Classification of Rocks | Petrology | Geology

The rocks can be classified into two main varieties, namely, Igneous rocks and Sedimentary rocks. But when these rocks are subjected to prolonged fluctuations of temperature and pressure, they are transformed to a new variety which is termed as Metamorphic rocks.

1. Igneous Rocks:

A change of temperature caused the formation of igneous rocks. It has already been noted that the earth, at its beginning was in a gaseous state. The loss of heat through radiation and gradual cooling led to solidification and hardening of the upper portion. Thus igneous rocks came to be formed in the past. The upper part was the first to solidify leading to the formation of igneous rocks. These are known as Primary rocks.

Further, since igneous rocks are not formed as layers or strata, these are also called unstratified rocks. Igneous rocks are formed even today in different regions of the earth and in different parts of the earths’ interior.

For instance, though the materials deep inside the earth are more or less in hardened state, these melt and turn liquid if for some reason the pressure from above decreases. These materials then wants to come out. That liquified materials of the interior is called magma.

As magma cools down with the passage of time it turns into rock. The rocks formed from magma are known by different names in different parts. The magma which escapes to the surface of the earth is called lava. The lava solidifies very quickly due to contact with the atmosphere. This type of rock is called extrusive igneous rock.

Due to rapid cooling, the minerals found in this type of rock solidify into very small-size crystals. Basalt is a rock of this type. It occurs widely in the south-western part of Deccan plateau and the Rajmahal Hills of Bihar.

At times the magma materials cannot escape out to the surface. These then get deposited in layers inside the earth’s crust which slowly cool down, and turn into rock. This type of rock is known as intrusive igneous rock. When magma solidifies at a considerable depth below the surface, it cannot lose heat easily through radiation. Hence, it cools down very slowly.

This slow cooling helps the formation of large crystals of various minerals in the rock. This type of rock is also known as plutonic rock. Granite and Dolerite are two examples of intrusive igneous rock. In India there are abundant deposits of such rocks in areas like Raniganj, Rajasthan, Bihar, Madhya Pradesh, Tamil Nadu, Karnataka, Meghalaya and Assam. In Assam, these rocks occur abundantly in and around Guwahati and North Cachar Hills.

On the basis of location, size and shape, intrusive igneous rocks are called by different names:

(а) Batholith:

Batholiths are the largest intrusive igneous rock bodies. They may be 50-80 km across and many hundred km in length. They are so huge that their bottoms are not visible. They form the core of the world’s mountain systems.

(b) Laccoliths:

Laccoliths are formed between horizontal layers of sedimentary rocks near the earth surface. They are also dome-shaped and take the shape of a mushroom. They are also of huge size and form the reservoir of magma producing eruption.

(c) Dykes:

Dykes are near vertical formation from a few metres to kilometres in length. They come into existence when liquid molten materials pass through cracks and get solidified into rocks.

(d) Sill:

Sometimes molten materials get solidified into a thin sheet in horizontal position between two rock layers. Such an intrusion is termed as sill.

(e) Stocks:

Batholiths of smaller size having somewhat rounded shape are called stocks.

2. Sedimentary Rocks:

With the passage of time, the primary rocks on the surface of the earth are weathered and reduced to bits and pieces by the physical and chemical actions of heat, cold, wind, rain, glacier, sea wave, etc. These broken down or weathered rock particles are carried away by river, wind, glacier, etc., to the oceans or lakes where these are deposited at the ocean floor of lakebeds.

Generally, the bigger and heavier particles are deposited below the finer particles. Due to pressure and chemical reaction, these sediments are turned into rock. This type of rock is called Sedimentary rock. Since the sediments are deposited in layers or strata it is also called stratified rock.

Sometimes the remains of animals and plants get embedded in the sediments and become rock-like substance due to pressure and chemical change. The remains are called fossils.

Sedimentary rocks which carry fossils in them are called fossiliferous rocks. Sandstone and limestone are two examples of sedimentary rock. Nearly 75 per cent of the earth’s landmass is made-up of sedimentary rocks. Sandstone deposits occur in many parts of Himalayas and the Aravalli Range of Rajasthan.

There are rich deposits of limestone in Cherraphunji area of Meghalaya and in North Cachar and Karbi Anglong District of Assam. The river valleys are generally made up of sedimentary rock deposits. Coal and mineral oil are two very important resources that are derived from the sedimentary rock deposits.

Although 75 per cent of the earth’s surface is covered with sedimentary rocks, they make up only about 5 per cent of the volume of the earth’s crust. Out of the total sedimentary rocks 8 per cent is limestone, 12 per cent sandstone and the rest 80 per cent shales.

Sedimentary rocks, although derived from pre-existing rocks, it is interesting to know that some of them are older than either igneous or the metamorphic rocks as found in Greenland in 1973.

3. Metamorphic Rocks:

At times, movements inside the earth cause sudden and marked change of pressure and temperature. These induce chemical and physical changes in igneous and sedimentary rocks. As a result, these rocks are transformed into a new kind of rock called Metamorphic rocks. This way, granitic igneous rocks are converted to gneiss and schist.

Sedimentary rocks such as sandstone and limestone are transformed into quartzite and marble respectively. Again mudstone, which is a sedimentary rock, is transformed into a metamorphic rock known as slate. In India, marble deposits are found in Rajasthan and Madhya Pradesh; quartzite in Rajasthan, Bihar, Madhya Pradesh, Tamil Nadu, etc.; and slate in Himachal Pradesh, Haryana, Bihar, etc.

Limited deposits of slate are found in Meghalaya, Nagaland, and Mizoram. Graphite which is a metamorphic rock formed of coal is found in Orissa and Andhra Pradesh.

The metamorphic rocks are recognised by their great hardness, specialised structures and interlocking crystals. In the process of change, new and precious minerals like gems, rubies and sapphires are born, their sizes are enlarged and crystals are re-arranged.