ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, September 4, 2020, 12:43 PM AKDT (Friday, September 4, 2020, 20:43 UTC)

MAKUSHIN VOLCANO (VNUM #311310)
53°53'24" N 166°55'30" W, Summit Elevation 5906 ft (1800 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

A few earthquakes were recorded throughout the week at Makushin Volcano. The earthquakes are part of a sequence that began on June 15, 2020 occurring within an area about 6 miles (10 km) east of the summit at a depth of about 5 miles (8 km). Seismicity has been steadily declining for months, with the rate of earthquakes nearing background levels this week. No signs of unusual activity have been noted at the surface. A clear web-camera view of the summit on Thursday showed typical steaming from the summit crater.

Makushin Volcano is monitored with a network of seismic instruments, web camera, GPS, satellite data, and regional infrasound and lightning detection instruments.

Makushin Volcano is located on northern Unalaska Island in the eastern Aleutian Islands. Makushin is a broad, ice-capped stratovolcano that rises to an elevation of 5906 feet (1800 m). The 1.86-mile-diameter (3 km) summit crater is the site of frequent steam and minor ash eruptions. Three large explosive eruptions occurred at Makushin between 6,000-9,000 years ago, depositing volcanic material many meters thick near the volcano, and several centimeters deep in the community of Unalaska and the port of Dutch Harbor, 16 miles (25 km) east of the volcano.

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 local earthquakes continued over the past week. The overall rate of earthquakes continues to slowly decline, but the level of seismic activity remains above the background rate. No surface changes were detected in satellite or web camera images.

Great Sitkin volcano is monitored by local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

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: ADVISORY
Current Aviation Color Code: YELLOW

Only infrequent small local earthquakes were detected at Semisopochnoi over the past week. Clouds obscured most satellite images of the surface. No explosive activity was indicated in seismic or regional infrasound sensors, and no ash emissions were detected in satellite imagery.

Semisopochnoi is monitored by local seismic sensors, satellite data, and regional infrasound and lightning detection instruments. 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-miles (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. 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 40 miles (65 km) northeast of Amchitka Island and 130 miles (200 km) west of Adak.

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, 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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