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India: Geological Structure

India: Geological Structure


INDIA: GEOLOGICAL STRUCTURE

It is essential to have a knowledge of geological time period before the study of India's geological structure. The geological history of the earth is divided into five main eras-Azoic (non-living era), Paleozoic, Mesozoic, Cenozoic and Neozoic era. The Azoic era is the era of the origin of the solid outer crust of the earth. At that time the continental part of the earth (Pangaea) was formed by the silica rich granite rocks of less density. The above-mentioned eras have been divided into several periods which can be seen in the following chart :

Era

Epoch

Period

Time of beginning of the era

1. Azoic

-

1. Pre-Cambrian or, Alogonican
2. Archaean

-

2. Palaeozoic

Primary 

1. Cambrian
2. Ordovician
3. Silurian
4. Devonian
5. Carboniferous
6. Permian

600 million years ago

3. Mesozoic

Secondary

1. Triassic
2. Jurassic
3. Cretaceous

225 million years ago

4. Cenozoic

Tertiary

1. Eocene
2. Oligocene
3. Miocene
4. Pliocene

70 million years ago

5. Neozoic


 

Quaternary

1. Pleistocene
2. Holocene (Modern age)

One million years ago


 
  • Pangaea was formed in the Azoic era. There is no evidence of the origin of organisms in this era.
  • The Silurian period is known as the period of vertebrates. Molluscs, corals and sharks originated in this period. 
  • The breaking up of Pangaea began in the Carboniferous period.It was divided into two parts because of the forces of gravity and buoyancy. The northern part was called Laurasia and the southern part, Gondwanaland. The middle part between these two changed into the Tethys Sea.
  • In the Jurassic period, Gondwanaland was broken up into the peninsular India, Madagascar, Australia, Antarctica etc.

The knowledge of the nature of rocks found in different parts of a country can be had by studying the geological structure of that country. Sedimentary rocks are found in the land formed by deposition of sediments from which fertile soil is made e.g., the Gangetic plain. Inversely, the soil made of old crystalline rocks is infertile, however, these rocks are important because of the presence of metallic minerals (iron, gold, manganese etc). Since there is a continuous deposition of animal remains between the deposited sedimentary rocks in the oceans, these areas provide the possibility of getting petroleum deposits, e.g., the Gulf of Khambhat, the Bombay High etc.

                         Both the oldest and the latest rocks are found in the geological structure of India. The oldest rocks of the Archaean period are found in the peninsular India which is a part of the oldest landmass Pangaea on one hand, while there is an abundance of the latest sedimentary rocks of the Quaternary epoch in its plains on the other.

Classification of the Indian Rocks

(1) Rocks of the Archaean System

  • These rocks have been formed as a result of the hot-molten earth becoming cold. These are the oldest and primary rocks. Their original form has been destroyed because of too much metamorphosis.
  • There are no fossils found in them.
  • Gneiss is formed because of the metamorphosis of the igneous rocks. The Bundelkhand gneiss is the oldest one.
  • The rocks of the Archaean system are found mainly in Karnataka, Tamil Nadu, Andhra Pradesh, Madhya Pradesh, Odisha, Chotanagpur plateau in Jharkhand and the south-eastern part of Rajasthan.

(2) Rocks Of Dharwar System

  • These rocks have been formed as a result of the erosion and sedimentation of the rocks of the Archaean system.
  • These are the oldest sedimentary rocks.
  • No fossil is found here. It is so because either there was no origin of species during their formation or the forms of the fossils got destroyed with the passage of time.
  • The Aravali mountain range which is one of the oldest fold mountains of the world has been made with these rocks. The rocks of this system are found in the districts of Dharwar and Shimoga in Karnataka.
  • The rocks of this system are found in the southern Deccan region from Karnataka to the Kaveri valley, districts of Bellary and Shimoga, Sasar mountain range in Jabalpur and Nagpur, and the Champaner mountain range in Gujarat.
  • In north India the rocks of this system are found in the Himalayan ranges of Ladakh, Zaskar, Garhwal and Kumaon, and the Shillong range of Assam plateau.
  • The rocks of this system are economically very important. All prominent metallic minerals (iron, gold, manganese etc.) are found in these rocks.

