The Eighties in America - Salem Press (2009)

scientific concern. A measurable harmonic tremor,
a signal of underground magma movement, indi-
cated that a large-scale eruption might follow. In ad-
dition, an expanding bulge on the upper northern
slope of the mountain created an avalanche hazard.
Several weeks of relative quiet ensued, however, as
small eruptions and bulge growth continued at a
steady rate.

The Eruption Scientists, officials, and the public
alike were shocked at the immensity of the May 18
cataclysm. Predictions about the nature and size of
eruptions had been based on evidence of previous
volcanic activity in the region, but the powerful lat-
eral blast that caused the mountain to collapse was
without precedent.
At 8:32a.m., an earthquake of 5.1 magnitude on
the Richter scale shook the mountain, collapsing its
northern flank and sending an avalanche surging
into Spirit Lake at the base of the volcano and west-
ward into the upper reaches of the North Fork
Toutle River. The symmetrical peak of the mountain
disintegrated into the largest debris avalanche in
recorded history. The removal of the north slope
allowed volcanic pressures below to explode in a
pyroclastic surge that propelled rock, ash, and gases
outward at speeds in excess of 650 miles per hour.
The blast expanded laterally across the terrain, dev-
astating the forest and its inhabitants in a fan-shaped

area of 230 square miles. Although blast tempera-

tures neared 660 degrees Fahrenheit, the granular

ash is thought to have asphyxiated, rather than

burned, a large number of the victims.

Within ten minutes of the initial earthquake, an

anvil-shaped Plinian column, or ash plume, had

erupted more than twelve miles into the strato-

sphere. Eruptions continued for nine hours, gener-

ating dense clouds of ash that spread at a rate of sixty

miles per hour and produced noticeable amounts of

fallout in eleven states. In regions near the volcano

in eastern Washington, daylight turned to darkness,

and travelers were stranded by poor visibility and

stalled automobile engines. Adding to the chaos

at the feet of the volcano, pyroclastic activity had

melted glaciers, creating lahars, or mudflows resem-

bling liquid concrete, that inundated major drain-

age systems. The flood damaged, and in some cases

destroyed, bridges, homes, and highways, as it made

its way some seventy miles to the Columbia River.

The shipping channel there was reduced by two-

thirds, necessitating a three-month dredging opera-

tion.

Impact Although two hundred people in the imme-

diate area survived the blast, fifty-seven lives were lost.

More than $1 billion in property and economic losses

resulted from the eruption, with erosion-control ex-

penditures alone topping $600 million. Soil erosion

resulting from the eruption

was magnified by the slash-

and-burn techniques that

had been used in the region

for timber salvage. The small

North Fork Toutle River car-

ried sediment downstream

in quantities exceeding the

load normally transported by

the Amazon River. The Army

Corps of Engineers worked

to control flooding by dredg-

ing streambeds and building

a system of dams, as loggers

harvested over 850 million

board feet of lumber from

downed trees. The volcano

itself continued to extrude

magma intermittently, build-

ing a total of three domes in-

side the cauldron between

670
Mount St. Helens eruption The Eighties in America

Cutaway View of Mount St. Helens Scenario

Summit

Ash Clouds

New Magma

Section

Blasted Away

Fractured Rock

Internal bulge

(Old Magma)

Ruptured Side Vent

Magma

Tunnel (Vent)

Landslides