NCERT Textbook - Water (Oceans) | Geography (Prelims) by Valor Academy - UPSC PDF Download

 Page 1


WATER (OCEANS)
This unit deals with
• Hydrological Cycle
• Oceans — submarine relief; distribution of temperature and
salinity; movements of ocean water-waves, tides and currents
UNIT
V
2024-25
Page 2


WATER (OCEANS)
This unit deals with
• Hydrological Cycle
• Oceans — submarine relief; distribution of temperature and
salinity; movements of ocean water-waves, tides and currents
UNIT
V
2024-25
WATER (OCEANS)
the ocean to land and land to ocean. The
hydrological cycle describes the movement of
water on, in, and above the earth. The water
cycle has been working for billions of years
and all the life on earth depends on it. Next to
air, water is the most important element
required for the existence of life on earth. The
distribution of water on earth is quite uneven.
Many locations have plenty of water while
others have very limited quantity. The
hydrological cycle, is the circulation of water
within the earth’s hydrosphere in different
forms i.e. the liquid, solid and the gaseous
phases. It also refers to the continuous
exchange of water between the oceans,
Figure 12.1 : Hydrological Cycle
C
an we think of life without water? It is
said that the water is life.  Water is an
essential component of all life forms that
exist over the surface of the earth.  The creatures
on the earth are lucky that it is a water planet,
otherwise we all would have no existence. W ater
is a rare commodity in our solar system. There
is no water on the sun or anywhere else in the
solar system.  The earth, fortunately has an
abundant supply of water on its surface. Hence,
our planet is called the ‘Blue Planet’.
HYDROLOGICAL CYCLE
Water is a cyclic resource. It can be used and
re-used. Water also undergoes a cycle from
CHAPTER
2024-25
Page 3


WATER (OCEANS)
This unit deals with
• Hydrological Cycle
• Oceans — submarine relief; distribution of temperature and
salinity; movements of ocean water-waves, tides and currents
UNIT
V
2024-25
WATER (OCEANS)
the ocean to land and land to ocean. The
hydrological cycle describes the movement of
water on, in, and above the earth. The water
cycle has been working for billions of years
and all the life on earth depends on it. Next to
air, water is the most important element
required for the existence of life on earth. The
distribution of water on earth is quite uneven.
Many locations have plenty of water while
others have very limited quantity. The
hydrological cycle, is the circulation of water
within the earth’s hydrosphere in different
forms i.e. the liquid, solid and the gaseous
phases. It also refers to the continuous
exchange of water between the oceans,
Figure 12.1 : Hydrological Cycle
C
an we think of life without water? It is
said that the water is life.  Water is an
essential component of all life forms that
exist over the surface of the earth.  The creatures
on the earth are lucky that it is a water planet,
otherwise we all would have no existence. W ater
is a rare commodity in our solar system. There
is no water on the sun or anywhere else in the
solar system.  The earth, fortunately has an
abundant supply of water on its surface. Hence,
our planet is called the ‘Blue Planet’.
HYDROLOGICAL CYCLE
Water is a cyclic resource. It can be used and
re-used. Water also undergoes a cycle from
CHAPTER
2024-25
WATER (OCEANS) 101
atmosphere, landsurface and subsurface and
the organisms.
About 71 per cent of the planetary water
is found in the oceans. The remaining is held
as freshwater in glaciers and icecaps,
groundwater sources, lakes, soil moisture,
atmosphere, streams and within life. Nearly
59 per cent of the water that falls on land
returns to the atmosphere through
evaporation from over the oceans as well as
from other places. The remainder runs-off on
the surface, infiltrates into the ground or a part
of it becomes glacier.
It is to be noted that the renewable water
on the earth is constant while the demand is
increasing tremendously. This leads to water
crisis in different parts of the world — spatially
and temporally. The pollution of river waters
has further aggravated the crisis.  How can
you intervene in improving the water quality
and augmenting the available quantity of
water?
