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Water (Oceans) – Class 11 Geography notes, important questions and MCQs

NCERT Class 11 Geography, chapter 12. Notes, key terms, important questions with answers and MCQs.

Chapter overview

About 71% of the Earth's surface is covered by water, most of it in the oceans. Water moves continuously through the hydrological cycle. The ocean floor has relief features such as the continental shelf, continental slope, deep-sea plains and trenches. The temperature and salinity of ocean water vary with latitude, depth and other factors.

Hydrological cycle and ocean floor relief

The hydrological cycle is the circulation of water through evaporation, condensation, precipitation, run-off, infiltration and transpiration; about 97% of the Earth's water is in the oceans.
Major relief divisions of the ocean floor are the continental shelf (shallow extension of continents), continental slope, deep-sea plain (abyssal plain) and oceanic deeps or trenches.
Minor features include mid-oceanic ridges, seamounts, guyots, submarine canyons and atolls.

Temperature and salinity

Ocean temperature decreases from the equator towards the poles and with depth; layers include a warm surface layer, the thermocline (where temperature falls rapidly) and a cold deep layer.
Salinity is the amount of dissolved salt in sea water, usually expressed as parts per thousand (‰); the average is about 35‰.
Salinity is affected by evaporation, precipitation, the inflow of rivers, freezing and thawing of ice, and ocean currents; the Dead Sea and the Red Sea have very high salinity, while the Baltic Sea has low salinity.

Key terms

Hydrological cycle: the continuous movement of water between the Earth and the atmosphere
Continental shelf: the shallow, gently sloping extension of a continent under the sea
Ocean trench: a long, narrow, very deep part of the ocean floor
Thermocline: a layer in which ocean temperature falls rapidly with depth
Salinity: the amount of dissolved salts in sea water

Important questions with answers

What is the continental shelf?2 marks

The continental shelf is the shallow, gently sloping extension of a continent beneath the sea, usually less than 200 m deep. It is important for fishing, oil and gas, and marine deposits.

What factors affect the salinity of ocean water?3 marks

Salinity increases with high evaporation and decreases with heavy precipitation and the inflow of fresh river water. Freezing of sea water increases salinity, while melting of ice decreases it. Ocean currents and the mixing of water also redistribute salinity.

Describe the relief of the ocean floor.5 marks

The ocean floor has features similar to those on land. Its major divisions are the continental shelf, the continental slope, the deep-sea plain and the oceanic deeps. The continental shelf is the shallow, gently sloping extension of the continent, usually up to about 200 m deep and varying in width; it is rich in fisheries, oil and gas. The continental slope begins at the edge of the shelf, where the gradient becomes steep and depths range from about 200 to 3,000 m; submarine canyons cut into it. The deep-sea plains or abyssal plains are gently sloping areas at depths of about 3,000 to 6,000 m, covered with fine sediments such as clays and silt. The oceanic deeps or trenches are the deepest parts of the oceans, long and narrow, found mainly near the margins of the Pacific Ocean, where plates are subducting; the Mariana Trench in the Pacific is over 11,000 m deep. Minor relief features include mid-oceanic ridges, which are long chains of underwater mountains formed where plates diverge, seamounts and flat-topped guyots of volcanic origin, submarine canyons, and atolls, which are ring-shaped coral reefs enclosing a lagoon.

Quick revision

71% of the surface is water; 97% in oceans.
Hydrological cycle processes.
Shelf, slope, deep-sea plain, trenches; minor features.
Temperature layers; salinity factors.

Exam tips

Draw a cross-section of the ocean floor and label its divisions.
Remember: average salinity is about 35 parts per thousand.

Practice MCQs

About what percentage of the Earth's surface is covered by water
1. 29%
2. 50%
3. 71%
4. 97%
Show answer
Answer: (c) 71%. Most of it is in the oceans.
The shallow extension of a continent under the sea is the
1. continental slope
2. trench
3. continental shelf
4. abyssal plain
Show answer
Answer: (c) continental shelf. It is rich in fish.
The average salinity of ocean water is about
1. 3‰
2. 35‰
3. 70‰
4. 350‰
Show answer
Answer: (b) 35‰. It means 35 grams of salt per 1,000 grams of water.
The layer where ocean temperature falls rapidly with depth is the
1. thermocline
2. troposphere
3. continental shelf
4. isohaline
Show answer
Answer: (a) thermocline. It lies below the warm surface layer.
The deepest part of the oceans is found in
1. ocean trenches
2. continental shelves
3. abyssal plains
4. lagoons
Show answer
Answer: (a) ocean trenches. The Mariana Trench is the deepest.
A flat-topped seamount is called a
1. canyon
2. atoll
3. guyot
4. ridge
Show answer
Answer: (c) guyot. Its top was eroded by waves.
Why is the salinity of the Baltic Sea low
1. high evaporation
2. high temperature
3. no rivers
4. large inflow of river water and low evaporation
Show answer
Answer: (d) large inflow of river water and low evaporation. Fresh water dilutes the sea.
Which process does NOT add water vapour to the atmosphere in the hydrological cycle
1. evaporation
2. transpiration
3. infiltration
4. sublimation
Show answer
Answer: (c) infiltration. Infiltration moves water into the ground.

All chapters of Class 11 Geography

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.