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ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, July 30, 2021, 1:13 PM AKDT (Friday, July 30, 2021, 21:13 UTC)
51°55'44" N 179°35'52" E,
Summit Elevation 2625 ft (800 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW
Low-level unrest continued at Semispochnoi volcano over the past week with no evidence of explosive activity since July 12. Seismicity remains above background with an increase in activity on Wednesday, July 28 through today. Clouds obscured views of the volcano most of the week although sulfur dioxide emissions were detected in satellite data on July 24 and robust steaming from the active vent was observed above the meteorological cloud layer on July 26. Web camera images were mostly obscured by fog, but clear views of the summit on July 30 showed minor steam emissions.
Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Cerberus and ash clouds under 10,000 ft above sea level are typical of recent activity at Semisopochnoi. New explosions could occur at any time with no warning.
Semisopochnoi is monitored by local seismic and infrasound sensors, satellite data, web cameras, and distant infrasound and lightning networks.
Remote 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 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.
GREAT SITKIN VOLCANO
52°4'35" N 176°6'39" W,
Summit Elevation 5709 ft (1740 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE
Unrest continued at Great Sitkin volcano over the past week. Seismicity remained above background this past week, however, no explosive activity was observed in seismic or local and regional infrasound data. Clouds obscured views of the volcano by satellite and web camera most of this week although moderately elevated surface temperatures and steam emissions were detected on July 27 and 28. A satellite radar image from July 27 showed that the dome in the summit crater has grown and is now approximately 130 m in diameter and 8 m high. AVO will continue to closely monitor this uplift feature. Satellite data from July 27 potentially showed minor fresh ash deposits.
Renewed explosive activity or lava effusion remain possible outcomes of the current period of unrest. This is, however, uncertain and a decline in unrest to background levels of activity is also possible.
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 th