The Weekly Volcanic Activity Report: May 21–27, 2025

New activity/unrest: Kanlaon, Philippines | Lewotobi, Indonesia.

New activity/unrest

Kanlaon, Philippines

10.4096°N, 123.13°E | Summit elev. 2422 m

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported continuing unrest at Kanlaon from May 20 to 26. The seismic network recorded 1 to 31 daily volcanic earthquakes. Daily sulfur dioxide emissions ranged from 653 to 1 836 tonnes per day. Gas-and-steam emissions generally rose 75 to 700 m above the crater rim and drifted northwest, west, southwest, and south-southwest.

According to Civil Defense PH, heavy rain on May 22 generated lahars that overflowed the Tamburong creek at the Tamburong Bridge in Barangay Biaknabato. The lahar deposits, consisting of mud and volcanic debris, blocked the Biaknabato Bridge and a portion of the Biaknabato Highway in La Castellana.

The Alert Level remained at 3 (on a scale of 0 to 5). The public was warned to stay 6 km (3.7 miles) away from the summit, and pilots were warned not to fly close to the volcano.

Geological summary: Kanlaon volcano (also spelled Canlaon) forms the highest point on the Philippine island of Negros. The massive andesitic stratovolcano is covered with fissure-controlled pyroclastic cones and craters, many of which are filled by lakes. The largest debris avalanche known in the Philippines traveled 33 km (20.5 miles) southwest from Kanlaon. The summit contains a 2 km (1.2 miles) wide, elongated northern caldera with a crater lake and a smaller but higher active vent, Lugud crater, to the south. Eruptions recorded since 1866 have typically consisted of phreatic explosions of small to moderate size that produce minor local ashfall.

Lewotobi, Indonesia

8.542°S, 122.775°E | Summit elev. 1703 m

In a special report on May 25, the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) stated that eruptive activity at Lewotobi Laki-laki had decreased after the larger May 18 eruption and subsequent ash plumes and crater incandescence observed from May 19 to 20. Seismicity indicated decreasing surface activity and magmatic recharge at depth from May 20 to 25, and tiltmeter data indicated deflation. No rumbling was heard and no incandescence was visible at the summit.

An eruptive event at 06:49 LT on May 23 produced a dense gray ash plume that rose around 300 m (980 feet) above the summit and drifted southwest and west. Later that day, a drone flight over the summit revealed that dark lava had filled the crater, covering about 87 000 m² (936 460 ft²).

At 09:00 LT on May 25, the Alert Level was lowered to 3 (on a scale of 1 to 4), and the public was warned to stay 6 km (3.7 miles) away from the center of Laki-laki. White steam-and-gas plumes rose as high as 800 m (2 625 feet) above the crater rim and drifted in various directions from May 25 to 28.

Geological summary: The Lewotobi edifice in eastern Flores Island is composed of the two adjacent Lewotobi Laki-laki and Lewotobi Perempuan stratovolcanoes (the “husband and wife”). Their summits are less than 2 km apart along a NW-SE line. The conical Laki-laki to the NW has been frequently active during the 19^th and 20^th centuries, while the taller and broader Perempuan has had observed eruptions in 1921 and 1935. Small lava domes have grown during the 20^th century in both of the summit craters, which are open to the north. A prominent cone, Iliwokar, occurs on the E flank of Perampuan.

Ongoing activity

Aira, Japan

31.5772°N, 130.6589°E | Summit elev. 1117 m

The Japan Meteorological Agency (JMA) reported ongoing eruptive activity at Minamidake Crater (Aira Caldera’s Sakurajima volcano) from May 19 to 28. The seismic network recorded 44 eruptive events and 38 explosions from May 19 to 23 that produced ash plumes rising up to 3 km (1.9 miles) above the crater rim and drifting north; the tallest plume followed an explosion at 15:00 LT on May 20.

Large blocks from the explosions were ejected as far as 1.2 km (0.75 miles) from the crater rim. Sulfur dioxide emissions were extremely high, averaging 11 200 tonnes per day on May 20 and 4 300 tonnes per day on May 22. Deformation data indicated continuing inflation. Only very small eruptive events were recorded from May 26 to 28.

The Alert Level remained at 3 (on a 5-level scale), and the public was warned to be cautious within 2 km (1.2 miles) of both the Minamidake and Showa craters.

