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/).

By late March, the lava flow that had been advancing down the NW flank along the Río Tabacón had stopped. However, a new flow was emerging from the active crater and had advanced a short distance over its predecessor. Lava extrusion from the summit area has produced more than 40 discrete flows.

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

Information Contacts: E. Malavassi R., Univ. Nacional, Heredia; J. Prosser, Dartmouth College.

Asama ejected incandescent tephra before dawn 8 April. Fine ash carried by W winds fell as much as 250 km away and turned snow-capped mountains gray 80 km from the volcano. Scattered brush fires were started by hot tephra in nearby foothills. During the day, columns of whitish vapor rose from the crater. Visible imagery from the NOAA 7 polar orbiting satellite at 1500 on 8 April showed remnants of a plume extending about 80 km to the ENE, probably at roughly 5 km altitude. No casualties or major damage were reported.

Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the horseshoe-shaped remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asama-yama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has an historical record dating back at least to the 11th century CE. Maekake has had several major plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.

Information Contacts: D. Haller, NOAA/NESDIS; UPI.

Lidar data from Fukuoka, Japan showed a significant decrease in peak values during the limited intervals when weather permitted observations. Broad, almost monolayer profiles were obtained. On 22 March, lidar at Hampton, Virginia showed a broader peak than it had on the 3rd, but about the same total amount of aerosol. From Mauna Loa, Hawaii, lidar detected only minor variations in total aerosol through March. In late April and early May, a lidar-equipped NASA aircraft will collect data on the El Chichón aerosols from high northern to high southern latitudes, and will coordinate with balloon launches from Palestine, Texas.

David Hofmann reported that a balloon launch from Laramie, Wyoming early 8 April detected remnants of the extensive cloud of tiny aerosols observed 28 January. About 20 particles per cm³ remained between 25 and 33 km altitude. A new layer of similar particles, probably about 1 week old, was observed at 20 km, an unusually low altitude. Particle concentrations were about 125/cm³, but the layer was only 200 m thick. The arctic airmass over Wyoming on 8 April lowered the tropopause to 9-10 km altitude, so the densest layers of the main El Chichón cloud were lower than usual. Counts of particles larger than 0.15 µm reached 13/cm³ at 12 km and were still 7/cm³ at 20 km. The layer terminated rather abruptly at 23 km.

Edward Brooks reported brilliant dawns and twilights and visible bands of volcanic aerosols over Jeddah, Saudi Arabia during several periods between late February and late March. In addition to colors observed shortly before sunrise and soon after sunset, caused by illumination of aerosols in the lower stratosphere, the presence of higher layers often resulted in unusual colors long before sunrise and after sunset. Early and late colors were both visible near dawn 21 February, but remained feeble. Brightly colored sunsets were observed 21-22 February, and another 2-stage dawn the 23rd. That evening, brown volcanic aerosols formed a layer at about 6° above the horizon. Clouds obscured the sky for the next several days, but many N-S bands of aerosols were visible at 1-3° elevation in the E sky early 28 February. During the first week in March, both early and late dawn colors were usually faint and were sometimes entirely absent. N-S bands of volcanic aerosols were present early 4 March at about 5° elevation. Clouds made observations difficult 10-14 March, but the return of clear weather revealed more bands of aerosols 15-19 and 22 March accompanying long, brilliant dawns and twilights.

Fred Schaaf saw several examples of Bishop's Ring in March from Millville. New Jersey, but frequent cloudiness limited his observations. Before sunset on 13 March, the sun was surrounded by a band about 6° wide that formed a ring with a radius of about 24-30°. On 15 March, the sun's brightness was considerably diminished by a haze that could not be accounted for by weather conditions or local pollution. A milky area bordered by a Bishop's Ring that was again about 24-30° in radius was visible an hour before sunset 20 March. A similar Bishop's Ring was observed before sunset 30 March. Sunset glows through the month were only weak to moderate and there was only one weak example of late glow indicating illumination of higher aerosols. Richard Keen reported that he had observed no unusual twilights from Boulder, Colorado since mid-January.

Geologic Background. The enormous aerosol cloud from the March-April 1982 eruption of Mexico's El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin. Descriptions of the initial dispersal of major eruption clouds remain with the individual eruption reports, but observations of long-term stratospheric aerosol loading will be found here.

Information Contacts: M. Hirono, Kyushu Univ., Japan; M. Osborn, NASA; T. DeFoor, MLO; D. Hofmann, Univ. of Wyoming; E. Brooks, Saudi Arabia; F. Schaaf, Millville NJ; R. Keen, Univ. of Colorado.

The following report from Juergen Kienle summarizes work by U.S., New Zealand, and Japanese scientists. "Surveillance of the activity associated with the anorthoclase phonolite lava lake continued during the 1982-83 austral field season. The summit crater was visited by New Zealand, U.S., and Japanese scientists on several occasions between November 1982 and February 1983. The approximately 100-m-long semicircular lava lake was still present. Its level had dropped by ~3 m, a loss of roughly 9,000 m³ of lava since the previous visit a year earlier. Since 1976, the lake area has stayed fairly constant. However, its level has been dropping at an average rate of ~2-3 m over the past 4 years.

