Guatemala's Volcán de Fuego was continuously active through September 2019; it has been erupting vigorously since 2002 with historical observations of eruptions dating back to 1531. These eruptions have resulted in major ashfalls, pyroclastic flows, lava flows, and damaging lahars. Large explosions with hundreds of fatalities occurred during 3-5 June 2018; after a brief pause, significant activity resumed and continued during April-September 2019, the period covered in this report. Reports are provided by the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH) and the National Office of Disaster Management (CONRED); aviation alerts of ash plumes are issued by the Washington Volcanic Ash Advisory Center (VAAC). Satellite data from NASA and other sources provide valuable information about heat flow and gas emissions.

Daily activity continued at a high level throughout April-September 2019 (table 19) with multiple ash explosions every hour, incandescent ejecta reaching hundreds of meters above the summit sending block avalanches down multiple ravines, and ash falling on communities on the SW flank and beyond. During April and part of May a lava flow was also active in the Seca ravine. Although explosive activity remained at a high level throughout the period, thermal activity began a decline in May that continued through September, noticeable in both the MIROVA radiative power data (figure 117), and monthly images of MODVOLC thermal alerts (figure 118).

Table 19. Activity summary by month for Fuego with information compiled from INSIVUMEH daily reports.

Figure 117. Thermal activity at Fuego increased steadily from January through April 2019, and then began a gradual decline through September as seen in this MIROVA graph of Radiative Power. The active lava flow in the Seca Ravine in April and early May likely contributed to the higher heat values during that time. Courtesy of MIROVA.

Figure 118. A steady decline in thermal activity at Fuego is apparent in the MODVOLC thermal alert images for April-September 2019. During April and early May a lava flow was active in the Seca ravine that extended as far as 1,000 m from the summit. Courtesy of MODVOLC.

Activity increased at the very end of March 2019. The rate of explosions increased to 14-32 events per hour by 31 March; ash plumes rose to 5 km altitude and resulted in ashfall in numerous nearby communities. An early morning lava flow that day reached 800 m down the Seca ravine. Continuous white and gray fumarolic plumes reached 4.1 to 4.4 km altitude during April 2019 and drifted generally W and SW. There were about 15-20 ash-bearing explosions per hour; the highest rate of 25 per hour occurred on 10 April. Plume altitudes were below 4.8 km for most of the month; on 28 and 29 April they rose to 5.0 and 4.9 km. For most of the month they drifted W and SW; the wind direction changed to the E during 10-16 April. Most days of the month ashfall was reported in the communities of Panimaché I y II, Morelia, Santa Sofía, Finca Palo Verde, San Pedro Yepocapa, Sangre de Cristo and El Porvenir on the W and SW flank. During 10-13 April when the wind direction changed to easterly, communities to the NE, E and SE of Alotenango, Ciudad Vieja, La Reunión, La Rochela, El Rodeo, Osuna, Ceilán and others on the N and E flanks were affected by ashfall. The Washington VAAC issued multiple daily advisories on 18 days in April, identifying short-lived ash plumes drifting with the prevailing winds.

Incandescent ejecta rose 200-300 m above the summit on most days (figure 119). During 23-25 April, ejecta rose 300-450 m above the summit. Six ravines were affected by the incandescent avalanche blocks nearly every day: the Seca, Taniluyá, Ceniza, Trinidad, Las Lajas, and Honda. The explosions caused rumbles, shock waves that rattled roofs, and sounds similar to a train locomotive at intervals of 5-20 minutes in nearby communities throughout the month. A lava flow was present in the Seca (Santa Teresa) ravine for most of the month; its length varied from 200 to 800 m. Special reports of lahars were issued seven times during April. On 4 April a moderate lahar descended the Seca ravine carrying centimeter- to meter-sized blocks, tree trunks and branches. During 9-11 April nine lahars were recorded in the Las Lajas, El Jute, Seca, Rio Mineral, Taniluya, and Ceniza ravines. The largest flows were 20 m wide and 3 m deep carrying blocks and debris up to 3 m in diameter; they were warm and thick with a strong sulfurous odor. Two more lahars were reported on 18 April in the Taniluya and Ceniza ravines carrying 1-2 m sized blocks in a warm, sulfurous flow.

Figure 119. Incandescent ejecta rose several hundred meters above the summit of Fuego on 30 April 2019 and sent large blocks down multiple ravines, typical activity for the entire month. Courtesy of CONRED (Boletín Informativo No. 1242019, martes, 30 de abril 2019, VOLCÁN DE FUEGO BAJO CONSTANTE MONITOREO).

During May 2019, primarily white fumaroles rose to 4.2-4.5 km altitude and drifted W, SW, and S; gray fumaroles were reported only during the first few days of the month. Generally, 15-20 ash explosions per hour occurred; the maximum was 26 on 17 May. Ash plume heights ranged from 4.5-4.8 km altitude nearly every day, drifting 10-25 km primarily W, SW, and S throughout the month, except for 6-8 May when plumes drifted NW and 18-19 May when wind directions changed and sent ash S and SE. Plumes drifted 25-30 km SE, S, and SW on 19 May. Ashfall was reported daily from communities on the W flank including Panimaché I and II, Morelia, Santa Sofía, El Porvenir, Los Yucales, Finca Palo Verde, Sangre de Cristo, and San Pedro Yepocapa, among others, and also from the E side including Ceilán and La Rochela when the wind direction changed. The Washington VAAC issued multiple daily ash advisories on 19 days during May.

Incandescent Strombolian activity continued sending ejecta 200-300 m above the summit during the first half of the month and 300-450 m high during the latter half (figure 120). Seven major ravines, the Seca, Taniluyá, Ceniza, Trinidad, El Jute, Las Lajas, and Honda were affected by block avalanches throughout the month. Intermittent explosions caused rumbles, shock waves that rattled roofs, and sounds similar to a train locomotive at frequent intervals on most days. The lava flow in the Seca ravine advanced from 300 m length on 2 May to 1,000 m long on 9 May. It was reported as being 500 m long on 18 May but was not active after that date. Numerous lahars descended multiple ravines in May. INSIVUMEH issued nine special reports of lahar activity on 3, 14, 16, 20, 23, and 27-29 May. They affected the Las Lajas, Ceniza, El Jute, El Mineral, and Seca ravines. The thick, pasty flows contained blocks of various sizes up to 3 m in diameter along with tree trunks and branches. Several were warm with a sulfurous smell (figure 121). SO₂ emissions remained low throughout April-September with only minor emissions recorded in satellite data on 1 April and 9 May 2019 (figure 122).

Figure 120. Incandescent ejecta at Fuego was captured on 27 May 2019 under a starry night sky by photographer Diego Rizzo in a 25-second exposure. Block avalanches are seen descending several ravines. NASA used the photo as an Astronomy Photo of the day and noted that the central plane of the Milky Way galaxy runs diagonally from the upper left, with a fleeting meteor just below, and the trail of a satellite to the upper right. The planet Jupiter also appears toward the upper left, with the bright star Antares just to its right. Much of the land and the sky were captured together in a single 25-second exposure taken in mid-April from the side of Acatenango volcano; the meteor was captured in a similar frame taken about 30 minutes earlier and added to this image digitally. Courtesy of NASA Astronomy Picture of the Day, copyright by Diego Rizzo.

Figure 121. Lahars were reported at Fuego nine separate times during May 2019. A steaming lahar descends a ravine at Fuego on 11 May 2019 (top). The Santa Teresa Canyon was clogged with debris from numerous past lahars on 22 May 2019. INSIVUMEH monitors the ravines continuously during the rainy season. Courtesy of CONRED (Boletín Informativo No. 1382019, sábado, 11 de mayo 2019, LLUVIAS GENERAN DESCENSO DE LAHARES EN EL VOLCÁN DE FUEGO and Boletín Informativo No. 1562019, miércoles, 22 de mayo 2019, SE REGISTRA DESCENSO DE LAHARES MODERADOS EN EL VOLCÁN DE FUEGO).

Figure 122. Weak SO2 emissions were recorded from Fuego on 1 April and 9 May 2019 by the TROPOMI instrument on the Sentinel 5P satellite. Courtesy of NASA Goddard Space Flight Center.

The fumarolic plumes were only white during June 2019, rising to 4.1-4.5 km altitude daily, drifting W or SW except during the first days of the month when variable winds sent the steam N, E, and SE. Explosions with ash took place 15-20 times per hour on most days with plumes rising to 4.5-4.8 km altitude and drifting primarily W or SW except for the first days of the month (figure 123). On most days, ash plumes drifted 15-20 km W and SW, except during 2-7 June when winds sent ash E, SE, N, and NW. Ashfall was reported virtually every day in Sangre de Cristo, Yepocapa, Morelia, Santa Sofía, and Panimache I and II. In addition, the communities of El Porvenir, Los Yucales, and Finca Palo Verde reported ashfall several days each week. During 2, 4, and 7 June, the N and SE winds caused ash to fall in Alotenango and San Miguel Dueñas. The Washington VAAC issued ash advisories on 15 days during June.

Figure 123. Emissions of both steam and ash rose from Fuego on 11 June 2019. Courtesy of Paul A. Wallace, University of Liverpool.

The height of the Strombolian ejecta varied from 200-300 m above the summit on many days in June , but also was sometimes stronger, rising 300-450 m. While block avalanches were reported in all seven barrancas (ravines) more than once (Seca, Taniluyá, Ceniza, Trinidad, El Jute, Las Lajas and Honda), on all days they were reported in the Seca, Taniluya, Ceniza, and Trinidad. Weak to moderate rumbles and shock waves rattled roofs every day, and train engine noises were heard every 5-10 minutes. Seven special reports of lahars were issued on days 2, 11, 21-23, and 30. They affected the Las Lajas, El Jute, Seca, El Mineral, and Ceniza ravines with thick, pasty flows containing blocks 1-3 m in size, shaking the ground as they flowed downstream.

During July 2019, white steam plumes rose daily from the summit of Fuego to an altitude of 4.1-4.3 km and drifted W and SW; higher plumes on 30 and 31 July rose to 4.5 km altitude. Fifteen to twenty ash explosions per hour were typical throughout the month and produced ash plumes that rose to 4.3-4.8 km altitude and drifted SW and W for 10-25 km before dissipating (figure 124). Near-daily ashfall was reported in Morelia, Santa Sofía, El Porvenir, Finca Palo Verde, San Pedro Yepocapa, Panimaché I y II, and Sangre de Cristo; La Rochela and Ceilán also reported ash on 4 and 6 July. Incandescent ejecta height varied from 150-450 m above the summit from day to day, sending block avalanches down all seven ravines on many days. Weak to moderate rumbles and shock waves rattled roofs every day, and train engine noises were heard every 5-15 minutes. On 19 July noises and vibrations were heard and felt 25 km away. Only one lahar was reported on 12 July in the Las Lajas ravine. It was warm, with a sulfurous odor, and carried volcanic ash, sand, and blocks 1-3 m in diameter that shook the ground as they flowed downstream. The Washington VAAC issued ash advisories on 13 days during July.

Figure 124. Steam-and-ash plumes rose from Fuego on 12 July 2019 in this image taken at dawn from Villa Flores San Miguel Petapa. Courtesy of Alex Cruz (cropped and color adjusted from original).

White steam plumes continued during August 2019, rising to an altitude of 4.1-4.5 km and drifting W and SW daily. Ash-bearing explosions continued also at a rate of about 15-20 per hour throughout the month, rising most days to between 4.5 and 4.7 km altitude. They drifted 15-20 km W or SW nearly every day before dissipating. Every day during the month, ashfall was reported in Morelia, Santa Sofía, El Porvenir, Finca Palo Verde, San Pedro Yepocapa, Panimaché I y II, Sangre de Cristo, and other communities on the SW flank. The Washington VAAC reported ash plumes at Fuego on 15 days during August (figure 125).

Figure 125. An ash emission at Fuego was recorded on 22 August 2019. Courtesy of William Chigna.

Incandescent ejecta also rose every day during August 2019 to 200-300 m above the summit, a few days were reported to 350-400 m. Every day, block avalanches descended the Seca, Taniluyá, Ceniza, and Trinidad ravines; most days blocks also traveled down the Las Lajas and Honda ravines, and many days they were also reported in the El Jute ravine (figure 126). Every 5-10 minutes, every day, weak and moderate rumbles sounding like a train engine shook buildings and rattled roofs in the nearby villages. On 13 August a small lava flow, 75-100 m long, was reported in the Seca ravine. Six lahars were reported on 3 August. They occurred in the Santa Teresa, Mineral, Ceniza, El Jute, and Las Lajas ravines. The thick pasty flows carried blocks 1-2 m in diameter, tree trunks and branches, and disrupted the roads between Siquinala and San Andres Osuna and El rodeo and El Zapote. The next day two more occurred in the Seca and Mineral drainages. From 17-20 August, six more lahars occurred, most in the Las Lajas drainage, but also in the Seca, Mineral and Ceniza ravines.

Figure 126. Incandescent blocks traveled down several ravines at Fuego on 2 August 2019. Courtesy of Publinews Guatamala.

There were no changes in the steam fumaroles during September 2019; plumes seldom rose over 4.3 km altitude and continued drifting W and SW. The ash explosion rate decreased somewhat and rates of 5-10 per hour were typical on many days. Ash plume heights remained constant around 4.5-4.7 km altitude most days, also drifting W and SW 15-20 km before dissipating (figure 127). While ashfall was reported daily in Panimaché I, Morelia, Santa Sofía, Porvenir, Palo Verde, Yepocapa and other communities on the SW flank for the first half of the month, it grew more intermittent during the second half of September. South-directed winds deposited ash on La Rochela villages and Ceylon on 25 September. The Washington VAAC issued aviation ash advisories on 11 days during the month. Strombolian ejecta mostly rose 200-300 m above the summit; occasionally it reached 300-400 m. On most days, block avalanches descended the Seca, Taniluyá, Ceniza, Trinidad, and Las Lajas ravines; occasionally they were reported in the El Jute and Honda ravines as well. Every day, rumbles and shock waves shook roofs in nearby villages every 5-10 minutes. Lahars were reported twice, on 2 ad 9 September, in the Seca and Rio Mineral drainages both days, dragging branches, tree trunks and blocks up to 2 m in diameter.

Figure 127. An ash plume drifts from the summit of Fuego on 16 September 2019, seen from the La Reunion webcam. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán de Fuego (1402-09), Semana del 14 al 20 de septiembre de 2,019).

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/ ); 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/); 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); NASA Astronomy Picture of the day (URL: https://apod.nasa.gov/apod/ap190527.html); 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/); Paul A. Wallace, Lecturer in Geology, University of Liverpool, Liverpool England (URL: https://www.liverpool.ac.uk/environmental-sciences/staff/paul-wallace/, Twitter: @Paul_A_Wallace, URL: https://twitter.com/Paul_A_Wallace/status/1138527752963993600); Alex Cruz, Photojournalist, Guatemala (Twitter: @ACruz_elP, URL: https://twitter.com/ACruz_elP/status/1149690904023691264/photo/1); William Chigna, Guatemala (Twitter: @William_Chigna, URL: https://twitter.com/William_Chigna/status/1164575009966370816); Publinews Guatemala, (Twitter: @PublinewsGT, URL: https://twitter.com/PublinewsGT/status/1157288917365903360).

Erta Ale, located in Ethiopia, contains multiple active pit craters both within the summit and the southeast calderas. On 17 January 2017 the active lava lake displayed intense spattering, fountaining, and rim overflows with lava flows that traveled as far as 1 km, forming a lava flow field. During April 2018 through March 2019 minor activity continued in both the summit and southeast calderas, and along the active lava flow to the E (BGVN 44:04). This report updates volcanism from April through October 2019. Information primarily comes from infrared satellite images and MODIS data.

Continued lava flow breakouts occurred from April through October 2019. On 4 May 2019 a lava flow outbreak was observed in satellite imagery NE of the summit caldera (figure 92). This outbreak continued to appear in clear-weather thermal satellite images through 13 June when it was seen south of its original location (figure 93). Faint incandescence is observed at the summit caldera between June and October 2019, though it is more pronounced in the months of August through October. On 28 June a second smaller lava flow outbreak occurred within 3.8 km of the summit location. The two lava flow outbreaks remained active at least through 18 June. The distal NE lava flow does not appear in very similar images from 17 August or 16 September 2019, but three proximal thermal anomalies are seen in the southeastern caldera within 4 km of the summit. The thermal anomalies remained within 5 km through October 2019.

Figure 92. Sentinel-2 thermal satellite imagery of Erta Ale volcanism on 4 May 2019 with thermal anomalies observed to the northeast of the summit caldera (bright orange). White plumes are seen rising from the summit with faint incandescence. Sentinel-2 satellite images with "False Color (Urban)" (bands 12, 11, 4) rendering; courtesy of Sentinel Hub Playground.

Figure 93. Sentinel-2 thermal satellite imagery of Erta Ale volcanism between 8 June and 21 October 2019. Lava flow outbreaks initially occur in the distal NE part of the lava flow, which then migrates slightly south. A second lava flow outbreak is seen less than 5 km of the summit caldera. Faint incandescence is seen at the summit caldera in each of these images. Sentinel-2 satellite images with "False Color (Urban)" (bands 12, 11, 4) rendering; courtesy of Sentinel Hub Playground.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed consistently high-power thermal anomalies during this reporting period (figure 94). Through July 2019 these thermal anomalies were detected at distances greater than 5 km from the summit. In early August 2019 there was an abrupt decrease in the distance that continued through late October 2019 (figure 94); this likely indicates when the distal NE outbreak ended and lava emissions from the closer SE locations increased (see satellite images in figure 93). The distance changes of MODIS thermal anomalies from the summit seen in MIROVA are corroborated by MODVOLC data, which show no distal NE thermal alert pixels after July 2019 (figure 95).

Figure 94. Two time-series plots of thermal anomalies from Erta Ale for the year ending on 24 October 2019 as recorded by the MIROVA system. The top plot (A) shows that the thermal anomalies were consistently strong (measured in log radiative power) and occurred frequently. The lower plot (B) shows these anomalies as function of distance from the summit, including a sudden decrease in the distance (measured in kilometers) in early August 2019 that reflects a change in lava flow outbreak location. Courtesy of MIROVA.

Figure 95. Locations of the thermal alerts at Erta Ale during November 2018-July 2019 (top) and August-October 2019 (bottom) identified by the MODVOLC system. A majority of the proximal (less than 5 km from the summit) thermal anomalies are found within the southeastern calderas while the distal (beyond 5 km) anomalies are northeast of the summit. Note that the distal NE anomalies are not present after July 2019. Two thermal alerts mark the location of the summit caldera (bottom map). Data courtesy of HIGP-MODVOLC Thermal Alerts System.

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

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/).

Eruptive activity at Karymsky has been frequent since 1996, with moderate ash explosions, gas-and-steam emissions, and thermal anomalies. The latest eruptive period began in mid-February 2019 (BGVN 44:05) when explosions resumed after more than four months of quiet, producing an ash plume that extended 55 km downwind. Intermittent explosive activity continued until 24 September 2019. The volcano is monitored by the Kamchatka Volcanic Eruptions Response Team (KVERT).

Ash plumes were reported during the second half of February and the first half of March 2019 (BGVN 44:05). During May-September 2019 similar activity continued, with ash plumes being generated at least every few days (table 12). Though not included in the weekly KVERT report as notable events, obvious ash plumes were also seen in Sentinel-2 imagery on 22 July and photographed from an aircraft on 23 July. Volcanologists doing fieldwork on 14 August observed an ash plume rising to 5 km altitude (figure 44). A week later, during 20-22 August, explosions generated ash plumes as high as 6 km altitude that were visible in satellite imagery (figure 45). Although not noted in KVERT reports, a photo from 9 September showed a plume blowing downwind directly from the summit crater (figure 46). No significant ash plumes were reported by KVERT after 24 August, but the last ash explosion was recorded on 24 September.

Table 12. Notable ash plumes reported from Karymsky during May-October 2019. All dates are in UTC. Courtesy of KVERT.

Figure 44. Aerial photo showing an ash plume rising to 5 km altitude from Karymsky 14 August 2019. Photo by D. Melnikov; courtesy of IVS FEB RAS, KVERT.

Figure 45. Satellite image from Sentinel-2 (natural color) of an ash plume at Karymsky on 21 August 2019. Courtesy of Sentinel Hub Playground.

Figure 46. Photo showing explosive activity at Karymsky at 1920 UTC on 9 September 2019. Photo by A. Manevich; courtesy of IVS FEB RAS, KVERT.

During May-October 2019, thermal anomalies were detected with the MODIS satellite instruments analyzed using the MODVOLC algorithm only on 25 July (2 pixels) and 21 August (10 pixels). Consistent with both observations, KVERT noted ash explosions on those dates. The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system, also based on analysis of MODIS data, detected numerous hotspots in May, none in June, 3 in July, 5 in August, and none in September or October. KVERT reported that a thermal anomaly was visible in satellite images on most, if not all, days when not obscured by clouds.

The Aviation Color Code remained at Orange (the second highest level on a four-color scale) until 3 October, when KVERT reduced it to Yellow, after which moderate gas-and-steam activity continued.

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/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); 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).

Recent activity at Shishaldin, located on Unimak Island within the Aleutian Islands, has included a lava eruption in the summit crater, thermal anomalies, elevated seismicity, and gas-and-steam and ash plumes (BGVN 41:11). This report describes minor gas-and-steam emissions, increased seismicity, thermal anomalies, lava fountaining accompanied by minor explosive activity, and a spatter cone. The primary source of information is the Alaska Volcano Observatory (AVO). This report updates activity through September 2019.

Volcanism was relatively low between March 2016 and early July 2019; increased seismicity and steam emissions were detected in December 2017, but the activity declined in February 2018. Elevated seismicity and some thermal anomalies accompanied by incandescence observed in satellite imagery (when not obscured by clouds) returned in mid-July 2019 (figure 12).

Figure 12. Summary graphic of MODVOLC thermal alerts measured over Shishaldin during July-September 2019. Courtesy of HIGP - MODVOLC Thermal Alerts System.

Elevated surface temperatures and low-level seismic tremors remained elevated through September 2019 (figure 13). Field crews reported an active lava lake and minor spattering within the summit crater on 23 July 2019 (figures 14 and 15). Satellite imagery showed the presence of a small spatter cone and some lava flows within the summit crater on 28 July. A small steam plume was observed in satellite imagery and webcam images on 29 July, 20 August, and 30 September.

Figure 13. Sentinel-2 satellite imagery of Shishaldin showing detected thermal anomalies between the months of July and September 2019. Top left: Satellite image on 19 July showing a gas-and-steam plume. Top center: On 29 July a thermal anomaly is detected in the summit crater. Top right: On 28 August, the thermal anomaly is still present. Bottom left: On 7 September, the thermal anomaly continues. Bottom right: On 24 September, the power of the thermal anomaly significantly decreases. Atmospheric penetration satellite image (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

Figure 14. Photo of surface lava within the summit crater at Shishaldin taken on 23 July 2019. Photo by David Fee (color corrected); courtesy of Alaska Volcano Observatory (AVO).

Figure 15. Photo of lava and a slightly growing spatter cone within the summit crater at Shishaldin taken on 23 July 2019. Photo by Dane Ketner (color corrected); courtesy of Alaska Volcano Observatory (AVO).

On 17 August 2019, a video taken by NOAA during an overflight showed repetitive minor explosive activity and low-level lava fountaining within the summit crater. This activity may have continued through 24 September, according to AVO. The spatter cone grew slightly in August and September, partially filling the summit crater. Accompanying lava flows also grew slightly during this time.

Satellite data from 3 September showed SO2 emissions and elevated surface temperatures. Satellite imagery and tiltmeter data recorded a collapse and slumping of the summit crater floor, which may have occurred on 19 September. In the last few weeks of September, seismicity and surface temperatures decreased to slightly above background levels.

