Classification of metamorphic rocks
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Classification of Metamorphic RocksA. INTRODUCTIONThe
Classification of Metamorphic Rocks
A. INTRODUCTION
The classification of metamorphic rocks is rather difficult because a particular metamorphic rock can be formed by the action of heat, pressure and chemical fluids on different pre-existing rocks. At the same time, one type of rock may give rise to a variety of metamorphic rocks under different kinds of metamorphism with varying degrees of metamorphism. For example, when argillaceous rocks (e.g.mudstone, shale or siltstone)/ are subjected to dynamothermal metamorphism, the low-temperature minerals like chlorite and muscovite are initially developed.
Characteristics of Contact Metamorphism
As stated earlier, the contact metamorphism involves the sharing of the heat and the hot fluids between the magma and the intruded rocks. The pressure (either directed or uniform) effects are almost negligible and recrystallisation is the major metamorphic effect. Hence, the calcareous rocks like limestone and the arenaceous rocks like sandstone also develop a granulose texture, when thermally metamorphosed. The limestone forms marble and the sandstone gets converted into a quartzite.
Characteristics of Cataclastic Metamorphism
As this metamorphism takes place when directed pressure leads to the crushing, fracturing and granulation of the affected rock, without appreciable rise in the temperature, the effects are largely structural, though they may be accompanied by some mineralogical: modifications. The harder minerals remain uncrushed while the less resistant minerals undergo grinding to form almost a powder.
Characteristics of Regional/ Dynamothermal metamorphism
The term regional metamorphism is applied to a metamorphism which affects wide areas years, predominantly the orogenic belts and continues for 100 to 150 million years. considerable rise in temperature and pressure. processes. The Pre-existing rocks are subjected to more or less complete recrystallisation and result in the development of the new structures. Foliation is characteristic feature of these rocks and the typical products are rocks like schists, gneisses, amphibolites and granulites.
Description of some metamorphic rocks
Slate
Argillaceous sediments such as shales, under the influence of high stress (directed pressure) with low to moderate temperature, are compressed and converted to slates, Minute flakes of chlorite and sericite grow with their cleavage surfaces at right angles to the direction of maximum compression, while the minerals like quartz recrystallise with their length parallel to it. The rock, therefore, possesses a preferential direction of splitting known as slaty cleavages (which is different from mineral cleavage),
Phyllite
Phyllite rock is developed when slate is further acted upon by continuing stresses and rising temperature. Minerals like chlorite and sericite continue to grow giving rise to large crystals of muscovite which are seen visible to the naked eye. The predominance of muscovite and chlorite imparts a bright lustre to the rock.
Quartzite and marble
During thermal metamorphism, the rocks are formed by the action of heat on the corresponding pre-existing rocks. Quartzite is formed from siliceous sandstone, banded haematite quartzite from ferruginous sandstone, marble from a limestone and dolomitic marble from a dolomite (Mg-carbonate).
Schist
Schist is a very common crystalline metamorphic rock/ generally consisting of medium sized mineral grains, which can be distinguished either with naked eye or with a hand lens. These rocks are formed by the dynamo-thermal or regional metamorphism of basic igneous rocks. A schist predominantly consists of dark minerals like biotite, hornblende and actinolite which are arranged parallel to each other, imparting schistosity to the rock.
e.g., mica schist, mica-garnet schist, hornblende schist and actinolite schist.
Gneiss
This type of rock characteristically develops due to the effects of dynamo-thermal or regional metamorphism on different rocks. They show banding of light and dark colours. The dark band is made up of minerals like biotite or hornblende, which are arranged parallel each other forming a schistose band, while the light coloured band is composed of quartz ( feldspathic minerals forming a granulose band. Due to alternation of schistose and granulose bands, the gneiss is resistant to breaking as compared to a schist.
