Dukono, a remote volcano on Indonesia’s Halmahera Island, has been erupting continuously since 1933, with frequent ash explosions and sulfur dioxide plumes (BGVN 48:06). This activity continued during June-November 2023, based on reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG; also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data.

Webcam photos posted to MAGMA Indonesia (a platform developed by PVMBG) showed that, white, gray, or dark plumes of variable densities were observed almost every day during the reporting period (figure 35), except when fog obscured the volcano. Plume heights ranged up to 2,500 m above the summit. Images during 7 October-27 November 2023. During the first week of November ashfall up to 0.5 mm thick fell in several areas downwind, including Mede, Popilo, Gorua (4.5 km ENE), Waro ino-Weri, Buwaele, Gura, Cina, Gamsungi, and Tobelo (15 km ENE). Banging noises were also heard several times in the same villages.

Figure 35. Webcam photos of Dukono showing ash plumes during October-November 2023. Courtesy of MAGMA Indonesia.

Thermal anomalies recorded by the MIROVA system were infrequent during this period except for approximately mid-October through mid-November 2023, corresponding to observations of ashfall. MODVOLC thermal alerts were recorded on 25 October, 10 November, and 15 November 2023. Weather clouds or steam plumes often prevented clear satellite views, but a distinct thermal anomaly at the center of the summit crater was observed in Sentinel-2 images on 24 June, 29 July, 7 October, 12 October, 27 October, 1 November, and 26 November. Sulfur dioxide plumes were detected by the TROPOMI instrument aboard the Sentinel-5P satellite (figure 36) on roughly half the days each month during the reporting period, with fewer in July and more in November.

Figure 36. Strong SO2 plumes from Dukono are depicted from 13 September and 16 October 2023 using data from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, have occurred since 1933. During a major eruption in 1550 CE, a lava flow filled in the strait between Halmahera and the N-flank Gunung Mamuya cone. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

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 (Multiplatform Application for Geohazard Mitigation and Assessment in Indonesia), Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); 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/); 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/).

The current eruption period of Ibu, a stratovolcano on Indonesia’s Halmahera Island, began in April 2008 and has primarily consisted of ash plumes and emissions varying from intermittent to daily, along with occasional thermal anomalies (BGVN 43:12, 48:06). This report updates activity through November 2023, based on daily reports from MAGMA Indonesia, advisories issued by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data. MAGMA Indonesia is a platform developed by PVMBG to improve timely public access to volcano and earthquake activity.

According to PVMBG, from June to November 2023 daily gray-and-white plumes of variable densities usually rose 200-1,000 m above the nested summit craters and drifted in multiple directions. Occasional larger explosions prompted PVMBG to issue 50 Volcano Observatory Notice for Aviation (VONA) ash plume alerts during the reporting period. VONAs were issued every month, ranging from 5 in June to 14 in October; all described the ash plumes as dense. A few plumes rose as high as 1,500 meters during August-November. Clear Sentinel-2 satellite views showed ash plumes on 8 and 13 August, but webcam images showed intermittent ash plumes similar to those seen in recent years.

The MIROVA satellite volcano hotspot detection system recorded frequent thermal anomalies of low to moderate power throughout the reporting period. Data from imaging spectroradiometers aboard NASA’s Aqua and Terra satellites and processed using the MODVOLC algorithm (MODIS-MODVOLC) recorded hotspots on 6 June, 3 August, 30 September, five days in October (5, 10, 15, 26, 31), and four days in November (5, 12, 13, 26). Sentinel-2 satellite images showed a summit crater hotspot in every clear view, along with thermal anomalies extending S from the crater in a partially obscured view on 21 November. White-to-grayish emissions were also visible on clear days.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

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 (Multiplatform Application for Geohazard Mitigation and Assessment in Indonesia), Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); 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/).

Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that grew within the youngest scarp, which is about 9 km wide and extends about 13 km from the W wall to the ocean on the E side. More than 150 eruptions, a majority of which have consisted of basaltic lava flows, have been recorded since the 17th century. The most recent eruption ended in October 2022, and was characterized by lava fountains, flows and strong sulfur dioxide emissions (BGVN 47:12). This report describes a new eruption with lava fountains, flows, and strong sulfur dioxide emissions during July through August 2023 using information from the Observatoire Volcanologique du Piton de la Fournaise (OVPF) and satellite data.

An increase in seismicity was observed beginning on 10 June; nine volcano-tectonic (VT)-type events were recorded on 10 June, eight on 11 June, and 40 on 12 June. These events had low (less than Mw 1) magnitudes, and some were located between 1.5-2 km deep under the Dolomieu crater. During June, 791 VT-type earthquakes, two deep earthquakes, 18 long-period (LP)-type earthquakes, and 163 rockfall events were recorded.

A seismic crisis began at 0736 on 2 July, which included an earthquake swarm and rapid deformation (figure 233). Volcanic tremor began at 0830, indicating the arrival of magma at the surface, and fissures opened on the E flank at around 0830 and 1230, and on the SE flank around 1750 inside the Enclos Fouqué caldera (figure 234). Lava flows from the two initial fissures moved ENE. The fissure on the SE flank was about 500 m long, trended NNW-SSE, and produced lava flows that traveled E. During the earthquake swarm, 578 shallow VT-type earthquakes were recorded below the S and N border of the Dolomieu crater. Seismicity then continued for several days, mainly below the summit crater; 1,754, 232, 70, 18, 33, and 12 shallow VT-type events were recorded on 2, 3, 4, 5, 6, and 7 July, respectively. There were 33 deep earthquakes detected below the E flank on 2 July after the onset of the eruption. Sulfur dioxide flux measured on 2 July was 10-20,000 tons/day (t/d) and then declined the following days to less than 1,000 t/d. During 3-6 July sulfur dioxide plumes of variable intensity were detected by the TROPOMI data using Sentinel-5P satellite images (figure 235). OVPF recommended a change in the Alert Level to 2-1, the lowest of two sub-levels in “Alert 2: ongoing eruption” (inside the Enclos Fouqué caldera) at 0850 on 2 July; Alert 2 is the third level on a four-color eruption scale.

Figure 233. Graph showing the spike in eruptive tremor in RSAM (Real-time Seismic Amplitude Measure) values at Piton de la Fournaise starting at around 0400 on 2 July 2023, using data from the SNE seismic station located at the summit of the volcano. After the spike, tremor was variable; two gradual decreases are visible in this RSAM graph and four notable spikes. Courtesy of OVPF-IPGP.

Figure 234. Map of Piton de la Fournaise showing the location of two eruptive vents (red lines) to the E and SE of the Enclos Fouqué and their flow fronts (red stars) updated on 3 July 2023. Courtesy of OVPF-IPGP.

Figure 235. Strong sulfur dioxide plumes were captured in Sentinel-5P satellite images using TROPOMI data at Piton de la Fournaise on 3 July 2023 (top left), 4 July 2023 (top right), 5 July 2023 (bottom left), and 6 July 2023 (bottom right), which accompanied the new eruptive activity. The plumes drifted NW, W, and SW and exceeded 2 Dobson Units (DU). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

The SE flank fissure was the most active of the two fissure areas by 0430 on 3 July, with lava flows traveling further to the E than from the higher E-flank fissures. The effusion rate generally fluctuated between 7-27 cubic meters per second (m³/s), averaging 12 m³/s based on satellite data. Field teams made visual observations during 0800-1000 on 3 July and noted that several eruptive fissures had opened on the E flank in Piton Vouvoul. The front of the lava flow had moved about 2 km from the highest point of the first fissure before stopping. Active lava fountaining built several cones along the SE-flank fissure. The main lava flow from the SE-flank fissure continued to advance, reaching 650 m in elevation in an area about 2.4 km from the nearest road by 0940 (figure 236). At 1012 there was a sharp decline in volcanic tremor amplitude that remained at lower levels. During 3-4 July the effusion rate fluctuated between 5-20 m³/s based on satellite data, and the flow front advanced at a rate of roughly 40 meters per hour (m/h) based on webcam images (figure 237). By 1424 on 4 July the lava flow was about 3.5 km long based on satellite image analysis (figure 238). On the morning of 6 July, eruptive activity was concentrated at a single vent located to the SE of Enclos Fouqué around 1.7 km elevation, where a cone continued to form (figure 239).

Figure 236. Photos of the active fissure on the SE flank of the Enclos Fouqué at Piton de la Fournaise showing the active lava flows at 0815 (top) and 0955 (bottom). Photos have been color corrected. Courtesy of OVPF-IPGP.

Figure 237. Photo of the active lava flow at Piton de la Fournaise moving SE of Enclos Fouqué at 2030 on 3 July 2023. Courtesy of OVPF-IPGP.

Figure 238. Map of Piton de la Fournaise showing the active fissure (red color) to the SE of Enclos Fouqué and the inactive fissure site (black color). The lava flow fronts are represented by the red stars updated on 4 July 2023. Courtesy of OVPF-IPGP.

