Mount St. Helens  (MSH)
Visitors Resource Packet
Compiled/Written by Lloyd & Doris Anderson
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Eruption of Mount St. Helens

See Also:  2a, 2c  and all of Section 3, but especially 3a, 3b and 3e.  (Much of the eruption story is told again and again in explaining what one is seeing in a visit to the mountain.)

Sources:   Many, especially Volcanic Eruptions of 1980 at Mount St. Helens; The First 100 Days.  Bruce L. Foxworthy & Mary Hill; Geological Survey Professional Paper 1249; USGPO, Washington, 1982; 125pp.  Quotes indicated “-VE”.  Also, see Sources, 3g.

Overview.  While not exceptional in recorded history, the volcanic eruption of  MSH in southwestern Washington in 1980 was the most violent of the century in the lower 48 states; the most  powerful of the approximately 50 volcanic eruptions worldwide that year; and, possibly, the most powerful of the preceding decade.  On May 18th, at 8:32 a.m., a strong earthquake and immense avalanche from the volcano’s north side was followed immediately by a major explosive eruption directed first northward and then upward.  The lateral blast, which lasted only the first few minutes of a nine-hour continuous eruption, spewed hot gas and rock particles from the volcano at hurricane speeds, its devastation reaching nearly 16 miles outward from the volcano’s center.  The mountain blew away 6.6 billion tons of material, more than a ton of MSH for every person on the earth, including 170 million cubic yards of glacial snow and ice, with the force of 20,000 Hiroshima-class atom bombs.  The story of the eruption involves:

·         the avalanche and its resulting deposits

·         the lateral blast which destroyed 230 square miles of forest and recreation areas

·         the loss of human, animal and plant life

·         its unusual impact on Spirit Lake

·         intensely hot pyroclastic flows

·         mudflows that clogged six river systems and blocked the shipping channel of the Columbia River

·         an ash plume that arose nearly 15 miles into the sky and rained ash around the world.

Most Studied Eruption.  The eruption did not occur in a remote, inaccessible third-world country.  It occurred within one-hundred miles of  four million people in the most technically advanced country in the world.  Dozens   were poised with cameras, radios and in airplanes to capture whatever would happen whenever it would happen.  Mount St. Helens obliged.

     While she is hidden about 85% of the time, MSH chose a sunny spring Sunday morning so all the world could witness her work and so human and biological casualties would be greatly reduced.  The great interest in the eruption was summarized as “the most intensively observed, photographed, documented, and reported series of geologic events in history.”   -VE, p2.  But this vast amount of information was of differing qualities and some was even erroneous as VE explains:  “The broad coverage...resulted in many different accounts....  Unfortunately, these accounts do not all agree or reflect the facts as they are now known.”  The US Geological Survey (USGS), the Federal agency responsible for geologic and hydrologic investigations and geologic hazard warnings, had the responsibility of observing, interpreting and documenting the volcanic activity and reporting it to the nation.  The USGS summarized its report thus:  “This report provides a scientifically sound, general description of the events and their effects, answering, What happened? and What does it all mean?  This paper also provides an accurate summary of events.”

Awakening. On March 20, 1980 MSH awakened after a 123 year slumber with a sharp earthquake.  Earthquakes occurred by the hundreds over the next week.  Large cracks began to appear in the ice and snow at the top (MSH had 11 glaciers).  On March 27th the volcano blasted a crater in its peak and shot a black plume of ash 7000’ into the sky.  Over the next two weeks daily eruptions of ash and steam enlarged the crater to 2000’x1000’ and 500’ deep and plumes rose to 20,000’.  Sunday, March 30th, was clear.  As many as 70 airplanes were flying around the volcano at the same time.  One official said the highway to the mountain looked like Seattle at rush hour.  As April wore on the eruptions decreased but the earthquakes continued at the daily rate of about 40 over 3.0.  They were a signal that magma was forcing its way up from miles below the mountain.  Each earthquake reported rock structures being broken by moving magma.  It was following paths of upward movement well established by previous periods of activity.

