The Weekly Volcanic Activity Report: May 31 – June 6, 2023

the weekly volcanic activity report

New activity/unrest was reported for 7 volcanoes from May 31 to June 6, 2023. During the same period, ongoing activity was reported for 22 volcanoes.

New activity/unrest: Ahyi, Mariana Islands (USA) | Karangetang, Sangihe Islands | Kilauea, Hawaiian Islands (USA) | Mayon, Luzon (Philippines) | Nyamulagira, DR Congo | Popocatepetl, Mexico | Rincon de la Vieja, Costa Rica.

Ongoing activity: Aira, Kyushu (Japan) | Bezymianny, Central Kamchatka (Russia) | Bulusan, Luzon (Philippines) | Cotopaxi, Ecuador | Ebeko, Paramushir Island (Russia) | Fuego, South-Central Guatemala | Great Sitkin, Andreanof Islands (USA) | Ibu, Halmahera | Iliamna, Alaska Peninsula | Krakatau, Sunda Strait | Lewotolok, Lembata Island | Merapi, Central Java | Nevado del Ruiz, Colombia | Nyiragongo, DR Congo | Sabancaya, Peru | Sangay, Ecuador | Santa Maria, Southwestern Guatemala | Semeru, Eastern Java | Semisopochnoi, Aleutian Islands (USA) | Sheveluch, Central Kamchatka (Russia) | Suwanosejima, Ryukyu Islands (Japan) | Taal, Luzon (Philippines).

New activity/unrest

Ahyi, United States

20.42°N, 145.03°E; summit elev. -75 m

Unrest at Ahyi Seamount continued during 30 May-7 June. A possible hydroacoustic signal was detected by pressure sensors on Wake Island (2,270 km E) during 2-3 June. No activity was visible in cloudy satellite images. The Aviation Color Code remained at Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level remained at Advisory (the second lowest level on a four-level scale).

Geological summary: Ahyi seamount is a large conical submarine volcano that rises to within 75 m of the sea surface about 18 km SE of the island of Farallon de Pajaros (Uracas) in the northern Marianas. Water discoloration has been observed there, and in 1979 the crew of a fishing boat felt shocks over the summit area of the seamount, followed by upwelling of sulfur-bearing water. On 24-25 April 2001 an explosive eruption was detected seismically by a station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 15 km) at a location near the southern base of Ahyi. An eruption in April-May 2014 was detected by NOAA divers, hydroacoustic sensors, and seismic stations.

Karangetang, Indonesia

2.781°N, 125.407°E; summit elev. 1797 m

Webcam images of Karangetang published in PVMBG daily reports periodically showed incandescence at Main Crater (S crater) and from material on the flanks of Main Crater during 31 May-6 June. Daily white gas-and-steam plumes were visible rising as high as 150 m above the summit and drifting in various directions. According to the Darwin VAAC ash plumes rose as high as 2.1 km (7,000 ft) a.s.l. and drifted SE and E during 2-4 June. 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. The Alert Level remained at 3 (on a scale of 1-4) and the public was warned to stay 2.5 km away from the craters on the S and SW flanks and 1.5 km away on the other 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.

Kilauea, United States

19.421°N, 155.287°W; summit elev. 1222 m

HVO reported that earthquake activity and changes in the patterns of ground deformation beneath Kilauea’s summit began to be detected during the evening of 6 June. The data indicated magma movement towards the surface, prompting HVO to raise the Volcano Alert Level to Watch (the second highest level on a four-level scale) and the Aviation Color Code to Orange (the second highest level on a four-color scale).

At about 0444 on 7 June incandescence in Halema’uma’u Crater was visible in webcam images indicting that a new eruption had begun. The Volcano Alert Level was raised to Warning and the Aviation Color Code was raised to Red. Lava flowed from fissures that had opened on the crater floor.

Geological summary: Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Mayon, Philippines

13.257°N, 123.685°E; summit elev. 2462 m

Daily visual and webcam observations of Mayon’s summit crater revealed more frequent rockfalls at the summit lava dome starting in the last week of April, indicating aseismic dome growth. The lava dome increased in volume by about 83,000 cubic meters during 3 February-9 May, with a total addition of nearly 164,000 cubic meters since 20 August 2022. Sulfur dioxide emission averages were as high as 576 tons per day on 29 April and 162 tons per day on 23 May. A total of 26 volcanic earthquakes had been recorded since 1 April. Electronic Distance Measuring (EDM), precise leveling, continuous GPS, and electronic tilt monitoring data showed that the volcano had been slightly inflated, especially on the NW and SE flanks, since 2020. Short-term inflation on the upper flanks had been detected since February.

