Mount St. Helens is an active
stratovolcano located in
Skamania County,
Washington, in the
Pacific Northwest region of the United States. It is 96 miles (154&_160;km) south of
Seattle and 53 miles (85&_160;km) northeast of
Portland, Oregon. Mount St. Helens takes its English name from the
British diplomat
Lord St Helens, a friend of explorer
George Vancouver who made a survey of the area in the late 18th century. The volcano is located in the
Cascade Range and is part of the
Cascade Volcanic Arc, a segment of the
Pacific Ring of Fire that includes over 160 active
volcanoes. This volcano is well known for its
ash explosions and
pyroclastic flows.
Mount St. Helens is most famous for its catastrophic eruption on May 18, 1980,[1] which was 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&_160;km) of railways, and 185 miles (300&_160;km) of highway were destroyed. The eruption caused a massive debris avalanche, reducing the elevation of the mountain's summit from 9,677&_160;feet (2,950&_160;m) to 8,365&_160;feet (2,550&_160;m) and replacing it with a mile-wide (1.5&_160;km-wide) horseshoe-shaped crater.[2] The debris avalanche was up to 0.7 cubic miles (2.9 km³) in volume. The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied.
As with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous summit, and off its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption.
Shaken by an earthquake measuring 5.1 on the Richter scale, the north face of this tall symmetrical mountain collapsed in a massive rock debris avalanche. Nearly 230 square miles of forest was blown down or buried beneath volcanic deposits. At the same time a mushroom-shaped column of ash rose thousands of feet skyward and drifted downwind, turning day into night as dark, gray ash fell over eastern Washington and beyond. The eruption lasted 9 hours, but Mount St. Helens and the surrounding landscape were dramatically changed within moments.
