The Weekly Volcanic Activity Report: July 26 – August 1, 2023

the weekly volcanic activity report

New activity/unrest was reported for 9 volcanoes from July 26 to August 1, 2023. During the same period, ongoing activity was reported for 18 volcanoes.

New activity/unrest: Bagana, Bougainville (Papua New Guinea) | Fagradalsfjall, Iceland | Kikai, Ryukyu Islands (Japan) | Mayon, Luzon (Philippines) | Rincon de la Vieja, Costa Rica | Sheveluch, Central Kamchatka (Russia) | Shishaldin, Fox Islands (USA) | Ubinas, Peru | Ulawun, New Britain (Papua New Guinea).

Ongoing activity: Aira, Kyushu (Japan) | Cleveland, Chuginadak Island (USA) | Dempo, Southeastern Sumatra | Ebeko, Paramushir Island (Russia) | Fuego, South-Central Guatemala | Great Sitkin, Andreanof Islands (USA) | Ibu, Halmahera | Klyuchevskoy, Central Kamchatka (Russia) | Krakatau, Sunda Strait | Lewotolok, Lembata Island | Lokon-Empung, Sulawesi | Merapi, Central Java | Piton de la Fournaise, Reunion Island (France) | Reventador, Ecuador | Sabancaya, Peru | Santa Maria, Southwestern Guatemala | Semeru, Eastern Java | Suwanosejima, Ryukyu Islands (Japan).

New activity/unrest

Bagana, Bougainville (Papua New Guinea)

6.137°S, 155.196°E | Summit elev. 1855 m

RVO reported that the eruption at Bagana continued during 18-29 July. Intermittent ash emissions drifted NNW, NW, and SW. On 26 July fine ashfall was reported on the coast of Torokina (20 km SW). The ash also drifted toward Laruma (25 km W) and Atsilima (27 km NW). During the night of 28 July a small explosive eruption at 2130 ejected lava fragments from the crater vents, according to reports from Torokina. A lava flow with two lobes was also reported. A second explosion occurred at 2157. Incandescence from the lava flow was observed from Piva as it descended the W flank around 2000 on 29 July. The Alert Level remained at 2 (on a four-level scale).

Geological summary: Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia’s youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue- shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.

Fagradalsfjall, Iceland

63.895°N, 22.258°W | Summit elev. 250 m

IMO reported that lava continued to erupt from main vent at Fagradalsfjall during 26 July through 2 August with no significant changes. Lava fountaining persisted in the active vent according to webcam images throughout the week. The lava effusion rate had decreased compared to the previous week, averaging 5 cubic meters per second during 23-31 July based on calculations from the University of Iceland, the Icelandic Institute of Natural History, and the National Land Survey of Iceland. They also estimated that the total erupted volume was about 15.9 million cubic meters, and the flow field covered an area of about 1.5 square kilometers. During 26 July through 2 August about 150 earthquakes were recorded in the eruptive area; the densest activity occurred around Keilir. The Aviation Color Code remained at Orange (the third level on a four-color scale).

Geological summary: 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.

Kikai, Ryukyu Islands (Japan)

30.793°N, 130.305°E | Summit elev. 704 m

JMA reported that minor eruptive activity was recorded at Satsuma Iwo-jima, a subaerial part of Kikai’s NW caldera rim, during 24-31 July. White gas-and-steam plumes rose 600 m above the crater rim. Surveillance cameras observed nightly incandescence. The Alert Level remained at 2 (on a 5-level scale), and residents were warned to stay 500 m away from the crater.

Geological summary: Kikai is a mostly submerged, 19-km-wide caldera near the northern end of the Ryukyu Islands south of Kyushu. It was the source of one of the world’s largest Holocene eruptions about 6,300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries. Post- caldera eruptions formed Iodake lava dome and Inamuradake scoria cone, as well as submarine lava domes. Historical eruptions have occurred at or near Satsuma-Iojima (also known as Tokara-Iojima), a small 3 x 6 km island forming part of the NW caldera rim. Showa-Iojima lava dome (also known as Iojima-Shinto), a small island 2 km E of Tokara-Iojima, was formed during submarine eruptions in 1934 and 1935. Mild-to-moderate explosive eruptions have occurred during the past few decades from Iodake, a rhyolitic lava dome at the eastern end of Tokara-Iojima.

