NCERT
Every NCERT book that matters for UPSC — open a book, then a chapter. Learn each chapter's concepts, then how UPSC tests them — with diagrams, flowcharts and the past questions to attempt.
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← Fundamentals of Physical Geography — all chaptersWater (Oceans)
Start here: why this chapter matters
In simple wordsSeen from space, the earth is mostly blue — oceans cover most of it. This chapter looks at that water in three ways: how it circulates (the water cycle), what the land under the sea looks like, and how warm and how salty the sea is and why.
Water is essential to all life, and the earth has it in abundance on its surface — which is why it is called the Blue Planet. The oceans store most of that water and most of the heat in the climate system; they shape weather (Chapters 8–11), feed ocean currents and tides (Chapter 13), and hold fisheries, minerals and oil.
- Builds on Chapters 3–4: trenches, ridges and seamounts are made by plate movements and volcanoes.
- Builds on Chapters 8 and 10: the sea heats slowly, evaporates and feeds rain.
- Leads into Chapter 13: waves, tides and currents move this water and its heat.
- UPSC: GS1 (ocean floor, temperature and salinity, water resources) and GS3 (water crisis, ocean resources).
| Word | What it means here |
|---|---|
| Hydrological cycle | The circulation of water in liquid, solid and gas forms between oceans, air, land, underground and living things |
| Continental shelf | The shallow, gently sloping, submerged edge of a continent |
| Trench (oceanic deep) | A long, narrow, steep-sided, very deep basin on the ocean floor |
| Seamount / guyot | An undersea volcanic mountain with a pointed top / a flat top |
| Thermocline | The layer in which water temperature falls rapidly with depth |
| Salinity | Grams of dissolved salt in 1,000 g (1 kg) of seawater, written ‰ (parts per thousand) |
| Halocline | The layer in which salinity rises sharply with depth |
At a glance — the six ideas to carry away
- The hydrological cycle moves water between oceans, air, land, underground and living things.
- Ocean floor: shelf → slope → deep-sea plain → trenches, plus ridges, seamounts, guyots, canyons, atolls.
- Shelf: ≤1°, ~80 km wide (Siberian shelf 1,500 km); slope 2–5°, 200–3,000 m; plain 3,000–6,000 m.
- Surface water averages ~27°C, falling towards the poles; NH oceans warmer than SH.
- Below the warm surface layer the thermocline cools rapidly; ~90% of ocean water lies below it.
- Salinity: open ocean 33–37‰; Red Sea 41‰; brackish below 24.7‰; halocline at depth.
1. The hydrological cycle
In simple wordsThe water in your glass today may once have been in a cloud over the Bay of Bengal, a Himalayan glacier, or a dinosaur's drinking pool. Water is never used up; it just keeps moving and changing form — vapour, liquid, ice — in a giant loop powered by the sun.
Water is a cyclic resource: it can be used and reused. The hydrological cycle describes the movement of water on, in and above the earth, between the oceans, atmosphere, land surface, subsurface and organisms, in liquid, solid and gaseous forms. It has run for billions of years and all life depends on it.
Reading NCERT Figure 12.1 — Hydrological cycle (page 100)
- A block diagram from snow-capped mountains to the ocean, with the sun over the sea.
- Arrows labelled: evaporation from the ocean → condensation into clouds → transport by wind (advection) inland → precipitation on the mountains; evapotranspiration from the land and vegetation back up to the clouds.
- On land: runoff in rivers to the ocean, penetration into soil (infiltration) and underground flow towards the sea.
- ‘Transport by wind (advection)’ is a detail only in the figure — it links this chapter to advection in Chapter 8.
| Component (where water is stored) | Processes (how it moves) |
|---|---|
| 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 |
Reading NCERT Table 12.1 — Components and processes of the water cycle (page 101)The table pairs each store with the processes that move water out of it. Learn the pairs; sublimation (ice turning straight into vapour) appears only here, not in the text. Note: hydration is not a water-cycle process (NCERT exercise Q1-i) — it is a chemical-weathering process (Chapter 5).
