Volcanoes and the Theory of Plate Tectonics

Major tectonic plates of the Earth.

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Only a few of the Earth’s active volcanoes are shown. (Sketch by Ellen Lougae.) PLATES EURASIAN NORTH AMERICAN JUAN DE FUCA PHILIPPINE CARIBBEAN PACIFIC COCOS NAZCA AUSTRALIAN EURASIAN ARABIAN INDIAN AFRICAN SOUTH AMERICAN SCOTIA ANTARCTIC EXPLANATION Plate boundary Active volcanoes

Volcanoes are not randomly distributed over the Earth’s surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world’s active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific “Ring of Fire.” In the past 25 years, scientists have developed a theory—called plate tectonics—that explains the locations of volcanoes and their relationship to other large-scale geologic features.

According to this theory, the Earth’s surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth’s hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes.