NCERT Textbook: Geomorphic Processes - UPSC PDF Download

 Page 1


GEOMORPHIC PROCESSES
CHAPTER
A
fter learning about how the earth was
born, how it evolved its crust and other
inner layers, how its crustal plates
moved and are moving, and other information
on earthquakes, the forms of volcanism and
about the rocks and minerals the crust is
composed of, it is time to know in detail about
the surface of the earth on which we live. Let
us start with this question.
Why is the surface of the earth uneven?
The earth’s crust is dynamic. You are well
aware that it has moved and moves vertically
and horizontally. Of course, it moved a bit faster
in the past than the rate at which it is moving
now. The differences in the internal forces
operating from within the earth which built up
the crust have been responsible for the
variations in the outer surface of the crust. The
earth’s surface is being continuously subjected
to external forces induced basically by energy
(sunlight). Of course, the internal forces are still
active though with different intensities. That
means, the earth’s surface is being
continuously subjected to by external forces
originating within the earth’s atmosphere and
by internal forces from within the earth. The
external forces are known as exogenic forces
and the internal forces are known as endogenic
forces. The actions of exogenic forces result in
wearing down (degradation) of relief/elevations
and filling up (aggradation) of basins/
depressions, on the earth’s surface. The
phenomenon of wearing down of relief
variations of the surface of the earth through
erosion is known as gradation. The endogenic
forces continuously elevate or build up parts
of the earth’s surface and hence the exogenic
processes fail to even out the relief variations
of the surface of the earth. So, variations remain
as long as the opposing actions of exogenic and
endogenic forces continue. In general terms,
the endogenic forces are mainly land building
forces and the exogenic processes are mainly
land wearing forces. The surface of the earth is
sensitive. Humans depend on it for their
sustenance and have been using it extensively
and intensively. So, it is essential to understand
its nature in order to use it effectively without
disturbing its balance and diminishing its
potential for the future. Almost all organisms
contribute to sustain the earth’s environment.
However, humans have caused extensive
damage to the environment through over use
of resources. Use we must, but must also leave
it potential enough to sustain life through the
future. Most of the surface of the earth had and
has been shaped over very long periods of time
(hundreds and thousands of years) and
because of its use and misuse by humans its
potential is being diminished at a fast rate. If
the processes which shaped and are shaping
the surface of the earth into varieties of forms
(shapes) and the nature of materials of which
it is composed of, are understood, precautions
can be taken to minimise the detrimental effects
of human use and to preserve it for posterity.
GEOMORPHIC PROCESSES
You would like to know the meaning of
geomorphic processes. The endogenic and
exogenic forces causing physical stresses and
chemical actions on earth materials and
© NCERT
not to be republished
Page 2


GEOMORPHIC PROCESSES
CHAPTER
A
fter learning about how the earth was
born, how it evolved its crust and other
inner layers, how its crustal plates
moved and are moving, and other information
on earthquakes, the forms of volcanism and
about the rocks and minerals the crust is
composed of, it is time to know in detail about
the surface of the earth on which we live. Let
us start with this question.
Why is the surface of the earth uneven?
The earth’s crust is dynamic. You are well
aware that it has moved and moves vertically
and horizontally. Of course, it moved a bit faster
in the past than the rate at which it is moving
now. The differences in the internal forces
operating from within the earth which built up
the crust have been responsible for the
variations in the outer surface of the crust. The
earth’s surface is being continuously subjected
to external forces induced basically by energy
(sunlight). Of course, the internal forces are still
active though with different intensities. That
means, the earth’s surface is being
continuously subjected to by external forces
originating within the earth’s atmosphere and
by internal forces from within the earth. The
external forces are known as exogenic forces
and the internal forces are known as endogenic
forces. The actions of exogenic forces result in
wearing down (degradation) of relief/elevations
and filling up (aggradation) of basins/
depressions, on the earth’s surface. The
phenomenon of wearing down of relief
variations of the surface of the earth through
erosion is known as gradation. The endogenic
forces continuously elevate or build up parts
of the earth’s surface and hence the exogenic
processes fail to even out the relief variations
of the surface of the earth. So, variations remain
as long as the opposing actions of exogenic and
endogenic forces continue. In general terms,
the endogenic forces are mainly land building
forces and the exogenic processes are mainly
land wearing forces. The surface of the earth is
sensitive. Humans depend on it for their
sustenance and have been using it extensively
and intensively. So, it is essential to understand
its nature in order to use it effectively without
disturbing its balance and diminishing its
potential for the future. Almost all organisms
contribute to sustain the earth’s environment.
However, humans have caused extensive
damage to the environment through over use
of resources. Use we must, but must also leave
it potential enough to sustain life through the
future. Most of the surface of the earth had and
has been shaped over very long periods of time
(hundreds and thousands of years) and
because of its use and misuse by humans its
potential is being diminished at a fast rate. If
the processes which shaped and are shaping
the surface of the earth into varieties of forms
(shapes) and the nature of materials of which
it is composed of, are understood, precautions
can be taken to minimise the detrimental effects
of human use and to preserve it for posterity.
GEOMORPHIC PROCESSES
You would like to know the meaning of
geomorphic processes. The endogenic and
exogenic forces causing physical stresses and
chemical actions on earth materials and
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 46
bringing about changes in the configuration
of the surface of the earth are known as
geomorphic processes. Diastrophism and
volcanism are endogenic geomorphic
processes. These have already been discussed
in brief in the preceding unit. Weathering, mass
wasting, erosion and deposition are exogenic
geomorphic processes. These exogenic
processes are dealt with in detail in this chapter.
