NCERT Class 11 Geography Chapter 13 Water (Oceans)

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NCERT Class 11 Geography Chapter 13 Water (Oceans)

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Water (Oceans)

Chapter: 13

GEOGRAPY [ PART – I ]

SHORT ANSWER TYPE QUESTIONS

Q.1. What are oceanic deeps?

Ans. The deeps are deeper than the abyssal plains. They occupy two percent of the oceanic areas. Although deeps are found in all major oceans, Mariana deep is the deepest one in the Pacific Ocean.

Q.2. Name the major oceanic water bodies.

Ans. Major Oceanic bodies are:

1. Pacific Ocean.

2. Atlantic Ocean.

3. The Indian Ocean.

4. Arctic Ocean.

Q.3. Which is the largest ocean of the world?

Ans. Pacific Ocean.

Q.4. Which are the two main banks of the world?

Ans. (i) The Doggar Bank (Northern sea).

(ii) The Grand Bank (Atlantic sea).

Q.5. Name the deepest trench of the world.

Ans. The Mariana Trench which is 11.02 km. deep. It lies in the Pacific Ocean.

Q.6. Name the largest barrier reef of the world. Where is it located?

Ans. The Great Barrier Reef is the largest reef of the world. It is found off the oceans of west coast of Australia.

Q.7. What is hydrography?

Ans. Hydrography is the investigation of seas and other bodies of water including charting ocean surface, study of tides, currents etc.

Q.8. How much percentage of earth is covered with water?

Ans. 71% of the surface of the earth is covered with water.

Q.9. What is the continuous evapotranspiration?

Ans. The continuous transfer of water state to gaseous state is referred as evapotrans-piration.

Q.10. What are the forms of precipitation?

Ans. The forms of precipitation are:

(i) rainfall.

(ii) snow fall.

(iii) hailstone.

Q.11. What is an abyssal plain?

Ans. Beyond the continental slope are the Abyssal Plains. They are located at a depth of 3000 to 6000 metres. They cover about 40% of the ocean floor. The deposits of fine mud containing bones and shells of minutes animals that live and die on the surface of sea are found. They are found in all the major oceans of the world.

SHORT ANSWER TYPE QUESTIONS

Q.1. What is the salinity of sea water?

Ans. 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 grams) dissolved in 1000 grams of sea water. Salinity of 24.7% has been considered as the upper limit to demarcate brackish water.

Q.2. Why do we call the earth a Blue Planet?

Ans. The earth is called a Blue Planet or Water Planet because of abundance of water found on its surface. The water bodies cover 71% of earth’s surface. If we take into account only the water surface of the earth then 43 per cent lies in the Northern Hemisphere and 57 per cent is the Southern Hemisphere.

Q.3. What is a continental margin?

Ans. The continental slope together with the continental shelf is called the continental margin. The continental margin is the zone of the ocean floor that seperates the thin oceanic crust from thick continental crust together the continental shelf continental slope and continental rise are called continental margin.

Q.4. List out the deepest trenches of various oceans.

Ans.

Ocean	Trade	Depth
1. Pacific	Challenger Deep	10.911 m
2. Pacific	Tonga	10882 m
3. Atlantic	Pureto Rice	8800 m
4. Southern Atlantic	Southern Sandwich	8428 m
5. Indian	Java	7725 m
6. Arctic	Eurosian basin	5450

Q.5. List the various direct and indirect uses of ocean to man.

Ans. Direct uses:

(i) Oceans are infinite store houses of fish.

(ii) Oceans are store houses of minerals.

(iii) Ocean provide the cheapest means of transportation. 

Indirect uses:

(i) Oceans control the climate.

(ii) Oceans can be used to produce tidal power and geo thermal energy.

Q.6. What is a thermocline?

Ans. The boundary region where there is a rapid decrease of temperature is called thermocline.

Thermocline is the second layer of ocean water which lies below the first layer and it is characterized by a rapid decrease in temperature with increasing depth.

Q.7. Examine the major relief features of the Atlantic.

Ans. The Atlantic Ocean is separated from Arctic Ocean by a submarine ridge extending from Greenland to Scotland.

The Mid-Atlantic Ridge (480-970 km wide) is a submarine mountain range. It is extending upto 16,100 km. It rises to the average height of 3050m and a few peaks of island.

The Mid-Atlantic Ridge divides the floor of the Atlantic Ocean into eastern and western sections. The greatest deep is the Milwanke Trench, north of Peurto Rico. Its depth is 8530 m.

Q.8. When you move into the ocean what thermal layers would you encounter? Why the temperature varies with the depth?