NCERT states that about 91% of the planet's water is in the oceans; the rest is freshwater in glaciers and ice caps, groundwater, lakes, soil moisture, the atmosphere, streams and living things. Of the water that falls on land, nearly 59% returns to the atmosphere by evaporation; the rest runs off, infiltrates the ground, or becomes glacier ice. Renewable water is constant while demand keeps rising — hence water crises in space and time, worsened by river pollution.
NCERT figures to handle with care
- 91% in the oceans: most standard sources (e.g. the US Geological Survey) put it at about 97%, with freshwater only about 2.5–3% — UPSC 1987 accepted 2.7% for freshwater.
- The ‘59%’ sentence is garbled in the book (it mixes evaporation over land and over the oceans). Remember only the idea: most rain falling on land evaporates back.
- ‘There is no water … anywhere else in the solar system’ is out of date: water ice exists on Mars's poles, on comets and on icy moons such as Europa (Beyond NCERT). Earth is special for its abundant liquid surface water.
Beyond NCERT (asked by UPSC)Of the freshwater, roughly two-thirds is locked in ice caps and glaciers and nearly one-third is groundwater; lakes, rivers, soil and air hold only about 1%. So after ice, the largest store of freshwater is groundwater (UPSC 2013). UPSC 2010 accepted ‘freshwater available for use is less than 1% of all water’ but rejected ‘95% of freshwater is in ice caps’.
⤵ One trip round the water cycle
Sun heats the oceanevaporation (and sublimation from ice)
Vapour rises and coolscondensation → clouds
Winds carry clouds inlandadvection
Precipitationrain and snow
Runs off in streamsback to the sea
Infiltrates the groundgroundwater → springs and the sea
Plants return waterevapotranspiration
2. Relief of the ocean floor: the major divisions
In simple wordsDrain the oceans and you would see a landscape as dramatic as any on land: a shallow ledge around each continent, a steep slope down to a vast flat plain, cut by deep trenches and crossed by the longest mountain ranges on earth.
The oceans fill the great depressions of the earth's outer layer and merge into one another. Geographers recognise five oceans — the Pacific, Atlantic, Indian, Southern and Arctic. Most of the ocean floor lies 3–6 km below sea level. Like the land, it has mountains, the deepest trenches and the largest plains, made by tectonic, volcanic and depositional processes.
Reading NCERT Figure 12.2 — Relief features of ocean floors (page 102)
- Three block diagrams, each about 200 km across:
- Continental margin (depth 0–2 km): continental shelf, shelf break, continental slope cut by a submarine canyon, and a continental rise at its foot.
- Deep ocean basin (0–10 km): abyssal plain, abyssal hills, a deep-sea trench, a guyot and seamounts.
- Mid-ocean ridge (0–4 km): a central rift with fractures cutting across the ridge.
- The continental rise, abyssal hills and fracture zones are labelled in the figure but not explained in the text: the rise is the gentle apron of sediment at the foot of the slope; abyssal hills are low hills on the deep plain; fracture zones are breaks that offset the ridge (Chapter 4's transform faults).
- Vertical scales are greatly exaggerated.
| Division | Slope / width | Depth | Key facts (NCERT) |
|---|---|---|---|
| Continental shelf | Average gradient 1° or less; average width ~80 km | From 30 m in places to 600 m | Shallowest part; ends at the shelf break. Almost absent off Chile and west Sumatra; the Siberian shelf (Arctic) is the widest at 1,500 km. Covered with sediments from rivers, glaciers, wind; long-buried deposits become fossil fuels |
| Continental slope | 2°–5° | 200–3,000 m | Links shelf and ocean basin; its boundary marks the end of the continent; canyons and trenches occur here |
| Deep-sea plain | Gently sloping; the flattest, smoothest regions on earth | 3,000–6,000 m | Covered with fine clay and silt |
| Oceanic deeps / trenches | Steep-sided, narrow | 3–5 km deeper than the surrounding floor | At the bases of continental slopes and along island arcs; with active volcanoes and strong earthquakes — key evidence of plate movement. 57 deeps explored: 32 Pacific, 19 Atlantic, 6 Indian |
⤵ Walking out from the coast
Land and coastlinethe continent
Continental shelf≤1°, ~80 km, to ~200 m — ends at the shelf break
Continental slope2–5°, down to ~3,000 m — the true edge of the continent
Deep-sea plain3,000–6,000 m, flat and muddy
Trench3–5 km deeper, near island arcs
Mid-ocean ridgemountain chain with a central rift
Exam traps
- UPSC 1988's coast sequence: land → continental shelf → continental slope → deep-sea plain — the shelf always comes before the slope. A continental shelf is a submerged part of the continent under shallow sea (UPSC 1980).
