The Alaska Volcano Observatory (AVO) has raised the Aviation Color Code for Semisopochnoi volcano to Orange and the Volcano Alert Level to Watch on July 12, 2021.
The decision was made due to the onset of continuous volcanic tremors and detections of explosive activity on newly installed local infrasound sensors.
Satellite observations made on the afternoon of July 12 showed emissions of a volcanic gas cloud (SO2), possibly containing volcanic ash.
Emissions occurred for tens of minutes beginning around 21:00 UTC (13:00 AKDT) and produced a low-altitude cloud that moved to the south at an altitude of less than 1.5 km (5 000 feet) above sea level.
Volcanic tremor continued for several hours before decreasing to low levels. No additional gas/ash emissions have been observed in satellite data or detected on the local infrasound network.
Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Cerberus and ash clouds under 3 km (10 000 feet) above sea level are typical of recent activity at Semisopochnoi.
Small explosions may occur without warning and could be undetected by regional infrasound sensors and cloudy weather conditions.
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.
Featured image: Semisopochnoi on June 8, 2021. Credit: Cyrus Read, USGS/AVO