Ans. When we move into ocean we well encounter the three thermal layers.

The temperature varies with the depth because the temperature structure of oceans over middle and low latitudes can be described as a three layer system. The first layer it is about 500m thick with the temperature 20° to 25° C.

Second layer: It is below the first layer and temperature decreases rapidly with increasing depth, third layer is very cold and extends upto the deep ocean floor.

Q.9. Name the most common relief features found on the ocean floors.

Ans. The most common relief features found on the ocean floors are:

1. Continental shelf.

2. Continental slope.

3. Submarine canyon.

4. Abyssal plains.

5. Submarine ridges.

6. Ocean deeps.

7. Troughs.

Q.10. What are sea mounts?

Ans. There are various mountains found on ocean floors. The mountains which are above 1000 metres in height and have a conical summits are known as sea mounts.

The mountains having flats summits are known as Guyots.

Many sea mounts are found in the Gulf of Alaska. Patton; Pratt, Faris etc. are well known sea mounts. Some of them attain a height  of 3000 metres.

Q.11. What is atoll?

Ans. There are low islands found in the tropical ocean consisting of coral reefs surrounding a central dipression. It may be a part of the sea (Lagoon) or sometimes form enclosing a body of fresh brackish or highly solim water.

Q.12. How have deep sea ridges been formed?

Ans. Deep sea ridges are elongated high features on the floors of the oceans. These ridges are either broad, gently sloping rises or steep sided narrow submarine mountains. The summits of the submarine ridges rise above sea level forming islands such as Andaman group of islands.

Formation: The submarine ridges áre formed by the following processes:

1. Large outpouring of basalt along-fissure line.

2. Rising of the crust by the rising convection currents.

3. Thickening and buckling of the basaltic crust by the downward movement of the convectional currents.

4. Formation of new oceanic crusts.

Q.13. Examine the reasons for horizontal variation of salinity.

Ans. The following are the reasons for horizontal variation of salinity in oceans:

1. Evaporation and precipitation: Where evaporation is high the salinity will be high in hot and dry regions. If somewhere rainfall is high the salinity will be low. Example, equatorial regions.

2. Fresh Water: If the source of fresh water is regular, the surface salinity will be low. Polar regions have the low salinity by the process of freezing and thawing of ice.

3. Wind is another factor responsible for the movement of water from one area to another also influences salinity.

4. The ocean currents which are moving laterally or vertically in different parts of the ocean contribute to the salinity variations.

Q.14. Distinguish between:

(i) Continental Shelf and Continental Slope.

Ans. 

Continental Shelf	Continental Slope
(i) It joins shore-line with continental slope.	(i) It joins continental shelf with Abyssal Plain.
(ii) Its slope is very gentle i.e., about 2 metres per km.	(ii) Its slope is very steep. The average gradient is about 4 degrees.
(iii) It is a shallow water area. Its depth is less than 200 metres.	(iii) Its depth varies from 200 to 2000 metres.
(iv) There are few canyons or deeps in the continental shelves.	(iv) Deep canyons are found on the continental slopes.
(v) They have large stores of sand and gravel.	(v) Very few land deposits are found on it.
(vi) They are of great use to man, particularly for fishing.	(vi) They are not of much use to man.

(ii) First and third layer of oceanic water.

Ans. 

The First Layer	The Third Layer
It represents the top layer of the warm oceanic water and it is about 500m thick and with temperature ranging between 20° and 25°C. This layer within the tropics is present throughout the year, but in mid-latitudes it develops only during summer.	It is very cold and extends upto the deep ocean’s floor. In the Arctic Ocean, the surface water temperature are close to 0°C and so the temperature change with depth is very slight.

(iii) Bank and Relief.

Ans. 

Bank	Reef
A bank is more or less flat topped elevation located on the continental margins. The depth of water over bank is relatively small but is adequate for navigation. The Dogger Bank in the North Sea and the Grand Bank in the North-Western Atlantic off Newfound land and George’s Bank off the eastern coast of U.S.A. are famous examples of Banks.	A reef is a rocky or coral elevation having a generally elongated shape. There are many well known types of reefs such as Fringing Reef, Barrier Reef, Coral Reef and Atoll Reef. The longest barrier reef extends off the Queensland coast of Australia.

(iv) Thermocline and Halocline.

Ans. 

Thermocline	Halocline
Below the halocline difference in the salinity is very small. Land salinity lies in the range of 34.6 to 34.9 per thousand for most of the ocean body. It is Thermocline zone.	A shallow surface layer of uniformly high salinity corresponds with the uniform warm layer. Below this layer is a zone of rapid decrease in the salinity, the halocline which corresponds with the thermocline.