According to MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS satellite instruments, more frequent thermal anomalies were detected in mid-July 2019 and remained elevated through early September (figure 16).

Figure 16. Thermal anomalies increased at Shishaldin from mid-July 2019 through early September and then abruptly stopped as recorded by MIROVA (log radiative power). Courtesy of MIROVA.

Geologic Background. The beautifully symmetrical volcano of Shishaldin is the highest and one of the most active volcanoes of the Aleutian Islands. The 2857-m-high, glacier-covered volcano is the westernmost of three large stratovolcanoes along an E-W line in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." A steady steam plume rises from its small summit crater. Constructed atop an older glacially dissected volcano, it is Holocene in age and largely basaltic in composition. Remnants of an older ancestral volcano are exposed on the west and NE sides at 1500-1800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is blanketed by massive aa lava flows. Frequent explosive activity, primarily consisting of strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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).

During September 2018 through June 2019, activity at Klyuchevskoy was characterized by weak thermal anomalies and moderate Strombolian-type explosions. Ash emissions were only reported on 1-2 July and 1 August during the period of July-September 2019. The volcano is monitored by the Kamchatkan Volcanic Eruption Response Team (KVERT) and is the primary source of information.

According to KVERT, moderate activity continued from July through at least the middle of September, with gas-and-steam emissions. At the beginning of July, KVERT reported incandescence in the crater. During 1-2 July, ash plumes drifted as far as 85 km E and SE. Ash plumes were visible blowing E in Sentinel-2 images on 17 and 19 July (figure 32); steam plumes were evident on some other days. KVERT reported that an ash emission was seen in webcam images on 1 August.

Figure 32. An ash plume can be seen blowing E from the summit crater of Klyuchevskoy in this Sentinel-2 natural color (bands 4, 3, 2) satellite image from 17 July 2019. Courtesy of Sentinel Hub Playground.

No thermal anomalies were detected with the MODIS satellite instruments analyzed using the MODVOLC algorithm. The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system, also based on analysis of MODIS data, detected no thermal anomalies in June, four scattered ones in July, and only one in August, all low power. According to KVERT, a weak thermal anomaly was detected throughout the reporting period, at least through mid-September, except for the numerous days when the volcano was obscured by clouds; the temperature of the anomalies had steadily decreased with time.

Instruments aboard NASA satellites detected high levels of sulfur dioxide near or directly above the volcano every day during the first week of July and on 12 July, but not on other days during the reporting period. However, the origin for the high levels may, at least in part, have been due to other active volcanoes in the area.

At the beginning of July, the Aviation Color Code (ACC) remained at Orange (the second highest level on a four-color scale). Because of decreased activity, KVERT lowered the ACC to Yellow on 30 August and to Green (the lowest on the scale) on 24 September.

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: 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/); 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/).

Heard Island, in the Southern Indian Ocean, is about 4,000 km from its closest point to Australia and about 1,500 km from the closest point in Antarctica. Because of the island's remoteness, monitoring is primarily accomplished by satellites. The Big Ben volcano has been active intermittently since 1910, if not before (BGVN 42:10), and thermal anomalies have been observed every month since June 2018 (BGVN 43:10, 44:04). The current reporting period is from April to September 2019.

During April-September 2019, only one thermal anomaly was detected with the MODIS satellite instruments analyzed using the MODVOLC algorithm, and that was on 10 June (2 pixels). The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system, also based on analysis of MODIS data, detected a few scattered thermal alerts in late May-early June and three in September; most were between 1-2 km of the summit and of low to moderate power.

The island is usually covered by heavy clouds, obscuring satellite views. However, Sentinel-2 satellite imagery detected cloud-obscured thermal anomalies during the reporting period, most likely due to a persistent lava lake and possibly lava flows (BGVN 41:08).

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

The eruption at Dukono, ongoing since 1933, is typified by frequent ash explosions and ash plumes (BGVN 43:04). This activity continued through at least September 2019. The data below were primarily provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Center for Volcanology and Geological Hazard Mitigation (CVGHM), and the Darwin Volcanic Ash Advisory Centre (VAAC).

According to PVMBG, during April-September 2019 the volcano continued to generate ash plumes almost every day that rose to altitudes of 1.5-3 km (table 20, figure 12). Ashfall was reported on 8 August at the Galela Airport, Maluku Utara, 17 km NW. The Alert Level remained at 2 (on a scale of 1-4), and the 2-km exclusion zone remained in effect.

Table 20. Monthly summary of reported ash plumes from Dukono for April-September 2019. The direction of drift for the ash plume through each month was highly variable, but did not extend for any notable distances during this reporting period. Data courtesy of the Darwin VAAC and PVMBG.

Figure 12. Satellite image from Sentinel-2 (natural color) of an ash plume at Dukono on 4 August 2019, with the plume blowing almost straight up. Courtesy of Sentinel Hub Playground.

Instruments aboard NASA satellites detected high levels of sulfur dioxide near or directly above the volcano on 11, 20-22 April; 17, 22, and 27 May; 15-18 August; and 23-24 and 29 September. However, the cause of the high levels may, at least in part, have been due to other active volcanoes in the area.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

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

Activity at Poás is characterized by weak phreatic explosions and gas-and-ash-emissions, with a hot acid lake that occasionally disappears (BGVN 44:05). During the current reporting period of May-September 2019, this weak activity continued. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and most of the material below comes from their weekly bulletins (Boletin Semanal Vulcanologia).

According to OVSICORI-UNA, a period of continuous emissions occurred during 30 April-1 May with plumes rising 300 m above the crater rim and drifting SW. Ash emissions were visible for a few hours on 30 April, and incandescence was visible at night. OVSICORI-UNA did not report any additional phreatic explosions in May until daily phreatic, geyser-type explosions were observed between 29 May and 1 June, which reached approximately 100 m above the vent. A phreatic explosion on 10 June reached approximately 20-30 m in height, and frequent small phreatic explosions (heights below 20 m) were reported through 16 June.

OVSICORI-UNA reported that on 12 June small geyser-like explosions ejected material less than 50 m high at a rate of about once per hour. At 0604 on 18 June an explosion that lasted about six minutes produced a plume of unknown height. Residents reportedly heard several loud noises during 0610-0615 and observed a plume rising from the crater. Ash fell in Cajón (12 km SW), San Luis de Grecia (11 km SW), Los Ángeles, San Miguel de Grecia (11 km SW), San Isidro (28 km SE), and San Roque (23 km SSE). Whitish ash deposits surrounding the crater, especially on the W and S sectors, were visible in webcam images. On 21 June frequent small phreatic explosions from vent A (Boca Roja) were visible during good viewing conditions ejecting material less than 10 m high.

No additional phreatic activity was reported by OVSICORI-UNA during rest of June or July. The small crater lake was still present on 5 July when visible in satellite imagery and as seen by visitors (figure 130), During the first part of August geyser-like explosions occurred on several days, and reached a maximum height of 50 m. This activity culminated on 17 August with about 30 explosions/day from the vent (Boca Roja). At least one event at 0650 on that day generated a 1-km-high plume of steam, gas, and fine particles. By 26 August, the geyser-type activity had ceased. Geyser-type phreatic explosions resumed on 12 September, reaching a maximum height of 30 m. The number of explosions increased up to 10-15 events/hour and then became continuous for a short time. A phreatic explosion occurred on 22 September at 2059 that generated a plume that rose 3 km above the crater rim and drifted NE. During 22-23 September explosions generated plumes that rose 1 km.

Figure 130. View of the Poás crater on 5 July 2019. The volcano is surrounded by cloud-cover, and there is some steam rising from the crater lake. Photo by Sheila DeForest (Creative Commons BY-SA license).

According to OVSICORI-UNA, during 16-26 September sulfur dioxide emissions drifted W and NE, causing a sulfur odor in Alajuela, Heredia, San José, and Cartago. Acidic rain was recorded at an official's house in the Poás Volcano National Park (PNVP) on 23 September and at the Universidad Nacional Costa Rica (UNA) in Heredia (23 km SE) on 26 September. On 30 September, at 0540, a 5-minute long phreatic explosion ejected sediment, and produced a plume that rose 2 km above the crater rim and drifted SW. Ashfall and a sulfur odor was reported in Trojas de Sarchi (10 km SW) and Grecia (16 km SSW). Officials closed the PNVP because of the eruption and ongoing elevated seismicity; the park remained closed the next day.

During the first week of August, strong evaporation had reduced the intracrater lake significantly, and by mid-September, the lake had disappeared. At the end of September, however, some water had begun to accumulate again.

General monitoring data. During April and May, OVSICORI-UNA took few gas measurements due to an unfavorable wind direction. An SO2 measurement during the first part of June was between 100 and 200 t/d. Flux remained low through July, with low SO2/CO2 ratios, and high H2S/SO2 ratios, which OVSICORI-UNA stated were consistent with water infiltration. At the end of July, SO2 concentrations significantly increased to 300-800 t/d, with H2S disappearing and the CO2/SO2 ratio declining, with some fluctuations. Levels remained high through most of August, but had decreased to about 300 t/d by the end of the month. They rose again in September, with fluctuations, and on 29 September were measured at about 1,000 t/d before falling to between 300-400 t/d.

According to OVSICORI-UNA weekly reports, seismicity was relatively low during the reporting period, with a few VTs and LPs and normal background tremor. No significant deformation occurred, except for some deflation in June and July.

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

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Sheila DeForest (URL: https://www.facebook.com/sheila.deforest).

Italy's Mount Etna on the island of Sicily has had historically recorded eruptions for the past 3,500 years and has been erupting continuously since September 2013 through at least September 2019. Lava flows, explosive eruptions with ash plumes, and Strombolian lava fountains commonly occur from its summit areas that include 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). The newest crater, referred to as the "cono della sella" (saddle cone), emerged during early 2017 in the area between SEC and NSEC. Varying activity that included several lava flows, Strombolian activity, and numerous ash plumes from most of the active summit vents and several flank fissures occurred during April-September 2019, the period covered in this report, with information provided primarily by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV).

Degassing of variable intensity was typical activity from all the vents at Etna during much of April 2019. Intermittent ash emission and Strombolian activity occurred at Bocca Nuova, especially during the last week. Minor ash emissions were reported from NEC and NSEC the last week as well. Most of the activity at the summit during May 2019 was focused around the New South East Crater (NSEC); repeated Strombolian activity was witnessed from the E vent near the summit throughout the month. Beginning on 30 May, two fissures opened on the N and SE flanks of NSEC and produced lava flows that traveled E and SE across the W wall of the Valle del Bove. The flows ceased during the first week of June; activity for the rest of that month consisted of intermittent explosions with small ash plumes from Voragine and Bocca Nuova. Discontinuous Strombolian explosions and isolated ash emissions from NEC, NSEC, and Bocca Nuova characterized activity during the first half of July 2019; the explosions intensified at NSEC later in the month. A lava flow emerged from the lower NE flank of NSEC on 18 July that lasted for several days. Explosions produced substantial ash plumes from the NSEC summit crater, causing ashfall nearby, and a new flow emerged from a fissure on the S flank of NSEC on 27 July.

Explosions with intermittent ash emissions during August 2019 were focused primarily on the North East Crater (NEC), with occasional ash emissions from Bocca Nuova. These continued into early September. Activity increased to include Strombolian explosions with the ash emissions at NEC, Bocca Nuova, and Voragine where a scoria cone formed deep within the crater from continued Strombolian activity. A lava flow emerged from the base of the scoria cone on 18 September and was active for about four days, sending branches of lava into multiple areas of the adjacent Bocca Nuova crater. Ash emissions at NEC continued during the end of the month. The multiple episodes of varying activity during the period were reflected in the MIROVA thermal energy data; spikes of thermal activity that corresponded to periods of lava effusion were apparent late May-early June, multiple times in July, and during the second half of September (figure 260).

Figure 260. The multiple episodes of varying activity at Etna from 11 December 2018 through September 2019 were reflected in the MIROVA thermal energy data; spikes of thermal activity were apparent in late April, late May-early June, multiple times in July, and during the second half of September. The largest energy spikes correlated with lava flows. Courtesy of MIROVA.

Activity during April-May 2019. During a site visit to the summit on 1 April scientists from INGV noted weak degassing from both pit craters, BN-1 and BN-2, within Bocca Nuova (BN); the Voragine (VOR) and North East Crater (NEC) were emitting abundant steam and gas emissions. The New Southeast Crater (NSEC) also had significant fumarolic activity concentrated primarily on the crater rim along with gas plumes visible from both the E vent and the 24 December 2018 flank fissure (figure 261). A brief episode of ash emission was observed from BN on the morning of 8 April. Persistent pulsating flashes of incandescence were noted at the E vent of NSEC during the second week. A new vent was observed in the inner wall of the Voragine crater during an inspection on 19 April, located immediately below the vent which formed on 12 January 2019 (figure 262). During the last week of April there were ten episodes of ash emission from BN, two from NEC, and one produced by the E vent at NSEC. Strombolian activity was observed on the morning of 28 April at BN-1, and persistent incandescence was visible from the E vent of NSEC. Early on 30 April both BN-1 and BN-2 were producing explosions every few seconds. Coarse ejecta (lapilli and bombs) rose higher than the crater rim; most fell back within the crater, but some material was observed on the rim the following day.

Figure 261. During a site visit to the summit of Etna on 1 April 2019 scientists from INGV noted weak degassing from both pit craters, BN-1 and BN-2, within Bocca Nuova (BN); Voragine (VOR) and North East Crater (NEC) were emitting abundant steam and gas emissions, and the New Southeast Crater (NSEC) also had significant fumarolic activity concentrated primarily on the crater rim along with gas plumes visible from both the E vent (bocca orientale) and the 24 December 2018 flank fissure. Courtesy of INGV, photos by Laboratorio di Cartografia FlyeEye Team (Report 15/2019, ETNA, Bollettino Settimanale, 01/04/2019 - 07/04/2019, data emissione 09/04/2019).

Figure 262. A new vent was observed at the W rim of Etna's Voragine crater on 19 April 2019. INGV scientists concluded that it likely formed during 17-18 April. It was located immediately below a pit crater that opened on 12 January 2019. Inset shows thermal image of the vents. Courtesy of INGV (Report 17/2019, ETNA, Bollettino Settimanale, 15/04/2019 - 21/04/2019, data emissione 24/04/2019).

Activity at the summit during May 2019 was focused around the New South East Crater (NSEC). Discontinuous Strombolian activity was observed at the E vent of NSEC early on 2 May accompanied by ash emissions from the summit vent that rose about 1,000 m (figure 263). Explosion frequency increased beginning on 5 May with weak and discontinuous ash emissions reported from the NSEC summit for the next several days; ash emissions were also observed from the Saddle vent and the NSEC E vent during 6-8 May. In addition to ash emissions and Strombolian activity continuing from both the summit and E vents at NSEC during the third and fourth weeks, overnight on 17-18 May several larger Strombolian explosions sent pyroclastic ejecta tens of meters above the crater rim (figure 264). The explosion intervals ranged from a few minutes to a few hours. The new vent that had formed at Voragine in mid-April coalesced with the 12 January vent during the second week of May; dilute ash was observed from the BN-1 vent on 23 May.

Figure 263. Strombolian activity at the E vent of NSEC at Etna was accompanied by ash emission on 2 May 2019. Left image is from the thermal camera at La Montagnola and the right image is from Tremestieri Etneo, taken by B. Behncke. Coutesy of INGV (Report 19/2019, ETNA, Bollettino Settimanale, 29/04/2019 - 05/05/2019, data emissione 07/05/2019).

Figure 264. Strombolian activity sent ejecta from a vent at Etna's NSEC crater on 14 May 2019 (a) and was captured by the Monte Cagliato thermal camera. Ash emission from the same vent was also visible that day (b) and on 17 May (c). Strombolian explosions from the E Vent of NSEC on 17 May (d) were captured by the EMOH (Montagnola) webcam. Courtesy of INGV (Report 21/2019, ETNA, Bollettino Settimanale, 13/05/2019 - 19/05/2019, data emissione 21/05/2019).

A fissure opened at the base of the N flank of NSEC shortly after midnight on 30 May 2019 at an elevation of about 3,150 m (figure 265). It produced mild explosive activity and a lava flow that spread towards the W wall of the Valle del Bove. By 0800 UTC the flow had reached an elevation of 2,050 m. A second fissure opened at 0335 the same morning at the base of the SE flank of NSEC at an elevation of 3,050 m. The lava flowed along the W wall of the Valle del Bove towards Serra Giannicola Grande and had reached an elevation of 2,260 m by 0815. Strong winds dispersed ash emissions from the fissures to the NE for much of the day; ashfall occurred in Linguaglossa (figure 266). The Toulouse VAAC reported an ash plume drifting ENE at 3.9 km altitude on 30 May. Samples of the ash that were collected and analyzed were shown to be about 70% lithic clasts, 25% crystals, and about 5% juvenile material. It became clear the next day that two vents along the SE-flank fissure initially produced separate flows that coalesced into a single flow which expanded along the W wall of Valle del Bove. By 0830 on 31 May that flow had reached an elevation of 1,700 m at the base of Serra Giannicola Grande. The fissure at the base of the N flank continued to propagate along the W wall of Valle del Bove also, and had reached an elevation of 2,050 near Monte Simone by 1030 on 31 May (figure 267). When the new eruptive activity began on 29 May, inclinometers measured slight but prolonged deflation of the volcano.

Figure 265. Two fissures opened at Etna during the early morning of 30 May 2019. One started from the base of the N flank of the NSEC/SEC complex and flowed E towards the Valle del Bove, and a second fissure with two vents opened on the SE flank of NSEC and flowed SE towards Serra Giannicola Grande. Mapping of the lava flows were done with drones, using the Sentinel 2 satellite images of 30 May and thermal images from 2 June taken at the Schiena dell'Asino. Courtesy of INGV (Report 23/2019, ETNA, Bollettino Settimanale, 27/05/2019 - 02/06/2019, data emissione 04/06/2019).

Figure 266. Lava flows broke out at Etna on both the N and SE flanks of NSEC on 30 May 2019. Ash emissions were also produced from the fissures. The northern flank fissure is seen from the (a) Monte Cagliato thermal camera (EMCT) and (b) the Montagnola high definition camera (EMOH). The fissure on the SE flank was seen from the Montagnola thermal (c) and high definition (d) (EMOH) webcams. Ash emissions and lava flows were visible on the flank (e) and ashfall was recorded in Linguaglossa (f). Courtesy of INGV (Report 23/2019, ETNA, Bollettino Settimanale, 27/05/2019 - 02/06/2019, data emissione 04/06/2019).

Figure 267. Images of the active lava flows at Etna on 31 May 2019 indicated the extent of the flow activity. Lava was flowing from two vents along a fissure on the SE flank (a and b, drone images courtesy of the FlyEye Team OE). The thermal image of the flow (c) is from Schiena dell'Asi, the visible photo (d) is also taken from Schiena dell'Asi by L. Lodato. The thermal (e) and visual (f) images of the active lava fields were taken from the Monte Cagliato (EMCT) thermal webcam and the Monte Cagliato (EMCH) high definition webcam. Courtesy of INGV (Report 23/2019, ETNA, Bollettino Settimanale, 27/05/2019 - 02/06/2019, data emissione 04/06/2019).

Activity during June-July 2019. The flow from the N flank of NSEC ceased advancing on 1 June 2019, but the active spattering continued from the fissure on the SE flank for a few more days. The SE-flank flow had reached 1,700 m elevation in the Valle del Bove by the afternoon of 2 June (figure 268). The intensity and frequency of the explosions decreased over the next few days, with the active flow front receding back towards the vent until it stopped moving on 6 June. The NE rim of the summit cone at NSEC appeared lowered by several meters after the eruption ceased. The lava flows and explosions of 30 May-2 June produced persistent SO₂ emissions that drifted E and N for over 800 km (figure 269).

Figure 268. During the morning of 1 June 2019 Strombolian and effusive activity at Etna continued from the fissure on the SE flank of NSEC (a and b, photos by M. Neri). By the evening of 1 June there was only one remaining arm of the flow that was active (c) as seen in the Monte Cagliato (EMCT) thermal webcam. The following evening, 2 June, another thermal image(d, photo by S. Scollo) showed the remaining active arm. Courtesy of INGV (Report 23/2019, ETNA, Bollettino Settimanale, 27/05/2019 - 02/06/2019).

Figure 269. Active lava flows and Strombolian activity at Etna during 30 May-2 June 2019 contributed to significant SO2 plumes that drifted E and NE from the volcano during this time, extending as far as 800 km from the source. Captured by the TROPOMI instrument on the Sentinel 5P satellite, courtesy of NASA Goddard Space Flight Center.

Activity for the rest of June 2019 moved to the other craters, mainly Voragine, after the flows ceased at NSEC. On the morning of 6 June there were sporadic ash emissions from NEC that quickly dissipated. A small ash plume appeared from Bocca Nuova (BN) on 11 June. An explosive sequence that began on 13 June from the crater floor of Voragine continued intermittently through the third week of the month (figure 270) and produced several small ash plumes. A new vent opened on the crater floor and produced a small ash plume; ejecta also landed on the crater rim several times. On 22 June small, discontinuous ash emissions were produced from BN-1; they dispersed rapidly, but intermittent explosions continued during the following week. By the end of the month, only BN was exhibiting activity other than degassing; incandescence from the crater was seen during the night of 24 June and three isolated ash emissions were seen in the webcams on 26 June.

Figure 270. An ash plume at Etna rose from the Voragine crater on 15 June 2019 during a series of intermittent explosions. Image taken from the Torre del Filosofo by M. Coltelli. Courtesy of INGV (Report 25/2019, ETNA, Bollettino Settimanale, 10/06/2019 - 16/06/2019, data emissione 18/06/2019).

Discontinuous Strombolian explosions and isolated ash emissions characterized activity during the first half of July 2019. Pulsating degassing from NEC produced ash emissions on 2 and 3 July (figure 271), and incandescence on 4 and 5 July. Intense degassing was observed at NSEC during 1-5 July, this turned into isolated ash emissions and Strombolian activity on 5 and 6 July from the E vent with explosions occurring every 1-5 minutes; the ejecta landed on the upper E flank. Dilute ash emissions were observed from Bocca Nuova on 6 July. NEC produced two major ash emissions on the evening of 8 July and the late morning of 13 July. The ash plumes quickly dispersed in the summit area. Strombolian activity at the E vent of NSEC was witnessed on 14 July. Explosive activity at Bocca Nuova remained deep within the crater during mid-July. Steam produced by the 13 June 2019 vent on the floor of Voragine occasionally contained dilute ash. During 15-17 July sporadic explosions were observed at NSEC accompanied by small puffs of ash that rapidly dispersed.

Figure 271. Surveillance cameras at Etna captured images of explosions with ash emissions from NEC on 2 (top) and 3 (bottom) July 2019. The left images are from Montagnola and the right images are from Monte Cagliato. Courtesy of INGV (Report 28/2019, ETNA, Bollettino Settimanale, 01/07/2019 - 07/07/2019, data emissione 09/07/2019).

Beginning early on 18 July, Strombolian activity increased at NSEC from an explosion every 1-2 minutes to multiple explosions per minute in the following hours. Continuous activity during the evening decreased sharply around 2200. About an hour later visual and thermal surveillance cameras on Monte Cagliato recorded the opening of a vent on the lower NE flank of NSEC; lava slowly advanced from the vent towards Valle del Leone (figures 272 and 273). Explosive activity resumed at the NSEC summit a few hours later, accompanied by occasional ash emissions from NEC and Bocca Nuova. Explosions tapered off briefly by noon on 19 July, but a sudden increase in explosive activity during the afternoon of 19 July produced Strombolian activity and sporadic ash emissions from three vents inside the NSEC crater. Ashfall was reported that evening in communities on the S flank of Etna. The Toulouse VAAC reported significant ash above the summit at 3.7 km altitude. Activity declined again later that evening at NSEC, but abundant ash emission began at NEC that lasted until the morning of 20 July. A new phase of explosive activity began at NSEC around 0700 on 20 July with an ash plume and an increase in lava emission from the vent on the NE flank (figure 274). By the evening of 20 July only a small amount of material was feeding the lava flow; the farthest advanced fronts were at an elevation around 2,150 m, above Monte Simone. A few small ash emissions were observed at Bocca Nuova on 21 July.

