BharatNotes Why this chapter matters for UPSC: This is Chapter 12 in the current rationalised NCERT Fundamentals of Physical Geography (Reprint 2026-27); it was Chapter 13 in pre-2023 editions, before "Minerals and Rocks" was dropped. It covers the hydrological cycle, the relief of the ocean floor, and the distribution of temperature and salinity. Mains GS1 has asked directly on salinity (2017), coral and global warming (2019) and ocean resources (2014, 2023); Prelims 2024 asked why the Red Sea is so salty.
Contemporary hook: India's coastline was re-measured at 11,098.81 km, up from the 7,516.6 km figure used since the 1970s (NCERT and older sources give 7,517 km). The revision was made by the National Hydrographic Office with the Survey of India at 1:250,000 scale and promulgated by a Ministry of Ports, Shipping and Waterways circular of 29 April 2025 (PIB, 4 December 2025).
🧠 First Principles — Read This First
About 91 per cent of the Earth's water sits in the oceans. That water is not still or uniform. It is part of a cycle, it lies over a floor with mountains and trenches, and it varies in heat and saltiness from place to place.
The hydrological cycle is the continuous movement of water between the oceans, the air, the land and living things. Water evaporates from the sea, condenses into clouds, falls as rain or snow, and returns through rivers, groundwater and ice. The total amount of renewable water stays roughly constant while demand keeps rising, which is why water shortages appear in particular places and seasons.
The ocean floor has its own relief. Next to each continent is a shallow ledge, the continental shelf. It ends in a steeper continental slope, which drops to wide, flat deep-sea plains, and in places to narrow trenches several kilometres deeper still. Ridges, seamounts and canyons rise from or cut into this floor, much as hills and valleys do on land.
Temperature in the ocean falls with depth. A warm surface layer sits over a zone of rapid cooling called the thermocline, and below that lies cold deep water. Think of a bucket of water left in the sun: the top warms first and the bottom stays cool.
Salinity is the amount of salt dissolved in seawater, measured in grams per kilogram (parts per thousand). Evaporation raises it; rain, river water and melting ice lower it.
Temperature and salinity together set how dense seawater is, and density decides which water sinks and which floats. That links this chapter to the next one, on ocean currents. UPSC asks for these mechanisms, not only the numbers: why one sea is saltier than another, and why ocean temperature differs from place to place.
PART 1 — Quick Reference
Table 1: Hydrological cycle (NCERT)
| Item | NCERT figure or statement |
|---|---|
| Share of planetary water in oceans | About 91% |
| Rest of the water | Glaciers and ice caps, groundwater, lakes, soil moisture, atmosphere, streams, living things |
| Water falling on land that returns to the atmosphere by evaporation | Nearly 59% |
| Remainder of water falling on land | Runs off, infiltrates the ground, or becomes glacier ice |
| Components (NCERT Table 12.1) | Storage in oceans; water in the atmosphere; storage in ice and snow; surface runoff; freshwater storage; groundwater storage |
| Processes (NCERT Table 12.1) | Evaporation, evapotranspiration, sublimation, condensation, precipitation, snowmelt runoff, stream flow, infiltration, groundwater discharge, springs |
Source: NCERT, Fundamentals of Physical Geography, Class XI, Ch. 12, Reprint 2026-27.
Table 2: Major divisions of the ocean floor (NCERT's four)
| Division | NCERT description | NCERT figures | Examples |
|---|---|---|---|
| Continental shelf | Extended margin of the continent under shallow seas and gulfs; ends at the shelf break | Gradient 1° or less; average width ~80 km; depth from 30 m to 600 m | Almost absent off Chile and west Sumatra; Siberian shelf (Arctic Ocean), the world's largest, up to 1,500 km wide |
| Continental slope | Joins shelf to ocean basin; its edge marks the end of the continent; canyons and trenches occur here | Gradient 2–5°; depth 200–3,000 m (NCERT's own exercise MCQ uses 200–2,000 m) | — |
| Deep sea plain (abyssal plain) | Flattest, smoothest regions of the world, covered with fine clay and silt | 3,000–6,000 m | — |
| Oceanic deeps (trenches) | Deepest parts; steep-sided, narrow basins at the bases of continental slopes and along island arcs, with active volcanoes and strong earthquakes | 3–5 km deeper than the surrounding floor; 57 deeps explored (32 Pacific, 19 Atlantic, 6 Indian) | Mariana Trench (Pacific) |
NCERT also says a major portion of the ocean floor lies 3–6 km below sea level. The shelf sediments, brought by rivers, glaciers and wind, become the source of fossil fuels over long periods.
