NCERT Textbook: Minerals & Rocks | Geography (Prelims) by Valor Academy - UPSC PDF Download

 Page 1


LANDFORMS
This unit deals with
• Rocks and minerals — major types of rocks and their
characteristics
• Landforms and their evolution
• Geomorphic processes — weathering, mass wasting, erosion
and deposition; soils — formation
UNIT
III
© NCERT
not to be republished
Page 2


LANDFORMS
This unit deals with
• Rocks and minerals — major types of rocks and their
characteristics
• Landforms and their evolution
• Geomorphic processes — weathering, mass wasting, erosion
and deposition; soils — formation
UNIT
III
© NCERT
not to be republished
MINERALS AND ROCKS
CHAPTER
T
he earth is composed of various kinds
of elements.  These elements are in solid
form in the outer layer of the earth and
in hot and molten form in the interior.
About 98 per cent of the total crust of the
earth is composed of eight elements like
oxygen, silicon, aluminium, iron, calcium,
sodium, potassium and magnesium (Table 5.1),
and the rest is constituted by titanium,
hydrogen, phosphorous, manganese, sulphur,
carbon, nickel and other elements.
Table 5.1 : The Major Elements of the Earth’s Crust
Sl. No. Elements By Weight(%)
1. Oxygen 46.60
2. Silicon 27.72
3. Aluminium 8.13
4. Iron 5.00
5. Calcium 3.63
6. Sodium 2.83
7. Potassium 2.59
8. Magnesium 2.09
9. Others 1.41
The elements in the earth’s crust are rarely
found exclusively but are usually combined with
other elements to make various substances.
These substances are recognised as minerals.
Thus, a mineral is a naturally occurring
organic and inorganic substance, having
an orderly atomic structure and a definite
chemical composition and physical
properties. A mineral is composed of two
or more elements. But, sometimes single
element minerals like sulphur, copper,
silver, gold, graphite etc. are found.
Though the number of elements making
up the lithosphere are limited they are
combined in many different ways to make up
many varieties of minerals.  There are at least
2,000 minerals that have been named and
identified in the earth crust; but almost all the
commonly occurring ones are related to six
major mineral groups that are known as major
rock forming minerals.
The basic source of all minerals is the hot
magma in the interior of the earth. When
magma cools, crystals of minerals appear and
a systematic series of minerals are formed in
sequence to solidify so as to form rocks.
Minerals such as coal, petroleum and natural
gas are organic substances found in solid,
liquid and gaseous forms respectively.
A brief information about some important
minerals in terms of their nature and physical
characteristics is given below :
PHYSICAL CHARACTERISTICS
(i) External crystal form — deter-
mined by internal arrangement of
the molecules — cubes, octahe-
drons, hexagonal prisms, etc.
(ii) Cleavage — tendency to break in
given directions producing
relatively plane surfaces — result
of internal arrangement of the
molecules — may cleave in one or
more directions and at any angle
to each other.
© NCERT
not to be republished
Page 3


LANDFORMS
This unit deals with
• Rocks and minerals — major types of rocks and their
characteristics
• Landforms and their evolution
• Geomorphic processes — weathering, mass wasting, erosion
and deposition; soils — formation
UNIT
III
© NCERT
not to be republished
MINERALS AND ROCKS
CHAPTER
T
he earth is composed of various kinds
of elements.  These elements are in solid
form in the outer layer of the earth and
in hot and molten form in the interior.
About 98 per cent of the total crust of the
earth is composed of eight elements like
oxygen, silicon, aluminium, iron, calcium,
sodium, potassium and magnesium (Table 5.1),
and the rest is constituted by titanium,
hydrogen, phosphorous, manganese, sulphur,
carbon, nickel and other elements.
Table 5.1 : The Major Elements of the Earth’s Crust
Sl. No. Elements By Weight(%)
1. Oxygen 46.60
2. Silicon 27.72
3. Aluminium 8.13
4. Iron 5.00
5. Calcium 3.63
6. Sodium 2.83
7. Potassium 2.59
8. Magnesium 2.09
9. Others 1.41
The elements in the earth’s crust are rarely
found exclusively but are usually combined with
other elements to make various substances.
These substances are recognised as minerals.
Thus, a mineral is a naturally occurring
organic and inorganic substance, having
an orderly atomic structure and a definite
chemical composition and physical
properties. A mineral is composed of two
or more elements. But, sometimes single
element minerals like sulphur, copper,
silver, gold, graphite etc. are found.
