Mount Agung, located on the E end of the island of Bali, Indonesia, rises above the SE rim of the Batur caldera. The summit area extends 1.5 km E-W, with the highest point on the W and a steep-walled 800-m-wide crater on the E. Recorded eruptions date back to the early 19th century. A large and deadly explosive and effusive eruption occurred during 1963-64, which was characterized by voluminous ashfall, pyroclastic flows, and lahars that caused extensive damage and many fatalities. More recent activity was documented during November 2017-June 2019 that consisted of multiple explosions, significant ash plumes, lava flows at the summit crater, and incandescent ejecta. This report covers activity reported during April-May 2022 and December 2022 based on data from the Darwin Volcanic Ash Advisory Center (VAAC).

Activity during 2022 was relatively low and mainly consisted of a few ash plumes during April-May and December. An ash plume on 3 April rising to 3.7 km altitude (700 m above the summit) and drifting N was reported in a Darwin VAAC notice based on a ground report, with ash seen in HIMAWARI-8 visible imagery. Another ash plume was reported at 1120 on 27 May that rose to 5.5 km altitude (2.5 m above the summit); the plume was not visible in satellite or webcam images due to weather clouds. An eruption was reported based on seismic data at 0840 on 13 December, with an estimated plume altitude of 3.7 km; however, no ash was seen using satellite imagery in clear conditions before weather clouds obscured the summit.

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

Information Contacts: 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/).

Tengger Caldera, located at the N end of a volcanic massif in Indonesia’s East Java, consists of five overlapping stratovolcanoes. The youngest and only active cone in the 16-km-wide caldera is Bromo, which typically produces gas-and-steam plumes, occasional ash plumes and explosions, and weak thermal signals (BGVN 44:05, 47:01). This report covers activity during January 2022-December 2023, consisting of mostly white gas-and-steam emissions and persistent weak thermal anomalies. Information was provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and satellite imagery. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to stay at least 1 km from the crater.

Activity was generally low during the reporting period, similar to that in 2021. According to almost daily images from MAGMA Indonesia (a platform developed by PVMBG), white emissions and plumes rose from 50 to 900 m above the main crater during this period (figure 24). During several days in March and June 2022, white plumes reached heights of 1-1.2 km above the crater.

Figure 24. Webcam image showing a gas-and-steam plume from the Bromo cone in the Tengger Caldera on 2 April 2023. Courtesy of MAGMA Indonesia.

After an increase in activity at 2114 on 3 February 2023, a PVMBG team that was sent to observe white emissions rising as high as 300 m during 9-12 February and heard rumbling noises. A sulfur dioxide odor was also strong near the crater and measurements indicated that levels were above the healthy (non-hazardous) threshold of 5 parts per million; differential optical absorption spectroscopy (DOAS) measurements indicated an average flux of 190 metric tons per day on 11 February. Incandescence originating from a large fumarole in the NNW part of the crater was visible at night. The team observed that vegetation on the E caldera wall was yellow and withered. The seismic network recorded continuous tremor and deep and shallow volcanic earthquakes.

According to a PVMBG press release, activity increased on 13 December 2023 with white, gray, and brown emissions rising as high as 900 m above Bromo’s crater rim and drifting in multiple directions (figure 25). The report noted that tremor was continuous and was accompanied in December by three volcanic earthquakes. Deformation data indicated inflation in December. There was no observable difference in the persistent thermal anomaly in the crater between 11 and 16 December 2023.

Figure 25. Webcam image showing a dark plume that rose 900 m above the summit of the Bromo cone in the Tengger Caldera on 13 December 2023. Courtesy of MAGMA Indonesia.

All clear views of the Bromo crater throughout this time, using Sentinel-2 infrared satellite images, showed a weak persistent thermal anomaly; none of the anomalies were strong enough to cause MODVOLC Thermal Alerts. A fire in the SE part of the caldera in early September 2023 resulted in a brief period of strong thermal anomalies.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); 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/).

Saunders is one of eleven islands that comprise the South Sandwich Islands in the South Atlantic. The active Mount Michael volcano has been in almost continuous eruption since November 2014 (BGVN 48:02). Recent activity has resulted in intermittent thermal anomalies and gas-and-steam emissions (BGVN 47:03, 48:02). Visits are infrequent due to its remote location, and cloud cover often prevents satellite observations. Satellite thermal imagery and visual observation of incandescence during a research expedition in 2019 (BGVN 28:02 and 44:08) and a finding confirmed by a National Geographic Society research team that summited Michael in November 2022 reported the presence of a lava lake.

Although nearly constant cloud cover during February 2023 through January 2024 greatly limited satellite observations, thermal anomalies from the lava lake in the summit crater were detected on clear days, especially around 20-23 August 2023. Anomalies similar to previous years (eg. BGVN 48:02) were seen in both MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS instruments and in Sentinel 2 infrared imagery. The only notable sulfur dioxide plume detected near Saunders was on 25 September 2023, with the TROPOMI instrument aboard the Sentinel-5P satellite.

Geologic Background. Saunders Island consists of a large central volcanic edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weather conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

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/); 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 (URL: https://dataspace.copernicus.eu/browser).

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).

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/).

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/).

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/).

Based on inspection of a photo taken on 13 April 2011 from the International Space Station of Tofua volcano (Tonga), it appeared that possible pumice rafts were floating near the island (BGVN 36:09). However, the source of the material was unknown.

The source, extent, and makeup of the material remains uncertain. Inquiries sent to Mark Belvedere and others in Tonga in late 2011 failed to identify any mariners or other observers who recall seeing either an eruption or material floating on the sea surface around March to April 2011.

If the rafts drifted from the Tongan region, as they have often done in the past, they may have originated from an eruption at one of the volcanoes of the Ha'apai and Vava'u Groups. Some of those, such as Late, Home Reef, Metis Shoal, and Falcon Island, have erupted frequently, with pumice rafts and emergent ephemeral islands (figure 2).

Figure 2. Map of the Tonga-Samoa area showing Tofua. Historically active volcanic centers are indicated by crosses. Stippled areas delineate water less than 1 km deep; diagonal and cross-ruled patterns indicate trench areas greater than 6 and 8 km, respectively. After Anonymous, 1979, with map credit to Tom Simkin (GVP).

Figure 2 came from a similar report in 1979, but in that (very different) case the initial problem was four active eruptive sources, any of which might explain the streaks and rafts of floating pumice drifting NE. The resulting uncertainty then revolved around which of those sources produced the bulk of the floating pumice (SEAN 04:06; Anonymous, 1979). Discovery of a large ephemeral island at Metis Shoal pointed to that as the primary source of the pumice rafts (SEAN 04:07 and 04:12).

NASA initiative and findings. Childs and others (2011), Chojnacki and others (2011), and Honaker and Childs (2011) point out the very practical goal of a "more timely warning system to divert maritime vessels from affected areas." They discussed the spectral signature of several pumice rafts from a remote-sensing perspective. They assessed the date of eruption onset, the volcano's name, coordinates, and the favored satellites to detect and track these rafts in their different environments.

For the limited cases they tested, MODIS best detected large scale pumice rafts and monitored them over time. Landsat 5, 7 and ALI best detected small rafts, especially in closed bodies of water such as lakes. The false-color composite improved the contrast of pumice rafts for visual identification. Thermal anomalies occurred over several large pumice rafts. They found a subpixel classification extremely effective at automatically identifying small areas of pumice.

Note that, for the case at hand, the authors did not know of or analyze the material on the sea surface.

References. Anonymous, 1979, Geophysical Events: Eos, Transactions, American Geophys. Union, 21 Aug 1079, p. 625.

Childs, LM, Chojnacki, PR, Coady, C., Geddes, Q, Honaker, LB, Lyddane, W, McGilloway, J, Scott, J, 2011, Pacific Ocean Disasters - Enhanced Detection and Monitoring of Pumice Rafts Using NASA EOS; Eos, Transaction of the Am. Geophys. Union, V44C-04; (URL: http://www.agu.org/meetings/)

Chojnacki, P, Lyddane, W, McGilloway, J, Geddes, Q, Honaker, L, Coady, C, and Scott, J, 2011, Implementing NASA Remote Sensing to Protect and Monitor our Waterways, [ley DEVELOP Team 5, posted 10 August 2011 in DEVELOP Virtual Poster Session with written transcript] Earthzine (URL: http://www.earthzine.org/2011/08/10/implementing-nasa-remote-sensing-to-protect-and-monitor-our-waterways/, https://www.youtube.com/watch?feature=player_embedded&v=sdTZFq8Kpg4).

Honaker, LB, Childs, L, 2011, Remote Sensing Monitoring of Pumice Rafts in the Pacific Ocean (URL: http://www.nianet.org/NIA/media/photo-gallery/Remote-Sensing-Monitoring-of-Pumice-Rafts-in-the-Pacific-Ocean.pdf).

Geologic Background. Reports of floating pumice from an unknown source, hydroacoustic signals, or possible eruption plumes seen in satellite imagery.

Information Contacts: Mark Belvedere, Treasure Island Eueiki Eco Resort, Vava'u, Tonga.

The previous report on Chaitén volcano, Chile (BGVN 35:12) noted that a second new lava dome was reported on 4 November 2008. The Alert Level remained at Red (the highest of the alert level system) from the onset of the eruption on 2 May 2008 through April 2010. This report will chronologically summarize the growth of the new lava domes and major eruptive events. In general, a gradual decline in activity occurred during November 2008-September 2011. The Alert Level stood at Yellow during May 2010-May 2011, and was then lowered to Green, where it remained through the end of 2011. Most of the information in this report comes from weekly to monthly reports from the Observatorio Volcanológico de los Andes del Sur-Servicio Nacional de Geología y Minería (OVDAS-SERNAGEOMIN) with collaboration from the Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI). Finally, the current status of the town of Chaitén (which was abandoned by all but a handful of residents who continued to live there without electricity or running water) will be discussed. The potential hazards from dome-collapse-generated block-and-ash flows and lahars from remobilized volcanic deposits were persistent throughout the reporting period.

