“Metamorphic Rocks” are formed from the older rocks when they are subjected to increased temperature, pressure, and shearing stresses at considerable depth in the earth’s crust. The older rocks may be either sedimentary, igneous, or other metamorphic rocks. During metamorphism, recrystallization takes place essentially in the solid-state and new minerals and new textures are produced.
Agents of Metamorphism
The agents which bring about metamorphic changes in the rock are (i) heat, (ii) uniform pressure, (iii) directed pressure or stress, and (iv) chemically active fluids and gases.
- Heat: Within the earth, temperature increases with depth, and as a result, appreciably high temperature exists at great depths. In the outer part of the earth, the common cause of elevated temperature is the intrusion of hot igneous bodies.
- Uniform Pressure: The static pressure on rocks is caused by deep burial. This pressure is due to the weight of the overlying layers of the rocks. As a temperature gradient exists within the earth, static pressure is usually associated with higher temperatures.
- Directed Pressure: Directed pressure or stress operates during folding movements that accompany mountain building. Generally, directed pressure plays an important role near the earth’s surface. As the depth increases, the effect of directed pressure decreases, and that of uniform pressure increases.
- Chemically Active Fluids and Gases: Chemically active fluids and gases when passing through the pores of the rocks, they bring about changes in their original composition. The source of these chemical agents is generally the intrusive igneous body within the country rocks.
Metamorphic rocks contain different types of minerals, and these minerals are largely controlled by the conditions of temperature, pressure, or stress. These minerals can be divided into three groups: (i) stress minerals, (ii) antistress minerals, and (iii) relict minerals. The three types of metamorphic minerals are as under;
- Stress Minerals: They are formed under the conditions of stress or directed pressure during regional metamorphism. They are usually flat, tabular, elongated, or flaky in nature and grow parallel to the direction of least pressure. Stress minerals play a major role in producing parallel structures and textures which are characteristic of many metamorphic rocks. Examples of stress minerals are micas, chlorite, talc, albite, amphiboles, kyanite, and staurolite.
- Antistress Minerals: They mainly develop under conditions of uniform pressure, such as in plutonic metamorphism. These minerals are often equidimensional in form. Examples of antistress minerals are pyroxenes, olivines, andalusite, sillimanite, cordierite, and spinel.
- Relict Minerals: During Metamorphism recrystallization of minerals takes place essentially in the solid-state. Complete recrystallization is not always possible and therefore certain original minerals continue to survive in the metamorphic rocks. These original minerals which have failed to react to the changed conditions of temperature and pressure, are called the “relict minerals”.
Structures of Metamorphic Rocks
Some Common Metamorphic Rocks