From 0500 on 4 June to 0500 on 5 June the number of rockfalls increased from an average of 5 events per day to 49 events per day. At 1000 on 5 June the Alert Level was raised to 2 (on a 0-5 scale). PHIVOLCS noted that although low-level volcanic earthquakes, ground deformation, and volcanic gas emissions indicated unrest, the steep increase in rockfall frequency may indicate increased dome activity. Residents were reminded to stay away from the 6-km-radius Permanent Danger Zone (PDZ).

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.

Nyamulagira, DR Congo

1.408°S, 29.2°E; summit elev. 3058 m

On 1 June the Observatoire Volcanologique de Goma (OVG) reported that seismicity at Nyamulagira remained at lower levels, similar to those recorded before the 17 May increase in activity. Incandescence above the crater had been absent for the past three days, but satellite imagery showed continuing lava effusion within the summit crater. The recent flows on the flanks covered an estimated 0.6 square kilometers.

Geological summary: Africa’s most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 13 km NNW of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.

Popocatepetl, Mexico

19.023°N, 98.622°W; summit elev. 5393 m

CENAPRED reported that during 30 May-6 June activity at Popocatépetl consisted of seismic tremor, a few explosions, emissions of steam and gas, with occasional ash, and ejections of incandescent material. There were 67-315 daily steam-and-gas emissions, sometimes containing minor amounts of ash, with the highest number recorded during 30-31 May. Overall activity decreased during the week. Explosions at 1423 and 1708 on 30 May produced gray ash plumes. During 30-31 May incandescent material was ejected from the vent short distances onto the flank. High-frequency tremor was recorded by the seismic network for around eight and a half hours, and was associated with nearly continuous emissions of steam, gas, and ash. Ashfall was reported in Ayapeango (22 km NW) and Acatzingo (100 km W), in the State of Mexico. A M1.6 volcano tectonic (VT) earthquake was recorded at 0952 on 31 May. The Washington VAAC stated that although ash emissions continued to be visible in satellite and webcam images drifting SSE, the intensity of the emissions had decreased.

CENAPRED stated on 1 June that tremor signals had significantly decreased during the previous few days, and on 2 June that overall activity had also decreased. Periods of high-frequency, low-amplitude tremor continued to be detected during the rest of the week. A period of tremor recorded during 1635-1850 on 3 June was associated with diffuse ash emissions that drifted SE. According to the Washington VAAC ash plumes during 1-3 June rose 5.5-6.7 km (18,000-22,000 ft) a.s.l., or as high as 1.3 km above the summit, and drifted SW, SSW, and SE. A minor explosion occurred at 0739 on 4 June. A minor explosion occurred at 1211 on 5 June and a M1.2 VT earthquake was recorded at 1818. On 6 June the Alert Level was lowered to Yellow, Phase Two (the middle level on a three-color scale).

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.

Rincon de la Vieja, Costa Rica

10.83°N, 85.324°W; summit elev. 1916 m

OVSICORI-UNA reported that increased eruptive activity continued at Rincón de la Vieja during 30 May-6 June. Seismic instruments continued to record low-magnitude volcano-tectonic earthquakes located S of the Pailas sector of Parque Nacional Rincón de la Vieja. Reports describing phreatic eruption events were issued almost daily. A phreatic event at 2215 on 30 May ejected incandescent material within the vicinity of the crater. A small phreatic eruption at 1753 on 31 May generated a small pyroclastic flow that traveled a short distance from the crater. Moderate phreatic eruptions were recorded during 1-2 June; the most energetic event occurred at 0902 and generated a plume of steam, gas, and ash that rose to 1.5 km above the crater and drifted N. OVSICORI-UNA raised the Alert Level from 2 to 3 (the third highest on a four-level scale) at 1650 on 2 June due to significant seismicity and significant emissions recorded during May.

