NCERT Class 11 Geography Chapter 14 Movements of Ocean Water

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NCERT Class 11 Geography Chapter 14 Movements of Ocean Water

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Movements of Ocean Water

Chapter: 14

GEOGRAPY [ PART – I ]

SHORT ANSWER TYPE QUESTIONS

Q.1. What are ocean currents?

Ans. Regular movements of the surface water of the oceans in the particular directions are called ocean currents.

Q.2. What are the main processes of cooling of ocean water?

Ans. Back radiation of heat from the sea surface, convection and evaporation.

Q.3. Name an important current of Atlantic Ocean.

Ans. Gulf Stream.

Q.4. Name the warm current which flows on the coast of Japan.

Ans. Kuroshio.

Q.5. Which current flows along the S.W. Coast of Africa?

Ans. Cold Benezuela Current.

Q.6. Name the two types of waves.

Ans. (i) Transverse waves. and 

(ii) Longitudinal waves.

Q.7. What are tides?

Ans. The periodic phenomenon of alternate rise and fall of sea water twice a day at constant intervals is called tides.

Q.8. What is the main cause of tides?

Ans. Tides are produced by gravitational interaction of the earth, moon and the sun.

Q.9. Define waves.

Ans. The oscillating motion of the sea surface is called waves. It is a swell on the surface of the ocean of sea. When the surface of the sea is disturbed by winds, waves are produced. In the waves the water itself does not move forward or backward but rises up and down. The highest part of the wave is called crest and depressions are called troughs.

Q.10. Describe the disadvantages of tides.

Ans. 1. Tides are sometimes harmful for ships and boats.

2. Tides check the formation of the deltas.

3. Marshes are formed due to accumulation of tidal water.

4. Tides also hinder fishing.

Q.11. When do tides occur?

Ans. On the first and the third quarter of the moon.

Q.12. Name the main types of wind produced waves.

Ans. 1. Long shore waves: When the waves break against the shore and move parallel to the shore.

2. Undertow: Seaward movement of water is known as undertow.

3. Rip current: Rip currents are localised streaks of returning water.

Q.13. What are the three things that affect the size of waves?

Ans. The three things which affect the size of the waves are:

1. Wind.

2. Origin.

3. Area.

Q.14. Where are the largest waves found?

Ans. The largest waves are found in the open oceans.

Q.15. When do the waves begin to break down?

Ans. The waves begin to the break down when the depth of water is less than half the wavelength of wave.

Q.16. What are the highest part and lowest part of the wave called?

Ans. The highest part of the wave is called crest and the lowest part is called trough.

Q.17. What factors cause the waves to decay?

Ans. 1. The friction of water and sea floor.

2. Stagnant deep water.

3. Depth of the water.

Q.18. Examine the relationship between earthquake, tsunami and waves.

Ans. Tsunami and waves are related to earthquakes. Tsunami is caused by the earthquake that occurs on the sea floor. Waves are also formed due to earthquakes.

Q.19. What is tidal bore?

Ans. A tidal bore is a high wall of water which occasionally rushes upstream from the funnel-shaped mouth of river. It is caused by the conflict of the currents of a tide and the currents of the river. As the currents of a tide flow upstream and the currents of the river flow downstream, they conflict and give birth to the tidal bore. It is very dangerous to small ships and boats.

Q.20. How can velocity of the ocean currents be measured?

Ans. The ocean currents flow like extensive rivers in the ocean. The rate of flow and the width of the currents, however, are not uniform. The Gulf Stream for example is 80 km wide and 1.6 km deep. The Florida Current has a velocity of 10 km. per hour of the surface and 3.5 km. at a depth of 200 m.

Q.21. What do you understand by the strength of ocean currents.

Ans. The strength of a current refers to the speed of the current. A fast current is considered strong. A current is usually strongest at the surface and decreases in strength (speed) with depth. Most currents are less than or equal to 5 knots. Usually the currents are strongest near the surface and may attain speed over 5 knots.

SHORT ANSWER TYPE QUESTIONS

Q.1. Where do waves in the ocean get their energy from? 

