Geography [2 Mark Question Answer]

The earth is said to be 4700 million years old and the rocks came into existence 3400 years ago. Since then these rocks have undergone various changes by which multiple transformation took place within the rocks. This continuous process of transformation of old rocks into new rocks is known as rock cycle.

Chemically formed sedimentary rocks are formed through chemical action of water. Some mineral like limestones and salts are dissolved into the water and after the evaporation of water form a particular type of rock which is formed by the chemical sedimentation of limestones into gypsum and salt into various types of nitrates.

Chemically-formed sedimentary rocks are produced through evaporation of water from solutions containing minerals. In this ways a rock like ‘gypsum’ is produced. Other examples are nitrates and potash etc.

Rain water, wind, ice or the running water are the agents involved in the formation of sedimentary rocks. These agents break and carry on loose material and deposit it in various low lying areas to convert into the solid sedimentary rocks.

There are three types of sedimentary rocks on the basis of the nature of sediments, their origin, composition and mode of formation. These are mechanically-formed sedimentary rocks, organically formed sedimentary rocks and chemically- formed sedimentary rocks.

Fossils are remains of vegetation and animals buried under the sediments. The sedimentary rocks are stratified rocks and are found in layers. These fossils are preserved in between these layers. But in Igneous rocks, the fossils are destroyed due to high temperature of lava.

It is believed that the earth was in a molten state in the beginning Igneous rocks were the first rocks to be formed due to cooling and solidification of molten matter. Hence these are called Primary Rocks.

Properties of metamorphic rocks :

1. These rocks are harder and more resistant than the original rocks.
2. These rocks have their minerals arranged in a series of bands.

Continuous deposition of sediments of silt, soil, debris etc. by water, wind and ice along lower landforms and solidification into different layers result in the formation of light sedimentary rocks.

On the basis of origin igneous rocks are divided into two groups i.e. Extrusive and Instrusive igneous rocks. Extrusive igneous rocks are found on the surface of the earth by cooling down of erupted magma. Intrusive igneous rocks are formed beneath the earth’s crust after solidification of magma.

Igneous rocks are formed by melting and cooling of magma originated from volcanic eruptions. These may be formed by diastrophism and volcanism. These rocks are strong, crystalline and dark in colour. These may be extrusive and intrusive on the surface and beneath the crust respectively.

It is the process of transformation of sedimentary and igneous rocks into metamorphic rocks through extreme temperature and compression by volcanism and diatrophism by physical and Chemical changes.

Laccoliths is formed by the intrusion of magma in the erupting channel just below the crust. Batholiths are deep seated dome shaped formation of magma intrusion, generally forming the base of mountain ranges, e.g., Ranchi Batholiths.