Shishaldin is located on the eastern half of Unimak Island, one of the Aleutian Islands. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. The previous eruption ended in May 2020 and was characterized by intermittent thermal activity, increased seismicity and surface temperatures, ash plumes, and ash deposits (BGVN 45:06). This report covers a new eruption during July through November 2023, which consisted of significant explosions, ash plumes, ashfall, and lava fountaining. Information comes from daily, weekly, and special reports from the Alaska Volcano Observatory (AVO) and various satellite data. AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

AVO reported that intermittent tremor and low-frequency earthquakes had gradually become more regular and consistent during 10-13 July. Strongly elevated surface temperatures at the summit were identified in satellite images during 10-13 July. On 11 July AVO raised the Aviation Color Code (ACC) to Yellow (the second color on a four-color scale) and Volcano Alert Level (VAL) to Advisory (the second level on a four-level scale) at 1439. Later in the day on 11 July summit crater incandescence was observed in webcam images. Observations of the summit suggested that lava was likely present at the crater, which prompted AVO to raise the ACC to Orange (the second highest color on a four-color scale) and the VAL to Watch (the second highest level on a four-level scale). The US Coast Guard conducted an overflight on 12 July and confirmed that lava was erupting from the summit. That same day, sulfur dioxide emissions were detected in satellite images.

A significant explosion began at 0109 on 14 July that produced an ash plume that rose to 9-12 km altitude and drifted S over the Pacific Ocean (figure 43). Webcam images and photos taken around 0700 from a ship SW off Unimak Island showed small lahar deposits, which were the result of the interaction of hot pyroclastic material and snow and ice on the flanks. There was also ashfall on the SW and N flanks. A smaller explosion at 0710 generated an ash plume that rose to 4.5 km altitude. Webcam images and pilot reports showed continued low-level ash emissions during the morning, rising to less than 4.6 km altitude; those emissions included a small ash plume near the summit around 1030 resulting from a small explosion.

Figure 43. Photo of a strong ash plume that rose to 9-12 km altitude on the morning of 14 July 2023. Lahar deposits were visible on the SW flank (white arrows). Photo has been color corrected. Courtesy of Christopher Waythomas, AVO.

Seismic tremor amplitude began increasing at around 1700 on 15 July; strongly elevated surface temperatures were also reported. An ash plume rose to 4.6 km altitude and drifted SSE at 2100, based on a satellite image. A continuous ash plume during 2150 through 2330 rose to 5 km altitude and extended 125 km S. At 2357 AVO raised the ACC to Red (the highest color on a four-color scale) and the VAL to Warning (the highest level on a four-level scale), noting that seismicity remained elevated for more than six hours and explosion signals were frequently detected by regional infrasound (pressure sensor) networks. Explosions generated an ash plume that rose to 4.9 km altitude and drifted as far as 500 km SE. Activity throughout the night declined and by 0735 the ACC was lowered to Orange and the VAL to Watch. High-resolution satellite images taken on 16 July showed pyroclastic deposits extending as far as 3 km from the vent; these deposits generated lahars that extended further down the drainages on the flanks. Ash deposits were mainly observed on the SSE flank and extended to the shore of Unimak Island. During 16-17 July lava continued to erupt at the summit, which caused strongly elevated surface temperatures that were visible in satellite imagery.

Lava effusion increased at 0100 on 18 July, as noted in elevated surface temperatures identified in satellite data, increasing seismic tremor, and activity detected on regional infrasound arrays. A significant ash plume at 0700 rose to 7 km altitude and continued until 0830, eventually reaching 9.1 km altitude and drifting SSE (figure 44). As a result, the ACC was raised to Red and the VAL to Warning. By 0930 the main plume detached, but residual low-level ash emissions continued for several hours, remaining below 3 km altitude and drifting S. The eruption gradually declined and by 1208 the ACC was lowered to Orange and the VAL was lowered to Watch. High-resolution satellite images showed ash deposits on the SW flank and pyroclastic deposits on the N, E, and S flanks, extending as far as 3 km from the vent; lahars triggered by the eruption extended farther down the flanks (figure 45). Lava continued to erupt from the summit crater on 19 July.

Figure 44. Photo of an ash-rich plume rising above Shishaldin to 9.1 km altitude on 18 July 2023 that drifted SE. View is from the N of the volcano and Isanotski volcano is visible on the left-hand side of the image. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Figure 45. Near-infrared false-color satellite image of Shishaldin taken on 18 July 2023 showing ash deposits on the N, E, and S flanks extending as far as 3 km from the vent due to recent eruption events. Courtesy of Matthew Loewen, AVO.

Elevated surface temperatures were detected in satellite images during 19-25 July, despite occasional weather cloud cover, which was consistent with increased lava effusion. During 22-23 July satellite observations acquired after the eruption from 18 July showed pyroclastic flow and lahar deposits extending as far as 3 km down the N, NW, and NE flanks and as far as 1.5 km down the S and SE flanks. Ash deposits covered the SW and NE flanks. No lava flows were observed outside the crater. On 22 July a sulfur dioxide plume was detected in satellite data midday that had an estimated mass of 10 kt. In a special notice issued at 1653 on 22 July AVO noted that eruptive activity had intensified over the previous six hours, which was characterized by an hours-long steady increase in seismic tremor, intermittent infrasound signals consistent with small explosions, and an increase in surface temperatures that were visible in satellite data. Pilots first reported low-level ash plumes at around 1900. At 2320 an ash plume had risen to 9 km altitude based on additional pilot reports and satellite images. The ACC was increased to Red and the VAL to Warning at 2343. Satellite images indicated growth of a significantly higher ash plume that rose to 11 km altitude continued until 0030 and drifted NE. During the early morning hours of 23 July ash plumes had declined to 4.6 k altitude. Seismic tremor peaked at 0030 on 23 July and began to rapidly decline at 0109; active ash emissions were no longer visible in satellite data by 0130. The ACC was lowered to Orange and the VAL to Watch at 0418; bursts of increased seismicity were recorded throughout the morning, but seismicity generally remained at low levels. Elevated surface temperatures were visible in satellite data until about 0600. On 24 July pilots reported seeing vigorous gas-and-steam plumes rising to about 3 km altitude; the plumes may have contained minor amounts of ash.

During 24-25 July low level seismicity and volcanic tremor were detected at low levels following the previous explosion on 23 July. Strongly elevated surface temperatures were observed at the summit crater in satellite data. Around 2200 on 25 July seismicity began to increase, followed by infrasound signals of explosions after 0200 on 26 July. An ash plume rose to 3 km altitude at 0500 and drifted ENE, along with an associated sulfur dioxide plume that drifted NE and had an estimated mass of 22 kt. Diffuse ash emissions were visible in satellite data and rose to 6.1-7.6 km altitude and extended 125 km from the volcano starting around 1130. These ash events were preceded by about seven hours of seismic tremor, infrasound detections of explosions, and five hours of increased surface temperatures visible in satellite data. Activity began to decline around 1327, which included low-frequency earthquakes and decreased volcanic tremor, and infrasound data no longer detected significant explosions. Surface temperatures remained elevated through the end of the month.

Seismicity, volcanic tremor, and ash emissions remained at low levels during early August. Satellite images on 1 August showed that some slumping had occurred on the E crater wall due to the recent explosive activity. Elevated surface temperatures continued, which was consistent with cooling lava. On 2 August small explosive events were detected, consistent with low-level Strombolian activity. Some episodes of volcanic tremor were reported, which reflected low-level ash emissions. Those ash emissions rose to less than 3 km altitude and drifted as far as 92.6 km N. Pilots that were located N of the volcano observed an ash plume that rose to 2.7 km altitude. Seismicity began to increase in intensity around 0900 on 3 August. Seismicity continued to increase throughout the day and through the night with strongly elevated surface temperatures, which suggested that lava was active at the surface.

An ash cloud that rose to 7.6-7.9 km altitude and drifted 60-75 km NE was visible in a satellite image at 0520 on 4 August. Pilots saw and reported the plume at 0836 (figure 46). By 0900 the plume had risen to 9.1 km altitude and extended over 100 km NE. AVO raised the ACC to Red and the VAL to Warning as a result. Seismic tremor levels peaked at 1400 and then sharply declined at 1500 to slightly elevated levels; the plume was sustained during the period of high tremor and drifted N and NE. The ACC was lowered to Orange and the VAL to Watch at 2055. During 5-14 August seismicity remained low and surface temperatures were elevated based on satellite data due to cooling lava. On 9 August a small lava flow was observed that extended from the crater rim to the upper NE flank. It had advanced to 55 m in length and appeared in satellite imagery on 11 August. Occasional gas-and-steam plumes were noted in webcam images. At 1827 AVO noted that seismic tremor had steadily increased during the afternoon and erupting lava was visible at the summit in satellite images.

Figure 46. Photo showing an ash plume rising above Shishaldin during the morning of 4 August 2023 taken by a passing aircraft. The view is from the N showing a higher gas-rich plume and a lower gray ash-rich plume and dark tephra deposits on the volcano’s flank. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Strong explosion signals were detected at 0200 on 15 August. An ash cloud that was visible in satellite data extended 100 km NE and may have risen as high as 11 km altitude around 0240. By 0335 satellite images showed the ash cloud rising to 7.6 km altitude and drifting NE. Significant seismicity and explosions were detected by the local AVO seismic and infrasound networks, and volcanic lightning was detected by the World Wide Lightning Location Network (WWLLN). A sulfur dioxide plume associated with the eruption drifted over the S Bering Sea and parts of Alaska and western Canada. Seismicity was significantly elevated during the eruption but had declined by 1322. A pilot reported that ash emissions continued, rising as high as 4.9 km altitude. Elevated surface temperatures detected in satellite data were caused by hot, eruptive material (pyroclastic debris and lava) that accumulated around the summit. Eruptive activity declined by 16 August and the associated sulfur dioxide plume had mostly dissipated; remnants continued to be identified in satellite images at least through 18 August. Surface temperatures remained elevated based on satellite images, indicating hot material on the upper parts of the volcano. Small explosions were detected in infrasound data on the morning of 19 August and were consistent with pilot reports of small, short-lived ash plumes that rose to about 4.3 km altitude. Low-level explosive activity was reported during 20-24 August, according to seismic and infrasound data, and weather clouds sometimes prevented views. Elevated surface temperatures were observed in satellite images, which indicated continued hot material on the upper parts of the volcano.

Seismic tremor began to increase at around 0300 on 25 August and was followed by elevated surface temperatures identified in satellite images, consistent with erupting lava. Small explosions were recorded in infrasound data. The ACC was raised to Red and the VAL to Warning at 1204 after a pilot reported an ash plume that rose to 9.1 km altitude. Seismicity peaked at 1630 and began to rapidly decline at around 1730. Ash plumes rose as high as 10 km altitude and drifted as far as 400 km NE. By 2020 the ash plumes had declined to 6.4 km altitude and continued to drift NE. Ash emissions were visible in satellite data until 0000 on 26 August and seismicity was at low levels. AVO lowered the ACC to Orange and the VAL to Watch at 0030. Minor explosive activity within the summit crater was detected during 26-28 August and strongly elevated surface temperatures were still visible in satellite imagery through the rest of the month. An AVO field crew working on Unimak Island observed a mass flow that descended the upper flanks beginning around 1720 on 27 August. The flow produced a short-lived ash cloud that rose to 4.5 km altitude and rapidly dissipated. The mass flow was likely caused by the collapse of spatter that accumulated on the summit crater rim.

Similar variable explosive activity was reported in September, although weather observations sometimes prevented observations. A moderate resolution satellite image from the afternoon of 1 September showed gas-and-steam emissions filling the summit crater and obscuring views of the vent. In addition, hot deposits from the previous 25-26 August explosive event were visible on the NE flank near the summit, based on a 1 September satellite image. On 2 and 4 September seismic and infrasound data showed signals of small, repetitive explosions. Variable gas-and-steam emissions from the summit were visible but there was no evidence of ash. Possible summit crater incandescence was visible in nighttime webcam images during 3-4 September.

Seismicity began to gradually increase at around 0300 on 5 September and activity escalated at around 0830. A pilot reported an ash plume that rose to 7.6 km altitude at 0842 and continued to rise as high as possibly 9.7 km altitude and drifted SSE based on satellite images (figure 47). The ACC was raised to Red and the VAL to Warning at 0900. In addition to strong tremor and sustained explosions, the eruption produced volcanic lightning that was detected by the WWLLN. Around 1100 seismicity decreased and satellite data confirmed that the altitude of the ash emissions had declined to 7.6 km altitude. By 1200 the lower-altitude portion of the ash plume had drifted 125 km E. Significant ash emissions ended by 1330 based on webcam images. The ACC was lowered to Orange and the VAL to Watch at 1440. Satellite images showed extensive pyroclastic debris flows on most of the flanks that extended 1.2-3.3 km from the crater rim.

Figure 47. Webcam image taken from the S of Shishaldin showing a vertical ash plume on 5 September 2023. Courtesy of AVO.

During 6-13 September elevated surface temperatures continued to be observed in satellite data, seismicity remained elevated with weak but steady tremor, and small, low-frequency earthquakes and small explosions were reported, except on 12 September. On 6 September a low-level ash plume rose to 1.5-1.8 km altitude and drifted SSE. Occasional small and diffuse gas-and-steam emissions at the summit were visible in webcam images. Around 1800 on 13 September seismic tremor amplitudes began to increase, and small explosions were detected in seismic and infrasound data. Incandescent lava at the summit was seen in a webcam image taken at 0134 on 14 September during a period of elevated tremor. No ash emissions were reported during the period of elevated seismicity. Lava fountaining began around 0200, based on webcam images. Satellite-based radar observations showed that the lava fountaining activity led to the growth of a cone in the summit crater, which refilled most of the crater. By 0730 seismicity significantly declined and remained at low levels.

Seismic tremor began to increase around 0900 on 15 September and rapidly intensified. An explosive eruption began at around 1710, which prompted AVO to raise the ACC to Red and the VAL to Warning. Within about 30 minutes ash plumes drifted E below a weather cloud at 8.2 km altitude. The National Weather Service estimated that an ash-rich plume rose as high as 12.8 km altitude and produced volcanic lightning. The upper part of the ash plume detached from the vent around 1830 and drifted E, and was observed over the Gulf of Alaska. Around the same time, seismicity dramatically decreased. Trace ashfall was reported in the community of False Pass (38 km ENE) between 1800-2030 and also in King Cove and nearby marine waters. Activity declined at around 1830 although seismicity remained elevated, ash emissions, and ashfall continued until 2100. Lightning was again detected beginning around 1930, which suggested that ash emissions continued. Ongoing explosions were detected in infrasound data, at a lower level than during the most energetic phase of this event. Lightning was last detected at 2048. By 2124 the intensity of the eruption had decreased, and ash emissions were likely rising to less than 6.7 km altitude. Seismicity returned to pre-eruption levels. On 16 September the ACC was lowered to Orange and the VAL to Watch at 1244; the sulfur dioxide plume that was emitted from the previous eruption event was still visible over the northern Pacific Ocean. Elevated surface temperatures, gas-and-steam emissions from the vent, and new, small lahars were reported on the upper flanks based on satellite and webcam images. Minor deposits were reported on the flanks which were likely the result of collapse of previously accumulated lava near the summit crater.

Elevated seismicity with tremor, small earthquakes, and elevated surface temperatures were detected during 17-23 September. Minor gas-and-steam emissions were visible in webcam images. On 20 September small volcanic debris flows were reported on the upper flanks. On 21 September a small ash deposit was observed on the upper flanks extending to the NE based on webcam images. Seismic tremor increased significantly during 22-23 September. Regional infrasound sensors suggested that low-level eruptive activity was occurring within the summit crater by around 1800 on 23 September. Even though seismicity was at high levels, strongly elevated surface temperatures indicating lava at the surface were absent and no ash emissions were detected; weather clouds at 0.6-4.6 km altitude obscured views. At 0025 on 24 September AVO noted that seismicity continued at high levels and nearly continuous small infrasound signals began, likely from low-level eruptive activity. Strongly elevated surface temperatures were identified in satellite images by 0900 and persisted throughout the day; the higher temperatures along with infrasound and seismic data were consistent with lava erupting at the summit. Around 1700 similarly elevated surface temperatures were detected from the summit in satellite data, which suggested that more vigorous lava fountaining had started. Starting around 1800 low-level ash emissions rose to altitudes less than 4.6 km altitude and quickly dissipated.

Beginning at midnight on 25 September, a series of seismic signals consistent with volcanic flows were recorded on the N side of the volcano. A change in seismicity and infrasound signals occurred around 0535 and at 0540 a significant ash cloud formed and quickly reached 14 km altitude and drifted E along the Alaska Peninsula. The cloud generated at least 150 lightning strokes with thunder that could be heard by people in False Pass. Seismicity rapidly declined to near background levels around 0600. AVO increased the ACC to Red and the VAL to Warning at 0602. The ash cloud detached from the volcano at around 0700, rose to 11.6 km altitude, and drifted ESE. Trace to minor amounts of ashfall were reported by the communities of False Pass, King Cove, Cold Bay, and Sand Point around 0700. Ash emissions continued at lower altitudes of 6-7.6 km altitude at 0820. Small explosions at the vent area continued to be detected in infrasound data and likely represented low-level eruptive activity near the vent. Due to the significant decrease in seismicity and ash emissions the ACC was lowered to Orange and the VAL to Watch at 1234. Radar data showed significant collapses of the crater that occurred on 25 September. Satellite data also showed significant hot, degassing pyroclastic and lahar deposits on all flanks, including more extensive flows on the ENE and WSW sections below two new collapse scarps. Following the significant activity during 24-25 September, only low-level activity was observed. Seismicity decreased notably near the end of the strong activity on 25 September and continued to decrease through the end of the month, though tremor and small earthquakes were still reported. No explosive activity was detected in infrasound data through 2 October. Gas-and-steam emissions rose to 3.7 km altitude, as reported by pilots and seen in satellite images. Satellite data from 26 September showed that significant collapses had occurred at the summit crater and hot, steaming deposits from pyroclastic flows and lahars were present on all the flanks, particularly to the ENE and WSW. A small ash cloud was visible in webcam images on 27 September, likely from a collapse at the summit cone. High elevated surface temperatures were observed in satellite imagery during 27-28 September, which were likely the result of hot deposits on the flanks erupted on 25 September. Minor steaming at the summit crater and from an area on the upper flanks was visible in webcam images on 28 September.

