Volcanoes are powerful displays of Earth in action. Scientists have determined that three forces within Earth can create a volcano. Volcanoes can occur where two of Earth's plates converge, or rub together. Volcanoes can also occur where two plates diverge, or move apart. Hot spots, areas in Earth's mantle that are hotter than neighboring areas, are also the sites of volcanoes.

There are three different forms of
volcanoes. The form of a volcano
depends on the composition of its
magma and the amount of water vapor
and other trapped gases. These two
things control the force of a volcano's eruption.

Shield volcanoes have sides with gentle slopes. They are made of basaltic lava. Cinder cone volcanoes have steep sides. They are made of tephra—lava cooled into different-sized pieces. Composite volcanoes have steep sides and are made of silica-rich lava and tephra. Their eruptive forces vary.

In this Virtual Lab you will explore some
volcanoes in the United States to learn
about their composition and their eruptive force.

Objectives:

• Explore different forms of volcanoes.
• Explain the effect of magma composition on the eruptive force of a volcano.
• Predict the eruptive force of a volcano.

Procedure:

1. Click the Video button. Watch the video about volcanoes.

2. Click A, B, or C on the map of the United States to select a volcano you want to investigate.

3. Click the For Reference button to review the three different forms of volcanoes.

4. Click the arrow below Volcano Form to select the form of the volcano shown. Record the form you selected in the Table.

5. Click the arrow below Silica Content to select the content of silica in the magma. Record your selection in the Table.

6. Click the arrow below Water Content to select the content of water in the magma. Record your selection in the Table.

If you have selected the correct volcano form and silica and water content of the magma,
an arrow displays below Eruptive Force. If no arrow displays, review your choices and adjust your selections.

7. Click the arrow below Eruptive Force to select the force of the volcanic eruption. Record your selection in the Table.

8. Click the Erupt button. If you have selected the correct Eruptive Force, watch the volcano erupt. If no eruption occurs, adjust your eruptive force selection. Record your observations in your Journal.

9. Click the Map button to return to the main screen and select another volcano to explore.