- NCERT's exercise asks for the ‘average depth of the continental slope’ with options 200–2,000 m etc.; the text says 200–3,000 m — pick 200–2,000 m, the closest option.
- NCERT names five oceans, then says the seas are parts of ‘these four large oceans’ — a leftover from when the Southern Ocean was not counted. The Arctic is the smallest (exercise Q1-v).
Beyond NCERT — the deepest trenches (exercise Q2-iii)The text asks you to list the deepest trenches but does not name them. Standard answers: Pacific — Mariana Trench (Challenger Deep, about 11 km, the deepest point on earth); Atlantic — Puerto Rico Trench (about 8.4 km); Indian — Java (Sunda) Trench (over 7 km); Southern — South Sandwich Trench. Other names UPSC has asked: Aleutian (North Pacific), Kermadec (South Pacific), Sunda (Indian), South Sandwich (South Atlantic) (UPSC 2000); Diamantina (Indian Ocean, south-west of Australia) (UPSC 2006).
3. Minor relief features
In simple wordsOn land there are hills, valleys and islands as well as plains and mountain ranges. The ocean floor has its own versions: undersea volcanoes (pointed or flattened), canyons cut by sediment-laden currents, and rings of coral that grow into islands.
| Feature | What it is (NCERT) | Example / number |
|---|---|---|
| Mid-oceanic ridge | Two chains of mountains separated by a large depression; peaks up to 2,500 m, some above the surface | Iceland (part of the Mid-Atlantic Ridge) |
| Seamount | Volcanic mountain with a pointed summit that does not reach the surface; 3,000–4,500 m tall | Emperor seamount, an extension of the Hawaiian Islands |
| Submarine canyon | Deep valleys, some like the Grand Canyon, cutting across shelves and slopes, often from the mouths of big rivers | Hudson Canyon — the best known |
| Guyot | A flat-topped seamount; shows gradual subsidence | More than 10,000 seamounts and guyots in the Pacific alone |
| Atoll | Low islands in tropical oceans: coral reefs around a central depression — a lagoon of sea, or of fresh, brackish or very salty water | — |
Exam trapAn oceanic deep is a major division, not a minor feature (NCERT exercise Q1-iii); seamounts, guyots and atolls are minor features. A seamount is pointed; a guyot is flat-topped.
Beyond NCERTGuyots get their flat tops from wave erosion when they stood at sea level; the ocean floor then carried them away from the hotspot or ridge and they sank. Atolls form the same way: a fringing coral reef keeps growing upward as its volcanic island slowly sinks, leaving a ring of coral round a lagoon (Darwin's subsidence theory). Lakshadweep's islands are mostly atolls.
4. Temperature of ocean waters
In simple wordsIn a swimming pool on a sunny afternoon, the top feels warm but your feet, a couple of metres down, feel cold. The sea is the same, only much deeper: a warm skin on top, then a zone where it gets cold fast, then a huge mass of near-freezing water below.
The oceans are heated by the sun, like land, but they heat and cool more slowly. Factors that affect surface temperature:
| Factor | Effect (NCERT) |
|---|---|
| Latitude | Surface temperature falls from the equator to the poles as insolation decreases |
| Unequal distribution of land and water | Northern-hemisphere oceans touch more land and receive more heat than southern ones |
| Prevailing winds | Offshore winds push warm surface water away, causing upwelling of cold water; onshore winds pile warm water against the coast |
| Ocean currents | Warm currents warm cold areas; cold currents cool warm areas — the Gulf Stream warms eastern North America and western Europe; the Labrador current cools north-eastern North America |
Enclosed seas in low latitudes are warmer than the open sea; enclosed seas in high latitudes are colder.
