When water combines with melted rocks in the magma chamber of the volcano, it pushes magma upward. Gas molecules form bubbles that expand as they rise. The pressure from the bubbles becomes stronger than the surface rock and it fractures, allowing the magma to reach the surface.
Magma is melted rock found under the Earth's surface. It is formed in the lower section of the Earth's crust and the upper section of the mantle. It contains crystals, rock fragments and dissolved gases. Magma that rises to the Earth's surface is called lava. When lava cools and hardens it turns into igneous rock.