Mount St. Helens Anniversary

May 18, 2001


Provided by: USGS
Summary author: USGS

On May 18, 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook Mount St. Helens. The bulge and surrounding area slid away in a gigantic rockslide and debris avalanche, releasing pressure, and triggering a major pumice and ash eruption of the volcano. Thirteen-hundred feet (400 m) of the peak collapsed or blew outwards. As a result, 24 square miles (62 square Km) of valley was filled by a debris avalanche, 250 square miles (650 square km) of recreation, timber, and private lands were damaged by a lateral blast, and an estimated 200 million cubic yards (150 million cubic meters) of material was deposited directly by lahars (volcanic mudflows) into the river channels. For more than nine hours a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20-25 km) above sea level. The plume moved eastward at an average speed of 60 miles per hour (95 km/hr), with ash reaching Idaho by noon. By early May 19, the devastating eruption was over. 57 people were killed.

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