Rock System Based on Geological History Of India

1. The Archaean Rock System.
2. The Purana Rock System.
3. The Dravidian Rock System.
4. The Aryan Rock System.

Archaean Rock System (Pre-Cambrian Rocks)

• Rocks formed prior to the Cambrian system.
• The Archaean rock system includes:

Archaean Gneisses and Schists [4 Billion Years]

Gneiss == Mineral composition varies from granite to gabbro.

Schists == mostly crystalline, include mica, talc, hornblende, chlorite, etc.

These rocks are:

• Oldest rocks [pre-Cambrian era] [formed about 4 billion years ago].
• Rocks formed due to solidification of molten magma – the earth’s surface was very hot then.
• Known as the ‘Basement Complex’ [They are the oldest and forms the base for new layers]
• Azoic or unfossiliferous,
• Foliated (consisting of thin sheets),
• Thoroughly crystalline (because they are volcanic in origin),
• Plutonic intrusions (volcanic rocks found deep inside).

Dharwar System [4 – 1 Billion Years]

• Formation period ranges from 4 billion years ago to – 1 billion years ago.
• Highly metamorphosed sedimentary rock-system. [formed due to metamorphosis of sediments of Archaean gneisses and schists].
• They are the oldest metamorphosed rocks.
• Found in abundance in the Dharwar district of Karnataka.
• Economically the most important rocks because they possess valuable minerals like high grade iron-ore, manganese, copper, lead, gold, etc.

Purana Rock System (1400 – 600 Million Years)

• Includes two divisions: the Cuddapah System and the Vindhyan System.

Cuddapah System

• Unfossiliferous clay, slates, sandstones and limestones was deposited in synclinal basins [depression between two folds {Fold mountain}].
• Outcrops best observed in Cuddapah district of Andhra Pradesh.
• These rocks contain ores of iron, manganese, copper, cobalt, nickel, etc.
• They contain large deposits of cement grade limestones.

Vindhyan System (1300-600 million years)

• This system derives its name from the great Vindhyan mountains.
• The system comprises of ancient sedimentary rocks (4000 m thick) superimposed on the Archaean base.
• Mostly Unfossiliferous.
• Large area of this belt is covered by the Deccan trap.
• The Vindhayan system have diamond bearing regions from which Panna and Golconda diamonds have been mined.
• It is devoid of metalliferous minerals but provides large quantities of durable stones, ornamental stones, limestone, pure glass making sand etc..

Dravidian Rock System (Palaeozoic)

• Formed about 600 – 300 million years ago.
• Found in the Extra Peninsular region (Himalayas and Ganga plain) and are very rare in Peninsular India. [The name ‘Dravidian’ doesn’t mean they are found in South India]
• Abundant fossils.
• The rocks of Cambrian, Ordovician, Silurian, Devonian and Carboniferous periods are fall under Dravidian system. (All these are not important, only Carboniferous is important)

Carboniferous rocks (350 million years)

• The Carboniferous rocks (350 million years) comprise mainly of limestone, shale and quartzite.
• Mount Everest is composed of Upper Carboniferous limestones.
• Coal formation started in the Carboniferous age.
• Carboniferous in geology means coal bearing. [most of the coal found in India is not of Carboniferous period; High quality coal of Great Lakes Region-USA, U.K and Ruhr region is Carboniferous coal].

Aryan Rock System

• Upper Carboniferous to the Recent.

Gondwana System

• The Gondwana System [derives its name Gonds, the most primitive people of Telangana and Andhra Pradesh]
• They are deposits laid down in synclinal troughs on ancient plateau surface.
• As the sediments accumulated, the loaded troughs subsided.
• Fresh water and sediments accumulated in these trough and terrestrial plants and animals thrived.
• This happened since Permian period (250 million years ago).

Gondwana Coal

• Gondwana rocks contain nearly 98 per cent of India’s coal reserves.
• Gondwana coal is much younger than the Carboniferous coal and hence it’s carbon content is low.
• They have rich deposits of iron ore, copper, uranium and antimony also.
• Sandstones, slates and conglomerates are used as building materials.

Jurassic System

• The marine transgression in the latter part of the Jurassic gave rise to thick series of shallow water deposits in Rajasthan and in Kuchchh.
• Coral limestone, sandstone, conglomerates and shales occur in Kuchchh.
• Another transgression on the east coast of the Peninsula is found between Guntur and Rajahmundry.

Deccan Trap

• Volcanic outburst over a vast area of the Peninsular India from the end of the Cretaceous till the beginning of the Eocene gave rise to Deccan Traps.
• Basaltic lava flowed out of fissures covering a vast area of about ten lakh sq km.
• These volcanic deposits have flat top and steep sides and therefore called ‘trap’ meaning a ‘stair’ or ‘step’ in Swedish.
• The process of weathering and erosion (denudation) since millions of years has reduced the Deccan Trap to almost half of its original size.
• Present Deccan Trap covers about 5 lakh sq km mainly in parts of Kuchchh, Saurashtra, Maharashtra, the Malwa plateau and northern Karnataka.
• Thickness of the Deccan Traps is 3,000 metres along the west which is reduced to 600-800 metres towards the south, 800 metres in Kuchchh and only 150 metres at the eastern limit.
• The weathering of these rocks for a long time has given birth to black cotton soil known as ‘regur’.

The Deccan Trap has been divided into three groups:

Tertiary System

• Eocene to Pliocene about 60 to 7 million years ago.
• The tertiary is the most significant period in India’s geological history because the Himalayas were born and India’s present form came into being in this period.

Primary References: NCERT Geography, Indian Geography by Kullar [Amazon and Flipkart]