A lateral eruption, also called a flank eruption or lateral blast if explosive, is a volcanic eruption that takes place on the flanks of a volcano instead of at the summit. Lateral eruptions are typical at rift zones where a volcano is breaking apart. Since it is easier for molten rock to flow laterally out the sides of weak flanks, the flank gives way before magma is pushed up through a conduit that feeds magma to the summit. These features are commonly found at shield volcanoes and produce basaltic lava flows and cinder cones.
Lateral blasts are understood to be created by immediate decompression of a magma chamber lying not far below the flanks of a volcano. This occurred during the 1980 eruption of Mount St. Helens in the U.S. state of Washington, and also along the base or flanks of Mount Pelée's lava dome during its 1902 eruption on the northern tip of the French overseas department of Martinique in the Lesser Antilles island arc of the Caribbean. As was observed during the Mount St. Helens eruption, the rapid decompression of magma coupled with the subsidence of the mountain flank can briefly subject the surrounding landscape to the full explosive force of the volcano, directed laterally, or diagonally, instead of vertically. Though only lasting a few seconds, this direct lateral exposure to the explosive forces, along with subsequent pyroclastic flows and lahars, scours out an area of devastation originating from the volcano roughly in the shape of a cone that can span hundreds of square kilometers. Areas close to the volcano are generally sterilized, with damage becoming gradually less severe further away from the volcano.
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