(v) Sea Mount and Guyot.

Ans. 

Sea Mount	Guyot
1. Sea Mounts are the mountains on the sea floor.	1. Guyots are hills found on the sea floor.
2. They are above 1000 metres in height.	2. They are also above 1000 metres in height.
3. They have conical summits.	3. They have flat tops.
4. They are of volcanic origin.	4. They are also of volcanic origin.
5. Some of them attain a height of 3000 metres.	5. An ordinary guyot has a diameter of 20-25 km.

(vi) Submarine Trench and Canyon.

Ans. 

Submarine Trench	Canyon
1. A submarine trench is a long narrow and steep-sided depression on the ocean bottom.	1. Canyon is a deep gorge on the continental slope.
2. Its depth varies from 7000 to 11000 metres.	2. It depth varies from 1800 to 2800 metres.
3. They are almost empty of sediments.	3. Coarse deposits are found on the canyons.
4. They are attributed to the endogenic forces which have produced mountains.	4. They are found near river mouth and may have been the result of erosion.

(vii) Shoal and Bank.

Ans. 

Shoal	Bank
1. Shoal is a bank of coastal sediment that rises almost to the surface of the sea.	1. It is a flat topped elevation located in the continental margins.
2. They are only 20 metres below sea level and so dangerous for navigation.	2. The depth of water over a bank though small but is adequate for navigation.
3. They are not of much use to man.	3. They are of much use to man because, they are the famous fishing ground such as Dogger Bank in North sea and Grand Bank in North-west Atlantic.

Q.15. Write short notes on the following:

(i) Continental Rise.

Ans. Continental Rise: The continental shelves give way to continental slopes. The slope abruptly is replaced by the continental rise, a surface of much gentler slope decreasing in slopiness towards the ocean basin floor. The continental rise generally has a moderate to low relief.

(ii) Mid-Oceanic Ridge.

Ans. Mid-Oceanic Ridge: A great submarine mountains chain extending from a total length of 64000 km is known as the mid-oceanic ridge. The ridge runs down the middle of the North and South Atlantic Ocean basins into Indian Ocean basin and then passes between Australia and Antarctica to enter the South Pacific Basin. The ridge in its entirety is a belt of 2000 to 2400 km wide in which the surface rises through a series of steps from abyssal plains on each side.

(iii) Ocean Basins.

Ans. Ocean Basins: The ocean basin is an extensive region of the basin floor, generally lying in a depth range of 2500 to 6000 m. It covers about 76.2 percent of the oceans’ areas. The ocean basin floor contains three types of features, abyssal plains, oceanic rise and sea mounts.

(iv) Continental slope, Abyssal Plains.

Ans. Continental Slope: It is an area of steep slope extending just after the continental shelf upto a considerable depth from where a gentle sea plain takes its form. The extent of the slope area is usually between 200-2000 metres depth. But sometimes it may extend to 36000 metres from the mean sea level. The continental slope along many coasts of the world is furrowed by deep canyon-like trenches. Continental slope covers 8.5 per cent of the total ocean area. There are five types of slopes:

(i) Fairly steep with the surface dissected by canyons.

(ii) Gentle slope with elongated hills and basins.

(iii) Faulted slopes.

(iv) Slopes with terraces and

(v) Slopes with sea mounts.

The Abyssal Plains: Beyond the continental slope are the Abyssal Plains. They are located at a depth of 3000 to 6000 metres. They cover about 40% of the ocean floor. The deposits of fine matter containing bones and shells of minute animals that live and die on the surface of sea are found. They are found in all the major oceans of the world.

(v) Submarine Canyons.

Ans. Submarine Canyons: They are deep valleys which have sharply oceanic floors. They occur along all the coasts of the world. Submarine canyons are of three types:

(i) The small gorges extend down from the edges of continental shelves to continental slopes.

(ii) The canyons situated at the mouths of rivers.

(iii) The canyons of dendritic appearance.

(vi) Mid Atlantic Ridge.

Ans. Mid Atlantic Ridge: The most striking feature of the Atlantic ocean is the presence of the mid-Atlantic Ridge. It extends from the north to the south parallel to the ‘S’ shape of the ocean itself. It divides the Atlantic Ocean into two deeper basins on either side. The ridge is about 14,000 km long and about 1500 metres high. The ridge has stretched features. The slopes on both sides are very gentle throughout the greater part of its length. It rises as a series of steps and becomes rugged near the crest.

