A volcano is a mountain that opens downward to a reservoir of molten rock (like a huge pool of melted rocks) below the earth’s surface. Unlike mountains, which are pushed up from the earth’s crust, volcanoes are formed by their buildup of lava, ash flows, and airborne ash and dust. When pressure from gases and molten rock becomes strong enough to cause an explosion, it erupts and starts to spew gases and rocks through the opening.