Report on Seguam (United States) — July 1993

Bulletin of the Global Volcanism Network, vol. 18, no. 7 (July 1993)
Managing Editor: Edward Venzke.

Seguam (United States) Ash eruptions to 2,400 m height; lava flow

Please cite this report as:

Global Volcanism Program, 1993. Report on Seguam (United States). In: Venzke, E (ed.), Bulletin of the Global Volcanism Network, 18:7. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN199307-311180.

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Seguam

United States

52.315°N, 172.51°W; summit elev. 1054 m

All times are local (unless otherwise noted)


Heavy ash eruptions rising 900-1,200 m were reported the morning of 31 July by the U.S. Coast Guard. A lava flow was also noted. According to these observers, the volcano was still erupting sporadically on 10 August, with light- to dark-gray ash plumes rising 2,400 m above the summit. Reports from Atka Island (~110 km W) indicate that weather conditions have frequently prevented observations.

Geologic Background. The elliptical, 11.5 x 24 km island of Seguam, lying between Amlia and Amukta Islands in the central Aleutians, contains two calderas with Holocene post-caldera cones. Growth of the basaltic-to-rhyolitic Wilcox volcano on the east side of the island during the late Pleistocene was followed by edifice collapse and an associated ignimbrite eruption about 9000 years ago, leaving a horseshoe-shaped caldera open to the west, inside which a rhyolitic cone was constructed. The 3 x 4 km wide westernmost caldera has a central scoria cone, Pyre Peak, which rises above the caldera rim and is the source of most of the historical eruptions of Seguam volcano. A very young basaltic field surrounds Pyre Peak, and lava flows partially fill the caldera and reach the southern coast. Older Holocene lava flows were erupted from vents within the eastern caldera, and a monogenetic Holocene cone forms Moundhill volcano on the eastern tip of the island.

Information Contacts: AVO.