A volcano is a mountain that sometimes shoots hot, liquid rock out of its top. Deep inside a volcano is an area filled with liquid rock. If pressure builds up in the chamber, the volcano may explode, and liquid rock will shoot out of the top. When the liquid rock is within earth, it is called magma, but when the volcano erupts and the liquid rock breaks through the earth, it is called lava.

When it is deep underground, magma is under immense pressure and contains high amounts of dissolved gases and water vapor. But when it gets out of earth and converts to lava, the pressure gets released and the dissolved gases escape. In this way, lava is essentially a degassed form of magma.