Portions of this report were initially synthesized and edited by Nick Legg (covering November 2008-March 2009) and Eduardo Guerrero (covering April 2009-July 2009), as part of a graduate student writing assignment in a volcanology class at Oregon State University under the guidance of professor Shan de Silva.

New lava domes observed. Initially occupying the caldera of Chaitén was the lava dome ('old dome') that existed prior to the 2 May 2008 eruption (and for the ~ 9,400 years since the previous eruption). A lava dome ('Dome 1') first observed on 21 May 2008 and a new lava dome ('Dome 2') confirmed on 4 November 2008 during an overflight (BGVN 35:12) were extruded on top of the old dome (figures 22 and 23). A third phase of dome extrusion ('Dome 3') was observed on 29 September 2009.

Figure 22. December 2008 aerial photograph of the Chaitén dome complex taken looking approximately SE into the caldera. The domes were emplaced on top of the old dome, emplaced ~9,400 years ago. Courtesy of OVDAS-SERNAGEOMIN.

Figure 23. Satellite photograph of Chaitén and surroundings, acquired on 30 September 2009. (a) Chaitén volcano (top right corner), Chaitén (Blanco) River, and their proximity to the town of Chaitén (bottom left). Note the significant lahar deposits at the mouth of the Chaitén river and cutting through the town of Chaitén. Inset index map shows Chaitén's location in Southern Chile. (b) Enlarged, annotated view of Chaitén volcano. Dome 1 is in the W side of the caldera and Dome 2 is in the NNE part. Dome 3 is not visible in this photograph. Satellite photograph courtesy of NASA Earth Observatory; index map modified from MapsOf.net.

November 2008-April 2010 (Red Alert). Following the confirmation of the existence of Dome 2 on 4 November 2008, a lateral explosion occurred on 17 November, directed WSW. The explosion was not constrained to either Dome 1 or 2 alone, but was associated with a collapse resulting from continued dome extrusion. By 6 December 2008, both active domes had exceed the height of the caldera rim (Dome 1 by ~ 250 m and Dome 2 by ~ 350 m). SERNAGEOMIN reported an increase in the growth of Dome 2, corresponding to a temporally slight increase in both hybrid (HB) and volcano-tectonic (VT) earthquakes.

Plumes continued to vary in color, indicating varying contributions of steam and ash from both new domes. By 9 January 2009, an observational overflight revealed that the inner caldera had been filled by Domes 1 and 2, and growth of sharp spines or pinnacles was reported.

On 19 January, a major collapse of the Dome 2 summit spines occurred, producing block-and-ash flows that traveled down the SE and E flanks of Chaitén. After the collapse, observers noted a decrease in seismicity and slowed dome extrusion.

During a flyby facilitated by ONEMI and the Chilean Air Force on 21 January 2009, researchers from the University at Buffalo-State University of New York (UB-SUNY) and the University of Chile acquired thermal images of the dome complex (figure 24). The thermal images indicated that, while Dome 1 still had areas of elevated temperature, the highest temperatures (greater than 150°C) corresponded to the pinnacles at the summit of Dome 2 (figure 24a). Images of pyroclastic flow and rockfall deposits resting within the E part of the caldera disclosed elevated temperatures there as well (higher than 80°C in places, figure 24b). Their research on thermal imaging of Chaitén will be showcased in an upcoming publication.

Figure 24. Thermal images of the dome complex of Chaitén acquired on 21 January 2009. (a) The summit of Dome 2, where the highest temperatures were recorded; the same area corresponded to the pinnacles and the area of the most rapid growth of Dome 2. (b) Pyroclastic flow and rockfall deposits within the E part of the caldera, emplaced just prior to 21 January, that were still elevated in temperature. The colors are scaled uniquely for both images with the temperature scales shown at right. The flyby was facilitated by ONEMI and the Chilean Air Force, the images were acquired by Patrick Whelley (UB) and Andrés Paves (University of Chile), and processing was performed by Marc Bernstein (UB); use of the thermal camera was courtesy of Eliza Calder under the UB-SUNY Research Foundation.

By 9 February 2009, a new high-standing pinnacle atop Dome 2 indicated the return of rapid dome growth. This did not coincide with significant increases in seismicity.

On 19 February, a large partial collapse generated pyroclastic flows that traveled down the Chaitén River towards the town of Chaitén (see annotated photo below). This event produced a plume reaching 9.1 km altitude according to the Buenos Aires Volcanic Ash Advisory Center (VAAC). The plume was white (indicating high water vapor content) at the top, and contained abundant ash at the base. The collapse created a scarp measuring approximately 500 m by 500 m. Increased seismicity occurred on the same day; background volcanic tremor occurred from 1028 until 1346, and swarms of earthquakes ranging from M 3.6-4.2 originated at depths of 3-5 km. Due to the amount of material deposited from the collapse (~10 x 10⁶ m³), SERNAGEOMIN reported a significantly higher than normal danger of lahars.

On 25 February 2009, dome growth focused on Dome 1, although seismicity had gradually decreased in frequency (excluding the outstanding events of mid-February discussed above) compared to November-December 2008 values. Despite the decrease, on the afternoon of 3 March, dome collapses occurred every ~ 40 minutes. Within the next few days, SERNAGEOMIN reported that "VT's almost disappeared." A gradual decrease in seismicity continued until 24 March, when the frequency of HB earthquakes increased briefly, but they decreased again by early April.

On 6 April, a prominent spine of lava was observed on the S part of Dome 1 (figure 25). It indicated that dome growth was still concentrated in the W part of the caldera (Dome 1). The next week, the same spine was reported to have a wider base, and the crater reportedly glowed at night. In early May, an aerial photograph captured a close view of a very fractured central lava spine of the dome complex (figure 26).

Figure 25. Images taken by Dirección General de Aeronáutica Civil (DGAC) and a Channel 13 cameraman on 6 April 2009 showing a prominent spine of lava that had grown in the S area of Dome 1. Courtesy of OVDAS-SERNAGEOMIN.

Figure 26. Aerial view of the dome complex in early May 2009 showing a very fractured central pinnacle. Courtesy of Javier Romero.

In the later parts of May 2009, dome growth and seismicity continued to be focused on the W part of the caldera, associated with Dome 1. Although there was no significant increase in seismicity until a slight increase in June, witnesses in the town of Chaitén reporting feeling tremors in May. Otherwise, activity through August consisted of continuous ash-and-steam emissions, small collapses of unstable portions of the domes, and resulting block-and-ash flows. Seasonal precipitation remobilized previously erupted material and lahars reached the town in July.

Seismicity remained slightly elevated (relative to April and May) through September. On 14 September, a surveillance camera captured a plume as wide as the caldera reaching ~ 1.5 km high. This plume was significantly wider than plumes in the previous months. On 29 September, witness reports prompted SERNAGEOMIN to fly past the caldera, and obervers saw evidence of a significant recent collapse.

SERNAGEOMIN's 29 September observation flight provided stunning views of the dome complex leading to the detection of a new dome, Dome 3, that had filled the 19 February 2009 collapse scar (figures 27 and 28). Dome 3 had a depression in its N sector and small central pinnacles (figure 28). The central pinnacle of the whole complex had disappeared, and a large active depression, elongated to the NNW, had formed E of Dome 3. This depression reportedly resulted from either a lateral explosion or a relatively slow and structurally controlled internal collapse.

Figure 27. Aerial views of Chaitén’s dome complex taken following the partial collapses of (a) 19 February and (b) 29 September 2009. (a) The dome complex following the 19 February partial collapse. Dimensions of the central pinnacle (*) and the collapse scarp (**) are given at the lower left. (b) The dome complex following the 29 September partial collapse, showing the first observation of Dome 3. Labels in all capital letters indicate structures or deposits, and labels in all lowercase indicate relative amounts of water vapor and ash in emitted plumes. Courtesy of OVDAS-SERNAGEOMIN; (a) photo by Paul Duhart, (b) photo and interpretation by Jorge Muñoz.

Figure 28. Aerial view of the center of Chaitén's dome complex on 29 September 2009. The then-newly ob served central depression is the most active area in the photograph, and a small new pinnacle, Proto-pinnacle 2, is seen near this area. Labels in all capital letters indicate structures or de pos its, and labels in all low er case indicate relative amounts of water vapor and ash in emitted plumes. Courtesy of OVDAS-SERNAGEOMIN; photo and interpretation by Jorge Muñoz.

Following the events of September, no major outstanding activity was reported through the rest of 2009. In late January 2010, two new telemetered seismic stations were added to the monitoring network, increasing the number of seismic stations around Chaitén to ten. A new observation camera was also installed ~ 800 m from the dome complex.

During the end of 2009 and January 2010, the growth rate of the dome complex slowed, and seismicity declined significantly, with larger earthquakes (stronger than M 3.5) being absent through at least March (figure 29). Following a few months of relatively calmer activity at Chaitén, and after at least a month without emissions of ash, the Alert Level was lowered to Yellow on 1 May 2010.

Figure 29. Daily earthquakes between M 3.5 and M 4.5 at Chaitén, from the onset of the eruption on 2 May 2008 to 1 April 2010 (a month prior to the lowering of the Alert Level to Yellow). Courtesy of OVDAS-SERNAGEOMIN.

1 May 2010-2 May 2011 (Yellow Alert). During the period of Yellow Alert, reported plume heights remained below 2.1 km altitude (Buenos Aires VAAC); compared to the prior, more active period when plumes regularly reached 3-4 km in height, this was a significant decline. Over this period, the Buenos Aires VAAC reported occasional emissions that included ash (table 2). OVDAS-SERNAGEOMIN began recording mainly VT events (interpreted as relating to rock fracturing) and long period (LP) events (interpreted as related to fluid dynamics in and beneath the volcanic edifice). Both types of seismicity remained low throughout the remainder of 2010 and into 2011. However, incandescence of the lava dome surface was observed at night in late January 2011.

Table 2. Emissions from Chaitén's lava dome complex during the period of Yellow Alert (1 May 2010-2 May 2011); '--' indicates information that was not reported. Courtesy of the Buenos Aires VAAC.

