NCERT Textbook - Minerals and Rocks Humanities/Arts Notes | EduRev

NCERT Textbooks (Class 6 to Class 12)

Humanities/Arts : NCERT Textbook - Minerals and Rocks Humanities/Arts Notes | EduRev

 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
2020-21
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
2020-21
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, and the
rest is constituted by titanium, hydrogen,
phosphorous, manganese, sulphur, carbon,
nickel and other elements.
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.
(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.
2020-21
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
2020-21
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, and the
rest is constituted by titanium, hydrogen,
phosphorous, manganese, sulphur, carbon,
nickel and other elements.
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.
(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.
2020-21
MINERALS AND ROCKS 41
(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.
(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 –
Metallic Minerals
These minerals contain metal content and can
be sub-divided into three types:
(i) Precious metals :  gold, silver , platinum
etc.
2020-21
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
2020-21
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, and the
rest is constituted by titanium, hydrogen,
phosphorous, manganese, sulphur, carbon,
nickel and other elements.
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.
(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.
2020-21
MINERALS AND ROCKS 41
(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.
(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 –
Metallic Minerals
These minerals contain metal content and can
be sub-divided into three types:
(i) Precious metals :  gold, silver , platinum
etc.
2020-21
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 42
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.
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
2020-21
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
2020-21
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, and the
rest is constituted by titanium, hydrogen,
phosphorous, manganese, sulphur, carbon,
nickel and other elements.
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.
(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.
2020-21
MINERALS AND ROCKS 41
(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.
(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 –
Metallic Minerals
These minerals contain metal content and can
be sub-divided into three types:
(i) Precious metals :  gold, silver , platinum
etc.
2020-21
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 42
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.
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
2020-21
MINERALS AND ROCKS 43
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).
EXERCISES
1. Multiple choice questions.
  (i) Which one of the following are the two main constituents of granite?
(a) Iron and nickel (c) Silica and aluminium
(b) Iron and silver (d) Iron Oxide and potassium
 (ii) Which one of the following is the salient feature of metamorphic rocks?
(a) Changeable (c) Crystalline
(b) Quite (d) Foliation
(iii) Which one of the following is not a single element mineral?
(a) Gold (c) Mica
(b) Silver (d) Graphite
 (iv) Which one of the following is the hardest mineral?
(a) Topaz (c) Quartz
(b) Diamond (d) Feldspar
 (v) Which one of the following is not a sedimentary rock?
(a) Tillite (c) Breccia
(b) Borax (d) Marble
2. Answer the following questions in about 30 words.
(i) What do you mean by rocks? Name the three major classes of rocks.
(ii) What is an igneous rock? Describe the method of formation and
characteristics of igneous rock.
 Fig 5.1 : Rock Cycle
2020-21
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