ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, July 31, 2020, 12:50 PM AKDT (Friday, July 31, 2020, 20:50 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

Small earthquakes have continued throughout the week at Makushin Volcano. The earthquakes are occurring within an area about 6 miles (10 km) east of the summit at a depth of about 5 miles (8 km) and are part of a sequence that began on June 15, 2020. Thus far, seismicity has been variable, though the earthquakes have generally decreased in both size and rate. At this time, the earthquake rate remains above background. No signs of unusual activity have been noted at the surface, apart from typical steaming from the summit crater lake.

The exact cause of this increased earthquake activity, in terms of possible volcanic or tectonic processes, remains uncertain at this time. Because of the distance and depth of the locations of the earthquakes from the summit of Makushin Volcano, and the absence of other forms of volcanic unrest such as increased steaming, gas emission, or ground deformation, there is no indication that the present earthquake sequence will necessarily lead to a volcanic eruption.

If an eruption were to occur, the main hazard would likely be from airborne ash and ash fall. Wind trajectory plots and hypothetical ash fall model information is available at https://www.avo.alaska.edu/activity/Makushin.php. These models are updated twice per day in a hypothetical mode and will be updated immediately if there is an eruption. Seismic data, web camera images, and information products are also available at this site.

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

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 observed in satellite and web camera data or detected on local and regional geophysical networks over the past week.

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 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.

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

Unrest continues at Great Sitkin. Small local earthquakes continued over the past week, declining in number from the previous week. Clear satellite and web camera views early in the week showed minor steam emissions and corresponding weakly elevated surface temperatures. No eruptive activity was detected by infrasound sensors.

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 26 miles (43 km) 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 1.2-1.9 mile (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 (7.6 km) 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 has continued this week at Semisopochnoi. Seismicity has declined over the week, with occasional small low-frequency earthquakes recorded by the local seismic network. The volcano has been obscured by clouds most of the week, although sulfur dioxide emissions were observed on July 26. 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.

VENIAMINOF VOLCANO (VNUM #312070)
56°11'52" N 159°23'35" W, Summit Elevation 8225 ft (2507 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Low-level unrest has continued this week at Veniaminof with intermittent seismic tremor and occasional low-frequency earthquakes. Overall clear satellite and web camera observations of the volcano this week showed weakly elevated surface temperatures and minor steam emissions from the intracaldera cone.

Eruptions at Veniaminof usually result in minor ash emissions, lava fountaining, and lava flows from the small cone in the summit caldera. Ash emissions are typically confined to the summit crater, but larger events can result in ash fall in nearby communities and drifting airborne ash.

Veniaminof volcano is monitored with a local real-time seismic network, which will typically allow AVO to detect changes in unrest that may lead to a more significant explosive eruption. AVO combines seismic, infrasound, lightning, and satellite data for rapid detection of such events.

Mount Veniaminof volcano is an andesitic stratovolcano with an ice-filled 10-km diameter summit caldera located on the Alaska Peninsula, 480 miles (775 km) southwest of Anchorage and 22 miles (35 km) north of Perryville. Veniaminof is one of the largest (77 cubic miles; ~300 cubic km) and most active volcanic centers in the Aleutian Arc and has erupted at least 13 times in the past 200 years. Recent significant eruptions of the volcano occurred in 1993-95, 2005, and 2013. These were Strombolian eruptions that produced lava fountains and minor emissions of ash and gas from the main intracaldera cone. During the 1993-95 activity, a small lava flow was extruded, and in 2013, five small lava flows effused from the intracaldera cone over about five months. Minor ash-producing explosions occurred nearly annually between 2002 and 2010. Previous historical eruptions have produced ash plumes that reached 20,000 ft (6 km)above sea level (1939 and 1956) and ash fallout that blanketed areas within about 25 miles (40 km) of the volcano (1939).

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

Pavel Izbekov, Acting Coordinating Scientist, UAFGI
peizbekov@alaska.gov (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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