Ans. Wind provides energy to the waves, wind causes waves to travel in the ocean and the energy is released on sharelines. The motion of the surface water seldom affects the stagment deep bottom water of the oceans. Waves continue to grow larger as they move and absorb energy from the wind. Most of the waves are caused by the wind driving against water. When a breeze of two knots or less blows over calm water small ripples form and grow as the wind speed increases until white caps appear in the breaking waves.

Q.2. What are waves?

Ans. Waves are oscillatory movements in water where particles moves in a vertical plane up and down. Waves are actually the energy which moves across the ocean surface.

Q.3. What are the icebergs?

Ans. Huge masses of floating ice are known as icebergs. These break from the ice sheets and reach the ocean. 1/10th of the icebergs remains above sea water. There are numerous icebergs in the north Atlantic. These originate from the glaciers of Greenland icebergs in the southern hemisphere originate from the Atlantic ice barrier. Alaska is also one of sources for icebergs. These are dangerous for navigation and many accidents take place.

Q.4. What is the interval of two tides?

Ans. Tides do not occur regularly every twelve hour and 25 minutes.

Reasons: The moon revolves around the earth from west to east and complete one revolution in 29 days. The moon shifts away and moves a little eastward from its position in 24 hours. Therefore it takes an extra time for the earth to bring the same point in the line of the moon everyday.

Extra time = 24 hrs × 60 minutes/29 = 50 minutes

So the tides are delayed by 50 minutes everyday. As the tide occurs twice a day each tide is delayed by 25 minutes. Hence high tide follow on interval of 12 hrs 25 minutes.

Q.5. What are tides?

Ans. The periodical rise and fall of sea water once or twice a day is called tides.

Q.6. Name the main types of wind produced waves.

Ans. (i) Long shore currents: When the waves break against the shore and move paralled to the shore.

(ii) Undertow: Sea ward movement of water is known as undertow.

(iii) Rip currents: Rip currents are local sides treaks of returning water.

Q.7. How are tides caused?

Ans. Tides are caused due to the following:

(i) The moons gravitational pull.

(ii) The sun’s gravitational pull.

(iii) Centrifugal force which is the force that acts to the counter balance the gravity. Together the gravitational pull and centrifugal force are responsible for creating the two major tidal bulges on the earth.

Q.8. How tides are related to navigation?

Ans. Tides are related to navigation as they help the navigators and their activities. Tidal heights are very important especially for harbours near river estuaries having shallow bars of the entrance which prevent ships and boats from entering into the harbours.

Q.9. How can velocity of a wave be determined?

Ans. The velocity of a wave can be determined as follows:

Velocity of a wave (V) = Wave Length (L)/Wave Period (T)

Every wave has a wave length, velocity, height and wave period. The distance between two successive crests or two successive troughs is called wave length. The time taken by a wave length to pass a fixed point is known as the wave period. The vertical distances between a trough and crest is the wave height.

Q.10. Name the four sets of factors which originate ocean currents.

Ans. Four sets of factors are:

1. Factors related to the earth’s rotation, gravitational force and force of deflection.

2. Factors originating outside the sea as atmospheric pressure, winds, insolation, evaporation and precipitation.

3. Factors originating within the sea such as pressure gradient, temperature difference, density and salinity, etc.

4. Factors modifying the ocean currents such as direction and shape of the coast.

Q.11. Name the factors that control the origin of the ocean currents.

Ans. The factors that control the origin of the ocean currents are:

1. Permanent winds or prevailing winds.

2. Variation in temperature.

3. The rotation of the earth.

4. Variation in salinity.

5. The direction of land masses.

The prevailing winds such as trade winds (westerlies) exert one way drag on sea surfaces over vast expanses of oceans. This drag produces a system of drift system.

Difference in density can also set currents in motion and these are described as thermocline currents.

Q.12. (i) Why do spring tides occur on only new moon and the full moon.

Ans. Spring tides occurs on new moon and full moon days because, on these days, the sun, the moon and the earth are in the same straight line and owing to the combined pull of the sun and the moon, the rise and fall of water is greater than it is on other days.

(ii) Why are tides delayed everyday by 56 minutes?

Ans. Tides occur twice a day and their interval is not exactly 12 hours but it is 12 hours and 25 minutes. The relative position of the moon and the earth is responsible for this variation. As we see in Fig. 14.5 the moon first appears at 1 and a part of the earth is under high tides. On second day, the moon has moved ahead at the position 2. To bring a place at the position 2, the earth will take an extra time of 50 minutes and hence this 50 minutes extra time delays the tides every day.