RELIEF OF THE OCEAN FLOOR
The oceans are confined to the great
depressions of the earth’s outer layer. In this
section, we shall see the nature of the ocean
basins of the earth and their topography. The
oceans, unlike the continents, merge so
naturally into one another that it is hard to
demarcate them. The geographers have divided
the oceanic part of the earth into five oceans,
namely the Pacific, the Atlantic, the Indian,
Southern ocean and the Arctic. The various
seas, bays, gulfs and other inlets are parts of
these four large oceans.
A major portion of the ocean floor is found
between 3-6 km below the sea level. The ‘land’
under the waters of the oceans, that is, the
ocean floor exhibits complex and varied
features as those observed over the land
(Figure 12.2). The floors of the oceans are
rugged with the world’s largest mountain
ranges, deepest trenches and the largest plains.
These features are formed, like those of the
continents, by the factors of tectonic, volcanic
and depositional processes.
Divisions of the Ocean Floors
The ocean floors can be divided into four
major divisions: (i) the Continental Shelf; (ii)
the Continental Slope; (iii) the Deep Sea Plain;
(iv) the Oceanic Deeps. Besides, these divisions
there are also major and minor relief features
in the ocean floors like ridges, hills, sea
mounts, guyots, trenches, canyons, etc.
Continental Shelf
The continental shelf is the extended margin
of each continent occupied by relatively
shallow seas and gulfs. It is the shallowest
part of the ocean showing an average gradient
of 1° or even less. The shelf typically ends at
a very steep slope, called the shelf break.
The width of the continental shelves vary
from one ocean to another. The average width
of continental shelves is about 80 km. The
shelves are almost absent or very narrow along
some of the margins like the coasts of Chile,
the west coast of Sumatra, etc. On the contrary,
the Siberian shelf in the Arctic Ocean, the
largest in the world, stretches to 1,500 km in
width. The depth of the shelves also varies. It
may be as shallow as 30 m in some areas
while in some areas it is as deep as 600 m.
The continental shelves are covered with
variable thicknesses of sediments brought
down by rivers, glaciers, wind, from the land
and distributed by waves and currents.
Massive sedimentary deposits received over a
long time by the continental shelves, become
the source of fossil fuels.
Table 12.1 : Components  and  Processes
    of the Water Cycle
Components Processes
Water storage Evaporation
in oceans Evapotranspiration
Sublimation
Water in the Condensation
atmosphere Precipitation
Water storage in Snowmelt runoff
ice and snow to streams
Surface runoff Stream flow freshwater
storage infiltration
Groundwater storage Groundwater
discharge springs
2024-25
Page 4


WATER (OCEANS)
This unit deals with
• Hydrological Cycle
• Oceans — submarine relief; distribution of temperature and
salinity; movements of ocean water-waves, tides and currents
UNIT
V
2024-25
WATER (OCEANS)
the ocean to land and land to ocean. The
hydrological cycle describes the movement of
water on, in, and above the earth. The water
cycle has been working for billions of years
and all the life on earth depends on it. Next to
air, water is the most important element
required for the existence of life on earth. The
distribution of water on earth is quite uneven.
Many locations have plenty of water while
others have very limited quantity. The
hydrological cycle, is the circulation of water
within the earth’s hydrosphere in different
forms i.e. the liquid, solid and the gaseous
phases. It also refers to the continuous
exchange of water between the oceans,
Figure 12.1 : Hydrological Cycle
C
an we think of life without water? It is
said that the water is life.  Water is an
essential component of all life forms that
exist over the surface of the earth.  The creatures
on the earth are lucky that it is a water planet,
otherwise we all would have no existence. W ater
is a rare commodity in our solar system. There
is no water on the sun or anywhere else in the
solar system.  The earth, fortunately has an
abundant supply of water on its surface. Hence,
our planet is called the ‘Blue Planet’.
