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Mount St. Helens is arguably
North America's most famous volcano. On May 18, 1980, Mount St. Helens
Volcano in Washington exploded violently. A magnitude
4.2 earthquake on March 20 was the first substantial indication of Mount
St. Helens' re-awakening. Earthquake activity increased during the following
weeks. With a thunderous explosion on March 27, Mount St. Helens began
to spew ash and steam.
Intense earthquake activity persisted at
the volcano during and between visible eruptive activity. As
early as March 31, seismographs began recording volcanic tremor, a type
of continuous, rhythmic ground shaking. Such continuous vibrations are
thought to reflect subsurface movement of fluids, eithergas or magma, and
suggested that magma and associated gases were on the move within the volcano.
Early on May 18,
following a magnitude 5.1 earthquake about 1 mile beneath the volcano,
the bulged, unstable north flank of Mount St. Helens suddenly began to
collapse, producing the largest landslide-debris avalanche recorded. Within
seconds, eruptions began. Page
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