Even though there are plenty of different reasons that cause a volcano to erupt, all come down to a pressure change inside the volcano that forces the magma to overflow the chamber it is held in.
Explanation:
There is a common type of eruption which is caused by the movement of tectonic plates. When one is brought under the other the magma and other stuff like sediment and sweater are forced within the chamber that eventually overflows provoking the volcano to erupt. This type of eruption causes thick lava and temperatures from 800 to 1000C.
There is a second type of eruption caused by tectonic plates caused by the movement of the plates moving away from each other allowing the magma to rise and fill the gap. It causes a "soft" explosion of thin lava of temperatures between 8000 to 1,200C.
