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).

During December [1997], lavas emitted beginning in September continued to flow down Arenal's W flank. They reached 1,400 m elevation and mass wasting carried some material to as low as 1,000 m elevation. December eruptive rates and intensities were low; also, the number of earthquakes and hours of tremor were both at or near the minimum values seen during the course of the year. This pattern continued into January 1998. Still, on infrequent occasions the active crater (Crater C) discharged plumes reaching at least 1 km in height above the crater.

Lavas vented in late January continued to flow in February, descending to 1,100 m elevation, and branching near 1,300 m elevation to form a new arm directed to the NW down the Tabacón river valley. During March, this new arm flowed down to reach 1,200 m elevation; the main channel extended to 1,000 m elevation; another arm branched off to the W at 1,400 m elevation and descended about 100 m.

Observers noted two pyroclastic flows during January-February. The first reached 1,100 m elevation on the SE flank. The second followed a similar path and reached 900 m elevation.

The number of low-frequency earthquakes (<4.0 Hz) during January and February, while still low, rose more than 25% over the number during December. The hours of tremor during January-February also remained low; during the latter month the dedicated seismic station (VACR) registered only 58 hours, the lowest monthly record in at least two years. During March, seismicity appeared to rise again, but the seismic system only functioned 18 days of the month. During this time the system recorded 80 hours of tremor.

OVSICORI-UNA scientists noted fumarolic activity in crater D as well as acid rain on the volcano's leeward flanks (towards the NW, W, and SE). In these sectors, some species of plants sustained visible leaf damage.

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. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Table 13 lists atmospheric lidar data from Hampton, Virginia for 8 April 1997 through 26 February 1998, and from Garmisch-Partenkirchen, Germany for 3 November 1997 to 14 April 1998. The aerosol backscatter measured at Hampton on 26 February 1998 shows a typical winter increase in stratospheric aerosol compared to measurements made the previous summer. The increase from summer to winter is generally a function of the difference in tropopause height between the two seasons. In this case there is a significant decrease in integrated stratospheric aerosol compared to measurements obtained during the winter of 1997 (Bulletin v. 22, nos. 1, 3).

Table 13. Lidar data collected for Virginia (April 1997-February 1998) and Germany (November 1997-April 1998) showing altitudes of aerosol layers. Backscattering rations from Hampton are for the ruby wavelength of 0.69 µm; those from Garmisch-Partenkirchen are for the Nd-YAG wavelength of 0.53 µm, with equivalent ruby values in parentheses. The integrated value shows total backscatter, expressed in steradians^-1, integrated over 300-m intervals from the tropopause to 30 km for both Virginia and Germany. Courtesy of Mary Osborne and Horst Jäger.

A graph of integral stratospheric aerosol backscatter (figure 5) shows how the stratospheric aerosol load had declined by the end of 1997 to pre-Pinatubo values. More observations are needed to decide whether a new background level has been reached or will be reached in the near future.

Figure 5. Graph showing the log of the lidar backscatter versus time at Garmisch-Partenkirchen, Germany for the latter two-thirds of 1991 through end-1997. The plotted data are preliminary 532 nm integral values of stratospheric aerosol backscatter (integrated from the tropopause or cirrus to the top of the aerosol layer) versus time. Labeled arrows indicate the eruptions of Pinatubo and Kliuchevskoi. Courtesy of Horst Jäger.

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 thorugh 1989. Lidar data and other atmospheric observations were again published intermittently between 1995 and 2001; those reports are included here.

Information Contacts: Mary Osborn, NASA Langley Research Center (LaRC), Hampton, VA 23665 USA; Horst Jäger, Fraunhofer -- Institut für Atmosphärische Umweltforschung, Kreuzeckbahnstrasse 19, D-82467 Garmisch-Partenkirchen, Germany.

No seismicity registered under the volcano during 2 March-5 April. On 5-7, 10, and 12-14 March, fumarolic plumes rose 50-300 m above the volcano. Fumarolic plumes on 16-20 and 22 March rose 50-200 m above the volcano and moved 5-10 km SSE. On 30-31 March and 1-4 April fumarolic plumes rose 100-500 m above the volcano.

Geologic Background. Prior to its noted 1955-56 eruption, Bezymianny had been considered extinct. The modern volcano, much smaller in size than its massive neighbors Kamen and Kliuchevskoi, was formed about 4700 years ago over a late-Pleistocene lava-dome complex and an ancestral edifice built about 11,000-7000 years ago. Three periods of intensified activity have occurred during the past 3000 years. The latest period, which was preceded by a 1000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large horseshoe-shaped crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

When scientists from the Alaska Volcano Observatory (AVO) conducted an overflight to Chiginagak on 11 March, the summit was visible but a thin cloud layer at about 1,700-1,900 m altitude obscured the fumarolic areas. Above the fumaroles, however, bulbous gray clouds penetrated through the thin cloud layer and extended to about 2,100 m altitude.

A strong sulfur smell was noticed 16-49 km downwind of the volcano. The gray clouds and sulfur smell supported observations from Pilot Point (60 km NW) that indicated continued vigorous fumarolic activity. Increased fumarolic activity has been reported at the volcano beginning as early as mid-1997 (BGVN 22:11 and 23:01). According to AVO, the increased activity did not imply an imminent eruption.