"An original tripartite array of short-period seismic stations was installed in December 1980. During the 1981-82 field season, this array was expanded by two stations. All stations have single-component vertical seismometers. The summit station also transmits acoustic data to monitor explosive gas discharge from the lava lake. Another data channel is used to monitor electromagnetic signals induced in a wire loop laid around the summit crater by the eruption of conducting magma in the static field of the earth.

"Over the past 2 years many of the microearthquakes we recorded were located immediately beneath the summit lava lake. For example, figure 3 shows the epicenters and hypocenters of events located between December 1981 and January 1982. All but one of these events were explosion earthquakes that positively correlated with acoustic and sometimes electromagnetic signals. Explosive gas discharges from the lava lake typically occurred 2-6 times per day. Observers living in the summit hut commonly reported hearing several explosions per day.

Figure 3. Locations (left) and E-W cross-section (right) of 75 earthquakes recorded 23 December 1981-18 January 1982 in map view. All shallow (<3 km) events were associated with an acoustic signal. Large triangles indicate positions of seismic stations. Inset shows the position of Ross Island. New determination of the velocity structure of Mt. Erebus will cause significant revision in earthquake locations.

"Over the past 2 years we have also recorded microearthquake swarms that show a negative correlation with acoustic and electromagnetic signals. Typical daily counts of about 20 events during quiet periods rise by factors of about 5-10 in swarm periods. Again, most of these events were located beneath the summit region at depths shallower than 3 km.

"On 8 October 1982 an unusual earthquake swarm was recorded from a new source region on Ross Island. On that day almost 700 events occurred near Abbott Peak, a station 10 km NNE of the summit of Mt. Erebus. At this time we do not have reliable magnitudes for the events, but the fact that some of them were recorded at Scott Base and Mt. Terror suggest that the largest events had a local magnitude of 2-3. Figure 4 shows epicenters and a hypothetical cross section of the October events. It is interesting to note that the epicentral area is located halfway between Mt. Bird and Mt. Erebus and roughly correlates with an area that apparently was hydrothermally active in 1908. T.W. Edgeworth, David R. Priestley, and J. Murray, all members of the 1908 Shackelton expedition, reported steam clouds in April 1908 and a major steam eruption ('geysir') on 17 June 1908, rising from a source region at the 600-m level on the SSW slope of Mt. Bird. A tall jet of steam erupted from the same place on 8 September 1908. Philip Kyle has investigated rock outcrops in this area in recent years but could not find any sign of hydrothermal activity. The 8 October earthquake swarm may be related to renewed magma movement (dike injection?) at depth between Mt. Erebus and Mt. Bird. Preliminary hypocentral determinations suggest a clustering of events at 12-15 km depth. [Data compiled] by the Japanese participants in the International Mt. Erebus Seismic Study (IMESS) [included] the number of earthquakes recorded per hour and day at Abbott Peak, September-November 1982. On 8-9 October, the Abbott seismic station also recorded volcanic tremor from presently unknown depths."

Figure 4. Epicenters of 92 earthquakes recorded at Erebus during 4-13 October 1982 in map view (left) and E-W cross section (right). New determination of the velocity structure of Mt. Erebus will cause significant revision in earthquake locations.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: J. Kienle and D. Marshall, Univ. of Alaska; P. Kyle, New Mexico Inst. of Mining. & Tech.; K. Kaminuma, Nat. Inst. of Polar Research, Tokyo; R. Dibble, Victoria Univ., Wellington, New Zealand.

A destructive S-flank fissure eruption began on 28 March, preceded by a series of strong earthquakes first felt during the night of 26-27 March. At about noon on the 27th, a strong smell of H₂S was noted from an old cone (Monte Silvestri) roughly 2 km S of the initial eruption, although H₂S is not normally present in that area. Seismicity continued through the following night. At about 0845 on 28 March a NNE-SSW-trending eruptive fissure opened from about 2,450 to 2,250 m altitude, roughly 4 km S (bearing ~170°) of the central crater (between the eruption fissure of 1910 and La Montagnola). The base and E side of this fissure fed several lava flows that initially moved to the SSE and SSW then turned S. Weak explosive activity along the entire fissure ejected modest quantities of lava fragments. By evening, the main flow had cut a road and overrun several buildings.

During the morning of 1 April, vigorous emission of gas, ash, and old lava, accompanied by occasional phreatic explosions, began from two explosion craters upslope at 2,700 m altitude. At the end of the day, explosions from the southern vent ejected lava fragments. On 2 April, nearly constant lava production fed numerous superposed flows that formed a 500-m-wide lava field extending to 1,900 m altitude. As of 3 April, the lava had not advanced below 1,450 m altitude, 3.5 km from the fissure. At least four principal effusive vents were active along the 750-m fissure, and from its upper part strong gas emission with sporadic explosions occurred at about 30 hornitos.

Bands of open fractures, oriented about N-S, extended from the central crater area to the eruptive fissure. A substantial widening was noted at the S rim of Bocca Nuova, site of frequent collapse activity since Etna's last eruption (from N flank fissures in March 1981). Strong vapor emissions from Bocca Nuova sometimes included abundant ash. There was no activity from the Northeast and Southeast craters.

