Ocean while the drift direction changed to NW closer to the volcano. The SO2 plumes measured in January (bottom center) were generally smaller than during the other months covered in this report. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 143. Sentinel-2 satellite imagery of Popocatépetl during November 2019-February 2020 provided evidence for ongoing dome growth and explosions with ash emissions. Top left: a ring of incandescence inside the summit crater on 8 November 2019 was indicative of the growth of dome 85 observed by CENAPRED. Top middle: incandescence on 8 December inside the summit crater was typical of that observed many times during the period. Top right: a dense, narrow ash plume drifted N from the summit on 17 January 2020. Bottom left: Snow cover made ashfall on 6 February easily visible on the E flank. On 11 February, the summit crater was incandescent and nearly all the snow was covered with ash. Bottom right: a strong thermal anomaly and ash emission were captured on 21 February. Bottom left and top right images use Natural color rendering (bands 4, 3, 2); other images use Atmospheric penetration rendering to show infrared signal (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Activity during September-November 2019. On 1 September 2019 minor ashfall was reported in the communities of Atlautla, Ozumba, Juchitepec, and Tenango del Aire in the State of Mexico. The ash plumes rose less than 2 km above the summit and incandescent ejecta traveled less than 100 m from the summit crater. Twenty-two minor and three moderate explosions were recorded on 4-5 September along with minor ashfall in Juchitepec, Tenango del Aire, Tepetlixpa, and Atlautla. During a flyover on 5 September, officials did not observe a dome within the crater, and the dimensions remained the same as during the previous visit (350 m in diameter and 150 m deep) (figure 144). Ashfall was reported in Tlalmanalco and Amecameca on 6 September. The following day incandescent ejecta was visible on the flanks near the summit and ashfall was reported in Amecameca, Ayapango, and Tenango del Aire. The five moderate explosions on 8 September produced ash plumes that rose as high as 2 km above the summit, and incandescent ejecta on the flanks. Explosions on 10 September sent ejecta 500 m from the crater. Eight explosions during 20-21 September produced ejecta that traveled up to 1.5 km down the flanks (figure 145). During an overflight on 27 September specialists from the National Center for Disaster Prevention (CENAPRED ) of the National Coordination of Civil Protection and researchers from the Institute of Geophysics of UNAM observed a new dome 30 m in diameter; the overall crater had not changed size since the overflight in early September.

Figure 144. CENAPRED carried out overflights of Popocatépetl on 5 (left) and 27 September (right) 2019; the crater did not change in size, but a new dome 30 m in diameter was visible on 27 September. Courtesy of CENAPRED (Sobrevuelo al volcán Popocatépetl, 05 y 27 de septiembre).

Figure 145. Ash plumes at Popocatépetl on 19 (left) and 20 (right) September 2019 rose over a kilometer above the summit before dissipating. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 19 y 20 de septiembre).

Fourteen explosions were reported on 2 October 2019. The last one produced an ash plume that rose 2 km above the summit and sent incandescent ejecta down the E slope (figure 146). Ashfall was reported in the municipalities of Atlautla Ozumba, Ayapango and Ecatzingo in the State of Mexico. Explosions on 3 and 4 October also produced ash plumes that rose between 1 and 2 km above the summit and sent ejecta onto the flanks. Additional incandescent ejecta was reported on 6, 7, 15, and 19 October. The communities of Amecameca, Tenango del Aire, Tlalmanalco, Cocotitlán, Temamatla, and Tláhuac reported ashfall on 10 October; Amecameca reported more ashfall on 12 October. On 22 October slight ashfall appeared in Amecameca, Tenango del Aire, Tlalmanalco, Ayapango, Temamatla, and Atlautla.

Figure 146. Incandescent ejecta at Popocatépetl traveled down the E slope on 2 October 2019 (left); an ash plume two days later rose 2 km above the summit (right). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 2 y 4 de octubre).

During 2-3 November 2019 there was 780 minutes of tremor reported in four different episodes. The seismicity was accompanied by ash emissions that drifted W and NW and produced ashfall in numerous communities, including Amecameca, Juchitepec, Ozumba, Tepetlixpa, and Atlautla in the State of México, in Ayapango and Cuautla in the State of Morelos, and in the municipalities of Tlahuac, Tlalpan, and Xochimilco in Mexico City. A moderate explosion on 4 November sent incandescent ejecta 2 km down the slopes and produced an ash plume that rose 1.5 km and drifted NW. Minor ashfall was reported in Tlalmanalco, Amecameca, and Tenango del Aire, State of Mexico. Similar ash plumes from explosions occurred the following day. Scientists from CENAPRED and the Institute of Geophysics of UNAM observed dome number 85 during an overflight on 5 November 2019. It had a diameter of 210 m and was 80 m thick, with an irregular surface (figure 147). Multiple explosions on 6 and 7 November produced incandescent ejecta; a moderate explosion late on 11 November produced ejecta that traveled 1.5 km from the summit and produced an ash plume 2 km high (figure 148). A lengthy period of constant ash emission that drifted E was reported on 18 November. A moderate explosion on 28 November sent incandescent fragments 1.5 km down the slopes and ash one km above the summit.

Figure 147. A new dome was visible inside the summit crater at Popocatépetl during an overflight on 5 November 2019. It had a diameter of 210 m and was 80 m thick. Courtesy of CENAPRED (Sobrevuelo al volcán Popocatépetl, 05 de noviembre).

Figure 148. Ash emissions and explosions with incandescent ejecta continued at Popocatépetl during November 2019. The ash plume on 1 November changed drift direction sharply a few hundred meters above the summit (left). Incandescent ejecta traveled 1.5 km down the flanks on 11 November (right). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 1 y 12 de noviembre).

Activity during December 2019-February 2020. Throughout December 2019 weak emissions of steam and gas were reported daily, sometimes with minor amounts of ash, and minor explosions were only reported on 21 and 27 December. On 21 December two new high-resolution webcams were installed around Popocatépetl, one 5 km from the crater at the Tlamacas station, and the second in San Juan Tianguismanalco, 20 km away. Ash emissions and incandescent ejecta 800 m from the summit were observed on 25 December (figure 149). Incandescence at night was reported during 27-29 December.

Figure 149. Incandescent ejecta moved 800 m down the flanks of Popocatépetl during explosions on 25 December 2019 (left); weak emissions of steam, gas, and minor ash were visible on 27 December and throughout the month. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 25 y 27 de diciembre).

Continuous emissions of water vapor and gas with low ash content were typical daily during January 2020. A moderate explosion on 9 January produced an ash plume that rose 3 km from the summit and drifted NE. In addition, incandescent ejecta traveled 1 km from the crater rim. A minor explosion on 21 January produced a 1.5-km-high plume with low ash content and incandescent ejecta that fell near the crater (figure 150). The first of two explosions late on 27 January produced ejecta that traveled 500 m and a 1-km-high ash plume. Constant incandescence was observed overnight on 29-30 January.

Figure 150. Although fewer explosions were recorded at Popocatépetl during January 2020, activity continued. An ash plume on 19 January rose over a kilometer above the summit (top left). A minor explosion on 21 January produced a 1.5-km-high plume with low ash content and incandescent ejecta that fell near the crater (top right). Smaller emissions with steam, gas, and ash were typical many days, including on 22 (bottom left) and 31 (bottom right) January 2019. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 19, 21, 22 y 31 de enero).

A moderate explosion on 5 February 2020 produced an ash plume that rose 1.5 km and drifted NNE. Explosions on 10 and 13 February sent ejecta 500 m down the flanks (figure 151). During an overflight on 18 February scientists noted that the internal crater maintained a diameter of 350 m and its approximate depth was 100-150 m; the crater was covered by tephra. For most of the second half of February the volcano had a continuous emission of gases with minor amounts of ash. In addition, multiple explosions produced ash plumes that rose 400-1,200 m above the crater and drifted in several different directions.

Figure 151. Ash emissions and explosions continued at Popocatépetl during February 2020. Dense ash drifted near the snow-covered summit on 6 February (top left). Incandescent ejecta traveled 500 m down the flanks on 13 February (top right). Ash plumes billowed from the summit on 18 and 22 February (bottom row). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl, 6, 15, 18 y 22 de febrero).

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

Information Contacts: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: http://www.cenapred.unam.mx/), Daily Report Archive http://www.cenapred.unam.mx:8080/reportesVolcanGobMX/BuscarReportesVolcan); 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/); 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).

The dacitic Santiaguito lava-dome complex on the W flank of Guatemala's Santa María volcano has been growing and actively erupting since 1922. Ash explosions, pyroclastic, and lava flows have emerged from Caliente, the youngest of the four vents in the complex, for more than 40 years. A lava dome that appeared within the summit crater of Caliente in October 2016 has continued to grow, producing frequent block avalanches down the flanks. Daily explosions with ash plumes and block avalanches continued during September 2019-February 2020, the period covered in this report, with information primarily from Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia) and the Washington VAAC (Volcanic Ash Advisory Center).

Constant fumarolic activity with steam and gas persisted from the Caliente dome throughout September 2019-February 2020. Explosions occurred multiple times per day, producing ash plumes that rose to altitudes of 3.1-3.5 km and usually drifted a few kilometers before dissipating. Several lahars during September and October carried volcanic blocks, ash, and debris down major drainages. Periodic ashfall was reported in communities within 10 km of the volcano. An increase in thermal activity beginning in November (figure 101) resulted in an increased number of observations of incandescence visible at night from the summit of Caliente through February 2020. Block avalanches occurred daily on the flanks of the dome, often reaching the base, stirring up small clouds of ash that drifted downwind.

Figure 101. The MIROVA project graph of thermal activity at Santa María from 12 May 2019 through February 2020 shows a gradual increase in thermal energy beginning in November 2019. This corresponds to an increase in the number of daily observations of incandescence at the summit of the Caliente dome during this period. Courtesy of MIROVA.

Constant steam and gas fumarolic activity rose from the Caliente dome, drifting W, usually rising to 2.8-3.0 km altitude during September 2019. Multiple daily explosions with ash plumes rising to 2.9-3.4 km altitude drifted W or SW over the communities of San Marcos, Loma Linda Palajunoj, and Monte Claro (figure 102). Constant block avalanches fell to the base of the cone on the NE and SE flanks. The Washington VAAC reported an ash plume visible in satellite imagery on 10 September at 3.1 km altitude drifting W. On 14 September another plume was spotted moving WSW at 4.6 km altitude which dissipated quickly; the webcam captured another plume on 16 September. Ashfall on 27 September reached about 1 km from the volcano; it reached 1.5 km on 29 September. Lahars descended the Rio Cabello de Ángel on 2 and 24 September (figure 102). They were about 15 m wide, and 1-3 m deep, carrying blocks 1-2 m in diameter.

Figure 102. A lahar descended the Rio Cabello de Ángel at Santa Maria and flowed into the Rio Nima 1 on 24 September 2019. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 21 al 27 de septiembre de 2019).

Througout October 2019, degassing of steam with minor gases occurred from the Caliente summit, rising to 2.9-3.0 km altitude and generally drifting SW. Weak explosions took place 1-5 times per hour, producing ash plumes that rose to 3.2-3.5 km altitude. Ashfall was reported in Monte Claro on 2 October. Nearly constant block avalanches descended the SE and S flanks, disturbing recent layers of fine ash and producing local ash clouds. Moderate explosions on 11 October produced ash plumes that rose to 3.5 km altitude and drifted W and SW about 1.5 km towards Río San Isidro (figure 103). The following day additional plumes drifted a similar distance to the SE. The Washington VAAC reported an ash emission visible in satellite imagery at 4.9 km altitude on 13 October drifting NNW. Ashfall was reported in Parcelamiento Monte Claro on 14 October. Some of the block avalanches observed on 14 October on the SE, S, and SW flanks were incandescent. Ash drifted 1.5 km W and SW on 17 October. Ashfall was reported near la finca Monte Claro on 25 and 28 October. A lahar descended the Río San Isidro, a tributary of the Río El Tambor on 7 October carrying blocks 1-2 m in diameter, tree trunks, and branches. It was about 16 m wide and 1-2 m deep. Additional lahars descended the rio Cabello de Angel on 23 and 24 October. They were about 15 m wide and 2 m deep, and carried ash and blocks 1-2 m in diameter, tree trunks, and branches.

Figure 103. Daily ash plumes were reported from the Caliente cone at Santa María during October 2019, similar to these from 30 September (left) and 11 October 2019 (right). Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 28 de septiembre al 04 de octubre de 2019; Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 05 al 11 de octubre de 2019).

During November 2019, steam plumes rose to 2.9-3.0 km altitude and generally drifted E. There were 1-3 explosions per hour; the ash plumes produced rose to altitudes of 3.1-3.5 km and often drifted SW, resulting in ashfall around the volcanic complex. Block avalanches descended the S and SW flanks every day. On 4 November ashfall was reported in the fincas (ranches) of El Faro, Santa Marta, El Viejo Palmar, and Las Marías, and the odor of sulfur was reported 10 km S. Incandescence was observed at the Caliente dome during the night of 5-6 November. Ash fell again in El Viejo Palmar, fincas La Florida, El Faro, and Santa Marta (5-6 km SW) on 7 November. Sulfur odor was also reported 8-10 km S on 16, 19, and 22 November. Fine-grained ash fell on 18 November in Loma Linda and San Marcos Palajunoj. On 29 November strong block avalanches descended in the SW flank, stirring up reddish ash that had fallen on the flanks (figure 104). The ash drifted up to 20 km SW.

Figure 104. Ash plumes rose from explosions multiple times per day at Santa Maria’s Santiaguito complex during November 2019, and block avalanches stirred up reddish clouds of ash that drifted for many kilometers. Courtesy of INSIVUMEH. Left, 11 November 2019, from Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 09 al 15 de noviembre de 2019. Right, 29 November 2019 from BOLETÍN VULCANOLÓGICO ESPECIAL BESTG# 106-2019, Guatemala 29 de noviembre de 2019, 10:50 horas (Hora Local).

White steam plumes rising to 2.9-3.0 km altitude drifted SE most days during December 2019. One to three explosions per hour produced ash plumes that rose to 3.1-3.5 km altitude and drifted W and SW producing ashfall on the flanks. Several strong block avalanches sent material down the SW flank. Ash from the explosions drifted about 1.5 km SW on 3 and 7 December. The Washington VAAC reported a small ash emission that rose to 4.9 km altitude and drifted WSW on 8 December, and another on 13 December that rose to 4.3 km altitude. Ashfall was reported up to 10 km S on 24 December. Incandescence was reported at the dome by INSIVUMEH eight times during the month, significantly more than during the recent previous months (figure 105).

Figure 105. Strong thermal anomalies were visible in Sentinel-2 imagery at the summit of the Caliente cone at Santa María’s Santiaguito’s complex on 19 December 2019. Image uses Atmospheric Penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Activity during January 2020 was similar to that during previous months. White plumes of steam rose from the Caliente dome to altitudes of 2.7-3.0 km and drifted SE; one to three explosions per hour produced ash plumes that rose to 3.2-3.4 km altitude and generally drifted about 1.5 km SW before dissipating. Frequent block avalanches on the SE flank caused smaller plumes that drifted SSW often over the ranches of San Marcos and Loma Linda Palajunoj. On 28 January ash plumes drifted W and SW over the communities of Calaguache, El Nuevo Palmar, and Las Marías. In addition to incandescence observed at the crater of Caliente dome at least nine times, thermal anomalies in satellite imagery were detected multiple times from the block avalanches on the S flank (figure 106).

Figure 106. Incandescence at the summit and in the block avalanches on the S flank of the Caliente cone at Santa María’s Santiaguito’s complex was visible in Sentinel-2 satellite imagery on 8 and 13 January 2020. Atmospheric penetration rendering images (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

The Washington VAAC reported an ash plume visible in satellite imagery at 4.6 km altitude drifting W on 3 February 2020. INSIVUMEH reported constant steam degassing that rose to 2.9-3.0 km altitude and drifted SW. In addition, 1-3 weak to moderate explosions per hour produced ash plumes to 3.1-3.5 km altitude that drifted about 1 km SW. Small amounts of ashfall around the volcano’s perimeter was common. The ash plumes on 5 February drifted NE over Santa María de Jesús. On 8 February the ash plumes drifted E and SE over the communities of Calaguache, El Nuevo Palmar, and Las Marías. Block avalanches on the S and SE flanks of Caliente dome continued, creating small ash clouds on the flank. Incandescence continued frequently at the crater and was also observed on the S flank in satellite imagery (figure 107).

