The Mount Bromo pyroclastic cone within the Tengger Caldera erupts frequently, typically producing gas-and-steam plumes, ash plumes, and explosions (BGVN 44:05). Information compiled for the reporting period of May-July 2019 is from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC).

The eruptive activity at Tengger Caldera that began in mid-February continued through late July 2019, including white-and-brown ash plumes, ash emissions, and tremors. During the months of May through June 2019, white plumes rose between 50 to 600 m above the summit. Satellite imagery captured a small gas-and-steam plume from Bromo on 5 June (figure 18). Low-frequency tremors were recorded by a seismograph from May through July 2019.

Figure 18. Sentinel-2 satellite image showing a small gas-and-steam plume rising from the Bromo cone (center) in the Tengger Caldera on 5 June 2019. Thermal (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

According to PVMBG and a Volcano Observatory Notice for Aviation (VONA), an ash eruption occurred on 19 July 2019; however, no ash column was observed due to weather conditions. A seismograph recorded five earthquakes and three shallow volcanic tremors the same day. In addition, rainfall triggered a lahar on the SW flank of Bromo.

On 28 July the Darwin VAAC reported that ash plumes originating from Bromo rose to a maximum altitude of about 3.9 km and drifted NW from the summit, based on webcam images and pilot reports. PVMBG reported that lower altitude ash plumes (2.4 km) on the same day were also recorded by webcam images, satellite imagery (Himawari-8), and weather models.

Geologic Background. The 16-km-wide Tengger caldera is located at the northern end of a volcanic massif extending from Semeru volcano. The massive volcanic complex dates back to about 820,000 years ago and consists of five overlapping stratovolcanoes, each truncated by a caldera. Lava domes, pyroclastic cones, and a maar occupy the flanks of the massif. The Ngadisari caldera at the NE end of the complex formed about 150,000 years ago and is now drained through the Sapikerep valley. The most recent of the calderas is the 9 x 10 km wide Sandsea caldera at the SW end of the complex, which formed incrementally during the late Pleistocene and early Holocene. An overlapping cluster of post-caldera cones was constructed on the floor of the Sandsea caldera within the past several thousand years. The youngest of these is Bromo, one of Java's most active and most frequently visited volcanoes.

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/).

Large areas of floating pumice, termed rafts, were encountered by sailors in the northern Tonga region approximately 80 km NW of Vava'u starting around 9 August 2019; the pumice reached the western islands of Fiji by 9 October (figure 7). Pumice rafts are floating masses of individual clasts ranging from millimeters to meters in diameter. The pumice clasts form when silicic magma is degassing, forming bubbles as it rises to the surface, which then rapidly cools to form solid rock. The isolated vesicles formed by the bubbles provide buoyancy to the rock and in turn, the entire pumice raft. These rafts are spread and carried by currents across the ocean; rafts originating in the Tonga area can eventually reach Australia. This report summarizes the pumice raft eruption from early August 2019 using witness accounts and satellite images (acquisition dates are given in UTC). Pending further research, the presumed source is the unnamed Tongan seamount (volcano number 243091) about 45 km NW of Vava'u, the origin of an earlier pumice raft produced during an eruption in 2001.

Figure 7. The path of the pumice from the unnamed Tongan seamount from 9 August to 9 October 2019 based on eye-witness accounts and satellite data discussed below, as well as additional Aqua/MODIS satellite images from NASA Worldview. Blue Marble MODIS/NASA Earth Observatory base map courtesy of NASA Worldview.

The first sighting of pumice was around 1430 on 9 August NW of Vava'u in Tonga (18° 22.068' S, 174° 50.800' W), when Shannon Lenz and Tom Whitehead on board SV Finely Finished initially encountered isolated rocks and smaller streaks of pumice clasts. The area covered by rock increasing to a raft with an estimated thickness of at least 15 cm that extended to the horizon in different directions, and which took 6-8 hours to cross (figure 8). There was no sulfur smell and the sound was described as a "cement mixer, especially below deck." There was also no plume or incandescence observed. Their video, posted to YouTube on 17 August, showed a thin surface layer of cohesive interconnected irregular streaks of pumice with the ocean surface still visible between them. Later footage showed a continuous, undulating mass of pumice entirely covering the ocean surface. Larger clasts are visible scattered throughout the raft. The pumice raft was visible in satellite imagery on this day NW of Late Island (figure 9). By 11 August the raft had evolved into a largely linear feature with smaller rafts to the SW (figure 10). Approximately four hours later, about 15 km to the WSW, Rachel Mackie encountered the pumice. Initially the pumice was "ribbons several hundred meters long and up to 20m wide. It was quite fine and like a slick across the surface of the water." By 2130 they were surrounded by the pumice, and around 25 km away the smell of sulfur was noted.

Figure 8. The pumice raft from the unnamed Tongan seamount on 9 August 2019 taken by Shannon Lenz and Tom Whitehead on board SV Finely Finished. The photos show the pumice raft extending to the horizon in different directions. Scattered larger clasts protrude from the relatively smooth surface that entirely obscures the ocean surface. Courtesy of Shannon Lenz and Tom Whitehead via noonsite.

Figure 9. The pumice raft from the unnamed Tongan seamount on 9 August 2019 (UTC) can be seen NW of Late Island of Tonga in this Aqua/MODIS satellite image. The dashed white line encompasses the visible pumice. The location of the pumice in this image is shown in figure 7. Courtesy of NASA WorldView.

Figure 10. The Sentinel-2 satellite first imaged the pumice from the unnamed Tongan seamount on 11 August 2019 (UTC). This image indicates the pumice distribution with the main raft towards the W and the easternmost area of pumice approximately 45 km away. The eastern tip of the pumice area is located approximately 30 km WNW of Lake islands in Tonga. The location of the pumice in this image is shown in figure 7. Natural color (bands 4, 3, 2) Sentinel-2 satellite image courtesy of Sentinel Hub Playground.

Michael and Larissa Hoult aboard the catamaran ROAM encountered the raft on 15 August (figure 11). They initially saw isolated clasts ranging from marble to tennis ball size (15-70 mm) at 18° 46′S, 174° 55'W. At around 0700 UTC (1900 local time) they noted the smell of sulfur at 18° 55′S, 175° 21′W, and by 0800 UTC they were immersed in the raft with visible clasts ranging from marble to basketball (25 cm) sizes. At this point the raft was entirely obscuring the ocean surface. On 16 and 21 August the pumice continued to disperse and drift NW (figures 12 and 13). On 20 August Scott Bryan calculated an average drift rate of around 13 km/day, with the pumice on this date about 164 km W of the unnamed seamount.

Figure 11. Images of pumice from the unnamed Tongan seamount encountered by Michael and Larissa Hoult aboard the catamaran Roam on 15 August. Left: Larissa takes photographs with scale of pumice clasts; top right: a closeup of a pumice clast showing the vesicle network preserving the degassing structures of the magma; bottom left: Michael holding several larger pumice clasts. The location of their encounter with the pumice is shown in figure 7. Courtesy of SailSurfROAM.

Figure 12. The pumice from the unnamed Tongan seamount (volcano number 243091) on 16 August 2019 UTC. The location of the pumice in this image is shown in figure 7. Natural color (bands 4, 3, 2) Sentinel-2 satellite image courtesy of Sentinel Hub Playground.

Figure 13. On 21 August 2019 (UTC) the pumice from the unnamed Tongan seamount (volcano number 243091) had drifted at least 120 km WNW of Late Island in Tonga. The location of the pumice in this image is shown in figure 7. Natural color (bands 4, 3, 2) Sentinel-2 satellite image courtesy of Sentinel Hub Playground.

An online article published by Brad Scott at GeoNet on 9 September reported the preliminary size of the raft to be 60 km2, significantly smaller than the 2012 Havre seamount pumice raft that was 400 km2. Satellite identification of pumice-covered areas by GNS scientists showed the material moving SSW through 14 August (figure 14).

Figure 14. A compilation of mapped pumice raft extents from 9 August (red line) through to 14 August (dark blue) from Suomi NPP, Terra, Aqua, and Sentinel-2 satellite images. The progression of the pumice raft is towards the SW. Courtesy of Salman Ashraf, GNS Science.

On 5 September the Maritime Safety Authority of Fiji (MSAF) issued a notice to mariners stating that the pumice was sighted in the vicinity of Lakeba, Oneata, and Aiwa Islands and was moving to the W. On 6 September a Planet Labs satellite image shows pumice encompassing the Fijian island of Lakeba over 450 km W of the Tongan islands (figure 15). The pumice entered the lagoon within the barrier reef and drifted around the island to continue towards the W. The pumice was imaged by the Landsat 8 satellite on 26 September as it moved through the Fijian islands, approximately 760 km away from its source (figure 16). The pumice is segmented into numerous smaller rafts of varying sizes that stretch over at least 140 km. On 12 September the Fiji Sun reported that the pumice had reached some of the Lau islands and was thick enough near the shore for people to stand on it.