Eruptive events were recorded during 3-4 June. The most energetic event occurred at 0624 on 3 June and generated a steam-and-gas plume that rose 1.5 km and drifted W. An eruptive event at 0526 on 4 June first generated emissions that rose to 500 m, followed at 0529 by a second plume of steam-and-gas rose to 3.5 km above the crater and drifted N. Steam-and-gas plumes with low ash content were generated from eruptive events during the nights of 4-5 June; the plumes rose 1-1.5 km above the crater and drifted W. At 0259 on 6 June a small phreatic eruption generated a plume that rose to 3 km and drifted NW.

During a press conference held by OVISOCORI-UNA, RSN, and CNE on 5 June, the public was reminded that although the recent volcanic activity is normal for Rincon de la Vieja, it is still necessary to remain cautious and that community emergency committees will remain activated. CNE maintained a Green Alert (first alert level on a four-color scale) for the districts of Dos Ríos (13 km N) and Aguas Claras (3 km NW) in the canton of Upala (22 km ENE), as well as the districts of Cañas Dulces (24 km ESE), Curubandé (18 km SW), and Mayorga (15 km W) in the canton of Liberia (21 km SW). The CNE reminded the public to stay away from the Azul, Pénjamo, and Azufrada rivers.

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.

Ongoing activity

Aira, 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 29 May-5 June. On 29 May sulfur dioxide emissions were high at 2,900 tons per day. Crater incandescence was observed nightly at both craters during 29 May-2 June, and very small eruptive events periodically occurred. Eruptive events at Minamidake at 0237 and 0454 on 4 June produced ash plumes that rose about 1.1 km above the crater rim. An explosion at 0012 on 5 June generated an ash plume that rose 1 km and drifted SE, and ejected blocks 500-700 m from the crater. At Showa, eruptive events at 0211, 0352, 0440, and 1436 on 5 June generated ash plumes that rose 1.3-1.5 km above the crater rim and drifted SE and E, or rose straight up; blocks were ejected as far as 300 m from the crater. The Alert Level remained at 3 (on a 5-level scale), and residents were 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.

Bezymianny, Russia

55.972°N, 160.595°E; summit elev. 2882 m

KVERT reported that eruptive activity at Bezymianny was generally characterized by lava effusion, gas-and-steam emissions, and lava dome incandescence during 25 May-1 June. A daily thermal anomaly was identified in satellite images. The Aviation Color Code remained at Yellow (the second lowest level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

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

Bulusan, Philippines

12.769°N, 124.056°E; summit elev. 1535 m

In a special advisory, PHIVOLCS reported that from 0500 on 31 May to 1500 on 1 June the seismic network at Bulusan recorded a total of 19 volcanic earthquakes. Out of those, five were located at depths of 2.7-6.6 km beneath the E part of the volcano and had local magnitudes of 1.8-2.7. Minor white steam emissions from the summit crater and active vents on the SE flank were occasionally visible. Ground deformation data from electronic tiltmeter stations continued to record short-term inflation of the SE flanks, first detected in December 2022. The Alert Level remained at 0 (the lowest level on a scale of 0-5) and PHIVOLCS reminded the public not to enter the 4-km-radius Permanent Danger Zone (PDZ).

Geological summary: Luzon’s southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit is unvegetated and contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded since the mid-19th century.

Cotopaxi, Ecuador

0.677°S, 78.436°W; summit elev. 5911 m

IG reported that moderate eruptive activity continued at Cotopaxi during 30 May-6 June. Seismic activity was mainly characterized by long-period earthquakes and tremors associated with daily emissions. Although weather clouds often obscured views, emissions were visible almost daily. During 30-31 May ash-and-gas emissions rose as high as 500 m above the summit and drifted W and NW. A tremor signal associated with an ash emission was detected on 1 June, though weather clouds prevented visual confirmation; ashfall was reported in San Ramón (108 km N) and San Agustín de Callo (16 km WSW). Multiple ash emissions were reported on 3 June; ash plumes rose as high as around 1 km above the summit and drifted SW, W, and NW. During 4-5 June several gas-and-ash emissions rose 400-800 m and drifted W and SW. Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE) maintained the Alert Level at Yellow (the second lowest level on a four-color scale).