3 May 2011-September 2011 (Green Alert). On 2 May 2011, the Alert Level was lowered to Green due to lower levels of activity since January 2011, including (1) seismicity remaining at low levels of occurrence and magnitude; (2) no significant emissions of ash; (3) a lack of dome growth and associated partial collapses; and (4) a lack of visual observations suggesting restlessness.

Since 3 May 2011, ash-free plumes rose no higher than 0.5 km; the exception was one plume in late May or early June. Seismicity remained low, with daily counts averaging fewer than 2 for LP events, less than 10 for VT events, and less than 1 for HB events.

Magma ascent prior to 2 May 2008 eruption. Wicks and others (2011) stated that "Because of the historically rare and explosive nature of rhyolite eruptions and because of the surprisingly short warning before the eruption of the Chaitén volcano, any information about the workings of the magmatic system at Chaitén, and rhyolitic systems in general, is important from both the scientific and hazard perspectives." There were only about 24 hours between the first felt seismicity in the town of Chaitén and the onset of the eruption. Such a short precursory period has been recorded for basaltic eruptions (e.g. Hekla volcano, Iceland; Soosalu and Einarsson, 2002), but not for silicic eruptions such as Chaitén (Castro and Dingwell, 2009).

Castro and Dingwell (2009) used petrological experiments to constrain the temperatures and decompression rates of the magma erupted explosively at Chaitén on 2 May 2008. Their results suggested that the magma rose from depths of at least 5 km in about four hours, shorter than the roughly 1-day period of seismicity that was felt in the town of Chaitén.

Wicks and others (2011) interpreted radar interferometry observations to indicate that the rapid ascent (as reported by Castro and Dingwell, 2009) of the Chaitén magma was controlled by pre-existing faults in the crust beneath Chile (figure 30). Specifically, they modeled a large, dipping, sill-like body (their "reservoir") residing under Minchinmávida volcano (~ 20 km E of Chaitén, "M" on figure 30) and rising towards the surface to the W of Chaitén (Morro Vilcún, "MV" on figure 30). The magma followed a path that intersected an inferred vertical conduit feeding Chaitén. They interpreted the rhyolitic reservoir as originating from either 1) a combination of tectonic stresses and magmatic overpressure draining rhyolitic magma from a mafic reservoir beneath Minchinmávida, or 2) an event similar to the M 9.5, 1960 Chilean earthquake creating permeability and a pressure gradient, allowing the overpressured magma to migrate to beneath Chaitén.

Figure 30. Cross-sectional model of the magmatic system that ultimately erupted at Chaitén (Wicks and others, 2011). The profile A-A' is approximately W-E; Morro Vilcún (MV), Chaitén (Ch), and Minchinmávida (M) are plotted at their relative positions on the surface. Stages a, b, and c are illustrated by a series of model cartoons. (a) The sill-like body (Reservoir) extends to the W, towards the surface, from the magma chamber beneath Minchinmávida; the dike (red body) begins propagation upwards, but at this stage has not intersected Chaitén's conduit. (b) Diking leads to an injection-caused earthquake (inflational, shown by the moment tensor solution "beachball diagram" where black indicates compression and white indicates tension) which occurred 2 hours before the 2 May 2008 eruption began. (c) As the eruption progressed and drained the shallow storage beneath Chaitén, the sill-like reservoir collapsed (shown by the moment tensor solution diagram). In all frames, the Liquiñe-Ofqui Fault Zone (LOFZ) is indicated beneath Minchinmávida volcano. CMT 1 refers to the centroid mean solution (CMT) of a moment magnitude 5.2 earthquake that occurred 2 hours prior to the main eruption on 2 May 2008. CMT 2 refers to an earthquake of magnitude 5.0 that occurred 19 hours after the onset of eruption. From Wicks and others (2011).

Restoring the town and damaged infrastructure. On 9 April 2011, the Chilean government reported that President Sebastián Piñera had announced the "North Chaitén Solution" plan. After restoring basic services (e.g. electricity) to the town of Chaitén in the first months of 2011, President Piñera stated that "everything will be definitively restored, electricity and light has already returned and ... all of the electricity poles are new." He also announced plans to dredge the harbor a second time (necessary due to the amount of remobilized volcanic material deposited in the bay) to allow boats to dock, a plan to install a floating dock, ongoing surveying for new paved roads to other surrounding cities, and plans for the town's school and aerodome.

References. Castro, J.M., and Dingwell, D.B., 2009, Rapid ascent of rhyolitic magma at Chaitén volcano, Chile: Nature, v. 461, p. 780-784 (DOI:10.1038/nature08458).

Soosalu, H., and Einarsson, P., 2002, Earthquake activity related to the 1991 eruption of the Hekla volcano, Iceland: Bulletin of Volcanology, v. 63, p. 536-544.

Wicks, C., de la Llera, J.C., Lara., L.E., and Lowenstern, J., 2011, The role of dyking and fault control in the rapid onset of eruption at Chaitén volcano, Chile, Nature, v. 478, pp. 374-377 (DOI:10.1038/nature10541).

Geologic Background. Chaitén is a small caldera (~3 km in diameter) located 10 km NE of the town of Chaitén on the Gulf of Corcovado. Multiple explosive eruptions throughout the Holocene have been identified. A rhyolitic obsidian lava dome occupies much of the caldera floor. Obsidian cobbles from this dome found in the Blanco River are the source of artifacts from archaeological sites along the Pacific coast as far as 400 km from the volcano to the N and S. The caldera is breached on the SW side by a river that drains to the bay of Chaitén. The first recorded eruption, beginning in 2008, produced major rhyolitic explosive activity and building a new dome and tephra cone on the older rhyolite dome.

Information Contacts: Observatorio Volcanológico de los Andes del Sur-Servico Nacional de Geologia y Mineria (OVDAS-SERNAGEOMIN), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637 / 1671, Santiago, Chile (URL: http://www.onemi.cl/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); MapsOf.net (URL: http://mapsof.net/); Marc Bernstien, Eliza Calder, and Patrick Whelley, University at Buffalo, State University of New York, 411 Cooke Hall, Buffalo, NY 14260; Andrés Paves, University of Chile, 2002 Blanco Encalada, Santiago, Chile; Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php); Javier Romero, Dirección de Vialidad - Ministerio de Obras Públicas, Puerto Montt; Gobierno de Chile, Santiago, Chile (URL: http://www.gob.cl/ or http://www.gob.cl/english/).

Microseismicity preceded and accompanied a jökulhlaup (a glacier-outburst flood) on 9 July 2011, as reported by the Iceland Met Office (IMO). The jökulhlaup escaped from under Mýrdalsjökull, the glacier that rests above Iceland's Katla volcano, its 10 x 14 km caldera, and environs (figure 4). IMO reported that microseismicity was registered near several ice cauldrons in the caldera for a few weeks prior to the event (figure 5). Peak harmonic tremor on 8 July coincided with rising water levels and increased water conductivity, as measured by the main flood gauge (figure 6; gauge is at red triangle on figure 4).

Figure 4. A map of road closures and restricted areas of Mýrdalsjökull glacier resulting from the 9 July 2011 jökulhlaup at Katla (see key, lower left). The town of Vík is shown near the bottom (in black), and the main road through the area is shown in red; the trace of Katla caldera is shown in black and labeled. The main flood gauge was on the bridge across the Múlakvísl river; both were destroyed in the jökulhlaup event (red triangle). Inset shows the geographic location of Katla and Mýrdalsjökull in the S of Iceland. Restricted areas map modified from ágúst Gunnar Gylfason of the National Commissioner of the Icelandic Police-Department of Civil Protection and Emergency Management; index map modified from Ginkgo Maps.

Figure 5. Map (top) and plot (bottom) of the seismicity recorded during 8-9 July 2011 at Katla. Colors indicate the timing of epicenters and their respective plotted magnitudes, recorded as late as 2250 on 9 July 2011, according to the scheme shown below the map. Black triangles indicate seismic monitoring stations. Courtesy of Iceland Met Office (IMO).

Figure 6. Running plots of (a) water level, (b) water temperature, and (c) water conductivity at the main flood gauge of the Múlakvísl river during 3-9 July 2011. The plots show rising water level and conductivity that were coincident with peak harmonic tremor. The plots stop abruptly (red vertical line) when the gauge was destroyed along with the bridge crossing the Múlakvísl river. Courtesy of Iceland Met Office (IMO).

IMO reported that, on the same day, the main flood gauge was damaged when flood waters reached the instrument near midnight; another station, normally not in the water, started recording rising water around 0400 on 9 July, and the water level there rose 5 m within 5 minutes (figure 7). When the flood reached the main road approximately one hour later, the main bridge over the Múlakvísl river was destroyed and the road was closed (red triangle, figure 41).

Figure 7. A running plot of water level at the second flood gauge (normally not submerged). The plot shows a significant rise in water level (5 m within 5 minutes). Courtesy of Iceland Met Office (IMO).

According to the news source Morgunblaðið, 200 people were safely evacuated, and allowed to return to their homes by that afternoon. Morgunblaðið reported that analysis of the flood waters indicated that the flood was caused by geothermal water, but that a sub-glacial eruption at Katla could not be ruled out. IMO stated that the harmonic tremor declined on 9 July, following the jökulhlaup event. After observational flights, new cracks and cauldrons were reported in the ice of Mýrdalsjökull glacier (figure 8).

Figure 8. Cracking and subsidence of the Mýrdalsjökull glacier around an ice cauldron above the Katla caldera. Widespread gray tephra deposited on the ice surface is due to the 2010 Eyjafjallajökull eruption (BGVN 35:03, 35:04). Courtesy of the Icelandic Coast Guard.

By 16 July, the National Commissioner of Icelandic Police in the Department of Civil Protection and Emergency Management reported that a new bridge had been built to replace the bridge destroyed in the jökulhlaup (figure 9).

Figure 9. Photograph of the remains of the bridge crossing of the Múlakvísl river, destroyed in the jökulhlaup event on 9 July 2011. The new bridge, constructed by the 16 July 2011, can be seen in the background. Courtesy of John A. Stevenson.

