Case Study - Mount St Helens

  • Located on a destructive plate boundary

  • In Skamania County, Washington (subduction zone)

  • North American and Pacific Plates collide

  • Composite cone/strato-volcano

  • By 1980, there had been no eruption for 120 years. Many believed the volcano was extinct

  • In mid March 1980, a magnitude 4 earthquake occurred

  • In the following 2 months, 10,000 ‘aftershocks’ occurred (or so many believed)

  • A state if emergency was declared. Home owners were evacuated. Only geologists and disaster tourists remained in the area

  • A bulge of 150m developed on the side of the volcano. This indicated a welling up of magma just beneath the surface

  • On May 8th 1980, another earthquake occurred (magnitude 5)

  • The bulge burst, causing a secondary vent eruption

  • This resulted in the largest landslide ever recorded on Earth (200km/hr), a lahar and a significant ash cloud.

  • A central vent eruption followed afterwards

  • The ash from the eruption travelled America in 3 days and the world in 15 days

  • Short term effects

  1. 57 people died

  2. Nearby river became clogged with mud, ash & debris

  3. Infrastructure was destroyed

  4. Thousands of animals and birds were killed

  5. Over 10 million fish were killed

  6. Agricultural land was ruined