Figure 272. Map of the summit craters of Etna showing the active vents and the lava flow of 19-21 July 2019. The base is modified from a 2014 DEM created by Laboratorio di Aerogeofisica-Sezione Roma 2. Black hatch marks indicate the crater rims: BN = Bocca Nuova, with NW BN-1 and SE BN-2; VOR = Voragine; NEC = North East Crater; SEC = South East Crater; NSEC = New South East Crater. Red circles indicate areas with ash emissions and/or Strombolian activity, yellow circles indicate steam and/or gas emissions only. Courtesy of INGV (Report 30/2019, ETNA, Bollettino Settimanale, 15/07/2019 - 21/07/2019, data emissione 23/07/2019).

Figure 273. Activity at Etna on 18 and 19 July 2019 included a new lava flow from a vent on the NE flank of NSEC and Strombolian activity at the NSEC summit vent. (a) Start of the flow from a vent on the NE flank of NSEC seen from the high-resolution camera at Monte Cagliato (EMCH) at 2307 UTC on 18 July. (b) Strombolian activity at the NSEC and glow of the new lava flow on the right seen from Tremestieri Etneo, 2347 that evening. (c) A new advancing lava flow and brown ash emission from NEC seen from the EMCH camera, 0338 on 19 July; (d) lava flow seen from the thermal camera at Monte Cagliato, 0700 on 19 July. Courtesy of INGV (Report 30/2019, ETNA, Bollettino Settimanale, 15/07/2019 - 21/07/2019, data emissione 23/07/2019).

Figure 274. Activity at Etna on 20 July 2019 included (a) ash emission from both NSEC and NEC craters at 0402 seen from Tremestieri Etneo, (b) ash from NSEC and the active flow on the SE flank at 0608 seen from the Monte Cagliato high-resolution camera, (c) ash emission from NSEC at 0700 seen by Tremesteieri Etneo, and (d) explosive activity at NSEC and the lava flow on the W wall of the Valle del Bove at 0700 seen from the Monte Cagliato thermal camera. Courtesy of INGV (Report 30/2019, ETNA, Bollettino Settimanale, 15/07/2019 - 21/07/2019, data emissione 23/07/2019).

Visible and thermal images taken on 24 July 2019 indicated only degassing at BN-1 and BN-2, and limited degassing from low-temperature fumaroles from the multiple vents at VOR (figure 275). After a few days of quiet, NSEC resumed discontinuous ash emissions on 25 July. A sudden increase in the amplitude of volcanic tremor was noted early on 27 July, which was followed a few hours later by the opening of a new eruptive fissure on the S flank of NSEC (figure 276). Explosive activity intensified and produced a dense ash-rich plume that dispersed to the E at an estimated altitude of 4.5-5 km. A thin layer of ash was reported in Giarre, Riposto, and Torre Archirafi. A lava flow emerged from the S portion of the fissure and expanded SW and S. By 1135 the most advanced front had reached and passed the N side of the base of the Barbagallo Mountians at an elevation of about 2,850 m. It continued to spread down into the area between Monte Frumento Supino and the pyroclastic cones of 2002-2003 (figure 277). A series of particularly strong explosions occurred from NSEC around midday, producing an ash plume that rose to 7.5 km altitude. By this time the most advanced lava fronts were located at an elevation of about 2,600 m, but they were rapidly advancing SSW towards Monte Nero, surrounding Monte Frumento Supino from the W. Explosive activity decreased significantly early in the morning on 28 July; flow activity also slowed around the same time. Occasional puffs of reddish-brown ash were noted from NEC during the morning as well. The explosions and the lava effusion ceased on the evening of 28 July. An isolated ash emission from Bocca Nuova in the early hours of 31 July was the last activity reported in July. A substantial SO₂ plume (6.59 DU) from the explosions on 27 July had drifted to the E coast of the Adriatic Sea by midday on 28 July and was detected in satellite instruments.

Figure 275. Degassing was the only activity occurring at the multiple vents at Etna's Voragine crater on 24 July 2019. The joined pit crater from the 12 January and 18 April 2019 vents is at the upper left; the newest vent formed 16 June 2019 is at lower left and appears cool in the thermal image inset a. Photo and annotations by S. Branca. Courtesy of INGV (Rep. N° 31/2019, ETNA, Bollettino Settimanale, 22/07/2019 - 28/07/2019, data emissione 30/07/2019).

Figure 276. A new eruptive fissure at Etna opened on the S flank of NSEC on 27 July 2019 (line of red circles). The base map is modified from a 2014 DEM created by Laboratorio di Aerogeofisica-Sezione Roma 2. Black hatch marks indicate the crater rims: BN=Bocca Nuova, with NW BN-1 and SE BN-2; VOR = Voragine; NEC = North East Crater; SEC = South East Crater; NSEC = New South East Crater. Red circles indicate areas with ash emissions and/or Strombolian activity, yellow circles indicate steam and/or gas emissions only. Courtesy of INGV (Report 31/2019, ETNA, Bollettino Settimanale, 22/07/2019 - 28/07/2019, data emissione 30/07/2019).

Figure 277. Lava flows and substantial ash emissions were reported at Etna on 27 July 2019. The lava flow at 1216 was located at about 2,600 m elevation (a). A thermal image of the S flank of NSEC showed the extent of the flow activity (b). A large ash plume formed after several explosions at NSEC at 1221 (c). Thermal images of the emissions were captured by the Montagnola (EMOT) webcam and by an INGV operator (d, e). Photos by S. Branca (a), B. Behncke (c), and E. Pecora (b, e). Courtesy of INGV (Report 31/2019, ETNA, Bollettino Settimanale, 22/07/2019 - 28/07/2019, data emissione 30/07/2019).

Activity during August-September 2019. Activity during August 2019 was focused primarily on the North East Crater (NEC), with occasional ash emissions from Bocca Nuova. The plumes were occasionally dense and dark brown from NEC. Weak emissions of dilute ash from NEC quickly dispersed on the morning of 4 August, followed by more intermittent ash emissions during 6-10 August; a few had significant concentrations of ash that drifted SE. Part of the N rim of NEC collapsed during the explosions of early August (figure 278). During a site inspection to the summit by INGV personnel on 16 August, continuous degassing at Bocca Nuova was interrupted every 10-15 minutes by explosions, but no ejecta was noted. Discontinuous emissions from NEC formed small ash plumes that rose a few hundred meters and remained in the summit area (figure 279). Thermal surveys that day indicated high temperatures of about 800°C along a 10-m-fracture zone on the northern rim of VOR. Ash emissions from NEC were persistent through 20 August when they decreased significantly; a few explosions had dilute ash emissions from Bocca Nuova that day and the next (figure 280). Sulfur dioxide emissions were notable during 19-22 August, drifting S and W hundreds of kilometers before dissipating. Isolated and dilute ash from NEC early on 28 August was interpreted by INGV as resulting from collapses along the inner crater walls. During site inspections on 27, 28, and 30 August, deep explosions from Bocca Nuova were heard, and degassing was observed at all of the summit vents.

Figure 278. Part of the N rim of the NEC crater at Etna collapsed during explosions in early August 2019. In this image from 10 August 2019 the collapsed N wall is shown by white arrows, the old crater rim is the dashed yellow line, and the new rim is the solid yellow line. Photo by Michele Mammino, courtesy of INGV (Report 33/2019, ETNA, Bollettino Settimanale, 05/08/2019 - 11/08/2019, data emissione 13/08/2019).

Figure 279. Discontinuous emissions at Etna on 16 August 2019 from the NEC crater formed small ash plumes that rose a few hundred meters and remained in the summit area (a). Smaller ash plumes remained within the crater (b and c). Courtesy of INGV (Report 34/2019, ETNA, Bollettino Settimanale, 12/08/2019 - 18/08/2019, data emissione 20/08/2019).

Figure 280. In the foreground weak degassing occurs on 21 August 2019 at Etna's BN-2 vent inside Bocca Nuova while a small ash plume in the background rises from NEC. Photo by F. Ciancitto, courtesy of INGV (Report 35/2019, ETNA, Bollettino Settimanale, 19/08/2019 - 25/08/2019, data emissione 27/08/2019).

Activity during September 2019 began with discontinuous and dilute ash emissions from NEC and Bocca Nuova, as well as episodes of Strombolian activity at both vents. This was followed by increased Strombolian activity, ash emissions, and a lava flow at Voragine. Isolated ash emissions occurred at NEC and VOR on 4 and 5 September. Sporadic deep explosions were heard from BN-1 during a site inspection on 7 September. Overnight during 7-8 September the visual webcams recorded incandescence at NEC and pyroclastic ejecta observed outside the crater rim that coincided with increased tremor activity. A more intense episode of Strombolian activity began the following evening at NEC. Activity was continuous from 1800 on 9 September to 0500 on 10 September, and produced dilute ash emissions that quickly dispersed (figure 281). Slight ashfall was reported in Piedimonte Etneo, Giarre-Riposto, and Rifugio Citelli. Continuous puffs of dilute ash were observed beginning at dawn on 11 September with sporadic ejecta again landing outside the crater rim. Significant SO₂ plumes were measured by satellite instruments on 10 and 11 September (figure 282).

Figure 281. Activity at Etna overnight during 9-10 September 2019 included Strombolian activity and dilute ash emissions from NEC that were observed from webcams on the S, W, and E flanks. Courtesy of INGV (Report 38/2019, ETNA, Bollettino Settimanale, 09/09/2019 - 15/09/2019, data emissione 17/09/2019).

Figure 282. Significant SO2 plumes from Etna were detected on 10 and 11 September 2019. Increased Strombolian activity was reported by INGV from the NEC crater during 9-11 September. Courtesy of NASA Goddard Space Center.

In addition to the Strombolian activity at NEC on 12 September, ash emissions began that morning at VOR. They increased in frequency and then transitioned to near-continuous Strombolian activity that produced ejecta which landed in the base of the adjacent Bocca Nuova crater. The explosions from the Strombolian activity were felt in Zafferana Etnea, Aci S. Antonio, Pedara, and neighboring areas. On 13 September the webcams observed multiple periods of continuous ash emissions from NEC and short, intense pulses of ash from VOR that accompanied Strombolian activity; coarse ejecta rose 20 m above and landed outside of the crater rim, producing impact craters on the W side of the summit between VOR and BN. The vent that sourced the Strombolian activity was located in the deepest part of the Voragine crater. By 15 September, continued ejecta had formed a scoria cone around the vent inside VOR (figure 283).

Figure 283. On 13 September 2019 Strombolian activity at Etna's NEC and VOR craters increased (a). INGV personnel observed an ash emission from NEC (b), a Strombolian explosion with ejecta from VOR (c), and impact craters from the ejecta around the rim (d). The continued activity at VOR produced a scoria cone inside the crater that grew noticeably between 13 (e) and 15 (f) September. Photos (a) and (e) courtesy of L. D'Agata, photo (f) by B. Behncke. Courtesy of INGV (Report 38/2019, ETNA, Bollettino Settimanale, 09/09/2019 - 15/09/2019, data emissione 17/09/2019).

Explosive activity inside VOR increased on the afternoon of 18 September 2019. Pyroclastic ejecta and ash erupted from several vents and reached heights of several tens of meters. A lava flow emerged from the W base of the scoria cone and headed S, advancing several hundred meters (figure 284). It then flowed over the saddle that divides VOR and BN, split into two branches, and entered Bocca Nuova. One stream poured into BN-1, and another stopped near the edge of the BN-2 pit crater. By 22 September the flow was cooling, but strong Strombolian activity continued inside Voragine. NEC was characterized by large-scale ash emissions during the end of September, including one in the morning of 27 September that sent a plume over the S flank of Etna before dissipating (figure 285). Strombolian activity continued within Bocca Nuova during the last week of the month.

Figure 284. Significant Strombolian and lava flow activity at Etna affected the Voragine crater on 18 and 19 September 2019. Visible and thermal images of the scoria cone (cono scorie) and lava flow (colata) inside Etna's large Voragine crater on 19 September 2019 (top) were taken from the southern edge of BN. Photo by F. Ciancitto. The bottom images were taken from the SW rim of BN on 18 September (left) by M. Tomasello and (right) 19 September by INGV personnel. Courtesy of INGV (Report 39/2019, ETNA, Bollettino Settimanale, 16/09/2019 - 22/09/2019, data emissione 24/09/2019).

Figure 285. An ash emission from Etna's NEC crater early on 27 September 2019 sent a plume drifting S before dissipating. It was captured by both the high-definition webcam of Bronte (EBVH, left) and the Milo (EMV) webcam. Courtesy of INGV (Report 40/2019, ETNA, Bollettino Settimanale, 23/09/2019 - 29/09/2019, data emissione 01/10/2019).

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/ ); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/aeroweb/info/vaac/).

Prior to renewed activity in June 2019, the most recent eruptive episode at Ubinas occurred between 13 September 2016 and 2 March 2017, with ash explosions that generated plumes that rose up to 1.5-2 km above the summit crater (BGVN 42:10). The volcano remained relatively quiet between April 2017 and May 2019. This report discusses an eruption that began in June 2019 and continued through at least August 2019. Most of the Information was provided by the Instituto Geofísico del Perú (IGP), Observatoria Vulcanologico del Sur (IGP-OVS), the Observatorio Volcanológico del INGEMMET (Instituto Geológical Minero y Metalúrgico) (OVI-INGEMMET), and the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Activity during June 2019. According to IGP, seismic activity increased suddenly on 18 June 2019 with signals indicating rock fracturing. During 21-24 June, signals indicating fluid movement emerged and, beginning at 0700 on 24 June, webcams recorded ash, gas, and steam plumes rising from the crater. Plumes were visible in satellite images rising to an altitude of 6.1 km and drifting N, NE, and E.

IGP and INGEMMET reported that seismic activity remained elevated during 24-30 June; volcano-tectonic (VT) events averaged 200 per day and signals indicating fluid movement averaged 38 events per day. Emissions of gas, water vapor, and ash rose from the crater and drifted N and NE, based on webcam views and corroborated with satellite data. According to a news article, a plume rose 400 m above the crater rim and drifted 10 km NE. Weather clouds often obscured views of the volcano, but an ash plume was visible in satellite imagery on 24 June 2019 (figure 49). On 27 June the Alert Level was raised to Yellow (second lowest on a 4-level scale).

Figure 49. Sentinel-2 satellite image in natural color showing an ash plume blowing north from Ubinas on 24 June 2019. Courtesy of Sentinel Hub Playground.

Activity during July 2019. IGP reported that seismic activity remained elevated during 1-15 July; VT events averaged 279 per day and long-period (LP) events (indicating fluid movement) averaged 116 events per day. Minor bluish emissions (magmatic gas) rose from the crater. Infrared imagery obtained by Sentinel-2 first showed a hotspot in the summit crater on 4 July.

According to IGP, during 17-19 July, gas-and-ash emissions occasionally rose from Ubinas's summit crater and drifted N, E, and SE. Beginning at 0227 on 19 July, as many as three explosions (two were recorded at 0227 and 0235) generated ash plumes that rose to 5.8 km above the crater rim. The Buenos Aires VAAC reported that, based on satellite images, ash plumes rose to an altitude as high as 12 km. The Alert Level was raised to Orange and the public were warned to stay beyond a 15-km radius. Ash plumes drifted as far as 250 km E and SE, reaching Bolivia. Ashfall was reported in areas downwind, including the towns of Ubinas (6.5 km SSE), Escacha, Anascapa (11 km SE), Tonohaya (7 km SSE), Sacohaya, San Miguel (10 km SE), Huarina, and Matalaque, causing some families to evacuate. The Buenos Aires VAAC reported that during 20-23 July ash plumes rose to an altitude of 7.3-9.5 km and drifted E, ESE, and SE.

IGP reported that activity remained elevated after the 19 July explosions. A total of 1,522 earthquakes, all with magnitudes under 2.2, were recorded during 20-24 July. Explosions were detected at 0718 and 2325 on 22 July, the last ones until 3 September. The Buenos Aires VAAC reported that an ash plume rising to an altitude of 9.4 km. and drifting SE was identified in satellite data at 0040 on 22 July (figure 50). Continuous steam-and-gas emissions with sporadic pulses of ash were visible in webcam views during the rest of the day. Ash emissions near the summit crater were periodically visible on 24 July though often partially hidden by weather clouds. Ash plumes were visible in satellite images rising to an altitude of 7 km. Diffuse ash emissions near the crater were visible on 25 July, and a thermal anomaly was identified in satellite images. During 26-28 July, there were 503 people evacuated from areas affected by ashfall.

Figure 50. Image of ash streaming from the summit of Ubinas on 22 July 2019 captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. Courtesy of NASA's Earth Observatory (Joshua Stevens and Kathryn Hansen).

Activity during August 2019. IGP reported that during 13-19 August blue-colored gas plumes rose to heights of less than 1.5 km above the base of the crater. The number of seismic events was 1,716 (all under M 2.4), a decrease from the total recorded the previous week.

According to IGP, blue-colored gas plumes rose above the crater and eight thermal anomalies were recorded by the MIROVA system during 20-26 August. The number of seismic events was 1,736 (all under M 2.4), and there was an increase in the magnitude and number of hybrid and LP events. Around 1030 on 26 August an ash emission rose less than 2 km above the crater rim. Continuous ash emissions on 27 August were recorded by satellite and webcam images drifting S and SW.

IGP reported that during the week of 27 August, gas-and-water-vapor plumes rose to heights less than 1 km above the summit. The number of seismic events was 2,828 (all under M 2.3), with VT signals being the most numerous. There was a slight increase in the number of LP, hybrid, and VT events compared to the previous week. The Alert Level remained at Orange.

Thermal anomalies. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected a large concentration of anomalies between 19 July until almost the end of August 2019, all of which were of low radiative power (figure 51). Infrared satellite imagery (figure 52) also showed the strong thermal anomaly associated with the explosive activity on 19 July and then the continuing hot spot inside the crater through the end of August.

Figure 51. Log radiative power MIROVA plot of MODIS thermal anomalies at Ubinas for the year ending on 4 October 2019. Thermal activity began in the second half of July. Courtesy of MIROVA.

Figure 52. Sentinel-2 satellite images (Atmospheric penetration rendering, bands 12, 11, 8A) showing thermal anomalies during the eruption on 19 July (left) and inside the summit crater on 29 July 2019 (right). A hot spot inside the crater persisted through the end of August. Courtesy of Sentinel Hub Playground.

Geologic Background. A small, 1.4-km-wide caldera cuts the top of Ubinas, Peru's most active volcano, giving it a truncated appearance. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front of Perú. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45 degrees. The steep-walled, 150-m-deep summit caldera contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3700 years ago extend 10 km from the volcano. Widespread plinian pumice-fall deposits include one of Holocene age about 1000 years ago. Holocene lava flows are visible on the flanks, but historical activity, documented since the 16th century, has consisted of intermittent minor-to-moderate explosive eruptions.

Information Contacts: Instituto Geofisico del Peru (IGP), Observatoria Vulcanologico del Sur (IGP-OVS), Arequipa Regional Office, Urb La Marina B-19, Cayma, Arequipa, Peru (URL: http://ovs.igp.gob.pe/); Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa (URL: http://ovi.ingemmet.gob.pe); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php?lang=es); 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); Instituto Nacional de Defensa Civil Perú (INDECI) (URL: https://www.indeci.gob.pe/); Gobierno Regional de Moquegua (URL: http://www.regionmoquegua.gob.pe/web13/); La Republica (URL: https://larepublica.pe/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/).

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

Activity at Santa Maria continued with little variation from previous months during March-August 2019, except for a short-lived increase in the frequency and intensity of explosions during early July that produced minor pyroclastic flows. Plumes of steam with minor magmatic gases rose continuously from both the S rim of the Caliente crater and from the summit of the growing dome throughout the period. They usually rose 100-700 m above the summit, generally drifting W or SW, and occasionally SE, before dissipating. In addition, daily explosions with varying amounts of ash rose to altitudes of around 2.8-3.5 km and usually extended no more than 25 km before dissipating. Most of the plumes drifted SW or SE; minor ashfall occurred in the adjacent hills almost daily and was reported at the fincas located within 10 km in those directions several times each month. Continued growth of the Caliente lava dome resulted in daily block avalanches descending its flanks to the base of the dome. The MIROVA plot of thermal energy during this time shows a consistent level of heat from early December 2018 through April 2019, very little activity during May and June, and a short-lived spike in activity from late June through early July that coincides with the increase in explosion rate and intensity. Activity decreased later in July and into August (figure 95).

Figure 95. Thermal activity at Santa Maria from 8 December 2018 through August 2019 was similar to previous months. A noticeable decrease in activity occurred during May and early June 2019 with a short-lived spike during late June and early July that corresponded to an increase in explosion rate and intensity during that brief interval. Courtesy of MIROVA.

Explosive activity increased slightly during March 2019 to 474 events from 409 events during February, averaging about 15 per day; the majority of explosions were weak to moderate in strength. The moderate explosions generated small block avalanches daily that sent debris 300 m down the flanks of Caliente dome; the explosions contained low levels of ash and large quantities of steam. Daily activity consisted mostly of degassing around the southern rim of the crater and within the central dome, with plumes rising about 100 m from the S rim, and pulsating between 100-400 m above the central dome, usually white and sometimes blue with gases; steam plumes drifted as far as 10 km. The weak ash emissions resulted in ashfall close to the volcano, primarily to the W and SW in the mountainous areas of El Faro, Patzulín, La Florida, and Monte Bello farms. During mid-March, residents of the villages of Las Marías and El Viejo Palmar, located S of the dome, reported the smell of sulfur. The seismic station STG3 registered 8-23 explosions daily that produced ash plumes which rose to altitudes between 2.7 and 3.3 km altitude. Explosions from the S rim were usually steam rich, while reddish oxidized ash was more common from the NE edge of the growing dome in the summit crater (figure 96). The constant block avalanches were generated by viscous lava slowly emerging from the growing summit dome, and also from the explosive activity. On the steep S flank of Santa Maria, blocks up to 3 m in diameter often produce small plumes of ash and debris as they fall.

Figure 96. Mostly steam rose from the S rim of the Caliente dome at Santa Maria throughout March-August 2019. On 1 March 2019, oxidized reddish ash from the growing dome was also part of the emissions (left). The dome continued to grow, essentially filling the inside of the summit crater of Caliente. Courtesy of INSIVUMEH (INFORME MENSUAL DE ACTIVIDAD VOLCÁNICA MARZO 2019, VOLCÁN SANTIAGUITO).

Late on 4 March 2019 an explosion was heard 10 km away that generated incandescence 100 m above the crater and block avalanches that descended to the base of the Caliente dome; it also resulted in ashfall around the perimeter of the volcano. Powerful block avalanches were reported in Santa María creek on 8 March. Ashfall was reported in the villages of San Marcos and Loma Linda Palajunoj on 14 March. Ash plumes on 18 March drifted W and caused ashfall in the villages of Santa María de Jesús and Calaguache. A small amount of ashfall was reported on 26 March around San Marcos Palajunoj. The Washington VAAC reported volcanic ash drifting W from the summit on 8 March at 4.6 km altitude. A small ash plume was visible in satellite imagery moving WSW on 11 March at 4.6 km altitude. On 20 March a plume was detected drifting SW at 3.9 km altitude for a short time before dissipating.

