US Forest Service

Mount St. Helens

Mount St. Helens National Monument

Mount St. Helens
Pacific Northwest Research Station
1220 SW 3rd Ave.
Portland, OR 97204

(503) 808-2100

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(FAQ 1) What happened during the big eruption of 1980?The May 18, 1980, eruption of Mount St. Helens created a real-time laboratory where scientists have studied volcanic processes and ecological responses for a quarter-century now (photo by USGS/CVO).


On the morning of May 18, 1980, after weeks of small tremors, a magnitude 5.1 earthquake shook beneath Mount St. Helens and triggered an enormous eruption. The eruption involved a complex series of events that unfolded over the next 12 hours, with many events going on simultaneously. These volcanic events buried some areas in debris avalanches and mudflows, scoured other areas with hot gases, blew down or scorched forests on slopes several miles away, and dusted forests farther away with volcanic ash.


Mount St. Helens’ north side collapsed in a debris avalanche

Scientists set up permanent plots in 1980 to observe how life responded after the blast (photo by Charlie Crisafulli).The entire northern side of the volcano collapsed in a massive debris avalanche. One lobe of the debris avalanche smashed into Spirit Lake, pushed the lake water up the surrounding slopes, and raised the lakebed 200 feet. As the water flowed back downhill, it dragged thousands of trees into the lake where the trees covered much of the surface. The second lobe of the debris avalanche surged over a 1,300-foot ridge and spilled into the South Coldwater Creek drainage. The third and largest lobe traveled 14 miles down the Toutle River valley, filling the valley to an average depth of 150 feet and leaving mounds of sediment in a bumpy pattern of hummocks.

A lateral blast ejected 660 °F steam and stones at more than 300 miles per hour

With the north side of the mountain gone, pressure was released on hot water within the volcano. The hot water burst into steam and blasted out the new opening in a powerful lateral blast. A hot stone-filled wind surged north at speeds over 300 miles per hour and temperatures of 660 F. This lateral blast toppled or snapped off trees over a 230-square-mile area north of the volcano, which later became known as the blowdown zone. On the outer fringes of the blowdown zone, the force of the lateral blast had diminished and trees remained standing but were seared by the hot air, leaving a band of standing dead trees referred to as the scorch zone.

Pyroclastic flows, rich in pumice, poured from the crater

Beginning about noon and lasting for several hours, super-hot (at least 1,300 ºF), fast-moving, pumice-rich pyroclastic flows poured from the crater and covered 6 square miles north of the volcano with pumice many feet deep. This sterile, desolate terrain was later called the Pumice Plain. Heat from the eruption melted snow and glaciers on the volcano’s slopes. The meltwater picked up soil, rocks, and logs, forming mudflows that traveled for tens of miles down river channels.

An ash column rose to 80,000 feet and circled the Earth

The towering column of ash rose for more than 9 hours and reached a height of about 80,000 feet. Wind carried ash mostly to the northeast where it darkened skies and covered the ground with gray, volcanic ash. Some ash remained aloft, and this part of the plume circled the Earth in 15 days. For more information on the 1980 eruption, visit

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US Forest Service - Pacific Northwest Research Station
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