Mount St. Helens National Volcanic Monument
Mount St. Helens is most notorious for its eruption on May 18, 1980, 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 that reduced the elevation of the mountain’s summit from 9,677 ft to 8,363 ft , replacing it with a 1 mile wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles in volume. The magma in St. Helens burst forth into a large-scale pyroclastic flow that flattened vegetation and buildings over 230 square miles. The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create lahars (volcanic mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying bridges and lumber camps. A total of 3,900,000 cubic yards of material was transported 17 miles south into the Columbia River by the mudflows. it ejected more than 0.67 cubic miles of material.
Mount St. Helens is geologically young compared with the other major Cascade volcanoes. It formed only within the past 40,000 years, and the pre-1980 summit cone began rising about 2,200 years ago. The volcano is considered the most active in the Cascades within the last 10,000 or so years.