Geological summary: The symmetrical, glacier-covered, Cotopaxi stratovolcano is Ecuador’s most well-known volcano and one of its most active. The steep-sided cone is capped by nested summit craters, the largest of which is about 550 x 800 m in diameter. Deep valleys scoured by lahars radiate from the summit of the andesitic volcano, and large andesitic lava flows extend to its base. The modern edifice has been constructed since a major collapse sometime prior to about 5,000 years ago. Pyroclastic flows (often confused in historical accounts with lava flows) have accompanied many explosive eruptions, and lahars have frequently devastated adjacent valleys. Strong eruptions took place in 1744, 1768, and 1877. Pyroclastic flows descended all sides of the volcano in 1877, and lahars traveled more than 100 km into the Pacific Ocean and western Amazon basin. Smaller eruptions have been frequent since that time.

Ebeko, Russia

50.686°N, 156.014°E; summit elev. 1103 m

KVERT reported that moderate activity at Ebeko was ongoing during 25 May-1 June. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E) explosions during 25 and 27-28 May generated ash plumes that rose as high as 3.5 km (11,500 ft) a.s.l. and drifted NE. A thermal anomaly was identified in satellite images on 26 May and 1 June. The Aviation Color Code remained at Orange (the second highest 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, Guatemala

14.473°N, 90.88°W; summit elev. 3763 m

INSIVUMEH reported that daily gas emissions rose from Fuego during 31 May-4 June. During 2-3 June minor explosions occurring at a rate of two per hour produced diffuse ash plumes that rose 450 m above the summit and drifted 10 km W and SE. Minor ashfall was reported in El Zapote (10 km SSE), La Rochela (8 km SSW), and San Andrés Osuna (12 km SSW).

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, United States

52.076°N, 176.13°W; summit elev. 1740 m

AVO reported that slow lava effusion continued at Great Sitkin during 30 May-6 June producing a thick lava flow confined to the summit crater. Seismicity remained low; a few local earthquakes were recorded daily. Slightly elevated surface temperatures were identified in satellite images during 30-31 May and 3-4 June. Satellite data during 5-6 June confirmed that the flow was expanding E. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest level 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, Indonesia

1.488°N, 127.63°E; summit elev. 1325 m

PVMBG reported that Ibu continued to erupt during 31 May-6 June. White-and-gray ash plumes of variable densities rose as high as 1 km above the summit and drifted in multiple directions during 31 May-1 June. According to the Darwin VAAC ash plume rose 2.1-2.4 km (7,000-8,000 ft) a.s.l. and drifted SE, E, and NE on 2 and 5 June. 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 radius, 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.

Iliamna, United States

60.032°N, 153.09°W; summit elev. 3053 m

AVO reported that seismic activity at Iliamna began to increase at around 1200 on 5 June. Initially earthquakes occurred about every one minute, then became more closely spaced. The source of the activity was possibly from movement of magma or hydrothermal fluids beneath the volcano, though similar activity had been observed before large mass movements or avalanches; AVO could not rule out either. The Aviation Color Code was raised to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level was raised to Advisory (the second lowest level on a four-level scale).

The earthquake activity culminated in an ice-rock avalanche just before 1714. There was no visual confirmation, but the signals matched historical observations associated with avalanches at Red Glacier on the E flank. Seismicity declined to background levels. The Aviation Color Code was lowered to Green (the lowest level on a four-color scale) and the Volcano Alert Level was lowered to Normal (the second lowest level on a four-level scale).

Geological summary: Iliamna is a prominentglacier-covered stratovolcano in Lake Clark National Park on the western side of Cook Inlet, about 225 km SW of Anchorage. Its flat-topped summit is flanked on the south, along a 5-km-long ridge, by the prominent North and South Twin Peaks, satellitic lava dome complexes. The Johnson Glacier dome complex lies on the NE flank. Steep headwalls on the S and E flanks expose an inaccessible cross-section of the volcano. Major glaciers radiate from the summit, and valleys below the summit contain debris-avalanche and lahar deposits. Only a few major Holocene explosive eruptions have occurred from the deeply dissected volcano, which lacks a distinct crater. Most of the reports of historical eruptions may represent plumes from vigorous fumaroles E and SE of the summit, which are often mistaken for eruption columns (Miller et al., 1998). Eruptions producing pyroclastic flows have been dated at as recent as about 300 and 140 years ago, and elevated seismicity accompanying dike emplacement beneath the volcano was recorded in 1996.