The temperature of the lava was less than 1,100°C and its chemistry (alkali basalt) [corrected from phonolitic tephrite] was similar to that from some of the more recent eruptions. An area of more than 1 km² was covered by lava and the volume emitted was estimated at about 8-10 x 10⁶ m³. The IIV considered the eruption to be a typical slow subterminal type. The last activity of this type on the S flank was in 1780. As of 8 April, effusive activity had diminished, but the eruption had not yet ended.

The lava destroyed ski lifts [the cable car system originally reported destroyed survived until March 1985] and destroyed or seriously damaged nine privately-owned huts and 11 small buildings owned by local authorities, including restaurants, chalets, mountain refuges, and a first aid station. Lava remained 8 km from the village of Nicolosi, its closest approach to a village or town.

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: R. Romano, IIV; M. Krafft, Cernay, France; UPI.

A Vanuatu Government team visited Hunter Island on 9 March at 1200. White vapor tinged with gray ash billowed to an altitude of approximately 900 m from the main active crater on the W side, and drifted to the W and NW. Fumaroles and two small superimposed craters on the E side were also fuming. Vegetation on the lower slopes of the E coast was burning, which suggested that the eruption had begun recently. By 2200, the fires had reached the central spine of the island and could be clearly seen from the anchorage on the NW coast.

[Later information revealed that human activity had started fires and no eruption had taken place.]

Further Reference. Maillet, P., Monzier, M., and Lefevre, C., 1987, Petrology of Matthew and Hunter volcanoes, South New Hebrides Island Arc (Southwest Pacific): JVGR, v. 30, p. 1-29.

Geologic Background. Hunter Island, the SE-most volcano of the New Hebrides arc, is a small 1-km-wide island consisting of a composite andesitic-to-dacitic cone topped by explosion craters and a lava dome. The island was named after the vessel that discovered it in 1798. A 100-m-deep, steep-sided crater occupies the NW part of the island, which contrasts with the southern cone, whose summit is filled by a lava dome. Several poorly documented eruptions have been noted since the 19th century. Large streams of lava were reported to be pouring from two craters on the eastern side of the island in 1895; the latest eruption apparently took place from the northern tip of the island. Fumarolic and solfataric areas are located at the northern tip of the island and the NE and SE coasts.

Information Contacts: A. Macfarlane, Dept. of Geology, Mines, and Rural Water Supplies, Vanuatu.

[With the following report, HVO scientists recognized that they were dealing with episodic eruptive behavior, which continued for the next three years. We have added numbered headers to help organize the reports. Originally referred to as "phases," the HVO staff later decided that the term "episode" was more appropiate (see SEAN 10:06). We have replaced the word "phase" throughout the text of earlier reports. Where the word "episode" was used in reports 8:3 to 10:6, we have substituted "period" to avoid confusion with the revised usage of "episode."]

EPISODE 3

"The 1983 eruption entered its third major episode of lava production in the early morning of 28 March. A 3.5-week quiescent period 4-28 March was interrupted only briefly by minor emission of spatter and pahoehoe lava on 21 March at vents S of Pu'u Kahaualea.

"Initially, on 28 March, the major eruptive activity occurred at a vent 700 m NE of Pu'u Kamoamoa, just inside the Hawaii Volcanoes National Park boundary. Vigorous extrusive activity at this vent produced a flow that extended nearly 5 km SE along the National Park boundary (figure 17). Eruption of this vent stopped at 2019 on 30 March.

"A vent S of Pu'u Kahaualea that was the source of the major flows of late February and early March resumed erupting 28 March, sporadically at first. Its eruptive activity became steady at approximately 1800 on 29 March. From then through 5 April, when this report was prepared, it supplied a flow that slowly extended 4 km [revised to 3 km in 8:4] NE along the rift zone to the vicinity of Kalalua. Another flow from this vent moved about 3 km SE on 4 and 5 April. The vent continued as the single locus of lava production. Its vigorous fountain, commonly 100 m high and at times estimated as high as 300 m, was visible from a number of vantage points along the highway from the Hawaii Volcanoes National Park to Hilo. Frothy scoria fragments, bombs of spatter, and thin spatter-fed flows built a prominent cone 60 m high [note growth to 80 m in 8:4], and a thin airfall pyroclastic blanket extended more than 1 km from the cone. Pele's hair fell as far as 17 km from the vent.

"The volume of basalt extruded since the eruption began on 3 January was on the order of 30 x 10⁶ m³ [to at least 50 x 10⁶ m³ by end of episode 3]. Flows of the present episode were dominantly aa. Lava temperatures ranging from 1112° to 1129°C were measured by thermocouple. Like the earlier 1983 lavas, those of late March-early April are slightly porphyritic, with scattered small plagioclase and olivine phenocrysts. The gas composition remained unchanged throughout the eruption, indicating that stored E rift magma remained the predominant source of the erupted lava."