August-December seismicity. IMO reported increased seismicity under Mýrdalsjökull in October (figure 10). They reported that 512 earthquakes occurred, with ~ 380 originating within the Katla caldera; a large portion (nearly 100) of those 512 earthquakes occurred on one day near the beginning of October (figure 11). The largest reported earthquake was M 4, with seven being larger than M 3. On 8 November, an M 3.2 earthquake that originated in the S most part of the caldera was felt by residents in the town of Vík.

Overall, following the July 2011 jökulhlaup event, seismicity has increased above background levels of the past year. The seismic peak is noticeable with respect to the number of earthquakes, their largest magnitudes, and the clustering under Katla (figures 10 and 11). The largest earthquakes were as large, or slightly larger, than the other earthquakes of M 3 or greater in earlier episodes of unrest (i.e., 1999 and 2002-2004, figure 10). The bulk of the 2011 seismic increase occurred over a shallow depth range (within 4 km of the surface, figure 12).

Figure 10. Plots of seismicity (greater than M 0.6) at Katla since 1999, showing the October 2011 seismicity in comparison with past episodes of non-eruptive unrest, such as in 1999 (sub-glacial eruption is uncertain in the GVP database) and 2002-2004. Plots (from the top) show: the monthly number of earthquakes (log scale); the magnitudes of earthquakes; cumulative number of earthquakes (red) and cumulative seismic moment (blue); and the focal depths of the located earthquakes. Courtesy of Iceland Met Office (IMO).

Figure 11. Seismic events (stronger than M 0.5) per day at Katla during December 2010-December 2011. Raw data is shown in blue, the 5 day moving average is shown in red, and events stronger than M 3.0 are indicated by gold stars. These trends highlight the increased seismicity of August-December 2011. Courtesy of the University of Edinburgh School of Geosciences.

Figure 12. Cumulative number of seismic events (stronger than M 0.5) at Katla since 23 November 2010. All events are shown in yellow, and events originating at depths greater than 4 and 10 km are shown in orange and red, respectively. During the August-December 2011 increase in seismicity, the majority of the recorded events originated from shallow depths (less than 4 km). Courtesy of the University of Edinburgh School of Geosciences.

Television filming sparks eruption fears. The Iceland Review reported that, in the early morning of 9 December, the Icelandic emergency hotline received calls from residents reporting bright lights on the slopes of Mýrdalsjökull. Callers feared that an eruption had started at Katla. The bright lights had also been noticed on a webcam by observers in Norway, who also enquired if there was an eruption. When the glacial slopes were inspected to find the cause of the lights, it was discovered that they were from film crews for the HBO series "Game of Thrones", who were filming in the early morning to capture the desired light conditions.

Geologic Background. Katla volcano, located near the southern end of Iceland's eastern volcanic zone, is hidden beneath the Myrdalsjökull icecap. The subglacial basaltic-to-rhyolitic volcano is one of Iceland's most active and is a frequent producer of damaging jökulhlaups, or glacier-outburst floods. A large 10 x 14 km subglacial caldera with a long axis in a NW-SE direction is up to 750 m deep. Its high point reaches 1380 m, and three major outlet glaciers have breached its rim. Although most recorded eruptions have taken place from fissures inside the caldera, the Eldgjá fissure system, which extends about 60 km to the NE from the current ice margin towards Grímsvötn volcano, has been the source of major Holocene eruptions. An eruption from the Eldgjá fissure system about 934 CE produced a voluminous lava flow of about 18 km3, one of the world's largest known Holocene lava flows. Katla has been the source of frequent subglacial basaltic explosive eruptions that have been among the largest tephra-producers in Iceland during historical time and has also produced numerous dacitic explosive eruptions during the Holocene.

Information Contacts: Einar Kjartansson, Iceland Met Office (IMO), Bústaðavegi 9, 150 Reykjavík, Iceland (URL: http://en.vedur.is/); National Commissioner of the Icelandic Police-Department of Civil Protection and Emergency Management, Skúlagata 21, 101 Reykjavík, Iceland (URL: http://www.almannavarnir.is/); Ginkgo Maps (URL: http://ginkgomaps.com/); Morgunblaðið, Hádegismóum 2, 110 Reykjavík, Iceland (URL: http://mbl.is/); Icelandic Coast Guard, Skógarhlíð 14, 105 Reykjavík, Iceland (URL: http://www.lhg.is/); John A. Stevenson (URL: http://all-geo.org/volcan01010/); The University of Edinburgh School of Geosciences (URL: http://www.ed.ac.uk/schools-departments/geosciences); The Iceland Review, Borgartúni 23, 105 Reykjavík, Iceland (URL: http://www.icelandreview.com/).

Lokon-Empung has been in a state of unrest since 2007 (BGVN 33:02). Between mid-February through mid-July 2011, occasional phreatic eruptions, modest ash plumes and elevated seismicity occurred, with a larger ash plume in July 2011 (BGVN 36:06). This report addresses seismic events from mid-July through 1 December 2011.

According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM), during 20-21 July 2011, seismicity and visual observations of Tompaluan crater in the saddle between the twin peaks of Lokon and Empung indicated that activity continued to be high. On 20 July plumes rose 100-500 m above Tompaluan crater, and during 21-24 July 2011 white plumes again rose 100-300 m. CVGHM noted that, since an eruption on 18 July, most data showed a decline in activity and therefore on 24 July the Alert Level was lowered to 3 (on a scale of 1-4). Residents and tourists were not permitted within 3 km of the crater. A news article (Straits Times) stated that on that same day about 5,000 residents that had evacuated returned home, and about 200 people remained in shelters.

CVGHM reported that during 24 July-8 August 2011 seismicity decreased at Tompaluan crater, with a drastic reduction on 26 July. According to a news article (BNO News, accessed on Daijiworld News), during 27 July-8 August white plumes rose 100-400 m above the crater. The article stated that at the end of August, Tompaluan crater erupted several times (12 times on 28 August). One explosion on 29 August 2011 ejected material 250 m above the crater. According to the article, activity decreased after 29 August. The article also noted that 222 people remained at temporary refugee camps because their homes were located within 3 km of the crater.

CVGHM reported that on 10 October 2011 white and gray plumes rose 100-300 m above Tompaluan crater. Based on information from CVGHM, the Darwin Volcanic Ash Advisory Centre (VAAC) reported that on 11 October an ash plume rose to an altitude of 2.1 km.

According to a news article (Kompas.com), a gray plume rose 1.2 km above Tompaluan crater and drifted SW on 26 October, followed by an explosion that sent incandescent material as far as 800 m away from Tompaluan crater. A second eruption produced a plume that rose 500 m above the crater.

Geologic Background. The Lokong-Empung volcanic complex, rising above the plain of Tondano in North Sulawesi, includes four peaks and an active crater. Lokon, the highest peak, has a flat craterless top. The morphologically younger Empung cone 2 km NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century. A ridge extending 3 km WNW from Lokon includes the Tatawiran and Tetempangan peaks. All eruptions since 1829 have originated from Tompaluan, a 150 x 250 m crater in the saddle between Lokon and Empung. These eruptions have primarily produced small-to-moderate ash plumes that sometimes damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Straits Times (URL: http://www.straitstimes.com); BNO News (URL: http://www.bnonews.com/); Kompas.com (URL: http://www.kompas.com/); Antara News (URL: http://www.antaranews.com/en/); Daijiworld News (URL: http://www.daijiworld.com/).

This report on Masaya presents a summary of activity through mid-2011. Our last report was issued in March 2009 (BGVN 34:03) and highlighted the intermittent plumes and explosions of 2006 and 2008.

From 2008-2010 activity generally consisted of degassing with sulfur dioxide (SO₂) fluxes typically under 1,200 tons per day. Instability of the S andW crater walls was a concern for the National Park and monitored by the agency INETER (Instituto Nicaragüense de Estudios Territoriales). Mass wasting, frequently triggered by heavy rain, occurred within the crater with debris occasionally blocking the active vents.

Throughout this 3-year period, fumarole temperatures ranged from 58 to 84°C and regular monitoring of the El Comalito cinder cone showed that degassing continued. Tremor sources shallowed during 2008-2010, rising from a 2008 depth of 26 km to a 2010 depth of ~ 1 km.

On 12 October 2010 incandescence occurred in the intra-crater area's largest opening (figure 24). Temperature at the points of incandescence reached 207°C. Differential optical-absorption spectroscopy (DOAS) measurements from vents registered SO₂ fluxes of 465 tons per day. SO₂ emissions increased throughout October 2010, reaching 586 tons per day. INETER reports contain plots with more detailed SO₂ data.

Figure 24. Incandescence seen in Masaya's Santiago crater on 12 October 2010. Note the crater's vertical walls and depth. Courtesy of INETER.

SO₂ fluxes in 2011. In January 2011, INETER's team measured SO₂ fluxes while in transit along the easternmost route on figure 25 (between the town of Ticuantepe and the community of San Juan). Those SO₂ measurements averaged 642 tons per day, an increase over 2010 that was attributed to increased gas and magma output.

Figure 25. Vehicle routes (heavy lines) used while recording Masaya's SO2 fluxes. The scale at bottom shows distance in meters. The topographic margin of Masaya's main caldera sits ~2 km E of the easternmost vehicle route. Courtesy of INETER.

During 7-30 March 2011 collaborators from the University of East Anglia, Heidelberg University, and Oxford University measured Santiago crater's SO₂ and other gas emissions. A Mini-DOAS mobile was one of the many instruments used to monitor the atmosphere and SO₂ fluxes (figures 26-28).

Figure 26. SO2 measurements underway at Masaya on 20 January 2011. The vehicle passed beneath Masaya's gas plume on the Southern Pan-American Highway. The laptop displays a well-defined red histogram representing SO2 measured along the plume transect. Courtesy of INETER.

Figure 27. Instruments used during the 7-30 March 2011 campaign to measure SO2 on a continuous basis from a viewing platform overlooking Masaya. Courtesy of INETER.

