Water (Oceans), Class 11 Geography: Hydrological Cycle, Relief, Temperature and Salinity

What Water (Oceans) is about

Weightage: This chapter is part of Unit V, Water (Oceans), of Fundamentals of Physical Geography, which carries 4 of the 70 theory marks in the school annual exam together with Movements of Ocean Water. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

Hydrological cycle

NCERT section Hydrological Cycle

Water is a cyclic resource: it can be used and reused. The hydrological cycle is the circulation of water within the hydrosphere in liquid, solid and gaseous phases, and the continuous exchange of water among oceans, atmosphere, land surface, subsurface and organisms. It has worked for billions of years, and all life depends on it; next to air, water is the most important element for life.

Where water is:

• About 91 per cent in the oceans.
• The rest is freshwater: glaciers and ice caps, groundwater, lakes, soil moisture, atmosphere, streams and living things.

Where rain on land goes: nearly 59 per cent returns to the atmosphere by evaporation; the rest runs off, infiltrates or becomes glacier.

Components and processes (Table 12.1):

Component	Processes
Water storage in oceans	Evaporation, evapotranspiration, sublimation
Water in the atmosphere	Condensation, precipitation
Water storage in ice and snow	Snowmelt runoff to streams
Surface runoff	Stream flow, freshwater storage, infiltration
Groundwater storage	Groundwater discharge, springs

Water crisis: renewable water is constant but demand is rising enormously, causing crisis in space and time; river pollution aggravates it.

Worked example

Why is there a water crisis even though the hydrological cycle keeps renewing water? (2 marks)

Try it first, then show the solutionSolution

1. The amount of renewable water on earth stays constant, and it is unevenly distributed.
2. Demand is rising tremendously, and pollution of rivers reduces usable water, so many places face shortages.

Answer: Constant, uneven supply versus rising demand and pollution.

Major divisions of the ocean floor

NCERT section Relief of the Ocean Floor

Oceans fill the great depressions of the earth's outer layer and merge into one another. Geographers recognise five oceans: Pacific, Atlantic, Indian, Southern and Arctic; seas, bays and gulfs are parts of them. Most of the floor lies 3-6 km below sea level. The floor is rugged, with the world's largest mountain ranges, deepest trenches and largest plains, formed by tectonic, volcanic and depositional processes (Figure 12.2).

Division	Features
Continental shelf	Extended margin of each continent under shallow seas and gulfs; shallowest part; gradient 1° or less; ends at the steep shelf break; average width 80 km; almost absent off Chile and west Sumatra; Siberian shelf (Arctic) the largest, 1,500 km wide; depth 30-600 m; covered with sediments from rivers, glaciers and wind; long-term deposits become fossil fuels
Continental slope	Links shelf and ocean basins; gradient 2°-5°; depth 200-3,000 m; its boundary marks the end of the continents; canyons and trenches occur
Deep sea plain	Gently sloping, the flattest and smoothest regions of the world; 3,000-6,000 m deep; covered with fine clay and silt
Oceanic deeps (trenches)	Deepest parts; steep-sided, narrow basins 3-5 km deeper than surrounding floor; at the bases of continental slopes and along island arcs; linked with active volcanoes and strong earthquakes, hence important for plate movements; 57 explored: 32 Pacific, 19 Atlantic, 6 Indian

Worked example

Why are continental shelves important economically, according to the chapter? (2 marks)

Try it first, then show the solutionSolution

1. Shelves receive variable thicknesses of sediment from rivers, glaciers and wind, spread by waves and currents.
2. Massive sedimentary deposits built up over a long time become the source of fossil fuels.

Answer: Their thick sediments become sources of fossil fuels.

Minor relief features

NCERT section Relief of the Ocean Floor

Feature	Description	Example
Mid-oceanic ridge	Two chains of mountains separated by a large depression; peaks up to 2,500 m, some rise above the surface	Iceland (part of the Mid-Atlantic Ridge)
Seamount	Mountain with a pointed summit rising from the sea floor but not reaching the surface; volcanic; 3,000-4,500 m tall	Emperor seamount, extension of the Hawaiian Islands
Submarine canyon	Deep valley, some comparable to the Grand Canyon; cuts across shelves and slopes, often from the mouths of large rivers	Hudson Canyon (best known)
Guyot	Flat-topped seamount; shows gradual subsidence in stages	More than 10,000 seamounts and guyots in the Pacific alone
Atoll	Low islands of tropical oceans: coral reefs around a central depression (a lagoon, or fresh, brackish or very saline water)	Tropical oceans

Worked example

Distinguish between a seamount and a guyot. (2 marks)

Try it first, then show the solutionSolution

1. A seamount is a volcanic mountain with a pointed summit that does not reach the ocean surface.
2. A guyot is a flat-topped seamount, showing gradual subsidence through stages.

