The three types, at the moment, are:

- The planet's tectonic plates, which move around something like broken eggshells on water, can move away from each other, allowing magma to seep up.
- The plates can also move towards each other, forcing eruptions.
- Plumes of magma well up from deep inside the Earth.

This fourth type is caused when one plate moves under another, squeezing the mantle and causing it to pop through the crust. These volcanoes are found mainly on the sea floor - ie Western Pacific.

http://www.msnbc.msn.com/id/14077362/