The Weekly Volcanic Activity Report: August 23 - 29, 2023

New activity/unrest was reported for 2 volcanoes from August 23 to 29, 2023. During the same period, ongoing activity was reported for 16 volcanoes.

New activity/unrest: Dempo, Southeastern Sumatra | Shishaldin, Fox Islands (USA).

New activity/unrest

Dempo, Southeastern Sumatra

4.016°S, 103.121°E | Summit elev. 3142 m

Webcam images of Dempo posted in the daily PVMBG reports showed possible patches of discolored water or material floating on the surface of the crater lake during 27-29 August. The Aviation Color Code was raised to Orange (the third color on a four-color scale). 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.

Geological summary: 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.

Shishaldin, Fox Islands (USA)

54.756°N, 163.97°W | Summit elev. 2857 m

AVO reported that the eruption at Shishaldin continued during 23-29 August. Low-level explosive activity likely continued during 23-24 August based on seismic and infrasound data. Elevated surface temperatures at the summit were identified in satellite data; webcam images were obscured by weather clouds. Seismic tremor began increasing at around 0300 on 25 August and was followed by elevated surface temperatures identified in satellite images, consistent with lava erupting at the summit. Small explosions were recorded in infrasound data. At 1204 AVO raised the Aviation Color Code to Red (the highest color on a four-color scale) and the Volcano Alert Level to Warning (the highest level on a four-level scale) in response to a pilot report of an ash plume at 8.5 km (28,000 ft) a.s.l. Seismicity peaked at 1630 and then began to rapidly decline at around 1730. Ash plumes rose as high as 10 km (32,000 ft) a.s.l. and drifted as far as 400 km NE. Ash plume altitudes began to decrease and by 2020 the plumes were rising as high 6.4 km (21,000 ft) a.s.l. and drifting NE. Ash emissions ended at 0000 on 26 August and seismicity was at low levels; AVO lowered the Aviation Color Code to Orange and the Volcano Alert Level to Watch. This large, ash-producing explosive event was the eighth to occur since the eruption started. Minor explosive activity within the summit crater was detected during 26-28 August and strongly elevated surface temperatures visible in satellite imagery. On 26 August a gas plume drifted NE. An AVO field crew working on Unimak Island observed a mass flow that descended the upper flanks beginning around 1720 on 27 August. The flow produced a short-lived ash cloud that rose to around 4.5 km (15,000 ft) a.s.l. and rapidly dissipated. The mass flow was likely caused by the collapse of lava spatter that had accumulated on the summit crater rim.

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

Ongoing activity

Aira, Kyushu (Japan)

31.593°N, 130.657°E | Summit elev. 1117 m

JMA reported ongoing activity at Minamidake Crater (Aira Caldera’s Sakurajima volcano) during 21-28 August. Very small eruptive events occasionally occurred at Minamidake and incandescence was observed at night. On 24 August sulfur dioxide emissions were extremely high at 3,300 tons per day. The Alert Level remained at 3 (on a 5-level scale), and the public was warned to stay 2 km away from both craters.

Geological summary: The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan’s most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu’s largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Cleveland, Chuginadak Island (USA)

52.825°N, 169.944°W | Summit elev. 1730 m

AVO reported that seismicity at Cleveland had decreased over the previous few weeks to background levels. The Aviation Color Code was lowered to Green (the lowest color on a four-color scale) and the Volcano Alert Level was lowered to Normal (the lowest level on a four-level scale). Slightly elevated surface temperatures and diffuse gas emissions from the summit crater continued to occasionally be observed, or normal behavior for Cleveland. Monitoring capabilities had been upgraded to a five-station real-time seismic network, or enough stations to located volcanic earthquakes, based on an AVO partnership with the AVERT (Anticipating Volcanic Eruptions in Real-Time) project at Columbia University.

