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

These Class 11 Geography Chapter 12 solutions cover Water (Oceans) from Fundamentals of Physical Geography (Unit V — Hydrosphere), the NCERT textbook continued for the 2026–27 session. The chapter explains the hydrological cycle, the relief features of the ocean floor (continental shelf, slope, deep-sea plains, trenches and minor features), and the horizontal and vertical distribution of temperature and salinity of ocean waters. Below you get every NCERT exercise question answered verbatim and step by step, plus key terms, extra practice, MCQs, Assertion–Reason and FAQs.

Class: 11 Subject: Geography Book: Fundamentals of Physical Geography Chapter: 12 Unit: V – Water (Oceans) Session: 2026–27
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Class 11 Geography Chapter 12 – Overview

Chapter 12, Water (Oceans), studies the hydrosphere — the water that makes Earth the ‘Blue Planet’. About 91 per cent of the planet’s water lies in the oceans, while the rest is held as freshwater in glaciers, groundwater, lakes, soil moisture, the atmosphere and streams. The chapter begins with the hydrological cycle, the continuous circulation of water among the oceans, atmosphere, land surface, subsurface and organisms in liquid, solid and gaseous forms. It then describes the relief of the ocean floor — the continental shelf, continental slope, deep-sea plains and oceanic trenches, along with minor features such as mid-oceanic ridges, seamounts, guyots, submarine canyons and atolls. Finally, it explains the factors affecting and the horizontal and vertical distribution of temperature (the three-layer warm–thermocline–cold structure) and salinity (the total content of dissolved salts, measured in parts per thousand) of ocean waters.

Key Concepts & Terms

Hydrological cycle: the continuous circulation of water within the Earth’s hydrosphere in its liquid, solid and gaseous phases, exchanging water among the oceans, atmosphere, land surface, subsurface and organisms.

Continental shelf: the shallow, gently sloping (about 1° gradient) extended margin of a continent; average width about 80 km, ending at the steep shelf break. Its thick sediments are a source of fossil fuels.

Continental slope: the steeply sloping zone (2–5°) connecting the shelf to the ocean basins; depth between 200 and 3,000 m, marking the true edge of the continents; site of canyons and trenches.

Deep-sea plain: gently sloping, flat and smooth regions of the ocean basins lying between 3,000 and 6,000 m, covered with fine clay and silt.

Oceanic deeps / trenches: the deepest, steep-sided, narrow parts of the ocean (3–5 km deeper than the surrounding floor), found at the base of slopes and along island arcs; associated with volcanoes and earthquakes.

Minor relief features: mid-oceanic ridge (two chains of mountains with a depression, e.g. Iceland), seamount (a volcanic peak below the surface), guyot (a flat-topped seamount), submarine canyon (a deep valley, e.g. Hudson Canyon) and atoll (a low coral-reef island in tropical seas).

Thermocline: the boundary zone (usually 100–400 m below the surface) where temperature falls rapidly with depth; about 90 per cent of ocean water lies below it where temperatures approach 0°C.

Three-layer temperature system: a warm top layer (about 500 m, 20–25°C), the thermocline layer (500–1,000 m thick), and a very cold bottom layer extending to the ocean floor.

Salinity: the total content of dissolved salts in seawater, measured as grams of salt per 1,000 g (1 kg) of seawater, expressed in parts per thousand (o/oo or ppt). Normal open-ocean salinity is 33–37 o/oo.

Halocline: a distinct zone in which salinity increases sharply with depth, causing denser, more saline water to sink and water to stratify by salinity.

NCERT Exercises — Full Solutions

All questions below are reproduced verbatim from the NCERT textbook’s end-of-chapter Exercises. Answers are original, written in exam-ready style.

1. Multiple choice questions.

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

ANSWER (b) Hydration. Evaporation, condensation and precipitation are processes of the hydrological cycle (Table 12.1). Hydration is a chemical weathering process in which minerals absorb water, so it is not part of the water cycle.

(ii) 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

ANSWER (b) 200–2,000 m. The depth of the continental slope region varies between 200 and 3,000 m; among the given options, 200–2,000 m is the correct range.

(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. Seamounts, atolls and guyots are minor relief features, whereas oceanic deeps (trenches) are one of the four major divisions of the ocean floor.