Explosion rates of 10-14 per day were typical for April 2019. Ash plumes rose to 2.7-3.2 km altitude. Block avalanches reached the base of the Caliente dome each day. Steam and gas plumes pulsated 100-400 m above the S rim of the crater (figure 97). Ashfall in the immediate vicinity of the volcano, generally on the W and SW flanks was also a daily feature. The Washington VAAC reported multiple small ash emissions on 2 April moving W and dissipating quickly at 4.9 km altitude. An ash plume from two emissions drifted WSW at 4.3 km altitude on 10 April, and on 22 April two small discrete emissions were observed in satellite images moving SE at 4.6 km altitude. Ashfall was reported on 13 and 14 April in the nearby mountains and areas around Finca San José to the SE. On 15 and 23 April, ash plumes drifted W and ashfall was reported in the area of San Marcos and Loma Lina Palajunoj.

Figure 97. Degassing from the Caliente dome at Santa Maria on 3 April (left, infrared image) and 13 April 2019 (right) produced steam-rich plumes with minor quantities of ash. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo:, Volcán Santiaguito, Semana del 30 de marzo al 05 de abril de 2019).

Constant degassing continued from the S rim of the crater during May 2019 while pulses of steam and gas rose 100-500 m from the dome at the center of the summit crater. Weak to moderate explosions continued at a rate of 8-12 per day. White and gray plumes of steam and ash rose 300-700 m above the crater daily. A moderate-size lahar on 16 May descended the Rio San Isisdro; it was 20 m wide and carried blocks 2 m in diameter. Ashfall was reported on the W flank around the area of San Marcos and Loma Lina Palajunoj on 21 and 24 May. INSIVIUMEH reported on 29 and 30 May that seismic station STG8 recorded moderate lahars descending the Rio San Isidro (a drainage to the Rio Tambor). The thick, pasty lahars transported blocks 1-3 m in diameter, branches, and tree trunks. They were 20 m wide and 1.5-2 m deep.

Weak to moderate explosions continued during June 2019 at a rate of 9-12 per day, producing plumes of ash and steam that rose 300-700 m above the Caliente crater. On 1 June explosions produced ashfall to the E over the areas of Calaguache, Las Marías and other nearby communities. Ash plumes commonly reached 3.0-3.3 km altitude and drifted W and SW, and block avalanches constantly descended the E and SE flanks from the dome at the top of Caliente. Ashfall was reported at the Santa María de Jesús community on 7 June. Ashfall to the W in San Marcos and Loma Linda Palajunoj was reported on 10, 15, 18, 20, and 22 June. Ashfall to the SE in Fincas Monte Claro and El Patrocinio was reported on 26 June. A few of the explosions on 28 June were heard up to 10 km away. On 29 June ash dispersed to the W again over the farms of San Marcos, Monte Claro, and El Patrocinio in the area of Palajunoj; the next day, ash was reported in Loma Linda and finca Monte Bello to the SW. The Washington VAAC reported ash emissions on 29 June that rose to 4.3 km and drifted W; two ash clouds were observed, one was 35 km from Santa Maria and the second drifted 55 km before dissipating.

With the onset of the rainy season, eight lahars were reported during June. The Rio Cabello de Ángel, a tributary of Río Nimá I (which flows into Rio Samalá) on the SE flank experienced lahars on 3, 5, 11, 12, 21, and 30 June (figure 98). The lahars were 15-20 m wide, 1-2 m deep, and carried branches, tree trunks and blocks 1-3 m in diameter. On 12 and 15 June, lahars descended the Río San Isidro on the SW flank. They were 1.5 m deep, 15-20 m wide and carried tree trunks and blocks up to 2 m in diameter.

Figure 98. Activity at Santa Maria on 12 June 2019 included explosions with abundant ash and lahars. This lahar is in the Rio Nimá I, and started in the Rio Cabello de Ángel. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito, Semana del 08 al 14 de junio de 2019).

An increase in the frequency and intensity of seismic events was noted beginning on 28 June that lasted through 6 July 2019. Explosions occurred at a rate of 5-6 per hour, reaching 40-45 events per day instead of the 12-15 typical of previous months. Ash plumes rose to 3.5-3.8 km altitude and drifted W, SW, and S as far as 10 km, and ashfall was reported in San Marcos Palajunoj, Loma Linda villages, Monte Bello farms, El Faro, La Mosqueta, La Florida, and Monte Claro. Activity decreased after 7 July back to similar levels of the previous months. As a result of the increased activity during the first week of July, several small pyroclastic flows (also known as pyroclastic density currents or PDC's) were generated that traveled up to 1 km down the S, SE, and E flanks during 2-5 and 13 July, in addition to the constant block avalanches from the dome extrusion and explosions (figure 99). As activity levels decreased after 6 July, the ash plume heights lowered to 3.3 km altitude, while pulsating degassing continued from the summit dome, rising 100-500 m.

Figure 99. An increase in explosive activity at Santa Maria during the first week of July 2019 resulted in several small pyroclastic flows descending the flanks, including one on 3 July 2019 (left). An ash emission on 19 July 2019 rose above the nearby summit of Santa Maria (right). Courtesy of INSIVUMEH (INFORME MENSUAL DE ACTIVIDAD VOLCÁNICA JULIO 2019, VOLCÁN SANTIAGUITO).

The Washington VAAC reported an ash plume on 2 July from a series of emissions that rose to 3.9 km altitude and drifted W. Satellite imagery on 4 July showed a puff of ash moving W from the summit at 4.3 km altitude. The next day an ash emission was observed in satellite imagery moving W at 4.9 km altitude. A plume on 11 July drifted W at 4.3 km for several hours before dissipating. Ashfall was reported on 2 July at the San Marcos farm and in the villages of Monte Claro and El Patrocinio in the Palajunoj area. On 4 and 6 July ash fell to the SW and W in San Marcos and Loma Linda Palajunoj. On 5 July there were reports of ashfall in Monte Claro and areas around San Marcos Palajunoj and some explosions were heard 5 km away. In Monte Claro to the SW ash fell on 7 July and sounds were heard 5 km away every three minutes. Incandescence was observed in the early morning on the SE and NE flanks of the dome. During 8 and 9 July, four to eight weak explosions per hour were noted and ash dispersed SW, especially over Monte Claro; pulsating degassing noises were heard every two minutes. Monte Bello and Loma Linda reported ashfall on 12, 16, 17, 19, and 20 July. On 15, 22, 26, and 29 July ash was reported in San Marcos and Loma Linda Palajunoj; 33 explosions occurred on 25 July. Two lahars were reported on 8 July. A strong one in the Rio San Isidro was more than 2 m deep, and 20-25 m wide with blocks as large as 3 m in diameter. A more moderate lahar affected Rio Cabello de Angel and was also 2 m deep. It was 15-20 m wide and had blocks 1-2 m in diameter.

Activity declined further during August 2019. Constant degassing continued from the S rim of the crater, but only occasional pulses of steam and gas rose from the central dome. Weak to moderate explosions occurred at a rate of 15-20 per day. White and gray plumes with small amounts of ash rose 300-800 m above the summit daily. Block avalanches descended to the base of the dome and sent fine ash particles down the SE and S flanks. Ashfall was common within 5 km of the summit, generally on the SW flank, near Monte Bello farm, Loma Linda village and San Marcos Palajunoj. Explosions rates decreased to 10-11 per day during the last week of the month. Degassing and ash plumes rose to 2.9-3.2 km altitude throughout the month.

On 1 August ash plumes drifted 10-15 km SW, causing ashfall in that direction. On 3 and 27 August ashfall occurred at Monte Claro and El Patrocinio in the Palajunoj area to the SW. On 7 and 31 August ashfall was reported in Monte Claro. San Marcos and Loma Linda Palajunoj reported ash on 11, 16, 19, and 23 August. On 21 August ashfall was reported to the SE around Finca San José. The Washington VAAC reported an ash plume visible in satellite imagery on 10 August 2019 drifting W at 4.3 km altitude a few kilometers from the summit which dissipated quickly. On 27 August a plume was observed 25 km W of the summit at 3.9 km altitude, dissipating rapidly. On 3 August a moderate lahar descended the Rio Cabello de Ángel that was 1 m deep, 15 m wide and carried blocks up to 1 m in diameter along with branches and tree trunks. A large lahar on 20 August descended Río Cabello de Ángel; it was 2-3 m high, 15 m wide and carried blocks 1-2 m diameter, causing erosion along the flanks of the drainage (figure 100).

Figure 100. A substantial lahar at Santa Maria on 20 August 2019 sent debris down the Río Cabello de Ángel in the vicinity of El Viejo Palmar (left), the spectrogram of the seismic signal lasted for 2 hours and 16 minutes (top right), and the seismograph was saturated with the lahar signal in red (bottom right). Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito, Semana del 17 al 23 de agosto de 2019).

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

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

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

Typical eruptive activity recorded at Stromboli by INGV during March-June 2019 was similar to activity of the past few years (table 6); two major explosions occurred in July and August with a fatality during the 3 July event. In the north crater area, both vents N1 and N2 emitted fine (ash) ejecta, occasionally mixed with coarser lapilli and bombs; most explosions rose less than 80 m above the vents, some reached 150 m. Average explosion rates ranged from 1 to 12 per hour. In the CS crater area continuous degassing and occasional intense spattering were typical at vent C, vent S1 was a low-intensity incandescent jet throughout the period. Explosions from vent S2 produced 80-150 m high ejecta of ash, lapilli, and bombs at average rates of 2-17 per hour.

After a high-energy explosion and lava flow on 25 June, a major explosion with an ash plume and pyroclastic flow occurred on 3 July 2019; ejecta was responsible for the death of a hiker lower down on the flank and destroyed monitoring equipment near the summit. After the explosion on 3 July, coarse ejecta and multiple lava flows and spatter cones emerged from the N area, and explosion rates increased to 4-19 per hour. At the CS area, lava flows emerged from all the vents and spatter cones formed. Explosion intensity ranged from low to very high with the finer ash ejecta rising over 250 m from the vents and causing ashfall in multiple places on the island. This was followed by about 7 weeks of heightened unrest and lava flows from multiple vents. A second major explosion with an ash plume and pyroclastic flow on 28 August reshaped the summit area yet again and scattered pyroclastic debris over the communities on the SW flank near the ocean.

Table 6. Summary of activity levels at Stromboli, March-August 2019. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

Thermal activity was low from March through early June 2019 as recorded in the MIROVA Log Radiative Power data from MODIS infrared satellite information. A sharp increase in thermal energy coincided with a large explosion and the emergence of numerous lava flows from the summit beginning in late June (figure 144). High heat-flow continued through the end of August and dropped back down at the beginning of September 2019 after the major 28 August explosion.

Figure 144. Thermal activity at Stromboli was low and intermittent from 12 November 2018 through early June 2019, based on this MIROVA plot of thermal activity through August 2019. A spike in thermal energy in late June coincided with a major explosion on 3 July and the emergence of lava from the summit area. Heightened activity continued from 3 July through 28 August with multiple lava flows emerging from both crater areas. Courtesy of MIROVA.

Activity during March-June 2019. Activity was low during March 2019. Low- to medium-intensity explosions occurred at both vents N1 and N2 in the north area. Ejecta was mostly coarse grained (lapilli and bombs) from N1 and fine-grained ash mixed with some coarse material from N2. Intense spattering activity was reported from N2 on 29 March. Explosion rates were reported at 5-12 per hour. At the CS area, medium-intensity explosions from both south area vents produced lapilli and bombs mixed with ash at a rate of 2-9 explosions per hour.

During a visit to the Terrazza Craterica on 2 April 2019, degassing was visible from vents N1, N2, C, and S2; activity continued at similar levels to March throughout the month. Low- and medium-intensity explosions with coarse ejecta, averaging 3-12 per hour, were typical at vent N1 while low-intensity explosions with fine-grained (ash) ejecta occurred at a similar rate from N2. Continuous degassing was observed at the C vent, and low-intensity incandescent jets were present at S1 throughout the month. Multiple emission points from S2 (as many as 4) produced low- to medium-intensity explosions at rates of 4-14 explosions per hour; the ejecta was mostly fine-grained mixed with some coarse material. Frequent explosions on 19 April produced abundant pyroclastic material in the summit area.

Low to medium levels of explosive activity at all of the vents continued during May 2019. Emissions consisted mostly of ash occasionally mixed with coarser material (lapilli and bombs). Rates of explosion were 2-8 per hour in the north area, and 5-16 per hour in the CS Area. Explosions of low-intensity continued from all the vents during the first part of June at rates averaging 2-12 per hour, although brief periods of high-frequency explosions (more than 21 events per hour) were reported during the week of 10 June. Strong degassing was observed from crater C during an inspection on 12 June (figure 145); by the third week, continuous degassing was interrupted at C by sporadic short-duration spattering events.

Figure 145. The Terrazza Craterica as seen from the Pizzo sopra la Fossa (above, near the summit) at Stromboli on 12 June 2019. In red are the two craters (N1 and N2) of the N crater area, in green is the CS crater area with 2 vents (C1 and C2) in the central crater and S2, the largest and deepest crater in the CS area, also with at least two vents. S1 is hidden by the degassing of crater C. Photograph by Giuseppe Salerno, courtesy of INGV (Report 25/2019, Stromboli, Bollettino Settimanale, 10/06/2019-16/06/2019).

Late on 25 June 2019, a high-energy explosion that lasted for 28 seconds affected vent C in the CS area. The ejecta covered a large part of the Terrazza Craterica, with abundant material landing in the Valle della Luna. An ash plume rose over 250 m after the explosion and drifted S. After that, explosion frequency varied from medium-high (17/hour) on 25 June to high (25/hour) on 28 June. On 29 June researchers inspected the summit and noted changes from the explosive events. Thermal imagery indicated that the magma level at N1 was almost at the crater rim. The magma level at N2 was lower and explosive activity was less intense. At vent C, near-constant Strombolian activity with sporadic, more intense explosions produced black ash around the enlarged vent. At vent S2, a pyroclastic cone at the center of the crater produced vertical jets of gas, lapilli, and bombs that exceeded 100 m in height (figure 146).

Figure 146. A high-energy explosion at Stromboli late on 25 June 2019 affected vent C in the CS Area (top row). The ejecta covered a large part of the Terrazza Craterica. An ash plume rose over 250 m after the explosion and drifted S. On 29 June (bottom row) thermal imagery indicated that the magma level at N1 was almost at the crater rim. At vent C, near-constant Strombolian activity was interrupted with sporadic, more intense explosions. At vent S2, a pyroclastic cone at the center of the crater produced vertical jets of gas, lapilli, and bombs that exceeded 100 m in height. Photo 2f by L. Lodato, courtesy of INGV (Rep 27/2019, Stromboli, Bollettino Settimanale, 24/06/2019-30/06/2019).

Activity during July 2019. A large explosion accompanied by lava and pyroclastic flows affected the summit and western flank of Stromboli on 3 July 2019. Around 1400 local time an explosion from the CS area generated a lava flow that spilled onto the upper part of the Sciara del Fuoco. Just under an hour later several events took place: lava flows emerged from the C vent and headed E, from the N1 and N2 vents and flowed N towards Bastimento, and from vent S2 (figure 147). The emergence of the flows was followed a minute later by two lateral blasts from the CS area, and a major explosion that involved the entire Terrazza Craterica lasted for about one minute (figure 148). Within seconds, the pyroclastic debris had engulfed and destroyed the thermal camera located above the Terrazza Craterica on the Pizzo Sopra La Fossa and sent a plume of debris across the W flank of the island (figure 149). Two seismic stations were also destroyed in the event. The Toulouse VAAC reported a plume composed mostly of SO₂ at 9.1 km altitude shortly after the explosion. They noted that ash was present in the vicinity of the volcano, but no significant ashfall was expected. INGV scientists observed the ash plume at 4 km above the summit.

Figure 147. A major eruptive event at Stromboli on 3 July 2019 began with an explosion from the CS area that generated a lava flow at 1359 (left). About 45 minutes later (at 1443:40), lava flows emerged from all of the summit vents (right), followed closely by a major explosion. Courtesy of INGV (Eruzione Stromboli. Comunicato straordinario del 4 luglio 2019).

Figure 148. A major explosion at Stromboli beginning at 1445 on 3 July 2019 was preceded by lava flows from all the summit vents in the previous 60 seconds (top row). This thermal camera (SPT) and other monitoring equipment on the Pizzo Sopra La Fossa above the vents were destroyed in the explosion (bottom row). Courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

Figure 149. The monitoring equipment at Stromboli on the Pizzo Sopra La Fossa above the summit was destroyed in the major explosion of 3 July 2019 (left, photo by F. Ciancitto). Most of the W half of the island was affected by pyroclastic debris after the explosion, including the town of Ginostra (right). Courtesy of INGV (Report 28/2019, Stromboli, Bollettino Settimanale, 01/07/2019 - 07/07/2019).

Two pyroclastic flows were produced as a result of the explosions; they traveled down the Sciara and across the water for about 1 km before collapsing into the sea (figure 150). A hiker from Sicily was killed in the eruption and a Brazilian friend who was with him was badly injured, according to a Sicilian news source, ANSA, and the New York Post. They were hit by flying ejecta while hiking in the Punta dei Corvi area, due W of the summit and slightly N of Ginostra, about 100 m above sea level according to INGV. Most of the ejecta from the explosion dispersed to the WSW of the summit. Fallout also ignited vegetation on the slopes which narrowly missed destroying structures in the town. Ejecta blocks and bombs tens of centimeters to meters in diameter were scattered over a large area around the Pizzo Sopra La Fossa and the Valle della Luna in the direction of Ginostra. Smaller material landed in Ginostra and was composed largely of blonde pumice, that floated in the bay (figure 151). The breccia front of the lava flows produced incandescent blocks that reached the coastline. High on the SE flank, the abundant spatter of hot pyroclastic ejecta coalesced into a flow that moved 200-300 m down the flank before cooling, crossing the path normally used by visitors to the summit (figure 152).

Figure 150. At the time of the major explosion of Stromboli on 3 July 2019 people on a German ship located about 2 km off the northern coast captured several images of the event. (a) Two pyroclastic flows traveled down the Sciara del Fuoco and spread over the sea up to about 1 km from the coast. (b) The eruption column was observed rising several kilometers above the summit as debris descended the Sciara del Fuoco. (c) Fires on the NW flank were started by incandescent pyroclastic debris. The photos were taken by Egon Karcher and used with permission of the author by INGV. Courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

Figure 151. Pumice filled the harbor on 4 July 2019 (left) and was still on roofs (right) on 7 July 2019 in the small port of Ginostra on the SW flank of Stromboli after the large explosion on 3 July 2019. Photos by Gianfilippo De Astis, courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

Figure 152. A small lava flow high on the SE flank of Stromboli formed during the 3 July 2019 event from abundant spatter of hot pyroclastic ejecta that coalesced into a flow and moved 200-300 m down the flank before cooling, crossing the path normally used by visitors to the summit. Photo by Boris Behncke taken on 9 July 2019, courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

INGV scientists inspected the summit on 4 and 5 July 2019 and noted that the rim of the Terrazza Craterica facing the Sciara del Fuoco in both the S and N areas had been destroyed, but the crater edge near the central area was not affected. In addition, the N area appeared significantly enlarged and deepened, forming a single crater where the former N1 and N2 vents had been located; an incandescent jet was active in the CS area (figure 153). Explosive activity declined significantly after the major explosions, although moderate overflows of lava continued from multiple vents, especially the CS area where the flows traveled about halfway down the southern part of the Sciara del Fuoco; lava also flowed E towards Rina Grande (about 0.5 km E of the summit). The main lava flows active between 3 and 4 July produced a small lava field along the Sciara del Fuoco which flowed down to an elevation of 210 m in four flows along the S edge of the scarp (figure 154). Additional block avalanches rolled to the coastline.

Figure 153. The summit craters of Stromboli were significantly altered during the explosive event of 3 July 2019. The rim of the Terrazza Craterica facing the Sciara del Fuoco in both the CS and N areas was destroyed, but the crater edge near the CS area was not affected. In addition, the N area was significantly enlarged and deepened, forming a single crater where the former N1 and N2 vents had been located; an incandescent jet was active in the CS area. Courtesy of INGV (Report 28/2019, Stromboli, Bollettino Settimanale, 01/07/2019 - 07/07/2019).

Figure 154. The main lava flows active between 3 and 4 July at Stromboli after the major explosion on 3 July 2019 produced a small lava field along the Sciara del Fuoco. Left: Aerial photo taken by Stefano Branca (INGV-OE) on 5 July; the yellow arrow shows a small overflow from the N crater area, the red arrow shows the largest overflow from the CS crater area. Right: Flows from the CS area traveled down to an elevation of 210 m in four flows along the S edge of the scarp. Additional block avalanches rolled to the coastline. Right photo by Francesco Ciancitto taken on 5 July 2019. Courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

During the second week of July lava flows continued; on 8 July volcanologists reported two small lava flows from the CS area flowing towards the Sciara del Fuoco. A third flow was noted the following day. The farthest flow front was at about 500 m elevation on 10 July, and the flow at the center of the Sciara del Fuoco was at about 680 m. An overflow from the N area during the evening of 12 July produced two small flows that remained high on the N side of the scarp; lava continued flowing from the CS area into the next day. A new flow from the N area late on 14 July traveled down the N part of the scarp (figure 155).

Figure 155. During the second week of July 2019 lava flows at Stromboli continued from both crater areas. Top left: Lava flows from the CS area flowed down the Sciara on 9 July while Strombolian activity continued at the summit, photo by P. Anghemo, mountain guide. Bottom left: A lava flow from the CS area at Stromboli is viewed from Punta dei Corvi during the night of 12-13 July 2019. Photo by Francesco Ciancitto. Right: The active flows on 10 July (in red) were much closer to the summit crater than they had been during 3-4 July (in yellow). Courtesy of INGV, top left and right photos published in Report 29/2019, Stromboli, Bollettino Settimanale, 08/07/2019 - 14/07/2019; bottom left photo published in 'Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019'.

A new video station with a thermal camera was installed at Punta dei Corvi, a short distance N of Ginostra on the SW coast, during 17-20 July 2019. During the third week of July lava continued to flow from the CS crater area onto the southern part of the Sciara del Fuoco, but the active flow area remained on the upper part of the scarp; block avalanches continuously rolled down to the coastline (figure 156). During visits to the summit area on 26 July and 1 August activity at the Terrazza Craterica was observed by INGV scientists. There were at least six active vents in the N area, including a scoria cone and an intensely spattering hornito; the other vents were ejecting coarse material in jets of Strombolian activity. In the CS area, a large scoria cone was clearly visible from the Pizzo, with two active vents generating medium- to high-intensity explosions rich in volcanic ash mixed with coarse ejecta (figures 157 and 158). Some of the finer-grained material in the jets reached 200 m above the vents. A second smaller cone in the CS area faced the southernmost part of the Sciara del Fuoco and produced sporadic low-intensity "bubble explosions." Effusive activity decreased during the last week of July; the active lava front was located at about 600 m elevation. Blocks continued to roll down the scarp, mostly from the explosive activity, and were visible from Punta dei Corvi.

Figure 156. Lava continued to flow from the CS area at Stromboli during the third week of July 2019, although the active flow area remained near the top of the scarp. Block avalanches continued to travel down the scarp. Image taken by di Francesco Ciancitto from Punta dei Corvi on 19 July 2019. Courtesy of INGV (Report 30/2019, Stromboli, Bollettino Settimanale, 15/07/2019 - 21/07/2019).

Figure 157. Thermal and visible images of Terrazza Craterica at the summit of Stromboli from the Pizzo Sopra La Fossa on 1 August 2019 showed significant changes since the major explosion on 3 July 2019. A large scoria cone was present in the CS area (left) and at least six vents from multiple cones were active in the N area (right). The active lava flow 'Trabocco Lavico' emerged from the southernmost part of the CS area (far left). Courtesy if INGV (Report 32/2019, Stromboli, Bollettino Settimanale, 29/07/2019 - 04/08/2019.

