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Report on Semisopochnoi (United States) — 15 September-21 September 2021


Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 15 September-21 September 2021
Managing Editor: Sally Sennert.

Please cite this report as:

Global Volcanism Program, 2021. Report on Semisopochnoi (United States) (Sennert, S, ed.). Weekly Volcanic Activity Report, 15 September-21 September 2021. Smithsonian Institution and US Geological Survey.

Weekly Report (15 September-21 September 2021)


United States

51.93°N, 179.58°E; summit elev. 1221 m

All times are local (unless otherwise noted)

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 15-21 September. Weather clouds obscured views of the volcano on most days. Several small daily explosions were recorded by local seismic stations; ash plumes were not visible in satellite images suggesting that they remained below 3 km (10,000 ft) a.s.l. and did not rise above the cloud deck. Overnight during 18-19 September small, discontinuous, low-level ash plumes were visible drifting 100 km SE. Ash emissions increased in frequency and intensity on 19 September. Ash clouds rose as high as 4.6 km (15,000 ft) a.s.l. and continued to drift 100 km SE. Sulfur dioxide emissions increased in the afternoon. AVO raised the Aviation Color Code to Red and the Volcano Alert Level to Warning at 2158. Explosions continued overnight and the next day, and ash plumes rose up to 4.6 km a.s.l. Plumes drifted 100 km NW. At 2012 on 20 September AVO lowered the Aviation Color Code to Orange noting the frequency of discrete explosions had decreased to a rate of about one per hour, though ash plumes from these events were still rising to 4.6 km a.s.l. and drifting NW. Sulfur dioxide plumes drifted N during 20-21 September.

Geological Summary. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide 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 Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical 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 eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

Source: US Geological Survey Alaska Volcano Observatory (AVO)