Report on Semisopochnoi (United States) — 4 December-10 December 2019
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 4 December-10 December 2019
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2019. Report on Semisopochnoi (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 4 December-10 December 2019. Smithsonian Institution and US Geological Survey.
51.93°N, 179.58°E; summit elev. 1221 m
All times are local (unless otherwise noted)
Strong tremor at Semisopochnoi was recorded by local seismic and regional infrasound networks beginning at 0026 on 7 December, heralding the start of an eruption and prompting AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch. The meteorological cloud deck over the volcano was at around 3 km (10,000 ft) a.s.l.; no ash signals were detected above this altitude. A period of intermittent explosions was detected, and afterwards seismicity remained elevated at least through 9 December.
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.