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/).

A large explosive and effusive eruption lasting about 11 months during 1963-64 at Indonesia's Mount Agung on Bali produced voluminous ashfall, devastating pyroclastic flows that caused extensive damage, and over 1,000 fatalities. The volcano remained largely quiet until renewed seismicity began in August 2017, the prelude to a new eruptive episode, which started in late November 2017 and is ongoing. Self and Rampino (2012) and Fontijn et al. (2015) published detailed summaries of historical activity at Agung prior to this new episode; a brief summary of their work is provided.

Information about the new eruptive episode comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Indonesian Center for Volcanology and Geological Hazard Mitigation (CVGHM), Badan Nasional Penanggulangan Bencana (BNPB) which is the National Board for Disaster Management, the Darwin Volcanic Ash Advisory Center (VAAC), and various sources of satellite data. The first two months of this new episode, through December 2017, are discussed in this report.

Summary of 1963-64 eruption. The February 1963 to January 1964 eruption, Indonesia's largest and most devastating eruption of the twentieth century, was a multi-phase explosive and effusive event that produced both basaltic andesite tephra and andesite lava (Self and Rampino, 2012). After a few days of felt earthquakes on 16 and 17 February 1963, explosive activity began at the summit on 18 February. This was followed the next day by the effusion of about 0.1 km³ of andesite lava which was extruded until 17 March 1963, when a large explosive eruption generated pyroclastic density currents (PDCs) and lahars that devastated wide areas N, SW, and SE of the volcano (figure 1) (Fontijn et al, 2015).

Figure 1. Map of Gunung Agung and vicinity, eastern Bali, showing the extent of the 1963 lava flow (cross-hatched), pyroclastic flow deposits (stippled), and lahar deposits (dark shading) of the 1963–1964 eruption (after unpublished map courtesy of Indonesian Volcanological Survey). Sg is Siligading village, where many fatalities occurred. Reproduced from Self and Rampino (2012, figure 3).

Explosive activity continued intermittently until a second explosive phase of similar intensity occurred two months later, beginning on 16 May 1963 with reported ash plumes reaching 10 km above the 3-km-high summit (figure 2). This phase produced the greatest proportion of the pyroclastic flow material from the eruption and led to additional death and destruction in villages at the foot of the volcano (Self and Rampino, 2012). Explosive outbursts continued intermittently until 17 January 1964. The total death toll of the eruption was estimated between 1,100 and 1,900 (see references in Fontijn et al., 2015). A total estimated volume of erupted magma was ca 0.4 km³ (Self and Rampino, 2012).

Figure 2. Photograph reported to be of the 16 May 1963 eruption column at Agung; the view is from the SW, perhaps near Rendang (shown on figure 1). Photo courtesy of the family of Denis Mathews, reproduced from Self and Rampino (2012, figure 2b).

Activity between 1964 and 2017. Almost no activity was reported from Agung during 1964-2017. Weak solfataric activity from within the summit crater was reported in 1989 (SEAN 14:07). MODVOLC thermal alerts were reported intermittently on one or two days during a few years (2001, 2002, 2004, 2006, 2008, 2012, 2013), but all of the alerts were located on the middle or lower flanks, suggesting their source was agriculture or forest fires, unrelated to volcanic activity. Chaussard et al. (2013) reported inflation centered on the summit at a rate of 7.8 cm/year between mid-2007 and early 2009, followed by slow deflation at a rate of 1.9 cm/year until mid-2011 (the last acquired data).

Summary of September-December 2017 Activity. Increases in seismic activity were first noted at Agung during mid-August 2017. Exponential increases in the rate of events during the middle of September led PVMBG to incrementally raise the Alert Level from I to IV (lowest to highest) between 14 and 22 September. Steam-and-gas emissions were intermittently observed 50-500 m above the summit crater from the end of September through October, with occasional bursts as high as 1,500 m. Seismicity dropped off almost as quickly as it rose, beginning on 20 October, and then continued a more gradual decrease through the end of the month and into November. The number and intensity of hot spots observed within the summit crater increased during September, then leveled off during October.

Ash emissions first appeared on 21 November, rising to 700 m above the summit. Ash density and heights of plumes increased several times during the rest of November to about 3,000 m. Ashfall as deep as 5 mm affected neighboring communities, and was reported several hundred kilometers from the summit; the international airport about 60 km SW was forced to close for a few days at the end of the month. Thermal data indicated effusion of lava into the summit crater at the end of November. After 30 November, emissions continued, primarily comprised of steam and gas, with intermittent plumes of dense ash, rising up to 2.5 km above the summit throughout December.

Activity during August-September 2017. In their monthly report of volcanic activity for August 2017, PVMBG noted that 49 volcanoes, including Agung, were listed at Alert Level 1, meaning "Normal", with no apparent increases in visual or seismic activity. The first signs of renewed unrest at Agung appeared as an increase in the rate of deep volcanic earthquakes (VA or Vulkanik Dalam) beginning on 10 August 2017. Shallow volcanic earthquakes (VB or Vulkanik Dangkal) began to increase two weeks later on 24 August, followed by an increase in the number of local tectonic earthquakes on 26 August (figure 3). Based on this increased seismicity, and an observation on 13 September of new solfataric activity at the bottom of the summit crater, PVMBG raised the Alert Level the following day from Level I (Normal) to Level II (Beware); the Aviation Color Code was raised to Yellow on a four-color scale (Green, Yellow, Orange, Red). The deeper earthquakes (VA) had a seismic amplitude range from 3-10 mm. The shallow earthquakes (VB) had an amplitude range of 2-7 mm. Otherwise, there was no surface expression of activity during September.

Figure 3. Seismic activity at Agung between 1 July and 13 September 2017. The Y-axis is the number of daily earthquakes. The increase in deep volcanic seismicity (VA, or Vulkanik Dalam) that began on 10 August 2017 was followed two weeks later by an increase in shallow volcanic seismicity (Vulkanik Dangkal or VB). Courtesy of PVMBG (Peningkatan Tingkat Aktivitas Gunung Agung, 14 September 2017).

The Agung Volcano Observatory (AVO) is located in Rendang village about 8 km SW. Webcams are located in Rendang and in Bukit Asah, about 8 km W. On 15 September 2017 a steam emission was observed rising 50 m above the crater rim. The AVO issued a VONA on 18 September noting a rapid increase in volcanic earthquake activity with a small hot spot detected in satellite data. This contributed to them raising the Alert Level again to Level III (Standby), resulting in a 6-km-radius exclusion zone activated around the summit, extending to 7.5 km on the N, SE, and SSW flanks where the pyroclastic flows of 1963 had caused the most damage. Many of the 50,000 village residents within the 6 km exclusion zone began voluntary evacuations. The communities affected included Jungutan (7 km S) and Buana Giri (12 km SE) villages in the Bebandem District, Sebudi Village (6 km SW) in the Selat Subdistrict, Besakih Village (12 km SW) in the Rendang Subdistrict, and Dukuh (4 km NE) and Ban (7.5 km NW) villages in the Kubu Subdistrict. About 9,500 people had voluntarily evacuated from the villages by 22 September 2017.

The observatory issued another VONA on 19 September 2017, reporting an 'ash cloud' at 0255 UTC (1055 Central Indonesia Time, or WITA). It was described as a dense, white plume moving to the W. Around the same time (0240 UTC) MODVOLC recorded ten thermal alerts on the N and E flanks. Bali's Regional Disaster Management Agency (BPBD) reported in Antara News on 19 September that the source of the smoke and ash were forest fires caused by excessively dry conditions.

A VONA issued by AVO in the morning of 22 September stated that a steam emission about 50 m above the summit drifted NW. During the evening of 22 September, PVMBG raised the Alert Level to Level IV (Caution), the highest of the four-level scale, based primarily on continuing increases in seismicity. They expanded the exclusion zone to 9 km around the summit, and to 12 km in the areas S, SE, and NNE. The number of evacuees had risen to nearly 35,000 people by 24 September. Steam-and-gas plumes were intermittently observed rising to 200 m above the crater rim during the rest of September. By 26 September, PVMBG reported increasing seismic activity with 579 deep volcanic (VA) quakes, 373 shallow quakes (VB), and 50 local tectonic events that day. Seismicity continued to escalate through the end of the month. By the end of September, the government was assisting with the logistics of evacuating tens of thousands of livestock, primarily cattle, as well as over 90,000 people from within and around the 9 km exclusion zone. MAGMA Indonesia reported that new steaming and thermal areas within the summit crater expanded during the last week of the month.

Activity during October 2017. Narrow steam plumes rose 50-200 m above the summit crater during the first half of October. The rate of earthquakes during the last week of September and the first week of October continued to fluctuate at high levels, averaging 1-3 per minute, and more than 600 per day. By the first week of October, shallow earthquakes alone had increased to more than 200 per day, suggesting the possibility of magmatic activity at shallow depth. Satellite data showed increasing steam emissions along the NE edge of the crater rim. Tiltmeter data showed sudden deflation on 1 October, followed by continued inflation through 5 October. AVO released a VONA on 7 October noting a steam plume rising 1,500 m above the summit crater at 1245 UTC and drifting E (figure 4).

Figure 4. A steam plume rose 1,500 m above the summit of Agung on 7 October 2017. Courtesy of PVMBG (Penurunan Status Gunungapi Agung, Bali Dari Level IV (awas) Ke Level III (siaga) Tanggal 29 Oktober 2017 Pukul 16.00 WITA).