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

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

The following is a summary of observations from scientists at the Observatoire du Piton de la Fournaise and Observatoires Volcanologiques (OVPF), Institut de Physique du Globe de Paris, and the Laboratoire des Sciences de la Terre, Université de la Réunion.

Narrative. An eruption broke out on Piton de la Fournaise (PdF) at 1505 on 9 March 1998, after an unusually long period of 63 months rest. PdF (figure 41) had an average eruption rate of more than one per year in the last several decades. For a time three fissure vents were simultaneously active. The eruption continued at one fissure vent (Piton Kapor) at least as late as 20 April 1998.

Figure 41. Schematic map of Piton de la Fournaise showing the 9 and 11-12 March vents, newly named scoria cones and related features, and the extent of lava flows as of 15 March. Courtesy of Thomas Staudacher, OVPF.

Following escalating seismicity seen over the past two years, a seismic swarm developed at 0338 on 9 March (figures 42, 43, 44, and 45). The swarm was under the edifice, centered slightly W of the small Bory crater, a feature that lies immediately W of the larger Dolomieu crater. In the first observation of its kind at PdF, hypocenters progressed towards the surface prior to the eruption (figure 44).

Figure 42. The number of seismic events accumulated annually at Piton de la Fournaise during 1996, 1997, and early 1998 (three separate curves). The seismic swarm at the end of November 1996 was not followed by an eruption. A significant change in the earthquake rate started in July 1997 and accelerated in early 1998. Courtesy of OVPF.

Figure 43. Located earthquakes at Piton de la Fournaise from 1957 on 6 March through 1857 on 10 March (top) and a vertical, E-W cross section showing hypocenters from 0000 on 8 March through 1200 on 9 March (bottom). Coordinates (labeled tic marks) for horizontal distances on the map and cross section are 5 km apart; this scale differs from the vertical scale on the cross section. Courtesy of OVPF.

Figure 44. During the seismic crisis on PdF hypocenters migrated upward during the pre-eruptive 36-hour period shown (0000 on 6 March-1200 on 9 March). This was the first observation of its kind at PdF; pre-eruptive seismicity had usually remained diffusely distributed within the whole edifice. Courtesy of Jean Battaglia and Nelly Rousseau, OVPF.

Figure 45. Pre-eruptive earthquake counts at Piton de la Fournaise and seismic moments for 8-9 March 1998 (times are GMT). Noteworthy points are labeled as follows: at A, focal depths of the volcano-tectonic events started at ~5 km below sea-level; at B they reached 3 km; at C, 2 km; and at D, 1 km. At E, there occurred the first long-period (near 1 Hz) event since 1993. Venting started at 1505 (1105 GMT). Courtesy of OVPF.

The summit deformed rapidly beginning around 1400. An example of clear and sudden inflation appears in figure 46, documenting changes in radial and tangential inflation at station "Bory." Another multi-component station ("Soufriere"; immediately N of Dolomieu crater) underwent similarly rapid, though larger amplitude, displacement beginning at 1410 and peaking at 1424 to 1429 (undergoing up to 200 µrad of tilt). Inflation at Soufriere station indicated migration of magma towards the N eruptive fissures. Surface venting started there at 1505.

Figure 46. Ground deformation at the summit of Piton de la Fournaise on 9 March during 1200-1700 (0800-1300 GMT). Surface venting began at 1505 (1105 GMT). The Bory two-component inclinometer, ~200 m S of Bory crater, measures tilt aligned radial and tangential to the volcano. The rapid inflation at 1011 GMT was linked to near-surface dike emplacement. Contact the authors for collateral inclinometer and extensometer data at other summit stations. Courtesy of OVPF.

EDM and GPS measurements showed concordant displacements at points around the summit (figures 47 and 48). The time-sequence of EDM data indicated that essentially all deformation occurred at the time of eruption. Consistent with the deformation, eruptive fissures developed between the reflectors to the NE and NW of the summit.

Figure 47. Automated electronic distance meter (EDM) measurements at Piton de la Fournaise taken from an instrument on the NW rim of the Enclos Fouqué caldera (star, labeled 1B10). The EDM computed distances and azimuths to 13 reflectors (triangles) on the flanks of the terminal cone. The numbers indicate centimeters of total displacement between 1000 and 1400 GMT on 9 March. Weather permitting, these measurements were made every hour and telemetered to the observatory in near real-time. Only reflectors E of the fissures underwent measurable relative motion, moving E up to 34 cm. Courtesy of OVPF.

Figure 48. GPS measurements at Piton de la Fournaise showing horizontal displacements in centimeters from GPS positioning in November 1997 and 15 March 1998. Courtesy of OVPF.

At 1505 on 9 March tilt on the northern summit inclinometer reversed and seismic tremor commenced, indicating the final stages of dyke emplacement and the onset of venting. Although at the time, bad weather impaired visual observation, venting was recognized, starting on a 150-m-long N-S fissure around 2,450 m elevation on the N flank of the terminal cone (figure 41). The fissure system quickly developed in an en echelon pattern stretching downslope to approximately 2,100 m elevation. Major venting migrated to the fissure's lower stretches where lava fountaining up to 50 m high fed a flow that descended E (towards an area of the N caldera called the Plaine des Osmondes). Vigorous venting continued through the night of 9 March.

