In chapter 4, you were introduced to some of the phenomena related to volcanoes and volcanic activity around the world. You learned that the eruptive behavior of volcanoes is related to aspects of the chemical composition of magmas - and the chemical composition of magmas varies with respect to plate tectonic setting.

These fundamental principles of volcanoes and volcanic processes were vividly illustrated for North American geologists and others worldwide during the catastrophic eruption of Mt. St. Helens in May 1980. Even though geologists knew Mt. St. Helens had a long eruptive history, prior to 1980 the volcano was in a dormant state; it had not erupted since 1857.

During the eruption, the summit of Mt. St. Helens lost nearly 400 m as this material was erupted skyward or blasted laterally toward the north. The gaping crater remains as evidence of the power of this eruption. The images on this page are computer-generated views of the volcano derived from a digital elevation model (DEM) of the mountain created following the 1980 eruption.

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The image above is a view from the northeast, looking into the crater. The broad apron extending outward from the entrance of the crater is composed of pyroclastic debris erupted from the mountain during the cataclysmic eruption of 18 May 1980 and smaller eruptions that emanated from the crater during the years following 1980.

The image below is from the southeast looking over the crater rim. This view shows a number of large debris flows on the flanks of the volcano. Much of this debris moved downslope during the eruption as the snow and ice mantle of the mountain melted and mixed with volcanic ash and debris to form lahars (volcanic mud flows) that streamed outward from the mountain for many miles. Additional debris flows occurred in the years following the eruption as the denuded slopes were subjected to heavy rainfall. Devoid of vegetation, the barren slopes easily became saturated with water and slid down hill under the influence of gravity.

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At the center of the crater, a large dome of lava has slowly extruded from beneath the volcano, and continues to grow slowly each year. Geologists expect this lava dome will eventually rebuild the volcano to something resembling its pre-1980 dimensions.

A wealth of additional information on Mt. St. Helens (both before and after the 1980 eruption) is available from the United States Geological Survey's Cascades Volcano Observatory.

Follow the link above to learn more about this fascinating volcano or take a virtual flight around Mt. St. Helens by clicking on the video link below.

Mount Saint Helens Fly By (1589.0K)

Points to Ponder:

1. Locate information on the diameter and depth of the crater in Mt. St. Helens.

2. From the data you collected in question1, calculate the volume of material that was blown out of the volcano to create this crater.

3. Assuming the material had a density of 2.7 grams per cubic centimeter, estimate how many tons of rock were blown out of Mt. St. Helens.

4. How does the magnitude of the Mt. St. Helens eruption compare to other famous eruptions (such as Krakatau, 1883 or Tambora, 1815)?