• The Earth's crust is primarily composed of rocks, which are aggregates of one or more minerals. These rocks undergo transformations leading to three general classes: igneous, sedimentary, and metamorphic.

Rocks:

•  A rock is a naturally occurring solid mass or crystals formed from various minerals. The study of rocks encompasses various sub-disciplines of geology, including petrology and mineralogy.
• Petrologists examine rocks comprehensively, considering aspects such as mineral composition, texture, structure, origin, occurrence, alteration, and relationships with other rocks.
• Approximately 98% of the Earth's crust comprises eight elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. The remaining portion consists of elements like hydrogen, phosphorus, manganese, sulfur, carbon, nickel, and other substances.

Important Minerals for Rock Formation:

• Silicate Mineral Group - Feldspar, quartz, and ferromagnesian
• Carbonate Mineral Group - Calcite and dolomite
• Sulphide Mineral Group - Pyrite, galena
• Metallic Mineral Group - Hematite, bauxite, and iron

Types of Rocks:

• Igneous Rocks
• Sedimentary Rocks
• Metamorphic Rocks

Igneous Rocks:

• Igneous rocks originate from magma and lava within the Earth's interior, classified as primary rocks.
• Formation occurs when magma cools and solidifies, either beneath the Earth's crust, resulting in plutonic (intrusive) igneous rocks, or on the Earth's surface, forming volcanic (extrusive) igneous rocks. 
• Examples include granite, gabbro, pegmatite, basalt, volcanic breccia, and tuff.
• Igneous rocks are further categorized based on texture, occurrence, and chemical composition.

Igneous Rocks Based on Texture:

• Coarse Grained: Large mineral grains formed through slow cooling at great depths.
•  Fine Grained: Small and smooth grains resulting from rapid surface cooling.
•  Intermediate Grained: Intermediate grain sizes due to conditions between slow and rapid cooling.
•  Porphyry: Formed through two stages of cooling, with large crystals (phenocrysts) embedded in a matrix of small crystals.

Igneous Rocks Based on Occurrence:

• Intrusive Igneous Rocks: Solidification of hot magma below the Earth's surface produces rocks such as dunite, diorite, gabbro, and granite. Further classified into Plutonic Rocks (coarse-grained, deep-seated, e.g., granite) and Hypabyssal Rocks (occurring at shallow depths with various forms like batholith, laccolith, phacolith, lopolith, sill, and dyke).
• Extrusive Igneous Rocks: Rapid cooling and solidification of molten lavas at the Earth's surface result in fine-grained crystals.

Igneous Rocks Based on Chemical Composition:

• Felsic or Silica Rocks: Acidic with over 65% to 85% silica content, including granite and granodiorite.
• Mafic or Basic Rocks: Basic with 45% to 55% silica content, dominated by ferro-magnesium minerals, e.g., gabbro, basalt, dolerite.
• Ultrabasic or Ultramafic Rocks: Containing less than 45% silica content, with peridotite as a typical example.

Economic Significance of Igneous Rocks:

• Examples like basalt, obsidian, and dacite have economic importance, contributing to the formation of black soil or regur soil through weathering.
• Used in construction as building stones, facing stones on buildings, and decorative materials.
• Basaltic materials are employed in road construction and contribute to the formation of fertile black soil, known as regur, in India.

Sedimentary Rocks:

• The term "sedimentary" is derived from the Latin word "sedimentum," signifying settling.
• Rocks on the Earth's surface undergo erosion by denudational agents, breaking into various-sized fragments.
•  These fragments are transported by diverse exogenous agents and subsequently deposited. 
• Through compaction, these deposits transform into rocks in a process known as lithification.
• Many sedimentary rocks retain distinct layers even after lithification, resulting in the presence of multiple layers in rocks like sandstone and shale.
• Sedimentary rocks cover approximately 75% of the Earth's crust but contribute only 5% to its formation. Fossils are commonly found in sedimentary rocks.

Stages in the Formation of Sedimentary Rocks:

• Weathering of any rock, whether igneous, sedimentary, or metamorphic.
• Transportation involves the movement of sediment by agents such as rivers, glaciers, wind, waves, or groundwater.
• Deposition occurs in various environments like glacial, arid, deltaic, etc.
• Lithification converts sediments into sedimentary rocks.
• Cementation is the process by which various cementing minerals tightly bind the mass together.

Sedimentary Rocks Based on Mode of Formation:

• Mechanically Formed Rocks: Fragments formed due to the mechanical or physical disintegration of previously formed rock, e.g., sandstone, conglomerate, limestone, shale, loess, etc.
• Organically Formed Rocks: Formed from organic sediments resulting from the disintegration or decomposition of plants and animals, e.g., geyserite, chalk, limestone, coal, etc.
• Chemically Formed Rocks: Formed when chemically active water interacts with country rocks, e.g., chert, limestone, halite, potash, etc.

