The Weekly Volcanic Activity Report: August 30 - September 5, 2023

New activity/unrest was reported for 3 volcanoes from August 30 to September 5, 2023. During the same period, ongoing activity was reported for 22 volcanoes.

New activity/unrest: Fukutoku-Oka-no-Ba, Volcano Islands (Japan) | Kaitoku Seamount, Volcano Islands (Japan) | Villarrica, Central Chile.

New activity/unrest

Fukutoku-Oka-no-Ba, Volcano Islands (Japan)

24.285°N, 141.481°E | Summit elev. -29 m

The Japan Coast Guard flew over Fukutoku-Oka-no-Ba on 15 August and confirmed that an eruption had started on 13 August and was ongoing, according to JMA. The island was horseshoe-shaped and about 1 km in diameter. Floating pumice extended about 60 km NW. The Alert Level was raised from “Volcanic Forecast” to “Volcanic Warning, Sea Area” based on a two-level scale for submarine volcanoes.

Geological summary: Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the island of Minami-Ioto. Water discoloration is frequently observed, and several ephemeral islands have formed in the 20th century. The first of these formed Shin-Ioto (“New Sulfur Island”) in 1904, and the most recent island was formed in 1986. The volcano is part of an elongated edifice with two major topographic highs trending NNW-SSE, and is a trachyandesitic volcano geochemically similar to Ioto.

Kaitoku Seamount, Volcano Islands (Japan)

26.127°N, 141.102°E | Summit elev. -95 m

The Japan Coast Guard flew over Kaitoku Seamount on 23 August and observed discolored water, according to JMA. The Alert Level was raised from “Volcanic Forecast” to “Volcanic Warning, Sea Area” based on a two-level scale for submarine volcanoes.

Geological summary: A submarine eruption was observed in 1984 from Kaitoku Seamount (Kaitoku Kaizan), a three-peaked submarine volcano 130 km NNW of Kita-Iojima. A submarine eruption had previously been reported in 1543 from a point about 40 km SW, which the Japan Meteorological Agency attributes to Kaitoku.

Villarrica, Central Chile

39.42°S, 71.93°W | Summit elev. 2847 m

SERNAGEOMIN reported that a long-period earthquake recorded at 1643 on 2 September was accompanied by a minor ash plume that rose to 180 m above Villarrica’s summit crater rim and dispersed SE. According to the Buenos Aires VAAC, periods of continuous gas-and-ash emissions were visible in webcam images from 1830 on 2 September to 0110 on 3 September. POVI reported that the lava lake was active and during 3-4 September lava fountaining was visible for the first time since 26 March. Fountains captures in webcam images at 2133 on 3 September and 0054 on 4 September rose as high as 60 m above the crater rim and ejected material onto the upper W flank. The Volcanic Alert level remained at Yellow (the second highest on a four-level scale) according to SERNAGEOMIN and the public was warned to stay 500 m away from the crater. SENAPRED maintained the Alert Level at Yellow (the middle level on a three-color scale) for the communities of Villarrica, Pucón (16 km N), Curarrehue, and Panguipulli.

Geological summary: The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic-andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Ongoing activity

Aira, Kyushu (Japan)

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

JMA reported ongoing activity at both Minamidake Crater and Showa Crater (Aira Caldera’s Sakurajima volcano) during 28 August-4 September. Incandescence at Minamidake was observed nightly. An explosion at 0640 on 31 August from the same crater ejected large blocks 400 m away and likely produced a plume that was obscured by weather clouds. Very small eruptive events continued to be detected during the rest of the week. A very small eruption at Showa Crater was also recorded on 4 September. 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.

Dukono, Halmahera

1.693°N, 127.894°E | Summit elev. 1229 m

PVMBG reported that the eruption at Dukono was ongoing during 30 August-4 September. Dense white-and-gray plumes rose as high as 400 m above the summit and drifted E, NW, and W during 31 August and 2-5 September; weather conditions sometimes prevented views. The Alert Level remained at Level 2 (on a scale of 1-4), and the public was warned to remain outside of the 2-km exclusion zone.

Geological summary: 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, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. 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.

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 24-31 August. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E), explosions during 27-31 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 during 27 and 29-30 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.

Epi, Vanuatu

16.68°S, 168.37°E | Summit elev. 833 m

On 31 August the Vanuatu Meteorology and Geo-Hazards Department (VMGD) reported that minor unrest continued at Epi. Volcanic seismicity was sustained, though no surficial activity was observed. The Alert Level remained at 1 (on a scale of 0-5) and the public was warned to stay outside of the Danger Zone, defined as a 2-km radius around the active vent.

