The Eruption

On March 20, 1980, a series of small earthquakes signaled the awakening of Mount St. Helens from a 123-year silence.

Over the next two months more than 10,000 earthquakes followed as magma moved into the volcano, wedging the volcano apart at a rate of five feet per day. The rising forest in less than five minutes. Heat from a rising plume of volcanic ash melted glacial ice creating cement-like slurries of rock and ash called mud flows. Superheated avalanches of hot gas, magma caused a visible swelling of the volcano’s north flank creating a feature that scientists called ‘the bulge’.

On May 18, 1980, a magnitude 5.1 earthquake triggered the collapse of the volcano’s bulging north flank and summit in a landslide of historic proportions. Magma trapped within the volcano, suddenly relieved of pressure, exploded outward in a lateral blast that blew down 230 square miles of forest in less than five minutes. Heat from a rising plume of volcanic ash melted glacial ice creating cement-like slurries of rock and ash called mud flows. Superheated avalanches of hot gas, ash, and pumice called pyroclastic flows flowed into the valley north of the crater. The resulting landscape was a seemingly gray wasteland.

Shaken by an earthquake measuring 5.1 on the Richter scale, the north face of this tall symmetrical mountain collapsed in a massive rock debris avalanche. Nearly 230 square miles of forest was blown over or left dead and standing. At the same time a mushroom-shaped column of ash rose thousands of feet skyward and drifted downwind, turning day into night as dark, gray ash fell over eastern Washington and beyond. The eruption lasted 9 hours, but Mount St. Helens and the surrounding landscape were dramatically changed within moments.

SOURCE: USGS <http://vulcan.wr.usgs.gov/Volcanoes/MSH/description_msh.html>