NCERT EXERCISES
1. Multiple choice
questions.
(i) Identify the element which is
not a part of the hydrological cycle
(a) Evaporation (b) Hydration (c) Precipitation (d) Condensation
Ans. (b) Hydration
(ii) The average depth of
continental slope varies between
(a) 2-20m (b) 200-2,000m (c)
20-200m (d) 2,000-20,000m
Ans. (b) 200-2,000m
(iii) Which one of the following
is not a minor relief feature in the oceans:
(a) Seamount (b) Atoll (c) Oceanic
Deep (d) Guyot
Ans. (c) Oceanic Deep
(iv) Salinity is expressed as the
amount of salt in grams dissolved in sea water per - (a) 10 gm (b) 1,000 gm (c)
100 gm (d) 10,000 gm
Ans. (b) 1,000 gm
(v) Which one of the following is
the smallest ocean?
(a) Indian Ocean (b) Arctic Ocean (c)
Atlantic Ocean (d) Pacific Ocean
Ans. (b) Arctic Ocean
2. Answer the following
questions in about 30 words.
(i) Why do we call the
earth a Blue Planet?
Ans. 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. Water 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 (About 71 % of
Surface of Earth is covered with Water). Hence, our planet is called
the ‘Blue Planet’.
(ii) What is a continental margin?
Ans. 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.
(iii) List out the deepest
trenches of various oceans.
Ans. 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. Mariana Trench is the
deepest trench (about 11 km deep) in the world located in Pacific Ocean.
(iv) What is a
thermocline?
Ans. The boundary region, between the surface waters of the ocean and the deeper layers, usually begins around 100 - 400 m below the sea surface and extends several hundreds of meters 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.
 |
| Thermocline |
(v) When you move into the
ocean what thermal layers would you encounter? Why the temperature varies with
depth?
Ans. The temperature-depth
profile for the ocean water shows how the temperature decreases with the
increasing depth. 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.
The average temperature of surface
water of the oceans is about 27°C and it gradually decreases from the equator
towards the poles and from surface to bottom. It is a well-known fact that the
maximum temperature of the oceans is always at their surfaces because they
directly receive the heat from the sun and the heat is transmitted to the
lower sections of the oceans through the process of convection. It results
into decrease of temperature with the increasing depth, but the rate of
decrease is not uniform throughout. The temperature falls very rapidly up
to the depth of 200m and thereafter, the rate of decrease of
temperature is slowed down.
(vi) What is salinity of
sea water?
Ans. All waters in nature, whether rainwater or ocean
water, contain dissolved mineral salts.
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/oo) or ppt.
The salinity for normal
Open Ocean ranges between 33o/oo and 37 o/oo. In the land locked Red
Sea, it is as high as 41o/oo, 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/oo.
3. Answer the following
questions in about 150 words.
(i) How are various
elements of the hydrological cycle interrelated?
Ans. Water is a cyclic
resource. It can be used and re-used. Water also undergoes a cycle from
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, atmosphere, land surface and subsurface and the organisms.
 |
| Hydrological Cycle |
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. Here is a Question for you - How can you intervene in
improving the water quality and augmenting the available quantity of water?
(ii) Examine the factors that
influence the temperature distribution of the oceans.
Ans. 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. This
process creates the spatial and vertical variations of temperature in various
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 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
from 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.
महासागरीय जल (अभ्यास प्रश्न - उत्तर)
Xtra Questions
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