Water (Oceans) class 11 geography MCQ & SAQ

Water (Oceans) class 11 geography NCERT SOLUTION:

i) Identify the element, which is not a part of the hydrological cycle

(a) Evaporation

(b) Hydration

(c) Precipitation

(d) Condensation

 

Answer: (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

Answer: (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

 

Answer: (c) Oceanic Deep

 

(iv) Salinity is expressed as the amount of salt in grams dissolved in seawater per

(a) 10 gm

(b) 1,000 gm

(c) 100 gm

(d) 10,000 gm

Answer: (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

 

Answer: (b) Arctic Ocean

 

2. Answer the following questions in about 30 words

(i) Why do we call the earth a Blue Planet?

 

Answer: It is estimated that 71 per cent of the total surface area of the earth is covered by water. Therefore, the earth has an abundant supply of water on its surface. Hence, the earth is called the ‘Blue Planet’.

 

(ii) What is a continental margin?

Answer: The continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust and is occupied by relatively shallow seas and gulfs.

 

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

Answer: 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.

The Mariana Trench with a maximum-known depth of 10,994 m is the deepest part of the Pacific Ocean as well as the world's oceans.

The Puerto Rico Trench has a maximum depth of 8,648 meters and is the deepest point in the Atlantic Ocean.

The maximum depth of the Sunda or the Java Trench in the Indian Ocean is 7,725 meters.

The deepest point in the Arctic Ocean is Litke Deep in the Eurasian Basin, at 5,450 m.

The Southern Ocean's deepest point is in the South Sandwich Trench at 7,235 meters sea level.

 

(iv) What is a thermocline?

Answer: A thermocline is the transition layer between warmer mixed water at the ocean's surface and cooler deep water below. The boundary usually begins about 100-400 meters below and extends several hundred meters downward.

 

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

Answer: We generally encounter three thermal layers, when we move into the ocean.

First Layer: It represents the top layer of warm oceanic water. It is about 500 m thick with temperature ranging between 20°C and 25°C.

Second Layer: This layer called thermocline layer lies below the first layer and is characterized by rapid decrease in temperature with increasing depth. It is 500 to 1,000 m thick.

Third Layer: This layer is very cold and extends up to the deep ocean floor. 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. The temperature falls very rapidly up to the depth of 200 m. Thereafter the thermal regime tracks the slow decay of radiogenic heat.

 

(vi) What is salinity of seawater?

Answer: Seawater is salty to taste because it contains a number of dissolved salts. Salinity is the term used to define the total content of dissolved salts in seawater. 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.

 

3. Answer the following questions in about 150 words

 

(i) How are various elements of the hydrological cycle interrelated?

Answer: The hydrological cycle describes the movement of water on, in, and above the earth. All the elements of water or hydrological cycle are interrelated. Water is present in the lithosphere, hydrosphere and atmosphere in the forms of solid, liquid and gas. Evaporation is the change of water from a liquid to gas. Water vapor in the atmosphere helps in the formation of clouds, which results in rain, hail, dew, snow or sleet. Some of the precipitation runs off over the surface in the form of streams and causes erosion and excess water causes floods. Some water soaks into the ground and is used by plants. Some amount of water reaches the deeper zones and percolates to maintain the streams during the dry season. The rivers return the water back to the oceans where it originated. This never-ending circulation of water from ocean to the atmosphere through the processes of Evaporation, Condensation, and Precipitation is called the hydrological cycle. Thus, the hydrological or water cycle interconnects the lithosphere, the hydrosphere, and the atmosphere.

(ii) Examine the factors that influence the temperature distribution of the oceans.

Answer: The factors, which affect the distribution of temperature of ocean water, are:

 Latitude: Ocean water is hottest at the equator and coldest at the poles because the amount of insolation decreases polewards. The average annual temperature at the equator is 26°C and 1°C at the 60°latitudes.

 Unequal distribution of land and water: The oceans in the northern hemisphere receive more heat than the oceans of the southern hemisphere due to their contact with larger extent of land.

 Prevailing Winds: Prevailing winds carry the surface water of the ocean with them. In order to compensate for the loss of water at the surface, cold water from below the surface comes up and reduces the surface temperature.

