The geological structure of a country helps in understanding the types and character of rocks and slopes, the physical and chemical properties of soils, the availability of minerals, and the surface and underground water resources.

Indian Rock System

• Geological Structure:
  • Refers to the arrangement and deposition of rocks in the earth’s crust
  • Result of earth movements or absence thereof
  • Also applies to morphological features of rocks
• Geological Time Scale:
  • Chronological dating of geological formations and life based on origin, evolution, and extinction
  • Developed by Giovanni Ardunia in 1760
  • Standardized at the International Geological Congress in 1881
  • India has its own Geological Time Scale, advocated by T.S. Holland
• Geological History of India:
  • Analyzes the geological structure and rock systems of India
  • Divided into Peninsular India and Extra Peninsular India
  • Major events include the formation of Peninsular India with the Earth’s crust
  • The Himalayas were uplifted in the tertiary period
  • The Indo-Gangetic plain was formed through aggradational processes in the Pleistocene period and continues to be formed through sedimentation

• Based on this complex and varied geological history, the Geological Survey of India has classified rock systems of the country into 4 major divisions:
  • The Archaean Rock System
  • The Purana Rock System
  • The Dravidian Rock System
  • The Aryan Rock System

Archaean Rock System (Pre-Cambrian Rocks)

• Earliest phase of tectonic evolution marked by cooling and solidification of upper crust
• Archaean era (prior to 2.5 billion years) in Precambrian period
• Gneisses and granites exposed on Peninsula represent this era
• Archaean refers to oldest rocks of earth’s crust
• Indian Craton (Block of Indian Subcontinent of Gondwanaland) consists of core Archaean rocks
• Archaean group of rocks consists of two systems:
  • Achaean System: granites and gneisses
  • Dharwar System: first sedimentary rocks
• Gneiss mineral composition varies from granite to gabbro
• Schists are mostly crystalline and include mica, talc, hornblende, chlorite, etc.
• Archaean Gneisses and Schists:
  • Oldest rocks, formed about 4 billion years ago in the pre-Cambrian era
  • Formed due to solidification of molten magma when earth’s surface was very hot
  • Known as the “Basement Complex” and form the base for new layers
  • Azoic or unfossiliferous
  • Foliated, consisting of thin sheets
  • Thoroughly crystalline due to volcanic origin
  • Plutonic intrusions, volcanic rocks found deep inside
• Dharwar System:
  • 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
  • Oldest metamorphosed rocks
  • Found abundantly in the Dharwar district of Karnataka
  • Economically important as they possess valuable minerals like high-grade iron-ore, manganese, copper, lead, gold, etc.

Purana Rock System

• The Cuddapah and Vindhyan rock systems are together known as the Purana rock system.
• They are formed by the erosion and deposition of Archean and Dharwar rocks, the process is believed to have taken place between 1400-600 million years ago.
• They are mostly sedimentary in nature.
• Cuddapah System:
  • Named after the Cuddapah district of Andhra Pradesh
  • Formed when sedimentary rocks like sandstone, limestone etc. were deposited in synclinal folds
  • Outcrops best observed in Cuddapah district of Andhra Pradesh
  • Rocks contain ores of iron, manganese, copper, cobalt, nickel, etc.
  • Large deposits of cement grade limestones present in these rocks.
• Vindhyan System (1300-600 million years):
  • System named after the Vindhyan mountains
  • Comprises ancient sedimentary rocks (4000 m thick) superimposed on the Archaean base
  • Mostly unfossiliferous rocks and large area covered by the Deccan trap
  • Diamond-bearing regions present in this system, from which Panna and Golconda diamonds have been mined
  • Devoid of metalliferous minerals but provides durable stones, ornamental stones, limestone, pure glass-making sand, etc.

Dravidian Rock System (Palaeozoic)

• Paleozoic rock formation is known as Dravidian systems in India, formed 600-300 million years ago.
• Dravidian rocks are mostly found in the extra-Peninsular regions of the Himalayas and the Gangetic plain.
• Major regions of Dravidian rocks include PirPanjal, Handwara, Lider valley, Annatnag of Kashmir, Spiti, Kangra & Shimla region of Himanchal Pradesh, Gharwal & Kumayun of Uttarakhand.
• Dravidian rocks include shales, sandstones, clays, quartzites, slates, salts, talc, dolomite, marble, etc.
• Dravidian period saw plenty of fossil evidences, indicating marine conditions in these Paleozoic rocks in India.
• The Cambrian, Ordovician, Silurian, Devonian, and Carboniferous Systems are included in the Dravidian system of geological formations.
• Dravidian period marks the beginning of coal formation (high-quality Carboniferous coal), but these formations were not found abundantly in India.
• It is the period when Pangaea was broken and the Tethys Sea came into existence.
• Carboniferous rocks (350 million years):
  • The Carboniferous rocks are 350 million years old and composed mainly of limestone, shale, and quartzite.
  • Mount Everest is made up of Upper Carboniferous limestones.
  • Coal formation began in the Carboniferous age.
  • The term Carboniferous means coal-bearing.
  • Most of the coal found in India is not from the Carboniferous period.
  • High-quality coal of Great Lakes Region-USA, U.K and Ruhr region is Carboniferous coal.