Answer: Seamount: pointed top; guyot: flat top (subsided).

Factors affecting ocean temperature

NCERT section Temperature of Ocean Waters

Ocean water is heated by solar energy, but it heats and cools more slowly than land.

Four factors:
1. Latitude: surface temperature falls from equator to poles because insolation decreases poleward.
2. Unequal distribution of land and water: northern hemisphere oceans receive more heat because they touch a larger extent of land.
3. Prevailing winds: offshore winds (land to sea) drive warm surface water away from the coast, causing upwelling of cold water, giving longitudinal variation; onshore winds pile up warm water near the coast and raise temperature.
4. Ocean currents: warm currents raise temperature in cold areas; cold currents lower it in warm areas. The Gulf Stream warms the east coast of North America and the west coast of Europe; the Labrador current cools the north-east coast of North America.

Enclosed seas: in low latitudes they are warmer than open seas; in high latitudes they are colder than open seas.

Worked example

How do offshore winds lower coastal sea temperatures? (2 marks)

Try it first, then show the solutionSolution

1. Winds blowing from land to sea push warm surface water away from the coast.
2. Cold water rises from below to replace it (upwelling), lowering the temperature near the coast.

Answer: They drive warm surface water away, causing cold upwelling.

Horizontal and vertical distribution of temperature

NCERT section Temperature of Ocean Waters

Vertical distribution:

• Maximum temperature is always at the surface, which receives the sun's heat; heat moves down by convection.
• Temperature falls very rapidly to about 200 m, then more slowly.
• Thermocline: the boundary between surface and deep water where temperature falls rapidly; begins around 100-400 m and extends several hundred metres down (Figure 12.3). About 90 per cent of ocean water lies below it, at temperatures approaching 0°C.

Three-layer system (middle and low latitudes):

Layer	Thickness	Temperature
1 Warm surface	About 500 m	20°-25°C; all year in the tropics, only in summer in mid-latitudes
2 Thermocline	500-1,000 m	Rapid decrease with depth
3 Deep cold	To the floor	Very cold

In the Arctic and Antarctic circles, surface water is near 0°C, so there is one cold layer from surface to floor.

Horizontal distribution (Figure 12.4):

• Average surface temperature about 27°C, decreasing poleward (NCERT gives a general rate of 0.5°C per degree of latitude).
• Averages: about 22°C at 20°, 14°C at 40°, 0°C near the poles.
• Northern hemisphere oceans are warmer: averages about 19°C (north) and 16°C (south), because of unequal land-water distribution.
• The highest temperature is not at the equator but slightly north of it.

Worked example

What is the thermocline and why does it matter? (2 marks)

Try it first, then show the solutionSolution

1. The thermocline is the boundary zone, beginning about 100-400 m down, where temperature falls rapidly with depth.
2. About 90 per cent of ocean water lies below it, at temperatures near 0°C, so it separates warm surface water from the cold deep ocean.

Answer: The layer of rapid temperature fall separating warm surface water from the cold deep ocean that holds 90 per cent of the water.

Salinity: meaning and factors

NCERT section Salinity of Ocean Waters

Salinity: the total content of dissolved salts in sea water, calculated as grams of salt in 1,000 g (1 kg) of sea water, expressed in parts per thousand (‰ or ppt). All natural waters contain some dissolved salts. 24.7‰ is the upper limit for brackish water.

Factors affecting salinity:
1. Evaporation and precipitation mainly control surface salinity (evaporation raises it, rain lowers it).
2. Fresh water from rivers in coastal regions, and freezing and thawing of ice in polar regions.
3. Wind moves water to other areas.
4. Ocean currents; salinity, temperature and density are interrelated, so a change in one affects the others.

