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Intense earthquake activity persisted at the volcano during and between visible eruptive activity.As early as March 31, seismographs also began recording occasional spasms of volcanic tremor, a type of continuous, rhythmic ground shaking different from the discrete sharp jolts characteristic of earthquakes.Helens was the one volcano in the conterminous United States most likely to reawaken and to erupt "perhaps before the end of this century." This prophetic conclusion was followed in 1978 by a more detailed report, in which Crandell and Mullineaux elaborated their earlier conclusion and analyzed, with maps and scenarios, the kinds, magnitudes, and areal extents of potential volcanic hazards that might be expected from future eruptions of Mount St. Collectively, these two publications contain one of the most accurate forecasts of a violent geologic event.A view to the north of the "two-tone" mountain--an appearance produced by prevailing easterly winds during the initial activity of Mount St. Mount Rainier is visible in background (Photograph by C. A magnitude 4.2 (Richter Scale) earthquake on March 20, 1980, at p.m.In contrast, the gently sloping shield volcanoes, such as those in Hawaii, typically erupt nonexplosively, producing fluid lavas that can flow great distances from the active vents.Although Hawaiian-type eruptions may destroy property, they rarely cause death or injury.Such continuous ground vibrations, commonly associated with eruptions at volcanoes in Hawaii, Iceland, Japan, and elsewhere, are interpreted to reflect subsurface movement of fluids, either gas or magma.
The initial explosions formed a 250-foot-wide crater within the larger, preexisting snow- and ice-filled summit crater, and new fractures broke across the summit area. Helens, from a measurement site about 2 miles to the northeast (Photograph by Peter Lipman).
Before 1980, snow-capped, gracefully symmetrical Mount St.
Helens was known as the "Fujiyama of America." Mount St.
Helens, located in southwestern Washington about 50 miles northeast of Portland, Oregon, is one of several lofty volcanic peaks that dominate the Cascade Range of the Pacific Northwest; the range extends from Mount Garibaldi in British Columbia, Canada, to Lassen Peak in northern California. Helens a composite volcano (or stratovolcano), a term for steepsided, often symmetrical cones constructed of alternating layers of lava flows, ash, and other volcanic debris.
Composite volcanoes tend to erupt explosively and pose considerable danger to nearby life and property.