Geological summary: A large caldera, with submarine post-caldera cones active in historical time, lies off the eastern coast of Epi Island. Epi Island itself, located slightly west of the main New Hebrides volcanic arc, largely consists of two Quaternary volcanoes, Mount Allombei on the west and Pomare (Tavani Kutali) on the east. Tavani Ruro, which forms an elongated eastern extension of Epi Island across a narrow isthmus, is related to Kuwae caldera to the east. Pomare volcano is the highest point on the island and has three well-preserved subsidiary cones to the east with youthful summit craters. Pomare volcano is truncated on its eastern side by the largely submarine East Epi caldera, which has been the source of all historical eruptions. Three small submarine basaltic and dacitic cones, known as Epi A, Epi B, and Epi C, are located along the northern rim of the breached caldera. Ephemeral islands were formed during eruptions in 1920 and 1953, and the summit of the shallowest cone, Epi B, was at 34 m below sea level at the time of a 2001 survey.

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 30 August-5 September, producing a thick flow in the summit crater that expanded E, based on a satellite image from 30 August and fieldwork. Seismicity was low; two earthquakes were detected during 2-4 September and five were recorded during 4-5 September. An AVO field geology team visited the volcano on 1 September and sampled the lava flow, did airborne photography and thermal imaging surveys, and measured gas emissions. During a visit on 3 September the team observed that the flows were warm and steaming, moving about 1 m every 3-4 days. Weakly elevated surface temperatures were identified in satellite data during 4-5 September. 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.

Ibu, Halmahera

1.488°N, 127.63°E | Summit elev. 1325 m

PVMBG reported that Ibu continued to erupt during 30 August-4 September. Daily white-and-gray ash emissions generally rose as high as 800 m above the summit and drifted E, NE, N, and SW. Gray ash plumes rose as high as 1.5 km on 31 August that drifted N and NE, and as high as 1 km on 5 September and drifted W and NW. The Alert Level remained at a 2 (the second highest level on a four-level scale), and the public was advised to stay outside of the 2 km hazard zone, and to stay 3.5 km away from the N area of the active crater.

Geological summary: 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.

Karangetang, Sangihe Islands

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

PVMBG reported that dense white gas-and-steam plumes from Karangetang were visible daily rising as high as 200 m and drifting multiple directions during 30 August-5 September. Weather clouds sometimes prevented views of the summit. According to news articles, incandescent lava avalanches from Main Crater (S crater) traveled as far as 1.5 km down the Batuawang and Kahetang drainages and as far as 1 km down the Batang, Timbelang, and West Beha drainages. Incandescence at the S and N craters continued to be visible. 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 24-31 August and a daily bright thermal anomaly was identified in satellite images. A plume of ash resuspended by strong winds drifted 95 km E at 3-3.5 km (10,000-11,500 ft) a.s.l., prompting KVERT to raise the Aviation Color Code to Orange (the third level on a four-color scale) at 1240 on 4 September, local time. By 1940 the plume had drifted as far as 170 km E, remining at the same altitudes; the Aviation Color Code was lowered back to Yellow at 1954 (local time). The Aviation Color Code was again raised to Orange for a few hours, during 1532-1808 local time on 5 September, due to plumes of resuspended ash drifting 120 km ENE. KVERT noted that Strombolian activity continued, feeding a lava flow that advanced down the Kozyrevsky drainage on the SW flank. Dates are based on UTC times; specific events are in local time where noted.

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 30 August-5 September. Almost daily white steam-and-gas plumes rose as high as 700 m above the summit and drifted W and NW. White-and-gray ash plumes rose as high as 700 m and drifted E, SW, and W on 1 September. A Volcano Observatory Notices for Aviation (VONA) described an ash plume rising 500 m and drifting SE on 3 September; a webcam image showed incandescent material at the crater and on the upper flank. 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 30 August-5 September, 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 115-196 rockfall events and 1-6 PDC events. There were 2-37 daily volcanic earthquakes; those totals included three tremor events, each with durations of 1-38 minutes, during 1-2 September. Sulfur dioxide emissions were measured on a few days and averaged between 1,673 and 2,247 tonnes per day, with the highest value recorded on 3 September. 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 25-31 August and seismicity remained at elevated levels. The SW lava dome produced a total of 118 lava avalanches that descended the S and SW flanks; two traveled as far as 1 km down the upper part of the Boyong drainage, 115 traveled as far as 2.3 km down the upper Bebeng drainage, and one traveled 500 m down the Senowo drainage. Morphological changes to the SW lava dome were due to continuing collapses of material. No changes were observed at the dome in the main crater. 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.

