Volcanoes are landforms that are made of molten material, gas, and rock that rise through a conduit from the earth's mantle and erupt onto the surface. Molten rock that flows out

Eruption of Mount St. Helens, Washington, 1980 (USGS/MSH-Rosenbaum J.)

on the surface is called lava. The shape and size of a volcano is determined by the volume of material, its composition and viscosity, the amount of gas, and in some cases the wind direction. Eruptions are hard to predict, and they can vary in eruptive style during the same episode and in subsequent eruptions.

Shield volcanoes are typical of intraplate eruption, where the lava is fluid and produces low-profile structures that in many instances are quite large because of the huge volume of material. They are the least violent volcanoes. A good example of a shield volcano is Mauna Loa on the island of Hawaii, which is produced by a mantle plume or hot spot on the ocean floor.

Composite or stratovolcanoes are among the most violent and are usually found adjacent to subduction zones. It is there that the sinking plate melts because of friction and being jammed down into hotter parts of the earth. The resultant molten rock rises upward through the continent, changing its compo sition as it dissolves some of the rock it passes through. Mount St. Helens in Washington, and many of the volcanoes found in the Andes Mountains of South America, are typical examples of composite volcanoes. Cinder cones are built up of molten material blasted into the atmosphere, where it cools and then falls to the surface around the vent, forming a volcanic edifice. If a strong wind is blowing during the eruption, more material may be blown to one side, forming an asymmetric cone. Often, huge volumes of lava flow out onto the surface through long cracks, forming flood lavas. Good examples of these are found in Idaho, India, and Scotland.

Resurgent volcanoes are the most violent. They are located over hot spots beneath continents, and when they erupt so much material is ejected that the entire volcanic structure collapses, leaving a huge caldera. The floor of the caldera, often containing a lake, lifts upward after the main eruptive phase, and hence its name. Lake Toba in Sumatra and the partially covered volcano that encircles most of Yellowstone National Park are examples of resurgent calderas.

Geologists are interested in volcanic eruptions because the molten material and gases are direct windows into the interior of the earth. Eruptions of these materials have been going on ever since the earth melted, during the early part of its history. Gases released from the interior during this time have made major contributions to the formation of the earth's atmosphere and oceans. Periods of intense volcanic eruption in subsequent periods of geologic history have thrown large amounts of ash and gas into the atmosphere, blocking sunlight and resulting in the alteration of climate, and possibly affecting life.

There are more than 500 active volcanoes on the earth that have erupted during recorded history. If a volcano has not erupted during recorded history but geologists think that there is a good chance that it will, it is said to be dormant. If a volcano will not erupt again, it is called extinct.

—Sidney Horenstein

See also: Geology, Geomorphology, and Geography; Oceans; Plate Tectonics


Ernst, Richard, and Kenneth Buchan, eds. 2001. Mantle Plumes: Their Identification through Time. Special Paper 352. Boulder: Geological Society of America; Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill Pub. Co.; Wright, T. L. and T. C. Pierson 1989. Living with Volcanoes. U.S. Geological Survey Circular 1073.

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