Highest salinity in water bodies (box): Lake Van, Turkey (330‰), Dead Sea (238‰), Great Salt Lake (220‰).

Worked example

A 2 kg sample of sea water contains 72 g of dissolved salts. Find its salinity and say whether it is brackish. (2 marks)

Try it first, then show the solutionSolution

1. Salinity = salt per 1,000 g = 72 ÷ 2 = 36 g per kg = 36‰.
2. Brackish water is up to 24.7‰; 36‰ is well above, so it is normal ocean water (within 33-37‰).

Answer: 36‰; not brackish.

Horizontal and vertical distribution of salinity

NCERT section Horizontal Distribution of Salinity

Horizontal distribution (Figure 12.5):

• Normal open ocean: 33-37‰.
• Red Sea (landlocked): as high as 41‰.
• Estuaries and the Arctic: 0-35‰, fluctuating seasonally.
• Hot, dry regions with high evaporation: sometimes 70‰.
• Pacific: variation due to its shape and size; falls from 35‰ to 31‰ in the western parts of the northern hemisphere because of melt water from the Arctic; after 15°-20°S it falls to 33‰.
• Atlantic: average about 36‰; highest between 15° and 20° latitude; maximum 37‰ between 20°N-30°N and 20°W-60°W; decreases northward.
• North Sea: high despite high latitude, because of saline water from the North Atlantic Drift.
• Baltic Sea: low, due to large river inflow.
• Mediterranean: high, due to high evaporation.
• Black Sea: very low, due to enormous fresh-water inflow from rivers.
• Indian Ocean: average 35‰; Bay of Bengal low (river water); Arabian Sea higher (high evaporation, little fresh water).

Vertical distribution:

• Surface salinity rises with loss of water to ice or evaporation, and falls with fresh-water input (rivers).
• Salinity at depth is fairly fixed (no water lost, no salt added).
• Lower-salinity water rests on top of higher-salinity, denser water; salinity generally increases with depth.
• Halocline: zone where salinity increases sharply.
• Higher salinity raises density; saline water sinks, giving stratification by salinity.

Worked example

Why is the Bay of Bengal less saline than the Arabian Sea? (2 marks)

Try it first, then show the solutionSolution

1. The Bay of Bengal receives a large influx of river water, which dilutes it.
2. The Arabian Sea has high evaporation and little fresh-water inflow, so its salinity is higher.

Answer: River inflow dilutes the Bay; evaporation concentrates the Arabian Sea.

Water (Oceans) in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• The Hydrological Cycle and Relief of the Ocean Floor: Shelf, Slope, Deep Plain, Trenches and Minor Features
• Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline

Water (Oceans): key facts

Water distribution

Oceans about 91%; rest freshwater; about 59% of water falling on land evaporates back

Renewable water is constant.

Ocean floor divisions

1. 1.Shelf (≤ 1°, 30-600 m, avg 80 km wide)
2. 2.slope (2°-5°, 200-3,000 m)
3. 3.deep sea plain (3,000-6,000 m)
4. 4.trenches (3-5 km deeper)

Most of the floor is 3-6 km deep.

Trenches explored

$57=32Pacific+19Atlantic+6Indian$

Linked with volcanoes and earthquakes.

Ocean temperature layers

1. 1.Surface (≈ 500 m, 20°-25°C)
2. 2.thermocline (500-1,000 m)
3. 3.cold deep water

90% of water below the thermocline, near 0°C.

Surface temperature averages

$Global\approx 27°C;20°\approx 22°C;40°\approx 14°C;poles\approx 0°C;NH19°C,SH16°C$

Highest slightly north of the equator.

Salinity

$Salinity(‰)=gramsofsaltper1,000gofseawater$

Brackish water up to 24.7‰.

Salinity values

Open ocean 33-37‰; Red Sea 41‰; Indian Ocean 35‰; Atlantic 36‰ (max 37‰); hot dry areas up to 70‰

Lake Van 330‰, Dead Sea 238‰, Great Salt Lake 220‰.

Salinity change by evaporation

$Newsalinity=salt(g)÷remainingwatermass(kg)$

Losing water raises salinity; adding fresh water lowers it.

Vertical salinity

$Salinityincreaseswithdepth;halocline=sharpincrease$

Denser saline water sinks: stratification.