 Ocean Currents: Warm ocean currents raise the temperature of the surrounding water, whereas cold currents lead to fall of temperature. For example, the Gulf Stream raises the temperature of eastern coast of North America and western coast of Europe.

Water (Oceans) class 11 geography MCQ 

1. Salinity refers to the total amount of

(a) sediments in the water.

(b) impurities present in the water.

(c) salt present in the water.

(d) heat present in the water.

► (c) salt present in the water.

 

2. Which one of the following is not a minor relief feature in the oceans:

(a) Seamount

(b) Atoll

(c) Oceanic Deep

(d) Guyot

► (c) Oceanic Deep

 

3. A major portion of sea level is found at:

(a) 10-12 kms below sea  level

(b) 6-9 kms below sea level

(c) 3-6 kms below sea level

(d) 1-2 kms below sea level

► (c) 3-6 kms below sea level

 

4. The Coral reefs are a major characteristic of:

(a) Indian Ocean

(b) Pacific Ocean

(c) Atlantic Ocean

(d) Mediterranean Sea

► (b) Pacific Ocean

 

5. A mid-ocean ridge is an underwater mountain system formed by

(a) plate tectonics.

(b) earthquake.

(c) ocean currents.

(d) submarine landslides.

► (a) plate tectonics.

 

6. Along the coasts, salinity is:

(a) Lowered

(b) Increased

(c) Stable

(d) Unaffected

► (a) Lowered

7. Which one of the following is the smallest ocean:

(a) Indian Ocean

(b) Arctic Ocean

(c) Atlantic Ocean

(d) Pacific Ocean

► (b) Arctic Ocean

 

8. The factor affecting the distribution of temperature of ocean water is

(a) evaporation.

(b) precipitation.

(c) hydration.

(d) ocean currents.

► (d) ocean currents.

 

9. The salinity increases with depth and there is a distinct zone called

(a) isohaline.

(b) halocline.

(c) salocline.

(d) thermocline.

► (b) halocline.

 

10. ________ is the extended margin of each continent occupied by relatively shallow seas and gulfs.

(a) Oceanic Shelf

(b) Continental shelf 

(c) Salocline.

(d) Hydra Shelf

► (b) Continental shelf 

 

11. The continental slope connects the continental shelf and _______

(a) the continental shelf

(b) the halocline

(c) the ocean basins

(d) none of the above

► (c) the ocean basins

 

12. ______ a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean

(a) Halcoline

(b) Seamount

(c) Oceanic Shelf

(d) None of the above

► (b) Seamount

 

13. The shelf typically ends at a very steep slope, called ________.

(a) Halcoline

(b) Seamount

(c) Oceanic Shelf

(d) Shelf Break

► (d) Shelf Break

 

14. The geographers have divided the oceanic part of the earth into ______ oceans.

(a) Two

(b) Three

(c) Four

(d) Five

► (d) Five

Question : Which one of the following is not a minor relief feature in the oceans:

(a) Seamount

(b) Atoll

(c) Oceanic Deep

(d) Guyot

 

Answer : C

 

Question : How long is the mid-ocean ridge?

(a) 14000 km.

(b) 1400 km.

(c) 140000 km.

(d) 1.4 km.

 

Answer : A

 

Question : Which of the following is a cold ocean Current ?

(a) Brazil current

(b) Gulf stream

(c) Kuroshio current 

(d) Humboldt current 

 

Answer : D

 

Question : What is the average salinity of the ocean water?

(a) 35 per thousand

(b) 350 per thousand

(c) 3.5 per thousand

(d) 3 per thousand

 

Answer : A

 

Question : Which one of the following is the smallest ocean:

(a) Indian Ocean

(b) Arctic Ocean

(c) Atlantic Ocean

(d) Pacific Ocean

 

Answer : B

 

Question : The geographers have divided the oceanic part of the earth into ______ oceans.

(a) Two

(b) Three

(c) Four

(d) Five

 

Answer : D

 

Question : The tides are primarily caused by-

(a) gravitational forces of the moon

(b) gravitational force of the sun

(c) gravitational forces of the earth

(d) none of these.

 

Answer : A

 

Question : Which is the deepest part of all the oceans?

