Mount St. Helens is located in Washington State approximately 96 miles south of Seattle Washington and 53 miles northeast of Portland Oregon. It is situated in the Cascade Mountain Range which is part of the Pacific Ring of Fire that contains over 150 active volcanoes on both sides of the Pacific Ocean.
Prior to the eruption, Mount St. Helens was the fifth highest peak in Washington State. Its symmetrical cone was highly visible and it resembled the snow-covered Mount Fuji in Japan. This majestic cone is now a crater.
Events Leading to the Eruption of Mount St. Helens
In March 1980, the University of Washington had completed a project of installing seismometers at various potential volcanoes in the Cascade Mountain Range. There had been no hint of activity at Mount St. Helens but that was soon to change.
On March 20th, 1980, an earthquake with the magnitude of 4.2 was recorded beneath Mount St. Helens. Earthquake activity accelerated with tremors occurring approximately 15 times per hour. On March 25th, heavier 4.0 earthquakes returned and were hitting Mount St. Helens around three times every hour. The epicenter of all these quakes was directly below the volcano.
When scientists flew over Mount St. Helens, they observed several changes to its surface. There were new fractures noted on the glaciers and evidence of rockslides and avalanches.
On March 27th, 1980, an explosion was heard at Mount St. Helens and a news team took an airplane tour to investigate. A new crater was discovered and there was a light covering of ash on the peak of the mountain. These were eruptions of steam were due to the ground water being heated by the magma inside the volcano.
During March, the steam eruptions were occurring approximately once an hour and then reduced to about one per day in April. The new crater continued to grow wider. As the eruptions started to slow down, the earthquake activity escalated but the tremors were starting to occur under a bulge that was building on the north flank of the mountain.
The bulge was quickly growing every day and according to the U.S. Geological Survey, “by May 17, more than 10,000 earthquakes had shaken the volcano and the north flank had grown outward at least 450 feet to form a noticeable bulge. Such dramatic deformation of the volcano was strong evidence that molten rock (magma) had risen high into the volcano”.
The Eruption of Mount St. Helens
On May 18th, 1980 an earthquake with a magnitude of 5.1 occurred. Within 15 to 20 seconds, the large bulge along with the summit of Mount St. Helens slid down the mountain. According to the U.S. Geological Survey, “...the volcano's bulge and summit slid away in a huge landslide - the largest on Earth in recorded history.” The landslide traveled 14 miles west along the North Fork Toutle River.
The landslide allowed the magma to be released and it triggered multiple powerful lateral explosions that blasted steam, volcanic gas, rocks and ash outward and upward to the north. This blast traveled at speeds up to 300 mph in the immediate area of the volcano. The blast cloud traveled at least 17 miles northward.
Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano. The ash plume rose as high as 12 to 16 miles above sea level and moved eastward at an approximate speed of 60 mph.
Gas and hot rocks were also blasted down the other three sides of Mount St. Helens. It quickly melted snow and ice on the volcano, resulting in water surges that mixed with soil, loose rock and debris. These formed lahars (volcanic mudflows) that destroyed river valleys, roads, houses on their downhill run.
Damage Resulting from the Eruption of Mount St. Helens
There were 57 people killed and many of their bodies have never been found, especially near the volcano where the debris was approximately 150 feet deep. Over 200 homes were destroyed along with 15 miles of railway and 185 miles of highway.
Over 7,000 bear, deer and elk were killed plus at least 12 million fish from a local hatchery.
Approximately 60,000 acres of trees were destroyed. Some of the trees further away from the mountain were blown over but were recovered and milled by logging companies.
The estimated loss was approximately $1 billion U.S. dollars.
Volcanologist David A. Johnston from the U.S. Geological Survey was stationed on Coldwater Ridge and reported the eruption as it happened but died from the hot ash cloud. In his honor, Coldwater Ridge was renamed Johnston Ridge.
On August 26, 1982, Congress declared the area around Mount St. Helens as the Mt. St. Helens National Volcanic Monument. This area of approximately 110,000 acres is being preserved for recovery and scientific study. The public are able to stop at several Visitor Centers in the area.
Sources:
Mount St. Helens: From the 1980 Eruption to 2000, U.S. Geological Survey
Mt. St. Helens Eruption (1980), San Diego State University
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