Figure 28. A compact, automatic meteorological station used to measure wind velocity, air humidity, and other parameters that could refine and enable comparisons with the gas measurements. Courtesy of INETER.

In addition to mobile DOAS and fixed gas monitoring systems, a small dirigible (Zeppelin) represented a novel monitoring approach. One potential use for the dirigible was as a platform from which to measure gas concentrations inside the volcanic plume at altitude. Unfortunately, when deployed on its trial launch, heavy winds quickly blew it out of control (figure 29).

Figure 29. The dirigible (Zeppelin) deployed at Masaya during 7-30 March 2011. The dirigible undergoing instrumental work (top), and floating above Santiago crater moments before being blown away by heavy winds (bottom). Courtesy of INETER.

Crater-wall collapse leads to 6 August 2011 Park closure. More than a dozen crater-wall collapses occurred at Santiago crater during June and July 2011. INETER geologist Marisol Echaverry López noted that the SW and W sides of the crater wall had severely eroded. Echaverry recommended that, should the situation worsen, nearby residents be evacuated since debris-covered vents could pressurize the system and lead to explosions. On 14 July, geologist Martha Ibarra found that debris shed from the steep walls was accumulating and the recent collapses had blocked two gas vents. The deep, steep wall of Santiago crater frequently collapsed along fracture zones.

On 6 August 2011, Masaya National Park officials alerted INETER that significant portions of the SW crater rim had collapsed and completely covered the active vent. The park closed for the day during inspections by INETER. The SW rim was the site of frequent failures and field investigators noted that gas emissions were blocked for ~ 10 minutes. No additional failures were observed and activity did not escalate.

During field investigations in September and October 2011, INETER described and measured temperatures from three new fumaroles within Santiago crater. These sites were located at the edges of debris fill within the crater, along the S and E walls and were degassing with temperatures from 48 to 74°C. SO₂ measurements from Mini-DOAS indicated decreasing emissions during this time period, from 518 tons per day in September to 153 tons per day in October 2011.

Geologic Background. Masaya volcano in Nicaragua has erupted frequently since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold" until it was found to be basalt rock upon cooling. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of observed eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Recent lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).

This report summarizes a news article by Lourdes Solidum-Montevirgen of the Philippines' Department of Science and Technology (Solidum-Montevirgen, 2011). The article noted that 20 years after one of the last century's biggest volcanic eruption (April-September 1991, BGVN 16:03-16:10), hunger and lahars continue to threaten Aeta communities around Pinatubo's foothills. The Aetas (an indigenous people who live in scattered, isolated mountainous parts of Luzon, Philippines) resided, in part, before the eruption in the towns of San Marcelino and Botolan, settlements almost destroyed by the 1991 eruption. The rainy season has resulted in lahar flows that continue to threaten these and nearby towns, displacing thousands of people. Agriculture continues to suffer badly, as hundreds of square kilometers of formerly arable land remain unproductive. Pinatubo is located NW of the capital of Manila (figure 39).

Figure 39. A map of major Philippine volcanoes, including Pinatubo. Courtesy of Lyn Topinka, US Geologic Survey.

Aetas were hardest hit because they were both uprooted from their homes and their way of life. Many remain in government resettlement areas, huddled in makeshift homes, tents, and evacuation dwellings. Many of them are recent refugees after part of a protective dike along the Bucao River collapsed during Typhoon Kiko in August 2009, flooding Botolan and ten villages, resulting in death and hunger. Typhoons that followed two months later (October 2009) broke down an additional 1-km portion of the dike, causing lahars and floodwaters to rise more than 1.5 m, displacing over 20,000 people in nine villages. Over 9,000 of these recent refugees remained in evacuation centers as they awaited dike repair. They have joined thousands of evacuees still huddled in the ten evacuation centers inside three resettlement sites that were created following Pinatubo's eruption.

There is still widespread devastation in Botolan and nearby towns where several square kilometers of lakes and farm lands were "desertified". It is doubtful whether a new bridge and the dike, when repaired, will hold lahar floods because the Bucao river is heavily silted. Botolan (population 51,675), the largest town in Zambales and closest to Pinatubo, also has the largest Aeta population in the province. In 2010, 160 km² (16,000 hectares) of the area nearest Pinatubo was declared as Aeta ancestral domain by the National Commission on Indigenous Peoples.

Farming has not yet resumed in many rice paddies and vegetable farms damaged by flash floods and lahars. Farm lands were covered with thick ash and reworked tephra, irrigation equipment ruined, roads and bridges destroyed, properties lost, trade and business centers collapsed. Overall, 364 communities and 2.1 million people were affected by the eruption, and more than 80,000 houses were lost. Roads and communications were damaged by pyroclastic flows and lahars. Some 800 km² of rice lands and almost 800,000 farm animals were lost. The cost to agriculture was estimated at P1.5 billion (~ $25 million US) and the cost of repairs to damaged infrastructure was P3.8 billion (~ $62 million US).

Reference. Solidum-Montevirgen, L., 2011, Hunger, lahar haunt homeless Aetas 20 years after Pinatubo, Malay Business Insight, 29 July.

Geologic Background. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Information Contacts: Lourdes Solidum-Montevirgen, Industrial Technology Development Institute-Food Processing Division, Department of Science and Technology, Phillippines; Malaya Business Insight (URL: http://www.malaya.com.ph).

Popocatépetl continued to be active during September 2010 to 13 December 2011 with explosions, tremor, and frequent gas-and-steam emissions occasionally containing ash (figure 59). This report continues the table in the previous report (BGVN 35:08) that tallies the seismic activity and ash emissions (table 21). Figure 60 shows the proximity of the volcano to population centers.

Figure 59. Photograph of the lava dome at Popocatépetl taken on 8 September 2011. Courtesy of CENAPRED.

Table 21. Reported plumes above Popocatépetl's summit crater that contained some ash between 5 October 2010 and 13 December 2011. Data provided by the MWO (Mexico City Meteorological Watch Office), Washington Volcanic Ash Advisory Center (VAAC), and Centro Nacional de Prevención de Desastres (CENAPRED), abbreviated CEN.

Figure 60. Map of Popocatépetl in relation to Mexico City and other large communities. Circular areas are approximate; they show some hazard zones, including the innermost "Red" zone, which is within 5 km of the summit and excludes the public from entry. Courtesy of CENAPRED.

During the reporting interval, there were a large number of MODVOLC thermal alerts for Popocatépetl.

Arana-Salinas and others (2010) discuss the Ochre Pumice Sequence, a major (VEI 6) event that occurred ~ 5,000 years ago. That unit contained a sequence of pyroclastic flow and fall deposits that covered ~ 300 km² directed NNE. The erupted magma amounts to a (dense rock equivalent) volume of ~2 km³. The authors stated that, depending on the wind direction, an equivalent event today would impact 15 million residents of Mexico City (the Capital), Puebla, Atlixco, and Cuautla and elsewhere, and it would severely damage infrastructure.

Reference. Arana-Salinas, L., Siebe, C., Macías, J.L, 2010, Dynamics of the ca. 4,965 yr ¹⁴C BP "Ochre Pumice" Plinian eruption of Popocatépetl volcano, México: Journal of Volcanology and Geothermal Research v. 192. p. 212-231.

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

Information Contacts: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: https://www.gob.mx/cenapred/); 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 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); MODVOLC, 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/).

Soputan volcano, Sulawesi, Indonesia (figure 4) was relatively quiet for more than two years following our last report in September 2008 (BGVN 33:09). Thermal anomalies appeared in late May 2011 and in late June 2011, Soputan re-commenced eruptive activity. This report covers activity at Soputan during 2011 (through 2 October). Unless otherwise noted, data was reported by the Center for Volcanology and Geological Hazard Mitigation (CVGHM).

Figure 4. A photograph of Soputan volcano, taken 6 March 2011 by Flickr account user Akhal-Téké. Index maps at left show the location of Soputan volcano on the island of Sulawesi (close-up, bottom) in Indonesia (top). Index maps modified from MapsOf.net (top) and Ginkgo Maps (bottom).

The first signs of the June-October eruption at Soputan occurred with some diffuse white plumes in June reaching 25-150 m above the crater. After an increase in seismicity during 21 June-2 July, CVGHM raised the Alert Level from 2 to 3 (on a scale from 1-4); climbing the slopes of the volcano was prohibited, and residents were discouraged from going within 6 km of Soputan's crater.

A Strombolian eruption, reported at 0603 on 3 July, generated an ash plume that rose 6 km altitude and drifted W. The eruption plume was captured in a NASA Earth Observatory satellite image (figure 5). A pyroclastic flow traveled up to 4 km W. A 10 pixel MODVOLC thermal alert was triggered at 0225 (UTC) on the same day (figure 5, table 8).

Figure 5. A natural color satellite image of the eruptive plume generated at Soputan on 3 July 2011. The plume is seen here drifting E over Laut Maluku (Molucca Sea); the brownish color of the plume indicates that it consisted of both gas and ash. The red outline highlights the area of the 10-pixel MODVOLC thermal alert (table 2) of the same day. Courtesy of NASA Earth Observatory.

Table 8. MODVOLC thermal anomalies recorded at Soputan in 2011. A 58 pixel thermal anomaly was recorded on 2 October 2011, but was omitted due to the sun-glint angle being below 12°. The University of Hawaii states that "If a pixel has a sun-glint angle of less than 12° it is potentially contaminated by sunglint and should not be trusted." Courtesy of HIGP Thermal Alerts System, University of Hawaii. [Note that the 21 May 2011 pixel originally reported below (deleted) was actually located at some distance from the volcano in the ocean, and was most likely due to sunglint.]