Mayon, Luzon (Philippines)

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

PHIVOLCS reported that the eruption at Mayon continued during 26 July through 1 August, with slow lava effusion from the summit crater feeding flows on the S, SE, and E flanks. The length of the lava flow in the Mi-Isi (S) drainage remained at 2.8 km, the flow in the Basud (E) drainage remained at 600 m long, and the flow in the Bonga (SE) drainage advanced to 3.4 km on 30 July. Collapses at the lava dome and from the lava flows produced incandescent rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) that descended the Mi-Isi, Bonga, and Basud drainages as far as 4 km. Each day seismic stations recorded 18-114 rockfall events, 0-1 PDC events, and 11-256 low-frequency volcanic earthquakes (LFVQs). During 29-31 July PDC and rockfall events were not detected due to a loss of power at the Anoling, Camalig Observation Station (VMAN). Sulfur dioxide emissions were reported on most days; variable amounts averaged between 1,258 and 4,113 tonnes per day, with the highest value recorded on 28 July. An ash emission was reported during 30-31 July. Beginning around 1800 on 31 July through the morning of 1 August eruptive activity was dominated by 57 events characterized by short, dark ash plumes that rose 100 m above the summit and drifted generally NE (called “ashing” by PHIVOLCS). In addition, there was an increase in the rate of lava effusion from the summit crater; new lava began to rapidly effuse from the crater, feeding the established lava flows on the S, E, and SE drainages. 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.

Rincon de la Vieja, Costa Rica

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

OVSICORI-UNA reported periodic small phreatic events at Rincón de la Vieja during 26-31 July. Small phreatic events were recorded at 1807 on 26 July, 0803 on 28 July, 1250 on 30 July, and 2136 on 31 July. The event on 28 July produced a gas-and-steam plume that rose 500 m above the crater; bad weather conditions on 30 July prevented a height estimation of the accompanying gas-and-steam plume.

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.

Sheveluch, Central Kamchatka (Russia)

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

KVERT reported that the eruption at Sheveluch continued during 20-28 July. Intense fumarolic activity was visible at the active dome, and thermal anomalies were identified in satellite images on 22, 25, and 27 July; the volcano was obscured by clouds during the other days of the week. Explosions generated ash plumes to 4.5 km (14,800 ft) a.s.l. at 0200 UTC on 28 July and 7.7-8 km (25,000-26,000 ft) a.s.l. at 0410 UTC. A resulting ash plume extended NE. Though the Aviation Color Code was briefly raised to Red (the highest on a four-color scale), about an hour later it was lowered back to Orange (the third level on a four-color scale). An ash plume on 28 July rose to 5.5-5.7 km (18,000-18,700 ft) a.s.l. and drifted ENE. 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 was continuing during 26 July through 1 August. Seismicity began to increase around 2200 on 25 July, followed by explosion signals detected using infrasound after 0200 on the 26th. A sustained ash cloud was reported at 6 km (15,000 ft) a.s.l. at 0351 on 26 July that drifted ENE, along with an associated sulfur dioxide plume that drifted NE. Diffuse ash emissions were seen in satellite imagery extending about 125 km from the volcano, though weather clouds began to obscure views beginning around 1130. Low-frequency earthquakes and volcanic tremor decreased, and significant explosions were no longer detected in infrasound data, following the activity early on 26 July. Gas-and-steam emissions from the summit crater were observed in webcam images during 29-30 July. Strong- to-moderately elevated surface temperatures continued to be observed in satellite data throughout the week. Satellite images on 1 August showed that some slumping had occurred on the E crater wall. The Volcano Alert Level remained at Watch (the second highest on a four-level scale) and the Aviation Color Code remained at Orange (the second highest on a four-level scale).

