ABSTRACT :
Welcome back to another interesting topic. This article will let you know all about “METAMORPHIC PETROLOGY”.It is also an important topic which you find everywhere in geology, with no delay let’s check out the topic.
HINT :
Metamorphic rocks are one of the 3 major rock types. It includes igneous rocks and sedimentary rocks.
DEFINITION :
Metamorphic rocks modify the pre-existing rocks under intense heat, pressure, and temperature.
ETYMOLOGY :
It is a greek word where “meta” means to change or turns and “morphe” means shape or form.
INTRODUCTION :
Metamorphic rocks are the changes from igneous rocks or sedimentary rocks. It is a cyclic process that can be observed in the “ROCK CYCLE”. We have already explained the rock cycle in the previous article igneous petrology.
Metamorphic rocks are generally formed from pre-existing sedimentary or igneous rocks at suitable conditions.
> Orthometamorphic rocks are those which are derived from igneous rocks.
> Parametamorphic rocks are those which are derived from sedimentary rocks.
»Let us know the origin classification, types, and uses of metamorphic rocks along with their texture and structure.
AGENTS OF METAMORPHISM :
These are the factors which places an important role occurring in rocks. Basically Change in the rock itself knows as metamorphism. The agents are the main cause and they are :
TEMPERATURE :
Rocks are normally made of minerals at a constant temperature at 100°c – 700°c later the metamorphism takes place due to various reasons and the temperature also raises to 1000°c – 1200°c. The mineral composition of the rock undergoes slight changes to adjust to the new temperatures.
The mentioned are the source of heat for metamorphism.
- Geothermal gradient (The internal heat).
- Magmatic heat (Around magmatic intruding).
- Radioactive disintegration.
PRESSURE :
Pressure is the dominating factor. Pressure occurs both vertically and horizontally. Based on this, Pressure is categorized into two;
1. Uniform Pressure :
It is also known as hydrostatic pressure. uniform pressure is due to the overlying weight of rocks vertically which can also change the mineralogical composition of the rock. This pressure acting vertically on rock is also called load metamorphism.Pressure increases with depth. It is also known as plutonic metamorphism.Ex:- GRANULITES
2. Directed Pressure :
It is also known as dynamic metamorphism or cataclastic metamorphism. It generally occurs near the earth’s surface due to orogenic movements.With increase in depth, Directed pressure decreases, and effect of uniform pressure increases directed pressure results in granulating the minerals and doesn't form any new minerals. It is due to orogenic activity occurring horizontally.Ex:- MYLONITES.
_Reginol metamorphism :
When both heat (temperature) and stress (pressure) dominates.
_Dynamothermal Metamorphism :
When the country rock is subjected to change both in mineral composition and texture.
CHEMICALLY ACTIVE FLUIDS :
Water passes through the pores of the rock and tends to change the actual composition of the chemicals. The intrusive igneous bodies within the country rocks are the source of these chemical agents. When the temperature rises, the pore fluids would expand and also may result in breaking the actual crystal boundaries.
Classification Of Metamorphic Rocks :
The metamorphic rocks are classified generally by various factors. This is the classification based on the presence or absence of layered structure also called foliation. It is of 2 types :
a) Foliated rocks :
It is produced during the reginol metamorphism. Foliated rocks are characterized by the parallel arrangement of the slaty minerals. This foliation is later changed into bands.
Ex :- GNEISSES, SCHISTS, SLATES, PHYLLITES, etc.
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| PHYLLITES |
b) Non – Foliated rocks :
It is produced by thermal metamorphism or contact metamorphism. Non-foliated rocks does not contain parallel arrangements of mineralogical constituents and layered structures of foliations. These rocks contain equidimensional mineral grains.
Ex :- MARBLE, QUARTZITE, HORNFELS, SOAPSTONE, AMPHIBOLES, etc
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| SOAP STONE |
METAMORPHIC GRADES :
The change of original rocks due to metamorphism is termed as metamorphic grades. Index minerals indicate the grade present in a set of minerals.The 3 grades are ;
a) Low Grade :
In this, the temperature ranges from 200°c to 400°c. The pressure range is high. The rocks are formed away from the intrusive igneous bodies.
Ex:- INDEX MINERALS – LAWSONITE, PREHNITE & LAUMONITE.
ROCKS – SLATE, PHYLLITE.
b) Medium Grade :
The temperature range of 650°c. Pressure is an important factor in determining the stability of various minerals formed in this grade.
Ex:- INDEX MINERALS – STAUROLITE, CORDIERITE.
ROCKS – SCHISTS.
c) High Grade :
Its temperature would be 580°c – 800°c above, under 3.5kb pressure. Rocks are formed close to their margin.
Ex :- INDEX MINERALS – HYPERSTHENE.
ROCKS – GRANULITES , GNEISS.
An increase in the grade of metamorphism is by the increase in grain size of rocks.
METAMORPHIC ZONES :
As temperature and pressure increase with increase in depth, the grade of metamorphism also varies with the level of depth. Based on this concept, zones of metamorphism is produced, There are 3 zones, They are
1.Epizone :
This zone of metamorphism takes place near the surface, with low temperature which is generally less then 300°c and has a strong shear stress cataclastic metamorphism takes place and rocks are mainly metamorphosed by dynamic metamorphism.
Ex:- SLATES, MICA, SCHISTS.
2.Mesozone :
This zone is present between epizone and katazone. Generally, reginol metamorphism takes place, with moderate temperature between 300 - 500°c. Dynamothermal metamorphism occurs in this zone.
Ex:- BIOTITE, GARNET, SCHISTS.
3.Katazone :
The bottom zone is known as katazone. It occurs in high temperatures with great depth hydrostatic stresses that are dominant in this zone. Plutonic metamorphism takes place.
Ex:- GNEISSES.