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).

Introduction. The Associated Press reported a sudden explosion had occurred at Mount Cameroon on 3 February 2012 (see subsection below). A review of MODVOLC thermal alerts based on satellite infrared data during 2001 to mid-2014 found few if any of the highest-level alerts. In contrast, there were numerous stronger alerts during 3 March to 17 July 2000 (activity described in BGVN 24:09, 25:06).

In (BGVN 26:11) we reported that during 26-27 June 2001, Limbe, an economically important coastal town with ~85,000 residents located on the S foot of the steep sided stratovolcano Mt. Cameroon (figure 4), was struck by a series of heavy rains leading to deadly floods and landslides. Figure 5 shows a photo taken in Limbe during the flood. Although many would classify the flood as a meteorological disaster and not a volcanological one, Mt. Cameroon is the tallest peak in Western Africa and orogenic uplift of warm most air over the massive edifice is a factor affecting the amount of rainfall. Limbe sits at the mouth of drainages coming down this side of the edifice. During July 2014, heavy rains again resulted in floods in Limbe (Ndaley, 2014). In strict eruptive-hazard terms, Mount Cameroon is the only volcanic peak in Western Africa with recent ongoing eruptions.

Figure 4. (Inset) Indicating the location of the Republic of Cameroon in Africa. (Main map) The location of Mount Cameroon, the main bulk of which is centered ~25 km from the coastline. Some other regional volcanic features and their ages are also indicated. Etinde is a prominent conical volcanic center on the SW flank of the larger structure of Mount Cameroon. The town of Limbe resides along the coast just S of the volcano. Taken from Suh and others (2008).

Figure 5. Flooding that inundated Clerks quarter, Limbe in 2001. The volcanic topography feeds a number of catchment basins into Limbe, which is also why the same town is also highly vulnerable to lava flows (Wantim and others, 2011). Photo taken from MIA-VITA literature.

This report discusses recent research on the 2000 eruption of Mount Cameroon and then summarizes news articles on a smaller 2012 explosion. After that, this report discusses vulnerability studies, including a United Nations project that examined and attempted to mitigate the risk to local communities.

The 2000 eruption. On the basis of first-hand knowledge of some of their co-authors, Wantim and others (2011) stated that the eruption occurred during 28 May and 20 July 2000. MODVOLC satellite-based thermal alerts were found during the interval 3 March 2000 to 17 July 2000. The seismic activity for this eruption lasted 3 months, up to September 2000 (Ateba and others, 2009). Multiple fissure segments produced lava that built four different lava flow fields at three sites they specify in their paper (Wantim and others, 2011).

Eruptions at site 1 (~ 3,930 m a.s.l) began in the night of 28 May 2000 with an explosive phase that produced only tephra and ballistic blocks (Suh and others, 2003 and Wantim and others, 2011). A ~850 m-long ′a′a flow field was emplaced at this site a month later, an observation supported by data from MODVOLC and multispectral images (Landsat ETM + and ASTER) analyzed in this study. The upper 2000 flow is one of the shortest (850 m) recorded for historical lava flows here despite having descended steep slopes (10-25°). Late emission of the lava and field observations of cone breach at the lava source suggest that the lava flow was fed by the drainage of a transient lava lake. That lake was presumably formed by lava fountaining in the eruptive cone (Wantim and others., 2011). The lava flow field covers a total surface area of (8 ± 2) × 104 m² with a total volume of (3.4 ± 0.8) × 105 m³. There were two lobes, ~4.5 m and ~8.7 m thick (Wantim and others., 2011).

Brief explosion--2012 news reports. Cameroon state radio and television reports stated that Mount Cameroon sent "ashes and flames" into the air in a brief explosion on 3 February 2012. A violent explosion lasted a couple of seconds and lightly injured two of the porters and guides on the mountain, according to a 6 February 2012 Associated Press report.

Lava flow hazard and risk, and weathering studies. According to Favalli and others (2010), Mt. Cameroon is one of the most active effusive volcanoes in Africa. About 500,000 people living or working around its fertile flanks are subject to significant threat from lava flow inundation. Therefore, this group initiated a scientific project to assess the hazards/risks associated with the volcano by simulating probable lava flow paths using the DOWNFLOW code, a routine that for lava-flow-hazard mapping that defines areas susceptible to inundation.

According to Wantim and others (2013), as for many other effusive volcanoes, only limited information exists on the relevant lava flow properties and emplacement dynamics for recent eruptions. This study provides new quantitative constraints for rheological and dynamic characteristics of lava flow effusion at Mount Cameroon during the 1982 and 2000 eruptions. These constraints were used to calibrate the FLOWGO thermo-rheological model for these lava flows. FLOWGO (Harris and Rowland, 2001) was the only model that enables full inversion of the thermo-rheological properties of lava flows. It can be constrained from channel morphology and down-flow evolution of crystal content.

Lava flow hazard and risk were assessed by simulating probable lava flow paths using the DOWNFLOW code (Tarquini and Favalli, 2011). That code incorporates digital elevation data and allows the definition of areas that are susceptible to inundation by lava flows originating from each vent; it has been used extensively to simulate lava flows at Mt. Etna and Nyiragongo volcanoes ( Favalli and others, 2005, 2006, 2009; Chirico and others 2009). The details of the modeling and the resulting maps they produced can be found in the cited references. Simulated lava flows from about 80,000 possible vents were used to produce a detailed lava flow hazard map. The lava flow risk in the area was mapped by combining the hazard map with digitized infrastructures (i.e., human settlements and roads).

