Ash emissions from the ongoing eruption at Semisopochnoi volcano, Alaska have increased in frequency and intensity, prompting the Alaska Volcano Observatory (AVO) to raise the Aviation Color Code to Red at 05:58 UTC on September 20, 2021.
Satellite images show an ash cloud at an estimated altitude of 4.6 km (15 000 feet) above sea level extending approximately 100 km (60 miles) to the southeast through 05:00 UTC.
Explosions have been observed throughout the day and increased sulfur dioxide gas emissions have been observed in satellite data.
"These observations represent an increase in eruptive activity and Aviation Color Code and Volcano Alert Level are being increased to RED/WARNING," AVO said.1
Increasing high clouds over Semisopochnoi will likely obscure satellite views of ash emissions.
Seismic and infrasound monitoring will continue to provide notice of ongoing explosive activity, however, it cannot determine the height or extent of ash emissions.
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 VOLCANO OBSERVATORY NOTICE FOR AVIATION (VONA) – Issued: 20210920/0558Z
2 Semisopochnoi – Geological summary – GVP
Featured image credit: AVO/USGS. Acquired on August 3, 2021
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