Rocks - Notes | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


ROCKS 
 The earth’s crust is composed of rocks.  
 A rock is an aggregate of one or more minerals. Rocks do not have definite 
composition of mineral constituents. 
Rock may be hard or soft and in varied colours. For example, granite is 
hard, soapstone is soft. Gabbro is black and quartzite can be milky white. 
Feldspar and quartz are the most common minerals found in rocks. 
There are many different kinds of rocks which are grouped under three 
families on the basis of their mode of formation.  
CLASSIFICATION OF ROCKS AS PER THEIR MODE OF 
FORMATION: 
(i) Igneous Rocks — solidified from magma and lava;  
(ii)  Sedimentary Rocks — the result of deposition of fragments of 
rocks by exogenous processes; 
(iii)   Metamorphic Rocks — formed out of existing rocks undergoing 
recrystallisation. 
IGNEOUS ROCKS 
As igneous rocks form out of magma and lava from the interior of the 
earth, they are known as primary rocks.  
The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma 
cools and solidifies.  
When magma in its upward movement cools and turns into solid form it is 
called igneous rock.  
Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of 
the examples of igneous rocks 
The process of cooling and solidification can happen in the earth’s crust or 
on the surface of the earth.  
CLASSIFICATION BASED ON COOLING AND SOLIDIFICATION  
1) Intrusive Rocks  
2) Extrusive Rocks  
 
Page 2


ROCKS 
 The earth’s crust is composed of rocks.  
 A rock is an aggregate of one or more minerals. Rocks do not have definite 
composition of mineral constituents. 
Rock may be hard or soft and in varied colours. For example, granite is 
hard, soapstone is soft. Gabbro is black and quartzite can be milky white. 
Feldspar and quartz are the most common minerals found in rocks. 
There are many different kinds of rocks which are grouped under three 
families on the basis of their mode of formation.  
CLASSIFICATION OF ROCKS AS PER THEIR MODE OF 
FORMATION: 
(i) Igneous Rocks — solidified from magma and lava;  
(ii)  Sedimentary Rocks — the result of deposition of fragments of 
rocks by exogenous processes; 
(iii)   Metamorphic Rocks — formed out of existing rocks undergoing 
recrystallisation. 
IGNEOUS ROCKS 
As igneous rocks form out of magma and lava from the interior of the 
earth, they are known as primary rocks.  
The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma 
cools and solidifies.  
When magma in its upward movement cools and turns into solid form it is 
called igneous rock.  
Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of 
the examples of igneous rocks 
The process of cooling and solidification can happen in the earth’s crust or 
on the surface of the earth.  
CLASSIFICATION BASED ON COOLING AND SOLIDIFICATION  
1) Intrusive Rocks  
2) Extrusive Rocks  
 
 
 
INTRUSIVE ROCKS: 
Sometimes, the molten matter is not able to reach the surface and instead 
cools down very slowly at great depths. Slow cooling allows big-sized 
crystals (large grains) to be formed. 
These rocks appear on the surface only after being uplifted and denuded. 
Eg . Granite  
EXTRUSIVE ROCKS: 
These are formed by rapid cooling of the lava thrown out during volcanic 
eruptions. Rapid cooling prevents crystallization, as a result such rocks are 
fine-grained. 
Eg . Basalt.  
The Deccan traps in the peninsular region is of basaltic origin. 
INTERMEDIATE ROCKS: 
These rocks occupy an intermediate position between the deep-seated 
plutonic bodies and the surface lava flows. 
Dyke rocks are semi-crystalline in structure. 
 
BASED ON THE PRESENCE OF ACID FORMING RADICAL, 
SILICON :  
ACID ROCKS:  
These are characterized by high content of silica—up to 80 per cent, while 
the rest is divided among aluminium, alkalis, magnesium, iron oxide, lime 
etc.. 
Page 3


ROCKS 
 The earth’s crust is composed of rocks.  
 A rock is an aggregate of one or more minerals. Rocks do not have definite 
composition of mineral constituents. 
Rock may be hard or soft and in varied colours. For example, granite is 
hard, soapstone is soft. Gabbro is black and quartzite can be milky white. 
Feldspar and quartz are the most common minerals found in rocks. 
There are many different kinds of rocks which are grouped under three 
families on the basis of their mode of formation.  
CLASSIFICATION OF ROCKS AS PER THEIR MODE OF 
FORMATION: 
(i) Igneous Rocks — solidified from magma and lava;  
(ii)  Sedimentary Rocks — the result of deposition of fragments of 
rocks by exogenous processes; 
(iii)   Metamorphic Rocks — formed out of existing rocks undergoing 
recrystallisation. 
IGNEOUS ROCKS 
As igneous rocks form out of magma and lava from the interior of the 
earth, they are known as primary rocks.  
The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma 
cools and solidifies.  
When magma in its upward movement cools and turns into solid form it is 
called igneous rock.  
Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of 
the examples of igneous rocks 
The process of cooling and solidification can happen in the earth’s crust or 
on the surface of the earth.  
CLASSIFICATION BASED ON COOLING AND SOLIDIFICATION  
1) Intrusive Rocks  
2) Extrusive Rocks  
 
