One of the five volcanoes in the Cascade Range in Washington State, USA.
Caused by the oceanic crust (Juan de Fuca) plate subducting under the continental crust (North American plate). The oceanic crust was destroyed and formed magma which rose to the surface.
Like the others, had been dormant for many years.
In March 1980 there were signs of an impending eruption, as first earthquakes occurred and then steam filled with ash exploded onto the white glacial summit of the mountain.
Residents had been told to leave and visitors were not allowed inside a 8km exclusion zone around the crater.
The eruption happened at 8:32am on 18th May.
The Effects
An earthquake measuring 5.1 on the Richter Scale caused a landscape on the north-east side of the mountain - the biggest landslide ever recorded and the sideways blast of pulverised rock, glacier ice and ash wiped out all living things 27km north of the crater,
In the outer 'blow-down' zone, the trees (some 500 year old cedars) were uprooted and tossed about like matchsticks.
57 people died including the vulcanologist David Johnston.
Rivers and Lakes Ash fell into rivers and lakes raising temperatures, while sediment and mud choked channels. This combined effect resulted in the death of all fish, including those in a hatchery and the loss of 250km of top-class salmon and trout rivers. Spirit Lake was filled in.
Communications and Services Floodwaters washed away several roads and railway bridges. Falling ash hindered the smooth running of car engines in three states. Electricity supplies were interrupted and telephone wires cut.
Forestry and Wildlife Every tree in the 250km squared forest, and lying within the 25km blast zone north of the volcano, was totally flattened and destroyed. Trees carried down by rivers in flood, caused a log jam 60km away. Some 10 million trees had to be replanted. As with trees, nothing survived within the blast zone.
Immediate Responses
Helicopter were mobilised to search and rescue those in the vicinity of the catastrophic blast, who could have been trapped in their homes or cars, or were being asphixilated by ash.
Those in the immediate vicinity could not have survived, but tourists caught in mudflows or by the lateral blast could be rescued.
Many people were stranded as the roads were blocked by ash, so the roads were cleared in three days so that traffic could flow.
The ash also posed a health risk, so President Carter promised to send two million more face masks.
Long-Term Responses
The ash improved soil fertility, allowing vegetation to return, so letting animals such as insects, birds and animals to survive there also.
Trees had been knocked over and killed by wind and heat, especially those near the volcano, so many were replanted.
Also, buildings and bridges had to be rebuilt, as many were damaged by falling trees and heat.