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Case study: Mt St Helens 1980
Mt St Helens is located in the North West of the USA in Washington State. It is part of the Cascade
Range of mountains. Mount St Helens erupted on the 18th May 1980; it was the worst eruption to
ever hit the USA.
The USA is an MEDC with a GDP per capita of over $25,000 in 1980. The area around the volcano is a
National Park which is mainly used for tourism, forestry, salmon fishing and agriculture therefore
the population density of the area is relatively low.
Mt St Helens is a stratovolcano, and attracts half a million visitors per year. Before 1980 Mt St Helens
hadn't erupted for 123 years but on March 20th, 1980, a magnitude 4 earthquake indicated that Mt
St Helens was resuming activity after a dormant period of 123 years. This activity attracted many
Mount St Helens is a volcano which lies near to as destructive plate boundary where the small Juan de
Fuca Plate is being subducted beneath the North American Plate. The Juan de Fuca plate being
subducted into the mantle is exposed to increased heat and friction causing the plate to melt. The
magma produced in the melting rises up through the North American plate, forming Mt St Helens.
Mt St Helens magma chamber had been growing for over 100 years. Scientists predicted that the
eruption would be vertical, however the event turned out to be a lateral blast meaning more
damage than expected was caused. This is thought to be as a result of something blocking the
magma ejecting out of the main vent, causing it to divert and form a rock bulge.
The trigger was a magnitude 5 earthquake underneath Mount St Helens on the 18th May at 8:32am.
This caused the bulge on the north face of the volcano to become unstable and collapse as an
avalanche. The volcano then went to erupt ash and produce pyroclastic flows.
On March 27th another earthquake led to a 65m carter opening on the summit, seismic events
continued into April.
Volcanologists from the United States geological survey were pressed for predictions. Ash samples
were tested after each mini release of steam and checked for sulphur dioxide levels a rise would
indicate that magma was rising. Scientists predicted that the volcano could erupt at any time and
hazard maps identified large areas of land at risk. By April the 3rd, the governor had declared a state
of emergency and ordered the evacuation of most of the citizens within five mile exclusion,
consisting of two high risk zones, where only scientists could enter.
A developing bulge was detected by tilt metres on the volcanoes north flank, and it soon became
clear to the naked eye that the volcanoes north face was swelling ominously. The bulge developed
as magma pushed up within the peak. Angle and slope distance measurements indicated it was
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It had pushed upwards and outwards by 100m by mid-April
and was over 135m by May 17th.
On May 18th at 8:32am an earthquake of Richter scale magnitude 5.1 triggered the biggest landslide
ever recorded. A mixture of rock, glacier ice and soil flowed as lahars into Spirit Lake causing waves
200m high, then down the North Fork of the Toutle River, filling the valley with 100m of debris.…read more
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Unemployment in the area rose by a factor of ten a few weeks after the eruption and
tourism in the region was crippled by the fact that the Mount Baker camp site was
completely destroyed and access to the area was heavily restricted.
However these effects were temporary and once the national park was reopened there was
a resurgence of visitors who had observed the eruption on the news, attracting an entire
new group of volcano based tourists.…read more