- Average surface temperature: about 27°C, falling towards the poles — about 22°C at 20°, 14°C at 40°, 0°C near the poles.
- Northern-hemisphere oceans are warmer (annual average ~19°C) than southern (~16°C), because of the land–water distribution.
- The highest temperature is not at the equator but slightly north of it.
- Temperature is highest at the surface (direct sunlight; heat passed down by convection) and falls with depth — very rapidly to about 200 m, then more slowly.
NCERT inconsistency — the rate of coolingNCERT says temperature falls ‘generally 0.5°C per latitude’, but its own numbers (27°C at the equator, 22°C at 20°, 14°C at 40°) mean about 0.25–0.4°C per degree. At 0.5°C per degree the 20° value would be about 17°C. Quote the averages; treat ‘0.5°C per degree’ with caution.
Reading NCERT Figure 12.4 — Spatial pattern of surface temperature of the oceans (page 105)
- The book's map is centred on the Pacific (Asia on the left, the Americas on the right), with isotherms at 28, 25, 20, 15, 10, 5 and 0°C.
- The warmest water (28°C) lies in the western Pacific around the Philippines and Indonesia and in the northern Indian Ocean — a detail only in the figure.
- Isotherms bend towards the equator along the west coasts of the Americas and Africa (cold currents, upwelling) and poleward along east coasts (warm currents off Japan, the USA and Brazil).
- In the North Atlantic the 5°C and 10°C lines swing far north towards Norway; in the north-west Atlantic they crowd together off Newfoundland, where the warm Gulf Stream meets the cold Labrador current.
- In the southern hemisphere the isotherms run almost parallel to latitude, with 0°C near 60°S.
5. The three-layer ocean and the thermocline
In simple wordsThe sea is like a layered drink: a warm, light layer floats on top, a thin ‘mixing’ layer in the middle where the temperature drops quickly, and a heavy, icy layer fills the bottom. The middle layer — the thermocline — acts like a lid between the warm top and the cold depths.
| Layer (low and middle latitudes) | Thickness / depth | Temperature (NCERT) |
|---|---|---|
| 1 · Warm surface layer | About 500 m thick | 20°–25°C; all year in the tropics, only in summer in middle latitudes |
| 2 · Thermocline | 500–1,000 m thick; begins around 100–400 m below the surface | Rapid fall in temperature with depth |
| 3 · Deep cold layer | Down to the ocean floor | Very cold, approaching 0°C; holds about 90% of the ocean's water |
| Polar waters (Arctic and Antarctic circles) | One layer from surface to floor | Near 0°C at the surface, little change with depth |
Reading NCERT Figure 12.3 — Thermocline (page 104)
- Temperature (0°–24°C) across the top; depth (0–4,500 m) down the side.
- The curve stays near 22–23°C to about 300 m, then plunges through the band marked Thermocline (about 300 to 1,000 m) to around 4–5°C.
- Below 1,000 m it creeps down slowly to about 2°C at 4,500 m — the deep, cold layer (shaded).
NCERT inconsistency — where the thermocline startsThe text gives three different depths: the boundary ‘begins around 100–400 m’; the warm top layer is ‘about 500 m thick’; and temperature ‘falls very rapidly up to 200 m’. Figure 12.3 draws the thermocline from about 300 to 1,000 m. The safe exam answer: a warm surface layer of a few hundred metres, a thermocline in which temperature drops rapidly, then a deep cold layer — and quote ‘500–1,000 m thick’ for the thermocline itself.
Beyond NCERT (asked by UPSC)India's meteorologists use Ocean Mean Temperature (OMT) — the average temperature of the upper ocean down to the depth of the 26°C isotherm (about 129 m in the south-western Indian Ocean in January–March) — to help forecast whether the monsoon will be above or below normal (UPSC 2020: both statements correct). It is a direct use of the vertical temperature structure in this section.