Any exogenic element of nature (like water,
ice, wind, etc.,) capable of acquiring and
transporting earth materials can be called a
geomorphic agent. When these elements of
nature become mobile due to gradients, they
remove the materials and transport them over
slopes and deposit them at lower level.
Geomorphic processes and geomorphic agents
especially exogenic, unless stated separately,
are one and the same.
A process is a force applied on earth
materials affecting the same.  An agent is a
mobile medium (like running water, moving ice
masses, wind, waves and currents etc.) which
removes, transports and deposits earth
materials. Running water, groundwater,
glaciers, wind, waves and currents, etc., can
be called geomorphic agents.
Do you think it is essential to distinguish
geomorphic agents and geomorphic
processes?
Gravity besides being a directional force
activating all downslope movements of matter
also causes stresses on the earth’s materials.
Indirect gravitational stresses activate wave and
tide induced currents and winds. Without
gravity and gradients there would be no
mobility and hence no erosion, transportation
and deposition are possible. So, gravitational
stresses are as important as the other
geomorphic processes. Gravity is the force that
is keeping us in contact with the surface and it
is the force that switches on the movement of
all surface material on earth. All the movements
either within the earth or on the surface of the
earth occur due to gradients — from higher
levels to lower levels, from high pressure to low
pressure areas etc.
ENDOGENIC PROCESSES
The energy emanating from within the earth is
the main force behind endogenic geomorphic
processes. This energy is mostly generated by
radioactivity, rotational and tidal friction and
primordial heat from the origin of the earth.
This energy due to geothermal gradients and
heat flow from within induces diastrophism
and volcanism in the lithosphere. Due to
variations in geothermal gradients and heat flow
from within, crustal thickness and strength,
the action of endogenic forces are not uniform
and hence the tectonically controlled original
crustal surface is uneven.
Diastrophism
All processes that move, elevate or build up
portions of the earth’s crust come under
diastrophism. They include: (i) orogenic
processes involving mountain building
through severe folding and affecting long and
narrow belts of the earth’s crust; (ii) epeirogenic
processes involving uplift or warping of large
parts of the earth’s crust; (iii) earthquakes
involving local relatively minor movements;
(iv) plate tectonics involving horizontal
movements of crustal plates.
In the process of orogeny, the crust is
severely deformed into folds. Due to epeirogeny,
there may be simple deformation. Orogeny is
a mountain building process whereas
epeirogeny is continental building process.
Through the processes of orogeny, epeirogeny,
earthquakes and plate tectonics, there can be
faulting and fracturing of the crust. All these
processes cause pressure, volume and
temperature (PVT) changes which in turn
induce metamorphism of rocks.
Epeirogeny and orogeny, cite the
differences.
Volcanism
Volcanism includes the movement of molten
rock (magma) onto or toward the earth’s
surface and also formation of many intrusive
and extrusive volcanic forms. Many aspects of
volcanism have already been dealt in detail
© NCERT
not to be republished
Page 3


GEOMORPHIC PROCESSES
CHAPTER
A
fter learning about how the earth was
born, how it evolved its crust and other
inner layers, how its crustal plates
moved and are moving, and other information
on earthquakes, the forms of volcanism and
about the rocks and minerals the crust is
composed of, it is time to know in detail about
the surface of the earth on which we live. Let
us start with this question.
Why is the surface of the earth uneven?
The earth’s crust is dynamic. You are well
aware that it has moved and moves vertically
and horizontally. Of course, it moved a bit faster
in the past than the rate at which it is moving
now. The differences in the internal forces
operating from within the earth which built up
the crust have been responsible for the
variations in the outer surface of the crust. The
earth’s surface is being continuously subjected
to external forces induced basically by energy
(sunlight). Of course, the internal forces are still
active though with different intensities. That
means, the earth’s surface is being
continuously subjected to by external forces
originating within the earth’s atmosphere and
by internal forces from within the earth. The
external forces are known as exogenic forces
and the internal forces are known as endogenic
forces. The actions of exogenic forces result in
wearing down (degradation) of relief/elevations
and filling up (aggradation) of basins/
depressions, on the earth’s surface. The
phenomenon of wearing down of relief
variations of the surface of the earth through
erosion is known as gradation. The endogenic
forces continuously elevate or build up parts
of the earth’s surface and hence the exogenic
processes fail to even out the relief variations
of the surface of the earth. So, variations remain
as long as the opposing actions of exogenic and
endogenic forces continue. In general terms,
the endogenic forces are mainly land building
forces and the exogenic processes are mainly
land wearing forces. The surface of the earth is
sensitive. Humans depend on it for their
sustenance and have been using it extensively
and intensively. So, it is essential to understand
its nature in order to use it effectively without
disturbing its balance and diminishing its
potential for the future. Almost all organisms
contribute to sustain the earth’s environment.
However, humans have caused extensive
damage to the environment through over use
of resources. Use we must, but must also leave
it potential enough to sustain life through the
future. Most of the surface of the earth had and
has been shaped over very long periods of time
(hundreds and thousands of years) and
because of its use and misuse by humans its
potential is being diminished at a fast rate. If
the processes which shaped and are shaping
the surface of the earth into varieties of forms
(shapes) and the nature of materials of which
it is composed of, are understood, precautions
can be taken to minimise the detrimental effects
of human use and to preserve it for posterity.