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.

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 26 July through 1 August. According to IGP there were 60 volcano-tectonic earthquakes recorded during 24-30 July indicating rock fracturing and 117 long- period earthquakes signifying the movement of gas and magma. In addition, there were five seismic signals associated with explosive events and 5-13 hours of seismic signals related to ash emissions. The Buenos Aires VAAC reported that intermittent ash emissions detected in satellite imagery rose to 6-7.3 km altitude and drifted NE on 26 July. Ash plumes on 27 July reached 5.5-6.4 km altitude and drifted NE and ENE based on webcam imagery. Ash emissions on 28 July rose to 6.4 km altitude and drifted E and SE. On 29 July small ash plumes rose to 6.7-7.9 km altitude and drifted SE, though they were mostly obscured by weather clouds. IGP reported that at 0957 on 29 July an explosion produced an ash plume that rose 2.5 km above the crater rim and drifted as far as 40 NE, E, and SE. As a result, significant ashfall was reported in the districts of Ubinas (6.5 km SSE) and Matalaque (17 km SSE). 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.

Ulawun, New Britain (Papua New Guinea)

5.05°S, 151.33°E | Summit elev. 2334 m

RVO reported that occasional ash emissions at Ulawun occurred during 25-26 July and drifted SE. Moderate seismicity was variable and was dominated by volcanic tremors. RSAM values ranged between 380 and 800. The Alert Level was raised to Stage 2 (the second highest on the four-level scale) on 27 July.

Geological summary: The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea’s most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

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 24-31 July. A very small eruption was reported on 24 July at Minamidake accompanied by occasional summit crater incandescence. An eruptive event at Showa at 0349 on 25 July produced an ash plume that rose 1 km above the crater rim and ejected blocks traveled up to 200 m from the vent. No incandescence was observed at this crater. Sulfur dioxide emissions averaged 1,600 tons per day on 26 July. 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 was low during 26 July through 1 August. Weather clouds obscured views of the volcano in satellite and webcam images, though minor steaming from the summit was observed during 27-31 July and 1 August. A single earthquake was detected on 26 July. During 28-30 July several earthquakes were detected by the local seismic network and weakly elevated surface temperatures were observed in satellite images. Many of these earthquakes were located less than 6 km below the surface. While these earthquakes are small (less than magnitude 2), they are unusual for Cleveland. The Volcano Alert Level remained at Advisory (the second level on a four-level scale) and the Aviation Color Code remained at Yellow (the second color on a four-color scale).

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.

Dempo, Southeastern Sumatra

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

PVMBG reported that the eruption at Dempo continued during 26 July-1 August. An ash plume at 2215 on 26 July rose 2 km (6,400 ft) above the summit crater and drifted S and SW, based on ground observations. 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.

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 20-27 July. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E), explosions generated ash plumes that rose as high as 4 km (11,500 ft) a.s.l. and drifted SE and E. Thermal anomalies were identified in satellite images during 24-26 July; 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 eruptive activity continued at Fuego during 26 July through 1 August. Low level gas-and-steam emissions drifted SW, W, and NW. Daily counts of weak to moderate explosions averaged 1-10 per hour. Sometimes explosions were accompanied by ash plumes that rose to 4.3-4.8 km (14,100-15,700 ft) a.s.l. and drifted 10-30 km W, NW, and SW. Explosions triggered weak and moderate avalanches that descended the Seca (W), Taniluyá (SW), Ceniza (SSW), La Lajas (SE), Trinidad (S), El Jute (ESE), and Honda (E) drainages. Weak shock waves from the explosions vibrated the roofs and windows of nearby houses. Sounds similar to a jet engine lasting 1-2 minutes were reported on 31 July and 1 August. Minor ashfall was reported in areas downwind, including Panimaché I (7 km SW), Panimaché II (8 km WSW), Morelia (9 km SW), Santa Sofía (12 km SW), El Porvenir (8 km SE), Yucales (12 km SW), Finca Palo Verde (10 km WSW), Sangre de Cristo (8 km W), Yepocapa (9 km WNW), and Acatenango (8 km E). During the night and early morning of 27-28 July incandescent pulses were observed 100-200 m above the crater. Explosions ejected incandescent material up to 150 m above the crater on 30 July. Lahars were reported descending the Las Lajas, El Jute, and Ceniza drainages during 29 July, transporting volcanic blocks up to 1.5 m in diameter, branches, and tree trunks. On 30 July a lahar down the Seca and Ceniza drainages carried blocks up to 3 m in diameter, tree trunks, and branches.