A few discrete seismic events were observed through the tremor during the next two days (10-11 March). The approximate locations of the events were SW of Bory crater. During 10-11 March venting continued in the N along two 100-m-long fissures. At the time, scientists lacked visual observations of the flow front due to cloud cover. Earthquakes at Piton de la Fournaise generally cease after an eruption has broken out, but in this case they continued, hence the impending opening of a new eruption fissure was forecast for the next few hours or days.

In accord with this forecast, during the night of 11 March until 0245 the next morning, a new, isolated eruptive fissure opened WSW of the Bory crater. The vent established itself S of the other erupting fissures, at ~2,200 m elevation (figure 41). Although lava escaped at a much lower rate here than along the northern vents, this southern fissure emitted lava along a zone ~100 m in length. Fountaining lava reached ~10 m high and fed a flow that by 0800 on 11 March had traveled 200-300 m downslope.

During the following days, eruptions continued at both the two northern fissures as well as the southern fissure. Estimated emission rates on the N were 30-50 m³/s and on the S at 5-10 m³/s. Issuing from the northern fissures, E-traveling lava descended to ~1,100 m elevation by 15 March. Here, ~4 km away from the vents, the flow front became stationary. Around the same time, lava issuing at the southern fissure reached an estimated length of 1,500 m. Maximum lava temperatures reached 1,167°C at the northern vents and 1,157°C at the southern vent.

Venting was progressively restricted to limited stretches of the three fissures where scoria cones started to grow. By 19 March the scoria cones were ~40 m high and 120 m long at the upper-elevation northern site, ~35 m high at the lower-elevation northern site, and 15 m high at the southwestern site.

Features at these cones were designated as the Maurice and Katia Krafft crater, Piton Kapor, and the Fred Hudson crater (figure 41). Activity at the three cones continued, but progressively decreased until venting was restricted to Piton Kapor by 31 March. Piton Kapor was still quite active as of 20 April 1998.

Preliminary petrography indicated that the lavas were mostly aphyric basalts carrying a small but variable number of millimeter-sized olivine crystals. Under the assumption that their composition lay close to the so-called "stationary basalts," modeling indicated that they vented at temperatures close to their liquidus.

Premonitory geophysical observations. Clear-cut long-term observations on the various surveillance networks that signaled an impending eruption were, as is customary at PdF, discrete and few. Increasing seismicity late in 1997 and accelerating in early 1998 were signs that an abnormal situation was developing. However, other crises, albeit of smaller intensities, occurred in November 1996 and July 1997 and did not result in an eruption. Small perturbations were seen on the deformation (inclinometry, geodesy, and extensometry) networks months before the present event but were not interpreted as premonitory. These signs most probably corresponded to magma intrusions within the edifice.

Surveillance network observations. It was only a few hours before the 9 March outbreak that short-term signs definitely signaled an impending eruption and civil authorities were warned of a maximum alert. Critical signs included seismic, tilt, and deformation data (summarized on figures 42 to 48). In addition, a total-field magnetometer network provided clear pre- and syn-eruptive signals that remain under interpretation. Measurements on about 50 of the approximately 100 microgravity-benchmark and GPS-array stations were repeated between 18 and 31 March with two Scintrex CG-3M gravimeters. The array was last surveyed in December 1997. A few stations showed variations of relatively small amplitude. Interpretations must await correction of the elevation changes and comparison with the recordings provided by the two permanent monitoring stations installed in December 1997. Radon stations did not show any unusual pattern either before or during the first stages of the outbreak as was hoped from previous behavior during intrusive events (BGVN 21:12).

The Observatoire Volcanologique du Piton de la Fournaise(OVPF) was built in 1979 after the devastation of the 1977 eruption owing to the financial help of the Institut National des Siences de l'Univers, France. The Observatory became operational in 1980; since then, tens of eruption have been closely observed and, most often, forecast sufficiently in advance to alleviate possible personal and material damages.

Besides the information contacts listed below, report contributors also included Kei Aki, Valérie Ferazzini, Louis-Philippe Ricard, Nelly Rousseau, Jean Battaglia, Nicolas Villeneuve, Philippe Kowalski, Philippe Catherine, Denis Wégerlé, Grégory Durand, Nadia Talibart, Jacques Lebreton, Maolidi Assoumani, Massimo Bonfiglio, Bernard Robineau, Jean-Lambert Join, Eric Delcher, Jean-Luc Folio, Jean-Luc Hoareau, Cécile Savin, Hamidou Nassor, Evelyn Maillot, Jean-Claude Lépine, Martine Hirn-Sapin, Christine Deplus, Pierre Briole, Sylvain Bonvalot, Jacques Zlotnicki, Germinal Gabalda, Philippe Labazuy, Alfred Hirn, Jean-Claude Delmond, Guy Aubert, Michel Diament, and Janine Gouin.

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: Thomas Staudacher, Observatoire Volcanologique du Piton de la Fournaise (OVPF), 14 RN3, le 27Km, 97418 La Plaine des Cafres, La Réunion, France; Patrick Bachèlery, Département des Sciences de la Terre, Université de la Réunion, BP 7151, 15 Avenue Rene Cassin, 97715 Saint Denis Cedex 9, La Réunion, France; Michel P. Semet and Jean-Louis Cheminée, Observatoires Volcanologiques, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France (URL: http://www.ipgp.jussieu.fr/).

