Why volcanoes occur in certain places Most volcanoes are found along a belt, called the Ring of Fire, that encircles the Pacific Ocean. Volcanic activity also occurs in such places as Hawaii, Iceland, and southern Europe and at the bottom of the sea. According to this theory, the earth's outer shell is divided into a number of rigid sections of rock, called plates. The plates slide or drift about continuously over a layer of partly melted rock. As the plates move, their boundaries collide, spread apart, or slide past one another.
Most volcanoes occur at the plate boundaries.
The map Where volcanoes occur in the print version of The World Book Encyclopedia shows the plate boundaries and the volcanic activity along them. Most volcanoes are formed where two plates collide. One of the plates is then forced under the other. As the plate sinks, friction and the earth's heat cause part of it to melt. This melted part then rises as magma.