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 26 July through 1 August, producing a thick lava flow in the summit crater. Seismicity remained slightly elevated throughout the week. Weather clouds often obscured satellite and webcam views, though a clear image from 29 July showed moderate steaming from the lava surface. During 27 and 29 July weakly elevated surface temperatures were observed in satellite data that was consistent with cooling lava. 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 26 July through 1 August. Near daily white-and-gray ash emissions rose 200-800 m above the summit and drifted in various directions. A gray ash plume rose 500 m above the summit and drifted N at 0913 on 27 July, according to ground observations. 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.

Klyuchevskoy, Central Kamchatka (Russia)

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

KVERT reported that the Strombolian eruption at Klyuchevskoy continued during 20- 27 July and a daily bright thermal anomaly was identified in satellite images. The new lava flow from 19 July continued to advance 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.

Krakatau, Sunda Strait

6.102°S, 105.423°E | Summit elev. 155 m

PVMBG reported that daily white gas-and-steam plumes rose 25-200 m above Krakatau’s summit during 26 July through 1 August and drifted NW, N, and NE. The Alert Level remained at 3 (on a scale of 1-4), and the public was warned to stay at least 5 km away from the crater.

Geological summary: The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan, and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

Lewotolok, Lembata Island

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

PVMBG reported that the eruption at Lewotolok continued during 26 July through 1 August. Daily white-and-gray plumes rose 5-500 m above the summit and drifted E, SE, NW, and W. On 27 July gray ash plumes rose 500 m above the summit and drifted NW. 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.

Lokon-Empung, Sulawesi

1.358°N, 124.792°E | Summit elev. 1580 m

PVMBG reported continuing daily gas-and-steam emissions at Lokon-Empung during 26 July through 1 August. White plumes with variable densities rose 25-200 m above the crater rim and drifted N, NW, and W. The Alert Level remained at 3 (on a scale of 1-4) and the public was reminded not to approach Tompaluan Crater within a radius of 2.5 km.

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

Merapi, Central Java

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

BPPTKG reported that the eruption at Merapi (on Java) continued during 21-27 July and seismicity remained at elevated levels. White gas-and-steam emissions rose 350 m above the summit at 0910 on 27 July. The SW lava dome produced a total of 254 lava avalanches that descended the W and S flanks; one avalanche traveled 1.5 km down the Sat/Putih drainage, 30 avalanches traveled a maximum distance of 1.8 km down the SW flank upstream from the Boyong drainage, and 222 traveled as far as 2 km down the Bebeng drainage. Morphological changes to the SW lava dome were due to continuing collapses. Based on analysis of an aerial photo taken on 24 June, the volume of the SW lava dome was approximately 2.5 million cubic meters. 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.