The ridge, though under the sea level, has many peaks projected out of the deep water above the surface of the ocean. These peaks are in fact, the islands of the Mid-Atlantic. Examples include Pico Island of Azores and Cape Verde Islands. Bermuda is a coral island. Ascension, Tristanda Cunha, St. Helena and Gough are volcanic islands.

Q.16. Compare the reliefs features of Pacific Ocean with with Indian Ocean.

Ans.

Relief features of the Pacific ocean	Relief-features of the Indian Ocean
1. Extention: It extends to 165 million sq. km.	1. Extention: It is extended from 20° E to 115° E longitude.
2. Ridges: There is absence of long ridges.	2. Ridges: There are a number of broad submarine ridges on its floor.
3. Ocean basins: The Pacific Ocean has a number of shallow basins. These are five in number.	3. Ocean basins: The central ridge divides the. ocean into six basins.
4. Ocean deeps: There are 32 deeps in this ocean. The deepest one is Mariana Trench (11022 m)	4. Ocean Deeps: Ocean deeps are almost absent from this ocean.
5. Marginal seas: Marginal seas are few.	5. Marginal seas: The true marginal seas are Red sea, Persian, Arabian, Bay of Bengal, etc.
6. Islands: It has more than 20,000 islands.	6. Islands: Continental islands are Sri Lanka and Madagascar.

Q.17. Compare the salinity of various oceans at different depths.

Ans. Salinity changes with depth but the way it changes depends on where the water body exists on the earth. Salinity at depths is very much fixed because there is no way that water is lost nor any way to add salt.

The salinity in the Red Sea is 41%, in Pacific Ocean the salinity is varied due to the shape. Salinity decreases are from 35% to 31% in the western portion.

The average salinity of Atlantic Ocean is around 36%. The highest salinity is recorded between 15° to 20° latitude. Maximum salinity 37° is observed between 20°-30° N and 20° W to 60°. The average salinity of Indian Ocean is 35%.

Q.18. Why is Indian ocean considered half ocean?

Ans. Atlantic ocean and Pacific ocean open out both north ward and south ward. Both are balanced along the equator. But Indian ocean is blocked on the north by Asia. Infact it extend only south ward. Therefore it is considered half ocean.

Q.19. What do you mean by Antipodal position?

Ans. The distribution of land and water on the globe is antipodal. Each continents is located diametrically opposite to an ocean. If there is a land in one part of the globe, there is water opposite to it on the other side of the earth. For example Arctic ocean is opposite to Antarctica continent. Europe and Africa are antipodal to Pacific ocean while North America is antipodal to Indian ocean.

LONG ANSWER TYPE QUESTIONS

Q.1. 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. It is always in movement and its changing starts from liquid to vapour and from vapour to ice and vice versa. The distribution of water on earth is quite varied in many locations. Some have plenty of water while others have very limited amount of water. The water cycle is the circulation of water within the earth’s hydrosphere involving changes in the physical state of water between liquid, solids and gaseous phases.

Q.2. Define continental shelf.

Ans. The portion of the land which are submerged under sea water constitutes continental shelf. The continental shelf is hallow and its depth is not more than 2000 metres. Its slope from land to sea is about 2 metres per km. The average depth is 70-75 km. Though it varies from a few metres to hundreds of km. In all about 7.5 per cent of total area of the oceans is covered by the continental shelves. The continental shelves may be of different types e.g. Glaciated shelves (example sea near New England); Broad river shelves (example, Yellow sea and the gulf of Siam) and coral shelves e.g. those of Eastern Australia.

The origin of the continental shelves is controversial:

1. Some scholars think that the shelves are the product of the deposition of rivers, winds or ocean waves.

2. But some continental shelves might be the result of subsidence of the coastal areas.

3. Diastrophic forces are also responsible for the origin and development of shelves. Old beaches and marines can be identified on the shelves.

The shelves are of great use to man:

1. Marine food comes almost entirely from them.

2. About 20 per cent of oil and gas of world is extracted from them.

3. They are large stores of sand and gravel.

4. They are the sites of productive fishing grounds.

Q.3. Describe the relief of the Indian Ocean floor.

Ans. The Indian ocean is smaller than the Pacific and Atlantic. It is said to be the half ocean because it is blocked on the north by Asia. It does not reach to Arctic Sea in the north. In the south it extends up to the continent of Antarctica. Its average depth is 4000 metres which is lesser than that of these two oceans.

Trenches: The only famous trench is the Lunda Trench which lies south of the island of Java and runs parallel to it.