Geological summary: The beautifully symmetrical Mount Cleveland stratovolcano is situated at the western end of the uninhabited Chuginadak Island. It lies SE across Carlisle Pass strait from Carlisle volcano and NE across Chuginadak Pass strait from Herbert volcano. Joined to the rest of Chuginadak Island by a low isthmus, Cleveland is the highest of the Islands of the Four Mountains group and is one of the most active of the Aleutian Islands. The native name, Chuginadak, refers to the Aleut goddess of fire, who was thought to reside on the volcano. Numerous large lava flows descend the steep-sided flanks. It is possible that some 18th-to-19th century eruptions attributed to Carlisle should be ascribed to Cleveland (Miller et al., 1998). In 1944 it produced the only known fatality from an Aleutian eruption. Recent eruptions have been characterized by short-lived explosive ash emissions, at times accompanied by lava fountaining and lava flows down the flanks.

Ebeko, Paramushir Island (Russia)

50.686°N, 156.014°E | Summit elev. 1103 m

KVERT reported that moderate explosive activity at Ebeko was ongoing during 17-24 August. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E), explosions during 19-22 August generated ash plumes that rose as high as 3.5 km (11,500 ft) a.s.l and drifted to the E and SE. A thermal anomaly was identified in satellite images on 21 August; weather clouds obscured views on other days. The Aviation Color Code remained at Orange (the third level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

Geological summary: The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Fuego, South-Central Guatemala

14.473°N, 90.88°W | Summit elev. 3763 m

INSIVUMEH reported that 3-11 explosions per hour were recorded at Fuego during 9-15 August, generating ash-and-gas plumes that rose as high as 1.1 km above the crater rim. The ash plumes drifted as far as 30 km SW during 23-24 August, causing ashfall in areas downwind including Morelia (9 km SW), Panimaché I and II (8 km SW), and Palo Verde (10 km WSW). During 25-29 August the plumes drifted E, NE, and N, causing ashfall in multiple areas including Alotenango (8 km ENE), La Reunion, El Porvenir (8 km ENE), Antigua Guatemala (18 km NE), San Miguel Dueñas (10 km NE), and Ciudad Vieja (13.5 km NE). Daily shock waves rattled structures in communities around the volcano and rumbling was sometimes heard. Daily block avalanches descended the Ceniza (SSW), Seca (W), Trinidad (S), Taniluyá (SW), Honda, Las Lajas (SE), and El Jute (ESE) drainages. During 23-25 August explosions ejected incandescent material as high as 200 m above the summit. Lahars descended the Ceniza drainage on 29 August, transporting volcanic blocks up to 1.5 m in diameter, branches, and tree trunks.

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

Great Sitkin, Andreanof Islands (USA)

52.076°N, 176.13°W | Summit elev. 1740 m

AVO reported that slow lava effusion continued at Great Sitkin during 23-29 August, producing a thick flow in the summit crater. Seismicity remained slightly elevated throughout the week. Weather clouds often obscured satellite and webcam views, though slightly elevated surface temperatures and steaming from the lava flow were visible during 22-23 August. Steaming from the flow was again visible on 28 August. The Volcano Alert Level remained at Watch (the third level on a four-level scale) and the Aviation Color Code remained at Orange (the third color on a four-color scale).

Geological summary: The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger parasitic 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 ancestral volcano and produced a submarine debris avalanche. Deposits from this and an 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. Historical eruptions have been recorded since the late-19th century.

Karangetang, Sangihe Islands

2.781°N, 125.407°E | Summit elev. 1797 m

According to 17 and 18 August news articles a total of 39 families (104 people) were able to return to Tatahadeng and Tarorane villages because activity at Karangetang had declined. Incandescent avalanches continued to descend the SW, S, and SE flanks, though the number of events and the distanced traveled were lower. PVMBG reported that dense white gas-and-steam plumes were visible on most days rising as high as 150 m and drifting NE, N, and NW during 23-29 August. Weather clouds sometimes prevented views of the summit. Webcam images published in the reports showed incandescence at the summit and from material on the flanks of Main Crater (S crater). According to a news source, incandescent lava avalanches traveled as far as 1.5 km down the Batuawang drainage and 1.8 km down the Kahetang and Keting drainages. Incandescent material was sometimes ejected up to 25 m above the summit. The Alert Level remained at 3 (on a scale of 1-4) and the public were advised to stay 2.5 km away from Main Crater with an extension to 3.5 km on the S and SE flanks.

