geosc 10: unit 3: plate tectonics ii mt. st. helens--a volcano blows its top the great eruption of...

GEOSC 10: Unit 3: Plate Tectonics II

Mt. St. Helens--A Volcano Blows its Top

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.

Photos from USGS, US Forest Service, and Richard & Cindy Alley. We borrowed many captions from the USGS, too, but with changes for you.


Before 1980, Mt. St. Helens was the queen of the Cascades, snow-capped, symmetric, and beautiful. But, geologists knew that the peak had also been the most active of the Cascades over many centuries.


In winter and spring of 1980, magma moved upwards beneath the volcano. (Magma is melted rock underground.) Small eruptions started, covering much of the snow with volcanic ash, as seen here. Mt. Adams, another Cascades peak, is visible behind and just to the left of Mt. St. Helens. This USGS picture was taken by Donald A. Swanson on April 10, 1980. No need to memorize numbers or dates here, but appreciate the power.


A "bulge" developed on the north side of Mount St. Helens as magma pushed up within the peak. Repeated measurement of the distance to the bulge (as shown here) found growth of up to five feet (1.5 meters) per day. By May 17, part of the volcano's north side had been pushed upwards and outwards over 450 feet (135 meters). You might not be surprised to learn that such a huge blister will tend to rupture and fall down. The view is from the northeast in this April 27, 1980 photo by Peter Lipman.


On May 18, 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook the peak, and the bulge and surrounding area slid away. The pressure release triggered a major eruption, as the magma bubbled and blew apart to create ash carried aloft by poison gas. The landslide from the bulge buried 24 square miles (62 square kilometers) of valley. Before going up as shown these photos, the blast first went sideways, gravely damaging 250 square miles (650 square kilometers). Fifty-seven people were killed or are still missing. USGS photos by Austin Post (left) and Robert Krimmel (right).


For more than nine hours a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20-25 kilometers) above sea level. The plume was blown eastward at an average speed of 60 miles per hour (95 kilometers/hour), with ash reaching Idaho by noon. By early May 19, the devastating eruption was over. This May 18 USGS photo of the ash plume is by Donald A. Swanson.


Map of the area affected by the May 18, 1980 eruption. North is to the top, and the little scale bar at the bottom is five miles--this was a big deal. The blast knocked over trees across most of the yellow area. Much of the bulge ended up in the black-lined debris-avalanche deposits, while melted snow and ice plus water pushed out of lakes

made the brown mudflow deposits. Thick layers of hot ash fell to make the red pyroclastic flow deposits.


After the May 18, 1980 eruption, Mount St. Helens' elevation was over 1300 feet lower than before, and the volcano had a one-mile-wide, horseshoe-shaped crater. This photo by Tom Casadevall of the USGS was taken four months after the eruption, on September 16, 1980. The yellow arrow points to a helicopter.


For weeks, volcanic ash covered the landscape around the volcano and for several hundred miles downwind to the east. Noticeable ash fell in eleven states. The total volume of ash (before its compaction by rainfall) was approximately 0.26 cubic mile (1.01 cubic kilometers), or, enough ash to cover a football field to a depth of 150 miles (240 kilometers). In this USGS photograph by Lyn Topinka from August 22, 1980, three months after the eruption, a helicopter stirs up ash while trying to land in the devastated area.


Mount St. Helens on May 17, 1980, one day before the devastating eruption. The view is from Johnston's Ridge, six miles (10 kilometers) northwest of the volcano. USGS photo by Harry Glicken. The next photo is from nearly the same place, four months later. Try looking at it, then flipping back to this one.


Mount St. Helens soon after the May 18, 1980 eruption, as viewed from a similar location as the previous photo, at Johnston's Ridge. USGS photo by Harry Glicken on Sept. 10, 1980.


BEFORE

AND AFTER

Another before-and-after pair. The blast lowered the peak by more than 1300 feet. These USGS (bottom) and US Forest Service (top) pictures are not exactly from the same place, but they’re close.


