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Home / IAS Preparation / Types Of Rocks: Geography NCERT Notes for UPSC

Types Of Rocks: Geography NCERT Notes for UPSC

Last Updated on Jun 20, 2024
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NCERT notes on different NCERT important topics are considered important as per UPSC exam point of view. One among such topics is “Types of Rocks” as it is one of the important topics which is important for the UPSC exam along with other competitive exams like banking PO, SSC, state civil services exams, and so on. In this article we will be covering topics like: types of rocks or classification of rocks, rock cycle, etc. Keeping all these things in mind, Testbook prepares the best quality NCERT Notes: Types of Rocks and covers all aspects related to this topic. By going through this note you can enhance your performance in UPSC and other competitive exams.

Types of Rocks (UPSC Geography) NCERT Notes: Download PDF Here!

Rocks In The Earth Crust

A rock is known to be a composition of different minerals. Formation of rocks occurs when different minerals aggregate or when a mixture of one or more minerals combine physically. They may vary in color, hardness and softness, size and many more. Like: granite is known to be hard whereas soapstone is soft. Gabbro is black in color and quartzite can be milky white

The most common types of minerals found in all types of rocks are feldspar and quartz. The branch of science which does the study of rocks is called petrology and those who do the study of rocks are called petrologists.

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Classification of Rocks
  1. As there are so many features which differentiate rocks from one another like their properties, the size of particles and mode of formation. By keeping all these features in mind, rocks may be classified into three types:
  2. Igneous Rocks: solidified from magma and lava
  3. Sedimentary Rocks: the result of
  4. deposition of fragments of rocks by exogenous processes
  5. Metamorphic Rocks: formed out of existing rocks undergoing recrystallisation

Also, check the NCERT Notes Geography on Endogenic Processes here.

Igneous Rocks
  1. Formation of igneous rocks occurs out of magma and lava from the interior of the earth. They are also called Primary rocks.
  2. As formation of igneous rocks occurs when magma cools down and gets solidified. Cooling of magma occurs when it is in upward movement and gets converted into solid form forming igneous rocks.
  3. Common examples of igneous rocks are: granite, gabbro, basalt.
  4. Based on texture of rocks and place and time taken in cooling of the molten matter, igneous rocks are classified into two types, they are:
    • Intrusive igneous rocks (Plutonic rocks)
    • Extrusive igneous rocks (Lava or Volcanic rocks)

Check the NCERT Notes Geography on Volcanoes here.

Classification of Igneous Rocks

Based on texture of rocks and place and time taken in cooling of the molten matter, igneous rocks are classified into two types:

A. Intrusive igneous rocks (Plutonic rocks):

When cooling of magma occurs at great depth, mineral grains formed in the rocks may be very large. Such kinds of igneous rocks are called Intrusive igneous rocks or plutonic rocks. Such rocks appear on the surface only after being uplifted and denuded.

B. Extrusive igneous rocks (Lava or Volcanic rocks):

When sudden cooling of magma occurs below the surface or lava above the surface, this process results in the small and smooth grains in rocks as rapid cooling prevents crystallisation, as a result, such rocks are fine-grained. Rocks which are formed through this process are extrusive rocks or volcanic rocks (e.g. Basalt). Basalt rocks are found in the Deccan traps in the Indian peninsular region. Such rocks have higher proportions of basic oxides, like iron, aluminium or magnesium, and because of this they are denser and darker in colour.

Based on the chemical properties, igneous rocks are of two types they are:

  1. Acidic Igneous rocks: They have high viscous properties and are composed of 65% or more of silica. They are colorful, hard and very strong in nature. Example of acidic igneous rock: Granite.
  2. Basic Igneous rocks: They have low viscous properties and are composed of less than 55% of silica and have more iron and magnesium. Their physical appearance is dark in color. Example of basic igneous rock: Basalt, Gabbro

Check the NCERT Notes Geography on Continental Drift for UPSC.

Sedimentary Rocks
  1. Sedimentary rocks are formed by the process called Lithification i.e. consolidation and compaction of sediments. Because of this reason they are layered or stratified differing in its thickness.
  2. Example of sedimentary rocks: sandstone, shale etc.
  3. Sediments are formed by a denudation process of all types of rocks i.e. weathering and erosion process of rocks.
  4. Sedimentary rocks occupy 75 percent of the earth’s crust but volumetrically occupy only 5 percent.
    1. These rocks are found only on the upper layer of the Earth crust.
    2. Based on mode of formation, sedimentary rocks are classified into three types, they are:
    3. Mechanically formed — sandstone, conglomerate, limestone, shale, loess.
    4. Organically formed — geyserite, chalk, limestone, coal.
    5. Chemically formed — limestone, halite, potash.