During October, explosion events continued between periods of low activity. Seismicity significantly increased starting at around 2100 on 2 October; around the same time satellite images showed an increase in surface temperatures consistent with lava fountaining. Small, hot avalanches of rock and lava descended an unspecified flank. In addition, a distinct increase in infrasound, seismicity, and lightning detections was followed by an ash plume that rose to 12.2 km altitude and drifted S and E at 0520 on 3 October, based on satellite images. Nighttime webcam images showed incandescence due to lava fountaining at the summit and pyroclastic flows descending the NE flank. AVO reported that a notable explosive eruption started at 0547 and lasted until 0900 on 3 October, which prompted a rise in the ACC to Red and the VAL to Warning. Subsequent ash plumes rose to 6-7.6 km altitude by 0931. At 1036 the ACC was lowered back to Orange and the VAL to Watch since both seismic and infrasound data quieted substantially and were slightly above background levels. Gas-and-steam emissions were observed at the summit, based on webcam images. Trace amounts of ashfall were observed in Cold Bay. Resuspended ash was present at several kilometers altitude near the volcano. During the afternoon, low-level ash plumes were visible at the flanks, which appeared to be largely generated by rock avalanches off the summit crater following the explosive activity. These ash plumes rose to 3 km altitude and drifted W. Trace amounts of ashfall were reported by observers in Cold Bay and Unalaska and flights to these communities were disrupted by the ash cloud. Satellite images taken after the eruption showed evidence of pyroclastic flows and lahar deposits in drainages 2 km down the SW flank and about 3.2 km down the NE flank, and continued erosion of the crater rim. Small explosion craters at the end of the pyroclastic flows on the NE flank were noted for the first time, which may have resulted from gas-and-steam explosions when hot deposits interact with underlying ice.

During 4 October seismicity, including frequent small earthquakes, remained elevated, but was gradually declining. Ash plumes were produced for over eight hours until around 1400 that rose to below 3.7 km altitude. These ash plumes were primarily generated off the sides of the volcano where hot rock avalanches from the crater rim had entered drainages to the SW and NE. Two explosion craters were observed at the base of the NE deposits about 3.2 km from the crater rim. Webcam images showed the explosion craters were a source of persistent ash emissions; occasional collapse events also generated ash. Seismicity remained elevated with sulfur dioxide emissions that had a daily average of more than 1,000 tons per day, and frequent small earthquakes through the end of the month. Frequent elevated surface temperatures were identified in satellite images and gas-and-steam plumes were observed in webcam images, although weather conditions occasionally prevented clear views of the summit. Emissions were robust during 14-16 October and were likely generated by the interaction of hot material and snow and ice. During the afternoon of 21 October a strong gas-and-steam plume rose to 3-4.6 km altitude and extended 40 km WSW, based on satellite images and reports from pilots. On 31 October the ACC was lowered to Yellow and the VAL was lowered to Advisory.

Activity in November was characterized by elevated seismicity with ongoing seismic tremor and small, low-frequency earthquakes, elevated surface temperatures, and gas-and-steam emissions. There was an increase in seismic and infrasound tremor amplitudes starting at 1940 on 2 November. As a result, the ACC was again raised to Orange and the VAL was increased to Watch, although ash was not identified in satellite data. An ash cloud rose to 6.1 km altitude and drifted W according to satellite data at 2000. By 0831 on 3 November ash emissions were no longer visible in satellite images. On 6 and 9 November air pressure sensors detected signals consistent with small explosions. Small explosions were detected in infrasound data consistent with weak Strombolian activity on 19 and 21 November. Seismicity started to decrease on 21 November. On 25 November gas-and-steam emissions were emitted from the vent as well as from a scarp on the NE side of the volcano near the summit. A gas-and-steam plume extended about 50 km SSE and was observed in satellite and webcam images on 26 November. On 28 November small explosions were observed in seismic and local infrasound data and gas-and-steam emissions were visible from the summit and from the upper NE collapse scarp based on webcam images. Possible small explosions were observed in infrasound data on 30 November. Weakly elevated surface temperatures and a persistent gas-and-steam plume from the summit and collapse scarps on the upper flanks. A passing aircraft reported the gas-and-steam plume rose to 3-3.4 km altitude on 30 November, but no significant ash emissions were detected.

Satellite data. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed a strong pulse of thermal activity beginning in July 2023 that continued through November 2023 (figure 48). This strong activity was due to Strombolian explosions and lava fountaining events at the summit crater. According to data from MODVOLC thermal alerts, a total of 101 hotspots were detected near the summit crater in July (11-14, 16-19, 23-24 and 26), August (4, 25-26, and 29), September (5, 12, and 17), and October (3, 4, and 8). Infrared satellite data showed large lava flows descending primarily the northern and SE flanks during the reporting period (figure 49). Sulfur dioxide plumes often exceeded two Dobson Units (DUs) and drifted in different directions throughout the reporting period, based on satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 50).

Figure 48. Graph of Landsat 8 and 9 OLI thermal data from 1 June 2024 showing a strong surge in thermal activity during July through November 2023. During mid-October, the intensity of the hotspots gradually declined. Courtesy of MIROVA.

Figure 49. Infrared (bands B12, B11, B4) satellite images show several strong lava flows (bright yellow-orange) affecting the northern and SE flanks of Shishaldin on 18 July 2023 (top left), 4 June 2023 (top right), 26 September 2023 (bottom left), and 3 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 50. Strong sulfur dioxide plumes were detected at Shishaldin and drifted in different directions on 15 August 2023 (top left), 5 September 2023 (top right), 25 September 2023 (bottom left), and 6 October 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ioto (Iwo-jima), located about 1,200 km S of Tokyo, lies within a 9-km-wide submarine caldera along the Izu-Bonin-Mariana volcanic arc. Previous eruptions date back to 1889 and have consisted of dominantly phreatic explosions, pumice deposits during 2001, and discolored water. A submarine eruption during July through December 2022 was characterized by discolored water, pumice deposits, and gas emissions (BGVN 48:01). This report covers a new eruption during October through December 2023, which consisted of explosions, black ejecta, discolored water, and floating pumice, based on information from the Japan Meteorological Association (JMA), the Japan Coast Guard (JCG), and satellite data.

JMA reported that an eruption had been occurring offshore of Okinahama on the SE side of the island since 21 October, which was characterized by volcanic tremor, according to the Japan Maritime Self-Defense Force (JMSDF) Iwo Jima Air Base (figure 22). According to an 18 October satellite image a plume of discolored water at the site of this new eruption extended NE (figure 23). During an overflight conducted on 30 October, a vent was identified about 1 km off the coast of Okinahama. Observers recorded explosions every few minutes that ejected dark material about 20 m above the ocean and as high as 150 m. Ejecta from the vent formed a black-colored island about 100 m in diameter, according to observations conducted from the air by the Earthquake Research Institute of the University of Tokyo in cooperation with the Mainichi newspaper (figure 24). Occasionally, large boulders measuring more than several meters in size were also ejected. Observations from the Advanced Land Observing Satellite Daichi-2 and Sentinel-2 satellite images also confirmed the formation of this island (figure 23). Brown discolored water and floating pumice were present surrounding the island.

Figure 22. Map of Ioto showing the locations of recorded eruptions from 1889 through December 2023. The most recent eruption occurred during October through December 2023 and is highlighted in red just off the SE coast of the island and E of the 2001 eruption site. A single eruption highlighted in green was detected just off the NE coast of the island on 18 November 2023. From Ukawa et al. (2002), modified by JMA.

Figure 23. Satellite images showing the formation of the new island formation (white arrow) off the SE (Okinahama) coast of Ioto on 18 October 2023 (top left), 27 November 2023 (top right), 2 December 2023 (bottom left), and 12 December 2023 (bottom right). Discolored water was visible surrounding the new island. By December, much of the island had been eroded. Courtesy of Copernicus Browser.

Figure 24. Photo showing an eruption off the SE (Okinahama) coast of Ioto around 1230 on 30 October 2023. A column of water containing black ejecta is shown, which forms a new island. Occasionally, huge boulders more than several meters in size were ejected with the jet. Dark brown discolored water surrounded the new island. Photo has been color corrected and was taken from the S by the Earthquake Research Institute, University of Tokyo in cooperation of Mainichi newspaper. Courtesy of JMA.

The eruption continued during November. During an overflight on 3 November observers photographed the island and noted that material was ejected 169 m high, according to a news source. Explosions gradually became shorter, and, by the 3rd, they occurred every few seconds; dark and incandescent material were ejected about 800 m above the vent. On 4 November eruptions were accompanied by explosive sounds. Floating, brown-colored pumice was present in the water surrounding the island. There was a brief increase in the number of volcanic earthquakes during 8-14 November and 24-25 November. The eruption temporarily paused during 9-11 November and by 12 November eruptions resumed to the W of the island. On 10 November dark brown-to-dark yellow-green discolored water and a small amount of black floating material was observed (figure 25). A small eruption was reported on 18 November off the NE coast of the island, accompanied by white gas-and-steam plumes (figure 23). Another pause was recorded during 17-19 November, which then resumed on 20 November and continued erupting intermittently. According to a field survey conducted by the National Institute for Disaster Prevention Science and Technology on 19 November, a 30-m diameter crater was visible on the NE coast where landslides, hot water, and gray volcanic ash containing clay have occurred and been distributed previously. Erupted blocks about 10 cm in diameter were distributed about 90-120 m from the crater. JCG made observations during an overflight on 23 November and reported a phreatomagmatic eruption. Explosions at the main vent generated dark gas-and-ash plumes that rose to 200 m altitude and ejected large blocks that landed on the island and in the ocean (figure 26). Discolored water also surrounded the island. The size of the new island had grown to 450 m N-S x 200 m E-W by 23 November, according to JCG.

Figure 25. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 10 November showing discolored water and a small amount of black floating material were visible surrounding the island. Photo has been color corrected. Photographed by JCG courtesy of JMA.

Figure 26. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 23 November showing a phreatomagmatic eruption that ejected intermittent pulses of ash and dark material that rose to 200 m altitude. Photo has been color corrected. Photographed by JCG courtesy of JMA.

The eruption continued through 11 December, followed by a brief pause in activity, which then resumed on 31 December, according to JMA. Intermittent explosions produced 100-m-high black plumes at intervals of several minutes to 30 minutes during 1-10 December. Overflights were conducted on 4 and 15 December and reported that the water surrounding the new island was discolored to dark brown-to-dark yellow-green (figure 27). No floating material was reported during this time. In comparison to the observations made on 23 November, the new land had extended N and part of it had eroded away. In addition, analysis by the Geospatial Information Authority of Japan using SAR data from Daichi-2 also confirmed that the area of the new island continued to decrease between 4 and 15 December. Ejected material combined with wave erosion transformed the island into a “J” shape, 500-m-long and with the curved part about 200 m offshore of Ioto. The island was covered with brown ash and blocks, and the surrounding water was discolored to greenish-brown and contained an area of floating pumice. JCG reported from an overflight on 4 December that volcanic ash-like material found around the S vent on the NE part of the island was newly deposited since 10 November (figure 28). By 15 December the N part of the “J” shaped island had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands (figure 27).

Figure 27. Photos of the new island formed off the SE (Okinahama) coast of Ioto on 4 December 2023 (left) and 15 December 2023 (right). No gas-and-ash emissions or lava flows were observed on the new land. Additionally, dark brown-to-dark yellow-green discolored water was observed surrounding the new land. During 4 and 15 December, the island had eroded to where the N part of the “J” shape had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands. Courtesy of Copernicus Browser.

Figure 28. Photo of new volcanic ash-deposits (yellow dashed lines) near the S vent on the NE coast of Ioto taken by JCG on 4 December 2023. White gas-and-steam emissions were also visible (white arrow). Photo has been color corrected. Courtesy of JMA.

References. Ukawa, M., Fujita, E., Kobayashi, T., 2002, Recent volcanic activity of Iwo Jima and the 2001 eruption, Monthly Chikyu, Extra No. 39, 157-164.

Geologic Background. Ioto, in the Volcano Islands of Japan, lies within a 9-km-wide submarine caldera. The volcano is also known as Ogasawara-Iojima to distinguish it from several other "Sulfur Island" volcanoes in Japan. The triangular, low-elevation, 8-km-long island narrows toward its SW tip and has produced trachyandesitic and trachytic rocks that are more alkalic than those of other volcanoes in this arc. The island has undergone uplift for at least the past 700 years, accompanying resurgent doming of the caldera; a shoreline landed upon by Captain Cook's surveying crew in 1779 is now 40 m above sea level. The Motoyama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island's high point. Many fumaroles are oriented along a NE-SW zone cutting through Motoyama. Numerous recorded phreatic eruptions, many from vents on the W and NW sides of the island, have accompanied the uplift.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo22-2.htm); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Asahi, 5-3-2, Tsukiji, Chuo Ward, Tokyo, 104-8011, Japan (URL: https://www.asahi.com/ajw/articles/15048458).

Puracé, located in Colombia, is a stratovolcano that contains a 500-m-wide summit crater. It is part of the Los Coconucos volcanic chain that is a NW-SE trending group of seven cones and craters. The most recent eruption occurred during March 2022 that was characterized by frequent seismicity and gas-and-steam emissions (BGVN 47:06). This report covers a brief eruption during November 2023 based on monthly reports from the Popayán Observatory, part of the Servicio Geologico Colombiano (SGC).

Activity during November 2022 through November 2023 primarily consisted of seismicity: VT-type events, LP-type events, HB-type events, and TR-type events (table 4). Maximum sulfur dioxide values were measured weekly and ranged from 259-5,854 tons per day (t/d) during November 2022 through April 2023. White gas-and-steam emissions were also occasionally reported.

SGC issued a report on 25 October that noted a significant increase in the number of earthquakes associated with rock fracturing. These earthquakes were located SE of the crater between Puracé and Piocollo at depths of 1-4 km. There were no reported variations in sulfur dioxide values, but SGC noted high carbon dioxide values, compared to those recorded in the first half of 2023.

SGC reported that at 1929 on 16 November the seismic network detected a signal that was possibly associated with a gas-and-ash emission, though it was not confirmed in webcam images due to limited visibility. On 17 November an observer confirmed ash deposits on the N flank. Webcam images showed an increase in degassing both inside the crater and from the NW flank, rising 700 m above the crater.

Table 4. Seismicity at Puracé during November 2022-November 2023. Volcano-tectonic (VT), long-period (LP), hybrid (HB), and tremor (TR) events are reported each month. Courtesy of SGC.

Geologic Background. Puracé is an active andesitic volcano with a 600-m-diameter summit crater at the NW end of the Los Coconucos Volcanic Chain. This volcanic complex includes nine composite and five monogenetic volcanoes, extending from the Puracé crater more than 6 km SE to the summit of Pan de Azúcar stratovolcano. The dacitic massif which the complex is built on extends about 13 km NW-SE and 10 km NE-SW. Frequent small to moderate explosive eruptions reported since 1816 CE have modified the morphology of the summit crater, with the largest eruptions in 1849, 1869, and 1885.

Information Contacts: Servicio Geologico Colombiano (SGC), Diagonal 53 No. 34-53 - Bogotá D.C., Colombia (URL: https://www.sgc.gov.co/volcanes).

Etna, located on the Italian island of Sicily, has had documented eruptions dating back to 1500 BCE. Activity typically originates from multiple cones at the summit, where several craters have formed and evolved. The currently active craters are Northeast Crater (NEC), Voragine (VOR), and Bocca Nuova (BN), and the Southeast Crater (SEC); VOR and BN were previously referred to as the “Central Crater”. The original Southeast crater formed in 1978, and a second eruptive site that opened on its SE flank in 2011 was named the New Southeast Crater (NSEC). Another eruptive site between the SEC and NSEC developed during early 2017 and was referred to as the "cono della sella" (saddle cone). The current eruption period began in November 2022 and has been characterized by intermittent Strombolian activity, lava flows, and ash plumes (BGVN 48:08). This report updates activity during July through October 2023, which includes primarily gas-and-steam emissions; during July and August Strombolian explosions, lava fountains, and lava flows were reported, based on weekly and special reports by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV) and satellite data.

Variable fumarolic degassing was reported at all summit craters (BN, VOR, NEC, and SEC) throughout the entire reporting period (table 15). The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system based on the analysis of MODIS data showed frequent low-to-moderate power thermal anomalies during the reporting period (figure 399). During mid-August there was a pulse in activity that showed an increase in the power of the anomalies due to Strombolian activity, lava fountains, and lava flows. Infrared satellite imagery captured strong thermal anomalies at the central and southeast summit crater areas (figure 400). Accompanying thermal activity were occasional sulfur dioxide plumes that exceeded 2 Dobson Units (DUs) recorded by the TROPOMI instrument on the Sentinel-5P satellite (figure 401).

Table 15. Summary of activity at the four primary crater areas at the summit of Etna during July-October 2023. Information is from INGV weekly reports.

Figure 399. Frequent thermal activity at Etna varied in strength during July through October 2023, as shown on this MIROVA plot (Log Radiative Power). There was a spike in power during mid-August, which reflected an increase in Strombolian activity. Courtesy of MIROVA.

Figure 400. Infrared (bands B12, B11, B4) satellite images showing strong thermal anomalies at Etna’s central and Southeast crater areas on 21 July 2023 (top left), 27 August 2023 (top right), 19 September 2023 (bottom left), and 29 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 401. Sulfur dioxide plumes that exceeded 2 Dobson Units (DUs) rose above Etna on 14 July 2023 (top left), 14 August 2023 (top right), 2 September 2023 (bottom left), and 7 October 2023 (bottom right). These plumes drifted NE, S, SE, and SW, respectively. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during July and August was relatively low and mainly consisted of degassing at the summit craters, particularly at SEC and BN. Cloudy weather prevented clear views of the summit during early July. During the night of 2 July some crater incandescence was visible at SEC. Explosive activity resumed at SEC during 9-10 July, which was characterized by sporadic and weak ash emissions that rapidly dispersed in the summit area (figure 402). INGV reported moderate Strombolian activity began at 2034 on 14 July and was confined to the inside of the crater and fed by a vent located in the E part of SEC. An ash emission was detected at 2037. A new vent opened on 15 July in the SE part of BN and began to produce continuous gas-and-steam emissions. During an inspection carried out on 28 July pulsating degassing, along with audible booms, were reported at two active vents in BN. Vigorous gas-and-steam emissions intermittently generated rings. On rare occasions, fine, reddish ash was emitted from BN1 and resuspended by the gas-and-steam emissions.