 
 
INTRUSIVE ROCKS: 
Sometimes, the molten matter is not able to reach the surface and instead 
cools down very slowly at great depths. Slow cooling allows big-sized 
crystals (large grains) to be formed. 
These rocks appear on the surface only after being uplifted and denuded. 
Eg . Granite  
EXTRUSIVE ROCKS: 
These are formed by rapid cooling of the lava thrown out during volcanic 
eruptions. Rapid cooling prevents crystallization, as a result such rocks are 
fine-grained. 
Eg . Basalt.  
The Deccan traps in the peninsular region is of basaltic origin. 
INTERMEDIATE ROCKS: 
These rocks occupy an intermediate position between the deep-seated 
plutonic bodies and the surface lava flows. 
Dyke rocks are semi-crystalline in structure. 
 
BASED ON THE PRESENCE OF ACID FORMING RADICAL, 
SILICON :  
ACID ROCKS:  
These are characterized by high content of silica—up to 80 per cent, while 
the rest is divided among aluminium, alkalis, magnesium, iron oxide, lime 
etc.. 
These rocks constitute the sial portion of the crust. Due to the excess of 
silicon, acidic magma cools fast and it does not flow and spread far away. 
This flow and cooling gives rise to High Mountains. 
 
BASIC ROCKS 
These rocks are poor in silica (about 40 per cent); magnesia content is up 
to 40 per cent and the remaining 40 per cent is spread over iron oxide, 
lime, aluminium, alkalis, potassium etc. 
Due to low silica content, the parent material of such rocks cools slowly 
and thus, flows and spreads far away. This flow and cooling gives rise to 
plateaus. 
Basalt is a typical example, others being gabbro and dolerite. 
Not being very hard, these rocks are weathered relatively easily. 
SEDIMENTARY ROCKS 
The word ‘sedimentary’ is derived from the Latin word sedimentum, 
which means settling.  
Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are 
exposed to denudational agents, and are broken up into various sizes of 
fragments.  
Such fragments are transported by different exogenous agencies and 
deposited.  These deposits through compaction turn into rocks.  This 
process is called lithification. 
 
  In many sedimentary rocks, the layers of deposits retain their characteristics 
even after lithification.  Hence, we see a number of layers of varying thickness 
in sedimentary rocks like sandstone, shale etc.  
Page 4


ROCKS 
 The earth’s crust is composed of rocks.  
 A rock is an aggregate of one or more minerals. Rocks do not have definite 
composition of mineral constituents. 
Rock may be hard or soft and in varied colours. For example, granite is 
hard, soapstone is soft. Gabbro is black and quartzite can be milky white. 
Feldspar and quartz are the most common minerals found in rocks. 
There are many different kinds of rocks which are grouped under three 
families on the basis of their mode of formation.  
CLASSIFICATION OF ROCKS AS PER THEIR MODE OF 
FORMATION: 
(i) Igneous Rocks — solidified from magma and lava;  
(ii)  Sedimentary Rocks — the result of deposition of fragments of 
rocks by exogenous processes; 
(iii)   Metamorphic Rocks — formed out of existing rocks undergoing 
recrystallisation. 
IGNEOUS ROCKS 
As igneous rocks form out of magma and lava from the interior of the 
earth, they are known as primary rocks.  
The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma 
cools and solidifies.  
When magma in its upward movement cools and turns into solid form it is 
called igneous rock.  
Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of 
the examples of igneous rocks 
The process of cooling and solidification can happen in the earth’s crust or 
on the surface of the earth.  
CLASSIFICATION BASED ON COOLING AND SOLIDIFICATION  
1) Intrusive Rocks  
2) Extrusive Rocks  
 
 
 
INTRUSIVE ROCKS: 
Sometimes, the molten matter is not able to reach the surface and instead 
cools down very slowly at great depths. Slow cooling allows big-sized 
crystals (large grains) to be formed. 
These rocks appear on the surface only after being uplifted and denuded. 
Eg . Granite  
EXTRUSIVE ROCKS: 
These are formed by rapid cooling of the lava thrown out during volcanic 
eruptions. Rapid cooling prevents crystallization, as a result such rocks are 
fine-grained. 
Eg . Basalt.  
The Deccan traps in the peninsular region is of basaltic origin. 
INTERMEDIATE ROCKS: 
These rocks occupy an intermediate position between the deep-seated 
plutonic bodies and the surface lava flows. 
Dyke rocks are semi-crystalline in structure. 
 