During the second half of the month, steam plumes were denser and rose more frequently to 200-500 m above the summit crater. BNPB flew drones over the summit on 20 and 29 October 2017 and captured 400 aerial photographs (figures 5 and 6). The images revealed a widening of the fracture zone on the E side of the summit crater, and a new fracture on the SE side.

Figure 5. A view into the summit crater of Agung on 20 October 2017, taken by a BNPB drone. Steam fumaroles rose from the NNE flank. N is to the left. Courtesy of PVMBG (Penurunan Status Gunungapi Agung, Bali Dari Level IV (awas) Ke Level III (siaga) Tanggal 29 Oktober 2017 Pukul 16.00 WITA).

Figure 6. A view into the summit crater of Agung on 29 October 2017, taken by a BNPB drone. The steam plumes rose from the NE corner of the summit crater. The NE rim of the crater slopes away to the upper left. Courtesy of PVMBG (Penurunan Status Gunungapi Agung, Bali Dari Level IV (awas) Ke Level III (siaga) Tanggal 29 Oktober 2017 Pukul 16.00 WITA).

PVMBG noted a decline in seismicity beginning on 20 October 2017 which continued through the end of the month (figure 7), leading them to lower the Alert Level from IV to III on 29 October, and reduce the exclusion zone to a 6 km radius, plus a 7.5 km area in the NNE, SE and SSW sectors. In their late October report, they observed that remote sensing thermal infrared data had detected an increase in the thermal energy beginning on 10 July 2017, in the form of an increased number of hot spots within the summit crater. During August and September, the number of hot spots had increased significantly and correlated with the increases in seismicity (figure 8). The intensity of the thermal anomalies then decreased during October. Inflation resumed in mid-August and peaked in mid-September. After that, the GPS data indicated deflation at lower levels, but uplift of 6 cm occurred near the summit. The deformation rate slowed after 20 October.

Figure 7. Daily seismic activity at Agung from 27 July-29 October 2017. Seismicity decreased noticeably on 20 October 2017, leading PVMBG to lower the Alert Level from IV to III on 29 October. Note that the vertical axis counting the number of daily seismic events ranges from 0 to 1,200, while in figure 3 the same axis ranges from 0 to 14. Courtesy of MAGMA Indonesia (Penurunan Status Gunungapi Agung, Bali dari Level IV (AWAS) ke Level III (SIAGA) Tanggal 29 Oktober 2017 pukul 16.00 WITA).

Figure 8. Satellite thermal imagery from Citra-Sentinel 2 revealed an increase in the number and intensity of hotspots within the summit caldera of Agung during September 2017, followed by a decrease in early October. Courtesy of PVMBG (Penurunan Status Gunungapi Agung, Bali Dari Level IV (awas) Ke Level III (siaga) Tanggal 29 Oktober 2017 Pukul 16.00 WITA).

Activity during November 2017. For the first three weeks of November, dense white steam plumes rose 50-500 m above the summit crater. A VONA issued late on 11 November reported a 500-m-high steam plume. Seismicity continued at a much lower rate than during late September-October, with tens of daily events as opposed to hundreds.

The first ash emission of the current eruption occurred on 21 November at 1705 local time; the plume rose to 700 m and drifted ESE (figure 9). Trace amounts of ashfall were reported in the Pidpid-Nawehkerti area about 9 km SE. At the time of the first ash emission, BNPB reported the number of evacuees living in temporary housing at about 25,000. The emission was preceded by a low-frequency tremor. Multiple volcanic ash advisories were issued by the Darwin VAAC on 21 November, although the ash was not visible in satellite imagery due to weather clouds. Continuous tremor with 2-5 mm amplitude was recorded the following three days, and ash-and-steam emissions rose 300-800 m above the summit crater.

Figure 9. The first reported ash emission from Agung in 53 years rose 700 m and drifted SE on 21 November 2017. Courtesy of PVMBG (Letusan Gunung Agung Selasa, 21 November 2017 Pukul 17.05 WITA).

A larger emission on 25 November sent black-gray ash plumes 2,000 m above the crater rim (figure 10) which then drifted W. The Darwin VAAC reported an ash plume visible in satellite imagery at 7.6 km altitude drifting WSW. Emissions continued later in the day, rising 4.6-6.7 km altitude and extending SE. Bright incandescence at the summit crater was observed that night. Ashfall was reported to the WSW in the villages of Menanga and Rendang (12 km SW) at the AVO Post, and also in Besakih Village, located in the upper part of Pempatan (8 km W). A number of international flights were cancelled from the I Gusti Ngurah Rai International Airport in Denpasar (60 km SW), affecting about 2,000 passengers, although the airport remained open.

Figure 10. An ash emission rose at least 1,500 m above the summit of Agung on 25 November 2017 and drifted W. Courtesy of PVMBG (Letusan Gunung Agung 25 November 2017 Pukul 17:30 Wita).

Around 0545 local time the following day (26 November), the intensity of the ash emissions increased; the top of the plume reached 3,300 m above the summit at 1100 local time, and was drifting SE and E (figure 11). Ashfall was reported in many areas downwind including North Duda (9 km S), Duda Timur (12 km S), Pempetan, Besakih, Sideman (15 km SSW), Tirta Abang, Sebudi (6 km SW), Amerta Bhuana (10 km SSW), and some villages in Gianyar (20 km WSW) (figure 12). The largest amount, deposits 5 mm thick, was reported in Sibetan (11 km SSE). Trace amounts of ash were also reported much farther away, in Nusa Penida (an island 40 km S), Lombok (100 km ESE), and Sumbawa, 250 km E on the island of West Nusa Tenggara. Explosions from the crater were audible 12.5 km away that evening. Incandescence at the summit was observed from Bukit Asah and Batulompeh. The Darwin VAAC reported continuous ash emissions to 7.9 km altitude drifting SE throughout most the day, increasing to 9.1 km later in the day; ashfall was also reported at the international airport.

Figure 11. A dense plume of ash rose 3,000 m above the summit of Agung and drifted ESE on 26 November 2017. Courtesy of PVMBG (Peningkatan Status Gunungapi Agung, Bali Dari Level III (siaga) Ke Level IV (awas), 27 November 2017).

Figure 12. Ash from an eruption of Agung on 26 November 2017 covered garden plants in Jungutan Village, 7 km SE. Courtesy of Reuters.

The airport in Denpasar was forced to close during 27-29 November 2017. On those days ash drifted in multiple directions at different altitudes; it was observed drifting E at 9.1 km altitude, SW at 7.6 km altitude, and was moving S below 6.1 km. This increase in emissions led PVMBG to raise the Alert Level from III to IV on 27 November. Pictures and video showed a white steam plume adjacent to a gray ash plume rising from the crater, suggesting two distinct sources (figure 13).

Figure 13. A white steam plume and dense gray ash both rose from the summit of Agung on 27 November 2017. Photo by K. Parwata, courtesy of Sutopo Purwo Nugroho, Twitter.

A single MODVOLC thermal alert appeared at the summit that day, along with a strong thermal anomaly in the MIROVA system data (figure 14) consistent with the appearance of new lava in the summit crater. The tiltmeter installed at the Yehkori station 4 km S of the summit showed continued inflation of up to 6 microradians between 22 and 27 November (figure 15). PVMBG also increased the exclusion zone to a radius of 8 km from the summit crater plus areas 10 km from the summit to the NNE, SE, S, and SW.

Figure 14. A MIROVA plot of satellite infrared data for the year ending 23 February 2018 showed the first thermal anomaly from Agung in late November 2017, consistent with the emergence of lava in the summit crater. Courtesy of MIROVA.

Figure 15. A steady inflation was measured by the tiltmeter located at the Yehkori station 4 km S of the summit of Agung between 22 November and 27 November. Courtesy of PVMBG (Peningkatan Status Gunungapi Agung, Bali Dari Level III (siaga) Ke Level IV (awas), 27 November 2017).

MAGMA Indonesia reported that beginning with the ash eruption on 21 November, lahars appeared in the Tukad Yehsa, Tukad Sabuh, and Tukad Beliaung drainages on the S flank, as well as Tukad Bara on the N flank. As of the end of November 2017, these lahars had impacted houses, roads, and agricultural areas. Although ash emissions increased, and lava was confirmed within the summit crater during the last week of November, the number of seismic events remained well below the values recorded during September and October (figure 16).

Figure 16. Seismicity at Agung decreased significantly beginning on 20 October 2017 and remained well below 200 daily events throughout November, even though ash emissions began on 21 November. Courtesy of PVMBG (Peningkatan Status Gunungapi Agung, Bali Dari Level III (siaga) Ke Level IV (awas), 27 November 2017).

Ash emissions were reported by PVMBG rising to 3,000 m above the summit and drifting S on 27 November (figure 17). Continuing ash emission during 28-29 November rose to 2,000-4,000 m above the summit and drifted WSW (figure 18). Continuous seismic tremors were recorded during 28 November-1 December.

Figure 17. Ash plumes from Agung rose to altitudes of around 6,000 m (3,000 m above the summit crater) and drifted S on 27 November 2017. Image courtesy of MAGMA Indonesia (Peningkatan Status Gunungapi Agung, Bali Dari Level Ill (SIAGA) ke Level IV (AWAS), 27 November 2017 10:07 WIB, Ir. Kasbani, M.Sc.).

