On the basis of mode of formation there are three types of rocks:
‘Ignis’ is a Latin word and its meaning is fire.
The cooling of intensely heated molten fluid liquid, known as magma, creates Igneous rocks.
It requires a higher amount of heat to melt the rocks under overlying pressure than on the surface. Magma forms at different depths not exceeding 40 km.
When magma comes at the surface of the earth it is called Lava.
Igneous rocks are formed below or on earth’s surface from solidified molten magma.
These rocks are parent rocks and also called as 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 of the earth is composed of Igneous Rocks.
These types of rocks are largely hard and massive because of their magmatic origin and are crystalline in appearances.
Extrusive igneous rocks
Extrusive igneous rocks are formed on the earth’s surface by the cooling of lava.
As the lava cools very quickly when it comes out of the earth’s interior, the mineral crystals that make up these rocks are very fine.
These rocks are also called volcanic rocks.
Examples of such rocks are Gabbro and Basalt.
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 intrusive rocks are referred to as plutonic rocks.
Shallow intrusive rocks are referred to as hypabyssals.
Examples of intrusive rocks are Granite and dolerite.
In the Himalaya and the Deccan Plateau, Granitic rocks are found.
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 form the core of the big 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
Acid igneous rocks contain more than 65 percent of silica.
Acid igneous rocks are light coloured very strong.
Granite is an example of acidic rock.
Basic igneous rocks have more iron and magnesium.
These rocks contain less than 55% of silica.
These are dark coloured and weak rocks.
examples of basic rocks are Gabbro, basalt, and dolerite.
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.
Sedimentary rocks have some kind of fossil in between their strata.
Fossil is the solid component or an impression of a prehistoric animal or plant that is buried in sedimentary rock strata.
These rocks are abundant on the surface of the earth, often at shallow depths.
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.
Sedimentation is the process by which rock-forming material is deposited.
It can settle in calmer lake or ocean waters or places where the transporter no longer has enough energy to carry them further.
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 loose, unconsolidated, soft rock material.
Sandstone, shale, limestone, and dolomite are examples of sedimentary rocks.
The sediments are mostly loose, unconsolidated, soft rock material, initially like sand and clay, but over time they get hardened by extreme pressure and cementation to form sedimentary rocks.
Examples of sedimentary rocks: sandstone, shale, calcareous and dolomite.
under suitable conditions sediments of various sizes get bound by cementing material.
Conglomerate is an example of such a sedimentary rock. This type of formation of consolidated material is termed as mechanically formed sedimentary rock.
The deposition of organic matter produced from plants and animals creates organic sedimentary rocks.
Coal and limestone are organic sedimentary rocks.
The sediments may also result from a chemical reaction. Examples: Gypsum, rock salt, and nitre.
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. For example, All river basins and their 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.
Under the impact of heat or pressure on sediments or igneous rocks, metamorphic rocks are formed.
Huge pressure and high temperature change the color, rigidity, and composition of all 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.
Molten magma changes surrounding rocks 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.