Nevados de Chillán is a large complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes that has had multiple historical eruptions dating back to the seventeenth century. The most recent eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early 2018. Explosions and pyroclastic flows continued into 2020; several lava flows appeared in late 2019. New dome growth began in late June 2020, accompanied by a new flow descending the N flank from the crater rim. This report covers continuing activity from November 2020-April 2021 when ongoing explosive events produced ash plumes, and growth of the dome inside the crater continued along with the lava flow extending farther down the N flank. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and from satellite data.

Activity at Nevados de Chillán during November 2020-April 2021 was similar to the previous few months. The flow that appeared on the N flank at the end of June lengthened by more than 200 m and increased significantly in volume. A second thermal anomaly on the edge of Nicanor crater at the head of the flow first appeared in satellite imagery on 14 July 2020 and persisted in all subsequent clear images through April 2021 along with the anomaly from the growing dome inside the crater. Tens of seismic explosive events were measured daily; many produced plumes of gas and ash. The dome inside the crater continued to grow even though explosive events intermittently destroyed parts of the dome. An increase in the flow rate was observed at the very end of April, and a new lava flow appeared on the NE flank at the beginning of May. Thermal activity shown in the MIROVA graph indicated a constant level of heat energy from July 2020 through the end of April 2021 (figure 69).

Figure 69. The MIROVA graph of Log Radiative Power at Nevados de Chillan from 8 July 2020 through April 2021 showed a very consistent pattern of thermal energy from a lava flow and growing dome until the end of April when an increase in the flow rate created a new lava flow. Courtesy of MIROVA.

During the first half of November 2020 there were 61 volcano-tectonic (VT) and 575 explosive events recorded. Plume heights did not exceed 1,140 m above the Nicanor crater and only a few of them contained identifiable ash (figure 70). Incandescence was observed at the E edge of crater on clear nights. SERNAGEOMIN reported that a small crater continued to grow on the inner E rim of the Nicanor crater and was identified in satellite images; during some explosions, two sources of emissions were apparent. During the second half of the month 70 VT and 573 explosive seismic events were recorded. Webcam images indicated that the plumes from the explosions remained at low altitude (less than 1,000 m above the crater rim), and little to no tephra was noted; the emissions were primarily gas. Most of the emissions originated from the central area of the dome inside the crater, but the second emission site, located at the E edge of the crater remained active.

Figure 70. A small ash emission rose to 1,280 m above the summit of Nevados de Chillan on 9 November 2020 and was typical of the explosive activity that occurred regularly throughout the reporting period. Courtesy of SERNAGEOMIN.

The lava flow (L5) on the N flank of Volcan Nuevo continued to advance during November 2020, reaching about 720 m from the rim of Nicanor crater by mid-month with an estimated speed of 1.4 m/day the flow only advanced about 10 m during the second half of November. Its size was estimated at about 580,000 m³ by the middle of the month. Dome growth inside the crater continued as well; it was measured in mid-November as 45 m high with a volume a little over 200,000 m³. During an overflight in mid-November SERNAGEOMIN scientists noted a fracture about 50 m long on the N side of the dome that extended NE-SW, was 10 m deep, and connected the dome to the central channel of the L5 lava flow. Inflation rates of 3-5 mm/month were recorded at GNSS stations around the active crater at mid-month. The vertical inflation increased to a rate of 12 mm/month during the second half of the month while the horizontal displacement remained low at 2 mm/month.

VT seismic events increased in frequency during December 2020 to 192 during the first half of the month and 236 during the second half. The numbers of explosions remained similar with 551 during the first two weeks and 582 during the second. Explosion plumes usually remained less than 1,200 m above the summit and contained moderate amounts of particulates (figure 71). Most of the explosions appeared to come from the dome inside the crater; later in the month a few explosions appeared to cause partial destruction of the NE corner of the dome and produced limited pyroclastic flows adjacent to the crater.

Figure 71. Plumes of gas and ash from Nevados de Chillan, along with thermal anomalies, were frequently detected by satellite instruments, as seen here on 9 (top) and 29 (bottom) December 2020. The plumes are drifting SE, and two thermal anomalies were present at the summit; the brighter one is from the center of the dome inside Nicanor crater and the dimmer one slightly to the NW is at the head of flow L5 that descends the N flank. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

The L5 lava flow was 750 m long and 20 m wide by 15 December, and it had grown 15 m by the end of the month. The rate of dome growth appeared to increase during the second half of December, with the dome growing over the crater rim on the NNE edge, causing block avalanches to descend the NNE flank. Deformation data indicated a high level of inflation, with a horizontal displacement rate of 5 mm/month and a vertical rate of 11 mm/month. The accumulated volume of new material from both the dome and the flow since late June 2020 was estimated at a little over 1,400,000 m³ at the end of December.

The number of seismic events continued to increase in January 2021 with 386 VT events recorded for 1-15 January and 244 events 16-31 January. For explosions, 659 events occurred during the first half of the month, and 676 were reported for the second half. The explosions from the dome rose up to 1,200 m above the crater during the first half of January and 1,400 m high during the second half. Dome growth continued with activity focused in the NNE part of the crater; fragments traveled up to 100 m down that flank. Increased flow from the fissure at the head of the L5 flow led to an increase in the flow rate at the head of the flow to about 1.7 m/day by mid-month when it was 788 m long. The inflation rate remained constant during the first half of the month. Intense continuous degassing was observed from the front of the L5 flow during 23-25 January (figure 72). This coincided with a rise in thermal radiance in the same area. By the end of the month the flow was 808 m long, and the central channel had enlarged to 40 m wide near the head of the flow. An increased flow rate observed in the webcams was accompanied by the development of lateral lobes at the head of the flow. A slight decrease in the rate of inflation was recorded near the end of the month.

Figure 72. Intense degassing and a rise in thermal radiance were observed at the head of the L5 flow at Nevados de Chillan during 23-25 January 2021. The left Sentinel-2 image from 25 January 2021 shows the thermal anomaly at the front of the L5 flow (top), the anomaly at the head of the flow, and the anomaly from the dome inside the Nicanor crater (bottom). The right image shows strong degassing at the L5 flow front at the base of the channel of the active flow. Courtesy of SERNAGEOMIN.

A bright thermal anomaly near the front of the L5 flow on 4 February 2021 was attributed by SERNAGEOMIN to the rupture of the crust and partial collapse of that area due to an increased flow rate (figure 73). Dome growth continued in February with ejecta from explosions reaching 70-100 m from the crater rim. The central channel of the L5 flow continued to widen from the inferred increase in flow rate (figure 74); it was 710 m long by the end of the month. The total flow length was estimated at 860 m by 15 February and 900 m long by the end of the month. Its volume measured on 15 February was about 1,700,000 m³. Inflation continued at a rate of 5-8 mm/month near the active crater, but a small deflation was recorded 11 km E. Eruption plumes rose no higher than 1,100 m above the crater during February 2021 and contained various amounts of particulate matter (figure 75). The number of VT seismic events decreased to 165 during the first half of the month and to 106 during the second half, but the number of explosive events was relatively constant at 598 during 1-15 February and 471 during 16-28 February.

Figure 73. A bright thermal anomaly appeared at the head of the L5 flow at Nevados de Chillan on 4 February 2021. SERNAGEOMIN attributed it to the rupture of the crust and partial collapse of that area due to increased flow. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 74. The growth of the L5 lava flow on the N flank of Nicanor crater and dome inside the crater at Nevados de Chillan was evident in these images comparing them from 1 December 2020 and 8 February 2021. Courtesy of SERNAGEOMIN.

Figure 75. A plume of steam and ash rose from the summit of Nevados de Chillan on 22 February 2021 after a VT seismic event that was located 3.3 km deep with a magnitude of 3.0. Courtesy of Volcanologia Chile.

Incandescence at night was often visible from the E side of the crater during March 2021. The explosion heights were up to 1,200 m during the beginning of the month, and below 1,000 m during the latter half when most of the plumes rose less than 500 m. There were 232 VT events and 534 explosions during 1-15 March and 214 VT events and 556 explosions during 13-31 March. The estimated volume of the dome on 6 March was a little over 300,000 m³. Dome growth continued with ejecta traveling up to 160 m from the dome; the L5 flow front was 80 m wide by the middle of the month and measured 950 m long in satellite imagery; detachment blocks fell from the front and sides of the flow. The rate of inflation decreased during the month. During an overflight on 24 March SERNAGEOMIN observers noted a significant increase in the quantity of pyroclastic debris inside Nicanor crater compared with the previous overflight on 2 December 2020. Blocks as large as 50 cm were scattered around the dome, and a few reached the adjacent Nuevo and Arrau craters. Ejecta was scattered up to 140 m from the crater rim. A collapse on the N rim had sent debris down that flank. The main fissure feeding the flow had grown to 20 m wide and almost 70 m long by the end of the month (figure 76). Measurements of the volume of effusive material on 24 March from a DEM indicated that the extruded volume since June 2020 of the flow was just over 1,900,000 m³ and for the dome was almost 412,000 m³.

Figure 76. Scientists doing fieldwork at Nevados de Chillan on 26 March 2021 collected samples of lava from the L5 flow on the N flank of Nicanor crater. The lava emerged from a fissure at the edge of the crater. The brown material on the left is the part of the dome that had grown over the NE rim of the crater and sent debris down the flank. Courtesy of SERNAGEOMIN.

Both explosive and effusive activity increased during April 2021, although the height of the plumes remained below 800 m. Continued dome growth beyond the NE edge of the crater had created an area of unconsolidated deposits on the NE flank. The dome extended 140 m beyond the crater by mid-month. The fissure connecting the dome and the N-flank L5 flow measured 60 m long, 25 m wide, and 8 m high by 15 April, and had grown to 80 m long and 32 m wide by the end of the month (figure 77). The total volume of extruded material from both the dome and the flow on 13 April was about 2,760,000 m³. Lava spines were reported during the second half of April. The lava dome continued to grow and reached 66 m high by mid-month. Most of the growth during the first half of the month was concentrated on the edges, with a depression in the center of the dome. During the second half of April the dome growth was concentrated on the W edge of the crater. The lava flow was about the same length at the end of April as it had been at the end of March. Horizontal inflation increased to 12.5 mm/month during the first half of April but was stable during the second half; the vertical component indicated inflation from all the stations, with the maximum deformation of 21 mm at station FRSC. An increase in the flow rate was observed beginning on 28 April (figure 78), and a new lava flow on the NE flank was observed on 4 May 2021.

Figure 77. An annotated image of the N flank of Nevados de Chillan from 6 April 2021 identified the central channel of the N-flank L5 flow and its front (dashed yellow line), the overall active flow area (red dashed line), detachment blocks falling off the active flow channel (derrumba de rocas), the lava dome (yellow circle), pyroclastic debris on the NE flank from the dome (Depositos piroclasticos provenientes del domo), and an eruptive column of gas and ash emerging from the dome inside Nicanor Crater. Courtesy of Volcanologia Chile.

Figure 78. Steam and ash emissions and thermal anomalies continued at Nevados de Chillan during April 2021, seen here in Sentinel-2 satellite images. On 15 April (left) a plume of steam and ash drifted SE from the Nicanor crater. On 28 April (right) a surge in flow activity produced a strong thermal anomaly at the front of the L5 flow on the N flank of the crater in addition to the anomalies from the head of the flow and the growing dome. Left image uses Natural color rendering (bands 4,3,2) and right image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

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

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Volcanología Chile (URL: https://www.volcanochile.com/joomla30/, Twitter: @volcanologiachl).

Stromboli, in the northeastern Aeolian Islands, includes the active Northern (N) and Central-South (CS) craters in the summit area at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW flank. The current eruption period began February 1934 and has been recently characterized by Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows (BGVN 46:02). This report updates activity consisting of similar eruptive events during January through April 2021 using information from daily and weekly reported by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Activity was consistent during this reporting period. The average explosion rates ranged from 2-28 per hour in the N crater and 1-8 per hour in the CS crater, though individual explosions varied in intensity (table 11). Ejected material rose up to 250 m above both the N and CS craters. Strombolian explosions were often accompanied by gas-and-steam emissions and frequent intense spattering in the N crater, depositing material on the Sciara del Fuoco. Lava fountains were also reported. On 18, 22, and 24 January four lava flows were detected in the N crater area. According to INGV, the average SO2 emissions measured 250-300 tons/day.

Table 11. Summary of activity at Stromboli during January-April 2021. Data courtesy of INGV.

Activity during January consisted of Strombolian explosions in the N and CS craters ranging from 7-18 per hour and 1-4 per hour, respectively. The frequency of explosive events was relatively low during 1-6 January (less than 10 events per hour), increased on 7 and 8 January (up to 18 events per hour), then dropped again during 9-10 January (12-14 events per hour). The N1 vent contained two points of emission, producing low-intensity explosions that ejected fine ash, coarse lapilli, and bombs. The N2 vent contained four points of emission that generated explosions of more variable intensity and ejected coarse lapilli. Spattering at this vent also constructed some hornitos, which in turn generated jets of incandescent material. Spattering was more energetic during 7-8 January and then started to decline to weak and less frequent intervals.

During 18, 22, and 24 January INGV reported four lava flows in the N crater beyond the edge of the N2 vent, extending onto the upper Sciara del Fuoco (figure 196). Two points of emission between vents N1 and N2 have been designated P1 and P2 (figure 197), the latter of which has been observed since October. The first lava flow originated from P1 and was reported on 18 January at 1115; incandescent blocks of material traveled a few tens of meters in the upper part of the Sciara before stopping (reaching an elevation of 700 m). A second lava flow, originating from P2 occurred between 1600 and 2100 on 18 January, stopped in the central part of the Sciara. The third lava flow also originated from P2 on 22 January between 1222 and 2100, extending for a few hundred meters in the upper and central part of the Sciara (figure 198); this flow had a greater volume compared to the first two and accumulated at the coastline. The fourth lava flow was detected on 24 January at 1956 from P2 and continued until the morning of 25 January; its volume was less than the third flow. In the CS crater at least two vents were active, one of which emitted only ash, while the other ejected coarse incandescent material 250 m above the crater.

Figure 196. Thermal webcam images showing the evolution of the lava flows (bright red, yellow, and green) at Stromboli during 18 (a-d), 22 (e-f), and 24 (g-h) January 2021 accompanied by some gas-and-steam emissions. Images captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 04/2021, Stromboli, Bollettino Settimanale, 18/01/2021 - 24/01/2021, data emissione 26/01/2021).

Figure 197. Thermal webcam image indicating the relative locations of the crater areas (N1, N2, and CS) and lava emission points (P1 and P2) on Stromboli. Image taken on 18 January 2021 captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 04/2021, Stromboli, Bollettino Settimanale, 18/01/2021 - 24/01/2021, data emissione 26/01/2021).

Figure 198. Webcam image of the effusive activity at Stromboli at 1920 (local time) on 22 January 2021. Image captured by the SCV surveillance camera. Courtesy of INGV.

