Mount St. Helens Eruptive Activity, 1980-1984
Lateral Blast
The sudden removal of the volcano's north flank released pressure on
the hydrothermal and magmatic system within the
volcano, triggering a devastating lateral blast to the
north. The abrupt pressure release, or "uncorking," of the volcano by
the avalanche can be compared in some ways to the removal of the cap
from a vigorously shaken bottle of soda pop, or to punching a hole in a
boiler tank under high pressure.
The northward-directed lateral blast of rock, ash, and hot gas
devastated an area of about 150 square miles. The blast stripped trees
from most hill slopes within six miles north of the volcano and leveled
nearly all vegetation for as far as 13 miles in a 180-degree arc north of
the mountain. The blast deposited blocks and smaller rock fragments and
organic debris over the devastated area in layers to more than three
feet in thickness. Surrounding this zone of toppled vegetation is a
narrow band of scorched but standing timber in which sandy
deposits are
as thick as four inches; this zone has an area of about 25 square miles.
Plinian Column
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Eruptive Activity
Living Lab Curriculum
Mount St. Helens