Mount St. Helens

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Mount St. Helens 18/05/1980

Info:

Located in the Cascade mountains,

Washington State, North-western USA

Before the eruption there was

2months of increased seismic activity

Causes:

Plate boundary:

It's a composite volcano formed at

a destructive plate boundary

Oceanic Juan de Fuca plate and

continental North American plate moved together

Juan de Fuca plate subducted under

the North American as it is denser

The magma created by the melting

of the plate rose through fissures in the earth's crust

It then cooled and over time Mount

St. Helens was formed

Cryptodome:

Solidified magma plugged the main

vent of the volcano

Magma went through a secondary

vent instead, creating a bulge (cryptodome) which grew 6ft bigger

everyday

Earthquake:

10,000 small earthquakes in the 2

months before the eruption

Caused by friction of the 2 plates

and magma movement

5.1 on Richter scale shook the

cryptodome free, causing Mount St. Helens to erupt laterally

(horizontally)

Effects of the eruption:

Primary:

All vegetation up to 21km away was

destroyed

24,000 animals died

57 people (including Harry Truman-

who refused to leave his house) died

Biggest landslide ever recorded

Spirit Lake filled with sediment

200 Houses destroyed

Secondary:

High unemployment as tourism and

logging industries collapsed

Flooding as river capacity

decreased due to ash build up

Fertile soils as ash increased

nutrients

Economic cost= $1.1bn

Responses:

Immediate:

Government set up 20 mile

exclusion zone

Most residents were evacuated

2 million face masks were sent

Roads cleared of ash

Long term:

Replanting of forest areas (10

million trees)

Roads rebuilt

Improved drainage- some drain

systems were blocked by ash so had to be cleared.

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