Q.13. Name the two warm currents and two cold currents of the North Atlantic Ocean and also explain the effects of these currents on the climate of the control areas along which they flow.

Ans. Two warm and two cold currents of the North Atlantic Ocean are as follows:

(a) Warm currents:

(i) Florida Currents.

(ii) Gulf Stream and North Atlantic drift.

(b) Cold currents:

(i) Canary current.

(ii) Labrador Current.

The climate of the coasts become warm or cold according to the currents passing through the coasts. United Kingdom and Labrador (U.K. N.America) are in the same latitude but remains unfrozen due to warm Gulf Stream and Labrador is frozen for nine months due to cold Labrador currents. Norway Coasts are unfrozen throughout the year due to North Atlantic Drift. Thus the currents affect the climate of the coastal areas.

Q.14. What is the importance of tide to man?

Ans. Tides are of many uses to man such as:

1. Tides keep the ocean waters in motion. Thus, the water neither freezes nor polluted.

2. Near the coastal towns, tides can wash away waste material.

3. At high tides, big ships can come near to the coasts or go up rivers, encourging trade.

4. Fishermen also take the advantage of the tides for sailing to the sea and returning to the harbour.

5. At high tides, many things, such as shells, conches etc. are washed ashore.

6. Tides are a vast store of energy. In some parts of the world huge reservoirs are built to store water at high tides. Which is used to generate hydro-electricity.

Q.15. Write the characteristics of waves in the oceanic water.

Ans. Characteristics of waves in the oceanic water are as under:

(i) The highest and lowest points of a wave are called the crest and trough.

(ii) Wave height is the vertical distance from the bottom of a trough to the top of a crest of a wave.

(iii) Wave amplitude is one half of the wave height.

(iv) Wave period is merely the time interval between two successive wave crests.

(v) Wave length is the horizontal distance between two successive crests.

(vi) Wave speed is the rate at which the wave moves through the water and in measured in knots.

(vii) Wave frequently is the number of wave passing a given point during a second time interval.

Q.16. Write a note on Gulf Stream Current and its significance to fishing.

Ans. Gulf Stream Current: It begins from the Florida strait and flows northward and eastward along the east coast of the U.S.A. It continues northward to the vicinity of the Grand Banks off Newfoundland. Here heavy fogs form along the boundary where it meets the cold Labrador Current. This mixing helps the planktons. So this area is considered as one of the best fishing grounds of the world.

This current is responsible for mild climate throughout the North West Europe. Even in winters Norwegian parts along the Atlantic are ice-free most of the time. From the Grand Bank the Gulf Stream Current flows eastward across the Atlantic. On reaching the British Isles it divides into two branches, the northern branch and southern branch and flows between Spain and Azores.

Q.17. Describe the main characteristics of Ocean currents.

Ans. (i) According to Farrel’s law ocean currents more to their right in the northern hemisphere and to their left in the southern hemisphere.

(ii) Warm currents move towards cold seas and cold currents move towards the warm seas.

(iii) In the lower latitudes warm currents flow on the eastern shores and cold an the western shores.

(iv) In the higher latitudes warm currents move along the western shores and cold currents along eastern shores.

(v) Cold and warm currents meet along the convergence and move out along divergence.

(vi) Cold currents move as sub-surface currents and warm currents move as surface currents.

Q.18. How are the tides caused?

Ans. The tides are caused:

1. due to the gravitational pull of the moon and of the sun on the earth.

2. due to gravitational pull waters of the oceans are attracted towards moon. Water being liquid rises higher than the solid crust of the earth. This is known as the high tide. When the level of the water falls, it is known as low tide.

3. due to the rotation of the earth tides occur twice a day at a place.

One tide occurs on the earth surface facing moon and the other tide occurs at the opposite side of the earth.

Q.19. Distinguish between:

(i) Warm and Cold currents.

Ans.

Warm currents	Cold currents
These currents flow from the low latitudes in tropical zones towards the high latitudes in the temperate and subpolar zones.	These currents flow from high latitudes to low latitudes.

(ii) Swash and Back Wash.

Ans.