The Jakarta Globe reported that, due to ash fall, the Indonesian Red Cross (Pa Merah Indonesia - PMI) distributed ~ 31,000 face masks to residents (figure 6). It also reported that Sutopo Purwo Nugroho, spokesman for the National Board for Disaster Managment (BNPB), said that "there is no need for evacuation because the nearest residents are living some 8 km from the mountain." Sam Ratui International Airport was closed for 3 hours (during 1200-1500) that afternoon, according to The Jakarta Globe. Following the eruption of 3 July, seismicity decreased, and the only reported activity was dense white plumes rising to 75 m above the crater on 18 July. The Alert Level was lowered to 2 on 19 July, allowing residents to come within 4 km of the crater.

Figure 6. Residents near Soputan with face masks they received from the Indonesian Red Cross (Pa Merah Indonesia - PMI). Courtesy of the Jakarta Globe.

Seismicity continued to decrease until 10 August. On 14 August, a plume containing ash rose to 1 km above the crater, and two other plumes rose to 1.3 km above the crater later in the day (figure 7). The Darwin Volcanic Ash Advisory Centre (VAAC) reported an ash plume that drifted more than 100 km W. The Alert Level was again raised to 3 on 14 August, once again prohibiting residents within 6 km of the crater.

Figure 7. An ash plume from Soputan rising to greater than 1 km above the crater on 14 August 2011. In the foreground, a resident of one of the local towns is working in their field. Courtesy of Andreas/AFP-Getty Images.

Following the eruptions of 14 August, seismicity decreased significantly, and small white plumes rose above the crater. The plumes steadily decreased from 200 m high above the crater (14-18 August) to, at most, 100 m above the crater (29 August-7 September). An early morning photograph captured an eruption on 15 August, showing a small plume and lava flows down the flank of Soputan (figure 8). On 8 September, the Alert Level was lowered to 2, allowing residents to come no closer than 4 km to the crater.

Figure 8. An early morning photograph of Soputan erupting on 15 August 2011. Lava flows down the flank of Soputan brightened the small eruptive plume billowing overhead. Courtesy of Andreas/AFP-Getty Images.

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is the only active cone in the Sempu-Soputan volcanic complex, which includes the Soputan caldera, Rindengan, and Manimporok (3.5 km ESE). Kawah Masem maar was formed in the W part of the caldera and contains a crater lake; sulfur has been extracted from fumarolic areas in the maar since 1938. Recent eruptions have originated at both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM) Diponegoro 57, Bandung, Jawa Barat 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Akhal-Téké, Flickr photostream (URL: http://www.flickr.com/photos/51873088@N04/); MapsOf.net (URL: http://mapsof.net/); Ginkgo Maps (URL: http://www.ginkgomaps.com/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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/); The Jakarta Globe, Citra Graha Building 11th Floor, Suite 1102, Jakarta 12950, Indonesia (URL: http://www.thejakartaglobe.com/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Andreas/AFP - Getty Images (URL: http://www.gettyimages.com/).

Our last report discussed Telica volcano's intermittent gas emissions from 2009 through early 2010 as well as installation of an early warning system (Sistema de Alerta Temprana, SAT) in March 2010 (BGVN 35:03). New information has been released by INETER (the Instituto Nicaragüense de Estudios Territoriales) detailing the escalation of activity that culminated in a major eruption in May 2011. This report also covers the field investigations from April 2010 through October 2011, seismic data from January 2010 through October 2011, SO₂ monitoring from 25 May 2011 through 13 September, and regular thermal measurements from the crater and flank fumaroles.

Volcanic activity at Telica during 2010 was characterized by diffuse degassing. Persistent gas emissions from this volcano have caused a legacy of hazards for local communities and have been linked to acute respiratory infections (Bellos and others, 2010; Freundt and others, 2006; Malilay and others, 1996). Field investigations in 2010 conducted by INETER focused on measuring temperatures from the crater as well as fumaroles located near the flanks (figure 20). Heavy rain and inaccessible roads limited visual and thermal monitoring to short field excursions in April, July, and August 2010.

Figure 20. (A) Telica's 700-m-wide double crater (gray to white central area) and E-flank fumaroles (diamond) were sites of thermal monitoring for 3 months in 2010. The seismic station TELN is located 30 m N of the orange diamond. (B) Typical diffuse degassing from the summit photographed on 14 January 2011. Courtesy of INETER.

Using a thermal camera, INETER determined the maximum temperatures within Telica's crater on 28 July and 18 August were 259°C and 251°C respectively. The four fumaroles located near seismic station TELN were measured three times in 2010. With a digital thermocouple, INETER determined that maximum temperatures from the flank fumaroles gradually increased from April to August: 72.3°C, 81°C, and 105°C. Minimum measured temperatures were irregular and ranged from 66.4 to 76°C.

Few earthquakes were sufficiently large to registered and be located during 2010 (table 2), but INETER reported that those recorded were smaller, and there was frequent tremor. The seismic network for this volcano was installed in 1994. The two local seismic stations, TEL3 and TELN, operated with 3-component sensors but during this reporting interval TELN was offline until July 2010. Microseismicity was reported during four months in 2010 (March, September, November, and December). The highest rate occurred in March, exceeding 150 events per day. In September, more than 120 events were recorded per day, and in November and December, the rate was 80 microseisms per day.

Table 2. Located earthquakes at Telica recorded in the interval from January 2010 through October 2011. Only months with earthquakes reported are shown. High values in May-June were during an eruption. Values are based on monthly reports from INETER.

INETER reported that field investigators encountered significant gas plumes from Telica's summit in April and July. The W edge of the crater and a small vent on the E interior wall were constant sources. There were notable rockfalls from the crater rim; an observation from 28 July 2010 mentioned the NE and SE walls in particular experience rockfalls of sufficient magnitude to increase the summit crater's size.

January 2011. Investigators from INETER visited Telica this month for instrument maintenance and monitoring activities. Rockfalls from the S crater wall were noted on 11 January by staff. According to Halldor Geirsson, Mel Rodgers (Univ. of South Florida) was in the vicinity during 25-31 January and noted strong degassing and occasional rockfalls. On 14 January thermal data was collected from the central crater and fumaroles on the outer flank. The maximum temperature recorded from the crater was 295°C. The four fumaroles located on the W flank (figure 20a) had recorded temperatures ranging from 68°C to 72°C.

Seismicity during January 2011 was generally high, with ~ 907 earthquakes recorded. Most were long-period (LP) with dominant frequencies of 1-3 Hz. From 19 to 23 January events were absent. Seismic tremor was recorded throughout January at 30-40 RSAM units with scattered intervals of greater than 100 RSAM units.

February 2011. Collaborative fieldwork was conducted on 26 February between INETER and scientists from the Institute of Renewable Energy (Spain). This team measured temperatures and took thermal images of the fumaroles located both within the crater and on the W flank. Maximum temperatures within the crater ranged from 62-75°C.

Elevated seismicity continued through February with 676 recorded events. These events were similar to those recorded in previous months; LP events had dominant frequencies of 1-3 Hz. Figure 21 presents an example of frequency analysis for one earthquake at Telica. Volcanic-tectonic (VT) events rarely occurred. Tremor was recorded at 30-40 RSAM units.

Figure 21. The seismic trace of a characteristic LP event (a) from Telica processed with spectrum-analysis software to obtain the dominant frequency ranges. This event was recorded on 1 February 2011 and lasted for more than 20 seconds (b, a zoom on the trace). The dominant frequencies (c, frequency vs. amplitude) ranged from 1 to 2 Hz with some signals to ~10 Hz. Courtesy of INETER.

Ashfall in March 2011. On 6 March residents living near Telica felt an earthquake during the night and during the following day they observed small plumes rising from the volcano. INETER scientists visited on 8 March for routine data collection and to investigate reports of fresh ashfall. Light ash was still visible on leaves and rooftops and appeared directed towards the N and SE. Residents also reported strong sulfurous odors during the explosive events. Field investigators found evidence of juvenile material along the SE rim of the crater.

INETER collected thermal data on 8 March from fumaroles located within the crater and near the seismic station (TELN) on the E flank. The maximum temperature measured within the crater was 137°C. Two fumaroles were identified within the crater on the W side; these sites had recorded temperatures in the range of 47-71°C. Thermal measurements near TELN ranged from 51-60°C. This site did not emit steam or other gases.

The INETER field crew noticed that degassing appeared to be more intense during the 8 March field visit compared to their previous 26 February visit. The view to the crater was often obscured from the point of view of seismic station TELN (figure 20b).

In their March report, INETER discussed the relevance of the new temperatures measured during the 8 March field campaign. The issue was the apparent decrease compared to temperatures recorded from Telica in January. INETER staff acknowledged the limits of monthly temperature readings but looked forward to longer-term correlations with seismic data.

Seismicity in March remained high with ~ 572 events recorded. The majority of the earthquakes were LP events with dominant frequencies between 1-3 Hz. Few VT events were recorded. Seismic tremor was between 30-40 RSAM units.

April 2011. New seismic data from an early warning system (Sistema de Alerta Temprana, SAT) was presented for the first time in INETER's April report. The new 6-station network contained five stations with 3-component sensors and one station with a single-component sensor. The network included two short-period stations, TEL3 and TEL4 (note that TEL4 was TELN), that continued to send data.

From late March through the second week of April, small explosions, LP earthquakes, and seismic swarms were detected. There was a three-day lull in early April, but VT earthquakes began occurring with increasing magnitudes and small explosions from the summit occurred at least once per week throughout the month. By 30 April, explosions were registered having 30-minute durations and were followed by periods of degassing and low-altitude ash plumes. For an interval in late April, seismicity was very high, with more than 600 events recorded each day with a range of M 0.1-3.3. A maximum of 380 RSAM units was recorded and seismic tremor ranged between 30-40 RSAM. The VT earthquakes were strong enough to be noticed by residents in local communities.

During field investigations on 11 April, INETER measured temperatures within the crater, recording a of maximum 254°C. One fumarole within the W wall of the crater had decreased temperature by 10°C while the other had increased by 1°C compared to values from March. The four fumaroles near TEL4 had recorded temperatures in the range of 53-69°C.

Eruptions in May 2011. The escalation of seismic activity and recurrence of ash plumes seen since March prompted INETER to issue an alert to civil defense on 13 May warning that eruptive activity was possible. During the first week of May seismicity increased to 500 microseismic events per day (in general, microseismic events in April occurred at a rate of ~ 220 per day) and VT events suddenly became rare (figure 22).