HYDROLOGICAL CYCLE
Water is a cyclic resource. It can be used and
re-used. Water also undergoes a cycle from
CHAPTER
2024-25
WATER (OCEANS) 101
atmosphere, landsurface and subsurface and
the organisms.
About 71 per cent of the planetary water
is found in the oceans. The remaining is held
as freshwater in glaciers and icecaps,
groundwater sources, lakes, soil moisture,
atmosphere, streams and within life. Nearly
59 per cent of the water that falls on land
returns to the atmosphere through
evaporation from over the oceans as well as
from other places. The remainder runs-off on
the surface, infiltrates into the ground or a part
of it becomes glacier.
It is to be noted that the renewable water
on the earth is constant while the demand is
increasing tremendously. This leads to water
crisis in different parts of the world — spatially
and temporally. The pollution of river waters
has further aggravated the crisis.  How can
you intervene in improving the water quality
and augmenting the available quantity of
water?
RELIEF OF THE OCEAN FLOOR
The oceans are confined to the great
depressions of the earth’s outer layer. In this
section, we shall see the nature of the ocean
basins of the earth and their topography. The
oceans, unlike the continents, merge so
naturally into one another that it is hard to
demarcate them. The geographers have divided
the oceanic part of the earth into five oceans,
namely the Pacific, the Atlantic, the Indian,
Southern ocean and the Arctic. The various
seas, bays, gulfs and other inlets are parts of
these four large oceans.
A major portion of the ocean floor is found
between 3-6 km below the sea level. The ‘land’
under the waters of the oceans, that is, the
ocean floor exhibits complex and varied
features as those observed over the land
(Figure 12.2). The floors of the oceans are
rugged with the world’s largest mountain
ranges, deepest trenches and the largest plains.
These features are formed, like those of the
continents, by the factors of tectonic, volcanic
and depositional processes.
Divisions of the Ocean Floors
The ocean floors can be divided into four
major divisions: (i) the Continental Shelf; (ii)
the Continental Slope; (iii) the Deep Sea Plain;
(iv) the Oceanic Deeps. Besides, these divisions
there are also major and minor relief features
in the ocean floors like ridges, hills, sea
mounts, guyots, trenches, canyons, etc.
Continental Shelf
The continental shelf is the extended margin
of each continent occupied by relatively
shallow seas and gulfs. It is the shallowest
part of the ocean showing an average gradient
of 1° or even less. The shelf typically ends at
a very steep slope, called the shelf break.
The width of the continental shelves vary
from one ocean to another. The average width
of continental shelves is about 80 km. The
shelves are almost absent or very narrow along
some of the margins like the coasts of Chile,
the west coast of Sumatra, etc. On the contrary,
the Siberian shelf in the Arctic Ocean, the
largest in the world, stretches to 1,500 km in
width. The depth of the shelves also varies. It
may be as shallow as 30 m in some areas
while in some areas it is as deep as 600 m.
The continental shelves are covered with
variable thicknesses of sediments brought
down by rivers, glaciers, wind, from the land
and distributed by waves and currents.
Massive sedimentary deposits received over a
long time by the continental shelves, become
the source of fossil fuels.
Table 12.1 : Components  and  Processes
    of the Water Cycle
Components Processes
Water storage Evaporation
in oceans Evapotranspiration
Sublimation
Water in the Condensation
atmosphere Precipitation
Water storage in Snowmelt runoff
ice and snow to streams
Surface runoff Stream flow freshwater
storage infiltration
Groundwater storage Groundwater
discharge springs
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 102
Continental Slope
The continental slope connects the continental
shelf and the ocean basins. It begins where the
bottom of the continental shelf sharply drops
off into a steep slope. The gradient of the slope
region varies between 2-5°. The depth of the
slope region varies between 200 and 3,000 m.
The slope boundary indicates the end of the
continents. Canyons and trenches are observed
in this region.
