Case Study - Mount St Helens

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  • Created by: Fiona
  • Created on: 14-05-13 19:55


Mount St Helens is a volcanic peak in the Cascade Mountains

It is a composite volcano (layers of ash from initial explosive phases of eruptions and subsequent layers of lava from the main erupton phases, andesitic lava)

Occurs on a destructive plate margin - the Juan de Fuca plate is being subducted by the North American plate

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In March 1980 there were a series of small earthquakes in the vicinity of Mount St Helens and a bulge appeared on the side of the volcano

On 18th May there was a 5.1 magnitude earthquake and this was followed by an eruption of volcanic material including rocks, ash, hot gases, lava and steam

The lateral blast covered an area 600km2 north of the volcano

There were pyroclastic flows, lahars (volcanic material mixed with water to create mudflows) and floods as a result of the eruption

540 million tonnes of ash were deposited over an area of 57,000km2

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Impacts - Social

57 people were killed, including a vulcanologist who had stayed to monitor the volcano, and Harry Truman, who refused to leave his home by Spirt Lake

200 homes were destroyed

Unemployment rose tenfold - people's livelihoods were disrupted

Social facilities were lost - for example Spirit Lake contained highly toxic gases from the eruption and this meant it was impossible for people to go fishing there


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Impacts - Environmental

240km2 of forest was lost

7,000 big game animals were killed, including deer and bears

Out of the 32 small mammal species that lived in the area, only 14 of them are known to have survived (mainly burrowing mammals eg gophers)

Sediment in Spirit Lake increased the bottom by 90m

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Impacts - Economic

In total the eruption cost $1.1 billion

$2.2 million was spent cleaning up ash in Yakima, a city in Washington

24km of railway and 300km of roads were destroyed, making transport very difficult

The timber industry were affected because of the loss of 240km2 of forest

Airports were forced to shut for 2 weeks due to the reduced visability and this led to 1000 flights being cancelled

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FEMA (the federal emergency management agency) coordinated the response

Both national and local organisations were involved, for example the US Army Corps of Engineers and local emergency services

Face masks were distributed to provide protection from the ash

Emergency shelters were provided for the homeless and the stranded

900,000 tonnes of ash were removed as part of the clean-up

Shipping channels had to be cleaned of sediment

45,000 acres of land was replanted

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Response Problems

When the earthquake occurred in March 1980, the USGS set up a system to monitor the volcano and the volcanic and seismic activity was reported daily

Using hazard maps from previous eruptions, warnings were issued to the people living in the area

In March an evacuation started involving 2,000 people

- These things all helped reduce the impacts, but there were some problems...

The scientists were unable to accuarately predict that the eruption was going to occur just before it did because there were no unusual changes or activity

Not everybody who lived in the area evacuated - for example Harry Truman refused to leave his house by Spirit Lake despite living in the danger zone

The lateral blast, due to a volcanic plug blocking the central vent, was unexpected and meant the eruption was different to the previous eruptions used to decide which areas were most in danger

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