Geological summary: The Aira caldera, located in the northern half of Kagoshima Bay, contains the active post-caldera Sakurajima volcano. Formation of the 17 by 23 km (10.6 by 14.3 miles) caldera occurred about 22 000 years ago during the eruption of the voluminous Ito pyroclastic flow. The smaller Wakamiko caldera formed in the northeastern part of Aira during the early Holocene, along with several post-caldera cones. Sakurajima began forming about 13 000 years ago on the southern rim of the caldera and initially formed an island, which was later joined to the Osumi Peninsula during the major eruption of 1914. Activity at the Kitadake summit cone ceased about 4 850 years ago, after which eruptions shifted to Minamidake. Frequent explosive eruptions since the 8th century have deposited ash on Kagoshima City, located only 8 km (5 miles) across the bay. The largest known historical eruption occurred between 1471 and 1476.

Great Sitkin, United States

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

The Alaska Volcano Observatory (AVO) reported that slow lava effusion continued to feed a thick flow in Great Sitkin’s summit crater from May 21 to 27. A radar image on May 22 confirmed that lava was slowly advancing. Small daily earthquakes were detected by the seismic network. Weather clouds sometimes obscured views of the volcano.

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 most of the northern part of Great Sitkin Island in the central Aleutians. A younger cone, capped by a small ice-filled summit caldera measuring 0.8 by 1.2 km (0.5 by 0.75 miles), was built within a larger late-Pleistocene or early Holocene collapse scarp. That collapse event truncated an older volcanic edifice and generated a large submarine debris avalanche, with deposits extending across the ocean floor to the north. The summit lies along the eastern rim of the younger scarp. Deposits from an earlier caldera-forming eruption of unknown age reach up to 6 m (19.7 feet) thick on the flanks of the island. The younger caldera was partially filled by lava domes formed in 1945 and 1974. Additionally, five older flank lava domes, two of which are located along the coast, were built along northwest- and north-northwest-trending alignments. Hydrothermal activity near the head of Big Fox Creek, to the south of the volcano, includes hot springs, mud pots, and fumaroles. Eruptions have been documented since the late 19^th century.

Home Reef, Tonga

18.992°S, 174.775°W | Summit elev. -10 m

The Tonga Geological Services reported that the eruption at Home Reef continued from May 11 to 24, though satellite observations suggested decreasing activity. Thermal anomalies were last detected on May 4, and no visible ash emissions were identified in a May 15 image, though discolored water surrounding the island suggested submarine activity.

Evidence for explosive activity during May 20 to 21 included new tephra deposits on the cone’s flanks and low-level ash plumes. The weather clouds often obscured views. Infrasound instruments did not detect any signals.

The Aviation Color Code remained at Yellow (the second lowest level on a four-level scale), the Maritime Alert Level remained at Orange (the third level on a four-level scale), with advice to stay at least 2 nautical miles (3.7 km) from the island, and the Alert Level for residents of Vava’u and Ha’apai remained at Green (the first level on a four-level scale).

Geological summary: Home Reef, a submarine volcano located midway between Metis Shoal and Late Island in the central Tonga islands, was first reported active in the mid-19^th century, when an ephemeral island formed. An eruption in 1984 produced a 12 km (7.5 miles) high eruption plume, large amounts of floating pumice, and an ephemeral island measuring 500 by 1 500 m (1 640 by 4 920 feet), with cliffs 30 to 50 m (98 to 164 feet) high that enclosed a water-filled crater. In 2006, an island-forming eruption produced widespread dacitic pumice rafts that drifted as far as Australia. Another island was built during a September to October 2022 eruption.

Karymsky, Russia

54.049°N, 159.443°E | Summit elev. 1513 m

The Kamchatkan Volcanic Eruption Response Team (KVERT) reported that, in general, moderate explosive activity continued at Karymsky from May 15 to 22. A thermal anomaly over the volcano was identified in satellite images on May 16 to 18, 20, and 22; 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; specific events are given in local time where noted.

Geological summary: Karymsky, the most active volcano in Kamchatka’s eastern volcanic zone, is a symmetrical stratovolcano built within a 5 km (3.1 miles) wide caldera that formed during the early Holocene. This caldera cuts into the southern side of the older Pleistocene Dvor volcano and lies just north of the mid-Pleistocene Polovinka caldera, which itself contains the smaller Akademia Nauk and Odnoboky calderas. Seismic activity preceding eruptions at Karymsky often originates beneath Akademia Nauk caldera, located immediately to the south. The Karymsky caldera formed approximately 7 600 to 7 700 radiocarbon years ago, and construction of the current stratovolcano began about 2 000 years later. The current eruptive period began roughly 500 years ago, following a 2 300-year quiescence. Much of the volcanic cone is covered by lava flows less than 200 years old. Historical eruptions have typically been vulcanian or vulcanian-strombolian in style, featuring moderate explosive activity and occasional lava flows from the summit crater.