Difficult as it may be to believe, these photos are from the same place, looking in the same direction! The trees on the left were obscuring a view of the beautiful snow-capped peak, which has been blown away, along with the trees, in the photo on the right. US Forest Service photos: http://www.fs.fed.us/gpnf/mshnvm/digital-gallery/25yearsofrecoverybeforeandafter.htm


These were towering coniferous trees before the eruption, but now are knocked over like matchsticks. Trees in the upper left weren’t knocked over, but were burned to death. USGS photo on Aug. 22, 1980 by Lyn Topinka.


The eruption melted glaciers, and the landslide pushed water out of Spirit Lake, together making a huge flood that destroyed many things including this logging camp on the South Fork Toutle River. Those yellow things in the upper-right are full-sized logging trucks destroyed by the flood (the arrow points at one). USGS photo taken on May 19, 1980, by Phil Carpenter.


Reid Blackburn's car, located approximately 10 miles from Mount St. Helens. Reid was a photographer for National Geographic as well as the Vancouver Columbian newspaper. Unfortunately, Reid did not make it out alive. A scholarship is now given by The National Press Photographers Association in Reid Blackburn’s honor. USGS Photograph taken on May 31, 1980, by Dan Dzurisin.


As noted for previous slides, the Mt. St. Helens eruption melted the glaciers on the peak, and also dumped immense amounts of rock material into Spirit Lake. The water displaced from the lake, and melted from the glaciers, mixed with volcanic ash and mud to make a giant mudflow that raced down the Toutle and Cowlitz Rivers, causing great destruction. This image and the next two, by Dr. Alley and his wife Cindy, show the situation miles downstream.


Another view of the damage caused by the immense mudflow that raced down the Toutle and Cowlitz Rivers. The broken trees attest to the power of the mud-filled flood.


Another view of the mudflow. In this one, a much younger Dr. Alley (this was 1980, remember) is visible in the lower-left of the picture, dwarfed by the high-mud mark on the tree. The tourists on the right are looking at the foundation of what had been a house.


In this picture taken about 10 years after the eruption, trees knocked over by the blast are scattered across the landscape. Some bark remains on these; they were slightly shielded by a hill and were not as strongly blasted as some.


In this image taken about 10 years after the eruption, members of a geological field trip attended by Dr. Alley take a walk through some standing former trees that were greatly shielded from the blast, which came over the hill from diagonally behind the camera position. Notice the green vegetation flourishing on the far hillside; salmonberry, fireweed and others moved back in quickly.


This third picture of Mt. St. Helens a decade after the blast shows deep gullying in a region without dead trees. Many dead trees were salvage-logged because of the value of the lumber, but such actions do have consequences for the landscape--without dead trees to dam the flow, rains feed little streams that rapidly wash away the loose soil, making it harder for new plants to grow.


Mt. St. Helens was a media star. T-shirts, coffee mugs, and vials of ash were among souvenirs sold to enthusiastic tourists. These ladies realized that people standing ankle-deep in Mt. St. Helens ash were spending $5 for a little vial of Mt. St. Helens ash, and so the ladies decided to gather some of their own. The colorful garb is possibly reflective of the era.


Can't Get Enough? For those interested in learning more about the Mt. St. Helens story, here are some additional links to explore.

The USGS fact sheet on the Mount St Helens event - pretty amazing stuff: http://vulcan.wr.usgs.gov/Volcanoes/MSH/May18/summary_may18_eruption.html

Summary text and pictures of the buildup to the eruption: http://vulcan.wr.usgs.gov/Volcanoes/MSH/May18/MSHThisWeek/framework.ht

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The Mount St. Helens National Volcanic Monument, which is a US Forest Service "national park"; http://www.fs.fed.us/gpnf/mshnvm/

Another U.S Forest Service site, with a fascinating view of what happened; http://www.fs.fed.us/gpnf/mshnvm/digital-gallery/25yearsofrecoverybeforeandaf

ter.htm

geosc 10: unit 3: plate tectonics ii mt. st. helens--a volcano blows its top the great eruption of...

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