The Volcano rock viscosity that is under the earth is the compression pressure, insert the crack up to the surface of the world by force is an ignition source or the force of the explosion occurred. What comes out from the mountain when Volcano is the stone steam viscosity dust stone and gas is going to come out of the crater (Rock, viscosity, if it is out on the surface of the world is called the lava. But if it is below the surface of the earth is called the trees).