On May 18, 1980, Mount St. Helens, Washington, exploded in a spectacular and devastating eruption that brought the volcano to the attention of the world. Few people realized that Mount St. Helens had long been the most active volcano in the Cascade Range of the Pacific Northwest. It has a rich and complex 300,000-year history and has produced both violent explosive eruptions of volcanic ash and pumice and relatively quiet outpourings of lava. The volcano's edifice was mostly built by lava domes and flows from numerous eruptions. Using evidence in these lavas and other deposits, U.S. Geological Survey (USGS) scientists have documented dozens of major individual eruptions of the volcano.
An extensive apron of ash and fragmented volcanic rocks surrounds Mount St. Helens and mostly fills the valleys draining its slopes. This material was transported by pyroclastic flows (searingly hot flows of ash and volcanic gases), lahars (volcanic mudflows), and debris avalanches (landslides). Farther away from the volcano, pumice and ash that fell during explosive eruptions form layers that bury the landscape to depths of 10 feet or more.
The pre-1980 eruptive history of Mount St. Helens is strongly episodic. Volcanologists have recognized and named four episodes of volcanic activity, called "stages"-- Ape Canyon, Cougar, Swift Creek, and Spirit Lake--separated by dormant intervals. The youngest stage, Spirit Lake, is further subdivided into six eruptive periods. Because the preservation of deposits and other geologic evidence is best for the youngest stages, the farther scientists look back in time the less detail they can infer for the history of volcanism at Mount St. Helens.
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