Kilauea, United States

19.421°N, 155.287°W | Summit elev. 1222 m

The Hawaiian Volcano Observatory (HVO) reported that the eruption within Kilauea’s Kaluapele summit caldera, characterized by episodic fountaining from two vents along the southwest margin of Halema’uma’u Crater, continued at variable levels from May 21 to 27. Incandescence at both the north and south vents was visible from May 20 to 25, along with intermittent spattering at both vents and occasional dome fountaining at the north vent. Large yellow flames from burning hydrogen gas sometimes rose from the north vent. The sulfur dioxide emission rate averaged around 1 200 tonnes per day, similar to rates recorded during prior pauses between high fountaining events. After midday on May 24, minor fountaining at the north vent as high as 65 m (213 feet) was accompanied by lava overflowing the cone onto the crater floor. Strong incandescence and periodic vigorous spattering at both vents was visible in the afternoon of May 24 and overnight from May 24 to 25. Lava fountaining at the north vent and an overspill of lava onto the crater floor also occurred overnight.

Narrow lava jets rose from the north vent on May 25 and became sustained fountains at 16:15 LT, which quickly began to cover the crater floor with lava flows. The activity escalated to fountains over 300 m (984 feet) tall within 30 minutes. Fountaining at the south vent began at 17:20 LT and grew rapidly, reaching sustained heights of about 250 m (820 feet). The fountains produced an eruptive plume heavily laden with tephra (ash, scoria, and Pele’s hair) that reached at least 4.3 km (2.7 miles) in height. Sulfur dioxide emission rates were estimated to be 50 000 to 75 000 tonnes per day based on webcam images from the Mauna Loa summit. Both vents produced lava flows that covered about half of the crater floor. After about six hours of sustained fountaining, the north vent stopped erupting at 21:48 LT, and the south vent stopped erupting at approximately 22:25 LT on May 25. Strands of Pele’s hair were present throughout the summit area of Hawai’i Volcanoes National Park and surrounding communities. 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: Kilauea overlaps the eastern flank of the massive Mauna Loa shield volcano on the Island of Hawaiʻi. Eruptions at Kilauea are prominent in Polynesian oral traditions, and written documentation since 1820 records frequent summit and flank eruptions, including long-term lava lake activity in Halemaʻumaʻu Crater until 1924. The 3 by 5 km (1.9 by 3.1 miles) summit caldera formed in multiple stages around 1 500 years ago and again during the 18^th century. Eruptions also occur along the lengthy East and Southwest Rift Zones, which extend to the ocean. About 90 percent of the volcano’s surface is covered by lava flows less than 1 100 years old, and approximately 70 percent is younger than 600 years. A prolonged eruption from the East Rift Zone from 1983 to 2018 produced lava flows that covered more than 100 km² (38.6 mi²), destroyed hundreds of homes, and created new coastline.

Kirishimayama, Japan

31.934°N, 130.862°E | Summit elev. 1700 m

The Japan Meteorological Agency (JMA) reported that increased unrest at Shinmoedake (Shinmoe peak, a stratovolcano of the Kirishimayama volcano group) from March through early May had stabilized. Volcanic earthquakes with epicenters beneath Shinmoedake began to increase on March 28, and tiltmeter data indicated inflation on March 30.

From April 22 to 26, a minor northwest tilt was observed in inclinometer data, and the number of earthquakes further increased. Volcanic tremor was detected on April 27 and from May 1 to 2. During the first half of May, seismicity remained high but did not significantly increase, and there were no changes detected in tiltmeter data. At 11:00 LT on May 27, the Alert Level was lowered to 2 (on a 5-level scale), and the public was warned to exercise caution within 2 km (1.2 miles) of Shinmoedake Crater.

Geological summary: Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km (12.4 x 18.6 miles). The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located southwest of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an east-west line of vents from Miike to Ohachi, and at Shinmoedake to the northeast. Frequent small to moderate explosive eruptions have been recorded since the 8^th century.

Lewotolok, Indonesia

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

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported that an eruption at Lewotolok was ongoing from May 21 to 27. Daily white steam-and-gas plumes rose as high as 700 m (2 297 feet) above the summit of the cone and drifted northwest and west.

