Mount St. Helens Eruptive Activity, 1980-1984
Plinian Column (Vertical
Eruption)
A vertically-directed ash column erupted from the newly formed
horseshoe-shaped crater within minutes of the lateral blast. Within ten
minutes, the ash column reached an altitude of more than 12 miles. Ash
from this eruption cloud was rapidly blown east-northeastward by the
prevailing winds, producing lightning and starting hundreds of small
forest fires, and causing darkness eastward for more than 125 miles.
Ash fell visibly over the Great Plains, and fine ash was detected by
systems used to monitor air pollution in several cities of the
northeastern United States. Some ash drifted around the globe within
about two weeks. The eruption subsided by late afternoon on May 18; by
early May 19 the eruption had stopped.
The air-fall ash deposited during the
nine hours of vigorous eruptive activity amounted to about 540 million
tons distributed over an area of more than 22,000 square miles. The
volume of uncompacted ash is equal to about 0.05 cubic mile of solid
rock, or only about ten percent of the amount of material that slid off
the volcano during the avalanche.
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