A rock is an aggregate of one or more minerals. The cooling of extremely fluid and hot liquid Magma forms igneous rocks.
Igneous rocks can be formed by two types of processes:
- Directly by cooling of magma in the interior of the earth.
- By cooling of lava above the surface of the earth when lava gets poured on the surface.
As the Igneous Rocks form the earth’s first crust and all other rocks are formed from these rocks, they are also known as the Primary Rocks or the parents of all rocks.
Characteristics of Primary Rocks:
- They can be easily identified with their texture, density, colour, and mineral composition. Its texture depends on the shape, size, time period to cool down and solidify, and the arrangement of crystals in the rock.
- Fossils are not found in these rocks. Fossils get destroyed due to the sheer heat these rocks produce.
- Usually do not react with acids. Comparatively fine grained and smooth textured.
- They are mostly hard rocks as water percolates with difficulty.
On the basis of location of occurrence, these rocks can be classified as Intrusive and Extrusive Igneous Rocks.
(A). Intrusive Igneous Rocks:
- 1. These are formed when magma cools and solidifies below the earth’s surface.
- 2. The rate of cooling below the surface of the earth is very slow, contributing to the development of large crystals in the rocks. E.g.: Granite, dolerite, etc.
(B). Extrusive Igneous Rocks:
- 1. These are formed on the earth's surface by the cooling of the lava.
- 2. The mineral crystals of extrusive igneous rocks are very fine as lava cools on the surface very rapidly. These rocks are also called Volcanic Rocks. E.g.: Basalt, Gabbro.
Hence, primary rocks are relevant in the southern part of India. Deccan Trap is a finest example.