Source: NCERT Ch. 12, pp. 101–102, Reprint 2026-27.
Table 3: Minor relief features (NCERT)
| Feature | NCERT description | Example |
|---|---|---|
| Mid-oceanic ridge | Two chains of mountains separated by a large depression; peaks up to 2,500 m, some above sea level | Iceland, part of the Mid-Atlantic Ridge |
| Seamount | Pointed volcanic mountain that does not reach the surface; 3,000–4,500 m tall | Emperor seamount, an extension of the Hawaiian Islands |
| Submarine canyon | Deep valley cutting across shelf and slope, often from the mouths of large rivers | Hudson Canyon, the best known |
| Guyot | Flat-topped seamount, showing gradual subsidence | More than 10,000 seamounts and guyots estimated in the Pacific alone |
| Atoll | Low tropical island of coral reefs around a central depression (lagoon, or enclosed fresh, brackish or very saline water) | Lakshadweep |
NCERT's exercise treats the oceanic deep as a major feature, not a minor one. Beyond NCERT: NOAA describes the global mid-ocean ridge system as nearly 65,000 km long, more than 90% of it underwater, with an average depth to the ridge top of 2,500 m (NOAA Ocean Exploration, "What is a mid-ocean ridge?").
Table 4: Deepest point of each ocean (beyond NCERT)
| Ocean | Deepest point | Depth (Five Deeps Expedition 2018–19) |
|---|---|---|
| Pacific | Challenger Deep, Mariana Trench | 10,924 ± 15 m |
| Atlantic | Brownson Deep, Puerto Rico Trench | 8,378 ± 5 m |
| Southern | Unnamed deep, South Sandwich Trench | 7,432 ± 13 m |
| Indian | Unnamed deep, Java (Sunda) Trench | 7,187 ± 13 m |
| Arctic | Molloy Hole | 5,551 ± 14 m |
Source: Bongiovanni, Stewart and Jamieson, "High-resolution multibeam sonar bathymetry of the deepest place in each ocean", Geoscience Data Journal 9(1), 2022, pp. 108–123 (NERC Open Research Archive). For Challenger Deep, the figure NOAA cites is 10,935 m (±6 m), from a 2021 pressure-based study of June 2020 dives (NOAA Institutional Repository). Older textbooks give 11,034 m, a 1957 sounding. Each figure carries an error margin, so quote one with its source rather than mixing them.
Table 5: Factors affecting ocean temperature (NCERT)
| Factor | Effect |
|---|---|
| Latitude | Surface temperature falls from the equator towards the poles as insolation decreases |
| Unequal distribution of land and water | Northern Hemisphere oceans receive more heat because they are in contact with more land |
| Prevailing wind | Offshore winds drive warm surface water away, so cold water upwells; onshore winds pile warm water against the coast and raise temperature |
| 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.
Table 6: Temperature data points (NCERT)
| Item | NCERT figure |
|---|---|
| Average surface temperature of the oceans | ~27°C |
| Fall in surface temperature with latitude | ~0.5°C per degree of latitude |
| Average at 20° / 40° latitude / near poles | ~22°C / ~14°C / ~0°C |
| Average annual temperature, NH / SH oceans | ~19°C / ~16°C |
| Highest temperature | Slightly north of the equator, not at it |
| Top layer (low and mid latitudes) | ~500 m thick, 20–25°C; all year in the tropics, only in summer in mid-latitudes |
| Thermocline | Begins ~100–400 m below the surface; 500–1,000 m thick |
| Below the thermocline | ~90% of the ocean's water; temperatures approach 0°C |
| Polar seas (Arctic and Antarctic circles) | One cold layer from surface to floor |
| Rate of cooling with depth | Very rapid down to ~200 m, slower below |
Source: NCERT Ch. 12, pp. 103–104, Reprint 2026-27.
Table 7: Factors affecting salinity (NCERT)
| Factor | Effect |
|---|---|
| Evaporation and precipitation | Main control of surface salinity; evaporation raises it, precipitation lowers it |
| River inflow (coasts) | Fresh water lowers salinity |
| Freezing and thawing of ice (polar regions) | Freezing raises salinity; melting lowers it |
| Wind | Moves water from one area to another |
| Ocean currents | Redistribute salinity; salinity, temperature and density are interrelated |
Definition: salinity is the grams of salt dissolved in 1,000 g (1 kg) of seawater, written ‰ or ppt. 24.7‰ is the upper limit used to define brackish water.