Similar explosive activity continued in February. The average explosion rates ranged from 5-28 per hour in the N crater and 1-6 per hour in the CS crater. The N1 vent continued to produce low-intensity explosions that ejected material less than 80 m high, and sometimes up to 250 m, that consisted of fine ash mixed with coarse lapilli and bombs. On the evening of 4 February the lava fountains exceeded 100 m above the vent. The N2 vent had stronger explosions that ejected coarse material 250 m high; intense and frequent spattering was also observed in this vent, particularly on 8, 10, and 13 February. Weak spattering was observed through 17 February. During 24-25 February observations were made by the HPHT Lab from INGV-Roma 1 and FlyEye Team of OE as part of the UNO Departmental Project. Drone images were taken to document the active vents and craters (figure 199). In addition, a Digital Surface Model (DSM) was created to map the summit craters (figure 200). Crater measurements showed that CS1 was greater than 90 m wide, CS2 was about 70 m, and CS3 has a diameter of 43 m. In the N crater, N1 and N2 measured 47 and 67 m in diameter, respectively.

Figure 199. Drone images of the summit crater at Stromboli documenting the craters (yellow outline) and the location of the active vents (red dots). White gas-and-steam emissions were also recorded in these images. Courtesy of INGV (Rep. No. 09/2021, Stromboli, Bollettino Settimanale, 22/02/2021 - 28/02/2021, data emissione 02/03/2021).

Figure 200. Digital Surface Model (DSM) of Stromboli’s summit crater area with a resolution of 10 cm. Each crater is outlined with a different color: CS1 (yellow), CS2 (orange), CS3 (dark red), N1 (light green), and N2 (dark green). The red dots represent active locations at the time of the survey. Courtesy of INGV (Rep. No. 11/2021, Stromboli, Bollettino Settimanale, 08/03/2021 - 14/03/2021, data emissione 16/03/2021).

During the week of 22-28 February two active vents in the N crater produced explosions that ejected lapilli and bombs up to 100-120 m high. Four vents were visible in N2, one of which was a hornito near the edge of the N1 crater that was characterized by degassing. Only two of these four vents were active, and the resulting explosions were small and contained coarse material. INGV also reported an extensive sulfur deposit on the hornito and on the N1 vent. In the CS crater ash sometimes mixed with coarse material was ejected 250 m high. Three vents were identified in the CS1 crater: one was inactive, one was degassing, and the largest vent was characterized by spattering and ejecta. In the SW section (CS2), there were at least four active vents: a hornito that produced some explosions and ejected coarse material, two vents behind the hornito that produced simultaneous explosions and ash emissions, and a deep crater further N with a vent on the floor that was characterized by small explosions and gas emissions.

Strombolian explosions persisted in March, though at an overall lower rate compared to February: 2-14 per hour in the N crater and 1-8 per hour in the CS crater. The N1 vent continued to generate low-intensity explosions that ejected material 80-250 m high, accompanied by minor ash emissions, while the N2 vent had similar explosions that ejected mixed coarse material. In the CS crater coarse material mixed with ash was ejected 250 m above the crater. An episode of stronger explosive activity lasting 3 minutes and 30 seconds early on 1 March consisted of about 10 explosions in the summit area (figure 201). Three strong explosive pulses were detected starting at 0232. At 0233 a strong explosion ejected material 350 m above the crater that fell along the upper part of the Sciara in the W and toward the Pizzo in the E. Two simultaneous explosions at 0235 from the CS crater and N1 lasted 22 seconds, ending the sequence. Resulting ejecta was deposited along the Sciara del Fuoco and toward Pizzo.

Figure 201. Thermal webcam images showing the explosive sequence at Stromboli on 1 March 2021 from 0232 (a) through 0236 (h) local time (the webcam timestampes are reported in UTC). Images captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 10/2021, Stromboli, Bollettino Settimanale, 01/03/2021 - 07/03/2021, data emissione 09/03/2021).

Field observations were conducted on 31 March and 2-3 April by INGV staff to measure any changes in the summit crater and describe the activity. According to INGV, N1 showed two vents with low-to-medium explosions that ejected mainly coarse material mixed with ash. Two vents in one cone at N2 had little activity, but a hornito produced gas-and-steam emissions. CS1 has two vents adjacent to each other; the northernmost exhibited gas-and-steam emissions. The southernmost vent was occasionally explosive and ejected coarse material. CS2 contained three emission points with a SW-NE orientation, showing explosive activity of varying intensities and ejecting material of different sizes. CS3 had little explosive activity; sporadic modest explosions generated reddish ash. The average rate of explosions in the N crater was 2-18 per hour, and in the CS crater it was 1-7 per hour. N1 continued to eject coarse material (lapilli and bombs) up to 250 m high while N2 ejected fine ash mixed with coarse material. After 12 April both the intensity and volume of ejecta decreased in N1. The N2 vent showed low-intensity explosions and fine ejecta through 15 April; coarse material and weak spattering characterized 17 April. In the CS crater, at least three emission points were observed that generated explosions and ejected coarse material was mixed with medium-sized ash up to 250 m high.

Intermittent, low-power thermal activity was detected during January through April, according to the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 202). During late January through early February, a cluster of stronger thermal anomalies were detected. Two thermal alerts were reported by the MODVOLC system on 21 January, around when the cluster of anomalies were detected by MIROVA. Sentinel-2 infrared satellite imagery showed a consistent thermal hotspot in both summit craters on clear days (figure 203).

Figure 202. Intermittent low thermal activity at Stromboli was recorded by the MIROVA system (Log Radiative Power) from MODIS satellite data during January through April 2021; a cluster of stronger anomalies occurred during late January through early February. A single low-power anomaly was detected in early March, with four in April. Courtesy of MIROVA.

Figure 203. Persistent thermal anomalies (bright yellow-orange) at Stromboli were visible in Sentinel-2 infrared satellite imagery from both summit craters during January through April 2021. On 7 January (top left) and 16 February (top right) 2021 three thermal anomalies were visible in the summit area. By 8 March (bottom left) and 27 April (bottom right) only two anomalies were observed. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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/); 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/); 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).

Infrequent phreatic explosions have occurred at the Sileri Crater Lake in the Dieng Volcanic Complex, with three explosions between 30 April and 2 July 2017, and one on 1 April 2018 (BGVN 42:10, 43:05). None were reported in 2019 and 2020. The volcano is monitored by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Centre for Volcanology and Geological Hazard Mitigation or CVGHM).

PVMBG reported that a phreatic explosion at the Sileri Crater Lake occurred at 1825 on 29 April 2021, ejecting rocks 200 m S and E and mud 400 m S and 300 m E. According to a news article, a local road was temporarily closed because rocks (about 10 cm in diameter) from the explosion were scattered along the road and the mud made conditions slippery. The closest residents are 1 km away. The Alert Level remained at 1 (on a scale of 1-4), and the public was warned to stay 500 m away from the crater rim.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Detik News (URL: https://news.detik.com/).

Karymsky, part of Kamchatka’s eastern volcanic zone, has had frequent eruptions since 1996 that have included ash explosions, ash plumes, gas-and-steam emissions, and thermal anomalies. Its most recent eruption began in April 2020 and has been characterized by ash explosions, ash plumes, ashfall, gas-and-steam emissions, and thermal anomalies (BGVN 45:10). This report covers activity from November 2020 through April 2021 and describes the end of the previous eruption in February 2021 and the start of a new eruption in April. Information comes from daily, weekly, and special reports from the Kamchatka Volcanic Eruptions Response Team (KVERT), the Tokyo Volcanic Ash Advisory Center (VAAC), and satellite data.

During the first half of November 2020 activity consisted of intermittent explosions accompanied by thermal anomalies. During 1-2 November an explosion sent ash plume to 6.5 km altitude that extended as far as 60 km ENE and 30 km SW. An ash explosion on 8 November generated an ash plume that rose to 8 km altitude and drifted 230 km NE. According to the Tokyo VAAC ash plumes were observed on 9 and 11 November that rose to 6 and 7 km, respectively. Intermittent thermal anomalies were observed in satellite data throughout the month.

Moderate gas-and-steam emissions were observed intermittently during December, sometimes accompanied by thermal anomalies. On 10 December, at 0845 local time, explosions generated ash plumes that rose to 6-7 km altitude and drifted NW. Ash explosions continued throughout the day, drifting as far as 220 km NW, W, and SW. Subsequent ash plumes were reported on 13 and 18 December that rose to 3.9 km altitude and drifted N, and 2.7 km altitude that drifted SW, respectively. Explosions on 26 and 30 December produced ash plumes that rose to 4-5 km altitude and drifted as far as 70 km NW. The Tokyo VAAC reported ash plumes to 5.2 km altitude that drifted NW and N on 27 December, to 3 km altitude that drifted SE on 29 December, and to 4.6 km altitude that drifted W on 31 December.

Similar ash explosions accompanied by thermal anomalies were reported during early January 2021. On 1 January the Tokyo VAAC reported an ash plume that rose up to 5.2 km altitude and drifted S, followed the next day by explosions that sent plumes to 5.5 km altitude and drifted 130 km SE. Some of the resulting ash deposits on the snow-covered flanks were observed in Sentinel-2 natural color satellite imagery (figure 55). KVERT reported that a thermal anomaly over a lava dome was visible in satellite images during 14-15, 20-24, and 27 January. Explosivity significantly decreased in February and activity was primarily characterized by moderate gas-and-steam emissions and a thermal anomaly that was last detected on 5 February, marking the end of the current eruption period.

Figure 55. Sentinel-2 natural color satellite images showing fresh ash deposits (dark gray) on the snowy flanks at Karymsky, occasionally accompanied by white gas-and-steam plumes, as seen on 3 (top left) and 28 (top right) December 2020 and 2 January (bottom left) 2020. Satellite images with “Natural Color” (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

During 2-6 April a thermal anomaly was detected in satellite data, according to KVERT. Explosions on 4 April at 1130 local time resulted in ash plumes that rose to 8.5 km altitude, which then drifted NE for 255 km during the day, marking the beginning of a new eruptive episode. On 11 April at 1745 ash explosions rose to 4 km altitude and drifted 67-115 km SE, according to a VONA (Volcano Observatory Notice for Aviation). KVERT continued to report weak thermal anomalies that were visible in satellite images during 9-12, 16-17, 22-23, and 29 April.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows three small clusters of low-to-moderate strength thermal anomalies during early November, early December 2020, and early January 2021 (figure 56), which each coincided with explosion events reported by KVERT. No thermal activity was detected after late January through April, according to the MIROVA graph, though KVERT noted thermal anomalies during early February and again in early April. A total of two thermal hotspots were detected by the MODVOLC thermal algorithm on 10 December, which was also visible in an infrared satellite image. Sentinel-2 infrared satellite images captured white gas-and-steam plumes rising from the summit during 10 November and 10 December; on 10 December the explosive events were accompanied by a strong thermal anomaly that was visible through the clouds (figure 57). Weaker thermal anomalies were observed in the summit crater on clear weather days on 25 December and 14 January 2021, which were also reported by KVERT (figure 57).

Figure 56. Small clusters of low-to-moderate strength thermal anomalies at Karymsky were detected during early November, early December 2020, and early January 2021 as seen in the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 57. Sentinel-2 infrared satellite images show strong degassing plumes from the summit crater of Karymsky on 10 November (top left) and 10 December (top right) 2020 both drifting W. On 10 December a strong thermal anomaly was visible at the summit but was mostly obscured by clouds. On 25 December (bottom left) 2020 and 14 January (bottom right) 2021 faint thermal anomalies were still visible in the crater, accompanied by some ash deposits (black color). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Langila, located at the western tip of Papua New Guinea’s New Britain Island, consists of a group of four small overlapping cones. Frequent mild-to-moderate explosive eruptions with ash emissions have been recorded since the 19th century from three active summit craters. The current eruption period began in October 2015 and has recently been characterized by low-level thermal activity and ash plumes (BGVN 45:11). Similar activity continued during this reporting period of November 2020 through April 2021 using information primarily from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite images.

The NASA Global Sulfur Dioxide page, using data from the TROPOMI instrument on the Sentinel-5P satellite, showed a few weak sulfur dioxide plumes during early December 2020 and late March 2021 that drifted in different directions (figure 22). The Darwin VAAC issued notices of ash plumes on 9 January 2021 to 4.9 km altitude that drifted W, on 13 January to 3 km that drifted WSW, and on 5 April to 1.5 km that drifted SW.

Figure 22. Small sulfur dioxide plumes were visible above Langila based on data from the TROPOMI instrument on the Sentinel-5P satellite. Faint plumes drifted W on 8 December (top left) 2020, N on 10 December (top right), SE on 18 March (bottom left) 2021, and NW on 20 March (bottom right). Small plumes were also present on most of those days originating from Manam (to the W) and Bagana (to the E) volcanoes. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

MIROVA recorded a single low-power thermal anomaly was detected during early November, followed by four more during early December (figure 23). A low-power cluster of thermal anomalies resumed in mid-March that continued through the month. Three more anomalies were recorded in late April. The latter part of this thermal activity was also detected in Sentinel-2 infrared satellite imagery. A single thermal anomaly was visible at the summit crater beginning in February 2021, and in March a second thermal anomaly appeared that continued to be observed through April (figure 24).

Figure 23. Few low-power thermal anomalies at Langila were detected during early November (1) and early-to-mid-December (4) 2020 as recorded by the MIROVA graph (Log Radiative Power). A cluster of low-power thermal anomalies were detected in mid-March 2021 that continued through the month, followed by three anomalies in late April. Courtesy of MIROVA.

Figure 24. Weak thermal anomalies were visible at the summit of Langila in infrared satellite imagery during February through April 2021. Though clouds obscured most of the view on 12 February (top left), a single faint thermal anomaly was observed. On 9 March (top right) two thermal anomalies were observed at the summit, which were also visible on 6 (bottom left) and 18 (bottom right) April. On 18 April, the western thermal anomaly seemed to have decreased in strength slightly. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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 E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N 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. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

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

Extensive lava flows, bomb-laden Strombolian explosions, and ash plumes from Mackenney crater have characterized the persistent activity at Pacaya since 1961. The latest eruptive period began with intermittent ash plumes and incandescence in June 2015; the growth of a new pyroclastic cone inside the summit crater was confirmed later that year and has continued, producing frequent loud Strombolian explosions rising above the crater rim and ongoing ash emissions. In addition, flank fissures have been the source of lava flows during 2019-2021. A significant increase in both effusive and explosive activity that began in February 2021 continued through mid-May. Activity during March-May 2021 is covered in this report with information provided by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), multiple sources of satellite data, and photographs from observers on the ground.

Summary of activity during March-May 2021. Incandescent explosions, ash emissions, and subsequent ashfall increased substantially at the beginning of March 2021 from the already increased levels during February. Explosions sent ejecta hundreds of meters high and hundreds of meters from the summit; ash plumes drifted tens to hundreds of kilometers and ashfall occurred almost daily in communities within tens of kilometers of Mackenney crater. The most extensive ash emissions forced closure of the International Airport in Guatemala City on 22 March. Ash emissions decreased during April and were intermittent into the first half of May, after which they tapered off.

Effusive activity also increased significantly during March 2021; by early in the month as many as three lava flows with multiple branches, all about 1 km long, were simultaneously active on multiple flanks. A new fast-moving flow appeared on the SW flank during the second half of March and rapidly reached 1.5 km in length, flowing NW then SW, ultimately extending over 3 km. It had multiple branches that caused vegetation fires, destroyed significant cropland, and crossed roads before stopping in mid-April. A new flow emerged along a similar path at the end of April and grew to over 2 km long in early May before activity at its source fissure ended on 17 May. High temperatures remained at many flow areas around the volcano for the rest of the month.

