On May 18, 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook Mount St. Helens.
On May 18, 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook Mount St. Helens.
United States Geological Survey - Source
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Eruption of Mount St. Helens

May 18, 1980. On that fateful day, Mount St. Helens Volcano in Washington exploded violently after 2 months of intense earthquake activity and intermittent, relatively weak eruptions, causing the worst volcanic disaster in the recorded history the United States.

The cataclysmic eruption and related events May 18 rank among the most significant geologic events in the United States during the 20th century. The processes, effects, and products of the chain of events were the most intensively studied and photographically documented of any explosive volcanic eruption in the world to date. The wealth of data on Mount St. Helens before, during, and after the May 18 eruption enabled geoscientists of the U.S. Geological Survey (USGS), the University of Washington, and other research institutions in the United States and abroad to put into perspective the devastating impact of suddenly unleashed volcanic energy.

In 1981 and 1982, the results of many of these studies were published by the USGS in two comprehensive volumes, Professional Papers 1249 and 1250, both dedicated to the memory of David A. Johnston, a USGS volcanologist killed while making scientific observations on May 18. Intermittently active through the 1980s, Mount St. Helens continues to receive intensive study. This booklet-updated and revised from the first edition (1984) on occasion of the 10th anniversary of the May 18, 1980, eruption-presents selected highlights of the volcano's eruptive history, reviews its activity in the past decade, and speculates about its possible future behavior. Materials cited in the Selected Readings provide more detailed information on topics that have been omitted or treated only briefly.

On May 18, 1980, the bulge on Mount St. Helens north flank slide away in a huge landslide which covered 23 square miles north and west of the volcano, and buried 14 miles of the North Fork Toutle River Valley to an average depth of 150 feet, with a maximum depth of 600 feet. The landslide depressurized the volcano's magma system, and triggered powerful explosions. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north, reached 17 miles northward from the volcano, and covered an area of 230 square miles, felling and singing trees. Over 4 billion board feet of timber - enough to build about 300,000 two-bedroom homes - was destroyed. The lateral blast produced an eruption column of ash and gas which rose more than 15 miles into the atmosphere in only 15 minutes. Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano. The plume spread across the United States in 3 days and circled the earth in 15. Beginning just after noon, pyroclastic flows - swift avalanches of hot ash, pumice, and gas - poured out of the crater at 50 to 80 miles per hour and spread as far as 5 miles to the north. Based on the eruption rate of these pyroclastic flows, scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m. Melting snow and ice which had covered the volcano melted sending surges of water that eroded and mixed with loose rock debris to form volcanic mudflows (lahars). Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying 27 bridges and over 200 homes. The lahars with the blast destroyed more than 185 miles of highways and roads, and 15 miles of railway. Fifty-seven people were killed.