Figure 402. Webcam image taken by the Monta Cagliato camera showing an ash emission rising above Etna’s Southeast Crater (SEC) on 10 July 2023. Photo has been color corrected. Courtesy of INGV (Report 28/2023, ETNA, Bollettino Settimanale, 03/07/2023 - 09/07/2023).

Around 2000 on 13 August INGV reported a sudden increase in volcanic tremor amplitude. Significant infrasonic activity coincided with the tremor increase. Incandescent flashes were visible through the cloud cover in webcam images of SEC (figure 403). Strombolian activity at SEC began to gradually intensify starting at 2040 as seismicity continued to increase. The Aviation Color Code (ACC) was raised to Yellow (the second lowest-level on a four-color scale) at 2126 and then to Orange (the second highest-level on a four-color scale) at 2129 due to above-background activity. The activity rapidly transitioned from Strombolian activity to lava fountains around 2333 that rose 300-400 m above the crater (figure 403). Activity was initially focused on the E vent of the crater, but then the vent located above the S flank of the cone also became active. A lava flow from this vent traveled SW into the drainage created on 10 February 2022, overlapping with previous flows from 10 and 21 February 2022 and 21 May 2023, moving between Monte Barbagallo and Monte Frumento Supino (figure 404). The lava flow was 350 m long, oriented NNE-SSW, and descended to an elevation of 2.8 km. Flows covered an area of 300,000 m² and had an estimated volume of 900,000 m³. The ACC was raised to Red at 2241 based on strong explosive activity and ashfall in Rifugio Sapienza-Piano Vetore at 1.7 km elevation on the S flank. INGV reported that pyroclastic flows accompanied this activity.

Figure 403. Webcam images of the lava fountaining event at Etna during 13-14 August 2023 taken by the Milos (EMV) camera. Images show the start of the event with increasing incandescence (a-b), varying intensity in activity (c-e), lava fountaining and pyroclastic flows (f-g), and a strong ash plume (g). Courtesy of INGV (Report 33/2023, ETNA, Bollettino Settimanale, 08/08/2023 - 14/08/2023).

Figure 404. Map of the new lava flow (yellow) and vent (red) at SEC (CSE) of Etna on 13 August 2023. The background image is a shaded model of the terrain of the summit area obtained by processing Skysat images acquired during on 18 August. The full extent of the lava flow was unable to be determined due to the presence of ash clouds. The lava flow extended more than 350 m to the SSW and reached an elevation of 2.8 km and was located W of Mt. Frumento Supino. CSE = Southeast Crater; CNE = Northeast Crater; BN = Bocca Nuova; VOR = Voragine. Courtesy of INGV (Report 34/2023, ETNA, Bollettino Settimanale, 14/08/2023 - 20/08/2023).

Activity peaked between 0240 and 0330 on 14 August, when roughly 5-6 vents erupted lava fountains from the E to SW flank of SEC. The easternmost vents produced lava fountains that ejected material strongly to the E, which caused heavy fallout of incandescent pyroclastic material on the underlying flank, triggering small pyroclastic flows. This event was also accompanied by lightning both in the ash column and in the ash clouds that were generated by the pyroclastic flows. A fracture characterized by a series of collapse craters (pit craters) opened on the upper SW flank of SEC. An ash cloud rose a few kilometers above the crater and drifted S, causing ash and lapilli falls in Rifugio Sapienza and expanding toward Nicolosi, Mascalucia, Catania, and up to Syracuse. Ashfall resulted in operational problems at the Catania airport (50 km S), which lasted from 0238 until 2000. By 0420 the volcanic tremor amplitude values declined to background levels. After 0500 activity sharply decreased, although the ash cloud remained for several hours and drifted S. By late morning, activity had completely stopped. The ACC was lowered to Orange as volcanic ash was confined to the summit area. Sporadic, minor ash emissions continued throughout the day. At 1415 the ACC was lowered to Yellow and then to Green at 1417.

During the night of 14-15 August only occasional flashes were observed, which were more intense during avalanches of material inside the eruptive vents. Small explosions were detected at SEC at 2346 on 14 August and at 0900 on 26 August that each produced ash clouds which rapidly dispersed into the atmosphere (figure 405). According to a webcam image, an explosive event detected at 2344 at SEC generated a modest ash cloud that was rapidly dispersed by winds. The ACC was raised to Yellow at 2355 on 14 August due to increasing unrest and was lowered to Green at 0954 on 15 August.

Figure 405. Webcam image of an ash plume rising above Etna’s SEC at 0902 (local time) on 26 August taken by the Montagnola EMOV camera. Photo has been color corrected. Courtesy of INGV (Report 35/2023, ETNA, Bollettino Settimanale, 21/08/2023 - 27/08/2023).

Activity during September and October was relatively low and mainly characterized by variable degassing from BN and SEC. Intense, continuous, and pulsating degassing was accompanied by roaring sounds and flashes of incandescence at BN both from BN1 and the new pit crater that formed during late July (figure 406). The degassing from the new pit crater sometimes emitted vapor rings. Cloudy weather during 6-8 September prevented observations of the summit craters .

Figure 406. Webcam image (top) showing degassing from Etna’s Bocca Nuova (BN) crater accompanied by nighttime crater incandescence at 0300 (local time) on 2 September 2023 by the Piedimonte Etneo (EPVH) camera and a photo of incandescence at BN1 and the new pit crater (bottom) taken by an observatory scientist from the E rim of BN during a survey on 2 September 2023. Courtesy of INGV (Report 36/2023, ETNA, Bollettino Settimanale, 28/08/2023 - 03/09/2023).

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

Information Contacts: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Suwanosejima is an 8-km-long island that consists of a stratovolcano and two active summit craters, located in the northern Ryukyu Islands, Japan. Volcanism over the past century has been characterized by Strombolian explosions, ash plumes, and ashfall. The current eruption began in October 2004 and has more recently consisted of frequent eruption plumes, explosions, and incandescent ejecta (BGVN 48:07). This report covers similar activity of ash plumes, explosions, and crater incandescence during July through October 2023 using monthly reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity during the reporting period was relatively low; only one low-power thermal anomaly was detected during mid-July and one during early August, based on a MIROVA (Middle InfraRed Observation of Volcanic Activity) Log Radiative Power graph of the MODIS thermal anomaly data. On two clear weather days, a thermal anomaly was visible in infrared satellite images (figure 81).

Figure 81. Infrared (bands B12, B11, B4) satellite imagery showing a thermal anomaly (bright yellow-orange) at the Otake crater of Suwanosejima on 23 September 2023 (left) and 18 October 2023 (right). Courtesy of Copernicus Browser.

Low-level activity was reported at the Otake crater during July and no explosions were detected. Eruption plumes rose as high as 1.8 km above the crater. On 13 July an ash plume rose 1.7 km above the crater rim, based on a webcam image. During the night of the 28th crater incandescence was visible in a webcam image. An eruptive event reported on 31 July produced an eruption plume that rose 2.1 km above the crater. Seismicity consisted of 11 volcanic earthquakes on the W flank, the number of which had decreased compared to June (28) and 68 volcanic earthquakes near the Otake crater, which had decreased from 722 in the previous month. According to observations conducted by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Toshima Village, and JMA, the amount of sulfur dioxide emissions released during the month was 400-800 tons per day (t/d).

Eruptive activity in the Otake crater continued during August and no explosions were reported. An eruptive event produced a plume that rose 1 km above the crater at 1447 on 12 August. Subsequent eruptive events were recorded at 0911 on 16 August, at 1303 on 20 August, and at 0317 on 21 August, which produced ash plumes that rose 1-1.1 km above the crater and drifted SE, SW, and W. On 22 August an ash plume was captured in a webcam image rising 1.4 km above the crater (figure 82). Multiple eruptive events were detected on 25 August at 0544, 0742, 0824, 1424, and 1704, which generated ash plumes that rose 1.1-1.2 km above the crater and drifted NE, W, and SW. On 28 August a small amount of ashfall was observed as far as 1.5 km from the crater. There were 17 volcanic earthquakes recorded on the W flank of the volcano and 79 recorded at the Otake crater during the month. The amount of sulfur dioxide emissions released during the month was 400-800 t/d.

Figure 82. Webcam image of an ash plume rising 1.4 km above Suwanosejima’s Otake crater rim on 22 August 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, August 2023).

Activity continued at the Otake crater during September. Occasionally, nighttime crater incandescence was observed in webcam images and ashfall was reported. An eruptive event at 1949 on 4 September produced an ash plume that rose 1 km above the crater and drifted SW. On 9 September several eruption events were detected at 0221, 0301, and 0333, which produced ash plumes that rose 1.1-1.4 km above the crater rim and drifted W; continuous ash emissions during 0404-0740 rose to a maximum height of 2 km above the crater rim (figure 83). More eruptive events were reported at 1437 on 10 September, at 0319 on 11 September, and at 0511 and 1228 on 15 September, which generated ash plumes that rose 1-1.8 km above the crater. During 25, 27, and 30 September, ash plumes rose as high as 1.3 km above the crater rim. JMA reported that large blocks were ejected as far as 300 m from the center of the crater. There were 18 volcanic earthquakes detected beneath the W flank and 82 volcanic earthquakes detected near the Otake crater. The amount of sulfur dioxide released during the month ranged from 600 to 1,600 t/d.

Figure 83. Webcam image of an ash plume rising 2 km above Suwanosejima’s Otake crater rim on 9 September 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, September 2023).

Activity during early-to-mid-October consisted of occasional explosions, a total number of 13, and ash plumes that rose as high as 1.9 km above the Otake crater rim on 29 October (figure 84). These explosions are the first to have occurred since June 2023. Continuous ash emissions were reported during 0510-0555 on 1 October. Explosions were recorded at 0304, 2141, and 2359 on 2 October, at 0112 on 3 October, and at 1326 on 6 October, which produced ash plumes that rose as high as 1 km above the crater rim and drifted SW and W. An explosion was noted at 0428 on 3 October, but emission details were unknown. A total of eight explosions were recorded by the seismic network at 1522 on 14 October, at 0337, 0433, 0555, 1008, and 1539 on 15 October, and at 0454 and 0517 on 16 October. Ash plumes from these explosions rose as high as 900 m above the crater and drifted SE. Eruptive events during 25-27 and 29-30 October generated plumes that rose as high as 1.9 km above the crater and drifted SE, S, and SW. Ash was deposited in Toshima village (3.5 km SSW). Eruptive activity occasionally ejected large volcanic blocks as far as 600 m from the crater. Nighttime crater incandescence was visible in webcams. Intermittent ashfall was reported as far as 1.5 km from the crater. There were 43 volcanic earthquakes detected on the W flank during the month, and 184 volcanic earthquakes detected near the Otake crater. The amount of sulfur dioxide emitted ranged between 400 and 900 t/d.

Figure 84. Webcam image of an ash plume rising 1.9 km above Suwanosejima’s Otake crater on 29 October 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, October 2023).

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Aira caldera, located in the northern half of Kagoshima Bay, Japan, contains the post-caldera Sakurajima volcano. Eruptions typically originate from the Minamidake crater, and since the 8th century, ash deposits have been recorded in the city of Kagoshima (10 km W), one of Kyushu’s largest cities. The Minamidake summit cone and crater has had persistent activity since 1955; the Showa crater on the E flank has also been intermittently active since 2006. The current eruption period began during March 2017 and has recently been characterized by intermittent explosions, eruption plumes, and ashfall (BGVN 48:07). This report updates activity during July through October 2023 and describes explosive events, ash plumes, nighttime crater incandescence, and ashfall, according to monthly activity reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity remained at low levels during this reporting period, according to the MIROVA (Middle InfraRed Observation of Volcanic Activity) system (figure 149). There was a slight increase in the number of anomalies during September through October. Occasional thermal anomalies were visible in infrared satellite images mainly at the Minamidake crater (Vent A is located to the left and Vent B is located to the right) (figure 150).

Table 30. Number of monthly explosive events, days of ashfall, area of ash covered, and sulfur dioxide emissions from Sakurajima’s Minamidake crater at Aira during July-October 2023. Note that smaller ash events are not listed. Ashfall days were measured at Kagoshima Local Meteorological Observatory and ashfall amounts represent material covering all the Kagoshima Prefecture. Data courtesy of JMA monthly reports.

Figure 149. Thermal activity at Sakurajima in the Aira caldera was relatively low during July through October 2023, based on this MIROVA graph (Log Radiative Power). There was an increase in the number of detected anomalies during September through October. Courtesy of MIROVA.

Figure 150. Infrared (bands B12, B11, B4) satellite images show a persistently strong thermal anomaly (bright yellow-orange) at the Minamidake crater at Aira’s Sakurajima volcano on 28 September 2023 (top left), 3 October 2023 (top right), 23 October 2023 (bottom left), and 28 October 2023 (bottom right). Vent A is located to the left and Vent B is to the right of Vent A; both vents are part of the Minamidake crater. Courtesy of Copernicus Browser.

JMA reported that during July, there were eight eruptions, three of which were explosion events in the Showa crater. Large blocks were ejected as far as 600 m from the Showa crater. Very small eruptions were occasionally reported at the Minamidake crater. Nighttime incandescence was observed in both the Showa and Minamidake crater. Explosions were reported on 16 July at 2314 and on 17 July at 1224 and at 1232 (figure 151). Resulting eruption plumes rose 700-2,500 m above the crater and drifted N. On 23 July the number of volcanic earthquakes on the SW flank of the volcano increased. A strong Mw 3.1 volcanic earthquake was detected at 1054 on 26 July. The number of earthquakes recorded throughout the month was 545, which markedly increased from 73 in June. No ashfall was observed at the Kagoshima Regional Meteorological Observatory during July. According to a field survey conducted during the month, the daily amount of sulfur dioxide emissions was 1,600-3,200 tons per day (t/d).

Figure 151. Webcam image showing a strong, gray ash plume that rose 2.5 km above the crater rim of Aira’s Showa crater at 1232 on 17 July 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, July 2023).

There were three eruptions reported at the Minamidake crater during August, each of which were explosive. The explosions occurred on 9 August at 0345, on 13 August at 2205, and on 31 August at 0640, which generated ash plumes that rose 800-2,000 m above the crater and drifted W. There were two eruptions detected at Showa crater; on 4 August at 2150 ejecta traveled 800 m from the Showa crater and associated eruption plumes rose 2.3 km above the crater. The explosion at 2205 on 13 August generated an ash plume that rose 2 km above the crater and was accompanied by large blocks that were ejected 600 m from the Minamidake crater (figure 152). Nighttime crater incandescence was visible in a high-sensitivity surveillance camera at both craters. Seismicity consisted of 163 volcanic earthquakes, 84 of which were detected on the SW flank. According to the Kagoshima Regional Meteorological Observatory there was a total of 7 g/m² of ashfall over the course of 10 days during the month. According to a field survey, the daily amount of sulfur dioxide emitted was 1,800-3,300 t/d.

Figure 152. Webcam image showing an eruption plume rising 2 km above the Minamidake crater at Aira at 2209 on 13 August 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, August 2023).

During September, four eruptions were reported, three of which were explosion events. These events occurred at 1512 on 9 September, at 0018 on 11 September, and at 2211 on 13 September. Resulting ash plumes generally rose 800-1,100 m above the crater. An explosion produced an ash plume at 2211 on 13 September that rose as high as 1.7 km above the crater. Large volcanic blocks were ejected 600 m from the Minamidake crater. Smaller eruptions were occasionally observed at the Showa crater. Nighttime crater incandescence was visible at the Minamidake crater. Seismicity was characterized by 68 volcanic earthquakes, 28 of which were detected beneath the SW flank. According to the Kagoshima Regional Meteorological Observatory there was a total of 3 g/m² of ashfall over the course of seven days during the month. A field survey reported that the daily amount of sulfur dioxide emitted was 1,600-2,300 t/d.

Eruptive activity during October consisted of 69 eruptions, 33 of which were described as explosive. These explosions occurred during 4 and 11-21 October and generated ash plumes that rose 500-3,600 m above the crater and drifted S, E, SE, and N. On 19 October at 1648 an explosion generated an ash plume that rose 3.6 km above the crater (figure 153). No eruptions were reported in the Showa crater; white gas-and-steam emissions rose 100 m above the crater from a vent on the N flank. Nighttime incandescence was observed at the Minamidake crater. On 24 October an eruption was reported from 0346 through 0430, which included an ash plume that rose 3.4 km above the crater. Ejected blocks traveled 1.2 km from the Minamidake crater. Following this eruption, small amounts of ashfall were observed from Arimura (4.5 km SE) and a varying amount in Kurokami (4 km E) (figure 154). The number of recorded volcanic earthquakes during the month was 190, of which 14 were located beneath the SW flank. Approximately 61 g/m² of ashfall was reported over eight days of the month. According to a field survey, the daily amount of sulfur dioxide emitted was 2,200-4,200 t/d.

Figure 153. Webcam image showing an ash plume rising 3.6 km above the Minamidake crater at Aira at 1648 on 19 October 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, October 2023).

Figure 154. Photo showing ashfall (light gray) in Kurokami-cho, Sakurajima on 24 October 2023 taken at 1148 following an eruption at Aira earlier that day. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, October 2023).

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 caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Nishinoshima is a small island in the Ogasawara Arc, about 1,000 km S of Tokyo, Japan. It contains prominent submarine peaks to the S, W, and NE. Recorded eruptions date back to 1973, with the current eruption period beginning in October 2022. Eruption plumes and fumarolic activity characterize recent activity (BGVN 48:10). This report covers the end of the eruption for September through October 2023, based on information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports, and satellite data.

No eruptive activity was reported during September 2023, although JMA noted that the surface temperature was slightly elevated compared to the surrounding area since early March 2023. The Japan Coast Guard (JCG) conducted an overflight on 20 September and reported white gas-and-steam plumes rising 3 km above the central crater of the pyroclastic cone, as well as multiple white gas-and-steam emissions emanating from the N, E, and S flanks of the crater to the coastline. In addition, dark reddish brown-to-green discolored water was distributed around almost the entire circumference of the island.

