On the basis of mode of formation there are three types of rocks:
The word igneous is derived from the Latin word ‘ignis’ meaning fire. Igneous rocks are formed by the cooling of highly heated molten fluid material, known as magma.
It requires a greater quantity of heat to melt the rocks under overlying pressure than at the surface.
Magma forms at different depths not exceeding 40 km.
When magma is ejected to the surface, it is called lava.
Igneous rocks are formed from solidified molten magma below or on the earth’s surface.
These are called the parent of all rocks or the ‘primary rocks’.
In simple words, all rocks can be described as of igneous origin because, at one time or another, they erupted to the surface.
About 95% of the volume of outermost 16 km of the earth is composed of them.
These types of rocks are largely hard and massive because of their magmatic origin and are crystalline in appearances.
On the basis of their mode of occurrence, igneous rocks can be classified as – extrusive or volcanic rocks and intrusive rocks.
Extrusive igneous rocks
Extrusive igneous rocks are formed by cooling of lava on the earth’s surface.
As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine.
These rocks are also called volcanic rocks.
Gabbro and basalt are very common examples of such rocks.
Deccan plateau’s regur soil in India is derived from lava.
Intrusive igneous rocks
Intrusive igneous rocks are formed when magma solidifies below the earth’s surface.
The rate of cooling below the earth’s surface is very slow which gives rise to the formation of large crystals in the rocks.
Deep-seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal.
Granite and dolerite are common examples of intrusive rocks.
The huge blocks of coarse granitic rocks are found both in the Himalaya and the Decean Plateau.
Common forms of intrusive igneous rocks are batholiths, sills and dykes etc.
Batholiths are huge masses of solidified magma.
These types of rocks vary in size; some are as much as several hundred kilometers across and thousands of kilometers thick.
They generally form the core of the major mountains.
Sill is the horizontal intrusion of solidified magma between the layers of preexisting rocks.
Dyke is similarly a more or less vertical formation from few metres to several kilometers in length and from few centimeters to hundreds of metres in thickness.
Acidic and Basic rocks
On the basis of chemical properties, igneous rocks are classified into acidic and basic rocks.
These are formed as a result of solidification of acidic or basic lava.
Acidic igneous rocks are composed of 65% or more of silica.
These rocks are light coloured, hard and very strong.
Granite is an example of an acidic rock.
Basic igneous rocks contain less than 55% of silica and have more of iron and magnesium.
These rocks are dark coloured and weak enough for weathering.
Gabbro, basalt and dolerite are examples of basic rocks.
Sedimentary Rocks are formed by successive deposition of sediments.
These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic rock or old sedimentary rock.
Sedimentary rocks have a layered or stratified structure. So these rocks are also called stratified rocks.
The thickness of strata varies from a few millimeters to several meters.
Generally, these rocks have some type of fossil between their strata.
Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks.
Sedimentary rocks are widely spread on the earth surface but to a shallow depth.
The individual rock particles are first broken from rocks and then transported by running water, ocean currents, and glaciers or even by the wind from one place to another.
The process by which rock-forming material is laid down is called sedimentation or deposition.
It may settle in calmer waters of lakes or oceans or at places where the transporting agent has no longer enough energy to carry them farther.
These are identified as riverine, lacustrine (formed by a lake), glacial or aeolian (formed by wind) sedimentary rocks with reference to their deposition near rivers, lakes, glacier or deserts respectively.
The sediments are often loose, unconsolidated, soft rock material, in the beginning like sand and clay, but in course of time, they get hardened to a compact material by excessive pressure and cementation to form sedimentary rocks.
Sandstone, shale, limestone, and dolomite are examples of sedimentary rocks.
Sediments of different sizes may get bound by cementing material under suitable conditions.
Conglomerate is an example of such a sedimentary rock.
This type of formation of consolidated material is termed as mechanically formed sedimentary rock.
The consolidation of organic matter derived from plants and animals forms sedimentary rocks of organic origin.
Coal and limestone are organic sedimentary rocks.
The sediments may also result from a chemical reaction.
Direct precipitation of minerals from their solution in water may give rise to sedimentary rocks of chemical origin.
Gypsum, rock salt, and nitre are examples of such sedimentary rocks.
Huge folded mountains of the world like the Himalayas, Andes etc. are made up of sedimentary rocks.
All the alluvial deposits of the world are also due to sedimentary accumulations.
All river basins, particularly their plains and deltas, e.g. Indo- Gangetic plain and Ganga-Brahmaputra delta are good examples of sedimentary accumulations.
Most rocks in course of time become metamorphic or changed forms of rocks.
Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks.
Tremendous pressure and high temperature change the colour, hardness, structure, and composition of all types of pre-existing rocks.
The rocks formed due to the operation of such processes are defined as the Metamorphic rocks.
Temperature, pressure, stress, and access to chemically reactive substances are the main agents, which are responsible for metamorphism.
Heat causes the minerals to recrystallize in the rock.
The process of change by heat is called thermal or contact metamorphism.
When molten magma or lava comes in contact with surrounding rocks, it bakes them and changes them into metamorphic rocks.
Similarly, the formation of the rocks due to tremendous pressure is known as dynamic or regional metamorphism.
Slate, gneiss, schist, marble, and diamond are good examples of these rocks.
These types of rocks are hard and tough in comparison to the parent rocks from which they are formed.
In India, marble is found in Rajasthan, Bihar and Madhya Pradesh, whereas slates are available in plenty in Orissa, Andhra Pradesh and Haryana.
In Kangra and Kumaun regions of Himalaya, slates of different colours are found.