Table 8: Salinity distribution (NCERT)
| Water body or region | NCERT figure or reason |
|---|---|
| Normal open ocean | 33–37‰ |
| Red Sea (NCERT calls it land-locked) | As high as 41‰ |
| Estuaries and the Arctic | 0–35‰, varying with season |
| Hot, dry regions with high evaporation | Sometimes reaches 70‰ |
| Atlantic Ocean | Average ~36‰; highest between 15° and 20° latitude; maximum 37‰ between 20°N–30°N and 20°W–60°W |
| Pacific Ocean | Falls from 35‰ to 31‰ in the western parts of the NH (Arctic meltwater); falls to 33‰ beyond 15°–20°S |
| Indian Ocean | Average 35‰; Bay of Bengal lower (river inflow); Arabian Sea higher (high evaporation, little fresh water) |
| North Sea | Higher despite high latitude, because the North Atlantic Drift brings saltier water |
| Baltic Sea | Low; large river inflow |
| Mediterranean Sea | High; high evaporation |
| Black Sea | Very low; large river inflow |
| Highest salinity in water bodies (NCERT box) | Lake Van, Turkey 330‰; Dead Sea 238‰; Great Salt Lake 220‰ |
Source: NCERT Ch. 12, pp. 104–105, Reprint 2026-27. Modern measurements of the Dead Sea differ from NCERT's 238‰; for the exam, use NCERT's figures.
Table 9: Coral reefs (beyond NCERT)
| Item | Detail | Source |
|---|---|---|
| Builders | Hard (stony) corals; soft corals do not build reefs | NOAA Corals Tutorial |
| Symbiosis | Zooxanthellae living in coral tissue photosynthesise; up to 90% of the organic material they produce passes to the coral | NOAA Corals Tutorial |
| Temperature | Many shallow-water reef corals grow best at 23–29°C | NOAA Corals Tutorial |
| Salinity | Corals need 32–42‰ | NOAA Corals Tutorial |
| Light | Most reef-building species live where light reaches, to about 30 m depth | NOAA Corals Tutorial |
| Latitude | Most reef-building stony corals lie between 35°N and 35°S | NOAA Corals Tutorial |
| Share of the ocean | Reefs cover about 1% of the ocean and shelter at least 25% of marine life | NOAA Corals Tutorial |
| Reef types | Fringing (the most common), barrier (separated from land by a lagoon), atoll (around a sunken volcanic island) | NOAA Corals Tutorial |
| India | Major reefs in the Gulf of Mannar, Palk Bay, Gulf of Kutch, Andaman and Nicobar Islands, and Lakshadweep. Lakshadweep reefs are atolls; the others are fringing reefs, and the Andamans also have a barrier reef on the west coast | Hoon, FAO (1997) |
PART 2 — Concepts & Narrative
The hydrological cycle
Water is a cyclic resource. It moves between the oceans, the atmosphere, the land surface and subsurface, and living organisms, changing between liquid, solid and gas as it goes. NCERT calls the Earth the "Blue Planet" because of its surface water.
The key quantities are in Table 1. The policy point NCERT draws from them is that renewable water is roughly constant while demand rises, so shortages appear in some places and seasons, and polluted rivers make them worse.
Relief of the ocean floor
The ocean floor is shaped by the same tectonic, volcanic and depositional processes as the land. NCERT divides it into four major divisions (Table 2) and a set of minor features (Table 3).
Reading a profile from the coast outward
Moving away from a coast, you first cross the continental shelf, then go down the continental slope, and reach the deep sea plain at 3–6 km. The shelf and slope together are the continental margin, and the slope boundary marks the true edge of the continent.
Trenches do not lie further out than the plains. NCERT places them "at the bases of continental slopes and along island arcs". They form where one plate descends beneath another, which is why they come with active volcanoes and strong earthquakes. NCERT notes that the shelf is almost absent off Chile and west Sumatra. West Sumatra faces the Sunda Trench, where the India Plate descends (USGS). Where there is no plate boundary near the coast, the slope gives way to the deep plain.
Ridges form along the boundaries where plates move apart. Iceland is a part of the Mid-Atlantic Ridge that rises above sea level.
So a "shelf → slope → plain → trench" sequence is not a universal coast-to-ocean order. A trench lies next to the slope at an active margin, not beyond the deep plain.