Deformation and seismicity. "The water tube tiltmeter at Uwekahuna Vault in the summit region showed the correspondence of summit subsidence with major extrusion episodes in the E rift zone (figure 18). Moderate summit re-inflation followed the extrusive episodes of early January and early March. The tiltmeter data in combination with levelling results indicated a cumulative volume of at least 70 x 10⁶ m³ [to 80 x 10⁶ m³ by the end of episode 3; 8:4] for magma withdrawn from the shallow summit region since the beginning of the eruptive/intrusive activity in early January.

"Since cessation of the initial earthquake swarm in early January, seismicity in the eruptive zone was characterized by unceasing harmonic tremor that waxed and waned in amplitude in concert with the eruptive activity. As determined from a seismic station near Pu'u Kamoamoa and from portable seismometer traverses, the tremor originated from a source within a few kilometers of the surface in a zone between Pu'u Kamoamoa and the vents S of Pu'u Kahaualea.

"Following the major outbreak of 25 February-4 March, tremor continued at a decreased level. On 21 March, the amplitude gradually increased from 0430 to 0630, remained moderately high for most of the day, and decreased to its previous low level on the following day.

"A gradual increase in amplitude occurred again beginning in the early morning of 27 March. By 0100 on 28 March the tremor amplitude increased by about 5 times at the Pu'u Kamoamoa station. Glow from active fountains was reported shortly thereafter. Tremor remained strong as of 5 April, at times reaching an amplitude greater than 10 times background for periods of a few minutes to several hours."

Addendum: Steven Brantley reported that lava fountaining from the vent S of Pu'u Kahaualea stopped at 0257 [but revised to 0247 in 8:4] on 9 April. By 0430, harmonic tremor had decreased to low levels, and the rate of summit deflation had decreased to less than 0.05 µrad/hour. The SE flow from this vent entered the Royal Gardens subdivision on 8 April. Approximately seven [six confirmed in 8:4] structures were destroyed before the flow stopped late in the afternoon of 9 April.

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

Information Contacts: E. Wolfe, A. Okamura, R. Koyanagi, and S. Brantley, HVO; UPI.

An earthquake swarm on the NE flank began 28 February. The majority of the events had foci above sea level (Kliuchevskoi's summit elevation is 4,850 m) and their maximum magnitude was 3. Based on the swarm's character, the IVP predicted that a flank eruption would start between 4 and 9 March. On 8 March a crater opened at 3,000 m altitude on the NE flank. Activity from the crater was purely effusive, producing an andesitic basalt flow that was 3 km long by 18 March.

Further References. Special issue on the 1983 eruption of Kliuchevskoi: Volcanology and Seismology, 1988, 148 pp. (English translation of Volcanology and Seismology, 1985, no. 1) (8 papers).

Panov, V.K., and Slezin, Y.B., 1985, The mechanism of formation of a lava field during the Predskazanny flank eruption, 1983, Klyuchevskoy volcano, Kamchatka: Volcanology and Seismology, no. 3, p. 3-13.

Tokarev, P.I., 1985, Prediction of lateral eruption of Kliuchevskoy volcano in March 1983: JVGR, v. 25, p. 173-180.

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

Information Contacts: B. Ivanov, IVP.

"After the increased explosive activity from Crater 2 in February, the level of activity returned to normal. White-grey vapour and ash emissions were seen on a few days, and occasional relatively small Vulcanian explosions were observed. Explosion and rumbling sounds were heard occasionally. The only volcano-seismic activity recorded was from Vulcanian explosions at Crater 2 which occurred at intervals of a few hours to about 4 days."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower eastern flank of the extinct Talawe volcano. Talawe is the highest volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila volcano was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the north and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit of Langila. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: C. McKee, RVO.

Marie Benson reported that Ol Doinyo Lengai erupted 1 and 9 January, and 17 and 20 February but not since then. During the 20 February eruption, lava was flowing toward Lake Natron (N of the volcano). Andrew Stirrat, and the Outward Bound expedition of which he was co-leader, were N and W of the volcano 13-16 March. They saw small gray plumes emerging from the summit area on 13-14 March, and the morning of the 15th but none later that day or on the 16th. Clear views of the summit were obtained during the evening of 14 March and throughout the night of 15-16 March but no glow was evident. An apparent thin layer of ash on the upper N flank was visible through binoculars but no fresh ash was recognized in the area visited by the expedition, which approached to within 2.5 km of the foot of the volcano, nor did they observe any new lava. A local guide who was near the volcano 7-10 March reported that the air was hazy and smelly and caused chemical burns on his arms after 6 hours of exposure, the only time he had encountered such problems in the previous 3 months. Madelon Kelly and others visited the area a few km E and N of the volcano 15 March but observed neither lava nor any apparent eruptive activity. Their photographs showed no plumes.

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

Information Contacts: M. Benson, USAID, Arusha; D. Miller, U.S. Embassy, Dar es Salaam; A. Stirrat, Outward Bound; M. Kelly, Tyrone, PA.

Seismicity continued to decline in the epicentral area of the major January earthquake swarm. Brief bursts of small shocks were occasionally recorded, but by early April, daily earthquake counts were approaching the pre-January background levels of 2-5 events of magnitude greater than or equal to 1 per day.

Geologic Background. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: D. Hill, USGS, Menlo Park, CA.