Figure 107. Incandescence at the summit and on the S flank of the Caliente cone at Santa María’s Santiaguito’s complex was frequent during February 2020, including on 2 (left) and 17 (right) February 2020 as seen in Sentinel-2 imagery. Atmostpheric Penetration rendering imagery (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

Geologic Background. Symmetrical, forest-covered Santa María volcano is part of a chain of large stratovolcanoes that rise above the Pacific coastal plain of Guatemala. The sharp-topped, conical profile is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank, and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four vents, with activity progressing W towards the most recent, Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); 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); 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/).

Historical eruptions at Chile's Villarrica, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. An intermittently active lava lake at the summit has been the source of Strombolian activity, incandescent ejecta, and thermal anomalies for several decades; the current eruption has been ongoing since December 2014. Continuing activity during September 2019-February 2020 is covered in this report, with information provided by the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN), and Projecto Observación Villarrica Internet (POVI), part of the Fundacion Volcanes de Chile, a research group that studies volcanoes across Chile.

A brief period of heighted explosive activity in early September 2019 caused SERNAGEOMIN to raise the Alert Level from Yellow to Orange (on a four-color scale of Green-Yellow-Orange-Red) for several days. Increases in radiative power were visible in the MIROVA thermal anomaly data during September (figure 84). Although overall activity decreased after that, intermittent explosions were observed at the summit, and incandescence continued throughout September 2019-February 2020. Sentinel-2 satellite imagery indicated a strong thermal anomaly from the summit crater whenever the weather conditions permitted. In addition, ejecta periodically covered the area around the summit crater, and particulates often covered the snow beneath the narrow gas plume drifting S from the summit (figure 85).

Figure 84. Thermal activity at Villarrica from 28 May 2019 through February 2020 was generally at a low level, except for brief periods in August and September 2019 when larger explosions were witnessed and recorded in seismic data and higher levels of thermal activity were noted by the MIROVA project. Courtesy of MIROVA.

Figure 85. Natural-color (top) and Atmospheric penetration (bottom) renderings of three different dates during September 2019-February 2020 show typical continued activity at Villarica during the period. Dark ejecta periodically covered the snow around the summit crater, and streaks of particulate material were sometimes visible on the snow underneath the plumes of bluish gas drifting S from the volcano (top images). Persistent thermal anomalies were recorded in infrared satellite data on the same dates (bottom images). Dates recorded are (left to right) 28 September 2019, 20 December 2019, and 1 January 2020. Natural color rendering uses bands 4,3, and 2, and Atmospheric penetration rendering uses bands 12, 11, and 8a. Courtesy of Sentinel Hub Playground.

SERNAGEOMIN raised the Alert Level from Green to Yellow in early August 2019 due to the increase in activity that included incandescent ejecta and bombs reaching 200 m from the summit crater (BGVN 44:09). An increase in seismic tremor activity on 8 September was accompanied by vigorous Strombolian explosions reported by POVI. The following day, SERNAGEOMIN raised the Alert Level from Yellow to Orange. Poor weather prevented visual observations of the summit on 8 and 9 September, but high levels of incandescence were observed briefly on 10 September. Incandescent ejecta reached 200 m from the crater rim late on 10 September (figure 86). Activity increased the next day with ejecta recorded 400 m from the crater, and the explosions were felt 12 km from the summit.

Figure 86. A new pulse of activity at Villarrica reached its maximum on 10 (left) and 11 (right) September 2019. Incandescent ejecta reached 200 m from the crater rim on 10 September and up to 400 m the following day. Courtesy of POVI (Volcan Villarrica, Resumen grafico del comportamiento, Septiembre 2019 a enero 2020).

Explosions decreased in intensity by 13 September, but avalanches of incandescent material were visible on the E flank in the early morning hours (figure 87). Small black plumes later in the day were interpreted by POVI as the result of activity from landslides within the crater. Fine ash deposited on the N and NW flanks during 16-17 September was attributed to wind moving ash from within the crater, and not to new emissions from the crater (figure 88). SERNAGEOMIN lowered the Alert Level to Yellow on 16 September as tremor activity decreased significantly. Activity continued to decrease during the second half of September; incandescence was moderate with no avalanches observed, and intermittent emissions with small amounts of material were noted. Degassing of steam plumes rose up to 120 m above the crater.

Figure 87. By 13 September 2019, a decrease in activity at Villarrica was apparent. Incandescence (red arrow) was visible on the E flank of Villarrica early on 13 September (left). Fine ash, likely from small collapses of new material inside the vent, rose a short distance above the summit later in the day (right). Courtesy of POVI (Volcan Villarrica, Resumen grafico del comportamiento, Septiembre 2019 a Enero 2020).

Figure 88. Fine-grained material covered the summit of Villarrica on 17 September 2019. POVI interpreted this as a result of strong winds moving fine ash-sized particles from within the crater and depositing them on the N and NW flanks. Courtesy of POVI (Volcan Villarrica, Resumen grafico del comportamiento, Septiembre 2019 a enero 2020).

Low-altitude degassing was typical activity during October-December 2019; occasionally steam and gas plumes rose 300 m above the summit, but they were generally less than 200 m high. Incandescence was visible at night when weather conditions permitted. Occasional Strombolian explosions were observed in the webcam (figure 89). During January and February 2020, similar activity was reported with steam plumes observed to heights of 300-400 m above the summit, and incandescence on nights where the summit was visible (figure 90). A drone overflight on 19 January produced a clear view into the summit crater revealing a 5-m-wide lava pit about 120 m down inside the crater (figure 91).

Figure 89. Activity continued at a lower level at the summit of Villarrica from October-December 2019. The 30-m-wide vent at the bottom of the summit crater (120 m deep) of Villarrica (left) was emitting wisps of bluish gas on 30 October 2019. Sporadic Strombolian explosions ejected material around the crater rim on 12 December (right). Courtesy of POVI (Volcan Villarrica, Resumen grafico del comportamiento, Septiembre 2019 a enero 2020).

Figure 90. Small explosive events were recorded at Villarrica during January and February 2020, including these events on 4 (left) and 18 (right) January where ejecta reached about 50 m above the crater rim. Courtesy of POVI (Volcan Villarrica, Resumen grafico del comportamiento, Septiembre 2019 a Enero 2020).

Figure 91. An oblique view into the bottom of the summit crater of Villarrica on 19 January 2020 was captured by drone. The diameter of the lava pit was calculated at about 5 m and was about 120 m deep. Image copyright by Leighton M. Watson, used with permission; courtesy of POVI (Volcan Villarrica, Resumen grafico del comportamiento, Septiembre 2019 a Enero 2020).

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

Information Contacts: 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/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Leighton M. Watson, Department of Earth Sciences at the University of Oregon, Eugene, OR 97403-1272, USA (URL: https://earthsciences.uoregon.edu/).

Semisopochnoi is a remote stratovolcano located in the western Aleutians dominated by an 8 km-wide caldera containing the small (100 m diameter) Fenner Lake and a three-cone cluster: a northern cone known as the North cone of Mount Cerberus, an eastern cone known as the East cone of Mount Cerberus, and a southern cone known as the South cone of Mount Cerberus. Previous volcanism has included small explosions, ash deposits, and gas-and-steam emissions. This report updates activity during September 2019 through March 2020 using information from the Alaska Volcano Observatory (AVO). A new eruptive period began on 7 December 2019 and continued until mid-March 2020 with activity primarily focused in the North cone of Mount Cerberus.

During September-November 2019, low levels of unrest were characterized by intermittent weeks of elevated seismicity and gas-and-steam plumes visible on 8 September, 7-8 October, and 24 November. On 6 October an SO₂ plume was visible in satellite imagery, according to AVO.

Seismicity increased on 5 December and was described as a strong tremor through 7 December. This tremor was associated with a small eruption on 7 December; intermittent explosions occurred and continued into the night. Increased seismicity was recorded throughout the rest of the month while AVO registered small explosions during 11-19 December. On 11-12 December, a gas-and-steam plume possibly containing some of ash extended 80 km (figure 2). Two more ash plumes were observed on 14 and 17 December, the latter of which extended 15 km SE. Sentinel-2 satellite images show gas-and-steam plumes rising from the North Cerberus crater intermittently at the end of 2019 and into early 2020 (figure 3).

Figure 2. Sentinel-2 satellite image showing a gray ash plume extending up to 17 km SE from the North Cerberus crater on 11 December 2019. Image taken by Hannah Dietterich; courtesy of AVO.

Figure 3. Sentinel-2 satellite images of gas-and-steam plumes at Semisopochnoi from late November 2019 through mid-March 2020. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

The month of January 2020 was characterized by low levels of unrest due to intermittent low seismicity. Small explosions were reported during 14-17 February and a gas-and-steam plume was visible on 26 February. Seismic unrest occurred between 18 February-7 March. Gas-and-steam plumes were visible on 1, 9, 14-17, 20, and 21 March (figure 4). During 15-17 March, small explosions occurred, according to AVO. Additionally, clear satellite images showed gas-and-steam emissions and minor ash deposits around North Cerberus’ crater rim. After 17 March the explosions subsided and ash emissions were no longer observed. However, intermittent gas-and-steam emissions continued and seismicity remained elevated through the end of the month.

Figure 4. Satellite image of Semisopochnoi showing degassing within the North Cerberus crater on 22 March 2020. Image taken by Matt Loewen; courtesy of AVO.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is 1221-m-high Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked 774-m-high Mount Cerberus volcano was constructed during the Holocene within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Cerberus appear younger than those on the southern side. Other post-caldera volcanoes include the symmetrical 855-m-high Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented historical eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time.

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

Ubinas, located 70 km from the city of Arequipa in Peru, has produced frequent eruptions since 1550 characterized by ash plumes, ballistic ejecta (blocks and bombs), some pyroclastic flows, and lahars. Activity is focused at the summit crater (figure 53). A new eruptive episode began on 24 June 2019, with an ash plume reaching 12 km altitude on 19 July. This report summarizes activity during September 2019 through February 2020 and is based on agency reports and satellite data.

Figure 53. A PlanetScope satellite image of Ubinas on 16 December 2019. Courtesy of PlanetLabs.

Prior to September 2019 the last explosion occurred on 22 July. At 2145 on 1 September moderate, continuous ash emission occurred reaching nearly 1 km above the crater. An explosion produced an ash plume at 1358 on the 3rd that reached up to 1.3 km above the summit; six minutes later ashfall and lapilli up to 1.5 cm in diameter was reported 6 km away, with ashfall reported up to 8 km away (figure 54 and 55). Three explosions produced ash plumes at 0456, 0551, and 0844 on 4 September, with the two later ash plumes reaching around 2 km above the crater. The ash plume dispersed to the south and ashfall was reported in Ubinas, Tonohaya, San Miguel, Anascapa, Huatahua, Huarina, and Matalaque, reaching a thickness of 1 mm in Ubinas.

Figure 54. An eruption at Ubinas produced an ash plume up to 1.3 km on at 1358 on 3 September 2019. Courtesy of INGEMMET.

Figure 55. Ash and lapilli fall up to 1.5 cm in diameter was reported 6 km away from Ubinas on 3 September 2019 (top) and an Ingemmet geologist collects ash samples from the last three explosions. Courtesy of INGEMMET.

During 8-9 September there were three explosions generating ash plumes to less than 2.5 km, with the largest occurring at 1358 and producing ashfall in the Moquegua region to the south. Following these events, gas and water vapor were continuously emitted up to 1 km above the crater. There was an increase in seismicity during the 10-11th and an explosion produced a 1.5 km high (above the crater) ash plume at 0726 on the 12th, which dispersed to the S and SE (figure 56). During 10-15 September there was continuous emission of gas (blue in color) and steam up to 1.5 km above the volcano. Gas emission, thermal anomalies, and seismicity continued during 16-29 September, but no further explosions were recorded.

Figure 56. An explosion at Ubinas on 12 September 2019 produced an ash plume to 1.5 km above the volcano. The ash dispersed to the S and SE. Courtesy of IGP.

Throughout October activity consisted of seismicity, elevated temperatures within the crater, and gas emissions reaching 800 to 1,500 m above the crater. No explosions were recorded. Drone footage released in early October (figure 57) shows the gas emissions and provided a view of the crater floor (figure 58). On the 15th IGP reported that the likelihood of an eruption had reduced.

Figure 57. IGP flew a fixed-wing drone over Ubinas as part of their monitoring efforts. This photograph shows gas emissions rising from the summit crater, published on 7 October 2019. Courtesy of IGP.

Figure 58. Drone image showing gas emissions and the summit crater of Ubinas. Image taken by IGP staff and released on 7 October 2019; courtesy of IGP.

Similar activity continued through early November with no reported explosions, and the thermal anomalies were no longer detected at the end of November (figure 59), although a faint thermal anomaly was visible in Sentinel-2 data in mid-December (figure 60). A rockfall occurred at 1138 on 13 November down the Volcanmayo gorge.

Figure 59. This MIROA Log Radiative Power plot shows increased thermal energy detected at Ubinas during August through November 2019. Courtesy of MIROVA.

Figure 60. Sentinel-2 thermal satellite image showing elevated temperatures in the Ubinas crater on 16 December 2019. Courtesy of Sentinel Hub Playground.

There were no explosions during January or February 2020, with seismicity and reduced gas emissions continuing. There was a small- to moderate-volume lahar generated at 1620 on 4 January down the SE flank. A second moderate- to high-volume lahar was generated at 1532 on 24 February, and three more lahars at 1325 and 1500 on 29 February, and at 1601 on 1 March, moved down the Volcanmayo gorge and the Sacohaya river channel. The last three lahars were of moderate to large volume.

Geologic Background. A small, 1.4-km-wide caldera cuts the top of Ubinas, Perú's most active volcano, giving it a truncated appearance. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45 degrees. The steep-walled, 150-m-deep summit caldera contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one of Holocene age about 1,000 years ago. Holocene lava flows are visible on the flanks, but historical activity, documented since the 16th century, has consisted of intermittent minor-to-moderate explosive eruptions.

Information Contacts: Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa, Peru (URL: http://ovi.ingemmet.gob.pe); Instituto Geofisico del Peru (IGP), Calle Badajoz N° 169 Urb. Mayorazgo IV Etapa, Ate, Lima 15012, Perú (URL: https://www.gob.pe/igp); 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/); Planet Labs, Inc. (URL: https://www.planet.com/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Yasur has remained on Alert Level 2 (on a scale of 0-4) since 18 October 2016, indicating "Major Unrest; Danger Zone remains at 395 m around the eruptive vents." The summit crater contains several active vents that frequently produce Strombolian explosions and gas plumes (figure 60). This bulletin summarizes activity during June 2019 through February 2020 and is based on reports by the Vanuatu Meteorology and Geo-Hazards Department (VMGD), visitor photographs and videos, and satellite data.

Figure 60. The crater of Yasur contains several active vents that produce gas emissions and Strombolian activity. Photo taken during 25-27 October 2019 by Justin Noonan, used with permission.

A VMGD report on 27 June described ongoing Strombolian explosions with major unrest confined to the crater. The 25 July report noted the continuation of Strombolian activity with some strong explosions, and a warning that volcanic bombs may impact outside of the crater area (figure 61).

Figure 61. A volcanic bomb (a fluid chunk of lava greater than 64 mm in diameter) that was ejected from Yasur. The pattern on the surface shows the fluid nature of the lava before it cooled into a solid rock. Photo taken during 25-27 October 2019 by Justin Noonan, used with permission.

No VMGD report was available for August, but Strombolian activity continued with gas emissions and explosions, as documented by visitors (figure 62). The eruption continued through September and October with some strong explosions and multiple active vents visible in thermal satellite imagery (figure 63). Strombolian explosions ejecting fluid lava from rapidly expanding gas bubbles were recorded during October, and likely represented the typical activity during the surrounding months (figure 64). Along with vigorous degassing producing a persistent plume there was occasional ash content (figure 65). At some point during 20-29 October a small landslide occurred along the eastern inner wall of the crater, visible in satellite images and later confirmed to have produced ashfall at the summit (figure 66).

Figure 62. Different views of the Yasur vents on 7-8 August 2019 taken from a video. Strombolian activity and degassing were visible. Courtesy of Arnold Binas, used with permission.

Figure 63. Sentinel-2 thermal satellite images show variations in detected thermal energy emitting from the active Yasur vents on 18 September and 22 December 2019. False color (bands 12, 11, 4) satellite images courtesy of Sentinel Hub Playground.

Figure 64. Strombolian explosions at Yasur during 25-27 October 2019. Large gas bubbles rise to the top of the lava column and burst, ejecting volcanic bombs – fluid chunks of lava, out of the vent. Photos by Justin Noonan, used with permission.

Figure 65. Gas and ash emissions rise from the active vents at Yasur between 25-27 October 2019. Photos by Justin Noonan, used with permission.

