Volcanic peak in the Cascade Range , southwestern Washington, U.S. Dormant since 1857, it erupted in May 1980 in one of the greatest volcanic explosions ever recorded in North America.
Sixty people and thousands of animals were killed in the May 18 event, and 10 million trees were blown down by the lateral air blast. At the event's end Mount St. Helens's volcanic cone had been completely blasted away; in place of its 9,680-ft (2,950-m) peak was a horseshoe-shaped crater with a rim reaching elevations of about 8,000 ft (2,400 m). Further eruptions have occurred since 1980, and a dome of lava has grown in the crater. Mount St. Helens National Volcanic Monument was established in 1982.