During March-October 1997 a series of phreatic explosions took place within Guagua Pichincha's caldera (figure 5). No precursory signals were detected prior to the activity. The intensity of these explosions peaked in May 1997; the last explosive signal was detected on 18 October 1997. This activity resembled phreatic explosions that occurred in 1981, 1990, and 1993.

Figure 5. Monthly counts of explosion signals at Guagua Pichincha detected by Instituto Geofisico seismic stations during 1997. Courtesy of the Instituto Geofisico.

Larger explosions on 15, 16, 18, 20, and 22 May, 22 and 23 July, and 18 October were detected by four short-period seismic stations located around the volcano. Tremor signals following these explosions had reduced displacements of 2. The largest explosion occurred on 29 May at 0654; its signal was recorded at eight sites, including seismic stations at the volcanoes Cotopaxi (58 km away), Cotacachi (60 km away), and Cayambe (70 km away). The accompanying tremor signal had a reduced displacement of 8.9 cm². An A-type fracture event located just outside the E caldera rim at 3 km depth preceded the explosion.

Following the 20 May explosion, volcanologists observed two new, white, 250-m-tall fumarolic plumes rising from the explosion crater. The crater showed evidence of recent collapses on its interior S and SW sides. Fine pulverized rock deposits covered more than 2 km² in the N part of the caldera bottom. Blocks up to 50 cm across were scattered over the caldera floor as far as 1 km from the crater; impact craters up to 2 m in diameter were formed. No juvenile material was found.

During 1997, the number of events at stations close to the caldera remained at normal values except during September-October, when a large number of events were detected at stations 1.0-1.2 km from the crater. However, at stations over 10 km away, the number of events remained at normal values. Low seismicity preceded phreatic activity in 1990 and 1993. The hypocenter locations of high-frequency events were at depths <5 km beneath the caldera floor (figure 6).

Figure 6. Epicenter map (top) and E-W cross-section (bottom) of high-frequency events at Guagua Pichincha during 1997. Courtesy of Instituto Geofisico.

A swarm of 26 local earthquakes (M <3) lasted less than 1 hour on 16 December 1997. This was the first such swarm detected at Guagua Pichincha since continuous seismic monitoring began in 1981. EDM deformation monitoring of the phreatic crater and outer flanks of the dome revealed no change with regard to the baseline established in 1988.

Thermocouple measurements of fumarole temperatures on the dome showed values of 120-120.7°C, the same as during prior measurements in 1995, but lower than those detected in February 1994 (138-139°C). Prior to 1994, fumarole temperatures were constant at 87°C. Analyses of spring water from the caldera and the surrounding area gave essentially the same results as in 1988.

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately to the W of Ecuador's capital city, Quito. A lava dome is located at the head of a 6-km-wide breached caldera that formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent in the breached caldera consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the central lava dome. One of Ecuador's most active volcanoes, it is the site of many minor eruptions since the beginning of the Spanish era. The largest historical eruption took place in 1660, when ash fell over a 1000 km radius, accumulating to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity, causing great economic losses.

Information Contacts: Mario Ruiz Romero, Instituto Geofísico de la Escuela Politécnica Nacional.

During 26-27 December a small seismic swarm at Irazú consisted of 220 earthquakes. At the swarm's peak 109 earthquakes occurred in 15 hours. The epicenters fell 20 km NNW of the summit, originating on a local fault. The largest earthquake, at 0154 on 27 December, was M 2.9. It had a focal depth of 5 km and an epicenter 20 km NW of the summit. For comparison, during the months of January, February, and March 1998, the respective counts consisted of 58, 59, and 70 local earthquakes.

During 20 and 22 February seven earthquakes took place, including one of M 2.3 and another of M 1.8. Both of these events had epicenters within 7 km of the summit; their respective focal depths were at 8 km and 1 km.

During January the lake in the active crater remained greenish yellow and lacked bubbling along its shores. These areas were not mentioned as active again during February-March, although the lake's color was later described as light green. The monthly fluctuations in lake level noted for December to March were under a meter. During early 1998 small landslides continued to occur along the crater's N, E, and W walls. During February, fumaroles remained active on the volcano's NW flanks; their visible outputs remained moderate and their temperatures measured 91°C.

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Seismicity remained above background level during 2 March-5 April and low-level Strombolian activity continued. As many as 70-100 gas-and-ash or gas-and-steam explosions occurred daily. Ash and steam rose 300-400 m above the crater during the first week of March.

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: Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry; Tom Miller, Alaska Volcano Observatory.

The E rift zone eruption at Kilauea remained steady during March. Seismicity was low, little inflation or deflation occurred at the summit, and magma moved through shallow conduits towards the E rift zone without disturbing the ground surface. The eruption has continued in this fashion since a brief surge in January (BGVN 22:12).

On 11 March glowing holes were observed in the Pu`u `O`o crater floor and in the crater vent; however, no lava escaped from the area. Researchers at the University of Hawaii also observed several large fissures and cracks within the cone edifice. Fumes issued from the cracks and surrounding area; during the last two weeks of March, profuse fumes obscured views of the crater vent. Skylights S of Pu`u `O`o cone revealed lava flowing toward the sea.