Figure 66. Planet Scope satellite images of Yasur show a change in the crater morphology between 20 and 29 October 2019. Copyright of Planet Labs.

Continuous explosive activity continued in November-February with some stronger explosions recorded along with accompanying gas emissions. Gas plumes of sulfur dioxide were detected by satellite sensors on some days through this period (figure 67) and ash content was present at times (figure 68). Thermal anomalies continued to be detected by satellite sensors with varying intensity, and with a reduction in intensity in February, as seen in Sentinel-2 imagery and the MIROVA system (figures 69 and 70).

Figure 67. SO2 plumes detected at Yasur by Aura/OMI on 21 December 2019 and 31 January 2020, drifting W to NW, and on 14 and 23 February 2020, drifting W and south, and NWW to NW. Courtesy of Global Sulfur Dioxide Monitoring Page, NASA.

Figure 68. An ash plume erupts from Yasur on 20 February 2020 and drifts NW. Courtesy of Planet Labs.

Figure 69. Sentinel-2 thermal satellite images show variations in detected thermal energy in the active Yasur vents during January and February 2020. False color (bands 12, 11, 4) satellite images courtesy of Sentinel Hub Playground.

Figure 70. The MIROVA thermal detection system recorded persistent thermal energy emitted at Yasur with some variation from mid-May 2019 to May 2020. There was a reduction in detected energy after January. Courtesy of MIROVA.

Geologic Background. Yasur, the best-known and most frequently visited of the Vanuatu volcanoes, has been in more-or-less continuous Strombolian and Vulcanian activity since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island, this mostly unvegetated pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide, horseshoe-shaped caldera associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); 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); Planet Labs, Inc. (URL: https://www.planet.com/); 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/); Justin Noonan (URL: https://www.justinnoonan.com/, Instagram: https://www.instagram.com/justinnoonan_/); Doro Adventures (Twitter: https://twitter.com/DoroAdventures, URL: http://doroadventures.com/).

Cleveland is a stratovolcano located in the western portion of Chuginadak Island, a remote island part of the east central Aleutians. Common volcanism has included small lava flows, explosions, and ash clouds. Intermittent lava dome growth, small ash explosions, and thermal anomalies have characterized more recent activity (BGVN 44:02). For this reporting period during February 2019-January 2020, activity largely consisted of gas-and-steam emissions and intermittent thermal anomalies within the summit crater. The primary source of information comes from the Alaska Volcano Observatory (AVO) and various satellite data.

Low levels of unrest occurred intermittently throughout this reporting period with gas-and-steam emissions and thermal anomalies as the dominant type of activity (figures 30 and 31). An explosion on 9 January 2019 was followed by lava dome growth observed during 12-16 January. Suomi NPP/VIIRS sensor data showed two hotspots on 8 and 14 February 2019, though there was no evidence of lava within the summit crater at that time. According to satellite imagery from AVO, the lava dome was slowly subsiding during February into early March. Elevated surface temperatures were detected on 17 and 24 March in conjunction with degassing; another gas-and-steam plume was observed rising from the summit on 30 March. Thermal anomalies were again seen on 15 and 28 April using Suomi NPP/VIIRS sensor data. Intermittent gas-and-steam emissions continued as the number of detected thermal anomalies slightly increased during the next month, occurring on 1, 7, 15, 18, and 23 May. A gas-and-steam plume was observed on 9 May.

Figure 30. The MIROVA graph of thermal activity (log radiative power) at Cleveland during 4 February 2019 through January 2020 shows increased thermal anomalies between mid-April to late November 2019. Courtesy of MIROVA.

Figure 31. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) confirmed intermittent thermal signatures occurring in the summit crater during March 2019 through October 2019. Some gas-and-steam plumes were observed accompanying the thermal anomaly, as seen on 17 March 2019 and 8 May 2019. Courtesy of Sentinel Hub Playground.

There were 10 thermal anomalies observed in June, and 11 each in July and August. Typical mild degassing was visible when photographed on 9 August (figure 32). On 14 August, seismicity increased, which included a swarm of a dozen local earthquakes. The lava dome emplaced in January was clearly visible in satellite imagery (figure 33). The number of thermal anomalies decreased the next month, occurring on 10, 21, and 25 September. During this month, a gas-and-steam plume was observed in a webcam image on 6, 8, 20, and 25 September. On 3-6, 10, and 21 October elevated surface temperatures were recorded as well as small gas-and-steam plumes on 4, 7, 13, and 20-25 October.

Figure 32. Photograph of Cleveland showing mild degassing from the summit vent taken on 9 August 2019. Photo by Max Kaufman; courtesy of AVO/USGS.

Figure 33. Satellite image of Cleveland showing faint gas-and-steam emissions rising from the summit crater. High-resolution image taken on 17 August 2019 showing the lava dome from January 2019 inside the crater (dark ring). Image created by Hannah Dietterich; courtesy of AVO/USGS and DigitalGlobe.

Four thermal anomalies were detected on 3, 6, and 8-9 November. According to a VONA report from AVO on 8 November, satellite data suggested possible slow lava effusion in the summit crater; however, by the 15th no evidence of eruptive activity had been seen in any data sources. Another thermal anomaly was observed on 14 January 2020. Gas-and-steam emissions observed in webcam images continued intermittently.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows intermittent weak thermal anomalies within 5 km of the crater summit during mid-April through November 2019 with a larger cluster of activity in early June, late July and early October (figure 30). Thermal satellite imagery from Sentinel-2 also detected weak thermal anomalies within the summit crater throughout the reporting period, occasionally accompanied by gas-and-steam plumes (figure 31).

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

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: 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); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

Nevados de Chillán, in the Chilean Central Andes, is a complex of late-Pleistocene to Holocene stratovolcanoes constructed along a NNW-SSE trend (figure 5). The Nuevo and Arrau craters, active during 1906-1945 and 1973-1986, respectively, are adjacent vents on the NW cone of a large stratovolcano complex 5 km SE of Cerro Blanco; the summit 1 km SE of Arrau is named Volcán Viejo (figure 6). A short eruption during August-September 2003 created a new fissure vent between the Nuevo and Arrau craters (BGVN 29:03, figure 3). Increased seismicity and fumarolic activity were recorded during December 2015, and a new eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater on the E flank of Nuevo cones (BGVN 41:06). This report adds information about the beginning of the event and continues with activity through September 2017. Information for this report is provided by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN) - Observatorio Volcanológico de Los Andes del Sur (OVDAS), Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Corporación Ciudadana Red Nacional de Emergencia (RNE), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Figure 5. This photograph, taken by an astronaut aboard the International Space Station on 11 June 2013, shows three of the largest features of the Nevados de Chillán volcanic complex: Cerro Blanco, Volcán Nuevo, and Volcán Viejo. North is to the lower right. New eruptive activity began in January 2016 from craters located in between Volcán Nuevo and Volcán Viejo.

Figure 6. Detailed location map identifying features of the Nevados de Chillán complex, and the warning zones around the volcano. The colors represent High (maroon), Medium (orange-red), and Low (gold) probabilities of pyroclastic material accumulation of more than one centimeter during a VEI 6 event. Circles with hatch marks inside represent craters. Stars are "Centro de emission,", blue ovals are hot springs. Diagonal cross-hatch is the area most susceptible to pyroclastic material greater than 6.4 cm in diameter in a radius of about 4 km around the active vents. The blue grid lines are spaced four km apart. Courtesy of SERNAGEOMIN, excerpted from Orozco et al. (2016).

Ash emissions at Nevados de Chillán began on 8 January 2016, and were intermittent through September 2017. Four new craters emerged in a NNE trend along the flanks of Volcán Nuevo and Volcán Arrau; two eventually merged into a single 100-m-diameter crater. Most plumes were brief pulses of steam and ash that rose 200-300 m above the craters. Larger events sent a few plumes as high as 2.2 km above the summit (to 5.4 km altitude). Strong prevailing winds quickly dissipated most ash plumes. Periods of multiple small explosions lasted for 1-2 weeks, separated by periods of relative quiet characterized by only steam-and-gas emissions from the active craters and nearby fumarolic centers. The first observable incandescence at the craters was noted in early March 2016. Incandescent bombs were thrown 300 m above the craters during July and September 2016, and 500 m high during March-May 2017 when blocks also fell with 500 m of the craters.

Activity during 2016. After the first explosion with ash emissions on 8 January 2016, nine more pulses of ash were emitted the next day, and small sporadic emissions were reported in the following days (figure 7). OVDAS researchers flew over the volcano on 9 January and concluded that the explosions came from a new crater on the E slope of Volcán Nuevo, about 40 m from the edge of the crater. Researchers from the University of Cambridge who visited the site on 13 January observed continuous degassing at the new 20-m-wide crater. The Buenos Aires VAAC noted puffs of steam and gas dissipating a few hundred meters above the summit (at 3.7 km altitude) in satellite imagery on 16 January 2016. ONEMI reported an ash emission on 29 January that originated from the Arrau crater (see figure 6). During an overflight on 30 January, OVDAS researchers saw occasional explosions from the new crater at Nuevo, as well as activity at a new 30-m-diameter crater about 50 m from the Arrau crater on its NE flank (figure 8). Several fumaroles were also identified on the E flank of Arrau crater.

Figure 7. Ash emission at Nevados de Chillán on 9 January 2016 from the edifice that contains the Nuevo and Arrau craters. The peak to the right is Volcán Viejo. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Advierten nuevo pulso volcánico en el Nevados de Chillán, 9 January 2016).

Figure 8. Photograph showing the Arrau crater at Nevados de Chillán observed during a flyover on 30 January 2016. Ash emissions from a new crater on the NE flank (at right) were reported on 29 January. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Otro cráter más se formó en Nevados de Chillán, 31 January 2016).

During the first two weeks of February 2016, there were 175 episodes of discrete tremor; webcams recorded explosions that ejected material from both craters. The Buenos Aires VAAC reported a brief ash emission on 3 February that dissipated quickly near the summit. During an overflight on 11 February coordinated with ONEMI, scientists identified a third crater, which created a 150-m-long NNE trend with the other two active craters identified during January. During the second half of February, emissions consisted mostly of steam plumes rising no more than 300 m above the crater.

Activity during March 2016 was characterized by steam plumes rising from the active craters; on 3 March, however, a small ash emission was observed. Incandescence was observed in the crater area on the night of 9 March. SERNAGEOMIN reported the beginning of an episode of long-period (LP) seismicity on 18 March, with a pulsating pattern of 3-4 events per minute. During the second half of March, LP and tremor activity was associated with ash emissions. Notably, a low-energy tremor on 30 March lasted for several hours, and concurrently a dense ash plume rose 200 m.

Ash emissions were observed on 7, 8, 9, 18, and 19 April 2016. Plumes were reported rising 400 m on 8 April, and 200 m on 18 and 19 April. Incandescence was observed along with the ash on 18 April. A significant explosion on 9 May 2016 generated an ash plume that rose 1,700 m above the summit (figure 9). The Buenos Aires VAAC reported the ash plume at 3.9 km altitude (700 m above the summit) drifting SE. An overflight by OVDAS on 9 May confirmed the presence of three active craters on the active summit, with the central one having enlarged by 50% since the previous overflight on 11 February. Only pulsating steam emissions were observed in the webcam during the remainder of May and June 2016.

Figure 9. An ash plume rises 1,700 m above the active crater area at Nevados de Chillán after an explosion in the early morning of 9 May 2016. Courtesy of SERNAGEOMIN.

Only steam emissions were reported during the first half of July 2016, but on 21 July an ash-laden emission sent incandescent bombs 300 m above the crater. The Buenos Aires VAAC reported that the webcam showed an ash emission to 3.4 km altitude (200 m above the craters) that day. Webcam Images obtained on 25 July showed debris from an explosion scattered 300 m down the NE flank. During the next few days, ash emissions were inferred from the seismic tremors, but weather conditions prevented direct observations.

During the first two weeks of August 2016, 14 explosions were recorded from the new craters on the E flanks of Nuevo and Arrau. The largest explosion, on 8 August, sent an ash plume 2 km above the crater, according to SERNAGEOMIN (figure 10). The Buenos Aires VAAC reported brief ash emissions on 1, 4, 8, and 9 August at altitudes of 3.7, 3.4, 4.3, and 3.7 km altitude, respectively. Fresh ashfall was visible on the flanks during a flyover on 12 August (figure 11). On the few days when the weather permitted observation of the summit during the remainder of the month, only steam plumes were observed rising no more than 400 m above the crater.

Figure 10. An ash emission at Nevados de Chillán rises 2 km above the active craters on 8 August 2016. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Volcán Nevados de Chillán presenta nuevo pulso eruptive, 8 August 2016).

Figure 11. Fresh ashfall coats the flanks of the active summit at Nevados de Chillán on 12 August 2016, after a large explosion on 8 August. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Registro aéreo muestra actual pulso eruptivo de volcán Nevados de Chillán, 12 August 2016).

Pulsating steam plumes, interrupted by periodic ash emissions, were typical during September 2016. During the first two weeks of the month, 37 recorded explosions were characterized by a high concentration of particulate material. The largest explosion, during the evening of 1 September, generated incandescent bombs for 20 minutes. Incandescence was observed during nighttime explosions a number of times. The Buenos Aires VAAC noted a pilot report of an ash cloud moving SW at 5.2 km altitude on 2 September. They also reported a weak emission of steam and gas with possible diffuse ash visible in the webcam that day. Another pilot report on 6 September indicated an ash cloud moving NE at 6.4 km altitude from a brief but intense emission event around 1420 UTC (figure 12). SERNAGEOMIN noted in their late September report that there had been six explosive episodes since January 2016, with the latest one that occurred during 1-10 September being the strongest.

Figure 12. An ash plume rises from Nevados de Chillán on 6 September 2016. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Volcán Nevados de Chillán registró nuevo pulso eruptive, 6 September 2016).

Explosive activity was recorded on 3, 7, and 8 October 2016 by SERNAGEOMIN; The events were low-energy episodes that emitted small quantities of ash. The Buenos Aires VAAC noted a pilot report on 3 October of an ash cloud moving SE near the summit. It was visible in the webcam but not in satellite imagery, and dissipated quickly. The tallest emission of those days rose to 300 m above the crater on 7 October. During an overflight on 22 October, the continued presence of the three craters along the E flanks of Nueva and Arrau reported previously was confirmed. In addition, the existence of a fourth crater was noted along the same trend as the others. The Buenos Aires VAAC noted ash emissions on 26 and 28 October rising to between 3.7 and 4.3 km altitude and dissipating quickly near the summit.

Seismic activity during the first half of November 2016 included 17 explosions from the active craters. An explosion on 18 November generated an ash plume that rose 1.2 km (figure 13). The Buenos Aires VAAC noted a pilot report of possible ash emissions between 4.6 and 6.1 km altitude on 17 and 27 November, although neither were identified in satellite data.

Figure 13. An ash emission rises 1.2 km above the active crater area at Nevados de Chillán on 18 November 2016. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Sernageomin emite reporte especial por actividad volcánica del complejo Nevados de Chillán, 18 November 2016).

Explosions associated with LP and tremor seismicity continued into December 2016. There were 14 explosive seismic events during the second half of the month, reported by SERNAGEOMIN. The largest occurred on 28 December. The Buenos Aires VAAC noted pilot reports of ash emissions that dissipated near the summit on 13, 28, and 29 December.

Activity during January-September 2017. Explosions related to LP and tremor seismicity increased again on 5 January 2017. The Buenos Aires VAAC reported a dark fumarolic plume drifting E at 4.5 km altitude on 6 January that was observed by a pilot and in the webcam. On 11 and 13 January, the webcam showed sporadic puffs of ash that dissipated very quickly. The largest event occurred on 15 January; the Buenos Aires VAAC reported a narrow plume of ash in satellite imagery at 3.9 km altitude moving W. The webcam also showed sporadic and small puffs that dissipated quickly. An event on 16 January produced an emission that rose 700 m above the crater according to SERNAGEOMIN. This was the last LP-associated explosion of the month. Scientists on a 20 January overflight noted low-intensity steam plumes from the Nuevo and Arrau craters, and from the Chudcún crater which formed in 2003 between them (see figure 6). Yellow and ocher-colored areas, indicating the presence of precipitated sulfur, were visible around the fumaroles and craters.

Low-level degassing rising less than 200 m above the crater was the only surface activity observed during February 2017. A new stage of explosive activity began on 7 March 2017 with emissions that rose as high as 300 m above the crater. The Buenos Aires VAAC noted a pilot report of an ash plume at 3.7 km altitude, and a short-lived puff of ash seen in the webcam. On 11 March, eight explosions sent incandescent blocks up to 0.5 km from the active craters, and emissions rose to 500 m above the crater. Another series of eight explosions on 14 March produced incandescent material and sent an ash plume 1.5 km above the craters. The Buenos Aires VAAC reported intermittent emissions rising up to 4.9 km altitude that day, followed by continuing steam emissions. The following day they noted a small plume near the volcano at 3.9 km altitude visible in satellite data.

