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As part of the Cascade Range of mountains, Mount St. Helens was formed by eruptive flows of magma as the Juan de Fuca tectonic plate is forced under the North American Plate. Beginning about 37,600 years ago, periodic eruptions and magma flows occurred in the area between Mount St. Helens and Mount Rainer. Huge eruptions occurred in the years 1480, 1482, and 1800, and smaller releases of steam and ash between 1831 and 1857. The large magma chamber beneath Mount St. Helens will likely continue to force lava upward and cause future eruptions. However, as history indicates, they could be several hundred years apart. The May 18, 1980 eruption was the first major eruption since 1800, and was 123 years after the last release of steam and ash in 1857.

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16y ago

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