Effects of Volatiles at High Pressure

Melting temperatures are lowered dramatically when H2O is added under pressure to a system of anhydrous silicate crystals. This is because a silicate melt can readily dissolve H2O, thereby reducing the volume of H2O from that occupied by a gas to that occupied by a liquid. The silicate crystals can dissolve only minute amounts of H2O. A system composed of an anhydrous phase plus H2O gas has a large volume just below the melting point, and the formation of hydrous melt will lead to a smaller volume above the melting point, so high pressure favours the formation of melt.