An eruption signature was detected at Semisopochnoi as strong tremor on local seismic and in regional infrasound networks on Adak Island, Alaska Volcano Observatory reported at 10:25 UTC on December 7, 2019.
The event started at 09:26 UTC (00:26 AKST) on December 7 and has continued with and ongoing but weaker local seismic signal. Any volcanic ash emissions are most likely low-level at this time.
The meteorological cloud deck has been around 3 000 m (10 000 feet) above sea level over Semisopochnoi and no ash signals have been detected above this height. Nothing has been detected in lightning data, AVO said.
The Volcano Alert Level was raised from Watch to Advisory and the Aviation Color Code from Yellow to Orange.
Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km (12.4 miles) wide at sea level and contains an 8-km-wide (5 miles) caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is 1221-m-high (4 005 feet) Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part.
The three-peaked 774-m-high (2 539 feet) Mount Cerberus volcano was constructed during the Holocene within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Cerberus appear younger than those on the southern side.
Other post-caldera volcanoes include the symmetrical 855-m-high (2 903 feet) Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera.
Most documented historical eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time.
This volcano is located within the Aleutian Islands, a UNESCO Biosphere Reserve property.
Featured image: Semisopochnoi in May 2017. Credit: Max Kaufman, USGS/AVO
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