The Weekly Volcanic Activity Report: August 27 – September 2, 2026

New activity/unrest: Karangetang, Indonesia | Klyuchevskoy, Russia | Krakatau, Indonesia | Ranakah, Indonesia | Sinabung, Indonesia.

Ongoing activity: Aira, Japan | Ambae, Vanuatu | Asosan, Japan | Etna, Italy | Great Sitkin, United States | Ibu, Indonesia | Iwatesan, Japan | Kilauea, United States | Lewotobi, Indonesia | Lewotolok, Indonesia | Mayon, Philippines | Poas, Costa Rica | Purace, Colombia | Rincon de la Vieja, Costa Rica | Sabancaya, Peru | Semeru, Indonesia | Sheveluch, Russia.

New activity/unrest

Karangetang, Indonesia

2.781°N, 125.407°E; Elev. 1,797 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) and a news source reported that an eruption at Karangetang’s North Crater (Kawah Dua) continued during 27 August-2 September. Daily white plumes rose as high as 500 m above the summit and drifted in various directions. Incandescence at the North Crater and from lava flows and/or incandescent avalanches on the flanks was visible in some webcam images throughout the week. Seismicity remained at high levels and rumbling sounds were occasionally heard. The Alert Level remained at 2 (on a scale of 1-4) and the public was advised to stay 1.5 km away from both craters, with an extension to 2.5 km on the NW and SW flanks of North Crater.

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

Klyuchevskoy, Russia

56.056°N, 160.642°E; Elev. 4,754 m

The Kamchatkan Volcanic Eruption Response Team (KVERT) reported that the eruption at Klyuchevskoy continued at the summit crater during 27 August-2 September. A daily thermal anomaly was identified in satellite images. Daily ash plumes rose as high as 7 km (23,000 ft) a.s.l. and drifted as far as 170 km NE and E during 27-30 August. 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: Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Krakatau, Indonesia

6.1009°S, 105.4233°E; Elev. 285 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported ongoing eruptive activity at Krakatau during 27 August-2 September. Ash-and-gas plumes that were a mixture of white, gray, and black colors rose 100-400 m above the summit and drifted mainly W, NW, N, and NE during 27-31 August. A webcam image on 27 August showed incandescent material being ejected above the crater. White plumes were visible rising as high as 200 m and drifting SW, W, and NW during 1-2 September. The Alert Level remained at 3 (on a scale of 1-4) and the public was advised to stay 3 km away from summit.

Geological summary: The renowned Krakatau (frequently misnamed as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older 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 the Rakata, Danan, and Perbuwatan cones 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 Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.

Ranakah, Indonesia

8.637°S, 120.5302°E; Elev. 2,288 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported increased unrest at Ranakah. Following the M 7.7 tectonic earthquake that occurred off the N coast Flores Island on 15 August there was an increase in the overall number of daily tectonic events. The number of deep volcanic earthquakes also increased, up to 35 events per day. Seismicity declined but remained high towards the end of August, until 1 September when over 100 deep volcanic earthquakes were recorded. Drone overflights revealed denser plumes rising from the solfataras W of the lava dome on 25 August and more diffuse plumes rising from the same area on 1 September. No significant changes at the lava dome were observed during the overflights despite earthquake-triggered landslides occurring in several areas at Ranakah on 18 and 20 August. At 2300 on 1 September the Alert Level was raised to 2 (on a scale of 1-4) and the public was warned to stay 1 km away from the crater.

Geological summary: The Anak Ranakah (Child of Ranakah) lava dome was formed in 1987 in an area without previous recorded eruptions at the base of the large older lava dome of Gunung Ranakah. An arcuate group of lava domes extending about 12 km west from Gunung Ranakah to Poco Mandosawa occurs on the outer N flanks of the poorly known Poco Leok caldera on western Flores Island.

Sinabung, Indonesia

3.17°N, 98.392°E; Elev. 2,460 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported that after a period of escalating unrest at Sinabung, which had been detected at least since May, a new eruption occurred on 31 August. During 1-30 August white-and-gray plumes rose as high as 1 km above the summit. Observers noted that the vegetation on the E flank yellowed and dried out. The seismic network recorded a total of one rockfall event, 14 earthquakes indicating emissions, two tornillo earthquakes, 29 low-frequency earthquakes, eight hybrid/multi-phase earthquakes, one shallow volcanic earthquake, and 141 deep volcanic earthquakes. On 28 August blue plumes descending the SE flank were observed.

