Classification of Metamorphic Rocks | Geography

Metamorphic rocks are classified on the basis of foliation into the following types: 1. Foliated Metamorphic Rocks 2. Non-Foliated Metamorphic Rocks.

1. Foliated Metamorphic Rocks:

i. Slate:

Slates are formed due to dynamic regional meta­morphism of shales and other argillaceous rocks. Slates are characterized by the ‘presence of numerous closely- spaced parallel planes of splitting or cleavage’ but the splitting planes of slates are not parallel to the bedding planes rather they form angle with the bedding planes.

Sometimes the angle between the splitting planes and bedding planes becomes obtuse angle. Such structure of slates is known as slaty cleavage (fig. 8.19) which is formed due to compressive pressure exerted on the rocks. The cleavage is always at right angle to the direction of compression. Slates, if subjected to further intense metamorphism due to immense compression, are changed to phyllites or fine-grained mica-schist.

‘Slates, in fact, may be regarded as a special type of fine-grained schist’ (S. W. Wooldridge and R.S. Morgan), Slates are not as much resistant to erosion as are schists and gneisses. They are of varied colours.

ii. Gneiss:

Gneisses are coarse-grained metamorphic rocks which are formed due to metamorphism of conglomer­ates (sedimentary rocks) and coarse gained granites (igneous). Feldspar is the most dominant mineral of gneisses. Like schists, gneisses are also foliated rocks but the foliation is open and is sometimes absent.

There are several types of banded gneisses, which sometimes pass into augen gneiss. The process of granitization or granitification means the transforma­tion of mica-schist to gneiss. Gneissic rocks produce, after weathering and erosion, rounded topography.

iii. Schist:

Schists are fine grained metamorphic rocks and are characterized by well-developed foliation. The word schist has been derived from French word schiste and German word schistose which means to split. When shale sedimentary rocks are subjected to intense compressive force and consequent folding and pres­sure, the clay and other minerals of the original shale rocks are changed to mica minerals due to high pres­sure and temperature and thus shales are changed to schists.

During the process of regional metamorphism the schists get foliated. Schists are named on the basis of dominant minerals e.g., mica-schists, hornblende schists, quartz schists etc.

Mica-schist is the comonest type of schist rocks because it is formed from argillaceous shale sedimentary rock which is a very common rock and is abundantly found on the earth’s surface. Mica- schist is composed of muscovite, biotite, plagioclase and sometimes garnet. Hornblend schists are formed from basaltic rocks and contain hornblende, plagioclase and some quartz minerals.

Green schists are composed of green minerals such as hornblende and chlorites, provided that the rocks are well foliated. If the schists rich in green minerals are poorly foliated, they are called greenstones. The term metabasite is used to name those schists which are formed from basalts or dolerites.

2. Non-Foliated Metamorphic Rocks:

i. Quartzites:

Quartzites are generally formed from sandstones which are dominated by the abundance of quartz min­eral. During the process of metamorphism the voids within the sandstones are compacted due to excessive compression and heat and are also filled with silica, with the result quartzites become very hard and resist­ant to erosion.

When quartzites lie over weaker sedi­mentary rocks like shales or limestones as caprocks, they form stupendous wall-like escarpments. Kaimur escarpment along the left bank of the Son river (in M.P. and Bihar), Bhander escarpments (Satna and Panna districts of M.P.), Rewa escarpments facing the Ganga plains etc. have been formed due to resistant caprocks of quartzitic sandstones resting over shale lithology.

‘The term quartzite is also extended to sandy rocks which have been subjected to cementation by silica deposited from solution. Such rocks are generally softer than the true metamorphic quartzite’s and often behave more like normal sandstones, breaking down into sandy soils’.

ii. Marbles:

Marbles are generally formed due to changes in limestones because of temperature changes. Lime­stones are transformed into marbles due to contact thermal metamorphism during volcanic activity. Lime­stones are also metamorphosed due to dynamic re­gional metamorphism wherein calcium carbonates and other finer particles are changed into calcite.

In fact, the metamorphism of limestones to marble involves a number of changes in the mineralogical characteristics of limestones. For example, the reaction between cal­cium carbonate of limestone during the process of metamorphism produces a new mineral known as wollastonite or calcium silicate.

The colour of marbles depends upon the nature of parent limestones. If the original limestones are devoid of any impurities, the resultant marbles become pure white in colour. The colour changes due to impurities of other materials in the parent limestones.

The marbles of Carrara region of Italy are pure white while the marbles exposed along both the banks of magnificent and stupendous gorge of the Narmada river at Bheraghat near Jabalpur (M.P.) show different grades of colour though white and pink colours dominate.

Dolomites and chalks are also metamorphosed to marbles due to excessive heat but these have only local importance. Marbles are more resistant to erosion than their parent limestones. Besides, they are eco­nomically valuable rocks because they are used as building materials for the construction of very impor­tant buildings as monuments. For example, Tajmahal of Agra and Dilwara temple of Mount Abu (Rajasthan) have been built of marbles.