Figure 239. Webcam image of the active vent at Piton de la Fournaise located SE of Enclos Fouqué at 0925 on 6 July 2023 (local time). Courtesy of OVPF-IPGP.

During an overflight on 7 July, a team from OVPF-IPGP determined that the lava flow had reached 1.8 km from the road but had not advanced since 5 July. The flow front did not extend any further to the E, but by 7 July active flows were moving through a lava tube. During 10-11 July flows traveled through tubes and were active at elevations about 1.3 km. Although clouds often prevented measurements, satellite analysis showed that lava flow rates fluctuated between 1.5-24 m³/s. The total volume of lava effused since the beginning of the eruption on 2 July was an estimated 5.5 million m³ by 11 July, based on the HOTVOLC and MIROVA systems.

On 12 July the top of the cone located SE from Enclos Fouqué partially filled. The flow front did not extend any further to the E and remained stalled 1.8 km from the road. Active flows traveled through lava tubes above 1.5 km elevation and continued to widen (increasing about 180 m since 7 July) and thicken, mainly affecting the N part of the flow and above 1.5 km elevation. Though clouds often prevented measurements, satellite analysis showed that effusion rates fluctuated between less than 1 and 13.5 m³/s. The total erupted volume since the start of the eruption was an estimated 6 ± 3 million m³ and by 22 July, the total volume of effused lava was an estimated 8.5 ± 3 million m³. The eruptive cone measured approximately 30 m high on the morning of 25 July. On 26 July the total volume of effused lava was an estimated 9.6 ± 3.4 million m³.

On 28 July the longest part of the flow remained stalled 1.8 km from the road and had solidified. An aerial overflight showed the active SE vent at Enclos Fouqué containing lava and accompanied by degassing (figure 240). By the end of July, there were 2,189 shallow VT-type earthquakes, 45 deep earthquakes, 111 LP-type earthquakes, and 235 rockfall events detected.

Figure 240. Aerial photos of the active vent at Piton de la Fournaise located SE of Enclos Fouqué at 0936 on 28 July 2023 in visible (left) and infrared (right) imagery. The bottom image uses FLIR imagery (Forward Looking Infrared). Courtesy of OVPF-IPGP.

Activity during August consisted of incandescence, lava flows, and seismicity. There were 60 shallow VT-type earthquakes detected below the summit crater, mainly located beneath the E rim of the Dolomieu crater and 330 rockfall events, a majority of which occurred inside the Dolomieu crater, along the cliffs of the East River, and around the active lava flows. During 2-3 August lava flows were visibly active and traveled mainly through tubes; incandescence from breakout flows were also occasionally visible in semi-clear views. These flows originated at distances 1.2-2.5 km from the eruptive cone and the lowest active flow zone was observed from the Piton des Cascades webcam at 1 km elevation. OVPF reported that the eruption ended at 0500 on 10 August as the amplitude of the volcanic tremor (an indicator of lava and gas emissions) had declined the previous week. According to IPGP, there was an estimated 11.7 ± 4 million m³ erupted on the surface as of 10 August (figure 241). An earthquake was felt by some residents on the island, mainly in the Salazie area at 2031 on 11 August.

Figure 241. Map showing the evolution of the lava flows erupted from Piton de la Fournaise during 3 July 2023 through 10 August 2023 using satellite data. The black lines represent the active fissures. The red triangle represents the primary eruptive vent on the SE flank of the volcano. Courtesy of OVPF-IPGP.

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

Information Contacts: Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Poás, located in Costa Rica, contains three summit craters along a N-S line. Its northern lake, Laguna Caliente, has been the site of frequent phreatic and phreatomagmatic eruptions since 1828. Eruptions often include geyser-like ejections of crater-lake water. The previous eruption occurred in April 2022 and was characterized by a phreatic explosion and an eruptive plume (BGVN 47:06). This report covers a new eruption during late July through September 2023 using reports from Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

OVSICORI-UNA reported that on 7 July volcanic tremor increased and fumarolic activity also intensified. An underwater image of the center of the lake showed a jet of gas-and-water mixed with sulfur rising 5 m above the bottom of the lake. The lake level began to decrease on 19 July by 1.2 m compared to the previous measurement taken on 6 July. A notable decrease in seismicity was reported during the week of 28 July; high amplitude levels were reported until 20 July. Pulses of short and intense tremor were recorded on 20 and 21 July. At 1652, on either 20 or 21 July (the date was not specified), a small hydrothermal eruption was reported. Sulfur dioxide emissions were measured at 149 ± 84 tons/day (t/d) during the last week of July.

A small phreatic eruption was recorded at 0153 on 3 August in the W part of the crater lake that ejected material 20 m above the water. Frequent, low-frequency volcanic earthquakes continued. Sulfur dioxide measurements were 150 ± 48 tons per day (t/d) during the first week of August. Another phreatic eruption was reported at 1655 on 5 August from three areas of the lake and ejected sediment and steam 50-100 m above the lake. At least two eruptive pulses were seen. Fumarolic degassing and lake convection were visible during 2-15 August. On 11 August at 0842 a small phreatic eruption at the center of the lake ejected material 5 m above the lake. Sulfur dioxide measurements were 175 ± 26 t/d during the week of 11 August. During the week of 18 August sporadic volcano-tectonic earthquakes were recorded to the N and E of the volcano and sulfur dioxide measurements decreased slightly to 120 ± 37 t/d. A small hydrothermal eruption was reported during the early morning of 21 August. Sulfur dioxide measurements and the lake level continued to fluctuate; the former was 152 ± 112 t/d during the week of 25 August.

OVSICORI-UNA reported a small phreatic eruption on 20 September at 1340 in the center of the crater lake. Sulfur dioxide measurements were 181 ± 50 t/d.

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

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

Sangay, located in Ecuador, has documented eruptions that date back to 1628 SE characterized by pyroclastic flows, lava flows, ash plumes, and lahars. The current eruption period began in March 2019 and has recently consisted of daily explosions, incandescent block avalanches, lava flows, and ash plumes (BGVN 48:03). This report describes similar activity of daily explosions, gas-and-ash emissions, light ashfall, crater incandescence, and incandescent material on the flanks, using information from Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE), the Washington Volcanic Ash Advisory Center (VAAC), and various satellite data.

During March through August 2023, IG-EPN reported daily explosions, gas-and-steam and ash plumes that rose as high as 9 km above the crater, and frequent crater incandescence, often accompanied by incandescent avalanches of material and lava flows that descended the flanks of the volcano. The highest ash plume rose 9 km above the crater on 21 April and drifted W. Explosions occurred nearly every day, ranging from 5 to nearly 2,200 per day, the most of which occurred on 20 June 2023 (figure 150). The average number of daily explosions during the reporting period was 425, and the highest monthly average number of daily explosions was 755, which occurred during June 2023.

Figure 150. Graph showing the number of daily explosions at Sangay during March through August 2023. The number of explosions markedly increased during mid-June and remained higher during June through August compared to March through May. There were some data gaps during April and May due to technical problems in seismic data. Data courtesy of IG-EPN (March-August 2023 daily reports).

Table 13. Monthly summary of explosions and plume heights recorded at Sangay during March through August 2023. The average number of explosions do not account for days where the monitoring instruments were offline. Data courtesy of IG-EPN (March-August 2023 daily reports).

During March, activity consisted of 7-166 daily explosions and gas-and-ash emissions that rose as high as 2 km above the crater during 22 and 24 March and drifted N and NW. The Washington VAAC reported that gas-and-ash emissions rose 500-1,500 m above the crater and drifted in multiple directions. Nighttime crater incandescence was occasionally observed, accompanied by some incandescent material on the SE flank. On 1 March a lava flow was visible on the SE flank, accompanied by incandescent block avalanches. According to MOUNTS sulfur dioxide measurements taken throughout the month, emissions ranged between 44 and 2,049 tons per day (t/d). On 3 March a lava flow was observed on the SE flank 600 m below the crater rim. A lava flow was also visible on the SE flank on 13 and 22 March, accompanied by crater incandescence. A lahar was reported on 14 March. During 18-20 and 31 March incandescent block avalanches descended the S and SE flanks (figure 151).

Figure 151. Webcam image showing a lava flow, an incandescent block avalanche, and crater incandescence from Sangay at 2123 on 18 March 2023. Courtesy of IG-EPN Daily report.

