Mount St Helens Case Study Notes

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

Mt St Helens 1980 Eruption


Mt St Helens erupted on the 18th of May 1980 in Washington State, USA. It's part of the Cascade Range Mountains. The eruption was catastrophic and the biggest one ever to hit the USA.


Mt St Helens is a volcano near the plate boundary between the Juan De Fuca and North American plates. This plate boundary was a destructive plate boundary because the plates were moving towards each other; the Juan De Fuca plate was being subducted below the North American plate. In the subduction zone, the Juan De Fuca plate was being melted in the mantle and the magama was rising up through the cracks in the North American plate, this formed Mt St Helens.

Mt St Helens erupted because of a magnitude 5 eruption near the plate boundary. The earthquake caused a bulge on the side of the volcano to collapse into an avalanche, this caused the volcano to erupt on that side because the rocks on that bulge were holding down magma in the magma chamber which was building up lots of pressure.

Predicting the eruption

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