Why do volcanoes form?

The primary ingredient for a volcano is magma. Magma is what we call melted rock. When rocks melt at depth they become less dense than the solid rocks around them and therefore they rise to the surface of the earth forming a volacano (for the same reason that a helium baloon rises through the more dense air around it).

The next obvious question is, what causes rocks to melt. This can happen in one of three ways; a rise in temperaure; a fall in pressure; or the addition of a substance that changes the melting point of a rock. Temperature rises are obvious, but the fall in pressure is just like spraying a deodorant. Pressing the nosel lowers the internal pressure, causing the liquid inside to undergo a state change to the gas / liquid mix you see come out of the can. An example of adding a substance to change the melting point is salting the road in winter, which lowers the melting temperature of ice.