Question

Describe the nature and mode of origin of the chief types of rock at the earth’s crust. How will you distinguish them?

Answer 1

The earth’s crust is composed of rocks. A rock is made up by aggregate of one or more minerals. Rock may be hard or soft and in varied colours. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.

There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation. They are:

1. Igneous Rocks
2. Sedimentary Rocks and
3. Metamorphic Rocks

1. Igneous Rocks: The igneous rocks are formed when magma cools and solidifies. Igneous rocks form out of magma and lava from the interior of the earth, therefore, they are known as primary rocks. When magma in its upward movement cools and turns into solid form it is called igneous rock. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks. Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.

2. Sedimentary Rocks: The word ‘sedimentary’ is derived from the Latin word sedimentum, which means ‘settling’. All types of rocks of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. This process is called lithification. In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale, etc.

3. Metamorphic Rocks: The word ‘metamorphic’ means ‘change of form’. These rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.