Deep Sea Plain
Deep sea plains are gently sloping areas of the
ocean basins. These are the flattest and
smoothest regions of the world. The depths
vary between 3,000 and 6,000m. These plains
are covered with fine-grained sediments like
clay and silt.
Oceanic Deeps or Trenches
These areas are the deepest parts of the
oceans. The trenches are relatively steep sided,
narrow basins.  They are some 3-5 km deeper
than the surrounding ocean floor. They occur
at the bases of continental slopes and along
island arcs and are associated with active
volcanoes and strong earthquakes. That is
why they are very significant in the study of
plate movements. As many as 57 deeps have
been explored so far; of which 32 are in the
Pacific Ocean; 19 in the Atlantic Ocean and 6
in the Indian Ocean.
Minor Relief Features
Apart from the above mentioned major relief
features of the ocean floor, some minor but
significant features predominate in different
parts of the oceans.
Mid-Oceanic Ridges
A mid-oceanic ridge is composed of two chains
of mountains separated by a large depression.
The mountain ranges can have peaks as high
as 2,500 m and some even reach above the
ocean’s surface. Iceland, a part of the mid-
Atlantic Ridge, is an example.
Seamount
It is a mountain with pointed summits, rising
from the seafloor that does not reach the surface
of the ocean. Seamounts are volcanic in origin.
These can be 3,000-4,500 m tall. The Emperor
seamount, an extension of the Hawaiian Islands
in the Pacific Ocean, is a good example.
Submarine Canyons
These are deep valleys, some comparable to
the Grand Canyon of the Colorado river. They
are sometimes found cutting across the
continental shelves and slopes, often extending
Figure 12.2 : Relief features of ocean floors
2024-25
Page 5


WATER (OCEANS)
This unit deals with
• Hydrological Cycle
• Oceans — submarine relief; distribution of temperature and
salinity; movements of ocean water-waves, tides and currents
UNIT
V
2024-25
WATER (OCEANS)
the ocean to land and land to ocean. The
hydrological cycle describes the movement of
water on, in, and above the earth. The water
cycle has been working for billions of years
and all the life on earth depends on it. Next to
air, water is the most important element
required for the existence of life on earth. The
distribution of water on earth is quite uneven.
Many locations have plenty of water while
others have very limited quantity. The
hydrological cycle, is the circulation of water
within the earth’s hydrosphere in different
forms i.e. the liquid, solid and the gaseous
phases. It also refers to the continuous
exchange of water between the oceans,
Figure 12.1 : Hydrological Cycle
C
an we think of life without water? It is
said that the water is life.  Water is an
essential component of all life forms that
exist over the surface of the earth.  The creatures
on the earth are lucky that it is a water planet,
otherwise we all would have no existence. W ater
is a rare commodity in our solar system. There
is no water on the sun or anywhere else in the
solar system.  The earth, fortunately has an
abundant supply of water on its surface. Hence,
our planet is called the ‘Blue Planet’.
HYDROLOGICAL CYCLE
Water is a cyclic resource. It can be used and
re-used. Water also undergoes a cycle from
CHAPTER
2024-25
WATER (OCEANS) 101
atmosphere, landsurface and subsurface and
the organisms.
About 71 per cent of the planetary water
is found in the oceans. The remaining is held
as freshwater in glaciers and icecaps,
groundwater sources, lakes, soil moisture,
atmosphere, streams and within life. Nearly
59 per cent of the water that falls on land
returns to the atmosphere through
evaporation from over the oceans as well as
from other places. The remainder runs-off on
the surface, infiltrates into the ground or a part
of it becomes glacier.
It is to be noted that the renewable water
on the earth is constant while the demand is
increasing tremendously. This leads to water
crisis in different parts of the world — spatially
and temporally. The pollution of river waters
has further aggravated the crisis.  How can
you intervene in improving the water quality
and augmenting the available quantity of
water?