GEOMORPHIC PROCESSES
You would like to know the meaning of
geomorphic processes. The endogenic and
exogenic forces causing physical stresses and
chemical actions on earth materials and
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 46
bringing about changes in the configuration
of the surface of the earth are known as
geomorphic processes. Diastrophism and
volcanism are endogenic geomorphic
processes. These have already been discussed
in brief in the preceding unit. Weathering, mass
wasting, erosion and deposition are exogenic
geomorphic processes. These exogenic
processes are dealt with in detail in this chapter.
Any exogenic element of nature (like water,
ice, wind, etc.,) capable of acquiring and
transporting earth materials can be called a
geomorphic agent. When these elements of
nature become mobile due to gradients, they
remove the materials and transport them over
slopes and deposit them at lower level.
Geomorphic processes and geomorphic agents
especially exogenic, unless stated separately,
are one and the same.
A process is a force applied on earth
materials affecting the same.  An agent is a
mobile medium (like running water, moving ice
masses, wind, waves and currents etc.) which
removes, transports and deposits earth
materials. Running water, groundwater,
glaciers, wind, waves and currents, etc., can
be called geomorphic agents.
Do you think it is essential to distinguish
geomorphic agents and geomorphic
processes?
Gravity besides being a directional force
activating all downslope movements of matter
also causes stresses on the earth’s materials.
Indirect gravitational stresses activate wave and
tide induced currents and winds. Without
gravity and gradients there would be no
mobility and hence no erosion, transportation
and deposition are possible. So, gravitational
stresses are as important as the other
geomorphic processes. Gravity is the force that
is keeping us in contact with the surface and it
is the force that switches on the movement of
all surface material on earth. All the movements
either within the earth or on the surface of the
earth occur due to gradients — from higher
levels to lower levels, from high pressure to low
pressure areas etc.
ENDOGENIC PROCESSES
The energy emanating from within the earth is
the main force behind endogenic geomorphic
processes. This energy is mostly generated by
radioactivity, rotational and tidal friction and
primordial heat from the origin of the earth.
This energy due to geothermal gradients and
heat flow from within induces diastrophism
and volcanism in the lithosphere. Due to
variations in geothermal gradients and heat flow
from within, crustal thickness and strength,
the action of endogenic forces are not uniform
and hence the tectonically controlled original
crustal surface is uneven.
Diastrophism
All processes that move, elevate or build up
portions of the earth’s crust come under
diastrophism. They include: (i) orogenic
processes involving mountain building
through severe folding and affecting long and
narrow belts of the earth’s crust; (ii) epeirogenic
processes involving uplift or warping of large
parts of the earth’s crust; (iii) earthquakes
involving local relatively minor movements;
(iv) plate tectonics involving horizontal
movements of crustal plates.
In the process of orogeny, the crust is
severely deformed into folds. Due to epeirogeny,
there may be simple deformation. Orogeny is
a mountain building process whereas
epeirogeny is continental building process.
Through the processes of orogeny, epeirogeny,
earthquakes and plate tectonics, there can be
faulting and fracturing of the crust. All these
processes cause pressure, volume and
temperature (PVT) changes which in turn
induce metamorphism of rocks.
Epeirogeny and orogeny, cite the
differences.
Volcanism
Volcanism includes the movement of molten
rock (magma) onto or toward the earth’s
surface and also formation of many intrusive
and extrusive volcanic forms. Many aspects of
volcanism have already been dealt in detail
© NCERT
not to be republished
GEOMORPHIC PROCESSES 47
processes and their respective driving forces.
It should become clear from this chart that for
each process there exists a distinct driving force
or energy.
As there are different climatic regions on
the earth’s surface owing to thermal gradients
created by latitudinal, seasonal and land and
water spread variations, the exogenic
geomorphic processes vary from region to
region.  The density, type and distribution of
vegetation which largely depend upon
under volcanoes in the Unit II and under
igneous rocks in the preceding chapter in this
unit.
What do the words volcanism and
volcanoes indicate?
EXOGENIC PROCESSES
The exogenic processes derive their energy
from atmosphere determined by the ultimate
energy from the sun and also the gradients
created by tectonic factors.
Why do you think that the slopes or
gradients are created by tectonic factors?
Gravitational force acts upon all earth
materials having a sloping surface and tend to
produce movement of matter in down slope
direction. Force applied per unit area is called
stress. Stress is produced in a solid by pushing
or pulling. This induces deformation. Forces
acting along the faces of earth materials are
shear stresses (separating forces). It is this
stress that breaks rocks and other earth
materials. The shear stresses result in angular
displacement or slippage. Besides the
gravitational stress earth materials become
subjected to molecular stresses that may be
caused by a number of factors amongst which
temperature changes, crystallisation and
melting are the most common. Chemical
processes normally lead to loosening of bonds
between grains, dissolving of soluble minerals
or cementing materials. Thus, the basic reason
that leads to weathering, mass movements, and
erosion is development of stresses in the body
of the earth materials.
As there are different climatic regions on
the earth’s surface the exogenic geomorphic
processes vary from region to region.
Temperature and precipitation are the two
important climatic elements that control
various processes.
All the exogenic geomorphic processes are
covered under a general term, denudation. The
word ‘denude’ means to strip off or to uncover.
