ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, June 4, 2021, 1:31 PM AKDT (Friday, June 4, 2021, 21:31 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

The level of unrest at Great Sitkin volcano has been at a low level throughout the past week. No additional ash emissions have been detected since a short-duration explosive event occurred on May 25 AKDT (May 26 UTC). Seismicity remains at low levels and satellite observations show no additional activity, although elevated surface temperatures have been detected when the summit has been free of cloud cover. The elevated surface temperatures are likely the result of still hot pyroclastic material around the vent.

The prognosis for renewed eruptive activity is uncertain, although the ongoing quiescence is suggestive of a gradual return to normal background conditions. It remains possible for the level of unrest at the volcano to change quickly, and if so, additional explosive events could occur in the coming days, weeks or months. AVO is monitoring the volcano closely and 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 at the north crater of Mount Cerberus volcano on Semisopochnoi Island has continued over the past week. Low-level seismic activity has occurred throughout most of the week and only minor steaming from the active vent has been occurring over the past several days as reported by AVO field crews working on the island. Last weekend (May 29-30), low-level ash emissions reaching 3,000 to 10,000 feet above sea level were observed by AVO personnel. Ash also was detected in GOES satellite data that showed several narrow, thin plumes extending towards the southwest over Amchitka Island. The ash emissions coincided with a period of relatively continuous seismic tremor that lasted for about 9 hours. Minor ash fall occurred on the southern part of Semisopochnoi Island, but there were no reports of ash fall on Amchitka Island southwest of the volcano where AVO personnel were located.

Satellite observations were mostly obscured by clouds over the past week, but occasional clear views of Semisopochnoi Island indicated elevated surface temperatures at north crater on several days. Emissions of sulfur dioxide gas also were detected in satellite data throughout the week. Overall, activity at north crater remains elevated and above background levels but the volcano appears to have entered a quiet period characterized by passive degassing following the short period of ash emission on May 29-30.

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 significant activity was detected in seismic and infrasound data over the past week. Satellite and web camera observations were mostly obscured by clouds but elevated surface temperatures were detected in the summit crater on June 1, and minor emissions of sulfur dioxide gas were detected on June 3. It is uncertain if these observations indicate a change in the level of unrest as the volcano is persistently active but difficult to observe due to cloud cover. The level of seismic activity does not suggest a change in the status of the volcano.

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:

Michelle Coombs, Scientist-in-Charge, USGS mcoombs@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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