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The May 18, 1980 eruption of Mount Saint Helens was the most destructive in the history
of the United States. Mt. Saint Helens is located in southwest Washington in the Cascade
Range, a mountain range dominated by periodically active volcanic peaks. In addition
to photos taken on May 18, 1980, this slide set includes images of pre-eruption activity
and post-eruption effects such as the blast area, mud flows, ash fall, and altered
terrain. The Juan de Fuca Plate is colliding with the North American Plate in the
Cascadia Subduction Zone off the coast of Oregon and Washington. Geologists and volcanologists
had been studying Mount Saint Helens in the 1970s and had predicted an eruption before
the end of the century. The mountain had been recognized as a dormant volcano that
had erupted intermittently from 1831 to 1857. The Klickitat Indians of the Pacific
Northwest called Mount Saint Helens "Louwala-Clough" or Smoking Mountain. However,
the beautiful snow-covered peak looked serene to the casual observer just days before
the start of eruptive activity. On March 20, 1980, seismographs in Washington and
Oregon began recording earthquakes and a rhythmic ground shaking known as harmonic
tremor (anindicator of the subsurface motion of gases or magma). A series of phreatic
(steam) eruptions began on March 27. Shortly after the start of eruptive activity
geologists noticed another sinister indicator of impending disaster. The north flank
of the mountain had begun to swell. Aerial photos indicated that this bulge increased
the height of some areas of the north side of the mountain by more than 450 feet.
On May 18, the bulge collapsed, triggering the eruption. The sequence of events on
May 18 began with a magnitude 5.1 earthquake. The shaking caused a massive landslide
on the unstable north side-one of the largest landslides in historic times. Removal
of this overburden resulted in the release of trapped gases that combined with steam
from water, melted ice, and snow. This massive uncorking of the volcano produced a
lateral blast that downed trees up to 19 miles from the volcano. The blast hurled
nearly a cubic mile of Mount Saint Helens across the landscape and into the atmosphere.
The mountain lost about one-seventh of its original height, dropping from 9,700 feet
to 8,400 feet. An ash cloud of new magmatic material and steam reached an altitude
of more than 16 miles in ten minutes. The cloud, driven by the prevailing winds, traveled
in an east-northeasterly direction and deposited ash in eleven states and reached
as far east as Wisconsin. Pyroclastic flows (gravity flows of volcanic material that
has been explosively ejected from a volcanic event) periodically poured out of thecrater
and down the north flank into the valley below. These flows, along wth hot ash and
debris, began melting the snow and ice that remained onthe flanks of the mountain.
The debris-filled water, together with ground water and rain water, raised Spirit
Lake 200 feet and created mudflows which moved down the Toutle River valley and the
Pine Creek drainage. The eruption resulted in scores of injuries and the loss of 60
lives. The lateral blast, debris avalanche, mudflows, and flooding caused extensive
damage. All buildings and related man-made structures in the vicinity of Spirit Lake
were damaged or buried. Two hundred houses were destroyed and many more were damaged
in Skamania and Cowlitz Counties, leaving many homeless. Many tens of thousands of
acres of forest land as well as recreational sites, trails, and four billion board
feet of salable timber were destroyed or damaged. More than 185 miles of highways
and roads and 15 miles of railways were destroyed or extensively damaged. Wildlife
suffered heavily in the area. Many agricultural crops were destroyed downwind of the
volcano. State and Federal agencies estimated that over 2.4million cubic yards of
ash, equivalent to 900,000 tons in weight, were removed from highways and airports
at a cost of $2.2 million. The total cost of the destruction and damage caused by
Mount Saint Helens was about $1 billion.
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