Weathering, mass wasting/movements, erosion
and transportation are included in denudation.
The flow chart (Figure 6.1) gives the denudation
precipitation and temperature exert influence
indirectly on exogenic geomorphic processes.
Within different climatic regions there may be
local variations of the effects of different climatic
elements due to altitudinal differences, aspect
variations and the variation in the amount of
insolation received by north and south facing
slopes as compared to east and west facing
slopes. Further, due to differences in wind
velocities and directions, amount and kind of
precipitation, its intensity, the relation between
precipitation and evaporation, daily range of
temperature, freezing and thawing frequency,
depth of frost penetration, the geomorphic
processes vary within any climatic region.
What is the sole driving force behind all
the exogenic processes?
Climatic factors being equal, the intensity
of action of exogenic geomorphic processes
depends upon type and structure of rocks. The
term structure includes such aspects of rocks
as folds, faults, orientation and inclination of
beds, presence or absence of joints, bedding
planes, hardness or softness of constituent
minerals, chemical susceptibility of mineral
constituents; the permeability or impermeability
Figure 6.1 : Denudational processes and their
driving forces
© NCERT
not to be republished
Page 4


GEOMORPHIC PROCESSES
CHAPTER
A
fter learning about how the earth was
born, how it evolved its crust and other
inner layers, how its crustal plates
moved and are moving, and other information
on earthquakes, the forms of volcanism and
about the rocks and minerals the crust is
composed of, it is time to know in detail about
the surface of the earth on which we live. Let
us start with this question.
Why is the surface of the earth uneven?
The earth’s crust is dynamic. You are well
aware that it has moved and moves vertically
and horizontally. Of course, it moved a bit faster
in the past than the rate at which it is moving
now. The differences in the internal forces
operating from within the earth which built up
the crust have been responsible for the
variations in the outer surface of the crust. The
earth’s surface is being continuously subjected
to external forces induced basically by energy
(sunlight). Of course, the internal forces are still
active though with different intensities. That
means, the earth’s surface is being
continuously subjected to by external forces
originating within the earth’s atmosphere and
by internal forces from within the earth. The
external forces are known as exogenic forces
and the internal forces are known as endogenic
forces. The actions of exogenic forces result in
wearing down (degradation) of relief/elevations
and filling up (aggradation) of basins/
depressions, on the earth’s surface. The
phenomenon of wearing down of relief
variations of the surface of the earth through
erosion is known as gradation. The endogenic
forces continuously elevate or build up parts
of the earth’s surface and hence the exogenic
processes fail to even out the relief variations
of the surface of the earth. So, variations remain
as long as the opposing actions of exogenic and
endogenic forces continue. In general terms,
the endogenic forces are mainly land building
forces and the exogenic processes are mainly
land wearing forces. The surface of the earth is
sensitive. Humans depend on it for their
sustenance and have been using it extensively
and intensively. So, it is essential to understand
its nature in order to use it effectively without
disturbing its balance and diminishing its
potential for the future. Almost all organisms
contribute to sustain the earth’s environment.
However, humans have caused extensive
damage to the environment through over use
of resources. Use we must, but must also leave
it potential enough to sustain life through the
future. Most of the surface of the earth had and
has been shaped over very long periods of time
(hundreds and thousands of years) and
because of its use and misuse by humans its
potential is being diminished at a fast rate. If
the processes which shaped and are shaping
the surface of the earth into varieties of forms
(shapes) and the nature of materials of which
it is composed of, are understood, precautions
can be taken to minimise the detrimental effects
of human use and to preserve it for posterity.
GEOMORPHIC PROCESSES
You would like to know the meaning of
geomorphic processes. The endogenic and
exogenic forces causing physical stresses and
chemical actions on earth materials and
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 46
bringing about changes in the configuration
of the surface of the earth are known as
geomorphic processes. Diastrophism and
volcanism are endogenic geomorphic
processes. These have already been discussed
in brief in the preceding unit. Weathering, mass
wasting, erosion and deposition are exogenic
geomorphic processes. These exogenic
processes are dealt with in detail in this chapter.
Any exogenic element of nature (like water,
ice, wind, etc.,) capable of acquiring and
transporting earth materials can be called a
geomorphic agent. When these elements of
nature become mobile due to gradients, they
remove the materials and transport them over
slopes and deposit them at lower level.
Geomorphic processes and geomorphic agents
especially exogenic, unless stated separately,
are one and the same.
A process is a force applied on earth
materials affecting the same.  An agent is a
mobile medium (like running water, moving ice
masses, wind, waves and currents etc.) which
removes, transports and deposits earth
materials. Running water, groundwater,
glaciers, wind, waves and currents, etc., can
be called geomorphic agents.
Do you think it is essential to distinguish
geomorphic agents and geomorphic
processes?
Gravity besides being a directional force
activating all downslope movements of matter
also causes stresses on the earth’s materials.
Indirect gravitational stresses activate wave and
tide induced currents and winds. Without
gravity and gradients there would be no
mobility and hence no erosion, transportation
and deposition are possible. So, gravitational
stresses are as important as the other
geomorphic processes. Gravity is the force that
is keeping us in contact with the surface and it
is the force that switches on the movement of
all surface material on earth. All the movements
either within the earth or on the surface of the
earth occur due to gradients — from higher
levels to lower levels, from high pressure to low
pressure areas etc.