Aryan Rock System

• The Aryan Group marks the beginning of the Upper Carboniferous period.
• The Aryan Group is classified in the following groups:
  • Upper Paleozoic Era – Upper Primary Epoch – Upper Carboniferous and Permian Period formation
  • Mesozoic Era – Secondary Epoch – Triassic, Jurassic, and Cretaceous period Formation (Gondwana Rock system, Deccan trap, Jurassic system)
  • Cenozoic Era – Tertiary Epoch – Paleocene, Eocene, Oligocene-Miocene, and Pliocene Period
  • Neozoic Era – Quaternary Epoch – Pleistocene and Holocene/recent Period.
• Gondwana System:
  • The Gondwana System is named after the Gonds, the most primitive people of Telangana and Andhra Pradesh.
  • The system consists of deposits laid down in synclinal troughs on ancient plateau surfaces.
  • Sediments accumulated in these troughs and terrestrial plants and animals thrived.
  • This happened since the Permian period, which was about 250 million years ago.
  • Gondwana rocks contain nearly 98% of India’s coal reserves.
  • Gondwana coal is much younger than Carboniferous coal and therefore has low carbon content.
  • They also have rich deposits of iron ore, copper, uranium, and antimony.
  • Sandstones, slates, and conglomerates from Gondwana rocks 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 occurred over a vast area of Peninsular India from end of Cretaceous to beginning of Eocene
  • Outburst gave rise to Deccan Traps, where basaltic lava flowed out of fissures covering about 10 lakh sq km
  • Deccan Trap deposits have flat top and steep sides, hence called ‘trap’ meaning ‘stair’ or ‘step’ in Swedish
  • Weathering and erosion since millions of years has reduced Deccan Trap to almost half of its original size
  • Present Deccan Trap covers about 5 lakh sq km in parts of Kuchchh, Saurashtra, Maharashtra, Malwa plateau and northern Karnataka
  • Thickness of 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
  • Weathering of Deccan Trap rocks for a long time has given birth to black cotton soil known as ‘regur’
• Tertiary System:
  • Tertiary period occurred from Eocene to Pliocene, about 60 to 7 million years ago
  • Characterized by two significant events: the final breaking up of the old Gondwana land and the upliftment of Tethys geosynclines or Himalayas
  • The Himalayas were born and India’s present form came into being during this period, making it the most significant period in India’s geological history
  • Tertiary Succession is fully spread over the Bengal and Ganges delta, East coast, and the Andaman Islands
  • Tertiary Succession is also found in the Salt Range, Potwar Plateau, outer Himalayan regions of Jammu and Punjab, Assam, Sind, and Baluchistan
  • Important rock systems of Tertiary period include Karewas of Kashmir, Bhangra, and Khadar of the Gangetic plains, etc.

• Geologic history of India is unique and diverse, with different regions containing rocks from different periods
• Indian Craton was once part of the supercontinent of Pangaea
• Breaking of Gondwanaland carton (225 my ago) and sliding towards the Eurasian craton created the Himalayan mountain region (65 my ago)
• Wide spared alluvial plain formation began in upper Pliocene & Pleistocene period as Indo-Ganga-Brahmaputra plain region
• Different parts of India contain rocks of almost every type of geological structure of different geologic periods
• Stratigraphy of India can be divided into several divisions such as Archean System, Dharwar System, Cudappah system, Vindhyan system, Dravidian system, and Aryan system (Gondwana, Jurassic, Deccan Trap, Tertiary and Quaternary rocks)
• India is marked by the oldest rocks to the newest alluvial formations found in these geological strata
• Oldest rocks of Archaean period found in peninsular India
• Sedimentary rocks found in land formed by deposition of sediments from the Indo-Gangetic plains
• Different sedimentary and metamorphic rocks also trace in the Great folded mountainous region.