Mount St Helens Earthquake

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  • Mount St Helens Earthquake
    • Background
      • Mt St Helens is located in the North West of the USA in Washington State.
      • Mount St Helens erupted on the 18th May 1980; it was the worst eruption to ever hit the USA. 
      • The area around the volcano is a National Park.
        • The population density of the area is relatively low.
    • Causes
      • Mount St Helens is a volcano which lies near to as destructive plate boundary.
        • The Juan de Fuca plate being subducted into the mantle is exposed to increased heat and friction causing the plate to melt.
      • Mt St Helens magma chamber had been growing for over 100 years.
      • The event turned out to be a lateral blast meaning more damage than expected was caused.
      • The trigger was a magnitude 5 earthquake underneath Mount St Helens on the 18th May at 8:32am.
        • The volcano then went to erupt ash and produce pyroclastic flows.
    • What happened?
      • On March 27th another earthquake led to a 65m carter opening on the summit, seismic events continued into April. 
      • April the 3rd, the governor had declared a state of emergency.
        • Ordered the evacuation of most of the citizens within five mile exclusion, consisting of two high risk zones,
      • 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.
        • The valley filled with 100m of debris.
    • Impacts
      • The environmental impacts
        • Melted 70 per cent of ice on Mt St Helens, resulting in a lahar mudslide travelling at 50kmph, sweeping up 20,000 trees.
        • Ash burst out the sides and covered 575km of land with ash and rock.
        • Volcanic ash covered three states.
          • Winds spread the ash around the whole globe.
        • 400 metres was blown off the top of the mountain.
          • Crater was left that was 500m deep.
      • The social impacts
        • Over 100 people ended up in nearby hospitals, with many suffering from extensive burns and broken bones.
        • Overall 57 people died, 53 of these being located out of the exclusion zones, areas which were deemed safe.
        • Ash that fell caused traffic chaos and airline flights to be cancelled.
        • Crops were ruined and so the livelihoods of loggers were devastated with large areas of trees being flattened.
      • The economic impacts
        • The cost of ash damaged to farmers crops and machinery totalled £100 million.
        • The lahar flow destroyed 27 bridges and 200 homes, and finally stopped 60km down river.
        • Many local businesses closed temporarily, however more jobs were created once tourism began to improve. 
        • Disruption of imports and exports as Washington state airport was closed.
        • Total cost of the damage was more than 1.1 billion US dollars.

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