(a) Mariana Trench

(b) Bonin Trench

(c) Sunda Trench

(d) Tonga Trench

 

Answer : A

 

Question : Which is the important property of ocean water?

(a) temperature

(b) salinity

(c) both of these

(d) none of these

 

Answer : C

 

Question : The continental slope connects the continental shelf and _______

(a) the continental shelf

(b) the halocline

(c) the ocean basins

(d) none of the above

 

Answer : C

 

Question : The Coral reefs are a major characteristic of:

(a) the Indian Ocean

(b) the Pacific Ocean

(c) the Atlantic Ocean

(d) the Mediterranean Sea

 

Answer : B

 

Question : The water which flows on top of ocean is

(a) more dense

(b) less dense

(c) coldest

(d) hottest

 

Answer : B 

 

Question : What is the special feature of the Atlantic Ocean?

(a) mid Atlantic ridge

(b) sea mount

(c) guyot

(d) bank

 

Answer : A

 

Question : How much part of the earth is covered by water?

(a) 76%

(b) 71%

(c) 7.1%

(d) 81%

 

Answer : B

 

Question : Along the coasts, salinity is:

(a) Lowered

(b) Increased

(c) Stable

(d) Unaffected

 

Answer : A

 

Question : Which is a result of condensation of water vapors?

(a) Rain

(b) Snowfall

(c) Hail

(d) Tiny water droplets

 

Answer : D

Question : Coriolis effect cause surface currents to turn anticlockwise in

(a) Southern Hemisphere

(b) Northern Hemisphere

(c) Eastern Hemisphere

(d) Western Hemisphere

 

Answer : A

Question : Which day of the year is celebrated as Water Day?

(a) 26 January

(b) 15 January

(c) 15 August

(d) 22 March

Answer : D

Question : The following are the types of tides ___

(a) Spring tide

(b) Autumn tide

(c) Neap tide

(d) Both a and c

Answer : D

Question : The biggest island of the Indian Ocean is

(a) Maldives

(b) Madagascar

(c) Lakshadweep

(d) Sumatra

Answer : B

Question : The process by which plants lose water is

(a) photosynthesis

(b) translocation

(c) transpiration

(d) transportation

Answer : C

Question : Which one is not a depositional landform developed by the running water?

(a) River terraces

(b) Alluvial

(c) Natural levees

(d) Delta

Answer : A

 

 

Question : The Labrador and Gulf stream are the examples of:

(a) Tides

(b) Ocean currents

(c) Tsunami

(d) Waves

Answer : B

Question : The largest Tsunami wave travels at the speed of

(a) more than 700 km per hour

(b) more than 500 km per hour

(c) more than 250 km per hour

(d) more than 100 km per hour

Answer : A

Question : Water is used for

(a) bathing

(b) cooking

(c) drinking

(d) all of these

Answer : D

Question : The place where underground water comes out on the surface of earth on its own is called

(a) rainwater

(b) spring

(c) sea level

(d) glacier

Answer : B

Question : Which one of the following is a warm ocean current ?

(a) East Australian current

(b) West Australian current

(c) Benguela current

(d) Peru current

 

Answer : A

Water (Oceans) class 11 geography SAQ

Why is earth called the ‘Blue Planet’?

The earth is called a Blue planet because of the abundance of water found on the surface of the earth. Water covers 71 % of the earth's surface.

If we take into account only the water surface of the earth, then 43% lies in the northern hemisphere and 57% in the southern hemisphere.

 

What are oceanic deeps?

The oceanic depths are deeper than the abyssal plains. They occupy 2% of the oceanic area. Although deeps are found in the Pacific ocean. Mariana deep is the deepest deep in the Pacific Ocean.

 

Name the most common features found on the ocean floor.

The most common features found on the ocean floor are:

1. Continental shelf

2. Continental slope

3. Submarine canyon

4. Abyssal plains

5. Submarine ridge

6. Ocean deeps

7. Troughs

 

Define the continental shelf.