Results show that the risk of lava flow inundation is greatest in the most inhabited coastal areas, specifically the town of Limbe (which constitutes the center of Cameroon's oil industry and an important commercial port). Buea, the second most important town in the area, has a much lower risk although it is significantly closer to the summit of the volcano. Non-negligible risk characterizes many villages and most roads in the area surrounding the volcano. In addition to the conventional risk mapping described above, the authors also present (1) two reversed risk maps (one for buildings and one for roads), where each point on the volcano is classified according to the total damage expected as a consequence of vent opening at that point; (2) maps of the lava catchments for the two main towns of Limbe and Buea, illustrating the expected damage upon venting at any point in the catchment basin. The hazard and risk maps provided here represent valuable tools for both medium/long-term land-use planning and real-time volcanic risk management and decision making.

The largely geochemical study of Che and others (2012), analyses the behavior and mobility of major and some trace elements during the physical and chemical development of landslide-prone soil profiles in Limbe, SW Cameroon. The soils result from in situ weathering of Tertiary basaltic and picrobasaltic rocks. Textural and chemical characterizations, together with two mass balance models are applied to understand the mobility and redistribution of elements during the weathering of pyroclastic cones and lava flows in the setting of Mt. Cameroon. This weathering is a major factor in the cohesion of steep slopes, and thus these studies address slope stability, another kind of volcano-related hazard that could occur even in times of volcanic quiescence.

Socio-economic vulnerability study. A United Nations project, MIA-VITA (Mitigate and Assess risk from Volcanic Impact on Terrain and human Activities) was started in Cameroon during 2011 (Apa and others, 2007; Bosi and others, 2011; European Commission, 2010). (The phrase 'Mia Vita' comes from the Italian, "My Life"). The project was based on the UN International Strategy for Disaster Reduction and a key expected outcome was to finding the best means to help local populations and authorities better perceive risks and thus reduce community vulnerability.

The program in Cameroon, as well in three other developing countries with active volcanoes, had several goals: (1) to assess the natural risk to local communities from the selected volcano, based on risk mapping and damage scenarios; (2) to improve crisis management, based on early warning systems and improved communications between government officials and the local populations, and (3) to reduce the vulnerability of populations in the wake of an eruption.

Besides Mount Cameroon, MIA-VITA also contributed to similar goals at Mount Merapi in Indonesia, Mount Kanlaon in Philippines, and Mount Fogo in Cape Verde. In the service of local citizens facing volcanic hazards, the MIA-VITA study also aimed to improve civil-defence, planning, and coordination and to reduce rumors and alarmist information.

MIA-VITA also integrates GIS capability with an analytic hierarchy method that yields volcanic risk maps. The approach is designed to solve complex multiple criteria problems using relative pairwise comparisons (Saaty, 1996; Wikipedia, 2014). To apply this approach, it is necessary to break down a complex unstructured problem into its component factors. The method incorporates both qualitative and quantitative criteria in the evaluation.

References. Apa, M.I., Kouokam, E:, Akoko, R.M, Thierry, P., and Buongiorno, M.F., 2007, Mt. Cameroon Socio-Economic Vulnerability and Resilience Assessment Through Traditional Survey Methods[FP7-ENV-2007-1] (URL: http://meetingorganizer.copernicus.org/EGU2011/EGU2011-3402.pdf http://meetingorganizer.copernicus.org/EGU2011/EGU2011-3402.pdf )

Bosi, V., Cristiani C. and Costantini, L., 2011, 3rd MIA-VITA Newsletter (Sept. 2011), MIA-VITA (URL: www.spinics.net/lists/volcano/msg02475.html )

Che, V.B., Fontijin, K., Ernst, G.G.J., Kervyn, M., Elburg, M, Van Ranst, E., Suh, C.E., 2012, Evaluating the degree of weathering in landslide-prone soils in the humid tropics: The case of Limbe, SW Cameroon; Geoderma, Vol. 170, pp. 378-389

Chirico G.D., Favalli, M., Papale, P., Pareschi, M.T., Boschi, E., 2009, Lava flow hazard at Nyiragongo volcano, D.R.C. 2. Hazard reduction in urban areas. Bull Volcanol 71:375-387. doi:10.1007/s00445-008-0232-z

European Commission, 2010 (31 March 2010), MIA-VITA--1st newsletter (URL: http://images.nationmaster.com/images/motw/africa/calabar_tpc_1996.jpg )

Favalli, M., Chirico, G.D., Papale, P., Pareschi, M.T., Boschi, E. (2009a) Lava flow hazard at Nyiragongo volcano, D.R.C. 1. Model calibration and hazard mapping. Bull Volcanol 71:363-374. doi:10.1007/s00445-008-0233-y

Favalli, M., Tarquini, S., Papale, P, Fomacai, A, and Boschi, E., 2011, Lava flow hazard and risk at Mt. Cameroon volcano, _Journal of Volcanology 2012 74:433-439. adsabs.harvard.edu/abs/2012BVol...74..423F

Favalli, M., Mazzarini, F., Pareschi, M.T., Boschi E (2009b) Topographic control on lava flow paths at Mount Etna, Italy: Implications for hazard assessment. J Geophys Res 114:F01019. doi:10.1029/2007JF000918

Favalli, M., Tarquini, S., Fornaciai, A., Boschi, E., 2009c, A new approach to risk assessment of lava flow at Mount Etna, Geology, 37(12):1111-1114. doi:10.1130/G30187A

Harris, AJL and Rowland, SK, 2001, FLOWGO: A kinematic thermorheological model for lava flowing in a channel. Bull Volcanol., . 63:20-44. doi:10.1007/s004450000120

Ndaley, Yannick Fonki, 2014, Heavy Rains Beat Limbe, Floods Put Residents In Distress. Eden Newspaper, 12 July 2014, (URL: http://edennewspaper.net/)

Saaty, T. L. (1996). Multicriteria decision making: The analytic hierarchy process. Pittsburgh, PA: RWS Publications, 479 pp. (ISBN 0962031712, 9780962031717)

Suh, C.E., Luhr, J.F., and Njome, M.S., 2008, Olivine-hosted glass inclusions from Scoriae erupted in 1954-2000 at Mount Cameroon volcano, West Africa, Journal of Volcanology and Geothermal Research, Volume 169, Issues 1-2, 1 January 2008, pp. 1-33, ISSN 0377-0273, http://dx.doi.org/10.1016/j.jvolgeores.2007.07.004.

