Mount St. Helens is an active stratovolcano located in Skamania County, Washington in the Pacific Northwest region of the United States. It lies 52 miles northeast of Portland, Oregon and 98 miles south of Seattle. Mount St. Helens takes its English name from the British diplomat Lord St Helens, a friend of explorer George Vancouver who surveyed the area in the late 18th century. The volcano is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire.
Mount St. Helens is best known 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; 200 homes, 47 bridges, 15 miles of railways, and 185 miles of highway were destroyed. A massive debris avalanche, triggered by a magnitude 5.1 earthquake, caused a lateral eruption that reduced the elevation of the mountain's summit from 9,677 ft to 8,363 ft, leaving a 1 mile wide, horseshoe-shaped crater. The debris avalanche was 0.6 cubic miles in volume.The 1980 eruption disrupted terrestrial ecosystems near the volcano.