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Tectonic Plates
It was thought for a very long time that the Earth's crust, or lithosphere, was one unbroken shell. It is actually made up of many irregular sections of rock, varying in size and shape. They also vary in thickness, from around 4 to 40 miles thick, although oceanic crust is thinner and denser than continental crust. There are many smaller plates and nine large plates; six of these are continental and three are oceanic. The size of the plate has absolutely no bearing on its impact on the Earth, and in fact the diminutive Juan de Fuca plate near Washington is responsible for many of the tremors and eruptions felt in that area.
Convergent Boundaries
Most of the Earth's volcanoes were formed at the convergent boundaries between two plates at areas known as subduction zones. At convergent boundaries two plates collide, and the denser of the two plates is driven under the other. As the denser plate is pushed down, it begins to melt in the heat, creating magma. This magma is less dense than the rock surrounding it, so it begins to move up towards the surface through cracks in the crust. Finally, the magma breaks through the surface of the continental plate, thus creating a volcano.
Divergent Boundaries
Divergent boundaries are created between two tectonic plates that are moving apart. This separation allows magma from the Earth's mantle to rise to the surface. New crust and volcanoes are created in this way, and oceans are made larger. Plates diverging can also create rifts on land, and can even completely separate landmasses. The majority of the volcanoes of this type are found under the ocean, although there are some in Africa's Rift Valley.
Hot Spots
The least common way for a volcano to form is over a so-called "hot spot" in the center of a tectonic plate. At a hot spot, magma is a lot closer to the surface of the Earth than it is elsewhere on the plate, and it begins to melt the rock above it. This in turn forms volcanic mountains. The islands of Hawaii formed gradually over a period of millions of years by a hot spot under the Pacific plate. Continents can also cover hot spots, and a good example is Yellowstone National Park. Here, a hot spot lies under the North American plate.