91�ȱ�

Mt. St. Helens 1980

A very common case study for volcanoes is the eruption of Mount St Helens in the USA in 1980. Other case studies include the eruption of Mount Etna in Sicily in 1974 and Heimaey eruption in Iceland in 1973.

The underlying causes

Mount St. Helens, Washington State, began its most recent series of eruptions in 1980 when a massive landslide and powerful explosive eruption created a large crater, and ended six years later after more than a dozen eruptions of lava built a dome in the crater.

The first sign of activity began in the spring of 1980 with a series of small earthquakes began. After thousands of additional earthquakes and steam explosions, a cataclysmic eruption occurred on 18 May 1980.

Mount St Helens lies close to a destructive plate boundary where the smaller Juan de Fuca plate is being forced into the mantle by the larger North American plate.

Friction and heat cause the plate to melt and, as it melts, molten rocks are formed. The molten rock builds up until it has the chance to reach the surface through cracks in the Earth’s crust.

Impact on landscape and population

Landscape

• The mountain was reduced from a height of 2950m to 2560m as the eruption created the largest landslide ever recorded.
• All plant and animal life within a 25km radius of the volcano was killed, including fully grown trees.
• Mudflows poured down the valleys choking rivers with rock debris, killing fish and ripping trees from their roots.

Population

• Sixty one people died due to mudflows, being crushed to death and poisonous gases, while 198 had to be rescued.
• Mudflows destroyed bridges, houses and logging camps.
• The explosion flattened buildings and trees and knocked out power supplies and telephones.
• Ash clouds resulted in airline flights being cancelled.
• Ash caused £100 million of damage to farm machinery and crops.

Methods of prediction and planning

Volcanoes are difficult to predict but, although they were unable to give a precise date scientists tried to predict the eruption of Mount St Helens by measuring the frequency of earthquakes on the mountain.

The greater the frequency, the nearer the eruption and measuring the size of the volcanic cone shows the build-up of magma in the vent. Scientists can also check for gas emissions (sulphur dioxide) and increased thermal activity at the crater. However, even before the eruption of Mount St Helens, scientists thought that the it might still be a few weeks away.

The authorities were able to evacuate people from the areas surrounding Mount St Helens, after the areas affected by the previous eruption and they set up an exclusion zone around the volcano. Emergency services were also on hand to rescue those people needing help.