Volcanoes are caused, in part, by movement of plate tectonics. The crust of the Earth is broken into rigid pieces, called tectonic plates, that float on a softer layer of rock inside the Earth. Most volcanoes occur near the edges of tectonic plates. Volcanoes caused by the movement along the edges of tectonic plates include subduction volcanoes and rift volcanoes.
Subduction volcanoes are caused when two tectonic plates collide and one tectonic plate dips below another. When the plate goes deeper into the Earth's mantle, or the layer of softer molten rock on which the plate floats, it's heated. Extreme heat releases fluids that heat the rock on top of it, producing magma, or molten rock, that rises to the surface and builds up in a magma chamber until it erupts out of the earth, creating a subduction volcano. Subduction volcanoes shoot lava from the Earth, breaking the crust of the Earth into shards of rock that fly through the air and rain down in the area around the volcano, causing a thick cloud of ash.
Rift volcanoes are caused when two tectonic plates move away from each other, opening a rift in the Earth's crust. This allows magma to ooze up from the mantle along the opening. There are more rift volcanoes than subduction volcanoes in the world, but since most of them are hidden on the ocean floor and cause much less damage than other types of volcanoes, they are not as well-known as subduction volcanoes.
Hot-spot volcanoes form in the middle of tectonic plates when magma melts a hole in a weak spot in the plate and oozes upward. Examples of hot-spot volcanoes and results of their eruptions are the volcanoes and volcanic formations that make up Hawaii, which lies in the center of the Pacific crustal plate. This type of volcano erupts often, but less forcefully than other volcanoes. Magma comes up to the surface and flows out, forming rivers and fountains of lava. Then the plate moves, and the existing volcano becomes extinct. Active volcanoes are formed over the hot spot of magma as plates move over them.