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

Klyuchevskoy, Central Kamchatka (Russia)

56.056°N, 160.642°E | Summit elev. 4754 m

KVERT reported that the explosive Strombolian eruption at Klyuchevskoy continued during 17-24 August and a daily bright thermal anomaly was identified in satellite images. Lava advanced down the Apakhonchich drainage on the SE flank. The Aviation Color Code remained at Yellow (the second level on a four-color scale).

Geological summary: Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka’s highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Lewotolok, Lembata Island

8.274°S, 123.508°E | Summit elev. 1431 m

PVMBG reported that the eruption at Lewotolok continued during 23-29 August. Daily white steam-and-gas plumes rose as high as 500 m above the summit and drifted W and NW, though the plumes in the webcam images in the reports appeared to contain ash. White-and-gray ash plumes rose as high as 500 m and drifted on 28 August. Volcano Observatory Notices for Aviation (VONAs) described ash plumes rising 700 m and drifting W and NW at 1409 on 23 August and 1603 on 24 August. The Alert Level remained at 2 (on a scale of 1-4) and the public was warned to stay at least 2 km away from the summit crater.

Geological summary: The Lewotolok (or Lewotolo) stratovolcano occupies the eastern end of an elongated peninsula extending north into the Flores Sea, connected to Lembata (formerly Lomblen) Island by a narrow isthmus. It is symmetrical when viewed from the north and east. A small cone with a 130-m-wide crater constructed at the SE side of a larger crater forms the volcano’s high point. Many lava flows have reached the coastline. Eruptions recorded since 1660 have consisted of explosive activity from the summit crater.

Mayon, Luzon (Philippines)

13.257°N, 123.685°E | Summit elev. 2462 m

PHIVOLCS reported that the eruption at Mayon continued during 23-29 August, with slow lava effusion from the summit crater feeding flows on the S, SE, and E flanks. The lengths of the lava flow in the Mi-Isi (S), Bonga (SE), and Basud (E) drainages remained at 2.8 km, 3.4 km, and 1.1 km, respectively. Collapses at the lava dome and from the lava flows produced incandescent rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) that descended the three drainages as far as 4 km. Each day seismic stations recorded 109-186 rockfall events and 1-6 PDC events. There were 2-56 daily volcanic earthquakes; those totals included 1-39 tremor events, each with durations of 1-36 minutes, during 23-27 August. Sulfur dioxide emissions were measured on a few days and averaged between 735 and 1,298 tonnes per day, with the highest value recorded on 26 August. The Alert Level remained at 3 (on a 0-5 scale) and residents were reminded to stay away from the 6-km-radius Permanent Danger Zone (PDZ). PHIVOLCS recommended that civil aviation authorities advise pilots to avoid flying close to the summit.

Geological summary: Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Merapi, Central Java

7.54°S, 110.446°E | Summit elev. 2910 m

BPPTKG reported that the eruption at Merapi (on Java) continued during 18-24 August and seismicity remained at elevated levels. The SW lava dome produced a total of 144 lava avalanches that descended the S and SW flanks; six traveled as far as 1.5 km down the upper part of the Boyong drainage and 138 traveled as far as 1.8 km down the upper Bebeng drainage. Morphological changes to the SW lava dome were due to continuing collapses of material. The Alert Level remained at 3 (on a scale of 1-4), and the public was warned to stay 3-7 km away from the summit based on location.

Geological summary: Merapi, one of Indonesia’s most active volcanoes, lies in one of the world’s most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Nevado del Ruiz, Colombia

4.892°N, 75.324°W | Summit elev. 5279 m

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that the eruption at Nevado del Ruiz continued at low-to-moderate levels during 22-28 August. Seismicity indicating the movement of fluids increased in number and magnitude compared to the week before; these seismic signals were most notable on 26 August and were associated with pulsating, or occasionally continuous, emissions of gas and ash. Seismicity indicating rock fracturing decreased compared to the previous week. These events were generally located below Arenas Crater and in areas within 8 km to the SE, NE, and NW at depths of less than 8 km. Ash-and-gas emissions continued, with the highest plumes rising as high as 1.3 and 2.6 km above the crater rim on 26 and at 1745 on 27 August, respectively. Plumes drifted NW, WNW, and SW and caused ashfall in Manizales (27 km NW) and Dosquebradas (40 km WSW) on 23 August, and on several other occasions during 26-28 August. The Alert Level remained at Yellow, Level III (the second level on a four-level scale).