The shelf is the part of the ocean floor most used by people. Shelf sediments become the source of fossil fuels. Sunlight reaches the bottom of the shallow water, which favours marine life. In India, the offshore oil and gas of the western shelf (Mumbai High) and coastal fisheries both depend on it.
NCERT's exercise asks for the deepest trenches of the oceans. Table 4 gives the current survey figures for each ocean.
Temperature of ocean waters
Ocean water is heated by the Sun like land, but it heats and cools more slowly. Heat enters at the surface and is carried downward by convection, so the maximum temperature is always at the surface. The horizontal pattern follows insolation and is modified by the four NCERT factors (Table 5). The Northern Hemisphere oceans are warmer on average (about 19°C against 16°C) because the Northern Hemisphere has more land, and the warmest belt lies slightly north of the equator.
Thermocline: the boundary layer, usually beginning 100–400 m below the surface and extending several hundred metres down, in which temperature falls rapidly with depth. It separates the warm surface water from the cold deep water. NCERT puts about 90% of the ocean's water below the thermocline, at temperatures approaching 0°C.
The three-layer system (NCERT)
In middle and low latitudes the ocean has three layers:
- Warm top layer, about 500 m thick, 20–25°C. In the tropics it exists all year; in mid-latitudes it forms only in summer.
- Thermocline layer, 500–1,000 m thick, in which temperature falls rapidly with depth.
- Cold deep layer, reaching down to the ocean floor.
Inside the Arctic and Antarctic circles the surface water is already close to 0°C, so temperature barely changes with depth and there is only one layer of cold water.
The two depth figures can look contradictory. "Begins 100–400 m down" describes where the thermocline starts in a temperature–depth profile. "Top layer about 500 m thick" is NCERT's generalised three-layer model. Use the figure that matches the question.
Beyond NCERT, the thermocline matters for marine life. It limits vertical mixing, so nutrients in deep water rise to the sunlit surface mainly where winds drive upwelling. NCERT describes this under the prevailing-wind factor: offshore winds push warm surface water away and cold water upwells.
Salinity of ocean waters
All natural water contains dissolved salts. Salinity is the total dissolved salt content of seawater (Table 7). At the surface it depends mainly on the balance between evaporation and precipitation, modified near coasts by river water and near the poles by the freezing and melting of ice. Most of NCERT's regional examples (Table 8) apply this one rule:
- More evaporation than fresh-water input means higher salinity. Examples: Red Sea, Mediterranean, Arabian Sea, and the subtropical Atlantic maximum at 20°N–30°N.
- More fresh-water input means lower salinity. Examples: Baltic and Black Seas, Bay of Bengal, estuaries, and the western North Pacific fed by Arctic meltwater.
- Imported water. The North Sea is saltier than its latitude suggests because the North Atlantic Drift brings in saltier water.
Vertical distribution. Surface salinity changes when water is lost to evaporation or ice, or when fresh water is added. At depth, salinity stays nearly constant because water is neither lost nor salt added there. Lower-salinity water rests above denser, higher-salinity water. Salinity generally increases with depth, and the zone where it rises sharply is the halocline. With other factors constant, saltier water is denser and sinks, so the ocean becomes stratified by salinity.
Halocline vs thermocline. The thermocline is a sharp change in temperature with depth; the halocline is a sharp change in salinity with depth. Both mark layers that resist mixing. The related term for density is the pycnocline, which is not in NCERT.
Coral reefs
Coral reefs: not in the current NCERT chapter, but tested in Mains GS1
The rationalised chapter mentions atolls only as a relief feature. Mains GS1 2019 (gs1-pyq-2019-07) asked for the impact of global warming on coral life, so the basics are in Table 9.
Bleaching. When corals are stressed by changes in temperature, light or nutrients, they expel the zooxanthellae living in their tissues and turn white. NOAA names rising ocean temperature from climate change as the leading cause. Runoff and pollution, very strong sunlight and extreme low tides can also cause bleaching. A bleached coral is not dead: it can recover if the stress is short, but dies if the stress continues. Cold water can also cause bleaching; NOAA records an event in the Florida Keys in January 2010.
Acidification. Carbon dioxide absorbed by the ocean lowers seawater pH. This reduces calcification in reef-building organisms, slowing coral growth and weakening reef structure (NOAA).
Atoll formation. In the sequence NOAA describes, a fringing reef forms around a volcanic island. The island sinks below sea level while the coral keeps growing upward, leaving a ring-shaped reef around a central lagoon. NOAA estimates that barrier reefs and atolls take 100,000 to 30 million years to form fully.