Although lava continued to travel in tubes from the Pu`u `O`o vent area to the ocean, surface flows have been sparse since early February (BGVN 23:02). Lava broke out of tubes on the Pulama Pali on 2 and 10 March, but both flows lasted less than a day. Small flows issued from weak points in the lava tubes on the coastal plain on 3-7, 10, and 14 March. Most of the breakouts were near the Waha`ula ocean entry.

Kilauea is one of five coalescing volcanoes that comprise the island of Hawaii. Historically its eruptions originated primarily from the summit caldera or along one of the lengthy E and SW rift zones that extend from the summit caldera to the sea. This latest Kilauea eruption began in January 1983 along the E rift zone. The eruption's early phases, or episodes, occurred along a portion of the rift zone that extends from Napau Crater on the uprift end to ~8 km E on the downrift end. Activity eventually centered on what was later named Pu`u `O`o. More than 223 hectares of new land have been added to the island and local communities have suffered more than $100 million in damages since the beginning of the eruption.

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: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii Volcanoes National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/); Ken Rubin and Mike Garcia, Hawaii Center for Volcanology, University of Hawaii, Dept. of Geology & Geophysics, 2525 Correa Rd., Honolulu, HI 96822 USA (URL: http://www.soest.hawaii.edu/GG/hcv.html).

During 2 March-5 April, seismicity under the volcano remained above background level and earthquakes at 25-30 km depth were recorded. Surface earthquakes were detected on 14 March from 0040-0105.

Fumarolic plumes rose 50-100 m above the volcano on 5, 7, 10, 13-15, 16, 18-20, and 22 March. On 30-31 March, and 1, 3, and 5 April the fumarolic plume rose 50-400 m above the volcano and moved 3-10 km SE. A gas-and-steam plume on 12 March rose 200-1,000 m and traveled more than 5 km ESE. On 17 March, a gas-and-steam plume rose 2-3 km above the volcano and drifted 5-10 km SE.

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

Information Contacts: Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

An 8 April 1998 report stated that during the past week Llaima increased its output of small explosions and ash emissions. The amplitude of seismic signals also increased, although the frequency of signals remained fixed at 1.5 Hz. Seismic amplitude (RSAM) values during March averaged about 25% above those of February. Daily RSAM estimates in March jumped to nearly 30 RSAM units on a few days but more frequently only reached about 10 RSAM units. A sample of the seismic record is shown on figure 9.

Figure 10. Sample seismic record at Llaima (Meli station) on 22 April 1998 beginning at 0400. The tic marks are at 1-minute intervals. Courtesy of OVDAS.

Geologic Background. Llaima, one of Chile's largest and most active volcanoes, contains two main historically active craters, one at the summit and the other, Pichillaima, to the SE. The massive, dominantly basaltic-to-andesitic, stratovolcano has a volume of 400 km3. A Holocene edifice built primarily of accumulated lava flows was constructed over an 8-km-wide caldera that formed about 13,200 years ago, following the eruption of the 24 km3 Curacautín Ignimbrite. More than 40 scoria cones dot the volcano's flanks. Following the end of an explosive stage about 7200 years ago, construction of the present edifice began, characterized by Strombolian, Hawaiian, and infrequent subplinian eruptions. Frequent moderate explosive eruptions with occasional lava flows have been recorded since the 17th century.

Information Contacts: Gustavo Fuentealba¹ and Paola Peña S., Observatorio Volcanológico de Los Andes del Sur (OVDAS), Manantial 1710-Carmino del Alba, Temuco, Chile; ¹Universidad de La Frontera (UFRO), Departamento Ciencias Fisicas, Universidad de la Frontera, Avda. Francisco Salazar 01145, Casilla 54-D, Temuco, Chile.

Measurements during a 28 February visit revealed higher-than-normal fumarolic temperatures in the summit area. The high temperatures were associated with a recent period of aridity, during which time fumarolic activity increased. Temperatures ranged from 318-748°C (figure 7).

Figure 7. Sketch of Momotombo's active crater showing fumarole temperatures on 28 February. Areas of fumarolic activity are gray. View is towards the S; the crater is ~150 m wide. Courtesy of A. Creusot.

Geologic Background. Momotombo is a young stratovolcano that rises prominently above the NW shore of Lake Managua, forming one of Nicaragua's most familiar landmarks. Momotombo began growing about 4500 years ago at the SE end of the Marrabios Range and consists of a somma from an older edifice that is surmounted by a symmetrical younger cone with a 150 x 250 m wide summit crater. Young lava flows extend down the NW flank into the 4-km-wide Monte Galán caldera. The youthful cone of Momotombito forms an island offshore in Lake Managua. Momotombo has a long record of Strombolian eruptions, punctuated by occasional stronger explosive activity. The latest eruption, in 1905, produced a lava flow that traveled from the summit to the lower NE base. A small black plume was seen above the crater after a 10 April 1996 earthquake, but later observations noted no significant changes in the crater. A major geothermal field is located on the south flank.

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

A 14 February visit to Cerro Negro's crater revealed a general decrease in fumarole temperatures since Alain Creusot last measured temperatures there on 23 December 1996 (BGVN 21:12). The highest temperature found on his latest visit was 340°C. For comparison, in October 1996 fumarole temperatures were as high as 700°C.

