Deep within the earth there is alot of rock. Some rock is so extremely hot that it tends to melt and change into a magma, or a thick flowing subtance. It also tends to rise; when it rises above the earth it does so through these vents (volcanos) releasing the magma and creating lava. Now your probably asking yourself why are some volcanic eruptions so enormous and devastating. This depends on the consistency of the magma. If it is thin and has no problem escaping to the surface there will be a smaller eruption. If the magma has a harder time getting out because it is thick, more pressure is placed to release it, causing a larger eruption!