Mount St. Helens is the youngest volcano in the Cascade Range. It had been dormant since 1857. It was a beautiful asymmetrical volcano with smooth slopes. Mount St. Helens had three minor steam explosions in 1898, 1903 and 1921 but was not believed to be a volcanic hazard. The volcano was teeming with wildlife, flora and fauna. It had become a leisure playground, used for hiking, camping, fishing, swimming and boating. Before the volcano, visitors could enjoy activities on the Toutle River and Spirit Lake. Some people said that Spirit Lake was so clear you could see straight to the bottom. Mountain goats graced the slopes, the volcano was rife with huckleberries, ferns and mosses.
Some intense activity began about two months before the Sunday morning, May 18, 1980 eruption at 8:32 a.m. The north slope of the volcano began to bulge; by May 18 the volcano had bulged a few hundred meters and had become very unstable. Then on that fateful morning an earthquake measuring 5.1 on the Richter scale shook the volcano. This caused the unstable slope to collapse as a landslide; this caused an immediate release of pressure on the magma (molten rock beneath the surface of the earth), below the volcano. The reduced pressure caused gas to come out of the magma and caused the catastrophic eruption.
57 people were killed directly from the eruption. The eruption also caused a plane crash, a traffic accident and 7 deaths from shoveling ash. The toll on the wildlife and fish was devastating as well. Almost 230 square miles of forest was blown down or buried. A cloud of ash rose thousands of feet skyward and drifted downwind turning the day into night. Ash fell over Eastern Washington and beyond.
The sudden collapse of the volcano’s north side released the tremendous pressure that had been building in the mountain. Expanding gasses and steam from the molten rock caused a series of huge explosions. The blast of rocks, ash and gasses swept across the land at speeds up to 670 miles an hour. Inside the blast temperatures reached 600 degrees Fahrenheit. The blast traveled as far as 16 miles north of the volcano. It spread over an area of about 150 square miles, stripping trees from hillsides as far away as 6 miles from the volcano. Further away, as the blast began to slow, trees were knocked over or snapped in half. Trees that were left standing around this blowdown zone were killed by the intense heat.
An enormous column of ash thrust 15 miles into the sky for the next nine hours. North of the volcano the pyroclastic flows deposited pumice and ash in layers as thick as 60 feet. The ash was blown eastward by the wind at about 60 miles per hour. Some of the ash reached the eastern United States within three days. Small particles entered the jet stream and circled the Earth within two weeks.
The eruption lasted for a very long nine hours. This nine hours changed the volcano and surrounding landscape in a devastating way. While the ash column was rising, other things were happening. The edges were collapsing onto the slopes of the newly formed crater. The superheated ash melted the snow and ice and began to flow down the slope. This mixture picked up rock debris — some as large as 20 feet in diameter as it sped from the volcano. The mudflow can travel at speeds as much as 30 miles per hour; easily destroying anything in its path; homes, trees, bridges and anything else. The large mudflow started about four hours after the eruption from the debris-avalanche deposit that formed from mudflows from the west, south and east sides of the volcano within minutes of the eruption.
Features of a debris avalanche deposit is made of rocks and rock layers that once filled the crater and formed the top 1,300 feet of volcano. The most obvious feature of the deposit is the hundreds of mounds and hills that geologists call hummocks. They range in size from 25 to 600 feet. The hummocks formed as whole pieces as the avalanching mountain came to rest and then slumped. This left behind an elevated area or hill. A second characteristic of this deposit is the volcano’s old rock layers. These old lava flows, pyroclastic flows (a mixture of volcanic gas, pumice and ash that swept down the north flank of the volcano at speeds of up to 100 miles an hour and temperatures of more than 1200 degrees fahrenheit), debris flows and ash layers were preserved in layers, even though they slid several miles from the volcano. The layers can be found right side up, upside down or tilted sideways. A third characteristic of the debris avalanche is the horse-shoe shaped crater that is left behind. These distinct characteristics have helped geologists document more than 200 other volcanic debris avalanche deposits around the world.
Several small streams that flowed into the Toutle River Valley were blocked when the debris avalanche slid from the volcano and into the North Fork of the Valley. The water backed up behind the edge of the debris avalanche deposit and formed new lakes. The three largest new lakes formed were Castle Lake, Coldwater Lake and Jackson Lake. Spirit Lake was dramatically changed by the debris avalanche. As the debris avalanche slammed into the south shore of the lake, a wave surged 600 feet up nearby hillsides. The avalanche swept soil and thousands of trees into the lake, thus raising the lake level by about 200 feet and the water temperature to 96 degrees Fahrenheit. The debris avalanche also created a very unstable natural dam that was holding back the waters of Spirit Lake. This created an urgent concern about the sudden release of water from these new and recreated lakes. $61 million dollars was spent between 1981 and 1985 on engineering solutions to prevent sudden lake breakouts.
The Toutle River valley was not left untouched, as well. The debris avalanche deposited 5.5 billion cubic yards of loose rock debris. Erosion of the material has been rapid. Huge channels more than 100 feet deep and 500 feet wide have been carved through the deposit. Acting much the same as a conveyor belt, the Toutle River transports sand and silt downstream. It drops some of the sediment on the river bottom along the way. During the winter of 1981, sediment from the avalanche deposit raised the Cowlitz River channel 20 feet. By depositing these large amounts of sediment, there is less water that the river can hold before it overflows its banks. Between 1980 and 1985 sediment was dredged from the Toutle and the Cowlitz Rivers, to prevent flooding.
Over the next five months, there were 5 smaller eruptions, followed by a series of 16 eruptions that formed the 270 meter (880 feet) high lava dome in the crater that was formed by the May 18th eruption.
For more information about Mount St. Helens you can access the website of the U.S. Geological Survey/Cascades Volcano Observatory. Some of the information in this article is courtesy of the US. Forest Service, Mount St. Helens National Volcanic Monument. If you are interested in seeing a webcam of the volcano in its current state you can click on this link for an up-to-date image of the volcano: http://www.fs.fed.us/gpnf/volcanocams/msh/