From the coasts towards the open sea, there often exists a stretch of the shallow sea. The bed of this shallow sea bordering the continental platform is known as the continental shelf. It is a continuation of the continental block submerged below the sea. It slopes gently towards the sea, the angle of slope usually being less than one degree. The depth of the shallow seas rushing over a continental shelf varies from low tide to about 100 fathoms. Along the eastern coast of India, there exists a fairly wide strip of the continental shelf.

 

What are oceanic deeps?

Troughs like depressions or trenches in the seafloor are called oceanic depths. Deeps occur mostly at the convergence of plates in subduction zones.

 

What are seamounts?

Seamounts are topographical features rising from the ocean floor. A seamount is an isolated peak, usually a volcano, with a pointed summit, usually lies below the ocean 

What is an abyssal plain?

The Abyssal plain is a deep-sea plain. It is a very large and relatively level area of the ocean floor, covered with a thin layer of sediment. Thus the abyssal plain is an area of the deep ocean floor having a flat bottom with a very faint slope. Characteristically situated at the foot of the continental rise, the abyssal plain is present in all ocean basins. It is formed by long-continued deposition of very fine sediments and, therefore, has a nearly perfect flatness.

 

Write a short note on the continental rise.

Continental rise is a submarine feature of the continental margins. Along its seaward margin, the continental shelf gives way to the continental slope. The slope is abruptly replaced by the continental rise. Continental rise is a surface of much gentler slope decreasing in steepness toward the ocean basin floor. It generally has a moderate to low relief.

 

What is runoff? How does it occur?

Runoff is the§ water flowing from a drainage area. It is that part of precipitation which run off the land surface into streams. Runoff occurs when precipitation, that does not have an opportunity to infiltrate into the soil, flows across the land surface. However, most of it enters the stream channel ultimately, which carries it to the oceans. A part of precipitation that infiltrates the soil percolates downward to the water table through springs. Broadly speaking, runoff is composed of water from both surface flow and seepage flow. It is an extremely important-segment of hydrological cycles.

 

Discuss the factors affecting the distribution of temperature of the ocean water.

The salinity of seawater varies from place to place. At the equator, the salinity is somewhat lower due to abundant rains, greater cloudiness and low rate of evaporation. In latitudes about 20° in both the hemispheres, where evaporation is more intense and precipitation is lower, salinity is greater and in the temperate latitudes, where evaporation is less and rains. are more abundant, salinity is lower.

In the Red Sea, where no river flows, the salinity is 40 to 41 per thousand. In the Black sea, in which numerous rivers discharge, the salinity is 17 to 18 per thousand. In the White sea. it is between 25 and 26 per thousand, while in the Baltic sea it is 3 to 4 per thousand on an average.

Fig. 13.4: Distribution of surface water salinity in oceans

 

Why is the seawater saline?

The seawater is saline because a number of salts are dissolved in it. The total weight of salts dissolved in the sea is 4800 million tons. Of these, sodium chloride, or common salt, accounts for 3800 million tons, sulfate for 3000 million tons, magnesium for 1600 million tons, potassium for 480 million tons and bromide for 83 million tons. The seawater may be less saline or more saline depending upon the extent of evaporation, river discharge and precipitation, but salt composition remains invariable.

 

What are the direct and indirect uses of oceans to man?

Oceans are directly or indirectly useful to man in many ways:

Direct uses of the oceans:

1. Oceans are the storehouse of fishes.

2. They are a storehouse of minerals.

3. They provide the cheapest route for transportation.

Indirect uses: Indirectly-oceans control the climate.

 

How is the depth of the seafloor measured?

The depth of the seafloor is measured with sound waves. A depth recorder is used for this purpose. The echo of the sound returning after striking the sea bottom forms the basis of it. It helps to measure the velocity and depth.

What do you understand about the hydrosphere?

The hydrosphere is the part of the earth submerged by oceans and seas. The vast oceans cover a total area of about 71% of the total earth’s surface. Earth is called a blue planet. Approximately 61% of the northern hemisphere and 81 % of the southern hemisphere is known as the water hemisphere. The distribution of land and water is antipolar on earth. The Arctic Ocean surrounds the north pole and the Antarctic continent encircles the south pole.

Q. 1. Name the major oceans of the world.

Ans. (1) The Pacific (2) The Atlantic (3) The Indian (4) The Arctic (5) Southern Ocean.

