Why do most volcanoes occur in designated narrow bands? Why not everywhere, such as in your backyard? Why are some explosive and some not? For that matter, why do volcanoes occur at all? Reasonable understanding of the answers to these questions has only been attained during the last hundred years or so.

An important clue to understanding volcanoes is knowing the location of the volcanic bands. Many of the world's active volcanoes are located around the edges of the Pacific Ocean: the west coast of the Americas, the east coast of Siberia, Japan, the Philippines, Indonesia, and in island chains from New Guinea to New Zealand--the so-called "Ring of Fire" (diagram to left). Recently, active volcanoes were also found in Iceland, the Kenya Rift Valley in eastern Africa, Italy, and Hawaii. Looking at the locations of these volcanoes through the glasses of plate tectonics, we also notice that most volcanoes occur near the edges of the large "plates" that comprise the solid surface of Earth. Looking even more closely, we also notice that the dangerous explosive volcanoes, such as Mount St. Helens and Mount Pinatubo, that make the evening news are located where plates are crunching together. The quieter, "effusive" volcanoes, like Iceland and Hawaii, are found mostly where plates are coming apart or in the middle of a plate.