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.