Tarquini, S and Favalli, M, 2011, Mapping and DOWNFLOW simulation of recent lava flow fields at Mount Etna. Journal of Volcanology and Geothermal Research, 204:27-39. doi:10.1016/j.jvolgeores.2011.05.001

Wantim, M.N. , Kervyn, M., Ernst, G.G.J, del Marmol, M.A., Suh, C.E.. and Jacobs, P., 2013, Numerical experiments on the dynamics of channelised lava flows at Mount Cameroon volcano with the FLOWGO thermo-rheological model, Journal of Volcanology and Geothermal Research, Volume 253, pp. 35-53, ISSN 0377-0273

Wikipedia, 2014, Multi-criteria decision analysis (URL: http://en.wikipedia.org/wiki/Multi-criteria_decision_analysis ).

Geologic Background. The massive steep-sided Mount Cameroon rises above the coast of west Cameroon, overlooking the Bight of Biafra, part of the Gulf of Guinea. The dominantly basaltic-to-trachybasaltic edifice forms a volcanic horst constructed above a basement of Precambrian metamorphic rocks covered with Cretaceous to Quaternary sediments. More than 100 small cinder cones, often fissure-controlled parallel to the long axis of the 1,400 km3 edifice, occur on the flanks and surrounding lowlands. A large satellitic peak, Etinde (also known as Little Cameroon), is located on the S flank near the coast. The first known reported activity was in the 5th century BCE by the Carthaginian navigator Hannon. Additional activity has frequently been reported since about 1800 CE, consisting of moderate explosive and effusive eruptions from both summit and flank vents. A 1922 SW-flank eruption produced a lava flow that reached the Atlantic coast, and a lava flow from a 1999 S-flank eruption stopped only 200 m from the sea. Explosive activity from two vents on the upper SE flank was reported in May 2000.

Information Contacts: MODVOLC Thermal Alert System, Hawai'i Instiute of Geophysics and Planetology (HIGP), (Univ. of Hawai'i, 2525 Corrrea Road, Honolulu, HI 96822 USA (URL: http://modis.higp.hawaii.edu/); Mary-Ann del Marmol, Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, 9000 Gent, Belgium; Associated Press, and Cameroon Radio Television, CRTV Siège, CRTV Mballa II B.P. 1634, Yaounde, Cameroon (URL: http://crtv.cm/).

Our last report (BGVN 35:08) described a flank eruption at Nyamuragira during 2-27 January 2010 that produced a new cone and 12-km-long lava flows. The final report of the GORISK Scientific Network (Kervyn and others, 2010) stated that this eruption ended by 27 January 2010. MODVOLC thermal alerts occurred regularly at Nyamuragira through 2 February 2010 and then ceased until early November 2011.

Nyamuragira began to erupt again on its flanks at 1755 on 6 November 2011, according to GORISK, after two days of unspecified "intense seismic activity." This report conveys information from a variety of sources credited below, but largely from Dario Tedesco and the GORISK Scientific Network. GORISK was an initiative of both the National Museum of Natural History (Luxembourg) and the Royal Museum for Central Africa (Belgium). GORISK inferred that the eruption lasted through April 2012 [but only noted degassing from a vent inside the pit crater after late February in their June 2012 eruption summary].

An early synopsis of the eruption that began on 6 November 2011 came from the Virunga National Park. The eruption was visible at Park headquarters. Park staff described the 6 November eruption as coming from a fissure on the volcano's NE flank. It produced slow-moving lava flows that advanced into unpopulated areas to the N. Park staff also took a video of fountaining at night. On 7 November the Park uploaded 39 seconds of their footage on Youtube. During the first week of the eruption the Park staff hiked through the bush, in places having to cut vegetation, crossing young forest and irregular lichen-covered volcanic topography on a 4 hour hike that enabled them to take a closer view.

The hikers described airfall scoria covering the landscape as they approached closer. Their log said that the "...eruption finally came into view, along with the roar of intensely spewing fire and lava, as well as lightning and thunder." The vent area was located 12 km ENE of the crater, close to one of the 1989 eruptive sites. The first fissure was oriented E-W, perpendicular to the rift, and emitted lava fountains up to 300 m high. The eruption site was described as a flat area cut by a 500- to 1,000-m-long fissure. Figure 40 shows a photo from around this time but a topographic high of new material had already grown. NASA Earth Observatory reported that lava flows had advanced as far as 11.5 km by 12 November 2011 (figure 41). On 12 November, the lava flow front was located 5 km from the Kelengera-Tongo road.

Figure 40. Lava fountaining at Nyamuragira during early November 2011. The venting fissure is also seen in the distance at left; tephra created the topographic high seen here, rising from a comparatively flat area. Exact date and look-direction unknown. Courtesy Dario Tedesco and the GORISK Scientific Network.