Figure 15. Planet Labs satellite images show Lakeba Island to the E of the larger Viti Levu Island in the Fiji archipelago. The top image shows the island on 7 July 2019 prior to the pumice raft from the unnamed Tongan seamount. The bottom image shows pumice on the sea surface almost entirely encompassing the island on 6 September. The location of the pumice in this image is shown in figure 7. Courtesy of Planet Labs.

Figure 16. Landsat 8 satellite images show the visible extent of the unnamed seamount pumice on 26 September 2019 (UTC), up to approximately 760 km from the Tongan islands. The pumice seen here extends over a distance of 140 km. The top image shows the locations of the other three images in the white boxes, with a, b, and c indicating the locations. White arrows point to examples of the light brown pumice rafts in these images, seen through light cloud cover. The island in the lower right is Koro Island, the island to the lower left is Viti Levu, and the island to the top right is Vanua Levu. The location of the pumice in this image is shown in figure 7. Landsat 8 true color-pansharpened satellite images courtesy of Sentinel Hub.

Pumice had reached the Yasawa islands in western Fiji by 29 September and was beginning to fill the eastern bays (figure 17). By 9 October bays had been filled out to 500-600 m from the shore, and pumice had also passed through the islands to continue towards the W (figure 18). At this point the pumice beyond the islands had broken up into linear segments that continued towards the NW.

Figure 17. These Sentinel-2 satellite images show the pumice from the unnamed Tongan seamount drifting towards the Yasawa islands of Fiji. The 24 September 2019 (UTC) image shows the beaches without the pumice, the 29 September image shows pumice drifting westward towards the islands, and the 9 October image shows the bays partly filled with pumice out to a maximum of 500-600 m from the shore. These islands are approximately 850 km from the Tongan islands. The Yasawa islands coastline impacted by the pumice shown in these images stretches approximately 48 km. The location of the pumice in this image is shown in figure 7. Sentinel-2 natural color (bands 4, 3, 2) satellite images courtesy of Sentinel Hub.

Figure 18. This Sentinel-2 satellite image acquired on 9 October 2019 (UTC) shows expanses of pumice from the unnamed Tongan seamount that passed through the Yasawa islands of Fiji and was continuing NWW, seen in the center of the image. The location of the pumice in this image is shown in figure 7. Sentinel-2 natural color (bands 4, 3, 2) satellite images courtesy of Sentinel Hub.

Geologic Background. A submarine volcano along the Tofua volcanic arc was first observed in September 2001. The newly discovered volcano lies NW of the island of Vava'u about 35 km S of Fonualei and 60 km NE of Late volcano. The site of the eruption is along a NNE-SSW-trending submarine plateau with an approximate bathymetric depth of 300 m. T-phase waves were recorded on 27-28 September 2001, and on the 27th local fishermen observed an ash-rich eruption column that rose above the sea surface. No eruptive activity was reported after the 28th, but water discoloration was documented during the following month. In early November rafts and strandings of dacitic pumice were reported along the coast of Kadavu and Viti Levu in the Fiji Islands. The depth of the summit of the submarine cone following the eruption determined to be 40 m during a 2007 survey; the crater of the 2001 eruption was breached to the E.

Information Contacts: GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/); Salman Ashraf, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/, https://www.geonet.org.nz/news/8RnSKdhaWOEABBIh0bHDj); Brad Scott, New Zealand GeoNet Project, a collaboration between the Earthquake Commission and GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.geonet.org.nz/, https://www.geonet.org.nz/news/8RnSKdhaWOEABBIh0bHDj); Scott Bryan, School of Earth, Environmental & Biological Sciences, Science and Engineering Faculty, Queensland University of Technology, R Block Level 2, 204, Gardens Point (URL: https://staff.qut.edu.au/staff/scott.bryan); Shannon Lenz and Tom Whitehead, SV Finely Finished (URL: https://www.noonsite.com/news/south-pacific-tonga-to-fiji-navigation-alert-dangerous-slick-of-volcanic-rubble/, YouTube: https://www.youtube.com/watch?v=PEsHLSFFQhQ); Michael and Larissa Hoult, Sail Surf ROAM (URL: https://www.facebook.com/sailsurfroam/); Rachel Mackie, OLIVE (URL: http://www.oliveocean.com/, https://www.facebook.com/rachel.mackie.718); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/); Fiji Sun (URL: https://fijisun.com.fj/2019/09/12/pumice-menace-hits-parts-of-lau-group/).

The current eruptive period of Popocatépetl began on 9 January 2005 and it has since been producing frequent explosions accompanied by ash plumes, gas emissions, and ballistic ejecta that can impact several kilometers away from the crater, as well as dome growth and destruction. This activity continued through March-August 2019 with an increase in volcano alert level during 28 March-6 May. This report summarizes activity during this period and is based on information from Centro Nacional de Prevención de Desastres (CENAPRED), Universidad Nacional Autónoma de México (UNAM), and various webcam and remote sensing data.

An overflight on 28 February confirmed that dome 82, which was first observed on 14 February, was still present and was 200 m in diameter. During March there were 3,291 observed low-intensity emissions, and 33 larger explosions that produced ash plumes to a maximum height of 5 km, accompanied by near-continuous emission of water vapor and volcanic gases. Explosions ejected blocks that fell on the flanks out to 1.2-2 km on 1, 10, 13, 17, 26, 27, and 29 March. The events on the 17th and 27th resulted in vegetation fires. Frequent sulfur dioxide (SO₂) plumes were detected by TropOMI (figure 130). An overflight on 7 March showed intense degassing and an ash plume at 1142, preventing visibility into the crater (figure 131). On 13 March Strombolian activity was observed for approximately 15 minutes at 0500, accompanied by incandescent ejecta that deposited mainly on the ESE flank.

An overflight on 15 March was taken by CENAPRED and UNAM personnel to observe changes to the crater after explosions on the 13th and 14th. They reported that dome 82 had been destroyed and the crater maintained its previous dimensions of 300 m in diameter and 130 m deep. An explosion on the 27th ejected incandescent rocks out to 2 km from the crater and produced a 3-km-high ash plume that dispersed to the NE. Ashfall was reported in Santa Cruz, Atlixco, San Pedro, San Andrés, Santa Isabel Cholula, San Pedro Benito Juárez, and in the municipalities of Puebla, Hueyapan, Tetela del Volcán, and Morelos.

On 28 March an explosion at 0650 generated a 2.5-km-high ash plume and ejecta out to 1 km from the crater, and a 130-minute-long event produced gas and ah plumes (figure 132). On this day the volcano alert level was increased from Yellow Phase 2 to Yellow Phase 3. On the 29th an ash plume rose to 3 km and was accompanied by ejecta that reached 2 km away from the crater. Later that day a 20-minute-long event produced ash and gas. During a surveillance flight on 30 March a view into the crater showed no dome present, and the crater size had increased to 350 m in width and 250-300 m in depth after recent explosions (figure 131). On this day Strombolian activity was also observed lasting for 14 minutes, producing an ash plume to 800 m and ejecta out to 300 m from the crater. Incandescence at the crater was often seen during nighttime throughout the month.

Figure 130. Significant SO2 plumes at Popocatépetl detected by the TROPOMI instrument on the Sentinel-5P satellite during 3-11 March 2019. SO2 plumes are frequently observed and these images show examples of plume drift directions on 3 March 2019 (top left), 6 March 2019 (top right), 7 March 2019 (bottom left), and 11 March 2019 (bottom right). Date, time, and measurements are provided at the top of each image. Courtesy of NASA Goddard Flight Center.

Figure 131. Activity at Popocatépetl and views of the crater during surveillance flights in March 2019. The top images show an ash plume (left) and a gas-and-steam plume (right) on 7 March. On 30 March (bottom left and right) no lava dome was observed in the crater, which was measured to be 350 m in diameter and 250-300 m deep. Courtesy of CENAPRED and Geophysics Institute of UNAM.

Figure 132. Explosive activity at Popocatépetl on 28 March 2019 producing ash plumes (top and bottom left) and ejecting incandescent ejecta out to 2 km from the crater at 1948. Courtesy of Carlos Sanchez/AFP (top), CENAPRED (bottom left and right), and Webcams de Mexico (bottom left).

There was a decrease in events during the next two months with 1,119 recorded low-intensity emissions and no larger ash explosions throughout April, followed by 1,210 low-intensity emissions and seven larger ash explosions through May (figure 133). Water vapor and volcanic gas emissions were frequently observed through this time and incandescence was observed some nights. A surveillance overflight on 26 April noted no new dome within the crater. On 6 May the alert level was lowered back to Yellow Phase 2. Another overflight on 9 May showed no change in the crater. An explosion at 1910 on 22 May produced an ash plume to 3.5 km above the crater with ashfall reported in Ozumba, Temamatla, Atlautla, Cocotitlán, Ayapango, Ecatzingo, Tenango del Aire and Tepetlixpa.

Figure 133. Graph showing the number of daily ash explosions and low-intensity emissions at Popocatépetl during March-August 2019. There was a decrease in the number of events during April and March, with an increase from March onwards. Data courtesy of CENAPRED.

