Location and Causes of Volcanoes
There are about 2,500 known active volcanoes
in the world. Nearly all of them are located in chains
stretching across the earth, often for many thousands of miles. Geologists believe that
this distribution is
related to a theory of the development of the earths surface called plate tectonics.
The plate tectonics theory says that the surface of the earth is
divided into segments, called plates, which
are moving relative to one another. Where two adjacent plates are moving away from each
other material
rises from beneath the plates to fill the gap, which is therefore marked by a line of
volcanoes.
In the opposite situation, in which two plates are moving together, a
string of volcanoes is also developed
by the melting of one plate as it descends beneath the other. The Pacific Ocean is nearly
surrounded by
such lines of collision between plates, along which are located two thirds of the world's
active volcanoes,
forming the "ring of fire."
A third type of volcanic chain is formed when a plate moves over what
appears to be a "hot spot" deep in
the earth. Volcanoes formed when over the hot spot are carried away by the motion of the
plate. The
Hawaiian Islands are an example of such a chain.