Rising Magma/The Bulge.  At times there would be a continuous shaking for up to half an hour.  This is called a harmonic tremor and indicates a flowing of magma up from below.  The epicenters of the earthquakes were pinpointed.  Most of them were shallow and under the mountain whose base is four miles across.  But there was an area without earthquakes.  It was cylindrical in shape, one to five miles below the mountain and one mile in diameter.  This was thought to be a magma chamber.  If it were that large it would have a volume equal to the top 3000’ of the mountain--enough magma to cover all of Southwest Washington with one foot of lava!  On the north side geologists began to notice a  bulging.  They didn’t know what to make of it.  It would continue until an area half a  mile wide and a mile up and down the slope of the north face had bulged 450’ to the north.  It was caused by magma pooling inside the mountain and wedging it apart seriously weakening the north face.

The Earthquake and Avalanche.  How seriously became apparent at 8:32 of May 18th when a sharp earthquake (5.1 magnitude) shook the north slope.  Geologists Keith and Dorothy Stoffel were passing over the area in a small plane at that exact moment and reported they saw the north face undulating like the waves of the ocean.  Then half a cubic mile of material plunged into the valleys below--the largest avalanche in recorded history.  One quarter of the material plowed into the Spirit Lake basin actually raising the lake bottom by 300’.  The material acted like a giant hand sloshing the water 850’ up the opposite ridge.  The lake that had been 180’ deep became 120’ deep but almost twice as broad.  Picking up speed as it plunged down the volcano, more of the material gathered so much force that it raced across the Toutle River valley, climbed 1100’ of ridge to the north and flew across the next valley, pounding its north side to bedrock and filling the steep ravine to valley profile.  But Johnston Ridge redirected most of the material down the Toutle River valley where it finally stopped 14 miles from where it started.  The debris avalanche had produced 600’ deep deposits at the SW corner of Spirit Lake and had buried the old highway, river and 23 square mile valley with an average of 150’ of material.

The Blast.  The avalanche was like removing the lid from a pressure cooker.  All the pressure it had been holding in let loose in a lateral blast to the north.  This took the professionals by surprise but they would later identify the remains of nearly 300 such explosions from the formations of other volcanoes.  This blast was phreatic (see 2A), the result of super heated water trapped within the mountain.  The blast began about 20 seconds after the avalanche began.  But the avalanche moved at the speed of something falling while the blast moved at the speed of something exploding.  The blast quickly swept ahead of the avalanche with great quantities of rock and debris and mowed down the forest in an arc greater than 120 degrees.  This included the thick stand on the north slopes of Spirit Lake.  When that enormous wave swept up the slope a few minutes later, the trees were already felled.  As the water ran back down the slope, it washed a million logs down the slopes and into the lake.  The avalanche also dammed the mouth of the lake.  This worried the scientists because the dam was made up of loose material that could give way and send a wall of water down the Toutle Valley that would greatly damage cities.  Engineers later brought in pumps to lower the lake and finally dug a tunnel to maintain the lake at the lower level.  The blast killed campers 14 miles from the mountain, knocked down 150 square miles of forest and cooked to death another 84 square miles of forest.  As more material was thrown from the north side of the mountain the blast began to redirect into the sky.  But this first stage that destroyed all life on the ground had lasted about eight minutes and expended the energy of 1000 Hiroshima-sized atom bombs.

Continuing Eruption.  The eruption continued for nine hours hollowing out a horseshoe-shaped crater during the morning hours and blasting magma into the sky during the afternoon hours.  The magma poured out of a hole in the crater 75' across.  Studies determined it to contain 10% water by volume.  This super-hot water extended the volcano's explosive activity until the pressure from below was released.  Scientists say what happened that day was the worst case scenario.  The ferocious blast, suffocating ash, searing heat and falling trees had killed 57 people, hundreds of black bears, thousands of deer and elk and hundreds of thousands of birds, fish and other animals.  It had also reduced the mountain’s height from 9677’ to 8364’.  But they forgot that it could have happened Monday morning and killed a thousand workers or that a new surge of pressure could have occurred during the eruption and squeezed more magma up from below.  A previous eruption was 13 times more powerful than this eruption.  The most destructive part of the 1980 eruption was the mudflows that rushed down six river systems, lifting bridges off their foundations, destroying roads, railroad tracks and several hundred homes.

Continuing Activity.  Cleanup of ash also caused great expense.  In all there were six explosive eruptions in 1980.  In 1982 the mountain began to build a dome in the crater.  This continued off and on through 1986.  The dome now stands over 1000’ high while the surrounding rim towers 2200’ above the crater floor.  Unusual earthquake activity in 1998 leaves the question of whether this eruptive cycle is over or not.  The cycle in the 1800’s lasted a quarter of a century.

Lloyd Anderson, 9/27/00

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