May 18, 1980
After remaining dormant since 1857, Mount St. Helens resumed volcanic activity on March 27, 1980. Mount St. Helens resumed substanitial seismic activity just seven days prior when on march 20, 1980 a magnitude 4.0+ earthquake, hit at 3:47pm pacific standard time (PST), the next seven days were filed with seismic activity which climaxed on March 25. The seismic activity began to decrease until on March 27, 1980 at 12:36 p.m. (PST), a loud boom was widely heard. Investigation by aerial observers confirmed the fact that a column of volcanic ash was being released from the mountain. This first eruption formed a 250-foot wide crater, and covered the southeastern slope of the volcano with windblown ash. Mount St. Helens continued throughout March and into mid April, when on April 22, eruptive blasts temproly stopped. A good portion of the seismic activity which occurred before the May 18, 1980 blast was composed of harmonic tremors, which are basically continour rhythmic shaking of the ground. Harmonic tremors often indicated the movement of magma or volcanic gasses below the soil surface. This movement of magma caused a huge bulge to form on the north side of the mountain. This bulge grew rapidly at a rate of five feet per day at times, and by may 12, 1980 parts of the north flank of the mountain were 450 feet higher than they were before Mount St. Helens resumed volcanic activity.
It was the collapse of this bulge on May 18,1980 which released all of Mount St. Helens' fury. At approximately 8:32 a.m. PDT a magnitude 5.1 earthquake, caused the bulged side of the volcano to give way. Triggering the largest landslide in the recorded history of man a massive 2.3-km cubed of rock rushed down the side of the mountain at speeds of 155 to 180 Mile per Hour. This debris avalanche flowed upward into the North Fork of the toutle river and fromed hummock deposits, it also flowed into spitit lake raising the lake bottem 295 feet, completely altering the once pristine, mountain lake. This debris avalanche rose the valley floor 150 feet in places.
At the same thime the avalanche was occuring on Mt. St. Helens, the released steam and volcanic gasses were surging from the mountain these gasses reached speeds as high as 670 mile per hour, this surge of hot gasses blew down trees and killed large animals. The dammage from this latteral blast reached as for as 19 miles and created a 230-square mile area of devistation. The dammage done by the lateral balst has been divided into three zones. The first destruction zone is the direct blast zone. In this zone everything was swept away and trees were snapped like toothpicks. The next zone is the channelized blast zone in this zone everything was flattened but not swept away, the blast was often channeled by different hills and topographic features. On the outter edge of the channelized blast zone lies the seared zone, in this area trees were left standing but were killed from the heat of the gasses which escaeped Mt. St. Helens.
Along with toxic gasses Mt. St. Helens emmited a large quantity of ash within ten minutes of the collapse of the nothe side of the volcano the ash column was over 12 miles high, this ash was swept in a north east direction by prevaling winds and the Jet stream. The ash soon reeached central Washington and continued traveling, east. Fine ash particles were detected in the northeast United States and eventually some of the ash made the trip completely around the world. It is estimated that during the nune hour eruptive period Mt. St. Helens Released approximatly 540 million tons of ash, this ash blanked Eastern Washington fruit orchards and farmlands adding important nutirents back into the soil.
The May 18, 1980 eruption also released Pyroclastic flows which were formed by the fallback and downslope movement of fragments and the frothing over of volcanic debris at the magma vent. Volcanic froth formed int his maner is called pumice. These pyroclastic flows continued throughout the day, as Mt. St. Helens produced new volcanic material. By the end of the eruption 17 separate pyroclastic flows had occurred, forming a large fan shapped aea, called the pumice plain. All of the surface water which once existed, in the pumice plain was flashed into steam, this flash heating cause explosions which sent debris over a mile in the air, these reuidal explosions lasted for several weeks after the initial eruption.
Further down steram the debris avalanche mixed with massive amounts of water, this ensuing surge of volcanic deposit and water ripped throught the Toutle River Drainage pulling up trees and choking out plants, even after flowing many miles from the eruption the mudflows maintained a temprature between 84 & 91 degrees fereinheight. These mudflows brought the level of the Toutle River 20 feet abouve normal, these mudflows left debris clinginf to eveything they passed, these mudlines aferaged from 33 to 66 feet deep. Although the mudflows had tremendous height, the average depth of the deposit in the Toutle River only averaged three feet deep. These mudflows were one of the most destructive aspects of the Mt. St. Helens eruption since they were able to reach into populated areas and decreased the Columbia river navigational channel forn 39 feet to a mere 13 feet, marroning large cargo ships, anc choking off all river trafic.
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