Figure 41. Satellite image of Nyamuragira on 15 November 2011 showing lava flowing away from the rift. The imager combines infrared and visible light; hot lava appears orange, and cooled lava appears black. Cooler clouds appear blue, and warm steam appears white and orange. Nyiragongo's lava lake is visible to the S. Image created by Jesse Allen, using EO-1 ALI data provided by the NASA EO-1 team. Courtesy NASA Earth Observatory.

For about a month, the park allowed overnight treks to the eruption site. A video featured on Youtube by Piet Schutter contains footage taken on 12 November; some scenes are at close range (looking up towards the eruption). That video shows both daylight and night scenes, features sound, and has people in the foreground, which helps establish scale.

The GORISK Scientific Team reported that satellite radar (InSAR) images acquired on 11 November 2011 revealed major ground deformation features associated with the eruption. Preliminary estimation of the observed deformation signal suggested an affected area spreading over 250 km². Pressure from the ascending magma caused the ground to rise more than 50 cm at the eruptive site where a spatter cone developed. Another 15 cm deformation was detected within the caldera, which was accompanied by deflation observed on the flanks.

An elongated spatter-and-scoria cone, referred to by scientists as the western cone and by locals as "Umoja," formed along the first fissure (figure 42). In early December 2011, a new cone formed on a new eruptive fracture to the E; this cone was referred to by scientists as the eastern cone and by locals as "Tuungane" (figure 42). During the next few days, the eruptive activity migrated to this new edifice. Satellite images acquired on 3 January 2012 showed fresh lava flowing to the NNE (figures 43 and 44).

Figure 42. Panoramic view of Nyamuragira and the two new cones of the November 2011-April 2012 eruption. Date of photo unknown (sometime between November 2011 and early June 2012). Photo courtesy Benoit Smets, GORISK Scientific Network.

Figure 43. False-color satellite image of Nyamuragira on 3 January 2012. The hot active lava was detected in shortwave and near-infrared light (bright red-orange). Nyiragongo's crater lava lake is visible to the S. Image acquired by the Advanced Land Imager (ALI) aboard the Earth Observing-1 (EO-1) satellite. Courtesy Jesse Allen, Robert Simmon, and EO-1 Team, NASA Earth Observatory.

Figure 44. Natural-color satellite image of Nyamuragira on 3 January 2012 showing close-up of outlined area in figure 43. Active lava is visible flowing N-NE, with older flows also visible to the N and NE. A sulfur-dioxide-rich plume extends to the SW from the central vent. Undisturbed vegetation is also visible. Image acquired by the Advanced Land Imager (ALI) aboard the Earth Observing-1 (EO-1) satellite. Courtesy Jesse Allen, Robert Simmon, and EO-1 Team, NASA Earth Observatory.

According to visiting scientists from the Afar Consortium Project, activity continued on 8 January 2012. The initial scoria cone appeared inactive and a second formed to the N of the first; both cones were about 300 m high. The second cone was extremely active for the duration of the observations (about 15 hours) with fountains over twice the height of the cone; lava flowed N. The observers, about 1.5 km away, felt the heat from the eruption and noted lapilli fall.

A group from Volcano Discovery observed the ongoing fissure eruption during 22-25 January 2012 from the newly formed cinder cones near the fissure. They reported three coalescent cones, the largest containing a small lava lake. The lake ejected spatter every few seconds, rising as high as 200 m above the summit. Some bombs reached the base of the cone. Lava flows from the vent extended several kilometers N. Numerous small breakouts formed secondary flows, and a large breakout ~2 km N of the cone fed a large lava flow ~20 m wide. Burning forests were reported to the NNE.

Preliminary estimates by GORISK for the 2011-2012 eruption indicated a volume of emitted lavas of at least 81.5 x 10⁶ m³. The lava flows did not reach inhabited areas and only affected vegetation in Virunga National Park. The 2011-2012 eruption was the biggest event at Nyamuragira since the 1991-1993 eruption, which lasted nearly 1.5 years and emitted an estimated ~131 x 10⁶ m³ of lava (Smets and others, 2010).

A report by Dario Tedesco stated that in March 2012, a series of explosion earthquakes were recorded by the seismic network of the Goma Volcano Observatory. Following this activity, the fissure eruptions suddenly stopped. Also in March the morphology of the pit crater began to change (figures 45 and 46). Beginning in late February 2012 through at least June 2012, degassing occurred in the caldera. The emission site was located inside the pit crater, but degassing occurred from all fractures inside the caldera. On several occasions, meteorological conditions caused sulfur odors to reach the city of Goma (~30 km S from Nyamuragira's crater).

Figure 45. View of Nyamuragira's pit crater on 20 January 2012. Direction unknown. Courtesy Dario Tedesco, International Organization of Migrants and Second University of Naples.

Figure 46. View of Nyamuragira's pit crater on 16 April 2012. Direction unknown. Courtesy Dario Tedesco, International Organization of Migrants and Second University of Naples.

MODVOLC thermal alerts had waned at the fissure area by late March 2012, suggesting the end of the fissure eruption in that time frame. The last alerts around the summit area had occurred on 2 February 2010.

References. Kervyn, F, d'Oreye, N, van Overbeke, A-C, 2010, GORISK: The combined use of Ground-Based and Remote Sensing techniques as a tool for volcanic risk and health impact assessment for the Goma region (North Kivu, Democratic Republic of Congo). Final Report. [Project SR/00/113] (URL: http://www.ecgs.lu/gorisk/wp-content/uploads/2010/11/GORISK_Final_Report_DISSEMINATION.pdf )

Smets, B., Wauthier, C., d'Oreye, N. (2010). A new map of the lava flow field of Nyamulagira (D.R. Congo) from satellite imagery. Journal of African Earth Sciences, 58 (5), 778-786. DOI:10.1016/j.jafrearsci.2010.07.005

Geologic Background. Africa's most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 13 km NNW of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.

