Volcanoes erupt because of density and pressure. The lower density of the magma relative to the surrounding rocks causes it to rise (like air bubbles in syrup). It will rise to the surface or to a depth that is determined by the density of the magma and the weight of the rocks above it.
An explosive eruption sends ash high up into the atmosphere forming a plume. If a magma has a lot of gas which becomes trapped in the magma, the pressure will build and build until eventually the magma erupts explosively out of the volcano. It's a bit like a bottle of fizzy soda.