Through the month of June there were 2,820 low-intensity emissions and 21 larger ash explosions recorded. Gas emissions were observed throughout the month. Two explosions on 3 June produced ash plumes up to 3.5 and 2.8 km, with ejecta out to 2 km S during the first explosion. On 11 June an explosion produced an ash plume to 1 km above the crater and ballistic ejecta out to 1 km E. Observers on a surveillance overflight on the 12th reported no changes within the crater

Explosions with estimated plume heights of 5 km occurred on the 14th and 15th, with the latter producing ashfall in the municipalities of San Pablo del Monte, Tenancingo, Papantla, San Cosme Mazatencocho, San Luis Teolocholco, Acuamanala, Nativitas, Tepetitla, Santa Apolonia Teacalco, Santa Isabel Tetlatlahuaca, and Huamantla, in the state of Tlaxcala, as well as in Nealtican, San Nicolás de los Ranchos, Calpan, San Pedro Cholula, Juan C. Bonilla, Coronango, Atoyatempan, and Coatzingo, in the state of Puebla.

On 17 June an explosion produced an ash plume that reached 8 km above the crater and dispersed towards the SW. An ash plume rising 2.5 km high was accompanied by incandescent ejecta impacting a short distance from the crater on the 21st, and another ash plume reached 2.5 km on the 22nd. Explosions on 26, 29, and 30 June resulted in ash plumes reaching 1.5 km above the crater and ballistic ejecta impacting on the flanks out to 1 km.

For the month of July there was an increased total of 5,637 recorded low-intensity emissions, and 173 larger ash explosions (figure 134). On 8 July an explosion produced ballistic ejecta out to 1.5 km and an ash plume up to 1 km above the crater. An ash plume up to 2.6 km was produced on the 12th. On 19 July a surveillance overflight observed a new dome (dome 83) with a diameter of 70 m and a thickness of 15 m (figure 135). Explosions on 20 July produced ashfall, and minor explosions that ejected incandescent ballistics onto the slopes. An event on the 24th produced an ash plume that reached 1.2 km, and ash plumes the following day reached 1 km. An overflight on 27 July confirmed that these explosions destroyed dome 83, and the crater dimensions remained the same (figure 136). The following day, ash plumes reached up to 1.6 km above the crater, and up to 2 km on the 29th. Minor ashfall was reported in the municipality of Ozumba on 30 June.

Figure 134. Examples of ash plumes at Popocatépetl on 1 July (top left), 18 July (top right and bottom left), and 30 July (bottom right) 2019. In the night time image taken on 18 July hot rocks are visible on the flank. Webcam images courtesy of CENAPRED and Webcams de Mexico.

Figure 135. A surveillance overflight at Popocatépetl on 19 July 2019 confirmed a new dome, dome number 83, with a width of 70 m and a thickness of 15 m. Courtesy of CENAPRED and Geophysics Institute of UNAM.

Figure 136. Photos of the summit crater of Popocatépetl taken during a surveillance flight on 27 July 2019 confirmed that the 83rd lava dome was destroyed by recent explosions and the crater maintained the same dimensions as previously measured. Courtesy of CENAPRED and Geophysics Institute of UNAM.

Throughout August the number of recorded events was higher than previous months, with 5,091 low-intensity emissions and 204 larger ash explosions (figure 137). Two explosions generated ash plumes and incandescent ejecta on 2 August, the first with a plume up to 1.5 km with ejecta impacting the slopes, and the second with an 800 m plume and ejecta landing back in the crater. Ashfall from the events was reported in in the municipalities of Tenango del Aire, Ayapango and Amecameca. On the 14th ashfall was reported in Juchitepec, Ayapango, and Ozumba. Explosions on 16 August produced ash plumes up to 2 km that dispersed to the WSW. Over the following two days ash plumes reached 1.2 km and resulted in ashfall in Cuernavaca, Tepoztlán, Tlalnepantla, Morelos, Ozumba, and Ecatzingo. Over 30-31 August ash plumes reached between 1-2 km above the crater and ashfall was reported in Amecameca, Atlautla, Ozumba, and Tlalmanalco. Incandescence was sometimes observed at the crater through the month.

Figure 137. Ash plumes at Popocatépetl on 7 August (top) and 26 August 2019 (bottom). Courtesy of CENAPRED and Webcams de Mexico.

The MODVOLC algorithm for MODIS thermal anomalies registered thermal alerts through this period, with 22 in March, three in May, five in July, and one in August. The MIROVA system showed that the frequency of thermal anomalies at Popocatépetl was higher in March, sporadic in April and May, low in June, and had increased again in July and August (figure 138). Elevated temperatures were frequently visible in Sentinel-2 thermal satellite data when clouds and plumes were not covering the crater (figure 139).

Figure 138. Thermal activity at Popocatépetl detected by the MIROVA system showed frequent anomalies in March, intermittent anomalies through April-May, low activity in June, and an increase in July-August 2019. Courtesy of MIROVA.

Figure 139. Sentinel-2 thermal satellite images frequently showed elevated temperatures in the crater of Popocatépetl during March-August 2019, as seen in this representative image from 7 May 2019. Sentinel2- atmospheric penetration (bands 12, 11, 8A) scene courtesy of Sentinel Hub Playground.

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

Information Contacts: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: http://www.cenapred.unam.mx/); Universidad Nacional Autónoma de México (UNAM), University City, 04510 Mexico City, Mexico (URL: https://www.unam.mx/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Webcams de Mexico (URL: http://www.webcamsdemexico.com/); Agence France-Presse (URL: http://www.afp.com/).

The ongoing eruption at Semeru weakened in intensity during 2018, with occasional ash plumes and thermal anomalies (BGVN 44:04); this reduced but ongoing level of activity continued through August 2019. The volcano is monitored by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC). The current report summarizes activity from 1 March to 31 August 2019. The Alert Level remained at 2 (on a scale from 1-4); the public was warned to stay 1 km away from the active crater and 4 km away on the SSE flank.

Based on analysis of satellite images, the Darwin VAAC reported that ash plumes rose to an altitude of 4-4.3 km on 19 April, 20 June, 10 July, and 13 July, drifting in various directions. In addition, PVMBG reported that at 0830 on 26 June an explosion produced an ash plume that rose around 600 m above the summit and drifted SW. A news article (Tempo.com) dated 12 August cited PVMBG as stating that the volcano had erupted 17 times since 8 August.

During March-August 2019 thermal anomalies were detected with the MODIS satellite instruments analyzed using the MODVOLC algorithm only on 5 July and 22 August. No explosions were recorded on those two days. Scattered thermal anomalies within 5 km of the volcano were detected by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system, also based on analysis of MODIS data: one at the end of March and 3-6 hotspots over the following months, almost all of low radiative power. Satellite imagery intermittently showed thermal activity in the Jonggring-Seloko crater (figure 37), sometimes with material moving down the SE-flank ravine.

Figure 37. Sentinel-2 satellite images showing the persistent elevated thermal anomaly in the Jonggring-Seloko crater of Semeru were common through August 2019, as seen in this view on 20 July. Hot material could sometimes be identified in the SE-flank ravine. Atmospheric penetration rendering (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Tempo.com (URL: https://www.tempo.com/).

Historical observations of eruptive activity from the glacier-covered Mount Michael stratovolcano on Saunders Island in the South Sandwich Islands were not recorded until the early 19th century at this remote site in the southernmost Atlantic Ocean, and remain extremely rare. With the advent of satellite observation technology, indications of more frequent eruptive activity have become apparent. Vapor emission is frequently reported from the summit crater, and AVHRR and MODIS satellite imagery has revealed evidence for lava lake activity in the summit crater (Lachlan-Cope and others, 2001). Limited thermal anomaly data and satellite imagery indicated at least intermittent activity during May 2000-November 2013, and from November 2014 through April 2018 (Gray and others, 2019). Ongoing observations, including photographs from two site visits in February and May 2019 suggest continued activity at the summit during most months through May 2019, the period covered in this report. Information, in addition to on-site photographs, comes from MIROVA thermal anomaly data, NASA SO₂ instruments, and Sentinel-2 and Landsat satellite imagery.

Near-constant cloud coverage for much of the year makes satellite data intermittent and creates difficulty in interpreting the ongoing nature of the activity. Gray and others (2019) concluded recently after a detailed study of shortwave and infrared satellite images that there was continued evidence for the previously identified lava lake on Mount Michael since January 1989. MIROVA thermal anomaly data suggest intermittent pulses of thermal energy in September, November, and December 2018, and April 2019 (figure 17). Satellite imagery confirmed some type of activity, either a dense steam plume, evidence of ash, or a thermal anomaly, each month during December 2018-March 2019. Sulfur dioxide anomalies were recorded in January, February, and March 2019. Photographic evidence of fresh ash was captured in February 2019, and images from May 2019 showed dense steam rising from the summit crater.

Figure 17. MIROVA thermal anomaly data from 19 September 2018 through June 2019 showed sporadic, low-level pulses of thermal energy in late September, November, and December 2018, and April 2019. Courtesy of MIROVA.

