Chapter 12: Water (Oceans) - Class 11 Geography Extra Notes

Water (Oceans)

Question. Differentiate between continental shelf and continental slope.

Ans: Continental shelf:

• 1. It is an extended margin of each continent occupied by shallow sea, gulfs, etc. It is a submerge part of the land adjoining the coast.
• 2. It is shallow and is about 800m deep with an average gradient 1° or even less.
• 3. Its edges extended up to the depth of 150-200 m. ‘
• 4. It contains the sediments deposited with land its average width is 80km.
• 5. The largest continental shelf is a Siberian shelf in Arctic Ocean which stretches up to 1500 km of width.
• 6. About one-fourth of the petroleum is produced from oil wells on the continental shelfs.
• 7. It is important for coastal transport and trade.

Continental slopes:

• 1. It represents the edge of continental block.
• 2. It connects continental shelf and ocean basin.
• 3. The depth varies from 200-300 m.
• 4. Its average gradient is 2-5°.
• 5. It has a steep slope linking continental shelf and deep sea §ow lying at an average depth of 3600m.
• 6. Canyons are formed here.

Question. The average temperature of water on oceans floor keeps on falling from equator to poles systematically. Explain.

Ans:

• 1. The average temperature of surface water of the- oceans is about 27°C and it gradually decreases from the equator towards the poles.
• 2. The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude.
• 3. The average temperature is around 22°C at 20° latitudes, 14° C at 40° latitudes and 0° C near poles.
  
  
  
• 4. The oceans in the northern hemisphere record relatively higher temperature than in the southern hemisphere.
• 5. The highest temperature is not recorded at the equator but slightly towards north of it.
• 6. The average annual temperatures for the northern and southern hemisphere are around 19° C and 16° C respectively. This variation is due to the unequal distribution of land and water in the northern and southern hemispheres.

Question. Explain about horizontal distribution of salinity.

Ans: Horizontal distribution of salinity:

• The salinity for normal Open Ocean ranges between 33% and 37%. In the land locked Red
• Sea records higher salinity due to high evaporation. Salinity is, however, very low in Black Sea due to enormous fresh water influx by rivers.
  
  
  
• The average salinity of the Indian Ocean is 35 %.
• The low salinity trend is observed in the Bay of Bengal due to influx of river water.
• On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.

Question. Explain about vertical distribution of salinity.

Answer: Vertical distribution of salinity:

• 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 decreased by the input of fresh waters, 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.’
  
  
  
• There is a marked difference in the salinity between the surface zones and the deep zones of the oceans.
• The lower salinity water rests above the higher salinity dense water.
• Salinity, generally, increases with depth and there is a distinct zone called the halocline, where salinity increases sharply.
• Other factors being constant, increasing salinity of seawater causes its density to increase.
• High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.

Question. Explain about vertical distribution of temperature.

Answer:

• The temperature structure of oceans over middle and low latitudes can be described as three-layer system from surface to the bottom.
• The first layer represents the top layer of warm oceanic water and it is about 500 m thick with temperatures ranging between 20° and 25° C. This layer, within the tropical region, is present throughout the year but in mid-latitudes it develops only during summer.
  
  
  
• The second layer called the thermocline layer lies below the first layer and is characterised by rapid decrease in temperature with increasing depth. The thermocline is 500 – 1,000 m thick.
• The third layer is very cold and extends up to the deep ocean floor. In the Arctic and Antarctic circles, the surface water temperatures are close to 0 C and so the temperature change with the depth is very slight.

Question. “Ocean seems to be water body but it has many types of landforms within it”. Justify the statements by giving some examples.

Ans: The statement is absolutely right. Like the surface of the earth ocean flow is neither level or flat, it is undulating and varying. It comprises of different types of landforms below it. Some of these are:

• Continental shelf: It is an extended margin of each continent occupied by shallow sea, gulfs, etc. It is a submerge part of the land adjoining the coast. Continental slopes: It represents the edge of continental block. It connects continental shelf and ocean basin. The depth varies from 200-300 m.
• Abyssal Plains (The Deep Sea Plains): These are the world’s §attest and smoothest regions. About 40% of the oceanic floor is flat. It is formed accumulation of sediments on the seafloor. Its depth varies from 3000-6000m.
  
