Volcanoes

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Magma, Lava, and Mountain Form
by Leslie Whitaker

Volcanic eruptions occur when magma from the Earth’s mantle begins to move near the Earth’s surface. Magma is rock which has been heated enough that it can flow. That flowing liquid can be thick and flow slowly like peanut butter or thin and flow quickly like warm molasses. As the magma moves it causes earthquakes and tries to push upward through weak spots in the Earth’s crust. When this magma reaches the surface of the crust, it flows as lava from vents or fumeroles on the volcano. Even when it does not reach the surface, the heat and pressure of the magma can cause the volcano to explode. These explosions occur when superheated water inside the volcano bursts into steam and forces rock, ash, and mud from the volcanoes interior up into the air and onto the surface of the mountain. The explosion of such superheated water is what caused the tremendous destruction of Mount St. Helens in 1980.

If the magma inside the volcano is thick, it does not flow easily and a great deal of pressure builds below the surface as the magma moves upward. When this pressure is great enough, a very large explosion occurs and large amounts of rock, ash, gas, and water are finally thrown from the volcano with great force. The result is that the explosion builds a sharply pointed mountain with the characteristic shape of a volcano. Mount Fuji and Mount St. Helens are examples of these types of cone shaped volcanoes. Unfortunately, these explosions can sometimes be so great that they actually tear down the mountain! In 1980, the eruption took over 1200 ft. of height off Mt. St. Helens, instead of building it higher.

If the magma is thin and flows easily, then it oozes out of the volcano as lava without building a very high pressure behind it. The result is less dramatic eruptions, lots of lava, and little gas and ash. The volcano built by these eruptions is called a shield volcano. The volcanoes of the Hawaiian islands are generally shield volcanoes. The name comes from their appearance of being war shields lying on the Earth.

Introduction - Magma, Lava, and Mountain Formation - Predicting Volcanoes - Tectonic Plates - Mt. St Helens - Mount St. Helens Recovery - Volcanoes: Good, Bad or Both