Figure 18. A dense plume of steam and ash rose from Agung and drifted away from this villager and his livestock on 28 November 2017. Courtesy of CNN.

With the increase in ash emissions during the last days of November 2017, satellite instruments also recorded significant releases of SO₂ (figure 19). MAGMA Indonesia reported on 1 December that satellite data also recorded high temperatures consistent with new lava within the crater on 27, 28, and 29 November 2017. They estimated the volume of lava in the crater to be about 20 million cubic meters, equivalent to about a third of the total crater volume. The base of the ash-and-steam plumes was often reddish during 29 November-5 December reflecting incandescence from the lava in the crater (figure 20).

Figure 19. The concentrations of SO2 emitting from Agung increased to levels that were easily detected by the Ozone Mapper Profiler Suite (OMPS) on the Suomi National Polar-orbiting Partnership (Suomi-NPP) satellite on 27 (top) and 28 (bottom) November 2017. The concentration of SO2 is measured in Dobson Units, a measure of the molecular density of the SO2 in the atmosphere. These NASA Earth Observatory images were created by Joshua Stevens, using OMPS data from the Goddard Earth Sciences Data and Information Services Center (GES DISC).

Figure 20. Incandescence appeared at the base of the ash-and-steam plume at Agung on 29 November 2017, consistent with lava effusion in the summit crater. Courtesy of MAGMA Indonesia (Perkembangan Terkini Aktivitas Gunung Agung (1 Desember 2017 21:00 WITA), 2 December 2017 07:55 WIB, Ir. Kasbani, M.Sc.).

By 29 November, continuous ash emissions were rising to 6.4 km altitude and drifting from the SW towards the S, becoming diffuse over the Denpasar region (figure 21). The plume was observed moving E at the same elevation on 30 November, lowering to 5.5 km later in the day. Although emissions were primarily steam and gas beginning on 30 November, pilot reports on 1 December indicated ash was still visible SE of Agung, and steam-and-ash emissions were continuing. Steam-only emissions were reported on 2 December rising less than 1,000 m above the summit.

Figure 21. Gas-and-ash emissions from Agung on 29 November 2017 were drifting both W and S in this false-color image generated by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. The image uses a combination of shortwave infrared light and natural color, making it easier to differentiate between ash, clouds, and forest. The plumes appear to rise from two vents in the volcano's summit crater. Courtesy of NASA Earth Observatory.

Activity during December 2017. Steam, gas, and ash emissions continued throughout December 2017. During the first two weeks, emissions were primarily steam and gas, rising up to 2,000 m (figure 22), and incandescence was often observed at the summit. Dense gray ash emissions were observed, however, during 1-2 December. BNPB noted on 5 December that 63,885 evacuees were distributed in 225 evacuation shelters. On 8 December at 0759 a brief event generated a dense ash plume that rose 2.1 km above the crater rim and drifted W (figure 23). Minor amounts of ash were deposited on the flanks, and lapilli were reported in Temakung. A second ash plume rose 3 km at 1457 later that day.

Figure 22. A burst of dense steam rose as high as 1,500 m from the crater of Agung on 5 December 2017 at 0848 local time (WITA) and drifted E, after which only a narrow diffuse plume remained. View is from the S. Courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 23. An eruption at Agung on 8 December 2017 at 0759 WITA sent a dense gray ash plume 2,100 m above peak to the W. View is from the S. Courtesy of Sutopo Purwo Nugroho, Twitter.

The Darwin VAAC reported multiple daily explosions during 8-15 December, creating ash plumes that drifted NW, W, and WSW at altitudes between 4.3 and 5.5 km. The explosions were visible in the webcams and from ground-based observers, and occasionally in satellite imagery when not blocked by weather clouds. VONA's were issued for events on 8 and 12 December. Multiple events during 11-12 December sent plumes rising up to 2.5 km above the crater rim and drifting NW and W (figure 24).

Figure 24. A small ash emission rose from the crater of Agung during the early morning of 11 December 2017. Courtesy of Sutopo Purwo Nugroho, Twitter.

The Darwin VAAC reported larger ash emissions to 7.6 km altitude on 15 and 16 December interspersed with lower altitude (5.5-6.1 km) plumes. Continuing, regular discrete emissions during 16-17 December rose to 6.1 km and drifted WNW. An overhead image of the summit crater of 16 December revealed that, since a similar photo was taken on 20 October, new lava had filled about 1/3 of the crater with an estimated 30 million cubic meters of material (figure 25).

Figure 25. Repeated overhead images of the Agung summit crater taken on 20 October and 16 December 2017 showed new lava filling about 1/3 of the crater with an estimated 30 million cubic meters of material. Posted on Twitter by Sutopo Purwo Nugroho for BNBP.

Discrete emissions to 5.5 km moving N and NNE were common during 18-21 December. Ash and steam drifted both E and W from the summit on 22 December. An ash emission on 23 December rose to 5.8 km and drifted NE, after which repeated emissions continued, rising to 4.6 km (figure 26). Ash fell on the flanks and in Tulamben, Kubu (9 km NE). In the morning of 24 December, a much larger plume drifting W at 10.7 km altitude was visible in satellite imagery. It dissipated after a few hours, and a separate plume was observed drifting NE at 5.5-5.8 km (figure 27); emissions continued throughout the day and into the next. PVMBG reported that the ash deposits from the NE-drifting plume were up to 3 mm thick (figure 28).

Figure 26. An event at Agung on 23 December 2017 sent a dense, gray plume to 2,500 m above the crater rim at 1157 WITA. View is from a village on the W flank, likely about 7 km from the summit. Courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 27. Agung erupted steam and ash with a plume height of 2,000-2,500 m on 24 December 2017 at 1005 WITA. Courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 28. Map showing distribution and thickness of volcanic ash and lapilli from the ash emissions at Agung that began on 24 December 2017 at 1005 WITA. A thin layer of ash was deposited in a narrow NE trending band on the NE side of Agung. Courtesy of Sutopo Purwo Nugroho, Twitter.

As of 25 December, BNPB reported just over 70,000 evacuees spread out in 239 shelters. Discrete ash emissions continued through the end of the month rising as high as 2 km above the crater rim and drifting in several different directions. The last VAAC report of 2017 indicated an ash plume drifting W at 4.3 km altitude on 31 December.

References: Chaussard E, Amelung F, Aoki Y, 2013, Characterization of open and closed volcanic systems in Indonesia and Mexico using InSAR time series. J Geophys Res Solid Earth, 118:3957–3969. DOI: 10.1002/jgrb.50288.

Fontijn K, Costa F, Sutawidjaja I, Newhall C G, Herrin J S, 2015, A 5000-year record of multiple highly explosive mafic eruptions from Gunung Agung (Bali, Indonesia): implications for eruption frequency and volcanic hazards. Bull Volcanol, 77: 59. DOI: 10.1007/s00445-015-0943-x.

Self S, Rampino M, 2012, The 1963–1964 eruption of Agung volcano (Bali, Indonesia). Bull Volcanol 74:1521–1536. DOI: 10.1007/s00445-012-0615-z.

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: 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/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.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/); Antara News (URL: https://bali.antaranews.com); Sutopo Purwo Nugroho, Head of Information Data and Public Relations Center of BNPB via Twitter (URL: https://twitter.com/Sutopo_PN); Cable News Network (CNN), Turner Broadcasting System, Inc. (URL: http://www.cnn.com/); Reuters (URL: http://www.reuters.com/).

An eruption at Bezymianny continued into April 2017 with ash plumes and lava flows (BGVN 42:06). Similar activity was reported from May through December 2017. Observations came from reports from the Kamchatka Volcanic Eruptions Response Team (KVERT) and Tokyo Volcanic Ash Advisory Center (VAAC) advisories.

KVERT reported on 26 May that activity had decreased after an explosion on 9 March and the effusion of several lava flows onto the dome flanks. Though gas-and-steam emissions continued, along with thermal anomalies identified in satellite images. The Aviation Color Code (ACC) was lowered to Yellow (the second lowest level on a four-color scale). Moderate gas-and-steam emissions continued throughout the reporting period.

On 15 June KVERT reported that the temperature of a thermal anomaly identified in satellite images had increased, and that the webcam recorded a gas-and-steam plume rising to an altitude of 4 km and drifting SSE. Hot avalanches of material originated from the lava dome. The next day, 16 June, a powerful explosion began at 1653 (local) that produced an ash cloud that rose to an altitude as high as 12 km and drifted 700 km E and SE. Nighttime incandescence from the lava dome was observed afterwards, and a lava flow emerged from the W flank of the dome. The ACC was raised to Red (the highest level on a four-color scale), but lowered back to Orange (the second highest level) about 5 hours later. At 2110 (local) the ash cloud was 212 x 115 km in size and drifting E; the leading edge of the cloud was about 245 km E. Strong gas-and-steam emissions and incandescence above the lava dome could be seen on 18 June (figure 23).

Figure 23. Photo of Bezymianny on 18 June 2017 showing the plume from a strong gas-and-steam emission, along with incandescence over the lava dome. Courtesy of A. Belousov, IVS FEB RAS.