During a flyover on 15 March, OVDAS scientists noted that two of the craters (craters 3 and 4) had merged into a single crater 100 m in diameter (figure 14). They also observed five explosions within the space of an hour, the highest resulting plume rose 900 m above the active crater. Webcam images during 16-17 March showed ash emissions rising to 2 km above the crater. The Buenos Aires VAAC reported an ash emission visible in satellite imagery at 5.5 km altitude moving SW on 16 March. For the remainder of the month, only weak degassing under 200 m above the crater was observed. Beginning on 24 March, low-level incandescence at night was reported for the rest of the month.

Figure 14. OVDAS scientists photographed two merged craters (3+4) at Nevados de Chillán on 15 March 2017. They also witnessed five explosions from one crater within an hour (yellow arrow). Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Complejo Volcánico Nevados de Chillán tiene cráter de 100 metros de diámetro, 24 March 2017).

Between 1 and 12 April 2017, there were 56 intermittent explosions marking a new phase of activity according to SERNAGEOMIN. The webcams around the complex imaged emissions up to 3 km above the crater throughout the month. The Buenos Aires VAAC reported sporadic emissions of ash visible in the webcam on 3 and 6-8 April. A faint emission at 3.7 km altitude was spotted in satellite imagery on 10 April. From 16 to 30 April, there were 79 intermittent explosions recorded. During dusk and dawn, incandescent material was observed traveling 600 m down the flanks, with some episodes lasting for 60 minutes. The Buenos Aires VAAC reported a brief ash emission and incandescent material visible in the webcam on 17 April, and sporadic ash emissions that rose to 3.9 km altitude on 21, 29, and 30 April (figure 15).

Figure 15. An ash emission on 30 April 2017 at Nevados de Chillán rose to 3.9 km altitude (700 m above the craters), and was photographed by a twitter user near the volcano. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Nuevo pulso eruptivo de volcán Nevados de Chillán preocupa en la región del Bío Bío, 30 April 2017).

Nine intermittent explosions occurred between 1 and 11 May 2017. The webcams showed emissions from the explosions rising generally 300 m above the craters according to SERNAGEOMIN. Intermittent explosions increased again during 27-31 May. Emissions rose to 1.5 km above the craters and incandescent blocks could be seen traveling 600 m down the flank. Periods of constant incandescence lasted for 30 minutes.

This explosive episode continued into June 2017, with 23 intermittent explosions between 1 and 5 June. The largest emission event on 5 June sent a plume 2.2 km above the craters (figure 16). The Buenos Aires VAAC observed the ash plume at 4.6 km altitude in satellite imagery. During 6-15 June, only steam emissions rising to 300 m were reported. Intermittent explosions on 20, 22, 25, and 26 June produced plumes that rose only 200 m above the craters; cloudy weather prevented observation from the webcams during these events.

Figure 16. Twitter users in Chile shared this image of an ash plume rising from the active craters at Nevados de Chillán with regional authorities on 5 June 2017. The Buenos Aires VAAC reported the plume rising to 4.6 km altitude. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Volcán Nevados de Chillán emite nuevo pulso eruptive, 5 June 2017).

No explosive events were observed in the webcams during the first half of July 2017; only steam plumes rising 200 m were reported. A single low-energy explosion was recorded on 31 July; the emission rose to only 100 m above the crater. During August 2017, there were 83 intermittent explosions associated with ash emissions recorded by SERNAGEOMIN. The emissions rose to about 300 m above the active craters; a few larger emissions rose 1,000 m. The Buenos Aires VAAC noted a pilot report of ash emissions on 17 August; the webcam captured a brief emission that dissipated rapidly.

About 150 intermittent explosions were reported during September 2017. The highest plumes, generally composed of steam and ash, rose 2,000 m above the craters. The Buenos Aires VAAC observed a narrow plume of ash in satellite imagery moving N at 3.9 km altitude and dissipating rapidly on 15 September, and a similar plume moving SE near the summit on 26 September 2017.

Reference: Orozco, G.; Jara, G.; Bertin, D. 2016. Peligros del Complejo Volcánico Nevados de Chillán, Región del Biobío. Servicio Nacional de Geología y Minería, Carta Geológica de Chile, Serie Geología Ambiental 28: 34 p., 1 mapa escala 1:75.000. Santiago.

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/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637/1671, Santiago, Chile (URL: http://www.onemi.cl/); Corporación Ciudadana Red Nacional de Emergencia (RNE, Citizen Corporation National Emergency Network), Avda. Vicuña Mackenna Nº3125, San Joaquín, Santiago de Chile, Chile (URL: http://www.reddeemergencia.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php?lang=es).

Located on an elevated plateau in central Java NW of Yogyakarta (figure 4), multiple craters within the Dieng Volcanic Complex (figure 5) have been intermittently active over the past 200 years. Brief phreatic eruptions took place at Sibanteng crater on 15 January 2009 (BGVN 34:04) and at Sileri crater on 26 September later that year (BGVN 34:08). Increased unrest during March-April 2013 (BGVN 38:08) consisted of elevated volcanic gas emissions from Timbang Crater that resulted in an increase in the Alert Level to as high as 3 on 27 March, then back to Level 2 on 8 May. There was a precautionary evacuation of local villages, but no eruption took place. Regular monitoring is done by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Centre for Volcanology and Geological Hazard Mitigation or CVGHM).

Figure 4. Topographic terrain map of central Java showing the Dieng Volcanic Province to the NW of Gunung Sumbing and Gunung Sindoro volcanoes. The volcano indicated by a red symbol N of Yogyakarta is Merapi. Courtesy of Peakery.

Figure 5. Topographic terrain map of the Dieng Volcanic Province on the Dieng plateau of central Java. The notable cone at bottom center is Bisma; the crater with a lake at center is Merdada, adjoining Kawah Sikidang to the SE. The frequently active Sileri area is immediately W of the more noticeable Pagerkandang crater N of Merdada. Courtesy of Peakery.

The alert status remained at Level 2 for about 15 months following the hazardous gas emissions in 2013. On 11 August 2014 the PVMBG noted that, due to decreased activity and no observable flow of gas in high concentrations from the crater, the Alert Level was lowered to 1 (on a scale of 1-4). No further activity was reported until late April 2017.

A phreatic event from Sileri Crater at 1303 on 30 April 2017 ejected material 10 m high and 1 m past the crater edge, forming a 1-2 mm thick deposit. Another emission at 0941 on 24 May consisted of gas and black "smoke" that rose 20 m.

The Disaster Management Authority, Badan Nacional Penanggulangan Bencana (BNPB), reported that there had been another phreatic eruption from the Sileri Crater lake at 1154 on 2 July 2017, ejecting mud and material 150 m high, and 50 m to the N and S. The event injured 11 of 18 tourists that were near the crater. According to a news article a helicopter on the way to assist with evacuations after the event crashed, killing all eight people (four crewmen and four rescuers) on board. PVMBG scientists visited the next day and observed weak white emissions rising 60 m.

PVMBG reported that during 8 July-14 September 2017 measurements indicated an increase in water temperature at Sileri Crater lake from 90.7 to 93.5°C. Soil temperatures also increased, from 58.6 to 69.4°C. At Timbang Crater temperatures in the lake increased from 57.3 to 62.7°C, and in the soil they decreased from 18.6 to 17.2°C. The report noted that conditions at Timbang Crater were normal.

Temperature increases at Sileri, along with tremor detected during 13-14 September, prompted PVMBG to raise the Alert Level to 2 (on a scale of 1-4). PVMBG warned the public to stay at least 1 km away from the crater rim, and for residents living within that radius to evacuate. However, after 20 September tremor and water temperatures both declined. The Alert Level was lowered back to 1 on 2 October, with a warning to stay at least 100 m from the crater rim.

Geologic Background. The Dieng plateau in the highlands of central Java is renowned both for the variety of its volcanic scenery and as a sacred area housing Java's oldest Hindu temples, dating back to the 9th century CE. The Dieng volcanic complex consists of two or more stratovolcanoes and more than 20 small craters and cones of Pleistocene-to-Holocene age over a 6 x 14 km area. Prahu stratovolcano was truncated by a large Pleistocene caldera, which was subsequently filled by a series of dissected to youthful cones, lava domes, and craters, many containing lakes. Lava flows cover much of the plateau, but have not occurred in historical time, when activity has been restricted to minor phreatic eruptions. Toxic gas emissions are a hazard at several craters and have caused fatalities. The abundant thermal features and high heat flow make Dieng a major geothermal prospect.

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/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); Peakery (URL: https://peakery.com/).

Italy's Mount Etna on the island of Sicily has had historically recorded eruptions for the past 3,500 years. Lava flows, explosive eruptions with ash plumes, and lava fountains commonly occur from its major summit crater areas, the North East Crater (NEC), the Voragine-Bocca Nuova (or Central) complex (VOR-BN), the South East Crater (SEC) (formed in 1978), and the New South East Crater (NSEC) (formed in 2011). A new crater, the SEC3 or "saddle cone" emerged during early 2017 from the saddle between SEC and NSEC.

After a major explosive event in December 2015 (BGVN 42:05), activity subsided for a few months before renewed Strombolian eruptions and lava flows affected all of the summit craters during late May 2016 (BGVN 42:09). These events were followed by a lengthy period of subsidence and intense fumarolic activity across the summit that lasted until a new eruptive episode began at the end of January 2017. The Osservatorio Etneo (OE), which provides weekly reports and special updates on activity, is run by the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV). This report uses information from INGV to provide a detailed summary of events between January and August 2017.

Summary of January-August 2017 Activity. Minor ash emissions began from a new vent in the saddle between NSEC and SEC on 20 January 2017, followed by Strombolian activity a few days later. Activity intensified at the end of February when the first of several lava flows emerged from this vent, and then from several other vents on the S flank of the new, rapidly growing cone during March and April. By mid-March 2017, Strombolian activity, ash emissions, and lava flows had created a cone higher than the adjacent NSEC and SEC cones. The last effusive episode at the end of April 2017 sent flows down both the N and S flanks of the new cone from multiple vents. Intermittent weak Strombolian activity at the new summit area was associated with abrupt tremor amplitude increases during May, but no additional flows were reported. During June-August, fumarolic activity persisted at several crater areas, and minor ash emissions were observed a few times, but no major eruptive activity took place. The sharp increase in heat flow resulting from the lava flows of March and April 2017 are clearly visible in the MIROVA thermal anomaly plot of log radiative power for the year ending on 12 October 2017 (figure 186).

Figure 186. Thermal anomalies at Etna (log radiative power) identified by the MIROVA system for the year ending on 12 October 2017. Major effusive eruptive events with lava flows and Strombolian activity occurred from late February through April 2017. Courtesy of MIROVA.

Activity during January-February 2017. Sporadic incandescence continued from the 7 August 2016 vent on the E side of VOR during January 2017, and minor ash plumes rose from the NSEC "saddle" vent on 20 January. Modest Strombolian activity began at the saddle vent that on 23 January and continued into February (figure 187). Small bombs were ejected onto the flank of NSEC and minor ash plumes quickly dissipated in the high winds near the summit. Also during February, steady subsidence continued at BN, especially in the BN-1 area (see figure 185, BGVN 42:09), where active degassing with minor amounts of ash was observed on 1 February (figure 187). Debris deposits from Strombolian activity at the saddle vent covered the S side of the pyroclastic cone and travelled to its base during the end of February.

Figure 187. Activity at Etna during the first week of February 2017. Left: Strombolian activity at the NSEC saddle vent; photo by B. Behncke. Right: degassing with minor ash emissions from the vent at the bottom of BN-1; photo by M. Ponte. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 30/01/2017-05/02/2017, No. 6/2017).

During the late afternoon of 27 February, the Strombolian activity that began on 20 January from the saddle vent between SEC and NSEC rapidly intensified, and lava emerged from the vent and flowed down the S flank of SEC (figure 188). It slowed after reaching the flat ground at the base of the cone, and expanded slowly SE toward the older cones of Monte Frumento Supino. Intense activity that evening sent shards and bombs 200 m above the vent while the flow continued. Ash from the Strombolian activity dispersed NE, with minor ashfall reported in Linguaglossa and Zafferana. A new cone of pyroclastic material that formed around the saddle vent quickly grew to about the same elevation as the NSEC and SEC crater rims, approximately 3,290 m (figure 189). The lava continued to flow until 2 March 2017, when it stopped at about 2,750 m elevation with an overall length of 2,180 m, covering an area of 306 x 10³ m², for a total volume of slightly less than 1 x 10⁶ m³.

Figure 188. An outline of the new lava flow at Etna that emerged from the saddle vent located between NSEC and SEC on 27 February 2017. It rapidly advanced down the steep S flank of SEC. Base map is a DEM image created by the INGV Cartography Laboratory. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 27/02/2017-05/03/2017, No. 10/2017).

Figure 189. Strombolian activity, the 27 February lava flow, and ash and vapor emissions from the new NSEC/SEC saddle vent at Etna on 28 February 2017 around 1730 local time. Photo by F. Ciancitto; courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 27/02/2017-05/03/2017, No. 10/2017).

Activity during March 2017. Sporadic ash emissions continued from the new saddle vent during early March 2017, accompanied by weak Strombolian activity during the night of 12-13 March. Intense degassing continued from VOR during March as well, with incandescent bursts visible on many clear nights. On the morning of 15 March the Montagnola webcam recorded a lava overflow from the saddle vent down the S flank of NSEC, and an intensification of explosive activity that caused the flow to reach the base of the complex at about 3,000 m elevation. During the day, it advanced towards Monte Frumento Supino; it had reached elevation 2,800 m by the late evening, overlapping significantly with the earlier flow from 27 February. Strombolian eruptions were nearly constant until late afternoon, and continued intermittently, along with ash emissions, for several days.

Shortly before 2300 UTC on 15 March (0100 on 16 March local time), a second new flow emerged from a vent near the base of the S flank of the new NSEC/SEC cone (at about 3,200 m elevation) and travelled SE (figure 190), splitting into two lobes. INGV personnel in the summit area reported a series of phreato-magmatic explosions at 0043 (just after midnight) along the lava front at an elevation of approximately 2,700 m along the W edge of the Valle del Bove. The contact of the active flow with the underlying snow caused several explosions. An INGV volcanologist suffered minor injuries during one of the explosions. Increased emissions also caused minor ashfall in Adrano and Santa Maria di Licodia (both about 17 km SW).

Figure 190. Explosions at Etna from a vent at the base of the new NSEC/SEC cone complex during the early morning of 16 March 2017 viewed from the Torre del Filosofo, 1 km S. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 13/03/2017-19/03/2017, No. 12/2017).

By the afternoon of 17 March 2017, the second flow had reached an elevation of about 2,600 m, near the base of the W slope of the Valle del Bove. INGV personnel at Monte Zoccolaro (1.5 km S) spotted a third flow on 18 March, located S of the other two (figure 191). The front had reached about 2,200 m elevation, and was responsible for some phreato-magmatic explosions during 18 and 19 March. Several avalanches of incandescent material reached the base of the slope at the edge of Valle del Bove as the flow fronts collapsed during 18 March. Two Landsat 8 Operational Land Imager images on 18 and 19 March captured evidence of the lava flows, an ash plume, and Strombolian activity during this episode (figure 192). By 19 March, the advance had slowed as the flows began to spread out over the valley floor. The flows into the Valle del Bove ceased on 20 March.

Figure 191. Thermal image of the W wall of the Valle del Bove at Etna on 18 March 2017, viewed from Monte Zoccolaro showing the activity of the three lava flows. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 13/03/2017-19/03/2017, No. 12/2017).

Figure 192. The eruption from Etna's NSEC/SEC cone on 18 and 19 March 2017 as captured from space. The upper image was taken on 18 March by the Operational Land Imager (OLI) on Landsat 8 as a natural-color image, and shows an ash plume and two columns of gas and steam drifting SE. The more northerly steam and gas plume and the ash plume are rising from the summit vent of the new NSEC/SEC cone, and the more southerly steam and gas plume is rising from the effusive vent at the base of the S flank of the NSEC/SEC cone. The lower image shows the thermal glow of active lava flows on the SE flank on 19 March 2017, and the Strombolian activity at the summit of the new cone (the yellow spot directly below the Mt. Etna label) surrounded by the city lights of Catania and the surrounding communities. An astronaut aboard the International Space Station took this image. Courtesy of NASA Earth Observatory.