Similar low-level activity was reported during October. Multiple white gas-and-steam emissions rose from the N, E, and S flanks of the central crater of the pyroclastic cone and along the coastline; these emissions were more intense compared to the previous overflight observations. Dark reddish brown-to-green discolored water remained visible around the circumference of the island. On 4 October aerial observations by JCG showed a small eruption consisting of continuous gas-and-steam emissions emanating from the central crater, with gray emissions rising to 1.5 km altitude (figure 129). According to observations from the marine weather observation vessel Keifu Maru on 26 October, white gas-and-steam emissions persisted from the center of the pyroclastic cone, as well as from the NW, SW, and SE coasts of the island for about five minutes. Slightly discolored water was visible up to about 1 km.

Figure 129. Aerial photos of gray emissions rising from the central crater of Nishinoshima’s pyroclastic cone to an altitude of 1.5 km on 4 October 2023 taken at 1434 (left) and 1436 (right). Several white gas-and-steam emissions also rose from the N, E, and S flanks of the central crater. Both photos have been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, October, 2023).

Frequent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity) during September (figure 130). Occasional anomalies were detected during October, and fewer during November through December. A thermal anomaly was visible in the crater using infrared satellite imagery on 6, 8, 11, 16, 18, 21, and 23 September and 8, 13, 21, 26, and 28 October (figure 131).

Figure 130. Low-to-moderate power thermal anomalies were detected at Nishinoshima during September through December 2023, showing a decrease in the frequency of anomalies after September, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 131. Infrared (bands B12, B11, B4) satellite images showing a strong thermal anomaly at the crater of Nishinoshima on 21 September 2023 (left) and 13 October 2023 (right). A strong gas-and-steam plume accompanied the thermal activity, extending NW. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kīlauea is on the island of Hawai’i and overlaps the E flank of the Mauna Loa volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 magma migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. As lava emerged from the fissures, the lava lake at Halema'uma'u drained and explosions sent ash plumes to several kilometers altitude (BGVN 43:10).

The current eruption period started during September 2021 and has been characterized by low-level lava effusions in the active Halema’uma’u lava lake (BGVN 48:01). This report covers three notable eruption periods during February, June, and September 2023 consisting of lava fountaining, lava flows, and spatter during January through September 2023 using information from daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Activity during January 2023. Small earthquake swarms were recorded on 2 January 2023; increased seismicity and changes in the pattern of deformation were noted on the morning of 5 January. At around 1500 both the rate of deformation and seismicity drastically increased, which suggested magma movement toward the surface. HVO raised the Volcano Alert Level (VAL) to Watch (the second highest level on a four-level scale) and the Aviation Color Code (ACC) to Orange (the second highest color on a four-color scale) at 1520.

Multiple lava fountains and lava effusions from vents in the central eastern portion of the Halema’uma’u crater began on 5 January around 0434; activity was confined to the eastern half of the crater and within the basin of the western half of the crater, which was the focus of the eruption in 2021-2022 (figure 525). Incandescence was visible in webcam images at 1634 on 5 January, prompting HVO to raise the VAL to Warning (the highest level on a four-level scale) and the ACC to Red (the highest color on a four-color scale). Lava fountains initially rose as high as 50 m above the vent at the onset of the eruption (figure 526) but then declined to a more consistent 5-6 m height in the proceeding days. By 1930 that same day, lava had covered most of the crater floor (an area of about 1,200,000 m²) and the lava lake had a depth of 10 m. A higher-elevation island that formed during the initial phase of the December 2020 eruption remained exposed, appearing darker in images, along with a ring of older lava around the lava lake that was active prior to December 2022. Overnight during 5-6 January the lava fountains continued to rise 5 m high, and the lava effusion rate had slowed.

Figure 525. A reference map of Kīlauea showing activity on 6 January 2023, based on measurements taken from the crater rim at approximately 0900. Multiple eruptive vents (orange color) are on the E floor of Halema’uma’u crater effusing into a lava lake (red color). Lava from these vents flowed laterally across the crater floorcovering an area of 880,000 m2. The full extent of new lava from this eruption (red and pink colors) is approximately 1,120,000 m2. An elevated part of the lake (yellow color) that is higher in elevation compared to the rest of the crater floor was not covered in lava flows. Courtesy of USGS, HVO.

Figure 526. Image of the initial lava fountain at the onset of Kīlauea’s eruption on 5 January 2023 from a newly opened vent in the Halema’uma’u crater at 0449. This lava fountain rose as high as 50 m and ejected lava across the crater floor. Courtesy of USGS, HVO.

On 6 January at 0815 HVO lowered the VAL to Watch and the ACC to Orange due to the declining effusion rates. Sulfur dioxide emission rates ranged from 3,000-12,500 tonnes per day (t/d), the highest value of which was recorded on 6 January. Lava continued to erupt from the vents during 6-8 January, although the footprint of the active area had shrunk; a similar progression has been commonly observed during the early stages of recent eruptions at Halema’uma’u. On 9 January HVO reported one dominant lava fountain rising 6-7 m high in the E half of the crater. Lava flows built up the margins of the lake, causing the lake to be perched. On 10 January the eastern lava lake had an area of approximately 120,000 m² that increased to 250,000 m² by 17 January. During 13-31 January several small overflows occurred along the margins of the E lake. A smaller area of lava was active within the basin in the W half of the crater that had been the focus of activity during 2021-2022. On 19 January just after 0200 a small ooze-out was observed on the crater’s W edge.

Activity during February 2023. Activity continued in the E part of Halema’uma’u crater, as well as in a smaller basin in the W part of the 2021-2022 lava lake (figure 527). The E lava lake contained a single lava fountain and frequent overflows. HVO reported that during the morning of 1 February the large E lava lake began to cool and crust over in the center of the lake; two smaller areas of lava were observed on the N and S sides by the afternoon. The dominant lava fountain located in the S part of the lava lake paused for roughly 45 minutes at 2315 and resumed by midnight, rising 1-2 m. At 0100 on 2 February lava from the S part was effusing across the entire E lava lake area, covering the crusted over portion in the center of the lake and continuing across the majority of the previously measured 250,000 m² by 0400. A small lava pond near the E lake produced an overflow around 0716 on 2 February. On 3 February some lava crust began to form against the N and E levees, which defined the 250,000 m² eastern lava lake. The small S lava fountain remained active, rising 1-6 m high during 3-9 February; around 0400 on 5 February occasional bursts doubled the height of the lava fountain.

Figure 527. An aerial visual and thermal image taken of Kīlauea’s Halema’uma’u crater on 2 February 2023. The largest lava lake is in the E part of the crater, although lava has also filled areas that were previously active in the W part of the crater. The colors of the map indicate temperature, with blues indicative of cooler temperatures and reds indicative of warmer temperatures. Courtesy of USGS, HVO.

A large breakout occurred overnight during 2100 on 4 February to 0900 on 5 February on the N part of the crater floor, equal to or slightly larger in size than the E lava lake. A second, smaller lava fountain appeared in the same area of the E lava lake between 0300 and 0700 on 5 February and was temporarily active. This large breakout continued until 7 February. A small, brief breakout was reported in the S of the E lava lake around midnight on 7 February. In the W lake, as well as the smaller lava pond in the central portion of the crater floor, contained several overflows during 7-10 February and intermittent fountaining. Activity at the S small lava pond and the small S lava fountain within the E lake declined during 9-10 February. The lava pond in the central portion of the crater floor had nearly continuous, expansive flows during 10-13 February; channels from the small central lava pond seemed to flow into the larger E lake. During 13-18 February a small lava fountain was observed in the small lava pond in the central portion of the crater floor. Continuous overflows persisted during this time.

Activity in the eastern and central lakes began to decline in the late afternoon of 17 February. By 18 February HVO reported that the lava effusions had significantly declined, and that the eastern and central lakes were no longer erupting. The W lake in the basin remained active but at a greatly reduced level that continued to decline. HVO reported that this decrease in activity is attributed to notable deflationary tilt that began early on the morning of 17 February and lasted until early 19 February. By 19 February the W lake was mostly crusted over although some weak lava flows remained, which continued through 28 February. The sulfur dioxide emission rates ranged 250-2,800 t/d, the highest value of which was recorded on 6 February.

Activity during March 2023. The summit eruption at Halema’uma’u crater continued at greatly reduced levels compared to the previous two months. The E and central vents stopped effusing lava, and the W lava lake remained active with weak lava flows; the lake was mostly crusted over, although slowly circulating lava intermittently overturned the crust. By 6 March the lava lake in the W basin had stopped because the entire surface was crusted over. The only apparent surface eruptive activity during 5-6 March was minor ooze-outs of lava onto the crater floor, which had stopped by 7 March. Several hornitos on the crater floor still glowed through 12 March according to overnight webcam images, but they did not erupt any lava. A small ooze-out of lava was observed just after 1830 in the W lava lake on 8 March, which diminished overnight. The sulfur dioxide emission rate ranged from 155-321 t/d on 21 March. The VAL was lowered to Advisory, and the ACC was lowered to Yellow (the second lowest on a four-color scale) on 23 March due to a pause in the eruption since 7 March.

Activity during April-May 2023. The eruption at Halema’uma’u crater was paused; no lava effusions were visible on the crater floor. Sulfur dioxide emission rates ranged from 75-185 t/d, the highest of which was measured on 22 April. During May and June summit seismicity was elevated compared to seismicity that preceded the activity during January.

Activity during June 2023. Earthquake activity and changes in the patterns of ground deformation beneath the summit began during the evening of 6 June. The data indicated magma movement toward the surface, prompting HVO to raise the VAL to Watch and the ACC to Orange. At about 0444 on 7 June incandescence in Halema’uma’u crater was visible in webcam images, indicating that a new eruption had begun. HVO raised the VAL to Warning and the ACC to Red (the highest color on a four-color scale). Lava flowed from fissures that had opened on the crater floor. Multiple minor lava fountains were active in the central E portion of the Halema’uma’u crater, and one vent opened on the W wall of the caldera (figure 528). The eruptive vent on the SW wall of the crater continued to effuse into the lava lake in the far SW part of the crater (figure 529). The largest lava fountain consistently rose 15 m high; during the early phase of the eruption, fountain bursts rose as high as 60 m. Lava flows inundated much of the crater floor and added about 6 m depth of new lava within a few hours, covering approximately 10,000 m². By 0800 on 7 June lava filled the crater floor to a depth of about 10 m. During 0800-0900 the sulfur dioxide emission rate was about 65,000 t/d. Residents of Pahala (30 km downwind of the summit) reported minor deposits of fine, gritty ash and Pele’s hair. A small spatter cone had formed at the vent on the SW wall by midday, and lava from the cone was flowing into the active lava lake. Fountain heights had decreased from the onset of the eruption and were 4-9 m high by 1600, with occasional higher bursts. Inflation switched to deflation and summit earthquake activity greatly diminished shortly after the eruption onset.

Figure 528. Photo of renewed activity at Kīlauea’s Halema’uma’u crater that began at 0444 on 7 June 2023. Lava flows cover the crater floor and there are several active source vents exhibiting lava fountaining. Courtesy of USGS, HVO.

Figure 529. Photo of a lava fountain on the SW wall of Kīlauea’s Halema’uma’u crater on 7 June 2023. By midday a small cone structure had been built up. The fissure was intermittently obscured by gas-and-steam plumes. Courtesy of USGS, HVO.

At 0837 on 8 June HVO lowered the VAL to Watch and the ACC to Orange because the initial high effusion rates had declined, and no infrastructure was threatened. The surface of the lava lake had dropped by about 2 m, likely due to gas loss by the morning of 8 June. The drop left a wall of cooled lava around the margins of the crater floor. Lava fountain heights decreased during 8-9 June but continued to rise to 10 m high. Active lava and vents covered much of the W half of Halema’uma’u crater in a broad, horseshoe-shape around a central, uplifted area (figure 530). The preliminary average effusion rate for the first 24 hours of the eruption was about 150 cubic meters per second, though the estimate did not account for vesiculated lava and variations in crater floor topography. The effusion rate during the very earliest phases of the eruption appeared significantly higher than the previous three summit eruptions based on the rapid coverage of the entire crater floor. An active lava lake, also referred to as the “western lava lake” was centered within the uplifted area and was fed by a vent in the NE corner. Two small active lava lakes were located just SE from the W lava lake and in the E portion of the crater floor.

Figure 530. A compilation of thermal images taken of Kīlauea’s Halema’uma’u crater on 7 June 2023 (top left), 8 June 2023 (top right), 12 June 2023 (bottom left), and 16 June 2023 (bottom right). The initial high effusion rates that consisted of numerous lava fountains and lava flows that covered the entire crater floor began to decline and stabilize. A smaller area of active lava was detected in the SW part of the crater by 12 June. The colors of the thermal map represent temperature, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

During 8-9 June the lava in the central lava lake had a thickness of approximately 1.5 m, based on measurements from a laser rangefinder. During 9-12 June the height of the lava fountains decreased to 9 m high. HVO reported that the previously active lava lake in the E part of the crater appeared stagnant during 10-11 June. The surface of the W lake rose approximately 1 m overnight during 11-12 June, likely due to the construction of a levee around it. Only a few small fountains were active during 12-13 June; the extent of the active lava had retreated so that all activity was concentrated in the SW and central parts of Halema’uma’u crater. Intermittent spattering from the vent on the SW wall was visible in overnight webcam images during 13-18 June. On the morning of 14 June a weak lava effusion originated from near the western eruptive vent, but by 15 June there were no signs of continued activity. HVO reported that other eruptive vents in the SW lava lake had stopped during this time, following several days of waning activity; lava filled the lake by about 0.5 m. Lava circulation continued in the central lake and no active lava was reported in the northern or eastern parts of the crater. Around 0800 on 15 June the top of the SW wall spatter cone collapsed, which was followed by renewed and constant spattering from the top vent and a change in activity from the base vent; several new lava flows effused from the top of the cone, as well as from the pre-existing tube-fed flow from its base. Accumulation of lava on the floor resulted in a drop of the central basin relative to the crater floor, allowing several overflows from the SW lava lake to cascade into the basin during the night of 15 June into the morning of 16 June.

Renewed lava fountaining was reported at the eruptive vent on the SW side of the crater during 16-19 June, which effused lava into the far SW part of the crater. This activity was described as vigorous during midday on 16 June; a group of observatory geologists estimated that the lava was consistently ejected at least 10 m high, with some spatter ejected even higher and farther. Deposits from the fountain further heightened and widened the spatter cone built around the original eruptive vent in the lower section of the crater wall. Multiple lava flows from the base of the cone were fed into the SW lava lake and onto the southwestern-most block from the 2018 collapse within Halema’uma’u on 17 June (figure 531); by 18 June they focused into a single flow feeding into the SW lava lake. On the morning of 19 June a second lava flow from the base of the eruptive cone advanced into the SW lava lake.

Figure 531. Nighttime photo of the upwelling area at the base of the spatter cone at Kīlauea’s Halema’uma’u crater on 17 June 2023. This upwelling feeds a lava flow that spreads out to the E of the spatter cone. Courtesy of M. Cappos, USGS.

Around 1600 on 19 June there was a rapid decline in lava fountaining and effusion at the eruptive vent on the SW side of the crater; vent activity had been vigorous up to that point (figure 532). Circulation in the lava lake also slowed, and the lava lake surface dropped by several meters. Overnight webcam images showed some previously eruptive lava still flowing onto the crater floor, which continued until those flows began to cool. By 21 June no lava was erupting in Halema’uma’u crater. Overnight webcam images during 29-30 June showed some incandescence from previously erupted lava flows as they continued to cool. Seismicity in the crater declined to low levels. Sulfur dioxide emission rates ranged 160-21,000 t/d throughout the month, the highest measurement of which was recorded on 8 June. On 30 June the VAL was lowered to Advisory (the second level on a four-level scale) and the ACC was lowered to Yellow. Gradual inflation was detected at summit tiltmeters during 19-30 June.

Figure 532. Photos showing vigorous lava fountaining and lava flows at Kīlauea’s Halema’uma’u crater at the SW wall eruptive vent on 18 June 2023 at 1330 (left). The eruption stopped abruptly around 1600 on 19 June 2023 and no more lava effusions were visible, as seen from the SW wall eruptive vent at 1830 on 19 June 2023 (right). Courtesy of M. Patrick, USGS.

Activity during July-August 2023. During July, the eruption paused; no lava was erupting in Halema’uma’u crater. Nighttime webcam images showed some incandescence from previously erupted lava as it continued to cool on the crater floor. During the week of 14 August HVO reported that the rate in seismicity increased, with 467 earthquakes of Mw 3.2 and smaller occurring. Sulfur dioxide emission rates remained low, ranging from 75-86 t/d, the highest of which was recorded on 10 and 15 August. On 15 August beginning at 0730 and lasting for several hours, a swarm of approximately 50 earthquakes were detected at a depth of 2-3 km below the surface and about 2 km long directly S of Halema’uma’u crater. HVO reported that this was likely due to magma movement in the S caldera region. During 0130-0500 and 1700-2100 on 21 August two small earthquake swarms of approximately 20 and 25 earthquakes, respectively, occurred at the same location and at similar depths. Another swarm of 50 earthquakes were recorded during 0430-0830 on 23 August. Elevated seismicity continued in the S area through the end of the month.

Activity during September 2023. Elevated seismicity persisted in the S summit with occasional small, brief seismic swarms. Sulfur dioxide measurements were relatively low and were 70 t/d on 8 September. About 150 earthquakes occurred during 9-10 September, and tiltmeter and Global Positioning System (GPS) data showed inflation in the S portion of the crater.

At 0252 on 10 September HVO raised the VAL to Watch and the ACC to Orange due to increased earthquake activity and changes in ground deformation that indicated magma moving toward the surface. At 1515 the summit eruption resumed in the E part of the caldera based on field reports and webcam images. Fissures opened on the crater floor and produced multiple minor lava fountains and flows (figure 533). The VAL and ACC were raised to Warning and Red, respectively. Gas-and-steam plumes rose from the fissures and drifted downwind. A line of eruptive vents stretched approximately 1.4 km from the E part of the crater into the E wall of the down dropped block by 1900. The lava fountains at the onset of the eruption had an estimated 50 m height, which later rose 20-25 m high. Lava erupted from fissures on the down dropped block and expanded W toward Halema’uma’u crater. Data from a laser rangefinder recorded about 2.5 m thick of new lava added to the W part of the crater. Sulfur dioxide emissions were elevated in the eruptive area during 1600-1500 on 10 September, measuring at least 100,000 t/d.

