A dramatic, mushroom-shaped eruption column, lit by the rising sun, rises above Alaska's Redoubt volcano on April 21, 1990. Clouds of this shape, which are produced when the upper part of an eruption column attains neutral buoyancy and is spread out above the troposphere-stratosphere boundary, are common during powerful explosive eruptions. This column at Redoubt, however, did not originate from an eruption at the summit crater, but is an ash column that is rising buoyantly above a pyroclastic flow sweeping down the volcano's north flank.
Photo by Joyce Warren, 1990 (courtesy of U.S. Geological Survey).