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See Also: 2a, 2c
and all of Section 3, but especially 3a, 3b and 3e. (Much of the eruption story is told again
and again in explaining what one is seeing in a visit to the mountain.)
Sources: Many, especially Volcanic Eruptions of 1980 at Mount St. Helens; The First 100 Days. Bruce L. Foxworthy & Mary Hill;
Geological Survey Professional Paper 1249; USGPO, Washington, 1982; 125pp. Quotes indicated “-VE”. Also, see Sources, 3g.
Overview. While not exceptional in recorded history,
the volcanic eruption of MSH in
southwestern Washington in 1980 was the most violent of the century in the
lower 48 states; the most powerful of
the approximately 50 volcanic eruptions worldwide that year; and, possibly, the
most powerful of the preceding decade.
On May 18th, at 8:32 a.m., a strong earthquake and immense avalanche
from the volcano’s north side was followed immediately by a major explosive
eruption directed first northward and then upward. The lateral blast, which lasted only the first few minutes of a
nine-hour continuous eruption, spewed hot gas and rock particles from the
volcano at hurricane speeds, its devastation reaching nearly 16 miles outward
from the volcano’s center. The mountain
blew away 6.6 billion tons of material, more than a ton of MSH for every person
on the earth, including 170 million cubic yards of glacial snow and ice, with
the force of 20,000 Hiroshima-class atom bombs. The story of the eruption involves:
·
the avalanche and its resulting deposits
·
the lateral blast which destroyed 230 square
miles of forest and recreation areas
·
the loss of human, animal and plant life
·
its unusual impact on Spirit Lake
·
intensely hot pyroclastic flows
·
mudflows that clogged six river systems and
blocked the shipping channel of the Columbia River
·
an ash plume that arose nearly 15 miles into the
sky and rained ash around the world.
Most Studied Eruption. The eruption did not occur in a remote,
inaccessible third-world country. It
occurred within one-hundred miles of
four million people in the most technically advanced country in the
world. Dozens were poised with cameras, radios and in airplanes to capture
whatever would happen whenever it would happen. Mount St. Helens obliged.
While she is hidden about 85% of the time, MSH chose a sunny spring
Sunday morning so all the world could witness her work and so human and
biological casualties would be greatly reduced. The great interest in the eruption was summarized as “the most
intensively observed, photographed, documented, and reported series of geologic
events in history.” -VE, p2. But this vast amount of information was of
differing qualities and some was even erroneous as VE explains: “The broad coverage...resulted in many
different accounts.... Unfortunately,
these accounts do not all agree or reflect the facts as they are now
known.” The US Geological Survey
(USGS), the Federal agency responsible for geologic and hydrologic
investigations and geologic hazard warnings, had the responsibility of
observing, interpreting and documenting the volcanic activity and reporting it
to the nation. The USGS summarized its
report thus: “This report provides a
scientifically sound, general description of the events and their effects,
answering, What happened? and What does it all mean? This paper also provides an accurate summary of events.”
Awakening. On March 20, 1980 MSH
awakened after a 123 year slumber with a sharp earthquake. Earthquakes occurred by the hundreds over
the next week. Large cracks began to
appear in the ice and snow at the top (MSH had 11 glaciers). On March 27th the volcano blasted a crater
in its peak and shot a black plume of ash 7000’ into the sky. Over the next two weeks daily eruptions of
ash and steam enlarged the crater to 2000’x1000’ and 500’ deep and plumes rose
to 20,000’. Sunday, March 30th, was
clear. As many as 70 airplanes were
flying around the volcano at the same time.
One official said the highway to the mountain looked like Seattle at
rush hour. As April wore on the eruptions
decreased but the earthquakes continued at the daily rate of about 40 over
3.0. They were a signal that magma was
forcing its way up from miles below the mountain. Each earthquake reported rock structures being broken by moving
magma. It was following paths of upward
movement well established by previous periods of activity.
Rising Magma/The Bulge. At times there would be a continuous shaking
for up to half an hour. This is called
a harmonic tremor and indicates a flowing of magma up from below. The epicenters of the earthquakes were
pinpointed. Most of them were shallow
and under the mountain whose base is four miles across. But there was an area without earthquakes. It was cylindrical in shape, one to five
miles below the mountain and one mile in diameter. This was thought to be a magma chamber. If it were that large it would have a volume equal to the top
3000’ of the mountain--enough magma to cover all of Southwest Washington with
one foot of lava! On the north side
geologists began to notice a
bulging. They didn’t know what
to make of it. It would continue until
an area half a mile wide and a mile up
and down the slope of the north face had bulged 450’ to the north. It was caused by magma pooling inside the
mountain and wedging it apart seriously weakening the north face.