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

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

Poás monthly reports from OVSICORI-UNA since November 1997, and as recently as March, have noted that its N crater lake has remained turquoise green and continued to host rafts of suspended sulfur. The lake's surface normally sits at ~2,300 m elevation. Although during 1997 the lake's surface reached a high stand, it descended during early 1998, dropping 3 m due to lack of rain. Mauricio Mora Fernandez provided plots of the lake water's pH, temperature, sulfate, and chlorine for the past several years (figures 67 and 68). Fernandez also reported that during February 1998 fumarolic activity continued in five areas within the active crater (figure 69). During April, he found a sixth fumarolic area on the dome's N slope.

Figure 67. Water pH and temperature measured in the N crater lake at Poás (right- and left-hand scales, respectively), 1993 to early 1998. The time scale is not linear. OVSICORI-UNA staff collected the lake geochemistry data. Courtesy of M. Mora Fernandez.

Figure 68. Aqueous sulfate and chlorine in the N crater lake at Poás, 1993 to early 1998. Time scale is not linear. OVSICORI-UNA staff collected the lake geochemistry data. Courtesy of M. Mora Fernandez.

Figure 69. The active crater at Poás viewed from the S. Numbers 1-5 correspond to areas with fumaroles active during February 1998; a sixth area, located N of the dome (on the side away from the view, not labeled), became active in April 1998. Courtesy of M. Fernandez.

Area 1, the fumarolic field located at the crater's S end, became active in May 1995 and remained comparatively stable thermally until at least March. During 1997-early 1998, the field extended S, SW, and W within the larger crater. During February 1998, the area's average temperature remained constant at ~92°C; during April, it attained 93°C. Steam and high concentrations of SO₂ and Cl gas escaped from the fumaroles; sulfur crystals were deposited around the vents. Mora noted that in the time since the fumaroles appeared, hydrothermal alteration became more rapid and reduced competency of the rock, leading up to two landslides in the area.

In area 2, the field W of the crater lake, a large landslide occurred during February. It took place at a spot where hydrothermal alteration resulted from three fumaroles that sent white gas plumes dominantly toward the SW. More fumaroles sprung up in this field during April.

In area 3, the field at the lake's N end, new fumaroles appeared during roughly the second half of 1997. These continued without important changes through April 1998; their emissions were white and not very vigorous.

In area 4, a field on the dome's E slope, small fumaroles produced white plumes. The emissions were not vigorous but their average February-April temperatures were 92-93°C. Some new fumaroles noted in this area during April had temperatures averaging 93°C.

In areas 5 and 6, fields located respectively on the dome's E and N slopes, vigorous fumaroles gave off mainly white plumes. During April, area 5 plumes had temperatures of 92°C and ascended to tens of meters before dispersing. Area 6, which became active in April, gave off plumes that covered the nearby slope with sulfur deposits.

OVSICORI-UNA reported that the pyroclastic cone in the crater discharged a plume that during January rose 400 m above the crater rim. They also noted that during February the rain collection network located around the active crater yielded samples with increased acidity. During this same month, residents 5.5 km SE of the crater reported occasional sulfur odors.

Seismic data from an OVSICORI-UNA station 2.7 km SW of the active crater revealed a noticeable rise in the duration of tremor during February and March 1998. Tremor generally occurred in discontinuous episodes, although one episode on 21 February carried on for 2.5 hours. Also, an anomalously large number of low-frequency earthquakes took place during February 1998 (figure 70)—a count of this magnitude was last seen in January 1996. In contrast, medium and high frequency earthquakes were not particularly abundant in February or March 1998 (figure 70). Many of the low-frequency earthquakes were attributed to continuous degassing.

Figure 70. Seismicity at Poás during January 1997-March 1998. Number of low-frequency earthquakes and hours of tremor (top); number of high- and medium-frequency earthquakes (bottom). Note that the scales are different. Courtesy of OVSICORI-UNA.

Geologic Background. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Eruptive activity at Tavurvur persisted during March following the 3 February eruption (BGVN 23:02), producing ash emissions, small pyroclastic flows, and relatively low but fluctuating seismicity. Seismicity peaked around20 March, when eruptions became more energetic, and was probably related to near-surface eruptive activity.

Deformation monitoring indicated steady inflation at Tavurvur. Readings from the Sulphur Creek water tube (3.5 km NW of Tavurvur) revealed a change of ~3 µrad tilt away from the volcano during March. Leveling and real-time GPS also showed continuing inflation.

Tavurvur continued to erupt throughout March and emitted ash at intervals of ~10 minutes to several hours; the rapidly convecting column sometimes rose 2-4 km. After emissions had ceased for more than 10-20 minutes, activity would often recommence with explosions that threw large numbers of blocks from the vent. Blocks up to 1 m in diameter were regularly thrown 1 km S and W of the vent, landing out to sea. Large blocks (~3-4 m across) littered the rim and upper slopes of Tavurvur, probably produced during larger-than-usual explosions on 7 and 8 March.

The 8 March explosion sent red oxide-covered lava blocks and boulders over the S crater rim and down the S flank of Tavurvur, where the flow traveled ~1 km. This mass was described as being "pushed" from the vent immediately prior to the explosion. At other times the ash plume underwent partial column collapse and sent short, billowing flows randomly down the cone's flanks. The flows deposited light gray dust ~50-150 m downslope in well-defined tongues.

During 18-26 March night glow became more evident; occasionally lava fountains sent glowing fragments 200-300 m above the crater rim for up to 5 minutes at a time. During 26-31 March intermittent ash emissions with discrete explosions after longer periods of quiescence resumed.

