Increase in eruptive activity at Semisopochnoi volcano, Alaska

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Eruptive activity at Semisopochnoi has gradually increased over the last 24 hours, the Alaska Volcano Observatory (AVO) reported at 21:48 UTC on April 22, 2021.

Satellite views of the volcano show a continuous plume extending 80 km (50 miles) to the south at approximately 2.4 km (8 000 feet) above sea level.

A regional infrasound array has detected this increase in activity.

The aviation color code and alert level remain at ORANGE/WATCH. However, AVO will continue to monitor the activity with satellite and regional infrasound data.

Small eruptions producing minor ash deposits within the vicinity of the volcano and ash clouds under 3 km (10 000 feet) above sea level are typical of activity at this volcano.

Ash emissions could be seen drifting to the south starting around 02:00 UTC on April 22, 2021. Ash plume is currently extending a bit beyond Amchitka Island (approximately 70 km) and appears to be following a trajectory at around 3 km (10 000 feet) a.s.l. Meteorological clouds are at 2.1 km (7 000 feet) a.s.l. Credit: NOAA/GOES-16, Hans Schwaiger

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 (5 miles) wide caldera. It formed as a result of the 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 1 221 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 (2 539 feet) high 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 (2 903 feet) high 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: NOAA/GOES-16, Hans Schwaiger

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