The high levels of activity are reflected in the MIROVA radiative power data for the period which show the increase in intensity to very high levels through mid-April, followed by a pulse in late April and early May that corresponds to explosions and lava flows. Thermal activity decreased significantly by the third week of May (figure 160). The MODVOLC thermal alert data shows a similar pattern with multiple alerts issued most days in March and for the first half of April, and another pulse of activity from 27 April-13 May. Significant sulfur dioxide emissions were recorded in satellite data several times in March and April and corresponded to periods of increased explosive and effusive activity (figure 161).

Figure 160. The ongoing eruption at Pacaya increased significantly in intensity in December 2020 and continued to increase through March 2021 as seen in this MIROVA log radiative power graph. Abundant ash emissions and extensive lava flows emerged from numerous fissures until activity decreased substantially in mid-May 2021. Courtesy of MIROVA.

Figure 161. Pulses of increased sulfur dioxide emissions at Pacaya were measured by the TROPOMI instrument on the Sentinel-5P satellite multiple times during March and April 2021, including (top row, left to right) on 5, 10, and 21 March, and (bottom row) 6, 8, and 16 April. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during March 2021. A notable increase in seismicity early on 1 March 2021 coincided with increased Strombolian activity. Observatorio Volcán de Pacaya geologists observed explosions sending ejecta 500 m above the rim of Mackenney crater accompanied by plumes of ash and gas that reached 3.5 km altitude and drifted W and SW. For most of March high levels of Strombolian activity sent ejecta 200-400 m high each day, sometimes higher, reaching 800 m on 3 March, 800-1,000 m on 5 March, and 700 m on 10 March (figure 162). Sounds as loud as a train locomotive or plane engine from the explosions were frequently reported, and ejecta was sometimes scattered 500-600 m from the cone. Explosive activity with ejecta and ash emissions were also reported from the fissure feeding the lava flow on the S flank 300 m below Mackenney crater. On 14 March, ejecta from the fissure sent block avalanches 1,300 m down the S flank.

Figure 162. Strombolian activity at Pacaya sent ejecta hundreds of meters above the summit and down the flanks on 4 March 2021 while effusion continued on the SW flank, also producing an intense glow. Image by Reuters photographer Josue Decavele taken from Los Rios. Courtesy of Reuters Pictures.

The increase in explosive activity also included an increase in dense ash emissions and resulting ashfall during March 2021. Ash plume heights ranged from 3 to 5.5 km altitude, and often drifted W, NW, or SW. The Washington VAAC reported an ash plume centered about 75 km WSW of the summit on 1 March. On 3 March a dense ash emission was drifting W from the summit at 3.7 km altitude. The next day ash was detected almost 100 km SW just off the Mexican Pacific coast before dissipating. The altitude of the ash emissions increased to 4.9 km on 5 March; puffs drifting W were visible in satellite images extending over 250 km from the summit the next day. Pulses of activity lasted between 15 minutes and 13 hours, and produced tephra fallout around the volcano, dense ash plumes that drifted 3-5 km, and finer ash plumes that drifted more than 60 km.

Explosions on 7 March caused lava fountains 100-500 m above the crater. The following day ash plumes were drifting 45 km SW at 3 km altitude. On 9 March ash plumes fanned out from the NW to the SW about 30 km from the summit before dissipating. From 11 March onward multiple daily discrete ash emissions extended at least 30-50 km WNW and SW from the summit at altitudes of 3.7-4.3 km altitude, and much farther on some days. The plumes reached 90 km WSW on 12 March, and 140 km W on 14 March. The next day, ash emissions extended over 100 km WSW, with remnants visible in satellite images almost 185 km away by the end of the day. On 16 March they drifted 170 km WNW at 4.3 km altitude and on 18 March the ash emissions were observed drifting SW at 3.4 km altitude extending 185 km from the summit. Dense gray-black emissions were accompanied by white steam emissions on 21 March (figure 163).

Figure 163. Dense dark gray ash emissions rose from the summit of Pacaya on 21 March 2021 causing significant ashfall around the region. In addition, white steam plumes surrounded the summit. Courtesy of CONRED.

Dense ash clouds seen on 22 March 2021 were drifting rapidly SSE at 4.9 km altitude as far as 75 km, SE at 6.1 km altitude, and visible in satellite imagery moving E at 7.6 km altitude up to 25 km from the volcano. The next day they were drifting NE at 3 km altitude up to 90 km away, and SW at 4.6 km altitude. A narrow ash plume was detected in visible satellite imagery on 28 March drifting about 80 km NW of the summit before dissipating. Over the next two days a plume was detected moving SW at 3 km altitude about 130-150 km from the summit. In addition, another plume was drifting NW at 4.3 km altitude on 31 March causing dense ash to cover the summit of Fuego that was visible on webcams. The lower plume was visible over 300 km SW of Pacaya before it dissipated (figure 164).

Figure 164. Haze from ash emissions at Pacaya extends for tens of kilometers across the region in multiple directions after many days of emissions, while a fresh ash plume rises above the volcano in the left foreground on 31 March 2021. Ash drifted NW up to 50 km and was reported in Sacatepéquez and Chimaltenango. In the middle right to the NW is the large Agua volcano, and behind it to the right are Fuego and Acatenango. Ash from Pacaya was visible in Fuego webcams that day. Courtesy of INSIVUMEH.

Communities all around Pacaya were affected by ashfall many times throughout March 2021 (figure 165, table 7). Most of the communities were within 10 km of the summit, but ashfall reached more than 20 km away multiple times. During the bigger ashfall events, blocks more than 6 cm in diameter fell on the flanks of the volcano, while lapilli (2 mm to 6 cm) fell up to 5 km away, and fine ash was observed up to 30 km away (figure 166). The most significant ashfall events occurred during 22-23 March when ash drifted tens of kilometers in multiple directions and caused the closure of La Aurora International Airport in Guatemala City (figure 167).

Figure 165. Communities all around Pacaya were affected by ashfall throughout March 2021. The red oval was the area where INSIVUMEH cautioned residents to be prepared for ashfall and lapilli after explosions on 3 March. All of the communities shown by yellow stars were affected by ashfall at some point during March. Courtesy of INSIVUMEH (Boletin Volcanologico especial BEPAC-41-2021, Eruption, Volcan Pacaya, 3 de marzo 2021, 11:55 horas).

Table 7. Communities reporting ashfall from Pacaya during March 2021. Information courtesy of INSIVUMEH.

Figure 166. Lapilli-size tephra (2 mm to 6 cm) from Pacaya was reported several times during March 2021 in communities as far as 5 km away, including this example on 16 March 2021 from Concepcion El Cedro (4 km NNW). Courtesy of INSIVUMEH (BOLETIN VULCANOLOGICO ESPECIAL BEPAC-56-2021, ACTUALIZACION DE ACTIVIDAD Y CAIDA DE TEFRA, 16 de marzo 2021, 09:05 horas).

Figure 167. A plane at the La Aurora international airport in Guatemala City was dusted with ash from Pacaya on 23 March 2021, forcing closure of the airport for much of the day. Photo by Moises Castillo/AP, courtesy of CNN.

Lava flow activity also increased significantly during March 2021. At the end of February, an active flow on the S flank remained about 1 km long, shedding incandescent blocks hundreds of meters from its advancing front. By 3 March, three flows with multiple branches were active on the SSW flank; they were 800-1,000 m long (figure 168). On 5 and 6 March two flows with many branches extended 300-500 m down the S flank (figure 169). Flows were active on the SW, S, and SE flanks on 7 March. The S-flank-flow with two branches reached 1 km long by 8 March and had incandescent blocks constantly falling of the leading edge. It increased steadily in length, reaching 1.8 km by 16 March (figure 170).

Figure 168. Three flows were active on the S and SW flanks of Pacaya on 3 March 2021, seen here with an infrared camera. Courtesy of INSIVUMEH.

Figure 169. On 5 March 2021 two main flows with multiple branches extended 300-500 m down the S flank of Pacaya causing very bright thermal signatures in satellite imagery. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 170. Two branches of the S-flank lava flow at Pacaya were each about 1.4 km long on 12 March 2021. Courtesy of INSIVUMEH (FOTOGRAFÍAS RECIENTES DE VOLCANES).

Two new flows emerged from the S and SE flanks on the morning of 18 March (figure 171). The S-flank flow grew to 500 m and part of it overflowed outside the plateau. The SE-flank flow was 400 m long in front of the village of Los Llanos, causing fires in the vegetation which continued for several days (figure 172). On 20 March the SE flank flow caused a strong thermal signature in satellite imagery with incandescent blocks falling downslope far beyond the front (figure 173). During the night of 20-21 March, a new flow appeared on the SW flank and grew to 500 m long; the flow on the SE flank reached 850 m. The following day the rapidly growing SW-flank-flow reached 1,500 m long, causing vegetation fires on ranches in Las Granadillas.

Figure 171. Two new flows emerged from the flanks of Pacaya on 18 March 2021 as seen in this FLIR thermal webcam image. The S-flank flow (center) grew to 500 m with two active branches. The SE flank flow (right) descended 400 m near the village of Los Llanos and burned vegetation. A third fissure higher on the SW flank (upper left) also had a short active flow. Courtesy of INSIVUMEH (Boletin Vulcanologico Especial BEPAC 58-2021, 18 March 2021).

Figure 172. Burning vegetation from a lava flow on Pacaya’s SE flank was controlled by CONRED workers on 20 March 2021. It was burning at the Los Llanos farmhouse, Finca el Muñeco, Villa Canales. Photo by Sergio Girón, courtesy of CONRED.

Figure 173. On 20 March 2021, a flow on the SE flank of Pacaya was about 400 m long with incandescent blocks falling several hundred meters downslope to the SE and causing fires in the vegetation. A strong thermal signature was also present from explosive activity inside Mackenney crater (top). Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Three flows were active on 22 March 2021, with existing flows on the SW (1,500 m) and SE flanks (300 m), and a new flow on the E (500 m) flank. By 25 March activity was focused on the SW-flank flow which had reached 2.5 km in length (figure 174). It was about 400 m wide and 2.5 m high, burning vegetation as it advanced, and causing damage on coffee and avocado plantations. By 31 March the flow exceeded 3 km in length with multiple active fronts. One of the flow fronts near the community of La Breña was still advancing, but the one at the Campo Alegre farm had stopped moving. The flow continued to cause fires, destroy crops and buildings, and block roads (figure 175).

Figure 174. A large flow on Pacaya’s SW flank had reached 2.5 km long by 25 March 2021 (left) and over 3 km long 5 days later on 30 March (right). It flowed W from a fissure on the W flank, then NW around a higher area before continuing SSW. The flow caused fires, destroyed crops and buildings, and blocked roads. Sentinel-2 images use Atmospheric rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 175. The large lava flow on Pacaya’s SW flank had traveled over 2.5 km by 27 March 2021 when this wide-angle drone image was taken. One of the fronts of the flow was near the community of La Breña and the other was near the Campo Alegre farm. Courtesy of CONRED.

Activity during April 2021. On 1 April 2021 remnant plumes from earlier ash emissions were moving SW over the Pacific about 400 km from the summit at 4.3 km altitude, while newer emissions were drifting S at 3.4 km altitude towards the coast. Continuous ash emissions were reported by the Washington VAAC through April 4 (figure 176) drifting tens of kilometers mostly SW at 3.5-4.5 km altitude. Ash drifted up to 20 km S and SW during the first week and caused frequent ashfall in communities on the SE, S, and SW flanks, with the most affected being Los Pocitos, El Rodeo, and El Patrocinio. A few moderate to strong explosions sent ejecta 100-500 m above the Mackenney crater. By 9 April ash emissions were more sporadic and tended to drift only 5-10 km SW, W, and NW, and no ashfall was reported. The VAAC reported occasional emissions observed in the webcam on 8 and 14 April. An ash plume was detected on 16 April moving NNW at 3.4 km altitude. Strombolian activity diminished and activity changed to primarily steam and gas plumes rising 200 m above the crater after this. A short episode of sporadic explosions during 24-29 April sent ejecta to 250 m above the crater, generated loud noises, and produced ash emissions that rose a few hundred meters and drifted several kilometers.

Figure 176. Daily explosions at Pacaya produced dense ash emissions rising to 3.5-4.5 km altitude during the first part of April, including on 2 April 2021 when the ash drifted S and SE. Multiple branches of the lava flow on the SW flank were also burning vegetation near Las Granadillas and Buena Vista (smoke plumes in the foreground). Courtesy of CONRED.

The SW-flank flow that began during 20-21 March remained active into early April and was 2.8-3 km long during the first week. It continued to advance during the second week and reached 3.7 km long with multiple active branches that were burning vegetation (figure 177). During 7-11 April it was advancing W and N in the area of La Breña and W and S in the area of El Patrocinio and El Rodeo on the Campo Alegre farm (figure 178). By 10 April this flow was 400 m from El Patrocinio and 250 m from San José El Rodeo. By 13 April it was burning avocado and coffee plantations 370 m from houses in El Patrocinio (figure 179). Another active front to the south was 250 m E of El Rodeo and had blocked the road between El Rodeo, El Caracol, and Los Pocitos. The seismic activity associated with the lava effusion decreased significantly beginning on 16 April.

Figure 177. Lava from Pacaya’s SW-flank flow was 300 m wide and extended more than 3 km by 7 April 2021; it was burning vegetation in its path as it advanced at about 5 meters per hour. Courtesy of CONRED.

Figure 178. The SW-flank flow at Pacaya continued to advance during the first half of April 2021 as seen here on 4 (left) and 9 (right) April. The communities of La Breña, El Patrocinio, and El Rodeo were the most affected. Sentinel-2 images use Atmospheric rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 179. By 14 April 2021 the SW-flank flow at Pacaya was 3.7 km long and several hundred meters wide. It had multiple active branches that came within a few hundred meters of the communities of El Patrocinio and San José El Rodeo and had burned significant acreage on coffee and avocado plantations. It also blocked the road between El Rodeo, El Caracol, and Los Pocitos. Courtesy of CONRED.

During 18-20 April 2021 the branch near La Breña stopped advancing, and by 21 April the branch near El Patrocinio had stopped (figure 180), although temperatures remained high and gas emissions from vents along the flow continued in many places through the end of April. A lava flow appeared on the SE flank on 27 April, following a few days of renewed explosive activity, and grew to 175 m by 29 April. INSIVUMEH reported another new flow on the N flank on 29 April (figure 181); it advanced rapidly to the NW around Cerro Chino, and then turned towards the SW, reaching 1.6 km long by later in the day when the leading edge was located about 100 m from La Breña with several active flow fronts.

Figure 180. The lava flow on the SW flank of Pacaya stopped advancing a few hundred meters before reaching El Patrocinio in San Vicente Pacaya, home to about 350 people, on 21 April 2021. Photo by Moises Castillo/Associated Press, courtesy of KTLA.

Figure 181. A lava flow emerged on the N flank of Pacaya on 29 April 2021 and advanced rapidly NW around Cerro Chino and then SW towards La Breña, reaching 1.6 km long by the end of the day. Courtesy of Colred Los Llanos.

