This unit deals with• Hydrological Cycle
• Oceans — submarine relief; distribution of temperature and salinity; movements of ocean
water-waves, tides and currents
The earth, fortunately has an abundant supply
of water on its surface. Hence, our planet is
called the ‘Blue Planet’.
HYDROLOGICAL CYCLE
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,
STUDY THE GIVEN TABLE SHOWING THE
WATER COMPOSITION ON THE EARTH
SURFACE AND ANSWER THE FOLLOWING
QUESTIONS
1. What is the % of water in the oceans?
2. Which component of the earth consists of
least percentage of water content?
3. what is the main use of soil moisture?
Study above table and answer the questions that fallow
1. What do you mean by evapotranspiration?
2. What are the different forms of water in the atmosphere?
The above table shows the distribution of water on the surface of the earth. 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
UNIT V CHAPTER -13 WATER (OCEANS)
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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
Divisions of the Ocean Floors
The ocean floors can be divided into four major divisions:
1) The Continental Shelf;
2) The Continental Slope;
3) The Deep Sea Plain;
4) 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
1. The continental shelf is the extended margin of each continent occupied by relatively shallow
seas and gulfs.
2. It is the shallowest part of the ocean showing an average gradient of 1° or even less.
3. The shelf typically ends at a very steep slope, called the shelf break.
4. The width of the continental shelves vary from one ocean to another.
5. The average width of continental shelves is about 80 km.
6. The shelves are almost absent or very narrow along some of the margins like the coasts of
Chile, the west coast of Sumatra, etc.
7. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world,
stretches to 1,500 km in width.
8. 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.
9. 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. 10.Massive
sedimentary deposits received over a long time by the continental shelves, become the
source of fossil fuels.
Continental Slope
1. The continental slope connects the continental shelf and the ocean basins.
2. It begins where the bottom of the continental shelf sharply drops off into a steep slope.
3. The gradient of the slope region varies between 2-5°.
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4. The depth of the slope region varies between 200 and 3,000 m.
Deep Sea Plain
1. Deep sea plains are gently sloping areas of the ocean basins.
2. These are the flattest and smoothest regions of the world.
3. The depths vary between 3,000 and 6,000 m.
4. These plains are covered with fine-grained sediments like
clay and silt.
Oceanic Deeps or Trenches
1. These areas are the deepest parts of the oceans.
2. The trenches are relatively steep sided, narrow basins.
3. They are some 3-5 km deeper than the surrounding ocean
floor.
4. They occur at the bases of continental slopes and along
island arcs and are associated with active volcanoes and
strong earthquakes.
5. That is why they are very significant in the study of plate
movements.
6. 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.
Mid-Oceanic
Ridges
1. A mid-oceanic
ridge is
composed of two
chains of
mountains
separated by a
large depression.
2. The mountain
ranges can have peaks as high as 2,500 m and some even reach above the ocean‘s surface.
3. 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 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.
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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
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 ON THE OCEANS
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 pole ward.
(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.
(v) 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 .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 characterized by
rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
The third layer is very cold and extends up to the deep ocean floor.
In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the
temperature change with the depth is very slight. Here, only one layer of cold water exists, which
extends from surface to deep ocean floor.
SALINITY OF OCEAN WATERS
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Salinity is the term used to define the total content of dissolved salts in sea water It is calculated as
the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater. It is usually expressed as parts
per thousand (o/) or ppt. Salinity is an important property of sea water. Salinity of 24.7°/oo has
been considered as the upper limit to demarcate ‗brackish water‘.
Factors affecting ocean salinity are mentioned below:
(i) The salinity of water in the surface layer of oceans depend mainly on evaporation
and precipitation.
(ii) Surface salinity is greatly influenced in coastal regions by the fresh water flow
from rivers, and in polar regions by the processes of freezing and thawing of ice.
(iii) Wind, also influences salinity of an area by transferring water to other areas.
(iv) The ocean currents contribute to the
salinity variations. Salinity, temperature
and density of water are interrelated. Hence,
any change in the temperature or density
influences the salinity of water in an area.
HORIZONTAL DISTRIBUTION OF SALINITY
The salinity for normal open ocean ranges
between 33 o/ oo. and 37 o/oo. In the land
locked Red Sea, it is as high as 41 o/00
while in the estuaries and the Arctic, the
salinity fluctuates from 0 - 35 o/oo,
seasonally. In hot and dry regions, where
evaporation is high, the salinity sometimes
reaches to 70 o/
The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent. Salinity
decreases from 35 o/oo on the western parts of the northern hemisphere because of the influx of
melted water from the Arctic region. In the same way, after 15° - 20° south, it decreases to 33 -
31 o/oo The average salinity of the Atlantic Ocean is around 36 0/00 The highest salinity is recorded
between 15° and 20° latitudes.
Maximum salinity (37 o/oo.) is observed between 20° N and 30° N and 20° W - 60° W. It gradually
decreases towards the north. The North Sea, in spite of its location in higher latitudes, records
higher salinity due to more saline water brought by the North Atlantic Drift. Baltic Sea records low
salinity due to influx of river waters in large quantity. The Mediterranean records higher salinity due
to high evaporation.
Vertical Distribution of Salinity
1. Salinity changes with depth, but the way it changes depends upon the location of the sea.
2 .Salinity at the surface increases by the loss of water to ice or evaporation, or decreased by the
input of fresh waters, such as from the rivers.
3. Salinity at depth is very much fixed, because there is no way that water is ‗lost‘, or the salt is
‗added.‘
4. There is a marked difference in the salinity between the surface zones and the deep zones of the
oceans.
5. The lower salinity water rests above the higher salinity dense water.
6. Salinity, generally, increases with depth and there is a distinct zone called the halocline,
where salinity increases sharply. Other factors being constant, increasing salinity of sea water
causes its density to increase. High salinity sea water, generally, sinks below the lower salinity
water. This leads to stratification by salinity