Activity during April was characterized by 5-316 daily explosions, frequent nighttime crater incandescence, and gas-and-ash emissions that rose 1-9 km above the crater and drifted in multiple directions. The Washington VAAC reported that gas-and-ash emissions rose 500-7,900 m above the crater. According to MOUNTS sulfur dioxide measurements taken throughout the month, emissions ranged between 22and 1,725 t/d. During 5-6 April a lava flow descended the SE flank, reaching 1.8 km below the crater rim. During 12-18 crater incandescence was visible in the upper part of the flanks and incandescent material descended the SE flank as far as 1.5 km below the crater rim. On 17 April light ashfall was reported in Macas and on 18 April light ashfall occurred in Morona, Sucúa, Sinaí, and Logroño (figure 152). There were some reports of roaring sounds in Chonta Punta. Two large explosions were detected during 20-21 April that generated initial eruption columns that rose 8 km above the crater. During the night and early morning of 20-21 April GOES-16 satellite images showed a wide ash cloud that reached 9 km above the crater and drifted W, according to IG-EPN. Ashfall was reported in Chimborazo, Bolivar, Guayas, and Los Ríos. More ashfall was reported in Guamote and ash remobilization in Babahoyo on 22 April. A pyroclastic flow occurred at 0814 on 24 April that descended the SE flank. Ash emissions that same day rose 6 km above the crater and drifted NE and NW. The Washington VAAC reported that gas-and-ash emissions rose 7.9 km above the crater and drifted W and E. There were some technical problems with the seismic instrumentation, so seismic counts could not be included on 24-25 and 28-30 April. On 25 April the Washington VAAC reported four ash emissions; the first rose 1.2 km above the crater and drifted NE, the second rose 6 km above the crater and drifted SW, the third rose 3.3 km above the crater and drifted NW, and the fourth rose 1.5 km above the crater and drifted E. A GOES-16 satellite image taken at 1640 showed a gas-and-ash plume rising 6 km above the crater and drifted SW. Ashfall was reported in Ishbug Utucun. Ash emissions rose 1.5 km above the crater and drifted E, W, SW, and SW on 26 April and SNGRE reported light ashfall in Chimborazo and Guamote.

Figure 152. Photo of an ash plume rising as high as 4 km above Sangay’s crater at 0730 on 18 April 2023 and drifted ESE. Photo has been color corrected. Photo by Jorge Duchi, courtesy of IG-EPN.

There were 304-600 daily explosions detected during May, in addition to occasional crater incandescence and gas-and-ash emissions that rose 300-2,000 m above the crater and drifted in different directions, according to both IG and the Washington VAAC. MOUNTS sulfur dioxide measurements taken throughout the month ranged between 9.7and 1,912 t/d. Technical problems with the seismic instruments occurred during 1-22 May, so seismic and explosion counts could not be collected. Cloudy weather often obscured clear views of the summit, but intermittent crater incandescence was visible. Light ashfall was reported in Palmira and during the night in Chauzán, both in Chimborazo on 5 May. An incandescent lava flow was observed on the SE flank 500 m below the crater rim on 15 and 17 May. By 18 May the lava flow reached 1 km below the crater rim. Constant ash emissions on 20 May rose less than 2 km above the crater and ashfall was reported in Chimborazo and light ashfall was reported in Guamote and Alausí on 21 May. During 22-23 May a lava flow descended the SE flank 1 km below the crater rim, then 1.8 km below the crater rim, respectively. A pyroclastic flow occurred on 24 May and descended the SE flank at 0920. During the morning of 29 May several pyroclastic flows descended the SE flank, one of which occurred at 0610 (figure 153). Light ashfall was reported in Cebadas. Nighttime crater incandescence was visible and lava flows were reported on the S and SE flanks. On the morning of 30 May another pyroclastic flow was reported moving down the SE flank. Light ashfall was observed in Cebadas on 31 May. Additionally, nighttime crater incandescence and a lava flow on the SE flank were visible.

Figure 153. Webcam image of a pyroclastic flow descending the SE flank of Sangay at 0615 on 29 May 2023. Image has been color corrected. Courtesy of IG-EPN.

IG-EPN reported 158-2,190 daily explosions during June, frequent crater incandescence, and gas-and-ash emissions that rose 400-1,800 m above the crater and drifted in different directions, according to both IG and the Washington VAAC. Sulfur dioxide emissions of 30 to 797 t/d were recorded throughout the month and reported by MOUNTS. Incandescent avalanches were reported 1 km below the crater rim on 5 June, accompanied by crater incandescence. During 9-12 June an incandescent avalanche descended the SE flank and was accompanied by crater incandescence. A GOES-16 satellite image taken on 15 June showed an ash cloud that rose 1.8 km above the crater and drifted W; light ashfall was reported in Palmira. A pyroclastic flow traveled 500 m below the crater rim on the SE on 19 June. On 20 June gas-and-ash emissions rose 561 m above the crater and drifted SW and light ashfall was reported in Llagos.

During July, 148-1,200 daily explosions were recorded, frequent crater incandescence, and intermittent gas-and-ash emissions that rose 400-2,000 m above the crater and drifted in multiple directions, according to both IG and the Washington VAAC. MOUNTS data for sulfur dioxide emissions showed that measurements ranged 22-515 t/d. During 3-6 and 12-13 July incandescent material descended the SE flank 1-1.8 km below the crater rim. On 6 July explosions ejected incandescent material above the crater rim. During 9-10 July incandescent material was ejected up to 1 km above the crater rim.

Activity persisted during August, with 33-943 daily explosions, occasional crater incandescence, and gas-and-ash emissions that rose 300-2,800 m above the crater and drifted in different directions, according to both IG and the Washington VAAC. The amount of sulfur dioxide emitted during the month, according to MOUNTS was 27-641 t/d. During 4 and 6-13 August crater incandescence was accompanied by incandescent material descending the SE flank as far as 1.8 km below the crater rim; some material also was visible on the S flank on 11 August. Gas-and-ash emissions rose 500-2,200 m above the crater and drifted W and NW and ashfall was reported in Cebadas on 17 August. That same day, incandescent material was visible 1 km below the crater rim. On 19 August a GOES-16 satellite image showed a gas-and-ash emission rise 1.2 km above the crater and drifted NW; during the morning SGR reported light ashfall in Palmira and ROVE reported light ashfall in Cebadas. During 22-30 August crater incandescence was accompanied by incandescent material 1-1.8 km below the crater rim on the S and SE flanks. At 1500 on 25 August light ashfall was observed in Cebadas.

Satellite data. Thermal activity was consistently strong throughout the reporting period due to crater incandescence, block avalanches, and lava flows that primarily affected the S and SE flanks. This activity was detected by the MIROVA hotspots detection system (figure 154) and the MODVOLC Thermal Alerts system, which recorded a total of 137 hotspots. Infrared satellite imagery showed crater incandescence, incandescent block avalanches, and lava flows descending the SE flank (figure 155). Frequent sulfur dioxide plumes were captured by the TROPOMI instrument on the Sentinel-5P satellite, many of which exceeded 2 Dobson Units (DUs) and drifted in different directions (figure 156).

Figure 154. Strong thermal activity at Sangay occurred frequently during March through August 2023 due to crater incandescence and lava flows on the flanks; there was a gap in activity during late April through early May. Courtesy of MIROVA.

Figure 155. Strong thermal activity was visible in infrared (bands B12, B11, B4) satellite images showing crater incandescence and a lava flow (bright yellow-orange) descending the SE flank at Sangay on 17 March 2023 (top left), 20 June 2023 (top right), 25 July 2023 (bottom left), and 19 August 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 156. Strong sulfur dioxide plumes from Sangay were frequently detected by the TROPOMI instrument on the Sentinel-5P satellite throughout the reporting period, as shown here on 2 March 2023 (top left), 9 April 2023 (top right), 21 May 2023 (bottom left), and 15 July 2023 (bottom right). Each of the plumes shown here exceed 2 Dobson Units (DUs) and drifted W, E, SW, and W, respectively. Sangay is located toward the bottom of the satellite images; Colombia’s Nevado del Ruiz is the topmost symbol that also frequently emits sulfur dioxide plumes. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within the open calderas of two previous edifices which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been eroded by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of an eruption was in 1628. Almost continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Servicio Nacional de Gestion de Riesgos y Emergencias (SNGRE), Samborondón, Ecuador (URL: https://www.gestionderiesgos.gob.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 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/); MOUNTS Project (Monitoring Unrest From Space), an operational monitoring system for volcanoes using Sentinel satellite data from ESA's Copernicus, Open Access Hub, hosted at UNAM and CT TU-Berlin (URL: http://www.mounts-project.com/home).

Following weak phreatic explosions at Great Sitkin in June-August 2018 and June 2019, an ash explosion on 25 May 2021 preceded the growth of a lava dome in the summit crater starting in mid-July 2021. Continued lava effusion overtopped the summit crater wall; lava flowed down the N, S, and W flanks (BGVN 46:08, 47:05). This activity subsided during January-April 2022, although slow lava effusion, accompanied by minor steam-and-gas emissions and weak seismicity, continued through April 2023 (BGVN 48:05). Great Sitkin (figure 26), located in Alaska’s Aleutian Island chain, is monitored by the Alaska Volcano Observatory (AVO) using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

Figure 26. Sentinel-2 satellite image of Great Sitkin Island on 6 June 2023. The width of the dark lava-covered crater area is about 1.2 km. Image rendered with highlight-optimized natural color (bands 4, 3, 2). Courtesy of Copernicus Browser.

