Introduction

Abstract

The word metamorphism stems from Greek roots and literally means ‘change of form’. Metamorphic rocks have been produced by physical and chemical changes that have affected other rocks. By now you will have met the other two fundamental divisions into igneous and sedimentary rocks. Each of the three major classes was produced by quite different processes in the Earth and as a result each has certain characteristic features. Igneous rocks were formed by the crystallisation of silicate melts. Since crystals in a melt can grow equally in all directions, the silicate minerals of such rocks interlock with each other in all directions. In the case of sedimentary rocks, particles or fragments are usually cemented together. The fragments were transported over the surface of the Earth and eventually deposited as loose sediment, and later packing and cementing of the grains produced solid rock. Sedimentary rocks are often arranged in parallel, sheet-like layers or beds and they may contain fossils. Metamorphic rocks are produced when other — pre-existing — rocks are affected, mainly by increases in pressure and temperature, which can happen only at considerable depths in the Earth’s crust. Metamorphic rocks were never molten at any stage in their development. Instead, they formed in the solid state under conditions of high stress, as a result of which their crystals may sometimes have grown in parallel alignment.