Plate Tectonics- Vulcanicity Case Study

Mt St Helens, USA 1980

Location

• Mount St. Helens is an active stratovolcano in Washington, in the Pacific North West region of the United States. 
• It is a composite volcano. It is located 96 miles South of the city of Seattle, and 53 miles North East of Oregon.
• It is close to the subduction boundary of the North American and Juan de Fuca (oceanic) tectonic plates.

Causes

• Mount St. Helens is part of the Pacific Ring Of Fire that includes over 600 active volcanoes.
• Mt St Helens erupted after many months of build-up activity including a massive bulge growth on the side of the mountain.
• The first indications of a major problem came on March 20, when a 4.2 magnitude earthquake was recorded beneath Mt. St. Helens. Three days later another 4.0 M was recorded, and that evening the earthquakes began occurring in swarms centred directly beneath the volcano, at a rate of about 15 per hour. By March 25, magnitude 4 events were shaking Mt. St. Helens at a rate of about 3 per hour. 
• The eruption began during a relatively quiet period in which no steam explosions had occurred for four days. On May 18, at 8:32 a.m., a 5.0-M earthquake triggered a very rapid series of events. 
• The entire northern slope above the bulge failed and the north flank of the volcano
  
  began to slide downward from almost the exact site of the east-west fracture at the summit. This gigantic landslide released a tremendous mass from above the hydrothermal system that had driven the precursor steam eruptions. 
• The abrupt loss of confining pressure above the heated groundwater caused a massive flashing to steam, which initiated a hydrothermal blast that was directed laterally through the landslide scarp. The lateral hydrothermal blast rapidly overtook the avalanche and devastated a fan-shaped area to the north, which was nearly 30 km wide over a distance of 20 kilometers. Trees were blown down like matchsticks.

Impacts

Social:

• Power supplies were cut off and that ash got into water supplies. Consequently, it led to contamination of the water and the spread of disease. 
• Loss of jobs in logging industry.
• 57 people were killed, 250 homes, 47 bridges, 15 miles of railways and 185 miles of highway were destroyed.

Economic:

• This was the deadliest and most economically destructive volcanic event in the history of the United States.
• Forest destroyed- This would have a great effect on the logging industry because there would not be as many trees for them to cut down. 
• Lodges would have had to be closed down. This would have an impact on the economy because if the lodges were closed down then there would be nowhere for tourists to come and stay there for they would lose money. 
• Ash settled 15cm deep this would have an effect because when it rained you would get lots of mud flows and it will but growing trees or crops for next year very hard.

Environmental:

• 1300 feet was blasted off the top of the mountain. This would have a great effect because that would all break up into ash and encircle the Earth, which would stop heat and light from the sun coming into the atmosphere. 
• 230 square miles of forest burned and got destroyed. This it would have a great effect on wildlife as 1000s of animals' homes would have been destroyed.

Response

• An immediate response to the eruption was evacuation. Other residents, who were more on the outer section of the blast zone, were evacuated on several premises such as hotels, campsites and research stations. 
• One man, an 84 year old innkeeper by the name of Harry Truman, became ‘famous’ after solidly refusing to leave. Part of the evacuation team and local authorities were desperately trying to persuade him, but he simply decided not to, claiming that he had lived there since he was 30 years of age.
• Locals witnessing the event called emergency services. The Federal Emergency Management Agency arrived not too long after, once permission had been granted owing to the vast situation. They brought with them helicopters, but unfortunately found them less helpful than they thought they would be. 
• Nevertheless, the emergency services reduced the amount of casualties and with the help of aid agencies, set up temporary rescue centres for those left homeless.
• Volcanic investigators and geologists also fled towards the scene, in order to proceed in as much research as possible. And because of the great amount of settling ash, street plows were called out and whilst on patrol, were advising people to stay indoors.
• Following the 1980 eruption a long-term response took place; the area was left to gradually return to its original state. And in 1982, President Ronald Reagan established the Mount St. Helens National Volcanic Monument, with a 100,000 acre area around the mountain.