Report on Shishaldin (United States) — 23 October-29 October 2019
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 23 October-29 October 2019
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2019. Report on Shishaldin (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 23 October-29 October 2019. Smithsonian Institution and US Geological Survey.
54.756°N, 163.97°W; summit elev. 2857 m
All times are local (unless otherwise noted)
Pilot and local observations as well as webcam views all indicated that the incandescence at Shishaldin’s summit crater was from low-level Strombolian activity during 21-22 October. Seismicity and infrasound signals were consistent with low-level eruptive activity, and a steam plume was persistent. By 23 October lava had filled the crater and during 23-24 October overflowed the N crater rim. A 200-m-long lava flow on the N flank melted snow and caused a lahar which traveled 2.9 km and stopped at about 1,230 m elevation. A smaller lava flows caused a 1-km-long lahar to descend the NE flank. Spatter accumulated around the crater rim and melted snow. On 25 October AVO noted that trace ash deposits had extended at least 8.5 km SE.
Low-level eruptive activity continued during 26-29 October. Seismicity remained elevated, with periods of high-amplitude tremor. Small explosions were recorded during 27-28 October. A satellite image from 28 October showed a 850-m-long SWIR anomaly on the NW flank from a lava flow. A central spatter cone was visible. Spatter deposits on the snow at the upper flanks was evident, and tephra deposits extended about 2.5 km N. The lahar on the NW flank branched extensively at lower elevations and was at least 5 km long. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.
Geologic Background. The beautifully symmetrical Shishaldin is the highest and one of the most active volcanoes of the Aleutian Islands. The glacier-covered volcano is the westernmost of three large stratovolcanoes along an E-W line in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." A steam plume often rises from its small summit crater. Constructed atop an older glacially dissected volcano, it is largely basaltic in composition. Remnants of an older ancestral volcano are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is blanketed by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century.