Water (Oceans): diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Water (Oceans): map work

The places on the official CBSE map list for this chapter. Find each one on an outline map of India and practise marking it.

How the school exam asks Water (Oceans)

The question patterns to expect, and the method that works for each.

Tricks to remember Water (Oceans)

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Ocean floor divisions

  'Shy Sea Plants Dive': Shelf, Slope, Plain, Deep.

  Use it when: Order of major divisions.

• Minor features

  'Rich Ships Can Get Anchored': Ridge, Seamount, Canyon, Guyot, Atoll.

  Use it when: Minor relief features.

• Temperature factors

  'Long Lazy Waves Curl': Latitude, Land-water, Winds, Currents.

  Use it when: Factors affecting ocean temperature.

• Trenches count

  '57 = 32 + 19 + 6' (Pacific, Atlantic, Indian).

  Use it when: Trench figures.

• Salinity extremes

  'Van 330, Dead 238, Salt Lake 220; Red Sea 41'.

  Use it when: Salinity values.

Water (Oceans): exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Water (Oceans)

The slips examiners see most often, and what to write instead.

• ✗Mistake: Listing oceanic deeps as a minor feature

  Trenches sound like small features.

  Wrong working

  Minor features: seamounts, guyots, trenches.

  Right working

  Oceanic deeps (trenches) are one of the four major divisions; minor features are ridges, seamounts, canyons, guyots, atolls.

  ✓Do this: Four majors: shelf, slope, plain, deep.

  Usually costs: 1 mark per MCQ.

• ✗Mistake: Expressing salinity in per cent

  Habit of using percentages.

  Wrong working

  Ocean salinity is 35 per cent.

  Right working

  35 parts per thousand (35‰): 35 g per 1,000 g.

  ✓Do this: Salt per thousand, not hundred.

  Usually costs: 1 mark.

• ✗Mistake: Saying salinity decreases with depth

  Students think salt settles out near the surface.

  Wrong working

  Surface water is the saltiest.

  Right working

  Salinity generally increases with depth; denser saline water sinks; the halocline marks a sharp rise.

  ✓Do this: Salty sinks.

  Usually costs: 1 mark.

• ✗Mistake: Placing the thermocline at the surface

  Confusing the layers.

  Wrong working

  The thermocline is the warm top layer.

  Right working

  The thermocline lies below the warm surface layer and is where temperature falls rapidly.

  ✓Do this: Thermo-CLINE = temperature decline.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up Baltic and North Sea salinity

  Both are northern European seas.

  Wrong working

  The Baltic Sea is saline because of the North Atlantic Drift.

  Right working

  North Sea: higher (North Atlantic Drift); Baltic: low (river inflow).

  ✓Do this: Baltic = brackish-ish; North = Drift.

  Usually costs: 1 mark.

NCERT exercise guide: Water (Oceans)

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Water (Oceans)

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   A submarine moves from the coast towards the open ocean. After a gently sloping zone with a gradient of 1° or less, the floor drops steeply at 2°-5°. The steep zone is the:
   (a) continental shelf
   (b) continental slope
   (c) deep sea plain
   (d) oceanic deep

   Hint

   The slope marks the end of the continents.

   Answer

   (b) Continental slope, which descends to about 200-3,000 m.

2. Q2 · 1 mark · MCQExam level

   A sonar survey finds a flat-topped submerged mountain in the Pacific that seems to have subsided gradually. It is a:
   (a) seamount
   (b) guyot
   (c) atoll
   (d) submarine canyon

   Hint

   A seamount has a pointed summit.

   Answer

   (b) Guyot: a flat-topped seamount.

3. Q3 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Trenches are significant in the study of plate movements.
   Reason (R): Trenches are associated with active volcanoes and strong earthquakes.
   Choose (a)-(d).

   Hint

   NCERT's 'that is why'.

   Answer

   (a).

4. Q4 · 2 marks · Short answerExam level

   An astronaut photographing the earth from space describes it as mostly blue. Explain why this description fits the earth but not the other planets of the solar system. (2 marks)

   Hint

   Water is rare in the solar system.

   Answer

   Most of the earth's surface is covered by water, so it looks blue from space; water is a rare commodity in the solar system, with none found on the sun or anywhere else in it, so only the earth is called the Blue Planet.