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: Ben Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Beginning at 1428 on 15 February, Rincón de la Vieja volcano discharged phreatic eruptions from the main crater. Ten eruptions took place in the first 15 hours of activity; only two followed in the subsequent 13 hours. During the course of the outburst subsidiary fumarolic activity also became more vigorous; it remained elevated until 18 February.

During 15-17 February numerous steam plumes rose hundreds of meters above the volcano. On 17 February one outburst sent a steam plume to a height of 2 km above the crater. This plume was seen by residents on the N and NE flanks of the volcano. A dozen eruptions around this time were small and lacked associated mudflows. An exception, at 0514 on 16 February, produced a modest mudflow that traveled about 9 km/hour and left a capping deposit of mud 30-cm thick in the upper reaches of the Pénjamo and Azul rivers. Rivers had been low in the region, attributed to the El Niño phenomena, with the result that the mudflow was relatively dry. The mudflow had a large impact on local fish and other stream organisms. Sediment from the mudflow was found 12.3 km from the main crater.

Inspecting the 16 February deposit near the summit on 1 March, scientists inferred from the scorching, burning, and other damage to vegetation on the NE flanks that there must have been several smaller eruptions around that time as well. Mudflows failed to develop due to the paucity of surface water in local drainages.

The 1 March visit also revealed the lake's temperature, 48°C, its color, light gray, the presence of suspended sulfur in the lake, and a haze of condensed gases above the lake. An outgassing fumarole on the SW wall made loud hissing noises (similar to gases exiting a high pressure valve) audible from the crater's rim. Columns of gas rose about 200 m above the crater before being blown E. Those inspecting the scene noted strong sulfurous odors, and experienced irritated skin and eyes. The material erupted was uniformly fine- to medium-grained, lacking either bombs, blocks, or impact craters. This contrasted with deposits left by previous eruptions in 1991 and 1995.

The local seismic station (RIN3) lies 5 km SW of the active crater. The station registered microearthquakes as follows: during January, 18 (including 3 of high frequency and 9 of low frequency); during February, 48 (including 1 of high frequency, 21 of low frequency); during March, 7. In assessing their records of the 48 February microearthquakes, seismologists recognized 20 eruptions including 11 comparatively high-intensity phreatic eruptions mainly registered on 15-18 February. Banded tremor occurred on 15 and 16 February during the main eruptive interval; the tremor prevailed for a total of ~6.5 hours. Low in frequency, the tremor had amplitudes that ranged between 1.0 and 37 mm. The larger amplitude registered during the eruption's initial phase, at 1428 on 15 February. Tremor amplitudes later declined to the 1-4 mm range. As with the 1991 and 1995 eruptions, seismic precursors were absent.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Sáenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Seismicity was about at background level during 2 March-5 April. Gas-and-steam plumes rose 100 m above the volcano on 7 and 13-15 March. On 16-18, 22, and 30-31 March, and 1 and 3 April, gas-and-steam plumes rose 100-500 m above the volcano. Clouds obscured observations of the volcano on several days in early April.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

This report condenses Scientific Reports of the Montserrat Volcano Observatory (MVO) covering February. During 1-14 February, seismic activity increased, heavy ashfalls reached the N part of the island, and dome growth continued. Activity during 15-28 February was dominated by rapid dome growth and elevated seismicity.

Visual observations. Low clouds during the first two weeks of February often hampered dome observations. However, on 6 February observers on a police boat reported continued growth in the 26 December collapse scar above the White River. By 10 February the growing dome almost completely filled the 26 December scar, approaching the volume prior to the collapse. In addition, two spines were observed on the dome's S side, and the talus slope below the growth area had grown considerably. Steam-and-ash venting continued and was vigorous during periods of elevated seismicity and rockfall.

Rockfalls and small pyroclastic flows occurred mainly on the Galways side of the dome, but a few small rockfalls were observed in the upper part of Tuitt's Ghaut. Fresh pyroclastic-flow deposits in the upper part of the White River were probably emplaced during the elevated activity of 5-6 February.

On 15 February several rockfalls and small pyroclastic flows traveled down the White River valley. Visibility was poor until 25 February when vigorous ash venting, rockfalls in the White River valley, and several stubby spines atop the dome were observed.

Seismicity. Earthquake activity during 1-14 February mainly consisted of rockfalls and hybrid earthquakes with some tremor. Most swarm events, including 21 locatable volcano-tectonic earthquakes, were concentrated below the dome complex's N sector and had shallow focal depths (2-4 km below the summit). During 15-28 February fewer rockfalls but comparatively more earthquakes and seismic swarms (table 27) occurred than in preceding weeks. The swarms were not followed by surface activity.

Table 27. Number of hybrid, long-period (LP), and volcano-tectonic (VT) events detected during earthquake swarms at Soufriere Hills during February 1998. Courtesy of MVO.

At the beginning of February, seismicity displayed a cyclic pattern with peak amplitudes occurring every 6-8 hours; by 14 February, the cycle had lengthened to 8-12 hours. By 22 February, the cycle was ~14 hours long. Peak amplitudes increased during 1-14 February; these peaks generally coincided with elevated rockfall activity. Towards the end of February, the peaks were dominated by hybrid earthquakes and tremor.