According to AVO, the slow, persistent, thick lava effusion continued during May-October 2023, expanding E into glacial ice but remaining confined to the summit crater. This activity was accompanied by weak seismic activity, minor steam emissions, and slightly elevated temperatures consistent with cooling lava. Weather clouds frequently obscured visibility. During the first week of September, an AVO field geology team visited the volcano and sampled the lava flow, did aerial photography (figures 27 and 28) and thermal imaging surveys, and measured gas emissions. The team observed that the flows were warm and steaming (figure 29), moving about 1 m every 3-4 days.

Figure 27. Aerial photo of Great Sitkin taken during a helicopter overflight on 1 September 2023, looking W at the active lava flows in the crater. The currently active lobe of the flow field is visible moving into the crater ice field on the lower left side of the image. Photo by Matt Loewen, courtesy of AVO.

Figure 28. Aerial photo of Great Sitkin taken during a helicopter overflight on 3 September 2023, looking NE at the active lava flows in the crater. Photo by Matt Loewen, courtesy of AVO.

Figure 29. Ground photo of the active blocky lava flow field at Great Sitkin taken during fieldwork on 1 September 2023. Photo is looking towards the vent region and main degassing source on the lava flow. Photo by Matt Loewen, courtesy of AVO.

Geologic Background. The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger volcano capped by a small, 0.8 x 1.2 km ice-filled summit caldera was constructed within a large late-Pleistocene or early Holocene scarp formed by massive edifice failure that truncated an older edifice and produced a submarine debris avalanche. Deposits from this and an even older debris avalanche from a source to the south cover a broad area of the ocean floor north of the volcano. The summit lies along the eastern rim of the younger collapse scarp. Deposits from an earlier caldera-forming eruption of unknown age cover the flanks of the island to a depth up to 6 m. The small younger caldera was partially filled by lava domes emplaced in 1945 and 1974, and five small older flank lava domes, two of which lie on the coastline, were constructed along northwest- and NNW-trending lines. Hot springs, mud pots, and fumaroles occur near the head of Big Fox Creek, south of the volcano. Eruptions have been recorded since the late-19th century.

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

Nyiragongo is within the Albertine Rift Zone along the western branch of the East African Rift System in the Democratic Republic of the Congo. A lava lake inside the 1.2 km summit crater has been active since at least 1971. Eruptions during the last 120 years have been associated primarily with the summit crater, but several flank lava flows have also been observed as recently as 2019 and 2021, with the latter reaching the city of Goma (15 km S) (BGVN 46:06). Besides lava extrusions, activity has included incandescence, gas and steam emissions, and seismicity. The current report summarizes data between April and September 2023 and is based on occasional communications from the Observatoire Volcanologique de Goma (OVG) and satellite data.

During the reporting period, volcanic activity was relatively low and confined to the persistent lava lake. OVG reported that the eruption continued as normal from 27 May to 11 June, with crater incandescence observed at 1900 on 4 June and 1800 on 10 June. SO₂ emissions were low. A subsequent OVG report indicated that activity continued low during 17-24 September. A diffuse sulfur dioxide plume with an estimated mass of 20 tons was identified in satellite data on 25 September.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) hotspot system recorded a surge of moderate to high-power hotspots in about the second week of June 2023, and then frequent hotspots throughout July, and a few scattered hotspots in August. The MODIS-MODVOLC thermal alerts system recorded hotspots only on 3 April, 10 July, and 24, 25, and 27 July (two pixels each day in July). Sentinel-2 satellites recorded lava effusion in the summit crater during the few satellite observation days of the reporting period, corresponding to the elevated thermal observations (figure 97).

Figure 97. Sentinel-2 infrared satellite imagery showed ongoing activity and lava effusion in the Nyiragongo summit crater on 11 June 2023 (left), 21 July 2023 (middle), and 26 July 2023 (right). Images rendered using bands B12, B11, B4. Courtesy of Copernicus Browser.

Geologic Background. The Nyiragongo stratovolcano contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

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

The Dempo stratovolcano in SE Sumatra, Indonesia, has had many small explosions reported since 1817. The active crater contains a 400-m-wide lake was the source of phreatic explosions in 2006, 2009, 2017, and most recently in May 2022. This report covers activity from June 2022 to December 2023, based on daily and special reports of the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), and satellite data. Throughout the reporting period, the Alert Level remained at 2 (on a scale of 1-4), and the public were reminded to stay 1 km away from the crater and as far as 2 km on the N flank.

PVMBG reported that at 2115 on 25 July 2023, an explosion produced a white-and-gray ash plume that rose at least 2 km above the summit and drifted S and SW. Additional ash explosions were recorded at 1547 and 2215 on 26 July, the latter rising 2 km above the summit crater and drifting S and SW, based on ground observations. Another explosion was reported by PVMBG at 2105 on 21 August, but no ash emissions were observed. Webcam images posted in daily MAGMA Indonesia reports showed possible patches of discolored water or material floating on the surface of the crater lake during 27-29 August (figure 8).

Figure 8. Webcam image of Dempo’s crater lake on 29 August 2023, showing material floating on surface. Courtesy of MAGMA Indonesia.

Most Sentinel-2 satellite views of Dempo during the reporting period were obscured by weather clouds. When observations were possible, the crater lake surface typically looked milky gray, indicating high levels of particulate matter. However, during several days in March and April 2023 the lake took on a blue appearance.

Geologic Background. Dempo is a stratovolcano that rises above the Pasumah Plain of SE Sumatra. The andesitic complex has two main peaks, Gunung Dempo and Gunung Marapi, constructed near the SE rim of a 3-km-wide amphitheater open to the north. The high point of the older Gunung Dempo crater rim is slightly lower, and lies at the SE end of the summit complex. The taller Marapi cone was constructed within the older crater. Remnants of seven craters are found at or near the summit, with volcanism migrating WNW over time. The active 750 x 1,100 m active crater cuts the NW side of the Marapi cone and contains a 400-m-wide lake at the far NW end. Eruptions recorded since 1817 have been small-to-moderate explosions that produced local ashfall.

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

Masaya, located about 20 km SE of Managua, the capital Nicaragua, is a broad, 6 x 11 km caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. Eruptions have originated from the Nindirí and Masaya cones, which were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. The Santiago crater contains a small lava lake that emits weak gas-and-steam plumes. The current eruption period began in October 2015 and more recently has been characterized by thermal activity in the summit crater (BGVN 48:03). This report covers similar activity from February through July 2023, consisting of continuing thermal activity and sulfur dioxide emissions, according to monthly reports from Instituto Nicaragüense de Estudios Territoriales (INETER) and satellite data.

Thermal activity has remained relatively consistent, according to a MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data, which showed occasional low-power thermal anomalies during the reporting period. There were six anomalies were detected during February, four during March, three during April, one during early June, and one during late July (figure 109). Infrared satellite imagery showed occasional hotspots in the Santiago crater’s lava lake throughout the reporting period (figure 110). The TROPOMI instrument on the Sentinel-5P satellite detected weak and small sulfur dioxide plumes that drifted generally W and SW on 8-9 February, 3-4, 13, and 18 March, 25 and 30 April, 11-12 May, 1-2 and 11 June, and 3 July.

Figure 109. Low-power thermal anomalies were occasionally detected at Masaya during February through July 2023, as shown on this MIROVA plot (Log Radiative Power). Six anomalies were recorded during February, four during March, three during April, one during early June, and one during late July. No hotspots were reported during May. Courtesy of MIROVA.

Figure 110. Infrared (bands B12, B11, B4) satellite images showed a persistent thermal anomaly (bright yellow-orange) at the active lava lake in Masaya’s Santiago crater on 17 February 2023 (top left), 3 April 2023 (top right), 23 May 2023 (bottom left), 17 June 2023 (bottom right). Occasional gas-and-steam emissions were also captured. Courtesy of Copernicus Browser.

Activity was relatively low throughout the reporting period. Seismic tremor was at 60 RSAM units during February and March. On 10 February FLIR temperature of the lava lake was measured from Santiago crater, recording values of 290°C, which was within the typical range of 200-500°C. INETER noted that the level of the lava lake had decreased compared to previous observations made during October 2022. On 15 March gas measurements were made with a Mobile DOAS technique, showing average sulfur dioxide values of 1,448 tons/day (t/d); the previous date that measurements were taken was on 14 November 2022 with a value of 1,222 t/d. On 25 April sulfur dioxide measurements decreased slightly to an average of 1,042 t/d. Temperature values of the lava lake from Santiago crater were 254°C on 26 April.