Figure 533. Photo of resumed lava fountain activity at Kīlauea’s Halema’uma’u crater on 10 September 2023. The main lava fountain rises approximately 50 m high and is on the E crater margin. Courtesy of USGS, HVO.

At 0810 on 11 September HVO lowered the VAL and ACC back to Watch and Orange due to the style of eruption and the fissure location had stabilized. The initial extremely high effusion rates had declined (but remained at high levels) and no infrastructure was threatened. An eruption plume, mainly comprised of sulfur dioxide and particulates, rose as high as 3 km altitude. Several lava fountains were active on the W side of the down dropped block during 11-15 September, while the easternmost vents on the down dropped block and the westernmost vents in the crater became inactive on 11 September (figure 534). The remaining vents spanned approximately 750 m and trended roughly E-W. The fed channelized lava effusions flowed N and W into Halema’uma’u. The E rim of the crater was buried by new lava flows; pahoehoe lava flows covered most of the crater floor except areas of higher elevation in the SW part of the crater. The W part of the crater filled about 5 m since the start of the eruption, according to data from a laser rangefinder during 11-12 September. Lava fountaining continued, rising as high as 15 m by the morning of 12 September. During the morning of 13 September active lava flows were moving on the N and E parts of the crater. The area N of the eruptive vents that had active lava on its surface became perched and was about 3 m higher than the surrounding ground surface. By the morning of 14 September active lava was flowing on the W part of the down dropped block and the NE parts of the crater. The distances of the active flows progressively decreased. Spatter had accumulated on the S (downwind) side of the vents, forming ramparts about 20 m high.

Figure 534. Photo of a strong lava fountain in the E part of Kīlauea’s Halema’uma’u crater taken on the morning of 11 September 2023. The lava fountains rise as high as 10-15 m. Courtesy of J. Schmith, USGS.

Vigorous spattering was restricted to the westernmost large spatter cone with fountains rising 10-15 m high. Minor spattering occurred within the cone to the E of the main cone, but HVO noted that the fountains remained mostly below the rim of the cone. Lava continued to effuse from these cones and likely from several others as well, traveled N and W, confined to the W part of the down-dropped block and the NE parts of Halema’uma’u. Numerous ooze-outs of lava were visible over other parts of the crater floor at night. Laser range-finder measurements taken of the W part of the crater during 14-15 September showed that lava filled the crater by 10 m since the start of the eruption. Sulfur dioxide emissions remained elevated after the onset of the eruption, ranging 20,000-190,000 t/d during the eruption activity, the highest of which occurred on 10 September.

Field crews observed the eruptive activity on 15 September; they reported a notable decrease or stop in activity at several vents. Webcam images showed little to no fountaining since 0700 on 16 September, though intermittent spattering continued from the westernmost large cone throughout the night of 15-16 September. Thermal images showed that lava continued to flow onto the crater floor. On 16 September HVO reported that the eruption stopped around 1200 and that there was no observable activity anywhere overnight or on the morning of 17 September. HVO field crews reported that active lava was no longer flowing onto Halema’uma’u crater floor and was restricted to a ponded area N of the vents on the down dropped block. They reported that spattering stopped around 1115 on 16 September. Nighttime webcam images showed some incandescence on the crater floor as lava continued to cool. Field observations supported by geophysical data showed that eruptive tremor in the summit region decreased over 15-16 September and returned to pre-eruption levels by 1700 on 16 September. Sulfur dioxide emissions were measured at a rate of 800 t/d on 16 September while the eruption was waning, and 200 t/d on 17 September, which were markedly lower compared to measurements taken the previous week of 20,000-190,000 t/d.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Tinakula is a remote 3.5 km-wide island in the Solomon Islands, located 640 km ESE of the capital, Honiara. The current eruption period began in December 2018 and has more recently been characterized by intermittent lava flows and thermal activity (BGVN 48:06). This report covers similar activity during June through November 2023 using satellite data.

During clear weather days (20 July, 23 September, 23 October, and 12 November), infrared satellite imagery showed lava flows that mainly affected the W side of the island and were sometimes accompanied by gas-and-steam emissions (figure 54). The flow appeared more intense during July and September compared to October and November. According to the MODVOLC thermal alerts, there were a total of eight anomalies detected on 19 and 21 July, 28 and 30 October, and 16 November. Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) detected a small cluster of thermal activity occurring during late July, followed by two anomalies during August, two during September, five during October, and five during November (figure 55).

Figure 54. Infrared (bands B12, B11, B4) satellite images showed lava flows mainly affecting the W flank of Tinakula on 20 July 2023 (top left), 23 September 2023 (top right), 23 October 2023 (bottom left), and 12 November 2023 (bottom right). Some gas-and-steam emissions accompanied this activity. Courtesy of Copernicus Browser.

Figure 55. Low-power thermal anomalies were sometimes detected at Tinakula during July through November 2023, as shown on this MIROVA plot (Log Radiative Power). A small cluster of thermal anomalies were detected during late July. Then, only two anomalies were detected during August, two during September, five during October, and five during November. Courtesy of MIROVA.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. It has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The Mendana cone is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Recorded eruptions have frequently originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Fuego is one of three large stratovolcanoes overlooking the city of Antigua, Guatemala. It has been erupting since January 2002, with observed eruptions dating back to 1531 CE. Typical activity is characterized by ashfall, pyroclastic flows, lava flows, and lahars. Frequent explosions with ash emissions, block avalanches, and lava flows have been reported since 2018. More recently, activity has been characterized by multiple explosions and ash plumes each day, ashfall, block avalanches, and pyroclastic flows (BGVN 48:09). This report describes similar activity of explosions, gas-and-ash plumes, and block avalanches during August through November 2023 based on daily reports from the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH) and various satellite data.

Multiple explosions each day were reported during August through November 2023 that produced ash plumes that rose to 4.9 km altitude and drifted as far as 30 km in different directions. The explosions also caused rumbling sounds of varying intensities, with shock waves that vibrated the roofs and windows of homes near the volcano. Incandescent pulses of material rose as high as 350 m above the crater, accompanied by block avalanches that descended multiple drainages. Light ashfall was often reported in nearby communities (table 29). MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent low-to-moderate power thermal activity during the reporting period (figure 175). A total of seven MODVOLC thermal alerts were issued on 11 August, 1, 13, and 23 September, and 10, 17, and 18 November. On clear weather days thermal anomalies were also visible in infrared satellite imagery in the summit crater (figure 176).

Table 29. Activity at Fuego during August through November 2023 included multiple explosions every hour. Ash emissions rose as high as 4.9 km altitude and drifted in multiple directions as far as 30 km, causing ashfall in many communities around the volcano. Data from daily INSIVUMEH reports.

Figure 175. Intermittent low-to-moderate power thermal activity was detected at Fuego during August through November 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 176. Infrared (bands B12, B11, B4) satellite images showing a persistent thermal anomaly at the summit crater of Fuego on 27 August 2023 (top left), 1 September 2023 (top right), 16 October 2023 (bottom left), and 30 November 2023 (bottom right). Courtesy of Copernicus Browser.

Activity during August consisted of 1-11 explosions each day, which generated ash plumes that rose to 4-4.8 km altitude and drifted 8-30 km W, NW, SW, N, NE, and E. Fine ashfall was reported in Panimaché I and II (8 km SW), Morelia (9 km SW), Santa Sofía (12 km SW), Yepocapa (8 km NW), Finca Palo Verde (10 km WSW), Sangre de Cristo (8 km WSW), Acatenango (8 km E), Aldeas, El Porvenir (11 km SW), La Reunión (7 km SE), San Miguel Dueñas (10 km NE), Ciudad Vieja (13.5 km NE), Antigua (18 km NE), Quisaché (8 km NW), and El Sendero. The explosions sometimes ejected incandescent material 50-250 m above the crater and generated weak-to-moderate block avalanches that descended the Santa Teresa (W), Seca (W), Taniluyá (SW), Ceniza (SSW), Las Lajas (SE), El Jute (ESE), Trinidad (S), and Honda (E) drainages. Lahars were reported in the Ceniza drainage on 8-9, 16, 26-27, and 29 August, carrying fine and hot volcanic material, branches, tree trunks, and blocks measured 30 cm up to 1.5 m in diameter. Similar lahars affected the Las Lajas, El Jute, Seca, and El Mineral (W) drainages on 27 August.

Daily explosions ranged from 3-11 during September, which produced ash plumes that rose to 4-4.8 km altitude and drifted 10-30 km SW, W, NW, S, and SE. The explosions were accompanied by block avalanches that affected the Seca, Taniluyá, Ceniza, Las Lajas, Honda, Santa Teresa, Trinidad, and El Jute drainages and occasional incandescent ejecta rose 50-300 m above the crater. Fine ashfall was reported in Panimaché I and II, Morelia, Palo Verde, Sangre de Cristo, Yepocapa, El Porvenir, Aldeas, Santa Sofía, Montellano, El Socorro, La Rochela (8 km SSW), La Asunción (12 km SW), San Andrés Osuna (11 km SSW), Guadalupe, La Trinidad (S). Lahars triggered by rainfall were detected in the Ceniza drainage on 3-4, 8, 13-14, 17, 20-21, 24, 26, 29-30 September, which carried fine and hot volcanic material, branches, tree trunks, and blocks measuring 30 cm to 3 m in diameter. Similar lahars were also detected in the Seca, El Mineral, Las Lajas, and El Jute drainages on 27 September.

There were 2-10 explosions recorded each day during October, which produced ash plumes that rose to 4-4.9 km altitude and drifted 10-30 km W, SW, S, NW, N, NE, and SE. Incandescent pulses of material rose 50-350 m above the crater. Many of the explosions generated avalanches that descended the Ceniza, Santa Teresa, Taniluyá, Trinidad, Seca, El Jute, Las Lajas, and Honda drainages. Ashfall was reported in Aldeas, Panimaché I and II, Morelia, Santa Sofía, El Porvenir, Sangre de Cristo, Yepocapa, Yucales, Palo Verde, Acatenango, Patzicía, Alotenango, La Soledad (11 km N), El Campamento, La Rochela, Las Palmas, and Quisaché. Lahars continued to be observed on 2-5, 7, 9, 11, and 21-22 October, carrying fine and hot volcanic material, branches, tree trunks, and blocks measuring 30 cm to 3 m in diameter. Similar lahars were also reported in the Seca and Las Lajas drainage on 2 October and in the Las Lajas drainage on 4 October. On 4 October lahars overflowed the Ceniza drainage toward the Zarco and Mazate drainages, which flow from Las Palmas toward the center of Siquinalá, resulting from intense rainfall and the large volume of pyroclastic material in the upper part of the drainage. On 9 October a lahar was reported in the Seca and Las Lajas drainages, and lahars in the Las Lajas and El Jute drainages were reported on 11 October. A lahar on 22 October was observed in the Seca drainage, which interrupted transportation between San Pedro Yepocapa and the communities in Santa Sofía, Morelia, and Panimaché.

During November, 1-10 daily explosions were recorded, sometimes accompanied by avalanches, rumbling sounds, and shock waves. Gas-and-ash plumes rose 4.5-4.8 km altitude and extended 10-30 km W, SW, S, E, SE, NW, and N. Incandescent pulses of material rose 50-200 m above the crater. Fine ashfall was reported in Panimaché I and II, Morelia, Yepocapa, El Porvenir, Palo Verde, Santa Sofía, Aldeas, Sangre de Cristo, Yucales, La Rochela, San Andrés Osuna, Ceilán (9 km S), Quisaché, Acatenango, La Soledad. Avalanches of material descended the Seca, Taniluyá, Ceniza, Las Lajas, El Jute, Honda, Santa Teresa, and Trinidad drainages.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

The Santiaguito lava dome complex of Guatemala’s Santa María volcano has been actively erupting since 1922. The lava dome complex lies within a large crater on the SW flank of Santa María that was formed during the 1902 eruption. Ash explosions, pyroclastic flows, and lava flows have emerged from Caliente, the youngest of the four vents in the complex for more than 40 years. A lava dome that appeared within Caliente’s summit crater in October 2016 has continued to grow, producing frequent block avalanches down the flanks. More recently, activity has been characterized by frequent explosions, lava flows, ash plumes, and pyroclastic flows (BGVN 48:09). This report covers activity during August through November 2023 based on information from Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia) and various satellite data.

Activity during August consisted of weak-to-moderate explosions, avalanches of material, gas-and-ash plumes, and incandescence observed at night and in the early morning. Weak degassing plumes rose 300-600 m above the crater. Frequent explosions were detected at a rate of 1-2 per hour, which produced gas-and-ash plumes that rose 200-1,000 m above the crater and drifted W, NW, SW, S, E, and NE. Two active lava flows continued mainly in the Zanjón Seco (SW) and San Isidro (W) drainages. Incandescent block avalanches and occasional block-and-ash flows were reported on the W, S, E, SE, and SW flanks, as well as on the lava flows. On 26 and 29 August, fine ash plumes rose to 3.5 km altitude and drifted E and NE, causing ashfall in Belén (10 km S) and Calaguache (9 km S), as well as Santa María de Jesús (5 km SE) on 29 August.

Daily degassing, weak-to-moderate explosions, gas-and-ash plumes, and nighttime and early morning incandescence in the upper part of the dome continued during September. Explosions occurred at a rate of 1-2 per hour. Gas-and-ash plumes rose 200-1,000 m above the crater and drifted SW, W, SE, and NW. Block avalanches descended the SW, S, SE, and E flanks, often reaching the base of the Caliente dome. These avalanches were sometimes accompanied by short pyroclastic flows, resulting in fires in some vegetated areas. Block-and-ash flows descended all flanks of the Caliente dome on 16 and 24 September following the eruption of gas-and-ash plumes that rose 700-1,000 m above the crater. Gray ash was primarily deposited in the drainages.

Continuous gas-and-steam emissions occurred in October, along with weak-to-moderate explosions, block avalanches, crater incandescence, and an active lava flow on the WSW flank. Explosions occurred at a rate of 1-4 per hour, that generated gas-and-ash plumes rose 200-1,000 m above the crater and drifted in different directions. Block avalanches traveled down the SW, S, SE, and E flanks, sometimes accompanied by small pyroclastic flows. On 21 and 25 October as many as 50 explosions occurred over the course of 24 hours.

Similar activity persisted during November, with frequent explosions, crater incandescence, and block avalanches. The active lava flow persisted on the WSW flank. Weak-to-moderate explosions occurred at a rate of 1-4 per hour. Incandescence was observed at night and in the early morning. Gas-and-ash emissions rose 700-900 m above the crater and drifted W, SW, S, and NW. Block avalanches were reported on the SW, W, S, SE, and E flanks, which deposited gray ash material in the drainages, sometimes reaching the base of the Caliente dome. Those avalanches were sometimes accompanied by small pyroclastic flows that reached the base of the dome on the W, SW, and S flanks. Ashfall was reported in Las Marías (10 km S), El Viejo Palmar (12 km SSW), El Patrocinio, and San Marcos (8 km SW) on 18 and 22 November. On 26 and 30 November ashfall was reported in San Marcos and Loma Linda Palajunoj (7 km SW).

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph showed frequent moderate-power thermal anomalies during the reporting period (figure 140). A total of 26 MODVOLC thermal alerts were issued on 6, 7, 7, 15, 16, and 21 August, 15 and 23 September, 19, 26, 27, and 29 October, and 2, 7, 11, 27, 28, and 29 November. Clouds covered the summit of the volcano on most days, so thermal anomalies could not be identified in most Sentinel infrared satellite images.

Figure 140. Moderate-power thermal anomalies were frequently detected at Santa María during August through November 2023, as shown on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Karangetang (also known as Api Siau), at the northern end of the island of Siau, Indonesia, contains five summit craters along a N-S line. More than 40 eruptions have been recorded since 1675; recent eruptions have included frequent explosive activity, sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters and collapses of lava flow fronts have also produced pyroclastic flows. The two active summit craters are Kawah Dua (the N crater) and Kawah Utama (the S crater, also referred to as the “Main Crater”). The most recent eruption began in early February 2023 and was characterized by lava flows, incandescent avalanches, and ash plumes (BGVN 48:07). This report covers similar activity through the end of the eruption during July through September 2023 using reports from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin VAAC (Volcano Ash Advisory Center), and satellite data.

Webcam images occasionally showed crater incandescence and lava flows on the flanks of Main Crater during July. Near daily white gas-and-steam plumes rose 50-400 m above the crater and drifted in multiple directions. A webcam image taken at 1732 on 1 July suggested that a pyroclastic flow descended the SE flank, as evident from a linear plume of gas-and-ash rising along its path (figure 66). Incandescent material extended about 1 km down the S flank and about 600 m down the SSW and SW flank, based on a Sentinel satellite image taken on 2 July (figure 67). During the evening of 3 July a lava avalanche descended the Kahetang drainage (SE), extending 1-1.8 km, and the Timbelang and Beha drainages, extending 700-1,000 m. There were 53 earthquakes also detected that day. According to a news article from 6 July the lava avalanche from 2 July continued down the SW flank of Main Crater toward the Batang, Timbelang, and Beha Barat drainages for 1.5 km. An avalanche was also visible on the S flank, affecting the Batuawang and Kahetang drainages, and extending 1.8 km. Incandescent avalanches were reported during 8-9 July, traveling 1.8 km toward the Kahetang, Batuawang (S), and Timbelang drainages (figure 68). PVMBG issued two VONAs (Volcano Observatory Notices for Aviation) at 0759 and 0850 on 10 July, which reported two pyroclastic flows that traveled about 2 km toward the Kahetang drainage (figure 69). There were also 55 earthquakes detected on 10 July. As a result, 17 residents from Bolo Hamlet, Tarorane Village, East Siau District, Sitaro Islands Regency, North Sulawesi were evacuated.

Figure 66. Webcam image showing a possible pyroclastic flow descending the SE flank of Karangetang at 1732 on 1 July 2023. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Figure 67. Incandescent avalanches of material and summit crater incandescence was visible in infrared (bands B12, B11, B4) satellite images at both the N and S summit craters of Karangetang on 2 July 2023 (top left), 16 August 2023 (top right), 25 September 2023 (bottom left), and 25 October 2023 (bottom right). The incandescent avalanches mainly affected the S flank and gas-and-steam plumes (blue color) were also sometimes visible. Courtesy of Copernicus Browser.

