Fig. 38 Gneiss Fig. 39 Marble

The role of water in metamorphism is determined by at least four variable geologically related parameters: rock pressure, temperature, water pressure and the amount of water present.

During a normal progressive metamorphism rock pressure and temperature are interdependent and the amount of water and the pressure of water are related to the sediments and to the degree of metamorphism in such a way that the excess of water characterizes the low-grade metamorphic rocks. The medium-grade metamorphic rocks are characterized by the absence of water.