Magma is molten rock within the Earth's crust. It consists mainly of the chemical elements silicon, aluminium, iron, calcium, sodium and potassium. These elements have very low volatility. The volatile components of the magma consist mainly of water vapour, carbon dioxide, sulphur dioxide, nitrogen, hydrogen, carbon monoxide, sulphur and chlorine. When the volcano erupts, the volatile components (gases) separate from the non-volatile components, the latter being expelled at the surface as lava.