During 20 June-29 September a daily thermal anomaly over Bezymianny was identified by KVERT in satellite images, when not obscured by clouds. A lava flow continued down the W flank of the dome, and incandescence from the dome was usually visible at night. Moderate gas-and-steam activity continued.

According to KVERT, by the first week of October the volcano had quieted somewhat, although moderate gas-steam activity continued. KVERT reported that a lava flow continued down the W flank of the lava dome through 4 October, but no mention was made of a lava flow in their reports after 4 October. Weak daily thermal anomalies were recorded when the volcano was not obscured by clouds. On 5 October, the ACC was lowered to Yellow.

On 18 December hot avalanches on the SE flank of the lava dome were recorded by a webcam, prompting KVERT to raise the ACC to Orange. A strong explosion that started at 1555 (local) on 20 December generated ash plumes that rose to an altitude of 10-15 km, prompting KVERT to raise the ACC to Red. Ash plumes identified in satellite data drifted at least 320 km NE. Later that day satellite images indicated decreased activity; the ACC was lowered back to Orange. Moderate gas-and-steam emissions continued on 29 December, and a lava flow likely effused onto the N flank of the lava dome. Thermal anomalies continued to be identified in satellite images. The ACC was lowered to Yellow.

Thermal anomalies. During May-December 2017 thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were only observed during a small portion of June and July 2017 (most days between 19-26 June, most days during the first week of July, 17-18 July, and 28 July). In contrast, the MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected numerous hotspots every month, with the most intense cluster during the middle of June through the middle of September. Virtually all MIROVA hotspots were within 5 km of the summit.

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

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences (KB GS RAS) (URL: http://www.emsd.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

Recent activity at Copahue through December 2016 consisted of gas and steam plumes with minor amounts of ash. Eruptive activity ended in late December 2016, but ash emissions began again in early June 2017. Distinct ash emissions decreased after July, and crater incandescence was no longer reported. However, persistent tremor and degassing with sporadic ash continued through 2017.

This report through December 2017 is based on information obtained from the Buenos Aires Volcanic Ash Advisory Center (VAAC), the Southern Andes Volcanological Observatory (OVDAS), and the Servicio Nacional de Geología y Minería (National Geology and Mining Service) (SERNAGEOMIN). Volcano Alert Levels are set by SERNAGEOMIN (on a four-color scale) and by the Chilean Oficina Nacional de Emergencia del Ministerio del Interior (National Office of Emergency of the Interior Ministry) (ONEMI), on a three-color scale), for alerts to individual communities in the region.

OVDAS-SERNAGEOMIN reported that webcams recorded an increase in ash emissions on 4 June 2017. There were no significant changes in the magnitude or number of earthquakes recorded by the seismic network. The report noted that due to inclement weather making visual observations difficult, the observatory did not know if the ash emission began in the early hours of 4 June, or the day before. On the same day, OVDAS-SERNAGEOMIN raised the Alert Level to Yellow; ONEMI set a Yellow Alert for the communities of Villarrica, Pucón, and Curarrehue in La Araucanía, and for Panguipulli in Los Ríos.

During 5-15 June 2017 the seismic network detected long-period earthquakes. Gas plumes constantly rose from El Agrio crater and on several days contained ash. The highest plume, detected on 5 June, rose 300 m and drifted E.

The Buenos Aires VAAC reported that on 1 July the webcam recorded a steam-and-gas plume with minor ash near the summit. Webcam and satellite images analyzed by the Buenos Aires VAAC showed that during 7-8 July steam plumes with minor amounts of ash rose to altitudes of 4-4.3 km altitude and drifted ESE. During 16-17 July similar plumes rose to altitudes of 3-3.4 km and drifted N and NW. According to ONEMI, OVDAS-SERNAGEOMIN reported that during 16-31 July surficial activity had decreased. The webcam recorded constant gas emissions with sporadic ash rising no more than 280 m from El Agrio crater. Crater incandescence was visible during clear weather. The Alert Level remained at Yellow, and SERNAGEOMIN recommended no entry closer than 1 km of the crater. ONEMI continued an Alert Level of Yellow for the municipality of Alto Biobío.

In August, activity continued to decrease. Degassing was constant and sometimes contained ash. Plumes did not exceed 500 m in height and incandescence was absent. During the first half of the month, 23 seismic events occurred, 20 of which were volcanic-tectonic; tremor associated with the degassing was constant. During the latter half of August, SERNAGEOMIN lowered the Alert Level to Green. Because gas emissions continued, SERNAGEOMIN suggested that the public stay beyond a radius of 500 m of the active crater.

SERNAGEOMIN reports for November and December indicated that some seismic activity continued. In November, 337 earthquakes occurred, 261 of which were volcanic-tectonic. Tremor associated with degassing continued, and incandescence was reported on some days. Based on satellite and webcam views, the Buenos Aires VAAC reported that during 21 and 24-27 November diffuse steam plumes containing minor amounts of ash rose and drifted E and NE. Plumes rose to altitudes of 3.3-3.6 km during 25-26 November.

On 2 December, one volcanic-tectonic earthquake occurred at 1758 local time. More than 20 volcanic-tectonic earthquakes occurred about 2245 on 5 December. The SERNAGEOMIN report for December noted persistent tremor associated with gas and ash emissions, and that constant gas plumes with sporadic ash rising to a maximum height of 1,300 m above the summit was recorded by the web camera. The Alert Level remained Green through December 2017.

Geologic Background. Volcán Copahue is an elongated composite cone constructed along the Chile-Argentina border within the 6.5 x 8.5 km wide Trapa-Trapa caldera that formed between 0.6 and 0.4 million years ago near the NW margin of the 20 x 15 km Pliocene Caviahue (Del Agrio) caldera. The eastern summit crater, part of a 2-km-long, ENE-WSW line of nine craters, contains a briny, acidic 300-m-wide crater lake (also referred to as El Agrio or Del Agrio) and displays intense fumarolic activity. Acidic hot springs occur below the eastern outlet of the crater lake, contributing to the acidity of the Río Agrio, and another geothermal zone is located within Caviahue caldera about 7 km NE of the summit. Infrequent mild-to-moderate explosive eruptions have been recorded since the 18th century. Twentieth-century eruptions from the crater lake have ejected pyroclastic rocks and chilled liquid sulfur fragments.

Information Contacts: Servicio Nacional de Geología y Minería, (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), 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/); 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/).

A central cone slightly lower than the summit caldera rim has been the site of numerous small-to-moderate historical eruptions recorded since the time of the Spanish conquistadors at Colombia's Galeras volcano. Persistent steam and gas, and occasional ash emissions from multiple vents around the summit have characterized activity for many years. Steam plumes are generally visible from two sites at the summit of the pyroclastic cone. Two small craters, known as Chavas and El Paisita, are located on the N and W rim of the larger central summit crater. Information for this report was gathered primarily from monthly technical reports provided by the Observatorio Vulcanológico y Sismológico de Pasto (OVSP) of the Sevicio Geologico Colombiano (SGC). Four webcams document the activity from the Observatorio Vulcanológico y Sismológico de Pasto (OVSP) located in Pasto (8 km ESE), from Consacá (11 km W), from the top of Galeras in the area called Barranco Alto (2.6 km NW), and from the SW flank at an area called Bruma.

The last time an Alert Level 1 (Red: imminent eruption or in progress) was issued was on 25 August 2010 when a plume of gas and ash rose 300 m above the summit and dispersed ash over numerous communities up to 30 km away. Seismicity decreased the following day, and steam and gas-only emissions returned. Fumarolic activity persisted throughout 2011, with only a single mention of possible low ash content in the plumes observed on 31 March and 1 April. Steam plumes rose a few hundred meters from the summit crater during January-May 2012. Seismic swarms were recorded in April and May.

An eruption with ash emissions began on 13 May 2012 and persisted until 30 January 2014 (BGVN 37:04, 38:03, 39:01). A summary of activity during that eruptive episode is provided below, along with additional information not previously reported. Activity after the end of that eruption, from February 2014 through December 2017, included only steam and gas emissions from the summit crater, and low levels of seismicity.

Activity during 2012. During January and February 2012, steam plumes rose 900-1,000 m above the summit, emerging from the El Paisita and Chavas vents at the N and W rims of the summit crater (figure 130). Plumes rose higher during March, reaching 1,900 m. VT seismic swarms were reported between 11 and 16 April 2012, and deformation sensors recorded inflation towards the W flank beginning in April. Most of the seismicity was located within the vicinity of the summit crater at depths less than 5 km. Steam plumes rose to 2,300 m above summit in April (figure 131).

Figure 130. Volcán Galeras, viewed at 1828 local time from Barranco Alto (2.6 km NW) on 16 February 2012, showed typical low-level steam plumes rising from vents on the N and W rims of the summit crater. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, febrero de 2012).

Figure 131. A substantial steam plume rose from Galeras in this image taken from OVSP (Observatorio Vulcanológico y Sismológico de Pasto) headquarters (8 km SE) on 20 April 2012 at 0738 local. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, abril de 2012).

Steam plumes rose less than 200 m above the summit at the beginning of May; a second swarm of VT seismic events on 9 and 10 May 2012 preceded a new sequence of ash emissions that began on 13 May. Pulsating plumes of ash rose less than 800 m and deposited material primarily on the upper NW flank. Inflation continued to be measured in the inclinometers on the W flank, coinciding with the area of the epicenters of the 9-10 May seismic swarm. Ash-bearing emissions were reported on 13, 14, 17, 26 (figure 132), 27, and 30 May.

