Mount St Helens in Washington (USA) is one of the several volcanic peaks in the Cascade Mountains. The Cascade Mountains are above a destructive plate margin, where the Juan de Fuca plate is being forced under the North American plate. Mount St Helens is a composite volcano.
From March 1980, there were signs that an eruption could happen soon, there were small earthquakes, a bulge appeared on the side of the volcano and ash and steam were released. On 18th May 1980 there was a large earthquake of magnitude 5.1 followed by a massive eruption of volcanic material - including rocks, ash, hot gases, steam and lava. The eruption covered a 600km2 fan shaped area north of the volcano (blast zone). Almost all vegetation and building were destroyed in the blast zone which reached up to about 30 km away from the volcano. Pyroclastic flows, lahars and floods (due to material being dumped in rivers and lakes, displacing water) were caused by the eruption. Shortly afterwards there was another eruption of ash and steam. About 540 million tonnes of ash were deposited over an area of 57000 km2. Impacts:
Economic: 1) The total cost of the damage was around $1.1 billion 2) Over 200 homes and cabins were destroyed and many more were damaged 3) Airports were temporarily closed (some for up to two weeks) because of reduced visibility. Over 1000 commercial flights were cancelled 4) A lot of money had to spent on clearing the ash from roads and runways, e.g. it cost $2.2 million to clear the ash in Yakima (a city 135km from the volcano) 5) 27 bridges, 24km of railways and around 300km of roads were destroyed 6) Rover traffic and shipping was disrupted, and 31 ships were stranded because mudflows dumped huge amounts of sediment in rivers 7) the timber industry was severely affected by the destruction of forests. Social: 1) 57 people were killed - mostly by inhaling volcanic ash 2) Hundreds of people lost their homes 3) Unemployment rose tenfold at first - although this did return to normal once clean up operations began 4) Social facilities and recreational sites were destroyed 5) some people experienced emotional stress Environmental: 1) Almost 240km2 of forest weer destroyed 2) Wildlife suffered - around 7000 big game animals (e.g. deer, elk and bear) and many thousands of birds and small mammals were killed. Around 12 million young salmon were also killed 3) Sediment dumped in Spirit Lake raised the lake bottom by over 90m and the water level by over 60m 4) Water quality was temporarily reduced |
The image below shows the extent of the ash fallout and the distance to which it travelled.
Response:
Emergency responses were cooridinated by FEMA (Federal Emergency Management Agency) and involved both national organisations (e.g. US Army Corps of Engineers) and local services (e.g. local emergency services). 1) Face masks were distributed in some areas to protect people from breathing in ash. 2) Emergency shelters were set up for stranded and homeless people 3) Ash clean-up operations were organised - around 900,000 tonnes of ash were removed from roads, buildings and airports 4) Shipping channels were cleared of sediment to restore shipping and some new channels were cut 5) 45500 acres of land have been replanted with more than 18.4 million trees 6) Domestic water supplies were monitored to ensure they weren't contaminated Management: 1) When the earthquakes began in March, the USGS (United States Geological Survey) started a round the clock monitoring system. It included gathering seismic data and measuring the rate of bulge, growth, ground temperature and sulphur dioxide gas emissions. Volcanic and seismic activity reports were issued daily 2) USGS scientists issued warnings to people living in the area and made recommendations about the locations of hazardous zones 3) Based on these recommendations, in March access to the volcano was restricted and the evacuation of around 2000 people started. 4) Despite the monitoring data, scientists couldn't accurately predict the eruption - on the day of the eruption there were no unusual changes that could have been taken as warning signs. |