Mount St. Helens

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< SECTION 1 OF 2 > Mount St. HelenS 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 km) south of Seattle, Washington, and 50 miles (80 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, at 8:32 am PDT, 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 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale, caused an eruption, reducing the elevation of the mountain’s summit from 9,677 ft (2,950 m) to 8,365 ft (2,550 m) and replacing it with a 1-mile (1.6 km) wide horseshoe-shaped crater. The earthquake was caused by a sudden surge of magma from the Earth’s mantle. The debris 3 avalanche was up to 0.7 cubic miles (2.9 km ) in volume. The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied. For more information see: http://en.wikipedia.Uorg/wiki/Mount_St._HeUlens Licensed under the CC-BY-SA: http://creativecommoUns.org/licenses/by-Usa/3.0/ PassageBank Passage provided under license from PassageBank.com. Use and duplication is restricted to licensees only. < SECTION 2 OF 2 > 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. Mount St. Helens is 45 miles (72 km) west of Mount Adams, in the western part of the Cascade Range. These “sister and brother” volcanic mountains are approximately 50 miles (80 km) from Mount Rainier, the highest of the Cascade volcanoes. Mount Hood, the nearest major volcanic peak in Oregon, is 60 miles (100 km) southeast of Mount St. Helens. 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 Holocene epoch (the last 10,000 or so years). For more information see: http://en.wikipedia.Uorg/wiki/Mount_St._HeUlens Licensed under the CC-BY-SA: http://creativecommoUns.org/licenses/by-Usa/3.0/ PassageBank Passage provided under license from PassageBank.com. Use and duplication is restricted to licensees only.

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