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Thurston County Connection
Thurston County Connection
September, 2014
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Back-to-School Safety 101:

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 The Eruption of Mount Saint Helens!

30th Anniversary of a landmark natural disaster.

Do you remember where you were at 8:30 the morning of May 18th, 1980? For many of us, the mention of that particular date will dredge up the memory of the incredible eruption of Mount Saint Helens. The eruption was the most significant volcanic explosion in the lower forty-eight states since the 1915 eruption of California's Lassen Peak.

Building Up to the Eruption


Prior to the eruption, there was about two months of increasing seismic activity at the mountain and authorities had declared some areas around Mt. St. Helens as “off limits”. The evacuation angered many who lived in the vicinity who felt it was unnecessary. However, scientists had expressed worry about a huge bulge that had grown on the mountain’s north slope and about the increased venting of steam. Teams of scientists moved in to monitor the mountain and devices were placed in strategic areas to measure the growth of the bulge, the gases that were escaping, the earthquakes under the mountain and other data.

May 18, 1980


Within 20 seconds of a magnitude 5.1 earthquake at 8:32 the morning of May 18th, the volcano's bulge and summit slid away in a huge landslide - the largest in recorded history. The landslide depressurized the volcano's magma system, triggering powerful explosions that ripped through the sliding debris. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north. This lateral blast of hot material accelerated to at least 300 miles per hour, then slowed as the rocks and ash fell to the ground and spread away from the volcano; several people escaping the blast on its western edge were able to keep ahead of the advancing cloud by driving 100 miles an hour or faster! The blast cloud traveled as far as 17 miles northward from the volcano and the landslide traveled about 14 miles west, down the North Fork Toutle River.

The lateral blast produced a column of ash and gas that rose more than 15 miles into the atmosphere in just 15 minutes. Less than an hour later, a second eruption column formed as magma erupted explosively from the new crater. Then avalanches of hot ash, pumice, and gas (pyroclastic flows) poured out of the crater at 80 miles per hour and spread as far as 5 miles to the north. 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, 250 miles away.

During the first few minutes of this eruption, parts of the blast cloud surged over the newly formed crater rim and down the west, south, and east sides of the volcano. The hot rocks and gas quickly melted some of the snow and ice capping the mountain, creating surges of water that eroded and mixed with loose rock debris to form volcanic mudflows (lahars). Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying roads and bridges.

Some Statistics from the Eruption


  • Mount St. Helens was reduced by more than 1,300 feet in height
  • Ash fell as far as 930 miles away
  • The debris avalanche and mudflows buried the Toutle valley to a depth of almost 50 meters
  • The explosive eruption lasted for 9 hours
  • 250 square miles of land was damaged


Fifty seven people (including innkeeper Harry R. Truman and geologist David A. Johnston) were killed or are still listed as missing. Hundreds of square miles were reduced to wasteland, causing over a billion U.S. dollars in damage. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but afterward the land passed to the United States Forest Service. The area was later preserved, as it was, in the Mount St. Helens National Volcanic Monument.

In all, Mt. St. Helens released 24 megatons of thermal energy. This is equivalent to 1,600 times the power of the atomic bomb dropped on Hiroshima.

For more information on Mount Saint Helens, please visit the Mount St. Helens National Volcanic Monument Web Site.

Information for this article is from the U. S. Forest Service and the U. S. Geological Survey.

By John Tennis

The eruption in May of 1980. The eruption in May of 1980.