  
  
• Sub Marine Ridges: Oceanic floors have sub-marine narrow and elongated ridges. They resemble mountain ridges on the earth surface. Peak of these ridges may rise above the sea level to form islands. Eg- Philippines Ice land is a mid-Atlantic ridge.
• Oceanic deeps of sub-marine trenches: Deep narrow steep sided depression found along the abyssal plain. The depth of these trenches may vary from 6,000 to 11,000 m. Example, Marina trenches is the deepest trench in Pacific Ocean.

Question. Explain hydrological cycle.

Ans:

• The hydrological cycle is a conceptual model that describes the storage and movement of water between the biosphere, atmosphere, lithosphere, and the hydrosphere.
• Water on our planet can be stored in any one of the following major reservoirs: atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields, and groundwater.
• Water moves from one reservoir to another by way of processes like evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, melting, and ground water flow.
  
  
  
• The oceans supply most of the evaporated water found in the atmosphere. Of this evaporated water, only 91% of it is returned to the ocean basins by way of precipitation. The remaining 9% is transported to areas over landmasses where climatologically factors induce the formation of precipitation.
• The resulting imbalance between rates of evaporation and precipitation over land and ocean is corrected by runoff and groundwater flow to the oceans

Question. Read the passage and answer the following questions:

Continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less. The shelf typically ends at a very steep slope, called the shelf break. 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. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width. The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m. The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers, wind, from the land and distributed by waves and currents. Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.

Q1) Continental Shelf ending at a steep slope is termed as

A) Shelf break B) Shelf gap C) Shelf Trench D) Shelf ridge

Ans- (A) shelf break

Q2) Continental shelves are present or have a very wider area along the coasts of

A) Sumatra B) Siberia C) Chile D) none of the above

Ans- B) Siberia

Q3) Continental shelf is the

A) Shallowest part of the ocean B) Deepest part of the ocean

C) Equal to depth of a Trench D) All of the above

Ans- A) Shallowest part of the ocean

GIST OF THE LESSON

HYDROLOGICAL CYCLE

Definition: Circulation of water within the earth' hydrosphere in different forms i.e. the liquid, solid, and gaseous phases.

Components: Oceans (71% of planetary water), freshwater in glaciers, icecaps, groundwater, 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.

Processes: Evaporation, condensation, precipitation, runoff, infiltration, and glacier formation. It is to be noted that the renewable water on the earth is constant while the demand is increasing tremendously. This leads to water crisis in different parts of the world — spatially and temporally.

RELIEF OF THE OCEAN FLOOR

Divisions of the Ocean Floors

The ocean floors can be divided into four major divisions:

• 1) The Continental Shelf;
• 2) The Continental Slope;
• 3) The Deep Sea Plain;
• 4) The Oceanic Deeps.

Besides, these divisions there are also major and minor relief features in the ocean floors like ridges, hills, sea mounts, guyots, trenches, canyons, etc.

Continental Shelf

• 1. The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs.
• 2. It is the shallowest part of the ocean showing an average gradient of 1° or even less.
• 3. The shelf typically ends at a very steep slope, called the shelf break.
• 4. The width of the continental shelves vary from one ocean to another.
• 5. The average width of continental shelves is about 80 km.
• 6. The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc.
• 7. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width.
• 8. The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.
• 9. The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers, wind, from the land and distributed by waves and currents.
• 10. Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.

Continental Slope

• 1. The continental slope connects the continental shelf and the ocean basins.
• 2. It begins where the bottom of the continental shelf sharply drops off into a steep slope.
• 3. The gradient of the slope region varies between 2-5°.
• 4. The depth of the slope region varies between 200 and 3,000 m.

