ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, March 26, 2021, 2:08 PM AKDT (Friday, March 26, 2021, 22:08 UTC)

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

Numerous small explosions from Seimisopochnoi were detected in regional infrasound data over the past week. None of these events produced an ash cloud that was visible in satellite data, however cloudy conditions obscured the volcano at times. Possible minor ash fall on the island was noted in a high-resolution satellite image collected on March 24. Plumes of sulfur dioxide gas were detected intermittently in satellite data throughout the week.

Small eruptions producing minor ash deposits within the vicinity of the volcano are typical of activity during unrest at Semisopochnoi since September 2018. Local seismic stations have been offline since November 11, 2020. New explosions could occur at any time with no warning.

Semisopochnoi is monitored remotely by satellite and lightning sensors. An infrasound array on Adak Island could detect explosive emissions from Semisopochnoi with a 13 minute delay 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.

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

Low-level eruptive activity continues at Veniaminof. Elevated seismicity continues to be observed in the limited available seismic data. Seismic activity reached a high level on March 23 and throughout the week consisted of periods of repeating low frequency events and intermittent tremor. Ash and volcanic gas emissions continued intermittently throughout the week and were visible at times in web camera and satellite data. Pressure waves from explosive activity were detected on local seismic stations and in regional infrasound data at times this past week, and reached a high level of on March 23. These explosions were heard by residents in the villages of Perryville located 21 miles (35 km) southeast, and in Chignik Lagoon located 34 miles (55 km) northeast, of the volcano. The audible noise produced by these periodic explosions occur when a gas-rich slug of magma reaches the surface and expands rapidly producing a sudden release of gas and fragmented magma. These audible explosions are common during eruptions at Veniaminof (and other similar volcanoes) and atmospheric conditions play a role in whether they are heard in nearby villages. Eruption of lava from a sub-glacial flank vent near the summit cone likely continues, as indicated by incandescence in web camera and satellite data, as well as highly elevated surface temperatures observed in satellite data.

Eruptive activity at Veniaminof usually consists of 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.

A limited amount of seismic data from the local network that had been offline since December 8, 2020, have been recently restored. While the network has not been fully restored the additional information will aid in the detection of changes in unrest that may lead to a more significant explosive eruption. The Alaska Volcano Observatory continues to monitor Veniaminof with satellite and webcam data and remote infrasound, regional seismic and lightning networks.

Mount Veniaminof volcano is an andesitic stratovolcano with an ice-filled 10-km diameter summit caldera located on the Alaska Peninsula, 775 km (480 mi) southwest of Anchorage and 35 km (22 mi) north of Perryville. Veniaminof is one of the largest (~300 cubic km; 77 cubic mi) and most active volcanic centers in the Aleutian Arc and has erupted at least 14 times in the past 200 years. Recent eruptions in 1993-95, 2005, 2013, and 2018 all occurred at the intracaldera cone and lasted for several months. These eruptions produced lava spattering and fountaining, minor emissions of ash and gas, and small lava flows into intracaldera icefield. Minor ash-producing explosions occurred nearly annually between 2002 and 2010. Previous historical eruptions have produced ash plumes that reached 15,000 to 20,000 ft above sea level (1939, 1956, and 2018) and ash fallout that blanketed areas within about 40 km (25 mi) of the volcano (1939, 2018).

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

The Aviation Color Code was raised to YELLOW and the Volcano Alert Level to ADVISORY earlier in the week on March 20, 2021 in response to satellite detections of increased volcanic gas emissions and elevated surface temperatures. Since that time, there have been no observations of activity observed in satellite data. However, seismic activity near Cleveland first noted on March 10, 2021 continues. An earthquake with a magnitude of approximately 4.3 occurred this morning in the vicinity of Cleveland Volcano at 14:41 UTC (6:41 am AKDT). This may be associated with volcanic unrest and represents a significant departure from background earthquake activity. The unrest could result in a future eruption, however that is not a certainty. Smaller earthquakes have continued to occur near Cleveland, as is expected for earthquakes of this size.

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

SUBSCRIBE TO VOLCANO ALERT MESSAGES by email: http://volcanoes.usgs.gov/vns/

FOLLOW AVO ON FACEBOOK: https://facebook.com/alaska.avo

FOLLOW AVO ON TWITTER: https://twitter.com/alaska_avo

CONTACT INFORMATION:

Matthew Haney, Acting Scientist-in-Charge, USGS
mhaney@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.

---