The 1980 eruption was the deadliest and most economically destructive volcanic event in the history of the United States. 57 people were killed; 250 homes, 47 bridges, 15 miles of railways, and 185 miles of highway were destroyed. A massive debris avalanche was triggered by an earthquake measuring 5.1 on the Richter scale. This caused the eruption, which reduced the height of the mountain's summit from to and replacing it with a wide horseshoe-shaped crater. The earthquake was caused by a sudden surge of magma from the Earth's mantle. The debris avalanche was up to 0.7 cubic miles (3.1 cubic kilometers) in volume. 