Figure 158. At the summit of Stromboli on 1 August 2019 two active vents inside a large cone in the CS area generated medium- to high-intensity explosions rich in volcanic ash mixed with coarse ejecta (left). There were at least six active vents in the N area (right), including a scoria cone and an intensely spattering hornito; the other vents were ejecting coarse material in jets of Strombolian activity. Courtesy of INGV (Report 32/2019, Stromboli, Bollettino Settimanale, 29/07/2019 - 04/08/2019).

Activity during August 2019. A small overflow of lava on 4 August 2019 from the N area lasted for about 20 minutes and formed a flow that went a few hundred meters down the Sciara del Fuoco. Observations made at the summit on 7 and 8 August 2019 indicated that nine vents were active in the N crater area, three of which had scoria cones built around them (figure 159). They all produced low- to medium-intensity Strombolian activity. In the CS area, a large scoria cone was visible from the summit that generated medium- to high-intensity explosions rich in volcanic ash, which sometimes rose more than 200 m above the vent. Lava overflowing from the CS area on 8 August was confined to the upper part of the Sciara del Fuoco, at an elevation between 500 and 600 m (figure 160). Occasional block avalanches from the active lava fronts traveled down the scarp. Ashfall was reported in the S. Bartolo area, Scari, and Piscità during the first week of August.

Figure 159. Nine vents were active in the N crater area of Stromboli on 7 August 2019, three of which had scoria cones built around them. They all produced low- to medium-intensity Strombolian activity (top). In the CS area (bottom), a large scoria cone was visible from the summit that generated medium- to high-intensity explosions rich in volcanic ash, which sometimes rose more than 200 m above the vent. Visible images taken by S. Consoli, thermal images taken by S. Branca. Courtesy of INGV (Report 33/2019, Stromboli, Bollettino Settimanale, 05/08/2019 - 11/08/2019).

Figure 160. Multiple Lava flows were still active on the Sciara del Fuoco at Stromboli on 7 August 2019. Top images by INGV personnel S Branca and S. Consoli, lower images by A. Di Pietro volcanological guide. Courtesy of INGV (Report 33/2019, Stromboli, Bollettino Settimanale, 05/08/2019 - 11/08/2019).

Drone surveys on 13 and 14 August 2019 confirmed that sustained Strombolian activity continued both in the N area and the CS area. Lava flows continued from two vents in the CS area; they ceased briefly on 16 and 17 August but resumed on the 18th, with the lava fronts reaching 500-600 m elevation (figure 161). A fracture field located in the southern part of the Sciara del Fuoco was first identified in drone imagery on 9 July. Repeated surveys through mid-August indicated that about ten fractures were identifiable trending approximately N-S and ranged in length from 2.5 to 21 m; they did not change significantly during the period. An overflight on 23 August identified the main areas of activity at the summit. A NE-SW alignment of 13 vents within the N area was located along the crater edge that overlooks the Sciara del Fuoco. At the CS area, the large scoria cone had two active vents, there was a pit crater, and two smaller scoria cones. A 50-m-long lava tube emerged from one of the smaller lava cones and fed two small flows that emerged at the top of the Sciara del Fuoco (figure 162).

Figure 161. Detail of a vent at Stromboli on 14 August 2019 located in the SW part of the Sciara del Fuoco at an elevation of 730 m. Flow is tens of meters long. Courtesy of INGV (COMUNICATO DI DETTAGLIO STROMBOLI del 20190816 ORE 17:05 LT).

Figure 162. Thermal and visual imagery of the summit of Stromboli on 23 August 2019 revealed a NE-SW alignment of 13 vents within the N area located along the crater edge that overlooks the Sciara del Fuoco. At the CS area, the large scoria cone had two active vents (1 and 2), there was a pit crater (3), and two smaller scoria cones (4). A 50-m-long lava tube formed from one of the smaller lava cones (5) and fed two small flows that emerged at the top of the Sciara del Fuoco. Photos by L. Lodato and S. Branca, courtesy of INGV (Report 35/2019, Stromboli, Bollettino Settimanale, 19/08/2019 - 25/08/2019).

INGV reported a strong explosion from the CS area at 1217 (local time) on 28 August 2019. Ejecta covered the Terrazza Craterica and sent debris rolling down the Sciara del Fuoco to the coastline. A strong seismic signal was recorded, and a large ash plume rose more than 2 km above the summit (figure 163). The Toulouse VAAC reported the ash plume at 3.7-4.6 km altitude, moving E and rapidly dissipating, shortly after the event. Once again, a pyroclastic flow traveled down the Sciara and several hundred meters out to sea (figures 164). The entire summit was covered with debris. The complex of small scoria cones within the N area that had formed since the 3 July explosion was destroyed; part of the N area crater rim was also destroyed allowing lava to flow down the Sciara where it reached the coastline by early evening.

Figure 163. A major explosion at Stromboli on 28 August 2019 produced a high ash plume and a pyroclastic flow. The seismic trace from the STR4 station (top left) indicated a major event. The ash plume from the explosion was reported to be more than 2 km high (right). The thermal camera located at Stromboli's Punta dei Corvi on the southern edge of the Sciara del Fuoco captured both the pyroclastic flow and the ash plume produced in the explosion (bottom left). Seismogram and thermal image courtesy of INGV (INGVvulcani blog, 30 AGOSTO 2019INGVVULCANI, Nuovo parossismo a Stromboli, 28 agosto 2019). Photo by Teresa Grillo (University of Rome) Courtesy of AIV - Associazione Italiana di Vulcanologia.

Figure 164. A pyroclastic flow at Stromboli traveled across the sea off the W flank for several hundred meters on 28 August 2019 after a major explosion at the summit. Photo by Alberto Lunardi, courtesy of INGV (5 SETTEMBRE 2019INGVVULCANI, Quando un flusso piroclastico scorre sul mare: esempi a Stromboli e altri vulcani).

At 1923 UTC on 29 August a lava flow was reported emerging from the N area onto the upper part of the Sciara del Fuoco; it stopped at mid-elevation on the slope. About 90 minutes later, an explosive sequence from the CS area resulted in the fallout of pyroclastic debris around Ginostra. Shortly after midnight, a lava flow from the CS area traveled down the scarp and reached the coast by dawn, but the lava entry into the sea only lasted for a short time (figure 165).

Figure 165. Lava flows continued for a few days after the major explosion of 28 August 2019 at Stromboli. Left: A lava flow emerged from the N crater area on 29 August 2019 and traveled a short distance down the Sciara del Fuoco. Incandescent blocks from the flow front reached the ocean. Photo by A. DiPietro. Right: A lava flow that emerged from the CS crater area around midnight on 30 August 2019 made it to the ocean around dawn, as seen from the N ridge of the Sciara del Fuoco at an altitude of 400 m. Photo by Alessandro La Spina. Both courtesy of INGV. Left image from 'COMUNICATO DI ATTIVITA' VULCANICA del 2019-08-29 22:20:06(UTC) – STROMBOLI', right image from INGVvulcani blog, 30 AGOSTO 2019 INGVVULCANI, 'Nuovo parossismo a Stromboli, 28 agosto 2019'.

An overflight on 30 August 2019 revealed that after the explosions of 28-29 August the N area had collapsed and now contained an explosive vent producing Strombolian activity and two smaller vents with low-intensity explosive activity. In the CS area, Strombolian activity occurred at a single large crater (figure 166). INGV reported an explosion frequency of about 32 events per hour during 31 August-1 September. The TROPOMI instrument on the Sentinel-5P satellite captured small but distinct SO₂ plumes from Stromboli during 28 August-1 September, even though they were challenging to distinguish from the larger signal originating at Etna (figure 167).

Figure 166. A 30 August 2019 overflight of Stromboli revealed that after the explosions of 28-29 August the N area had collapsed and now contained a single explosive vent producing Strombolian activity and two smaller vents with low intensity explosive activity. In the CS area, a single large crater remained with moderate Strombolian activity. No new lava flows appeared on the Sciara del Fuoco, only cooling from the existing flows was evident. Courtesy of INGV (Report 35.6/2019, Stromboli, Daily Bulletin of 08/31/2019).

Figure 167. Small but distinct SO2 signals were recorded from Stromboli during 28 August through 1 September 2019; they were sometimes difficult to discern from the larger signal originating at nearby Etna. Courtesy of NASA Goddard Space Flight Center.

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

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/aeroweb/info/vaac/); AIV, Associazione Italiana di Vulcanologia (URL: https://www.facebook.com/aivulc/photos/a.459897477519939/1267357436773935; ANSA.it, (URL: http://www.ansa.it/sicilia/notizie/2019/07/03/-stromboli-esplosioni-da-cratere-turisti-in-mare); The New York Post, (URL: https://nypost.com/2019/07/03/dozens-of-people-dive-into-sea-to-escape-stromboli-volcano-eruption-in-italy/).

Our previous report about Arenal discussed ongoing sporadic eruptive behavior, preliminary information about the 24 May 2010 dome collapse, and the higher frequency of rockfalls through September 2010 (BGVN 35:07). Since October 2010, volcanic activity at Arenal appears to be decreasing. Events like the explosion on 24 July 2010, discussed below (see figure 110) have become rare. Reports from Costa Rica's Volcanological and Seismological Observatory and National University (OVSICORI-UNA) include direct observations of summit activity, seismic analysis, and acid-rain data and provide the basis for this report covering the 24 May, 2010 event in addition to activity from October 2010 to May 2011.

Figure 110. At 0538 on 24 July 2010 (local time) an ash explosion at Arenal was recorded seismically and its resulting cloud was photographed. In the lower left-hand corner is the seismic trace of the event, which began suddenly and saturated the record (seismic station VACR; OVSICORI-UNA). Courtesy of Phil Slosberg (OVSICORI-UNA).

Incandescent avalanche of 24 May 2010. Sudden activity down Arenal's SW flank on 24 May 2010 produced long, incandescent avalanches and pyroclastic flows, forcing the National Park to evacuate visitors on this day. No injuries or damage to infrastructure had been reported during Arenal's activity in May 2010. Previous pyroclastic events had also caused evacuations in June 2009, June 2008, and September 2007.

Beginning at noon on 24 May, incandescent avalanches descended from the summit dome. They affected a sector that has been subject to avalanches in the last 3 years (see figure 111). A field investigation by OVSICORI on 31 May found that material fell from the summit down to 1,200 m elevation and accumulating in a toe 400 m x 80 m. The majority of blocks surpassed 2 m in diameter. Deposits from the dome collapse were still hot when they arrived at the forest that borders Río Agua Caliente. The OVSICORI-UNA field report of 31 May 2010 contains photos and additional details. Several sections of the river scarp show signs of being struck and eroded by direct impact of the incandescent blocks that arrived with high speed. The dome that supplied the block-and-ash flows became visibly deflated but activity culminated through the week with the formation of a new dome toward the E side of the summit. The formation and destruction of domes at the top of Crater C is very common. These domes reach ten's of meters in size and frequently collapse violently, especially when they are destabilized at the crater rim.

Figure 111. Changes in morphology at Arenal's Crater C are visible owing to the 24 May 2010 dome collapse. Located on the eastern side of the summit, the point of failure was attributed to the "Unstable area." Courtesy of E. Duarte (OVSICORI-UNA).

Decreasing activity. The number of explosive events peaked in February 2010, became regular up to October, but since mid-October they have become sporadic. No lava flows or night-time incandescence was observed on the flanks. Gas emission continued at the active Crater C and fumarolic activity was continuous at Crater D, the pre-1968 summit crater.

Acid-rain affected Arenal's flanks and the NE, E, and SE flanks showed a loss of vegetation. These conditions plus the high amounts of rainfall aggravated erosion on the steep slopes; rockfalls and landslides continued to occur in these valleys: Calle de Arenas, Manolo, Guillermina, and Río Agua Caliente. OVSICORI-UNA released a report on acid-rain measurements that began on 9 April 2003 and ended on 30 November 2010; data from four stations showed generally decreasing acidity with time (figure 112). The trend steadily increased from pH ~4 to ~4.5 for all stations. Although irregular spikes are recorded, the low outliers were generally less acidic with time.

Figure 112. Variation of the pH (level of acidity) of rain-water collected from four stations on Arenal. Data points represent measurements from 9 April 2003 to 30 November 2010. Courtesy of OVSICORI-UNA.

Waldo Taylor assessed seismic data from the local network. The 2010 mid-year ICE report discussed seismicity and the general trend shown in table 26. The large spike in seismic events from 2009 dropped off abruptly the following year.

Table 26. Earthquakes counted at Arenal during 2005-2010. Courtesy of ICE.

Gerardo J. Soto discussed Arenal seismicity. "In general terms, the average magnitude increased from 2.0 in 2006 to 2.3 in 2010. The biggest was M 4.1 in 1 November 2009. Mean [focal] depth deepened from 5.5 km in 2006 to about 2 km in 2010. Most of them were between 2 and 5 km deep in 2009-2010, and down to 9 km deep in 2010.

"The number of [respective] earthquakes from September through December 2010 decreased monthly [in the sequence] 24, 12, 9, 3. Epicenters shifted from SE to NW quadrangle of the volcano through time.

"We preliminarily interpret this as a possible withdrawal of magma below the volcano, [on the basis of] focal mechanisms."

Secondary hazards. With Arenal's decrease in explosive activity, no ash collection has been possible this year (2011). A network of seven stations exists for regular sampling. The most effusive event occurred in 1968 when roughly 2 x 10⁵ metric tons of ash fell on the flanks. Later, a hydroelectric project was completed in the 1970s and filled the basin below the volcano with 2.416 x 10⁶ m³ of water (the maximum storage capacity), forming Lake Arenal. From 1992 to 1997, the annual sediment load into the lake contained 1.4% remobilized material from Arenal.

Future activity at Arenal within the next 100 years may include large eruptions with the potential to produce 10 million metric tons of volcanic sediments; within the next 200 years an extreme event could contribute 10⁷ metric tons of volcaniclastics to Lake Arenal (Soto, 1998). The distribution of volcaniclastic sediments is largely controlled by the Río Agua Caliente, a drainage connecting tributaries from Arenal's southern flank. Roughly every 2-5 years there are relatively large debris flows along this river. As recently as the first week of May 2011, intense flooding damaged a bridge by severely undermining the concrete abutments (G.J. Soto, personal communication).

Satellite thermal alerts. Since 15 September 2010 there have been no MODVOLC satellite thermal alerts through February 2011.

References. Soto, G.J., 1998, Cálculo de ceniza eyectada por el Volcán Arenal y ceniza caída en el embalse durante el período 1992-1997; Informe OSV.98.05.ICE, 18 pp. (in Spanish)

OVSICORI-UNA, 2010, Cambios Morfológicos y Avalanchas Incandescentes del 24 de Mayo en el Volcán Arenal. (in Spanish) (URL: http://www.ovsicori.una.ac.cr/vulcanologia/informeDeCampo/2010/InfcampAremayo10.pdf)

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: Phil Slosberg and Eliecer Duarte, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Gerardo J. Soto, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica; Waldo Taylor, Sismológico y Vulcanológico de Arenal y Miravalles (OSIVAM), Oficina de Sismología y Vulcanología (OSV), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

The Grotto vent cluster contains an assemblage of black smoker vents that lie within the Main Endeavour Field on the northern Juan de Fuca ridge (Bemis, 2001; Rona and others, 2001, 2010a; Bobbitt, 2007) (figure 4). New imagery of submarine plume behavior and properties was achieved with a new acoustic system that extends underwater observational distances beyond those of light to image buoyant plumes of submarine black smokers in 3-dimensions and image areas of diffuse flow seeping from the sea floor in 2-dimensions (Rona, 2011; Rona and others, 2010a, 2010b, and 2011).

Figure 4. Map of Main Endeavour Field, Juan de Fuca Ridge (grid system in meters), showing the location of the Grotto Vent at grid coordinates of about 6115 and 4920. Note scale-the entire Endeavour Field is only ~400 m long. According to Merle (2006) Grotto vent resides at 47.95°N latitude, 129.10°W longitude, and at a depth of ~2,196 m.

The Cabled Observatory Vent Imaging Sonar (COVIS) was installed in September 2010 (Light, 2011). Operations were initiated with in situ sensors in the NEPTUNE (North-East Pacific Time-Series Underwater Networked Experiments) Canada Program cabled observatory on the Main Endeavour Field (MEF) of the Juan de Fuca Ridge, nearly 370 km (200 nautical miles) off British Columbia, Canada, in the NE Pacific Ocean (figures 5 and 6). NEPTUNE is a Canadian research facility designed for regional-scale underwater ocean investigations focusing on continuous monitoring of temperature, chemistry, biodiversity, and motion. This data will be broadcast via the Internet for scientists, students, educators and the public to collaborate and promote investigations into: underwater volcanic processes; earthquakes and tsunamis; minerals, metals, and hydrocarbons; ocean-atmosphere interactions; climate change; greenhouse gas cycling in the ocean; marine ecosystems; long-term changes in ocean productivity; marine mammals; fish stocks; pollution and toxic blooms. The public can gain a more in-depth understanding of the seafloor, while ocean scientists can run deep-water experiments from labs and universities anywhere around the world.

Figure 5. Map of NEPTUNE Canada Program's six submarine sites with multiple sensors connected to a high-speed optical cable linked with University of Victoria in British Columbia, Canada. The Main Endeavour Field, labeled as Endeavor (in red), one of the instrumented sites, is ~350 km WSW from Port Alberni. Over the project's 25-year lifespan, Endeavor will collect data for underwater volcanic processes, seismicity, plate tectonics, hydrothermal vent systems, and deep sea ecosystems. Courtesy of NEPTUNE Canada (2011).

During a research cruise in September-October 2010, scientists from the University of Washington and Rutgers University connected COVIS to the NEPTUNE Canada cable system for the first time and initiated data acquisition on 29 September 2010. COVIS, equipped with a customized multibeam sonar, 400/200 kHz projectors, and a rotator system to orient acoustic transducers, was positioned to acquire acoustic data from a fixed site on the floor of the ridge's axial valley at a range of tens of meters from the Grotto vent cluster in the MEF (figure 6).

Figure 6. COVIS acoustic image, oriented NE on the left to NW on the right, made at 0600 UTC on 11 October 2010, looking S at black smoker plumes and areas of diffuse flow draped over bathymetry of the Grotto vent cluster (Jackson and others, 2003) in the Main Endeavour Field, Juan de Fuca Ridge. The image was made when tidal currents were minimal (e.g., near slack tide). The larger plume is from the N tower edifice at the NW end, and the smaller plumes are from the NE end of Grotto vent at the in-situ experiments. The legend (at the upper left) specifies isosurfaces of plume volume scattering strengths (in decibels per meter) related to particle content and temperature-density discontinuities. The vertical color bar (at the far right) gives normalized decorrelation of backscatter (0-1) due to diffuse flow from the sea floor at 0.8-sec lag. The plumes decrease in acoustic backscatter intensity as they mix with surrounding seawater with height (in meters) above vents. From Rona (2011).

The purpose of the COVIS experiment was to acoustically image, quantify, and monitor seafloor hydrothermal flow on time scales of hours (response to ocean tides) to weeks-months-years (response to volcanic and tectonic events); this advances our understanding of these interrelated processes. According to Rona and others (2003), net volume flux of a plume can be calculated by integrating the vertical flux through a plume cross-section, which can then be converted to heat and particle flux if coordinated with in-situ measurements of temperature and particle properties (concentration, size distribution, density). To achieve this, COVIS acquired acoustic data from a projector mounted on a tripod ~4 m above the seafloor at a fixed position. A computer controlled, 3- degrees-of-freedom (yaw, pitch, and roll), positioning system was used to point the sonar transducers providing a large coverage area at the site. Sonar data is collected at ranges of tens of meters from targets to make three types of measurements: 1) volume backscatter intensity from suspended particulate matter and temperature fluctuations in black smoker plumes which was used to reconstruct the size and shape of the buoyant portion of a plume; 2) Doppler phase shift which was used to obtain the flow rise velocity at various levels in a buoyant plume; 3) scintillation which was used to image the area of diffuse flow seeping from the seafloor.

References. Bemis, K.G., Rona, P.A., Jackson, D.R., Jones, C., Mitsuzawa, K., Palmer, D., Silver, D., and Gudlavalletti, R., 2001, Time-averaged images and quantifications of seafloor hydrothermal plumes from acoustic imaging data: a case study at Grotto Vent, Endeavour Segment Seafloor Observatory, Abstract OS21B-0446 presented at American Geophysical Union, Fall Meeting 2001, San Francisco, CA, December.

Bobbitt, A., 2007, NeMO 2007 Cruise Report: Axial Volcano, Endeavour Segment, and Cobb Segment, Juan de Fuca Ridge, R/V Atlantis Cruise AT 15-21, August 3-20, 2007, Astoria, Oregon, to Astoria Oregon, Jason dives J2-286 to J2-295, unpublished report (URL: http://www.pmel.noaa.gov/vents/nemo/NeMO2007-cruise-report.pdf)

Jackson, D.R., Jones, C.D., Rona, P.A., and Bemis, K.G., 2003, A method for Doppler acoustic measurement of black smoker flow fields, Geochemistry Geophysics Geosystems (G3), v. 4, no. 11, p. 1095 (DOI: 10.1029/2003GC000509, 2003).

Light, R., Miller, V., Rona, P., and Bemis, K., 2010, Acoustic Instrumentation for Imaging and Quantifying Hydrothermal Flow in the NEPTUNE Canada Regional Cabled Observatory at Main Endeavour Field (unpublished paper - URL: http://www.apl.washington.edu/projects/apl_presents/topics/covis/covis.php).

Light, R., Miller, V., Jackson, D.R., Rona, P.A., and Bemis, K.G., 2011, Cabled observatory vent imaging sonar (abstract of presentation), Journal of the Acoustical Society of America, v. 129, no. 4, p. 2373.

Merle, S. (compiler), 2006, NeMO 2006 Cruise Report, NOAA Vents Program, Axial Volcano and the Endeavour Segment, Juan de Fuca Ridge, R/V THOMPSON Cruise TN-199, August 22 - September 7, 2006. Seattle WA to Seattle WA; ROPOS dives R1008 - R1014 (URL: http://www.pmel.noaa.gov/vents/nemo2006/nemo06-crrpt-final.pdf).

NEPTUNE Canada, 2011, Transforming Ocean Science; Ocean Networks Canada. (URL: http://www.neptunecanada.ca/about-neptune-canada/neptune-canada-101/)

Rona, P.A., Bemis, K.G., Jackson, D.R., Jones, C.D., Mitsuzawa, K., Palmer, D.R., and Silver, D., 2001, Acoustic Imaging Time Series of Plume Behavior at Grotto Vent, Endeavour Observatory, Juan de Fuca Ridge, Abstract OS21B-0445 presented at American Geophysical Union, Fall Meeting 2001, San Francisco, CA, December.

Rona, P.A., Jackson, D.J., Bemis, K.G., Jones, C.D., Mitsuzawa, K., Palmer, D.R., and Silver, D., 2003, A New Dimension in Investigation of Seafloor Hydrothermal Flows, Ridge 2000 Events, v. 1, no. 1, p. 26 (URL: http://ridge2000.bio.psu.edu).

Rona, P.A., Bemis, K.G., Jones, C., Jackson, D. R., Mitsuzawa, K, and Palmer, D. R., 2010a, Partitioning Between Plume and Diffuse Flow at the Grotto Vent Cluster, Main Endeavour Vent Field, Juan de Fuca Ridge: Past and Present, Abstract OS21C-1519 presented at American Geophysical Union, Fall Meeting 2010, San Francisco, Calif., December.

Rona, P., Light, R., Miller, V., Jackson, D., Bemis, K., Jones, C., and KenneyM., 2010b, Cabled Observatory Vent Imaging Sonar (COVIS) Connected to NEPTUNE Canada Cabled Observatory (poster abstract), 2010 R2K (Ridge 2000) Community Meeting, Portland, OR, 29-31 October 2010 (URL: http://ridge2000.marine-geo.org/community-meeting/october-2010/2010-r2k-community-meeting).