After satellite imagery and thermal anomaly data in late September 2018 showed evidence for eruptive activity (BGVN 43:10, figure 16), a single thermal anomaly in MIROVA data was recorded in mid-November 2018 (figure 17). A rare, clear Sentinel-2 image on 2 December revealed a dense steam plume over the active summit crater; the steam obscured the presence of any possible thermal anomalies beneath (figure 18).

Figure 18. Sentinel-2 images of Mount Michael on Saunders Island on 2 December 2018 revealed a dense steam plume over the summit crater that was difficult to distinguish from the surrounding snow in Natural Color rendering (bands 4,3,2) (left), but was clearly visible in Atmospheric Penetration rendering (bands 12,11, 8a) (right). Courtesy of Sentinel Hub Playground.

Clear evidence of recent activity appeared on 1 January 2019 with both a thermal anomaly at the summit crater and a streak of ash on the snow (figure 19). Steam was also present within the summit crater. A distinct SO₂ anomaly appeared in data from the TROPOMI instrument on 14 January (figure 20).

Figure 19. A thermal anomaly and dense steam were recorded at the summit of Mount Michael on Saunders Island on 1 January 2019 in Sentinel-2 Satellite imagery with Atmospheric Penetration rendering (bands 12, 11, 8a) (left). The same image shown with Natural Color rendering (bands 4,3,2) (right) shows a recent streak of brown particulates drifting SE from the summit crater. Courtesy of Sentinel Hub Playground.

Figure 20. A distinct SO2 plume was recorded drifting NW from Saunders Island by the TROPOMI instrument on the Sentinel 5-P satellite on 14 January 2019. Courtesy of NASA Goddard Space Flight Center.

Multiple sources of satellite data and sea-based visual observation confirmed activity during February 2019. SO₂ emissions were recorded with the TROPOMI instrument on 10, 11, 15, and 16 February (figure 21). A Landsat image from 10 February showed a dense steam plume drifting NW from the summit crater, with the dark rim of the summit crater well exposed (figure 22). Sentinel-2 images in natural color and atmospheric penetration renderings identified a dense steam plume drifting S and a thermal anomaly within the summit crater on 15 February (figure 23). An expedition to the South Sandwich Islands between 15 February and 8 March 2019 sponsored by the UK government sailed by Saunders in late February and observed a stream of ash on the NNE flank beneath the cloud cover (figure 24).

Figure 21. Faint but distinct SO2 plumes were recorded drifting away from Saunders Island in various directions on 10, 11, 15, and 16 February 2019. Courtesy of NASA Goddard Space Flight Center.

Figure 22. The dark summit crater of Mount Michael on Saunders Island was visible in Landsat imagery on 10 February 2019. A dense steam plume drifted NW and cast a dark shadow on the underlying cloud cover. Courtesy of Sentinel Hub Playground.

Figure 23. At the summit of Mount Michael on Saunders Island, Sentinel-2 images in Natural Color (bands 4,3,2) (left) and Atmospheric Penetration (bands 12, 11, 8a) (right) renderings identified a dense steam plume drifting S and a thermal anomaly within the summit crater on 15 February 2019. Courtesy of Sentinel Hub Playground.

Figure 24. Recent ash covered the NNE flank of Mount Michael on Saunders Island in late February 2019 when observed by an expedition to the South Sandwich Islands sponsored by the UK government. Courtesy of Chris Darby.

Faint SO₂ emissions were recorded twice during March 2019 (figure 25), and a dense steam plume near the summit crater was visible in Landsat imagery on 23 March (figure 26). Two thermal anomalies were captured in the MIROVA data during April 2019 (figure 17).

Figure 25. Faint SO2 plumes were recorded on 1 and 11 March 2019 emerging from Saunders Island. Courtesy of NASA Goddard Space Flight Center.

Figure 26. A dense steam plume drifted E from the summit crater of Mount Michael at Saunders Island on 25 March 2019. Landsat-8 image courtesy of Sentinel Hub Playground.

A volcano-related research project "SSIVOLC" explored the South Sandwich Islands volcanoes during 15 April-31 May 2019. A major aim of SSIVOLC was to collect photogrammetric data of the glacier-covered Mount Michael (Derrien and others, 2019). A number of still images were acquired on 17 and 22 May 2019 showing various features of the island (figures 27-30). The researchers visually observed brief, recurrent, and very weak glow at the summit of Mount Michael after dark on 17 May, which they interpreted as reflecting light from an active lava lake within the summit crater.

Figure 27. The steep slopes of an older eroded crater on the E end of Saunders island in the 'Ashen Hills' shows layers of volcanic deposits dipping away from the open half crater. In the background, steam and gas flow out of the summit crater of Mount Michael and drift down the far slope. Drone image PA-IS-03 taken during 17-22 May 2019, courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

Figure 28. A dense steam plume drifts away from the summit of Mount Michael on Saunders Island in this drone image taken during 17-22 May 2019. The older summit crater is to the left of the dark patch in the middle of the summit. North is to the right. Image SU-3 courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

Figure 29. This close-up image of the summit of Mount Michael on Saunders Island shows steam plumes billowing from the summit crater, and large crevasses in the glacier covered flank, taken during 17-22 May 2019. The old crater is to the left. Image TL-SU-1 courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

Figure 30. A dense plume of steam rises from the summit crater of Mount Michael on Saunders Island and drifts over mounds of frozen material during 17-22 May 2019. The older crater is to the left, and part of the Ashen Hills is in the foreground. Image TL-SU-2 courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

References: Lachlan-Cope T, Smellie J L, Ladkin R, 2001. Discovery of a recurrent lava lake on Saunders Island (South Sandwich Islands) using AVHRR imagery. J. Volcanol. Geotherm. Res., 112: 105-116.

Gray D M, Burton-Johnson A, Fretwell P T, 2019. Evidence for a lava lake on Mt. Michael volcano, Saunders Island (South Sandwich Islands) from Landsat, Sentinel-2 and ASTER satellite imagery. J. Volcanol. Geotherm. Res., 379:60-71. https://doi.org/10.1016/j.volgeores.2019.05.002.

Derrien A, Richter N, Meschede M, Walter T, 2019. Optical DSLR camera- and UAV footage of the remote Mount Michael Volcano, Saunders Island (South Sandwich Islands), acquired in May 2019. GFZ Data Services. http://doi.org/10.5880/GFZ.2.1..2019.003

Geologic Background. Saunders Island is a volcanic structure consisting of a large central edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young constructional Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of parasitic cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Vapor emission is frequently reported from the summit crater. Recent AVHRR and MODIS satellite imagery has revealed evidence for lava lake activity in the summit crater.

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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); Chris Darby (URL: https://twitter.com/ChrisDDarby, image at https://twitter.com/ChrisDDarby/status/1100686838568812544).

Pacaya is one of the most active volcanoes in Guatemala, with activity largely consisting of frequent lava flows and Strombolian activity at the Mackenney crater. This report summarizes continued activity during February through July 2019 based on reports by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH) and Sistema de la Coordinadora Nacional para la Reducción de Desastres (CONRED), visiting scientists, and satellite data.

At the beginning of February activity included Strombolian explosions ejecting material up to 5 to 30 m above the Mackenney crater and a degassing plume up to 300 m. Multiple lava flows were observed throughout the month on the N, NW, and W flanks, reaching 350 m from the crater and resulting in avalanches from the flow fronts. Strombolian activity continued with sporadic to continuous explosions ejecting material 5-75 m above the Mackenney crater. Degassing produced plumes up to 300 m above the crater, and incandescence from the crater and lava flows were seen at night. Daniel Sturgess of Bristol University observed activity on the 24th, noting a 70-m-long lava flow with individual blocks from the front of the flow rolling down the flanks (figure 108). He reported that mild Strombolian explosions occurred every 10-20 minutes and ejected blocks, up to approximately 4 m in diameter, as high as 5-30 m above the crater and towards the northern flank.

Figure 108. An active lava flow on the NW flank of Pacaya on 24 February 2019 with incandescence visible in lower light conditions. Courtesy of Daniel Sturgess, University of Bristol.

Similar activity continued through March with multiple lava flows reaching a maximum of 200 m N and NW, and avalanches descending from the flow fronts. Ongoing Strombolian explosions expelled material up to 75 m above the Mackenney crater. Degassing produced a white-blue plume to a maximum of 900 m above the crater (figure 109) and incandescence was noted some nights.

Figure 109. A degassing plume at Pacaya reaching 350 m above the crater and dispersing to the S on 19 March 2019. Courtesy of CONRED.

During April lava flows continued on the N and NW flanks, reaching a maximum length of 300 m, with avalanches forming from the flow fronts. Degassing formed plumes up to 600 m above the crater that dispersed with various wind directions. Strombolian activity continued with explosions ejecting material up to 40 m above the crater. On the 2nd and 3rd weak rumbles were heard at distances of 4-5 km. Similar activity continued through May with lava flows reaching 300 m to the N, degassing producing plumes up to 600 m above the crater, and Strombolian explosions ejecting material up to 15 m above the crater.

