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Volcanic eruptions occur only in certain places
and do not occur randomly.
That's because the Earth's outermost shell -- the lithosphere -- is broken
into a series of slabs known as lithospheric or tectonic plates. These plates are rigid, but
they float on the hotter, softer layer in the Earth's mantle. There are 16 major plates.
As the plates move about, they spread apart,
collide, or slide past each other. Volcanoes occur most frequently at plate boundaries.
Some volcanoes, like those that form the Hawaiian Islands,
occur in the interior of plates at areas called
hot spots. Although most of the active volcanoes we see on land occur where plates collide, the
greatest number of the Earth's volcanoes are hidden from view, occurring on the ocean floor along
spreading ridges.
Mount St. Helens is typical of more than 80 percent of the volcanoes that have formed on land.
Known as
subduction zone volcanoes, they occur along the edges of continents where one plate dives, or
subducts, beneath a second plate. When the subducting plate reaches about 100 kilometers (60 miles) into the
Earth's hot mantle, it triggers partial melting of the overlying plate and forms new magma. Some of the
magma rises and erupts as volcanoes.
-- Excerpts from:
Volcanoes! -- U. S. Department of the Interior, U. S. Geological Survey, Teaching Packet;
Map modified from: Tilling, Heliker, and Wright, 1987, and Hamilton, 1976
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