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Mount Saint Helens Eruption

Before the eruption

http://volcano.und.nodak.edu/vwdocs/msh/gifs/4612.gif

On May 18th, 1980 at 8:32 AM on a quiet Sunday morning Mount Saint Helens finally blew its top. An earthquake at a measure of 5.1 on the Richter Scale shook the volcano. Immediately the whole north face of the mountain broke free and slid downward as a giant rock avalanche.

http://volcano.und.nodak.edu/vwdocs/msh/gifs/4559.gif

In seconds, as the rock slid off the mass of hot lava inside the mountain, pressure in the lava dropped, and water that had been dissolved in the lava turned into superheated steam, which formed bubbles that violently expanded and fragmented the lava into a fine powder ash. This mass of superheated steam and ash blasted upward and outward over the top of the avalanche, roaring to the north and west at speeds reaching hundreds of miles an hour.

http://www.cotf.edu/ete/modules/volcanoes/vmsh9pict.html

Mudflows started crashing down the west south and east sides of the volcano. The mudflows sloshed from side-to-side, up to 30 MPH, as they rushed through forests and clearcuts, crushing and burying everything in thier paths. They all left their mark on the land.

http://volcano.und.nodak.edu/vwdocs/msh/ov/ovd/ovdmf.html

Within a short time pyroclastic flow, hot mixtures of volcanic gas, pumice, and ash swept down the north flank of the volcano at speeds up to 100 miles an hour. The temperatures reached over 1200 degrees Farenheit. Also at the start of the eruption, an enormous cloud of ash hurrled 15 miles into the sky. The volcano continued to eject ash for about nine hours. The plume deposited ash and pumice on top of the blast deposit downwind from the volcano. The ash was blown eastward by the wind at about 60 miles per hour. Some of the ash reached the eastern United States within 3 days. Small particles entered the jet stream and circled the Earth within two weeks.

The ash fell at Sand Point, Idaho, more than 300 miles northeast of Mount Saint Helens. Ash continued to fall several days after the eruption had subsided. It damaged or destroyed many agricultural crops downwind from the eruption. The air had so much ash it made the pictures appear fuzzy as if they had not been developed correctly.

http://www.cotf.edu/ete/modules/volcanoes/vmsh13pict.html

Mount Saint Helens viewed from the north on June 4, 1980. The huge U-shaped basin formed by the landslide and the initial blast was nearly two miles long and one mile wide. The mountain was lowered 1,300 feet. Ash covered slopes, and extensive mudflows are visible in the photograph. Mudflows dumped more than 75 million cubic yards of sediment into rivers, valleys, and reservoirs. Several pyroclastic flows left about 0.05 cubic mile of deposits in this area. About 230 square miles were devastated by the eruption.

http://www.cotf.edu/ete/modules/volcanoes/vmsh10pict.html

Volcanic activity at Mount St. Helens declined significantly after the May 18 blast. Smaller explosive eruptions continued throughout the remainder of 1980, with a final one in early 1982. A volcanic dome was formed on the floor of the central crater by a series of small lava eruptions, the last of these occurring in 1986. Minor steam explosions continued into the early 1990s. The only activity at present is the rumble of small earthquakes, but the situation may change at any time. Mount St. Helens sleeps, but the sleep is fitful. Given its violent past, Mount St. Helens will awake again in the not too distant future.

3 years after the eruption

http://www.cotf.edu/ete/modules/volcanoes/vmsh20pict.html

To the nation--and especially to those living nearby--the May 18 eruption seemed of gigantic proportions. The crown and heart of a whole mountain had been blasted away, and the surrounding countryside devastated. The energy released by the eruption was estimated at ten megatons, which is an explosion thousands of times stronger than the Hiroshima bomb. Thousands of deer, elk, bear, and smaller animals were dead -- in addition to 57 humans. Two hundred thirty-two square miles of forest were destroyed. Numerous buildings, bridges, roads, and machines were destroyed, and farms and communities up to a thousand miles away were partially buried in ash. One hundred sixty-nine lakes and more than 3,000 miles of streams had either been marginally damaged or destroyed.

http://vulcan.wr.usgs.gov/Photo/Pictograms/before_after_spiritlake.html

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Copyright © 2004 Earlham College. Revised April 18, 2004 . Send corrections or comments to parkero@earlham.edu