Figure 132. An ash emission rose from Galeras at 0802 local time on 26 May 2012 and was recorded by the Barranco Alta webcam on the NW flank. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, mayo de 2012).

Ash emissions continued during June-August 2012. Plume heights during the period ranged from 1,300-2,500 m above the summit. Plumes recorded on 12 and 17 June (figure 133) resulted in ashfall in Sandoná (14 km NW) and Samaniego (32 km NW), Mapachico (9 km NE), and Genoy (7 km NNE). Additional days with reports of ash emissions included 5, 6, 8, 19, 22, 27 and 29 June. Ash-bearing emissions were reported on at least 16 days during July with reports of ashfall in Maragato, Chorillo (18 km W) and Genoy. Ash plumes rose to 2,500 m above the summit during at least nine different days of August, and ashfall was reported again in the Genoy area.

Figure 133. Seismogram and spectrogram of a tremor (TRE) event recorded at 1605 local time on 17 June 2012 that was associated with an ash emission from Galeras as viewed from the Barranca (upper left), OVSP (upper and lower right), and Consacá (lower left) webcams (11 km W). Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, junio de 2012).

Tremor associated with gas and ash emissions persisted throughout September 2012; another VT seismic swarm was reported on 28 September. Ash-bearing emissions were reported during at least seven days of the month, and reached 2,000 m above the crater (figure 134). During at least 16 days of October, tremors were associated with ash emissions that rose as high as 1,800 m. On 19 October, fine-grained ashfall was reported by personnel of the Observatory who were working on the upper NE flank.

Figure 134. Gas and ash emissions at Galeras on 12 September 2012 were recorded photographically from the El Vergel Shelter in Pasto around 1805 local time, at most of the digital seismograph stations around the volcano, and also at the analog recorder at the Anganoy station (upper right) in Pasto (Provided by Architect Darío Gómez of the Municipal Council for Risk and Disaster Management (DMGRD) of the municipality of Pasto). Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, septiembre de 2012).

Gas and ash plumes rose 1,000-1,300 m during November and December 2012 and were also associated with tremor signals. The most significant emissions were observed on 1, 7, 14, 22, 23, 29 and 30 November, and 17 (figure 135), 19, 21, 26, 27 and 29 December.

Figure 135. Ash emissions rose from Galeras on the morning of 17 December 2012 as seen in this series of images from the OVSP webcam while seismographs recorded tremor-type events. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, diciembre de 2012).

Activity during 2013. Continuous inflation towards the western flank was measured beginning in April 2012. Similar deformation processes continued at Galeras during much of 2013. The 'Crater' inclinometer located about 0.8 km E of the summit crater showed the most significant amount of westward inflation (figure 136).

Figure 136. Resultant vectors for the electronic inclinometers at Galeras for the period between 25 October 2012 and 31 January 2013 show 2,962.1 microradians (µrad) of movement to the W at the 'Crater' inclinometer as well as movement to the N and SW at several other instruments. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, enero de 2013).

Eruptive activity continued in a similar manner to 2012 throughout 2013. During January, ash-bearing emissions rose up to 1,000 m at least nine times and drifted in various directions. The emission event of 22 January caused ashfall in Sandoná (13 km NW). During February, the most notable seismic activity was several tremor events associated with ash emissions. Plume heights remained below 1,500 m and were observed on at least 11 days of the month. There were reports of ashfall in San Isidro, the upper part of the municipality of Sandoná, NW of the volcano, during the morning of 24 February. Most of the ash emissions during March 2013 were deposited on the upper NW flank. The Crater, Cobanegra, and Calabozo inclinometers continued to show movement associated with inflation towards the W flank during March and April. Gas and ash plumes reached 1,000 m above the summit on 6, 7, 11, 22 and 25 March. Activity was similar during April, with plumes rising to 1,200 m and seismic tremors associated with ash and gas emissions reported on at least 13 days (figures 137 and 138).

Figure 137. Seismograms registered a tremor-type event (TRE) on 5 April 2013 at Galeras that was associated with ash emissions captured in the Barranca webcam. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, abril de 2013).

Figure 138. Gas and steam emissions rose from the crater at the summit of the pyroclastic cone at Galeras on 24 April 2013. Image taken from the caldera rim at the summit. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, abril de 2013).

Seismic activity decreased somewhat during May 2013, although tremor signals associated with ash and gas emissions were noted on at least eight occasions. The pulsating ash plumes were small, and deposited material mostly on the NW flank. The deformation network recorded stability at the Crater inclinometer for the first time in many months. SGC noted a seismic swarm during the evening of 22 May that included a tremor event that lasted for 11 minutes and possibly included ash emissions.

Emissions during June 2013 were mostly steam that rose to 1,300 m, but ash plumes were reported on seven days. The frequency of seismic activity remained steady during July, but the amount of energy released increased significantly. The Crater inclinometer showed deflation. Ash and gas plumes were noted on 6, 12, 13, 17 and 22 July rising as high as 1,500 m. Seismic frequency and energy both decreased during August and September 2013, and inclinometers showed little change in deformation. Plume heights, mostly gas and steam, remained below 500 m. Tremors associated with ash emissions were reported on five days of August and on 3, 11 and 14 September.

Seismicity increased in both amplitude and frequency during October and November 2013. The majority of the VT seismicity was located on the NE flank at 5-10 km depth. Steam plume heights remained below 600 m; emissions reported on 8 and 11 October included ash (figure 139). In addition to steam plumes observed throughout November, ash plumes were reported rising to 1,000 m on 17, 23, and 30 November. Seismicity decreased during December 2013 while deformation remained stable. Ash plumes were reported on 4, 13, 26, 27, and 31 December associated with tremor events (figure 140).

Figure 139. Ash emissions rose from the summit crater at Galeras on 11 October 2013. They were photographed by Mr. Mario Alberto Caicedo, Radio and TV Analyst, from the RTVC Galeras station, at the caldera rim near the summit. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, octubre de 2013).

Figure 140. Seismograms recorded frequencies associated with tremor (TRE) events on 4 December 2013 while the Barranca webcam recorded ash emissions. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, diciembre de 2013).

Activity during 2014. Tremor events during 11-14, 21, 23, and 27-30 January 2014 were associated with ash and gas emissions (figure 141) that reached 850 m above the summit. During the early hours of 11, 13, and 23 January, incandescence was observed at the crater. The last confirmed ash emission of the year occurred on 30 January 2014.

Figure 141. Emissions of steam and ash on 29 January 2014 were captured by the Bruma webcam (SW of the cone) while seismograms registered tremor events. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, enero de 2014).

A decrease in both frequency and energy levels of seismicity were reported during March 2014. SGC noted several tremor-type seismic events associated with gas emissions; steam plumes rose up to 1,000 m above the summit. Although they reference "gas and ash" emissions in a few photographs, only steam is visible in the photographs from March. Reports of activity by SGC for April and May 2014 refer to only steam plumes rising 1,000 m from the summit from the vents on the N and W sides of the crater rim. No further reports are available for Galeras for 2014.

Activity during 2015-2017. Throughout 2015, SGC reported only steam plumes rising from the two vents at the summit of the Galeras pyroclastic cone, known as the Chaldean fumarole fields (Las Chavas) on the W rim, and the El Paisita on the N rim (figure 142). Plume heights were as high as 700 m in January, but dropped below 200 m by May, where they remained for the rest of the year. Inflation to the W began again in September 2014 and continued through May 2015.

Figure 142. Steam plumes rose a few hundred meters above the summit of the pyroclastic cone at Galeras on 9 April 2015. This type of activity was typical for all of 2015. Photo from the Barranco webcam NW of the summit. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, abril de 2015).

Minor variations in seismic frequency and energy levels fluctuated throughout 2016 and 2017, but there were no reported particulate emissions. Steam emissions from the two primary vents at the summit crater (Las Chavas and El Paisita) rarely rose more than 200 m above the summit, often drifting NW.

An inspection of the summit crater by SGC on 25 August 2016 revealed a deep vent with several points of gas emissions (figure 143), including areas on the N wall (El Paisita) and the E wall (Las Alterada). The W wall (Las Chavas) had a cave-like entrance of 50 m diameter with fumarolic activity on the back wall and the ceiling that condensed into a sulfur-rich water on the floor of the opening. The El Pinta vent had no observed emissions. A rare 200-m-high steam plume rose from the crater in October 2016, but otherwise activity remained very low at Galeras throughout 2017 (figure 144).

Figure 143. An inspection of the summit crater at Galeras by SGC on 25 August 2016 revealed a deep vent with several points of gas emissions including areas on the N wall (El Paisita) and the E wall (Las Alterada). The W wall (Las Chavas) had a cave-like entrance of 50 m diameter with fumarolic activity on the back wall and the ceiling that condensed into a sulfur-rich fluid on the floor of the opening. The El Pinta vent had no emissions. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, agosto de 2016).

Figure 144. Low-level steam emissions seen from the Bruma webcam SW of the summit of Galeras on 3 August 2017 were typical activity for the entire year. Courtesy of SGC (Informe mensual de actividad de Los Volcanes Galeras, Cumbal, Doña Juana, Azufral, Las Ánimas, Chiles Y Cerro Negro, agosto de 2017).