"Southern crater became more active in March after showing increasing activity in late February. Weak explosion sounds were heard on most days until 14 March, accompanying weak to moderate white-grey vapour and ash emissions. Weak ejections were reported on 12 and 13 March, when weak crater glow was seen. A period of somewhat stronger emissions was reported 19-24 March, including low to moderate explosion sounds on 23 and 24 March, and continuous vapour and ash emissions were noted on 26 and 29 March. Weak rumbling was heard from the 29th to the month's end, and deep booming sounds were reported on the 31st.

"Generally steady activity of weak to moderate white-grey emissions occurred at Main crater until about 23 March. These emissions were usually not accompanied by sound effects, but on 2 and 3 March weak explosion sounds were heard. From the 24th to the end of the month emissions were reported to be moderate, and included brown ash from the 27th. Light ashfalls were reported on about 30% of days from locations on the E and SE flanks.

"A steady increase in the daily number of volcanic earthquakes took place in March, from about 1200 at the beginning of the month to about 2100 at month's end. Event amplitudes showed a slight stepwise increase at mid-month. A marked brief increase in seismic amplitudes was also noted on 4 and 5 March.

"Tilt measurements showed steady changes of about 1 µrad down to the NW from the observatory, on the SW flank (figure 1)."

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: C. McKee, RVO.

A Vanuatu government team arrived at Matthew Island on 10 March at 0700. The only activity noted was emission of wispy, white vapor from the central crater in the island's W (main) edifice.

Further Reference. Maillet, P., Monzier, M., and Lefevre, C., 1987, Petrology of Matthew and Hunter volcanoes, South New Hebrides Island Arc (Southwest Pacific): JVGR, v. 30, p. 1-29.

Geologic Background. Isolated Matthew Island is composed of two low andesitic-to-dacitic cones separated by a narrow isthmus. Matthew Island was discovered in 1788 by a ship captain, who named the island after the owner of his vessel. Only the triangular eastern portion of the small, 0.6 x 1.2 km wide island was present prior to the 1940s, when construction of the larger western segment began; it consists primarily of lava flows. The 177-m-high western cone contains a crater that is breached to the NW and is filled by a lava flow whose terminus forms the NW coast.

Information Contacts: A. Macfarlane, Dept. of Geology, Mines, and Rural Water Supplies, Vanuatu.

". . . Observations in December 1972 indicated three main areas of steaming ground in which temperatures as high as 59°C were recorded at depths of 0.25 m. Numerous fumarolic ice towers were scattered throughout the summit area (Lyon and Giggenbach, 1974). Two members of the U.S. Antarctic Research Program climbed the mountain in January 1983. No change was noted in the number, size, and distribution of ice towers and steaming ground from the 1972 reports. Measured ground temperature temperatures were also similar to those in 1972. There was no evidence of any change in the activity over the last 10 years."

Reference. Lyon, G.L., and Giggenbach, W.F., 1974, Geothermal activity in Victoria Land, Antarctica: New Zealand Journal of Geology and Geophysics, v. 17, p. 511-521.

Further Reference. Keys, J.R., McIntosh, W.C., and Kyle, P.R., 1983, Volcanic activity of Mount Melbourne, North Victoria Land: Antarctic Journal of the United States, 1983 review, v. 18, no. 5, p. 10-11.

Geologic Background. Mount Melbourne is a large undissected stratovolcano along the western coast of the Ross Sea in Antarctica's northern Victoria Land. The 2732-m-high glacier-clad edifice lies at the center of a volcanic field containing both subglacial and subaerial vents along a dominantly N-S trend. A large number of scoria cones, lava domes, viscous lava flows, and lava fields are exposed at the summit and upper flanks. A number of very young-looking cones are located at the summit and on the flanks. Tephra layers are found within and on top of ice layers, and the most recent eruption was estimated to have occurred between 1862 and 1922. The volcano displays fumarolic activity that is concentrated along a NNE-SSW line cutting through the summit area and along a line of phreatomagmatic craters on the southern rim of the summit crater. Prominent ice towers and pinnacles were formed from steam condensation around fumarolic vents.

Information Contacts: P. Kyle, New Mexico Inst. of Mining and Tech.

"A team of four HVO scientists, five scientists from the USGS Water Resources Division, plus Civil Defense and other government officials from the Commonwealth of the North Mariana Islands visited Pagan 5-15 March.

"There were two very minor ash eruptions on 7 and 15 March; ashfall was confined to the summit cone. During the remainder of the visit, activity was limited to degassing. The gases were essentially atmospheric in composition, much different than the May 1981 gases, which had a high magmatic component. Seismic monitors showed varying amounts of B-type events and harmonic tremor. More numerous and stronger seismic events preceded the ash eruptions of 7 and 15 March.

"HVO scientists established a second EDM array (one had been installed in May 1981) and a tilt network, and installed a seismic event counter and two-component tiltmeter. Both EDM arrays showed minor deflation 5-15 March. Reoccupation of the original EDM line showed that 25 cm of net inflation had occurred on the higher slopes of the volcano between May 1981 and March 1983, but the lack of other measurements between those dates prevented determination of shorter-term deformation trends.

