wikipedia.org:
Mount St. Helens is most notorious for its catastrophic eruption on May 18, 1980, at 8:32 a.m. PDT,[2[[http://en.wikipedia.org/wiki/Mount_St._Helens#cite_note-USDA-2|]]] the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale caused an eruption that reduced the elevation of the mountain's summit from 9,677 ft (2,950 m) to 8,365 ft (2,550 m), replacing it with a 1 mile (1.6 km) wide horseshoe-shaped crater.[3[[http://en.wikipedia.org/wiki/Mount_St._Helens#cite_note-USDATeacherCorner-3|]]] The debris avalanche was up to 0.7 cubic miles (2.9 km3) in volume. The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied.