Volcanoes are used as an escape route for Magma. While the magma flows through volcanoes, it builds up gases pressure underground. These gases cause the lava to be pushed up and out of the cracks in the crust. Lava is magma that has risen to the surface of the Earth. As soon as the lave meets air, it instantly begins to cool and harden. This causes it to turn into volcanic rock. Slowly, over time this rock build up makes the volcano grow bigger.