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Mount St. Helens Volcano, Cascade Range, southern Washington, U.S.A. (46.20°N, 122,18°W). All times are local (GMT—7 hours). Without giving a sign that its internal forces had built to the breaking point, Mount St. Helens erupted with a tremendous explosion on May 18, at 0823, projecting ash into the stratosphere. A magnitude 5 earthquake struck the mountain apparently seconds before the eruption and may have prompted the blast, according to U.S. Geological Survey geologist Bob Christiansen.