Why do volcanoes erupt? It seems like a pretty simple question when you first pose it, so let’s look into the answer, shall we. The simplest answer would seem to be that magma, being less dense than the rock surrounding it, tries to float to the surface. If the magma contains water and dissolved gasses, when the magma reaches the surface the water and dissolved gasses will suddenly expand into steam and gas, causing a violent eruption. There are several factors that will trigger a volcanic eruption, but three predominate: the buoyancy of the magma, the pressure from the exsolved gases in the magma and the injection of a new batch of magma into an already filled magma chamber.