Study the NCERT Notes Geography on Earthquake here.

Classification of Sedimentary Rocks:

A. Mechanically Formed Sedimentary Rocks:

Such sedimentary rocks are formed by agents like running water, wind, ocean currents, ice, etc.

Such soil also acts as a best reservoir of groundwater, petroleum and many more liquids. Example: Sandstone, claystones, shales.

B. Chemically Formed Sedimentary Rocks: 

Such rocks are formed when mineral constituents in solution become supersaturated and inorganically precipitate. Example: limestone and rocks composed of evaporite minerals, such as halite, sylvite, barite and gypsum.

C. Organically Formed Sedimentary Rocks: 

Such rocks are formed when remains of plants and animals are buried under sediments, and due to heat and pressure from overlying layers, their composition changes.

Example: coal and limestone.

Read the NCERT Notes on Environment here.

Metamorphic Rocks
  1. Rocks which are formed under the action of volume, pressure, and temperature (PVT) changes are called metamorphic rocks.
  2. The process when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks is called a metamorphic process.
  3. Because of the process of thermal metamorphism, the materials of rocks get chemically modified and recrystallized.
  4. Types of metamorphism:
    1. Thermal metamorphism: Metamorphism occurs because of the influence of heat, it is called Thermal Metamorphism. They are of two types:
    2. Contact metamorphism: When rocks come in contact with hot intruding magma and lava and the rock materials recrystallize under high temperatures, then such metamorphism is called contact metamorphism.
    3. Regional metamorphism: When rocks undergo recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both then it is called regionaL metamorphism.
    4. Dynamic metamorphism: Metamorphism occurs without any appreciable chemical change, then it is called dynamic metamorphism. For example: granite is converted into gneiss; clay and shale are transformed into schist.

Check NCERT Notes on Minerals and their Characteristics here.

Rock Cycle
  1. Process by which old rocks are transformed into new ones is called the rock cycle. The region behind this cycle is that rocks do not remain in their original form for a long time, so rocks keep on transforming.
  2. As igneous rocks are primary rocks and transformation of primary rocks lead to the formation of other rocks like sedimentary and metamorphic rocks.
  3. Fragments of igneous rocks and metamorphic rocks lead to the formation of sedimentary rocks.
  4. Sedimentary rocks further undergo the fragmentation process and can act as a source of sedimentary rocks.
  5. The crustal rocks (igneous, metamorphic and sedimentary) are further carried down into the mantle (interior of the earth) through a process called subduction and the same meltdown and turn into molten magma, the source for igneous rocks.

Some Rock Forming Minerals

List of some rock forming minerals are given below:

Feldspar: It occupies half the crust and is light in color. Its main constituents are silicon, oxygen, sodium, potassium, calcium, aluminium.

  1. Quartz: It has two major elements, silicon and oxygen and the structure is hexagonal in nature. Bauxite: A hydrous oxide of aluminium, it is the ore of aluminium and non crystalline in nature.
  2. Cinnabar: they are mercury sulphide and brownish colour.
  3. Dolomite: A double carbonate of calcium and magnesium and used in cement and iron industries.
  4. Gypsum: It is hydrous calcium sulphate and mainly used in industries of cement, fertiliser.
  5. Haematite: Red ore of iron.
  6. Magnetite: Black ore (or iron oxide) of iron.

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We hope the above NCERT Notes on Geography Topic Types of Rocks proves to be useful for UPSC aspirants and for preparation of other competitive exams. For more notes and updated details on this topic, download the Testbook App and get exam-ready in no time!

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Types of Rocks NCERT Notes FAQs

Sedimentary rocks are formed in layers i.e. of different strata. They are made up of fossils of plants and animals, so they are fossiliferous in nature. They are generally porous in nature and allow water to percolate through them.

Sedimentary rocks are considered to be the weakest rock as they are formed from the fossils of plants and animals.

Some of the economical significance of sedimentary rocks are listed below:Sedimentary rocks contain important minerals like hematite iron ore, phosphates, building stones, coals, petroleum and material used in the cement industry are found, which gives economical values. They also contain important minerals like bauxite, manganese, and tin. They are also a source of the richest soil.

Example of igneous rocks are: Granite, Gabbro, Pegmatite, Basalt, Volcanic, Breccia, Tuff

Some of the common causes of metamorphism are: A. Orogenic Movements: Movements which occur because of an interplay of folding, warping and high temperatures. B. Lava Inflow: When streams of molten rock that pour or ooze from an erupting vent called lava flow. C. Geodynamic Forces: Forces because of plate tectonics called geodynamic forces.
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