Krakatau, Indonesia

6.102°S, 105.423°E; summit elev. 155 m

PVMBG reported that at 1434 on 6 June a dense gray ash plume from Anak Krakatau rose around 500 m above the summit and drifted NW. 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, Indonesia

8.274°S, 123.508°E; summit elev. 1431 m

PVMBG reported that the eruption at Lewotolok continued during 31 May-6 June. Ash plumes were periodically visible through the week. Dense white-and-gray ash plumes rose as high as 600 m above the summit and drifted W and NW on 1 June. Ash plumes on 2 June rose as high as 1 km and drifted W and SW. A dense ash plume rose 550 m and drifted SW on 5 June. 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 in all directions.

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.

Merapi, Indonesia

7.54°S, 110.446°E; summit elev. 2910 m

BPPTKG reported that the eruption at Merapi (on Java) continued during 26 May-1 June and seismicity remained at elevated levels. The SW lava dome produced 155 lava avalanches that traveled as far as 2 km down the SW flank (upstream in the Bebeng and Boyong drainages) and one that traveled 500 m NW (upstream of the Senowo River). Morphological changes to the SW lava dome due to continuing collapses of material were evident in webcam images. 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 during 31 May-6 June and was characterized by periodic gas, steam, and ash emissions, and thermal anomalies at the lava dome in Arenas Crater. Seismicity fluctuated at low levels; on 31 May SGC stated that during the past several days seismicity had decreased compared to the previous weeks. Daily gas-and-steam emissions were visible in webcam images and contained ash on most days; emissions rose as high as 2 km above the crater and mainly drifted NW. Ash emissions were confirmed in satellite images on the other days according to the Washington VAAC. A significant thermal anomaly was observed within the crater on 31 May. That same day a sulfur odor was reported in Cerro Gualí. Minor ashfall was reported in the municipalities of Villamaría (28 km NW) and Manizales (28 km NW) on 4 June. The Alert Level was remained at Orange, Level II (the second highest 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.

Nyiragongo, DR Congo

1.52°S, 29.25°E; summit elev. 3470 m

The Observatoire Volcanologique de Goma (OVG) characterized activity at Nyiragongo during 27 May-4 June as normal. Sulfur dioxide emissions were low. Faint incandescence at the crater was observed at 1900 on 4 June. The Alert Level remained at Yellow.

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

Sabancaya, Peru

15.787°S, 71.857°W; summit elev. 5960 m

Instituto Geofísico del Perú (IGP) reported moderate levels of activity at Sabancaya during 29 May-4 June with a daily average of 24 explosions. Gas-and-ash plumes rose as high as 2.1 km above the summit and drifted NW and W. Six thermal anomalies originating from the lava dome in the summit crater were identified in satellite data. Minor inflation continued to be detected near Hualca Hualca (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.

Sangay, Ecuador

2.005°S, 78.341°W; summit elev. 5286 m

IG reported that the eruption at Sangay continued at a high level during 30 May-6 June, though weather clouds often prevented visual observations. The seismic network recorded 504-528 explosions per day during 30 May-2 June and 158-384 per day during the rest of the week. Periods of occasional-to-frequent ash plumes were reported almost daily. Incandescence at the crater was visible during 31 May and 3-4 June; incandescent material traveled 1 km down the SE flank. Ashfall was reported on 1 June in Cebadas Parish, Chimborazo Province (33 km WNW). On 4 June an ash plume rose to 1.1 km above the crater and drifted W and SW. During 4-5 June incandescent material traveled 1 km down the SE flank. Several steam-and-ash plumes rose as high as 1.1 km above the summit and drifted W. Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE) maintained the Alert Level at Yellow (the second lowest 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.

Santa Maria, Guatemala

14.757°N, 91.552°W; summit elev. 3745 m

INSIVUMEH reported that the eruption at Santa María’s Santiaguito lava-dome complex continued during 31 May-6 June. Effusion from the Caliente dome complex fed lava flows that descended the San Isidro and Zanjón Seco drainages on the W and SW flanks; the main lava flow was 4.3 km long and remained active. Avalanches of material from the growing dome and occasional explosions descended all flanks of the dome, and avalanches from the margins of lava flows descended the S and SW flanks. Incandescence from the dome and lava flows was visible during the nights and early mornings. An average of 1-2 explosions per hour were recorded on most days, generating ash-and-steam plumes that rose up to 1 km above the dome and on some days drifted S and SE.