Prior to the eruption on 31 August a swarm was detected that consisted of 85 volcanic earthquakes, one earthquake indicating an emission, and one harmonic tremor. At 1017 a dense black ash plume rose around 3.5 km above the summit and drifted E and SE. The eruption lasted just over an hour based on seismic data. At 1230 the Alert Level was raised to 3 (the second highest level on a scale of 1-4). Though most of the exclusion zone remained at 3 km from the summit, it was increased to 6 km on the SE flank. Hotspots on the N flank were visible. According to a news source ashfall was reported in Berastagi City and residents were urged to wear masks while outside to avoid breathing in ash. Schools in high-risk areas implemented virtual learning for one day.

Later that day, during 1800-2400, dense gray-and-brown plumes rose 200-500 m above the summit and drifted NE and E. On 1 September white-and-gray ash plumes rose as high as 600 m above the summit and drifted E, SE, and S, causing ashfall in areas to the SE. News sources reported that residents of Kuta Tengah Village and Payung Village (5 km SSW) were preemptively moved to evacuation centers following increased volcanic activity. Two flights on the Banda Aceh-Medan route were affected, one delayed and one canceled, due to ash in the air. At 0720 on 2 September a gray-to-brown ash plume rose about 1 km above the summit and drifted E and SE.

Geological summary: Gunung Sinabung is a Pleistocene-to-Holocene stratovolcano with many lava flows on its flanks. The migration of summit vents along a N-S line gives the summit crater complex an elongated form. The youngest crater of this conical andesitic-to-dacitic edifice is at the southern end of the four overlapping summit craters. The youngest deposit is a SE-flank pyroclastic flow 14C dated by Hendrasto et al. (2012) at 740-880 CE. An unconfirmed eruption was noted in 1881, and solfataric activity was seen at the summit and upper flanks in 1912. No confirmed historical eruptions were recorded prior to explosive eruptions during August-September 2010 that produced ash plumes to 5 km above the summit.

Ongoing activity

Aira, Japan

31.5772°N, 130.6589°E; Elev. 1,117 m

The Japan Meteorological Agency (JMA) reported ongoing eruptive activity at Minamidake Crater (Aira Caldera’s Sakurajima volcano) during 24-31 August. Crater incandescence was visible in nighttime webcam images during 24-28 August. An explosion at 0951 on 25 August generated an ash plume that rose 400 n above the summit, into weather clouds. Volcanologists conducted a field survey on 27 August and measured very high sulfur dioxide emissions, averaging 3,400 tons per day. There were 22 eruptive events recorded during 28-31 August generating ash plumes that rose as high as 2 km above the crater rim. The Alert Level remained at 3 (on a 5-level scale), and the public was warned to be cautious within 1 km of both the Minimadake and Showa 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 caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Ambae, Vanuatu

15.389°S, 167.835°E; Elev. 1,496 m

The Vanuatu Meteorology and Geohazards Department (VMGD) and the Wellington Volcanic Ash Advisory Center (VAAC) reported that the eruption at Ambae continued during 27 August-2 September. According to the VAAC multiple daily plumes during 27-30 August were identified in satellite data and webcam images rising 1.5-7.6 km (5,000-25,000 ft) a.s.l. and drifting W, SE, and E. An ash plume rose to 12.5 km (41,000 ft) a.s.l. on 1 September and drifted ESE. VMGD noted that the ash plume also contained very high amounts of sulfur dioxide. Ashfall was reported in areas to the N, E, and S parts of the island, and reached the N part of Pentecost Island. The Alert Level remained at 3 (on a scale of 0-5), and the public was warned to stay outside of Danger Zone B, defined as a 3-km radius around the active vents in Lake Voui, and to stay away from drainages during heavy rains.

Geological summary: The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone with numerous scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Asosan, Japan

32.8849°N, 131.085°E; Elev. 1,592 m

The Japan Meteorological Agency (JMA) reported that unrest at Asosan showed a downward trend since 17 August based on seismic and gas emission data. At 1600 on 1 September the Alert Level was lowered to 2 (on a scale of 1-5) and the public was warned not to enter the area around the crater.