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

ANSWER (b) 1,000 gm. Salinity is the amount of salt (in grams) dissolved in 1,000 g (1 kg) of seawater, expressed as parts per thousand (o/oo).

(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. Of the oceans listed, the Arctic Ocean is the smallest and shallowest, while the Pacific is the largest.

2. Answer the following questions in about 30 words.

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

ANSWER The Earth is called the ‘Blue Planet’ because, unlike anywhere else in the solar system, it has an abundant supply of water on its surface — about 91 per cent of which lies in the oceans — giving it a blue appearance from space.

(ii) What is a continental margin?

ANSWER A continental margin is the submerged outer edge of a continent that separates the thin oceanic crust from the thick continental crust. It includes the continental shelf, the continental slope and the continental rise.

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

ANSWER Trenches are the deepest parts of the oceans; 57 deeps have been explored so far — 32 in the Pacific, 19 in the Atlantic and 6 in the Indian Ocean. The deepest known is the Mariana Trench (Pacific), followed by the Puerto Rico Trench (Atlantic) and the Java/Sunda Trench (Indian Ocean).

(iv) What is a thermocline?

ANSWER The thermocline is the boundary zone — usually beginning 100–400 m below the surface — in which the temperature of ocean water decreases rapidly with increasing depth, separating the warm surface water from the cold deep water.

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

ANSWER Moving downward you meet three layers: a warm top layer (about 500 m, 20–25°C), the thermocline where temperature falls rapidly, and a very cold bottom layer near 0°C. Temperature varies with depth because the Sun’s heat directly warms only the surface and is carried down slowly by convection.

(vi) What is salinity of sea water?

ANSWER Salinity is the total content of dissolved salts in seawater. It is the amount of salt (in grams) dissolved in 1,000 g of seawater, 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 is the continuous circulation of water among the oceans, atmosphere, land and living organisms in liquid, solid and gaseous phases, and its elements form one connected chain. Evaporation and transpiration change liquid water from oceans, lakes, rivers and plants into water vapour that rises into the atmosphere. Condensation turns this vapour into clouds, which return water to the surface as precipitation (rain, snow, hail). About 59 per cent of the water that falls on land returns to the atmosphere through evaporation. The remaining precipitation either flows over the surface as run-off into streams and back to the ocean, infiltrates the ground to become groundwater, or is stored as ice in glaciers and snow. Snowmelt later feeds streams again. Thus storage (oceans, ice, groundwater) and transfer processes (evaporation, condensation, precipitation, run-off, infiltration) are linked so that the renewable water on Earth stays constant while moving endlessly between its reservoirs.

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

ANSWER The distribution of temperature of ocean water is controlled by several factors: (i) Latitude: the temperature of surface water decreases from the equator towards the poles because the amount of insolation received decreases poleward. (ii) Unequal distribution of land and water: oceans in the northern hemisphere receive more heat than those in the southern hemisphere because they are in contact with a larger extent of land. (iii) Prevailing winds: winds blowing from land towards the ocean drive warm surface water away from the coast, causing cold water to upwell from below; onshore winds, on the contrary, pile up warm water near the coast and raise its temperature. (iv) Ocean currents: warm currents raise the temperature in cold areas (e.g. the Gulf Stream off North America and Europe), while cold currents lower it in warm areas (e.g. the Labrador Current). Enclosed seas in low latitudes are warmer, and those in high latitudes colder, than open seas.

Project Work

(i) Consult the atlas and show ocean floor relief on the outline of the world map.

ANSWER This is a map activity to be done in your practical notebook. On an outline world map, mark and shade the major relief features using your atlas: the broad continental shelves (e.g. the Siberian shelf in the Arctic), the mid-oceanic ridges (the Mid-Atlantic Ridge running north–south through the Atlantic, and the ridges of the Indian and Pacific Oceans), the major trenches (Mariana in the Pacific, Puerto Rico in the Atlantic, Java in the Indian Ocean) and the deep-sea plains. Use a key with different colours/symbols for shelves, ridges and trenches.

(ii) Identify the areas of mid oceanic ridges from the Indian Ocean.

ANSWER Using the atlas, identify and mark the mid-oceanic ridges of the Indian Ocean: the Central Indian Ridge running roughly north–south, which branches near the equator into the Carlsberg Ridge (extending towards the Gulf of Aden in the north-west) and the South-West Indian Ridge and South-East Indian Ridge in the south. Together they form an inverted ‘Y’ shape that divides the Indian Ocean floor.