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, Indonesia

8.108°S, 112.922°E; summit elev. 3657 m

PVMBG reported that the eruption at Semeru continued during 31 May-6 June. Steam-and-gas plumes were occasionally visible, though weather clouds often obscured visual observations. White-and-gray ash plumes rose 300-500 m above the summit and drifted W and NW on 3 June. 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.

Semisopochnoi, United States

51.93°N, 179.58°E; summit elev. 1221 m

AVO reported that low-level unrest continued at Semisopochnoi during 30 May-6 June. Seismicity remained low and few earthquakes were detected. Minor steam emissions were occasionally visible in webcam images on 31 May and 2 and 4 June. The Volcano Alert Level remained at Advisory (the second highest on a four-level scale) and the Aviation Color Code remained at Yellow (the second highest color on a four-color scale).

Geological summary: Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island’s northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

Sheveluch, Russia

56.653°N, 161.36°E; summit elev. 3283 m

KVERT reported that the eruption at Sheveluch was ongoing during 25 May-1 June. Intense fumarolic activity at the active crater was likely associated with growth of Karan lava dome. A thermal anomaly over the active crater and Karan dome area was identified in satellite images during 25-30 May; weather clouds obscured the volcano on the other days. Plumes of ash, originally deposited during the 10-13 April eruption and resuspended by strong winds, were visible in satellite images drifting 120 km ESE during 27-28 May. The Aviation Color Code remained at Orange (the second highest 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, Japan

29.638°N, 129.714°E; summit elev. 796 m

JMA reported that the eruption at Suwanosejima’s Ontake Crater continued during 29 May-5 June. At 1407 on 30 May an explosion generated an ash plume that rose 800 m. Incandescence at the crater was visible at night during 2-5 June. Four eruptive events occurred during 4-5 June. Ash plumes rose to 1.1 km and drifted E and S at 1455 and 2327 on 4 June, respectively. At 1037 and 2349 on 5 June ash plumes rose 1-1.1 km and drifted E. The Alert Level remained at 3 (on a 5-level scale) and residents were warned to stay 2 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.

Taal, Philippines

14.002°N, 120.993°E; summit elev. 311 m

In a 2 June advisory PHIVOLCS reported continuing low-level unrest at Taal. Starting at 0635 on 2 June relatively weak but continuous volcanic tremor located at shallow depths along the Daang Kastila fissure was recorded by all 15 seismic stations of the Taal Volcano Network. At the same time webcams recorded upwelling in Main Crater Lake on Taal Volcano Island (TVI) and more intense thermal anomalies in the N portion of the lake. Pronounced inflation in the SW part of Taal Volcano Island was detected towards the end of May, following a longer phase of deflation in that same sector. Sulfur dioxide gas emissions had slightly increased during the previous two weeks, averaging 5,360 tonnes per day during 22 May-1 June, higher than the 3,000 tonnes per day average recorded during 1 April-21 May. Emissions averaged 5,831 tonnes per day on 1 June. PHIVOLCS stated that a new phase of magma degassing at depth was likely driving the increased shallow hydrothermal activity.

At 2230 on 3 June visible upwelling of volcanic fluids in the lake produced voluminous steam-rich plumes that rose 3 km above TVI. Significant vog was detected in the caldera and reported by residents in the municipalities of Balete (E of Taal Lake), and Laurel and Agoncillo (both W of Taal Lake), Batangas. Upwelling of how fluids in the lake continued during 4-7 June and steam-rich plumes rose as high as 3 km above the lake and drifted NE and NNE. Sulfur dioxide gas emissions averaged 9,391 tonnes per day on 5 June and 7,680 tonnes per day on 6 June; significant vog persisted over the Taal region. The Alert Level remained at 1 (on a scale of 0-5), and PHIVOLCS reminded the public that the entire Taal Volcano Island was a Permanent Danger Zone (PDZ).

Geological summary: Taal is one of the most active volcanoes in the Philippines and has produced some of its most powerful historical eruptions. Though not topographically prominent, its prehistorical eruptions have greatly changed the landscape of SW Luzon. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, and several eruptive centers lie submerged beneath the lake. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all observed eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones. Powerful pyroclastic flows and surges have caused many fatalities.


Smithsonian / US Geological Survey Weekly Volcanic Activity Report – May 31 – June 6, 2023 –
Managing Editor: Sally Kuhn Sennert


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