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USGS/Cascades Volcano Observatory, Vancouver, Washington 
Eruption of Mount St. Helens

 
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 widely heard in the region 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. The sudden removal of the upper part of the volcano by the landslides triggered the almost instantaneous expansion (explosion) of steam and gases within the volcano. The abrupt pressure release uncorked the volcano.

 
Although the lateral blast began some seconds later than the debris avalanche, the blast's velocity was much greater, so that it soon overtook the avalanche.

 
The blast was widely heard hundreds of miles away in the Pacific Northwest. A strong, vertically directed explosion of ash and steam began very shortly after the lateral blast and rose very quickly. In less than 10 minutes, the ash column reached an altitude of more than 12 miles and began to expand into a mushroom-shaped ash cloud.

 
The 5.1-magnitude earthquake caused the gravitational collapse of Mount St. Helens' north flank, which produced the debris avalanche and triggered the ensuing violent lateral and vertical eruptions. The blast, ash, lightning, lava, mudflows and floods casued by the eruption caused widespread and extensive damage.

 

URL for CVO HomePage is: <http://vulcan.wr.usgs.gov/home.html>                                                                                                  12/21/01, Lyn Topinka