Lava flows continued out to 300 m in length to the N and NW during June (figures 110 and 111). Strombolian activity ejected material up to 30 m above the crater and degassing resulted in plumes that reached 300 m. During July there were multiple active lava flows that reached a maximum of 300 m in length on the N and NW flanks (figure 112). Avalanches generated by the collapse of material at the front of the lava flows were accompanied by explosions ejecting material up to 30 m above the crater.

Figure 110. An active lava flow on Pacaya on 9 June 2019 with incandescent blocks rolling down the flank from the flow front. Courtesy of Paul Wallace, University of Liverpool.

Figure 111. Activity at Pacaya on 22 June 2019 with a degassing plume dispersed to the W and a 300-m-long lava flow. Photos by Miguel Morales, courtesy of CONRED.

Figure 112. Two lava flows were active to the N and NW at Pacaya on 20 July 2019. Photos courtesy of CONRED.

During February through July multiple lava flows and crater activity were detected in Sentinel-2 satellite thermal images (figures 113 and 114) and relatively constant thermal energy was detected by the MIROVA system with a slight decrease in the energy and frequency of anomalies during June (figure 115). The thermal anomalies detected by the MODVOLC system for each month from February through July spanned 6-30, with six during June and 30 during April.

Figure 113. Sentinel-2 thermal satellite images of Pacaya show lava flows to the N and NW during February through April 2019. There was a reduction in visible activity in early March. False color (urban) satellite images (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 114. Sentinel-2 thermal satellite images of Pacaya showing lava flow and hot avalanche activity during June and July 2019. False color (urban) satellite images (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 115. MIROVA log radiative power plot of MODIS thermal infrared at Pacaya during October 2018 through July 2019. Detected thermal energy is relatively stable with a decrease through June and subsequent increase during July. Courtesy of MIROVA.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.php); 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/); Daniel Sturgess, School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, United Kingdom (URL: http://www.bristol.ac.uk/earthsciences/); Paul Wallace, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, United Kingdom (URL: https://www.liverpool.ac.uk/environmental-sciences/staff/paul-wallace/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Frequent historical eruptions at Volcán de Colima date back to the 16th century and include explosive activity, lava flows, and large debris avalanches. The most recent eruptive episode began in January 2013 and continued through March 2017. Previous reports have covered activity involving ash plumes with extensive ashfall, lava flows, lahars, and pyroclastic flows (BGVN 41:01 and 42:08). In late April 2019, increased seismicity related to volcanic activity began again. This report covers activity through July 2019. The primary source of information was the Centro Universitario de Estudios e Investigaciones de Vulcanologia, Universidad de Colima (CUEIV-UdC).

On 11 May 2019, CUEIV-UdC reported an explosion that was recorded by several monitoring stations. A thermal camera located south of Colima captured thermal anomalies associated with the explosion as well as intermittent degassing, which mainly consisted of water vapor (figure 131). A report from the Unidad Estatal de Protección Civil de Colima (UEPCC), and seismic and infrasound network data from CUEIV-UdC, recorded about 60 high-frequency events, 16 landslides, and 14 low-magnitude explosions occurring on the NE side of the crater during 11-24 May. Drone imagery showed fumarolic activity occurring on the inner wall of this crater on 22 May (figure 132).

Figure 131. Gas emissions from Colima during the 11 May 2019 eruption as seen from the Naranjal station. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 17 mayo 2019 no 121).

Figure 132. A drone photo showing fumarolic activity occurring within the NE wall of the crater at Colima on 22 May 2019. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 24 mayo 2019 no 122).

Small explosions and gas-and-steam emissions continued intermittently through mid-July 2019 concentrated on the NE side of the crater. An overflight on 9 July 2019 revealed that subsidence from the consistent activity slightly increased the diameter of the vent; other areas within the crater also showed evidence of subsidence and some collapsed material on the outer W wall (figure 133). During the weeks of 19 and 26 July 2019, monitoring cameras and seismic data recorded eight lahars.

Figure 133. A drone photo of the crater at Colima on 8 July 2019 shows continuing fumarolic activity and evidence of a collapsed wall on the W exterior side. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 12 julio 2019 no 129).

Geologic Background. The Colima volcanic complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the 4320 m high point of the complex) on the north and the 3850-m-high historically active Volcán de Colima at the south. A group of cinder cones of late-Pleistocene age is located on the floor of the Colima graben west and east of the Colima complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide caldera, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, and have produced a thick apron of debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions (most recently in 1913) have destroyed the summit and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Centro Universitario de Estudios e Investigaciones de Vulcanologia, Universidad de Colima (CUEIV-UdC), Colima, Col. 28045, Mexico; Centro Universitario de Estudios Vulcanologicos y Facultad de Ciencias de la Universidad de Colima, Avenida Universidad 333, Colima, Col. 28045, Mexico (URL: http://portal.ucol.mx/cueiv/); Unidad Estatal de Protección Civil, Colima, Roberto Esperón No. 1170 Col. de los Trabajadores, C.P. 28020, Mexico (URL: http://www.proteccioncivil.col.gob.mx/).

Masaya, in Nicaragua, contains a lava lake found in the Santiago Crater which has remained active since its return in December 2015 (BGVN 41:08). In addition to this lava lake, previous volcanism included explosive eruptions, lava flows, and gas emissions. Activity generally decreased during March-July 2019, including the number and frequency of thermal anomalies, lava lake levels, and gas emissions. The primary source of information for this report comes from the Instituto Nicareguense de Estudios Territoriales (INETER).

On 21 July 2019 a small explosion in the Santiago Crater resulted in some gas emissions and an ash cloud drifting WNW. In addition to the active lava lake (figure 77), monthly reports from INETER noted that thermal activity and gas emissions (figure 78) were decreasing.

Figure 77. Active lava lake visible in the Santiago Crater at Masaya on 27 June 2019. Photo by Sheila DeForest (Creative Commons BY-SA license).

Figure 78. Gas emissions coming from the Santiago Crater at Masaya on 29 June 2019. Photo by Sheila DeForest (Creative Commons BY-SA license).

On 15 May and 22 July 2019, INETER scientists used a FLIR SC620 thermal infrared camera to measure temperatures of fumaroles on the Santiago Crater. In May 2019 the temperature of fumaroles had decreased by 48°C since the previous month. Between May and July 2019 fumarole temperatures continued to decline; temperatures ranged from 90° to 136°C (figure 79). Compared to May 2019 these temperatures are 3°C lower. INETER reports that the level of the lava lake has been slowly dropping during this reporting period.

Figure 79. FLIR (forward-looking infrared) and visible images of the Santiago Crater at Masaya showing fumarole temperatures ranging from 90° to 136°C. The scale in the center shows the range of temperatures in the FLIR image. Courtesy of INETER (March 2019 report).

According to MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS satellite instruments, frequent thermal anomalies were recorded from mid-March through early May 2019, with little to no activity from mid-May to July 2019 (figure 80). Sentinel-2 thermal images show high temperatures in the active lava lake on 10 March 2019 (figure 81). Thermal energy detected by the MODVOLC algorithm showed 14 hotspot pixels with the most number of hotspots (7) occurring in March 2019.

Figure 80. Thermal anomalies were relatively constant at Masaya from early September 2018 through early May 2019 and then abruptly decreased until mid-June 2019 as recorded by MIROVA. Courtesy of MIROVA.

Figure 81. Sentinel-2 thermal satellite image showing a detected heat signature from the active lava lake at Masaya on 10 March 2019. The lava lake is visible (bright yellow-orange). Approximate diameter of the crater containing the lava lake is 500 m. Thermal (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); 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); Sheila DeForest (URL: https://www.facebook.com/sheila.deforest).

The acid lake of Rincón de la Vieja's active crater has generated intermittent weak phreatic explosions regularly since 2011, continuing during the past year through at least August 2019. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and the information below comes from its weekly bulletins between 4 March and 2 September 2019. Clouds often prevented webcam and satellite views. The current report describes activity from March through July 2019.

OVSICORI-UNA reported that weak events occurred on 19 March at 1851 and on 29 March 2019 at 2043. A two-minute-long phreatic explosion on 1 April at 0802 produced a plume that rose 600 m above the crater rim. Continuous emissions were visible during 3-4 April, rising 200 m above the crater rim. On 3 April, at 1437, a small explosion was detected. An explosion on 10 April at 0617 produced a gas-and-steam plume that rose 1 km above the crater rim and drifted SE. On 12 April at 0643, a plume rose 500 m. Another event took place at 0700 on 13 April, although poor weather conditions prevented visual observations. On 14 April, OVSICORI-UNA noted that aerial photographs showed a milky-gray acid lake at a relatively low water level with convection cells of several tens meters of diameter in the center and eastern parts of the lake.

According to an OVSICORI-UNA bulletin, a small phreatic explosion occurred on 1 May. Another explosion on 11 May at 0720 produced a white gas-and-steam plume that rose 600 m above the crater rim. Phreatic explosions were recorded on 14 May at 1703 and on 17 May at 0357, though dense fog prevented visual confirmation of both events with webcams. On 15 May a local observer noted a diffuse plume of steam and gas, material rising from the crater, and photographed milky-gray deposits on the N part of the crater rim ejected from the event the day before. A major explosion occurred on 24 May.