Popocatepetl, Mexico

19.023°N, 98.622°W | Summit elev. 5393 m

CENAPRED reported that eruptive activity continued at Popocatépetl during 29 August-4 September. Long-period events totaling 30-99 per day were accompanied by steam-and-gas plumes that sometimes contained minor amounts of ash. Periods of volcanic tremor (20-389 minutes) were recorded daily; low- to medium-amplitude, high-frequency tremor during 31 August-2 September was accompanied by continuous gas-and-steam emissions that contained minor amounts of ash, rose 1-2 km above the crater rim, and drifted WSW. A few volcano-tectonic earthquakes were recorded during 30 August-1 September. Minor and moderate explosions were recorded at 1848 on 30 August, at 0032, 1614, and 1702 on 31 August, at 1524 and 1755 on 1 September, at 0442, 0720, 2221 on 3 September, at 1745 on 4 September, and at 0758, 0859, and 1000 on 5 September. Ashfall was reported in Ozumba (18 km W), Atlautla (16 km W), Tepetlixpa (21 km W), and Ecatzingo (15 km SW) in the State of México and in Cuernavaca (65 km WSW), Temixco (67 km WSW), Huitzilac (67 km W), Tepoztlán (49 km W), and Jiutepec (59 km SW) in the State of Morelos on 1 and 5 September. The Alert Level remained at Yellow, Phase Two (the middle level on a three-color scale) and the public was warned to stay 12 km away from the crater.

Geological summary: 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.

Reventador, Ecuador

0.077°S, 77.656°W | Summit elev. 3562 m

IG-EPN reported that the eruption at Reventador was ongoing during 29 August-5 September. Seismicity was characterized by 23-48 daily explosions, long-period earthquakes, volcano-tectonic events, harmonic tremor, and tremor associated with emissions. Crater incandescence was visible overnight during 29 August-3 September; incandescent material was sometimes ejected onto the flanks and then descended as far as 500 m. Daily ash-and-gas plumes rose 600-1,000 m above the crater rim and drifted NW, W, and SW. Weather conditions sometimes prevented views of the volcano. Secretaría de Gestión de Riesgos maintained the Alert Level at Orange (the second highest level on a four-color scale).

Geological summary: Volcán El Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic stratovolcano has 4-km-wide avalanche scarp open to the E formed by edifice collapse. A young, unvegetated, cone rises from the amphitheater floor about 1,300 m to a height comparable to the rim. It has been the source of numerous lava flows as well as explosive eruptions visible from Quito, about 90 km ESE. Frequent lahars in this region of heavy rainfall have constructed a debris plain on the eastern floor of the scarp. The largest recorded eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Rincon de la Vieja, Costa Rica

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

OVSICORI-UNA reported that small phreatic events and gas-and-steam emissions continued at Rincón de la Vieja during 29 August-5 September. Three small eruptive events generated steam-and-gas plumes that rose 2-3 km above the crater rim during 28-29 August. Multiple events produced steam-and-gas plumes during 30-31 August. At 1526 on 1 September an eruptive event generated a steam-and-gas plume that rose 2 km above the crater rim and ejected material onto the flanks. Small events were detected in infrasound data during 2-3 September. At 1251 on 4 September a steam-and-gas plume that rose 1 km above the crater rim and drifted W. 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.

Sangay, Ecuador

2.005°S, 78.341°W | Summit elev. 5286 m

IG-EPN reported a high level of eruptive activity at Sangay during 29 August-5 September, with seismic stations recording 355-723 daily explosions. Nightly webcam images showed incandescent material at the crater, explosions ejecting material above the crater, and incandescent material descending the SE flank as far as 1.8 km. Several ash-and-gas plumes rose as high as 2 km above the crater rim and drifted mainly SW and W each day except for 1 September, when weather clouds prevented views. Secretaría de Gestión de Riesgos maintained the Alert Level at Yellow (the second highest level on a four-color scale).

Geological summary: 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.

Semeru, Eastern Java

8.108°S, 112.922°E | Summit elev. 3657 m

PVMBG reported that eruptive activity continued at Semeru during 30 August-5 September. At 0843 and 0916 on 30 August dense gray-to-white ash plumes rose 500-800 m above the summit and drifted NW and SW, respectively. White gas-and-steam emissions rose as high as 100 m above the summit and drifted in variable directions during 1-3 September; weather clouds sometimes hindered views. The Alert Level remained at 3 (third highest on a scale of 1-4). The public was warned to stay at least 5 km away from the summit in all directions, 13 km from the summit to the SE, 500 m from the banks of the Kobokan drainage as far as 17 km from the summit, and to avoid other drainages including the Bang, Kembar, and Sat, due to lahar, avalanche, and pyroclastic flow hazards.