Eclogite
It is a rock which is made up of equidimensional minerals like the pyroxenes and the garnets. It gives an equigranular aspect to the rock. It is formed from basic igneous rocks by the plutonic metamorphism at high uniform pressure and high temperature.
metamorphism,
type of metamorphism,
thermal metamorphism,
contact metamorphism,
regional metamorphism,
dynamothermal metamorphism,
slaty cleavage,
slate,
schist,
gneissose,
granulose,
maculose,
marble,
quartzite,
hornblende schist,
A. INTRODUCTION
The classification of metamorphic rocks is rather difficult because a particular metamorphic rock can be formed by the action of heat, pressure and chemical fluids on different pre-existing rocks. At the same time, one type of rock may give rise to a variety of metamorphic rocks under different kinds of metamorphism with varying degrees of metamorphism. For example, when argillaceous rocks (e.g.mudstone, shale or siltstone)/ are subjected to dynamothermal metamorphism, the low-temperature minerals like chlorite and muscovite are initially developed.
Characteristics of Contact Metamorphism
As stated earlier, the contact metamorphism involves the sharing of the heat and the hot fluids between the magma and the intruded rocks. The pressure (either directed or uniform) effects are almost negligible and recrystallisation is the major metamorphic effect. Hence, the calcareous rocks like limestone and the arenaceous rocks like sandstone also develop a granulose texture, when thermally metamorphosed. The limestone forms marble and the sandstone gets converted into a quartzite.
Characteristics of Cataclastic Metamorphism
As this metamorphism takes place when directed pressure leads to the crushing, fracturing and granulation of the affected rock, without appreciable rise in the temperature, the effects are largely structural, though they may be accompanied by some mineralogical: modifications. The harder minerals remain uncrushed while the less resistant minerals undergo grinding to form almost a powder.
Characteristics of Regional/ Dynamothermal metamorphism
The term regional metamorphism is applied to a metamorphism which affects wide areas years, predominantly the orogenic belts and continues for 100 to 150 million years. considerable rise in temperature and pressure. processes. The Pre-existing rocks are subjected to more or less complete recrystallisation and result in the development of the new structures. Foliation is characteristic feature of these rocks and the typical products are rocks like schists, gneisses, amphibolites and granulites.
Description of some metamorphic rocks
Slate
Argillaceous sediments such as shales, under the influence of high stress (directed pressure) with low to moderate temperature, are compressed and converted to slates, Minute flakes of chlorite and sericite grow with their cleavage surfaces at right angles to the direction of maximum compression, while the minerals like quartz recrystallise with their length parallel to it. The rock, therefore, possesses a preferential direction of splitting known as slaty cleavages (which is different from mineral cleavage),
Phyllite
Phyllite rock is developed when slate is further acted upon by continuing stresses and rising temperature. Minerals like chlorite and sericite continue to grow giving rise to large crystals of muscovite which are seen visible to the naked eye. The predominance of muscovite and chlorite imparts a bright lustre to the rock.
Quartzite and marble
During thermal metamorphism, the rocks are formed by the action of heat on the corresponding pre-existing rocks. Quartzite is formed from siliceous sandstone, banded haematite quartzite from ferruginous sandstone, marble from a limestone and dolomitic marble from a dolomite (Mg-carbonate).
Schist
Schist is a very common crystalline metamorphic rock/ generally consisting of medium sized mineral grains, which can be distinguished either with naked eye or with a hand lens. These rocks are formed by the dynamo-thermal or regional metamorphism of basic igneous rocks. A schist predominantly consists of dark minerals like biotite, hornblende and actinolite which are arranged parallel to each other, imparting schistosity to the rock.
e.g., mica schist, mica-garnet schist, hornblende schist and actinolite schist.
Gneiss
This type of rock characteristically develops due to the effects of dynamo-thermal or regional metamorphism on different rocks. They show banding of light and dark colours. The dark band is made up of minerals like biotite or hornblende, which are arranged parallel each other forming a schistose band, while the light coloured band is composed of quartz ( feldspathic minerals forming a granulose band. Due to alternation of schistose and granulose bands, the gneiss is resistant to breaking as compared to a schist.
Eclogite
It is a rock which is made up of equidimensional minerals like the pyroxenes and the garnets. It gives an equigranular aspect to the rock. It is formed from basic igneous rocks by the plutonic metamorphism at high uniform pressure and high temperature.
metamorphism,
type of metamorphism,
thermal metamorphism,
contact metamorphism,
regional metamorphism,
dynamothermal metamorphism,
slaty cleavage,
slate,
schist,
gneissose,
granulose,
maculose,
marble,
quartzite,
hornblende schist,
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