Deep Sea Plain

• 1. Deep sea plains are gently sloping areas of the ocean basins.
• 2. These are the flattest and smoothest regions of the world.
• 3. The depths vary between 3,000 and 6,000 m.
• 4. These plains are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

• 1. These areas are the deepest parts of the oceans.
• 2. The trenches are relatively steep sided, narrow basins.
• 3. They are some 3-5 km deeper than the surrounding ocean floor.
• 4. They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes.
• 5. That is why they are very significant in the study of plate movements.
• 6. 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.

Mid-Oceanic Ridges

• 1. A mid-oceanic ridge is composed of two chains of mountains separated by a large depression.
• 2. The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean‘s surface.
• 3. Iceland, a part of the mid Atlantic Ridge, is an example.

Seamount

• It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000-4,500 m tall. The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.

Submarine Canyons

• These are deep valleys, some comparable to the Grand Canyon of the Colorado River. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. The Hudson Canyon is the best known submarine canyon in the world.

Guyots

• It is a flat topped seamount. They show evidences of gradual subsidence through stages to become flat topped submerged mountains. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

Atoll

• These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression. It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.

TEMPERATURE OF OCEAN WATERS

• Ocean waters get heated up by the solar energy just as land.
• The process of heating and cooling of the oceanic water is slower than land.

Factors Affecting Temperature Distribution ON THE OCEANS

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

• (i) Latitude: the temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases pole ward.
• (ii) Unequal distribution of land and water: the oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
• (iii) Prevailing wind: the winds blowing from the land towards the oceans drive warm surface water away from the coast resulting in the upwelling of cold water from below. It results into the longitudinal variation in the temperature. Contrary to this, the onshore winds pile up warm water near the coast and this raises the temperature.
• (iv) Ocean currents: warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America.
• (v) Horizontal and Vertical Distribution of Temperature: 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 meters downward .This boundary region, from where there is a rapid decrease of temperature, is called the thermocline. About 90 per cent of the total volume of water is found below the thermocline in the deep ocean. In this zone, temperatures approach 0° C. The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom. The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and 25° C. This layer, within the tropical region, is present throughout the year but in mid -latitudes it develops only during summer. The second layer called the thermocline layer lies below the first layer and is characterized by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick. The third layer is very cold and extends up to the deep ocean floor. In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature change with the depth is very slight. Here, only one layer of cold water exists, which extends from surface to deep ocean floor.

SALINITY OF OCEAN WATERS

• Salinity is the term used to define the total content of dissolved salts in sea water It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater. It is usually expressed as parts per thousand (o/) or ppt. Salinity is an important property of sea water. Salinity of 24.7°/oo has been considered as the upper limit to demarcate  ̳brackish water‘.

Factors affecting ocean salinity are mentioned below:

• (i) The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation.
• (ii) Surface salinity is greatly influenced in coastal regions by the fresh water flow from rivers, and in Polar Regions by the processes of freezing and thawing of ice.
• (iii) Wind, also influences salinity of an area by transferring water to other areas.
• (iv) The ocean currents contribute to the salinity variations. Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of water in an area.

MULTIPLE CHOICE QUESTIONS

1. Which of the following is the deepest part of the world’s oceans?

• (A) Mariana Trench (B) Tonga Trench
• (C) Java Trench (D) Philippine Trench
• Ans- (A) Mariana trench

2. The salinity of ocean water is generally measured in which unit?

• (A) Grams per liter (B) Parts per million
• (C) Parts per thousand (D) Kilograms per cubic meter
• Ans – (C) parts per thousand

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

• (A) 10-12 km below sea level
• (B) 6-9 km below sea level
• (C) 3-6 km below sea level
• (D) 1-2 km below sea level
• Ans – (C) 3-6 km below sea level

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

• (A) Two (B) Three (C) Four (D) Five
• Ans – (D) five

5. What is the average salinity of seawater?

• (A) 25‰ (B) 30‰ (C) 35‰ (D) 40‰
• Ans – (C) 35‰

6. Which of the following statements is true about the continental shelf?

• (A) It is the steep slope at the edge of the continent.
• (B) It is the gently sloping area next to the continent.
• (C) It is the deepest part of the ocean.
• (D) It is a mountain range under the ocean.
• Ans – (B) It is the gently sloping area next to the continent.