Strombolian activity and ash emissions ceased at the summit vent of the NSEC/SEC cone between 20 and 22 March 2017 leaving a new pyroclastic cone that rose above the adjacent NSEC and SEC cones (figure 193). Once the Strombolian activity had ended, yet another lava flow emerged from the base of the cone at an elevation of about 3,010-3,030 m, and spread into several segments, one of which flowed W around Monti Barbagallo (near the former Torre del Filosofo) and then turned SW following the valley between Monti Barbagallo and Monte Frumento Supino. By 26 March the front of this flow segment had reached an elevation of 2,300 m and travelled about 2.5 km from the vent. A second segment of the flow travelled E of Monti Barbagallo, following the earlier flows that had been active along the W slope of the Valle del Bove; it slowed and broke into several additional segments, reaching 1.3 km from the vent on 26 March, and advancing through the first week of April.

Figure 193. The new pyroclastic cone 'cono di scorie' between the SEC and NSEC rises above and between both older craters at Etna shortly after 22 March 2017. It first emerged during the eruption of 27 February to 1 March 2017, and then continued to increase in size until 22 March 2017 from extensive Strombolian activity. The dotted white line separates the South East Crater (SEC) from the New South East Crater (NSEC). "Bocca effusive" is the effusive vent that fed the lava flows beginning on 22 March, and the new lava is the dark material with fumarolic emissions in the foreground. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 20/03/2017-26/03/2017, No. 13/2017).

Activity during April 2017. The active lava flow continued WSW towards the cones of the 2002-2003 eruption from the vent at the base of the NSEC/SEC cone until it stopped advancing sometime during the night between 8 and 9 April (figure 194). Another new flow then emerged from the same vent on 10 April and was active for just over 24 hours. This flow travelled SE to the W edge of the Valle del Bove and moved a few hundred meters along the edge before stopping during the day of 12 April.

Figure 194. The lava flow at Etna that emerged from the base of the NSEC/SEC cone complex on 22 March 2017 flows WSW towards the cones of the 2002-2003 eruption during the first week of April 2017. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 27/03/2017-02/04/2017, No. 14/2017).

During the evening of 13 April 2017, Strombolian activity at the summit crater of the NSEC/SEC cone accompanied the emergence of flows from three vents along the S flank at elevations of approximately 3,200 m, 3,150 m, and 3,010 m which headed S and SE. The upper flows were active for only a few hours, but the lower flow continued SE towards the Valle del Bove and had overlapped the 10-11 April flow by the next day. The active front of the flow was at an elevation of 2,400 m on the western slope of the Valle del Bove, just north of the Serra Giannicola Grande. A flyover on 14 April revealed the extent of the fracture system on the flank of the NSEC/SEC complex from which the numerous flows emerged (figure 195). The flow rate diminished during the day of 15 April, and the flow stopped sometime during the next night.

Figure 195. Thermal images of the fracture system affecting the S flank of the NSEC/SEC cone at Etna on 14 April 2017 showing the pyroclastic cone 'Cono di scorie', a collapsed portion of the cone 'Porzione collassata', and the three eruptive vents 'Frattura eruttiva' that opened on 13 April (at 3,200 m, 3,150 m and 3,010 m elevation). Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 10/04/2017-16/04/2017, No 16/2017).

A thermal anomaly appeared at the S edge of the NSEC/SEC summit vent, which INGV began calling SEC3, on the morning of 19 April. Weak Strombolian activity from the vent was followed by the emergence of a lava flow from the S side of the crater rim that flowed down the S flank of the cone. Dense, brown ash emissions about an hour later accompanied the re-opening of three vents on the S flank from which new lava flows emerged (figure 196). Lava jets rose tens of meters above the crater rim for about an hour in the afternoon. The lava flows from the three vents formed into two branches moving down the S flank (figure 197), then turned E and spread over the W slope of the Valle del Bove; by 20 April they had reached an elevation of 1,950 m. Explosive activity ceased at SEC3 that afternoon, and the flows stopped advancing sometime during the night of 20-21 April. Observations of the summit of SEC3 on 22 April revealed a N-S trending graben formed in the S rim of the summit crater about 100 m long, 10 m wide, and several tens of meters deep.

Figure 196. The new SEC3 cone at Etna lies in the former saddle between SEC and NSEC. The red circles indicate the positions of the three eruptive vents (V1, V2, and V3) that opened on 19 April 2017 on the S flank of the cone. Lava from the vents is flowing E toward the Valle del Bove in this N-looking photo taken by Mauro Coltelli on 20 April 2017. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 17/04/2017-23/04/2017, No. 17/2017).

Figure 197. Lava flows from the summit crater of the new cone (SEC3) at Etna on 20 April 2017. Photo by Salvatore Allegra/Anadolu Agency/Getty Images/CFP, published in Globaltimes, 20 April 2017.

The next eruptive episode began late in the day on 26 April 2017, with a slow-moving lava flow that emerged from the summit vent of SEC3. The flow made it part way down the S flank before another flow from the same vent covered it and reached the base of the flank. Strombolian activity began at the summit vent during the late evening while the flow continued to spread SE toward the Valle del Bove (figure 198). Strombolian activity intensified during the early hours of 27 April and a new vent opened at the summit immediately N of the first one. At around 0220, two new eruptive fractures opened on the N flank of SEC3, from which lava flowed N toward the Valle del Leone (figure 199). At daybreak, an ash plume was visible about 1.5 km above the summit drifting E. Phreato-magmatic explosions were observed in the Valle del Leone when the northern lava flow encountered snow on the ground. Strombolian activity ceased around noon and the flows on both the N and S flanks had ceased by the following morning.

Figure 198. Lava flows down the S flank of SEC3 at Etna during the early morning of 27 April 2017, heading SE towards the Valle del Bove. Strombolian activity occurred from both of the summit vents, and an ash plume rose from the summit. Photo taken from the roof of the INGV-Osservatorio Etneo located 27 km S of the volcano. Courtesy of INGV (Attivita' dell'Etna, 20 Aprile-14 Giugno 2017).

Figure 199. Lava flows from both the N and S flanks of SEC3 at Etna on 27 April 2017. a) the two lava flows are clearly visible from the Monte Cagliato thermal camera (EMCT) in this view looking W. b) a phreato-magmatic explosion in the Valle del Leone from the lava flow encountering snow on the N side of SEC3. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 24/04/2017 - 30/04/2017, No. 18/2017).

Activity during May-August 2017. Intense degassing with incandescence at night continued from the vent at VOR throughout April and into May 2017. At NEC, degassing continued from the large fumarole field at the bottom of the summit crater. No further lava flows erupted during May 2017, however, there were several short, high-energy tremor episodes in the area around SEC3. During May, more than 35 episodes of transient increases in tremor amplitude were recorded by INGV seismic instruments (figure 200). During 15-18 May, there were 11 episodes of Strombolian activity from the northern SEC3 summit vent, repeated at regular intervals of about every 8-9 hours. Lava fragments were ejected outside the crater rim and rolled down the flanks (figure 201). Each episode was accompanied by a sharp increase in volcanic tremor amplitude. Eight additional episodes of weak and discontinuous Strombolian activity occurred between 25 and 28 May at intervals ranging from 3 to 14 hours, each lasting about an hour, and accompanied by increased tremor amplitude. A short sequence of dense ash emissions from BN-1 on the morning of 31 May was the only ash plume reported during May.

Figure 200. During the month of May 2017, more than 35 episodes of transient increases in the amplitude of tremor were recorded by the seismic instruments at Etna. Some, but not all, of these episodes were accompanied by Strombolian activity at the N vent at the SEC3 summit. Courtesy of INGV (Attivita' dell'Etna, 20 Aprile-14 Giugno 2017).

Figure 201. The summit of the new NSEC/SEC complex at Etna on 16 May 2017 as viewed from the NW. The blue arrow indicates the eruptive vent that produced discontinuous Strombolian activity during May. Photo by M. Cantarero; courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 15/05/2017-21/05/2017, No. 21/2017).

Weak and discontinuous Strombolian activity resumed at NSEC on 6 June 2017, along with a sudden increase in tremor. The activity lasted until 9 June and included four episodes of roughly one hour each. Very little material fell outside the crater rim during these events. Vigorous degassing and nighttime incandescence continued at the VOR vent during June. INGV-OE personnel inspected the summit on 23 and 29 June, and 2 July 2017. High temperatures (around 600°C) were recorded at the VOR vent on 23 June. The other fumarolic areas, especially in the fracture field between NEC and VOR, were around 250°C, cooler than when last measured on 31 August 2016. Occasional weak ash emissions began on 24 June from SEC3; they lasted for a few days and quickly dissipated near the top of the cone. They ceased late in the evening of 28 June.

In a survey by drone on 4 July 2017, INGV-OE personnel noted widespread degassing along the rim and E side of the SEC3 crater. The vent that had formed during 27 February-26 April appeared to be blocked (figure 202). During the late morning of 9 July, the vent that had formed during 26-27 April emitted a small amount of red-gray ash. The next day a small amount of ash emerged from the base of BN-1. Incandescence was frequently observed at night from the VOR vent and from the NSEC. Degassing was observed regularly throughout the month at the VOR vent, the bottom of BN-1, and NEC (figure 203).

Figure 202. Detailed view of the summit of the new SEC3 cone at Etna on 4 July 2017 taken by an INGV-OE drone. 1) eruptive vent active during 27 February-26 April; 2) eruptive vents active during 26-27 April; a) closeup of the bottom of one of the 26-27 April vents, from which a small amount of reddish-gray ash emerged on 9 July. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 3/07/2017-9/07/2017, No. 28/2017).

Figure 203. Panoramic photos of the summit craters of Etna on 27 July 2017. VOR, seen from the northwestern edge, continued with strong degassing from the 7 August 2016 vent on the E rim; the NEC, seen from the fracture that cuts the southern rim, had modest, diffuse degassing from the fracture zone within the crater; and BN, seen from the eastern edge, had moderate degassing occurring from the vent at the base of BN-1 throughout the month. Courtesy of INGV-OE (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 24/07/2017-30/07/2017, No. 31/2017).

Occasional weak, diffuse ash emissions continued during August 2017 from the bottom of BN-1. INGV-OE scientists attributed this to collapse at the base of the crater. Limited degassing was noted at NEC, but persistent degassing continued from the 7 August 2016 vent at VOR, and from a vent on the E side of NSEC in addition to a vent at the SEC3 summit (figures 204 and 205).

Figure 204. Areas of persistent degassing and fumarolic activity at Etna during August 2017. The black hatch lines outline the crater rims: BN = Bocca Nuova, which contains the NW vent (BN-1) and the SE vent (BN-2); VOR = Voragine; NEC = North East Crater; SEC = South East Crater; NSEC = New South East Crater. Yellow circles indicate the locations of the degassing mouths of VOR, BN, and both the "Cono della sella" (saddle cone, or SEC3) and the E vent at NSEC. The base map is from a 2014 DEM of the summit from INGV Aerogeophysics Laboratory - Section 2. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 31/07/2017-06/08/2017, No. 32/2017).

Figure 205. Aerial photographs of the summit crater area of Etna taken on 16 August 2017. a) view from ENE; b) view from the SE. Weak fumarolic activity is visible from the E vent of the New South East Crater (NSEC). More intense and continuous degassing emerges from the Central Crater (VOR and BN). See figure 204 for additional label explanations. Photos by Piero Berti; courtesy of Butterfly Helicopter Services and INGV-OE (Bollettino settimanale sul monitoraggio vulcanico, geochimico e sismico del vulcano Etna, 14/08/2017-20/08/2017, No. 34/2017).

Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV-OE), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.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/); Global Times, http://www.globaltimes.cn/galleries/774.html.

Volcán de Fuego has been erupting continuously since 2002. Historical observations of eruptions date back to 1531, and radiocarbon dates are confirmed back to 1580 BCE. These eruptions have resulted in major ashfalls, pyroclastic flows, lava flows, and damaging lahars. Fuego was continuously active from January to June 2016. Daily explosions that generated ash plumes to within 1 km above the summit (less than 5 km altitude) were typical. In addition, there were ten eruptive episodes that included Strombolian activity, lava flows, pyroclastic flows, and ash plumes rising above 5 km altitude (BGVN 42:06). Every month, lahars flowed down several drainages. This report continues with a summary of similar activity during July-December 2016. In addition to regular reports from the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH), the Washington Volcanic Ash Advisory Center (VAAC) provides aviation alerts. Locations of many towns and drainages are listed in table 12 (BGVN 42:05).

Activity during July-December 2016 was very similar to the previous six months. Background activity included daily explosions, and ash emissions that often generated minor ashfall in communities within 15 km, generally to the SW. Strombolian activity sent material 300 m above the crater, and block avalanches down the flanks. Six eruptive episodes occurred during the second half of 2016, with characteristics very similar to the ten that occurred during the first half of the year (table 13). The episodes usually lasted around 48 hours. During the eruptive episodes, the amplitude and frequency of explosions increased to several per hour, and ash emissions that rose 1-3 km above the summit crater (4.8-7.8 km altitude) distributed ash tens of kilometers away. Diffuse ash plumes were often visible in satellite imagery several hundred kilometers from the volcano. Each episode was also accompanied by Strombolian activity that sent incandescent material 200-500 m above the summit crater, creating lava flows that descended major drainages. Episodes 11 and 16, in July and December, also included pyroclastic flows. The thermal signature recorded in the University of Hawaii's MODVOLC thermal alert system closely correlated with the increased heat flow from the lava flows during the eruptive episodes. Numerous lahars descended major drainages after heavy rains during August, but no damage was reported. A modest lahar was reported near the end of September.

Table 13. Eruptive episodes at Fuego during 2016. Details of episodes 1-10 are described in BGVN 42:06, episodes 11-16 are discussed in this report. The eruptive episode number is just for 2016 and was assigned by the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH).

Activity during July 2016. Explosions of incandescent material from the summit crater of Fuego were constant during July 2016, according to INSIVUMEH. On 5 July, an increase in the number of explosions per hour led to an ash plume rising to 4.5 km altitude and drifting W and SW. Incandescent blocks reached the vegetation on the W flank a few hundred meters from the summit. Ashfall was reported in the villages of Morelia, Santa Sofia, Sangre de Cristo (all around 10-12 km SW), and San Pedro Yepocapa (9 km NW). Another increase in activity on 15 July resulted in eight weak-to-moderate explosions per hour, which generated ash plumes that rose to about 4.3-4.8 km altitude and drifted more than 15 km W and SW; ash fell on the flanks in those directions. The Washington VAAC reported ash emissions rising to 4.6-5.2 km altitude, and MODVOLC issued one thermal alert. On 17 July, the Washington VAAC reported an ash emission drifting about 18 km W at 4.9 km altitude. Another ash emission was observed in satellite imagery on 19 July, at 5.2 km altitude drifting NW. The Washington VAAC also reported that the webcam showed lava on the flank near the summit that day.

Eruptive episode 11 began on 28 July 2016 and lasted for about 48 hours. Moderate-to-strong explosions expelled ash plumes to 5.5 km altitude that eventually drifted more than 250 km SW, W, and NW. The INSIVUMEH webcam at Finca La Reunion (SE) captured an image of the ash plume accompanied by a pyroclastic flow which descended the Santa Teresa ravine (barranca) around midday on 29 July (figure 50). Incandescent material was ejected about 500 m above the crater and fed two lava flows; one traveled 1.5 km down the Santa Teresa ravine, and the other traveled 3 km down the Las Lajas ravine. Some of the villages that reported ashfall included Sangre de Cristo, San Pedro Yepocapa, and Patzún (27 km NW). The Washington VAAC observed the ash plume in the early morning of 29 July extending 30 km WNW from the summit at 5.8 km altitude. By late morning, the plume had risen to 6.7 km altitude and was visible 150 km NW. The plume altitude dropped later in the day to 5.2 km, and the drift direction changed more toward the W. The farthest edge of the plume was faintly visible over 250 km W before it dissipated that evening. Incandescent explosions continued into the night, but had subsided by the next morning. A MODVOLC thermal anomaly signal first appeared on 26 July and persisted through 31 July; there were 17 thermal alert pixels reported on 29 July.

Figure 50. An ash plume rises from the summit of Fuego on 29 July 2016 while a small pyroclastic flow descends a drainage on the SE flank, as seen from the Finca la Reunion webcam. Eruptive episode 11 lasted from 28 to 30 July. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Julio 2016).