Figure 68. Webcam image showing crater incandescence and lava flows from Main Crater descending Karangetang at 1936 on 8 July 2023. Courtesy of MAGMA Indonesia.

Figure 69. Webcam image showing a pyroclastic flow descending the SE flank of Karangetang at 0850 on 10 July 2023. Courtesy of MAGMA Indonesia.

An incandescent avalanche of material descended 1-1.8 km down the Kahetang drainage and 1 km down the Batang drainage on 14 July. During 18-29 July lava avalanches continued to move 1-1.8 km toward the Kahetang drainage, 700-1,000 m toward the Batuawang and Batang drainages, 700-1,000 m toward the Timbelang and Beha Barat drainage, and 1.5 km toward the West Beha drainage. Gray-and-white plumes accompanied the lava avalanches. During 20 July crater incandescence was visible in the gas-and-steam column 10-25 m above the crater. The Darwin VAAC reported that ash plumes rose to 2.4 km altitude at 1710 on 21 July, at 1530 on 22 July, and at 0850 on 23 July, which drifted NE and E. According to a news article, there were 1,189 earthquakes associated with lava avalanches recorded during 24-31 July.

Incandescent avalanches originating from Main Crater and extending SW, S, and SE persisted during August. Frequent white gas-and-steam plumes rose 25-350 m above the crater and drifted in different directions during August. Incandescent avalanches of material traveled S as far as 1.5 km down the Batuawang drainage, 1.8-1.9 km down the Kahetang drainage, and 2-2.1 km down the Keting drainage and SW 800-1,500 m down the Batang, Timbelang, and Beha Barat drainages. Occasional gray plumes accompanied this activity. According to a news article, 1,899 earthquakes associated with lava avalanches were recorded during 1-7 August. Incandescent ejecta from Main Crater was visible up to 10-25 m above the crater. Nighttime crater incandescence was visible in the N summit crater. There were 104 people evacuated from Tatahadeng and Tarorane during the first week of August, based on information from a news article that was published on 9 August. According to a news article published on 14 August the frequency of both earthquakes and lava avalanches decreased compared to the previous week; there were 731 earthquakes associated with avalanches detected during 8-15 August, and 215 during 24-31 August . Lava avalanches descending the Batang and Timbelang drainages continued through 24 August and the Batuawang, Kahetang, and Keting through 30 August. A news article published on 17 August reported pyroclastic flows due to collapsing accumulated material from lava flows.

Near-daily white gas-and-steam plumes rose 25-300 m above the crater and drifted in multiple directions during September. According to news articles, lava avalanches from Main Crater continued toward the Batuawang, Kahetang, and Keting drainages, reaching distances of 1-1.8 km. Lava avalanches also descended the Batang, Timbelang, and Beha Barat drainages as far as 1 km from Main Crater. Main Crater and N Crater incandescence were visible as high as 10 m above the crater. During 1-7 September the number of earthquakes associated with avalanches declined, although effusive activity continued. During 8-15 September lava effusion at Main Crater was not visible, although sounds of avalanches were sometimes intense, and rumbling was also occasionally heard. According to a news article published on 26 September, avalanches were no longer observed.

On 29 November PVMG lowered the Volcano Alert Level (VAL) to 2 (the second lowest level on a scale of 1-4) due to declining activity. Seismic data and visual observations indicated that effusion had decreased or stopped, and lava avalanches were no longer observed.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed strong thermal activity during July through August 2023, which was mainly characterized by incandescent avalanches of material and lava flows (figure 70). During August, the frequency and intensity of the thermal anomalies declined and remained relatively low through December. There was a brief gap in activity in late September. According to data recorded by the MODVOLC thermal algorithm, there were a total of 22 during July and 19 during August. Infrared satellite images showed summit crater incandescence at both the N and S craters and occasional incandescent avalanches of material affecting mainly the S flank (figure 67).

Figure 70. Strong thermal activity was detected at Karangetang during July through August 2023, as recorded by this MIROVA graph (Log Radiative Power). The frequency and intensity of the thermal anomalies declined during August and remained relatively low through December. A brief gap in activity was visible in late September. Courtesy of MIROVA.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented (Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

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.esdm.go.id/v1); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Antara News, Jalan Antara Kav. 53-61 Pasar Baru, Jakarta Pusat 10710, Indonesia (URL: antaranews.com).

On 3 December, satellite imagery indicated a plume rising to a height of 5-6 km from Alaid. The nearest seismic station, located in the town of Severo-Kurilsk (Paramushir Island), 25 km E of Alaid, recorded the beginning of local seismic activity at about the same time as the satellite report. A large snow storm obscured the volcano, and no visual reports of eruptive activity were received from the coast guard, ships, or aircraft in the area.

Geologic Background. The highest and northernmost volcano of the Kuril Islands, Alaid is a symmetrical stratovolcano when viewed from the north, but has a 1.5-km-wide summit crater that is breached open to the south. This basaltic to basaltic andesite volcano is the northernmost of a chain constructed west of the main Kuril archipelago. Numerous pyroclastic cones are present the lower flanks, particularly on the NW and SE sides, including an offshore cone formed during the 1933-34 eruption. Strong explosive eruptions have occurred from the summit crater beginning in the 18th century. Reports of eruptions in 1770, 1789, 1821, 1829, 1843, 1848, and 1858 were considered incorrect by Gorshkov (1970). Explosive eruptions in 1790 and 1981 were among the largest reported in the Kuril Islands.

Information Contacts: Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; NOAA/NESDIS Satellite Analysis Branch; Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

During late 1996, Arenal's ongoing Strombolian eruptions continued. OVSICORI-UNA noted that late-1996 avalanches had traveled down the N and SW flanks. For example, one at 1109 on 11 December traveled down the N flank to ~1,300 m elevation. Acidic rainfall also continued to damage vegetation.

Some noteworthy September eruptions and their associated pyroclastic flows were mentioned in BGVN 21:09. Others also occurred late in 1996; one at 0926 on 11 December reached a point on the SW flank at 1,230 m elevation. Another pyroclastic flow occurred at 1630 on 30 December reaching 1,000 m elevation.

Observations during 19-24 November. Multiple pyroclastic flows with distinct pulses were reported by Steve O'Meara, Tippy D'Auria, and Robert Benward who watched Arenal for much of five days (19-24 November 1996) from 3.8 km due N of the summit (location determined by GPS); they found it very active with eruptions occurring intermittently from five distinct vents. The group noted long-duration fountaining and jetting; on 20 November these processes were continuous for over 30 minutes. The group saw multiple eruptions discharging plumes to heights >1 km, observed ashfall, and heard window-rattling explosion noises. They watched an advancing incandescent summit lava flow lobe whose front frequently calved off and produced spectacular incandescent rockfalls and avalanches. Also, they witnessed tumbling boulders and noted periodically strong snorting and huffing noises at the summit vents associated with the escape of incandescent gas.

Sheared from the lava flow front, some of the boulders were easily the size of small cars. As the boulders fell they made audible impact bursts and broke into smaller pieces as they rolled and bounced down the mountain (sometimes reaching the 700-m elevation). The scene of the falling and breaking boulders looked similar to a fireworks display.

The source of the lava lobe appeared to be the new spatter cone on the N side of Crater C, which the flow all but encompassed. The main lobe branched into three separate ones, each a source of rock falls. A lava lobe also appeared to be moving E into the saddle between craters C and D. A new lobe was moving down Crater C's NW side.

The source of the pyroclastic flows was unclear to the group, though they thought it might be the new spatter cone. On video footage some of the pyroclastic flows were very distinct. Many of the flows were also accompanied by a sluggish, reddish, small ash eruption from what appears to be one of the five vents, though these eruptions could also have issued from the spatter cone. Pyroclastic flows traveled 100-1000 m from the summit, but stayed within well-defined channels on the W side of the latest 1996 lava flow. O'Meara was somewhat concerned to encounter an active campsite at Laguna Cedeño--right in the path of some of the pyroclastic flows--and only 2 km from the summit.

Table 19 shows a sample of some of the kinds of observations recorded during the O'Meara group's stay. During their visit they also considered the positions of the sun and moon. The group suggested that the largest eruptions may have had a possible correlation to moon phase and tidal pulls as well as moon rise and set times. It appeared that the more violent eruptions of Vent 1 occurred at intervals of 6.5 hours. Using that hypothesis, the group claimed to have successfully and confidently estimated vent 1's major blasts to within 2 minutes of the events.

Table 19. A sample of the field notes, from 20 November 1996, describing Arenal activity during the O'Meara group's 5-day visit (complete observations available upon request). Courtesy of Steven O'Meara, Tippy D'Auria, and Robert Benward.

Behavior at the five vents during the group's visit were as follows. Vents 1 and 3 were twin summit vents that appeared to be centered in Crater C. Vent 1 was the stronger of the two when they erupted separately. Both sent plumes vertically and produced prolonged roars and blasts.

Vent 2, located on the E side of Crater C, was the strongest and loudest, producing most of the house- and window-shaking eruptions. Many times the ejecta would blow out of this vent at a 45° angle toward Crater D. Sometimes, E- and N-arcing ejecta fell about a quarter of the way down the volcano's slope. This vent may have been the eastern lava lobe's source.

Vent 4 was centered on the new summit spatter cone N of Crater C. The vent ejected occasional spatter, though some long-enduring fountaining-degassing episodes also took place. Whereas vent 4 had been active when the group arrived, it was almost inactive by 22 November, though the lava flow around it continued to advance.

Vent 5, on the W side of the summit, produced very strong ash eruptions, but these events were mostly associated with muffled blasts at the observation site. Variously directed eruptions sometimes sent ejecta E, at a 45° angle toward the W, and at other times vertically.

Seismic data, recent reports, and a hazards caution. During September and November OVSICORI-UNA noted that both tremor and earthquakes were common, though not in unusual abundance (tremor duration, 300 and 237 hours; number of earthquakes, 875 and 471; respectively). Earthquakes were more abundant during October, a month when 1,094 registered, more than at any time during 1996. At the OVSICORI-UNA station, tremor duration in the previous months of 1996 had ranged between 261 and 434 hours; during October it was also moderately high (381 hours).

Some Arenal reports and abstracts that may not appear in typical European and North American literature indexes have come out recently. Five technical reports on Arenal appeared in a volume in November 1996 (ICE, 1996). The reports include discussions of 1) the volcano's behavior during the year 1993, 2) the spectral character of seismic signals and the P-wave velocity in the upper part of the edifice, 3) possible eruptions in 1915 and 1922, 4) the hazards, costs, and hazard perceptions associated with a moderate eruption, and 5) how meta-igneous, granulite facies xenoliths in Arenal lavas could provide evidence for a mafic basement. In addition, at least one of about 50 papers at a recent conference in Perú discussed hazards from Arenal (Laurent, 1996).

As a caution to people visiting or acting as guides at the popular volcano it should be noted that Melson and others (1997) have looked at the frequency of eruptions in a sample of over 196 days between 1987 and 1994. They found that hazardous pyroclastic eruptions occurred at wide-ranging intervals, from as little as a minute to over a day apart. However, 96% had a recurrence interval of <100 minutes. During these eruptions, ballistic blocks and bombs and infrequent pyroclastic flows posed the greatest hazards.

References. ICE Boletín, 1996, año 6, no. 11-12, Dirección de Ingeniería Civil, Departamento de Ingeniería Geológica, 78 p.

Laurent, K., 1996, Impacto de los desastres ocasionados por los volcanes Arenal e Irazú sobre la infraestructura energética de Costa Rica y medidas actuales para mitigar sus efectos: El Segundo Seminario Latinamericano "Volcanes, Sismos Y Prevencion" en Lima-Arequipa, Perú, del 4 al 9 de noviembre de 1996, J-C Thouret (coodinator), 178 p.

Melson, W.G., O'Hearn, T., Funk, V., Barquero, J., Barboza, V., Saenz, R., Fernandez, E., McNutt, S., and Benoit, J., 1997, Probabilistic volcanic hazard assessment, Arenal volcano, Costa Rica, 1987-95: unpublished report, Smithsonian Institution, Washington DC, U.S.A., 10 p.

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

Information Contacts: E. Fernández, E. Duarte, V. Barboza, R. Van der Laat, E. Hernandez, M. Martinez, and R. Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; G.J. Soto and J.F. Arias, Oficina de Sismología y Vulcanología del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica; Steven O'Meara, PO Box 218, Volcano, HI 96785, USA.

Lidar data for part of 1996 (mid-May through the month of September) over Germany (table 8) indicated a possible aerosol layer centered between 14.7 and 21.6 km altitude. The possible layer's center often resided at ~19 km. The "ci" in Table 9 stands for cirrus. Cirrus in the tropopause region usually obscures the lower boundary of the aerosol layer.

Table 8. Backscattering ratios from German lidar data for the Nd-YAG wavelength of 0.53 µm, 16 May-30 September 1996. The equivalent backscattering ratios for ruby are in parentheses (ruby wavelength, 0.69 µm). Courtesy of Horst Jäger.

Geologic Background. The enormous aerosol cloud from the March-April 1982 eruption of Mexico''s El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin thorugh 1989. Lidar data and other atmospheric observations were again published intermittently between 1995 and 2001; those reports are included here.

Information Contacts: Horst Jäger, Fraunhofer -- Institut für Atmosphärische Umweltforschung, Kreuzeckbahnstrasse 19, D-8100 Garmisch-Partenkirchen, Germany.

On 5-6 and 17 December and on 4, 9, and 14-15 January, fumarolic plumes were observed reaching as high as 100 m above the crater. The plumes extended 10 km downwind.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

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

During the last four years, quiet prevailed at Piton de la Fournaise (figure 35), with unusually low seismic activity and no eruptions. During late November, however, scientists at the Observatoire Volcanologique du Piton de la Fournaise noted an increase in radon flux followed a day later by an increase in seismicity. This also accompanied changes in tilt, local extension, and larger-scale distance movements. These observations led the scientists to infer that there had been a magma intrusion.

Figure 35. Sketch map of Piton de la Fournaise and vicinity showing the locations of observation stations. Notice that the topographic contour intervals on this map are uneven. Courtesy of OVPDLF.

Seismicity. In September a 16-km-deep swarm of earthquakes was recorded ~5 km NW of the summit. Seismicity continued to increase under the summit, and a M 2.3 event was registered on 18 October. A short but intense seismic crisis (figure 36), which began at 2126 on 26 November, consisted of 134 registered events. Among them, there were 37 earthquakes larger than M 1 and two in the range of M 2-2.4. The strongest one took place at 2149, an event nearly coincident with changes recorded by inclinometers and extensometers and inferred to correspond to the beginning of the principal magma movement. The seismic activity decreased at 2220 and ended with a final event of M 1.9 at 2310. After that, seismicity remained low. All events during this seismic crisis originated at about sea level ( ~2.5 km below the summit) at the Soufrière region, slightly N of the summit.

Figure 36. Cumulative seismic moment during January-November 1996 at Piton de la Fournaise. Thin dotted line shows cumulative seismic moment under the summit; heavier line represents volcano-wide cumulative seismic moment minus the cumulative seismic moment under the summit. Courtesy of OVPDLF.

Tilt and extension. Although not shown in figure 37, a small amount of ground movement might have taken place prior to the prominent tilt event at about 2149 on 26 November. The tilt accompanied inflation of the summit region, and tilts of 16, 8, and 4 µrad were recorded at stations Soufrière, Dolomieu, and Bory, respectively. Stations Chapelle and Chateau Fort at the base of the cone tilted less than 1 µrad. Displacements of less than 0.1 mm were also recorded by extensometers at stations Magne, Chateau Fort, Soufriere, and Dolomieu.

Figure 37. Tilts at stations Bory, Soufriére, and Dolomieu at Piton de la Fournaise. "Rad" and "tgt" refer to radial and tangential components, respectively. Courtesy of OVPDLF.

Distance change. Distances between station Piton Partage and different prisms on the cone were measured automatically by a TM3000 instrument (figure 38). During 25-26 November, a decrease up to 10 mm occurred for prisms on the profile between Soufrière and Puy Mi-Cote (prisms 1M20, 2M54, 2M53, 2M52, 2M42, and 1M40). In addition, the distance decreased slightly for one prism at Magne (2M26); no distance change was observed for the prisms between Chapelle and Bory and for one prism at the E side of the cone (2M31).

Figure 38. Distance changes between station Piton Partage and different prisms on the Piton de la Fournaise volcanic cone. Courtesy of OVPDLF.

Radon. None of the six radon stations transmitted by radio showed significant changes during the seismic crisis; however, about 36 hours before the crisis, two of the stations, Chateau Fort and Cratere Catherine, recorded distinct Gaussian-like anomalies for a period of 15 hours. These changes were significantly above the background signal of 1-5 counts per hour (figure 39). Thus increased radon flux might have been a precursor to the intrusion.

Figure 39. Radon flux at stations Chateau Fort and Cratere Catherine at Piton de la Fournaise. Courtesy of OVPDLF.

Geologic Background. Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three scarps formed at about 250,000, 65,000, and less than 5,000 years ago by progressive eastward slumping, leaving caldera-sized embayments open to the E and SE. Numerous pyroclastic cones are present on the floor of the scarps and their outer flanks. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest scarp, which is about 9 km wide and about 13 km from the western wall to the ocean on the E side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures outside the scarps.

Information Contacts: Thomas Staudacher; Patrick Bachèlery; Valéry Ferrazzini, and Kei Aki, Observatoire Volcanologique du Piton de la Fournaise (OVPDLF), 14 RN3, le 27Km, 97418 La Plaine des Cafres, La Réunion, France.

During 12-19 November a white-to-gray column was observed rising 70-200 m above the crater; winds then dispersed it to the S over the volcano's flanks. On 20 November the column rose to 500 m and drifted E; the following day the column's height was 200 m and oriented SW. From 25 November to 12 December the column was again at 50-200 m height and blown toward the S and SW. During this observation period the Fuego-Acatenango seismic network recorded a few earthquakes up to M 1.

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

Information Contacts: Otoniel Matías, Seccion Vulcanologia, INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia of the Ministerio de Communicaciones, Transporte y Obras Publicas), 7A Avenida 14-57, Zona 13, Guatemala City, Guatemala.

