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Series of small explosions and tremor bursts at Semisopochnoi volcano, alerts raised

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A series of small explosions and tremor bursts have been detected in seismic data at Semisopochnoi over the past 7 hours and are continuing, representing a significant uptick in activity, the Alaska Volcano Observatory (AVO) reported at 09:19 UTC on February 15, 2020.

No ash plumes have been observed, however low-level ash emissions may accompany these minor explosions. Any ash emissions may be currently obscured by cloud cover at approximately 3 km (10 000 feet) above sea level.

In light of this most recent activity, AVO has raised the Aviation Color Code to ORANGE and the Alert Level to WATCH.

Seismicity was low over the past week, with weak tremor observed over the past day, AVO reported on February 14. Nothing significant was observed in predominantly cloudy satellite views. Explosive activity has occurred intermittently at Semisopochnoi over the last year and could resume without advance warning.

Semisopochnoi is monitored with an on-island seismic network and remotely by satellite and lightning sensors. Furthermore, an infrasound array on Adak Island may detect explosive emissions from Semisopochnoi with a 13-minute delay if atmospheric conditions permit.

Geological summary

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 on December 11, 2020. Credit: Copernicus EU/Sentinel-2

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