Water Oceans - Solutions

 CBSE Class 11 Geography

NCERT Solutions
Chapter 13
Water (Oceans)

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) 2-20 m
(b) 200-2,000 m
(c) 20-200 m
(d) 2,000-20,000 m.

Ans. (b) 200-2,000 m

(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. (b) Atoll

(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. Planet Earth has been called the "Blue Planet" due to the abundant water on its surface. Here on Earth, we take liquid water for granted; after all, our bodies are mostly made of water. However, liquid water is a rare commodity in our solar system. 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. Hence, our planet is called the 'Blue Planet'.

(ii) What is a continental margin?

Ans. The continental margin is the shallow water area found in proximity to continent. The continental marginconsists of three different features: the continental rise, the continental slope, and the continental shelf. Continental margins constitute about 28% of the oceanic area. The continental margin is the extended portion 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.

(iii) List out the deepest trenches of various oceans.

Ans. 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. Some important trenches of the world are as follows:
(a) Mariana Trench: The Challenger Deep in the Mariana Trench is the deepest known point in Earth's oceans. Mariana Trench is the world's deepest trench. It lies in Pacific Ocean. It is 11034 km below the ocean.
(b) Puerto Rico Trench: The Puerto Rico Trench is located on the boundary between the Caribbean Sea and the Atlantic Ocean. It is deepest trench in Atlantic Ocean.
(c) Sunda Trench: The Sunda Trench, earlier known as, and sometimes still indicated as the Java Trench, is located near Sumatra, formed where the Australian Plate subducts under a part of the Eurasian Plate. It is deepest trench in Indian Ocean.

(iv) What is a thermocline?

Ans. A thermocline (sometimes metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below.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.

(v) When you move into the ocean what thermal layers would you encounter? Why the temperature varies with depth?

Ans. The temperature of the ocean decreases as we go deeper due to the lack of thermal radiation from the sun. Water is generally incompressible. Thus, we cannot increase the intra-molecular collisions by applying pressure to water. Therefore the lack of thermal radiation far out weights the increased pressure with respect to temperature. The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom.

(a) The surface layer of the ocean is known as the epipelagic zone and extends from the surface to 200 meters (656 feet). It is also known as the sunlight zone because this is where most of the visible light exists. With the light come heat. This layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° C and 25° C. This layer, within the tropical region, is present throughout the year but in mid-latitudes it develops only during summer.

(b) Thermocline, oceanic water layer in which water temperature decreases rapidly with increasing depth. A widespread permanent thermoclineexists beneath the relatively warm, well-mixed surface layer, from depths of about 200 m (660 feet) to about 1,000 m (3,000 feet), in which interval temperatures diminish steadily. It lies below the first layer and is characterised by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.

(c) Hyplomnion – The third layer, the lower layer of water characterised by a uniform temperature that is cooler than other layers.The third layer is very cold and extends upto the deep ocean floor. In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature variation with the depth is very slight.

(vi) What is salinity of sea water?

Ans. The concentration of salt in seawater (salinity) is about 35 parts per thousand. Stated in another way, about 3.5 percent of the weight of seawater comes from the dissolved salts; in a cubic mile of seawater, the weight of the salt (in the form of sodium chloride) would be about 120 million tons.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(%) or ppt. Salinity is an important property of sea water. Salinity of 24. 7% has been considered as the upper limit to demarcate 'brackish water'. Salinity changes with depth, but the way it changes depends upon the location of the sea. Salinity at the surface increases by the loss of water to ice or evaporation, or decreases by the input of fresh water, such as from the rivers. Salinity at depth is very much fixed, because there is no way that water is 'lost', or the salt is 'added.'

3. Answer the following questions in about 150 words.
(i) How are various elements of the hydrological cycle interrelated?

Ans. The various elements of the hydrological cycle are air and water. Water on the earth exists in three states-liquid, gas and solid. Water is a cyclic resource. It can be used and re-used. Water also undergoes a cycle from atmosphere, land surface and sub surface 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 ice-caps, 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. 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.

(ii) Examine the factors that influence the temperature distribution of the oceans.
The factors which affect the distribution of temperature of ocean water are explained below:
(a) Latitude: The ocean water gets heated by the absorption of solar radiation. The sun's rays are always vertical at the equator, but because of the spherical shape of the earth, with increasing distance from the equator the rays become more and more slanting.The temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward. 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.
(b) Unequal distribution of land and water: The uneven and unequal distribution of Earth’s land and oceans causes there to be moisture circulation in order to maintain balance. Their distribution determines large-scale atmospheric and oceanic circulation patterns. Their dissimilar heat capacities, with Oceans having a much greater capacity relative to land, drives continental-scale convective atmosphere transport. 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.
(c) Prevailing wind: Temperature of the surface water of the oceans and seas is also affected by the prevailing winds. When the warm air masses from over the heated land areas in the tropical regions move over the oceans, their surface temperatures are immediately raised. The effect of such winds is especially marked on the landlocked or partially enclosed seas. 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. As a result, there is longitudinal variation in the temperature. On the contrary, the onshore winds pile up warm water near the coast and this raises the temperature.
(d) Ocean currents: The ocean currents also exercise dominant control over the tempera­ture of the ocean water. All the warm ocean currents originate in the tropical oceans. When they reach the higher latitude oceans, temperature of the surface water registers an increase of several degrees. Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. Gulf stream 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.
(e) Salinity:

  • Salinity determines compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation and humidity.
  • It also influences the composition and movement of the sea: water and the distribution of fish and other marine resources.

Saline water absorbs more heat and its temperature rises much higher than fresh water.
All these factors influence the temperature of the ocean currents locally.