The seismic swarm that began on 1 August (BGVN 21:08-21:10) continued during December and the first half of January. There were 2-16 earthquakes recorded each day.

Geologic Background. Iliamna is a prominentglacier-covered stratovolcano in Lake Clark National Park on the western side of Cook Inlet, about 225 km SW of Anchorage. Its flat-topped summit is flanked on the south, along a 5-km-long ridge, by the prominent North and South Twin Peaks, satellitic lava dome complexes. The Johnson Glacier dome complex lies on the NE flank. Steep headwalls on the S and E flanks expose an inaccessible cross-section of the volcano. Major glaciers radiate from the summit, and valleys below the summit contain debris-avalanche and lahar deposits. Only a few major Holocene explosive eruptions have occurred from the deeply dissected volcano, which lacks a distinct crater. Most of the reports of historical eruptions may represent plumes from vigorous fumaroles E and SE of the summit, which are often mistaken for eruption columns (Miller et al., 1998). Eruptions producing pyroclastic flows have been dated at as recent as about 300 and 140 years ago, and elevated seismicity accompanying dike emplacement beneath the volcano was recorded in 1996.

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

Although no visual observations were made, during December and 1-20 January seismicity remained above background in a manner that suggested continued low-level Strombolian eruptions. Seismic data indicated that up to 300 explosions occurred each day.

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

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

The seismicity at Kliuchevskoi remained above background levels during December and 1-20 January. Fumarolic plumes were observed during December rising 100-1,200 m above the volcano and extending 5-15 km downwind. On 28 December and 4 January, gas-and-steam explosions rose to 200-300 m above the crater, and plumes extended 10-20 km to the NW.

An increase in eruptive activity was first noticed at 1740 on 7 January 1997 from the town of Kliuchi, ~30 km NE of the volcano. An ash-and-steam plume was observed rising 2,500-3,000 m above the crater and extending 20 km SE. Seismic activity, while still elevated, did not show an increase. An AVO analysis of a satellite image taken early on the morning of 8 January indicated that the plume had subsided. On 9 and 11 January, gas-and-steam explosions, possibly with minor ash, rose to 300-700 m above the crater, and the plumes traveled 10-15 km to the W or SW. On 13-14 and 16 January gas-and-steam plumes reached a height of 300-600 m and extended 10 km E. On 15 January, a gas-and-steam explosion rose 1,200 m above the crater, and its plume moved 15 km SE.

Geologic Background. Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

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

Eruptive activity at Crater 2 continued during October-December. The activity pattern of 7-26 October was similar to that during January-September. Emissions included thin, white to thick, gray vapor-and-ash clouds. The ash clouds rose ~1-2 km above the crater. Ashfall was observed on 8-22 October on the N, NW, and SE sides of the volcano. Weak, steady, red glows were seen on the nights of 7, 12-13, 20-21, and 26 October. Projections of red incandescent lava fragments occurred on the nights of 8-10 and 17 October. After 26 October eruptive activity declined to emissions of thin, weak, white vapor. During November and December, activity at Crater 2 consisted of emissions of thin, weak, white vapor, and, occasionally, moderate ash clouds. Light ashfall was observed on 5 and 11-12 November on the N, NW, and SE sides of the volcano.

Crater 3 remained quiet during October-December. Thin, weak, white vapor began to emit on 1 December. Seismic monitoring was conducted only between 11 November and 4 December. Seismicity was low during this period.

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

Information Contacts: B. Talai, I. Itikarai, and P. de Saint Ours, RVO.

A series of large eruptions took place during October and November, and culminated with a paroxysmal phase on 3 December. The paroxysm accounted for 13 deaths in a coastal village called Budua Old near SW Valley (figure 7).

Figure 7. Sketch map of Manam Island, showing the distribution of the four valleys. After Palfreyman and Cooke, 1976.

After the last large eruption in 1992 (BGVN 17:09-17:11), Main Crater remained inactive with weak vapor emissions, while South Crater showed intermittent phases of weak to moderate, mainly Strombolian, activity. Mild Vulcanian explosions resumed at both craters in mid-September 1996 (BGVN 21:09), and Strombolian eruptions returned to both craters in October. During October and November, five strong phases of activity occurred at Main Crater at intervals of 7-12 days; four of them were preceded or accompanied by moderate activity at South Crater (figure 8). From early October to 10 November, the first four phases progressively increased in strength, and were accompanied by a large buildup in seismic amplitude (figure 9). Like the strongest phase of the 1992 eruption (BGVN 17:09-17:11), the first four phases of eruptions at Main Crater produced pyroclastic flows and lava flows in NE Valley.

Figure 8. A rough (qualitative) estimate of the level of October-December volcanic activity at Manam. Activity is shown as a relative scale that is arbitrary, with the highest intensity artificially set at "100". Courtesy of RVO.

Figure 9. Relative seismic amplitude during October-December at Manam. Courtesy of RVO.

At South Crater, ash-laden emissions, roaring sounds, and fluctuating glows occurred on 19 November, and moderate Strombolian projections (200 m high) and ash-laden emissions (~700 m high) were observed on 20-21 November. The fifth phase of eruptions at Main Crater took place on 20-22 November. On 20 November, Main Crater produced frequent roaring sounds, bright fluctuating night glows, and up to ~800-m-high brown emissions, indicating deep-seated Strombolian activity. Strombolian eruptions were sub-continuous until 1800 on 21 November, with a 3-km high, dark, convoluting column. Several lava tongues traveled down the upper part of NE Valley, and reached an altitude of 600-700 m. On 22 November two lava flows moved down to an altitude of ~200 m in the central and N parts of the valley. On the afternoon of 22 November, the fifth eruptive phase declined, and the Strombolian activity changed to 2-3 km high, thick, gray emissions, which were accompanied by jet sounds. Frequency, volume, and height of the emissions progressively decreased during the next two days.

On 28 November, eruptive activity at South Crater increased with the emissions of weak to moderate, gray-brown plumes (up to 500 m high) and occasional Strombolian projections (~50 m high). On 29 November, emissions became sub-continuous. In the next two days Strombolian projections progressively rose as high as 100-200 m above the crater. At Main Crater, white to gray ash clouds were occasionally released, and glow was seen at night from two locations on the NE rim of the crater. On 30 November, emissions at Main Crater were thick and continuous, but no strong activity was observed.

Strombolian explosions at South Crater increased in strength (occasionally up to heights of 400 m) and frequency (at 1-2 minute intervals) on the morning of 2 December, died out in the afternoon, then resumed in the night. On the morning of 3 December, Strombolian explosions occurred at ~1 minute intervals with ~200-m-high projections, and light gray plumes rose ~500 m high. Seismicity increased but was still in the normal range. Strombolian projections were sub-continuous until 1300, and reached as high as 300 m above the crater. At 1430 explosions took place every a few seconds with heavy scoria falls on the upper cone, similar to the situations in the strongest phases of the 1984, 1987, and 1992 eruptions (SEAN 09:02, 09:03, 12:06, and BGVN 17:09-17:11), which sent pyroclastic flows and lava flows into the SE and SW Valleys.

Large pyroclastic flows started to move into SE Valley at about 1500, and into SW Valley around 1505. The central eruption column became thicker as it incorporated the ash cloud elutriated above the pyroclastic flows. At 1510 ash clouds that were generated by the pyroclastic flows moved down SE Valley and extended over the ground at 400-600 m above sea level. At 1515 the dark central billowing pillar reached an altitude of ~5 km, and spread W of the island. Dense ash and scoria falls caused darkness in the W part of the island for 90 minutes. Pyroclastic flows continuously moved into both SE and SW Valleys at short intervals. At 1520 one large pyroclastic flow in SW Valley reached the sea and was followed by many others. Approximately at that time, pyroclastic flows overran the village of Budua Old, resulting in 13 deaths. The devastated area extended ~1.5 km on either side of the central channel of SW Valley. In SE Valley, pyroclastic flows started to reach the sea around 1530. The overriding ash clouds progressed <100 m offshore. However, within ~10 minutes bubbles were seen piercing the water surface as far as 500 m offshore, indicating that underwater propagation of the hot pyroclastic flows reached quite a distance from the shoreline.

The central column was very dense. Eruption sounds were muffled to a continuous deep roar. Although emitted at South Crater, some pyroclastic flows moved into NE Valley and descended to mid-slope, but none was able to override the 100-m vertical valley walls that protect the cultivated and inhabited flanks of the island. At 1605 the pitch of the roaring sound slightly decreased, indicating the beginning of decline in eruptive activity. By 1615 pyroclastic flows no longer reached the sea, and at 1630 all pyroclastic flows stopped. At 1645-1700 ash began to fall again on the downwind (W-NW) side of the island. At night Strombolian activity continued at South Crater, with ~3 loud explosions every hour. At 0925 on 4 December, a large hot avalanche moved down SW Valley to an altitude of ~20 m. Explosions occurred at South Crater with decreasing strength and frequency until 7 December.

At Main Crater, two vents were active with intermittent, thick, dark, convoluting clouds on the morning of 3 December. Moderately thick, gray clouds were observed rising 500-1,000 m above the crater, with weak night glows until 15 December.

The paroxysmal eruption on 3 December changed the configuration of South Crater by sculpting a 100-m-deep, V-shaped crack NW-SE across the summit. There was an absence of high-frequency earthquakes during and after the paroxysm. This suggested that the crack, which had a similar alignment to one pair of valleys, may have pre-dated the eruption, having been covered by earlier eruption debris. Blocks up to 4 m in size from the crater wall were found in pyroclastic-flow deposits in the lower part of SW Valley.

Neither the start of the 1996 eruption nor its strong phases were anticipated by monitoring results. Ground deformation, monitored by the water tube tiltmeters at Tabele Observatory (figure 7), showed steady conditions between April and mid-September. Resumption of activity at both craters in mid-September accompanied a hardly discernible radial deflation (~0.5 µrad). From then to 8-12 November, the tiltmeters accumulated a barely significant ~1 µrad radial inflation, and thereafter a slight deflation (~1 µrad). However, a remarkable deflation of ~1.5 µrad was recorded during the paroxysm at South Crater.

Reference. Palfreyman, W.D., and Cooke, R.J.S., 1976, Eruptive history of Manam volcano, Papua New Guinea in Johnson R.W. (ed.), Volcanism in Australasia, Elsevier, Amsterdam, p. 117-131.

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

Information Contacts: B. Talai, I. Itikarai, and P. de Saint Ours, RVO; NOAA/NESDIS Satellite Analysis Branch; Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

On 25 November, a fumarolic plume was observed rising to a height of 1 km above the crater.

Geologic Background. Massive Mutnovsky, one of the most active volcanoes of southern Kamchatka, is formed of four coalescing stratovolcanoes of predominantly basaltic composition. Multiple summit craters cap the volcanic complex. Growth of Mutnovsky IV, the youngest cone, began during the early Holocene. An intracrater cone was constructed along the northern wall of the 1.3-km-wide summit crater. Abundant flank cinder cones were concentrated on the SW side. Holocene activity was characterized by mild-to-moderate phreatic and phreatomagmatic eruptions from the summit crater. Explosive eruptions have been common since the 17th century, with lava flows produced during the 1904 eruption.

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

On 23 December a visit to the crater found that the maximum fumarole temperature was only 606°C, indicating a decrease by another 30°C since the previous measurements on 27 November (BGVN 21:11).

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

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

Reports by Otoniel Matías of INSIVUMEH described volcanic activity after the 11 November eruption (BGVN 21:11) until 12 December. Seismic data were registered at a local one-component station (exact location undisclosed).

Activity during 11-30 November 1996. Heightened explosive Strombolian activity from MacKenney crater on 11 November was accompanied by abundant lava flows that covered most of the SW flank and traveled as far as 2 km from the volcano. The ash column reached 2 km, and deposited a 7-cm-thick layer of coarse tephra (1-3 cm in size) over the village of El Caracol (~4 km SW of the summit). Ash transported SW by strong winds fell in Escuintla and, as some local newspapers reported, in the Mexican city of Tapachula (300 km NW). At 2010 of the same day the eruption started dying down.

For the remainder of the month there was continuous emission of white-blue "smoke" at variable heights (50-300 m) above the crater. After the 11 November eruption seismicity was dominated by type-A volcano-tectonic swarms. During the next few days the swarm's foci deepened from 0.2-1 km to 5-10 km and then to 15-20 km and finally on 18 November, to 25-35 km. On 12 November a lava tongue ~2.5 km long was flowing S toward Cerro Buena Vista. On 13 November another small lava flow ~450 m long was observed on the SW flank.

During 15-18 November there were explosions of variable intensity every 1-3 minutes; the strongest explosions lasted 25-50 seconds and had amplitudes of 35-40 mm (peak-to-peak) and frequencies of 3-5 Hz. These explosions expelled pyroclastic material 75-150 m above the crater, depositing most material inside the crater.

On 20 November the seismic activity became characterized by harmonic tremor and type-B earthquakes, indicating magma movement toward the crater. The same day an eyewitness reported a red glow inside the main crater. Early on 21 November the white-blue column rose >500 m above MacKenney crater, but later that day decreased to ~300 m.

During 26-28 November the seismicity became characterized by alternating periods of activity followed by quiet for about 12 hours. The seismic activity shifted between seismo-tectonic swarms (one event every 2-10 minutes) and continuous tremor, suggesting that magma was ascending from depth and the confining rock was adjusting to the new stress field. After 28 November the activity was again dominated by seismo-tectonic events. Originating at depths of 0.2-1 km and up to M 1, there were 467 such events during a 24-hour observation period.

Activity during 1-12 December 1996. The white-blue column observed during November was also reported throughout this period at heights of 70-250 m. On 2, 3, and 4 December small explosions every 1-8 minutes were accompanied by ejection of incandescent material up to 100 m above MacKenney crater. This Strombolian activity was associated with harmonic tremor, although such tremors had appeared since the night of 1 December.

On 5 December puffs of steam lasting 5-7 minutes built up a column that rose 400 m. On 6 December, steam-and-ash expulsions of variable intensity turned brown in color and were observed at intervals of 2-10 minutes.

During 7-10 December the seismicity was characterized by tremor (mean amplitude, 2-5 mm; frequency, 3-4 Hz; and duration, 2-15 minutes) and type-A seismic events at 0.3-1 km depth. On 10-11 December steaming was observed on the N, W, and SW flanks of the MacKenney cone.

On 12 December white-blue emissions reached 300 m above the crater and drifted slowly SW for 5 km. That day, low-amplitude, low-frequency (0.5-1 Hz) harmonic tremor alternated with pulses of disharmonious tremor with amplitude 3-10 mm and 4-6 Hz frequency. During these pulses, steam-and-ash emissions occurred in association with considerable mass wasting inside the crater.

Geologic Background. Eruptions from Pacaya 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 older Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1,500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate scarp inside which the modern Pacaya volcano (Mackenney cone) grew. The NW-flank Cerro Chino crater 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 covered the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit.

Information Contacts: Otoniel Matías, Seccion Vulcanologia, INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia of the Ministerio de Communicaciones, Transporte y Obras Publicas), 7A Avenida 14-57, Zona 13, Guatemala City, Guatemala.

The current episode of eruptive activity, which began on 15 September (BGVN 21:08-21:10), persisted [through 3 January]. On 2 December, infrared video taken by the Alaska State Troopers confirmed that the E summit vent was more active than the W vent. The source of intense steaming low on the N flank, which had been intermittently visible to aerial and ground observers for several weeks, was not a new flank vent, but simply a site where lava was in contact with ice or meltwater. Meltwater channels extended down to the low pass between Pavlof and Pavlof Sister, then to the NW into the Cathedral River drainage. On the early morning of 4 December, seismic activity abruptly declined to about background, the lowest level after the onset of the eruption. The substantial decrease in seismicity implied that eruptive activity probably abated.

After a few days of quiescence, seismicity sharply increased on 10 December, accompanying intense long eruption pulses. Steam plumes reached an altitude of 8,500 m, and ash plumes rose to 7,700 m. On 11 December, pilots reported a steam plume at 8,700 m altitude and an ash cloud at 5,200 m. Satellite imagery indicated that a thick 20-km-wide plume extended as far as 160 km SE and a thinner, more diffuse part of the plume then turned E, extending 105 km. Seismicity declined on 12 December. However, lava fountaining from the summit vents and intermittent bursts of steam and ash to below 6,100 m continued; two lava flows were still active on the N flank. Seismicity decreased to about background levels on the evening of 13 December, but observers in Cold Bay, 60 km SW, reported lava fountaining prior to the seismic decrease. There was no steam or ash visible on the morning of 15 December.

Seismic activity began to build again around midnight on 25 December. In the early morning of 27 December seismicity quickly and steadily increased, reaching the highest level to date in this eruption episode. Early morning satellite images and pilot observations showed a summit hot spot, an active lava flow, and an ash plume extending tens of kilometers downwind (NW). That afternoon observers reported discontinuous bursts of ash and steam rising several hundred meters above the summit. Ground observers in Nelson Lagoon, 80 km NE, reported vigorous fire fountaining and a lava flow visible at night. On 28 December, visual reports in the afternoon from pilots and ground observers in Cold Bay indicated plumes reaching altitudes of 3,700-4,900 m and extending for up to 32 km WNW.

Seismicity started to decline in the afternoon, and the volcano was quiet during the night. On the morning of 29 December, pilots and ground observers in Cold Bay reported no eruptive activity. On 2 January, a pilot report indicated steam and ash drifting S from the summit. On the morning of 3 January, an observer in Cold Bay spotted a small burst of ash rising just above the summit. The eruptive pause continued during the week of 11-17 January with very low levels of seismicity.

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

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

During the four-month interval of September-December 1996 OVSCICORI-UNA reported the following N crater lake temperatures: 40, 35, 31, and 29°C, respectively. There were also fluctuations in the N crater lake's surface height. Defining a lake surface height increase with respect to the height in August as positive (+), during the four-month interval the progressive shifts were as follows: +13 cm, -54 cm, -31 cm, and +144 cm. Figures 62 and 63 show temperature and pH for the N crater lake and some chemical data for rainfall during the first eleven months of 1996.

Figure 62. The pH and temperature of the N crater lake at Poás. Data points show discrete values taken at the date shown. Courtesy of OVSICORI-UNA.