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

Sheveluch, Central Kamchatka (Russia)

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

KVERT reported that the eruption at Sheveluch continued during 24-31 August. Intense fumarolic activity was visible at the active dome, and daily thermal anomalies were identified in satellite images. A plume of resuspended ash drifted 650 km SE during 30-31 August. Plumes of resuspended ash drifted 110 km E at altitudes of 2.5-3 (8,200-10,000 ft) a.s.l. on 4 September. 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.

Shishaldin, Fox Islands (USA)

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

AVO reported that the eruption at Shishaldin continued during 30 August-5 September. Weather conditions sometimes prevented observations. Daily, small, repetitive explosions were recorded in seismic and infrasound data, though high winds occasionally masked the signals. Elevated surface temperatures at the summit were identified in satellite data on most days. Hot deposits on the NE flank, emplaced during 25-26 August, were visible in a 1 September satellite image, along with steam emissions obscuring the summit vent. Possible incandescence at the summit was visible in nighttime webcam images during 3-4 September, and small steam emissions were visible in daytime images.

Seismicity began to gradually increase at around 0300 on 5 September and activity escalated around 0830. A pilot in the vicinity of the volcano reported an ash plume at about 7.6 km (25,000 ft) a.s.l. at 0842 that was continuing to rise. The ash plume was large and may have risen as high as 9.7 km (32,000 ft) a.s.l. and drifted SSE based on satellite images. 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) at 0901. Seismic amplitude decreased rapidly at around 1100, and remained low, and the altitude of ash emissions observed in satellite images also decreased to an estimated 4.5 km (15,000 ft) a.s.l. By 1200 the lower-altitude portion of the ash plume had drifted 125 km E. Significant ash emissions ended by 1330 based on webcam images. At 1440 AVO lowered the Aviation Color Code to Orange and the Volcano Alert Level to Watch. This event marks the ninth period of elevated eruptive activity resulting in significant ash emissions and mass flows of volcanic debris on the volcano’s flanks since the onset of the current eruption.

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.

Suretamatai, Banks Islands (Vanuatu)

13.8°S, 167.47°E | Summit elev. 921 m

On 31 August the Vanuatu Meteorology and Geohazards Department (VMGD) reported that activity at Suretamatai continued at a level of “minor unrest.” No notable volcanic emissions were identified in the most recent satellite data, though steam emissions from areas around the cone and the Sulphur River continued to be locally observed. The public was reminded that the danger zone was near the cone and the Sulphur River, though the risk to the public was low.

Geological summary: Suretamatai volcano (also known as Soritimeat) forms much of Vanua Lava Island, one of the largest of Vanuatu’s Banks Islands. The younger lavas overlie a number of small older stratovolcanoes that form the island. In contrast to other large volcanoes of Vanuatu, the dominantly basaltic-to-andesitic Suretamatai does not contain a youthful summit caldera. A chain of small stratovolcanoes oriented along a NNE-SSW line gives the low-angle volcano an irregular profile. The youngest cone, near the northern end of the chain, is the largest and contains a lake of variable depth within its 900-m-wide, 100-m-deep summit crater. Activity reported during the 19th century consisted of moderate explosive eruptions.

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 28 August-4 September. Eruptive events produced volcanic plumes that rose as high as 700 m above the crater rim. An eruptive event at 1949 on 4 September produced an ash plume that rose 1 km above the crater rim and drifted 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 28 August-3 September. There were daily averages of 78 volcano-tectonic earthquakes indicating rock fracturing and 42 long-period earthquakes signifying the movement of gas and magma. In addition, seismic signals associated with ash emissions were recorded for an average of 30 minutes per day, with a maximum of 1 hour on 3 September. Webcams recorded ash-and-gas emissions rising as high as 600 m above the crater rim and drifting as far as 5 km E and SE during 28 August-4 September. 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.

Yasur, Vanuatu

19.532°S, 169.447°E | Summit elev. 361 m

On 31 August the Vanuatu Meteorology and Geohazards Department (VMGD) reported that activity at Yasur continued at a high level of “major unrest,” as defined by the Alert Level 2 status (the middle level on a scale of 0-4). Recent satellite observations indicated an increase in steam, gas, and ash emissions from the summit crater. Explosions continued, with some ejecting bombs that landed back in and around the crater. The public was reminded to not enter the restricted area within 600 m around the boundaries of the Permanent Exclusion Zone, defined by Danger Zone A on the hazard map.

Geological summary: Yasur has exhibited essentially continuous Strombolian and Vulcanian activity at least since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island in Vanuatu, this pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide open feature associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

References:

Smithsonian Institution / US Geological Survey – Weekly Volcanic Activity Report – August 30 – September, 2023 – Managing Editor: Sally Sennert.

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