6. Salinity of ocean waters
In simple wordsLeave a bowl of salt water in the sun: the water evaporates but the salt stays, so it gets saltier. Pour fresh water in and it gets less salty. The sea's saltiness is a running balance between water taken out (evaporation, freezing) and fresh water added (rain, rivers, melting ice).
All natural water contains dissolved salts. Salinity is the total dissolved salt in seawater, measured as grams of salt in 1,000 g (1 kg) of seawater and written as parts per thousand (‰ or ppt). Water below 24.7‰ is regarded as brackish.
⤵ What raises and lowers salinity
Surface salinitya balance of water lost and water gained
Raised byhigh evaporation · formation of sea ice · saltier water brought by currents
Lowered byheavy rain · river inflow · melting ice · fresher water brought by currents
Winds move water from place to placeand salinity, temperature and density all affect one another
| Water body | Salinity (NCERT) | Why |
|---|---|---|
| Open ocean | 33–37‰ | — |
| Red Sea (landlocked) | 41‰ | Hot, dry, high evaporation |
| Hot, dry regions | up to 70‰ | Very high evaporation |
| Estuaries and the Arctic | 0–35‰, seasonally | River water, freezing and melting |
| Atlantic Ocean | average ~36‰; maximum 37‰ at 20°–30°N, 20°–60°W | Highest between 15° and 20°; decreases northward |
| Indian Ocean | average 35‰ | Low in the Bay of Bengal (rivers); high in the Arabian Sea (evaporation, little fresh water) |
| Pacific Ocean | 35 → 31‰ in the west of the NH; 33‰ beyond 15°–20°S | Arctic melt water; its shape and size |
| North Sea | Higher than expected for its latitude | Saltier water brought by the North Atlantic Drift |
| Baltic Sea | Low | Large river inflow |
| Mediterranean Sea | High | High evaporation |
| Black Sea | Very low | Enormous river inflow |
Reading NCERT Figure 12.5 — Surface salinity of the world's oceans (page 106)
- Isohalines at 30–37‰, with the Tropic of Cancer, equator and Tropic of Capricorn marked.
- Closed 37‰ cells sit in the subtropical North Atlantic and off Brazil in the South Atlantic; 36‰ rings surround them and also appear in the South Pacific and Arabian Sea.
- Salinity is lower near the equator (34–35‰, heavy rain) than in the subtropics — the subtropical highs (Chapter 9) are where evaporation beats rain.
- The lowest values: 30–32‰ in the Bay of Bengal and seas off south-east Asia, and about 33‰ in the North Pacific and near Antarctica.
- Around 60°S the lines run straight at about 34‰.
Reading the box on page 104 — highest salinity in water bodiesNCERT lists Lake Van, Turkey (330‰), the Dead Sea (238‰) and the Great Salt Lake (220‰). These are inland salt lakes, far saltier than any ocean. Different sources give quite different values for them (the Dead Sea is often quoted near 340‰), so learn NCERT's numbers and order for the exam, but do not rely on them elsewhere.
Vertical distribution. Surface salinity rises when water is lost to ice or evaporation and falls when fresh water is added; deep salinity is almost fixed, since water is not lost and salt is not added there. Salinity generally increases with depth, with a sharp increase in a zone called the halocline. Saltier water is denser and sinks below fresher water — the ocean is stratified by salinity.
Exam traps and an NCERT slip
- Both salinity and density increase with depth (UPSC 1981); seawater is densest where depth and salinity are greatest (UPSC 1979).
- The Red Sea is very salty because it gets almost no rain and no rivers flow into it (UPSC 2024: both true).
- The Bay of Bengal is less salty than the Arabian Sea, despite being in the same ocean.
- NCERT's salinity paragraph refers to ‘Table 12.4’, which does not exist in the chapter.
NCERT exercise — answer outlines
Try each question yourself first, then open the outline to compare. The multiple-choice questions are in the self-check below.
Why do we call the earth a Blue Planet? (30 words)
Oceans cover most of its surface and it has an abundant supply of liquid water — rare in the solar system — so from space it looks blue. Water makes life possible.
What is a continental margin? (30 words)
The submerged edge of a continent between the shoreline and the deep-sea floor: the continental shelf, the continental slope and the continental rise at its foot (Figure 12.2). The slope's boundary marks the end of the continent.