Figure 63. The chemistry of rainwater falling on Cerro Pelon at Poás. The ion concentration (for SO2+4 and Cl- in mg/liter) and pH (± 0.5 units) during the interval 26 January-28 November. Bars and points show discrete values taken at the date shown. Courtesy of OVSICORI-UNA.

Also during September-October, the accessible fumaroles on the pyroclastic cone maintained a maximum temperature of 93-94°C. Gas columns in the interval rose 500 m above the crater floor.

The number of low-frequency Poás earthquakes registered as follows: 2,351 (September), 2,043 (October), 1,704 (November, adjusted because of 14 days without data). December earthquakes have not yet been reported.

September contained the largest number of earthquakes recorded in any month in 1996. Low-frequency tremor, not seen since May, appeared for ~8 hours in September, also the most seen up to that point in 1996. Continuing the trend, tremor occurred for 28 hours during October. No tremor was recorded in November. For comparison, tremor had reached over 300 hours in December 1995.

Deformation during August and September 1996 was very low. Two lines in the sector S of the crater contracted by an average of ~5 mm (2-3 ppm), a shift considered insignificant. Although electronic tilt measurements were absent for August-September, two leveling lines along the crater's S border have lacked significant tilt since 1995.

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

Information Contacts: E. Fernández, E. Duarte, V. Barboza, R. Van der Laat, E. Hernandez, M. Martinez, and R. Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA).

Popocatépetl erupted on 25 December 1996 beginning around 1145 as seen in satellite imagery and discussed in pilot reports from American Airlines. Comparison of satellite imagery at 1145 and 1215 suggested that the eruption was not continuous.

About 1145 on 25 December the ash cloud was bounded by the following points: 19.0°N, 98.6°W; 19.2°N, 98.8°W; 19.3°N, 98.6°W; and 19.1°N, 98.4°W. At around that time GOES-8 data indicated that the eruption's ash plume moved N and E while dispersing rather quickly in both infrared and visible imagery. At 1645 that day the ash plume appeared nearly linear. It formed a band ~50 km wide trending WNW-ESE (from 20.2°N, 99.2°W to 19.6°N, 97.1°W) and covering a distance of ~230 km.

The plume from the prior day's eruption was indistinguishable by 26 December, based on GOES-8 infrared imagery. A SIGMET valid during 2300-2400 on 25 December indicated only near-source ash, suggesting that by this time conditions had return to normal. No additional eruptions were seen around that time.

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: NOAA/NESDIS Synoptic Analysis Branch, USA.

Strong Strombolian eruptions occurred at Tavurvur on 4-5 October, but activity was generally low during November and December. Before the eruptions in October, activity consisted of weak to moderate emissions of white vapor and occasional ash clouds that rose ~600 m above the crater. Some of the ash emissions were accompanied by roaring noises. Light ashfall was observed to the N and NW. Three moderate explosions occurred at 1209 on 3 October, and at 0017 and 0219 on 4 October, producing dark gray ash clouds 3-4 km high.

Strombolian eruptions started with emissions of dark gray ash clouds at 1200 on 4 October. At that time, real-time seismic amplitude measurement (RSAM) values were 30-50. Eruptive activity gradually increased, with more frequent emissions of thick, dark gray ash clouds. The activity rapidly increased at 1430, and peaked at 1450 with an RSAM value of ~680. Activity quickly declined after only 20 minutes, to an RSAM value of 300 by about 1700. Activity soon increased again, and reached another peak at about 2000 with an RSAM value of 800. Frequent loud explosions rattled windows and doors 7-8 km from the crater, and were heard as far as 40 km away. Red incandescent lava fragments were frequently projected ~1 km above the crater. Some of the ejecta were 1-2 m in diameter during the peak eruptions. The hot and plastic ejecta were deformed during flight.

Beginning at 2000, the eruptive activity slightly decreased and RSAM values dropped to ~710 in two hours, then the activity increased again and peaked at about 2320. According to the RSAM data, activity remained almost unchanged from then until 0820 the next morning with a constant RSAM value of ~860. From 0820 on 5 October, the activity began to decline, accompanied by frequent explosions. The frequency of explosions peaked at 1700 on 5 October (~150 explosions/hour), and decreased exponentially, with only ~20 explosions/hour at 0700 on 6 October. The last relatively large explosion occurred on 25 October.

During the strong phase of Strombolian eruptions, a significant amount of lava was erupted. Effusive activity began around 0130 on 5 October. Lava flowed to the S of the vent, covering a large area of coconut plantation and burying two houses. Three lobes of the lava flow moved into the sea, ~1.6 km from the vent. The volume of lava was estimated at 4-5 x 10⁶ m³, the largest amount produced at Tavurvur in more than 200 years. Effusive activity occurred at Tavurvur during the 1994 eruption (BGVN 19:08-19:10), with a lava flow of ~0.4 x 10⁶ m³.

Ash clouds produced by the Strombolian eruptions and the subsequent large explosions rose 3-4 km above the crater. Light ash fell on the N, NW, W, SW, and SE areas of the caldera. Moderate, damp ashfall was observed on Matupit Island 1.5 km W of the vent.

A low level of eruptive activity occurred at Tavurvur during November and December, with very weak to moderate emissions of white vapor. However, emissions of small volumes of pale gray ash took place twice in December, the first on 6-7 December. Pale gray emissions with a very low ash content began on the morning of 6 December, then gradually changed to weak, white vapor the next day. The ash clouds rose ~3 km above the crater before being blown to the NE, NW, and W. Very light ashfall was observed on 7 December in Rabaul Town, ~5 km NW of the crater. The second ash emission began on 27 December and was continuing at the end of the month.

During October-December, 27 high-frequency earthquakes were recorded. Two events on 24 December were felt with an intensity of III. During November and December, seismicity was generally low; the only seismic increases were associated with the two ash-emission episodes.

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: B. Talai, I. Itikarai, and P. de Saint Ours, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Since the end of the last rainy season, the vapor plume continuously present over the volcano has reduced in size compared to recent years. Late in 1996, fumaroles outside the crater decreased significantly. Fumaroles inside the crater concentrated in the lower part of the S and SE walls and had temperatures estimated to be in the 650-700°C range.

During the night and early morning on 24 December several incandescent areas were observed in the crater. These areas were similar to those reported over the past 15 years. Incandescence was visible from the top of the volcano at 500 m distance, and from the rim of the 1976 crater, a feature 400 m in diameter and 100-150 m deep. During 1996 small local earthquakes were detected monthly.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may have been from other Marrabios Range volcanoes.

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

Santa María's large SW-flank crater that formed in a major eruption in 1902 contains Santiaguito, a dacite dome active almost continuously since 1922. In accord with this pattern, a small explosion was observed on 14 October. During 19 November-12 December 1996 several explosions of moderate-to-high intensity occurred almost daily. These explosions, three per hour, expelled ash in columns that rose variably 300-1,000 m above the active Caliente cone. The ash plumes, white-to-dark-gray in color, remained 8-15 minutes above the volcano before being blown W or SW or both directions. Light ashfalls were reported in the Rosario Palajunoi Estate (15 km from the volcano), La Finca Estate (~7 km SSW of the cone), over the woods in Siete Orejos area, but mostly in the proximity of Caliente cone.

Some of the explosions triggered small collapses and avalanches of blocks and ash down the Nimà Segundo river (SE flank) and along a channel opened by lava flows on the E flank of the volcano.

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

Information Contacts: Otoniel Matías, INSIVUMEH.

On 5-6 December, and on 7 and 14-15 January, the usual fumarolic emissions were observed above the volcano.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. 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 occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. 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: Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia.

The following condenses the daily Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 9 December 1996-10 January 1997.

Visual observations during 9-31 December On 9 December it was noted that a crack on Galway's wall had opened 33 cm in five days. One side of the crack had moved by 7 cm, consistent with the wall being pushed outwards. Helicopter inspections on 10 December detected 20-m-deep cracks along and on top of this wall, making it very unstable. On 11 December a new dome appeared to the S of the 1 October dome, between Castle Peak and Galway's Wall (see map in BGVN 21:11). New fractures 100 m long and 1 m wide were seen at the E end of Galway's Wall.

On 14 December the new dome volume was estimated at 500,000 m³ ; having grown over a period of 2-3 days, its extrusion was comparable to the initial rates for the 1 October dome. On 15 December the new dome's top was at 910 m, higher than the October 1 dome at that time; growth had occurred along a linear structure oriented ESE. By 16 December the top of the dome was estimated at 920 m. Observations that day showed that the dome had nearly filled the scar left by the explosion in September and that a new spine had grown from its top.

On 17 December the new dome started to overflow the September explosion scar; this caused several moderate-sized rockfalls and small pyroclastic flows into the Tar River that traveled ~250 m from the dome. That day the new dome was 909 m high, its surface rubbly with coarse blocks, and its shape conical with a flat top and two spines. Comparison of recent dome surveys with previous results showed that older material near the new dome rose 80 m, a volume change of perhaps 7 x 10⁶ m³ since the beginning of December. On 17 December several large steam clouds ascended above the volcano, probably caused by steam venting from the new dome.

On 19 December the new dome's E face was near-vertical and appeared very unstable. Discrete pulses of rockfalls and small pyroclastic flows from the dome occurred only a few minutes apart; these descended as far as 1 km along the gully on the S side of Castle Peak creating many small ash clouds that rose 300 m above the crater and drifted slowly W. A dome survey carried out using laser-ranging binoculars estimated the new dome's volume at approximately 800,000 m³, yielding an extrusion rate of 0.5 m³/s. That evening both the E side of the new dome and part of the pre-September dome failed, causing moderately large pyroclastic flows. These flows traveled down the Tar River and over its fan reaching to within 40 m of the sea; ash clouds rose 3 km and were carried SW. As the flows were generated observations from the airport suggested that fresh lava emerged into the dome nearly as quickly as it was lost in the flows. The next day, ground and helicopter observations indicated a reactivation of the 1 October dome growth. Many small rockfalls descended the 1 October dome's N side, fewer from its S side. Small pyroclastic flows generated ash clouds characterized by little convection, possibly suggesting that colder material was involved. This was confirmed by observations during the night using an infrared imaging system. This imaging system also showed that the entire 1 October dome was active.

During the following days, rockfalls and pyroclastic flows caused many more ash clouds which deposited ash in Plymouth; associated clouds displayed robust convection, suggesting that hot, fresh material was involved. A helicopter inspection on 22 December confirmed that the activity was restricted to the 1 October dome and that there was no sign of activity on the 11 December dome.

On 23 December heavy rain caused mudflows in Fort Ghaut that carried half-meter diameter boulders into the sea. On 25 December some uplift was observed on the N flank of the October 1 dome, perhaps due to an injection of fresh lava. On 26 December satellite imagery showed ash ~100 km WSW at a height of 1-2 km.

Ground and helicopter observations on 26 December showed a significant amount of new material on the top of the dome, darker in color and smoother than the older material. On 29 December, glowing all over the NE flank and avalanching of incandescent blocks were observed. Incandescence in daylight suggested that this dome lava may have been hotter than previous dome lavas. A considerable amount of material was observed on 31 December falling down the N flank of the pre-September dome towards Farrell's Wall. The new material at the top of the dome had changed texture, and looked more slabby than before.

Visual observations during 1-10 January 1997. The first seven days of January were characterized by numerous rockfalls and small pyroclastic flows from the 1 October dome, mainly down its NE and E sides. Much of this activity was channeled into the Tar River valley by way of either an erosion chute cutting across the top of Castle Peak or one to its N. At times of peak activity the pyroclastic flows occurred every few minutes and the largest traveled ~300 m past the Tar River Soufriere. Many of the rockfalls and flows generated ash clouds that drifted W and SW, forming a semi-continuous ash plume observed at altitudes of 1.3-1.6 km. On 3 January the plume was reported at 2 km altitude, and on 4 January satellite observations detected the plume 360 km W of Montserrat.

Theodolite measurements of the dome on 5 January showed that although the height had remained relatively constant at ~900 m since 1 January, a new lobe of lava at the top of the dome was ~50 m thick. It was calculated that 4.6 x 10⁶ m³ of material was added between 25 December and 5 January, an extrusion rate of 4.4 m³/s. This was the highest sustained extrusion rate yet measured during this eruption. A helicopter inspection on 5 January revealed material slowly accumulating against the N crater wall; only 7 m of ridge remained above the divide to Tuitt's Ghaut. On 6 January the glowing dome appeared less steep in its upper part.

On 8 January several pyroclastic flows originating from behind Castle Peak moved down the Tar River Valley to reach beyond the Tar River Estate House; at least one pyroclastic flow reached the sea. Further growth was observed on the NW side of the 1 October dome, but was still contained inside the 17-18 September scar. Several new glowing channels eroded by the pyroclastic flows on the E side of the dome were visible. On 10 January a new, unstable-looking extrusion was observed in the middle of the heavily eroded chute crossing Castle Peak. This new extrusion was butterfly shaped and composed of slabs of fresh lava.

Seismicity and seismically detected mass wasting. Seismic activity during 9-11 December was characterized by swarms of shallow volcano-tectonic earthquakes, at times large enough to be felt close to the volcano. A few rockfalls from the dome and some landslides from the Galway's Wall were also detected by the seismic network, indicating that the wall became increasingly unstable during intense earthquake activity. However, a lack of seismicity on 12 December was accompanied by more landslides on Galway's Wall. During the following days rockfalls occurred sporadically, but their number slightly increased after 16 December, as the 11 December dome kept growing. Also, a few landslides from Galway's Wall suggested continued slow deformation. Seismicity increased on 20 December with a shallow volcano-tectonic earthquake swarm that reached the level of intensity of the early December swarms, although maximum magnitudes were not as large as before. Several rockfalls were also detected, mostly from the 1 October dome.

On 22 December the volcano-tectonic seismicity died out, rockfall signals continued, and hybrid seismicity reached April levels. This and increases in the quantity of ash and pyroclastic flows were taken as an indication that the dome growth rate had increased, but poor visibility prevented dome observations. By 24 December hybrid events and continuous tremor dominated the records, but by 27 December banded tremor reached a maximum. Banded tremor, which was last seen between late July and mid-September, had taken place associated with large pyroclastic flows and the 17-18 September explosion.

On 28 December large hybrid events and rockfall signals dominated, but regularly spaced bands of continuous seismic tremor returned on 30 December. Lower in amplitude than before, the banded tremor occurred at ~10-hour intervals. By 31 December the activity was again dominated by banded tremor episodes ~5 hours apart, and by hybrid earthquakes and rockfall signals. This pattern of seismicity continued during the first seven days of January. Volcano-tectonic earthquakes returned on 4 January with signals similar to the November and December events, but possibly from slightly greater depths (2-3 km). From 8 to 10 January, in correspondence with increased dome activity, the seismicity became dominated by rockfall and pyroclastic-flow signals.

COSPEC, EDM, and other measurements. COSPEC measurements were made on 27 and 28 December. The data from 27 December averaged 350 metric tons/day (t/d) but reached ~400 t/d shortly after one of the peaks in seismic tremor. The average fluxes on 28 December, and on 1, 4, 9, and 10 January were 325, 300, 400, 1,130, and 390 t/d, respectively. The increase in emission of SO₂ measured on 9 January was probably due to a partial collapse of the dome.

EDM measurements carried out on the E triangle on 10 and 13 December suggested a continuation of the shortening trend started several weeks earlier. On 16 December, the S triangle had been unchanged since 4 December; on 18 December the lines N of the volcano had also not changed significantly since 5 November. The average shortening on the lines to Castle Peak during 20-22 December was 6 cm; such high rates of deformation had occasionally been seen in the past. In contrast, the shortening seen there during 26-28 December was small (2.4 mm). This drop in the deformation rate roughly coincided with the appearance of new material at the surface on the 1 October dome.

Gravity measurements on the E flank on 22 December showed no significant changes since July 1996. Changes at stations on the upper slope were consistent with the mass added to the dome.

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

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

Microseismic events, which were only detected locally, appeared 540 times during September, leading to the largest monthly total yet seen in 1996. The totals for October and November were 308 and 220 (the latter was extrapolated from 17 days of recording). Monthly microearthquake totals were essentially zero for the first four months of 1996 and generally grew steadily through September. The seismic station (VTU) lies 0.5 km SW of the active crater. The cumulative dry-tilt for the first 10 months of 1996 measured 10 µrad. The temperature and pH, however, remained relatively stable for the two available measurements during 1996 (BGVN 21:08).

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

Information Contacts: E. Fernández, E. Duarte, V. Barboza, R. Van der Laat, E. Hernandez, M. Martinez, and R. Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

The following summarizes observations of eruptive activity during 11 September 1996-13 January 1997, based on descriptions by volcano guides and a visit to the volcano by Werner Keller in January 1997.

On 11 September 1996 a group of mountain climbers observed intense degassing of water vapor and reported that the small lava pond on the crater floor was not visible. On 14 September (BGVN 21:09) there was emission of ash accompanied by a dull rumbling noise. Guide Claudio Marticorena of Pucon was close to the summit with a group of tourists at the time of the ash emissions and reported that lava blocks tens of centimeters in diameter were ejected above the rim of the summit crater.

October and November were characterized by a notable rise of the magma column within the central crater pit, which was almost completely filled to its rim. Mountain guides Victor Sepulveda and Claudio Marticorena reported a vigorously convecting lava lake 50 m in diameter with fountaining from several areas of the lake. Frequent bursts ejected spatter and incandescent bombs beyond the summit crater, onto the upper flanks of the cone every few seconds. This activity lasted until mid-November 1996, followed by a rapid subsidence of the magmatic column and accompanied by strong vapor emission later that month. In December, the characteristic nocturnal crater glow observed at Villarrica during the past years disappeared.

Fumarolic emissions from the summit crater diminished in early January 1997, and on 4 January Sepulveda noted that the inner crater pit was again completely visible, for the first time since late November 1996. At that date, the central pit was ~100 m deep, with two small degassing vents at the bottom. No incandescent lava was visible in either of the vents, but gas emissions produced a distinct noise. The S part of the intracrater platform left after the 1984-85 eruption had collapsed into the central pit. On 13 January, mountain guides noted incandescent lava within the central pit: this suggested a new rise of the magma column.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Boris Behncke, Geomar Research Center for Marine Geosciences, Wischhofstrasse 1-3, 24148 Kiel, Germany; Werner Keller, Wiesenstrasse 8, 86438 Kissing, Germany.