Information Contacts: Dario Tedesco, International Organization of Migrants (I.O.M.), Goma, DRC, and Second University of Naples, DISTABIF, Caserta, Italy; GORISK Scientific Team [an International scientific team for the study and monitoring of active volcanoes and their corresponding hazards in the Virunga Volcanic Province] (URL: http://terra.ecgs.lu/rnvt/); Virunga National Park, Democratic Republic of Congo (URL: http://virunga.org/); Jesse Allen and Robert Simmon, NASA Earth Observatory (URL: http://earthobservatory.nasa.gov); Volcano Discovery (URL: http://www.volcanodiscovery.com/); and Afar Consortium Project (URL: http://www.see.leeds.ac.uk/afar/).

A recent partial survey conducted by David K. Lynch and Paul M. Adams of Red Island, previously known as Red Hill, in Southern California, USA, has resulted in the discovery of five steaming hot vents on the SW flank of the northern Salton Buttes volcanic field. Lynch and Adams sent us a report, which follows. We also include some remarks from related literature and note geothermal power plants in the region.

Background. Red Island is part of the Salton Buttes, a collection of five late Quaternary rhyolitic volcanic necks in the Salton Sea Geothermal Field (SSGF) (figure 1). The SSGF rests within a topographic low, and has a geothermal gradient that averages ~0.3°C/m, reaching a maximum of 4.3°C/m (Lynch and others, 2013). This high geothermal gradient results from the shallow magma body of the spreading center between the San Andreas and Imperial faults. As shown in figure 2, the SSGF lies at the head of the Gulf of California, on the boundary of the Pacific and North American plates (Elders and Sass, 1988). Consequently, the SSGF's unique geology creates the perfect setting for hot geothermal fluids to seep to the surface, and has been slated as a site for geothermal electricity-generating plants. There are no previous Bulletin reports on the Salton Buttes.

Figure 1. The Salton Trough is a result of crustal stretching and sinking associated with regional extensional tectonics including the San Andreas Fault (SAF) and the East Pacific Rise (EPR, the spreading center shown at the bottom of the map).This sketch shows the boundary between the Pacific and North American plates, with the rectangle indicating the Salton Trough. The S end of the Salton Trough (as defined by the box) begins adjacent to the Sea of Cortez, the body of water separating the Baja California peninsula from mainland Mexico. The SSGF is within the Salton Trough. Other abbreviations include Gorda Rise, GR; Mendocino Triple Junction, MTJ; and Rivera Triple Junction, RTJ. Taken from Elders and Sass (1988).

Figure 2. Sketch map showing location of Salton Sea and the Salton Buttes volcanic area study area. For scale, the N-S distance from the S end of the Salton Sea to the USA-Mexico border is ~100 km. The lake is receding but its 2014 surface elevation is close to -69 m. Courtesy of Lynch and others (2013).

The Salton Buttes reside near the SE end of the Salton Sea. The Sea resides on the floor of the Salton Trough, chiefly in Imperial County, California. This briny water body is about 56 x 10 km. The Buttes lie along a NNE trending line spanning a distance of 7 km. The Salton Trough was filled, in part, by sediments carried by the Colorado River, which eventually built up and blocked the river's flow. The river was diverted away from the Salton Trough, yet, in 1905, heavy rainfall caused nearby levees to collapse, creating the Salton Sea (Morris, 2008).

Until recent work by Lynch and others (2011) and Schmidt and others (2013), the Salton Buttes were thought to have been formed by extruded magma during the late Pleistocene, ~16,000 BP. Age dates for some lavas are now dated to closer to 2,000 BP, much younger than originally understood, bringing closer scrutiny of the Buttes by the U.S. Geological Survey (USGS) California Volcano Observatory and other agencies concerned with geological threats in California (Lynch and Adams, unpublished draft).

Red Island consists of two conjoined volcanoes of related, yet distinctly different, geology. They are 2.5 km SSW of the Mullet Island fumaroles, an area discussed further by Lynch and others (2013).

Mullet Island fumaroles: As the briny water level of the Salton Sea began dropping in 1983, a number of fumarole fields were exposed subaerially for the first time since 1945. The Salton Sea overlaps the SSGF and, as a result, an interaction of rising gas and hot water with sediments has produced a number of hot, fumarolic gryphons (mud volcanoes) and salses (bubbling water in calderas of gryphons) (Lynch and others, 2013). Over-pressured subsurface gas cause the upward migration of fluidized sediment, creating these gryphons, as seen in figure 3.

Figure 3. View of a steaming spatter cone, one of the first stages of a gryphon's development. Once the rising mud becomes more viscous and covers the spatter cone completely, a composite gryphon is formed. The height of these gryphons can range from a few centimeters to ~2 m. Mullet Island can be seen at the top left of the image. Taken from Lynch and others (2013).

The NW trending alingment of these geothermal features is suggestive of a fault, most likely the Calipatria fault. Other fumarole fields are still below water level or are being exposed as the lake recedes.

Red Island vents. Before the work on Mullet Island fumaroles was published, Michael McKibben, while on a 2008 Desert Symposium field trip (Reynolds and others, 2008), mentioned a 'volcanic hotspot' on the SW flank of the N volcano at Red Island in a private communication to Lynch and Adams. However, a swift search of the area during the field trip did not reveal its location. As a result of this observation, Lynch and Adams performed a partial survey of the SW flank of the N volcano on 6, 7, and 29 November 2013, which resulted in the discovery of the five hot steaming vents on the summit of the south-facing slope. Lynch and Adams found that the vents were distributed along an ~80 m long line trending N65E, which they recognized as a possible fault.