Activity during May 2021. Sporadic emissions of steam and gas with occasional ash were typical from Mackenney crater at the beginning of May 2021. Possible ash emissions were seen in satellite data on 1 May drifting W at 3.4 km altitude. Dense plumes, some with abundant ash, were reported on 8 May drifting W and S. Strombolian activity on 10 May from the NW-flank fissure was feeding the flow which began on 29 April; it sent ejecta 50-150 m high, and loud noises were heard. The Washington VAAC reported minor amounts of ash observed in satellite images moving SW from the summit during 10-13 May, when intermittent pulses of dense ash were reported drifting W and SW from the crater. Intermittent ash emissions rose to 3.7 km altitude on 14 May and were observed about 100 km SW before dissipating. Ash plumes drifted up to 5 km W on 15 and 16 May, causing ashfall during 16 and 17 May in El Patrocinio and El Rodeo (figure 182). During 18-21 May constant steam and gas, and periodic ash, emissions drifted 5-10 km NW and W at about 3 km altitude with ashfall reported in communities such as San Francisco de Sales, Concepción El Cedro, Aldea El Patrocinio, and San Miguel Petapa. For the remainder of May, small quantities of ash accompanied dense steam and gas emissions that rose 200-700 m above the summit and drifted W, SW, and S up to 5 km. El Patrocinio, El Rodeo, and other fincas in that area within 10 km reported ashfall on 26 May.

Figure 182. Pulses of dense ash emissions from the summit of Pacaya were noted on 16 May 2021 by a team of volcanologists from Boise State and Michigan Tech Universities. Steam and gas from still-hot lava flows rose from the flanks. Courtesy of Geo_Sci_Jerry.

The N-flank flow that began on 29 April 2021 continued to advance into early May. It had originally flowed NW, then curved around Cerro Chino and headed W. It was 2 km long and advancing in the vicinity of La Breña on 3 May. On 5 May incandescent ejecta was observed at the fissure feeding the flow, which had advanced to the S of La Breña where incandescent blocks continued to fall off the front of the advancing flow. On 6 May the flow reached 2.3 km in length on the W flank, with only one of the fronts continuing to advance slowly. Small explosions were reported at the fissure. The lava flow continued to advance laterally in places as incandescent material spilled over the edges. Explosions from the fissure on 9 May threw material 15 m away as the flow continued moving slowly W (figure 183). By 11 May the flow was no longer advancing at its front but was still expanding due to overflows along its edges. Explosions from the fissure on 14 May launched ejecta 40 m (figure 184), and the flow front again moved slowly westward; by then it was about 2.3 km long (figure 185). Activity at the fissure ceased by 17 May.

Figure 183. Strombolian explosions at the fissure feeding the W-flank lava flow at Pacaya were visible on the night of 8 May 2021. Although the lava flowed rapidly, it didn’t advance significantly after the first week of May; instead the lava flowed laterally and spread out over the flanks in several places until activity at the fissure ceased on 17 May. Copyrighted photo by David Rojas, used with permission.

Figure 184. The fissure on the NW flank of Pacaya was still active on 14 May 2021. Explosions produced ash and ejecta that rose 40 m above the fissure. Courtesy of CONRED.

Figure 185. The flow on the NW flank of Pacaya was also still active on 14 May 2021. It was over 2 km long and still actively flowing but no longer advancing. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.

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/ ); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.php, https://twitter.com/ConredGuatemala/status/1393207685756203011); Colred Los Llanos, Coordinadora local para la reduccion de desastres, Los Llanos, Villa Canales (URL: https://www.facebook.com/Colred-Los-Llanos-102105058094847); 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/); 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); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Reuters Pictures (URL: https://twitter.com/reuterspictures/status/1367472450418704387); CNN (URL: https://www.cnn.com/2021/03/23/americas/guatemala-airport-volcano-closure-latam-intl/index.html); KTLA, (URL: https://ktla.com/news/nationworld/lava-from-guatemalas-pacaya-volcano-threatens-small-communities-that-live-nearby/); David Rojas, (URL: https://twitter.com/DavidRojasGt/status/1391592159221063680); Geo_Sci_Jerry (URL: https://twitter.com/SciJerry/status/1394083192773222406).

Etna is located on the island of Sicily, Italy, and has had eruptions that date back 3,500 years. Its most recent eruptive period began in September 2013 and more recently has been characterized by frequent Strombolian explosions, effusive activity, and ash emissions. Activity has commonly originated from the summit areas, including the Northeast Crater (NEC), the Voragine-Bocca Nuova (or Central) complex (VOR-BN), the Southeast Crater (SEC, formed in 1978), and the New Southeast Crater (NSEC, formed in 2011). Another crater, referred to as the "cono della sella" (saddle cone), developed during early 2017 in the area between SEC and NSEC. This report covers activity from December 2020 through March 2021, consisting of frequent Strombolian explosions of variable intensity, effusive activity, ash emissions, and ashfall. Information for this report comes from weekly and special reports by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV).

Summary of activity during December 2020-March 2021. Intra-crater Strombolian explosions that varied in frequency and intensity throughout the reporting period, and the accompanying ash plumes that rose to a maximum altitude of 11 km, primarily originated from the Southeast Crater (SEC), Voragine Crater (VOR), and occasionally the Northeast Crater (NEC) and Bocca Nuova Crater (BN). Beginning in mid-February a series of short lava fountaining events occurred in the SEC that continued through March. These episodes were also characterized by accompanying ash plumes, incandescent ejecta, and lava flows.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows strong and frequent thermal anomalies throughout the reporting period (figure 319). Some of these anomalies were markedly high in mid-December, mid-January, and mid-March. According to the MODVOLC thermal algorithm, a total of 190 alerts were detected in the summit craters during December through March; thermal anomalies were reported for nine days in December, eleven days in January, fifteen days in February, and sixteen days in March. Frequent Strombolian activity contributed to distinct SO₂ plumes that drifted in multiple directions (figure 320).

Figure 319. Strong and frequent thermal anomalies at Etna were detected during December 2020 through March 2021, as reflected in the MIROVA data (Log Radiative Power). Some thermal anomalies were significantly high in mid-December, mid-January, and mid-March. Courtesy of MIROVA.

Figure 320. Distinct SO2 plumes from Etna were detected by the TROPOMI instrument on the Sentinel-5P satellite on multiple days during December 2020 to March 2021 due to frequent Strombolian explosions, including 22 December (top left) 2020, 20 January (top right), 21 February (bottom left), and 7 March (bottom right) 2021. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during December 2020. During December, INGV reported intra-crater Strombolian explosions in the SEC, NEC, and BN. During the more intense SEC explosions material was ejected onto the flanks. Gas-and-steam emissions were reported in the VOR. A field survey on 13-14 December showed that the SEC was an irregular ellipse 150 x 230 m open to the SW. On 13 December Strombolian activity intensified at 2020 and around 2300 evolved to lava fountains which lasted through 2350, though explosions continued. Collapses of the SW part of the SEC at 2315 resulted in pyroclastic flows that traveled up to 2 km, covering the Monte Frumento Supino cone (SSW flank). Around that time two fissures opened on the SW flank of the SEC and produced lava flows until about 2350 (figure 321). A third minor pyroclastic flow went down the SSW flank at 2330. Two lava fountains were seen during 0050-0110 on 14 December.

Figure 321. Photos of Strombolian activity at the Southeast Crater at Etna on the evening of 14 December 2020 (left) seen from Tremestieri Etneo (20 km S) and a thermal image showing the Bocca Nuova, Voragine, and three active vents in the SEC seen from the Montagnola (EMOV) thermal camera at 0949 (UTC) on 15 December (right). Courtesy of INGV (Report 52/2020, ETNA, Bollettino Settimanale, 14/12/2020 – 20/12/2020, data emissione 22/12/2020).

During a field inspection on 14 December scientists noted that the two lava flows on the S and SW flanks were cooling; the S flow had widened near the base of the SEC and formed four main lobes, one of which had stopped just NW of the cones that formed in 2002-2003 (figure 322). The SW flow traveled SSW, branched, curved around the W part of Monte Frumento Supino, and then stopped. An explosion in the easternmost SEC vent generated an ash plume at 1352 that rose to 4 km altitude and drifted S. Additionally, on 14 December sporadic ash explosions resumed in the VOR; incandescent ejecta was visible at night. On 15 December a new lava flow formed on the SW flank of the SEC at 0924 that advanced a few hundred meters. Eruptive activity briefly stopped in the E vent of the SEC during the afternoon of the 15th and during 16-18 December exhibited strong degassing and nighttime incandescence.

Figure 322. Map of the Southeastern Crater (SEC) at Etna showing active lava flows and the cono della sella (red dot). The light green hatch mark represents the location of the eruptive fissure that opened on the SEC flank. The lava flow extended about 2 km SW, and by 14 December had formed four main lobes. The black arrow represents the direction of the pyroclastic flow after the collapse of the SW portion of the SEC cone. This map uses ground observations and thermal image analysis on a PlanetScope satellite image. Courtesy of INGV (Report 51/2020, ETNA, Bollettino Settimanale, 07/12/2020 – 13/12/2020, data emissione 15/12/2020).

Seismic tremor amplitude gradually increased on 20 December, though weather conditions prevented observations. On 21 December at 1008 Strombolian activity increased in the SEC from the central and easternmost vents. Activity evolved to lava fountaining that lasted an hour, as well as an ash plume that rose to 10 km altitude and drifted NE. An active lava flow was still visible on the SW part of the cone which had collapsed on 13 December. A second flow was observed at 1521 on the S slope of the SEC that descended toward the Valle del Bove. Activity continued through the night (figure 323), and on 22 December at 0350 Strombolian activity increased in the central and easternmost vents; around 0415 a lava flow from the SW flank traveled W, overlapping cooling lava from 21 December. Lava fountaining began again at 0519 and fed three lava flows: one from the S flank traveled SW for 2.8 km and was 600 m wide, branching off to the W and E of Monte Frumento Supino, a second that traveled 2.8 km E toward the Valle del Bove, and a third that originated at the E vent of the SEC that traveled 1.3 km ENE toward the Valle del Leone (figure 324). At 0520 a few small phreatic explosions in the Valle del Bove were due to the lava flow interacting with snow. By 0600 the lava fountains gradually subsided and stopped, though Strombolian explosions persisted at varying intensities. On 24 December at 0830 explosive activity in the E vent of the SEC gradually increased, ejecting material above the crater rim and emitting ash that drifted E.

Figure 323. Photo of Etna’s Southeast Crater showing a new episode of lava fountaining during the early morning on 22 December 2020 viewed from Tremestieri Etneo, south of the volcano. Photo by Boris Behncke, INGV.

Figure 324. Thermal webcam images showing (top left) Strombolian explosions and (top right) lava fountaining in the Southeast Crater seen from the Nicolosi (ENT) and Montagnola (EMOT) cameras on 22 December 2020. Lava flows were visible traveling toward the Valle del Bove and Valle del Leone seen from the Monte Cagliato (EMCT) (bottom left) and Schiena della’asino (ESR) (bottom right). Courtesy of INGV (Report 53/2020, ETNA, Bollettino Settimanale, 21/12/2020 – 27/12/2020, data emissione 29/12/2020).

On 29 December at 0750 there was a gradual increase in explosive activity in the E vent of the SEC, producing ash emissions that drifted ENE. Around 0900 Strombolian activity further intensified, ejecting coarse material onto the E flank of SEC (figure 325), but by 1000 the explosions had decreased. Intra-crater Strombolian activity in the NEC, VOR, and BN continued with sporadic ash emissions through the rest of the month; explosions in the VOR intensified, ejecting material above the crater rim.

Figure 325. Photos of the Strombolian activity at the Voragine (left background) and Southeast Crater (right foreground) at Etna on the evening of 28 December (left) and ash emissions rising from the SEC on the morning of 29 December (right) 2020. Photos by Boris Behncke, INGV.

Activity during January 2021. Activity in January continued with intra-crater Strombolian explosions of variable intensity in the SEC, NEC, VOR, and BN with sporadic ash emissions. On 4 and 6 January at least two episodes of intense Strombolian explosions produced continuous ash plumes that drifted E and ejected coarse pyroclastic material. A lava flow on 17 January breached the SEC at 0740 and traveled to the base of the cone toward the Valle del Bove (figure 326); the lava effusion rate increased at 0819, and the flow reached an elevation of 3 km by 1000. Volcanic tremor amplitude and Strombolian activity intensified at 2000 on 18 January, evolving into lava fountains through 2130. A lava flow emerged in the E vent of the SEC at 2015 and moved 2 km ESE toward the Valle del Bove. Lava fountaining produced a plume that drifted SE, resulting in ashfall in Fleri and Acicastello (figure 327). During 2130-2147 a second lava flow on the N side of the SEC reached a length of 1.3 km. By 19 January the explosions decreased in intensity and the lava flows had begun to cool. On 20 January a new lava flow on the N side of the SEC traveled ENE at 0140, overlapping the previous flow on the 18th; by 1830 it was no longer active. The VOR was characterized by almost continuous Strombolian explosions that ejected material above the crater rim. Satellite imagery from 27 January showed that a small lava flow from a vent in the N section of the VOR was pouring into the BN. The BN also produced Strombolian explosions that often ejected material above the crater rim. At night, summit crater incandescence was observed in the NEC.

Figure 326. Map of the summit craters of Etna showing the active vents and lava flow field on 18 January 2021. The base is modified from a 2014 DEM created by Laboratorio di Aerogeofisica-Sezione Roma 2. The hatch marks indicate the crater rims: BN = Bocca Nuova; VOR = Voragine; NEC = North East Crater; SEC = South East Crater. Red circles indicate areas with ash emissions and/or Strombolian activity. Yellow circles indicate steam and/or gas emissions only. The red shape highlights the active lava flow on 18 January and the yellow and orange shape highlights the cooling lava flow from 17 January. Courtesy of INGV (Report 04/2021, ETNA, Bollettino Settimanale, 18/01/2021 – 26/01/2021, data emissione 26/01/2021).

Figure 327. Photos of lava fountains and an ash plume in the SEC at Etna that resulted in ashfall on the SE flank (top) as well as in Fleri (bottom left) and Acicastello (bottom right). Photo a was taken from Tremestieri on the S side of the volcano. Courtesy of INGV (Report 04/2021, ETNA, Bollettino Settimanale, 18/01/2021 – 26/01/2021, data emissione 26/01/2021).

Activity during February 2021. Variable Strombolian activity continued into February at all four summit craters; the last time this occurred was during 1998-1999. The most intense, almost continuous, Strombolian explosions at the SEC originated from two vents in the eastern top of the cone; less intense activity occurred at the S vent. Intra-crater Strombolian activity at the NEC sometimes produced nighttime incandescence. Explosions at the BN sometimes ejected coarse material above the crater rim. A field inspection on 5 February showed that three scoria cones had been built around vents at the bottom of the crater. Another nearby cone occasionally produced dense emissions. Intra-crater lava flows continued to spill into the BN from the VOR, overlapping those formed in late January. On 6 February around 0530 Strombolian activity intensified in the E vent of the SEC and produced an ash plume that drifted E.

