Geography Quiz: Geological Structure and Rock Systems of India

In geology a rock is any natural aggregate of one or more minerals that forms part of the earth's crust, whether it is hard like granite or soft like clay, and rocks are classified as igneous, sedimentary or metamorphic according to how they were formed.

From the Upper Carboniferous onwards, thick beds of plant material accumulated in troughs and freshwater basins on the Gondwana landmass and were buried under sandstone and shale, giving the coalfields of the Damodar, Mahanadi, Godavari, Son and Wardha valleys.

The sandstones, shales and limestones of this Purana system, which stretch across central India into the Son valley, contain the conglomerate beds in which diamonds are found, and the Panna mines of Madhya Pradesh as well as the diamonds once traded from the Kurnool area are associated with them.

Named after the Dharwar region of Karnataka, it is the oldest metamorphosed sedimentary system in India and is described as highly metalliferous, yielding iron ore, manganese, gold, copper, lead, zinc and mica; the Champion gneiss of the Kolar gold field belongs to it.

Limestone is a sedimentary rock composed mainly of calcium carbonate, and when it is subjected to heat and pressure its grains recrystallise into the dense, even-textured rock called marble.

At the close of the Cretaceous, highly fluid lava welled out of long cracks in the crust in the north-western Deccan and spread as sheet after sheet over Maharashtra and parts of Gujarat and Madhya Pradesh; the flat, step-like arrangement of these flows is what the word trap describes.

Sediment settles in layers at ordinary temperatures, so the remains of organisms buried with it are preserved as the layers are compacted and cemented, which is why sedimentary rocks are also called stratified rocks and why the history of life is read from them.

Lignite, or brown coal, is a low-rank coal formed from vegetation buried relatively recently, and the Neyveli field of Tamil Nadu is a Tertiary deposit, as are the petroleum-bearing rocks of Assam and Gujarat.

Bhangar is the older alluvium occupying the higher terraces above the present flood level of the rivers, and its soils often hold calcareous concretions known as kankar.

Charnockite is a granitic rock of the Archaean Basement Complex and is found in the Nilgiri and Palni hills of the south.