BASED ON THE PRESENCE OF ACID FORMING RADICAL, 
SILICON :  
ACID ROCKS:  
These are characterized by high content of silica—up to 80 per cent, while 
the rest is divided among aluminium, alkalis, magnesium, iron oxide, lime 
etc.. 
These rocks constitute the sial portion of the crust. Due to the excess of 
silicon, acidic magma cools fast and it does not flow and spread far away. 
This flow and cooling gives rise to High Mountains. 
 
BASIC ROCKS 
These rocks are poor in silica (about 40 per cent); magnesia content is up 
to 40 per cent and the remaining 40 per cent is spread over iron oxide, 
lime, aluminium, alkalis, potassium etc. 
Due to low silica content, the parent material of such rocks cools slowly 
and thus, flows and spreads far away. This flow and cooling gives rise to 
plateaus. 
Basalt is a typical example, others being gabbro and dolerite. 
Not being very hard, these rocks are weathered relatively easily. 
SEDIMENTARY ROCKS 
The word ‘sedimentary’ is derived from the Latin word sedimentum, 
which means settling.  
Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are 
exposed to denudational agents, and are broken up into various sizes of 
fragments.  
Such fragments are transported by different exogenous agencies and 
deposited.  These deposits through compaction turn into rocks.  This 
process is called lithification. 
 
  In many sedimentary rocks, the layers of deposits retain their characteristics 
even after lithification.  Hence, we see a number of layers of varying thickness 
in sedimentary rocks like sandstone, shale etc.  
These rocks consist of a number of layers or strata. These rocks have 
fossils of plants and animals. 
These rocks are generally porous and allow water to percolate through 
them. 
Eg . Alluvial deposits in the Indo-Gangetic plain  
DEPENDING UPON THE MODE OF FORMATION, SEDIMENTARY 
ROCKS ARE CLASSIFIED INTO THREE MAJOR GROUPS: 
 MECHANICALLY FORMED  
Formed by mechanical agents like running water, wind, ocean currents, 
ice, etc. 
sandstone, conglomerate, limestone, shale, loess etc. are examples; 
CHEMICALLY FORMED  
Water containing minerals evaporate at the mouth of springs or salt lakes 
and give rise to Stalactites and stalagmites  
chert, limestone, halite, potash etc. are some examples. 
ORGANICALLY FORMED  
The remains of plants and animals are buried under sediments and due to 
heat and pressure from overlying layers, their composition undergoes a 
change. 
geyserite, chalk, limestone, coal etc. are some examples;  
NOTE: 
Plant remains give rise to coals of different grades depending upon the 
proportion of carbon and the degree of overlying pressure. 
The peat and lignite (brown coal) is the first stage of coal having below 45 
per cent of carbon; the bituminous variety is the next stage with 60 per 
cent carbon. 
 
METAMORPHIC ROCKS 
The word metamorphic means ‘change of form’.  
Page 5


ROCKS 
 The earth’s crust is composed of rocks.  
 A rock is an aggregate of one or more minerals. Rocks do not have definite 
composition of mineral constituents. 
Rock may be hard or soft and in varied colours. For example, granite is 
hard, soapstone is soft. Gabbro is black and quartzite can be milky white. 
Feldspar and quartz are the most common minerals found in rocks. 
There are many different kinds of rocks which are grouped under three 
families on the basis of their mode of formation.  
CLASSIFICATION OF ROCKS AS PER THEIR MODE OF 
FORMATION: 
(i) Igneous Rocks — solidified from magma and lava;  
(ii)  Sedimentary Rocks — the result of deposition of fragments of 
rocks by exogenous processes; 
(iii)   Metamorphic Rocks — formed out of existing rocks undergoing 
recrystallisation. 
IGNEOUS ROCKS 
As igneous rocks form out of magma and lava from the interior of the 
earth, they are known as primary rocks.  
The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma 
cools and solidifies.  
When magma in its upward movement cools and turns into solid form it is 
called igneous rock.  
Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of 
the examples of igneous rocks 
The process of cooling and solidification can happen in the earth’s crust or 
on the surface of the earth.  
CLASSIFICATION BASED ON COOLING AND SOLIDIFICATION  
1) Intrusive Rocks  
2) Extrusive Rocks  
 
 
 
INTRUSIVE ROCKS: 
Sometimes, the molten matter is not able to reach the surface and instead 
cools down very slowly at great depths. Slow cooling allows big-sized 
crystals (large grains) to be formed. 
These rocks appear on the surface only after being uplifted and denuded. 
Eg . Granite  
EXTRUSIVE ROCKS: 
These are formed by rapid cooling of the lava thrown out during volcanic 
eruptions. Rapid cooling prevents crystallization, as a result such rocks are 
fine-grained. 
Eg . Basalt.  
The Deccan traps in the peninsular region is of basaltic origin. 
INTERMEDIATE ROCKS: 
These rocks occupy an intermediate position between the deep-seated 
plutonic bodies and the surface lava flows. 
Dyke rocks are semi-crystalline in structure. 
 