Swash	Back Wash
(i) It is the movement of a turbulent layer of water upslope of a beach as a result of the breaking of a wave.	(i) It is the movement of marine water down a beach, under gravitational influence after breaking of a wave.
(ii) It moves and deposits pebbles and conches along the beach.	(ii) It gives a seaward movement to the beach material.

(iii) Spring Tides and Neap Tides.

Ans.

Spring Tides	Neap Tides
(i) It is a tide with a range considerably increased from that of the mean tidal levels.	(i) It is tides with a range considerably lower than that of the mean tidal level.
(ii) It occurs twice each month around the time of the new moon and full moon.	(ii) It occurs twice each month during Lunar quadrature.
(iii) The high tides are somewhat lower than the average.	(iii) The high tides are somewhat lower and low tides are somewhat higher than average.
(iv) It is due to complementary gravitational effects caused when the earth, moon and sun are in a straight line.	(iv) Here tidal production forces do not supplement each other because the moon, the earth and the sun are at right angles. So the velocity of tidal currents slows down.

(iv) Swell and Surf.

Ans.

Swell	Surf
(i) These are wind generated waves in the open sea.	(i) These are also wind generated waves but near the beach.
(ii) Swell moves in a uniform pattern of equivalent period and height.	(ii) This is a foaming water which results from the highly dynamic wave activity.
(iii) They are generated in the open oceans.	(iii) They are found in the bracken zone of a shoreline.

LONG ANSWER TYPE QUESTIONS

Q.1. Give an account of the currents of Indian Ocean.

Ans. Ocean Currents in South Indian Ocean: The circulatory cycle as shown in fig. 14.6 in the southern Indian ocean is anti-clockwise. South Equatorial Current flows from east to west and it receives a part of the South Equatorial Current of the Pacific Ocean, which emerging out of the Indonesian Group of Islands enters Indian Ocean.

In the west, South Equatorial Current has two branches. One branch passes through Africa and Madagascar. It is called Mozambique Current. Another branch flows east of Madagascar parallel to the Mozambique Current. It is called Madagascar Current. These two branches meet each other south of Madagascar and form Agulhas Current. It later meets the South Pacific Current which flows from west to east. A branch of it flows along the western coast of Australia from south to north and meets the South Equatorial Current. The current along Western Australia is known as West Australian Current.

Ocean Currents in North Indian Ocean:

Summer: In summer, the prevailing winds blow from sea towards land and are known as South-West Monsoon. Under its influence a branch of South Equatorial current turns towards Africa in the west. The current turns towards east from the Gulf of Aden, and takes a round of the Bay of Bengal washing the western and the eastern coasts of India.

Winter: The direction of monsoon winds in winter is just the reverse of that in summer. It is north-east in winter. Hence the an ocean current starting from the strait of Malacca flows along the coast in the Bay of Bengal. After crossing Sri Lanka its flows along the coasts of western India from south to north. It takes a round of Gulf of Aden in the west and begins to flow towards south where its direction is from west to east. Hence, it called South Indian Current.

Q.2. If there were no ocean currents what would have happened to the world? Discuss.

Ans. The currents are useful to man and deeply affect the physical and economic environment of the areas where they flow. If there were no ocean currents then the following advantages would not be availed:

1. Effect on climate: The climates of coasts becomes warm or cool according as the current washing the coast is warm or cool.

2. Temperature: Oceanic currents also affect the temperature.

3. Rainfall: The currents of the oceans are helpful to bring the rain. If the currents were not present then rainfall would not materialize in the adjoining areas.

4. Effects on the marine ocean: Ocean currents sweep a large refuge off the coast which is the food for planktons.

5. Effect on Trade: The absence of ocean currents will adversely affect the trade and transport.

Q.3. What are the causes of currents?

Ans. The causes of currents are as under:

1. Prevailing winds: Prevailing winds influence the ocean currents more than any other single factor. These winds blow in a particular direction and drag the surface water due to the force of friction. This leads to the formation of ocean currents.

2. Temperature difference: Warm water is lighter and rises but the cold water is denser and sinks. This makes the difference in the levels of a sea. Warm equatorial water moves polewards along the surface while the cold water from the poles move along the bottom towards the equator.

3. Difference in salinity: Waters of high salinity are denser and heavier while the waters of low salinity are lighter. As a result light water with less salinity rushes towards the high salinity water and a current is formed.

