Factors of Metamorphism

Abstract

Temperature and pressure are the physical factors controlling the process of metamorphism. Metamorphism, in general, refers to the reactions between neighbouring minerals of a rock, in response to conditions of temperature and pressure prevailing at depth. A certain mineral paragenesis, formed at some given temperature and pressure, becomes unstable if subjected to different conditions; the minerals react to form a new paragenesis in equilibrium at the new conditions. If carbonates and H₂O- or OH — bearing minerals take part in the reaction, CO₂ and H₂O are liberated. The higher the temperature of metamorphism, the smaller is the amount of CO₂ and H₂O combined in the stable minerals. Therefore, a fluid phase composed of volatile constituents is always present during metamorphism. At supercritical conditions, which generally are realized in metamorphism except at very low temperature, the fluid is a gas of high density having many properties of a liquid. Water e. g. at 500^°C and 2000 bars has a density of 0.69 g/cm³; at 400^°C and the same pressure its density is 0.97 g/cm³. Volatile constituents already existed in the rocks even before metamorphism, occupying pores and minute cracks or being adsorbed on the grain boundaries. Even in the case of igneous rocks, a sufficient amount of H₂O must have been present during metamorphism, either originally contained in the rocks or introduced along minute cracks. Otherwise the metamorphism of basalts to amphibolites or chlorite-epidote greenschists is impossible.