Brief notes on the Classification of Metamorphic Rocks

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The third great group of rocks is called the metamorphic rocks. The term metamorphic means transformation of pre-existing rocks (igneous, sedimentary or even metamorphic rocks) into new types by the action of heat, pressure, stress, and hot and chemically active migrating fluids (magma).

To quote Arthur Holmes, “When crystal rocks are buried and come under the influence of any combination of the following:

(a) The intense pressure or stress differences set up by gravity in association with other processes responsible for earth movement;

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(b) The increased temperature associated with nearby igneous activity, or caused by internal friction or through the passage of hot gases;

(c) The chemical changes stimulated by the through-passage of chemically active hot gases and liquids- they respond by changes in structure and mineral composition and so become transformed into (altogether) new types of rocks”.

It was Lyell who gave the name ‘metamorphism’ in 1833 to the chief processes responsible for the rock transformation.

The internal processes of the earth such as heat, pressure and earth movements that bring about the re-crystallization of rocks, with or without change of composition are included in ‘metamorphism’.

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In fact, metamorphic processes consist of the thermal, dynamic and geo- chemical and their various combinations.

Metamorphism may be classified as under:

(a) Thermal or contact metamorphism.

(b) Regional metamorphism.

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Thermal or contact metamorphism:

Thermal metamorphism is caused by the molten magma rising through the earth’s crust giving off heat, and also by the solutions and gases that can modify the country rock already present.

The major effect of metamorphism is to wholly or partially melt the rock being affected. By this process the mineral crystals are re-crystallized. Such metamorphism produces rocks whose minerals are segregated in wavy bands.

This effect is known as foliation. In case the banding is very fine, the individual minerals show flattened, platy structure. The rocks tend to flake along these bands. Such rocks are called schists.

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Where the bands are broad, the altered rock is known as gneiss. Granite being coarse-grained is changed into gneiss. Shale, a fine grained sedimentary rock is metamorphosed as schist.

Some shales are changed into another metamorphic rock called slate which is more massive. It has a tendency to break apart along flat surfaces.

Since contact metamorphism is on a more local scale than those produced by earth movements, it is also known as local metamorphism. It is important to note that under the impact of heat and pressure, the metamorphic rocks become harder, denser and darker.

As a result of metamorphism new minerals are produced, and the rock texture becomes entirely changed. Sometimes the igneous intrusion brings about changes in the margins of the surrounding rocks, due to intense heat.

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Such margins of the metamorphosed rocks are called aureoles where new mineral are formed. Metamorphic rocks of this kind are called hornfels which consists of a mosaic of grains.

Regional Metamorphism:

When rocks throughout an extensive region are transformed by all the agencies of metamorphism, it is known as regional metamorphism. Regional metamorphism is closely associated with mountain building and intrusive movement.

When igneous as well as sedimentary rocks are buried deep in the earth’s crust in the orogenic belt, the intense heat and enormous pressure caused by compressional forces during the period of mountain building alter their original form.

The resulting metamorphic rocks are entirely changed in appearance and composition. Such rocks are much harder and more compact than their parent types.

However, this is more applicable in case of pre-existing sedimentary rather than igneous rocks. The moving tectonic plates also cause metamorphism of the pre-existing rocks.

The regional metamorphism may be divided into two sub-types, e.g. dynamic meta­morphism, and static regional metamorphism.

In the former type the high pressure caused by the compressive forces plays dominant role, while in the latter type the rocks at greater depths are metamorphosed due to intense pressure exerted by the huge mass of overlying rocks.

However, such a classification appears to be rather superficial, because in both the cases heat and pressure, more or less act together.

It may be noted that the most characteristic feature of the metamorphic rocks is the presence of parallel arrangement of their constituent minerals, all of which have their longer axis pointing in the same direction.

This type of structure is known as foliation. The courses grained metamorphic rocks in which foliation are not perfectly developed are known as gneisses, whereas fine-grained metamorphic rocks, in which more perfect parallel structures are present, are known as schists.

Fine-grained clayey rocks when metamorphosed under heat and pressure are converts into slates characterised by the presence of closely-spaced parallel planes of splitting known as cleavage.

Slates may be regarded as a special type of fine-grained schist. Metamorphic rocks are classified into two broad groups: (a) Foliated and (b) Non-foliated.

Some common metamorphic rock types:

Gneiss (pronounced as nice) – Gneisses are among the most abundant metamorphic rocks. They represent a high grade of metamorphism. Various acid igneous rocks such as granites and rhyolites are morphosed into gneisses. They are coarse-grained and crystalline.