Geological summary: Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America’s deadliest eruption.

Rincon de la Vieja, Costa Rica

10.83°N, 85.324°W | Summit elev. 1916 m

OVSICORI-UNA reported that small phreatic events continued to be recorded at Rincón de la Vieja during 23-29 August. Four small events were recorded during 1900 om 24 August and 0828 on 25 August. The event at 0828 produced a steam-and-gas plume that rose 3 km above the crater rim and drifted NW. Four small events were also recorded during 27-28 August; the event at 0813 on 28 August lasted two minutes and generated a steam-and-gas plume that rose 2.5 km above the crater rim. The Alert Level remained at Level 3, Orange, the third level on a four-level scale.

Geological summary: Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the “Colossus of Guanacaste,” it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Santa Maria, Southwestern Guatemala

14.757°N, 91.552°W | Summit elev. 3745 m

INSIVUMEH reported that eruptive activity continued at Santa Maria’s Santiaguito lava dome complex during 23-29 August. Incandescence from the dome was visible during most nights and early mornings, and occasionally from the SW lava flow. Lava extrusion continued and caused dome collapses and occasional short pyroclastic flows. Lava flows remained active in the Zanjón, Seco, and San Isidro drainages. Daily weak-to-moderate explosions generated gas-and-ash plumes that rose 700-1,000 m above the dome and drifted in multiple directions. Some explosions also triggered incandescent avalanches that descended the dome’s flanks in all directions, and into the Zanjón, Seco, and San Isidro drainages. Ashfall was reported in Belén (10 km S), Calaguache (9 km S), Santa María de Jesús (5 km SE) during 25-26 and 28-29 August.

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

Sheveluch, Central Kamchatka (Russia)

56.653°N, 161.36°E | Summit elev. 3283 m

KVERT reported that the eruption at Sheveluch continued during 17-24 August. Intense fumarolic activity was visible at the active dome, and daily thermal anomalies were identified in satellite images. The Aviation Color Code remained at Orange (the third level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

Geological summary: The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka’s largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Suwanosejima, Ryukyu Islands (Japan)

29.638°N, 129.714°E | Summit elev. 796 m

JMA reported that the eruption at Suwanosejima’s Ontake Crater continued during 21-28 August. Eruptive events produced volcanic plumes that rose as high as 1.4 km above the crater rim and produced ashfall in Toshima village (3.5 km SSW). Events at 0544, 0742, 0824, 1424, and 1704 on 25 August produced ash plumes that rose 1.1-1.2 km above the crater rim and drifted NE, W, and SW. The Alert Level remained at 2 (on a 5-level scale) and the public was warned to stay at least 1 km away from the crater.

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

Ubinas, Peru

16.355°S, 70.903°W | Summit elev. 5672 m

Instituto Geofísico del Perú (IGP) reported that the eruption at Ubinas continued during 22-27 August. There were daily averages of 229 volcano-tectonic earthquakes indicating rock fracturing and 54 long-period earthquakes signifying the movement of gas and magma. In addition, seismic signals associated with ash emissions were recorded for an average of eight hours per day, with a maximum of 17 hours on 26 August. An explosion at 1757 on 25 August generated an ash-and-gas plume that rose 4.2 km above the crater rim and drifted up to 25 km in multiple directions. According to the Buenos Aires VAAC diffuse ash-and-gas puffs, both daily and with periods of continuous emissions, reached 6.1-7.3 km (20,000-24,000 ft) a.s.l. (up to 1.6 km above the summit) and drifted multiple directions. The Alert Level remained at Orange (the third level on a four-color scale) and the public was warned to stay 4 km away from the crater.

Geological summary: A small, 1.4-km-wide caldera cuts the top of Ubinas, Perú’s most active volcano, giving it a truncated appearance. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45 degrees. The steep-walled, 150-m-deep summit caldera contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one of Holocene age about 1,000 years ago. Holocene lava flows are visible on the flanks, but historical activity, documented since the 16th century, has consisted of intermittent minor-to-moderate explosive eruptions.

References:

Smithsonian Institution / US Geological Survey – Weekly Volcanic Activity Report, August 23 – 29, 2023 – Managing Editor: Sally Sennert.

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