During the morning of 15 February explosive activity at the SEC intensified, with activity continuing at the E vents. Sporadic and sometimes violent explosions were also observed at the saddle cone; intra-crater explosive activity continued in the BN, VOR, and NEC. On 16 February at 1700 lava began advancing down the E flank of the SEC for a few kilometers. A partial cone collapse at 1705 produced a pyroclastic flow that traveled 1.5 km along the W wall of the Valle del Bove. The activity changed to lava fountains around 1710, rising 500-600 m high and generating an ash plume that rose to 6-10 km altitude and drifted S (figure 328). Centimeter-sized lapilli and ash was observed in Nicolosi (16 km S), Mascalucia (19 km S), and as far as Catania (29 km SSE) while fine ashfall was reported in Syracuse (60-80 km SSE). Lava flows continued to advance into the Valle del Bove, reaching an elevation of 2 km by 1759. Another lava flow from the SEC traveled N toward the Valle del Leone; smaller lava flows traveled N and S, reaching 2.9 km elevation. Explosive activity decreased and lava fountaining stopped between 1800 and 1838, though ashfall continued; by 2025 the lava flows had stopped. Strombolian activity persisted at the SEC overnight during 16-17 February and stopped by 0715 on 17 February, though sporadic explosions were reported in the VOR at 0420, 0435, 0444.

Figure 328. Photos during 15-16 February of Strombolian activity at the summit craters at Etna on 15 February 2021 (top left); a pyroclastic flow that occurred at the beginning of the eruptive event on 16 February at 1805 (top right); an eruption plume that was a result from the eruptive event on 16 February, seen from the S (bottom left); map of the lava flows on 16 February showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (light green and dark green), as well as the maximum length (4 km) and volume (2.6 million cubic meters) (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

An eruptive event began at 2330 on 17 February, about 30 hours after the previous one, with a lava flow from the E vents in the SEC, followed by lava fountaining at 0100 on the 18th that rose 600-700 m (figure 329). The lava flow advanced toward the Valle del Bove, the NE, SE, and SW through the saddle vent (“bocca della sella”), covering an area of about 1 km. A second flow on the N flank of the SEC moving toward the Valle del Leone was about 1 km long. Another flow was reported on the S side of the SEC. The resulting ash plume drifted SE, causing ashfall in Zafferana, Etna, and Acireale. The lava fountains ended between 0140-0155 on 18 February, though the lava flows continued to advance.

Figure 329. Activity at Etna during 17-18 February 2021 included lava flows and fountaining. The initial lava flow is seen in a thermal camera image just before midnight from Monte Cagliato on the E side of the volcano (top left). Lava fountains that rose 600-700 m high and lava flows are seen from Milos shortly after midnight (top right). An eruption plume seen from Milos at 0020 on 18 February was accompanied by nighttime incandescence, lava fountains, and lava flows (bottom left). A map of the lava flows on 17-18 February shows the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (yellow and green), as well as the maximum length (4.1 km) and volume (4 million cubic meters) of the flows (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

During the morning of 19 February a lava flow effused from the E vents in the SEC at 0855, followed by a rapid increase in explosions and renewed lava fountaining (figure 330). A line of 4-5 vents produced “fan-shaped” lava fountains at 0953. An ash plume rose to 10 km altitude and drifted SE, causing ashfall in some towns. The lava flow that descended toward the Valle del Bove interacted with snow, causing strong explosions, and were accompanied by rockfalls on the flanks of the SEC. By 1110 the explosive activity had stopped.

Figure 330. Thermal images of the lava flow at Etna around 0900 (local) on 19 February 2021 taken with the thermal camera in Monte Cagliato (top left). Later lava fountains reached 600-700 m high, based on the thermal image from Monte Cagliato (top right). A strong ash plume was observed from Pisano (SE) (bottom left). A map of the lava flows on 19 February showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (orange and green), as well as the maximum length (3.8 km) and volume (4 million cubic meters) (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

Weak Strombolian activity was visible in the late afternoon of 20 February (figure 331). At 2230 a small lava flow from the E vent in the SEC descended 150-200 m into the Valle del Bove. By 2300 the activity had changed to pulsating lava fountains. Beginning at 0100 on 21 February more western vents became active and the E vents ejected lava 600-800 m high. At 0128 lava fountains were ejecting lava up to 1 km high and were sustained for about 10 minutes (figure 331). At the same time, a lava flow from the saddle vent moved a few hundred meters SW. An ash plume rose to 10 km altitude, resulting in ashfall on the SW flank. At 0200 the lava fountains decreased in intensity and by 0220 explosive activity stopped. Periodic ash emissions rose from both the S and E vents later in the evening. A lava flow in the SEC advanced 1 km toward the Valle del Bove. Lava fountains and Strombolian explosions continued at multiple vents. Activity intensified again during 0218-0220 on the 22nd, with lava fountains over 1 km high sending incandescent material onto the flanks. Lava flows in the Valle del Bove reached 3.5-4 km from the crater. During 0430-0515 about 20 strong explosions from SEC vents ejected incandescent bombs that landed at the base of the cone. The NEC was characterized by strong degassing and crater incandescence, often accompanied by Strombolian activity.

Figure 331. Images of weak Strombolian activity in the eastern vents of the SEC at Etna at sunset on 20 February 2021 (top left). Thermal image from the Bronte thermal camera showing strong Strombolian activity at 0131 (local) on 21 February (top right). A strong ash plume at 0205 on 21 February was observed from Tremestieri Etneo (bottom left). A map of the lava flows during 20-21 February showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (red and green), as well as the maximum length (3.2 km) and volume (2.9 million cubic meters) (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

During the evening on 22 February weak Strombolian explosions were visible in the SEC. The frequency and intensity of the explosions increased and by 2210 material was ejected onto the flanks. Jets of lava were ejected 300 m high at 2305, and by 2327 lava fountains were reported from a second SEC vent. Lava overflowed the crater at 2328 toward the Valle del Bove. Within the first hour of 23 February lava fountains rose more than 1.5 km and an ash plume reached 10 km altitude, causing ashfall to the NW. Lava overflowed the S vent and descended SW. At 0115 the lava fountains decreased. Strombolian activity intensified again at 0450, accompanied by ash emissions. Two lava flows traveled SW and SE, the latter of which reached 1.7-1.8 km elevation. By 1000 the lava flows were no longer active; the flow on the SW flank had traveled a few hundred meters, overlapping the previous flows.

The lava fountaining episodes continued; Strombolian activity at the two vents in the SEC increased during the late afternoon on 24 February that evolved into lava fountains reaching 400 m above the crater. Ash emissions also persisted in the SEC. Lava overflows from the crater headed ESE toward the Valle del Bove as far as 2-4 km and in the S area of the SEC. During 1900-2122 the lava fountains reached 500 m high and a resulting ash plume rose as high as 11 km altitude. A second lava flow traveled SW and at 2100 a pyroclastic flow descended 1 km into the Valle del Bove. The lava fountains in the SEC stopped by 2335, though the lava flow remained active in the SW and E sections.

Weak Strombolian activity on 28 February was visible at 0810 that evolved to lava fountains at 0839, feeding lava flows that traveled E toward the Valle del Bove. The fountains abruptly intensified at 0902 with jets of lava rising 700 m above the crater rim. An ash plume rose as high as 11 km altitude and drifted ESE, resulting in ashfall to the E (figure 332). A small lava flow at the S part of the SEC began at 0909, followed by a pyroclastic flow at 0920. The lava fountains ended abruptly at 0933, though the lava descending E remained active. By 1526 the lava flow in the Valle del Bove was no longer active.

Figure 332. Photo of a strong ash plume rising above Etna’s Southeast Crater on the morning of 28 February 2021 that drifted ESE, with ashfall visible. Taken from Tremestieri Etneo. Photo by Boris Behncke, INGV.

Activity during March 2021. Weak Strombolian activity resumed on 2 March at 1145 in the SEC, which increased in intensity at 1234 with ash emissions. From 1324 to 1550 lava fountains generated an ash plume 9 km above the crater, depositing ash and lapilli in Nicolosi, Aci San Antonio (18 km SE), Pedara (15 km SSE), and Catania (29 km SSE). On 4 March Strombolian explosions increased at 0200 and produced ash emissions that dispersed NE (figure 333). At the same time, Strombolian activity from VOR ejected material above the crater. Degassing persisted in the NEC. Around 0320 the Strombolian explosions in the SEC evolved to lava fountains and at 0515 a lava flow from the E section of the base of the cone was traveling toward the Valle del Bove. Strombolian activity in VOR changed to lava fountains at 0859 that were 300 m high. An ash plume rose 11 km above the crater, depositing ash and lapilli in Fiumefreddo (19 km ENE), Linguaglossa (17 km ESE), and the area of Reggio Calabria. Lava fountains continued.

Figure 333. Photos of the beginning an eruptive episode characterized by an early lava flow originating from Etna’s Southeast Crater (right foreground) and an explosion at the Voragine Crater (left background) on 4 March 2021 (left). Dense gray ash plumes and white degassing plumes were visible from several summit vents on 4 March (right). Taken from Tremestieri Etneo. Photos by Boris Behncke, INGV.

Another eruptive episode on 7 March starting between 0100 and 0200 included Strombolian explosions and minor lava effusions at the E base of the SEC that descended into the Valle del Bove. At 0430 an increase in Strombolian activity generated an ash plume that rose to 5 km altitude and drifted E. The lava reached an elevation of 2.8 km altitude by 0450. Strombolian activity intensified again at 0520 and the lava flow advanced to 2.7 km elevation. Lava fountains at 0720 generated another ash plume that rose to 10 km altitude and drifted E. INGV-OE personnel reported ash and lapilli deposits in Milo (11 km ESE), Fornazzo (10 km ESE), Trepunti (17 km ESE), Giarre (17 km ESE), Macchia di Giarre (16 km ESE), Mascali (18 km E), Riposto (19 km ESE), and Torre Archirafi (20 km ESE). Strombolian activity resumed at 1050 and was over by 1500.

Similar Strombolian activity in the SEC on 10 March changed to lava fountaining and a large eruption plume that rose to at least 9 km altitude and drifted ENE (figure 334). Ash and lapilli were reported in Mascali, Giarre, and Fiumefreddo. A lava flow from the S vent reached an elevation of 1.8 km. By 0430 on 10 March the lava fountaining had stopped, though sporadic ash emissions continued until 0700. On 12 March Strombolian activity in the SEC and accompanying ash emissions began again. As the activity intensified, lava overflowed the E part of the SEC, descending toward the Valle del Bove. Lava fountaining was observed up to 500 m and generating an ash plume that rose to 6 km altitude and drifted E. Within an hour, lava had advanced from an elevation of 2.8 km to 2 km. By 0939 the ash plume had risen to 9-10 km altitude and resulted in ashfall in Fleri, Milo, Fornazzo , Giarre, Santa Venerina (15 km SE), and Torre Archirafi (20 km ESE) (figure 335). Lava fountaining had stopped at 1050, though weak Strombolian activity and ash emissions persisted until 1115. The lava flow advanced as far as 1.7 km elevation while a second lava flow expanded on the W slope of the Valle del Bove for an average length of 3 km and a volume of roughly 1 million cubic meters. Strombolian activity continued in the NEC, BN, and VOR, producing minor ash emissions.

Figure 334. Photo of the nighttime lava fountaining activity at Etna during 9-10 March 2021. Courtesy of INGV Youtube channel.

Figure 335. Photo of an ash plume rising above Etna’s Southeast Crater on the morning of 12 March 2021. Taken from Tremestieri Etneo. Photo by Boris Behncke, INGV.

On 14 March Strombolian activity began at 2110 that evolved into lava fountaining at 0048 on the 15th (figure 336). Lava traveled toward the Valle del Bove as an ash plume drifted E (figure 337). By 0343 lava fountaining had stopped, though weak Strombolian activity and lava flows continued. On 17 March at 0155 weak Strombolian activity was observed, changing into lava fountaining at 0319. An ash plume drifted SE and a lava flow was moving toward the Valle del Bove, the latter of which overlapped the one from 15 March. Due to cloud cover, observations were limited and discontinuous. Fountaining activity stopped at 0717 and was followed by explosive activity. Weather conditions cleared the summit on 18 March at 2142, showing explosions in the SEC and a lava flow in the Valle del Bove. On 19 March at 0734 explosive activity was visible in the SEC, which intensified at 0915, accompanied by ash emissions. Lava fountaining started at 0935 with an accompanying ash plume that drifted ENE. By 1136 lava fountaining had stopped and changed to Strombolian activity, which gradually decreased. Only sporadic explosions were visible with minor ash emissions by 1350; lava flows along the Valle del Bove were reported in the late morning.

Figure 336. Photo of a lava fountain episode at Etna’s Southeast Crater during the night of 14-15 March 2021. Taken from Tremestieri Etneo. Photo by Boris Behncke, INGV.

Figure 337. A map of the lava flows on 15 March 2021 showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (blue), as well as the maximum length (2.7 km) and volume (1.1 million cubic meters) (bottom right). Courtesy of INGV (Report 12/2021, ETNA, Bollettino Settimanale, 15/03/2021 – 21/03/2021, data emissione 23/03/2021).

Though weather conditions often prevented a clear view of the summit, weak Strombolian activity was reported in the SEC at 2005 on 23 March, which had evolved into lava fountaining at 0330 on 24 March (figure 338). At 0335 a lava flow from the SEC was seen branching toward the Valle del Bove and the SE. A pyroclastic flow followed the lava at 0336, descending into the Valle del Bove. The lava fountains generated an ash plume that rose to 6-7 km altitude and drifted SSE, resulting in ashfall on the S slope and in Catania. Lava fountaining gradually decreased at 0700 and by 0945, it had stopped; the lava flows continued to advance. Intra-crater Strombolian activity continued in the NEC, BN, and VOR, accompanied by sporadic weak ash emissions. After the fountains stopped, another ash plume was seen rising to 4.5 km altitude and drifting SE. At night, ashfall was reported in Milia and Trecastagni (16 km SE). The explosions had stopped by 1347. By 25 March the two active lava flows had stopped.

Figure 338. Photos of the lava fountain episode and incandescent Strombolian activity at Etna’s Southeast Crater during 23 (left) and 24 (right) March 2021. Taken from Tremestieri Etneo. Photos by Boris Behncke, INGV.

On 30 March weak Strombolian activity in the SEC resumed around 0607 with a single ash explosion that quickly dispersed near the summit (figure 339). Over the course of the day activity at the SEC gradually changed from degassing to continuous weak Strombolian activity at about 1830 from at least two active vents. This activity increased during the night, throwing lava above the crater rim accompanied by sporadic ash emissions. Several lava flows effused from the S base vent. The main part of the flow traveled toward the Valle del Bove with other smaller flows descending to the S and SW. Two other vents at the S base had opened by the evening, one of which ejected spatter a few tens of meters high. Throughout the night, periods of lava fountaining were detected while the main lava flow descended the W wall of the Valle del Bove. Strombolian activity intensified at 1850 and produced an ash plume that rose to 4 km altitude and drifted SSW. At 0000 there was a gradual transition from Strombolian activity to lava fountaining.

Figure 339. Photo of an ash plume rising from Etna’s Southeast Crater on the morning of 30 March 2021. Photo by Boris Behncke, INGV.

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

Information Contacts: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/ ); Boris Behncke, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy; MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA 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/).