4. Coriolis effect: Due to the Coriolis effect the moving water turns and flows elliptical path known as gyre. Water circulates in these gyres in clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.

5. Shape of the coast: Shape of the coast diverts ocean currents. For example the Cape Sao Reque of Brazil diverts the South Equatorial Current as Brazilian Current.

Q.4. Give an account of the currents of the Pacific Ocean. 

Ans. The main currents of Pacific Ocean are as:

1. North Equatorial Current: It is a warm current. It flows south of the equator from east to west.

2. South Equatorial Current: It is also a warm current. It flows south of equator from east to west.

3. Kuroshivo Current: It is a warm current. It flows along the coast of Japan.

4. New South Wales Current: It is also called East Australian Current. It is a warm current. It flows south along the east coast of Australia.

5. Antarctic Drift: It is a cold current. It flows round the globe from west to east near the Antarctic Ocean.

6. Humbolt Current: It is also known as Peruvian Current. It is a cold current. It flows from south to north along the west coast of South America. It is a branch of Atlantic Drift.

7. California Current: It is a cold current. It flows along the coast of California and Mexico.

8. Kuril Current: It is a cold current. From Bering Strait, it flows south along the coast of Kuril Islands.

Q.5. Classify tides based on frequency of their occurrence and height.

Ans. The tides can be classified on the basis of frequency as:

1. Semi-Diurnal Tides: The most common tidal pattern featuring two high tides and two low tides each day.

2. Diurnal Tides: There is only one high tide and one low tide during each day. Successive high tides and low tides are approximately of the same height.

3. Mixed Tides: Tides having variation in heights are known as mixed tides.

On the basis of height, tides can be classified as under:

1. Spring Tides: Tides of certain periods of time are unusually lower or higher than normal height. When sun, moon and the earth are in a straight line the tidal range will be high. These are called spring tides.

2. Neap Tides: When the moon is at first and last quarter, the sun and the moon make a right angle to the earth. The attraction of the sun and the moon tends to balance each other. As a result tides with lowest amplitude occur which are termed as neap tides.

Q.6. How do currents affect the temperature? How does it affect the temperature of coastal areas in the N.W. Europe?

Ans. Impacts of currents on temperature varies depending on weather currents are warm or cold.

(i) Cold currents: Cold currents bring cold water in to warm water areas. Then currents are usually found on the west coast of continents in the low and middle latitudes (Both Hemisphere) and on the east coast the higher latitudes in the Northern Hemisphere.

(ii) Warm currents: Warm currents bring warm water into cold water areas and are usually observed on the east coast of continents in the low and middle latitudes (both hemisphere). In the northern hemisphere they are found on the west coasts of continents in high latitudes.

In North-West Europe warm currents exist. They increase the temperature in coastal areas of N.W. Europe.

Q.7. Differentiate between warm currents and cold currents.

Ans.

Warm Currents	Cold Currents
1. Warm currents bring warm water into cold water areas and are usually observed on the east costs of continents in the low and middle latitudes.	1. Cold currents bring cold water into warm water areas. These currents are usually found on the west coasts of the continents in the low and middle latitudes.
2. In the Northern Hemisphere they are found in west coasts of continents in high latitudes.	2. They are also found on the east coasts in the higher latitudes in the Northern Hemisphere.
3. They have temperature, higher than that of the surrounding water and raise the temperature of water through which they pass.	3. Their temperature is lower than that of surrounding water and bring a fall in temperature wherever they go.

Q.8. Describe the geographical effects of tides.

Ans. Advantages:

(i) Tides keep the coasts clear of all the waste and refuse of ports.

(ii) Tides keep the water in motion and so ocean water does not freeze.

(iii) Tides help some of the rivers in making nevigable such as St. Lawrence Hoogly and Hudson.

(iv) Tidal force may be used as a source of electricity such as in France and Japan.

(v) Tides bring many big ships to reach the ports located in shallow waters. At high tides ship can enter the harbour such as at Kolkata, London etc.

Disadvantages:

(i) Tides are sometimes harmful for ships and boats.

(ii) Tides check the formation of the deltas.

(iii) Marshes are formed due to accumulation of tidal water.

(iv) Tides also hinder fishing.