The Earthquake and Avalanche. How seriously became apparent at 8:32 of May
18th when a sharp earthquake (5.1 magnitude) shook the north slope. Geologists Keith and Dorothy Stoffel were
passing over the area in a small plane at that exact moment and reported they
saw the north face undulating like the waves of the ocean. Then half a cubic mile of material plunged
into the valleys below--the largest avalanche in recorded history. One quarter of the material plowed into the
Spirit Lake basin actually raising the lake bottom by 300’. The material acted like a giant hand
sloshing the water 850’ up the opposite ridge.
The lake that had been 180’ deep became 120’ deep but almost twice as
broad. Picking up speed as it plunged
down the volcano, more of the material gathered so much force that it raced
across the Toutle River valley, climbed 1100’ of ridge to the north and flew
across the next valley, pounding its north side to bedrock and filling the
steep ravine to valley profile. But
Johnston Ridge redirected most of the material down the Toutle River valley
where it finally stopped 14 miles from where it started. The debris avalanche had produced 600’ deep
deposits at the SW corner of Spirit Lake and had buried the old highway, river
and 23 square mile valley with an average of 150’ of material.
The Blast. The avalanche was like removing the lid from
a pressure cooker. All the pressure it
had been holding in let loose in a lateral blast to the north. This took the professionals by surprise but
they would later identify the remains of nearly 300 such explosions from the
formations of other volcanoes. This
blast was phreatic (see 2A), the result of super heated water trapped within
the mountain. The blast began about 20
seconds after the avalanche began. But
the avalanche moved at the speed of something falling while the blast moved at
the speed of something exploding. The
blast quickly swept ahead of the avalanche with great quantities of rock and
debris and mowed down the forest in an arc greater than 120 degrees. This included the thick stand on the north
slopes of Spirit Lake. When that
enormous wave swept up the slope a few minutes later, the trees were already
felled. As the water ran back down the
slope, it washed a million logs down the slopes and into the lake. The avalanche also dammed the mouth of the
lake. This worried the scientists
because the dam was made up of loose material that could give way and send a
wall of water down the Toutle Valley that would greatly damage cities. Engineers later brought in pumps to lower
the lake and finally dug a tunnel to maintain the lake at the lower level. The blast killed campers 14 miles from the
mountain, knocked down 150 square miles of forest and cooked to death another
84 square miles of forest. As more
material was thrown from the north side of the mountain the blast began to
redirect into the sky. But this first
stage that destroyed all life on the ground had lasted about eight minutes and
expended the energy of 1000 Hiroshima-sized atom bombs.
Continuing Eruption. The eruption continued for nine hours
hollowing out a horseshoe-shaped crater during the morning hours and blasting
magma into the sky during the afternoon hours.
The magma poured out of a hole in the crater 75' across. Studies determined it to contain 10% water
by volume. This super-hot water
extended the volcano's explosive activity until the pressure from below was
released. Scientists say what happened
that day was the worst case scenario.
The ferocious blast, suffocating ash, searing heat and falling trees had
killed 57 people, hundreds of black bears, thousands of deer and elk and
hundreds of thousands of birds, fish and other animals. It had also reduced the mountain’s height
from 9677’ to 8364’. But they forgot
that it could have happened Monday morning and killed a thousand workers or
that a new surge of pressure could have occurred during the eruption and
squeezed more magma up from below. A
previous eruption was 13 times more powerful than this eruption. The most destructive part of the 1980
eruption was the mudflows that rushed down six river systems, lifting bridges
off their foundations, destroying roads, railroad tracks and several hundred
homes.
Continuing Activity. Cleanup of ash also caused great
expense. In all there were six
explosive eruptions in 1980. In 1982
the mountain began to build a dome in the crater. This continued off and on through 1986. The dome now stands over 1000’ high while the surrounding rim
towers 2200’ above the crater floor.
Unusual earthquake activity in 1998 leaves the question of whether this
eruptive cycle is over or not. The
cycle in the 1800’s lasted a quarter of a century.
Lloyd Anderson,
9/27/00
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