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: Servicio Geologico Colombiano (SGC), Diagonal 53 No. 34-53 - Bogotá D.C., Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

The most recent eruptive period at Heard began in September 2012 (BGVN 38:01). Direct observations are rare at this remote volcano, but the presence of lava flows can frequently be discerned using infrared satellite data. Thermal anomalies were intermittent, with some episodes of clearly stronger activity, during 2016 and through September 2017 (BGVN 42:10).

During all of 2017, MODIS infrared satellite data analyzed using the MODVOLC algorithm showed anomalies near the summit only on 2, 16, and 26 September, and on 1 and 22 October. The MIROVA system also detected numerous hotspots within 5 km of the volcano through late October. One additional significant anomaly was identified on approximately 12 November 2017 (figure 31). No further significant anomalies were noted through February 2018.

Figure 31. Low to moderate power thermal anomalies in MODIS data were identified by the MIROVA system in September and October, with another on approximately 12 November 2017. Courtesy of MIROVA.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben because of its extensive ice cover. The active Mawson Peak forms the island's high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported at this isolated volcano, but observations are infrequent and additional activity may have occurred.

Information Contacts: 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/).

A series of three explosions at Kanlaon on 18 June 2016 sent ash plumes as high as 3 km above the crater and caused minor ashfall in neighborhoods W, SW, and NW of the volcano (BGVN 42:01). This was followed by steam plumes through 25 July 2016. The active Lugud crater (figure 4) has been the source of 21 reported eruptions since 1969; the latest eruption took place in December 2017. Information summarized here for activity from September 2016 through December 2017 was provided by the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Figure 4. Photo looking down from the rim into the historically active Lugud crater at Kanlaon on 7 March 2010. Courtesy of Billy Lopue, used under Creative Common BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/).

PHIVOLCS reported on 5 May 2017 that since the last phreatic eruption in June 2016 there had been a general decline in activity: seismicity was at baseline levels, no significant deformation had been detected since August 2016, sulfur dioxide emissions were low, and no steaming had been observed since 29 September 2016. The Alert Level was lowered to 0 (on a scale of 0-5), though the public was warned to not enter the 4-km-radius Permanent Danger Zone (PDZ).

Between 24 June and 18 August 2017 the seismic network detected 244 volcanic earthquakes. The PHIVOLCS report noted that the increased seismic activity could be followed by phreatic explosions at the summit crater, despite the absence of visible degassing or steaming from the active vent. The Alert Level was raised to 1. The number of daily volcanic earthquakes increased after 18 August. In their 15 November report, PHIVOLCS indicated that during the previous 24 hours there had been 279 deep volcanic earthquakes recorded (compared to five the day before). This prompted them to raise the Alert Level to 2 (moderate level of unrest), where it remained for the rest of the year. The next day, the number recorded was 217. After that the daily number of volcanic events dropped considerably, especially after 21 November. Based on PHIVOLCS reports, the number of daily volcanic earthquakes during the first eight days of December 2017 varied from one to seven.

On 9 December an approximately 10-minute-long, low-energy phreatic explosion began at 0947 that was heard as far away as La Castellana, Negros Occidental (15 km SW). A plume of voluminous steam and dark ash rose 3-4 km above the summit vent (figure 5), and minor amounts of ash fell in Sitio Guintubdan (23 km W), and barangays W of the volcano (Ara-al, Sag-ang, and Ilihan). The eruption was preceded by the resumption of degassing at the summit crater at 0634, detectable as continuous low-energy tremor during periods when the summit was not visible; degassing was last observed September 2016.

Figure 5. Photo of the 9 December 2017 plume rising from Kanlaon as seen from Barangay Manghanoy, La Castellana, Negros Occidental, about 15 km SW. Photo by Ms. Ritchel Demerin Villanueva; posted by PHIVOLCS on Facebook.

Only three volcanic earthquakes were detected on 10 December, but then the number increased to 155 the next day. The number of daily events earthquakes increased again to 578 on 13 December, rose to 1,007 the next day, and peaked at 1,217 on the 15 December. The earthquake count dropped to 149 on 16 December before returning to six or fewer through 19 December. White steam plumes rose 800 and 300 m above the crater on 13 and 14 December, respectively. White plumes were diffuse on 15 December; weather clouds prevented views of the summit area during 16-18 December. Sulfur dioxide emissions were 603-687 tons per day during 13-14 December.

PHIVOLCS reported that during 19-20 December there were 412 volcanic earthquakes. A low-energy, explosion-type earthquake was detected at 0233 on 21 December associated with gas emissions from the summit area. Later in the day steam plumes rose 400 m and drifted NE. The number of daily volcanic earthquakes increased to 957 the next day and then decreased to less than 20 per day during 22-23 December. The daily earthquake count increased to 382 and 776 events on 24 and 25 December, respectively, decreased to 82 on 26 December, and the dropped to three or fewer over the last days of the year. Weather clouds often prevented observations , but white plumes rose 300 m and drifted NE, NW, and SW on 21 December, and 700 m on 26 December. A steam plume on 30 December was seen rising 500 m above the crater rim and drifting SW. On 30 December 2017, sulfur dioxide levels were measured at an average of 1,946 tonnes/day.

Geologic Background. Kanlaon volcano (also spelled Canlaon) forms the highest point on the Philippine island of Negros. The massive andesitic stratovolcano is covered with fissure-controlled pyroclastic cones and craters, many of which are filled by lakes. The largest debris avalanche known in the Philippines traveled 33 km SW from Kanlaon. The summit contains a 2-km-wide, elongated northern caldera with a crater lake and a smaller but higher active vent, Lugud crater, to the south. Eruptions recorded since 1866 have typically consisted of phreatic explosions of small-to-moderate size that produce minor local ashfall.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Billy Lopue, flickr (URL: https://www.flickr.com/photos/21905294@N03/).

After an explosive eruption during January-September 2011, Shinmoe-dake (Shinmoedake), a stratovolcano of the Kirishimayama volcano group, was quiet except for gas-and-steam plumes and slowly decreasing seismicity that returned to baseline levels by May 2012 (BGVN 37:07). The following report summarizes events through December 2017, and relies primarily on reports from the Japan Meteorological Agency (JMA).

On 22 October 2013, JMA reported that no eruptions had been detected at the volcano since the eruption on 7 September 2011. Earthquake activity and sulfur dioxide emissions were both below the detection limit. The Alert Level was lowered from 3 to 2 (on a scale of 1-5).

According to JMA, an eruption began at 0534 on 11 October 2017, prompting the agency to raise the Alert Level to 3 (figure 21). Ash plumes rose 300 m above the crater rim (2 km altitude) and drifted NE. Volcanic tremor amplitude increased and inflation was detected. Ashfall was noted in at least four towns in the Miyazaki (to the E) and Kagoshima (to the SW) prefectures. Based on JMA notices, pilot observations, and satellite data, the Tokyo Volcanic Ash Advisory Center (VAAC) reported that ash plumes rose to an altitude of 1.8-2.1 km on 11 October and 3.4 km on 12 October.

Figure 21. An ash plume rises from the Shinmoedake crater at Kirishimayama after its eruption on 11 October 2017. Courtesy of Tomoaki Ito / Kyodo News.

Gas measurements taken during field surveys on 12 and 13 October showed that the sulfur dioxide flux was 1,400 tonnes/day, an increase from 800 tonnes/day measured on 11 October. Volcanic tremor fluctuated but the amplitude was slightly lower. During 0823-1420 on 14 October, an event produced a tall plume which rose 2.3 km above the crater rim. Another event, at 1505, generated a grayish-white plume that rose 1 km and then blended into the weather clouds. Ashfall was reported in Kirishima (22 km SW) in the Kagoshima prefecture, in Kobayashi (14 km NE) in the Miyazaki prefecture, and reaching as far as Hyuga city (92 km NE). An increase in low-frequency earthquakes was recorded on 16 October.

The eruption lasted almost continuously until the morning of 17 October. The eruption plume usually rose several hundred meters about the crater rim, though on 14 October the plume rose as high as 2.3 km. Sulfur dioxide flux exceeded 10,000 tonnes/day. Cloudy weather conditions prevented webcam views during 19-20 October. Plumes rose 200-600 m on 21, 23, and 24 October. During an overflight on 24 October, scientists observed a white plume rising from the active vent on the E side of the crater, and puddles in multiple low areas of the crater.

Activity during 25 October-20 November 2017 activity continued to be slightly elevated. White plumes rose 100-500 m above the crater rim, though weather clouds sometimes prevented visual observations. Almost daily field surveys by JMA revealed no particular changes in the fumarolic and fissure areas near the cracks on the W flank, or to the thermally anomalous zone below the crack. Sulfur dioxide fluxes were as high as 200 tonnes/day. The Alert Level remained at 3.

Geologic Background. Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Associated Press (URL: https://www.ap.org/en-us); Kyodo News (URL: https://english.kyodonews.net).

Since an eruptive episode in May 2007, Loopevi has been quiet except for a thick gray plume on 24 February 2008 and a short-lived increase in activity in December 2014 (BGVN 32:05, 34:08, 40:05). This report covers activity during January 2015-December 2017. Data were primarily drawn from reports issued by the Vanuatu Geohazards Observatory (VGO) and the Wellington Volcanic Ash Advisory Center (VAAC).

