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Kasatochi volcano in the central Aleutian Islands erupted unexpectedly on 7–8 August 2008. Kasatochi has received little study by volcanologists and has had no confirmed historical eruptions. The island is an important nesting area for seabirds and a long‐term biological study site of the U.S. Fish and Wildlife Service. After a notably energetic preeruptive earthquake swarm, the volcano erupted violently in a series of explosive events beginning in the early afternoon of 7 August. Each event produced ash‐gas plumes that reached 14–18 km above sea level. The volcanic plume contained large amounts of SO2 and was tracked around the globe by satellite observations. The cumulative volcanic cloud interfered with air travel across the North Pacific, causing many flight cancelations that affected thousands of travelers. Visits to the volcano in 2008–2009 indicated that the eruption generated pyroclastic flows and surges 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. Pyroclastic flow deposits contain abundant accidental lithic debris derived from the inner walls of the Kasatochi crater. Juvenile material is crystal‐rich silicic andesite that ranges from slightly pumiceous to frothy pumice. Fine‐grained pyroclastic surge and fall deposits with accretionary lapilli cover the lithic‐rich pyroclastic flow deposits and mark a change in eruptive style from episodic explosive activity to more continuous ash emission with smaller intermittent explosions. Pyroclastic deposits completely cover the island, but wave erosion and gully development on the flanks have begun to modify the surface mantle of volcanic deposits.