Q. 2. Name the most common features of the Oceanic land.

Ans. (1) Continental shelf (2) Submarine canyons (3) Continental slope (4) Deep sea plains. (5) Submarine ridges. (6) Ocean deeps.

Q. 3. Name some important Submarine canyons.

Ans. (1) Oceanographer Canyon. (2) Indus Canyon. (3) Bering Canyon.

Q . 4. What is Hydrosphere ?

Ans. The part of earth submerged under water.

Q. 5. Why is earth called a watery planet or ‘blue planet’ ?

Ans. Due to excess water.

Q. 6. Which hemisphere is called the water hemisphere ?

Ans. Southern Hemisphere.

Q. 7. Which instrument records the depth of oceans ?

Ans. Sonic depth record (Echo sound recording method).

Q. 8. What are the economic advantages of shelves ?

Ans. These are rich fishing grounds. These are sources of oil and gas; sand and gravel.

Q. 9. What is an abyssal plain ? What is its area ?

Ans. Deep sea plain.  About 77% area of the oceans.

Q. 10. Name an important submarine ridge.

Ans. Mid-Atlantic ridge.

Q. 11. What is an ocean trench ?

Ans. A long, narrow, steep sided depression.

Q. 12. Name two important trenches.

Ans. Mariana trench, Kuril trench.

Q. 13. How many ocean deeps are there in the oceans ?

Ans. 57 deeps.

Q. 14. Which is the largest ocean ?

Ans. Pacific ocean.

Q. 15. What is continental Margin ?

Ans. It is the margin of each continent occupied by shallow seas and gulfs.

Q. 16. What do you understand about the Hydrosphere ?

Ans. Hydrosphere: The part of the earth submerged 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 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 the ‘Water hemisphere’ due to excess water. The distribution of land and water is Antipodal on the earth. The  Arctic ocean surrounds the north pole and the Antarctica continent encircles the south pole.

Q. 17. What do you mean by Antipodal position ?

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

Q. 18. Explain the characteristics and formation of submarine canyons.

Ans. Submarine Canyons. Narrow deep ‘V’ shaped valleys cut deep into the continental shelf and slope are called submarine canyons. These occur close to the river mouths around all the coasts of the world such as river Hudson, Mississippi, Indus, Ganges and Congo. There are about 102 submarine canyons in this world.

Types of Canyons : There are three types of canyons :

1. Small gorges found on the shelf and slope such as oceanographer canyon in the New England region.

2. Canyons which  begin at the mouth of the river and extend over the shelf such as the canyons of Mississippi river.

3. Deep dissected canyons such as along the coast of California.

Origin: The submarine canyons have been formed by different aerial processes and submarine processes. (1) By a rise or fall  of sea level in the Pleistocene period. (2) Canyons are river-cut valleys which have been submerged due to down faulting. (3) Marine erosion has formed the canyons at great depth. (4) Erosion by Turbidity currents along the edge of the shelf has resulted in the formation of canyons.

Q. 19. What do you mean by continental shelf ?

Ans. Continental shelf is a shallow zone around a continent with an average depth of 150 to 200 meters. It is a shallow platform submerged under water. In fact, continental shelf is an extension of continents.

Q. 20. Which is the deepest place in the oceans in the world ?

Ans. The deepest place in the world lies near Guam islands in  Mariana Trench. Its depth is 11022 meters. If Mt. Everest was to be submerged in this trench, its peak will still remain 2 km below the sea level.

Q. 21. List the various direct and indirect uses of the ocean to man.

Ans. Direct uses : 

(i) Oceans are an infinite storehouse of fish.

(ii) Oceans are a storehouse of minerals.

(iii) Oceans 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. 22. Why is the Indian Ocean considered half ocean ?

Ans. The Atlantic Ocean and Pacific Ocean open out both northward and southward. Both are balanced along the equator. But the Indian Ocean is blocked on the North by Asia. In fact it extends only southward. Therefore it is considered  half ocean.

Water (Oceans) class 11 geography LONG QUESTION

Prepare a map of the Atlantic Ocean floor, mentioning ocean basins and mid-Atlantic ridges.