Extra Practice Questions

Short Answer Type Questions

Q1. What proportion of the planet’s water is found in the oceans, and where is the rest held?

ANSWERAbout 91 per cent of the planet’s water is found in the oceans. The remaining 9 per cent is held as freshwater in glaciers and icecaps, groundwater, lakes, soil moisture, the atmosphere, streams and within living organisms.

Q2. What is a shelf break?

ANSWERThe shelf break is the very steep slope at which the gently sloping continental shelf ends and drops off sharply into the continental slope. It marks the outer edge of the continental shelf.

Q3. Name the four major divisions of the ocean floor.

ANSWERThe ocean floor is divided into four major divisions: (i) the Continental Shelf, (ii) the Continental Slope, (iii) the Deep Sea Plain, and (iv) the Oceanic Deeps or Trenches.

Q4. Why is salinity at depth nearly fixed while surface salinity varies?

ANSWERSurface salinity changes because water is added by rivers and rainfall or lost by evaporation and freezing. At depth, salinity is almost fixed because there is no way for water to be lost or salt to be added, so it stays nearly constant.

Q5. What is an atoll?

ANSWERAn atoll is a low island found in tropical oceans, consisting of coral reefs surrounding a central depression. The depression may be part of the sea (a lagoon) or may enclose a body of fresh, brackish or highly saline water.

Long Answer Type Questions

Q1. Describe the major relief features of the ocean floor.

ANSWERThe ocean floor has four major divisions. The continental shelf is the shallow, gently sloping (about 1°) extended margin of a continent, with an average width of about 80 km, ending at the steep shelf break; its thick sediments are a source of fossil fuels. The continental slope connects the shelf to the ocean basins, has a gradient of 2–5° and a depth of 200–3,000 m, and marks the real boundary of the continents, with canyons and trenches in this zone. The deep-sea plains are the flattest, smoothest regions of the world, lying between 3,000 and 6,000 m and covered with fine clay and silt. The oceanic deeps or trenches are the deepest, steep-sided, narrow parts of the oceans, 3–5 km deeper than the surrounding floor, found at the base of slopes and along island arcs and linked with volcanoes and earthquakes; 57 deeps have been explored.

Q2. Explain the horizontal and vertical distribution of salinity in the oceans.

ANSWERHorizontal distribution: the salinity of normal open ocean ranges between 33 and 37 o/oo. In the landlocked Red Sea it is as high as 41 o/oo, while in estuaries and the Arctic it fluctuates between 0 and 35 o/oo seasonally; in hot, dry regions evaporation can raise it to 70 o/oo. The Pacific shows variation due to its shape and large extent; the Atlantic averages about 36 o/oo with the highest salinity between 15° and 20° latitudes; the Indian Ocean averages 35 o/oo, low in the Bay of Bengal (river influx) but high in the Arabian Sea (high evaporation). Vertical distribution: salinity generally increases with depth through a sharp zone called the halocline; higher-salinity, denser water sinks below lower-salinity water, leading to stratification by salinity. Surface salinity rises with evaporation/freezing and falls with freshwater input, while salinity at depth is nearly fixed.

Q3. Discuss the three-layer thermal structure of ocean waters and the factors that affect ocean temperature.

ANSWERIn the middle and low latitudes the temperature of oceans shows a three-layer system. The first layer is the warm top layer, about 500 m thick with temperatures of 20–25°C; it is present all year in the tropics but only in summer in mid-latitudes. The second layer, the thermocline, lies below it and is 500–1,000 m thick, marked by a rapid fall in temperature with depth. The third layer is very cold and reaches the deep ocean floor. In the Arctic and Antarctic, surface water is near 0°C, so only one cold layer exists. Temperature is influenced by latitude (insolation decreases poleward), the unequal distribution of land and water, prevailing winds (causing upwelling or piling of warm water) and ocean currents (warm currents raise and cold currents lower temperature). The average surface temperature is about 27°C, falling by roughly 0.5°C per degree of latitude towards the poles.