Economic Significance of Sedimentary Rocks:

• Sedimentary rocks are valuable for construction material, with limestone and coal being well-known examples of sedimentary rocks of organic origin. 
• While economically less mineral-rich, sedimentary rocks serve as sources for hematite iron ore, phosphates, various building stones, coal, and materials for making cement. 
• Petroleum, formed by the decay of marine organisms, is also found in the muds of sedimentary rocks, which also yield nutrient-rich soils.

Metamorphic Rocks:

• "Metamorphic" denotes a change of form. Metamorphic rocks form due to pressure, volume, and temperature changes.
• Metamorphism involves the recrystallization and reorganization of materials within already consolidated rocks.
• These rocks are exceptionally hard and lack fossils. Metamorphism occurs when rocks are forced to lower levels through tectonic processes.
•  Examples of metamorphic rocks include gneissic, granite, syenite, slate, schist, marble, quartzite, etc.
• Metamorphic rocks formed through igneous rocks are termed meta-igneous or ortho-metamorphic rocks, such as gneiss from granite, amphibolite from basalt, and schist from basalt.
• Metamorphic rocks formed by the further metamorphosis of metamorphic rocks, are phyllite from slate, schist from phyllite, serpentine from gabbro. Metamorphism can be categorize on the

Metamorphism

• On the Basis of Nature of Agents
• Thermal Metamorphism
• On the Basis of Place or Area
• Contact Metamorphism
• Hydro Metamorphism
• Dynamic Metamorphism
• On the Basis of Nature of Agents
• Regional Metamorphism
• Hydro-Thermal

Nature of Agents in Metamorphism:

• Thermal Metamorphism: In this type of metamorphism, the materials of rocks undergo chemical alteration and recrystallization due to heat.
• Dynamic Metamorphism: Dynamic metamorphism involves mechanical disruption and reorganization of the original minerals within rocks. This occurs through breaking and crushing without any significant chemical changes.
• Hydro Metamorphism: Alterations in the composition of rocks due to hydrological factors define hydro metamorphism. It includes pressure exerted on underlying rocks by the storage of an immense volume of water in large reservoirs, known as hydrostatic metamorphism.
• Hydro-Thermal Metamorphism: Minor changes in the physical and chemical composition of rocks occur due to the weight and pressure of water mass, as well as chemically active hot gases and water vapor.

Place or Area in Metamorphism:

• Contact Metamorphism: This type involves rocks coming into contact with hot intruding magma and lava. The rock materials recrystallize under high temperatures, and new materials often form from magma or lava.
• Regional Metamorphism: In regional metamorphism, rocks undergo recrystallization due to deformation caused by temperature, pressure, or both. During this process, some rock grains or minerals may arrange themselves in layers or lines.

Economic Significance of Metamorphic Rocks:

• Gneiss: Widely used as a building stone and in statues in the regions where it occurs.
• Coal: Transforms into anthracite and graphite, containing over 90% carbon.

Causes of Metamorphism:

• Orogenic (Mountain Building) Movements: These movements involve interplay of folding, warping, and high temperature, giving existing rocks a new appearance.
• Lava Inflow: Molten magmatic material in the Earth's crust influences surrounding rocks under intense temperature and pressure, causing changes.
• Geodynamic Forces: The omnipresent geodynamic forces, such as plate tectonics, also play a crucial role in metamorphism.

Foliation:

• Definition: Foliation, also known as lineation, refers to the parallel arrangement of minerals or grains in metamorphic rocks.
•  It involves the organization of minerals from different groups into alternating thin to thick layers, giving the rock a distinctive appearance with light and dark shades. 
• The presence of banding characterizes this structure, and rocks exhibiting banding are referred to as banded rocks. 
• The types of metamorphic rocks are determined by the original rocks that underwent metamorphism.

Rock Cycle:

• Continuous Transformation: The rock cycle is a continuous and dynamic process in which old rocks undergo transformation to become new ones.
• Primary Role of Igneous Rocks: Igneous rocks, as primary rocks, play a foundational role in the rock cycle, serving as the starting point for the formation of other rock types.
• Transformation Potential: Igneous rocks can undergo changes to become metamorphic rocks.
• Formation of Sedimentary Rocks: Fragments derived from both igneous and metamorphic rocks come together to form sedimentary rocks.

• Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of sedimentary rocks.
• The crustal rocks such as igneous, metamorphic and sedimentary once formed may be carried down into the mantle through subduction process.
• The same melt down due to increase in temperature in the interior and turn into molten magma i.e., the original source for igneous rocks

Prelims Facts

• Which type of rock is known as Primary rock?
  • - Igneous rock [BPSC (Pre) 2011, UPPSC (Pre) 2017)
• Which type of rock contains coal and petroleum?
  • Sedimentary IMPPSC (Pre) 2022]
• The rocks found in Himalayan system are mainly
  • - Sedimentary [JPSC (Pre) 2013]
•  In which rocks are metallic minerals generally found ? Igneous rock and Metamorphic rock
  • [MPSC (Pre) 2013]
• The percentage of sand is high in which type of rocks?
  • -Arenaceous rocks (MPSC (Mains) 2015]
• Sandstone is Flaky rock, because
  • -It is formed underwater (MPPSC (Pre) 1993]
• The rocks formed deep inside the Earth as a result of the freezing of lava are called
  • Plutonic rocks [CGPSC (Pre) 2011
• Breaking down of rock in situ is known as Weathering [UPPSC (Pre) 2016
  • Sandston is an example of Sedimentary rocks (UPPSC (Pre) 2014
•  Rocks formed due to alteration of original structure due to heat and excessive pressure are called
  • - Metamorphic rocks (MPPSC (Pre) 2016
•  Which is a coarse grained igneous rock that contains quartz and feldspar?
  • Granite [BPSC (Pre) 2000
• Which of the following rock systems in India is the latest one?
  • - Gondwana [CGPSC (Pre) 2016
• The origin of Metamorphic rocks occurs from Both Igneous and Sedimentary rock [JKPSC (Pre) 2011

Self Check

1. The rocks formed by the condensation of liquid substances of the Earth are called

(a) sedimentary rocks

(b) metamorphic rocks

(c) igneous rocks

(d) None of the above

2. Sandstone is an example of

(a) non-foliated rock

(b) sedimentary rock

(c) igneous rock

(d) metamorphic rock

3. Which of the following is a plutonic rock?

(a) Granite

(b) Gneiss

(c) Gypsum

(d) None of these

4. Which type of rocks may contain fossils?

Nagaland PSC (Pre) 2016

(a) Igneous

(b) Sedimentary

(c) Hypabyssal

(d) Metamorphic

5. Which of the following is not an example of metamorphic rock?

UKPSC (Pre) 2022

(a) Quartzite

(b) Marble

(c) Gneiss

(d) Dolomite

6. Which one among the following is a mechanically formed sedimentary rock?

(a) Salt rock

(b) Limestone

(c) Sandstone

(d) Gypsum

7. Which of the following rocks contains coal and petroleum?

MPPSC (Pre) 2022

(a) Granite

(b) Igneous

(c) Metamorphic

(d) Sedimentary

8. A large body of material that cools in the deeper depth of the Earth's crust and develops in the forms of large domes is known as

(a) Batholiths

(b) Lacoliths

(c) Lopoliths

(d) Phacoliths

9. There are three types of rock formations such as igneous, sedimentary and metamorphic. In which rock formations are metallic minerals generally found?

MPSC (Pre) 2013

(a) Igneous rocks and sedimentary rocks

(b) Sedimentary rocks and metamorphic rocks

(c) Igneous rocks and metamorphic rocks

(d) In igneous rocks only

10. Observe the following statements.

1. Arenaceous rocks are the types of sedimentary rocks.
2. The percentage of sand is high in arenaceous rocks.

Which of the statements) given above is/are correct?

(a) Only statement 1 is correct.

(b) Only statement 2 is correct.

(c) Both the statements are correct.

(d) Both the statements are incorrect.

11. Which of the following statements) is/are correct?

1. The major constituent mineral of granite rocks is quartz.
2. The major constituent mineral of sandstone rocks is feldspar.
3. The major constituent mineral of limestone rocks is dolomite.

Select the correct answer by using the codes given below.

(a) 1, 2 and 3 

(b) Only 3

(c) 1 and 2 

(d) 2 and 3

12. Consider the following statements about rocks.

1. Shale becomes slate through metamorphosis.
2. Shale is converted to slate when it is subjected to tremendous pressure and high temperature.

Which of the statements) given above is/are correct?

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

13. Which of the following statements) is/are correct?

1. Rocks do not remain in their original form for long and undergo transformation.
2. Transformation of rocks is caused by weathering, erosion and metamorphic action.

Codes

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

14. Which of the following statements is incorrect?

MPPSC (Pre) 2015

(a) Calcium is converted into graphite due to the heat of lava.

(b) Anthracite is a type of metamorphic rock.

(c) Mineral oil deposits are found in the carbonaceous rocks.

(d) Dyke rocks are made of lava.

15. Consider the following statements about the igneous rocks.

UPPSC (Pre) 2014

1. Igneous rocks are normally crystalline in structure.
2. They occur in strata and contain fossils.
3. These rocks have no silica.

Which of the statements) given above is/are correct?

(a) 1 and 3

(b) 1 and 2 

(c) 2 and 3

 (d) Only 1

Know Right Answer

1 (c)

2 (b)

3 ( a)

4 (b)

5 (d)

6 (c)

7 (d)

8 (a)

9 (c)

10 (c)

11 (c)

12 (c)

13( c)

14 (a)

15 (d)