What is Metamorphism?
Metamorphism is the mineralogical and structural adjustment of solid rocks to physical and chemical conditions which have been imposed at depths below the surface zones of weathering and cementation, and which differ from the conditions under which the rocks in question originated. Thus metamorphism is the response of the solid rocks to pronounced changes of temperature, pressure and chemical environment.
What happens when metamorphism takes place?
Metamorphism stands midway between diagenesis and general melting of rocks. Important features of metamorphic changes:
i) The bulk chemical composition of the metamorphic rock is the same as that of the rock from which it formed. Thus metamorphic changes are isochemical changes.
ii) The structural and textural characteristics of the metamorphic rocks are the outcome of the structure and texture of the pre-existing rocks and the temperature-pressure condition of the metamorphic changes.
iii) The changes in metamorphism take place is an essentially solid medium.
Metamorphism occurs for which class of rocks?
Metamorphic rocks which are derived from igneous rocks are known as Orthometamorphic rock, and those which are derived from the sedimentary rocks, are known as para metamorphic rocks.
Which agent of metamorphism can cause?
The agents which are mostly responsible for bringing about metamorphic changes are as follows:
Temperature – Metamorphism
It may be supplied by geothermal gradient, magmatic heat, frictional heat and by radioactive disintegration. The temperature range within which metamorphic changes take place is from 200 to 700 degree C. However, in certain cases, a temperature of 1000 to 1200 degree C may be encountered.
Temperature accelerates the process of reaction, increases the volume of the rocks, remove volatiles and moisture contents of the rocks.
The following types of metamorphism are said to be the result of temperature effect on rocks.
Metamorphism occurs at temperatures between
i) Pyrometamorphism. At 800 to 1000 degree C, in the immediate vicinity of intrusives.
The induration, backing, burning and fritting effects of lava flows and intrusions on neighboring rocks is known as Caustic metamorphism or optalic metamorphism.
ii) Contact metamorphism. It occurs around larger intrusives at comparatively low temperature. it includes:
a) Normal contact Metamorphism. Where rocks are simply crystallized without new mineral formation.
b) Pneumatolytic, additive or metasomatic. The composition of the rocks is vastly modified depending on the addition of material from magmatic emanations.
c) Injection metamorphism. Here with the intrusion of magma or its residual liquid may alter the intruded rock substantially.
iii) Auto metamorphism. It is the mineralogic readjustment of an igneous assemblage to the falling temperature as the body of the igneous rock cools. It includes uralitization, serpentinization etc.
iv) Retrograde metamorphism. Also known as diaphoresis, where mineralogical rearrangement of high-temperature assemblage to a low-temperature one take place.
Metamorphism due to pressure
a) Uniform pressure. It is the hydrostatic pressure which increases with depth. Uniform pressure and temperature can both dominate together at great depths. there is a reduction in the volume of the rock and a change in the mineralogical composition. It is known as Plutonic Metamorphism, e.g., Granulites, Eclogites.
Load Metamorphism. It is due to the vertically acting stress of superincumbent rock masses aided by high temperature.
b) Directed pressure. It is produced mostly by orogenic movement. It dominates at or near the surface. It results in crushing and granulation of minerals, without the formation of any mineral. It is also known as dynamic metamorphism or cataclastic metamorphism, e.g., Mylonites.
Water both heat and stress dominates, the metamorphism is known as Regional Metamorphism. here the country rocks are subjected to changes both in mineral composition and texture. It is also known as dynamo thermal metamorphism.
Chemically active fluids.
These are from the following sources:
a) Meteoric water.
b) Juvenile water.
Water carries minerals in some cases in solution and also serves as a medium in which chemical changes occur with ease. Chemical activity is more pronounced in the vicinity of the igneous intrusions.
Grades of Metamorphism
The degrees of metamorphism or grades depend upon the extent to which the agents were in operation during the process. According to the temperature, pressure condition, there are usually three grades of metamorphism and accordingly, there are three zones:
1 Epizone. It is the zone of low-grade metamorphism, where temperature ranges from 100 to 300 degree C, pressure is low to moderate. It is characterized by the presence of the hydrous minerals.
Sericite, muscovite, chlorite, biotite, talc, actinolite, epidote, andalusite etc.
Rocks. Slates, phyllites, chlorite-schists, muscovite-schists, biotite-schists.
2. Mesozone. This is the zone of medium grade metamorphism, where the temperature ranges from 300 to 500 degree C, pressure is moderately high. It occurs at an intermediate depth, i.e., between 5 to 10 miles.
Minerals. Biotite, andalusite, cordierite, quarz, hypersthene, almandine, orthoclase. ilmenite etc.
Rocks. Phyllites and mica-schists.
3. Katazone. It is the zone of high grade metamorphism where the temperature ranges from 500 to 650 degree C, pressure is high. It occurs at a depth of 9 to 13 miles. It is characterized by anhydrous and anti-stress minerals.
Minerals. Biotite, alkalifeldspar, plagioclase, quartz, garnet, silimanite, kyanite, etc.
Rocks. Gneisses of various types, hornfels etc.
In case of Regional metamorphism, the following grades have been identified:
- Zone of Chlorite
- Zone of biotite
- Zone of Garnet
- Zone of staurolite
- Zone of kyanite
- Zone of sillimanite
These zones are according to the progressive grade of regional metamorphism.