Seismic tremor was at 50 RSAM units during May and consisted of one volcano-tectonic (VT) event and 347 low-frequency (LF) events. On 5 May INETER recorded a temperature of the lava lake from Santiago crater of 270°C and reported that the level of the lava lake was lower compared to previous observations made in April. Sulfur dioxide measurements taken on 15 May were 1,438 t/d. During June and July, seismic tremor was maintained at 50 RSAM units and included one VT event and 221 LP-type events during June, and three LP-type events during July. On 28 June measurements of the lava lake had increased slightly to 315°C and the level of the lava lake continued to decrease. The average values of released sulfur dioxide emissions were 1,056 t/d on 19 June and 1,012 t/d on 14 July.

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

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); 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, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Manam is a 10-km-wide island that consists of two active summit craters: the Main summit crater and the South summit crater and is located 13 km off the northern coast of mainland Papua New Guinea. Frequent mild-to-moderate eruptions have been recorded since 1616. The current eruption period began during June 2014 and has more recently consisted of ash plumes and thermal activity (BGVN 48:07). This report covers activity during January through June 2023 primarily using various satellite data.

Weak and intermittent sulfur dioxide plumes were detected using the TROPOMI instrument on the Sentinel-5P satellite, some of which exceeded at least two Dobson Units (DU) and drifted in different directions (figure 96). Few low-power thermal anomalies were recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system; two anomalies were captured during January, one during late March, one during early April, one during late May, and one during early June (figure 97). On clear weather days, thermal activity was captured in infrared satellite images in both the Main and South summit craters (figure 98).

Figure 96. Distinct sulfur dioxide plumes were visible rising above Manam based on data from the TROPOMI instrument on the Sentinel-5P satellite on 23 March 2023 (left) and 15 May 2023 (right). Plumes drifted NE and SE, respectively. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 97. Few thermal low-power thermal anomalies were detected at Manam during January through June 2023, as shown in this MIROVA graph (Log Radiative Power). Two anomalies were captured during January, one during late March, one during early April, one during late May, and one during early June. Courtesy of MIROVA.

Figure 98. Infrared (bands B12, B11, B4) satellite images show a consistent thermal anomaly (bright yellow-orange) in both the Main (the northern crater) and South summit craters on 14 January 2023 (top left), 3 February 2023 (top right), 24 April 2023 (bottom left), and 28 June 2023 (bottom right). The two craters are just under 400 m apart. Gas-and-steam emissions occasionally accompanied the thermal activity. Courtesy of Copernicus Browser.

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

Information Contacts: 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, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Popocatépetl, located 70 km SE of Mexico City, Mexico, contains a 400 x 600 m-wide summit crater. Records of activity date back to the 14th century; three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. The current eruption period began in January 2005, characterized by numerous episodes of lava dome growth and destruction within the summit crater. Recent activity has consisted of daily gas-and-ash emissions, explosions, and ashfall (BGVN 48:04). This report updates activity during April through July 2023, which consisted of similar activity, according to daily reports from México's Centro Nacional de Prevención de Desastres (CENAPRED) and various satellite data.

Daily gas-and-steam emissions, containing some amount of ash, persisted during April through July 2023. CENAPRED reported the number of low-intensity gas-and-ash emissions or “exhalations” and the minutes of tremor, which sometimes included harmonic tremor in their daily reports (figure 213). A total of 36 volcano-tectonic (VT) tremors were detected throughout the reporting period. The average number of exhalations was 104 per day, with a maximum number of 315 on 31 May 2023. Frequent sulfur dioxide plumes that exceeded two Dobson Units (DU) and drifted in multiple directions were visible in satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 214).

Figure 213. Graph showing the number of daily “exhalations” (in blue, left scale), and the number of minutes of tremor (in gold, right scale) at Popocatépetl each day during April through July 2023. The maximum number of daily exhalations was 315 on 31 May 2023; the maximum duration of 1,425 minutes of tremor was detected on 29 May 2023. Data from CENAPRED daily reports.

Figure 214. Strong sulfur dioxide plumes were detected at Popocatépetl and drifted in different directions on 6 April 2023 (top left), 22 May 2023 (top right), 11 June 2023 (bottom left), and 15 July 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during April consisted of gas-and-steam and ash emissions, with 139 explosions and six VT-type events detected. An average number of 169 exhalations occurred each day, which mostly consisted of water vapor, volcanic gases, and a small amount of ash. On most days, webcam images showed nighttime crater incandescence and incandescent material that was deposited on the upper flanks. Two large explosions at 0256 and 0342 on 3 April ejected material 2.4 km from the crater. Light ashfall was reported in Amecameca (20 km NW). On 12 April at 0353 a moderate explosion ejected material 1.5-2 km onto the S flank. Light ashfall was reported in Tochimilco (16 km SSE) and Atlixco (25 km SE) on 20 April and in Atlautla (16 km W), Tlalmanalco (27 km NW), San Pedro Benito Juárez and Atlixco on 21 April. Explosions at 0533 (figure 215) and 0559 on 21 April ejected incandescent material on the slopes as far as 2.3 km N from the crater. A moderate explosion at 0109 on 26 April ejected material on the N flank 1 km from the crater and at 0116 on 27 April ejected incandescent material was deposited onto the upper flanks. According to the Washington VAAC, ash plumes identified in daily satellite images rose to 5.8-7.3 km altitude and drifted NE, E, and SE.

Figure 215. Webcam image of an explosion at Popocatépetl at 0534 on 21 April 2023. Incandescent material was deposited on the upper flanks to the N. Courtesy of CENAPRED daily report.

During May there was an average of 131 exhalations each day, and a monthly total of 5 VT-type events, and 123 explosions, 85 of which were minor explosions and 37 were described as moderate. According to the Washington VAAC, daily ash plumes were identified in satellite images that rose to 6.1-7.3 km altitude and drifted E and SE. The National Center for Communications and Civil Protection Operations (CENACOM) reported light ashfall in Ozumba (18 km W), Tlalmanalco, and Temamatla (33 km NW) on 5 May and in Amecameca, Ayapango (24 km WNW), Temamatla, Tenango del Aire (29 km NW), Tlalmanalco, Tepetlixpa (20 km W), and Cocotitlán (34 km NW) on 6 May; moderate ashfall was reported in Tlalmanalco and Tenango del Aire. On 15 May light ashfall was reported in Puebla (43 km E) and Atlixco and moderate ashfall was reported in areas S of the volcano. CENAPRED reported that activity intensified during mid-May after the formation of small-to-medium lava domes on the summit crater floor, followed by their destruction. Periods of high-frequency tremor lasted more than 12 hours during 16-17 May and more than 10 hours during 17-18 May.

During the morning of 17 May CENACOM reported light ashfall in Puebla, Atlixco, Cholula (35 km E), and San Martín Texmelucan (35 km NE) and in Atlixco and Cholula during the morning of 18 May. Light ashfall was reported in Tétela del Volcán (20 km SW) during the morning of 19 May. A period of high-frequency tremor began around 1800 on 19 May and lasted roughly 10 hours until about 0400 on 20 May and was accompanied by gas-and-steam and ash plumes that drifted NNW. Continuous incandescent ejecta was deposited onto the flanks as far as 1.5 km from the crater.

According to CENACOM on 20 May the Benito Juárez International Airport closed during 04300-1000 and the Felipe Ángeles International Airport closed during 0600-1100 so that ash could be cleared from the runways. Ash was deposited in multiple areas downwind including in the municipalities: Tétela del Volcán, Venustiano Carranza (66 km NW), Gustavo A. Madero (73 km NW), Azcapotzalco (78 km NW), Tlalpan (62 km NW), Iztapalapa (58 km NW), Zumpango, Tlalmanalco, Ecatepec, Chicoloapan (48 km NW), Amecameca, Ayapango, Ozumba, Ecatzingo (15 km SW), Atlautla, Chalco (38 km NW), Temamatla, Tenango del Aire, Juchitepec (28 km NW), Cocotitlán, Tepetlixpa, Tonanitla, Tecámac, Jaltenco, Chiconcuac, Acolman, Valle de Chalco (44 km NW), Iztapaluca, La Paz (50 km NW), and Nezahualcóyotl (56 km NW). Ashfall in Puebla municipalities included Huejotzingo (28 km NE), Nealtican (21 km E), Chignahuapan (108 km NE), Puebla Capital (44 km E), San Martín Texmelucan (35 km NE), and San Felipe Teotlalcingo (26 km NE).

Almost 19 hours of high-frequency tremor recorded during 20-21 May was accompanied by continuous gas-and-steam and ash emissions and occasional incandescent ejecta deposited short distances onto the flanks. According to the Washington VAAC, activity intensified at 1453 on 20 May due to a large, dense ash plume that was observed in webcam images (figure 216). By 1551 the ash plume was visible in satellite images and rose to 8.2 km altitude and at 2041 the ash plume rose to 9.1 km altitude and drifted ENE over the Gulf of Mexico. At 2136 the ash plume rose to 9.7 km altitude and remained at that altitude at least through 0341 on 21 May, drifting NE and ENE. High-frequency tremor was almost constant for over 23 hours during 21-22 May. Ash plumes at 0951 on 21 May rose to 9.1 km altitude and at 1436 ash plumes rose to 8.5 k altitude. Satellite images showed a large, dense ash plume that drifted 388 km NE over the Bay of Campeche. According to CENAPRED, on 21 May the Alert Level was raised from Yellow, Phase 2 to Yellow, Phase 3, the highest of the three sub-phases. Ashfall was reported in San Andrés Cholula (36 km E), San Pedro Cholula (34 km E), Cuautlancingo (38 km E), Amozoc (61 km E), Puebla Capital (44 km E), Zacatlán (121 km NE), Tetela de Ocampo (121 km NE), and Chignahuapan (108 km NE).