ENDOGENIC PROCESSES
The energy emanating from within the earth is
the main force behind endogenic geomorphic
processes. This energy is mostly generated by
radioactivity, rotational and tidal friction and
primordial heat from the origin of the earth.
This energy due to geothermal gradients and
heat flow from within induces diastrophism
and volcanism in the lithosphere. Due to
variations in geothermal gradients and heat flow
from within, crustal thickness and strength,
the action of endogenic forces are not uniform
and hence the tectonically controlled original
crustal surface is uneven.
Diastrophism
All processes that move, elevate or build up
portions of the earth’s crust come under
diastrophism. They include: (i) orogenic
processes involving mountain building
through severe folding and affecting long and
narrow belts of the earth’s crust; (ii) epeirogenic
processes involving uplift or warping of large
parts of the earth’s crust; (iii) earthquakes
involving local relatively minor movements;
(iv) plate tectonics involving horizontal
movements of crustal plates.
In the process of orogeny, the crust is
severely deformed into folds. Due to epeirogeny,
there may be simple deformation. Orogeny is
a mountain building process whereas
epeirogeny is continental building process.
Through the processes of orogeny, epeirogeny,
earthquakes and plate tectonics, there can be
faulting and fracturing of the crust. All these
processes cause pressure, volume and
temperature (PVT) changes which in turn
induce metamorphism of rocks.
Epeirogeny and orogeny, cite the
differences.
Volcanism
Volcanism includes the movement of molten
rock (magma) onto or toward the earth’s
surface and also formation of many intrusive
and extrusive volcanic forms. Many aspects of
volcanism have already been dealt in detail
© NCERT
not to be republished
GEOMORPHIC PROCESSES 47
processes and their respective driving forces.
It should become clear from this chart that for
each process there exists a distinct driving force
or energy.
As there are different climatic regions on
the earth’s surface owing to thermal gradients
created by latitudinal, seasonal and land and
water spread variations, the exogenic
geomorphic processes vary from region to
region.  The density, type and distribution of
vegetation which largely depend upon
under volcanoes in the Unit II and under
igneous rocks in the preceding chapter in this
unit.
What do the words volcanism and
volcanoes indicate?
EXOGENIC PROCESSES
The exogenic processes derive their energy
from atmosphere determined by the ultimate
energy from the sun and also the gradients
created by tectonic factors.
Why do you think that the slopes or
gradients are created by tectonic factors?
Gravitational force acts upon all earth
materials having a sloping surface and tend to
produce movement of matter in down slope
direction. Force applied per unit area is called
stress. Stress is produced in a solid by pushing
or pulling. This induces deformation. Forces
acting along the faces of earth materials are
shear stresses (separating forces). It is this
stress that breaks rocks and other earth
materials. The shear stresses result in angular
displacement or slippage. Besides the
gravitational stress earth materials become
subjected to molecular stresses that may be
caused by a number of factors amongst which
temperature changes, crystallisation and
melting are the most common. Chemical
processes normally lead to loosening of bonds
between grains, dissolving of soluble minerals
or cementing materials. Thus, the basic reason
that leads to weathering, mass movements, and
erosion is development of stresses in the body
of the earth materials.
As there are different climatic regions on
the earth’s surface the exogenic geomorphic
processes vary from region to region.
Temperature and precipitation are the two
important climatic elements that control
various processes.
All the exogenic geomorphic processes are
covered under a general term, denudation. The
word ‘denude’ means to strip off or to uncover.
Weathering, mass wasting/movements, erosion
and transportation are included in denudation.
The flow chart (Figure 6.1) gives the denudation
precipitation and temperature exert influence
indirectly on exogenic geomorphic processes.
Within different climatic regions there may be
local variations of the effects of different climatic
elements due to altitudinal differences, aspect
variations and the variation in the amount of
insolation received by north and south facing
slopes as compared to east and west facing
slopes. Further, due to differences in wind
velocities and directions, amount and kind of
precipitation, its intensity, the relation between
precipitation and evaporation, daily range of
temperature, freezing and thawing frequency,
depth of frost penetration, the geomorphic
processes vary within any climatic region.
What is the sole driving force behind all
the exogenic processes?
Climatic factors being equal, the intensity
of action of exogenic geomorphic processes
depends upon type and structure of rocks. The
term structure includes such aspects of rocks
as folds, faults, orientation and inclination of
beds, presence or absence of joints, bedding
planes, hardness or softness of constituent
minerals, chemical susceptibility of mineral
constituents; the permeability or impermeability
Figure 6.1 : Denudational processes and their
driving forces
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 48
etc. Different types of rocks with differences in
their structure offer varying resistances to
various geomorphic processes. A particular
rock may be resistant to one process and non-
resistant to another. And, under varying
climatic conditions, particular rocks may
exhibit different degrees of resistance to
geomorphic processes and hence they operate
at differential rates and give rise to differences
in topography. The effects of most of the
exogenic geomorphic processes are small and
slow and may be imperceptible in a short time
span, but will in the long run affect the rocks
severely due to continued fatigue.
Finally, it boils down to one fact that the
differences on the surface of the earth though
originally related to the crustal evolution
continue to exist in some form or the other due
to differences in the type and structure of earth
materials, differences in geomorphic processes
and in their rates of operation.
Some of the exogenic geomorphic processes
have been dealt in detail here.
WEATHERING
Weathering is action of elements of weather and
climate over earth materials.  There are a
number of processes within weathering which
act either individually or together to affect the
earth materials in order to reduce them to
fragmental state.