Though the number of elements making
up the lithosphere are limited they are
combined in many different ways to make up
many varieties of minerals.  There are at least
2,000 minerals that have been named and
identified in the earth crust; but almost all the
commonly occurring ones are related to six
major mineral groups that are known as major
rock forming minerals.
The basic source of all minerals is the hot
magma in the interior of the earth. When
magma cools, crystals of minerals appear and
a systematic series of minerals are formed in
sequence to solidify so as to form rocks.
Minerals such as coal, petroleum and natural
gas are organic substances found in solid,
liquid and gaseous forms respectively.
A brief information about some important
minerals in terms of their nature and physical
characteristics is given below :
PHYSICAL CHARACTERISTICS
(i) External crystal form — deter-
mined by internal arrangement of
the molecules — cubes, octahe-
drons, hexagonal prisms, etc.
(ii) Cleavage — tendency to break in
given directions producing
relatively plane surfaces — result
of internal arrangement of the
molecules — may cleave in one or
more directions and at any angle
to each other.
© NCERT
not to be republished
MINERALS AND ROCKS 41
(iii) Fracture — internal molecular
arrangement so complex there are
no planes of molecules; the crystal
will break in an irregular manner,
not along planes of cleavage.
(iv) Lustre — appearance of a material
without regard to colour; each
mineral has a distinctive lustre like
metallic, silky, glossy etc.
(v) Colour — some minerals have
characteristic colour determined
by their molecular structure —
malachite, azurite, chalcopyrite etc.,
and some minerals are coloured by
impurities.  For example, because
of impurities quartz may be white,
green, red, yellow etc.
(vi) Streak —  colour of the ground powder
of any mineral.  It may be of the
same colour as the mineral or may
differ — malachite is green and gives
green streak, fluorite is purple or
green but gives a white streak.
(vii) Transparency — transparent: light
rays pass through so that objects
can be seen plainly; translucent
— light rays pass through but will
get diffused so that objects cannot
be seen; opaque — light will not pass
at all.
(viii) Structure — particular arrange-
ment of the individual crystals;
fine, medium or coarse grained;
fibrous — separable, divergent,
radiating.
(ix) Hardness — relative resistance
being scratched; ten minerals are
selected to measure the degree of
hardness from 1-10.  They are:
1. talc;  2. gypsum; 3. calcite;
4. fluorite;  5. apatite; 6. feldspar;
7. quartz;  8. topaz; 9. corundum;
10. diamond.  Compared to this for
example, a fingernail is 2.5 and
glass or knife blade is 5.5.
(x) Specific gravity — the ratio between
the weight of a given object and
the weight of an equal volume of
water; object weighed in air and
then weighed in water and divide
weight in air by the difference of the
two weights.
SOME MAJOR MINERALS AND
THEIR CHARACTERISTICS
Feldspar
Silicon and oxygen are common elements in
all types of feldspar and sodium, potassium,
calcium, aluminium etc. are found in specific
feldspar variety. Half of the earth’s crust is
composed of feldspar. It has light cream to
salmon pink colour. It is used in ceramics and
glass making.
Quartz
It is one of the most important components of
sand and granite. It consists of silica. It is a
hard mineral virtually insoluble in water.  It is
white or colourless and used in radio and radar.
It is one of the most important components of
granite.
Pyroxene
Pyroxene consists of calcium, aluminum,
magnesium, iron and silica.  Pyroxene forms
10 per cent of the earth’s crust. It is commonly
found in meteorites.  It is in green or black
colour.
Amphibole
Aluminium, calcium, silica, iron, magnesium
are the major elements of amphiboles. They
form 7 per cent of the earth’s crust.  It is in
green or black colour and is used in asbestos
industry. Hornblende is another form of
amphiboles.
Mica
It comprises of potassium, aluminium,
magnesium, iron, silica etc. It forms 4 per cent
of the earth’s crust.  It is commonly found in
igneous and metamorphic rocks. It is used in
electrical instruments.
Olivine
Magnesium, iron and silica are major elements
of olivine. It is used in jewellery. It is usually a
greenish crystal, often found in basaltic rocks.
Besides these main minerals, other minerals
like chlorite, calcite, magnetite, haematite,
bauxite and barite are also present in some
quantities in the rocks.
