Rock System and it's classification
Indian Rock System: Geological Structure
- Geological structure refers to the arrangement and deposition of rocks in the earth's crust, a result of earth movements or the absence thereof.
- It also applies to the morphological features of rocks, e.g., Gondwana structure.
Geological Time Scale
- Chronological dating of geological formations and life according to their time, place of origin, evolution, and extinction.
- Giovanni Ardunia developed the Geological Time Scale in 1760.
- Standard Geological Time Scale developed at the International Geological Congress in 1881, Italy.
- The Indian Geological Time Scale was advocated by T.S. Holland.
Geological History of India
- The geological structure and rock systems of India are analyzed with reference to their geographical locations and geological history.
- The main physiographic divisions of India used for geological referencing are:
- Peninsular India
- Extra Peninsular India
- Major events in India's geological history include:
- 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.
Rock Systems of India
- Based on India's complex geological history, the Geological Survey of India classifies the rock systems into four major divisions:
- The Archaean Rock System
- The Purana Rock System
- The Dravidian Rock System
- The Aryan Rock System
Major Rock Systems in India
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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.
- The Archaean rocks consist of:
- Archaean System: Granites and Gneisses.
- Dharwar System: First sedimentary rocks.
Archean Rock System -
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.
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Dharwar System
- Formed between 4 billion to 1 billion years ago, highly metamorphosed sedimentary rock system.
- Oldest metamorphosed rocks, found mainly in Dharwar, Karnataka.
- Economically important for containing valuable minerals like high-grade iron ore, manganese, copper, lead, and gold.
Dharwar Rock System -
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.
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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.
Cuddappah Rock System -
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.
Other Rock Systems Overview
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