"Scientists from the Water Resources Division installed equipment to transmit data from the two-component tiltmeter (including periodic temperature measurements), the seismic event counter, and a rain gauge to Hawaii via the GOES West satellite. They also performed a water resource evaluation and sampled volcanic gases.

"Stratigraphy of the tephra deposits indicated that Pagan had erupted at least four and perhaps as many as seven times since May 1981. The volume of the post-May 1981 tephra deposits is minor in comparison to that of the May 1981 deposit. It is difficult to assign eruption dates to each tephra layer because of the sporadic nature of observations on the island. However, it is probable that a single lava flow and one of the tephra layers was produced on 11 June, 1981 (6:6). Other eruptions were observed in November 1981 (6:11) and January through February 1982 (7:2). The date of emplacement of the uppermost and thickest tephra layers is uncertain. However, comparison of December 1982 aerial photographs with those taken in August 1982 suggested that these layers were emplaced during that interval. In addition, during late September-October 1982, residents of Saipan (roughly 300 km to the S) reported a dark cloud, similar to the one ejected in May 1981, drifting to the S.

"The most recent eruptive products were slightly richer in phenocrysts than products of the May 1981 eruption. A preliminary microprobe analysis indicated that February 1982 eruption material was less differentiated than that of May 1981 but similar in composition to the 1925 magma (6:6).

"The activity seen by USN personnel on 10 December 1982 was much less intense than that of March 1983. The burning seen along the S and SW slopes was on another edifice on the opposite (S) end of the island, and was due to a brush fire, unrelated to eruptive activity.

"Early in 1982, there was speculation that Pagan may have been the source for the 'Mystery Cloud' of volcanic aerosols in the stratosphere (7:1-3). The volume of individual tephra layers does not by itself suggest that Pagan was the source of the aerosols. However, they contain a large fraction of lithic material, suggestive of powerful gas jetting and erosion of the vent; lidar studies indicated that the source was at the approximate latitude of Pagan; and Pagan is the only volcano thought to have been in eruption at that latitude and at that time. Thus, at present, Pagan remains the best possible source for the 'Mystery Cloud' of early 1982." [Later work on Nimbus 7 satallite data by Arlin Krueger revealed a large SO2 cloud originating in the vicinity of Nyamuragira Volcano, Zaire in late December 1984. This eruption is now thought to be a more likely source for the `Mystery Cloud.']

Geologic Background. Pagan Island, the largest and one of the most active of the Mariana Islands volcanoes, consists of two stratovolcanoes connected by a narrow isthmus. Both North and South Pagan stratovolcanoes were constructed within calderas, 7 and 4 km in diameter, respectively. The 570-m-high Mount Pagan at the NE end of the island rises above the flat floor of the northern caldera, which may have formed less than 1000 years ago. South Pagan is a 548-m-high stratovolcano with an elongated summit containing four distinct craters. Almost all of the historical eruptions of Pagan, which date back to the 17th century, have originated from North Pagan volcano. The largest eruption of Pagan during historical time took place in 1981 and prompted the evacuation of the sparsely populated island.

Information Contacts: N. Banks, HVO.

A fissure extending from the E side across the summit of the eroded cone at the S end of the crater lake emitted gases that were hotter in late March than several months earlier. On 22 March, Jerry Prosser measured a temperature of 800°C on the side of the cone and 890°C at the summit. Cheminée and others had reported variable but generally falling temperatures between June 1981 (940°C) and December 1982 (731°C). Prosser also noted that the crater lake was consistently warmer than 60°C, its highest temperature since just prior to the 1978 eruption.

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

Information Contacts: J. Prosser, Dartmouth College.

This paragraph is primarily from a report by Jorge Barquero H. and Juan de Dios Segura. During the night of 6 February, residents of towns (Dos Ríos de Upala, Colonia Blanca, and Colonia Libertad) 8 km N and NE of the volcano heard strong rumblings and observed the rise of a large eruption column from the crater. Personnel from the Proyecto de Investigaciones Vulcanológicas climbed the volcano 19 February. The odor of sulfur was stronger than it had been during their previous ascent in November 1982. Phreatomagmatic eruptions had ejected bombs, lapilli, and ash, as well as blocks 10-100 cm in diameter that formed impact craters. Tephra fell SE, S, and SW of the vent to a distance of about 1.5 km. Destruction, primarily to vegetation, was greatest to the SE and S. The tephra had a high water content because the vent contained a lake. Strong rains and rapid erosion since the eruption made it difficult to calculate the original depth of the airfall deposits, although in some places SE of the vent they were 4 cm thick. The eroded ash washed into a ravine, producing a small mudflow in a NE flank river (Río Pénjamo), causing the deaths of thousands of fish 7-8 February, possibly because of the acidity of the water. The pH of the cold lake was 3.5 on 19 February and 4.1 on 5 March.