Sources: NOAA National Ocean Service, Corals Tutorial and "What is coral bleaching?"; V. Hoon, Coral Reefs of India, FAO (1997).
The ocean in the climate system
Two measured quantities show the ocean's role in climate change:
- Heat. Ocean warming accounted for 91% of the heating of the climate system between 1971 and 2018. Land warming, ice loss and atmospheric warming accounted for about 5%, 3% and 1% respectively (IPCC AR6 WGI Summary for Policymakers, A.4.2, 2021).
- Carbon. The ocean CO₂ sink was 2.9 ± 0.4 GtC per year in 2014–2023, or 26% of total CO₂ emissions. Over the same decade the land sink took about 30% and the atmosphere kept about 48% (Global Carbon Budget 2024, Earth System Science Data, 2025).
The carbon taken up lowers seawater pH (ocean acidification), which links back to corals.
India's ocean resources (beyond NCERT)
- Coastline: 11,098.81 km (re-assessed; MoPSW circular of 29 April 2025, reported by PIB on 4 December 2025). NCERT and older sources give 7,516.6 km (~7,517 km), measured at a much coarser scale in the 1970s. In the revised figures, Gujarat (2,340.62 km) and Andaman and Nicobar Islands (3,083.50 km) have the longest stretches.
- EEZ: over 2 million km² (MoES, Draft Blue Economy Policy, February 2021). That is about three-fifths of India's land area of ~3.29 million km², so the EEZ is smaller than the land area.
- Fisheries: total fish production was over 184 lakh tonnes in 2023-24, up from 95 lakh tonnes in 2013-14. India is the second-largest fish producer, with about 8% of world output (Ministry of Fisheries, reported by All India Radio, 15 February 2025). This figure includes inland fisheries, which produce most of India's fish.
- Deep-sea minerals: India became a pioneer investor in 1987. Its application for polymetallic nodules in the Central Indian Ocean was approved on 17 August 1987 by the Preparatory Commission, and India signed its exploration contract with the International Seabed Authority in 2002 (Statement by India, ISA Council, 10 August 2017).
- Deep Ocean Mission: approved by the CCEA on 16 June 2021 at an estimated Rs 4,077 crore over 5 years, with MoES as the nodal ministry. Components include a manned submersible for three people to 6,000 m, an integrated system for mining polymetallic nodules from 6,000 m in the central Indian Ocean, a search for hydrothermal sulphides along Indian Ocean mid-oceanic ridges, and an OTEC-powered desalination study (PIB, 16 June 2021).
- Draft Blue Economy Policy: released by MoES for public comment in February 2021, with seven thematic areas, including coastal and deep-sea mining and offshore energy.
PART 3 — UPSC Integration
PYQ pattern (from the question banks)
- Mains GS1 2017 (gs1-pyq-2017-14, 15 marks): "Account for variations in oceanic salinity and discuss its multi-dimensional effects." Tables 7 and 8 and the vertical-salinity section answer the first half. The effects (density, stratification and the link to currents) answer the second.
- Mains GS1 2019 (gs1-pyq-2019-07, 10 marks): "Assess the impact of global warming on the coral life system with examples."
- Mains GS1 2014 (gs1-pyq-2014-02, 10 marks): "Critically evaluate the various resources of the oceans which can be harnessed to meet the resource crisis in the world."
- Mains GS1 2023 (gs1-pyq-2023-16, 15 marks): "Comment on the resource potentials of the long coastline of India and highlight the status of natural hazard preparedness in these areas."
- Mains GS3 2023 (gs3-pyq-2023-05, 15 marks): impact on India and the Indian Ocean region of the roughly one-metre sea-level rise by 2100 predicted by the IPCC. Ocean heat uptake (91% of the added heat) is the physical starting point.
- Prelims 2024 (bank id geo-083): two statements on the Red Sea, that it receives very little precipitation and that no water enters it from rivers. Both were correct, and together they explain its high salinity.
- Prelims 2026 (prelims-2026-gs1-075): Deep Ocean Mission (nodal ministry, Matsya-6000, Samudrayaan).
Mains frameworks
- Salinity answers: controls (evaporation–precipitation balance, rivers, ice, wind, currents) → horizontal pattern with NCERT examples → vertical pattern and halocline → effects (density and stratification, deep circulation, marine life, salt production).
