Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

Rocks [arr] Material that form crust of earth

Igneous Rocks

Formed through cooling & solidification of molten material magma ( Erupted from volcanoes, molten material moves towards surface of earth through crack)
Normally crystalline in structure, do not occur in strata & do not contains fossils
Can be subdivided on the basis of mineral composition

Basic rocks contain high proportion of basic oxides mainly of iron, aluminium & magnesium

When they contain high proportion of silica, they are said to be acidic, which are less dense & lighter in color than basic rocks for ex. Granite

Most igneous rocks are extremely hard & resistant hence are quarried for road making & polished as monuments & grave stones
Are parent / primary rocks as all other rocks are derived from it.

In terms of origin they can be divided into mainly 2 classes viz.

Plutonic Rocks	formed due to solidification of magma (interior of earth) They have cooled & solidified slowly so that large, easily recognized crystals are formed till some depth inside earth crust Examples [arr] Granite, Diorite & Gabbro Exposed at the surface by denudation & erosion
Volcanic Rocks	formed due to solidification of lava (interior of earth) Molten rocks poured out of volcanoes as lavas They have solidified rapidly on the earth’s surface hence have small crystals For ex. Basalt (which yield Regur or black soil)

Sedimentary Rocks

Formed due to deposition of layers of sediment usually along the water bodies over a long period of time
Sediment is deposited layer by layer in form of strata hence also known as stratified rocks
Process of turning sediments into hard rock layers by pressure is known as lithification
Rocks may be fine grained or coarse, soft or hard & material forming them may be brought by streams, glaciers, winds or even animals
May be derived from Igneous, Metamorphic or Sedimentary rocks
Hence, Sedimentary rocks are most varied in formation of all rocks
They are non-crystalline & often contains fossils of animals, plants & other microorganisms

May be classified under 3 categories with respect to their origin & composition viz

Mechanically formed	Formed from the accumulation of materials derived from other rocks Made from sand grains with tremendously varying texture, composition & colour. For ex : Sandstone Mainly quarried for building purposes or for making grindstones Finer sedimentary materials form clay, widely used for making bricks, shale or mudstone. Sand & gravel may occur in uncemented form
Organically formed	Formed from the remains of living organisms such as corals or shellfish, whose fleshy part has been decomposed, leaving behind the hard cells. Most common rocks formed due to this process are Calcareous type for ex. Limestone & Chalk Carbonaceous rocks are also organically formed but from vegetative matter – Swaps and forests Pressure of overlying sediments has compressed the plants remains into compact masses of carbon which eventually becomes Peat, Lignite or Coal
Chemically formed	Formed chemically from solutions of one another For ex. Gypsum (Calcium sulphate) is obtained from evaporation of salt lakes having high salinity Same way Potash & Nitrates may be formed

Metamorphic Rocks

Formed when original structure of igneous & sedimentary rocks partially or wholly changes under the action of heat & pressure
Contains no fossils
No stratification
For ex. Clay [arr] Slate

Igneous to Metamorphic