Geological summary: The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan’s most active volcanoes. It was the location of Japan’s first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu’s most popular tourist destinations.

Etna, Italy

37.748°N, 14.999°E; Elev. 3,357 m

The Sezione di Catania – Osservatorio Etneo (INGV) reported that eruptive activity at Etna’s summit craters during 24-30 August was characterized by gas emissions and periodic minor ash emissions at Northeast Crater, and minor gas emissions at Southeast Crater. The volcano Alert Level remained at Yellow (the second lowest level on a four-level scale) according to the Dipartimento della Protezione Civile.

Geological summary: Mount Etna, towering above Catania on the island of Sicily, has one of the world’s longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Great Sitkin, United States

52.076°N, 176.13°W; Elev. 1,740 m

The Alaska Volcano Observatory (AVO) reported that slow lava effusion continued to feed a thick flow in Great Sitkin’s summit crater during 27 August-2 September based on webcam and satellite images. The flow gradually expanded E. Minor earthquakes were detected daily, likely associated with rockfalls on the growing dome. Weather clouds obscured webcam and satellite views during most of the week. 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 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 older edifice and produced a submarine debris avalanche. Deposits from this and an even 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. Eruptions have been recorded since the late-19th century.

Ibu, Indonesia

1.4941°N, 127.6324°E; Elev. 1,357 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported that activity at Ibu continued during 27 August-2 September, with daily eruptive events recorded by the seismic network. Gray ash plumes rose 300-800 m above the summit and drifted in multiple directions, especially towards the N, NE, and SE. Incandescence at the summit was visible in some nighttime webcam images. The Alert Level remained at 2 (the second lowest level on a four-level scale) and the public was advised to stay 2 km away from the active crater and 3.5 km away from the N crater wall opening.

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.

Iwatesan, Japan

39.853°N, 141.001°E; Elev. 2,038 m

The Japan Meteorological Agency (JMA) reported that unrest at Iwatesan had ceased. Deformation detected by multiple methods had stopped and seismicity was at a low level. At 1100 on 27 August the Alert Level was lowered to 1 (the lowest level on a 1-5 scale).

Geological summary: Viewed from the east, Iwatesan volcano has a symmetrical profile that invites comparison with Fuji, but on the west an older cone is visible containing an oval-shaped, 1.8 x 3 km caldera. After the growth of Nishi-Iwate volcano beginning about 700,000 years ago, activity migrated eastward to form Higashi-Iwate volcano. Iwate has collapsed seven times during the past 230,000 years, most recently between 739 and 1615 CE. The dominantly basaltic summit cone of Higashi-Iwate volcano, Yakushidake, is truncated by a 500-m-wide crater. It rises well above and buries the eastern rim of the caldera, which is breached by a narrow gorge on the NW. A central cone containing a 500-m-wide crater partially filled by a lake is located in the center of the oval-shaped caldera. A young lava flow from Yakushidake descended into the caldera, and a fresh-looking lava flow from the 1732 eruption traveled down the NE flank.

Kilauea, United States

19.421°N, 155.287°W; Elev. 1,222 m

The Hawaiian Volcano Observatory (HVO) reported that the eruption within Kilauea’s Kaluapele summit caldera, characterized by episodic fountaining, incandescence, and intermittent spatter from craters along the SW margin of Halema`uma`u Crater, continued at variable levels during 27 August-2 September. Incandescence or flaming at the N vent was periodically recorded in webcam images. The Volcano Alert Level remained at Advisory (the second lowest level on a four-level scale) and the Aviation Color Code remained at Yellow (the second lowest color on a four-color scale).

Geological summary: Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the Island of Hawai`i. 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 Halema`uma`u 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.

Lewotobi, Indonesia

8.542°S, 122.775°E; Elev. 1,703 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported that eruptive activity continued at Lewotobi Laki-Laki during 27 August-2 September. At 0830 on 27 August a gray ash plume rose 2.5 km above the summit and drifted SW and W. According to a news source the eruption lasted just over two minutes and was accompanied by a booming sound. From that time through 31 August daily ash plumes rose 200-700 m above the summit and drifted NW, W, and SW. The Alert Level remained at 3 (on a scale of 1-4), and the public was warned to stay 5 km away from the center of Laki-Laki.