RELIEF OF THE OCEAN FLOOR
The oceans are confined to the great
depressions of the earth’s outer layer. In this
section, we shall see the nature of the ocean
basins of the earth and their topography. The
oceans, unlike the continents, merge so
naturally into one another that it is hard to
demarcate them. The geographers have divided
the oceanic part of the earth into five oceans,
namely the Pacific, the Atlantic, the Indian,
Southern ocean and the Arctic. The various
seas, bays, gulfs and other inlets are parts of
these four large oceans.
A major portion of the ocean floor is found
between 3-6 km below the sea level. The ‘land’
under the waters of the oceans, that is, the
ocean floor exhibits complex and varied
features as those observed over the land
(Figure 12.2). The floors of the oceans are
rugged with the world’s largest mountain
ranges, deepest trenches and the largest plains.
These features are formed, like those of the
continents, by the factors of tectonic, volcanic
and depositional processes.
Divisions of the Ocean Floors
The ocean floors can be divided into four
major divisions: (i) the Continental Shelf; (ii)
the Continental Slope; (iii) the Deep Sea Plain;
(iv) the Oceanic Deeps. Besides, these divisions
there are also major and minor relief features
in the ocean floors like ridges, hills, sea
mounts, guyots, trenches, canyons, etc.
Continental Shelf
The continental shelf is the extended margin
of each continent occupied by relatively
shallow seas and gulfs. It is the shallowest
part of the ocean showing an average gradient
of 1° or even less. The shelf typically ends at
a very steep slope, called the shelf break.
The width of the continental shelves vary
from one ocean to another. The average width
of continental shelves is about 80 km. The
shelves are almost absent or very narrow along
some of the margins like the coasts of Chile,
the west coast of Sumatra, etc. On the contrary,
the Siberian shelf in the Arctic Ocean, the
largest in the world, stretches to 1,500 km in
width. The depth of the shelves also varies. It
may be as shallow as 30 m in some areas
while in some areas it is as deep as 600 m.
The continental shelves are covered with
variable thicknesses of sediments brought
down by rivers, glaciers, wind, from the land
and distributed by waves and currents.
Massive sedimentary deposits received over a
long time by the continental shelves, become
the source of fossil fuels.
Table 12.1 : Components  and  Processes
    of the Water Cycle
Components Processes
Water storage Evaporation
in oceans Evapotranspiration
Sublimation
Water in the Condensation
atmosphere Precipitation
Water storage in Snowmelt runoff
ice and snow to streams
Surface runoff Stream flow freshwater
storage infiltration
Groundwater storage Groundwater
discharge springs
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 102
Continental Slope
The continental slope connects the continental
shelf and the ocean basins. It begins where the
bottom of the continental shelf sharply drops
off into a steep slope. The gradient of the slope
region varies between 2-5°. The depth of the
slope region varies between 200 and 3,000 m.
The slope boundary indicates the end of the
continents. Canyons and trenches are observed
in this region.
Deep Sea Plain
Deep sea plains are gently sloping areas of the
ocean basins. These are the flattest and
smoothest regions of the world. The depths
vary between 3,000 and 6,000m. These plains
are covered with fine-grained sediments like
clay and silt.
Oceanic Deeps or Trenches
These areas are the deepest parts of the
oceans. The trenches are relatively steep sided,
narrow basins.  They are some 3-5 km deeper
than the surrounding ocean floor. They occur
at the bases of continental slopes and along
island arcs and are associated with active
volcanoes and strong earthquakes. That is
why they are very significant in the study of
plate movements. As many as 57 deeps have
been explored so far; of which 32 are in the
Pacific Ocean; 19 in the Atlantic Ocean and 6
in the Indian Ocean.
Minor Relief Features
Apart from the above mentioned major relief
features of the ocean floor, some minor but
significant features predominate in different
parts of the oceans.
