NCERT Solutions for Class 11 Geography Fundamentals of Physical Geography Chapter 12: Water (Oceans)

Chapter at a glance

Exercise Questions

Question 1 (i)

Identify the element which is not a part of the hydrological cycle
(a) Evaporation (b) Hydration (c) Precipitation (d) Condensation

Answer

Hydration is a chemical weathering process, not part of the water cycle.

2. Hydration

Question 1 (ii)

The average depth of continental slope varies between
(a) 2–20 m (b) 20–200 m (c) 200–2,000 m (d) 2,000–20,000 m

Answer

The continental slope begins at the edge of the shelf (about 200 m). Of the options, 200–2,000 m fits best (the textbook gives the slope region as 200–3,000 m).

3. 200–2,000 m

Question 1 (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

Oceanic deeps (trenches) are part of the major relief divisions of the ocean floor.

3. Oceanic Deep

Question 1 (iv)

Salinity is expressed as the amount of salt in grams dissolved in sea water per
(a) 10 gm (b) 100 gm (c) 1,000 gm (d) 10,000 gm

Answer

3. 1,000 gm (parts per thousand, ‰)

Question 1 (v)

Which one of the following is the smallest ocean:
(a) Indian Ocean (b) Arctic Ocean (c) Atlantic Ocean (d) Pacific Ocean

Answer

2. Arctic Ocean

Question 2 (i)

Why do we call the earth a Blue Planet?

Answer

About 71 % of the earth’s surface is covered with water, mostly oceans. Seen from space, the vast water bodies make the earth appear blue; hence it is called the Blue Planet. Water also makes life possible on earth.

Because about 71 % of its surface is covered by water (oceans), which makes it look blue from space.

Question 2 (ii)

What is a continental margin?

Answer

The continental margin is the zone between the shore of a continent and the deep-sea basin. It includes the continental shelf, continental slope and continental rise, and deep oceanic trenches.

The submerged edge of a continent between the shoreline and the deep ocean basin — shelf, slope and rise.

Question 2 (iii)

List out the deepest trenches of various oceans.

Answer

Pacific: Mariana; Atlantic: Puerto Rico; Indian: Java (Sunda); Southern: South Sandwich; Arctic: Molloy Deep.

Question 2 (iv)

What is a thermocline?

Answer

A thermocline is the boundary layer in the ocean, usually from 100–400 m to about 500–1,000 m below the surface, where temperature decreases rapidly with increasing depth. It separates warm surface water from cold deep water.

A layer below the warm surface water (roughly 500–1,000 m deep) where temperature falls rapidly with depth.

Question 2 (v)

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

Answer

Thermal layers

1. Top layer: warm surface water, about 500 m thick, with temperatures of 20°–25°C (in tropics); present year-round in tropics, only in summer in mid-latitudes.
2. Thermocline: temperature decreases rapidly with depth.
3. Deep layer: very cold, extending to the ocean floor, with temperatures near 0°C.

Why it varies: the ocean is heated by the sun from above; solar energy penetrates only the upper layers. Deeper water receives no sunlight, and cold, dense polar water sinks and spreads along the ocean floor. So temperature falls with depth.

Warm surface layer, thermocline (rapid fall) and cold deep layer; because the sun heats only the upper water and cold dense water sinks to the bottom.

Question 2 (vi)

What is salinity of sea water?

Answer

Salinity is the total amount of dissolved salts in sea water, expressed as grams of salt per 1,000 g of sea water (parts per thousand, ‰). Average ocean salinity is about 35 ‰. Water with salinity of 24.7 ‰ is considered the upper limit of brackish water.

The amount of dissolved salt in grams per 1,000 g of sea water (‰); average about 35 ‰.

Question 3 (i)

How are various elements of the hydrological cycle interrelated?

Answer

The hydrological cycle is the continuous circulation of water among the oceans, atmosphere and land.

1. Evaporation from oceans and water bodies, and transpiration from plants (evapotranspiration), send water vapour into the atmosphere.
2. Rising vapour cools and condenses into clouds.
3. Clouds release water as precipitation (rain, snow, hail).
4. On land, some water infiltrates to become groundwater, some flows as surface runoff in rivers, some is stored in glaciers and lakes, and some evaporates again.
5. Rivers and groundwater return water to the oceans, and the cycle repeats.

Each element depends on the previous one: the amount of evaporation controls precipitation; precipitation controls runoff and groundwater; runoff returns water to the oceans for evaporation. The total amount of water remains constant, only its form and place change. Human activities (deforestation, urbanisation) can alter the balance, causing floods or water scarcity.

Evaporation and transpiration supply vapour, which condenses and falls as precipitation; this infiltrates or runs off and returns to the oceans to evaporate again — each stage feeds the next, keeping total water constant.

Question 3 (ii)

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

Answer

1. Latitude: surface water temperature decreases from the equator towards the poles, because insolation decreases poleward.
2. Unequal distribution of land and water: the northern hemisphere’s oceans receive more heat because they are in contact with larger land masses, so they are warmer than those of the southern hemisphere.
3. Prevailing winds: offshore winds blowing from land drive warm surface water away, causing upwelling of cold water; onshore winds pile up warm water near coasts. E.g. trade winds make western parts of oceans warmer than eastern parts in the tropics.
4. Ocean currents: warm currents (e.g. Gulf Stream) raise the temperature of cold areas, while cold currents (e.g. Labrador, Peru) lower the temperature of warm areas.
5. Enclosed seas: in low latitudes they record higher temperatures than open oceans (e.g. Red Sea); in high latitudes, lower.
6. Depth: temperature decreases with depth.

Latitude, unequal distribution of land and water, prevailing winds (upwelling), ocean currents, enclosed seas and depth control ocean temperature.

Topic checklist

• Hydrological cycle
• Relief of the ocean floor: major divisions and minor features
• Temperature of ocean waters: distribution and factors
• Salinity: factors and distribution

Total questions answered: 13 of 13