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Locations of Volcanoes Imagine taking a world map, closing your eyes, and putting your finger down on the map anywhere at random. If you were instantly transported to that spot on Earth and were to look around, do you think you would be able to see a volcano? Or even more exciting, would you see an erupting volcano? Probably not, because most volcanoes, especially active ones, occur in only a few well-defined narrow bands across the face of Earth--something like wild animals confined to reservations. Actually, at most locations on Earth, you would only see a lot of sea water (but that's another story!). But even if you picked a place on land, in most cases there would be no volcano nearby. That is probably just as well, because volcanoes, like wild animals, can be very dangerous and unpredictable neighbors. Sometimes volcanic eruptions are quiet outpourings of lava or playful displays of fire fountains that can safely be viewed from a reasonable distance. Other eruptions are so destructive that everything within a thousand miles can be annihilated within minutes. 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 been attained only during the last hundred years or so.
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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, and 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.
Let's carefully examine the edges
of colliding plates to see why volcanoes might be found there. Remember that the plates
move because heat flowing up from Earth's core causes mantle rock to slowly flow in giant
convection currents. The rigid plates are carried on the currents, crunching into each
other. Plate edges may be either oceanic crust or continental crust. So when
plates collide, we have only three possibilities: oceanic-oceanic, oceanic-continental, or
continental-continental collisions.