Weathering is defined as mechanical
disintegration and chemical decom-
position of rocks through the actions of
various elements of weather and climate.
As very little or no motion of materials
takes place in weathering, it is an in-situ or
on-site process.
Is this little motion which can occur
sometimes due to weathering synonymous
with transportation?  If not, why?
Weathering processes are conditioned by
many complex geological, climatic, topographic
and vegetative factors.  Climate is of particular
importance. Not only weathering processes
differ from climate to climate, but also the depth
of the weathering mantle (Figure 6.2).
Figure 6.2 : Climatic regimes and depth of weathering
mantles (adapted and modified from Strakhov, 1967)
Activity
Mark the latitude values of different
climatic regimes in Figure 6.2 and
compare the details.
There are three major groups of weathering
processes : (i) chemical; (ii) physical or
mechanical; (iii) biological weathering processes.
Very rarely does any one of these processes ever
operate completely by itself, but quite often a
dominance of one process can be seen.
Chemical Weathering Processes
A group of weathering processes viz; solution,
carbonation, hydration, oxidation and
reduction act on the rocks to decompose,
dissolve or reduce them to a fine clastic state
through chemical reactions by oxygen, surface
and/or soil water and other acids.  Water and
air (oxygen and carbon dioxide) along with
heat must be present to speed up all chemical
reactions. Over and above the carbon dioxide
present in the air, decomposition of plants and
animals increases the quantity of carbon
dioxide underground. These chemical
reactions on various minerals are very much
similar to the chemical reactions in a laboratory.
Solution
When something is dissolved in water or acids,
the water or acid with dissolved contents is
© NCERT
not to be republished
Page 5


GEOMORPHIC PROCESSES
CHAPTER
A
fter learning about how the earth was
born, how it evolved its crust and other
inner layers, how its crustal plates
moved and are moving, and other information
on earthquakes, the forms of volcanism and
about the rocks and minerals the crust is
composed of, it is time to know in detail about
the surface of the earth on which we live. Let
us start with this question.
Why is the surface of the earth uneven?
The earth’s crust is dynamic. You are well
aware that it has moved and moves vertically
and horizontally. Of course, it moved a bit faster
in the past than the rate at which it is moving
now. The differences in the internal forces
operating from within the earth which built up
the crust have been responsible for the
variations in the outer surface of the crust. The
earth’s surface is being continuously subjected
to external forces induced basically by energy
(sunlight). Of course, the internal forces are still
active though with different intensities. That
means, the earth’s surface is being
continuously subjected to by external forces
originating within the earth’s atmosphere and
by internal forces from within the earth. The
external forces are known as exogenic forces
and the internal forces are known as endogenic
forces. The actions of exogenic forces result in
wearing down (degradation) of relief/elevations
and filling up (aggradation) of basins/
depressions, on the earth’s surface. The
phenomenon of wearing down of relief
variations of the surface of the earth through
erosion is known as gradation. The endogenic
forces continuously elevate or build up parts
of the earth’s surface and hence the exogenic
processes fail to even out the relief variations
of the surface of the earth. So, variations remain
as long as the opposing actions of exogenic and
endogenic forces continue. In general terms,
the endogenic forces are mainly land building
forces and the exogenic processes are mainly
land wearing forces. The surface of the earth is
sensitive. Humans depend on it for their
sustenance and have been using it extensively
and intensively. So, it is essential to understand
its nature in order to use it effectively without
disturbing its balance and diminishing its
potential for the future. Almost all organisms
contribute to sustain the earth’s environment.
However, humans have caused extensive
damage to the environment through over use
of resources. Use we must, but must also leave
it potential enough to sustain life through the
future. Most of the surface of the earth had and
has been shaped over very long periods of time
(hundreds and thousands of years) and
because of its use and misuse by humans its
potential is being diminished at a fast rate. If
the processes which shaped and are shaping
the surface of the earth into varieties of forms
(shapes) and the nature of materials of which
it is composed of, are understood, precautions
can be taken to minimise the detrimental effects
of human use and to preserve it for posterity.
GEOMORPHIC PROCESSES
You would like to know the meaning of
geomorphic processes. The endogenic and
exogenic forces causing physical stresses and
chemical actions on earth materials and
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 46
bringing about changes in the configuration
of the surface of the earth are known as
geomorphic processes. Diastrophism and
volcanism are endogenic geomorphic
processes. These have already been discussed
in brief in the preceding unit. Weathering, mass
wasting, erosion and deposition are exogenic
geomorphic processes. These exogenic
processes are dealt with in detail in this chapter.
Any exogenic element of nature (like water,
ice, wind, etc.,) capable of acquiring and
transporting earth materials can be called a
geomorphic agent. When these elements of
nature become mobile due to gradients, they
remove the materials and transport them over
slopes and deposit them at lower level.
Geomorphic processes and geomorphic agents
especially exogenic, unless stated separately,
are one and the same.
A process is a force applied on earth
materials affecting the same.  An agent is a
mobile medium (like running water, moving ice
masses, wind, waves and currents etc.) which
removes, transports and deposits earth
materials. Running water, groundwater,
glaciers, wind, waves and currents, etc., can
be called geomorphic agents.
Do you think it is essential to distinguish
geomorphic agents and geomorphic
processes?
Gravity besides being a directional force
activating all downslope movements of matter
also causes stresses on the earth’s materials.