© NCERT
not to be republished
Page 4


LANDFORMS
This unit deals with
• Rocks and minerals — major types of rocks and their
characteristics
• Landforms and their evolution
• Geomorphic processes — weathering, mass wasting, erosion
and deposition; soils — formation
UNIT
III
© NCERT
not to be republished
MINERALS AND ROCKS
CHAPTER
T
he earth is composed of various kinds
of elements.  These elements are in solid
form in the outer layer of the earth and
in hot and molten form in the interior.
About 98 per cent of the total crust of the
earth is composed of eight elements like
oxygen, silicon, aluminium, iron, calcium,
sodium, potassium and magnesium (Table 5.1),
and the rest is constituted by titanium,
hydrogen, phosphorous, manganese, sulphur,
carbon, nickel and other elements.
Table 5.1 : The Major Elements of the Earth’s Crust
Sl. No. Elements By Weight(%)
1. Oxygen 46.60
2. Silicon 27.72
3. Aluminium 8.13
4. Iron 5.00
5. Calcium 3.63
6. Sodium 2.83
7. Potassium 2.59
8. Magnesium 2.09
9. Others 1.41
The elements in the earth’s crust are rarely
found exclusively but are usually combined with
other elements to make various substances.
These substances are recognised as minerals.
Thus, a mineral is a naturally occurring
organic and inorganic substance, having
an orderly atomic structure and a definite
chemical composition and physical
properties. A mineral is composed of two
or more elements. But, sometimes single
element minerals like sulphur, copper,
silver, gold, graphite etc. are found.
Though the number of elements making
up the lithosphere are limited they are
combined in many different ways to make up
many varieties of minerals.  There are at least
2,000 minerals that have been named and
identified in the earth crust; but almost all the
commonly occurring ones are related to six
major mineral groups that are known as major
rock forming minerals.
The basic source of all minerals is the hot
magma in the interior of the earth. When
magma cools, crystals of minerals appear and
a systematic series of minerals are formed in
sequence to solidify so as to form rocks.
Minerals such as coal, petroleum and natural
gas are organic substances found in solid,
liquid and gaseous forms respectively.
A brief information about some important
minerals in terms of their nature and physical
characteristics is given below :
PHYSICAL CHARACTERISTICS
(i) External crystal form — deter-
mined by internal arrangement of
the molecules — cubes, octahe-
drons, hexagonal prisms, etc.
(ii) Cleavage — tendency to break in
given directions producing
relatively plane surfaces — result
of internal arrangement of the
molecules — may cleave in one or
more directions and at any angle
to each other.
© NCERT
not to be republished
MINERALS AND ROCKS 41
(iii) Fracture — internal molecular
arrangement so complex there are
no planes of molecules; the crystal
will break in an irregular manner,
not along planes of cleavage.
(iv) Lustre — appearance of a material
without regard to colour; each
mineral has a distinctive lustre like
metallic, silky, glossy etc.
(v) Colour — some minerals have
characteristic colour determined
by their molecular structure —
malachite, azurite, chalcopyrite etc.,
and some minerals are coloured by
impurities.  For example, because
of impurities quartz may be white,
green, red, yellow etc.
(vi) Streak —  colour of the ground powder
of any mineral.  It may be of the
same colour as the mineral or may
differ — malachite is green and gives
green streak, fluorite is purple or
green but gives a white streak.
(vii) Transparency — transparent: light
rays pass through so that objects
can be seen plainly; translucent
— light rays pass through but will
get diffused so that objects cannot
be seen; opaque — light will not pass
at all.
(viii) Structure — particular arrange-
ment of the individual crystals;
fine, medium or coarse grained;
fibrous — separable, divergent,
radiating.
(ix) Hardness — relative resistance
being scratched; ten minerals are
selected to measure the degree of
hardness from 1-10.  They are:
1. talc;  2. gypsum; 3. calcite;
4. fluorite;  5. apatite; 6. feldspar;
7. quartz;  8. topaz; 9. corundum;
10. diamond.  Compared to this for
example, a fingernail is 2.5 and
glass or knife blade is 5.5.
(x) Specific gravity — the ratio between
the weight of a given object and
the weight of an equal volume of
water; object weighed in air and
then weighed in water and divide
weight in air by the difference of the
two weights.
SOME MAJOR MINERALS AND
THEIR CHARACTERISTICS
Feldspar
Silicon and oxygen are common elements in
all types of feldspar and sodium, potassium,
calcium, aluminium etc. are found in specific
feldspar variety. Half of the earth’s crust is
composed of feldspar. It has light cream to
salmon pink colour. It is used in ceramics and
glass making.