The Atlantic Ocean covers about one-sixth of the earth’s area. Resembling ‘S’ in shape, the ocean is bounded on the west by North America and South America, and On the east by Europe and Africa. To the south, it extends up to Antarctica and in the north, it is bounded by Greenland and Iceland.

The mid-Atlantic ridge, running from north to south in ‘S’ form is about 14,450 km. long and about 4000 m. high. The ridge has a broad fracture in the middle and slopes on both sides gently. The mid-oceanic ridge in the North Atlantic is known as Dolphin Ridge, and that in the South Atlantic as the Challenger Ridge. A number of islands are located on the ridge.

The mid-Atlantic ridge divides the ocean into two major basins, namely, the East and West Atlantic basins. Some other basins in the Atlantic are the North American basin, The Labrador basin, The Brazil basin. Argentina, Agulhas, Angola, and the West.

 

What type of relief features exist in the ocean basin floor?

The ocean basin is an extensive region of the basin floor, generally lying at a depth ranging between 2,500 and 6,000 m. It covers about 76.2% of the ocean’s area.

The ocean basin floor contains three types of relief features:

1. abyssal plains and hills,

2. oceanic rise and

3. seamounts.

An abyssal plain is an area of the deep ocean floor having a flat bottom with a very faint slope. Characteristically situated at the foot of the continental rise, the abyssal plain is present in all ocean basins. Abyssal plains are surfaces formed by long-continued deposition of very fine sediments and, therefore, have a nearly perfect flatness. Abyssal hills are small hills rising to heights of a few tens of meters to a few hundred meters above the sea basin floor.

The ocean rise is an area hundreds of km. in the breadth over which the surface rises several hundred meters above the surrounding abyssal plain. Within the rise, the relief may range from subdued to very rugged.

Seamounts are the isolated peaks that rise 1000 ms. or more above the seafloor. Many of the seamounts are conspicuously flat-topped and extremely steep riches, named guyot.

 

Identify various layers of the ocean based on the level of salinity.

The salinity of the water is the ratio of the total weight of dissolved

solids to the weight of water. It is variable in quantity, differentiating in value from place to place over the oceans and at various depths. The average salinity is 35 per thousand. The relationship of salinity to depth is analogous with the three-layer temperature systems.

1. First and the topmost layer is the shallow surface layer of high salinity (35.0 to 36.5 per thousand)

2. Below this layer is a zone of the rapid decrease in salinity, called a halocline. It corresponds with the thermocline.

3. Below the halocline, differences in salinity are very small and salinity lies in the range of 34.6 to 34.9 per thousand for most of the ocean body. Thus, the salinity decreases with the increasing depth.

 

Discuss the longitudinal variation in the salinity of ocean water.

The salinity of seawater varies from place to place. At the equator, the salinity is somewhat lower due to abundant rains, greater cloudiness and low rate of evaporation. In latitudes about 20° in both the hemispheres, where evaporation is more intense and precipitation is lower, salinity is greater and in the temperate latitudes, where evaporation is less and rains. are more abundant, salinity is lower.

In the Red Sea, where no river flows, the salinity is 40 to 41 per thousand. In the Black sea, in which numerous rivers discharge, the salinity is 17 to 18 per thousand. In the White sea. it is between 25 and 26 per thousand, while in the Baltic sea it is 3 to 4 per thousand on an average.

Fig. 13.4: Distribution of surface water salinity in oceans

 

Describe the hydrological cycle in brief.

The hydrological cycle is the continuous circulation of water from the earth’s surface to the atmosphere. Water from the oceans is evaporated and lifted into the atmosphere. It is eventually condensed and is returned back to the earth’s surface in the form of rain, hail, snow or sleet.

Some of the precipitation, after wetting the foliage and ground, runs off over the surface to the streams. It is the water that sometimes causes erosion and is the main contribution to floods. Of the precipitation that soaks into the ground, some are available for growing plants and for evaporation. Some reach the deeper zones and percolate through springs and seeps to maintain streams during the dry period. The streams, in turn, eventually return the water back to the oceans where it originated. It is because of this never-ending circulation that the process has come to be known as the hydrological cycle.