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 19th and 20th centuries, while the taller and broader Perempuan has had observed eruptions in 1921 and 1935. Small lava domes have grown during the 20th century in both of the summit craters, which are open to the north. A prominent cone, Iliwokar, occurs on the E flank of Perampuan.

Lewotolok, Indonesia

8.274°S, 123.508°E; Elev. 1,431 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported continuing eruptive activity at Lewotolok during 27 August-2 September. Daily ash plumes with various color combinations of white, gray, and black were visible rising 300-800 m above the summit and drifting NW and W. The lava flow on the SSE flank had lengthened to 1.8 km by the beginning of the week and did not advance by the end of the week. A lava flow on the NE flank was 600 m long and advanced to 1.1 km long by 1 September. The Alert Level remained at 2 (on a scale of 1-4). The public was warned to stay 2 km away from the summit with an extension to 3 km on the S and SE flanks.

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

Mayon, Philippines

13.257°N, 123.685°E; Elev. 2,462 m

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that the eruption at Mayon continued at low levels during 27 August-2 September. Each day the seismic network recorded 6-26 volcanic earthquakes. Sulfur dioxide emissions (SO₂) averaged 5,005-5,878 tonnes per day (t/d) during 26-30 August, peaked at a very high rate of 7,115 t/d on 31 August, and then slightly decreased to 6,981 and 6,931 t/d on 1 and 2 September, respectively. SO₂ emission had averaged 3,844 t/d since the Alert level was lowered to 2 on 5 August.

Six ash emissions, each lasting as long as 47 minutes, were recorded on 26 August; emissions rose as high as 1 km and drifted ESE. A weak explosion at 0900 on 27 August generated a grayish ash plume that rose 500 m above the summit and drifted ENE. The event lasted about one minute. At 0908 on 28 August a grayish ash plume rose 150 m above the summit and drifted ENE. On 30 August an ash emission was recorded for about one minute. Weather clouds obscured views on the other days. The Alert Level remained at 2 (on a 0-5 scale) and the public was 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 periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents 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.

Poas, Costa Rica

10.2°N, 84.233°W; Elev. 2,697 m

The Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA) reported several small eruptions at Poás during 27 August-2 September with a few notable events. An eruption at 0235 on 27 August was comprised of two pulses based on seismic and acoustic data. The event ejected incandescent ballistics 600 m from the crater onto the E terrace. An eruptive event at 0048 on 2 September produced a small ash plume. The Volcanic Alert Level remained at 2 (the second lowest level on a four-level scale).

Geological summary: The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world’s most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Purace, Colombia

2.3095°N, 76.3948°W; Elev. 4,650 m

The Servicio Geologico Colombiano (SGC) reported intensifying eruptive activity at Puracé during 26 August-2 September. Signals associated with fluid movement dominated the seismicity and were characterized as tremor-type signals and, to a lesser extent, long-period events located beneath Puracé volcano at depths less than 2 km. Sulfur dioxide flux stayed above the average baseline level, averaging 760 tonnes per day (t/d) on 27 August and 1,338 t/d on 26 August. Soil carbon dioxide concentrations rose throughout the reporting period. Slow ground deformation persisted in an area between the Puracé, Piocollo, and Curiquinga volcanoes. As many as eight daily ash emissions were recorded by the seismic network, though adverse weather conditions often prevented visual confirmation. Ash emissions rose as high as 3,250 m above the summit during 26-27 August. Eruption noises and ashfall were reported in the town center of Coconuco, Cristales, Campamento, and 20 de Julio, in the municipality of Coconuco. During 28-29 August ashfall was reported in the villages of Chiligló, Alto de Anambío (7 km NW), and Cristales, in the municipality of Puracé. Ash plumes rose up to 240 m above the summit during 31 August-1 September.

