Indian Rock System: Geological Structure, Time Scales, History, and Rock Classifications in India

• Peninsular India
• Extra Peninsular India

• Peninsular India was part of the ancient landmass since the formation of the Earth's crust.
• The upheaval of the Himalayas during the tertiary period.
• The formation of the Indo-Gangetic plain during the Pleistocene period, which continues through sedimentation in the floodplains of rivers.

Archaean Rock System

• The earliest tectonic evolution marked by the cooling and solidification of the earth's upper crust in the Archaean era (Precambrian Period, over 2.5 billion years ago).
• These rocks form the core of the Indian Craton (part of Gondwanaland).
• Introduced by J.D. Dana in 1782, 'Archaean' refers to the oldest rocks in the earth's crust.
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• The Archaean rocks consist of:
  • Archaean System: Granites and Gneisses.
  • Dharwar System: First sedimentary rocks.

Archaean Gneisses and Schists

• These are the oldest rocks (formed about 4 billion years ago).
• Rocks formed from the solidification of molten magma when the earth’s surface was extremely hot.
• Known as the ‘Basement Complex’ (they form the base for newer layers).
• Fossil-free (azoic), foliated (thin sheets), crystalline, and volcanic in origin.
• Volcanic rocks found deep inside are known as plutonic intrusions.

Dharwar System

• Formed between 4 billion to 1 billion years ago, highly metamorphosed sedimentary rock system.
• Oldest metamorphosed rocks, found mainly in Dharwar, Karnataka.
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• Economically important for containing valuable minerals like high-grade iron ore, manganese, copper, lead, and gold.

Purana Rock System

• Consists of the Cuddapah and Vindhyan rock systems, formed between 1400 to 600 million years ago.
• Formed by the erosion and deposition of Archean and Dharwar rocks.
• Primarily sedimentary in nature.

Cuddapah System

• Named after Cuddapah district, Andhra Pradesh, where large outcrops are found.
• Formed by sedimentary rocks deposited in synclinal folds between mountain ranges.
• Contains iron ores, manganese, copper, cobalt, nickel, and large deposits of cement-grade limestone.
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Vindhyan System (1300-600 million years)

• Named after the Vindhyan mountains, consisting of ancient sedimentary rocks superimposed on the Archaean base.
• Contains unfossiliferous rocks with large areas covered by the Deccan trap.
• Known for diamond-bearing regions, including Panna and Golconda.
• Lacks metalliferous minerals but provides durable and ornamental stones, limestone, and glass-making sand.

Dravidian Rock System (Paleozoic)

• Formed during the Paleozoic era (600-300 million years ago), also known as the Carboniferous rock system.
• Found mainly in extra-Peninsular regions like the Himalayas and Gangetic plain.
• Includes shales, sandstones, clays, quartzites, and more.
• Marine conditions prevailed during this period, with fossil evidence found in formations from this era.
• Though the period marked the beginning of coal formation, India lacks significant Carboniferous coal deposits.

Carboniferous Rocks (350 million years ago)

• Composed primarily of limestone, shale, and quartzite.
• Mount Everest consists of Upper Carboniferous limestone.
• Coal formation began during the Carboniferous age, though most Indian coal is not from this period.

Aryan Rock System Overview

• The Upper Carboniferous period marks the beginning of the Aryan Group, spanning from the Upper Carboniferous to the Holocene period.
• The Aryan Group of Rock Formation is classified into:
  • Upper Paleozoic Era: Upper Carboniferous & Permian Period formation.
  • Mesozoic Era: Triassic, Jurassic & Cretaceous period formations (Gondwana Rock system, Deccan trap, Jurassic system).
  • Cenozoic Era: Paleocene, Eocene, Oligocene-Miocene & Pliocene Period.
  • Neozoic Era: Pleistocene & Holocene/recent Period.

Gondwana System

• Named after the Gonds, the primitive people of Telangana and Andhra Pradesh.
• Deposits laid down in synclinal troughs on ancient plateau surfaces.
• Accumulation of sediments led to subsidence of troughs, supporting freshwater and terrestrial life.
• Originated during the Permian period (250 million years ago).

Gondwana Coal

• Contains nearly 98% of India’s coal reserves.
• Coal is younger than Carboniferous coal, with lower carbon content.
• Rich in iron ore, copper, uranium, and antimony.
• Sandstones, slates, and conglomerates are used as building materials.

Jurassic System

• Marine transgression in the latter part of the Jurassic resulted in shallow water deposits in Rajasthan and Kuchchh.
• Deposits include coral limestone, sandstone, conglomerates, and shales.
• Another transgression is observed on the east coast between Guntur and Rajahmundry.

Deccan Trap

• Volcanic outburst from the end of the Cretaceous to the beginning of the Eocene led to the formation of the Deccan Traps.
• Basaltic lava covered about ten lakh sq km, with flat-topped, steep-sided formations.
• Weathering and erosion have reduced the Deccan Trap to about 5 lakh sq km.
• Thickness ranges from 3,000 meters in the west to 150 meters at the eastern limit.
• The weathering process has created black cotton soil known as ‘regur’.
• Divided into three groups:
  • Upper Trap: Found in Maharashtra and Saurashtra.
  • Middle Trap: Found in Central India and Malwa.
  • Lower Trap: Includes inter-trappean beds.

Tertiary System

• Spanning from Eocene to Pliocene (about 60 to 7 million years ago), marked by significant geological events including the breakup of Gondwana and uplift of the Himalayas.
• Significant for the formation of the Himalayas and the shaping of India’s present form.
• Spread across Bengal and Ganges delta, East coast, Andaman Islands, Salt Range, Potwar Plateau, Jammu and Punjab, Assam, Sind, and Baluchistan.
• Includes important rock systems like Karewas of Kashmir, Bhangra, and Khadar of the Gangetic plains.

Geological History Summary

• The Indian Craton was once part of the supercontinent Pangaea.
• Following the breakup of Gondwanaland (225 million years ago) and its movement towards the Eurasian craton, the Himalayas formed (65 million years ago).
• The Indo-Ganga-Brahmaputra plain began to form during the upper Pliocene & Pleistocene period.
• India’s geological strata encompass rocks from nearly all geological periods, with significant variations across regions.