ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, May 22, 2020, 10:56 AM AKDT (Friday, May 22, 2020, 18:56 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

Small earthquakes were detected at Great Sitkin volcano throughout the past week. Earthquake activity remains above background levels and the degree of seismic unrest has not changed appreciably for a number of weeks. Satellite and web camera views of the volcano were obscured by clouds throughout the week, except for May 15 when minor steaming at the summit was observed in satellite data. There were no additional observations of outward unrest this week.

Great Sitkin volcano is monitored by local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks. Should conditions at the volcano change, this should be apparent in these data streams and AVO will respond accordingly.

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 decapitated volcano and a younger parasitic cone with a 2-3 km diameter summit crater. A steep-sided lava dome, emplaced during an eruption in 1974, occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974. That eruption produced a lava dome and 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: ADVISORY
Current Aviation Color Code: YELLOW

Unrest continues at Semisopochnoi. Seismicity remains elevated and numerous small, low-frequency earthquakes and occasional periods of continuous seismic tremor were detected over the past week. No eruptive activity was indicated in seismic or regional infrasound data. Clouds obscured satellite views of the volcano throughout the week and there were no observations of emissions or other outward signs of unrest. There is no indication that activity is increasing towards an eruption in the short-term. It remains possible for conditions at the volcano to escalate and explosive activity could occur with minimal precursory unrest.

Semisopochnoi is monitored with a local seismic network and remotely by satellite, infrasound and lightning sensors.

Remote Semisopochnoi volcano occupies the largest, young volcanic island in the western Aleutians. The volcano is dominated by an 8-km (5-mile) 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. The last 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 65 km (40 mi) northeast of Amchitka Island and 200 km (130 mi) west of Adak.

SHISHALDIN VOLCANO (VNUM #311360)
54°45'19" N 163°58'16" W, Summit Elevation 9373 ft (2857 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Unrest at Shishaldin Volcano has continued over the past week. Satellite and web camera observations of the volcano have been obscured by clouds for much of the week, although slightly elevated surface temperatures at the summit were observed on May 16, 17 and 19. Minor steam emissions were observed on May 15, 16, 19 and 20 and minor amounts sulfur dioxide gas were detected in satellite data near the volcano on May 16. The level of seismic activity remains above background with nearly continuous, weak tremor and occasional low-frequency earthquakes occurring over the past week. No evidence of eruptive activity has been observed this week.

Although the current level of unrest at the volcano remains low, it is possible for activity to escalate at any time with little warning. Thus, ash emission, lava effusion and the possibility of lahars in the drainages extending from the volcano remain plausible outcomes of escalating unrest. Shishaldin is monitored by local seismic and infrasound sensors, satellite data, web cameras, a telemetered geodetic and tilt network, and distant infrasound and lightning networks.

Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetric cone with a base diameter of approximately 16 km (10 mi). A 200-m-wide (660 ft) funnel-shaped summit crater typically emits a steam plume and occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, with at least 54 episodes of unrest including over 24 confirmed eruptions since 1775. Most eruptions are relatively small, although the April-May 1999 event generated an ash column that reached 45,000 ft above sea level.

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:

Dave Schneider, Acting Scientist-in-Charge, USGS
djschneider@usgs.gov (907) 786-7497

David Fee, Coordinating Scientist, UAFGI
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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