Figure 22. Telica's seismicity recorded during May 2011: epicenters shown on map and cross section (roughly aligned with map) were located with the new, early warning system (the 6 stations shown as blue triangles). Note that the focal depths are clustered under the volcano at depths mainly below 5 km. Courtesy of INETER.

Large explosions from Telica were registered at midnight on 13 May. After that, INETER reported that residents in three different communities observed pink-colored ash had fallen, and residents had also felt earthquakes. Frequent explosions of abundant gas and falls of coarse-to-fine ash occurred 14-15 May. Seismicity during 14 May was dominated by M 1.0-3.3 events with depths between 1 and 5 km (figure 23). Ash fell over the community of La Quemada, located 4 km N of the volcano. Residents heard loud noises from the volcano.

Figure 23. This minor explosion at Telica was observed on 16 May 2011. Courtesy of Halldor Geirsson (Pennsylvania State University).

On 16 May observers first saw gray ash clouds rising from Telica's summit. Later, this activity visibly escalated and ashfall was observed in continuous plumes (figure 23). The highest plumes reached 1.2 km altitude and ash fell to the SE over communities. By mid-May the number of seismic events had increased to 800 microseismic events per day, most of which were explosions with a few VT events (figure 24). INETER noted that seismic stations recorded up to 140 RSAM units on 16 May.

Figure 24. The number of seismic events registered per day during March-May 2011. This record was dominated by microseismicity and explosions from Telica. Courtesy of INETER.

Fieldworkers from INETER and others, deployed a large tarp, collected ~ 80 kg of tephra during 16-18 May. Preliminary assessments determined the nature of material that landed on the tarp included dominant lithics, fragmented rock and crystalline material of 0.5-1.0 mm diameter, and round fragments of pink-colored tephra (as opposed to the gray, sand-size grains from a small event on 12 May). These observations were also reported in field notes by Halldor Geirsson and in an abstract by Witter and others (2011). During a lull in activity on 16 May 2011, the team visited Telica's summit. Upon approaching the N rim, they heard no sounds coming from the crater, and they measured temperatures on the crater floor of ~ 395°C. They observed fresh, inward-directed rockfalls from the crater's rim and found the N wall unstable and dangerous, seemingly on the verge of falling. The team also observed a dark area on the SE wall and floor of the crater, which suggested that recent explosions had concentrated tephra on these surfaces.

Washington VAAC 15 and 17 May. The Washington Volcanic Ash Advisory Center (VAAC) reported that the GOES-13 satellite detected at least two plumes on 15 and 17 May 2011. This satellite imagery confirmed gas-rich plumes on both days at ~ 1.8 km altitude, but ash content could not be determined from available images. INETER reported that these events were accompanied by elevated seismicity on 15 May and associated ashfall occurred 4 km N of the summit. After three emissions of gas on 17 May, ash and tephra fell SE of the summit. The last time plumes from Telica appeared in VAAC reports was in early months of 2007 when plumes reached an altitude of ~ 1.5 km for three days in January and February drifting SW.

Explosion with sustained ash plumes. On 18 May INETER posted two online reports ("Volcanic Communications" 5 and 6) indicating the peak of activity starting on that day. After two hours of sustained explosive activity producing ash plumes ~ 600 m high, the largest explosion suddenly occurred at 1350 on the 18th. A column of ash rose to an altitude of 2.6 km and was maintained for six minutes (figure 25). More than 15 explosions were seismically recorded (with a maximum of 350 RSAM units). The episodic explosions produced ash, steam, and, at times, lightning within the plume. Temperatures within the crater were gauged at a maximum 432°C, and flank fumaroles were measured to be 60-126°C.

Figure 25. A sustained ash plume issued from Telica during the peak of activity in 21 May 2011. Courtesy of Halldor Geirsson.

In a 19 May statement to news agencies, the municipal committee for disaster prevention and mitigation (COMUPRED) reported the evacuation of 390 families from nine villages near the volcano. The villages included Agua Fría (150 m from the edifice) and Los Patos, the most distant of the villages at 8 km from the edifice.

On 19 May, public meetings were held that included INETER, civil defense, and national disaster response (SINAPRED) representatives. A widely discussed issue was how the heavy ashfall and volcanic gases were affecting water quality. Officials favored monitoring local wells within 5 km of Telica's edifice.By 20 May, Telica's explosions became infrequent: three were registered that day. Only ash and resulting plumes rose to 500-800m altitude. Microseismicity remained high (850 events) and 60 earthquakes (M 0.8-2.2) were located at depths of 0.7-2.2 km (table 2).

On 20 May, explosions were also infrequent but four large events from the crater emitted plumes with heights of 500-700 m. In addition, at 1500 on the 20th, one large, continuous explosion occurred that lasted 36 minutes. Observers described a plume containing gas, steam, tephra, and small rocks. The largest ballistics did not reach farther than the crater rim and lightning was observed in the plume. INETER noted that wind conditions allowed the plume to reach 2 km altitude. Nine hundred microseismic events were recorded on 21 May and 57 earthquakes (M 0.5-2.0) were located with average depth of 1 km.

Well monitoring on 21 May revealed slight changes in local water quality. Sites located NE and within 5 km of the summit showed elevated quantities of sulfates, chlorides, alkalinity and PH.

New SO₂ monitoring efforts. On 22 May, Universidad Tecnológica de Chalmers (Switzerland) and SNET (El Salvador) installed two portable Mini-DOAS stations (Differential Optical Absorption Spectrometer) at Los Angeles and Mendoza to measure SO₂ levels.. These stations were installed downwind from Telica, SW of the edifice (figure 26). While the fixed stations collected data, traverses were made across the plume with mobile Mini-DOAS.

Figure 26. Location map of SO2 monitoring stations, Los Angeles and Mendoza, located SW of Telica Volcano (green stars). The 22 May 2011, Mini-DOAS traverse is highlighted in red. Courtesy of INETER.

The first Mini-DOAS results from fixed stations were obtained on 24 May. Data from the Los Angeles and Mendoza monitoring stations showed that flow of SO₂ oscillated between 50 and 150 tons/day. Data collected from traverses below the plume on 23-27 May were processed by the Universidad Tecnológica de Chalmers (Switzerland). The traverse results compared well with the fixed stations: reported values ranged from 40 to 130 tons/day. Peak SO₂ values from the fixed stations appeared as follows: Mendoza station, 420 tons/day on (28 May); Los Angeles station, 194 tons/day (30 May).

May 2011 eruption declines. INETER reported that ash explosions became infrequent during 23-30 May. That said, a cluster of eight explosions occurred sequentially on 24 May and created plumes reaching 600 m above the crater. Microseismicity remained high throughout the rest of the month. During 23-24 May, the largest number of earthquakes were located. Approximately 100 earthquakes (M 0.7-2.5) occurred with hypocenters at depths of 1-15 km.

According to the information supplied by the Directiorate of Meteorology of INETER, on 24 May ash expelled from Telica drifted SE at 8-15 km/hour at altitudes of 1.5 km. In their 24 May report, INETER warned that eruptive conditions could continue during the remainder of the month. They recommended the authorities of the Institute Nicargüense of Aeronaútica Civil (INAC) to caution air traffic about persistent and dispersed volcanic ash.

SO₂ monitoring in June. On 3 June INETER conducted field investigations and measured SO₂ with Mini-DOAS and Mobile DOAS. There were eight successful on-land traverses below the plume, each covering 18 km. Mobile DOAS data indicated a decrease in SO₂: the maximum value recorded was 39 tons/day. SO₂ flux from the two fixed stations, Mendoza and Los Angeles, also showed reduced levels during the early part of the month but an increase appeared from both sites in 13-15 June. INETER suggested that the low SO₂ flux in early June may have been influenced by local wind patterns. Observers in the area noticed that the summit plume was very dispersed during this time. Wind velocities reported by NOAA were as low as 1.2 m/s on 3 June.

During a field visit by INETER on 14 June, the investigators managed to count 17 explosions that expelled ash and gas. The explosions occurred within short intervals of time, from two to three minutes and the longest interim was 10 minutes. The field team visited fumaroles S of the TEL4 seismic station and recorded temperatures from three fumaroles with values ranging from 64-76°C.

Field data collected on 30 June included a maximum temperature of 590°C from the crater (figure 27). The team observed incandescence within the crater and from a new vent near the NE wall. There were jetting and collapse sounds emitting from the crater.

Figure 27. Telica's crater temperatures measured January-October 2011. Note multiple measurements taken during the peak of activity in May and two measurements in September. The value measured in February was obtained with an infrared camera (Institute of Renewable Energy). Courtesy of INETER.

During June the number of earthquakes diminished but seismicity remained high. Approximately 500 earthquakes were registered per day. Approximately 100 earthquakes (up to M 2.8) were located (half the number located in May) at a maximum depth of 28 km (table 2). The majority of the events were volcanic-tectonic (VT) and doublet earthquakes (paired events). The dominant frequencies of the earthquakes shifted in June to 4.0-8.0 Hz.

Routine monitoring in July. During fieldwork on 12 July, INETER measured SO₂ flux with Mobile DOAS. Five traverses, each one 8.5 km in distance, were recorded. The average value of SO₂ was higher than the previous month, 484 tons/day. In their monthly report, INETER discussed the strong impact of inferred wind speed on their new gas measurements. During the month wind patterns were variable with speeds average ~ 5.8 m/s. They commented that the plume was noticeably less dispersed when they conducted the gas measurements.

On 22 July routine fieldwork was conducted at Telica. Residents of La Joya had heard loud jetting noises and at night saw incandescence at the summit. During the day, the team also heard jetting but did not see any explosive activity or feel earthquakes. Crater temperatures averaged 265°C (five measurements), very low compared to the previous month (figure 27). Temperatures taken from fumaroles S of the seismic station ranged from 68-72°C (three fumaroles).