All attempts at identifying vents were made before sunrise, when the air temperature was at its lowest diurnal value (~8°C). This provided a recognizable thermal contrast between cold and hot rocks. Lynch and Adams noted that it was unlikely that warm air coming from the vents could have been felt on a hot or windy day, as the vents appeared unremarkable from a relatively close distance (a few meters), "among the uneven field of loose, jutting volcanic rocks." To locate the vents, Lynch and Adams employed the following three tactics:

1. An Agena ThermoVision 470 infrared camera was used to look for areas that were warmer than the background (e.g., figure 4, lower). Absolute temperatures from the camera may have been off by 3-4°C due to systematic errors, but the image records relative temperatures between different parts of scene. were preserved. The vent seen in figure 4 may have been deliberately covered with a pile of rocks.

2. They reached into holes and crevasses to check for heat.

3. They measured rock temperatures using a Martin P. Jones & Associates, Inc., Model 9910 TE Infrared Thermometer. Because it was small enough to be placed deeper in the vents, the temperatures from this thermometer were higher than the IR camera temperatures.

Figure 4. (Top) Image of a hotspot (designated H3) seen in visible wavelength light. (Bottom) Image of the same spot taken with an Agena ThermoVision 470 IR camera. Yellow patches in the center and lower right of the image indicate bad pixels in the IR camera. Taken from Lynch and Adams (2013).

Lynch and Adams found one vent "by feeling hot air coming from it," one "by noticing wet rock," one "by seeing its steam cloud," and two by locating them with the IR camera. Once located, all vents were found to be steaming (figure 5) and surrounded by rocks wet from condensation.

Figure 5. Steam cloud from H1; still taken from a video of the hotspot at Red Island, Salton Buttes. Taken from Lynch and Adams (2013).

No surface deposits (e.g., sulfur) were seen, aside from water and greenish algae. The temperatures 1-2 m within the vents were 35-38°C. According to Lynch and Adams, these hotspots may "represent heat from original volcanism, or recent magma intrusions that have not reached the surface." The distribution of these vents, distinguished as H1, H2, H3, H4, H5, is shown in figure 6.

Figure 6. Vent locations marked on an image of Red Island, Salton Buttes, from Google Earth. Taken from Lynch and Adams (2013).

The team was relatively confident that no additional vents were located within the area extending 225 m to the S and W, although a more complete survey must be undertaken to investigate seismicity and movement/deformation of the area from GPS networks. However, other "warm spots" not associated with venting or outgassing were found on the SW flank of the N volcano. They were ~5-10°C warmer than ambient temperatures and may represent weak signals from the warm interior of the volcano. More likely, however, the warmer temperatures are due to emissivity variations in rock layers, or normal temperature distributions that occur in crevasses where rocks are not able to radiate heat into the cold night sky.

Geothermal electricity-generating plants. According to the Geothermal Energy Association, currently there are three major geothermal production sites in the Imperial Valley, totaling in 16 plants. Figure 7 shows one of these sites, which hosts seven geothermal plants with running capacities ranging from 5-45 MW (Geothermal Energy Association). Despite the fact that this site alone has contributed enough electricity to power ~100,000 homes, geothermal energy only accounts for 4.4% of all system power in California (Matek and Gawell, 2014). The SSGF is considered the best opportunity for increasing the production of geothermal energy in California. The unique geology of the Salton Sea area allows geothermal fluids to seep to the surface, allowing a range of capacity from 1,700 to 2,900 MW.

Figure 7. This CalEnergy geothermal site is located on the edge of the Salton Sea, and currently has seven running geothermal power plants. Taken from The Center for Land Use Interpretation.

References. Elders, W and Sass, J, 10 November 1988, The Salton Sea Scientific Drilling Project; Journal of Geophysical Research, v. 03, no. B11, pp. 12,953-12,968.

Lynch, D., Hudnut, K., and Adams, P., 2013, Development and growth of recently-exposed fumarole fields near Mullet Island, Imperial County, California; Geomorphology, v. 195, pp. 27-44.

Lynch, D.K., Schmitt, A.K.., Rood, D., and Akciz, S, 2011, Radiometric Dating of the Salton Buttes, Proposal to the Southern California Earthquake Center.

Matek, B., and Gawell, K., February 2014, Report on the State of Geothermal Energy in California; Geothermal Energy Association, 2014.

Morris, R., 2008, Welcome to the Salton Trough, California State University Long Beach Geology.

Reynolds, R., Jefferson, G., Lynch, D., 2008, Trough to Trough: The Colorado River and the Salton Sea, Proceedings of the 2008 Desert Symposium, Robert E. Reynolds (ed.), California State University, Desert Studies Consortium and LSA Associates, Inc.

Schmitt, A, Martin, A, Stockli, D, Farley, K, Lovera, O, 2013, (U-­-Th)/He zircon and archaeological ages for a late prehistoric eruption in the Salton Trough (California, USA), Geology, January 2013, v. 41, pp. 7-10.

Geologic Background. The Salton Buttes consist of five small rhyolitic lava domes extruded onto Quaternary sediments of the Colorado River delta at the SE margin of the Salton Sea. Their age has variously been considered to be late Pleistocene or early Holocene based on different dating techniques. Recent paleomagnetic dating calibrated by radiocarbon ages suggests that the domes were erupted during an interval of about 500 years between about 2,300 and 1,800 years ago, with the possible exception of Mullet Island at the northern end of the field, which could be as much as 5,000 years older. The present-day saline Salton Sea was formed in the early 20th century by unintended flooding into the basin formerly occupied by Pleistocene Lake Cahuilla Lake during diversion of the Colorado River for irrigation purposes. The Salton Sea geothermal field produces saline brines.

