ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, May 28, 2021, 12:09 PM AKDT (Friday, May 28, 2021, 20:09 UTC)

GREAT SITKIN VOLCANO (VNUM #311120)
52°4'35" N 176°6'39" W, Summit Elevation 5709 ft (1740 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

A short-duration explosive event occurred on May 25 AKDT (May 26 UTC) following a day-long increase in local seismic activity. The Aviation Color Code and Volcano Alert Level was increased to ORANGE/WATCH on May 25 at 7:43 PM AKDT (03:43 UTC on May 26) in response to the elevated seismic activity as well as observations of robust steaming, sulfur dioxide gas emissions, and elevated surface temperatures observed the previous week. A 1–2 minute-long explosive event occurred on May 25 at 9:04 PM AKDT (05:04 UTC on May 26) and the Aviation Color Code and Volcano Alert Level were increased to RED/WARNING. The ash cloud rose to an altitude of ~15,000 ft above sea level and drifted eastward over the Bering Sea. This event did not result in ashfall on local communities. Following the explosion, seismic activity decreased greatly, and satellite observations showed no additional ash emissions. As a result of this marked decrease in activity the Aviation Color Code and Volcano Alert Level were decreased to ORANGE/WATCH on May 26 at 8:31 AM AKDT (16:31 UTC) and to YELLOW/ADVISORY on May 27 at 12:58 PM AKDT (20:58 UTC). Seismicity remains at low levels and satellite observations show no additional activity.

The prognosis for renewed eruptive activity is uncertain. Additional explosive events, the eruption of lava, or a return to non-eruptive behaviors are all possible. AVO will report on significant changes and observations in monitoring data should they occur.

Great Sitkin is monitored with a local real-time seismic network, which will typically allow AVO to detect changes in unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption would be accomplished using a combination of seismic, infrasound, lightning, and satellite data.

Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 43 km (26 miles) east of the community of Adak. The volcano is a composite structure consisting of an older dissected volcano and a younger parasitic cone with a 3-km-diameter summit crater. A steep-sided lava dome, emplaced during the most recent significant eruption in 1974, occupies the center of the crater. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.

SEMISOPOCHNOI VOLCANO (VNUM #311060)
51°55'44" N 179°35'52" E, Summit Elevation 2625 ft (800 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

Unrest continued over the past week. Seismic data from the island was restored on May 26 by AVO field crews who repaired the satellite uplink on Amchitka Island. Seismic data had been offline since November 11, 2020. Periods of seismic tremor (continuous shaking) were observed once the data was restored. Tremor amplitude was moderate at times, as has been the case since unrest began in 2018, but no explosive activity was observed in local seismic or regional infrasound data. Satellite observations were mostly obscured by clouds over the past week, but steaming from the north crater of Mount Cerberus (the active vent) was observed on May 25. A comparison of satellite radar (which can seen the ground through clouds) between May 15 and 27 show no evidence of changes in shape of the active vent.

Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Cerberus and ash clouds under 10,000 ft above sea level are typical of recent activity at Semisopochnoi. This type of activity is not always detected by regional infrasound sensors and often not observed due to cloudy weather conditions. New explosions could occur at any time with no warning.

Semisopochnoi is monitored by a local seismic network, and remotely by satellite and lightning detection sensors. An infrasound array on Adak Island may detect explosive emissions from Semisopochnoi with a slight delay (approximately 13 minutes) if atmospheric conditions permit.

Remote Semisopochnoi volcano occupies the largest, young volcanic island in the western Aleutians. The volcano is dominated by an 5-mile (8-km) diameter caldera that contains a small lake and a number of post-caldera cones and craters. The age of the caldera is not known with certainty but is likely early Holocene. Prior to 2018, the previous known eruption of Semisopochnoi occurred in 1987, probably from Sugarloaf Peak on the south coast of the island, but details are lacking. Another prominent, young post-caldera landform is Mount Cerberus, a three-peaked cone cluster in the southwest part of the caldera. The island is uninhabited and part of the Alaska Maritime National Wildlife Refuge. It is located 40 miles (65 km) northeast of Amchitka Island and 130 miles (200 km) west of Adak.

CLEVELAND VOLCANO (VNUM #311240)
52°49'20" N 169°56'42" W, Summit Elevation 5676 ft (1730 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

No activity was detected in seismic and infrasound data over the past week. Satellite and web camera observations were mostly obscured by clouds.

Episodes of lava effusion and explosions can occur without advance warning. Explosions from Cleveland are normally short duration and only present a hazard to aviation in the immediate vicinity of the volcano. Larger explosions that present a more widespread hazard to aviation are possible, but are less likely and occur less frequently.

Cleveland volcano is monitored by only two seismic stations, which restricts AVO's ability to precisely locate earthquakes and detect precursory unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption may be possible using a combination of seismic, infrasound, lightning, and satellite data.

Cleveland volcano forms the western portion of Chuginadak Island, a remote and uninhabited island in the east central Aleutians. The volcano is located about 45 miles (75 km) west of the community of Nikolski, and 940 miles (1500 km) southwest of Anchorage. The most recent significant period of eruption began in February, 2001 and produced 3 explosive events that generated ash clouds as high as 39,000 ft (11.8 km) above sea level. The 2001 eruption also produced a lava flow and hot avalanche that reached the sea. Since then, Cleveland has been intermittently active producing small lava flows, often followed by explosions that generate small ash clouds generally below 20,000 ft (6 km) above sea level. These explosions also launch debris onto the slopes of the cone producing hot pyroclastic avalanches and lahars that sometimes reach the coastline.

OTHER ALASKA VOLCANOES

Information on all Alaska volcanoes is available at : http://www.avo.alaska.edu.

For definitions of Aviation Color Codes and Volcano Alert Levels, see: http://www.avo.alaska.edu/color_codes.php

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CONTACT INFORMATION:

Kristi Wallace, Acting Scientist-in-Charge, USGS
kwallace@usgs.gov (907) 786-7497

David Fee, Coordinating Scientist, UAF
dfee1@alaska.edu (907) 322-4085



The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

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