The Mountain Blows its Top

Grades 5-8

Adapted from the USGS Learning Web Lesson Plans

Background | Materials | Instructions


Background

A volcano is a vent in the surface of the Earth through which magma and gasses erupt. Volcanic eruptions are among the Earth's most powerful and destructive forces, but volcanoes are also creative. Volcanoes have also shaped the Earth's landscape, as many of our mountains, islands, and plains have been built by volcanic eruptions.

Mount St. Helens was one of the most powerful volcanic eruptions in recent memory. The bulge that developed on the north flank of Mount St. Helens was evidence of changes occurring inside the volcano. Magma was moving closer to the surface and inflating, or deforming the side of the volcano.

A 5.1 magnitude earthquake on May 18, 1980 shook the volcano, including the bulge area. This shaking caused a sudden collapse of the volcano’s north flank and triggered a large avalanche. The removal of this large mass of rock by the avalanche caused a sudden release of pressure inside the volcano and a violent eruption occurred.

Objective

Students will observe fault movements on a model of the earth's surface.

Time Needed

1 or 2 Class periods

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Materials Needed

• 1,500 ml beaker

• Damp sand

• Several small balloons

• Rubber bands

• Bunsen burner or hot plate

• Straight pin

• A bottle of soda water

• Basin or bowl

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Instructions

  1. Put about ½ inch of damp sand in the bottom of the beaker and level the surface of the sand. Partially inflate a balloon, secure it with a rubber band, and place the balloon on top of the sand in the beaker. Cover the balloon with sand to a depth of about 1 ½ inches. Level the surface of the sand.
  2. Explain that inflation caused a bulge to develop on the north side of Mount St. Helens. The inflated balloon represents the magma rising within Mount St. Helens and the sand represents the surface of the Volcano.
  3. Place the beaker on the Bunsen burner or the hot plate. Heat the beaker until the balloon begins to expand (the surface of the sand should bulge).
  4. Observe the changes in the shape of the surface of the sand. What happens to the “land” as the “magma chamber” expands?
  5. Compare a soda bottle to a magma chamber. When soda is opened it fizzes because the gasses are escaping. Shake a soda bottle and open it (in a bowl or basin). The soda will “erupt” when the cap is suddenly removed. When the avalanche was triggered on Mount St. Helens the “cap” was taken off allowing the volcanic gasses to erupt.

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