(3) Rocks of Cuddapah System

  • These rocks have been formed as a result of the erosion and sedimentation of the rocks of Dharwar system. These are also sedimentary rocks.
  • These rocks have been named after the district of Cuddapah (Kadapa) in Andhra Pradesh where these are semi-circular in expansion.
  • These are famous for sandstone, limestone, marble, asbestos etc. 
  • The Cuddapah rocks are also found in Rajasthan.

(4) Rocks of the Vindhyan System

  • These have been formed after the Cuddapah rocks by the deposition of silt of river valleys and shallow oceans. Thus these rocks are also sedimentary rocks.
  • The evidences of fossils of microorganisms are found in this structure.
  • These rocks are found in the Vindhyas, e.g. the Malwa plateau, the Semari range in the Son valley, Bundelkhand etc.
  • This structure is famous for house-building rocks. The Sanchi Stupa, the Red Fort, the Jama Masjid etc are built with the red sandstone of this structure. Besides, limestone, china clay, dolomite etc are also found in this structure.
  • The diamond mines of Golconda in Karnataka and Panna in Madhya Pradesh are found in this structure.

(5) Rocks of Gondwana System

  • The word Gondwana has originated from the Gond region of Madhya Pradesh.
  • 98% of coal in India is found in this structure.
  • These rocks have been formed between the Carboniferous and Jurassic periods
  • Several cracks were formed in the peninsular India during the Carboniferous period. Valley-like depressions were made because of the sinking of land among these cracks. Coal was formed by the burying down of the vegetation of that period. This coal is now found mainly in the river valleys of the Damodar, the Son, the Mahanadi, the Godavari, the Wardha etc.

(6) The Deccan Trap 

  • The volcanic action in the peninsular India began in the last period (Cretaceous period) of the Mesozoic era. Thus, the Deccan trap has been formed as a result of fissure eruption.
  • This structure is made up of basalt and dolerite rocks.These rocks are very hard and their weathering has resulted in the formation of the black soil.
  • This structure is found in the most parts of Maharashtra and some parts of Gujarat, Madhya Pradesh and Tamil Nadu.

(7) Rocks of the Tertiary System

  • These rocks have been formed between the Eocene and Pliocene period.
  • The Himalayan mountain range has developed during this period.
    1. The Great Himalayas were formed during the Oligocene period.
    2. The Lesser Himalayas were formed during the Miocene period.
    3. The Shiwaliks were formed during the Pliocene and Upper Pleistocene period. 
  • Mineral oil in Assam, Rajasthan and Gujarat is found in the structures of the Eocene and Oligocene period.

The Tertiary epoch has been divided chronologically into four parts-
(a) Eocene
(b) Oligocene
(c) Miocene
(d) Pliocene

(8) Rocks of the Quaternary System

  • These rocks are found in the plains of the Indus and the Ganga.
  • The Quaternary epoch is divided chronologically into two parts- Pleistocene and Holocene periods.
  • During the Upper and Middle Pleistocene periods, old alluvial soil was formed which is known as 'bangar'.
  • The formation of the alluvial soil began at the end of the Pleistocene period and it is still going on in the present Holocene period. It is known as 'khadar'.
  • he Kashmir valley was formed during the Pleistocene period. This valley was a lake in the beginning. The continuous deposition of soil gave rise to the present form (valley) which is known as 'kareva'.

Deposition of the Pleistocene period are found in the Thar desert.The 'Rann of Kachchh' was previously a part of the ocean. It was filled by the sedimentary deposits during the Pleistocene and Holocene period.

India: Geological Rock System

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