Guatemala's Volcán de Fuego has been erupting vigorously since 2002; reported eruptions date back to 1531. These eruptions have resulted in major ashfalls, pyroclastic flows, lava flows, and damaging lahars, including a series of explosions and pyroclastic flows in early June 2018 that caused several hundred fatalities. Activity consisting of explosions with ash emissions, block avalanches, and lava flows has continued since 2018; activity during December 2020-March 2021 is covered in this report. Daily reports are provided by the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH); aviation alerts of ash plumes are issued by the Washington Volcanic Ash Advisory Center (VAAC). Satellite data provide valuable information about thermal anomalies and ash emissions.

The many hourly explosions at Fuego throughout December 2020-March 2021 produced vibrations that rattled roofs and windows in the communities around the volcano every day, sometimes heard and felt as far as 20 km away. The explosions produced incandescent block avalanches that descended the flank ravines (barrancas), with a few of the blocks traveling as far as the vegetation near the bottom. The Seca, Ceniza, and Taniluya ravines were most often affected, but blocks were also reported many times in the Trinidad, Santa Teresa, El Jute, Las Lajas, and Honda ravines. Incandescent ejecta could be seen rising 100-300 m above the summit on most nights. Ash plumes rose to 4.4-4.8 km altitude every day and usually drifted W and SW; the Washington VAAC issued 2-5 ash advisories daily. Ashfall was a near-daily occurrence throughout the period. Effusive activity from 13-15 February produced two lava flows; a series of pyroclastic flows on 14 February affected the Ceniza canyon. For several days after the effusive activity, strong explosions caused ashfall in communities up to 50 km away. The MIROVA graph of thermal anomalies showed persistent high heat levels throughout the period with a brief spike to higher levels during mid-February when the lava flows were active (figure 141). MODVOLC thermal alerts were issued on multiple days each month including eight days in December 2020, 11 days in January 2021, 12 days in February, and seven days in March. Sentinel-2 satellite data showed thermal anomalies inside the summit crater five or six times each month, in all available non-cloudy images.

Figure 141. Consistently high levels of thermal anomalies continued at Fuego during July 2020-March 2021. A brief spike in mid-February 2021 corresponded to two lava flows and a pyroclastic flow. Courtesy of MIROVA.

Explosive activity continued at Fuego during December 2020. Seven to eleven explosions per hour were typical; a few days had 10-15 explosions per hour. Gas and ash emissions rose to 4.4-4.8 km every day with ash plumes drifting usually W and SW 10-15 km, occasionally to 20-25 km (figure 142). Plumes drifted over 10 km N and NE on 6 December, 20-25 km S and SW on 13 and 14 December, and 30 km E, SE, and N during 28-31 December. Vibrations were heard and felt up to 15 km away on the W and SW flanks on 7 December. Ashfall was reported almost daily in multiple communities including Panimache I and II, Morelia, Santa Sofia, Los Yucales, Sangre de Cristo, Finca Palo Verde, and San Pedro Yepocapa. In addition, ashfall was reported on 10 December in Ojo de Agua and Santa Isabel, on 14 Dec in Ojo de Agua and Santa Emilia, in Santa Emilia on 20 and 21 December, and in Chimaltenango to the N on 31 December.

Figure 142. An ash plume rose from the summit of Fuego early on 9 December 2020 while blocks descended multiple ravines and resuspended ash on the flanks. Photo by Fredy Arnoldo Esquit Chiquitá, 07:56 am hora local, courtesy of INSIVUMEH.

The ash plume drift direction continued to be N and NW on 1 and 2 January 2021 resulting in ashfall reported in San Pedro Yepocapa, La Soledad, and San Miguel Duenas. According to INSIVUMEH, plumes drifted 20-25 km those days. In addition to ashfall in Panimache I, Morelia, Santa Sofia, and Yucales most days of the month, ashfall was reported in La Rochela on 3 and 6 January and Ceilan on 6 January. Ashfall was reported to the N in Acatenango on 10 January after activity increased; rumbling was heard 20 km away. Explosions produced ejecta which rose 300 m and sent incandescent blocks around the crater rim and onto the upper flanks. High levels of activity continued the next day and produced ashfall in San Pedro Yepocapa, Santa Sophia, Morelia, Panimache II, El Porvenir Yepocapa, Sangre de Cristo, and at finca Palo Verde. Pulses of incandescent ejecta rising 100-300 m were common during the second half of January and ashfall continued on many days in the same communities to the W and SW. Remobilized ash triggered by incandescent blocks descending the ravines was reported in the last week of January. The number of explosions per hour was 6-12 on many days and they produced noises as loud as a train engine that lasted for several minutes at a time.

Explosive activity during February 2021 remained the same as previous months, with 7-15 explosions per hour, train engine noises that lasted for 3-10 minutes, and gas and ash plumes that rose usually to 4.5-4.8 km altitude and drifted W, SW, and S. Rumblings that rattled windows and roofs were heard 15-20 km away on 5 and 10 February; incandescent blocks descended the ravines for hundreds of meters (figure 143). Near-daily reports of ashfall in communities to the W, SW, and S continued; most affected were Panimache I, Morelia, Santa Sofia, Porvenir, Finca Asuncion, Rochela, Santa Sofia, Yucales, Sangre de Cristo, Palo Verde and Yepocapa. In addition Ceilan, El Zapote, and El Rodeo reported ashfall on 5 February when winds carried ash to S and SE.

Figure 143. Incandescent block avalanches could be seen descending a ravine on the NW flank of Fuego in Sentinel-2 satellite imagery on 3 February 2021. A diffuse ash plume drifts S from the summit. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

In a special report issued on 13 February INSIVUMEH noted that the seismic station had registered a change in the eruptive pattern on 12 February. During the night a lava flow emerged from the summit and traveled 1,000 m down the Ceniza ravine on the SW flank (figure 144). It produced incandescent blocks at the leading edge that fell farther, reaching the vegetation. Loud noises similar to a train engine were audible 8 km from the volcano. At 2100 on 13 February a second flow began in the Seca ravine that grew to 500 m long. Incandescent ejecta rose 200 m above the crater and constant loud noises were reported. By this time the Ceniza flow had reached 1,500 m. The following morning both flows remained active; the barranca Ceniza flow was 1,300 m long and the barranca Seca flow was 500 m long. Persistent explosions of ejecta to 200 m above the crater continued along with loud noises. The incandescent blocks spalling off the front of the flows remobilized ash that drifted S, SE, and SW.

Figure 144. Two lava flows were active on the flanks of Fuego on 13 February 2021. A flow in the Ceniza ravine on the SW flank grew to 1,500 m long, while a 500-m-long flow descended the Seca ravine on the NW flank. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Beginning at 1020 on 14 February 2021 a series of pyroclastic flows were observed in the Ceniza ravine. They lasted for three minutes and traveled several hundred meters. Ashfall was reported in Alotenango, El Porvenir, and Finca La Reunion. By the end of the day the Ceniza lava flow was active for 800 m and the Seca flow reached 200 m. Seismic energy decreased noticeably the next day along with a decrease in the flow rate and thermal energy. Explosions continued with ash plumes drifting E, NE, and N up to 50 km resulting in ashfall in Porvenir and Alotenango. INSIVUMEH considered the effusive eruption over by the evening of 15 February, and noted a decrease in the rate of explosions to 12-14 per hour (figure 145).

Figure 145. Although explosions at Fuego on 15 February 2021 had decreased in frequency, they still produced ash plumes and blocks rolling down the ravines that caused plumes of resuspended ash. Courtesy of INSIVUMEH (BOLETIN VULCANOLOGICO ESPECIAL BEFGO 023-2021, Guatemala, 15 de febrero de 2021, 15:30 horas).

Loud explosions continued 16 February and produced abundant ash that drifted E, NE, and N. The Washington VAAC reported intermittent ash emissions seen in satellite images moving ESE at 4.9 km altitude extending around 110 km from the summit before dissipating. Ashfall was reported in Celian, San Andres Ozuna, Rochela, Zapote, and El Rodeo. On 17 February ash plumes rose to 4.5-4.8 km altitude and drifted N, NE, and E as far as 50 km and caused ashfall in many communities, including as far away as Guatemala City. The wind changed to the E and SE later in the day, and plumes drifted 30-40 km over the departments of Sacatepequez, Escuintla, and Guatemala. Ash plumes from Pacaya were also affecting the same areas that day. The following day ash plumes were drifting 40 km SW. For the remainder of February ashfall affected the same communities to the SW and W as earlier in the month. The incandescent ejecta that rose 350 m above the summit on 28 February produced a strong thermal anomaly in satellite data that also showed incandescent blocks descending all the ravines around the summit (figure 146).

Figure 146. Incandescent ejecta was observed 350 m above the summit of Fuego on 28 February 2021 and produced a strong thermal anomaly shown in this Sentinel-2 satellite image. Also visible is incandescent ejecta around all the ravines near the summit and a small ash plume drifting WNW. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Explosive activity continued throughout March 2021, producing ash plumes that rose to 4.5-4.8 km altitude and drifted mostly W and SW (figure 147). This resulted in ashfall most days in the same communities as before that were located 10-20 km away. The loud rumblings continued daily, lasting for 2-5 minutes at a time and rattling windows and roofs all around the volcano. Incandescent ejecta rose 100-300 m and the blocks traveled down all of the ravines, sometimes reaching the vegetation.

Figure 147. Numerous ash emissions at Fuego during March 2021 were captured in Sentinel-2 satellite images along with the frequent thermal anomalies. Ash plumes drifted W on 5 and 20 March (top row) and NW on 25 and 30 March 2021 (bottom row). Images for 5 and 25 March use Natural color rendering (bands 4, 3, 2). Images for 20 and 30 March show Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: 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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Kavachi is an active submarine volcano in the SW Pacific, located in the Solomon Islands south of Gatokae and Vangunu islands. Volcanism has been characterized by phreatomagmatic explosions that ejected steam, ash, and incandescent bombs. The previous report described discolored plumes extending from a single point during early September 2020 (BGVN 45:10); similar activity was recorded for this reporting period covering October 2020 through April 2021 using satellite data.

Activity at Kavachi is most frequently observed through Sentinel-2 satellite imagery and has recently been characterized by discolored submarine plumes. On 2 October 2020 a slight yellow-green discoloration in the water was observed extending NE from a specific point (figure 23). Similar faint discolored plumes were intermittently recorded on 27 October, 1 November 2020, and 25 January 2021, which each extended NE, SW, and SW, respectively, from a point source above the summit where previous activity has occurred. Intermittent discolored plumes were also visible during March 2021 (figure 24). The plume discoloration on 1 March extended S from the origin point. On 11 March, the discoloration remained near the origin point. A narrow plume extended several kilometers W on 26 March, followed by a short plume seen towards the NW on 31 March. The only plume seen in April was a broad diffuse area of discoloration extending S on the 10th (figure 24). No discoloration near the volcano was observed in May.

Figure 23. Sentinel-2 satellite images of a discolored plume (light yellow-green) at Kavachi beginning on 2 October 2020 (top left) that extended NE. Additional plumes were visible during clear weather on 27 October (top right) that extended NE, on 1 November (bottom left) 2020 that extended SW, and strongly on 25 January 2021 (bottom right) that extended SW. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 24. Sentinel-2 satellite imagery of discolored plumes (light yellow-green) at Kavachi during March-April 2021. On 1 March (top left) the plume was observed extending S with a strongly discolored origin point. On 11 March (top right) the plume remained close to the origin point and did not seem to extend outward. On 26 March (bottom left) the plume was narrow and strongly extended W for several kilometers. On 10 April (bottom right) the plume extended S. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

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

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

The volcanic Semisopochnoi Island in the western Aleutian Islands contains a group of cones within a caldera complex (figure 5). The active Cerberus center has three summit craters, with the current activity originating from North Cerberus. Since September 2018, typical activity has produced minor ash deposits within the vicinity. This bulletin summarizes activity that occurred from April 2020 through May 2021 based on information given by the Alaska Volcano Observatory (AVO), supplemented by satellite data.

Figure 5. This satellite image of Semisopochnoi Island, Alaska, shows the major surface features with an 8-km-wide caldera in the center. As of 2021, Mount Cerberus is the most active of three cones within the caldera complex. The North, East, and South Cerberus craters are indicated, with a faint gas plume dispersing NE from the active North crater on 22 August 2020. Base satellite image from Sentinel-2 using Natural color (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

Intermittent small explosions occurred at Semisopochnoi during early 2020. An AVO Volcano Activity Notice for Aviation (VONA) issued on 1 April reported no indication of activity over the previous two weeks and seismicity at background levels. Satellite data show detectable sulfur dioxide (SO₂) emission and visible steam plumes. Low-level unrest continued into early June with occasional small earthquakes, including a few small low-frequency events and episodic tremor, occasional steam plumes, and detectable SO₂ emissions. An increase in tremor was detected around 12-13 June, and infrasound and seismicity indicated rapid degassing events on 17 and 19 June, with activity declining again by the 20th. AVO noted that clear satellite images acquired on the 21st showed minor ash deposits near the crater, likely from the elevated activity during the previous week, and vigorous gas and steam emission (figure 6). Steam and gas emission continued through to the end of the month then intermittently through July. A 200-km-long SO₂ plume was detected on 15 July and low-level unrest continued.

Figure 6. Minor ash deposits are visible on the Semisopochnoi North Cerberus Crater and a steam plume is shown dispersing ESE on 21 June 2020. Sentinel-2 satellite image with Natural color (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

A gas plume was reported on 7 August and seismicity was above background to the 9th, after that seismicity was at very low levels with no more significant events detected. Infrequent small earthquakes were detected through September and minor steam emissions on the 22nd. Seismicity remained low throughout October. No eruptive activity had detected since mid-June and seismicity had declined to very low levels prior to seismic data transmission failing on 11 November. Due to the lack of data, on 20 November the Aviation Color Code and Alert Level were reduced to Unassigned.

There were no reports of activity during December 2020 or January 2021. A satellite image acquired on 7 February showed several small ash deposits extending at least 3 km from the North Cerberus Crater, likely produced by a small explosion the previous week (figure 7). Steam emission prevented views into the crater and clouds obscured the volcano over the following week.

Figure 7. This Landsat 8 image acquired on 7 February 2021 at Semisopochnoi shows several linear ash deposits from the North Cerberus Crater. This reflects low-level explosive activity. Landsat 9 true Color – pansharpened scene. Courtesy of Sentinel Hub Playground.

On 10 March a satellite image revealed a recently emplaced ash deposit that extended 1.5 km from the crater, with a steam plume being blown to the E (figure 8). Several similar small ash deposits had been noted by AVO in the previous weeks. No activity was observed or detected through 18 March, other than a possible gas plume that day. At 0350 on the 19th a small explosion was detected by infrasound monitoring. Another small explosion was detected at 0230 on the 21st, followed by a series of smaller explosions. During 22-23 March three explosions were detected. Cloud cover prevented visual observation of these events, but possible SO₂ plumes were detected and a confirmed plume on the 23rd indicated further unrest. A probable ash deposit and plume were imaged on the 24th (figure 9). Activity continued with intermittent explosions and SO₂ plumes detected through the 27th.

Figure 8. This 10 March 2021 WorldView-3 satellite image shows ash deposits from low-level explosive activity at the Cerberus North Crater at Semisopochnoi. The ash extends to 1.5 km from the vent and has been partly remobilized by wind. A plume emanating from the crater is being blown to the E. Figure by Hannah Dietterich, courtesy of AVO.