Mid-Oceanic Ridges
A mid-oceanic ridge is composed of two chains
of mountains separated by a large depression.
The mountain ranges can have peaks as high
as 2,500 m and some even reach above the
ocean’s surface. Iceland, a part of the mid-
Atlantic Ridge, is an example.
Seamount
It is a mountain with pointed summits, rising
from the seafloor that does not reach the surface
of the ocean. Seamounts are volcanic in origin.
These can be 3,000-4,500 m tall. The Emperor
seamount, an extension of the Hawaiian Islands
in the Pacific Ocean, is a good example.
Submarine Canyons
These are deep valleys, some comparable to
the Grand Canyon of the Colorado river. They
are sometimes found cutting across the
continental shelves and slopes, often extending
Figure 12.2 : Relief features of ocean floors
2024-25
WATER (OCEANS) 103
from the mouths of large rivers. The Hudson
Canyon is the best known submarine canyon
in the world.
Guyots
It is a flat topped seamount. They show
evidences of gradual subsidence through
stages to become flat topped submerged
mountains. It is estimated that more than
10,000 seamounts and guyots exist in the
Pacific Ocean alone.
Atoll
These are low islands found in the tropical
oceans consisting of  coral reefs surrounding
a central depression. It may be a part of the
sea (lagoon), or sometimes form enclosing a
body of fresh, brackish, or highly saline water.
TEMPERATURE OF OCEAN WATERS
This section deals with the spatial and vertical
variations of temperature in various oceans.
Ocean waters get heated up by the solar energy
just as land. The process of heating and cooling
of the oceanic water is slower than land.
Factors Affecting Temperature Distribution
The factors which affect the distribution of
temperature of ocean water are :
(i) Latitude : the temperature of surface water
decreases from the equator towards the
poles because the amount of insolation
decreases poleward.
(ii) Unequal distribution of land and water :
the oceans in the northern hemisphere
receive more heat due to their contact
with larger extent of land than the oceans
in the southern hemisphere.
(iii) Prevailing wind : the winds blowing from
the land towards the oceans drive warm
surface water away form the coast
resulting in the upwelling of cold water
from below. It results into the longitudinal
variation in the temperature. Contrary to
this, the onshore winds pile up warm
water near the coast and this raises the
temperature.
(iv) Ocean currents : warm ocean currents
raise the temperature in cold areas while
the cold currents decrease the
temperature in warm ocean areas.  Gulf
stream (warm current) raises the
temperature near the eastern coast of
North America and the West Coast of
Europe while the Labrador current (cold
current) lowers the temperature near the
north-east coast of North America.
All these factors influence the temperature
of the ocean currents locally. The enclosed seas
in the low latitudes record relatively higher
temperature than the open seas; whereas the
enclosed seas in the high latitudes have lower
temperature than the open seas.
Horizontal and Vertical Distribution
of Temperature
The temperature-depth profile for the ocean
water shows how the temperature decreases
with the increasing depth. The profile shows  a
boundary region between the surface waters
of the ocean and the deeper layers. The
boundary usually begins around 100 - 400 m
below the sea surface and extends several
hundred of metres downward (Figure 12.3).
This boundary region, from where there is a
rapid decrease of temperature, is called the
thermocline. About 90 per cent of the total
volume of water is found below the thermocline
in the deep ocean. In this zone, temperatures
approach 0° C.
The temperature structure of oceans over
middle and low latitudes can be described as
a three-layer system from surface to the bottom.
The first layer represents the top layer of
warm oceanic water and it is about 500m
thick with temperatures ranging between 20°
and 25° C. This layer, within the tropical
region, is present throughout the year but in
mid latitudes it develops only during summer.
The second layer called the thermocline
layer lies below the first layer and is characterised
by rapid decrease in temperature with increasing
depth. The thermocline is 500 -1,000 m thick.
The third layer is very cold and extends
upto the deep ocean floor. In the Arctic and
2024-25