Activity during August 2016. Weak and moderate explosions that generated ash plumes characterized activity during August 2016. Although a few strong explosions were recorded, there were no distinct eruptive episodes documented by INSIVUMEH. Constant rains, however, led to several lahars descending the major ravines. Persistent steam plumes rose to 4.2 km and drifted W and SW. Weak-to-moderate explosions with ash reached 4.3 to 4.8 km altitude, and drifted more than 15 km SW and W before dissipating. Ashfall was reported primarily in the communities of Sangre de Cristo, Yepocapa, Morelia, Hagia Sophia, and Panimaché I and II. Incandescent material was ejected 300 m above the crater, and generated weak-to-moderate avalanches within the crater.

The Washington VAAC reported an ash plume visible in satellite imagery on 7 August at 4.9 km altitude extending about 10 km WNW from the summit. On 11 August, a narrow plume was spotted in both visible and multispectral imagery extending about 80 km W at the same altitude. A puff of volcanic ash appeared in clear satellite and webcam images drifting W at 4.9 km on 19 August. A series of ash emissions were spotted on 20 August in satellite imagery. The head of the plume was about 35 km W of the summit. The highest altitude plume reported by the Washington VAAC during August was at 5.8 km on 25 August, drifting 25 km W. A single MODVOLC thermal alert was also recorded that day. On 15 and 16 August moderate-to-large lahars descended the Las Lajas and El Jute ravines, carrying blocks as large as 3 m in diameter, tree trunks, branches, and other debris. Another lahar recorded on 28 August descended the Santa Teresa tributary of the Pantaleón River, where the residents noted the warm temperature of the debris.

Activity during September 2016. Two eruptive episodes took place during September 2016. Episode 12 began on 6 September and lasted about 48 hours. Moderate-to-strong explosions generated ash plumes that rose to 5 km altitude and drifted 25 km W and SW. Incandescent material rose to 300 m above the crater and fed two lava flows, one traveled 1.2 km down the Las Lajas ravine (figure 51), and the other travelled 500 m down the Taniluyá. The Washington VAAC reported an ash plume, identified in satellite imagery, on 7 September moving WSW at 4.9 km altitude. MODVOLC thermal alerts were issued during 4-8 September, with 10 alerts appearing on 8 September.

Figure 51. On 7 September 2016, the Operational Land Imager (OLI) on Landsat 8 captured this image of lava flowing down the Las Lajas and Santa Teresa ravines at Fuego during eruptive episode 12. The image is a composite of natural color (OLI bands 4-3-2) and shortwave Infrared (OLI band 7). Shortwave infrared light (SWIR) is invisible to the naked eye, but strong SWIR signals indicate increased temperatures. Courtesy of NASA Earth Observatory.

A bright hotspot in satellite imagery was reported by the Washington VAAC on 25 September 2016. A modest lahar descended the Santa Teresa ravine on 26 September, carrying 50-cm-diameter blocks, branches, and tree trunks; it was 10 m wide and 1 m high. Eruptive episode 13 began the next day, 27 September 2016, with moderate-to-strong explosions, and an ash plume that rose to 5 km altitude and drifted more than 20 km W and SW (figure 52). Incandescent material rose 300 m above the crater, feeding two lava flows. Lava traveled 1.5 km down the Las Lajas ravine (figure 53) and 1.8 km down the Santa Teresa ravine. A fissure developed on the S flank of the crater rim, and new fumarolic activity was observed during the day. Constant rumbling noises were audible in the areas of Finca Palo Verde, Sangre de Cristo, and San Pedro Yepocapa on the W and SW flanks. The Washington VAAC reported an intense hotspot in shortwave imagery. Activity subsided on 28 September. A strong multi-pixel thermal alert signal appeared in the MODVOLC data from 24-29 September.

Figure 52. An ash emission rises to 5 km altitude on 27 September 2016 at Fuego during eruptive episode 13. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Septiembre 2016).

Figure 53. Lava flows down the Las Lajas barranca (ravine) at Fuego on 28 September 2016 during eruptive episode 13. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Septiembre 2016).

Activity during October 2016. Six to ten explosions per day were recorded at Fuego during October 2016. Some of them generated ashfall on the SW flank. Episode 14, which began at the end of the month, produced three lava flows and strong explosions with an ash plume that rose to 7 km altitude and drifted N and NW. The Washington VAAC reported multiple ash emissions at 5.2 km altitude on 3 October, with the furthest one extending 35 km S. The next day, ash emissions were observed at 4.9 km altitude and drifted 22 km SSE. Pyroclastic flows were seen in an INSIVUMEH webcam on 10 October. They also reported ashfall in nearby communities that day.

Incandescent material rose 150-200 m above the summit crater on 28 October, and lava traveled 500 m down the Las Lajas ravine. Episode 14 began the next day with a strong explosion that generated an ash plume to 7 km altitude that drifted 110 km W and NW. Constant loud rumbling was reported up to 15 km from the volcano, and ashfall was reported in San Pedro Yepocapa, Sangre de Cristo, and La Conchita. Three incandescent lava fountains were seen in the early hours of 30 October (figure 54). The first, 450 m above the crater, fed a 2-km-long flow in the Las Lajas ravine. The second fountain rose to 250 m and fed a flow that traveled 300 m down the Santa Teresa canyon. The third fountain rose 200 m and formed a flow that traveled down the Taniluyá drainage for 500 m. Activity declined during the night of 30 October, but weak and moderate avalanches of incandescent material continued into the first part of the next day.

Figure 54. Three Strombolian fountains at Fuego feed three lava flows on 30 October 2016 during eruptive episode 14. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Octubre 2016).

The first ash emissions of episode 14 were visible in satellite imagery on 29 October, extending roughly 45 km NNW from the summit. By early the next day, the ash emissions were detected at 7.3 km altitude, based on a pilot report. They extended about 110 km NNW from the summit. Later in the day, the plume had lowered to 5 km altitude and drifted 15 km N and NW. A single MODVOLC thermal alert was reported on 13 October, but a lengthy series of multi-pixel alerts were generated during 24-31 October, including 19 pixels on 30 October at the peak of episode 14.

Activity during November 2016. Activity during November 2016 remained at background levels until the third week of the month; explosions increased in amplitude and frequency to as many as 15 per hour, leading to episode 15 which began on 20 November. The background levels of the second and third weeks included incandescent material rising to 300 m above the crater, causing avalanches down the flanks around the crater rim and continuous explosions of weak-to-moderate energy that generated ash plumes rising to altitudes between 4.3 and 4.7 km that drifted W and SW.

The Washington VAAC reported ash emissions in satellite imagery every day from 8 to15 November 2016. A plume was seen on 8 November rising to 4.6 km altitude and drifting 25 km SW. The next day, a plume at the same altitude drifted 45 km NW. On 10 November, a faint plume was seen in visible imagery, extending about 25 km NNW. A larger plume was visible in morning imagery on 11 November at 5.5 km altitude extending 35 km WSW. The next day, at the same altitude, a diffuse plume was visible 10 km W of the summit. Multiple emissions were spotted drifting W from the summit at 4.6-4.9 km altitude on 13 and 14 November. Two single MODVOLC thermal alerts were reported on 12 and 14 November. A hotspot was detected in satellite imagery on 15 November, along with continuing emissions to 4.6 km altitude that drifted within 10 km SW of the summit. On 17 November ashfall was reported in Morelia, Santa Sofia, and Panimache I and II. Emissions on 18 November rose to 4.7 km altitude and drifted 10 km SW, and on 20 November they rose to 4.9 km altitude and drifted 24 km from the summit (figure 55).

Figure 55. Strombolian eruption and ash emission at Fuego, looking S from Acatenango summit on the early morning of 18 November 2016. Photo copyright by Martin Rietze, used with permission.

Eruptive episode 15 began on 20 November with strong explosions that caused ash plumes to rise to 5 km altitude and drift as far as 175 km SSW and W, generating ashfall again in Morelia, Santa Sofia, and Panimache I and II. Three lava flows emerged from the 300-m-high Strombolian ejections; one traveled 1 km down the Trinidad ravine, one descended 2 km down barranca Ceniza, and the third flowed 2.5 km down barranca Las Lajas (figure 56). Numerous clouds of volcanic ash rose from block avalanches in Las Ceniza ravine on 20 and 21 November.

Figure 56. Eruptive episode 15 at Fuego occurred during 20-21 November 2016. An ash plume rose to 5 km altitude (top) before Strombolian activity 300 m high sent flows down three major ravines (bottom). These views from the rooftop of a hostel in Antigua (18 km NE) show the ravines in daylight during the afternoon of 20 November, and again around midnight that night as the incandescent material traveled downward. Photos copyright by Martin Rietze, used with permission.

The Washington VAAC reported an extremely large hotspot on 20 November 2016 (local time) in infrared imagery, along with ash emissions at 4.9 km altitude drifting SW to 65 km. Emissions at 3.8 km persisted into the night. By early morning on 21 November, they were visible extending 175 km W. A lengthy period of multi-pixel MODVOLC thermal alerts coincided with eruptive episode 15 during 17-23 November, and included 26 pixels on 21 November 2016. Eruptive activity decreased to background levels by 22 November, and only weak explosions and fumarolic activity were reported for the rest of the month.

Activity during December 2016. Weak-to-moderate explosions and ash plumes characterized background activity at Fuego during December 2016. Minor ashfall was regularly reported in communities located 8-12 km SW. Activity increased somewhat during 15-16 December, and eruptive episode 16 was recorded during 20-21 December. During episode 16, Strombolian activity created three lava flows that descended major ravines, and a large pyroclastic flow traveled 3.5 km from the summit, burning vegetation in its path.

The Washington VAAC reported an ash emission on 5 December at 5.8 km altitude drifting N. On 8 December, intermittent ash plumes were drifting W over the East Pacific Ocean at 6.1 km altitude. Remnants over 450 km W were seen in multispectral imagery by early on 9 December. Multiple new detached plumes continued moving WNW between 5.5 and 6.1 km altitude on 9 December. They were 80 km NW by late afternoon. New discrete emissions at 4.6 km altitude appeared in satellite imagery on 10 December, drifting W up to 130 km before dissipating.

During the afternoon of 20 December 2016, eruptive episode 16 began with moderate-to-strong ash emissions producing an ash plume that rose to 4.7 km altitude and drifted more than 15 km W and SW. Incandescent material rose 400 m above the crater, and bombs fell more than 300 m away. Block avalanches were concentrated in the Ceniza and Trinidad ravines. By the evening of 20 December, three lava flows had formed, in the Santa Teresa, the Taniluyá, and the Las Lajas ravines (figure 57). By the morning of 21 December, they were 2.5, 2.0, and 1.8 km long, respectively. During that day, strong explosions generated ash plumes that rose to 5.2 km altitude and drifted 18 km S, SW, W, and NW. Some of the communities that reported ash from this event included Panimaché, Morelia, Santa Sofia, Sangre de Cristo, San Pedro Yepocapa, and Palo Verde.

Figure 57. Landsat image showing the locations of three lava flows at Fuego during eruptive episode 16 on 21 December 2016. Image courtesy of USGS/NASA, annotations courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Diciembre 2016).

Around 1000 on 21 December, pyroclastic flows that descended the Taniluyá ravine generated an ash plume that rose 2 km and drifted W and SW. The flows traveled 3.5 km and were estimated to be 300 m wide. They descended the ravine at high speed and high temperature, burning everything in their path (figure 58). These were the first pyroclastic flows in several months. By the end of eruptive episode 16, the lava flow in the Taniluyá ravine had reached 2.8 km in length (figure 59).

Figure 58. A pyroclastic flow descends the Taniluyá ravine around 1000 local time on 21 December 2016 at Fuego during eruptive episode 2016. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Diciembre 2016).

Figure 59. Both a pyroclastic flow (3.5 km long, yellow outline) and a lava flow (2.8 km long, red outline) descended the Taniluyá ravine at Fuego during eruptive episode 16, from 20 to 21 December 2016. The white arrows indicate the ravine. The orange outline indicates the area where vegetation was destroyed by the pyroclastic flows. Courtesy of INSIVUMEH (Informe Mensual De La Actividad Del Volcán Fuego, Diciembre 2016).

During episode 16, the Washington VAAC reported ash emissions rising to 5.2 km (about 1.4 km above the summit crater) altitude and drifting about 230 km SW. Continuing ash emissions on 23 December were visible in satellite imagery moving 45 km SW from the summit at 4.3 km altitude. Intermittent diffuse ash emissions extended up to 30 km WSW and NW from the summit during 28-31 December at 4.3-5.2 km altitude.

MODVOLC thermal alerts were intermittent throughout December. They were recorded on 8 (2), 11 (2), 12, 14 (2), 16 (3), and 18 (3) December prior to episode 16. The biggest interval of multi-pixel alerts was during episode 16 from 20-22 December, and included 14 alerts on 21 December 2016. Additional single alerts were recorded on 25 and 29 December.

Geologic Background. Volcán Fuego, one of Central America's most active volcanoes, is also one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. The scarp of an older edifice, Meseta, lies between Fuego and Acatenango to the north. Construction of Meseta dates back to about 230,000 years and continued until the late Pleistocene or early Holocene. Collapse of Meseta may have produced the massive Escuintla debris-avalanche deposit, which extends about 50 km onto the Pacific coastal plain. Growth of the modern Fuego volcano followed, continuing the southward migration of volcanism that began at the mostly andesitic Acatenango. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks. Frequent vigorous historical eruptions have been recorded since the onset of the Spanish era in 1524, and have produced major ashfalls, along with occasional pyroclastic flows and lava flows.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); 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: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); Martin Rietze (URL: http://www.mrietze.com/index.htm).

The remote island of Heard in the southern Indian Ocean is home to the Big Ben stratovolcano, which has had confirmed intermittent activity since 1910. The nearest continental landmass, Antarctica, lies over 1,000 km S. Visual confirmation of lava flows on Heard are rare; thermal anomalies detected by satellite-based instruments provide the most reliable information about eruptive activity. Thermal alerts reappeared in September 2012 after a four-year hiatus (BGVN 38:01), and have been intermittent since that time. Information comes primarily from MODVOLC and MIROVA thermal anomaly data, but Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) also provides reports from research expeditions. The independent March-April 2016 Cordell Expedition also provided recent ground-based observations mentioned in this report, which covers activity through September 2017.

Expeditions during January-April 2016. Scientists aboard the CSIRO Research Vessel Investigator observed an eruption of Big Ben on 31 January 2016. Vapor was seen emanating from the peak and lava flowed down the flank over a glacier (see figure 23, BGVN 41:08, and video link in Information Contacts). The research team, lead by the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS), was conducting a study of the link between active volcanoes on the seafloor and the mobilization of iron by hydrothermal systems which enriches and supports life in the Southern Ocean.

During a private expedition from 22 March to 11 April 2016, scientists and engineers from the 2016 Cordell Expedition documented changes to the island and its life since a prior visit in 1997, and tested radio operations. On 23 March the team was able to photograph the usually cloud-covered Mawson Peak, the summit of Big Ben (figure 24). Steam was visible above the flat upper surface, possibly a crater rim or fissure. They estimated a height of about 45 m of an edifice rising above the adjacent slope. The ground at the site of the team campsite, near Atlas Cove on the NW side of the island, was covered with lava flows (figure 25). While the expedition had to cancel a planned expedition to the summit, rocks collected from the shoreline confirmed the diversity of volcanic rocks on the island (figure 26).

Figure 24. Mawson Peak is the summit of Big Ben volcano on Heard Island in the southern Indian Ocean. This photograph, taken on 23 March 2016 from Altas Cove on the NW side of the island by the 2016 Cordell Expedition, shows steam from a possible crater or vent area at the summit, and lava flows covered with a dusting of snow around the otherwise glacier-covered peak. Courtesy of Robert W. Schmieder, 2016 Cordell Expedition, used with permission.

Figure 25. Lava flows cover the ground near the 2016 Cordell Expedition campsite at Atlas Cove on the NW side of Heard Island in March 2016. Courtesy of Robert W. Schmieder, 2016 Cordell Expedition, used with permission.

Figure 26. Rock samples collected at Heard by the 2016 Cordell Expedition during 23 March-11 April 2016 attest to the volcanic activity of the island. Top: A conglomerate sampled from the east shore of Stephenson Lagoon with mostly volcanic rock fragments, including vesicular basalt (dark brown, lower center) and clasts of volcanic breccia containing fragments of lava (large clast on right side). Sample is about 25 cm long. Bottom: A variety of textures was typical in the volcanic rocks collected on the islands. Courtesy of Robert W. Schmieder, 2016 Cordell Expedition, used with permission.

At the southern end of Sydney Cove, near Magnet Point on the northern tip of Laurens Peninsula (the NW side of the island), the team identified a small islet, with dimensions of about 40 x 120 m and nearly vertical sides about 100 m high. Columnar jointing in the volcanic rocks is well exposed at the base and on the nearly flat upper surface (figure 27).

