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Mt St Helens Case Study Geography

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.

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.

Mount St Helens, Washington State, NW USA is located in the Cascade mountain range and prior to its eruption in 1980 it had been active for over 100 years. The volcano sits on a destructive boundary where the Juan de Fuca plate meets the North American plate.

Mount St Helens erupted on May 18th 1980 following a period of activity which began in March 1980 with an earthquake measuring 4.0 on the richter scale. What followed was 3 months of seismic activity as magma rose within the mountain. As the magma rose, a large bulge grew on the north flank of the volcano, this was due to a blockage in the main vent resulting in the growth of a cryptodome (mound of viscous lava) in the side of the volcano.

On May 18th, an earthquake measuring 5.1 on the richter scale caused a landslide on the northern flank of the volcano, which in turn exposed the cryptodome below, resulting in a sudden release of pressure and a cataclysmic eruption in the form of a lateral (sideways) blast. The blast zone consisted of 230 square miles with the eruption leaving a 'lunor' landscape in its wake.

Watch the short video clip below to remind yourself of the nature of the lateral blast:

The effects of the eruption included:
* laval flows and ash filling in Spirit Lake and log jams and ash blocking the channel of the Toutle River;
* 57 people died in the eruption - most from poisonous gases;
* large number of wildlife were killed by the blast and the volcanic ash with nothing surviving in the blast zone
* flooding resulting from blocked rivers washed away road and rail bridges
* crops were ruined and livelihoods of loggers were devastated with large areas of trees being flattened like matchsticks.

For your exam you will need to learn a detailed case study of a volcanic eruption, using Mount St Helens as your eruption. You will need to be able to discusscauses and effects of the eruption and the responses of people to the event.It is important that you learn some place specific detail / facts and figures to put into your exam answer in order to reach the highest marks.

Through the use of class notes and independent research you now need to create your case study. You task is set out below and there are a number of links for you to follow up for further information.

TASK: Your task is to write an article for a magazine. You should give your work the title "Volcanic Fury - the 1980 eruption of Mount St Helens" and you need to ensure that you include labelled diagrams / pictures in your work. You need to ensure that you structure your work using the sub-heading given on the task sheet (which can be downloaded here).

The following websites should provide useful information and photographs to help you, but you should also make good use of your video notes and information from classwork.

USGS Background Information on Mt St Helens
Mount St Helens National Volcanic Monument - includes tourist information related to Mount St Helens and a useful digital library with pre and post eruption images (useful for comparions / exploring effects).
Global Volcanism Programme - St Helens (basic facts)
Wikipedia - 1980 eruption of Mount St Helens - includes some very useful information on aftermath, including impacts such as cost etc. and a good overview of the build up to disaster - worth exploring!
Mount St Helens - from the 1980 eruption to 2000 (USGS)
Vegetation around the volcano - before and after (comparative photographs)

To view Mount St Helens in Google Earth download this .kmz file (you will need Google Earth on your computer to be able to view this).

See this fantastic panorama from the top of Mount St Helens after the eruption.

Photograph courtesy of the USGS

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