Eruptive activity continues at Mount Cerberus on Semisopochnoi Island, Alaska


Eruptive activity continues at the north crater of Mount Cerberus on Semisopochnoi Island, the Alaska Volcano Observatory (AVO) reported at 20:01 UTC on September 8, 2021.

Seismicity at the volcano remains elevated and variable, with periods of continuous tremor, and occasional short-lived explosions lasting several minutes, AVO said.1

These explosive events are generating small ash clouds that typically rise from 3 – 4.6 km (10 000 – 15 000 feet) above sea level and dissipate within an hour or two (as observed in satellite).

Periods of lower-altitude ash emissions, interspersed with robust steaming, were observed in web camera images on September 7.

Image of eruption plume from Mount Cerberus, Semisopochnoi Island, Aleutian Islands, caught by the webcam at station CEPE at 09:05 LT on September 7, 2021. Credit: AVO/USGS

These ash clouds were blown horizontally by the wind with little vertical uplift at altitudes below 1.5 km (5 000 feet) a.s.l.

Sulfur dioxide emissions continue to be observed in satellite data (once per day).

Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Cerberus and ash clouds usually under 3 km (10 000 feet) above sea level have characterized the recent activity, which shows no signs of abating, the observatory added.

Small explosions may continue to occur and could be difficult to detect, especially during poor weather conditions.

Semisopochnoi Island is monitored by local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

Semisopochnoi, Alaska on September 1, 2021. Credit: Copernicus EU/Sentinel-2, TW

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 (4 005 feet) high 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.2


1 ALASKA VOLCANO OBSERVATORY DAILY UPDATE – USGS – Wednesday, September 8, 2021, 20:01 UTC

2 Semisopochnoi, Geological summary – GVP

Featured image credit: AVO/USGS

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