Mount St Helens is an active stratovolcano (volcanic cone is composed of layers of ash and lava) located on the west coast of North America in Washington State.
It is located along the Pacific Ring of Fire at the destructive plate boundary between the Juan de Fuca plate and the North American plate.
It lay dormant for 123 years until March 1980. Two months later on 18 May 1980 a devastating eruption and landslide occurred.
Fig. 18 Mount St Helens before and after the eruption. In the ‘after’ photo note the new dome growing within the crater.
Predicting the eruption
Geologists had set up a network of monitoring equipment on the volcano: • Laser measurements monitored the shape of the cone. • Heat sensors took the ground temperature. • Seismometers measured the vibrations caused by moving magma. • Gas sensors measured sulfur dioxide, carbon dioxide and radon emissions.
During March 1980, there were many small earthquakes and steam-venting episodes as magma moved below the volcano.
Slowly a huge bulge (volcanic dome) and a fracture system formed on the north slope of the volcano. Deep underground, huge volumes of magma were being pushed up into the volcano. The dome in the crater grew by up to 1.8 m per day.
The earthquake activity continued and caused avalanches of snow and ice. People were evacuated from their homes. The side of the volcano was pushed upwards and outwards by over 135 m.