Figure 9. An ash deposit is present between the dashed lines, deposited on snow (red) in this Planet Labs near-IR false color satellite image acquired on 24 March 2021. The deposit extends over 8 km ESE across Semisopochnoi from the North Cerberus Crater and a plume is also visible in the same area. Image courtesy of AVO.

Several small low-altitude ash and gas plumes were detected in satellite images on 30 March and 1 April. Cloud cover prevented satellite views until 12 April, when new ash deposits and low-level ash emissions were observed extending at least to the coastline, accompanied by weak infrasound signals. Low-level activity was also detected the following day. Sustained ash emission that began on the morning of the 15th (figure 10) produced a plume extending more than 350 km E to altitudes of 6 km; activity continued through the next day with a change in direction to the N at around 3 km altitude. Ash emission continued over the following days with a VONA released on the 22nd reporting an ash plume reaching 3 km and extending about 75 km S (figure 11). Through to the end of April ash and SO₂ plumes were either observed or noted as probably occurring under cloudy conditions.

Figure 10. This Sentinel-3 satellite scene acquired on 15 April 2021 shows plumes from Semisopochnoi dispersed over 330 km from the vent. The insert shows a zoomed-in view of the island and the proximal ash plume. Original image by Hannah Dietterich, AVO.

Figure 11. This Planet Labs satellite image acquired on 22 April 2021 shows an ash plume produced by the North Cerberus Crater and dispersing S. Ash deposits are visible on the flanks of the cone. Figure by Hannah Dietterich, AVO.

The volcano was often obscured during the first week of May, with activity possibly continuing at a low level without detection. A gas plume was detected on the 11th, and an ash plume is visible in satellite images acquired on the 17th (figure 12). Small explosions and SO₂ emissions were detected through 21 May. An ash emission reaching 3 km altitude that was seen by an AVO field crew on 29 May was also observed in satellite data moving SW. Elevated temperatures were detected in the North Cerberus Crater. Ash emissions were produced again on the 30th and observed by an AVO field crew (figure 13). Seismic data transmission was restored on 26 May.

Figure 12. Satellite images of Semisopochnoi acquired on 17 and 29 May (top), and a photograph taken on 29 May 2021 (bottom) show weak activity at the North Cerberus Crater, including ash emission, gas emission, and elevated temperature on the crater floor. Sentinel-2 color infrared (vegetation, bands 8, 3, 4) scene at the top left and the false color (urban, bands 12, 11, 4) scene at the top right courtesy of Sentinel Hub Playground. Photo courtesy of Hannah Dietterich, AVO.

Figure 13. Minor ash emissions produced on 30 May 2021 at Semisopochnoi’s North Cerberus Crater around 1320 local time, taken from a helicopter during field work. Both top and bottom-left photos are taken from the SE. Photos courtesy of Hannah Dietterich, 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 Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank of Cerberus appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical 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 eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

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/); 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); Planet Labs, Inc. (URL: https://www.planet.com/).

Piton de la Fournaise is located on the French island of Réunion in the western Indian Ocean. Its previous most recent eruption occurred during February into April 2020, characterized by fissure eruptions, fountaining, and significant lava flows (BGVN 45:05). This report covers May through December 2020, describing the new eruption in early December that was characterized by lava fountains and flows, using information from the Observatoire Volcanologique du Piton de la Fournaise (OVPF) and various satellite data.

Slight deformation was recorded after the end of the April eruption, but overall activity during May-November 2020 was low, with no eruptive events, according to OVPF. Starting around 16 June seismicity resumed, which included 77 shallow volcano-tectonic earthquakes during the month and occasional rockfall events in the Dolomieu Crater. This increase in seismicity was accompanied by inflation at the base and summit of the volcano. Shallow volcano-tectonic earthquakes continued to be reported under the Dolomieu Crater during July-November accompanied by rockfall events. In late September the number of shallow volcano-tectonic earthquakes increased markedly to 1,648, but then decreased to 129 in October and only four in November.

OVPF reported that during 0510-0554 on 4 December a seismic swarm of about 101 volcano-tectonic earthquakes was accompanied by minor, but rapid, inflation just below the center and N rim of the Dolomieu Crater. Seismicity decreased after 0600, but inflation continued through 6 December. A second seismic crisis began at 0228 on 7 December, accompanied by rapid inflation. Fissures opened on the WSW flank of the Dolomieu Crater at 0440 at elevations ranging from 2.2-2.3 km and spanning a 700-m-long area; lava began to erupt from these fissures during 0455-0500 (figure 202). Scientists on an overflight at 0700-0730 observed lava fountains rising 15 m high from the three active fissures and short lava flows (figure 203). By 1700 the fissure at an elevation of 2.3 km was the most active, with five small vents, while the other two were showing less intense activity. Satellite data via the HOTVOLC platform showed a lava flow rate of 5 and 30 m³/s during 7 December. The eruption period ended at 0715 on 8 December, following a gradual decrease in tremor and a three-hour phase of seismic signals that indicated degassing. Twenty-one volcano-tectonic earthquakes were recorded during that day under the W rim of the Dolomieu Crater. Another six earthquakes were reported during the morning of 9 December through 0900. Surficial activity was no longer visible.

Figure 202. Photo of the active fissure vents on the WSW flank of the Dolomieu Crater and the lava fountains accompanied by degassing at Piton de la Fournaise at 0730 on 7 December 2020. Courtesy of OVPF-IPGP (Bulletin d'activité du lundi 7 décembre 2020).

Figure 203. Photo of the lava fountains up to 15 m high at Piton de la Fournaise during 7-8 December 2020. Courtesy of OVPF-IPGP.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed brief, but significant, thermal activity during early December, reflecting the new eruption. This thermal activity was also visible in Sentinel-2 thermal satellite imagery on 7 December 2020, showing lava flows and possibly lava fountains from the fissures on the SW and W flanks (figure 204). Accompanying this activity were SO₂ emissions that were detected by the Sentinel-5P/TROPOMI instrument (figure 205).

Figure 204. Sentinel-2 infrared satellite image of the thermal activity (bright yellow-orange) on the S and SW flanks of Piton de la Fournaise on 7 December 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Figure 205. Image of the SO2 emissions that occurred during the eruption at Piton de la Fournaise on 7 December 2020 detected by the Sentinel-5P/TROPOMI satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The massive Piton de la Fournaise basaltic shield volcano on the French island of Réunion in the western Indian Ocean is one of the world's most active volcanoes. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three calderas formed at about 250,000, 65,000, and less than 5000 years ago by progressive eastward slumping of the volcano. Numerous pyroclastic cones dot the floor of the calderas and their outer flanks. Most historical eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest caldera, which is 8 km wide and breached to below sea level on the eastern side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures on the outer flanks of the caldera. The Piton de la Fournaise Volcano Observatory, one of several operated by the Institut de Physique du Globe de Paris, monitors this very active volcano.

Information Contacts: Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); 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).

Heard is a remote island located in the southern Indian Ocean that contains the Big Ben stratovolcano, which has had intermittent activity since 1910. More recent activity since 2012 through October 2020 has been characterized by thermal anomalies in the summit crater and lava flows, primarily identified based on information from satellite data (BGVN 45:11). This report covers similar activity that continued during November 2020 and January 2021.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows a total of three thermal anomalies of varying power during November 2020 (figure 46). Sentinel-2 thermal satellite imagery shows a single thermal anomaly on 9 November 2020 and later, on 11 November two strong thermal anomalies, possibly two lava flows, were observed descending the S and SW flanks (figure 47). These thermal anomalies were also detected by the MIROVA system. Weaker thermal anomalies were observed on 18 and 20 January 2021 in the summit crater. No new thermal activity was detected after November through April 2021 by the MIROVA system.

Figure 46. Only three thermal anomalies at Heard were detected during November 2020, according to the MIROVA system, shown in this Log Radiative Power graph. The strongest thermal anomaly represents the two possible lava flows that were observed in Sentinel-2 infrared satellite data. No thermal anomalies were observed during December through April 2021. Courtesy of MIROVA.

Figure 47. Sentinel-2 infrared satellite imagery of Heard Island’s Big Ben volcano showed a thermal anomaly (bright yellow-orange) on clear weather days on 9 (top left) and 11 (top right) November 2020, along with 18 (bottom left) and 20 (bottom right) January 2021. On 11 November two strong thermal anomalies, possibly representing different lava flows, were observed descending to the S and SW flanks, though much of the activity was covered by clouds. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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

Information Contacts: 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).

Eruptive activity continued in November. The volcano erupted 17 times during 14 daylight hours of observation 15-16 November. Intervals between eruptions varied from 15 to 150 minutes. Most were accompanied by loud explosions and rumbling. Clouds, often containing ash, rose 1,300-1,700 m above the summit. Summit eruptions caused light ashfall (grain size 0.25-0.50 mm) 4 km W, and threw incandescent blocks 900 m down the flanks. Very frequent rockfalls occurred on the NW side of the dome. Measurements indicated an SO₂ flux of <100 t/d.

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. Fernández, OVSICORI; B. Gemmel, H. Mango, K. Roggensack, R. Stoiber and other geologists, Dartmouth College.

Lidar data from Mauna Loa, Hawaii continued to show a gradual decline in stratospheric aerosols (figure 49). Integrated backscattering on 24 November was the lowest measured since the 1982 increases associated with the eruptions of El Chichón and the "Mystery Cloud" (probably from the initial explosive phase of the December 1981-January 1982 Nyamuragira eruption in Zaire). Values measured in Virginia and Germany remained relatively stable. The low integrated backscattering recorded 9 October off the coast of North Carolina was attributed at least partly to the high tropopause that night.

Figure 49. Average monthly lidar profiles from Mauna Loa, Hawaii, June-November 1987. The dotted line superimposed on each profile represents the average 5-22 November 1985 data, before the arrival of Ruiz aerosols.

Figure 50. Lidar data from various locations, showing altitudes of aerosol layers during October-November 1987. Note that some layers have multiple peaks. Data from the Cape hatteras area are from a NASA airborne mission taking correlative measurements with the SAGE II satellite. Backscattering ratios are for the ruby wavelength of 0.69 µm. Integrated values show total backscatter, expressed in steradians-1, integrated over 300-m intervals from 16-33 km at Mauna Loa and from the tropopause to 30 km at Hampton, Virginia and the NASA airborne mission. Altitudes of maximum backscattering ratios and coefficients are shown for each layer at Mauna Loa. The 22 October and 12 November data from Hampton replace previously published values.

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: William Fuller, NASA Langley Research Center, Hampton, VA 23665 USA; Thomas DeFoor, Mauna Loa Observatory, P. O. Box 275, Hilo, HI 96720 USA; Horst Jäger, Fraunhofer-Institut für Atmosphärische Umweltforschung, Kreuzeckbahnstrasse 19, D-8100 Garmisch-Partenkirchen, West Germany.

Bagana continued to emit moderate-strong white vapors during November. Weak glow from the summit was seen on 6, 13, and 14 November. The number of long-duration seismic events (interpreted as rockfalls from the lava flow) showed a slight increase during the first part of the month to about 40/day, but the activity declined to 10/day by the end of the month.

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

Information Contacts: P. Lowenstein, RVO.

At 1359 on 16 November, pilots Dave Holman and Jay Brown (U.S. Coast Guard) noted steam with some ash being emitted from the summit vent of a volcano that they believed to be Carlisle. The plume rose to 2,500 m altitude . . . and trailed 30 km ENE. Of the five islands in the immediate area, the pilots were able to see the three closest to the Bering Sea (Kagamil, Uliaga, and Carlisle), but could not see the islands closer to the Pacific Ocean (Herbert or Chuginadak). Although their report strongly suggested that Carlisle was the source of the activity, the possibility that it was from Mt. Cleveland (10 km SE on Chuginadak Island), site of recent ash emission (SEAN 12:6-8), could not be eliminated. Steam (but no ash) was emerging from Carlisle's summit when it was observed on 28 August.

Geologic Background. Carlisle Island is a steep-sided, conical stratovolcano across the Carlisle Pass strait from Mount Cleveland. Radar images suggest that this uninhabited, 7-km-wide island may contain two closely spaced volcanic cones (Myers, in Wood and Kienle 1990). Like nearby Herbert volcano, no geologic studies have been conducted on the volcano. Eruptions have been reported since the 18th century, but are very poorly documented. A variety of names was attached to Carlisle on early hydrographic maps, and Miller et al. (1998) noted that some 18th and 19th century eruptions reported at the closely spaced volcanoes of the "Islands of the Four Mountains" area could refer to Carlisle as well as Cleveland, Uliaga, or Kagamil volcanoes.

Information Contacts: J. Reeder, ADGGS.

An increase in seismicity began 3 November, associated with weak inflation NE of Dolomieu summit crater. An eruptive phase followed on 6 November at 2111, on the N flank of the cone. After a discernable decrease in tremor and continuing seismic activity, a second eruptive phase was registered at 2147. Activity lasted until about 2330. During these two phases, a series of three fractures had opened ~1 km N of Dolomieu. The main aa flow had stopped advancing by the time a new fissure opened at 0040 on 7 November, 1 km NE of the earlier fissures. About 1.6 x 10⁶ m³ of aphyric basalt had been extruded by the end of all activity at 0600 on 8 November.

On 29 November an earthquake swarm of ~20 events was recorded. An eruption began the next day when a new fissure opened at 0805 in Enclos Caldera S of the central cone. A second fissure opened at 0932. On 1 December, there was moderate activity during a single event and weak tremor.

Geologic Background. The massive Piton de la Fournaise basaltic shield volcano on the French island of Réunion in the western Indian Ocean is one of the world's most active volcanoes. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three calderas formed at about 250,000, 65,000, and less than 5000 years ago by progressive eastward slumping of the volcano. Numerous pyroclastic cones dot the floor of the calderas and their outer flanks. Most historical eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest caldera, which is 8 km wide and breached to below sea level on the eastern side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures on the outer flanks of the caldera. The Piton de la Fournaise Volcano Observatory, one of several operated by the Institut de Physique du Globe de Paris, monitors this very active volcano.

Information Contacts: P. Bachélery, Univ de la Réunion; OVPDLF (translated from a report inLAVE Bulletin no. 11).

Small explosions began in the crater of the summit cinder cone (Mihara-yama) at 1047 on 16 November. Bombs were ejected to 1.6 km distance, instruments in the summit area were damaged, and nearby windows were cracked. The sounds were heard from as far as the Honshu mainland (at least 30 km away). The crater was obscured by clouds, but within the next hour aerial observers saw eruption columns rising to 5,000 m height. Tephra fell primarily to the E, and totaled a few thousand metric tons. Explosions were heard at irregular intervals until 1546. At about 1700, a lava fountain a few meters high was observed within the crater, but no flows formed. The next day, observations indicated that the crater floor had subsided 25 m since before the eruption.

Summit glow was observed at 0329 on 18 November, and ashfall to the SW soon followed. At about 0500, a gray ash column rose an estimated 1,600 m. Five hours later, at 1004, there were a few small explosions and a small amount of ash fell to the NW. Small eruption columns were observed until evening. The next day, helicopter observations indicated that the crater floor had subsided an additional 150 m [but see 13:1]. From 19 to 27 November, plumes a few hundred meters high occasionally rose from Mihara-yama. On 21 November, 5,200 of the 10,000 Oshima Island residents participated in evacuation drills that used six ships, 20 planes, and 145 motor vehicles. A white plume was the only activity at the volcano that day.