Gray ash plumes rose 100 to 500 m (328 to 1 640 feet) above the summit and drifted southeast, northwest, and west on May 23 and from May 25 to 27. Clear nighttime webcam images showed incandescent material at the summit cone and on the upper south flank. The Alert Level remained at 2 (on a scale of 1 to 4), and the public was warned to stay 2 km (1.2 miles) away from the vent and 2.5 km (1.6 miles) away on the south, southeast, and west flanks.

Geological summary: Lewotolok (also known as Lewotolo) is a stratovolcano located at the eastern tip of a narrow peninsula that extends north into the Flores Sea from Lembata (formerly Lomblen) Island. The volcano appears symmetrical when viewed from the north and east. A small cone with a 130 m (427 feet) wide crater, constructed on the southeastern side of a larger crater, marks the summit. Numerous lava flows have reached the coastline. Since 1660, eruptions have been characterized by explosive activity from the summit crater.

Marapi, Indonesia

0.38°S, 100.474°E | Summit elev. 2885 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported that eruptive activity at Marapi (on Sumatra) continued from May 21 to 27, characterized by occasional dense ash plumes. White plumes rose 100 to 200 m (328 to 656 feet) above the summit and drifted in multiple directions from May 21 to 22 and on May 24. Eruptive events were recorded at 12:27 LT on May 23 and at 14:57 LT on May 26, though weather conditions prevented visual confirmation.

On May 25, white-and-brown plumes rose 100 to 250 m (328 to 820 feet) above the summit and drifted east and southeast. At 08:22 LT on May 27, a dense white-and-gray ash plume rose 1.2 km (3 937 feet) above the summit and drifted southeast. The Alert Level remained at 2 (on a scale of 1 to 4), and the public was warned to stay 3 km (1.9 miles) away from the active crater.

Geological summary: Gunung Marapi, not to be confused with the more widely known Merapi volcano on Java, is the most active volcano on the island of Sumatra. This large complex stratovolcano rises about 2 000 m (6 562 feet) above the Bukittinggi Plain in the Padang Highlands. The broad summit area contains several partially overlapping craters constructed within the 1.4 km (0.87 miles) wide Bancah caldera. The summit craters are aligned along an east-northeast to west-southwest trend, with volcanic activity having migrated westward over time. Since the late 18^th century, more than 50 eruptions, typically small-to-moderate explosive events, have been documented. No lava flows outside the summit area have been recorded in historical time.

Merapi, Indonesia

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

The Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG) reported that the eruption at Merapi (on Java) continued from May 16 to 22. Seismicity remained at high levels. The southwest lava dome produced 41 lava avalanches that traveled as far as 2 km (1.2 miles) southwest down the Bebeng drainage, 10 that traveled as far as 1.8 km (1.1 miles) southwest down the Krasak drainage, and 73 that traveled as far as 2 km (1.2 miles) west down the Sat/Putih drainage. Small morphological changes to the southwest lava dome resulted from continuing effusion and minor collapses.

The Alert Level remained at 3 (on a scale of 1 to 4), and the public was warned to stay 3 to 7 km (1.9 to 4.3 miles) away from the summit, based on location.

Geological summary: Merapi, one of Indonesia’s most active volcanoes, rises above one of the world’s most densely populated regions and dominates the landscape immediately north of the city of Yogyakarta. It is the youngest and southernmost volcano in a chain extending north-northwest to Ungaran. The development of “Old Merapi” during the Pleistocene ended with a major edifice collapse around 2 000 years ago, leaving behind a large arcuate scarp that cuts into the older Batulawang volcano. Growth of the current, steep-sided “Young Merapi” edifice began southwest of this scarp and remains active today. Due to frequent eruptions, its summit area remains mostly unvegetated. Repeated dome growth and collapse events have generated pyroclastic flows and lahars that have devastated agricultural areas on the western and southern flanks, causing significant fatalities over time.

Poas, Costa Rica

10.2°N, 84.233°W | Summit elev. 2697 m

The Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA) reported continuing gas-and-steam emissions at Poás from May 21 to 27; ash content of the plumes was either very low or not present. Sulfur dioxide gas emissions remained at high levels, though they had decreased compared to the previous week. The emissions detected in satellite data drifting southwest on May 20 were estimated to be 598 tonnes per day.