Figure 216. Webcam image showing a strong ash plume rising above Popocatépetl at 1454 on 20 May 2023. Courtesy of CENAPRED daily report.

According to Gobierno de Puebla, the Hermanos Serdán International Airport in Puebla closed at 2300 on 21 May until 0700 on 22 May. The Washington VAAC reported that during the early morning of 22 May continuing ash emissions rose to 3.7 km above the summit and drifted E and ENE. An accompanying notable sulfur dioxide plume (figure 214) drifted as far as Cancun (1,295 km E). Ashfall was reported in San Andrés Cholula, San Pedro Cholula, Cuautlancingo, Amozoc, Zacatlán, Tetela de Ocampo, San Nicolás de los Ranchos (15 km NE), Palmar de Bravo (115 km SE), Tepeaca (76 km E), Izúcar de Matamoros (51 km S), Epatlán (51 km SE), Teopantlán (52 km SE), Tlapacoy (144 km NE), Huatlatlauca Chignahuapan (72 km SE), Juchitepec, Hueyapan (17 km SW), Xochitepec, and Tlaxcala. At 1651 on 22 May the Hermanos Serdán International Airport suspended operations because of ash on the runway. During 22-23 May tremor remained nearly continuous for more than 20 hours. On 23 May CENACOM reported ashfall in Nealtican, Tianguismanalco (22 km SE), Atlixco, San Diego la Mesa (39 km SE), Huaquechula (30 km SE), and Atzizihuacán (23 km S). Ash plumes rose as high as 3.7 km above the summit and drifted E, according to the Washington VAAC.

Continuous ash emissions rose to 7.6-10.7 km altitude and drifted SE, S, and SSE during 24-25 May, based on data from the Washington VAAC. Ashfall was reported in Nealtican, San Pedro Cholula, San Andrés Cholula, Tzicatlacoyan (65 km E), Tianguismanalco, Atlixco, Huaquechula, Ocoyucan (33 km SE), San Diego La Mesa Tochimiltzingo (39 km SE), San Juan Atzompa (71 km SE), Tehuitzingo (85 km SE), Tepexi de Rodríguez (88 km SE), Atzitzihuacán (23 km S), and Tilapa (48 km S). At 0045 on 25 May light ashfall was recorded in Atlixco, San Pedro Cholula, and Puebla Capital and at 0600 light ashfall was recorded in Tetela del Volcán. Incandescent material was ejected onto the flanks close to the crater on 25 May. A drone overflight conducted on 25 May showed that ash and incandescent material had significantly filled the inner crater. Air quality alerts were issued in Puebla and there was a reported increase in illnesses relating to ash exposure.

During 26-30 May gas-and-steam and ash rose to 5.2-7.9 km altitude and drifted SE, E, and ESE, according to the Washington VAAC. At 0330 on 26 May light ashfall was recorded in Tetela del Volcán. Later that day, ashfall was recorded in Hueyapan, Tetela del Volcán, Cuautla (43 km SW), Jantetelco, Jonacatepec (43 km SW), Tlacotepec (110 km SE), Atzitzihuacán, Chietla (56 km S), Huaquechula, Tlapanalá (39 km SE), Tepeojuma (39 km SE), and Nopalucan (87 km NE). On 27 May ashfall was recorded in Jonacatepec de Leandro Valle (43 km SSW), Tepalcingo, Yecapixtla (30 km SW), Zacualpan de Amilpas (32 km SSW), Hueyapan, Jantetelco, Ecatzingo, and Tenango del Aire. On 29 May light ashfall was reported in Ecatzingo, Tetela del Volcán, Hueyapan, Morelos, Nopalucan, Puebla Capital, San Andrés Cholula, Cuautlancingo, Amozoc, San Pedro Benito Juárez, and Tianguismanalco. Light ashfall was reported during 30-31 May in Ayapango and Acatzingo.

An average of 70 exhalations consisting of gas-and-steam and ash emissions were reported each day during June. Seismicity was characterized by a monthly total of 12 VT-type events. There were 31 explosions were recorded, 18 of which were described as minor explosions and 13 were described as moderate explosions. According to the Washington VAAC frequent ash plumes rose to 5.5-7 km altitude and drifted SW, SSW, SE, SSE, S, and W. On 6 June the Alert Level was lowered to Yellow, Phase 2 (the middle level on the three-color scale).

On 12 June light ashfall was reported in Hueyapan, Tetela del Volcán, Temoac (32 km SSW), Zacualpan de Amilpas, Jonacatepec de Leandro Valle, and Morelos, on 13 June in Hueyapan, Yecapixtla, Ayala, Jantetelco, and Morelos, on 14 June ashfall in Jantetelco and Morelos, on 15 June light in Hueyapan, Tetela del Volcán, Yecapixtla, Ayala, Amecameca, and Atlautla, and on 16 June in Amecameca, Ayapango, Chalco, Ecatzingo, Temamatla, Tepetlixpa, Tlalmanalco, and Tenango del Aire. On 17 June CENAPRED noted a moderate explosion at 0337 ejected material as far as 2.5 km from the crater. That same day, light ashfall was reported in Ixtapaluca (60 km NNW), Valle de Chalco, La Paz, Nezahualcóyotl, Amecameca, Atlautla, Ayapango, Cocotitlan, Chalco, Ecatzingo, Temamatla, Tenango del Aire, Tepetlixpa, and Tlalmanalco. Light ashfall was observed in Tepoztlan (49 km W), Cuernavaca (63 km WSW), Ocuituco (24 km SW), Cuautla (43 km SW), Atlatlahucan, Jiutepec (59 km SW), Emiliano Zapata (62 km SW), Morelos, Ixtapaluca, La Paz, Chalco Valley, Nezahualcóyotl, Chicoloapan, Atlautla, Ecatzingo, and Tonatico on 19 June, in Atlatlahucan, Morelos, Ozumba, Tenango del Aire, Tepetlixpa, Juchitepec, and Ixtapan on 20 June, and in Cuernavaca on 21 June. Small bursts of incandescent ejecta from the crater were visible during the night of 20 June. At 0312 on 22 June a moderate explosion ejected incandescent material as far as 1.5 km from the crater and generated an ash plume that rose 2 km above the crater. On 22 June light ashfall was reported in Hueyapan and Morelos. Two minor explosions at 0405 and 0745 on 23 June generated ash plumes that rose 500 m above the crater and incandescent material was ejected short distances from the crater. Moderate ashfall was reported in Ozumba, Juchitepec, and Tepetlixpa on 26 June. Ashfall was reported in Amecameca and Ayapango on 27 June. At 0850 on 28 June CENACOM reported light ashfall in Ixtapaluca, Valle de Chalco, and Nezahualcóyotl and at 1927 light ashfall was reported in Amecameca de Juárez, Ozumba, and Temamatla and moderate ashfall in Tenango del Aire. On 30 June light ashfall was observed in Atlautla, Chalco, and Telalmanalco, and moderate ashfall was observed in Amecameca and Cocotitlan.

Similar activity continued during July with an average of 47 exhalations each day, and a monthly total of 13 VT-type events and 31 explosions. There were 17 minor explosions and 14 moderate explosions recorded throughout the month. Daily ash plumes were reported by the Washington VAAC, based on webcam and satellite images; plumes rose as high as 2.2 km above the summit and drifted NE, SW, W, and NW. Two moderate explosions recorded during 1-2 July produced a gas-and-ash plume that rose 1.6 km above the crater and drifted SSW, SW, WSW, and NW (figure 217). CENACOM reported light ashfall in Ixtapaluca and Valle de Chalco on 1 July and in Atlautla, Ecatzingo, Yecapixtla, Ocuituco, Tetela del Volcán, Hueyapan, Cuautla, and Ayala on 2 July. During 2-3 July gas-and-ash plumes rose 1.3 km above the crater and drifted SW and W. An ash plume at 0843 on 10 July rose 1 km above the summit and drifted as far as 28 km NW. and at 0930 CENACOM reported light ashfall in Amecameca, Ayapango, Temamatla, and Tenango del Aire. By noon, the plume had drifted 185 km NW and the Secretaría de Gestión Integral de Riesgos y Protección Civil (SGIRPC) of the City of México reported ashfall in Milpa Alta (46 km WNW), Xochimilco (56 km WNW), Coyoacán (65 km WNW), Tlalpan, La Magdalena Contreras (70 km WNW), Álvaro Obregón (72 km WNW), Cuajimalpa (80 km WNW), Tláhuac (49 km NW), and Iztapalapa. At 0850 on 11 July CENACOM reported light ashfall in Ozumba and Juchitepec. Light ashfall was reported in Valle de Chalco, Ixtapaluca, La Paz, Nezahualcóyotl, Amecameca, Atlautla, Ayapango, Cocotitlán, Temamatla, Tepetlixpa, Tenango del Aire, Juchitepec, Chapultepec (100 km WNW), Calimaya (108 km W), Milpa Alta (46 km WNW), Tláhuac (49 km NW), Iztapalapa, and Tlalpan on 13 July and in Valle de Chalco, Amecameca, Ayapango, Atlautla, and Tenango del Aire on 14 July. On 23 July moderate ashfall was reported in Ozumba, Atlautla, Chalco, Cocotitlán, Tenango del Aire, and Temamatla. Light ashfall was observed during the morning of 25 July in Amecameca, Ayapango, and Tenango del Aire.