The hydrological cycle is mathematically expressed as:

RF = RO + ET, where RF-includes all types of precipitation, RO is runoff and ET is evapotranspiration.

Runoff occurs when precipitation, that does not have an opportunity to infiltrate into soil, flows across the land surface. However, most of it enters the stream channel ultimately, which carries it to the oceans. A part of precipitation that infiltrates the soil percolates downward to the water table through springs. Broadly speaking, runoff is composed of water from both surface flow and seepage flow. It is an extremely important segment of the hydrological cycle. Rainwater that reaches the soil surface is wholly or partly absorbed by the soil in the process of infiltration. The amount of rainfall entering the soil depends upon the s* rate of rainfall and infiltration rate of the soil.

Q. 1. What are tides? How are these formed?

Ans. Tides. The sea level is always changing. The water of the sea rises and falls twice a day. These are called tides. ‘The regular rise and fall of sea water is called tides’. When the sea water rises, it is known as flood or high tide or incoming tide. When the sea water falls, it is known as ebb or low tide or outgoing tide. In open seas, the height of the tide is about one meter, but the highest tidal range is found up to a height of 20 meters in the Bay of Fundy (North America).

Characteristics of Tides :

(1) The height of tides varies at different places.

(2) The period of high tide or low tide varies at different places.

(3) The seawater rises for 6 hours 13 minutes and falls also for the same period.

(4) Tides do not occur regularly at a place.

Causes of origin. According to Newton, tides are caused due to the gravitational pull of the moon and of the sun on the earth. Due to gravitational pull, waters of the ocean are attracted towards the moon. Water being liquid rises higher than the solid crust of the earth.

This is known as high tide. At the level of the waterfalls, it is known as low tide.

Due to the rotation of the earth, tides occur twice a day at a place.

One tide occurs in the waters of the earth facing the moon; the other tide occurs at the diametrically opposite side of the earth. This is due to the centrifugal force of the earth. The moon is very close to the earth and its gravitational pull is 5/11 times greater than that of the sun. The sun, owing to its greater distance from the earth, does not exert much influence.

Fig. 4. Relation between gravitational forces and tides

Q. 2. Describe the different types of Tides.

Ans. Types of Tides. Tides vary in their frequency, direction and movement spatially from place to place and also from time to time. Tides may be grouped into various types based on their frequency in one day or 24 hours or based on height.

(A) Tides based on Frequency

1. Semi-diurnal Tide. The most common tidal pattern, featuring two high tides and two low tides each day. Successive high or low tides are approximately the same height.

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

3. Mixed Tide. Tides having variations in height are known as mixed tides. These tides generally occur along the west coast of North America and on many islands of the Pacific Ocean.

(B) Tides based on height. The height of rising water (high tide) varies appreciably over space and time. Besides attraction of the Sun and the Moon, the narrowness of bays, and depth of water also influence the height of tides. Spring tides and neap tides come under this category.

1. Spring Tides. Tides at certain periods of time are unusually lower or higher than normal height. The position of both the sun and the moon in relation to the Earth has direct bearing on tide height. When all three are in a straight line, the tidal range will be higher. These are called spring tides and they occur twice a month, one during the full moon period and another during the new moon period.

2. Neap Tides result in less extreme tidal conditions. Normally there is a seven day interval between spring tides and neap tides. At this time the forces of the Sun and Moon tend to counteract one another. The Moon’s attraction, though more than twice as strong as the Sun’s, is diminished by the counteracting force of the Sun’s gravitational pull.

Q.3. How are waves caused? What are their characteristics?

Ans. Waves. Waves are actually energy, not the water as such, which move across the ocean surface. Water particles only travel in a small circle as a wave passes. Wind provides the energy to the waves. Wind causes waves to travel in the ocean and the energy is released on shorelines.

The motion of the surface water seldom affects the stagnant deep bottom waters of the oceans. As a wave approaches the beach, it slows down. This is due to the friction occurring between the dynamic water and the beach i.s. sea floor, And, when the depth of water is less than half the wavelength of the wave, the wave breaks.

The largest waves are found in the open oceans. Waves continue to grow larger as they move and absorb energy from the wind. When the wave height becomes one seventh the size of the wave length, the wave will fall over.