Incandescence during August to October 2011. Halldor Geirsson noted that incandescence was seen in August 2011 (by Mel Rodgers and INETER staff). Further anomalous activity was not reported that month. A night visit took place on 9 September. The INETER team observed incandescence from the crater and measured temperatures with a thermal camera recording a maximum 458?C. No jetting sounds were heard.

On 13 September INETER measured SO₂ with Mobile DOAS. There were seven traverses along an 8.5-km stretch of road to cross below Telica's gas plume. SO₂ flux was significantly lower than the previous month with an average of 81 tons/day.

On 13 October, SO₂ traverses were attempted, but no gas was detected. Wind patterns had been disrupted by a low-pressure system that caused major flooding along Nicaragua's W coast.

During INETER's 27 October field visit, the team observed incandescence during the day and fragments of molten spatter were released during moderate gas explosions (figure 28). They also observed minor gas emissions and loud jetting persisted from the crater. INETER took five measurements of crater temperature, which averaged 280?C. Temperatures from three fumaroles S of seismic station TEL4 were 66-72°C. Vegetation was noticeably affected by volcanic gases; numerous dead plants were photographed during the 27 October visit.

Figure 28. Two photos both showing the same scene, centered on active vents within Telica's summit crater on 27 October 2011. Original photo (left); black and white copy (right). Gas explosions and spatter escaped from these vents during the field visit. Courtesy of INETER.

References. Bellos, A., Mulholland, K., O'Brien, K.L., Qazi, S.A., Gayer, M., Checchi, F., 2010, The burden of acute respiratory infections in crisis-affected populations: a systematic review: Conflict and Health, v. 4, no. 3.

Freundt, A., Kutterolf, S., Schmincke, H.-U., Hansteen, T., Wehrmann, H., Peréz, W., Strauch, W., Navarro, M., 2006, Volcanic hazards in Nicaragua: Past, present, and future, in Rose, W.I., Bluth, G.J.S., Carr, M.J., Ewert, J.W., Patino, L.C., and Vallance, J.W., eds., Volcanic hazards in Central America: Geological Society of America Special Paper 412, p. 141-165.

Malilay, J., Real, M.G., Vanegas, A.R., Noji, E., and Sinks, T., 1996, Public Health Surveillance after a Volcanic Eruption: Lessons from Cerro Negro, Nicaragua, 1992: Bulletin of Pan American Health Organization, v. 30, no. 3.

Witter, M.R., Geirsson, H., La Femina, P.C., Roman, D.C., Rodgers, M., Muñoz, A., Morales, A., Tenorio, V., Chavarria, D., Feineman, M.D., Furman, T., and Longley, A., 2011, May 2011 eruption of Telica Volcano, Nicaragua: Multidisciplinary observations, Abstract V53E-2670, Fall Meeting, AGU, San Francisco, California.

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

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Halldor Geirsson, The Pennsylvania State University, Department of Geosciences, 536 Deike Building, University Park, PA 16802, USA; La Prensa: (URL: http://www.laprensa.com.ni/2011/05/19/departamentos/60948); Mel Rodgers, University of South Florida, Department of Geology, 4202 East Fowler Ave., SCA528, Tampa, FL 33620, USA.

Following two M 3 earthquakes in the region on 13 December 2011, fishermen in Salif City, Yemen, reported an eruption in the Zubair island group that began as late as 18 December 2011. Moderate Resolution Imaging Spectroradiometer (MODIS) images of the area also first revealed a plume on 18 December, and this and later MODIS images fixed the vent's location at a spot in the N portion of Yemen's Jebel Zubair (Zubair Group, figure 1). A new island emerged in this vicinity and was large enough to resolve in satellite imagery by 23 December 2011. The latitude and longitude given in the title for Jebel Zubair (15.05°N, 42.18°E) indicate the largest island of the Zubair Group (figure 1); the new island emerged at approximately 15.158°N, 42.101°E.

Figure 1. Map and index map of the 10 islands of the Zubair Group (Yemen) with our indication of the site of the new eruption and its associated emergent island. The islands emerge in the southern Red Sea, dotting an elongate region of about 8 x 27 km. Islands represented by gray shading; other features identified by legend at left. Plotted earthquake epicenters are given in table 1. The cross section at bottom is along line A-B. Modified from Gass and others (1973); index map modified from MapsOf.net.

Initial reports. According to an article published in the Yemen Observer on 19 December, the fishermen who first reported the eruption stated that it was near Saba island (figure 1). They stated that they could see the eruption from 3 hours travel time away. The fishermen reported that the volcano had been "popping up red lava that reached 20-30 meters high." The same day, an EOS-AURA Ozone Monitoring Istrument (OMI) image showed an SO₂ cloud in the area (figure 2). According to Volcano Discovery, a reader from Yemen confirmed the reported eruption, and added that an earthquake was felt on 19 December. Two other seismic events in the Zubair Group were recorded by the Seismological and Volcanological Observatory Center (SVOC) on 13 December (table 1, figure 1).

Figure 2. An SO2 cloud over the area of the Zubair island group captured by the AURA satellite's OMI imager between 1023 and 1205 on 19 December 2011. Scale at right is in Dobson Units (DU). The mass of SO2 depicted on this image was 0.403 kilotons (kt); the area of the cloud was ~45,352 km2; the maximum SO2 values on the image occured at 15.28°N and 14.28°E and reached 1.4 DU. Courtesy of Simon Carn and NASA Global Sulfur Dioxide Monitoring Aura/OMI.

Table 1. Seismic events recorded in the Zubair Group in December 2011. Courtesy of the Seismological and Volcanological Observatory Center (SVOC).

On 20 December, the Toulouse Volcanic Ash Advisory Center (VAAC) reported a white plume that may have contained some unidentifiable ash (as reported that day from an aircraft in the area). Their report included a remark that the eruption seemed to be a continuing submarine eruption that began on 18 December. They stated that the plume was not identifiable in their satellite data. On 22 December, the Emirates News Agency published an article reporting that the head of SVOC stated that, based on preliminary data, there was no danger to marine navigation.

Small plumes were visible on MODIS imagery beginning on 18 December (figure 3). While cloud cover and dust plumes rendered the images speculative on a few days, others provided a clear view of the plumes, and highlighted their origin (figure 3f). The plumes did not appear to originate from one of the Zubair islands, but instead from just N of Rugged and ~ 1.5 km SW of Haycock islands (figure 4; also see "Eruption site" on figure 1). The lack of plumes prior to 18 December 2011, and the persistence of plumes after, indicates that the eruption began breaking the surface of the Red Sea sometime during 17-18 December.

Figure 3. Satellite images of the Zubair Group captured during 17-22 December 2011. Images b-f show small plumes (circled) emanating from ~1.5 km SW of Haycock and just N of Rugged. A dust plume somewhat obscures the visibility in (b) and clouds are present in (c) and (d). Pixel resolution in each image is 250 m. All images acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard NASA's Aqua satellite except (d), which was acquired by the MODIS instrument aboard NASA's Terra satellite. Images courtesy of NASA's Land Atmosphere Near Real-time Capability for EOS (LANCE).

Figure 4. NASA Earth Observatory images captured by the Advanced Land Imager (ALI) aboard NASA's Earth Observing-1 (EO-1) satellite on (a) 24 October 2007 and (b) 23 December 2011. The 23 December 2011 image shows that an apparent new island is the eruption site, less than 1 km to the N of Rugged Island. An eruptive plume is seen rising and drifting to the N. Courtesy of Jesse Allen and Michon Scott, NASA Earth Observatory.

New island. Finally, following approximately a week of widespread speculation on the exact location of the plume's source, NASA Earth Observatory published a high resolution satellite image of a new eruption (acquired 23 December 2011), clearly showing the off-island source of the eruptive plumes (figure 4b). From comparison with an image acquired on 24 October 2007, the 23 December 2011 image clearly shows that the eruption site was less than 1 km due N of Rugged Island, and was an apparent new island (figure 4). Their report stated that "The image . . . shows an apparent island where there had previously been an unbroken water surface." As of 28 December 2011, all information seems to point to the formation of a new, as yet unnamed, island in the Red Sea.

Reference. Gass, I.G., Mallick, D.I.J., and Cox, K.G., 1973, Volcanic islands of the Red Sea, Journal of the Geological Society of London, v. 129, p. 275-310 (DOI: 10.1144/gs

Geologic Background. The 5-km-long Jebel Zubair Island is the largest of a group of small islands and submerged shoals that rise from a shallow platform in the Red Sea rift. The platform and eruptive vents forming the islands and shoals are oriented NNW-SSE, parallel to the rift. An early explosive phase was followed by a brief period of marine erosion, then by renewed explosive activity accompanied by the extrusion of basaltic pahoehoe lava flows. This latest phase of activity occurred on the morphologically youngest islands of Zubair, Centre Peak, Saba, and Haycock. Historical explosive activity was reported from Saddle Island in the 19th century. Spatter cones and pyroclastic cones were erupted along fissures that form the low spine of Zubair Island. Eruptions that began in late 2011 built two new islands, increasing the total number in the group to 12.

Information Contacts: MapsOf.net (URL: http://mapsof.net/); The Yemen Observer, P.O. Box 19183, Sana'a, Rep. of Yemen (URL: http://www.yobserver.com/); Seismological and Volcanological Observatory Center (SVOC), P.O. Box 87175, Dhamar, Yemen (URL: http://www.nsoc.org.ye/); Simon Carn, NASA Global Sulfur Dioxide Monitoring, Aura/OMI (URL: https://so2.gsfc.nasa.gov/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo France, 42 Avenue Gaspard Coriolis, 31057 Toulouse Cedex 1, France (URL: http://www.meteo.fr/vaac/eindex.html); Emirates News Agency, 3790 Abu Dhabi, United Arab Emirates (URL: http://wam.org.ae/); NASA's Land Atmosphere Near Real-time Capability for EOS (LANCE) (URL: http://lance.nasa.gov/); NASA Earth Observatory (Jesse Allen and Michon Scott), NASA Goddard Space Flight Center (URL: http://earthobservatory.nasa.gov/).