MCQs & Assertion–Reason

1. About what percentage of the planet’s water is found in the oceans?

(a) 59%    (b) 71%    (c) 91%    (d) 97%

2. The average width of the continental shelves is about:

(a) 8 km    (b) 80 km    (c) 800 km    (d) 1,500 km

3. A flat-topped seamount is called a:

(a) atoll    (b) guyot    (c) ridge    (d) canyon

4. Iceland is an example of a feature that is part of the:

(a) Continental shelf    (b) Deep-sea plain    (c) Mid-Atlantic Ridge    (d) Oceanic trench

5. The deep-sea plains generally lie at a depth between:

(a) 200 and 600 m    (b) 1,000 and 2,000 m    (c) 3,000 and 6,000 m    (d) 6,000 and 11,000 m

6. The salinity of normal open ocean water ranges between:

(a) 0–5 o/oo    (b) 24–30 o/oo    (c) 33–37 o/oo    (d) 40–70 o/oo

7. The zone in which salinity increases sharply with depth is called the:

(a) thermocline    (b) halocline    (c) shelf break    (d) lagoon

8. The average temperature of the surface water of the oceans is about:

(a) 14°C    (b) 19°C    (c) 22°C    (d) 27°C

9. The highest salinity among water bodies is recorded in:

(a) the Dead Sea    (b) Lake Van (Turkey)    (c) the Great Salt Lake    (d) the Red Sea

10. Which one of the following processes is NOT part of the water cycle?

(a) Evaporation    (b) Condensation    (c) Precipitation    (d) Hydration

Answer key: 1-(c), 2-(b), 3-(b), 4-(c), 5-(c), 6-(c), 7-(b), 8-(d), 9-(b), 10-(d).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: The Earth is called the Blue Planet.

Reason: The Earth has an abundant supply of water on its surface, most of which lies in the oceans.

A-R 2. Assertion: The temperature of ocean water decreases with increasing depth.

Reason: The Sun’s heat directly warms only the surface and is transmitted slowly to lower layers by convection.

A-R 3. Assertion: Salinity at depth is nearly fixed.

Reason: At depth there is no way for water to be lost or for salt to be added.

A-R 4. Assertion: Oceanic deeps are minor relief features of the ocean floor.

Reason: Trenches occur at the bases of continental slopes and along island arcs.

A-R 5. Assertion: The Bay of Bengal records lower salinity than the Arabian Sea.

Reason: The Bay of Bengal receives a large influx of fresh water from rivers, while the Arabian Sea has high evaporation and low freshwater influx.

Answer key: 1-(A), 2-(A), 3-(A), 4-(D), 5-(A).

Exam Tips & Common Mistakes

How to score full marks in this chapter

Memorise the key numbers: 91% of water in oceans, continental shelf gradient ~1° and average width 80 km, slope gradient 2–5° (depth 200–3,000 m), deep-sea plains 3,000–6,000 m, normal salinity 33–37 o/oo, and surface temperature falling ~0.5°C per degree of latitude. For the relief question, draw a labelled cross-section from shelf to trench. Clearly distinguish major divisions (shelf, slope, deep-sea plain, trenches) from minor features (ridge, seamount, guyot, canyon, atoll). For temperature and salinity questions, always list the four factors and explain the layered structure (thermocline / halocline).

Common mistakes to avoid

  • Listing oceanic deeps (trenches) as a minor feature — they are a major division.
  • Confusing the thermocline (rapid fall of temperature) with the halocline (sharp rise of salinity).
  • Writing salinity per 100 g instead of per 1,000 g of seawater.
  • Mixing up continental shelf (gentle, ~1°) with continental slope (steep, 2–5°).
  • Calling hydration a part of the water cycle — it is a weathering process.
  • Forgetting that the highest ocean temperature occurs slightly north of the equator, not exactly at it.

Frequently Asked Questions

What is Chapter 12 of Class 11 Geography (Fundamentals of Physical Geography) about?

Chapter 12, Water (Oceans), deals with the hydrological cycle, the relief features of the ocean floor (continental shelf, slope, deep-sea plains, trenches and minor features), and the horizontal and vertical distribution of temperature and salinity of ocean waters.

What is the difference between the thermocline and the halocline?

The thermocline is the boundary zone where ocean temperature decreases rapidly with depth, while the halocline is the zone where salinity increases sharply with depth. Both cause the ocean water to become layered (stratified).

How many NCERT exercise questions are there in Class 11 Geography Chapter 12?

The end-of-chapter Exercises have five multiple-choice questions, six short (30-word) questions, two long (150-word) questions, and two Project Work tasks — all answered step by step on this page.

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