Report on Semisopochnoi (United States) — 18 September-24 September 2019
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 18 September-24 September 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, 18 September-24 September 2019. Smithsonian Institution and US Geological Survey.
51.93°N, 179.58°E; summit elev. 1221 m
All times are local (unless otherwise noted)
AVO lowered the Volcano Alert Level for Semisopochnoi to Advisory and the Aviation Color Code to Yellow on 18 September noting a decrease in seismic activity over the past few weeks and an absence of tremor since 15 September. Low-levels sulfur dioxide emissions persisted, and the crater lake continued to fill with water.
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.