The fixed Differential Optical Absorption Spectroscopy (DOAS) station recorded slightly lower averages compared to the previous week, averaging around 309 tonnes per day from May 17 to 23, and a transect using a mobile DOAS instrument recorded 3 000 tonnes per day on May 22, similar to the previous week. Intense incandescence continued to be visible at both Boca A and Boca C vents.

On May 21, the temperature at Boca A was measured with an infrared sensor mounted on a drone, revealing a maximum temperature of 470 °C (878 °F). Water continued pooling at Boca C. Pulses in acoustic data from May 26 to 27 likely originated from bubbling at Boca C. The volcano’s Alert Level remained at 3 (the second-highest level on a four-level scale).

Geological summary: Poás, one of Costa Rica’s most active volcanoes, is a broad, heavily vegetated stratovolcano with three craters aligned along a north–south trend. Its multi-hued summit crater lakes are easily accessible from the capital city, San José. A fissure trending north–south extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost summit crater lake, Botos, last erupted approximately 7 500 years ago. The more prominent northern crater lake, Laguna Caliente, is geothermally heated and among the most acidic natural lakes on Earth, with a pH near zero. Frequent phreatic and phreatomagmatic eruptions have occurred at Laguna Caliente since the first recorded event in 1828, often producing geyser-like ejections of crater-lake water.

Semeru, Indonesia

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

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported that activity continued at Semeru from May 21 to 27, with multiple daily eruptive events recorded by the seismic network. Daily white-and-gray ash plumes rose 400 to 1 200 m (1 312 to 3 937 feet) above the summit and drifted in multiple directions.

The Alert Level remained at 2 (the second lowest level on a scale of 1 to 4). The public was warned to stay at least 3 km (1.9 miles) away from the summit in all directions, 8 km (5 miles) from the summit to the southeast, 500 m (1 640 feet) from the banks of the Kobokan drainage as far as 13 km (8.1 miles) 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 and one of the most active volcanoes on Java, lies at the southern end of a volcanic massif that extends north to the Tengger caldera. The steep-sided volcano, also known 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 cuts along a north–south trend through the summit area, and cinder cones and lava domes are present on the eastern and northeastern flanks. The summit topography is complex due to shifting crater locations from northwest to southeast. Frequent eruptions in the 19^th and 20^th centuries were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive events that produced pyroclastic flows reaching the lower flanks.

Sheveluch, Russia

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

The Kamchatkan Volcanic Eruption Response Team (KVERT) reported that lava extrusion may have continued at Sheveluch’s “300 years of RAS” dome on the southwest flank of Old Sheveluch and at the Young Sheveluch dome from May 15 to 22. Thermal anomalies over the domes were identified almost daily in satellite images; weather clouds sometimes obscured views, especially during the second half of the week.

The Far Eastern Branch of the Russian Academy of Sciences (FEB RAS) reported that explosive activity was recorded by infrasound instruments on May 16, with ash plumes that rose as high as 4.7 km (15 420 feet) above the summit and drifted northeast. KVERT noted that ash plumes drifted 835 km (519 miles) northeast and 185 km (115 miles) southeast from May 16 to 18.

The Aviation Color Code remained at Orange (the second highest level on a four-color scale). Dates are based on UTC; specific events are in local time where noted.

Geological summary: The high, isolated massif of Sheveluch (also spelled Shiveluch) rises above the lowlands north-northeast of the Kliuchevskaya volcano group. The 1 300 km³ (312 mi³) andesitic volcano is one of the largest and most active in Kamchatka, with at least 60 large eruptions during the Holocene. The summit of the roughly 65 000-year-old Stary Shiveluch is truncated by a broad 9 km (5.6 miles) wide late-Pleistocene caldera breached to the south. Many lava domes are present on the outer flanks. The Molodoy Shiveluch lava dome complex was built during the Holocene within the breached caldera, and Holocene dome extrusion has also occurred on the flanks of Stary Shiveluch. Widespread tephra deposits from these eruptions serve as important markers for dating volcanic activity in Kamchatka. Frequent dome collapses, including a major one in 1964, have produced debris avalanches that cover much of the caldera floor.

References:

¹ Smithsonian Institution / US Geological Survey – GVP – Weekly Volcanic Activity Report May 21–27, 2025 – Managing Editor: Sally Sennert

Teo Blašković

I'm a dedicated researcher, journalist, and editor at The Watchers. With over 20 years of experience in the media industry, I specialize in hard science news, focusing on extreme weather, seismic and volcanic activity, space weather, and astronomy, including near-Earth objects and planetary defense strategies. You can reach me at teo /at/ watchers.news.