Information Contacts: David Lynch, Earthquake Science Center, USGS- Pasadena; Paul Adams, Thule Scientific, Topanga, CA (URL: http://thulescientific.com/Research.html); The Center for Land Use Interpretation, Culver City, CA (URL: http://clui.org/); and Geothermal Energy Association, Washington, D.C. (URL: http://geo-energy.org/).

This report summarizes activity from Santa María's active cone, Santiaguito, during October 2011-June 2014. Ash explosions, ashfall, and incandescent avalanches were observed throughout this time period. During the rainy season (April-September), lahars were frequently reported within the major drainages in the southern sector of the volcano. The sources for this report were Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hidrologia (INSIVUMEH), Washington Volcanic Ash Advisory Center (VAAC), and Coordinadora Nacional para la Reducción de Desastres (CONRED).

Recurrent ash explosions. INSIVUMEH and the Washington VAAC reported frequent ash explosions from Santiaguito's active dome, Caliente, during October 2011-June 2014 (figure 35). Ash plumes were typically in the range of 500 m above the dome with exceptional cases in the range of 4,000 m, such as the explosive event on 9 May 2014. Significant ash plumes were known to drift as far as the Guatemala-Mexico border (such as activity during 5-6 November 2011 when ash extended 18-28 km SE of the summit). Degassing from the Caliente dome also generated frequent, diffuse, white plumes that rose to heights around 200 m above the summit.

Figure 35. The Santiaguito dome complex of Santa María includes four major domes: El Brujo, El Monje, La Mitad, and Caliente (active since 1922). This photo was taken from the INSIVUMEH observatory located on Finca El Faro, ~6 km S of the active dome. Modified from Ball and others (2013).

Ashfall from explosions and rumbling noises from explosions and avalanches were frequently reported in communities nearby (table 5). Following activity on 9 May 2014, ashfall triggered evacuations affecting ~130 people. CONRED and INSIVUMEH reported that ash had extended up to 20 km from the summit reaching the communities of Las Marías, San Marcos (10 km SW), Palajunoj (18 km SSW), El Faro (SW flank), La Florida (5 km S), Patzulín, and Quetzaltenango (18 km WNW).

Table 5. Ashfall from explosions at Santa María's active dome, Santiaguito, was reported in numerous communities during November 2011-June 2014. Courtesy of INSIVUMEH.

Avalanches and pyroclastic flows originating from Caliente dome were reported throughout late 2011 through June 2014 (table 6). A pyroclastic flow observed on 9 May 2014 traveled ~7 km from the active lava dome (figure 35). Approximately 1 million cubic meters of tephra was deposited within the Nimá I drainage. Secondary explosions occurred along the flowpath associated with hot deposits in contact with river water.

Table 6. A summary of significant pyroclastic flows from Santa María's Santiaguito occurred during February 2012-May 2014. Courtesy of INSIVUMEH.

Figure 36. Looking approximately N toward Santa María's Santiaguito cone, this photo has been annotated to show surveyed distance measurements (in meters, here "mts.") measured along the slope between the summit and base of Santiaguito as well as the main pathway along the Nimá I drainage. The pyroclastic flow from 9 May 2014 traveled more than 6 km from the active dome (red dotted line). The length of the active lava flow on 11 May 2014 was 152 m. Courtesy of Gustavo Chigna, INSIVUMEH, and the International Volcano Monitoring Fund (IVM Fund).

Lahars. During 2012-2014, lahars began flowing down Santa María's SE drainages during the onset of the rainy season (table 7). INSIVUMEH reported that many of these events were triggered by heavy rainfall and were frequently contained within the Nimá I drainage (figure 37). Lahars following the nearby rivers Nimá II, San Isidro, and Tambor and merged with the larger river, Samalá. These primary drainages are located S and SW of the active dome (see map in figure 28 of BGVN 24:03; note that Río San Isidro is an intermittent stream located between the Tambor and Nimá II rivers), three of which were included in a hazard map prepared by INSIVUMEH in collaboration with Japan International Cooperation Agency (JICA) in 2003 (figure 38). INSIVUMEH and CONRED released public announcements when Río Samalá was threatened by lahars (for example: 21 May 2012, 23 June 2012, and 6 June 2014) that included specific warnings for the Castillo de Armas bridge; the bridge supports the Interamerican Highway where it passes through the town of San Sebastián.

Table 7. During April 2012- June 2014, weak-to-strong flowing lahars were frequently triggered by heavy rainfall, mainly during April-September each year. Courtesy of INSIVUMEH.

Figure 37. This set of two images of the Nimá I drainage shows a small-sized lahar that flowed from Santiaguito cone at 1615 on 7 October 2013 (left image was before (Antes); right image was during (Durante) the lahar flow). Looking upstream, this view is focused on a narrow section of Nimá I that was filled by a 12-m-wide and 1.5-m-high lahar. The rock wall on the right-hand side of the drainage (~3 m high) became a ramp for the lahar and was half-covered by the flow as the gray mass wrapped around the narrow corner in a fast and turbulent flow. Courtesy of Gustavo Chigna, INSIVUMEH and the IVM Fund.

