Mt. St. Helens is most notorious for its major 1980 eruption, the deadliest and most economically destructive volcanic event in the US history. 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 feet to 8,363 feet, leaving a 1 mile wide horseshoe-shaped crater. Today, almost 40 years after the eruption, people can walk on the rim of the crater and enjoy spectacular views.