Jorge Barquero H., J. Bruce Gemmell, and Jerry Prosser climbed the volcano on November 1982, and Gemmell provided the following report. "Rincón de la Vieja is a large composite volcano with a series of collapse craters aligned ENE-WSW. Its main cone is covered with thick vegetation but three craters to the W are not vegetated. The most recently active crater (250 m in diameter) is 1 km NW of the main cone. No activity or gas emissions were seen in this crater and a cold yellowish-green lake covered the crater floor. No steam was rising from the lake but two areas of brown discoloration near its center may have indicated subaqueous vents. The area around the summit craters was covered with accessory blocks of andesitic lava and tuff breccias, in addition to juvenile andesitic breadcrust bombs, lapilli, and ash from the most recent recorded eruptions in 1966-70. Numerous mudpots, hot springs, and steam vents occurred in two main areas (Aguas Termales and Sitio Hornillas), on the S flank at about 900 m elevation."

Further Reference. Barquero, J., and de Diós Segura, J., 1983, La Actividad del Volcán Rincón de la Vieja: Boletín de Vulcanología, no. 13, p. 5-10.

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: J. Barquero H. and J. de Dios Segura, Univ. Nacional, Heredia; J.B. Gemmell and J. Prosser, Dartmouth College; R. Sáenz R., Ministro de Energía y Minas; La República, San José.

Viewing Crater Lake from the air, pilot K. Newton had reported the color was gray on 5 March, but was reverting to blue-green 3 days later. When NZGS personnel visited Ruapehu on 17 March, Crater Lake was gray. They found no evidence of recent eruptions; neither ash deposits nor surge marks. Upwelling over the N vent area was slight. Over the central vent no upwelling was visible, but thin black sulfur strands appeared in midafternoon.

Lake water temperature measured at the outlet was 23°C, 4° lower than on their last visit, 28 February. Concentrations of both chlorine and magnesium had risen slightly; the Mg/Cl ratio remained 0.104.

The horizontal deformation survey showed that the distance between 2 stations on opposite sides of the crater had decreased an additional 8 mm since 28 February, for a total contraction of 22 mm since 10 February.

Since seismicity increased 23 February, there have been 9 B-type earthquake sequences, the three reported last month plus others on 4, 7, 8, 9, 12, and 14 March. These sequences typically began with a high-frequency roof rock (tectonic) earthquake of about M 2 at a relatively shallow depth. Within a minute or so, this was followed by a deeper B-type (volcanic) earthquake of magnitude 2.9-3.4, at a depth between the focus of normal magmatic events (about 1 km depth) and those in the roof rock. No significant volcanic tremor has occurred since the earthquake series began. The lake's color changes appeared to correlate with the earthquake series.

The largest B-type earthquake in the series, [M_L 3.25], occurred at 1406 on 12 March. According to J.H. Latter, earthquakes at Ruapehu have not in the past exceeded this magnitude in a closed-vent situation, as this appears to be, without an accompanying eruption.

The seismicity and deflation were tentatively interpreted by the NZGS as indicating a decreasing magmatic or gas pressure at a deep level below the N vents (or, less likely, intrusion occurring beyond the crater, resulting in compression of the crater rim). As long as the present seismicity persists, they consider the probability of eruption to remain higher than usual.

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

Information Contacts: P. Otway, NZGS, Wairakei; J. Latter, DSIR, Wellington.

A study of the geochemical evolution of Carbet l'Echelle spring, continued during 1981 and 1982 (figure 10). The water temperature decreased from 62.5°C in January 1981 to 50° in September 1982, while the concentration of HCO₃ ions continued to increase (+ 30%). Moreover, although large variations in Cl concentration have been observed since 1979, the background level of Cl began to fall in September 1981 and was reduced by half in one year. Likewise, the concentration of F ions, which has oscillated around a constant value since the beginning of the survey, decreased slightly in late 1982. Water temperatures and chemistry remained nearly constant at the other springs monitored by this study.

Figure 10. Changes in temperature (°C), HCO3 and Cl concentration (103 moles/liter), and F concentration (105 moles/liter) for Carbet l'Echelle spring, 500 m from the summit of Soufrière Guadeloupe, January 1981-September 1982.

Further References: Bigot, S., and Hammouya, G., 1987, Surveillance hydrogéochimique de la Soufrière de Guadeloupe, 1979-1985: Diminuition d'activité ou confinement?: C.R. Acad. Sci. Paris, t. 304, Série II, no. 13, p. 757-760.

Observations volcanologiques: Rapport d'Activité des Observatoires Volcanologiques des Antilles (Guadeloupe-Martinique) - November 1976-April 1977, April 1977-December 1978, 1979, 1980, 1981, 1982, and 1983-1984: Institut de Physique du Globe, Paris.

Geologic Background. La Soufrière de la Guadeloupe volcano occupies the southern end of Basse-Terre, the western half of the butterfly-shaped island of Guadeloupe. Construction of the Grand Découverte volcano about 0.2 million years ago (Ma) was followed by caldera formation after a plinian eruption about 0.1 Ma, and then by construction of the Carmichaël volcano within the caldera. Two episodes of edifice collapse and associated large debris avalanches formed the Carmichaël and Amic craters about 11,500 and 3100 years ago, respectively. The presently active La Soufrière volcano subsequently grew within the Amic crater. The summit consists of a flat-topped lava dome, and several other domes occur on the southern flanks. Most historical eruptions have originated from NW-SE-trending fissure systems that cut across the summit and upper flanks. A relatively minor phreatic eruption in 1976-77 caused severe economic disruption when Basse-Terre, the island's capital city, which lies immediately below the volcano, was evacuated.