Quartz
It is one of the most important components of
sand and granite. It consists of silica. It is a
hard mineral virtually insoluble in water.  It is
white or colourless and used in radio and radar.
It is one of the most important components of
granite.
Pyroxene
Pyroxene consists of calcium, aluminum,
magnesium, iron and silica.  Pyroxene forms
10 per cent of the earth’s crust. It is commonly
found in meteorites.  It is in green or black
colour.
Amphibole
Aluminium, calcium, silica, iron, magnesium
are the major elements of amphiboles. They
form 7 per cent of the earth’s crust.  It is in
green or black colour and is used in asbestos
industry. Hornblende is another form of
amphiboles.
Mica
It comprises of potassium, aluminium,
magnesium, iron, silica etc. It forms 4 per cent
of the earth’s crust.  It is commonly found in
igneous and metamorphic rocks. It is used in
electrical instruments.
Olivine
Magnesium, iron and silica are major elements
of olivine. It is used in jewellery. It is usually a
greenish crystal, often found in basaltic rocks.
Besides these main minerals, other minerals
like chlorite, calcite, magnetite, haematite,
bauxite and barite are also present in some
quantities in the rocks.
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 42
Metallic Minerals
These minerals contain metal content and can
be sub-divided into three types:
(i) Precious metals :  gold, silver, platinum
etc.
(ii) Ferrous metals : iron and other metals
often mixed with iron to form various
kinds of steel.
(iii) Non-ferrous metals : include metals
like copper, lead, zinc, tin, aluminium
etc.
Non-Metallic Minerals
These minerals do not contain metal content.
Sulphur, phosphates and nitrates are examples
of non-metallic minerals. Cement is a mixture
of non-metallic minerals.
ROCKS
The earth’s crust is composed of rocks.  A
rock is an aggregate of one or more minerals.
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. Rocks do not have definite
composition of mineral constituents.
Feldspar and quartz are the most common
minerals found in rocks.
Petrology is science of rocks. A petrologist
studies rocks in all their aspects viz.,
mineral composition, texture, structure,
origin, occurrence, alteration and
relationship with other rocks.
As there is a close relation between rocks
and landforms, rocks and soils, a geographer
requires basic knowledge of rocks.  There are
many different kinds of rocks which are
grouped under three families on the basis of
their mode of formation. They are: (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. You already know what
magma is. When magma in its upward
movement cools and turns into solid form it is
called igneous rock. The process of cooling and
solidification can happen in the earth’s crust
or on the surface of the earth.
Igneous rocks are classified based on
texture.  Texture depends upon size and
arrangement of grains or other physical
conditions of the materials.  If molten material
is cooled slowly at great depths, mineral grains
may be very large.  Sudden cooling (at the
surface) results in small and smooth grains.
Intermediate conditions of cooling would result
in intermediate sizes of grains making up
igneous rocks. Granite, gabbro, pegmatite,
basalt, volcanic breccia and tuff are some of
the examples of igneous rocks.
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.
Depending upon the mode of formation,
sedimentary rocks are classified into three major
groups: (i) mechanically formed — sandstone,
conglomerate, limestone, shale, loess etc. are
examples; (ii) organically formed— geyserite,
chalk, limestone, coal etc. are some examples;
(iii) chemically formed — chert, limestone, halite,
potash etc. are some examples.
© NCERT
not to be republished
Page 5


LANDFORMS
This unit deals with
• Rocks and minerals — major types of rocks and their
characteristics
• Landforms and their evolution
• Geomorphic processes — weathering, mass wasting, erosion
and deposition; soils — formation
UNIT
III
© NCERT
not to be republished
MINERALS AND ROCKS
CHAPTER
T
he earth is composed of various kinds
of elements.  These elements are in solid
form in the outer layer of the earth and
in hot and molten form in the interior.
About 98 per cent of the total crust of the
earth is composed of eight elements like
oxygen, silicon, aluminium, iron, calcium,
sodium, potassium and magnesium (Table 5.1),
and the rest is constituted by titanium,
hydrogen, phosphorous, manganese, sulphur,
carbon, nickel and other elements.
Table 5.1 : The Major Elements of the Earth’s Crust
Sl. No. Elements By Weight(%)
1. Oxygen 46.60
2. Silicon 27.72
3. Aluminium 8.13
4. Iron 5.00
5. Calcium 3.63
6. Sodium 2.83
7. Potassium 2.59
8. Magnesium 2.09
9. Others 1.41
The elements in the earth’s crust are rarely
found exclusively but are usually combined with
other elements to make various substances.