The groundmass of the ejecta contained abundant microphenocrysts of pyroxene and plagioclase. This suggested to volcanologists that the rock was not fresh magma, but was derived from the lava lake that has been cooling inside Mihara-yama crater since the November 1986 eruption.

Seismicity. Seismicity in the summit region had been gradually increasing since January 1987 (figure 12). A significant peak at the end of September 1987 was not accompanied by eruptive activity. Seismicity rapidly increased in the middle of November, reaching 543 events/day on 13 November and 586 on 14 November. Within a few days after the eruption on 16 November, seismicity rapidly declined to <50 events/day.

Figure 12. Number of earthquakes per day in the summit region of Izu-Oshima island, May-November 1987. Courtesy of Earthquake Research Institute, Univ of Tokyo.

Seismographs installed in the summit area two weeks before the eruption showed that the earthquakes were very shallow (less than ~400 m depth), and almost entirely confined to the area underneath the summit crater. Periods of both episodic and continuous tremor occurred in the months before the eruption. Tremor became continuous just before the eruption.

Ground deformation. Levelling and tilt data suggested that almost the entire summit region of Oshima has been subsiding since the November 1987 eruption. No clear ground deformation precursors were seen prior to the eruption. A stepwise change at all tiltmeters around the island occurred almost simultaneously with the start of the second series of eruptions at 0329 on 18 November (figure 13). The change is consistent with a center of inflation roughly 5 km NW of Mihara-yama. The volumetric strainmeter in the NW part of the island recorded an expansion consistent with the tilt (figure 13).

Figure 13. Map showing monitoring instrumentation on Izu-Oshima Island. Vectors show downward direction and magnitude of stepwise change in tilt, 18 November at 0326.

Fumarolic activity. A clear increase in fumarolic activity was noted before the eruption. A ring of fumaroles around the summit cone was observed at the end of July, and the activity of this ring increased with time. A second ring formed around the first ring in November. Fumarolic activity increased rapidly 5-10 days before the eruption.

Further References. Abe, K. and Takahashi, M., 1987, Description of the November 21, 1986 fissure eruption on the caldera floor of Izu-Oshima volcano, Japan: analysis of a series of photographs: Bulletin of the Earthquake Research Institute, Tokyo, v. 62, p. 149-162.

Aramaki, S., ed., 1988, The 1986-1987 eruption of Izu-Oshima volcano: Earthquake Research Institute, University of Tokyo, 61 p. (9 papers).

Endo, K., Chiba, T., Taniguchi, H., Sumita, M., Tachigawa, S., Miyahara, T., Uno, R., and Miyaji, N., 1988, Izu-Oshima 1986-1987 eruptions and the eruptive products: Proceedings, Kagoshima International Conference on Volcanoes, p. 119-122.

Special Issue — Oshima volcano, Izu: Journal of Geomagnetism and Geoelectricity (Terra Science Publishing Co.), v. 42, p. 139-363 (in English).

Special Issue — The 1986 eruption of Isu-Oshima volcano: Bulletin of the Geological Survey of Japan, v. 38 (1987), p. 601-753 (12 papers) (in Japanese with English abstracts and captions).

Special Issue — The 1986 eruption of Isu-Oshima volcano: Bulletin of the Volcanological Society of Japan, v. 33 (2nd series) (1988), (in Japanese with English abstracts and captions).

Watanabe, H., the 1986-1987 eruption of Izu-Oshima volcano: Proceedings, Kagoshima International Conference on Volcanoes, p. 37-40.

Geologic Background. Izu-Oshima volcano in Sagami Bay, east of the Izu Peninsula, is the northernmost of the Izu Islands. The broad, low stratovolcano forms an 11 x 13 km island and was constructed over the remnants of three dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous historical eruptions. More than 40 cones are located within the caldera and along two parallel rift zones trending NNW-SSE. Although it is a dominantly basaltic volcano, strong explosive activity has occurred at intervals of 100-150 years throughout the past few thousand years. Historical activity dates back to the 7th century CE. A major eruption in 1986 produced spectacular lava fountains up to 1600 m height and a 16-km-high eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: Y. Ida, H. Watanabe, K. Yamaoka, S. Aramaki, and H. Glicken, Earthquake Research Institute, Univ of Tokyo; Kyodo radio, Tokyo.

Lava production continued into early December. . .. Lava flowed into the sea 1-25 November, the W lobe continuously (in the national park, W of Kupapau Point), and the eastern intermittently (E of Kupapau Point), as in October. Activity at the coast declined to a trickle on 26 November. The next day lava broke out at several points between 240 and 170 m elevation, indicating blockage within the tube system. Several flows advanced over the prominent south flank fault scarp, reaching 60 m elevation. By 2 December, the middle lobe had reached the previously covered coast highway and by 7 December lava was again flowing into the ocean near Kupapau Point, on top of previous flows. In Royal Gardens subdivision, lava movement was intermittent through November, primarily slow ooze-outs on top of older flows. However, lava re-entered Royal Gardens in early December, and one lobe destroyed the last remaining house in the SE part of the subdivision before stagnating on 4 December.

Shallow tremor continued at a low level . . . near the active vent and Pu`u `O`o. Accompanying microshocks caused by rockfalls, degassing, or thermal compensation in recent lava flows occurred several to 100 times a day. Intermediate and deep tremor activity in summit region was intermittent. Minor bursts of short-period events beneath the summit were recorded for a week in mid-November, and were followed a week later by a 2-day swarm of long-period events. Many of the month's earthquakes occurred at intermediate depths . . . under the summit and S flanks of Kilauea.

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: C. Heliker and R. Koyanagi, HVO.

A moderate increase in activity began the second week of November; a weak glow from Crater 2 was noted, accompanied by low rumbling noises and a weak-moderate white-gray plume. Seismicity showed no significant change and there was no activity from Crater 3.

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 E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N 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. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: P. Lowenstein, RVO.

Activity remained at the same level in November that it had reached after the moderate increase during the first weeks of October. At Southern Crater a fluctuating night glow accompanied by rumbling noises suggested that Strombolian activity was occurring within the crater. Ejections of incandescent material just over the crater rim were seen on 4 and 16 November. Main Crater emitted a moderate plume of white and blue vapor and displayed a steady night glow 12-27 November. Seismicity showed no significant change, remaining at 1,200-1,400 small-amplitude B-type events/day. Tilt measurements were steady.

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

Information Contacts: P. Lowenstein, RVO.

A short but intense sequence of earthquakes occurred in the Lake Rotomahana area of the [Tarawera] rift on 16 November. The largest event (M_L 3.8) occurred at 1835, in the middle of the sequence; events continued until about 2000. A geodetic survey of the Lake Rotomahana strain monitoring pattern was completed ~20 minutes before the earthquake sequence commenced. Selected stations were reoccupied three days later but no significant co-seismic deformation was detected. All the earthquakes appeared to be of tectonic origin. Similar swarms were recorded 22-23 February 1986 and in February 1983.

The 17-km-long Tarawera Rift was the site of a vigorous eruption in 1886 that ejected ~0.7 km³ of basaltic magma in ~4 hours (Nairn and others, 1986); large phreatic explosions occurred from Lake Rotomahana, which has grown substantially since that eruption. Phreatic explosions have been recorded [16] times between 1896 and 1973 in the [Waimangu] thermal area to the SW, along the rift.

Reference. Nairn, I.A., Cole, J.W., Houghton, B.F., and Wilson, C.J.N., 1986, Tarawera 1886 eruption: International Volcanological Congress Handbook, 1-9 February 1986, p. 111-121.

Geologic Background. The massive, dominantly rhyolitic Okataina Volcanic Centre is surrounded by extensive ignimbrite and pyroclastic sheets produced during multiple caldera-forming eruptions. Numerous lava domes and craters erupted from two subparallel NE-SW-trending vent lineations form the Haroharo and Tarawera volcanic complexes. Lava domes of the Haroharo complex, at the northern end of the Okataina Volcanic Centre, occupy part of the 16 x 26 km Pleistocene Haroharo caldera, which formed incrementally between 300,000 and 50,000 years before present (BP). The oldest exposed rocks on the caldera floor are about 22,000 years old. The Tarawera complex at the southern end of Okataina consists of 11 rhyolitic lava domes and associated lava flows. The oldest domes were formed as late as about 15,000 years BP, and the youngest were formed in the Kaharoa eruption about 800 years BP. The NE-SW Tarawera vent lineation extends from the two dacitic cones of Maungaongaonga and Mangakakaramea on the SW to Mount Edgecumbe on the NE. Construction of the Haroharo and Tarawera complexes impounded lakes Rotoiti, Totoehu, Okataina, and Tarawera against the outer margins of the Okataina ring structure. A major hydrothermal area is located at Waimangu; the world-renowned Pink and White Terrace siliceous sinter deposits were destroyed during the major basaltic explosive eruption of 1886.

Information Contacts: B. Scott, NZGS Rotorua; S. Sherburn, DSIR Geophysics, Wairakei.

Minor eruptive activity from a cone in the SW part of the caldera was observed at 1410 on 16 November by airplane pilots Dave Holman and Jay Brown (Coast Guard). Steam with some ash rose to ~2,100 m altitude and a thin plume drifted 20 km ENE at 1,500-2,100 m altitude. [On 1 December] at 1300 and 1510, pilot T. Madsen (Aleutian Air) observed a light gray plume drifting 25 km ESE at 1,200 m altitude. He also noted that the cone appeared to be higher than the caldera rim (900-950 m elevation), indicating that it had grown since he last saw it over a month ago.

Geologic Background. The broad, basaltic Okmok shield volcano, which forms the NE end of Umnak Island, has a dramatically different profile than most other Aleutian volcanoes. The summit of the low, 35-km-wide volcano is cut by two overlapping 10-km-wide calderas formed during eruptions about 12,000 and 2050 years ago that produced dacitic pyroclastic flows that reached the coast. More than 60 tephra layers from Okmok have been found overlying the 12,000-year-old caldera-forming tephra layer. Numerous satellitic cones and lava domes dot the flanks of the volcano down to the coast, including 1253-m Mount Tulik on the SE flank, which is almost 200 m higher than the caldera rim. Some of the post-caldera cones show evidence of wave-cut lake terraces; the more recent cones, some of which have been active historically, were formed after the caldera lake, once 150 m deep, disappeared. Hot springs and fumaroles are found within the caldera. Historical eruptions have occurred since 1805 from cinder cones within the caldera.

Information Contacts: J. Reeder, Alaska Division of Geological and Geophysical Surveys (ADGGS).

The volcano had apparently been quiet for several weeks after new flank ash deposits were seen 4 September; it was inactive and covered with fresh snow on 20 September. A NOAA 9 satellite image on 30 September at 0525 showed a plume extending about 20 km S from Pavlof. Weather clouds obscured later activity.

Eruptions began again in mid-October and continued in November (table 4). Dark ash was steadily emitted to 250 m above the summit on the morning of 5 November. The next day wind blew ash down the SE flank for 120 m; the plume trailed about 30 km SE. Continued ash emission was observed on 6, 7, and 9 November. Plumes reached to 3.6 km altitude (~1 km above the summit) and drifted a maximum of 35 km from the volcano. During observations on 16 and 27 November only white steam was emitted.

Geologic Background. The most active volcano of the Aleutian arc, Pavlof is a 2519-m-high Holocene stratovolcano that was constructed along a line of vents extending NE from the Emmons Lake caldera. Pavlof and its twin volcano to the NE, 2142-m-high Pavlof Sister, form a dramatic pair of symmetrical, glacier-covered stratovolcanoes that tower above Pavlof and Volcano bays. A third cone, Little Pavlof, is a smaller volcano on the SW flank of Pavlof volcano, near the rim of Emmons Lake caldera. Unlike Pavlof Sister, Pavlof has been frequently active in historical time, typically producing Strombolian to Vulcanian explosive eruptions from the summit vents and occasional lava flows. The active vents lie near the summit on the north and east sides. The largest historical eruption took place in 1911, at the end of a 5-year-long eruptive episode, when a fissure opened on the N flank, ejecting large blocks and issuing lava flows.

Information Contacts: J. Reeder, ADGGS; M. Matson, NOAA/NESDIS.

Seismicity remained at a very low level during November; only 34 events were recorded. The eight events large enough to be located occurred in the Greet Harbour, Vulcan-Matupit Island areas . . . . Tilt measurements indicated ongoing very slow subsidence in the Greet Harbour and Vulcan areas. Seismicity has remained somewhat elevated in 1987, although from August to October seismicity has been at its lowest levels since the 1983-85 crisis.

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the 688-m-high asymmetrical pyroclastic shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: P. Lowenstein, RVO.

Since a strong seismic crisis in July, seismicity has remained at elevated levels, . . . . Minor ash emission has occasionally been seen since August. During November, high- and low-frequency earthquake activity increased. The number of high-frequency events rose slightly, to 330 . . ., and low-frequency events increased to 933 . . ., but energy release was relatively low. Shallow (explosion) seismicity declined to 111 recorded shocks. . . . No ash was emitted and deformation measurements showed low to moderate changes. The average rate of SO₂ emission was ~1,500 t/d. Occasional minor ash emission has occurred since . . .ash emission in June.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: E. Parra, INGEOMINAS, Manizales.

Intermittent ash emission continued in November and small eruption earthquakes were recorded on 31 October, and 8, 11(?), 15, 16, and 20 November. An eruption was observed at the time of the 31 October earthquake from Ohope (50 km S) and Matata (>55 km SW) at about 0810. A fast-rising "billowing" black column emerged to replace the usual white steam column and rose to >2,500 m. Ash fallout occurred N of the island.

During a 30 October inspection by geologists, minor to moderate ash emission was continuous and incandescent ash emerged at low pressure and without noise from Hitchhiker Vent in Congress Crater (on the E side of 1978 Crater). Since 7 September, 230 mm of fresh tephra had accumulated on the rim of 1978 Crater. Block ejecta consisted of altered country rock and smaller tephra were lithic-dominated with no fresh scoria component noted. Temperatures at sites ~120 m W of 1978 Crater (on Donald Mound) had continued to increase and generally ranged between 550 and 650°C.

Deflation continued near Congress Crater, with only a slight possible rise at the E end of Donald Mound, formerly an area of rapid inflation. Large changes had occurred in magnetic values since the last survey 11 June. Measurements indicated deep-seated (>700 m depth) heating below Donald Mound and in the areas N and S. A strong shallow (on the order of 100-200 m deep) heating center was found below the W edge. A general cooling was recorded over the remainder of the crater. A new small vent on the W edge of 1978 Crater floor was first sighted during a visit on 20 November, emitting a brown plume with fine ash. The 5-m-diameter vent had not been present on 15 October aerial photos nor was it noted during the 30 October visit. Hitchhiker Vent was emitting a light brown ash plume in semicontinuous pulses. Only a few meters of ash and a few new blocks (>0.2 m) had fallen on 1978 Crater rim since 30 October.

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: I. Nairn, NZGS Rotorua; J. Cole, Victoria Univ, Wellington.