Mount St. Helens is most notorious for its catastrophic eruption on May 18, 1980, at 8:32 am PDT, Info compliments of mt. st Helens Facebook page

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  of railways, and 185 miles of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale, caused an eruption, reducing the elevation of the mountain's summit from 9,677 ft  to 8,365 ft ) and replacing it with a 1 mile) wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles  in volume. The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied.