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The following points highlight the four main forms and types of metamorphism. The types are:- 1. Contact Metamorphism 2. Regional Metamorphism 3. Hydro-Metamorphism 4. Hydro-Thermo-Metamorphism.
Type # 1. Contact Metamorphism:
Contact metamorphism takes place when the mineral composition of the surrounding rocks known as aureoles is changed due to intense heat of the intruding magmas. This process of metamorphism is called contact metamorphism because of the fact that metamorphism occurs when the rocks come in contact with the intruding magmas. This process is also called as thermal metamorphism because the rocks are changed in their forms due to high temperature of the intruding magmas.
Such metamorphism occurs during volcanic activity when the physical properties of the surrounding rocks are changed due to intense heat of the rising magmas of dykes (fig. 8.16). Sometimes, the rocks coming in contact with the intruding magmas are also changed in their chemical composition due to some water and water vapour associated with the intruding magmas. Lime-stones are changed to marbles due to contact metamorphism.
As stated above, the rocks surrounding the igneous intrusions are altered due to intense heat of mag-mas.
The margins of the altered rock around igneous intrusions are called aureoles the width of which (i.e., the dimension of metamorphosed rocks) depends upon mainly two factors e.g.:
(i) The temperature of intruding magma, and
(ii) The depth of magma intrusions in the crust.
Type # 2. Regional Metamorphism:
When the rocks are altered in their forms in extensive area the process is called regional metamorphism. Such metamorphism is also known as dynamic metamorphism because pressure plays dominant role in the alteration of the form of the rocks though temperature is also an important factor.
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The sedimentary rocks are folded due to compressive forces during the period of mountain building. This process results in intense pressure and heat which ultimately alter the original form of the concerned rocks. Dynamic metamorphism leads to crystallization in the rocks and if the rocks are already crystallized, they are recrystallized. Regional metamorphism is a characteristic feature of mountainous area.
Regional metamorphism is further divided into two sub-types viz.:
(i) Dynamic regional metamorphism, when the rocks are metamorphosed due to compressive forces and resultant high pressure caused by convergent horizontal movements (fig. 8.17), and
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(ii) Static regional metamorphism, when the rocks are metamorphosed at greater depth due to intense pressure and weight of overlying rocks (superincumbent load) (fig. 8.18).
Type # 3. Hydro-Metamorphism:
The alteration in the composition of the rocks due to hydrological factor takes place in a number of ways e.g.:
(i) When the chemically active water (solvent) passes through the country-rocks, there occur several chemical changes in the rocks due to varied chemical reactions,
(ii) The storage of immense volume of water in big reservoirs exerts high pressure on the underlying rocks and thus the rocks are altered in their forms due to pressure of overlying huge volume of water. Such type of metamorphism is known as hydrostatic metamorphism.
Type # 4. Hydro-Thermo-Metamorphism:
The minor alteration in the physical and chemical composition of the rocks caused by the weight and pressure of water mass and chemically active hot gases and water vapour is called hydro-thermo-metamorphism which is, in fact, geographically less important.