Report on Semisopochnoi (United States) — 12 February-18 February 2020
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 12 February-18 February 2020
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2020. Report on Semisopochnoi (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 12 February-18 February 2020. Smithsonian Institution and US Geological Survey.
51.93°N, 179.58°E; summit elev. 1221 m
All times are local (unless otherwise noted)
A series of explosions and tremor bursts at Semisopochnoi were detected by the seismic network beginning on 14 February, prompting AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch. Tremor bursts lasting from three to twelve minutes every few hours were recorded the next day. Ash plumes were not visible, though a weather cloud deck persisted between 1.5 and 4.6 km (5,000 and 15,000 ft) a.s.l. Seismic data continued to record small explosions and tremor bursts lasting from six to ten minutes every few hours on 16 February, but by 17 February there were almost no events recorded. Weather clouds continued to obscure views of the volcano.
Geologic Background. 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 was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank of Cerberus 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 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.