Mount St. Helens is most notorious for its major eruption on May 18 , 1980 , the deadliest and most economically destructive volcanic event in U.S. history . Fifty-seven people were killed ; 250 homes , 47 bridges , of railways , and 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 to , leaving a wide horseshoe-shaped crater . The debris avalanche was up to in volume . The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for the eruption 's aftermath to be scientifically studied
its major eruption on May 18 , 1980
1980 eruption of Mount St. Helens
Mount St. Helens