OVSICORI-UNA recorded a significant phreatic 10-minute-long explosion that began on 11 June at 0343, but plumes were not visible due to weather conditions. No further phreatic events were reported in July.

Seismic activity was very low during the reporting period, and there was no significant deformation. Short tremors were frequent toward the end of April, but were only periodic in May and June; tremor almost disappeared in July. A few long-period earthquakes were recorded, and volcano-tectonic earthquakes were even less frequent.

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

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/).

Sakurajima rises from Kagoshima Bay, which fills the Aira Caldera near the southern tip of Japan's Kyushu Island. Frequent explosive and occasional effusive activity has been ongoing for centuries. The Minamidake summit cone has been the location of persistent activity since 1955; the Showa crater on its E flank has also been intermittently active since 2006. Numerous explosions and ash-bearing emissions have been occurring each month at either Minamidake or Showa crater since the latest eruptive episode began in late March 2017. This report covers ongoing activity from January through June 2019; the Japan Meteorological Agency (JMA) provides regular reports on activity, and the Tokyo VAAC (Volcanic Ash Advisory Center) issues tens of reports each month about the frequent ash plumes.

From January to June 2019, ash plumes and explosions were usually reported multiple times each week. The quietest month was June with only five eruptive events; the most active was March with 29 (table 21). Ash plumes rose from a few hundred meters to 3,500 m above the summit during the period. Large blocks of incandescent ejecta traveled as far as 1,700 m from the Minamidake crater during explosions in February and April. All the activity originated in the Minamidake crater; the adjacent Showa crater only had a mild thermal anomaly and fumarole throughout the period. Satellite imagery identified thermal anomalies inside the Minamidake crater several times each month.

Table 21. Monthly summary of eruptive events recorded at Sakurajima's Minamidake crater in Aira caldera, January-June 2019. The number of events that were explosive in nature are in parentheses. No events were recorded at the Showa crater during this time. Data courtesy of JMA (January to June 2019 monthly reports).

There were eight eruptive events reported by JMA during January 2019 at the Minamidake summit crater of Sakurajima. They occurred on 3, 6, 7, 9, 17, and 19 January (figure 76). Ash plume heights ranged from 600 to 2,100 m above the summit. The largest explosion, on 9 January, generated an ash plume that rose 2,100 m above the summit crater and drifted E. In addition, incandescent ejecta was sent 800-1,100 m from the summit. Incandescence was visible at the summit on most clear nights. During an overflight on 18 January no significant changes were noted at the crater (figure 77). Infrared thermal imaging done on 29 January indicated a weak thermal anomaly in the vicinity of the Showa crater on the SE side of Minamidake crater. The Kagoshima Regional Meteorological Observatory (KRMO) (11 km WSW) recorded ashfall there during four days of the month. Satellite imagery indicated thermal anomalies inside Minamidake on 7 and 27 January (figure 77).

Figure 76. Incandescent ejecta and ash emissions characterized activity from Sakurajima volcano at Aira during January 2019. Left: A webcam image showed incandescent ejecta on the flanks on 9 January 2019, courtesy of JMA (Explanation of volcanic activity in Sakurajima, January 2019). Right: An ash plume rose hundreds of meters above the summit, likely also on 9 January, posted on 10 January 2019, courtesy of Mike Day.

Figure 77. The summit of Sakurajima consists of the larger Minamidake crater and the smaller Showa crater on the E flank. Left: The Minamidake crater at the summit of Sakurajima volcano at Aira on 18 January 2019 seen in an overflight courtesy of JMA (Explanation of volcanic activity in Sakurajima, March 2019). Right: Two areas of thermal anomaly were visible in Sentinel-2 satellite imagery on 27 January 2019. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

Activity increased during February 2019, with 15 eruptive events reported on days 1, 3, 7, 8, 10, 13, 14, 17, 22, 24, and 27. Ash plume heights ranged from 600-2,300 m above the summit, and ejecta was reported 300 to 1,700 m from the crater in various events (figure 78). KRMO reported two days of ashfall during February. Satellite imagery identified thermal anomalies at the crater on 6 and 26 February, and ash plumes on 21 and 26 February (figure 79).

Figure 78. An explosion from Sakurajima at Aira on 7 February 2019 sent ejecta up to 1,700 m from the Minamidake summit crater. Courtesy of JMA (Explanation of volcanic activity in Sakurajima, February 2019).

Figure 79. Thermal anomalies and ash emissions were captured in Sentinel-2 satellite imagery on 6, 21, and 26 February 2019 originating from Sakurajima volcano at Aira. Top: Thermal anomalies within the summit crater were visible underneath steam and ash plumes on 6 and 26 February (closeup of bottom right photo). Bottom: Ash emissions on 21 and 26 February drifted SE from the volcano. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

The number of eruptive events continued to increase during March 2019; there were 29 events reported on numerous days (figures 80 and 81). An explosion on 14 March produced an ash plume that rose 3,500 m above the summit and drifted E. It also produced ejecta that landed 800-1,100 m from the crater. During an overflight on 26 March a fumarole was the only activity in Showa crater. KRMO reported 14 days of ashfall during the month. Satellite imagery identified an ash plume on 13 March and a thermal anomaly on 18 March (figure 82).

Figure 80. A large ash emission from Sakurajima volcano at Aira was photographed by a tourist on the W flank and posted on 1 March 2019. Courtesy of Kratü.

Figure 81. An ash plume from Sakurajima volcano at Aira on 18 March 2019 produced enough ashfall to disrupt the trains in the nearby city of Kagoshima according to the photographer. Image taken from about 20 km away. Courtesy of Tim Board.

Figure 82. An ash plume drifted SE from the summit of Sakurajima volcano at Aira on 13 March (left) and a thermal anomaly was visible inside the Minamidake crater on 18 March 2019 (right). "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

A decline in activity to only ten eruptive events on days 7, 13, 17, 22, and 25 was reported by JMA for April 2019. An explosion on 7 April sent ejecta up to 1,700 m from the crater. Another explosion on 13 April produced an ash plume that rose 2,200 m above the summit. Most of the eruptive events at Sakurajima last for less than 30 minutes; on 22 April two events lasted for almost an hour each producing ash plumes that rose 1,400 m above the summit. Ashfall at KRMO was reported during seven days in April. Two distinct thermal anomalies were visible inside the Minamidake crater on both 12 and 27 April (figure 83).

Figure 83. Two thermal anomalies were present inside Minamidake crater at the summit of Sakurajima volcano at Aira on 12 (left) and 27 (right) April 2019. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

There were 15 eruptive events during May 2019. An event that lasted for two hours on 12 May produced an ash plume that rose 2,900 m from the summit and drifted NE (figure 84). The Meteorological Observatory reported 14 days with ashfall during the month. Two thermal anomalies were present in satellite imagery in the Minamidake crater on both 17 and 22 May.

Figure 84. An ash plume rose 2,900 m above the summit of Sakurajima at Aira on 12 May 2019 (left); incandescent ejecta went 1,300 m from the summit crater on 13 May. Courtesy of JMA (Explanation of volcanic activity in Sakurajima, May 2019).

During June 2019 five eruptive events were reported, on 11, 13, and 24 June; the event on 11 June lasted for almost two hours, sent ash 2,200 m above the summit, and produced ejecta that landed up to 1,100 m from the crater (figure 85). Five days of ashfall were reported by KRMO.

Figure 85. A large ash plume on 11 June 2019 rose 2,200 m above the summit of Sakurajima volcano at Aira. Courtesy of Aone Wakatsuki.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Mike Day, Minnesota, Twitter (URL: https://twitter.com/MikeDaySMM, photo at https://twitter.com/MikeDaySMM/status/1083489400451989505/photo/1); Kratü, Twitter (URL: https://twitter.com/TalesOfKratue, photo at https://twitter.com/TalesOfKratue/status/1101469595414589441/photo/1); Tim Board, Japan, Twitter (URL: https://twitter.com/Hawkworld_, photo at https://twitter.com/Hawkworld_/status/1107789108754038789); Aone Wakatsuke, Twitter (URL: https://twitter.com/AoneWakatsuki, photo at https://twitter.com/AoneWakatsuki/status/1138420031258210305/photo/3).

After a large, deadly explosive and effusive eruption during 1963-64, Indonesia's Mount Agung on Bali remained quiet until a new eruption began in November 2017 (BGVN 43:01). Lava emerged into the summit crater at the end of November and intermittent ash plumes rose as high as 3 km above the summit through the end of the year. Activity continued throughout 2018 with explosions that produced ash plumes rising multiple kilometers above the summit, and the slow effusion of the lava within the summit crater (BGVN 43:08, 44:02). Information about the ongoing eruptive episode comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Indonesian Center for Volcanology and Geological Hazard Mitigation (CVGHM), the Darwin Volcanic Ash Advisory Center (VAAC), and multiple sources of satellite data. This report covers the ongoing eruption from February through May 2019.

Intermittent but increasingly frequent and intense explosions with ash emissions and incandescent ejecta characterized activity at Agung during February through May 2019. During February, explosions were reported three times; events on seven days in March were documented with ash plumes and ashfall in surrounding villages. Five significant events occurred during April; two involved incandescent ejecta that traveled several kilometers from the summit, and ashfall tens of kilometers from the volcano. Most of the five significant events reported in May involved incandescent ejecta and ashfall in adjacent villages; air traffic was disrupted during the 24 May event. Ash plumes in May reached altitudes over 7 km multiple times. Thermal activity increased steadily during the period, according to both the MIROVA project (figure 44) and MODVOLC thermal alert data. MAGMA Indonesia reported at the end of May 2019 that the volume of lava within the summit crater remained at about 25 million m³; satellite information indicated continued thermal activity within the crater. Alert Level III (of four levels) remained in effect throughout the period with a 4 km exclusion radius around the volcano.

Figure 44. Thermal activity at Agung from 4 September 2018 through May 2019 was variable. The increasing frequency and intensity of thermal events was apparent from February-May. Courtesy of MIROVA.

Steam plumes rose 30-300 m high daily during February 2019. The Agung Volcano Observatory (AVO) and PVMBG issued a VONA on 7 February (UTC) reporting an ash plume, although it was not visible due to meteoric cloud cover. Incandescence, however, was observed at the summit from webcams in both Rendang and Karangasem City (16 km SE). The seismic event associated with the explosion lasted for 97 seconds. A similar event on 13 February was also obscured by clouds but produced a seismic event that lasted for 3 minutes and 40 seconds, and ashfall was reported in the village of Bugbug, about 20 km SE. On 22 February a gray ash plume rose 700 m from the summit during a seismic event that lasted for 6 minutes and 20 seconds (figure 45). The Darwin VAAC reported the plume visible in satellite imagery moving W at 4.3 km altitude. It dissipated after a few hours, but a hotspot remained visible about 10 hours later.

Figure 45. An ash plume rose from the summit of Agung on 22 February 2019, viewed from the Besakih temple, 7 km SW of the summit. Courtesy of PunapiBali.

Persistent steam plumes rose 50-500 m from the summit during March 2019. An explosion on 4 March was recorded for just under three minutes and produced ashfall in Besakih (7 km SW); no ash plume was observed due to fog. A short-lived ash plume rose to 3.7 km altitude and drifted SE on 8 March (UTC) 2019. The seismic event lasted for just under 4 minutes. Ash emissions were reported on 15 and 17 March to 4.3 and 3.7 km altitude, respectively, drifting W (figure 46). Ashfall from the 15 March event spread NNW and was reported in the villages of Kubu (6 km N), Tianyar (14 km NNW), Ban, Kadundung, and Sukadana. MAGMA Indonesia noted that two explosions on the morning of 17 March (local time) produced gray plumes; the first sent a plume to 500 m above the summit drifting E and lasted for about 40 seconds, while the second plume a few hours later rose 600 m above the crater and lasted for 1 minute and 16 seconds. On 18 March an ash plume rose 1 km and drifted W and NW. An event on 20 March was measured only seismically by PVMBG because fog prevented observations. An eruption on 28 March produced an ash plume 2 km high that drifted W and NW. The seismic signal for this event lasted for about two and a half minutes. The Darwin VAAC reported the ash plume at 5.5 km altitude, dissipating quickly to the NW. No ash was visible four hours later, but a thermal anomaly remained at the summit (figure 47). Ashfall was reported in nearby villages.

Figure 46. Ash plumes from Agung on 15 (left) and 17 (right) March 2019 resulted in ashfall in communities 10-20 km from the volcano. Courtesy of PVMBG and MAGMA Indonesia (Information on G. Agung Eruption, 15 March 2019 and Gunung Agung Eruption Press Release March 17, 2019).

Figure 47. A thermal anomaly was visible through thick cloud cover at the summit of Agung on 29 March 2019 less than 24 hours after a gray ash plume was reported 2,000 m above the summit. "Atmospheric Penetration" rendering (bands 12, 11, and 8A) courtesy of Sentinel Hub Playground.

The first explosion of April 2019 occurred on the 3rd (UTC); PVMBG reported the dense gray ash plume 2 km above the summit drifting W. A few hours later the Darwin VAAC raised the altitude to 6.1 km based on infrared temperatures in satellite imagery. The seismic signal lasted for three and a half minutes and the explosion was heard at the PGA Post in Rendang (12 km SW). Incandescent material fell within a radius of 2-3 km, mainly on the S flank (figure 48). Ashfall was reported in the villages of Telungbuana, Badeg, Besakih, Pempatan, Teges, and Puregai on the W and S flanks (figure 49). An explosion on 11 April also produced a dense gray ash plume that rose 2 km above the summit and drifted W. A hotspot remained about six hours later after the ash dissipated.

Figure 48. Incandescent ejecta appeared on the flanks of Agung after an eruption on 4 April 2019 (local time) as viewed from the observation post in Rendang (8 km SW). Courtesy of Jamie Sincioco.

Figure 49. Ashfall in a nearby town dusted mustard plants on 4 April 2019 from an explosion at Agung the previous day. Courtesy of Pantau.com (Photo: Antara / Nyoman Hendra).

PVMBG reported an eruption visible in the webcam early on 21 April (local time) that rose to 5.5 km altitude and drifted SW. The ash spread W and S and ash fell around Besakih (7 km SW), Rendang (8 km SW), Klungkung (25 km S), Gianyar (20 km WSW), Bangli (17 km WNW), Tabanan (50 km WSW), and at the Ngurah Rai-Denpasar Airport (60 km SW). About 15 hours later a new explosion produced a dense gray ash plume that rose to 3 km above the summit and produced incandescent ejecta in all directions as far as 3 km away (figure 50). The ash spread to the S and ashfall was reported in Besakih, Rendang, Sebudi (6 km SW), and Selat (12 km SSW). Both of the explosions were heard in Rendang and Batulompeh. The incandescent ejecta from the explosions remained within the 4-km exclusion zone. A satellite image on 23 April showed multiple thermal anomalies within the summit crater (figure 51). A dense gray plume drifted E from Agung on 29 April (30 April local time) at 4.6 km altitude. It was initially reported by ground observers, but was also visible in multispectral satellite imagery for about six hours before dissipating.

Figure 50. An explosion at Agung on 21 April 2019 sent incandescent eject 3,000 m from the summit. Courtesy of MAGMA Indonesia (Gunung Agung Eruption Press Release April 21, 2019).

Figure 51. Multiple thermal anomalies were still present within the summit crater of Agung on 23 April 2019 after two substantial explosions produced ash and incandescent ejecta around the summit two days earlier. "Atmospheric Penetration" rendering (bands 12, 11, and 8A) courtesy of Sentinel Hub Playground.

PVMBG reported an eruption on 3 May 2019 that was recorded on a seismogram with a signal that lasted for about a minute. Satellite imagery reported by the Darwin VAAC showed a growing hotspot and possible ash near the summit at 4.3 km altitude moving NE. A few days later, on 6 May, a gray ash plume rose to 5.2 km altitude and drifted slowly W before dissipating; it was accompanied by a seismic signal that lasted for about two minutes. Explosions on 12 and 18 May produced significant amounts of incandescent ejecta (figure 52). The seismic signal for the 12 May event lasted for about two minutes; no plume was observed due to fog, but incandescent ejecta was visible on the flanks and the explosion was heard at Rendang. The Darwin VAAC reported an ash plume from the explosion on 17 May (18 May local time) at 6.1 km altitude in satellite imagery moving E. They revised the altitude a short while later to 7.6 km based on IR temperature and movement; the plume drifted N, NE, and E in light and variable winds. A few hours after that it was moving NE at 7.6 km altitude and SE at 5.5 km altitude; this lasted for about 12 hours until it dissipated. Ashfall was reported in villages downwind including Cutcut, Tongtongan, Bonyoh (20 km WNW), and Temakung.

Figure 52. Explosions on 12 (left) and 18 (right) May (local time) 2019 produced substantial ejecta on the flanks of Agung visible from a distance of 10 km or more in PVMBG webcams. The ash plume from the 18 May event resulted in ashfall in numerous communities downwind. Courtesy of PVMBG (Information Eruption G. Agung, May 13, 2019, Information Eruption G. Agung, May 18, 2019).

The initial explosion on 18 May was captured by a webcam at a nearby resort and sent incandescent ejecta hundreds of meters down the NE flank within 20 seconds (figure 53). Satellite imagery on 3, 8, 13, and 18 May indicated multiple thermal anomalies growing stronger at the summit. All of the images were captured within 24 hours of an explosive event reported by PVMBG (figure 54).

Figure 53. The 18 May 2019 explosion at Agung produced an ash plume that rose to over 7 km altitude and large bombs of incandescent material that traveled hundreds of meters down the NE flank within the first 20 seconds of the explosion. Images taken from a private webcam located 12 km NE. Courtesy of Volcanoverse, used with permission.

