The Mt. St. Helens Eruption
Today is the 40th anniversary of the eruption of Mt. St. Helens. Located in southern Washington’s Skamania County, the stratovolcano is 50 miles northeast of Portland and 100 miles south of Seattle. In early 1980 the mountain was 9,677 feet high, making it the fifth-highest peak in the state. Rising 5,000 feet from its base, the symmetrical cone dwarfed the immediate region, earning St. Helens the nickname “Mount Fuji of America.” The beauty lay dormant for more than a century, its last activity tapering to zero in the 1850s.
She started to rouse from the deep slumber in March 1980. Hundreds of earthquakes started to shake the area around the mountain, indicating magma was likely moving underneath. On March 27 a series of eruptions and pyroclastic flows, albeit relatively small in scale, began. By the end of the month, the volcano was erupting hundreds of times per day.
In April, a bulge appeared on the north flank of the mountain. Prior to this, geologists thought St. Helens would likely continue to vent in the traditional way: upward. The bulge hinted at a potential “lateral blast,” which would cause a disaster to the region.
On the morning of May 18, at 8:32 a.m., a 5.1 magnitude earthquake centered just below the north flank occurred. The temblor caused the flank to slide off the mountain. Geologist David Johnston, manning a scientific outpost six miles away, radioed headquarters, saying, “Vancouver! Vancouver! This is it!” Many geologists felt this outpost was easily in the safe zone, but no further radio contact was received. His body was never found. Johnston had been hit by the largest landslide ever recorded, the first stage of the disaster that unfolded on that mid-May morning.
The video above shows the literal destruction of a nearly-10,000-foot mountain. It’s hard for a human brain to grasp that magnitude. The landslide traveled at 110 to 155 miles per hour. Entire forests were flattened. The slide caused a 600-foot wave in Spirit Lake. The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet. Thirteen miles down the North Fork of the Toutle River, 600 feet of debris covered the valley. The effects of the landslide were colossal, but they were just the beginning.
According to the U.S. Geological Survey, the landslide abruptly released the pent-up pressure of the volcanic system, “uncorking” the magma, resulting in a lateral blast. And it was monumental:
“Although the lateral blast began some seconds later than the debris avalanche, the blast’s velocity was much greater, so that it soon overtook the avalanche. Calculations have shown that the blast’s initial velocity of about 220 miles an hour quickly increased to about 670 miles an hour. The average velocity did not surpass the speed of sound in the atmosphere (about 735 miles an hour)…In some areas near the blast front, however, the velocity may have approached, or even exceeded, the supersonic rate for a few moments.”
Click here to listen to a recording of the eruption. From 140 miles away. Simply incredible.
As the avalanche and initial pyroclastic flow were still advancing, a huge ash column grew to a height of 12 miles above the crater in less than 10 minutes. It ejected tephra into the atmosphere for 10 straight hours. The ash clouds near the volcano caused lightning storms, which sparked forest fires on trees that hadn’t been obliterated already. The eruption cloud eventually rose to 80,000 feet and deposited ash as far east as Minnesota and Oklahoma. Parts of Washington were covered with half a foot of soot. The cloud spread across the United States in three days and circled Earth in 15 days.
The hot, exploding material also broke apart and melted nearly all of the mountain’s glaciers along with most of the overlying snow. As in many previous St. Helens’ eruptions, this created huge lahars – volcanic mudflows – that affected three of the four drainage systems on the mountain. An hour and a half after the eruption, mudflows had moved 27 miles. Some spots reported walls of mud 12 feet high. The Columbia River’s channel depth was decreased from 40 feet to just 14 because of the lahars. The Columbia River is 50 miles from Mt. St. Helens.
The devastation was incomprehensible. Officially, 57 people lost lives directly due to the eruption. Damages amounted to $3.4 billion in 2020 dollars. Thermal energy released during the eruption was equal to 26 megatons of TNT. That’s 1,600 times more powerful than the atomic bomb dropped on Hiroshima.
The mountain, in the morning, stood at 9,677 feet. By noon its height was 8,363 feet. Over 1,300 feet of the massive mountain was gone. 40 years later, log jams still float in the surrounding lakes, an eerie reminder of the power of our planet.
Further Reading and Exploration
Eruptions of Mount St. Helens: Past, Present, and Future, U.S. Geological Survey Special Interest Publication – Robert I. Tilling, Lyn Topinka, and Donald A. Swanson
Geology of Mount St. Helens National Volcanic Monument – USGS
Mount St. Helens – PBS NOVA documentary