These substances are recognised as minerals.
Thus, a mineral is a naturally occurring
organic and inorganic substance, having
an orderly atomic structure and a definite
chemical composition and physical
properties. A mineral is composed of two
or more elements. But, sometimes single
element minerals like sulphur, copper,
silver, gold, graphite etc. are found.
Though the number of elements making
up the lithosphere are limited they are
combined in many different ways to make up
many varieties of minerals.  There are at least
2,000 minerals that have been named and
identified in the earth crust; but almost all the
commonly occurring ones are related to six
major mineral groups that are known as major
rock forming minerals.
The basic source of all minerals is the hot
magma in the interior of the earth. When
magma cools, crystals of minerals appear and
a systematic series of minerals are formed in
sequence to solidify so as to form rocks.
Minerals such as coal, petroleum and natural
gas are organic substances found in solid,
liquid and gaseous forms respectively.
A brief information about some important
minerals in terms of their nature and physical
characteristics is given below :
PHYSICAL CHARACTERISTICS
(i) External crystal form — deter-
mined by internal arrangement of
the molecules — cubes, octahe-
drons, hexagonal prisms, etc.
(ii) Cleavage — tendency to break in
given directions producing
relatively plane surfaces — result
of internal arrangement of the
molecules — may cleave in one or
more directions and at any angle
to each other.
© NCERT
not to be republished
MINERALS AND ROCKS 41
(iii) Fracture — internal molecular
arrangement so complex there are
no planes of molecules; the crystal
will break in an irregular manner,
not along planes of cleavage.
(iv) Lustre — appearance of a material
without regard to colour; each
mineral has a distinctive lustre like
metallic, silky, glossy etc.
(v) Colour — some minerals have
characteristic colour determined
by their molecular structure —
malachite, azurite, chalcopyrite etc.,
and some minerals are coloured by
impurities.  For example, because
of impurities quartz may be white,
green, red, yellow etc.
(vi) Streak —  colour of the ground powder
of any mineral.  It may be of the
same colour as the mineral or may
differ — malachite is green and gives
green streak, fluorite is purple or
green but gives a white streak.
(vii) Transparency — transparent: light
rays pass through so that objects
can be seen plainly; translucent
— light rays pass through but will
get diffused so that objects cannot
be seen; opaque — light will not pass
at all.
(viii) Structure — particular arrange-
ment of the individual crystals;
fine, medium or coarse grained;
fibrous — separable, divergent,
radiating.
(ix) Hardness — relative resistance
being scratched; ten minerals are
selected to measure the degree of
hardness from 1-10.  They are:
1. talc;  2. gypsum; 3. calcite;
4. fluorite;  5. apatite; 6. feldspar;
7. quartz;  8. topaz; 9. corundum;
10. diamond.  Compared to this for
example, a fingernail is 2.5 and
glass or knife blade is 5.5.
(x) Specific gravity — the ratio between
the weight of a given object and
the weight of an equal volume of
water; object weighed in air and
then weighed in water and divide
weight in air by the difference of the
two weights.
SOME MAJOR MINERALS AND
THEIR CHARACTERISTICS
Feldspar
Silicon and oxygen are common elements in
all types of feldspar and sodium, potassium,
calcium, aluminium etc. are found in specific
feldspar variety. Half of the earth’s crust is
composed of feldspar. It has light cream to
salmon pink colour. It is used in ceramics and
glass making.
Quartz
It is one of the most important components of
sand and granite. It consists of silica. It is a
hard mineral virtually insoluble in water.  It is
white or colourless and used in radio and radar.
It is one of the most important components of
granite.
Pyroxene
Pyroxene consists of calcium, aluminum,
magnesium, iron and silica.  Pyroxene forms
10 per cent of the earth’s crust. It is commonly
found in meteorites.  It is in green or black
colour.
Amphibole
Aluminium, calcium, silica, iron, magnesium
are the major elements of amphiboles. They
form 7 per cent of the earth’s crust.  It is in
green or black colour and is used in asbestos
industry. Hornblende is another form of
amphiboles.
Mica
It comprises of potassium, aluminium,
magnesium, iron, silica etc. It forms 4 per cent
of the earth’s crust.  It is commonly found in
igneous and metamorphic rocks. It is used in
electrical instruments.
Olivine
Magnesium, iron and silica are major elements
of olivine. It is used in jewellery. It is usually a
greenish crystal, often found in basaltic rocks.