Figure 54. Satellite images from 3, 8, 13, and 18 May 2019 at Agung showed persistent and increasing thermal anomalies within the summit crater. All images were captured within 24 hours of explosions reported by PVMBG. "Atmospheric Penetration" rendering (bands 12, 11, and 8A) courtesy of Sentinel Hub Playground.

PVMBG issued a VONA on 24 May 2019 reporting a new ash emission. They indicated that incandescent fragments were ejected 2.5-3 km in all directions from the summit, and the seismic signal lasted for four and a half minutes (figure 55). A dense gray ash plume was observed from Tulamben on the NE flank rising 2 km above the summit. Satellite imagery indicated that the plume drifted SW and ashfall was reported in the villages of Besakih, Pempatan, Menanga, Sebudi, Muncan, Amerta Bhuana, Nongan, Rendang, and at the Ngurah Rai Airport in Denpassar. Additionally, ashfall was reported in the districts of Tembuku, Bangli, and Susut (20 km SW). The Darwin VAAC reported an ash plume visible in satellite imagery at 4.6 km altitude along with a thermal anomaly and incandescent lava visible in webcam imagery. The remains of the ash plume were about 170 km S of the airport in Denpasar (60 km SW) and had nearly dissipated 18 hours after the event. According to a news article several flights to and from Australia were cancelled or diverted, though the International Gusti Ngurah Rai (IGNR) airport was not closed. On 31 May another large explosion produced the largest ash plume of the report period, rising more than 2 km above the summit (figure 56). The Darwin VAAC reported its altitude as 8.2 km drifting ESE visible in satellite data. It split into two plumes, one drifted E at 8.2 km and the other ESE at 6.1 km altitude, dissipating after about 20 hours.

Figure 55. A large explosion at Agung on 24 May 2019 produced incandescent ejecta that covered all the flanks and dispersed ash to many communities to the SW. Courtesy of PVMBG (Gunung Agung Eruption Press Release 24 May 2019 20:38 WIB, Kasbani, Ir., M.Sc.).

Figure 56. An explosion at Agung on 31 May 2019 sent an ash plume to 8.2 km altitude, the highest for the report period. Courtesy of Sutopo Purwo Nugroho, BNPB.

Geologic Background. Symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The volcano, whose name means "Paramount," rises above the SE caldera rim of neighboring Batur volcano, and the northern and southern flanks extend to the coast. The summit area extends 1.5 km E-W, with the high point on the W and a steep-walled 800-m-wide crater on the E. The Pawon cone is located low on the SE flank. Only a few eruptions dating back to the early 19th century have been recorded in historical time. The 1963-64 eruption, one of the largest in the 20th century, produced voluminous ashfall along with devastating pyroclastic flows and lahars that caused extensive damage and many fatalities.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); 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); The Jakarta Post, Mount Agung eruption disrupts Australian flights, (URL: https://www.thejakartapost.com/news/2019/05/25/mount-agung-eruption-disrupts-australian-flights.html); PunapiBali (URL: http://punapibali.com/, Twitter: https://twitter.com/punapibali, image at https://twitter.com/punapibali/status/1098869352588288000/photo/1); Jamie S. Sincioco, Phillipines (URL: Twitter: https://twitter.com/jaimessincioco. Image at https://twitter.com/jaimessincioco/status/1113765842557104130/photo/1); Pantau.com (URL: https://www.pantau.com/berita/erupsi-gunung-agung-sebagian-wilayah-bali-terpapar-hujan-abu?utm_source=dlvr.it&utm_medium=twitter); Volcanoverse (URL: https://www.youtube.com/channel/UCi3T_esus8Sr9I-3W5teVQQ); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN ).

Frequently active, Indonesia's Mount Kerinci on Sumatra has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838. Intermittent explosions with ash plumes, usually multiple times per month, have characterized activity since April 2018. Similar activity continued during February-May 2019, the period covered in this report with information provided primarily by the Indonesian volcano monitoring agency, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, notices from the Darwin Volcano Ash Advisory Center (Darwin VAAC), and satellite data. PVMBG has maintained an Alert Level II (of 4) at Kerinci for several years.

On 13 February 2019 the Kerinci Volcano Observatory (KVO), part of PVMBG, noted a brownish-white ash emission that was drifting NE about 400 m above the summit. The seismicity during the event was dominated by continuous volcanic tremor. A brown ash emission was reported on 7 March 2019 that rose to 3.9 km altitude and drifted NE. Ash also drifted 1,300 m down the SE flank. Another ash plume the next morning drifted W at 4.5 km altitude, according to KVO. On 10, 11, and 13 March KVO reported brown ash plumes drifting NE from the summit at about 4.0-4.3 km altitude. The Darwin VAAC observed continuous ash emissions in satellite imagery on 15 March drifting W at 4.3 m altitude that dissipated after about 3 hours (figure 10). A gray ash emission was reported on 19 March about 600 m above the summit drifting NE; local news media noted that residents of Kayo Aro reported emissions on both 18 and 19 March (figure 11). An ash emission appeared in satellite imagery on 25 March (figure 10). On 30 March the observatory reported two ash plumes; a brown emission at 0351 UTC and a gray emission at 0746 UTC that both drifted NE at about 4.4 km altitude and dissipated within a few hours. PVMBG reported another gray ash plume the following day at a similar altitude.

Figure 10. Sentinel-2 satellite imagery of Kerinci from 15 (left) and 25 (right) March 2019 showed evidence of ash plumes rising from the summit. Kerinci's summit crater is about 500 m wide. "Geology" rendering (bands 12, 4, 2), courtesy of Sentinel Hub Playground.

Figure 11. Dense ash plumes from Kerinci were reported by local news media on 18 and 19 March 2019. Courtesy of Nusana Jambi.

Activity continued during April with a brown ash emission reported on 3 April by several different agencies; the Darwin VAAC and PVMBG daily reports noted that the plume was about 500 m above the summit (4.3 km altitude) drifting NE. KVO observed two brown ash emissions on 13 April (UTC) that rose to 4.2 km altitude and drifted NE. Satellite imagery showed minor ash emissions from the summit on 14 April; steam plumes 100-500 m above the summit characterized activity for the remainder of April (figure 12).

Figure 12. A dilute ash emission rose from the summit of Kerinci on 14 April 2019 (left); only steam emissions were present on a clear 29 April in Sentinel-2 imagery (right). "Geology" rendering (bands 12, 4, 2), courtesy of Sentinel Hub Playground.

Ashfall on the NE and S flanks within 7 km of the volcano was reported on 2 May 2019. According to a news article, at least five villages were affected late on 2 May, including Tanjung Bungo, Sangir, Sangir Tengah, Sungai Rumpun, and Bendung Air (figures 13 and 14). The smell of sulfur was apparent in the villages. Brown ash emissions were observed on 3 and 4 May that rose to 4.6 and 4.1 km altitude and drifted SE. The Darwin VAAC reported an emission on 5 May, based on a pilot report, that rose to 6.7 km altitude and drifted NE for about an hour before dissipating. A brown ash emission on 10 May rose 700 m above the summit and drifted SE. Satellite imagery captured ash emissions from the summit on 14 and 24 May (figure 15). For the remainder of the month, 300-700-m-high dense steam plumes were noted daily until PVMBG reported white and brown plumes on 26 and 27 May rising 500-1,000 m above the summit. Although thermal anomalies were not reported during the period, persistent weak SO₂ emissions were identified in TROPOMI instrument satellite data multiple times per month (figure 16).

Figure 13. Ashfall was reported from five villages on the flanks of Kerinci on 2 May 2019. Courtesy of Uzone.

Figure 14. An ash plume at Kerinci rose hundreds of meters on 2 May 2019; ashfall was reported in several nearby villages. Courtesy of Kerinci Time.

Figure 15. Ash emissions from Kerinci were captured in Sentinel-2 satellite imagery on 14 (left) and 24 (right) May 2019. The summit crater is about 500 m wide. "Geology" rendering (bands 12, 4, 2), courtesy of Sentinel Hub Playground.

Figure 16. Weak SO2 anomalies from Kerinci emissions were captured by the TROPOMI instrument on the Sentinel-5P satellite multiple times each month from February to May 2019. Courtesy of NASA Goddard Space Flight Center.

Geologic Background. Gunung Kerinci in central Sumatra forms Indonesia's highest volcano and is one of the most active in Sumatra. It is capped by an unvegetated young summit cone that was constructed NE of an older crater remnant. There is a deep 600-m-wide summit crater often partially filled by a small crater lake that lies on the NE crater floor, opposite the SW-rim summit. The massive 13 x 25 km wide volcano towers 2400-3300 m above surrounding plains and is elongated in a N-S direction. Frequently active, Kerinci has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Nuansa Jambi, Informasi Utama Jambi: (URL: https://nuansajambi.com/2019/03/20/gunung-kerinci-semburkan-asap-tebal/); Kerinci Time (URL: https://kerincitime.co.id/gunung-kerinci-semburkan-abu-vulkanik.html); Uzone.id (URL: https://news.uzone.id/gunung-kerinci-erupsi-5-desa-tertutup-abu-tebal).

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/).