Seismic activity associated with rock fracturing (volcano-tectonic earthquakes, VT) was low in both number and magnitude, with signals located beneath Piocollo and Puracé at depths less than 4 km. Beginning at 1620 on 29 August, however, both the number and magnitude of VT earthquakes increased and by 1500 on 30 August the seismic network recorded 472 events. They were mainly located 10-15 km NE of Puracé volcano at depths of 6-12 km. There were 18 events with magnitudes equal or great than 2, with the largest a M 4.3, and at least seven were felt by residents in the municipalities of Puracé, Totoró, Sotará, and Popayán. About 150 more VT events were recorded by the afternoon of 31 August, and around 180 more by the early afternoon of 2 September, all located in a similar area, and a few felt by residents. The Alert Level remained at Orange (the second highest level on a four-color scale).

Geological summary: Puracé is an active andesitic volcano with a 600-m-diameter summit crater at the NW end of the Los Coconucos Volcanic Chain. This volcanic complex includes nine composite and five monogenetic volcanoes, extending from the Puracé crater more than 6 km SE to the summit of Pan de Azúcar stratovolcano. The dacitic massif which the complex is built on extends about 13 km NW-SE and 10 km NE-SW. Frequent small to moderate explosive eruptions reported since 1816 CE have modified the morphology of the summit crater, with the largest eruptions in 1849, 1869, and 1885.

Rincon de la Vieja, Costa Rica

10.8314°N, 85.3364°W; Elev. 1,729 m

The Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA) reported that small phreatic eruptions at Rincón de la Vieja were recorded daily by the seismic and infrasound networks during 26 August-2 September. A total of 20 phreatic eruptions were detected from the morning of 28 August to 0512 on 1 September, with eight of those generating lahars on the flanks. The most energetic event occurred at 1637 on 31 August and also produced a steam plume that rose 1 km. The Alert Level remained at 3 (Caution) and the Aviation Color Code remained Orange.

Geological summary: Rincón de la Vieja is a volcanic complex in the Guanacaste Range of NW Costa Rica. Sometimes referred to as the Rincon de la Vieja-Santa María Volcanic Complex, it consists of a slightly arcuate 20-km-long ridge of 12 craters and pyroclastic cones 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. The Santa María cone, the highest peak of the complex, is located on the E side of the ridge and has a lake within the 400-m-diameter 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 reported eruptions possibly dating back to the 16th century, have been from the active crater, near the center of the complex, with an acidic 300-m-diameter lake.

Sabancaya, Peru

15.787°S, 71.857°W; Elev. 5,960 m

The Instituto Geofísico del Perú’s (IGP) Centro Vulcanológico Nacional (CENVUL) reported continuing eruptive activity at Sabancaya during 24-30 August. The seismic network detected 71 seismic events associated with the movement of magmatic fluids, along with earthquakes indicating internal rock fracturing; no volcanic explosions occurred during the period. Gas, steam, and ash emissions periodically rose as high as 900 m above the summit and drifted less than 10 km NE, E, and SE. Moderate sulfur dioxide (SO₂) emissions averaged 1,138 tons per day, and nine thermal anomalies (each 2 MW or less) associated with the lava dome were identified. The Alert Level remained at Orange (the third level on a four-color scale) and the public was warned to stay outside of a 12 km radius from the summit.

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 observed eruptions date back to 1750 CE.

Semeru, Indonesia

8.108°S, 112.922°E; Elev. 3,657 m

The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported that activity continued at Semeru during 27-2 September, with daily eruptive events recorded by the seismic network. White-and-gray ash plumes rose 300-1,000 m above the summit and drifted in multiple directions during 27-29 August and 31 August-2 September. The Alert Level remained at 3 (the second highest level on a scale of 1-4) and the public was warned to stay at least 5 km away from the Jonggring Saloko Crater in all directions, 13 km from the summit on the SE flank along the Kobokan drainage, and 500 m from the banks of the Kobokan drainage as far as 17 km SE of the summit.

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

Sheveluch, Russia

56.653°N, 161.36°E; Elev. 3,283 m

The Kamchatkan Volcanic Eruption Response Team (KVERT) reported continuing lava effusion and explosive activity at the lava domes along the N crater floor of Young Sheveluch (on the SW flank of Old Sheveluch) during 27 August-2 September. A thermal anomaly was identified in satellite images on most days; weather clouds occasionally obscured views. 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 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.

References:

¹ Smithsonian Institution / US Geological Survey Weekly Volcanic Activity Report – GVP – August 27 – September 2, 2026 – Managing Editor: Sally Sennert