Rona, P., 2011, Sonar images hydrothermal vents in seafloor observatory, EOS Transactions, American Geophysical Union, v. 92, no., 20, p. 169-170.

Rona, P.A., Benis, K.G., Jones, C.D., and Jackson, D.R., 2011, Multibeam sonar observations of hydrothermal flows at the Main Endeavour Field (abstract of presentation), Journal of the Acoustical Society of America, v. 129, no. 4, p. 2373.

Geologic Background. The Endeavour Segment (or Ridge) lies near the northern end of the Juan de Fuca Ridge, west of the coast of Washington and SW of Vancouver Island. The northern end is offset to the east with respect to the West Valley Segment, which extends north to the triple junction with the Sovanco Fracture Zone and the Nootka Fault. The 90-km-long, NNE-SSW-trending segment lies at a depth of more than 2000 m and is the site of vigorous high-temperature hydrothermal vent systems that were first discovered by scientists in 1981. Five major vent fields that include sulfide chimneys and black smoker vents, first seen from the submersible vehicle Alvin in 1984, are spaced at about 2-km intervals in a 1-km-wide axial valley at the center of the ridge. Preliminary uranium-series dates of Holocene age were obtained on basaltic lava flows, and other younger "zero-age" flows were sampled. Seismic swarms were detected in 1991 and 2005.

Information Contacts: Peter Rona, Institute of Marine and Coastal Sciences and Department of Earth and Planetary Sciences, Rutgers University, New Brunswick, NJ; NEPTUNE Canada (URL: http://www.oceannetworks.ca/).

Gudmundsson and others (2010a) noted that the last day of sustained activity at Eyjafjallajökull took place on 22 May 2010. By 23 June 2010, the Iceland Meteorological Office (IMO) and the University of Iceland Institute of Earth Sciences (IES) ceased issuing regular status reports. In addition to discussing the eruption and its final stages, this report also cites a small sample of abstracts and papers from the numerous conferences, sessions, and publications that have thus far emerged on the eruption.

The eruption's initial phase, 20 March-12 April 2010, occurred at Fimmvörðuháls, a spot on the E flanks of Eyjafjallajökull (figure 16, and "F" and "E" on figure 17). Venting at Fimmvörðuháls took place on an exposed ridge cropping out in a region with extensive glaciers to the E and W. Eruptions began in the initially ice-capped summit crater of Eyjafjallajökull on 14 April 2010 (BGVN 35:03 and 35:04). After melting overlying portions of the icecap, the summit crater then emitted clouds of fine-grained ash that remained suspended in the atmosphere for long distances. The ash blew both over the Atlantic and for considerable intervals passed directly over Europe, halting flights of most commercial aircraft for nearly a week in a controversial shutdown with economic impacts in the billions.

Figure 16. Index map showing Iceland, some major plate-tectonic features and generalized spreading directions, and the location of Eyjafjallajökull volcano. Note proximity of Eyjafjallajökull to Katla and to the volcanoes of the Vestmann island area (Vestmannaeyjar), Surtsey and Heimaey. Courtesy of USGS.

Figure 17. A shaded-relief map showing Eyjafjallajökull (E), and 9 km to its E, the flank vent Fimmvörðuháls (F). Stars indicate 2010 eruptive sites (map scale at top left). Glaciers cover extensive portions of both Eyjafjallajökull and Katla volcanoes (light pattern). During 14-29 April 2010 many earthquakes struck with epicenters along the N-S axis of Eyjafjallajökull (black dots). The map includes a small slice of the Atlantic ocean along the lower left-hand margin. Two of four geodetic (GPS) stations are shown (STE2 and THEY). Revised from a map by Sigmundsson and others (2010).

In terms of satellite thermal data on the overall eruption, the MODVOLC system measured extensive (multi-pixel) daily alerts during 21 March-21 May 2010, but the alerts became absent thereafter.

Venting at Fimmvörðuháls. At a 15-19 September 2010 conference on the eruption, Höskuldsson and others (2010a) characterized the course of events during the 20 March to 12 April basaltic Fimmvörðuháls flank eruption at Eyjafjallajökull as follows: "At the beginning the eruption featured as many as 15 lava fountains with maximum height of 150 m. On March 24 only four vents were active with fountains reaching to heights of 100 m. On March 31 and April 1 the activity was characterized by relatively weak fountaining through a forcefully stirring pool of lava. The vents were surrounded by 60-80 m high ramparts and the level of lava stood at approximately 40 m. This high stand led to opening of a new fissure trending northwest from the central segment of the original fissure. As activity on the new fissure intensified, the discharge from the original fissure declined and stopped on April 7.

"The intensity of the lava fountains varied significantly on the time scale of hours and was strongly influenced the level of the lava pond in the vents, producing narrow, gas-charged, piston-like fountains during periods of low lava levels, but spray-like fountains when the lava level was high . . ..

"The eruption produced a fountain-fed lava flow field with an area of about 1.3 km². Initially (20-25 March), the lava advanced towards northeast, but on March 26 the lava began advancing to the west and northwest, especially after April 1 when the activity became concentrated on the new fissure. The flow field morphology is dominantly 'a'a, but domains of pahoehoe and slabby pahoehoe are present, particularly in the western sector of the flow field. The advance of the lava from the vents was episodic; when the lava stood high the lava surged out of the vents, but at low stand there was a lull in the advance. The lava discharged from the vents through open channels as well as internal pathways. The open channels were the most visible part of the transport system, feeding lava to active 'a'a flow fronts and producing spectacular lava falls when cascading into deep gullies just north of the vents. The role of internal pathways was much less noticeable, yet an important contribution to the overall growth of the flow field as it fed significant surface breakouts emerging on the surface of what otherwise looked like stagnant lava. When activity stopped on April 12 the fissure had issued about 0.025 km³ of magma, giving a mean discharge of 13 m³/s."

Summit eruption. The second eruption occurred within the initially ice-covered caldera of Eyjafjallajökull. Opening of the ice cover and explosivity into the atmosphere was amplified by magma-ice interaction that produced a fine ash capable of suspension in the atmosphere for prolonged periods.

Höskuldsson and others (2010b) described the eruption at Eyjafjallajökull's summit (beginning 14 April 2010) as consisting of three phases (table 2). They also stated that at the summit the "Total amount of tephra produced in the eruption is about 0.11 km³ and that of lava 0.025 km³ DRE [dense-rock equivalent]. Average discharge rate in the eruption was about 40 m³/s DRE or about 4 times that of Fimmvörðuháls eruption."

Table 2. Three phases of the eruption at Eyjafjallajökull volcano's summit beginning 14 April 2010 as summarized and condensed by Höskuldsson and others (2010b).

The summit area was still steaming and geothermally active, and the eruption channel was still very hot in October 2010 (figure 18). Investigators expected that cooling to ambient temperatures would take a few years . As noted below, during June 2010, hot lava could still be seen in cracks in the cooled rock on Fimmvörðuháls, and inside craters, but that was not the case at the ice-engulfed summit caldera.

Figure 18. The summit crater complex of Eyjafjallajökull taken after the first winter snow, as seen from the air at 0810 on 9 October 2010. The scene helps explain the high degree of water and ice interaction with the erupting lavas. Snow had melted from numerous ash and lava-covered surfaces (black areas). Although portions of the crater emitted steam, evidence of substantial ongoing lava emissions were absent at this point in time. Photo courtesy of Ólafur Sigurjónsson, IMO.

According to Gudmundsson and others (2010b) the summit eruption produced 0.1-0.2 km³ (dense rock equivalent) of tephra. IES reported that by 11 June 2010 a lake about 300 m in diameter had formed in the large summit crater, and by 23 June water was slowly accumulating in the crater because ice was no longer in contact with hot material.

Intrusion triggering. Sigmundsson and others (2010) noted that the 2010 eruptions came after 18 years of intermittent volcanic unrest. The deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single source. Deformation was rapid before the flank eruption (0.5 mm per day after 4 March 2010), but negligible during it.

During the summit eruption (beginning 14 April 2010) gradual contraction of a source, distinct from the pre-eruptive inflation sources, was evident from geodetic data. Thus, clear signals of volcanic unrest may occur over years to weeks, indicating reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

Figure 19 shows a cross-sectional model of the shallow crust by Sigmundsson and others (2010) based deformation and seismic analyses of the 2010 event. A previous issue of the Bulletin (BGVN 35:03) contained an alternate model by Paul Einarsson.

Figure 19. Schematic E-W cross-section across the Eyjafjallajökull summit area, with deformation sources plotted at their best-fit depth (vertical exaggeration of 2). Gray shaded background indicates source-depth uncertainties (95% confidence interval), which overlap. Courtesy of Sigmundsson and others (2010).

Processed satellite image. Vincent J. Realmuto created two composite figures generated from the MODIS-Terra satellite data acquired 15 April 2010 at 1135 UTC (figure 20). Outlined in black in each image are Iceland on the upper left side (W), Faroe Islands in the center, Scotland and N Ireland in the lower center, and part of the Scandinavian peninsula on the right side (E). An ash plume can be seen in each image extending from Iceland SW toward Europe. The left-hand image is the true-color RGB (red-green-blue) composite and the right-hand image is a false-color composite; in the right-hand rendition the ash plume appears red and the ice-rich clouds appear blue. The right-hand image puts obvious emphasis on the ash plume and shows it streaming and more or less intact for several hundreds of kilometers E of Iceland.

Figure 20. Graphics generated from the MODIS-Terra satellite data acquired 15 April 2010 at 1135 UTC. The left-hand graphic is a true-color RGB (red-green-blue) composite, and the right-hand image is a false-color composite of Bands 32, 31, and 29 (12, 11, and 8.5 um, respectively) displayed in red, green, and blue, respectively. These data were processed with the decorrelation stretch (D-stretch), a technique for enhancing spectral contrast based on principal components analysis. In this rendition the ash plume appears red and the ice-rich clouds appear blue. The D-stretch was based on scene statistics and was intended to be a quick method for discriminating material that may be volcanic in origin. Courtesy of Vincent J. Realmuto, Jet Propulsion Laboratory, California Institute of Technology.

Conference field trip. Following The Atlantic Conference on Eyjafjallajökull and Aviation in Iceland, 15-16 September 2010 (discussed below), a field trip brought scientists to accessible areas on the volcano, including the flank vent on Fimmvörðuháls ridge where the eruption began. John and Liudmila Eichelberger provided some photographs from this trip (figure 21). The same base map appeared in BGVN 35:03, with the key and other data. The horseshoe shape of the lava distribution in this figure is the feature imaged by an ASTER satellite thermal signature as active lava flows on 19 April 2010 in BGVN 35:03.

Figure 21. (Central panel) Map showing fissures at Fimmvörðuháls (thin red lines) and the distribution of new scoria and lava deposited at various points in time (shaded areas) during 21 March-7 April 2010. Marked arrows on the map give locations of labeled photos (A-E) taken 18 September 2010. (A) Fresh lava (darker) seen looking N. In the distance appear fresh black lava flows, some portions of which formed the lava falls down the valley walls. (B) View showing the elongate ridge as seen from the upslope perspective (people in the distance for scale). (C, looking down) Glowing lava (~1.5 m long and ~0.3 m wide) at the bottom of a fissure. This photo was taken with a flash, otherwise the fissure walls would have been very dark. (D) The fracture indicated on the map as it appeared near the rim of the ridge of newly erupted lava. (E) The same fracture seen in D from another perspective. Courtesy of John and Ludmilla Eichelberger.

More on conferences and publications. Recently, several conferences have been held and many publications have been issued relevant to the eruption. What follows is a mere sample of the available resources, many of which emphasized plume research. At the American Geophysical Union (AGU) 2010 Fall Meeting, several sessions focused on the 2010 eruption (eg., Carn and others, 2010; see References for the link to abstracts volume).

The Workshop on Ash Dispersal Forecast and Civil Aviation held in Geneva, 18-20 October 2010, addressed the characteristics and range of application of different volcanic ash transport and dispersal models (VATDM), identifying the needs of the modeling community, investigating new data acquisition strategies, and discussing how to improve communication between the volcanology community and operational agencies (eg., Bonadonna and others, 2011).

The Cities on Volcanoes conference (COV-6; Tenerife, Canary Islands, Spain, 31 May-4 June 2010) included both papers (eg. Fischer and others, 2010) and a forum on the "Assessment of volcanic ash threat: learning and considerations from the Eyjafjallajökull eruption."

In addition, several other papers relevant to the eruption were presented during this meeting, as well as at the Annual Meeting of the American Meteorological Society (AMS) in Seattle, WA, in January 2011, and at the European Geosciences Union (EGU) 2011 General Assembly in Vienna, Austria.

The journal Atmospheric Chemisrty and Physics published multiple issues with a section entitled "Atmospheric implications of the volcanic eruptions of Eyjafjallajökull, Iceland 2010." These and other papers discussed various means of plume detection, and in some cases, sampling, including on the ground, in ultralight aircraft, and on satellites; models of plume dispersion were evaluated (Flentje and others, 2010; Emeis and others, 2011; Vogel and others, 2011; Fischer and others, 2010).

According to Loughlin (2010), scientists from the British Geological Survey found large ash particles from the eruption in the United Kingdom. Most of the very small ash particles in volcanic plumes fell as clusters of particles known as aggregates. The aggregation could have resulted from a number of mechanisms, including electrostatic attraction, particle collisions, condensation of liquid films and secondary mineralization. The process of aggregation effectively removed very small particles from the plume and was therefore one variable on how long ash particles stay in the atmosphere. Ripley (2010) and Chivers (2010) published articles on the U.K. Met Office's tracking and prediction of movements of volcanic ash based on observations from the Eyjafjallajökull eruption.

Gislason and others (2011) reported on analyses of two sets of fresh, comparatively dry ash samples that fell in Iceland and were collected rapidly on 15 and 27 April, during more and less explosive phases, respectively. Both sets of samples were kept dry and analyzed swiftly to minimize issues with hydration and alteration, particularly to salts on the ash surfaces. The ash was dominantly glass of andesitic composition (57-58% SiO₂). They found the ash particles especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers.

References. Bonadonna, C., Folch, A., and Loughlin, S., 2011, Future Developments in Modeling and Monitoring of Volcanic Ash Clouds, Eos, Transactions of the American Geophysical Union (AGU), v. 92, no. 10; pp. 85-86, DOI: 10.1029/2011EO100008 (URL: http://www.agu.org/pub/eos/).

Carn, S.A., Karlsdottir, S., and Prata, F., 2010, The 2010 Eruption of Eyjafjallajokull: A Landmark Event for Volcanic Cloud Hazards I, II, and III, Abstracts V41E, V53F, and V54C presented at 2010 Fall Meeting, American Geophysical Union, San Francisco, CA, 13-17 December 2010 (URL: http://www.agu.org/meetings/fm10/program/index.php).

Chivers, H., 2010, Dark Cloud: VAAC and predicting the movement of volcanic ash, Meterological Technology International, June 2010, pp. 62-65.

Emeis, S., Forkel, R., Junkermann, W., Schäfer, K., Flentje, H., Gilge, S., Fricke, W., Wiegner, M., Freudenthaler, V., Groß, S., Ries, L., Meinhardt, F., Birmili, W., Münkel, C., Obleitner, F., and Suppan, P., 2011, Measurement and simulation of the 16/17 April 2010 Eyjafjallajökull volcanic ash layer dispersion in the northern Alpine region, Atmospheric Chemistry and Physics, v. 11, pp. 2689-2701.

Fischer, C., van Haren, G., Pohl, T., Vogel, A., and Weber, K., 2010, Airborne in-situ measurements of the volcanic ash dust plume over a part of Germany caused by the volcano eruption of the Eyjafjallajökull (Iceland) by means of an optical particle counter and a light

sport aircraft, Abstract, Session 1.3, p. 229, Cities on Volcanoes 6 Conference (URL: http://www.citiesonvolcanoes6.com/ver.php).

Flentje, H., Claude, H., Elste, T., Gilge, S., Köhler, U., Plass-Dülmer, C., Steinbrecht, W., Thomas, W., Werner, A., and Fricke W., 2010, The Eyjafjallajökull eruption in April 2010 - detection of volcanic plume using in-situ measurements, ozone sondes and lidar-ceilometer profiles, Atmospheric Chemistry and Physics, v. 10, pp. 10085-10092, DOI: 10.5194.

Gasteiger, J., Groß, S., Freudenthaler, V., and Wiegner, M., 2011, Volcanic ash from Iceland over Munich: mass concentration retrieved from ground-based remote sensing measurements, Atmospheric Chemistry and Physics, v. 11, pp. 2209-2223.

Gislason, S.R., Hassenkam, T., Nedel, S., Bovet, N., Eiriksdottir, E.S., Alfredsson, H.A., Hem, C.P., Balogh, Z.I., Dideriksen, K., Oskarsson, N., Sigfusson, B., Larsen, G., and Stipp, S.L.S., 2011, Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment, Proceedings of the National Academy of Sciences, v. 108, no. 18, p. 7303-7312.

Gudmundsson, M. T., Pedersen, R., Vogfjörd, K., Thorbjarnardóttir, B., Jakobsdóttir, S., and Roberts, M.J., 2010a, Eruptions of Eyjafjallajökull Volcano, Iceland, Eos, Transactions of the American Geophysical Union (AGU), v. 91, no. 21, p. 190, DOI: 10.1029/2010EO210002.

Gudmundsson, M.T., Thordarson, T., Hoskuldsson, A., Larsen, G., Jónsdóttir, I., Oddsson, B., Magnusson, E., Hognadottir, T., Sverrisdottir, G., Oskarsson, N., Thorsteinsson, T., Vogfjord, K., Bjornsson, H., Pedersen, G.N., Jakobsdottir, S., Hjaltadottir, S., Roberts, M.J., Gudmundsson, G.B., Zophoniasson, S., and Hoskuldsson, F., 2010b, The Eyjafjallajökull eruption in April-May 2010; course of events, ash generation and ash dispersal, EOS, Transactions of the American Geophysical Union (AGU), V. 91, no. 21, Abstract V53F-01, 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 December (URL: http://www.agu.org/cgi-bin).

Heue, K.-P., Brenninkmeijer,C.A.M., Baker, A. K., Rauthe-Schöch, A., Walter, D., Wagner, T., Hörmann, C., Sihler, H., Dix, B., Frieß, U., Platt, U., Martinsson, B. G., van Velthoven, P.F.J., Zahn, A., and Ebinghaus, R., 2011, SO2 and BrO observation in the plume of the Eyjafjallajökull volcano 2010: CARIBIC and GOME-2 retrievals, Atmospheric Chemistry and Physics, v. 11, pp. 2973-2989.

Höskuldsson, A., Magnusson, E., Guðmundsson, M.T., Sigmundsson, F., and Sigmarsson, O., 2010a, The 20 March to 12 April basaltic Fimmvörðuháls flank eruption at Eyjafjallajökull volcano, Iceland: Course of events, abstract of presentation in Program of the Eyjafjallajökull and Aviation Conference (15-16 September 2010) and associated Eyjafjallajökull Eruption Workshop (Hotel Hvolsvellir, 17-19 September 2010); (URL: http://en.keilir.net/keilir/conferences/eyjafjallajokull/volcanological-workshop).

Höskuldsson, Á., Larsen, G., Gudmundsson, M.T., Oddsson, B., Magnússon, E., Sigmarsson, O., Óskarsson, N., Jónsdóttir, I., Sigmundsson, F., Einarsson, P., Hreinsdóttir, S., Pedersen, R., Högnadóttir, Þ., Thordarson, T., Hayward, C., Hartley, M., Meara, R., Arason, Þ., Karlsdóttir, S., and Petersen, G.N., 2010b, The Eyjafjallajökull eruption April to May 2010: Magma fragmentation, plume and tephra transport, and course of events, abstract of presentation in Program of the Eyjafjallajökull and Aviation Conference (15-16 September 2010) and associated Eyjafjallajökull Eruption Workshop (17-19 September 2010); (URL: http://en.keilir.net/keilir/conferences/eyjafjallajokull/volcanological-workshop).

Laursen, L., 2010, Iceland eruptions fuel interest in volcanic gas monitoring, Science, v. 328, no. 5977, p. 410-411.

Loughlin, S., 2010, Modelling of Iceland volcanic ash particles, news item from British Geological Survey (URL: http://www.bgs.ac.uk/research/highlights/IcelandAshParticles.html?src=sfb).

Ripley, T., 2010, Cloud Busting: How the UK is tracking the volcanic ash cloud, Meterological Technology International, June 2010, pp. 6-10.

Schumann, U., Weinzierl, B., Reitebuch, O., Schlager, H., Minikin, A., Forster, C., Baumann, R., Sailer, T., Graf, K., Mannstein, H., Voigt, C., Rahm, S., Simmet, R., Scheibe, M., Lichtenstern, M., Stock, P., Rüba, H., Schäuble, D., Tafferner, A., Rautenhaus, M., Gerz, T., Ziereis, H., Krautstrunk, M., Mallaun, C., Gayet, J.-F., Lieke, K., Kandler, K., Ebert, M., Weinbruch, S., Stohl, A., Gasteiger, J., Groß, S., Freudenthaler, V., Wiegner, M., Ansmann, A., Tesche, M., Olafsson, H., and Sturm, K., 2011, Airborne observations of the Eyjafjalla volcano ash cloud over Europe during air space closure in April and May 2010, Atmospheric Chemistry and Physics, v. 11, pp. 2245-2279.

Sigmundsson, F., Hreinsdóttir, S., Hooper, A., Árnadóttir, T., Pedersen, R., Roberts, M.J., Óskarsson, N., Auriac, A., Decriem, J., Einarsson, P., Geirsson, H., Hensch, M., Ófeigsson, B.G., Sturkell, E., Sveinbjörnsson, H., and Feigl, K.L., 2010, Letter: Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption, Nature, v. 468, pp. 426-430.

Stohl, A., Prata, A.J., Eckhardt, S., Clarisse, L., Durant, A., Henne, S., Kristiansen, N.I., Minikin, A., Schumann, U., Seibert, P., Stebel, K., Thomas, H.E., Thorsteinsson, T., Tørseth, K., and Weinzierl, B., 2011, Determination of time- and height-resolved volcanic ash emissions and their use for quantitative ash dispersion modeling: the 2010 Eyjafjallajökull eruption, Atmospheric Chemistry and Physics, v. 11, pp. 4333-4351.

Vogel, A., Weber, K., Fischer, C., van Haren, G., Pohl, T., Grobety, B., and Meier, M., 2011, Airborne in-situ measurements of the Eyjafjallojökull ash plume with a small aircraft and optical particle spectrometers over north-western Germany - comparison between the aircraft measurements and the VAAC-model calculations, European Geophysical Union General Assembly, Geophysical Research Abstracts, v. 13, p. EGU2011-13253.

Geologic Background. Eyjafjallajökull (also known as Eyjafjöll) is located west of Katla volcano. It consists of an elongated ice-covered stratovolcano with a 2.5-km-wide summit caldera. Fissure-fed lava flows occur on both the E and W flanks, but are more prominent on the western side. Although the volcano has erupted during historical time, it has been less active than other volcanoes of Iceland's eastern volcanic zone, and relatively few Holocene lava flows are known. An intrusion beneath the S flank from July-December 1999 was accompanied by increased seismic activity. The last historical activity prior to an eruption in 2010 produced intermediate-to-silicic tephra from the central caldera during December 1821 to January 1823.