Figure 217. Webcam image of a gas-and-ash plume rising above Popocatépetl at 1953 on 1 July 2023. Courtesy of CENAPRED daily report.

MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed frequent low-to-moderate thermal anomalies during the reporting period; intermittent and short breaks in activity were visible during late May, early June, and mid-July (figure 218). A strong thermal anomaly was detected in mid-May. According to data from MODVOLC thermal alerts, a total of 38 hotspots were detected at the summit crater in April (3, 4, 6, 14, 15, 28, 29, and 30), May (2, 5, 10, and 22), June (3, 10, 15, 17, 19, 21, 22, 24, and 27), and July (5 and 16). Thermal activity in the summit crater was also visible in infrared satellite data and was sometimes accompanied by ash plumes, as shown on 26 April and 21 May (figure 219).

Figure 218. Frequent low-to-moderate power thermal anomalies were detected at Popocatépetl during April through July 2023. Intermittent and short breaks in activity were noted during late May, early June, and mid-July. Courtesy of MIROVA.

Figure 219. Infrared (bands B12, B11, B4) satellite images show a persistent thermal anomaly (bright yellow-orange) in the summit crater of Popocatépetl on 26 April 2023 (top left), 21 May 2023 (top right), 5 June 2023 (bottom left), and 15 July 2023 (bottom right). Strong ash plumes drifted E on 26 April and 21 May. On 21 May a significant thermal anomaly was visible at the summit crater. Courtesy of Copernicus Browser.

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

Information Contacts: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: http://www.cenapred.unam.mx/, Daily Report Archive https://www.gob.mx/cenapred/archivo/articulos); Secretaría de Gestión Integral de Riesgos y Protección Civil (SGIRPC), 18 norte 406, Col. Barrio los Remedios Puebla, Pue. C.P. 72377, México (URL: https://sg.puebla.gob.mx/); 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 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/).

The Fagradalsfjall volcanic system on the Reykjanes Volcanic Zone in Iceland first erupted on 19 March 2021, following more than a year of earthquake activity and inflation/deflation periods. This was the first volcanic activity on the system after about 6,000 years of quiescence. Although the Fagradalsfjall fissure swarm has previously been considered a split or secondary swarm of the Krýsuvík-Trölladyngja volcanic system, as of September 2022 Icelandic volcanologists managing the Catalogue of Icelandic Volcanoes made the decision to identify Fagradalsfjall as a distinct and separate system.

The fissure eruption that started in August 2022 near the border of the previous lava flow field N of Fagradalsfjall in Meradalir was characterized by lava flows, fountains, and sulfur dioxide emissions (BGVN 47:09). This report covers a new eruption period that began on 10 July 2023 and ended in early August, characterized by similar activity. Information comes from Icelandic Meteorological Office (IMO), the Institute of Earth Sciences, and various satellite data.

Inflation began in early April in the western Reykjanes Peninsula, reaching a total of 3 cm, and at a rate of about 1 cm per month. Data indicated possible accumulation of magma at 15 km depth. In June, more than 1,000 earthquakes were recorded, more of which were located beneath Reykjanestá, NE of Fagradalsfjall and SW of Kleifarvatn. Seismicity intensified during 3-4 July. An earthquake swarm began at 1400 on 4 July and by 1150 on 5 July, more than 1,800 earthquakes were detected in the vicinity of the July 2022 dike intrusion (figure 37). The depth of the earthquakes became shallower within the first few hours of the swarm; by 5 July the depths were at 2-3 km. Seven of those earthquakes exceeded an M 4, and the largest was an M 4.8 event that was recorded at 0821 on 5 July. On 5 July at 1055 IMO raised the Aviation Color Code (ACC) to Orange (the third level on a four-color scale). By 1330 on 7 July, more than 7,000 earthquakes were detected in the swarm that began on 4 July. The epicenters were aligned NE-SW between Fagradalsfjall and the Keilir cone, NNE from the 2021 and 2022 eruptions, and mostly concentrated just N of the Litli Hrútur hill. Deformation data (GPS and radar interferometry) showed uplift in the same area, which IMO suggested was a magmatic dike intrusion that reached 1 km depth by early on 6 July. Seismicity decreased during 6-7 July and the rate of deformation slowed; an analysis showed that by 9-10 July the dike had propagated 1 km further NE. Small earthquakes were reported in Kleifarvatn, the largest of which was an M 4.6, and caused some rock falls in Kleifarvatn and Trolladyngja.

Figure 37. Map showing the locations of the earthquakes (circles) at Fagradalsfjall from 30 July 2022 to 5 July 2023. The red circles represent earthquakes in 2023 and the blue circles represent the earthquake swarm that occurred during July-August 2022. The orange line represents the vertical projection of the dike intrusion that occurred in 2022 and the lighter orange color shows the extent of the lava flow field from 2022. The map also shows the magnitude for earthquakes greater than 1, and the size of the circles are proportional to the magnitude of the earthquakes. Courtesy of IMO.

Tremor was detected at 1425 on 10 July and continued to intensify, leading to a new fissure eruption oriented NE-SW, beginning at 1640 (figure 38). Webcam images showed gas-and-steam emissions and incandescence, but no major ash emissions. The fissure was about 200-m-long, primarily located in a depression, and was characterized by lava fountains (figure 39) and lava flows originating from three short fissures on the E and NE flanks of Litli-Hrútur and moving NE and S. Gas-and-steam emissions drifted NW. As a result, IMO raised the ACC to Red (the highest level on a four-color scale) at 1707. At 1724 the ACC was lowered back to Orange. According to estimate from the Institute of Earth Sciences, the fissure quickly reached about 900-m-long, based on drone footage.

Figure 38. Photo of the start of the new fissure eruption at Litli-Hrútur, part of the Fagradalsfjall volcanic system taken on 10 July 2023. Gas-and-steam emissions accompanied the lava flow. Courtesy of Elísabet Pálmadóttir, IMO.

Figure 39. Photo of the lava fountains emanating from Litli-Hrútur, part of the Fagradalsfjall volcanic system taken the night of 10 July 2023. People are visible at the bottom middle of this photo for scale. Courtesy of Vilhelm Gunnarsson via Almannavarnadeildar ríkislögreglustjóra.

Tremor levels peaked between 2100 on 10 July and 0000 on 11 July, and steadily declined through 1100. The intensity of the eruption notably decreased with fewer active lava fountains by 1250 on 11 July. Only one vent with an elongated crater and multiple lava fountains were active by 1635. Gas-and-steam plumes rose as high as 4 km above the vent. Lava flows mostly traveled SE and flowed into a shallow valley S of Litli-Hrútur. On 11 July IMO measured the amount of sulfur dioxide with a remote sensing device, DOAS (Differential Optical Absorption Spectroscopy). The amount of sulfur dioxide ranged around 66-80 kg/s.

On 14 July the advancing edge of the lava flow connected with the 2021 lava field in the NE part of Meradalir. The main vent was elongated and measured roughly 22-m-tall on 15 July. During 13-17 July the lava flow rate was an estimated 12.7 m³/sec, and by 18 July the total erupted volume was about 8.4 million m³. The flow advanced an average of 300-400 m per day. Flow thicknesses averaged 10 m; some areas reached 20 m. Lava filled the main cone and occasionally spilled over onto the flanks at around 2330 on 18 July. Around 0251 on 19 July, a breach was reported high on the NE rim, followed by lava that flowed down the flank. Spatter was ejected past the crater rim. Lava fountaining increased around 0259 and lava flowed short distances E. Around 0412 sections of the NW wall of the cone collapsed, which drained the crater and sent lava flows to the N and W. Around the same time, there was a significant decrease in the rate at the S-moving lava flow and by 0530 it was fully crusted over.