They are characterised by well-marked foliation resulting from the layers of various mineral groups. The banding is the result of the separation of light and darker minerals in crude layers, particularly quartz and feldspars.

Gneisses may also be formed from clastic sedimentary rocks such as shales. There are wide variations in the structure, mineralization composition an appearance of gneisses.

Schist:

Schist’s are the metamorphic rocks which are medium-grained in which separate grains are visible. They are derived from fine grained shale and basalt.

In this type of metamorphosed rock foliation is due to the parallel arrangement of various platy minerals such as micas, graphite and talc. Besides, quartz, sodium plagioclase and feldspar enter into its mineral composition. Shale’s can give rise to mica-schist or horn-bende schist.

Slates:

Slates are metamorphosed form of fine-grained shale under conditions of low temperature and low pressure. Foliation is well developed in a very fine-grained rock which is not crystalline. This is called slaty cleavage.

Under increased heat and pressure slate may convert into schist. The most important quality of slate rock is that it splits into thin plates which are used for roofing purposes. It is highly durable.

As regards its mineral composition it consists of clay minerals, chlorite and some mica. Slate is found in a variety of colours. This rock is generally black or gray in colour. Such rocks are dense as well as brittle.

Slate is closely associated with mountain building. So that it generally occurs in mountain regions. It may be pointed out that slates are harder and more resistant to weathering and erosion than the un-metamorphosed original rocks.

Marbles:

Marbles are the metamorphosed form of limestone, calcite and dolomite. They belong to the group of non-foliated metamorphic rocks. The crystals are generally large and interlocked to form a dense crystalline rock.

Because of the presence of impurities they are of different colours such as, white, pink, brown or blue-gray. If the limestone does not contain impurities, it becomes white marble after it is metamorphosed.

Marbles are formed due to contact or thermal metamorphism which is associated with volcanic activity.

Limestone’s are also converted into marbles under the impact of dynamic/regional metamorphism Marble rocks at Bheraghat, Jabalpur (Madhya Pradesh) are characterised by different colours such as, white pink and gray.

Marble rock is well-known for its softness and its effervescence with hydrochloric acid. Remember that the Taj Mahal, well-known for its beauty, was built of pure white marble obtained from the marble quarries of Makrana, Rajasthan.

Quartzites:

Sandstone is metamorphosed into quartzite. It is the most resistant type of rock. Quartz is the most dominant mineral in the mineral composition of quartzite. This rock is medium-grained.

As regards the mineral composition, quartzites are composed of re-crystallized quartz, feldspars and sometimes minor muscovite. Conglomerate and siltstone are also metamorphosed into quartzite.

During the process of metamorphism, the open spaces within the above- mentioned sedimentary rocks are filled with silica.

This type of rock is un-foliated because quartz grains do not form platy crystals. Pure quartzite is while in colour, but impurities like iron oxide or some other minerals often lend various grades of red, brown, green and other colours.

Amphibolite:

Amphibolite is a coarse-grained metamorphic rock. When basalt, gabbro and other rocks that are rich in iron and magnesium are metamorphosed, they form amphibolite.

Its mineral composition consists of minerals such as mica, quartz, garnet etc. However, in case mica and other platy minerals are present in this metamorphosed rock, it is characterised by foliation.

Hornfels:

When rocks like shale come into contact with molten magma a fine-grained, non-foliated metamorphic rock is formed. It is hard as well as dense. They are generally dark coloured and resemble limestone or basalt.

Salient features of metamorphic rocks:

1. Metamorphic rocks are formed by the metamorphism of all the three types of rocks, e.g. igneous, sedimentary and even metamorphic.

2. Metamorphism brings about changes in the texture and mineral composition of pre­existing rocks.

3. Metamorphic rocks are harder and more resistant to erosion than their parent rocks.

4. Metamorphic rocks are characterised by the complete absence of fossils, because fossils embedded in the sedimentary rocks are destroyed by intense heat and pressure.

5. Metamorphic rocks such as, mica-schist split along the bedding planes.

6. Coarse-grained metamorphic rocks are almost non-foliated, e.g., gneiss derived from granites. On the contrary, fine-grained metamorphic rocks (schist derived from shale’s) are marked by perfect foliation.

7. Some of the metamorphic rocks are impervious. They are marble and slate. Gneiss, a type of metamorphic rock, is pervious.

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