Based on a pilot observation and webcam views, the Wellington VAAC reported that a short-lived steam-and-gas plume beginning at 0500 on 13 January 2017 produced a that rose no higher than 3 km in altitude and drifted SE. That same day VGO reported that the Volcanic Alert Level (VAL) was raised to 3 (on a scale of 0-5); it was lowered to Level 2 on 17 January and then to Level 1 on 20 February.

Steam plumes were again observed on 23 September by the web camera, prompting VGO to raise the VAL to 2, indicating major unrest (danger around the crater rim and specific area, considerable possibility of eruption, chance of flank eruption). Observation flights on 30 September and the first week of October showed that the activity was occurring only in the active craters below the summit crater (figure 24). Photographs and thermal infrared images taken during the flights confirmed that activity consisted of hot volcanic gas and steam. VGO reported that photos and satellite images acquired at the end of November confirmed that gas-and-steam emissions were continuing.

Figure 24.Aerial view of the active cone at Lopevi on 3 October 2017. Courtesy of VGO.

The unrest continued through at least December 2017, and the VAL remained at 2. The Wellington VAAC noted that on 20 December a low-level plume was visible in satellite and webcam images drifting NW at an altitude of 1.5 km.

Geologic Background. The small 7-km-wide conical island of Lopevi, known locally as Vanei Vollohulu, is one of Vanuatu's most active volcanoes. A small summit crater containing a cinder cone is breached to the NW and tops an older cone that is rimmed by the remnant of a larger crater. The basaltic-to-andesitic volcano has been active during historical time at both summit and flank vents, primarily along a NW-SE-trending fissure that cuts across the island, producing moderate explosive eruptions and lava flows that reached the coast. Historical eruptions at the 1413-m-high volcano date back to the mid-19th century. The island was evacuated following major eruptions in 1939 and 1960. The latter eruption, from a NW-flank fissure vent, produced a pyroclastic flow that swept to the sea and a lava flow that formed a new peninsula on the western coast.

Information Contacts: Vanuatu Geohazards Observatory (VGO), Department of Geology, Mines and Water Resources of Vanuatu (URL: http://www.geohazards.gov.vu/, http://www.vmgd.gov.vu/vmgd/index.php/geohazards/volcano); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html).

Reventador has exhibited historical eruptions with numerous lava flows and explosive events since the 16th century. Eruptive activity has been continuous since 2008. Persistent ash emissions and incandescent block avalanches characterized activity during 2016; occasional pyroclastic and lava flows were also reported (BGVN 42:11). Similar activity continued during January-September 2017; information for this period is provided primarily by the Instituto Geofisico-Escuela Politecnicia Nacional (IG-EPN) of Ecuador and also from satellite-based MODIS infrared data.

Summary of activity, January-September 2017. Activity remained high at Reventador during January-September 2017. The strongest (4 km long) pyroclastic flow since 2002 occurred in late June along with a large lava flow that traveled over 2.5 km, the longest since 2008. Visual observations of ash emissions and block avalanches were often difficult due to weather conditions that obscured views of the summit certain times of the year (figure 60, table 9). Thermal alerts and anomalies recorded by satellite instruments complemented the visual information reported by IG-EPN (figure 61) and showed near-continuous activity as well. Variation in the frequency of the different types of seismic events fluctuated throughout the period (figure 62) and generally corresponded to variations in the surface activity.

Figure 60. Activity at Reventador during January-September 2017 included MODVOLC alerts (red), ash emissions (gray) and block avalanches (blue) reported many times each month. The number of cloudy days (yellow) affected the number of observed events during most months. Data courtesy of IG-EPN, compiled from daily reports.

Table 9. High levels of activity at Reventador during January-September 2017 were evident from the numbers of MODVOLC thermal alerts, and days with reported ash emissions and block avalanches. Cloudy weather impacted observations of activity during most months. Compiled from IG-EPN daily reports, VAAC reports, and MODVOLC data.

Figure 61. MIROVA thermal anomalies for Reventador for the year ending 29 September 2017 show a persistent record of heat flow from the volcano. Significant cloudiness during certain times of the year affected the completeness of the MODIS infrared satellite data on which this is based. Courtesy of MIROVA.

Figure 62. Frequency of daily seismic events at Reventador between 6 January and 14 September 2017. LP: Long Period, EXPL: Explosions, TRESP: Tremors. A significant tremor event took place during the lava flow event of late June, and LP seismic events peaked during the eruptive activity of late August. Courtesy of IG-EPN (Informe Especial del Volcán El Reventador, 2017, N° 4, Continúa la erupción, alternancia de actividad efusiva y explosive, 14 de septiembre del 2017).

Ash emissions occurred many times each month, with the highest plumes exceeding 3,000 m above the summit of the pyroclastic cone inside the caldera. The number of block avalanches reported each month increased steadily throughout the period, with blocks falling hundreds of meters from the summit on all flanks numerous times. Pyroclastic flows were reported a few times most months; the largest event in June sent flows nearly 4 km. Four lava flow events were recorded during the period; on 3 April, a flow traveled 1,600 m down the SW flank, a small flow in early June travelled 200 m down the NE flank, the large flow of 23 June-1 July traveled over 2.5 km down the NE flank, and multiple flows overflowed the summit crater and traveled in five different directions on 24 August 2017.

Activity during January-May 2017. Steam, gas, and ash emissions were reported during 10 of the 12 clear days of January 2017 when observations could be made. The plume heights varied up to 3,000 m above the 3,600-m-altitude summit. Ashfall was reported in El Chaco (30 km SW) on 18 January; nine MODVOLC thermal alerts were reported during the month.

Clearer skies during February 2017 resulted in observations of gas, steam, and ash emissions during 18 days of the month. The plume heights ranged from 900-2,000 m above the summit crater. On 7-8 February, in addition to steam and ash emissions rising 1,500 m and drifting W, block avalanches were observed traveling 1,000-1,500 m down all the flanks. A pyroclastic flow also traveled 800 m down the S flank. On 13 February at 0806 local time, the pilot of a plane from Aerogal observed a vertical plume that reached 2,000 m above the summit; nearby lookouts reported explosion sounds, and slight ashfall was observed in Gonzalo Pizarro in the Sucumbíos province (about 40 km NE). Incandescence appeared at the summit six times in February, triggering 13 MODVOLC thermal alerts.

Ash plume heights in March 2017 ranged from 500-2,000 m during the 18 days they were observed. Although incandescence was seen at the summit seven times, block avalanches were observed on the flanks only twice, on 11 and 23 March, traveling 1,000 m down the flanks each time. A pyroclastic flow traveled 500 m from the summit on 16 March.

Activity increased significantly during April 2017; ash emissions, ranging from 200-2,000 m high were recorded on 21 days, and block avalanches were observed 12 days, traveling 600-1,500 m down the SE flank most of the time. The largest event, on 20 April, sent large blocks 1,800 m down all the flanks. A lava flow moved 1,600 m down the SW flank on 3 April. On 10 April, multiple emissions of steam and gas with moderate ash content reached 2,000 m above the summit crater. On 24 April, a 1,300-m-high ash plume was witnessed during a flyover.

Block avalanches continued at a high rate during May 2017, traveling 500-800 m down all the flanks on at least 10 days of the month. Ash emissions persisted and were observed on 18 of the 25 clear days, rising from 300 to over 800 m. In the early hours of 26 May, a cloud of material was observed on the S flank, likely from a pyroclastic flow.

Activity during June 2017. The technical staff of IG-EPN visited the NE flank of Reventador to monitor activity during 29 May-1 June 2017. They observed a small lava flow on the NE flank, several explosions and emissions associated with both the N and S vents at the summit, pyroclastic flows, 'chugging' (audible, closely spaced intermittent gas emissions), and the projection of ballistic material.

The new lava flow was located on the upper NE flank; the only movement they detected was collapsing of the front of the flow, which sent blocks down to the base of the cone. Explosions with ash emissions from the two vents generally occurred every 15-30 minutes. Gas and ash emissions generally rose 1-2 km high, and the larger explosions produced pyroclastic flows. The sounds of the explosions were audible 5-8 km from the volcano. The researchers used a thermal camera to record a small pyroclastic flow that lasted for about 1 minute and 16 seconds and reached 800 m in length. They also observed avalanche blocks from the S vent that rolled 1,200 m down the flank. The thermal camera measured temperatures as high as 521°C.

During a flyover on 7 June 2017, scientists observed recent pyroclastic flows around all the flanks, the largest ones, on the N and S flanks, reached 1.2 km. Volcanic bombs were visible around the periphery of the crater rim. The lava flow observed a few days earlier by the ground crew extended 200 m down the NNE flank, and did not appear to be associated with either of the summit vents. Several explosions were witnessed from the two vents at the summit crater (figure 63).

Figure 63. Two active vents were visible at the summit crater of the central cone at Reventador on 7 June 2017. Top: Steam and ash emerged from the N vent at the summit crater, and fumarolic activity rose from the NE flank in this view to the NE. Bottom: A lava flow created a pale scar on the NE flank (foreground), while ash and steam emissions rose from the summit crater in this view looking SW. Photos by P Ramón, courtesy of IG-EPN (Informe Especial No. 2-Volcan El Reventador, Observaciones entre 29 de mayo -01 junio y 7 de junio 2017, 26 junio 2017).