7. 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

8. Identify the element which is not a part of the hydrological cycle:

• (A) Evaporation (B) Hydration
• (C) Precipitation (D) Condensation
• Ans – (B) Hydration

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

• (A) Plate tectonics. (B) Earthquake.
• (C) Ocean currents. (D) Submarine landslides.
• Ans – (A) Plate tectonics.

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

• (A) Evaporation. (B) Precipitation. (C) Hydration. (D) Ocean currents.
• Ans – (D) Ocean currents

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

• (A) isohaline. (B) halocline. (C) salocline. (D) thermocline.
• Ans –(B) halocline.

12. Which one of the following is not minor relief feature in the oceans?

• (A) sea mount (B) oceanic deep (C) atoll (D) guyot
• Ans –(B) oceanic deep

13. Salinity is expressed as the amount of salt in gram dissolved in sea water per

• (A) 10 gm (B) 100 gm (C) 1000gm (D) 10000 gm
• Ans –(C) 1000gm

14. Which one of the following is the smallest ocean?

• (A) Indian ocean (B) Atlantic ocean (C) Arctic ocean (D) Pacific ocean
• Ans –(C) Arctic ocean

15. Which of the following reservoir contains the largest amount of water

• (A) Ice cap and glacier (B) Streams and rivers
• (C) Lakes (D) Atmosphere
• Ans – (A) Ice cap and glacier

16. Which of the following ocean as the largest number of ocean deeps

• (A) Pacific (B) Indian
• (C) Atlantic (D) Arctic
• Ans – (A) Pacific

17. Which one of the following is not the major division of ocean floor-

• (A) Continental shelf (B) Continental slope
• (C) Mid oceanic ridge (D) Oceanic deeps
• Ans – (C) Mid oceanic ridge

18. The average depth of the deep sea plain varies between

• (A) 300-1000 m (B) 1000-3000m
• (C) 3000-6000 m (D) 6000- 8000 m
• Ans –(C) 3000-6000 m

19. Which of the following shelf is the largest in the world-

• (A) The Siberian shelf in the Atlantic Ocean
• (B) The European shelf in the Mediterranean Sea
• (C) The North American shelf in the Pacific Ocean
• (D) The African shelf in the Indian Ocean
• Ans –(A) The Siberian shelf in the Atlantic Ocean

20. The boundary from where there is a rapid decrease of temperature with the increasing

depth in the ocean , called-

• (A) Pycnocline (B) thermocline
• (C) halocline (D) isocline
• Ans –(B) thermocline

21. About ________ percent of the total volume of water is found below the thermocline in the deep ocean.

• (A) 25 (B) 60 (C) 75 (D) 90
• Ans –(D) 90

22. The thickness of the thermocline is –

• (A) 500- 1000 m (B) 1000-2000m (C) 2000-3000m (D) 4000-5000m
• Ans –(A) 500- 1000 m

23. Which of the following water bodies has the highest salinity in the world-

• (A) Van lake (B) Dead sea (C) Great salt lake (D) Assal lake
• Ans –(A) Van lake

24. The salinity for normal open ocean ranges between-

• (A) 15 ‰ – 22 ‰ (B) 22 ‰ – 32 ‰
• (C) 33 ‰ - 37 ‰ (D) 38 ‰ – 45 ‰
• Ans –(C) 33 ‰ - 37 ‰

25. The average salinity of the Indian ocean is

• (A) 31 ‰ (B) 35 ‰
• (C) 38 ‰ (D) 41 ‰
• Ans –(B) 35 ‰