Under the earths crust is the mantle. The heat and pressure causes the rock to a liquid called magma. The magma finally rises since it is lighter than rock. Then it comes together in chambers called magma chambers. After a long while, the magma rises up through vent in the earth. Finally, The magma reaches the earths surface and it is called lava. lava. As the magma rises it collects in magma chambers. This fuels the volcanoes so that it can bust through the earths surface. That is what fuels the volcano. As the magma gets closer and closer to the earths surface, the pressure gets lower, but still when it reaches the surface it can still explode. Once the magma reaches the gases expand, and shoots the lava in to the air.