Figure 38. Volcanic hazard map (#3 of 5 published in a series) for Santa María focused on the region S of Santiaguito dome. The basemap is from 2001-2002 aerial survey photos and the hazard assessments conducted during 2001-2003 in collaboration with the Japan International Cooperation Agency (JICA). The three drainages (Río Nimá I, Río Nimá II, and Río Samalá labeled in red text) were added by GVP staff. Major towns, farms, and the INSIVUMEH observatory (OVSAN) are labeled; hazard zones are indicated with color coding; the blue semicircle and linear corridor indicates the extent of the study area; the area encompassed by the red semi-circle is at risk for volcanic ballistics. Other hazards include pyroclastic flows (orange shading), lava flows (pink), lahars (blue), ashfall (orange outline), and debris avalanches (yellow and green outlines). Courtesy of INSIVUMEH.

The most damaging lahar during this reporting period occurred on 6 June 2014. The lahar flowed in pulses down the Nimá I drainage with crests 5-9 m high reaching a maximum width of 80 m. The Santiaguito Observatory (OVSAN) was forced to evacuate when the lahar overflowed the banks and spread across the facility grounds; important scientific equipment was damaged and also washed away. The lahar also flowed into a nearby farm.

Reference. Ball, J.L., Calder, E.S., Hubbard, B.E., and Bernstein, M.L., 2013, An assessment of hydrothermal alteration in the Santiaguito lava dome complex, Guatemala: implications for dome collapse hazards, Bulletin of Volcanology, 75:676.

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/inicio.html); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié; 21-72, Zona 13, Guatemala City, Guatemala (URL: http://www.conred.org/); and Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20748, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

The year 2014 marks the 3rd decade of largely non-explosive activity at Villarrica, historically one of the most active volcanoes in the Andes. Villarrica has been relatively quiet since our last report, which discussed events from April 2010 to October 2010 (BGVN 35:10). This report covers the time period from November 2010 to December 2013.

During this reporting period, comparative quiet prevailed. There were occasional cases reported of spattering lava, small white plumes, minor ash emissions (up to 50 m above the crater rim), and nighttime incandescence reflected off of the plumes according to Proyecto Observación Villarrica Internet (POVI) and Observatorio Volcanológico de los Andes del Sur (OVDAS-SERNAGEOMIN). Satellite thermal radiance during the reporting interval suggested often low radiance, with rare cases of high incandescence consistent with turbulence and fountaining in the deep, 40 m wide lava lake.

On 17 September 2011 remobilized tephra rose ~500 m above the crater, which according to POVI, was likely caused by a sudden impact when a snow cornice detached and fell into the crater. On 19 September 2011, a rapid rise in the level of the lava lake caused much of the snow and ice to melt, especially on the southern inner wall. Strombolian explosions from the crater were observed on 26 September 2011, and tephra deposits on the E edge of the crater were noted. On 27 September 2011 incandescence from the lava lake was reflected in the cloud cover above.

The period from November 2011 to March 2012 saw very little explosive activity. Two small ash emissions occurred on 7 March. Incandescence from the crater was observed from the town of Pucon (16 km N) during 7-8 March. During 7-9 March, lava spattering from the lava lake was observed for the first time that year. Four small ash emissions were observed during 13-14 March. On 20 March a large, white plume was visible above the crater. The observer postulated that due to the humid atmospheric conditions that day, the steam condensate in the visible plume remained conspicuous both to a height of 1,500 m above the crater as well as 20 km SW of the crater.

According to POVI, an ash plume rose 50 m above Villarrica on 19 April 2012. Incandescence from Villarrica's crater subsided in mid-April and was undetected by satellite and ground observations at least through 10 November 2012.

On 30 January 2013, weak incandescence was observed in the near-infrared spectrum from the ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) instrument on the Terra satellite. POVI reported that satellite images of Villarrica acquired on 25 July revealed a weak thermal anomaly. On 29 July 2013 observers photographed the crater and described a thermal anomaly on the S edge of the crater rim, in the same area from which a lava flow originated on 29 December 1971. They also heard deep degassing sounds. A second photograph showed a diffuse gas plume rising from the bottom of the crater, and ash and lapilli on the snow on the inner crater walls.

Analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) band 21 (3.929-3.989 μm) satellite images from 2003 to 2013 highlights three main cycles of activity. These were characterized by convective lava fountains and Strombolian explosions from the lava pit, located ~ 40-150 m below the rim of the crater, according to POVI. The last time MODIS infrared sensors detected elevated thermal radiance was in early 2012 (figure 8).

Figure 28. Elevated thermal radiance in Watts per square meter detected at Villarrica using MODIS band 21 (3.929-3.989 μm) from 2003 through 2013. Courtesy of POVI and NASA MODIS.

In accord with the thermal radiance data seen in figure 28, OVDAS-SERNAGEOMIN maintained an Alert Level of Green for Villarrica from the period of 5 March 2012 to 30 December 2013, characterizing Villarrica as active but stable with no immediate threat. The seismicity reports from OVDAS-SERNAGEOMIN during the period of July 2013 to December 2013 showed the monthly number of earthquakes recorded ranged from 439 to 1,433. The reduced displacement of the tremors recorded fluctuated throughout July 2013- December 2013 from 0.6 cm² to 9.9 cm². During this period of time, the amount of SO₂ emissions recorded by a scanning DOAS spectrometer OVDAS-SERNAGEOMIN varied from 156 tons/day to 888 tons/day. The height above the crater rim of the steam-gas plumes ranged from 150 m to 1,500 m. MODIS did not record any thermal anomalies during this period of time.

Figure 29. Aerial image of the Villarrica crater at dawn on 14 October 2013. Copyrighted image taken by Diego Spatafore.

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: Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); and Observatorio Volcanológico de los Andes del Sur Servicio Nacional de Geologia y Mineria (OVDAS SERNAGEOMIN), Santiago, Chile (URL: http://www2.sernageomin.cl/ovdas).