<P> Mount St. Helens is most notorious for its major 1980 eruption , 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 8,363 ft ( 2,549 m ) , leaving a 1 mile ( 1.6 km ) wide horseshoe - shaped crater . The debris avalanche was up to 0.7 cubic miles ( 2.9 km ) in volume . The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied . </P>
<P> Mount St. Helens is most notorious for its major 1980 eruption , 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 8,363 ft ( 2,549 m ) , leaving a 1 mile ( 1.6 km ) wide horseshoe - shaped crater . The debris avalanche was up to 0.7 cubic miles ( 2.9 km ) in volume . The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied . </P>
mt st helens height before and after eruption
mt st helens height before and after eruption
9,677 ft ( 2,950 m ) 8,363 ft ( 2,549 m )
9,677 ft ( 2,950 m ) 8,363 ft ( 2,549 m )