Report on Shishaldin (United States) — 28 April-4 May 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 28 April-4 May 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on Shishaldin (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 28 April-4 May 2004. Smithsonian Institution and US Geological Survey.
54.756°N, 163.97°W; summit elev. 2857 m
All times are local (unless otherwise noted)
AVO raised the Concern Color Code at Shishaldin from Green to Yellow on 3 May due to unusual seismic activity at Shishaldin during the previous week. Seismicity changed at the volcano from discrete earthquakes to more continuous ones. Tremor was observed for the first time since the most recent eruption ended in May 1999. Airwaves (acoustical waves traveling in air) accompanying earthquakes were recorded by the seismic network, suggesting that the source of seismicity had become more shallow. AVO reported that there were no indications that an eruption was imminent or even likely. Satellite data showed no significant increase in ground temperature at the volcano, nor had there been reports of increased steaming. However, AVO warned that activity at Shishaldin could increase rapidly and increased the frequency of their seismic-data analysis.
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.