List out the deepest trenches of various oceans. (30 words)
Not named in the text (Beyond NCERT): Pacific — Mariana (Challenger Deep ~11 km, deepest on earth); Atlantic — Puerto Rico (~8.4 km); Indian — Java/Sunda (over 7 km); Southern — South Sandwich. NCERT: 57 deeps — 32 Pacific, 19 Atlantic, 6 Indian.
What is a thermocline? (30 words)
The boundary layer below the warm surface water where temperature falls rapidly with depth; it begins around 100–400 m and is 500–1,000 m thick. About 90% of ocean water lies below it, near 0°C.
When you move into the ocean what thermal layers would you encounter? Why the temperature varies with depth? (30 words)
(1) A warm surface layer (~500 m, 20–25°C); (2) the thermocline (rapid cooling); (3) a cold deep layer near 0°C. Sunlight heats only the surface; heat passes down slowly by convection, so temperature falls with depth.
What is salinity of sea water? (30 words)
The total dissolved salts in seawater, as grams of salt per 1,000 g (1 kg) of water, written ‰ (ppt). Open oceans: 33–37‰; below 24.7‰ is brackish.
How are various elements of the hydrological cycle interrelated? (150 words)
Intro: the cycle moves water between oceans, atmosphere, land, subsurface and organisms in three phases; total water is constant.
Links: the sun drives evaporation (and sublimation) from oceans and evapotranspiration from land and plants → vapour condenses into clouds → winds carry it inland (advection) → precipitation as rain or snow → snow stored as ice later melts into streams → surface runoff to rivers, lakes and the sea, or infiltration to groundwater, which flows slowly to the sea and emerges in springs.
Interdependence: each store feeds the next; a change in one (deforestation reduces evapotranspiration and infiltration; warming increases evaporation and melts ice) changes the others.
Numbers: most water in the oceans (NCERT 91%; standard ~97%); most rain on land evaporates back.
Diagram: Figure 12.1 with labelled arrows.
Conclude: renewable water is constant while demand grows — hence water crises.
Links: the sun drives evaporation (and sublimation) from oceans and evapotranspiration from land and plants → vapour condenses into clouds → winds carry it inland (advection) → precipitation as rain or snow → snow stored as ice later melts into streams → surface runoff to rivers, lakes and the sea, or infiltration to groundwater, which flows slowly to the sea and emerges in springs.
Interdependence: each store feeds the next; a change in one (deforestation reduces evapotranspiration and infiltration; warming increases evaporation and melts ice) changes the others.
Numbers: most water in the oceans (NCERT 91%; standard ~97%); most rain on land evaporates back.
Diagram: Figure 12.1 with labelled arrows.
Conclude: renewable water is constant while demand grows — hence water crises.
Examine the factors that influence the temperature distribution of the oceans. (150 words)
Latitude: insolation decreases poleward — about 27°C at the equator, 22°C at 20°, 14°C at 40°, 0°C near the poles.
Land–water distribution: NH oceans touch more land and are warmer (~19°C vs ~16°C in the SH); the warmest water is slightly north of the equator.
Prevailing winds: offshore winds cause upwelling of cold water; onshore winds pile up warm water.
Ocean currents: Gulf Stream warms eastern North America and western Europe; Labrador current cools north-eastern North America; isotherms bend equatorward on cold-current coasts (Figure 12.4).
Enclosed seas: warmer than open seas in low latitudes, colder in high latitudes.
Depth: warm surface layer, thermocline, cold deep layer; polar seas have one cold layer.
Diagram: a temperature–depth profile.
Land–water distribution: NH oceans touch more land and are warmer (~19°C vs ~16°C in the SH); the warmest water is slightly north of the equator.
Prevailing winds: offshore winds cause upwelling of cold water; onshore winds pile up warm water.
Ocean currents: Gulf Stream warms eastern North America and western Europe; Labrador current cools north-eastern North America; isotherms bend equatorward on cold-current coasts (Figure 12.4).
Enclosed seas: warmer than open seas in low latitudes, colder in high latitudes.
