Volcanism is the eruption of molten rock (magma) onto the surface of a planet. A volcano is the vent through which magma and gases are discharged.Magma that reaches the surface is called “lava.” Volcanos are named for Vulcan — the Roman god of fire!olcanism is the eruption of molten rock (magma) onto the surface of a planet. A volcano is the vent through which magma and gases are discharged.