Mount St. Helens

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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