Ridges: There are a number of broad submarine ridges on the floor of Indian Ocean. A prominent ridge runs from Kanyakumari to Antarctica. It is situated in the middle and divides the ocean into two basins on either side. It is called the Lakshadweep-Chagos Ridge in the north, the St. Paul Ridge in the middle and the Amsterdam St. Plateau in the south, where it widens out considerably. There are two minor and parallel ridges which run north-westward. These are known as the Sacotra-Chagos Ridge and the Seychelles Ridges. The Madagascar Ridge and Prince Edward Crozet Ridges are other ridges. The Carlsberg Ridge divides Arabian sea into two parts. The Andaman and Nicobar Ridge extends from the mouth of the Irrawaddy to the Nicobar Islands.

Basins: The chief basins in the Indian Ocean are Central Basin, Arabian Basin, South Indian Basin, Mascarene Basin, West Australian and South Australian Basins.

Islands: The Andaman and Nicobar, Sri Lanka, Madagascar and Zanzibar islands are the parts of the continental blocks. The Lakshadweep Island is a coral island. The Mauritius and Reunion islands to the east of Madagascar are the volcanic origin.

Q.4. Prepare a map of Atlantic Ocean floor mentioning ocean basins and the mid-Atlantic ridge.

Ans. The Atlantic Ocean covers about 1/6th of the earth’s total surface. On the east it is bounded by Europe and Africa, on the west by both North America and South America, in the north by Greenland, Iceland and other smaller islands and in the south it is open upto Antarctica.

Continental Shelf: Continental shelf occurs all around the Atlantic Ocean but it varies in width. It is wider in the north than in the south.

Seas: There are numerous marginal seas on both sides of the Atlantic Ocean. Most of them are located in shelves like Hudson Bay, the Baltic Sea and the North Sea.

Ridge: Special feature of the Atlantic Ocean is the Mid-Atlantic Ridge. It extends from north to south parallel to the ‘S’ shape of the ocean itself. It divides the Atlantic into two deeper basins on either side. The ridge is about 1400 km long and about 4000 metres high. The slopes on both sides are very gentle. It rises as a series of steps.

Islands: The ridge has many peaks projecting themselves above the water surface. Such a island is Pico Island of Azores. Cape Verde Island of Bermuda is a coral island. Ascension, Tristan Dacunha, St. Helena, Gough are volcanic islands.

Trough and Trenches: The Atlantic Ocean lacks in trough and trenches. North Gayman and Puerto Rico are the two troughs and Ramanche and South Sandwich are the two trenches in the Atlantic Ocean.

Q.5. Examine the factors that influence the temperature distribution of the oceans.

Ans. Factors which affect the temperature distribution are as under:

1. Latitude: The temperature of surface water decreases from equator towards the poles because the sun’s rays become more and more inclined and thus the amount of insolation decreases poleward.

2. 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 their counterparts in the Southern Hemisphere.

3. Prevailing winds: The winds blowing from the land towards the ocean drive surface water away from the coasts resulting into upwelling of cold bottom water from below.

4. Ocean Currents: Warm ocean currents increase the temperature in cold areas while cold currents decrease the temperature in warm ocean. Example: gulf stream, a warm current increases the temperature of eastern coast of North America and west coast of Europe.

5. Other factors are local weather conditions like storms, cyclones, etc.

6. Salinity: The temperature of ocean water varies with salinity. The greater the salinity the higher is the temperature.

Q.6. What do you understand by hydrosphere?

Ans. Hydrosphere: The part of the earth sub-merged by oceans and seas is known as Hydrosphere. The vast oceans cover an area of about 361 million sq. km which is 71% of the total area of the earth surface. Many scientists called the earth as a watery planet. Nearly 61% of the Northern hemisphere and 81% of the southern hemisphere is covered by water. The southern hemisphere is known as water hemisphere due to excess of water. The distribution of land and water is antipolal on the earth. A Arctic ocean surrounds the north pole and the Antarctica continent circles the south pole.

Q.7. Why are the ocean deeps or tranches believed to be of tectonic origin?

Ans. A long narrow and steep sided depression on the ocean bottom is called a trench. There are believed to have resulted from down faulting of the earth’s crust. Therefore these are of techtonic origin. These are generally found along the fold mountains or the island chains.

Q.8. Explain the features of the sea mounts.

Ans. (i) It is the mount with a pointed submit rising from the sea floor but do not reach the surface of ocean. For example emperor sea mount, an extension of Hawaiin islands. The tallest sea mount between Soman and New Zealand.

(ii) Sea mounts are volcanic in origin and can be 3000 to 4000 m tall.