Indirect gravitational stresses activate wave and
tide induced currents and winds. Without
gravity and gradients there would be no
mobility and hence no erosion, transportation
and deposition are possible. So, gravitational
stresses are as important as the other
geomorphic processes. Gravity is the force that
is keeping us in contact with the surface and it
is the force that switches on the movement of
all surface material on earth. All the movements
either within the earth or on the surface of the
earth occur due to gradients — from higher
levels to lower levels, from high pressure to low
pressure areas etc.
ENDOGENIC PROCESSES
The energy emanating from within the earth is
the main force behind endogenic geomorphic
processes. This energy is mostly generated by
radioactivity, rotational and tidal friction and
primordial heat from the origin of the earth.
This energy due to geothermal gradients and
heat flow from within induces diastrophism
and volcanism in the lithosphere. Due to
variations in geothermal gradients and heat flow
from within, crustal thickness and strength,
the action of endogenic forces are not uniform
and hence the tectonically controlled original
crustal surface is uneven.
Diastrophism
All processes that move, elevate or build up
portions of the earth’s crust come under
diastrophism. They include: (i) orogenic
processes involving mountain building
through severe folding and affecting long and
narrow belts of the earth’s crust; (ii) epeirogenic
processes involving uplift or warping of large
parts of the earth’s crust; (iii) earthquakes
involving local relatively minor movements;
(iv) plate tectonics involving horizontal
movements of crustal plates.
In the process of orogeny, the crust is
severely deformed into folds. Due to epeirogeny,
there may be simple deformation. Orogeny is
a mountain building process whereas
epeirogeny is continental building process.
Through the processes of orogeny, epeirogeny,
earthquakes and plate tectonics, there can be
faulting and fracturing of the crust. All these
processes cause pressure, volume and
temperature (PVT) changes which in turn
induce metamorphism of rocks.
Epeirogeny and orogeny, cite the
differences.
Volcanism
Volcanism includes the movement of molten
rock (magma) onto or toward the earth’s
surface and also formation of many intrusive
and extrusive volcanic forms. Many aspects of
volcanism have already been dealt in detail
© NCERT
not to be republished
GEOMORPHIC PROCESSES 47
processes and their respective driving forces.
It should become clear from this chart that for
each process there exists a distinct driving force
or energy.
As there are different climatic regions on
the earth’s surface owing to thermal gradients
created by latitudinal, seasonal and land and
water spread variations, the exogenic
geomorphic processes vary from region to
region.  The density, type and distribution of
vegetation which largely depend upon
under volcanoes in the Unit II and under
igneous rocks in the preceding chapter in this
unit.
What do the words volcanism and
volcanoes indicate?
EXOGENIC PROCESSES
The exogenic processes derive their energy
from atmosphere determined by the ultimate
energy from the sun and also the gradients
created by tectonic factors.
Why do you think that the slopes or
gradients are created by tectonic factors?
Gravitational force acts upon all earth
materials having a sloping surface and tend to
produce movement of matter in down slope
direction. Force applied per unit area is called
stress. Stress is produced in a solid by pushing
or pulling. This induces deformation. Forces
acting along the faces of earth materials are
shear stresses (separating forces). It is this
stress that breaks rocks and other earth
materials. The shear stresses result in angular
displacement or slippage. Besides the
gravitational stress earth materials become
subjected to molecular stresses that may be
caused by a number of factors amongst which
temperature changes, crystallisation and
melting are the most common. Chemical
processes normally lead to loosening of bonds
between grains, dissolving of soluble minerals
or cementing materials. Thus, the basic reason
that leads to weathering, mass movements, and
erosion is development of stresses in the body
of the earth materials.
As there are different climatic regions on
the earth’s surface the exogenic geomorphic
processes vary from region to region.
Temperature and precipitation are the two
important climatic elements that control
various processes.
All the exogenic geomorphic processes are
covered under a general term, denudation. The
word ‘denude’ means to strip off or to uncover.
Weathering, mass wasting/movements, erosion
and transportation are included in denudation.
The flow chart (Figure 6.1) gives the denudation
precipitation and temperature exert influence
indirectly on exogenic geomorphic processes.
Within different climatic regions there may be
local variations of the effects of different climatic
elements due to altitudinal differences, aspect
variations and the variation in the amount of
insolation received by north and south facing
slopes as compared to east and west facing
slopes. Further, due to differences in wind
velocities and directions, amount and kind of
precipitation, its intensity, the relation between
precipitation and evaporation, daily range of
temperature, freezing and thawing frequency,
depth of frost penetration, the geomorphic
processes vary within any climatic region.
What is the sole driving force behind all
the exogenic processes?
Climatic factors being equal, the intensity
of action of exogenic geomorphic processes
depends upon type and structure of rocks. The
term structure includes such aspects of rocks
as folds, faults, orientation and inclination of
beds, presence or absence of joints, bedding
planes, hardness or softness of constituent
minerals, chemical susceptibility of mineral
constituents; the permeability or impermeability
Figure 6.1 : Denudational processes and their
driving forces
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 48
etc. Different types of rocks with differences in
their structure offer varying resistances to
various geomorphic processes. A particular
rock may be resistant to one process and non-
resistant to another. And, under varying
climatic conditions, particular rocks may
exhibit different degrees of resistance to
geomorphic processes and hence they operate
at differential rates and give rise to differences
in topography. The effects of most of the
exogenic geomorphic processes are small and
slow and may be imperceptible in a short time
span, but will in the long run affect the rocks
severely due to continued fatigue.
