THE AUGUST 7, 2008 ERUPTION OF KASATOCHI VOLCANO: A MAJOR EXPLOSIVE ERUPTION AND ITS AFTERMATH AT A SMALL ISLAND VOLCANO IN THE CENTRAL ALEUTIAN ISLANDS OF ALASKA

Kasatochi Volcano is a 3 x 3 km island in the central Aleutian Islands of Alaska that erupted spectacularly on August 7, 2008. Kasatochi is seismically unmonitored, has received little study by volcanologists, and has had no confirmed historical eruptions. The island is a long-term study site of the US Fish and Wildlife Service and a major nesting area for seabirds. After a brief period of precursory seismicity the volcano erupted violently in a series of three explosive events beginning early afternoon August 7. Each of these events produced ash and gas plumes that reached 13 to 15 km above sea level. The gas plume contained large amounts of SO2 and was tracked around the globe by satellite data. Ash clouds from the eruption interfered with air travel across the North Pacific and about 45 flights between Anchorage and Seattle and numerous intra-Alaska flights were cancelled affecting many thousands of travelers.

Brief visits to the volcano in August 2008 and summer 2009 indicated that the eruption generated lithic-rich pyroclastic flows that swept all flanks of the island, accumulated several tens of meters of pyroclastic debris and increased the diameter of the island by about 800 m. Juvenile material in the pyroclastic deposits is slightly to highly pumiceous, crystal-rich andesite (58-59 wt. % SO2) containing plagioclase, ortho- and clinopyroxenes, amphibole, and Ti-magnetite. Fine-grained, 1-2 m thick, pyroclastic surge and fall deposits with abundant accretionary lapilli cover the pyroclastic-flow deposits, and mark a change in eruptive style from episodic explosive activity to more continuous ash emission with smaller intermittent explosions.

A focus of our work in 2009 was to document and monitor geomorphic changes occurring on the island since the eruption and to evaluate the geoecological impacts of a major volcanic disturbance in an Aleutian Island setting. Pyroclastic deposits completely cover the formerly lush volcanic island, but wave erosion of the coastline, rill and gully development on the flanks, and localized surviving vegetation has begun to return the island to its pre-eruptive condition. Rapid coastal erosion is exhuming buried talus fields, and may, within a year or two, provide suitable nesting habitat for seabirds that formerly exceeded several hundred thousand individuals.