Information Contacts: Institute of Earth Sciences (IES), University of Iceland, Sturlugata 7, Askja , 101 Reykjavík (URL: http://www.earthice.hi.is/); Icelandic Meteorological Office (IMO) (URL: http://en.vedur.is/earthquakes-and-volcanism/articles/nr/1884); U.K. Meteorological Office (URL: http://www.metoffice.gov.uk); ármann Höskuldsson, Institute of Earth Sciences (IES), University of Iceland, Sturlugata 7, Askja , 101 Reykjavík (URL: http://www.earthice.hi.is); 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/); Sue C. Loughlin, The British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Scotland, UK (URL: http://www.bgs.ac.uk/); Vincent J. Realmuto, Jet Propulsion Laboratory, California Institute of Technology, M/S 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109 USA; John Eichelberger, U.S. Geological Survey, Volcano Hazards Program, Reston, VA (URL: http://volcanoes.usgs.gov/); Ludmilla Eichelberger, Global Volcanism Program, National Museum of Natural History, 10^th and Constitution Ave., NW, Washington, DC 20560 USA; Iceland Review (URL: http://icelandreview.com/icelandreview/daily_news/).

In April 2010 the lake within Irazú's crater dwindled to only a few centimeters depth and from May to August the lake was dry enough to allow plants to grow up to 10 cm high. Water began to accumulate in September 2010 but disappeared again during the following month. Since November 2010 water returned to the crater and as late as April 2011, a shallow turquoise-blue lake was maintained. Continuous monitoring of acid rain on Irazú's flanks reflected contributions from Turrialba. Often called Irazú's "twin volcano," Turrialba is less than 10 km to the ENE and during the past 4 years it has caused a region-wide increase in acid rain. Covering January 2004 through September 2007, the last Bulletin report on Irazú (BGVN 32:11) highlighted decreasing lake levels, fumarolic changes, and minor mass wasting on the crater walls during January 2004 to March 2007 (see table 8 for a summary of lake changes).

Table 8. Changing lake conditions based on observations of Irazú's crater. Double asterisks indicate times when the lake disappeared; "--" fills cells where no data is available; lake levels are reported qualitatively except for the 7 October to 12 March 2010 time interval when absolute values were measured. This summary is based on ICE data and OVSICORI Monthly Reports.

On 22 July 2010 a team of investigators from Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) descended to the dry crater floor. They documented changes in vegetation, fumaroles, and clay deposition on the crater floor. Photos taken during prior trips provided comparisons with previous conditions (figure 14). Rockfalls and minor mass wasting had been occurring regularly and the long runout of debris across the crater floor was visible during this investigation. Most of the debris fell from the E and SW walls. On the NE side of the dry crater a rocky area emitted low temperature (24°C) sulfur-smelling gases from three aligned vents. Higher temperatures (86°C) were measured from fumaroles on the N side of the crater but they appeared to be releasing gas with less energy than observed in the past years when bubbles were visible within the lake. Another interesting finding was a waterfall on the inside of the crater on the SW wall; this small waterfall did not have sufficient volume to pool on the crater floor and instead soaked directly into the surrounding clay.

Figure 14. Views taken from Irazú's S rim. (top) The crater on 24 April 2004 contained a turquoise lake. (bottom) A repeat photo taken on 22 July 2010 shows the lake had disappeared; the former lake level and the clay base on the crater floor are marked. Since November 2010 water had accumulated and as of April 2011, was several meters deep. Courtesy of Eliecer Duarte, OVSICORI-UNA.

The water level in Irazú's crater has been variable throughout time; the Bulletin recorded a dry crater during February 1977 and June 1987 (SEAN 12:07), and April 1990 (BGVN 15:04). Factors highlighted during the IAVCEI CVL-7 ("Commission of Volcanic Lakes" Costa Rica, 10-19 March 2010) included complex connections with Turrialba, seasonal effects, infiltration within the crater, and the role of mass wasting. The mechanism for the recent disappearance of the lake is still under investigation by OVSICORI-UNA and ICE investigators (Guillermo Alvarado, personal communication).

Erosion. Mass wasting had been an ongoing process for at least 10 years. Material is primarily shed from the E and SW walls and the lake contained islands of black and red material formed from the debris. In February 2003 a major rockslide into the lake caused the water color to change from green to shades of red. An analysis of seismicity during that month showed no correlation to these slope failures (BGVN 28:12). Cracks along the NW rim formed and widened since December 2007; these cracks caused blocks up to 3 x 20 m to fall from the rim in March 2008.

Local gas measurements. Since the large phreatic explosion in December 1994 (BGVN 19:12), the NW fumarole has been releasing low gas emissions regularly. Different temperature measurements recorded since June 2010 ranged between 90°C to 86°C. To monitor changes in sulfur dioxide output from Irazú, a network of three stations collected rain samples from sites along the volcano's flanks.

The pH data from September 2004 through July 2010 were plotted in the OVSICORI-UNA July 2010 monthly report. The results correlate pH changes to much larger degassing events occurring at Turrialba, a neighboring volcano that began major degassing in 2007. Only the "Borde Sur" station was sampling continuously but the other two stations reflected similar trends in acidity. Despite irregular fluctuations, a decreasing pH trend began in 2007. The lowest point of the trend was measured by "Borde Este" at approximately pH 3.25. Where there "Pacayas" station data began, the trend appeared to have stabilized between pH 3.25 and 4.75.

References. D. Rouwet, R.A. Mora-Amador, C.J. Ramírez-Umaña, G. González, Seepage of "aggressive" fluids reduce volcano flank stability: the Irazú and Turrialba case, Costa Rica, Abstract, CVL 7 Workshop Costa Rica, IAVCEI-Commission of Volcanic Lakes, March 2010.

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. Duarte, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); G. Alvarado and G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San Jose, Costa Rica.

This is the first Bulletin report on Cerro Machín volcano, the site of seismic unrest for many years, most recently, 1992, 1999, 2002, 2004, 2008, 2009, and 2010. This activity did not lead to eruptions. Instrumental monitoring by INGEOMINAS began in 1987 and has determined Machín's background seismicity ranged from 1 to10 earthquakes/day, but during intervals of unrest, seismicity sometimes reached several hundred earthquakes per day.

This is a small but explosive volcano located at the S end of the Ruiz-Tolima massif, 185 km NNE of the Nevado del Huila volcano and 147 km WSW of Bogotá, the capital (figure 1). (Tolima volcano, not shown, lies ~22 km NNE of Machín.)

Figure 1. Map of Colombia showing the location of the Machín volcano. Note the Departments (states) of Tolima (1) and Huila (2) are shaded regions. Courtesy of the IFRC and Relief Web.

Machín caldera contains three dacitic domes; the 3-km-wide caldera is breached to the S. According to Mendez and others (2002), there have been six eruptions within the past 10,000 years. In the same report, the authors noted geomorphological similarities between Machín and Pinatubo prior to its large 1991 eruption. The seismic events have drawn increased attention to Machín from the Volcanic and Seismological Observatory of Manizales, Colombia Institute of Geology and Mining (INGEOMINAS).

According to news articles published in mid-May 2004, INGEOMINAS reported that there had been an increase in seismicity at Machín in April. About 60 earthquakes were recorded daily (in comparison to the 1-10 earthquakes normally recorded); however, no surface changes were seen at that time at the volcano.

There was no further significant seismic activity until the first week of January 2008 when INGEOMINAS reported unusual seismicity at Machín during 6-8 January. Long-period earthquakes were detected S of the main lava dome. On 7 January, the volcano-tectonic seismic signals were occasionally felt and reported by nearby residents. The simultaneous occurrence of both types of seismic signals was unusual for Machín. Again, the activity diminished to the previous background levels until 9 November when INGEOMINAS reported a cluster of ~375 earthquakes, the majority of which were located towards the E sector and below the dome of the volcano with depths between 2.5 and 5 km. The earthquake activity occurred underneath the central and E parts of the lava dome complex in the summit caldera and fumarolic activity in the area increased. During 8-10 November 2008, Machín registered 1,210 volcano-tectonic earthquakes, 9 of which were M 2.5. According to news articles, approximately 400-450 people evacuated to shelters or other safe areas. There were also reports of landslides that blocked a highway.

Table 1 and figure 2 detail the local villages in proximity to Machín.

Table 1. Villages in proximity to Machín and the respective distances from the caldera (approximate). Taken from web sources such as Google Earth.

Figure 2. A regional map showing population centers and paved and unpaved roads. Courtesy of INGEOMINAS.

On 10 November the seismic activity of the volcano diminished to background conditions. On 17 December INGEOMINAS reported that a swarm of 98 earthquakes occurred at Machín SE of the lava domes at depths of 2-6 km. The largest earthquake was M 2.6 at a depth of ~4 km.

There were two significant seismic events at Machín during 2009. On 31 July there was in increase in seismic activity, which consisted of ~200 events. Initially the increase was gradual, however, during the last hour the activity increased abruptly and included an earthquake of M 2.7. This subsided to a background level until early December when INGEOMINAS detected 54 earthquakes, some M ~ 1.3. Authorities issued a "Yellow" alert (Yellow; "changes in the behavior of volcanic activity") for Machín. The Tolima Regional Emergency Committee conducted evacuation training with local communities as a precaution.

INGEOMINAS reported that on 24 July 2010 a seismic crisis at Machín was characterized by volcano-tectonic earthquakes. An M 2.6 earthquake was located S of the main lava dome at a depth of ~4 km. The next day an M 4.1 volcano-tectonic earthquake occurred 0.8 km S of the main dome at a depth of ~3.9 km. The Yellow alert remained in effect following the increase in registered seismic activity in the area. On 29 July the number of volcano-tectonic events again increased; the earthquakes were a maximum M 1.7 and between 3 and 4 km depth, S of the main dome.

On 17 September 2010, INGEOMINAS again reported increased seismicity. About 140 volcano-tectonic earthquakes as large as M 1.85 were located S and SW of the main lava dome at depths of 2-4 km. On 4 October there was an M 3.5 tectonic earthquake located 0.37 km S of the main dome at a depth of ~4.14 km. Residents near the volcano felt this earthquake. The Alert Level remained at Yellow.

On 3 December 2010 about 340 volcano-tectonic earthquakes with low magnitudes were located SW of the main lava dome, at an average depth of 4 km. The largest event, a M 3.7 earthquake located SW of the dome at a depth of about 3.5 km, was felt by local residents. On 31 December INGEOMINAS reported a period of increased seismicity. A total of 346 volcano-tectonic events no stronger than M 2.1 were located S and SW of the main lava dome.

On 1 January 2011 seismicity again increased, and at the time of the report, 367 events had been detected. The low-magnitude events were located S and SW of the main dome at depths between 2.5 and 4.5 km. The largest event, M 2.3, was located S of the dome at a depth of about 3.3 km and felt by residents near the volcano and in the municipality of Cajamarca, 8 km SSW. On 13 January an increased number of earthquakes were located to the W and SW of the main dome at depth of 2.5-3.5 km. The largest event registered M ~2.6 and was reported to have been felt by residents near the volcano.

Since 1989, INGEOMINAS noted a gradual increase in seismicity has been following the events closely in order to report any changes on the volcano's activities (figure 3). All the local emergency committees were activated in the area near Machín volcano in addition to the regional emergency committees in Tolima District.

Figure 3. Map showing potential hazards from hypothetical future activity at Machín. Thicknesses of potential ash fall to the W are shown (in cm) as modeled by computer-aided dispersion modeling (VAFTAD); PF stands for pyroclastic flow deposits. Adaped from INGEOMINAS (2007).

References. Méndez, RA; Cortés, GP; and Cepeda, H; [Calvache, ML, Project Chief], 2002, Evaluacíon de la Amenaza Volcánica Potencial del Cerro Machín (Departamento del Tolima, Colombia), Manizales, Sept. 2002, INGEOMINAS, 66 p. (in Spanish).

Méndez, RA, Cortés, GP, and Cepeda, H., 2007, Evaluacíon amenazas potencial de volcan Cerro Machín [Large map in Spanish taken from 2002 report of same name. Name in English, 'Evaluation of potenial hazards from volcan Cerro Machín'] Mapa Amenaza Volcán Machín, INGEOMINAS (URL: http://intranet.ingeominas.gov.co/manizales/images/5/55/MAPA_AMENAZA_VOLCAN_MACHIN.jpg)

Geologic Background. The small Cerro Machín stratovolcano lies at the southern end of the Ruiz-Tolima massif about 20 km WNW of the city of Ibagué. A 3-km-wide caldera is breached to the south and contains three forested dacitic lava domes. Voluminous pyroclastic flows traveled up to 40 km away during eruptions in the mid-to-late Holocene, perhaps associated with formation of the caldera. Late-Holocene eruptions produced dacitic block-and-ash flows that traveled through the breach in the caldera rim to the west and south. The latest known eruption of took place about 800 years ago.

Information Contacts: Instituto Colombiano de Geologia y Mineria (INGEOMINAS), Observatorio Vulcanológico y Sismológico de Manizales, Manizales, Colombia; Relief Web (URL: https://reliefweb.int/); International Federation of Red Cross And Red Crescent Societies (IFRC) (URL: http://www.ifrc.org/); Caracol Radio; El Tiempo:Portafolio (URL: http://columbiareports.com).

Occasional, typically minor phreatic eruptions occurred at Poás through at least early February 2011 (BGVN 35:12). They emerged from the active crater lake, Lago Caliente. The Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA) illuminated intervals of phreatic eruptions and relations on the chemistry of Lago Caliente's waters over a period of more than 30 years (figure 94). This report includes photos of phreatic eruptions in 2009, 2010, and early 2011, and reviews events through March 2011.

Figure 94. Plots of the sulfur, chlorine, and fluorine concentrations, as well as the temperature, pH, and gas volumes in the Lago Caliente waters at Poás, with respect to time. The data on the time axis extends from early 1978 to late 2009. Arrows along the top indicate periods with frequent phreatic eruptions. Notice the low pH, often well below pH 1.5. Courtesy of OVSICORI-UNA.

Volcanic gases and associated condensate and rainfall led to increasing areal extent and degree of damage to vegetation in nearby areas. In studying the Lago Caliente's waters, Martinez and others (2011) found in solution a variety of oxo-anions of sulfur called polythionates (S_nO₆^-2, where n can be 20 or larger), which they found to vary in concentration from undetectable to 8,000 mg/L. They considered polythionates to be "highly relevant for monitoring purposes at Poás, in particular because they may signal impending phreatic eruptions."

More on the 25 December 2009 phreatic eruption. A previous report (BGVN 35:12) discussed a phreatic eruption on 25 December 2009 but some further comments are worth adding. As previously noted (BGVN 35:12), "Steam and lake water mixed with sediment and blocks were ejected 550-600 m above Laguna Caliente and fell in the vicinity of the lake, within the crater." No mention was previously made of a 24 December 2009 phreatic eruption discussed by OVSICORI-UNA. It took place in the morning at 0808 and all erupted material fell back in the crater.

Photos taken on 25 December 2009 and recently posted on the Picasa website have come to our attention. The four photos on figure 95 come from a set of nine taken from the S rim. The earliest of the set depict a very tranquil lake with steaming at or near the dome (not shown here). The next photo, taken 129 seconds after that tranquil scene, portrays the advancing eruption (figure 95a). The subsequent two photos (figure 95b and c) captured the interval closest to the peak of the eruptive vigor.

Figure 95. Four sequential photos taken looking N at Poás of a phreatic eruption from the center of Lago Caliente on 25 December 2009. The time intervals between the four photos was as follows: photos (a) to (b), 5 sec; photos (b) to (c), 5 sec; and photos (c) to (d), 11 sec. Photo descriptions below: (a) The earliest available photo of the eruption cloud, which, based on the next photo in this set, was clearly still emerging energetically. It advanced with the leading portions of the plume chiefly dark. At the plume's base, white steam clouds mask the lake. (b and c) The shots taken closest to the maximum point of the eruption's thrust phase, with dark material still conspicuous. White tufts expanded and began to cap most of the advancing jets. The clouds engulfing the base of the plume now contain more discolored zones. (d) As the plume evolves and the vigorous exhalative part of the eruption ends or wanes, a steam-rich cloud envelops the eruption cloud. Note the gray-colored rain falling out of the plume. Taken from Cindy and JM's Gallery (undated) on the Picasa photo sharing website (see References and Information Contacts below).

An exact assessment of the photos is complicated by several factors. There were shifts in the focal length of the lens (documented in camera metadata found on the website). Also, in detail, the camera's time record indicated 0252 hrs, clearly incorrect for this daylight scene. That problem is reconciled by a photo featured in the OVSICORI-UNA report, which showed a plume photo by another photographer at a stage nearly identical to figure 95b and the text indicated the eruption occurred at 0952 hrs local time.

An email response from Cindy Doire provided these comments about witnessing the phreatic eruption.

"We arrived at the volcano early in the morning. We were one of the first to arrive that day. Our group and a few other tourists were looking at it and NOTHING was happening. The people finished looking and started leaving that spot. It was just about 4 of us still there, when suddenly the volcano started to erupt. There was NO warning at all. Even the rangers were surprised. At the beginning, white steam (gas?) shot up, then black rock and dirt started exploding out. I believe that everything that shot up, fell back into the crater . . . the gas could be smelled and was strong . . .."

In an email to GVP regarding the 25 December 2009 eruption, Eliecer Duarte commented: "It seems that this [25 December 2009] eruption opened a more permanent vent at the bottom of the lake. Since that event the frequency of phreatic ones increased and remained like this for [a] year and a half. We still have dozens of smaller ones daily.

More on crater degassing. Field visits during 2010 and 2011 allowed scientists to see the expanding effects of Poás volcanic gases on vegetation (figures 96 and 97). Dry conditions resulted in winds carrying the gases considerable distances from the volcano. The area most affected was an elongate zone downwind of the active crater and extending ~4 km SW. Figure 97 portrays transitional zones with intermediate effects.

Figure 96. A commercial airline pilot and amateur photographer took this and other photos of Poás on 28 April 2010. The active crater and its discolored lake (Lago Caliente) reside at the right-hand side of this shot. It is part of an elongate zone of barren rock stretching ~4 km across the otherwise lushly vegetated landscape. As is typical, the plume's orientation on this day lies directly over the barren zone. From "Len" (undated), (see Reference below).

Figure 97. Oblique view highlighting the area to the S of Poás (note volcano's crater lakes, including the active "Lago Caliente") On color versions of this figure, the pink rhombuses show sites for collecting acid rain. Providencia is shown in the lower left. The crater lake at upper right, "Botos" is ~0.5 km across in the long direction but the scale on this image varies with distance towards the foreground. Courtesy E. Duarte, OVSICORI-UNA.

Starting just beyond the elongate zone of harsh effects, the areas of discolored vegetation had increased impact and areal extent. One such impacted area was a nature preserve called Providencia, which is seen in figure 97 to the left of Poás. Farther from the volcano lies Cerro Pelón (2.5 km distance and direction SW of the crater) , which also showed the effects of chemical burning from volcanic gases (figure 97).

In the past, activity centers have migrated within the crater. OVSICORI-UNA reported that, for at least the past year (ending March 2011), the points of degassing have been concentrated in the hot crater lake and dome (figure 98). The emanating steam and gases, often carried by wind, have affected areas up to several hundred meters around the crater (figures 96-98).

Figure 98. The active crater at Poás, showing pronounced steam release both from fractures in the dome as well as from the lake's surface. Conditions like this (with more or less steam) often prevailed in recent times (including just a few seconds prior to the eruption sequence shown in figure 95). The crater lake (Lago Caliente) rests behind (N of) the dome and steam clouds. Courtesy E. Duarte, OVSICORI-UNA.

OVSICORI-UNA reported that through at least March 2011 small phreatic eruptions occurred daily at Lago Caliente. These eruptions sometimes only reached the lake's surface, but at other times reached a few meters above the lake, and occasionally, tens of meters above the lake. The majority of the erupted sediments fell back into the lake. The fine sediments sometimes remained suspended in the lake water and caused its gray color. The majority of eruptions occurred in the central part of the crater, with a few originating slightly more to the N or S of the center. Because of the phreatic activity and high temperature of the lake (57°C), strong evaporation occurred and plumes traveled long distances in the wind (figure 99).

Figure 99. At Poás, a phreatic eruption at Lago Caliente reaching several meters high, in a manner typical of daily activity during recent months. View from the active crater's N side (opposite the viewpoint). Photo taken sometime in January 2011. Courtesy E. Duarte, OVSICORI-UNA.

A comparison of vegetation in the area between Cerro Pelón and Providencia (designated "F1" in figure 97) made during August 2010 to January 2011 found that most plant species were resistant at certain levels of acidification. However, when their tolerance thresholds were reached, the affected species decayed quickly and were sometimes unable to recover. Certain species, including eucalyptus, pine, alder, and cypress, were particularly sensitive to the volcanic gases. Minor effects from gases were observed on Cypress trees as far as 9 km SW of the emission source. OVSICORI-UNA reports contained several photos showing more details on the effects of acidic gases on vegetation. One of their later reports, from April 2011, discussed ongoing phreatic eruptions and dome temperature of 560°C.

References. Cindy and JM's Gallery, undated, "Poas volcano eruption, December 25th, 2009" [9 photos] Picassa (URL: https://picasaweb.google.com/cjmdoire); [includes camera-related metadata].

Len (Barfbag), undated, "Wednesday, April 28, 2010, Mt Poas, Costa Rica" ; in Viewsfrom the left seat, A look at the airline world ... ride along in the cockpit (URL: http://viewsfromtheleftseat.blogspot.com/2010/04/mt-poas-costa-rica.html)

Martínez, M., van Bergen, M.J., Fernández, E., and Takano, B., 2011, Polythionates monitoring at the acid crater lake of Poás Volcano, IAVCEI-COMMISSION OF VOLCANIC LAKES, CVL7 Workshop, Costa Rica, 10-19 March 2010, Online Abstracts volume (May 2011), p. 12 (URL: http://www.ulb.ac.be/sciences/cvl/)

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

Information Contacts: E. Duarte and E. Fernández, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Cindy Doire (address withheld by request).

This report on Ranau, a Pleistocene caldera that lies along the Great Sumatran fault, is based on accounts of fish kills, including one on 4 April 2011. The fish died near hot springs in Lake Ranau, a large caldera lake, and their deaths were attributed to seismically induced H₂S releases by the Center of Volcanology and Geological Hazard Mitigation (CVGHM). CVGHM reported the surface area of Lake Ranau to be ~127 km², and noted that the Lake Ranau complex is geothermally active, with hot springs that emerge at the foot of Mount Seminung on the banks of Lake Ranau. In addition to the 2011 event, fish kills have been recorded in Lake Ranau (figure 1) for the past five decades (table 1).

Figure 1. Photo of Lake Ranau with Mount Seminung in the background. Posted by blogger "masternewstoday" in May 2011.

Table 1. Previous fish kills in Lake Ranau reported during the past five decades. (Note that there is no mention of any correlation between seismicity and geochemical anomalies.) Courtesy of CVGHM.

Reports stated that the 4 April 2011 fish kill was large in scale. According to the head of a nearby village, Sugih Sane, the event began with turbulent water in Lake Ranau that lasted for approximately 30 minutes. Local residents reported that the fish kill occurred during a relatively short time in portions of the lake surrounding hot springs. At the time of the incident, the water in the affected areas appeared milky white, and wind spread the smell of sulfur to surrounding areas.

Geochemistry. Scientists conducted field work near the three hot springs Kota Batu, Ujung, and Way Wahid during 16-19 April 2011. At that time they reported the following: No dead algae were found on the lake's surface. There was no smell of sulfur, the water was clear, and the water around the hot springs was bubbling and warm. * Dead fish were no longer present. The pH of the lake water was 7.74, and the temperature was 26.1°C. The water near the hot springs had a pH of 6.32-7.06, with a temperature of 47.8-62°C. The water of the river that empties into Lake Ranau (input) had a pH of 8.07-8.10, and the lake water discharge (output) had a pH of 7.86. The result of ambient gas examination showed no gases associated with magmatic gases, such as CH₄, CO₂, CO, and H₂S, in the vicinity of the hot springs discharge. The degree to which the above measurements were anomalous was unstated.

Seismicity. Seismic data recorded during 16-20 April 2011 showed microearthquake activity around Lake Ranau. The earthquakes were located along a fault line oriented in the SE-NW direction along Lake Ranau, at depths of 0.6 and 10 km below the surface of the lake. The Berkelulusan location coincides with the location of the Kota Batu hot springs. Prior to the fish kill at Lake Ranau on 4 April, an M 5.1 earthquake was recorded on 29 March 2011 in Bengkulu, ~160 km W of Lake Ranau.