Finally, it boils down to one fact that the
differences on the surface of the earth though
originally related to the crustal evolution
continue to exist in some form or the other due
to differences in the type and structure of earth
materials, differences in geomorphic processes
and in their rates of operation.
Some of the exogenic geomorphic processes
have been dealt in detail here.
WEATHERING
Weathering is action of elements of weather and
climate over earth materials.  There are a
number of processes within weathering which
act either individually or together to affect the
earth materials in order to reduce them to
fragmental state.
Weathering is defined as mechanical
disintegration and chemical decom-
position of rocks through the actions of
various elements of weather and climate.
As very little or no motion of materials
takes place in weathering, it is an in-situ or
on-site process.
Is this little motion which can occur
sometimes due to weathering synonymous
with transportation?  If not, why?
Weathering processes are conditioned by
many complex geological, climatic, topographic
and vegetative factors.  Climate is of particular
importance. Not only weathering processes
differ from climate to climate, but also the depth
of the weathering mantle (Figure 6.2).
Figure 6.2 : Climatic regimes and depth of weathering
mantles (adapted and modified from Strakhov, 1967)
Activity
Mark the latitude values of different
climatic regimes in Figure 6.2 and
compare the details.
There are three major groups of weathering
processes : (i) chemical; (ii) physical or
mechanical; (iii) biological weathering processes.
Very rarely does any one of these processes ever
operate completely by itself, but quite often a
dominance of one process can be seen.
Chemical Weathering Processes
A group of weathering processes viz; solution,
carbonation, hydration, oxidation and
reduction act on the rocks to decompose,
dissolve or reduce them to a fine clastic state
through chemical reactions by oxygen, surface
and/or soil water and other acids.  Water and
air (oxygen and carbon dioxide) along with
heat must be present to speed up all chemical
reactions. Over and above the carbon dioxide
present in the air, decomposition of plants and
animals increases the quantity of carbon
dioxide underground. These chemical
reactions on various minerals are very much
similar to the chemical reactions in a laboratory.
Solution
When something is dissolved in water or acids,
the water or acid with dissolved contents is
© NCERT
not to be republished
GEOMORPHIC PROCESSES 49
called solution.  This process involves removal
of solids in solution and depends upon
solubility of a mineral in water or weak acids.
On coming in contact with water many solids
disintegrate and mix up as suspension in
water. Soluble rock forming minerals like
nitrates, sulphates, and potassium etc. are
affected by this process. So, these minerals are
easily leached out without leaving any residue
in rainy climates and accumulate in dry
regions. Minerals like calcium carbonate and
calcium magnesium bicarbonate present in
limestones are soluble in water containing
carbonic acid (formed with the addition of
carbon dioxide in water), and are carried away
in water as solution. Carbon dioxide produced
by decaying organic matter along with soil
water greatly aids in this reaction. Common
salt (sodium chloride) is also a rock forming
mineral and is susceptible to this process of
solution.
Carbonation
Carbonation is the reaction of carbonate and
bicarbonate with minerals and is a common
process helping the breaking down of
feldspars and carbonate minerals. Carbon
dioxide from the atmosphere and soil air is
absorbed by water, to form carbonic acid that
acts as a weak acid. Calcium carbonates and
magnesium carbonates are dissolved in
carbonic acid and are removed in a solution
without leaving any residue resulting in cave
formation.
Hydration
Hydration is the chemical addition of water.
Minerals take up water and expand; this
expansion causes an increase in the volume of
the material itself or rock. Calcium sulphate
takes in water and turns to gypsum, which is
more unstable than calcium sulphate. This
process is reversible and long, continued
repetition of this process causes fatigue in the
rocks and may lead to their disintegration.
Many clay minerals swell and contract during
wetting and drying and a repetition of this
process results in cracking of overlying
materials. Salts in pore spaces undergo rapid
and repeated hydration and help in rock
fracturing. The volume changes in minerals
due to hydration will also help in physical
weathering through exfoliation and granular
disintegration.
Oxidation and Reduction
In weathering, oxidation means a combination
of a mineral with oxygen to form oxides or
hydroxides.  Oxidation occurs where there is
ready access to the atmosphere and
oxygenated waters. The minerals most
commonly involved in this process are iron,
manganese, sulphur etc. In the process of
oxidation rock breakdown occurs due to the
disturbance caused by addition of oxygen. Red
colour of iron upon oxidation turns to brown
or yellow. When oxidised minerals are placed
in an environment where oxygen is absent,
reduction takes place. Such conditions exist
usually below the water table, in areas of
stagnant water and waterlogged ground. Red
colour of iron upon reduction turns to greenish
or bluish grey.
These weathering processes are inter-
related. Hydration, carbonation and oxidation
go hand in hand and hasten the weathering
process.
Can we give iron rusting as an example
of oxidation? How essential is water in
chemical weathering processes? Can
chemical weathering processes dominate
in water scarce hot deserts?
Physical Weathering Processes
Physical or mechanical weathering processes
depend on some applied forces. The applied
forces could be: (i) gravitational forces such as
overburden pressure, load and shearing stress;
(ii) expansion forces due to temperature
changes, crystal growth or animal activity;
(iii) water pressures controlled by wetting and
© NCERT
not to be republished