Metamorphic Rocks

Metamorphism is defined as the changes that take place in the rocks due to pronounced changes in temperature, pressure and chemical environment. The rocks, which are formed by this process, are called as ‘metamorphic rocks’. The term “Metamorphism" is derived from the Greek: Meta -meaning change (due to Pressure and Temperature), Morph meaning form (structure). The original rock which changes its form because it has undergone metamorphism is known as protolith. Sedimentary and igneous rocks are Protoliths that undergo metamorphism. The changes in the protoliths are due to pressures and temperatures it is subjected to. The mineral composition of the original rock may completely change resulting in new textures which are completely different from those under which the original rock formed.

You can find the rocks and its properties file in references which is developed by Dr. Sandeep A. Meshram, Associate Professor in Geology, COEP Technological University, Pune

Agents of Metamorphism :

Important agents of metamorphism are:

• Temperature
• Pressure
• Chemically active fluids

Temperature

• Heat is generated is the crust of the earth due to geothermal gradient. The temperature goes on rising with increasing depth in the crust of the earth. On an average the rise in temperature is approximately at the rate of 2.00 to 2.20 C per 100 m depth.
• Thus at considerable depth the temperature is very high. Heat may also be available around the area of magmatic intrusion. Radioactive minerals constantly change from one isotope to other and emit heat.

Pressure

• At certain depth in the crust of the earth pressure is available due to the load of overlying layers of rocks. Pressure may also be available due to tectonic forces. Pressure is of two types: (a) Uniform pressure and (b) Directed pressure or stress.
• Uniform pressure, which changes the size, acts equally upon solids and liquids. Directed pressure or stress, which changes shape, can act only on solids. It cannot act upon liquids. It gets absorbed in liquids.
• The rocks in the upper past of the earth crust are solid in state. Due to rise in temperature, at certain depth in the crust of the earth, the rocks become semi plastic and plastic in conditions. As a result of this, directed pressure is not present at a great depth in the crust of the earth and only uniform pressure is present.

Chemically Active Fluids

• Water, which is under influence of gravity, keeps moving downward. It dissolves most of the soluble matter inside the earth as it universal solvent.
• It forms chemically active fluid which acts as catalyst for chemical processes.

Types of Metamorphism

• Cataclastic metamorphism
• Dynamic metamorphism
• Contact metamorphism or Thermal metamorphism
• Dynamothermal metamorphism
• Hydrothermal metamorphism
• Plutonic metamorphism or subduction metamorphism
  • Cataclastic Metamorphism / Shock / Directed Pressure Metamorphism - The metamorphism in which only the directed pressure or lateral stress plays dominant role, is called ‘Cataclastic Metamorphism’. Such as in a meteorite impact, the energy released is converted to heat and the high pressure is converted to shock waves into the earth at the point of impact. Example Quartz polymorphs to coesite. This type of metamorphism operates mainly in upper part of the earth’s crust where the temperature is moderately low but enough to raise the temperature to the melting temperature of the rock impacted and pressure is high.
  • Dynamic Metamorphism - In areas where stresses caused by earth movements such as folding and faulting, the temperature in such rocks may be moderately low and pressure is high. This type of metamorphism results from mechanical deformation of rock bodies which during faulting slide past each other. In the narrow fault zone the heat generated by friction during sliding, fragment the crushed rocks into powder or rubble. Example: mylonite.
  • Contact Metamorphism or Thermal Metamorphism : The rock ‘hornfels’, is an example produced by contact metamorphism. Rocks formed by contact or thermal metamorphism are fine-grained and show no foliation. The metamorphic aureole or the zone surrounding the igneous intrusion is a small area that is heated by the igneous intrusion. Therefore metamorphism occurs in the area adjacent or in contact with this intrusion. Temperature is dominant and Pressure is low. Beyond this zone of contact, the rocks are not metamorphosed. In contact metamorphism the temperature increases in all directions, directed towards the magma intrusion but shows variations in temperature between the deeper surrounding rocks and the magma intrusion. The metamorphism that is brought about by temperature alone is called as ‘thermal’ metamorphism. In rocks such as limestone thermal metamorphism causes melting of crystal of calcite. During recrystallization of this melt a new rock i.e. marble is formed. Limestone Thermal metamorphism Marble