Cause of the fish kill. CVGHM concluded that, based on the results of the field work (location of dead fish near hot springs, sulfur smell carried by wind up to 3 km away, absence of dead algae, and changing color of the lake water to milky white during the event), the fish kill in Lake Ranau was caused by the release of H₂S gas into the lake water, which caused imbalances in lake water chemistry. They said that hydrothermal gas was trapped over time and escaped to the surface after the pressure due to tectonic disturbances. CVGHM concluded that the M 5.1 earthquake in Bengkulu on 29 March 2011 led to increased pressure on the fault in the vicinity of Lake Ranau; then, H₂S gas was released to the surface in the vicinity of the hot springs. According to CVGHM, the occurrence of microearthquakes is a result of the fault in the vicinity of Lake Ranau, and are neither dangerous nor destructive. However, CVGHM asked residents to report future fish kills to the local government.

Geologic Background. Ranau is an 8 x 13 km Pleistocene caldera partially filled by the crescent-shaped Lake Ranau. The caldera lies along the Great Sumatran Fault that extends the length of Sumatra. Incremental formation of the caldera culminated in the eruption of the voluminous Ranau Tuff about 0.55 million years ago. A morphologically young post-caldera stratovolcano, Gunung Semuning, was constructed within the SE side of the caldera to a height of more than 1,200 m above the lake surface. The volcano has not been mapped in sufficient detail to determine the age of its latest eruptions, although fish kills and sulfur smells in the late 19th and early 20th centuries may be related to volcanism.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); Masternewstoday (URL: http://hot-breaking-news-masternewstoday.blogspot.com).

Low-frequency earthquakes and tremor were reported at Rincón de la Vieja during the first half of 2008 (BGVN 33:07). Since then, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) had issued intermittent reports of activity through January 2011. Those reports are summarized in the following sections, with much of the discussion centered around fumaroles and behavior of the geothermally warmed lake in the active crater. Occasional, typically small phreatic eruptions had occurred here in past years, for example in the 1990s (eg., BGVN 21:02, 21:03, 22:01, and 23:03) but were absent in the current reporting interval (last half of 2008 through January 2011).

August 2008. OVSICORI-UNA reported that the level of the lake was at a high level, with a bluish color, generated convection cells with evaporation, and had sulfur particles visible on it's surface. Sulfur deposition and fumarolic activity continued along the SW wall.

March 2009. In mid-March 2009, scientists visited the S and SW flank, collected samples, and noted some temperatures of 75-78°C. Because the visit occurred during the dry season, most areas encountered were dry. The scientists examined an area of acidification to the W of Von Seebach crater, ~3 km SW of the active crater. Strong winds common in that direction sometimes carried volcanic gases. Consequently, most of this narrow expanse only contained patches of grassland and shrubs that barely covered the rocky surface.

October 2009. OVSICORI-UNA reported that seismographic station RIN3, located ~5 km SW of the main crater, registered volcano-tectonic events and tremor lasting for minutes.

Weak ongoing fumarolic activity during 2010 through January 2011. OVSICORI-UNA reported that the level of the crater lake remained high during 2010, with constant evaporation. Geochemical, seismic, and deformation data did not show significant changes in physico-chemical parameters during 2010. The changing color of the lake, from blue to gray, was attributed to intense rains and fumarolic activity in the crater.

Later reporting. Reports during 2010 through at least January 2011 described fumarolic activity along the S and SW walls of the crater, with sulfur deposition and moderate gas discharge. The lake remained a gray color, with sulfur particles in suspension. Figure 15 shows a photo taken in April of the crater looking at the SW wall with fumarolic activity along with sulfur deposition. In April 2010, OVSICORI-UNA reported that the temperature of the lake was 49°C. A fumarole sometimes seen active along the N flank had stopped discharging gas.

Figure 15. Photo of the active crater lake of Rincón de la Vieja on 29 April 2010 showing yellow sulfur deposits and fumarolic activity along the SW wall of the crater. This kind of activity was typical throughout the reporting interval (last half of 2008 through January 2011). Photo by E. Fernandez, OVSICORI-UNA.

OVSICORI-UNA reported that 2010 was unusual in that four domestic volcanoes were active: Arenal, Poás, Turrialba, and Rincón de la Vieja. Irazú was comparatively inactive (see separate report in this issue of the Bulletin).

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

Information Contacts: E. Fernández, W. Sáenz, E. Duarte, M. Martínez, S. Miranda, F. Robichaud, T. Marino, M. Villegas, and J. Barquero, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

This report first describes activity seen at Shiveluch during December 2010-March 2011. Data from that interval included several ash plumes visible as they blew to over 100 km from the volcano. Thermal imagery analysis showed the character of the dome and the path of pyroclastic-flow deposits during that interval. After that, we provide a follow-up to the 27 October 2010 eruption (BGVN 35:11), adding some previously unmentioned details. That eruption destroyed the dome's SE sector and generated pyroclastic flows.

During December 2010-March 2011, KVERT reported that Shiveluch both underwent moderate seismicity and emitted bright thermal anomalies conspicuous in satellite imagery (figure 27). Details of significant explosions and ash plumes during that time appear on table 10. Figure 28 shows a photo with the distant skyline dominated by a long Shiveluch ash plume.

Table 10. An inexhaustive synopsis of significant plumes at Shiveluch visible on satellite imagery from December 2010 through 26 March 2011 (times and dates are UTC). Courtesy KVERT.

Figure 27. Satellite thermal anomalies recorded at Shiveluch during December 2010-March 2011. Data from KB GS RAS, with cooperation from Alaska Volcano Observatory (AVO).

Figure 28. A panoramic photo showing a long ash plume from Shiveluch, seen in the distant parts of the photo (volcano is on the left). Photo taken on 24 February 2011 from N slope of Kliuchevskoi volcano by Yuri Demyanchuk.

More on the 27 October 2010PFs. As previously reported, an explosive eruption on 27 October 2010 (BGVN 35:11) vented at the dome and destroyed its SE portion, generating pyroclastic flows laden with many fragments of dome material (figure 29). The associated eruptive plume extended more than 1,500 km from the volcano. The pyroclastic flows traveled SSE in a radial direction, as far as 20 km from the source.

Figure 29. Two images showing the lava dome of Shiveluch. Photo (a) was taken before the eruption, on 7 October 2010. Photo (b) was taken a few days after the eruption, on 2 November 2010 and discloses enormous losses to the mass of the dome toward the SE (free face). The large ash clouds from the dome document ongoing explosions, processes associated with continued rebuilding of the lava dome. Both photos courtesy of Yuri Demyanchuk.

Near the dome, visiting scientists found agglomerate deposits of fragmental dome material spread widely down the SE slope. The character of the deposits was similar to debris avalanches, since so much dome material suddenly traveled down slope. The pyroclastic flow deposits retraced numerous upslope tributaries along the Kabeku River. The deposits filled small valleys and other low-lying areas, leveling landscapes that had prior to the eruption been rough (figure 30).

Figure 30. Photo showing the fresh pyroclastic flow deposits filling Bekesh river valley to the point where the valley had become nearly flat in transverse profile. In the background appears the steaming, Shiveluch with its recently broken lava dome. Photo taken 2 November 2010 by Alexander Ovsyannikov.

Figures 31a and b, satellite images, illustrate the trail of hot material descending to the S. They formed a large, complex, and widely distributed deposit following the recent collapse of the lava dome. A sub-circular area about ~4 km in diameter at about 9-14 km distance from the dome may reflect denser deposition (figure 31a). The images make clear that pyroclastic flow deposits descended yet farther, leaving dense, thermally radiant tracks over narrower valleys trending to the SE. The images are from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer). Figure 31b shows the flow's heat signature as measured in thermal infrared energy. The white area at the lava dome was very hot, while the red areas on the edge of the flow were merely warmer than the surrounding snow.

Figure 31. (a) False-color ASTER satellite image of Shiveluch showing the visible-wavelength information that discloses the remnants of the 27 October 2010 pyroclastic flow. Image taken 25 February 2011. (b) The hot pyroclastic flow appears in this ASTER image made using thermal infrared wave lengths. The white area at the lava dome is very hot, while the red areas on the edge of the flow are simply warmer than the surrounding snow. Image taken on 25 January 2011. Courtesy of NASA Earth Observatory.

Fieldwork in the distal area revealed that the most powerful pyroclastic flow went into the headwaters of two narrow valleys, then merged into a single stream down into the Kabeku Valley river almost to its confluence with the Bekesh river (5 km N of the Kluchi-Ust'-Kamchatsk road, figures 32 and 33).

Figure 32. Images (a) and (b) show Shiveluch deposits of pyroclastic flows in the Bekesh river valley. Note person in distance in center of photo for scale. Courtesy Yuri Demyanchuk and Alexander Manevich.

Figure 33. Results of pyroclastic surges, with small trees and shrubs knocked over and stripped of bark. Trees and shrubs showed signs of scorching up to 3-4 m high. Deposits of pyroclastic surges were found on the sides of the Bekesh river valley. Image taken 2 November 2010. Courtesy of Yuri Demyanchuk.

Water in the bed of the Bekesh river ran down the same path as thick pyroclastic flows and continued to be fed by melting snow on the upper slopes. Water also seeped through the loose pyroclastic flow deposit, resulting in large amounts of steam escaping at the surface in the form of fumaroles, degassing pipes, and zones of jetting emissions. This created the impression that the river water was boiling; on its surface rose a wall of steam (figure 34). Walking over the pyroclastic flow deposit was difficult and potentially dangerous, since the deposit's upper portion remained hot and gas saturated (figure 34b).

Figure 34. At Shiveluch, fresh pyroclastic-flow deposits occurring on the Bekesh river. (a) Steam and gas pervade the atmosphere as the river makes its way across the fresh pyroclastic-flow deposits. (b) The still-hot deposits emitting abundant steam and gas. Photos courtesy of Yuri Demyanchuk.

Reference. Ovsyannikov, A., Manevich, A., 2010, Eruption Shiveluch in October 2010, Bulletin of Kamchatka Regional Association (Educational-Scientific Center); Earth Sciences (in Russian), IV&S FEB RAS, Petropavlovsk-Kamchatsky, 2010, vol. 2, no. 16, ISSN 1816-5532 (Online).

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: Kamchatka Volcanic Eruptions Response Team (KVERT), Institute Volcanolohy and Seismology Far East Division, Russian Academy of Sciences (IVS FED RAS), Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences (KB GS RAS) (URL: http://www.emsd.iks.ru/index-e.php). 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/); Y. Demyanchuk, A. Ovsyannikov, A. Manevich (IVS FED RAS); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/); Tokyo Volcanic Ash Advisory Centre (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/).

In discussing the week ending on 12 September, "Earthweek" (Newman, 1997) incorrectly claimed that a volcano named "Mount Pinukis" had erupted. Widely read in the US, the dramatic Earthweek report described terrified farmers and a black mushroom cloud that resembled a nuclear explosion. The mountain's location was given as "200 km E of Zamboanga City," a spot well into the sea. The purported eruption had received mention in a Manila Bulletin newspaper report nine days earlier, on 4 September. Their comparatively understated report said that a local police director had disclosed that residents had seen a dormant volcano showing signs of activity.

In response to these news reports Emmanuel Ramos of the Philippine Institute of Volcanology and Seismology (PHIVOLCS) sent a reply on 17 September. PHIVOLCS staff had initially heard that there were some 12 alleged families who fled the mountain and sought shelter in the lowlands. A PHIVOLCS investigation team later found that the reported "families" were actually individuals seeking respite from some politically motivated harassment. The story seems to have stemmed from a local gold rush and an influential politician who wanted to use volcanism as a ploy to exclude residents. PHIVOLCS concluded that no volcanic activity had occurred. They also added that this finding disappointed local politicians but was much welcomed by the residents.

PHIVOLCS spelled the mountain's name as "Pinokis" and from their report it seems that it might be an inactive volcano. There is no known Holocene volcano with a similar name (Simkin and Siebert, 1994). No similar names (Pinokis, Pinukis, Pinakis, etc.) were found listed in the National Imagery and Mapping Agency GEOnet Names Server (http://geonames.nga.mil/gns/html/index.html), a searchable database of 3.3 million non-US geographic-feature names.

The Manila Bulletin report suggested that Pinokis resides on the Zamboanga Peninsula. The Peninsula lies on Mindanao Island's extreme W side where it bounds the Moro Gulf, an arm of the Celebes Sea. The mountainous Peninsula trends NNE-SSW and contains peaks with summit elevations near 1,300 m. Zamboanga City sits at the extreme end of the Peninsula and operates both a major seaport and an international airport.

[Later investigation found that Mt. Pinokis is located in the Lison Valley on the Zamboanga Peninsula, about 170 km NE of Zamboanga City and 30 km NW of Pagadian City. It is adjacent to the two peaks of the Susong Dalaga (Maiden's Breast) and near Mt. Sugarloaf.]

References. Newman, S., 1997, Earthweek, a diary of the planet (week ending 12 September): syndicated newspaper column (URL: http://www.earthweek.com/).

Manila Bulletin, 4 Sept. 1997, Dante's Peak (URL: http://www.mb.com.ph/).

Simkin, T., and Siebert, L., 1994, Volcanoes of the world, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Information Contacts: Emmanuel G. Ramos, Deputy Director, Philippine Institute of Volcanology and Seismology, Department of Science and Technology, PHIVOLCS Building, C. P. Garcia Ave., University of the Philippines, Diliman campus, Quezon City, Philippines.

Xinhua News Agency filed a news report on 27 February under the headline "Volcano erupts in Somalia" but the veracity of the story now appears doubtful. The report disclosed the volcano's location as on the W side of the Gedo region, an area along the Ethiopian border just NE of Kenya. The report had relied on the commissioner of the town of Bohol Garas (a settlement described as 40 km NE of the main Al-Itihad headquarters of Luq town) and some or all of the information was relayed by journalists through VHF radio. The report claimed the disaster "wounded six herdsmen" and "claimed the lives of 290 goats grazing near the mountain when the incident took place." Further descriptions included such statements as "the volcano which erupted two days ago [25 February] has melted down the rocks and sand and spread . . . ."

Giday WoldeGabriel returned from three weeks of geological fieldwork in SW Ethiopia, near the Kenyan border, on 25 August. During his time there he inquired of many people, including geologists, if they had heard of a Somalian eruption in the Gedo area; no one had heard of the event. WoldeGabriel stated that he felt the news report could have described an old mine or bomb exploding. Heavy fighting took place in the Gedo region during the Ethio-Somalian war of 1977. Somalia lacks an embassy in Washington DC; when asked during late August, Ayalaw Yiman, an Ethiopian embassy staff member in Washington DC also lacked any knowledge of a Somalian eruption.

A Somalian eruption would be significant since the closest known Holocene volcanoes occur in the central Ethiopian segment of the East African rift system S of Addis Ababa, ~500 km NW of the Gedo area. These Ethiopian rift volcanoes include volcanic fields, shield volcanoes, cinder cones, and stratovolcanoes.

Information Contacts: Xinhua News Agency, 5 Sharp Street West, Wanchai, Hong Kong; Giday WoldeGabriel, EES-1/MS D462, Geology-Geochemistry Group, Los Alamos National Laboratory, Los Alamos, NM 87545; Ayalaw Yiman, Ethiopian Embassy, 2134 Kalorama Rd. NW, Washington DC 20008.

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Information Contacts: Erol Erkmen, Tuvpo Project Alp.

In December 2002 information appeared in Mongolian and Russian newspapers and on national TV that a volcano in Central Mongolia, the Har-Togoo volcano, was producing white vapors and constant acoustic noise. Because of the potential hazard posed to two nearby settlements, mainly with regard to potential blocking of rivers, the Director of the Research Center of Astronomy and Geophysics of the Mongolian Academy of Sciences, Dr. Bekhtur, organized a scientific expedition to the volcano on 19-20 March 2003. The scientific team also included M. Ulziibat, seismologist from the same Research Center, M. Ganzorig, the Director of the Institute of Informatics, and A. Ivanov from the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences.

Geological setting. The Miocene Har-Togoo shield volcano is situated on top of a vast volcanic plateau (figure 1). The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Pliocene and Quaternary volcanic rocks are also abundant in the vicinity of the Holocene volcanoes (Devyatkin and Smelov, 1979; Logatchev and others, 1982). Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Figure 1. Photograph of the Har-Togoo volcano viewed from west, March 2003. Courtesy of Alexei Ivanov.

Observations during March 2003. The name of the volcano in the Mongolian language means "black-pot" and through questioning of the local inhabitants, it was learned that there is a local myth that a dragon lived in the volcano. The local inhabitants also mentioned that marmots, previously abundant in the area, began to migrate westwards five years ago; they are now practically absent from the area.

Acoustic noise and venting of colorless warm gas from a small hole near the summit were noticed in October 2002 by local residents. In December 2002, while snow lay on the ground, the hole was clearly visible to local visitors, and a second hole could be seen a few meters away; it is unclear whether or not white vapors were noticed on this occasion. During the inspection in March 2003 a third hole was seen. The second hole is located within a 3 x 3 m outcrop of cinder and pumice (figure 2) whereas the first and the third holes are located within massive basalts. When close to the holes, constant noise resembled a rapid river heard from afar. The second hole was covered with plastic sheeting fixed at the margins, but the plastic was blown off within 2-3 seconds. Gas from the second hole was sampled in a mechanically pumped glass sampler. Analysis by gas chromatography, performed a week later at the Institute of the Earth's Crust, showed that nitrogen and atmospheric air were the major constituents.

Figure 2. Photograph of the second hole sampled at Har-Togoo, with hammer for scale, March 2003. Courtesy of Alexei Ivanov.

The temperature of the gas at the first, second, and third holes was +1.1, +1.4, and +2.7°C, respectively, while air temperature was -4.6 to -4.7°C (measured on 19 March 2003). Repeated measurements of the temperatures on the next day gave values of +1.1, +0.8, and -6.0°C at the first, second, and third holes, respectively. Air temperature was -9.4°C. To avoid bias due to direct heating from sunlight the measurements were performed under shadow. All measurements were done with Chechtemp2 digital thermometer with precision of ± 0.1°C and accuracy ± 0.3°C.

Inside the mouth of the first hole was 4-10-cm-thick ice with suspended gas bubbles (figure 5). The ice and snow were sampled in plastic bottles, melted, and tested for pH and Eh with digital meters. The pH-meter was calibrated by Horiba Ltd (Kyoto, Japan) standard solutions 4 and 7. Water from melted ice appeared to be slightly acidic (pH 6.52) in comparison to water of melted snow (pH 7.04). Both pH values were within neutral solution values. No prominent difference in Eh (108 and 117 for ice and snow, respectively) was revealed.

Two digital short-period three-component stations were installed on top of Har-Togoo, one 50 m from the degassing holes and one in a remote area on basement rocks, for monitoring during 19-20 March 2003. Every hour 1-3 microseismic events with magnitude <2 were recorded. All seismic events were virtually identical and resembled A-type volcano-tectonic earthquakes (figure 6). Arrival difference between S and P waves were around 0.06-0.3 seconds for the Har-Togoo station and 0.1-1.5 seconds for the remote station. Assuming that the Har-Togoo station was located in the epicentral zone, the events were located at ~1-3 km depth. Seismic episodes similar to volcanic tremors were also recorded (figure 3).

Figure 3. Examples of an A-type volcano-tectonic earthquake and volcanic tremor episodes recorded at the Har-Togoo station on 19 March 2003. Courtesy of Alexei Ivanov.

Conclusions. The abnormal thermal and seismic activities could be the result of either hydrothermal or volcanic processes. This activity could have started in the fall of 2002 when they were directly observed for the first time, or possibly up to five years earlier when marmots started migrating from the area. Further studies are planned to investigate the cause of the fumarolic and seismic activities.

At the end of a second visit in early July, gas venting had stopped, but seismicity was continuing. In August there will be a workshop on Russian-Mongolian cooperation between Institutions of the Russian and Mongolian Academies of Sciences (held in Ulan-Bator, Mongolia), where the work being done on this volcano will be presented.

References. Devyatkin, E.V. and Smelov, S.B., 1979, Position of basalts in sequence of Cenozoic sediments of Mongolia: Izvestiya USSR Academy of Sciences, geological series, no. 1, p. 16-29. (In Russian).

Logatchev, N.A., Devyatkin, E.V., Malaeva, E.M., and others, 1982, Cenozoic deposits of Taryat basin and Chulutu river valley (Central Hangai): Izvestiya USSR Academy of Sciences, geological series, no. 8, p. 76-86. (In Russian).

Geologic Background. The Miocene Har-Togoo shield volcano, also known as Togoo Tologoy, is situated on top of a vast volcanic plateau. The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Information Contacts: Alexei V. Ivanov, Institute of the Earth Crust SB, Russian Academy of Sciences, Irkutsk, Russia; Bekhtur andM. Ulziibat, Research Center of Astronomy and Geophysics, Mongolian Academy of Sciences, Ulan-Bator, Mongolia; M. Ganzorig, Institute of Informatics MAS, Ulan-Bator, Mongolia.

An eruption at Mount Elgon was mistakenly inferred when fumes escaped from this otherwise quiet volcano. The fumes were eventually traced to dung burning in a lava-tube cave. The cave is home to, or visited by, wildlife ranging from bats to elephants. Mt. Elgon (Ol Doinyo Ilgoon) is a stratovolcano on the SW margin of a 13 x 16 km caldera that straddles the Uganda-Kenya border 140 km NE of the N shore of Lake Victoria. No eruptions are known in the historical record or in the Holocene.

On 7 September 2004 the web site of the Kenyan newspaper The Daily Nation reported that villagers sighted and smelled noxious fumes from a cave on the flank of Mt. Elgon during August 2005. The villagers' concerns were taken quite seriously by both nations, to the extent that evacuation of nearby villages was considered.

The Daily Nation article added that shortly after the villagers' reports, Moses Masibo, Kenya's Western Province geology officer visited the cave, confirmed the villagers observations, and added that the temperature in the cave was 170°C. He recommended that nearby villagers move to safer locations. Masibo and Silas Simiyu of KenGens geothermal department collected ashes from the cave for testing.

Gerald Ernst reported on 19 September 2004 that he spoke with two local geologists involved with the Elgon crisis from the Geology Department of the University of Nairobi (Jiromo campus): Professor Nyambok and Zacharia Kuria (the former is a senior scientist who was unable to go in the field; the latter is a junior scientist who visited the site). According to Ernst their interpretation is that somebody set fire to bat guano in one of the caves. The fire was intense and probably explains the vigorous fuming, high temperatures, and suffocated animals. The event was also accompanied by emissions of gases with an ammonia odor. Ernst noted that this was not surprising considering the high nitrogen content of guano—ammonia is highly toxic and can also explain the animal deaths. The intense fumes initially caused substantial panic in the area.

It was Ernst's understanding that the authorities ordered evacuations while awaiting a report from local scientists, but that people returned before the report reached the authorities. The fire presumably prompted the response of local authorities who then urged the University geologists to analyze the situation. By the time geologists arrived, the fuming had ceased, or nearly so. The residue left by the fire and other observations led them to conclude that nothing remotely related to a volcanic eruption had occurred.

However, the incident emphasized the problem due to lack of a seismic station to monitor tectonic activity related to a local triple junction associated with the rift valley or volcanic seismicity. In response, one seismic station was moved from S Kenya to the area of Mt. Elgon so that local seismicity can be monitored in the future.

Information Contacts: Gerald Ernst, Univ. of Ghent, Krijgslaan 281/S8, B-9000, Belgium; Chris Newhall, USGS, Univ. of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; The Daily Nation (URL: http://www.nationmedia.com/dailynation/); Uganda Tourist Board (URL: http://www.visituganda.com/).