5. Q5 · 3 marks · Short answerHarder exam level

   Compare the salinity of the Baltic Sea and the North Sea and explain. (3 marks)

   Hint

   Rivers versus a warm current.

   Answer

   The Baltic Sea has low salinity because of a large influx of river water. The North Sea, though also at high latitude, has higher salinity because the North Atlantic Drift brings more saline water into it.

6. Q6 · 3 marks · Short answerHarder exam level

   What is the halocline, and why does salinity increase with depth? (3 marks)

   Hint

   Density.

   Answer

   The halocline is the zone where salinity increases sharply with depth. Salinity at depth is fairly fixed, while surface water is diluted by rain and rivers. Higher salinity means higher density, so saline water sinks below less saline water, stratifying the ocean by salinity.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose a new port is planned on a coast where winds blow from land to sea most of the year. What would you expect of the sea temperature there, and why? (3 marks)

   Hint

   Offshore winds and upwelling.

   Answer

   Cooler than expected for its latitude. Offshore winds drive warm surface water away from the coast, causing upwelling of cold water from below, creating longitudinal variation in temperature.

8. Q8 · 5 marks · Long answerHarder exam level

   Describe the major divisions of the ocean floor. (5 marks)

   Hint

   Four divisions with figures.

   Answer

   Continental shelf: shallow margin, ≤ 1°, average 80 km wide (Siberian shelf 1,500 km; absent off Chile), 30-600 m, sediments become fossil fuels. Continental slope: 2°-5°, 200-3,000 m, marks the end of continents, canyons. Deep sea plain: 3,000-6,000 m, flattest, clay and silt. Trenches: deepest, 3-5 km deeper than the floor, near island arcs, volcanoes and earthquakes; 57 explored (32 Pacific, 19 Atlantic, 6 Indian).

9. Q9 · 2 marks · Short answerHOTS / competency

   Why is the highest ocean surface temperature found slightly north of the equator rather than at it? (2 marks)

   Hint

   Land and water.

   Answer

   Northern hemisphere oceans touch a larger extent of land and receive more heat, so they are warmer (average 19°C against 16°C in the south); this shifts the warmest belt slightly north of the equator.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What share of the earth's water is in the oceans?Show answer

   About 91 per cent.

2. 2.What is the average width of continental shelves?Show answer

   About 80 km.

3. 3.Which is the largest continental shelf?Show answer

   The Siberian shelf in the Arctic Ocean, about 1,500 km wide.

4. 4.Give an example of a seamount.Show answer

   The Emperor seamount, an extension of the Hawaiian Islands.

5. 5.Which is the best-known submarine canyon?Show answer

   The Hudson Canyon.

6. 6.What is the average salinity of the Indian Ocean?Show answer

   35‰.

7. 7.What salinity marks the upper limit of brackish water?Show answer

   24.7‰.

8. 8.What is the halocline?Show answer

   The zone where salinity increases sharply with depth.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Blue Planet; oceans hold about 91% of water; 59% of rain on land evaporates back.
• ☐Five oceans; most floor 3-6 km deep.
• ☐Shelf ≤ 1°, avg 80 km, Siberian 1,500 km, 30-600 m; slope 2°-5°, 200-3,000 m; plain 3,000-6,000 m; trenches 3-5 km deeper; 57 deeps.
• ☐Minor: ridges (Iceland), seamounts (Emperor), canyons (Hudson), guyots, atolls.
• ☐Temperature factors: latitude, land-water, winds (upwelling), currents (Gulf Stream, Labrador).
• ☐Layers: warm 500 m (20°-25°C), thermocline 500-1,000 m, cold deep; 90% below thermocline near 0°C.
• ☐Surface average 27°C; NH 19°C, SH 16°C; warmest slightly north of the equator.
• ☐Salinity = g per 1,000 g (‰); brackish ≤ 24.7‰; open ocean 33-37‰; Red Sea 41‰.
• ☐Low: Baltic, Black Sea, Bay of Bengal; high: Mediterranean, Red Sea, Arabian Sea, North Sea (drift).
• ☐Salinity increases with depth; halocline; stratification.
• ☐Map: Black, Baltic, Caspian, Mediterranean, North, Red Seas; Bay of Fundy.

Water (Oceans): frequently asked questions