Geological Structure and Rock Systems of India Notes

India is geologically three countries in one. The Peninsular block south of the Narmada is built of the oldest rocks on earth, rigid and long since worn down. The Himalayan region in the north is made of young, folded, still-rising rocks. Between them lies the Indo-Gangetic plain, a trough filled with alluvium brought down by rivers. Examiners test this chapter because every rock system is tied to a mineral: ask where India's coal comes from and the answer is a rock system, ask where its diamonds come from and the answer is another. Learn each system with its age, its characteristic rock and its economic use, and the questions answer themselves.

The three families of rock

Igneous rocks form when molten magma cools and are called primary rocks, because all other rocks are ultimately derived from them. Magma cooling below the surface gives intrusive rocks with large crystals, such as granite; lava cooling on the surface gives extrusive rocks with fine grains, such as basalt. Sedimentary rocks are formed by the deposition and compaction of sediment in layers, which is why they are also called stratified rocks; they may be mechanically formed like sandstone and shale, organically formed like limestone and coal, or chemically formed like gypsum and rock salt. Fossils occur only in sedimentary rocks. Metamorphic rocks are older rocks changed in place by heat and pressure: granite becomes gneiss, sandstone becomes quartzite, limestone becomes marble, shale and clay become slate, and coal becomes graphite. The movement of material from one family to another is the rock cycle.

The Archaean system

The Archaean rocks are the oldest in India and form what geologists call the Basement Complex, over which every later system rests. They are gneisses and granites, without any fossils, and they underlie about two-thirds of Peninsular India, appearing at the surface in the Bundelkhand, Bengal and Nilgiri gneisses. One name from this system is asked repeatedly: charnockite, the granitic rock of the Nilgiri and Palni hills, named after Job Charnock, whose tombstone in Kolkata was cut from it.

The Dharwar system

Named after the Dharwar region of Karnataka, this is the oldest metamorphosed sedimentary system in India, and by far the most valuable. It is highly metalliferous: iron ore, manganese, gold, copper, lead, zinc and mica all come from Dharwar rocks. The Champion gneiss of the Kolar gold field belongs to this system, and so do the Aravalli and the Iron Ore series. If a question asks which system is richest in metallic minerals, the answer is always Dharwar.

The Purana group: Cuddapah and Vindhyan

The Cuddapah system, named after the district in Andhra Pradesh, consists of shales, sandstones and limestones laid down in a synclinal basin. Its metallic content is poor, but it holds great reserves of cement-grade limestone, along with asbestos and barytes. The Vindhyan system, named after the Vindhyan range and stretching across central India to the Son valley, is famous for two things. Its diamond-bearing conglomerates gave the Panna mines of Madhya Pradesh and the old Golconda diamonds of the Kurnool area, and its red sandstone built the Qutub Minar, the Red Fort, Fatehpur Sikri and the Sanchi stupa. The Vindhyan rocks are largely unfossiliferous.

The Gondwana system

Deposited from the Upper Carboniferous onwards in troughs and freshwater basins on the old Gondwana landmass, this system carries almost the whole of India's coal. The coalfields of the Damodar, Mahanadi, Godavari, Son and Wardha valleys are all Gondwana. Because these coals are younger than the Carboniferous coals of Europe and North America, they have a lower carbon content, so Indian coal is mostly bituminous rather than anthracite. The system takes its name from the Gond country of central India and also preserves the plant fossils that first suggested the former union of the southern continents.

The Deccan Trap

At the close of the Cretaceous, lava poured out of long fissures rather than from central volcanoes and spread in sheet after sheet over the north-western Deccan, covering much of Maharashtra and parts of Gujarat and Madhya Pradesh. The resulting basalt is arranged in flat, step-like layers, which is the meaning of the word trap. Weathering of this basalt produces the black cotton soil, called regur, that makes the Deccan India's cotton belt. The Trap is also the source of building stone and of secondary minerals such as agate.

The Tertiary and Quaternary systems

The Tertiary is the era of the Himalaya: the Tethys sediments were folded and raised into mountains in a series of movements, and the youngest Himalayan range, the Shiwaliks, preserves a famous collection of vertebrate fossils. Economically the Tertiary rocks are India's fuel rocks, holding the petroleum of Assam and Gujarat and the lignite, or brown coal, of Neyveli in Tamil Nadu. The Quaternary brought the alluvium of the Indo-Gangetic plain, divided into the older Bhangar on higher ground, which carries lime nodules called kankar, and the newer Khadar of the flood plains. The Karewas of Kashmir, lake deposits of the same age, are the beds on which saffron is grown.

Rock systems at a glance

How the three divisions differ

Write the contrast in a line each. The Peninsular block is made of Archaean and Purana rocks, is rigid and stable, has no young fold mountains and carries most of India's metallic minerals. The extra-Peninsular Himalayan region is made of Tertiary sedimentary rocks, is unstable and earthquake-prone, and carries fuels rather than metals. The Indo-Gangetic plain is a structural trough between them, floored by Quaternary alluvium of enormous depth, with no mineral wealth but the most fertile soil in the country.