Mt. St. Helens & Volcanic Hazards; More to Worry About
(right) Mount St. Helens by Mike Doukas, USGS (Public Domain).
Take a Tour of Mount St. Helens
The great eruption of Mt. St. Helens in May of 1980 is ancient history for most of you, from before you were born. That is often the way with geological disasters—they are far enough apart that we forget... and the reminder is often unpleasant.
The eruption blasted out at over 300 miles per hour and over 600ºF, followed by even hotter blasts at more than 1300ºF. Deaths included 57 people, nearly 7000 large animals (deer, elk, bear), countless smaller creatures, and enough trees to supply lumber for 300,000 homes. Most eruptions build volcanoes, but a few really dramatic ones blow the top off - this one lowered the peak by more than 1300 feet.
Mt. St. Helens, in southwestern Washington, was in some ways the queen of the Cascades Range. Beautifully symmetric, and snow-capped, it had been called the Fujiyama of the Pacific Northwest. Scores of people flocked to St. Helens’ flanks to hike, camp, ski, and generally enjoy the environment. But, all that changed in 1980.
(It may seem weird to you that we are about to focus on an event from before most of you were born, from 1980, when larger volcanic eruptions have happened more recently. But, St. Helens is the largest eruption that has occurred in the lower-48 states of the USA since before the country was formed, is the easiest eruption site to get to and observe, and really is awesome. Professor Alley’s elder daughter, Janet, was a ranger there one summer and recommends that you include Ape Cave if you visit. Our goal here is to help you see just how immense the eruption's effects really were—and, we strongly recommend that you visit if you can.)
Mt. St. Helens has also been the most active of the Cascades volcanoes over the most recent centuries. In early 1980, the volcano clearly was “waking up”. Earthquakes shook it almost continuously, including special “harmonic tremors”, similar to those sometimes caused by fluid flow in pipes, which showed that liquid rock was moving up from below. Small eruptions occurred, and hot springs and fumaroles (steam or gas vents) became increasingly active. The north side of the mountain was bulging, blowing up like a balloon as the magma moved into it. Scientists were scrambling to study the volcano and predict its course. They recommended evacuation for safety, and most people (but not all, including some scientists) were moved out of the way. Penn State professor Barry Voight warned that the huge bulge on the north side of the volcano would fail, unleashing a giant landslide and a devastating eruption.
On the morning of May 18, 1980, Professor Voight’s prediction came frighteningly, awesomely true. The bulge failed. A large earthquake either caused, or was caused by, the failure of the north side of the mountain in a giant landslide. Like pulling the cap off a hot, well-shaken soda bottle, the liquid beneath flashed into a froth, driving an eruption 12 miles (20 km) high. A shock wave knocked over full-grown trees in an area of 20 x 10 miles (32 x 16 km). The landslide eventually poured more than 100 million cubic yards of rock material down the Toutle and Cowlitz Rivers, raising the floor of the North Fork of the Toutle as much as 600 feet (200 m), and sweeping roads and houses downstream, with the debris reaching and clogging the shipping channels of the Columbia River. The Toutle floor now sat higher than the smaller streams that fed it, and lakes began to form; only quick work by the Army Corps of Engineers prevented those lakes from overtopping the mud that was damming them, then cutting quickly down through the mud and releasing further floods. (We will revisit the dangers of such mud-dammed lakes in Module 5.)
In total, the Corps of Engineers spent $250 million removing mud dams, clearing shipping channels, and doing other critical work. 57 people were killed in the blast and landslide; some were buried under hundreds of feet of steaming mud and their bodies were never recovered.
President Jimmy Carter scowled at the disaster from a helicopter. Disaster planners pontificated. And in the shadows of the other Cascades volcanoes, people continued building houses in regions of known volcanic hazard.
The Mt. St. Helens Volcanic Memorial today has little in common with conditions pre-1980. The center of the volcano was lowered more than 1/2 mile (nearly 1 km) during the eruption, with the missing rock spread over the surrounding countryside, forming a visible layer as far as 900 miles (1500 km) away. (Professor Alley and his wife Cindy were driving in Alberta, Canada during the summer of 1980, on a great, seven-week, see-the-national-parks-in-a-Chevette-with-a-tent honeymoon when a secondary eruption of Mt. St. Helens put enough ash in the air to halt traffic because of reduced visibility, hundreds of miles from the volcano.) Many of the trees knocked over by the blast still lie there—hundred-foot-long toothpicks pointing in the direction of the searing winds of the blast. Among these dead trees, however, salmonberry, fireweed, and young firs are pushing skyward, elk are grazing, and coyotes are searching for rodents. In some places, salvage-logging of the downed trees was allowed. In some of those places, erosion accelerated, large gullies developed, and the return of vegetation was slowed. In the crater of the volcano, a new lava dome has formed. (Go back and see the slideshow for photos of some of these details.)
Optional Viewing
Below, you'll see two time-lapse images of Mount St. Helens, taken many years after the great eruption and showing how the volcano continued to evolve. Watch them if you're interested. The first one shows it squeezing slowly upwards like toothpaste from a very hot tube, and amazingly, a glacier sits behind it, fed by the great snowfall and the avalanches down into the crater, and partially protected from melting by the debris that falls into it with the avalanches. The second one shows changes in the lava dome and Crater Glacier. The heat of the volcano does melt some of the glacier’s ice from below, but not fast enough to balance the supply of snow from above.
Video: Time-lapse images of Mount St. Helens dome growth 2004-2008 (1:27)
Video: Time series of dome & glacier growth at Mount St. Helens, WA (00:20)
Want to see more?
Visit the Mount St. Helens National Volcanic Monument website. While you are not required to review this, you may find it interesting and possibly even helpful in preparing for the quiz!