BASED ON THE PRESENCE OF ACID FORMING RADICAL, 
SILICON :  
ACID ROCKS:  
These are characterized by high content of silica—up to 80 per cent, while 
the rest is divided among aluminium, alkalis, magnesium, iron oxide, lime 
etc.. 
These rocks constitute the sial portion of the crust. Due to the excess of 
silicon, acidic magma cools fast and it does not flow and spread far away. 
This flow and cooling gives rise to High Mountains. 
 
BASIC ROCKS 
These rocks are poor in silica (about 40 per cent); magnesia content is up 
to 40 per cent and the remaining 40 per cent is spread over iron oxide, 
lime, aluminium, alkalis, potassium etc. 
Due to low silica content, the parent material of such rocks cools slowly 
and thus, flows and spreads far away. This flow and cooling gives rise to 
plateaus. 
Basalt is a typical example, others being gabbro and dolerite. 
Not being very hard, these rocks are weathered relatively easily. 
SEDIMENTARY ROCKS 
The word ‘sedimentary’ is derived from the Latin word sedimentum, 
which means settling.  
Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are 
exposed to denudational agents, and are broken up into various sizes of 
fragments.  
Such fragments are transported by different exogenous agencies and 
deposited.  These deposits through compaction turn into rocks.  This 
process is called lithification. 
 
  In many sedimentary rocks, the layers of deposits retain their characteristics 
even after lithification.  Hence, we see a number of layers of varying thickness 
in sedimentary rocks like sandstone, shale etc.  
These rocks consist of a number of layers or strata. These rocks have 
fossils of plants and animals. 
These rocks are generally porous and allow water to percolate through 
them. 
Eg . Alluvial deposits in the Indo-Gangetic plain  
DEPENDING UPON THE MODE OF FORMATION, SEDIMENTARY 
ROCKS ARE CLASSIFIED INTO THREE MAJOR GROUPS: 
 MECHANICALLY FORMED  
Formed by mechanical agents like running water, wind, ocean currents, 
ice, etc. 
sandstone, conglomerate, limestone, shale, loess etc. are examples; 
CHEMICALLY FORMED  
Water containing minerals evaporate at the mouth of springs or salt lakes 
and give rise to Stalactites and stalagmites  
chert, limestone, halite, potash etc. are some examples. 
ORGANICALLY FORMED  
The remains of plants and animals are buried under sediments and due to 
heat and pressure from overlying layers, their composition undergoes a 
change. 
geyserite, chalk, limestone, coal etc. are some examples;  
NOTE: 
Plant remains give rise to coals of different grades depending upon the 
proportion of carbon and the degree of overlying pressure. 
The peat and lignite (brown coal) is the first stage of coal having below 45 
per cent of carbon; the bituminous variety is the next stage with 60 per 
cent carbon. 
 
METAMORPHIC ROCKS 
The word metamorphic means ‘change of form’.  
These rocks form under the action of pressure, volume and temperature 
(PVT) changes.  
Metamorphism is a process by which already consolidated rocks undergo 
recrystallisation and reorganisation of materials within original rocks.  
DYNAMIC METAMORPHISM: 
Mechanical disruption and reorganisation of the original minerals within 
rocks due to breaking and crushing without any appreciable chemical 
changes is called dynamic metamorphism.  
THERMAL METAMORPHISM: 
The materials  of rocks chemically alter and recrystallise due to thermal 
metamorphism.   
TYPES OF THERMAL METAMORPHISM: 
1) Contact metamorphism  
2) Regional metamorphism.  
In contact metamorphism the rocks come in contact with hot intruding 
magma and lava and the rock materials recrystallise under high 
temperatures.  Quite often new materials form out of magma or lava are 
added to the rocks.  
In regional metamorphism, rocks undergo recrystallisation due to 
deformation caused by tectonic shearing together with high temperature 
or pressure or both.  
In the process of metamorphism in some rocks grains or minerals get 
arranged in layers or lines.  Such an arrangement of minerals or grains in 
metamorphic rocks is called foliation or lineation.