Thermal imagery taken during the 7 June overflight revealed three emission centers at the summit; the two vents inside the crater that produced explosions with ash, larger bombs, and pyroclastic flows, and a fissure on the NE flank about 70 m below the summit that produced the lava flow (figure 64). The highest temperatures were measured in the N vent (Vento Norte).

Figure 64. Thermal imagery taken during the overflight of Reventador on 7 June 2017 revealed three emission centers at the summit; the two vents inside the crater (Vento Sur, Vento Norte) produced explosions with ash, larger bombs and pyroclastic flows, and a fissure on the NE flank (fisurales) that produced a small lava flow (flujo de lava). Inset photos show visible image (top right) and thermal image (bottom right) of summit. Courtesy of IG-EPN (Informe Especial No. 2-Volcan El Reventador, Observaciones entre 29 de mayo -01 junio y 7 de junio 2017, 26 junio 2017).

In a special report on 23 June 2017, IG-EPN noted that Reventador had averaged about 50 daily explosions in recent months, as well as a similar number of LP earthquakes. During 22-24 June, a continuous seismic tremor was recorded (figure 62), along with more episodic tremors that included small explosions. Surface activity included pyroclastic flows down all the flanks, and ash plumes that rose about 2.5 km and drifted W. The pyroclastic flows sent material as far as 4 km to the E of the cone, into the headwaters of the El Reventador River (figure 65). IG-EPN reported that the pyroclastic flows generated during this event were the strongest since 2002.

Figure 65. A large pyroclastic flow on 23 June 2017 traveled down the NE flank of Reventador at 0757 local time, as viewed from the Copete webcam on the SE edge of the caldera. Courtesy of IG-EPN (Informe Especial No. 1-Volcan El Reventador, Cambio en la actividad eruptive, 23 junio 2017).

The tremors were associated with a new emission of lava that advanced rapidly down the NE flank of the cone and was active until 1 July. It traveled about 2.65 km before stopping, and was nearly 250 m wide near the base (figure 66). IG-EPN reported that the lava flow was the longest since 2008 and covered and area of just under 0.5 km². In addition to pyroclastic flows and a lava flow, a significant SO₂ plume was released on 24 June 2017 (figure 67). Ash emissions were reported on 22 days during June. Plume heights ranged substantially from less than 200 m to over 2,000 m. Block avalanches traveling up to 800 m down the flanks were reported on ten days, and 20 MODVOLC thermal alerts were issued.

Figure 66. The lava flow and pyroclastic flows of 23 June-1 July 2017 at Reventador were measured in an overflight on 21 July by IG-EPN. dC is the diameter of the summit crater (168 m). The width of the flow was about 120 m partway down the flank, and 246 m at its widest point. It traveled a distance of 2.65 km (F1) from the summit. The pyroclastic flow was measured at 3.95 km (Pf) from the summit. Inset thermal image shows lava flow during the same overflight. Photo by St. Almeida, courtesy of IG-EPN (Erupción de junio de 2017 del volcán El Reventador, Reporte de erupción, volcán El Reventador, 2017-01, Publicado el 19 de septiembre de 2017).

Figure 67. An SO2 plume captured by the OMI instrument on the Aura satellite on 24 June 2017 drifted WNW from Reventador. It coincided in time with an eruptive episode that also produced several pyroclastic flows and a 2.65-km-long lava flow. Courtesy of NASA Goddard Space Flight Center.

Activity during July-September 2017. There were fewer observations of ash emissions during July, on only 17 days, with plume heights ranging from 200-1,500 m (figure 68). Twelve MODVOLC thermal alerts were issued and block avalanches were reported on nine different days moving 200-800 m down all the flanks. A pyroclastic flow reported on 6 July traveled 800 m down the E flank. By the time of the 21 July overflight by IG-EPN, the two summit vents had merged, block avalanches surrounded the rim, and the still-warm flow was visible on the NE flank (figure 69). A visit by IG-EPN scientists on 1 August confirmed the continuing audible explosions, as well as the cooling of the late June lava flow (figure 70).

Figure 68. A dense ash plume rose 1.5 km above the summit crater and drifted N at Reventador during a flyover by IG-EPN on 21 July 2017. Glacier-covered Volcán Cayambe appears in the distance to the NW (right of the ash plume). Courtesy of IG-EPN (Erupción de junio de 2017 del volcán El Reventador, Reporte de erupción, volcán El Reventador, 2017-01, Publicado el 19 de septiembre de 2017).

Figure 69. Thermal and visible images of Reventador on 21 July 2017 reveal a single strong thermal anomaly at the summit, block avalanches and bombs around the rim, and a still warm lava flow on the NE flank, dark brown in the visible image on the right. Photo by Almeida, courtesy of IG-EPN (Erupción de junio de 2017 del volcán El Reventador, 2017-01, Publicado el 19 de septiembre de 2017).

Figure 70. Ash emissions and the cooling lava flow on the NE flank of Reventador on 1 August 2017. Top: An ash-laden emission rose from the summit of the cone; the fresh dark brown lava flow is visible on the lower flank. Bottom: The same image from the thermal camera showed the residual heat from the lava flow (lower right), active heat from the ash emission, and a warm area on the upper flank (upper left), likely from block avalanches or a smaller flow. Photo and Image by M. Almeida, courtesy of IG-EPN (Erupción de junio de 2017 del volcán El Reventador, Reporte de erupción, volcán El Reventador, 2017-01, Publicado el 19 de septiembre de 2017).

The frequency of eruptive activity increased substantially during August 2017. Ash emissions were reported on 29 days of the month most rising over 500 m; block avalanches occurred on at least 25 days sending debris as far as 1,000 m down all the flanks. Pyroclastic flows were reported twice, during 11-12 and 23-24 August (figure 71). Lava flows descended multiple flanks simultaneously on 23 August (figure 72).

Figure 71. A pyroclastic flow descended the SE flank of Reventador during the early morning of 24 August 2017 in this image taken by the IG Copete webcam. Courtesy IG-EPN (Informe del estado del Volcan Reventador No. 236, Jueves, 24 de agosto de 2017).

Figure 72. Lava flows descended multiple flanks of Reventador simultaneously on 23 August 2017 in this infrared image. Five lava flows emerged from both the N and S vents at the summit of the central cone. Ln-1 flowed NE from the N vent and Ln-2 flowed ENE from the N vent. Three flows emerged from the S vent, Ls-1 flowed WSW, Ls-2 flowed ESE, and Ls-3 flowed S. Image by M. Almeida, processing by M.-F. Naranjo, courtesy of IG-EPN (Informe Especial del Volcán El Reventador, 2017, N° 4, Continúa la erupción, alternancia de actividad efusiva y explosive, 14 de septiembre del 2017).

The Washington VAAC issued 114 aviation alerts during August 2017 and 123 during September, indicating a continued level of high eruptive activity; plume heights were reported as high as 3,500 m above the summit, and block avalanches covered most of the upper cone down to 900 m a number of times during both months (figure 73).

Figure 73. Explosions with rolling incandescent blocks descend 900 m on all sides of Reventador on 11 September 2017 in this image from the Copete webcam. Courtesy of IG-EPN (Informe Especial del Volcán El Reventador, 2017, N° 4, Continúa la erupción, alternancia de actividad efusiva y explosive, 14 de septiembre del 2017).

Geologic Background. Volcán El Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic stratovolcano has 4-km-wide avalanche scarp open to the E formed by edifice collapse. A young, unvegetated, cone rises from the amphitheater floor to a height comparable to the rim. It has been the source of numerous lava flows as well as explosive eruptions visible from Quito, about 90 km ESE. Frequent lahars in this region of heavy rainfall have left extensive deposits on the scarp slope. The largest recorded eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Instituto Geofísico (IG), Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); 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 Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

In 2016 and the first quarter of 2017, activity at Semeru was characterized by numerous ash explosions and thermal anomalies (BGVN 42:05). Thermal anomalies became consistent after mid-May 2017, increasing over the next few months and continuing through December 2017. The information below comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as the Center for Volcanology and Geological Hazard Mitigation, or CVGHM), the Darwin Volcanic Ash Advisor Center (VAAC), and MODIS thermal sensors aboard satellites. The Alert Level since February 2012 has remained at Yellow (Waspada, or Alert).

According to PVMBG monthly reports, Semeru did not show any change of activity during the reporting period. Presumably, this included numerous ash explosions and thermal anomalies indicating the presence of lava flows or dome growth. A Darwin VAAC ash advisory stated that an ash explosion on 7 June at 0020 UTC generated a plume that rose 4 km in altitude and drifted 13 km SW a day later.

Thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were not observed between 19 November 2016 and 6 June 2017. On 6 June, a single hotspot was recorded, coincident with the ash explosion. The next hotspot occurred on 2 August, followed by anomalous pixels on three additional days through 13 August, but none during the rest of August. The number rose to 7-12 days per month during September-December, many of which were multi-pixel events.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected only two distinct MODIS hotspots during April through the middle of May 2017. After mid-May, the number rose dramatically and every month through December numerous hotspots were detected, almost all within 5 km of the volcano.

Figure 31. MODIS satellite thermal anomaly data at Semeru analyzed by the MIROVA system for the year ending 8 January 2018. Courtesy of MIROVA.

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

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