Lava from the main vent had grown to about 90 x 40 m during 19-26 July and continued to advance SSW, according to a VONA (figure 40). Sulfur dioxide plumes rose 1-2 km above the crater rim. Seismicity decreased since the eruption began and was mainly concentrated at the N end of the dike and to the E of Keilir. The effusion rate averaged 8 m³/sec during 18-23 July and the total erupted volume was about 12.4 million m³, based on calculations from the University of Iceland, the Icelandic Institute of Natural History, and the National Land Survey of Iceland (figure 41). The lava flow field covered an area of about 1.2 km² (figure 42). According to a news article, part of the N crater rim collapsed just before 1200 on 24 July, which directed lava flows down a new channel, mainly to the S, but also moving E.

Figure 40. A hazard map of Litli-Hrútur, part of the Fagradalsfjall volcanic system, updated on 20 July 2023 that shows the hazard area (light pink), the lava flow extent as of 18 July 2023 (dark pink), and earlier lava flow fields from 2021 and 2022 (orange). The location of the vent is marked with a red dot, and the dike propagation is highlighted by the dark blue dotted line, extending from Keilir to Meradalahnúkar. Courtesy of IMO.

Figure 41. Graphs showing the evolution of the inundated area (top), volume (middle), and effusion rate (bottom) of the lava flow from 18-23 July 2023 at Litli-Hrútur, part of the Fagradalsfjall volcanic system. By 23 July lava covered an area of 1.15 km2, had an erupted volume of 12.4 million m3, and had an effusion rate of 8 m3/s. Courtesy of IES.

Figure 42. Map showing the extent of the lava flow field for the eruption at Litli-Hrútur, part of the Fagradalsfjall volcanic system on 23 July 2023 (red) compared to the previous eruptions in 2021 (green) and 2022 (orange). The S part of the 2023 flow intersects with the previous 2022 lava flow field. Courtesy of IES.

During 23-31 July the lava effusion rate decreased to 5 m³/sec and the total erupted volume was about 15.9 million m³. The flow field covered an area of 1.5 km² by 31 July. During 26 July through 2 August about 150 earthquakes were recorded in the eruptive area, many of which occurred around Keilir. At 1706 on 5 August IMO lowered the ACC to Yellow (the second level on a four-color scale), noting that activity had declined during the previous few days and very minor crater activity was visible in webcam images. Tremor decreased during the previous 36 hours and reached background levels by 1500 on 5 August. Gas-and-steam plumes generally rose 1-2 km above the vent, except on 1 August, where plumes rose more than 2.5 km above the vent. According to news articles, a notable amount of gas-and-steam emissions rose from the vent on 4 August. Activity was last seen on 5 August and a thermal anomaly in the crater was last identified in satellite images on 6 August.

Satellite data. Data from the TROPOMI instrument on the Sentinel-5P satellite, available on maps from the NASA Global Sulfur Dioxide page, showed two sulfur dioxide plumes exceeding 2 DU (Dobson Units) on 11 and 23 July 2023 and drifted W and SE, respectively (figure 43). Strong thermal activity was detected in MODIS data shown on the MIROVA (Middle InfraRed Observation of Volcanic Activity) graphs showing the beginning of the eruption in early July due to lava effusion (figure 44). In late July, the power of the thermal anomalies gradually declined as activity slowed; residual heat remained low through September. Infrared satellite images also showed the start of the fissure eruption and thermal activity expanding NE and SW through July (figure 45). By early August, the lava flows began to cool.

Figure 43. Two strong sulfur dioxide plumes that exceeded 2 DUs (Dobson Units) were detected above Fagradalsfjall on 11 and 23 July 2023 and drifted W and SE, respectively (left and right, respectively), based on data from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 44. Strong thermal activity began at Fagradalsfjall in early July 2023, according to the MIROVA graph (Log Radiative Power) and continued through early August due to a new eruption characterized by lava flows and fountains. Lower, possibly residual thermal activity persisted through September. Courtesy of MIROVA.

Figure 45. Infrared (bands B12, B11, B4) satellite imagery showed the onset of a new fissure eruption at Litli-Hrútur, part of the Fagradalsfjall volcanic system on 11 (top left), 13 (top right), and 18 (bottom left) July 2023 and 2 August 2023 (bottom right). The bright yellow-orange color represents the lava flows and fountains as it expanded NE and SW throughout July. By early August, thermal activity declined and began to cool. Courtesy of Copernicus Browser.

Geologic Background. Although the Fagradalsfjall fissure swarm has previously been considered a split or secondary swarm of the Krýsuvík–Trölladyngja volcanic system, as of September 2022 Icelandic volcanologists managing the Catalogue of Icelandic Volcanoes made the decision to identify it as a distinct separate system. The recent eruptions and related reports have been reassigned here, and other content will be prepared and adjusted as appropriate.

Information Contacts: Icelandic Meteorological Office (IMO), Bústaðavegur 7-9 105 Reykjavík, Iceland (URL: http://en.vedur.is/); Institute of Earth Sciences, Sturlugata 7 101 Reykjavík, Iceland (URL: http://www.earthice.hi.is/); Almannavarnadeildar ríkislögreglustjóra, Skúlagata 21, Reykjavík, Iceland (URL: https://www.almannavarnir.is/); Icelandic National Broadcasting Service (RUV), Efstaleiti 1 150 Rekyjavík, Iceland (URL: http://www.ruv.is/); 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/).

Dukono, a remote volcano on Indonesia’s Halmahera Island, has been erupting continuously since 1933, with frequent ash explosions and sulfur dioxide plumes (BGVN 48:06). This activity continued during June-November 2023, based on reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG; also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data.

Webcam photos posted to MAGMA Indonesia (a platform developed by PVMBG) showed that, white, gray, or dark plumes of variable densities were observed almost every day during the reporting period (figure 35), except when fog obscured the volcano. Plume heights ranged up to 2,500 m above the summit. Images during 7 October-27 November 2023. During the first week of November ashfall up to 0.5 mm thick fell in several areas downwind, including Mede, Popilo, Gorua (4.5 km ENE), Waro ino-Weri, Buwaele, Gura, Cina, Gamsungi, and Tobelo (15 km ENE). Banging noises were also heard several times in the same villages.

Figure 35. Webcam photos of Dukono showing ash plumes during October-November 2023. Courtesy of MAGMA Indonesia.

Thermal anomalies recorded by the MIROVA system were infrequent during this period except for approximately mid-October through mid-November 2023, corresponding to observations of ashfall. MODVOLC thermal alerts were recorded on 25 October, 10 November, and 15 November 2023. Weather clouds or steam plumes often prevented clear satellite views, but a distinct thermal anomaly at the center of the summit crater was observed in Sentinel-2 images on 24 June, 29 July, 7 October, 12 October, 27 October, 1 November, and 26 November. Sulfur dioxide plumes were detected by the TROPOMI instrument aboard the Sentinel-5P satellite (figure 36) on roughly half the days each month during the reporting period, with fewer in July and more in November.

Figure 36. Strong SO2 plumes from Dukono are depicted from 13 September and 16 October 2023 using data from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, have occurred since 1933. During a major eruption in 1550 CE, a lava flow filled in the strait between Halmahera and the N-flank Gunung Mamuya cone. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

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 (Multiplatform Application for Geohazard Mitigation and Assessment in Indonesia), Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); 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/); 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/).

The current eruption period of Ibu, a stratovolcano on Indonesia’s Halmahera Island, began in April 2008 and has primarily consisted of ash plumes and emissions varying from intermittent to daily, along with occasional thermal anomalies (BGVN 43:12, 48:06). This report updates activity through November 2023, based on daily reports from MAGMA Indonesia, advisories issued by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data. MAGMA Indonesia is a platform developed by PVMBG to improve timely public access to volcano and earthquake activity.

According to PVMBG, from June to November 2023 daily gray-and-white plumes of variable densities usually rose 200-1,000 m above the nested summit craters and drifted in multiple directions. Occasional larger explosions prompted PVMBG to issue 50 Volcano Observatory Notice for Aviation (VONA) ash plume alerts during the reporting period. VONAs were issued every month, ranging from 5 in June to 14 in October; all described the ash plumes as dense. A few plumes rose as high as 1,500 meters during August-November. Clear Sentinel-2 satellite views showed ash plumes on 8 and 13 August, but webcam images showed intermittent ash plumes similar to those seen in recent years.

The MIROVA satellite volcano hotspot detection system recorded frequent thermal anomalies of low to moderate power throughout the reporting period. Data from imaging spectroradiometers aboard NASA’s Aqua and Terra satellites and processed using the MODVOLC algorithm (MODIS-MODVOLC) recorded hotspots on 6 June, 3 August, 30 September, five days in October (5, 10, 15, 26, 31), and four days in November (5, 12, 13, 26). Sentinel-2 satellite images showed a summit crater hotspot in every clear view, along with thermal anomalies extending S from the crater in a partially obscured view on 21 November. White-to-grayish emissions were also visible on clear days.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

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 (Multiplatform Application for Geohazard Mitigation and Assessment in Indonesia), Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); 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/).