Figure 27. Distinctive columnar jointing in the volcanic rocks is visible around the base and on the top of a small islet in Sydney Cove off the NW end of Heard Island in this image taken during the 23 March-11 April 2016 Cordell Expedition. Courtesy of Robert W. Schmieder, 2016 Cordell Expedition, used with permission.

Satellite thermal and visual data, 2012-2017. The most consistent source of information about eruptive activity at Heard comes from satellite instruments in the form of visual and thermal imagery, and thermal anomaly detection. From the time that renewed activity was detected in MODVOLC data in late September 2012 through September 2017, either the MODVOLC or MIROVA systems have consistently detected thermal signals, with only a few short breaks. A four-month span from mid-July to mid-November 2014, and a two-month gap during February and March 2015 are the only periods longer than a month when no thermal signal was reported. Continuous MIROVA information from late January 2016 through September 2017 shows intermittent but persistent thermal anomalies throughout the period (figure 28).

Figure 28. A continuous MIROVA signal from 27 January 2016 through 6 October 2017 shows persistent low-level thermal activity through the period with intervals of increased activity during late January 2016, July-August 2016, late September-November 2016, early February 2017, and September 2017. Courtesy of MIROVA.

The moderate signal at the very end of January 2016 coincides with the CSIRO expedition observing the lava flows on the flank of Big Ben. Low-level MIROVA anomalies were recorded in April and early May 2016. Activity picked up during June, and strengthened through July and August 2016. Late September through November 2016 was a period with heightened activity as well. From December 2016 through August 2017, intermittent low-to-moderate intensity anomalies were recorded every month. Activity appeared to increase briefly during early February and September 2017. On 4 February 2017, Landsat 8 captured a rare clear view that showed fresh lava and debris flows emanating from the summit on top of the snow (figure 29). The longest flow is estimated to be 1,300 m long. False-color infrared imagery of the same image of Mawson Peak also reveals two vents separated by about 250 m (figure 30). Subsequent imagery on 20 and 27 February also detected thermal anomalies at the summit. The visual imagery of the lava flows on 4 February 2017 corresponds to the early February spike in MIROVA thermal anomaly data.

Figure 29. Lava and debris flows radiate away from Mawson Peak on Heard Island in this Landsat 8 OLI image captured on 4 February 2017. MIROVA thermal anomaly data show a spike in activity at the same time. Courtesy of NASA and Bill Mitchell (CC-BY).

Figure 30. False-color infrared imagery of Mawson Peak, Heard Island, 4 February 2017. Two vents are visible in red-yellow, separated by about 250 m. Data source: Landsat 8 OLI/TIRS bands 7-6-5. Image courtesy of Bill Mitchell (CC-BY), data from NASA/USGS.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben because of its extensive ice cover. The historically active Mawson Peak forms the island's high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported at this isolated volcano, but observations are infrequent and additional activity may have occurred.

Information Contacts: Commonwealth Scientific and Industrial Research Organisation (CSIRO) (URL: http://www.csiro.au/); CSIROscope, CSIRO Blog, Big Ben Erupts: Australia's active volcano cluster blows its lid (URL: https://blog.csiro.au/big-ben-erupts/); Robert W. Schmieder, 2016 Cordell Expedition, 4295 Walnut Blvd., Walnut Creek, CA 94596, Post Expedition report to the Australian Antarctic Division (AAD) (URL: http://www.cordell.org/, http://www.heardisland.org/); 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/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); Bill Mitchell, The Inquisitive Rockhopper, Big Ben eruption update 2017-02-27 (URL: https://inquisitiverockhopper.wordpress.com/2017/02/).

During March 2014-March 2017, activity at Ibu consisted of lava-dome growth, occasional weak emissions containing ash (figure 11), and frequent thermal anomalies (BGVN 40:11 and 42:05). Ongoing activity between April and August 2017 consisted primarily of intermittent ash explosions. Data come from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC).

Figure 11. Photo of an ash explosion from Ibu's central vent in November 2014. Courtesy of Tom Pfeiffer, Volcano Discovery.

On 3 April 2017, at 0757 (local), an explosion produced an ash plume that rose to an altitude of 1.7 km and drifted S. Seismic signals indicated explosions and avalanches. During the rest of April through August, occasional explosions generated weak ash plumes that generally rose to altitudes of 1.5-1.8 km (0.2-0.5 km above the volcano) and drifted in various directions (table 2).

Table 2. Ash plume data for Ibu, April-August 2017. Courtesy of PVMBG and Darwin VAAC.

Between April and August 2017, thermal anomalies (based on MODIS satellite instruments analyzed using the MODVOLC algorithm) were recorded 2-5 days per month, with no monthly trend. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected numerous hotspots each month; all except one were within 3 km of the volcano, and all were of low or moderately-low power.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); 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/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/).

Recent activity at the large Gunung Marapi stratovolcano on Sumatra has consisted of small ash plumes, with eruptions of a single day to periods of a few months. Ashfall around the active crater rim (figure 5) and thin layers of ash deposits seen in the crater wall (figure 6) provide evidence of both the recent and very long history of explosive activity. Since 2011 there have been eruptive episodes during August-October 2011, March-May 2012, 26 September 2012, February 2014, and 14 November 2015. As reported by the Indonesian Center of Volcanology and Geological Hazard Mitigation (PVMBG), another series of explosions took place on 4 June 2017.

Figure 5. Photo taken at the rim of the active Verbeek Crater at Marapi on 17 April 2014. The most recent eruption prior to this photo was during 3-26 February 2014. Courtesy of Axel Drainville.

Figure 6. Photo showing the rim and interior wall of the Verbeek Crater at Marapi on 17 April 2014. Courtesy of Axel Drainville.

Four explosions on 4 June lasted less than one minute each, and generated ash plumes above the summit (figure 7) and drifted E. The explosions occurred at 1001 local time (0301 UTC), 1011, 1256, and 1550. Dense ash-and-steam plumes from each explosion rose 300 m, at least 700 m, 200 m, and 250 m above the crater, respectively. The Darwin VAAC reported ash at about 3.6 km altitude extending 37 km ENE, based on satellite imagery. Ejected bombs were deposited around the crater, and minor ashfall was reported in the Pariangan District (8 km SSE), Tanah Datar Regency. Seismicity increased after the explosions. The Alert Level remained at 2 (on a scale of 1-4); residents and visitors were advised not to enter an area within 3 km of the summit.

Figure 7. Photos of ash plumes rising from Marapi on 4 June 2017. The upper right image appears to show a smaller white plume to the right. Photos by PVMBG, posted to Twitter by Sutopo Purwo Nugroho (BNPB).

The broad summit area with multiple craters is a popular destination for hiking expeditions. A video posted by YouTube user "SiGiTZ" documented the experience of one group during visits on 30 April 2016 and on 11 May 2017. The video provides excellent views from 2016 of the entire crater complex and of the Verbeek Crater, from which a steam-and-gas plume appears to be rising. A video posted by YouTube user "yogi antula" included a television broadcast from the Anak Borneo Channel of a video from climbers in the crater area during the 4 June explosions, taken from approximately 400-500 m away. In that video, a significant dark ash plume can be seen rising from Bungsu-Verbeek crater complex, along with a smaller white plume from a closer location. The news report was concerned with 16 hikers known to be on the mountain; there were no later reports of anyone being injured.

References: SiGiTZ, 1 August 2017, Expedisi puncak Gunung Marapi Bukittingi Sumbar Mei 2017 (URL: https://www.youtube.com/watch?v=pVxhWAbo2VA).

yogi antula, 5 June 2017, Video amatir pendakian saat Gunung Marapi Erupsi – 4 Juni 2017 (by Anak Borneo Channel) (URL: https://www.youtube.com/watch?v=8GAY6lsTLEE).

Geologic Background. Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra's most active volcano. This massive complex stratovolcano rises 2,000 m above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in historical time.

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/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); Sutopo Purwo Nugroho, Badan Nasional Penanggulangan Bencana (BNPB) (URL: https://twitter.com/Sutopo_BNPB); Axel Drainville, Flickr.com, with Creative Commons license Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0, https://creativecommons.org/licenses/by-nc/2.0/) (URL: https://www.flickr.com/photos/axelrd/).

The most recent eruption began on 27 November 2012 along two fissures a few kilometers S of the main Tolbachik edifice, within the Tolbachinsky Dol lava plateau (BGVN 37:12). Monitoring is done by the Kamchatkan Volcanic Eruption Response Team (KVERT); they recorded an end date for this eruption as 15 September 2013.

Activity reported through February 2013 included Strombolian fire fountains (figure 14), voluminous lava flows on the surface (figure 15 and 16) and under the ice and snow cover (figure 17), ash explosions, and the building of cinder cones (BGVN 37:12). Satellite imagery in early June 2013 revealed both a lava pond at the active vent and a large lava flow lower down the flank, with multiple flow-front breakouts (figure 18). Cinder cones continued to grow along the S fissure through 16-22 August 2013, and lava flows remained active (figure 19), but then gas-and-ash plumes weakened and seismicity decreased during the last week of the month (BGVN 38:08).

Figure 14. Lava fountain in the cinder cone at Tolbachik on 24 January 2013. Photo by Yu. Demyanchuk; courtesy of IVS FEB RAS and KVERT.

Figure 15. Photo of lava flow at Tolbachik on 25 January 2013. Photo by Yu. Demyanchuk; courtesy of IVS FEB RAS and KVERT.

Figure 16. Lava flows moving ESE at Tolbachik on 25 February 2013. Photo by Yu. Demyanchuk; courtesy of IVS FEB RAS and KVERT.

Figure 17. Photo of a lava flow intruding under deep snow at Tolbachik on 25 February 2013. Photo by Yu. Demyanchuk; courtesy of IVS FEB RAS and KVERT.

Figure 18. False-color image of Tolbachinsky in shortwave infrared and near-infrared light (combined with green light), taken on 6 June 2013 by the Advanced Land Imager on the Earth Observing-1 satellite. Hot surfaces glow in shortwave infrared wavelengths. The active vent and lava flow are bright red, along with scattered lava "breakouts"at the front of the flow. High temperature surfaces in the scene also glow in near infrared light, revealing a lava pond in the active vent and fluid lava in the center of the lava flow. Courtesy of NASA (image by Jesse Allen and Robert Simmon, caption by Robert Simmon).

Figure 19. Photo of lava flow front adjacent to the Kruglenkaya slag cone at Tolbachik on 16 August 2013. Photo by D.V. Melnikov; courtesy of IVS FEB RAS and KVERT.

Seismicity continued to decrease during 22-24 August 2013, and KVERT noted on 27 August that no incandescence had been seen in recent days, and there were no current ash plumes. Satellite data did still show a large thermal anomaly in the northern area of Tolbachinsky Dol, which KVERT attributed to the lava flows remaining hot. The MODIS thermal anomaly data recorded in the MODVOLC system identified the latest hotspot on 27 August 2013. According to the Kamchatkan Volcanic Eruption Response Team (KVERT), the Aviation Color Code (ACC) was lowered from Orange to Yellow on 27 August 2013.

When the ACC was lowered to Green on 31 January 2014, KVERT reported that weak seismic activity and episodes of tremor continued, gas-and-steam activity was sometimes observed, and satellite data continued to show a weak thermal anomaly. However, they also stated that "probably its active phase was finishing in September 2013." The KVERT website recorded an end date of 15 September 2013. The new lava flows were still noticeable in visible satellite imagery more than a year after the eruption ended (figure 20).

Figure 20. Satellite image from Landsat/Copernicus showing the final extent of new lava flows on the SSW flank of Tolbachik on 30 December 2014. The new lava flows extend across the center of the image, with the main edifice at top right. Color and contrast have been adjusted to enhance the contrast between fresh darker lava and faded older deposits. Courtesy of Google Earth.

Geologic Background. The massive Tolbachik basaltic volcano is located at the southern end of the dominantly andesitic Kliuchevskaya volcano group. The massif is composed of two overlapping, but morphologically dissimilar volcanoes. The flat-topped Plosky Tolbachik shield volcano with its nested Holocene Hawaiian-type calderas up to 3 km in diameter is located east of the older and higher sharp-topped Ostry Tolbachik stratovolcano. The summit caldera at Plosky Tolbachik was formed in association with major lava effusion about 6500 years ago and simultaneously with a major southward-directed sector collapse of Ostry Tolbachik volcano. Lengthy rift zones extending NE and SSW of the volcano have erupted voluminous basaltic lava flows during the Holocene, with activity during the past two thousand years being confined to the narrow axial zone of the rifts. The 1975-76 eruption originating from the SSW-flank fissure system and the summit was the largest historical basaltic eruption in Kamchatka.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); 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/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); Google Earth (URL: https://www.google.com/earth/).

Ubinas is an active stratovolcano in southern Peru about 70 km E of the city of Arequipa. Holocene lava flows cover its flanks, and the historical record since the mid-1500's contains evidence of minor explosive eruptions, debris avalanches, tephra deposits, phreatic outbursts, and pyroclastic flows and lahars. An eruptive episode that began with phreatic explosions on 1 September 2013 lasted through 27 February 2016, producing numerous small ash emissions, several large explosions with ash plumes that rose above 10 km altitude, large SO₂ anomalies, evacuations, and several millimeters of ashfall in surrounding villages. Significant MIROVA thermal anomalies first appeared in mid-June 2015 and persisted through January 2016. A smaller eruptive episode described below began on 13 September 2016 and continued with intermittent explosive activity through 2 March 2017. Information is provided by the Instituto Geofísico del Perú, Observatoria Vulcanologico del Sur (IGP-OVS), the Observatorio Volcanológico del INGEMMET (Instituto Geológical Minero y Metalúrgico) (OVI-INGEMMET), and the Buenos Aires VAAC (Volcanic Ash Advisory Center).

After activity subsided at the end of February 2016, Ubinas remained quiet through August 2016, with only sporadic steam and gas emissions, and very low levels of seismicity. Seismicity increased again beginning on 9 September, and the first ash emission of a new episode was reported on 13 September 2016. An explosion on 3 October released a significant ash plume that rose 2 km above the 5,672-m-summit. Four additional explosions with minor ash emissions were reported in November, and one occurred on 6 December. Webcams captured images of sporadic low-density ash emissions throughout February 2017, with the last report of possible emissions on 2 March 2017. Emissions of steam and gas and seismicity decreased throughout April 2017, and IGP-OVS lowered the alert level to Green by the end of May. Ubinas remained quiet through September 2017.

Activity during April-December 2016. After the small ash emission of 27 February 2016, seismicity at Ubinas dropped to very low levels of a few events per day (BGVN 41:10, figure 40). Sporadic steam emissions with small quantities of bluish magmatic gases rose no more than a few hundred meters above the summit during March-August 2016; there were no reports of ash emissions. A small seismic swarm of about 100 earthquakes was recorded on 5 April. The first "tornillo" type earthquakes seen in several months appeared beginning on 4 June, indicating to IGP-OVS the beginning of a new eruptive cycle. The lagoon that had formed at the bottom of the summit crater due to rains earlier in the year began to disappear as the dry season approached (figure 41).

Figure 41. A view down into the steep-sided summit crater at Ubinas shows remnants of a disappearing lake after the rainy season, during the second quarter of 2016. Photo by Melquiades Álvarez; courtesy of OVS (Reporte Annual Volcan Ubinas, 2016).

Beginning on 9 September 2016, both OVI and OVS noted an increase in seismic activity of LP, hybrid, and VT-type events (figure 42). On 13 September, OVS reported that steam plumes rose higher than 1,000 m above the summit for the first time in many months, and a minor ash emission was observed. OVI reported possible ash emissions in weekly reports on 12, 17, and 24 September. Emissions of bluish gas and steam were typical for the remainder of September (figure 43).

Figure 42. An increase in several types of seismicity at Ubinas first appeared on 9 September 2016 after several months of quiet. This was followed by an ash emission on 13 September, and an explosion with ash on 3 October. Courtesy of IGP-OVS (Reporte N°31-2016, Actividad del volcán Ubinas, Resumen actualizado de la principal actividad observada del 01 al 18 de octubre).

Figure 43. Bluish SO2-rich gas and steam emissions increased in frequency during the second half of September 2016 at Ubinas, as seen in this image taken from the village of Ubinas on 27 September 2016 by Melquiades Álvarez. Courtesy of IGP-OVS (Reporte Annual Volcan Ubinas, 2016).

Both OVI and OVS reported ash emissions from explosions on 3 October 2016 (figure 44). Seismic tremor, associated with ash emissions, lasted for nine and a half hours. The ash plume drifted NE, E, SE, and SW up to 2 km above the summit, according to OVS. Fumarolic activity then returned, with steam and bluish gases rising no more than 1,500 m above the crater rim for the remainder of October. The Buenos Aires VAAC noted the eruption reported by IGP, but was not able to identify volcanic ash from satellite data under clear skies. After peaking in early October at several hundred events per day, seismicity declined to below 50 events on 21 October, and then rose slightly to around 200 events per day for the rest of the month. Steam and gas emissions remained less than 500 m above the summit.