Depth: warm surface layer, thermocline, cold deep layer; polar seas have one cold layer.
Diagram: a temperature–depth profile.
What to remember
Numbers
Water in the oceans (NCERT)~91% (standard estimate ~97%)
Ocean floor mostly3–6 km below sea level
Continental shelf≤1°; ~80 km wide; 30–600 m deep
Widest shelfSiberian shelf, 1,500 km
Continental slope2–5°; 200–3,000 m
Deep-sea plain3,000–6,000 m
Trenches3–5 km deeper than the surrounding floor
Deeps explored57 = 32 Pacific + 19 Atlantic + 6 Indian
Mid-ocean ridge peaksup to 2,500 m
Seamounts3,000–4,500 m tall
Seamounts + guyots in the Pacificover 10,000
Average ocean surface temperature~27°C (NH ~19°C, SH ~16°C)
Warm surface layer / thermocline~500 m (20–25°C) / 500–1,000 m thick
Water below the thermocline~90%
Open-ocean salinity33–37‰ (brackish below 24.7‰)
Red Sea / Atlantic / Indian Ocean41‰ / ~36‰ / 35‰
Atlantic maximum37‰ at 20°–30°N, 20°–60°W
One-liners
- Salinity = grams of salt per 1,000 g of seawater (‰).
- Hydration is not part of the hydrological cycle.
- Shelf → slope → deep-sea plain → trench.
- A seamount is pointed; a guyot is flat-topped.
- Iceland is part of the Mid-Atlantic Ridge.
- Thermocline = rapid fall in temperature; halocline = rapid rise in salinity.
- Bay of Bengal fresher (rivers); Arabian Sea saltier (evaporation).
- North Sea saltier than its latitude suggests — North Atlantic Drift.
Traps to avoid
- An oceanic deep is a major division, not a minor relief feature.
- NCERT says 91% of water is in the oceans; standard sources say ~97%.
- NCERT lists five oceans, then calls them ‘four large oceans’.
- Text says 0.5°C fall per degree of latitude, but its own figures imply less.
- The thermocline's starting depth differs between text (100–400 m, 200 m, 500 m) and figure (~300 m).
- The highest ocean temperature is slightly north of the equator, not at it.
- Black Sea and Baltic are low-salinity; Red Sea and Mediterranean are high-salinity.
- Both salinity and density increase with depth.
- The Arctic is the smallest ocean.
How to strategise
Prelims — what UPSC asks
- Ocean-floor basics: what a continental shelf is (1980), the sequence from coast to deep sea (1988), trenches and their oceans (2000 — Aleutian, Kermadec, Sunda, South Sandwich; 2006 — Diamantina).
- Salinity and density: both increase with depth (1979, 1981); Red Sea salinity (2024).
- Freshwater distribution: share of freshwater (1987), less than 1% usable, not 95% in ice (2010), groundwater as the largest store after ice (2013). NCERT's 91% is not the standard figure — learn both.
- Ocean temperature with depth: Ocean Mean Temperature and the 26°C isotherm (2020).
- Expect UPSC to test the reasons for regional salinity (rivers, evaporation, currents) and the order of ocean-floor features.
Mains — where it appears and how to answer
- GS1: variations in oceanic salinity and their effects (2017); the global freshwater crisis — availability and access (2023).
- Answer frame for salinity: define (‰) → factors (evaporation, precipitation, rivers, ice, winds, currents) → horizontal pattern (subtropical maxima, equatorial dip, enclosed seas) → vertical pattern (halocline) → effects (density, currents, marine life, desalination).
- Use the ocean-floor profile or the thermocline graph as a 30-second sketch; label the numbers.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Identify the element which is not a part of the hydrological cycle:
2. The average depth of continental slope varies between:
3. Which one of the following is not a minor relief feature in the oceans?
4. Salinity is expressed as the amount of salt in grams dissolved in sea water per:
5. Which one of the following is the smallest ocean?
6. Why does the Bay of Bengal have lower surface salinity than the Arabian Sea?
7. A flat-topped submarine mountain that shows evidence of gradual subsidence is a:
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.