Information Contacts: S. Bigot, Lab. de Géochemie, Paris; G. Hammouya, Lab. de Physique du Globe; J. Le Mouel, J. Cheminée, IPG, Paris.

CVO personnel report that frequent rockfalls from the toe of the February lobe and changes to its morphology, coupled with elevated SO₂ emission and small seismic events, may reflect continuing endogenous dome growth. Poor weather limited visibility and restricted access to the crater through March and early April.

A new incandescent radial fissure was observed on the NE side of the February lobe during a night overflight 23 March and remained visible through the end of the month. ln the past, such fissures have typically formed during periods of rapid endogenous growth. By 31 March a mound of rubble estimated from brief aerial observations to be roughly 50-60 m in diameter and 20-30 m high had developed on the lobe, in the area where a spine had been extruded in late February. Frequent rockfalls from the E end of the lobe were continuing on 11 April, but conditions in the crater prevented geologists from mapping changes to the lobe front. Gas and ash explosions, some of moderate size, were observed or detected seismically roughly 1-2 times per day through early April. Most appeared to originate from fumaroles at the top of the dome. The main fumarole, a conical pit tens of meters in diameter at the surface and tens of meters deep, was located at the head of the E-flank notch that was the source of the February lobe.

Rates of SO₂ emission remained elevated through early April, averaging 150 ± 95 t/d in March, only 20 t/d less than the February mean. Early April values ranged from 135 to 180 t/d. In contrast, rates in the months prior to January were only about 35 t/d. Deformation of the N and W sides of the dome continued but did not accelerate in March. Measurements of the deformation of the active E side of the dome have not been possible.

Extremely small events, felt by field crews in the crater but recorded only by a single seismometer 1 km N of the dome, continued through March. The number of larger earthquakes remained above background levels, averaging about half a dozen per day. This value oscillated considerably, without obvious trends or apparent correlations with variations in activity at the dome, but dropped substantially after the early March increase (SEAN 08:03). Seismic energy release exceeded previous post-extrusion periods. In the past, 90-95% of cumulative energy release had occurred during extrusion episodes, but this pattern changed in October 1982, and seismic energy equivalent to two typical extrusions has been released since then. However, seismic energy accompanying extrusion of the February lobe was unusually low, comparable to that associated with smaller extrusions such as in October 1981. Energy release in early April exceeded February rates.

Geologic Background. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older St. Helens edifice, but few lava flows extended beyond the base of the volcano. The modern edifice was constructed during the last 2200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers.

Information Contacts: T. Casadevall, C. Newhall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

"A notable seismic crisis occurred 21-23 March. From mid-January until 21 March, seismicity had been fluctuating between 400 and 1200 B-type volcanic shocks of moderate amplitude per day, in an apparently cyclic manner with a period of 16 days. On 21 March, shocks occurred at the rate of about 800/day until 1700, when their amplitude decreased sharply. At 1930 their frequency started to increase, and from 2000 they became subcontinuous (1800/day) and moderate in amplitude. This high level of seismicity continued for about 38 hours before gradually decreasing again.

"The volcano was obscured by heavy cloud for most of this period. The first visual observations were made at 0730 on 22 March when 4-6 almost perfect smoke rings, reportedly pale-grey in colour, were ejected rapidly to about 300 m above the summit crater. Consistently strong white vapour emissions from the summit crater commenced on 25 March, contrasting with the usually weak to moderate white emission. A report on 26 March suggests a brief interval of dark grey emission, but seismic records show no changes that could indicate eruptive activity. The decline in seismicity on 23 March was followed by almost 2 weeks of exceptionally low levels interrupted only by a brief increase, 30-31 March.

"It is interesting to note that the high level of seismicity closely followed a heavy rainfall of 65 mm. It also occurred only three days after a major tectonic earthquake (ML 7.7) in the Solomon Sea, felt at MM V in the vicinity of Ulawun. It is possible that the earthquake opened microfissures in the volcanic edifice allowing ground water to penetrate and interact phreatically with shallow magma. The cyclic variations in seismicity since mid-January and increasing instability in March are believed to indicate that an eruption may occur in the near future."

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: C. McKee, RVO.

Aerial inspection by NZGS personnel on 10 March revealed no evidence of eruptive activity since 7 January. A white steam plume was rising to about 600 m altitude from the SE part of 1978 Crater. For 200 m to the N, there were moderate emissions from vents in deep gullies and from two fumaroles. Very little emission was originating from Donald Mound. Most of the Mound was covered with yellow sublimates, but a central zone was gray.

Since 7 January the number of low-frequency (B-type) events has increased, especially 9-15 February (more that 25/day; maximum, 42) and 22 February-4 March (more than 21/day). High-frequency (volcano-tectonic) events usually numbered fewer than 5/day, except for 6 on 29 January, 7 on 6 February, and 10 on 21 February. Wide-band seismic events were recorded on 19 and 24 February, and 2 and 6 March. They lasted 4-40 minutes with peak-to-peak amplitudes up to 70 mm.

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

Information Contacts: B. Scott, NZGS, Rotorua.