Besides these main minerals, other minerals
like chlorite, calcite, magnetite, haematite,
bauxite and barite are also present in some
quantities in the rocks.
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 42
Metallic Minerals
These minerals contain metal content and can
be sub-divided into three types:
(i) Precious metals :  gold, silver, platinum
etc.
(ii) Ferrous metals : iron and other metals
often mixed with iron to form various
kinds of steel.
(iii) Non-ferrous metals : include metals
like copper, lead, zinc, tin, aluminium
etc.
Non-Metallic Minerals
These minerals do not contain metal content.
Sulphur, phosphates and nitrates are examples
of non-metallic minerals. Cement is a mixture
of non-metallic minerals.
ROCKS
The earth’s crust is composed of rocks.  A
rock is an aggregate of one or more minerals.
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. Rocks do not have definite
composition of mineral constituents.
Feldspar and quartz are the most common
minerals found in rocks.
Petrology is science of rocks. A petrologist
studies rocks in all their aspects viz.,
mineral composition, texture, structure,
origin, occurrence, alteration and
relationship with other rocks.
As there is a close relation between rocks
and landforms, rocks and soils, a geographer
requires basic knowledge of rocks.  There are
many different kinds of rocks which are
grouped under three families on the basis of
their mode of formation. They are: (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. You already know what
magma is. When magma in its upward
movement cools and turns into solid form it is
called igneous rock. The process of cooling and
solidification can happen in the earth’s crust
or on the surface of the earth.
Igneous rocks are classified based on
texture.  Texture depends upon size and
arrangement of grains or other physical
conditions of the materials.  If molten material
is cooled slowly at great depths, mineral grains
may be very large.  Sudden cooling (at the
surface) results in small and smooth grains.
Intermediate conditions of cooling would result
in intermediate sizes of grains making up
igneous rocks. Granite, gabbro, pegmatite,
basalt, volcanic breccia and tuff are some of
the examples of igneous rocks.
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.
Depending upon the mode of formation,
sedimentary rocks are classified into three major
groups: (i) mechanically formed — sandstone,
conglomerate, limestone, shale, loess etc. are
examples; (ii) organically formed— geyserite,
chalk, limestone, coal etc. are some examples;
(iii) chemically formed — chert, limestone, halite,
potash etc. are some examples.
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MINERALS AND ROCKS 43
Metamorphic Rocks
The word metamorphic means ‘change of form’.
These rocks form under the action of pressure,
volume and temperature (PVT) changes.
Metamorphism occurs 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 or the
underlying rocks are subjected to great
amounts of pressure by overlying rocks.
Metamorphism is a process by which already
consolidated rocks undergo recrystallisation
and reorganisation of materials within original
rocks.
Mechanical disruption and reorganisation
of the original minerals within rocks due to
breaking and crushing without any
appreciable chemical changes is called dynamic
metamorphism. The materials  of rocks
chemically alter and recrystallise due to
thermal metamorphism.  There are two types
of thermal metamorphism — contact meta-
morphism and 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.  Sometimes minerals or materials of
different groups are arranged into alternating
thin to thick layers appearing in light and dark
shades. Such a structure in metamorphic
rocks is called banding and rocks displaying
banding are called banded rocks. Types of
metamorphic rocks depend upon original
rocks that were subjected to metamorphism.
Metamorphic rocks are classified into two
major groups — foliated rocks and non-foliated
rocks.  Gneissoid, granite, syenite, slate, schist,
marble, quartzite etc. are some examples of
metamorphic rocks.
ROCK CYCLE
Rocks do not remain in their original form for
long but may undergo transformation.  Rock
cycle is a continuous process through which
old rocks are transformed into new ones.
Igneous rocks are primary rocks and other
rocks (sedimentary and metamorphic) form
from these primary rocks.  Igneous rocks can
be changed into metamorphic rocks.  The
fragments derived out of igneous and
metamorphic rocks form into sedimentary
rocks.  Sedimentary rocks themselves can turn
into fragments and the fragments can be a
source for formation of sedimentary rocks.  The
crustal rocks (igneous, metamorphic and
sedimentary) once formed may be carried
down into the mantle (interior of the earth)
through subduction process (parts or whole
of crustal plates going down under another
plate in zones of plate convergence) and the
same melt down due to increase in
temperature in the interior and turn into
molten magma, the original source for
igneous rocks (Figure 5.1).
 Fig 5.1 : Rock Cycle
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