Semisopochnoi

Lava continues to slowly erupt within the summit crater at Great Sitkin Volcano. Cloudy conditions obscured satellite and webcam views of the volcano throughout the week. Seismic activity remains low, with the detection of occasional small volcanic earthquakes and seismic signals from small rockfalls within the crater.

The current lava eruption began in July 2021 and has filled most of the summit crater and advanced into the valleys below. There have been no explosions at Great Sitkin Volcano since an event in May 2021. The volcano is monitored using local seismic and infrasound sensors, satellite data, and web cameras, and regional 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 (42 km) 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 ~1 mile (1.6 km)-diameter summit crater. A steep-sided lava dome, emplaced in the crater during an eruption in 1974, has been mostly buried by the ongoing eruption. The 1974 eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft (7.6 km) above sea level. A poorly documented eruption also occurred in 1945, 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.

Matt Haney, Scientist-in-Charge, USGS [email protected] (907) 786-7497

David Fee, Coordinating Scientist, UAFGI [email protected] (907) 378-5460

Contact AVO: https://avo.alaska.edu/contact

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.

A small explosion occurred Sunday evening (February 22) at Semisopochnoi volcano and was detected by seismic and infrasound sensors on Semisopochnoi Island. Weather conditions were cloudy at the time, and no eruption cloud or deposits were detected subsequently in satellite or webcam data. Seismicity quickly returned to background levels, and no additional activity has been detected. This unheralded event posed little hazard, as it was localized to Mount Young, which last erupted in 2023. The north cone of Mount Young—the likely source of the explosion—hosts a crater lake and has vigorous fumaroles that produce a small gas plume. The Aviation Color Code and Volcano Alert Level at the volcano were not changed following this event and remain at GREEN and NORMAL.  

Semisopochnoi is monitored by a local seismic and infrasound network, local web cameras, regional lightning and infrasound sensors, and satellite data.   

Semisopochnoi volcano occupies the largest, young volcanic island in the western Aleutians. The volcano is dominated by a 5-mile (8 km) diameter caldera that contains a small lake and several 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 historical eruption of Semisopochnoi volcano 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 Young, a three-peaked cone cluster in the southwest part of the caldera. Mount Young has been intermittently active since 2018. 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.

Matt Haney, Scientist-in-Charge, USGS [email protected] (907) 786-7497

David Fee, Coordinating Scientist, UAFGI [email protected] (907) 378-5460

Contact AVO: https://avo.alaska.edu/contact

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.

Signs of volcanic unrest at Shishaldin Volcano continue. Seismic and infrasound activity remains elevated, with numerous small earthquakes, frequent infrasound detections, and periods of volcanic tremor being observed throughout the week. Minor steaming was also consistently observed in daytime webcam views, and sulfur dioxide emissions were seen in satellite data most days. Barely elevated surface temperatures, probably related to the gas plume, were also seen occasionally at the volcano's summit in satellite views.

Shishaldin Volcano is monitored by local seismic and infrasound sensors, web cameras, and a telemetered geodetic network. In addition to the local monitoring network, the Alaska Volcano Observatory uses nearby geophysical networks, regional infrasound and lightning data, and satellite data to monitor the volcano. 

Shishaldin Volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a conical stratovolcano with a base diameter of approximately 10 miles (16 km). It is one of the most active volcanoes in the Aleutian volcanic arc, with at least 54 episodes of unrest, including over 28 confirmed eruptions since 1824. Most eruptions are relatively small, although activity during the 1999 and 2023 eruptions generated ash columns that reached up to 46,000 ft (16 km) above sea level.

Matt Haney, Scientist-in-Charge, USGS [email protected] (907) 786-7497

David Fee, Coordinating Scientist, UAFGI [email protected] (907) 378-5460

Contact AVO: https://avo.alaska.edu/contact

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.