Piton de la Fournaise, Reunion Island (France)

21.244°S, 55.708°E | Summit elev. 2632 m

OVPF reported that the eruption that began on 2 July at Piton de la Fournaise was ongoing during 26 July through 1 August, though weather conditions sometimes obscured views. The active cone was about 30 m tall and located on the upper part of Grandes Pentes, SE of Enclos Fouqué, at approximately 1,720 m a.s.l. Volcano-tectonic earthquake events showed an overall decreasing trend throughout the week and remained very low relative to the onset of the eruption. Lava was mainly transported through lava tubes and fed flows that extended 1,200-2,500 m from the cone. The longest part of the flow remained stalled 1.8 km from the road, and by 28 July it had solidified. Breakouts were visible in areas between 900 m and 1,100 m elevation. The total volume of lava effused since the beginning of the eruption through 26 July was an estimated 9.6 +/- 3.4 million cubic meters.

Geological summary: 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.

Reventador, Ecuador

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

IG-EPN reported that the eruption at Reventador was ongoing during 26 July through 1 August. Seismicity was characterized by 26-59 daily explosions, long-period earthquakes, harmonic tremor, and tremor associated with emissions. Weather clouds often hindered visual observations, though crater incandescence was visible on most nights and early mornings, and material was seen descending the flanks. Gas-and-ash plumes rose 600-1,200 m above the crater rim and drifted NW, W, and SW during 25- 26 July. During 26-27 July incandescent material was ejected from the vent and descended the flanks. During the morning of 27 July gas-and-ash emissions rose less than 500 m above the crater rim and drifted W. During the morning of 28 July an incandescent avalanche of material was reported descending the flanks of the volcano. Three gas-and-ash emissions were recorded during the afternoon of 28 July rising to less than 600 m above the crater rim and drifting W. During the morning of 29 July crater incandescence was visible and incandescent material was ejected as far as 400 m onto the flanks. On 30 July a gas-and-ash plume rose less than 400 m above the crater rim and drifted NW. During 30-31 July gas-and-ash plumes rose 200-400 m above the crater rim and drifted NE and SW. Incandescent material was ejected 600 m from the crater, accompanied by blocks rolling down the S flank during 31 July-1 August. Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE) 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.

Sabancaya, Peru

15.787°S, 71.857°W | Summit elev. 5960 m

Instituto Geofísico del Perú (IGP) reported that the eruption at Sabancaya continued during 24-30 July with a daily average of 12 explosions. Gas-and-ash plumes rose as high as 2.3 km above the summit and drifted E, SE, and NE. Six thermal anomalies from the lava dome in the summit crater were detected using satellite data. Minor inflation was detected near the Hualca Hualca sector (4 km N). The Alert Level remained at Orange (the second highest level on a four-color scale) and the public were warned to stay outside of a 12 km radius.

Geological summary: Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning "tongue of fire" in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

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 26 July through 1 August. Weak degassing was reported 500-700 m above the dome. Daily weak to moderate explosions generated gas-and- ash plumes 200-1,000 m above the dome, which drifted NW, W, and SW, and triggered incandescent avalanches down the W, S, E, and SE flanks. Lava effusion at Caliente dome fed a lava flow on the WSW flank, and occasionally produced both avalanches and pyroclastic flows that traveled short distances down the W, S, and E flanks, especially moving toward the Zanjón Seco and San Isidro drainages on the Wand SW flanks. Incandescence was observed at the crater and along lava flow margins during the night and early morning.

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.

Semeru, Eastern Java

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

PVMBG reported that eruptive activity continued at Semeru during 26 July through 1 August. White-and-gray ash emissions of variable densities rose 400-800 m above the summit and drifted N and NE during 27 and 29 July, and S, SE, SW, and W during 30-31 July. At 0515 on 31 July a white-to-gray ash plume rose 1 km (3,200 ft) above the summit and drifted SE and S. On 1 August white-and-gray ash emissions rose 500-700 m above the summit and drifted SE, S, and SW. 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.

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 24- 31 July. An eruptive event reported on 31 July produced an eruption plume that rose 1.2 km above the crater rim. No explosions or ejecta were observed during this time period. Nighttime crater incandescence was visible in webcam images. 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.


Smithsonian Institution / US Geological Survey Weekly Volcanic Activity Report – July 26 – August 1, 2023 – Managing Editor: Sally Sennert


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