SAINT HELENS, MOUNT

volcanic peak in the Cascade Range, southwestern Washington, U.S., that erupted on May 18, 1980, in one of the greatest volcanic explosions ever recorded in North America. Mount St. Helens had been dormant since 1857. An explosive steam eruption on March 27, 1980, was followed by alternating periods of quiescence and minor eruption. Pressure from rising magma within the volcano caused extensive fissures and the growth of a bulge on the north flank of the peak. On the morning of May 18 an earthquake with a 5.1 Richter magnitude triggered a gigantic landslide on the mountain's north face. The north slope fell away in an avalanche that was followed and overtaken by a lateral air blast, which carried a high-velocity cloud of superheated ash and stone outward more than 12 miles (20 km) from Mount St. Helens' summit. The avalanche and lateral blast were followed by mudflows, pyroclastic flows, and floods that buried the river valleys east of Mount St. Helens in deep layers of mud and debris for as far away as 17 miles (27 km). Meanwhile, simultaneously with the avalanche, a vertical eruption of gas and ash formed a column more than 12 miles high that produced ash falls as far east as central Montana. Sixty people and thousands of animals were killed in the May 18 event, and ten million trees were blown down by the lateral air blast. At the event's end Mount St. Helens' volcanic cone had been completely blasted away; in place of its 9,680-foot (2,950-metre) peak was a horseshoe-shaped crater with a rim reaching elevations of about 8,000 feet (2,400 m). Further eruptions have occurred since the May 18 explosion, and a dome of lava has grown intermittently in the crater.