Contents: •Introduction •Agents of metamorphism •Types of metamorphism

•Thermal metamorphism

•Cataclastic metamorphism

•Dynamo thermal metamorphism

•Plutonic metamorphism

•Conclusions

•References

Introduction Metamorphism may be defined as the response in the solid rocks to pronounced changes of temperature , pressure and chemical environment.

The rocks which subjected to metamorphism, may be either igneous , sedimentary, or other metamorphic rocks.

The changes in three factors of temperature pressure and chemical environment, upset the physical and chemical equilibrium of a mineral assemblages and effort to establish new mineral equilibrium.

Agents of metamorphism: The agents which are mostly responsible for bringing about metamorphic changes are

1).Temperature 2).Hydrostatic (or)uniform pressure 3).Directed pressure4).Chemically active fluids

Agents of metamorphism: The agents which are mostly responsible for bringing about metamorphic changes are

1).Temperature 2).Hydrostatic (or)uniform pressure 3).Directed pressure4).Chemically active fluids

Agents of metamorphism: The agents which are mostly responsible for bringing about metamorphic changes are

1).Temperature 2).Hydrostatic (or)uniform pressure 3).Directed pressure4).Chemically active fluids

Volume changes

Agents of metamorphism: The agents which are mostly responsible for bringing about metamorphic changes are

1).Temperature 2).Hydrostatic (or)uniform pressure 3).Directed pressure4).Chemically active fluids

Changes in shape

Agents of metamorphism: The agents which are mostly responsible for bringing about metamorphic changes are

1).Temperature 2).Hydrostatic (or)uniform pressure 3).Directed pressure4).Chemically active fluids

•Fluids present in the original rock or introduced from the magma, create metamorphic rocks with water and gas molecules

•Carbon dioxide, hydro-fluoric acid, bromine, fluorine and some other gases may present and can induce many changes in rocks

Types of metamorphism: •Depending upon the dominance of one or more agencies , the process of metamorphism can be broadly classified into several type

Thermal metamorphism

Cataclastic metamorphism

Dynamo thermal metamorphism

Plutonic metamorphism

Hydrothermal metamorphism

Burial metamorphism

Shock metamorphism

Thermal metamorphism: Temperature dominated, pressure and fluids place only secondary role

Pyrometamorphism:

This metamorphism resulted by the effect of highest degree of heat, without fusion. The original minerals are forced to recrystallise or re rearrange themselves.

Optalic metamorphism:

The indurating burning and fritting effects of lava-flows on neibouring rocks is known as caustic or optalic metamorphism.

Kimmeridge on the Dorset coast of England

Burned shale

Contact metamorphism:

Contact metamorphism caused due to local heating of rocks by the intrusion of hot igneous body to the surrounding rocks

Pneumatolytic metamorphism Injection metamorphism

The Purcell Sill, in Montana.

Intruded Igneous rock

Marble

Lime stone

Thermal metamorphism

Cataclastic metamorphism:

It is a type of metamorphism in which only the directed pressure plays dominant role. They operate mainly in the upper part of the earth’s crust where the temperature is moderately low.

Due to this metamorphism the rocks are crushed, grained ,and deformed. New rocks thus formed are called cataclastic rocks.

Some times parallel lenticular and banded structures forms by cataclastic metamorphism.

Cataclastic metamorphism

Conversion of shale into slate

Devils Punchbowl County Park, southern California.

Crush breccia

Shattered and pulverized due to directed pressure

Reru valley in the Zanskar range. This range is located in the north. East of India in the state of Jammu and Kashmir

Mylonite

Augen structure

Dynamo-thermal metamorphism:

Dynamo-thermal metamorphism is due to the co-operation of directed pressure and high temperature.

High temperature facilitates the recrystallisation, directed pressure facilitates deforming effects.

New parallel textures and structures are usually oriented perpendicular to the direction of greatest stress

both temperature and directed pressure

regional metamorphism

Clay Regional metamorphism Mica schist

schist

Acid igneous rocks Regional metamorphism Schist (or) gneiss

Flaser granite

Augen structure

Meta-conglomerate

Plutonic metamorphism:

The plutonic metamorphism is meant the changes which are produced in rocks by great heat and uniform pressure. The changes necessarily at depths where the directed pressure become less.

Rocks formed from this metamorphism give place to granulose, even grained, direction less structures.

Formation of anti-stress minerals of small specific volume and high density, will be favored by these conditions.

Rocks from locality at Kodaikanal,

Eclogite

The specimen from Queen Canyon in the northern White Mountains of California.

sericite

Hydrothermal metamorphism:

Hydrothermal metamorphism is defined as the alteration due to the super-heated fluids passes through the permeable rock.

Chapel Hill - Hillsborough area, Florida

composted of sericite and quartz with red and yellow mottling.

Burial metamorphism:

Sedimentary rocks buried more than a few kilometers, experience not only an increase in pressure but a gradual rise in temperature resulting in formation of metamorphic rocks.

Some times processes such as compaction, solution, cementation, accompanied by chemical changes.

At low temperature At high temperature

When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock. These ultrahigh pressures can produce metamorphic changes.

Shock metamorphism:

Northwest Territories of Canada

Shatter cones in limestone

conclusions: Different structural, textural, mineralogical changes due to

metamorphism is depends on the metamorphic agents.

Orogenic activities plays an important role in the metamorphism

During the metamorphism recrystallisation takes place and new minerals and new textures may produced.

Minerals and rocks are stable only under the conditions at which they form.

Each rock can be metamorphosed by several different combinations of heat, pressure and fluids.

References : 1)G.W.Tyrrell(1958), Principles of petrology Publishers Chapman and Hall, P:105-1202)Donald.W.Hyndman(1972), Petrology of igneous and metamorphic rocks, Published by McGraw-Hill, P:541-545

3)P.K.Mukerjee(1931), A Test book of Geology Published by Sri D.Chakraborty, P:122-129.