Ground deformation. Two GPS occupations of LEESNET (includes sites at Old Towne, Waterworks, St. Georges Hill, and Lees Yard) were made during 1-14 February. No movement within this network was detected. Meanwhile, GPS surveys at Harris, Hermitage, Lees Yard, Perches, St. Georges Hill, Old Towne, Blakes, and Lookout Yard North confirmed that the Hermitage and Perches sites continued to move NNE. Sites on the volcano's N and NW flanks remained relatively stable.

Electronic tiltmeters were installed at Hermitage and on Gages Mountain to provide data on deformation of the volcano's NE flank. The EDM reflector on the N crater wall (Peak B) was shot from Windy Hill during 15-28 February. During 25 January-late February a 5-cm shortening occurred on this line. Lines between the Lees Yard reflector and sites at MVO south and the Waterworks Estate did not show any movement.

Volume measurements. A 10 February theodolite survey of the dome from Garibaldi Hill and the Delta petrol station revealed that the dome's highest point was 970 m. On 27 February, theodolite measurements from Garibaldi Hill and the old observatory in Old Towne showed that the highest point on the dome had reached 997 m. More theodolite measurements on 1 March from South Soufriere Hills and Perches Mountain gave a height of 1011 m, revealing 14 m of vertical growth in only 2 days.

Environmental monitoring. Sulfur dioxide diffusion tube measurements during 1-14 February showed raised (10-12 ppb) SO₂ levels in Plymouth and at St. Georges Hill and low (0-0.6 ppb) levels at Weekes, MVO south, and Lawyers. During 15-28 February SO₂ levels at Plymouth, MVO south, and Lawyers were higher than earlier in the month, but levels at St. Georges Hill were reduced by half. The site in Plymouth showed very high values (30.2 ppb) because it was surrounded by ~30-cm-thick tephra deposits and redeposited debris from nearby pyroclastic-flow deposits.

The mass of fine ash deposited in N Montserrat during several 28 January-7 February ashfalls was calculated using an array of ash collection trays. The mass totaled more than 1 kg/m²; most of this ash was produced during episodes of ash venting and rockfall activity. At most locations the ash collected during 3-5 February accounted for more than 50% of the local monthly ash accumulation.

Dust Trak monitoring at four fixed sites to measure airborne particles revealed elevated values (0.05-0.38 mg/m³) during ashfalls on 4-5 February. Levels were even higher (0.11-0.43 mg/m³) on 7 February due to resuspension of the ash. Sites in the S part of the island showed higher concentrations than in the N. During 15-28 February, no major ash fall occurred and levels were low (3) at all sites; however, a diffuse volcanic plume was occasionally blown N, causing light ash fall and hazy conditions.

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, P. O. Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/).

Beginning at about 0900 on 26 March, an unusual cloud or steam plume in the vicinity of Spurr volcano was observed from Anchorage (125 km E). However, seismicity remained at normal levels and nothing unusual was noted in satellite images of the area. The level of concern remained at green, indicating normal seismic and fumarolic activity.

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

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Scientists visited Telica's crater on 7 February. They observed a slight increase in fumarolic activity and an active collapse zone on the S crater rim. Light incandescence seen at night had an estimated temperature of 550°C.

Geologic Background. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

OVSICORI-UNA scientists have taken sporadic samples of the chemistry, pH, and temperature of Turrialba's fumaroles (figures 2 and 3). During January, fumaroles had low emissions but the temperature of one fumarole remained fixed at 90°C (figure 3). Small landslides down the N and S sides of the crater walls covered fumaroles on the crater floor during January; however, during this time new fumaroles also appeared on the crater floor as well.

Figure 2. Chlorine and sulfate in Turrialba fumarolic condensate at [nine] sampling dates during late 1996-early [1997]. For sampling and analytical methods, contact the authors. Courtesy of OVSICORI-UNA.

Figure 3. The pH and temperature of Turrialba fumarolic condensate at four sampling dates during the interval late 1996 to early 1998. Courtesy of OVSICORI-UNA.

The local seismic station ("VTU," located 500 m S of the active crater) was out of service during September-December 1997. After that, the station registered microearthquakes as follows: January, 53; February, 83; and March 96. Two of the February earthquakes, one high- and one low-frequency, also registered on the more distant seismic station IRZ2, ~15 km from the active crater. Besides the 96 microearthquakes registered during March, several more low- and high-frequency earthquakes also took place.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Luis Hernan Ecueñique, a manager in charge of "Las Cavernas," a tourist attraction 8 km from Villarrica's active crater, noted that during late March through at least early April there had been an ascent of magma in the central crater. Erupted material reached ~100 m from the crater's edge. Local tour guides had also informed him that explosions had deposited tephra on the N flanks. Measurements within "Las Cavernas" (which are lava tubes) indicated the air temperature rose by about 2°C.

A digital seismic station 21 km from the crater failed to detect either an increase in the number of seismic events or a shift in their character; the system did register a minor increase in event amplitude.

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

Information Contacts: Gustavo Fuentealba¹ and Paola Peña S., Observatorio Volcanológico de Los Andes del Sur (OVDAS), Manantial 1710-Carmino del Alba, Temuco, Chile; ¹Universidad de La Frontera (UFRO), Departamento Ciencias Fisicas, Universidad de la Frontera, Avda. Francisco Salazar 01145, Casilla 54-D, Temuco, Chile.