Figure 44. An explosion at Ubinas on 3 October 2016 created a significant ash plume that rose 2,000 m above the crater rim, and drifted NE, E, SE, and SW. Photos by Melquiades Álvarez, courtesy of IGP-OVS (Reporte Annual Volcan Ubinas, 2016).

Three explosions with minor ash and gas (mostly SO₂) were reported by IGP-OVS on 8 November (local time). NASA Goddard Space Flight Center reported a significant SO₂ emission associated with this event. The ash plume rose to about 1,500 m above the crater rim (about 7.2 km altitude). Seismicity remained high, with 250-350 events per day for several days after the explosion before declining back to around 150 events per day by 15 November. Another explosion, with minor ash emissions that rose 500 m, was reported by both OVS and OVI on 17 November 2016. After a small spike in seismicity between 23 and 29 November, the number of seismic events dropped below 50 per day. OVS reported a small ash emission that rose 100 m above the summit and drifted NW on 6 December 2016. OVI noted a modest increase in seismicity between 6 and 15 December, but only sporadic emissions of water vapor and gas were detected for the remainder of the month.

Activity during January-September 2017. Gas and steam emissions remained below 500 m above the crater rim during January 2017. OVS reported an explosion at 0223 on 24 January, but could not confirm ash emissions due to darkness. Occasional emissions of steam and gas rose as high at 2 km above the summit crater, but they generally remained below 500 m. OVI observed five lahars during January, but no damage was reported. Seismicity remained below 60 events per day during the month, except for a few days during 8-12 January when the frequency increased to 100-150 events per day.

OVS reported sporadic low-density ash emissions throughout February 2017 (figure 45). They were accompanied, occasionally, by water vapor and bluish gas, and did not rise more than 1,500 m above the summit crater. Weather clouds obscured the summit for much of the month. OVI reported minor ash emissions on 4, 10, 14, and 18 February (figure 46). Seismicity fluctuated throughout the month from values as high as almost 70 events per day (8 February) to fewer than 10 events per day (10-19 February).

Figure 45. Sporadic emissions of ash along with steam and magmatic gases were recorded in the IGP-OVS webcams at Ubinas on 4 and 9 February 2017. Courtesy of IGP-OVS (Reporte 03-2017 - Actividad del volcán Ubinas, Resumen actualizado de la principal actividad observada del 01 al 15 de febrero de 2017).

Figure 46. The OVI webcam captured a clear image of the 4 February 2017 ash emission. Courtesy of OVI (Reporte Semanal de Monitoreo: Volcan Ubinas, Reporte 06, Semana del 30 de enero al 05 febrero de 2017).

OVS reported only magmatic gas and steam emissions (with no ash) during March 2017, with plumes rising to a maximum height of 300 m above the summit crater. OVI noted possible diffuse ash emissions on 1 and 2 March, but only steam and gas emissions for the remainder of the month. They reported variable seismicity with the frequency of daily events ranging from less than 10 per day to almost 70, averaging about 30 events per day.

Seismic energy decreased significantly during April 2017. Sporadic steam emissions reached maximum heights of only a few hundred meters above the crater. This relative quiet enabled OVS scientist Melquiades Álvarez to make a brief inspection of the summit crater on 14 April where he observed intermittent steam emissions rising from the base of the summit crater (figure 47). No ash emissions were reported during April. OVI reported that the number of seismic events dropped consistently during April from a high of 20 daily events on 1 April, to fewer than 5 events per day at the end of the month.

Figure 47. A view into the summit crater at Ubinas on 14 April 2017 revealed only sporadic steam emissions. Photo by Melquiades Álvarez; courtesy of IGP-OVS (Reporte 07-2017-Actividad del volcán Ubinas, Resumen actualizado de la principal actividad observada del 01 al 15 de abril de 2017).

The reduction in activity continued during May 2017; steam and gas emissions became more sporadic and were rarely reported rising above 500 m over the summit crater. IGP-OVS reduced the alert level from Yellow to Green (2 to 1 on a 4-level scale) during the second half of the month. Seismicity reported by OVI fluctuated between 2 and 14 daily events. Ubinas remained quiet from June through September 2017, with only occasional minor fumarolic activity of steam or magmatic gas plumes that rose a few hundred meters above the summit crater (figure 48). Frequency of seismic events remained below 20 events per day through August and dropped to less than 10 per day in September.

Figure 48. Virtually no emissions of any kind were reported from Ubinas after mid-July 2017, as seen in this image from the second half of August 2017. Courtesy of IGP-OVS (Reporte 16-2017-Actividad del volcán Ubinas, Resumen actualizado de la principal actividad observada del 16 al 31 de agosto de 2017).

Geologic Background. A small, 1.4-km-wide caldera cuts the top of Ubinas, Perú's most active volcano, giving it a truncated appearance. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45 degrees. The steep-walled, 150-m-deep summit caldera contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one of Holocene age about 1,000 years ago. Holocene lava flows are visible on the flanks, but historical activity, documented since the 16th century, has consisted of intermittent minor-to-moderate explosive eruptions.

Information Contacts: Instituto Geofisico del Peru, Observatoria Vulcanologico del Sur (IGP-OVS), Arequipa Regional Office, Urb La Marina B-19, Cayma, Arequipa, Peru (URL: http://ovs.igp.gob.pe/); Observatorio Volcanologico del INGEMMET, (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa (URL: http://ovi.ingemmet.gob.pe); 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?lang=es).

A previous report on Wrangell noted that the heat flux from a crater on the N side of the summit rim had increased by an order of magnitude between 1964 and 1986 (SEAN 11:04). Wrangell has several active fumarolic areas in its summit caldera. These fumaroles frequently produce steam plumes that are mistaken for eruptive activity. The Alaska Volcano Observatory (AVO) receives several reports per year from pilots and local residents who observe larger than normal steam clouds over the summit. Although there have been some events possibly involving wind-blown ash, there have been no recent confirmed eruptions.

Activity during 1996-2000. According to Neal and McGimsey (1997), a pilot reported a suspicious cloud around 18 January 1996 rising about 1.5 km near Wrangell. The National Weather Service (NWS) confirmed that a robust steam plume had been visible over the volcano for several weeks.

McGimsey and Wallace (1999) reported that, on 3 June 1997, a pilot reported steam rising from the summit. On 24 June another report described a steam plume rising about 200 m above the summit. This sighting was not observed on satellite imagery.

McGimsey and others (2004) reported that on the morning of 14 May 1999, a NWS observer in Gulkana (about 75 km WNW) reported anomalous steam emissions containing a small amount of ash. During clear weather at approximately 0930 local time, a rapidly billowing grayish-white plume rose to about 900 m above the N summit crater. The observer stated that at this time of year, on clear days, a small, wispy, steam plume is usually visible above Wrangell in the early morning, and dissipates by early afternoon. On this day, the plume developed quickly, was abnormally voluminous, and had a grayish color.

A pilot had also observed the activity and noticed that more "dirt" surrounded the N crater than usual, and that on the upper part of the Chestnina Glacier high on the SW flank, blocks of ice were chaotically jumbled (higher relief between blocks) and that the glacier surface was much more crevassed than he had ever previously seen. He also observed that one of two known fumaroles at 3,350 m elevation on the S flank, which typically issue steam through ice holes, was surrounded by a sizeable patch of bare rock, a new development since his last recent flight over the area. The pilot further reported that he had observed no sign of flowage or melting events high on the flank, but that he had not flown over the lower reaches of the glacier. As of 1700 that day the NWS observer in Gulkana could still see a small steam plume and with binoculars could see that the snow around the summit area appeared to be light gray and that this was a definite color contrast and not an effect from shadows.

According to Neal and others (2004), a Trans Alaska Pipeline worker reported an unusually strong, white steam plume on 18 March 2000 between 0500 and 0600 local time. Later that day a National Park Service (NPS) employee in Kenny Lake reported robust steaming during the previous month from multiple sources on the SW flank between approximately 600-1,500 m below the summit. AVO found no anomalies in satellite imagery and concluded that no significant unrest had occurred.

Activity during 2002-2003. Neal and others (2005) reported that on 1 August 2002, AVO received several calls reporting a dark cloud drifting downwind from the general summit area and a dark deposit high on its snow-covered flank. AVO seismologists, however, checked data from the Wrangell seismic network and, based on a lack of correlative seismicity, concluded that no eruption or explosion had occurred. AVO also consulted with a local NPS geologist, who suggested that high winds had lofted fine-grained material exposed in the area near the summit fumaroles. On 4 August, an AVO geologist traveling in the area verified that a diffuse, light gray stripe extended a short distance down the flank of the volcano, emanating from the W caldera rim.

Subsequently, a local resident presented AVO with a video showing the waning portion of the event and his written observations. The witness described multiple dark billowing black ash puffs; the wind was from the E and the puffs were not rising over the summit. By the time he had returned to a good vantage point to film, about 10-12 minutes later, the billowing had stopped and the puffs had "turned a more grayish color."

According to the authors, the video showed discrete, light gray "puffs" that moved downwind and retained their individual integrity. There were no other weather clouds in the vicinity. A light gray, relatively motionless and irregular-shaped cloud sat near the caldera rim. A breeze could be observed at ground level (indicated by motion in the trees) but at altitude, clouds were not shearing rapidly. High on the snow-covered flank, a gray-colored swath extended from a high point at the W caldera rim near Wrangell's crater. The end of the video footage showed two distinct dark areas on the rim that were normally snow-covered. The resident's son reported a similar but more vigorous event on 2 August at about the same time of the day, but AVO received no further inquiries or reports.

AVO concluded that no volcanic process of significance had occurred. However, the authors stated "these observations remain enigmatic: lack of any seismicity would seem to preclude a phreatic or magmatic eruption and yet the pulsatory, 'puffing' nature of the dirty clouds is difficult to reconcile with a wind phenomenon."

McGimsey and others (2005) reported that NPS geologist Danny Rosenkrans contacted AVO with photographs taken by a local resident on 11 June 2003 showing an unusual towering cloud over the summit. Although the authors acknowledged that it could simply have been a common cumulus cloud, they noted that the absence of cumulus clouds in the area over nearby Mts. Drum and Sanford suggested that calm weather conditions permitted steam emissions from the known summit fumaroles to coalesce and form the plume-like cloud.

McGimsey and others (2005) also reported that on 18 September 2003 the Center Weather Service Unit called with a Pilot Weather Report of a steam plume 600-700 m over the volcano. The pilot reported no ash or sulfur smell. AVO scientists checked satellite imagery and seismograms and found nothing unusual.

Activity during 2007. McGimsey and others (2011) stated that an M 8.2 earthquake in the Kurile Islands on 13 January 2007 may have triggered seismicity at Wrangell and other nearby volcanoes. There were no reports of steaming immediately following this event; however, two weeks later, on 7 February, a relatively large local earthquake was recorded on the Wrangell network that was followed another two weeks later by steaming from the summit. According to the authors, this was the first report of Wrangell steaming in several years.

The authors also mentioned additional episodes of steaming in March 2007. On 25 March, a resident living about 80 km N of the summit reported a strong sulfur odor, an occurrence the resident stated was rare in his 15 years of living in the area. Earlier that day, the Wrangell network had recorded several multi-station seismic events. The authors note that several months later, local residents sent AVO photographs taken on 20 June of steaming from Wrangell and a deposit of ash extending from the W crater many hundreds of meters down the SW flank (figure 2). According to the authors, this ash was likely redistributed from the summit craters by strong winds. No anomalous seismic activity was observed.

Figure 2. View of the northwest flank of Wrangell volcano on 20 Jun e2007 showing a dark stripe of probable redistributed ash extending from West Crater. The photo was taken at Mile 20 of the Tok Cutoff (Hwy 1), between Gakona and Slana. Strong north winds were reported. Note the steam plume rising from skyline saddle near North Crater (left). Photo by Norma Traw, courtesy of AVO.

Activity during 2010. A report by Neal and others (2014) noted that no significant eruptive activity or restlessness had occurred in 2010. However, the authors stated that AVO had received a report of possible vapor emission from the summit area. In May 2010, a single LIDAR swath taken during a summit overflight by glaciologists from the Geophysical Institute, University of Alaska-Fairbanks, depicted the topography of North Crater, a long-known fumarolic source on the NW rim of the ice-filled summit caldera. According to the authors, there are several secondary depressions, including a complex, kidney-bean shaped pit about 20 m deep and 200 m across, located in the center of North crater. This result is broadly consistent with previously recorded surveys of North Crater using photogrammetric techniques.

Neal and others (2014) reported that in early November 2010, a long-time local resident called AVO to report "more activity at the Mount Wrangell summit than he had ever seen before." He sent AVO several images of the volcano taken on 2 November and assured AVO that when the activity in question began, there had been no weather clouds in the area. He noted about ten "bursts" from the summit and said this was unusual compared to the typical steady emissions often seen. The authors stated that AVO reviewed available seismic and satellite data and, finding no evidence of volcanic signals, concluded that the phenomenon was most likely weather-related.

Activity during 2012. According to Herrick and others (2014), no eruptive activity or significant unrest had occurred in 2012, but as in previous years AVO received reports of fumarolic activity high on its flanks. The authors noted that, because of seismic station outages, AVO had removed Wrangell from its monitored list on 27 January 2012, where it remained for at least through the rest of the year. At the same time, the Aviation Color Code and Volcano Alert Level were downgraded from Green/Normal to Unassigned.

Herrick and others (2014) reported that on 11 March 2012, local observers noted "puffs of steam." AVO analysts using satellite images detected small plumes above known fumaroles. On 20 March 2012, a citizen noticed unusually rigorous steaming and described it as looking like "a pressure cooker shot through with nails." Steam rose from both the summit and a location on the SW flank at an elevation of about 3 km. Other calls to AVO registered concern about the significant plumes. Because no other evidence of significant volcanic unrest was detected, AVO concluded these events were likely generated by normal fumarolic activity.

References. Neal, C., and McGimsey, R. G., 1997, 1996 volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 97-0433, 34 p.

McGimsey, R. G., and Wallace, K. L., 1999, 1997 volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 99-0448, 42 p.

McGimsey, R. G., Neal, C. A., and Girina, O., 2004, 1999 Volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 2004-1033, 49 p.

McGimsey, R. G., Neal, C. A., Dixon, J. P., Malik, N., and Chibisova, M., 2011, 2007 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2010-5242, 110 p. Available online at http://pubs.usgs.gov/sir/2010/5242/.

Neal, C. A., McGimsey, R. G., and Chubarova, O., 2004, 2000 Volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 2004-1034, 37 p.

Neal, C. A., McGimsey, R. G., and Girina, O., 2005, 2002 Volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 2004-1058, 55 p., available online at http://pubs.usgs.gov/of/2004/1058/.

McGimsey, R. G., Neal, C. A., and Girina, O., 2005, 2003 volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report 2005-1310, 62 p., http://pubs.usgs.gov/of/2005/1310/.

McGimsey, R. G., Neal, C. A., Dixon, J. P., Malik, N., and Chibisova, M., 2011, 2007 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2010-5242, 110 p. Available online at http://pubs.usgs.gov/sir/2010/5242/.

Neal, C. A., Herrick, J., Girina, O. A., Chibisova, M., Rybin, A., McGimsey, R. G., and Dixon, J., 2014, 2010 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands - Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2014-5034, 76 p., http://dx.doi.org/10.3133/sir20145034/.

Herrick, J. A., Neal, C. A., Cameron, C. E., Dixon, J. P., and McGimsey, R. G., 2014, 2012 Volcanic activity in Alaska: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2014-5160, 82p., http://dx.doi.org/10.3133/sir20145160/.

Geologic Background. With a diameter of 30 km at 2000 m elevation, Mount Wrangell is one of the world's largest continental-margin volcanoes. The massive andesitic shield volcano has produced fluid lava flows as long as 58 km and contains an ice-filled caldera 4-6 km in diameter and 1 km deep, located within an ancestral 15-km-wide caldera. Most of the edifice was constructed during eruptions between about 600,000 and 200,000 years ago. Formation of the summit caldera followed sometime between about 200,000 and 50,000 years ago. Three post-caldera craters are located at the broad summit, along the northern and western caldera rim. A steep-sided flank cinder cone, Mount Zanetti, is located 6 km NW of the summit. The westernmost cone has been the source of infrequent historical eruptions beginning in the 18th century. Increased heat flux in recent years has melted large volumes of ice in the northern crater.

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/).