Mount St. Helens

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.

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.

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.

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 http://vulcan.wr.usgs.gov/.