- Ocean resources: shelf (fisheries, oil and gas) → deep sea (polymetallic nodules, hydrothermal sulphides) → energy (tidal, OTEC) → governance (ISA contract, Deep Ocean Mission, draft Blue Economy Policy) → limits (environmental impact assessment, cost of technology).
- Coral and warming: symbiosis → thermal stress and bleaching → acidification → India's reef sites → recovery depends on how long the stress lasts.
Cross-paper relevance
- GS1 (Physical geography): ocean-floor relief, temperature and salinity distribution, coral reefs.
- GS3 (Environment, Economy, S&T): ocean heat and carbon uptake, acidification, sea-level rise (GS3 2023); Deep Ocean Mission and the blue economy.
- GS2: UNCLOS and the International Seabed Authority, where India holds exploration contracts.
Exam Strategy
Prelims fact-traps:
- Shelf width: NCERT's average is ~80 km, and the Siberian shelf reaches 1,500 km. Shelf depth varies from 30 m to 600 m, so "always 200 m" is wrong.
- Slope depth: NCERT's text says 200–3,000 m, but its exercise MCQ offers 200–2,000 m as the answer. Know both.
- Major vs minor features: oceanic deeps are a major division. Seamount, guyot and atoll are minor features (NCERT exercise iii).
- Enclosed seas: warmer than open seas only in low latitudes, colder in high latitudes.
- Warmest ocean belt: slightly north of the equator, not on it.
- Highest salinity (NCERT box): Lake Van 330‰ > Dead Sea 238‰ > Great Salt Lake 220‰. Among the seas, the Red Sea reaches 41‰.
- Brackish water limit: 24.7‰.
- Bay of Bengal vs Arabian Sea: the Bay of Bengal is less saline (river inflow); the Arabian Sea is more saline (evaporation).
- North Sea: high salinity despite its latitude, because of the North Atlantic Drift.
- Smallest ocean: the Arctic (NCERT exercise v).
- Coastline: a question set after 2025 may use 11,098.81 km. NCERT still prints the old 7,517 km figure.
Mains patterns: "Account for variations…", "Examine the factors…" (temperature, NCERT exercise 3.ii) and "Critically evaluate resources…". Each needs mechanism first, then NCERT examples, then an Indian dimension.
Practice Questions
- Mains GS1 2017 (gs1-pyq-2017-14): Account for variations in oceanic salinity and discuss its multi-dimensional effects. (15 marks)
- Mains GS1 2019 (gs1-pyq-2019-07): Assess the impact of global warming on the coral life system with examples. (10 marks)
- Prelims 2024 (bank id geo-083): Consider the following statements: (1) The Red Sea receives very little precipitation in any form. (2) No water enters the Red Sea from rivers. Which of the statements given above is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2. Answer: (c).
- Practice (UPSC-pattern, not a past paper): Consider the following statements: (1) In high latitudes, enclosed seas record higher temperatures than open seas. (2) About 90% of the ocean's water lies below the thermocline. (3) In polar seas the ocean has a three-layer temperature structure. Which of the statements given above is/are correct? (a) 1 and 2 only (b) 2 only (c) 2 and 3 only (d) 1, 2 and 3. Answer: (b). Per NCERT, enclosed high-latitude seas are colder, and polar seas have a single cold layer.
- Practice (UPSC-pattern, not a past paper): Examine the factors that influence the distribution of temperature in the oceans, with examples. (150 words)
📦 Revision Capsule
Hard Facts
- About 91% of planetary water is in the oceans; nearly 59% of water falling on land returns by evaporation (NCERT)
- Four major divisions: continental shelf, continental slope, deep sea plain, oceanic deeps (NCERT)
- Shelf: gradient 1° or less, average width ~80 km, depth 30–600 m; Siberian shelf up to 1,500 km
- Slope: 2–5°, 200–3,000 m (exercise MCQ: 200–2,000 m); deep sea plain 3,000–6,000 m
- 57 deeps explored: 32 Pacific, 19 Atlantic, 6 Indian (NCERT)
- Challenger Deep ~10,935 m (NOAA-cited 2021 study); 10,924 m (Five Deeps); old figure 11,034 m
- Surface average ~27°C, falling ~0.5°C per degree of latitude; NH oceans ~19°C, SH ~16°C
- Top layer ~500 m at 20–25°C; thermocline 500–1,000 m thick; ~90% of water below it
- Open ocean salinity 33–37‰; Red Sea up to 41‰; brackish limit 24.7‰
- Lake Van 330‰, Dead Sea 238‰, Great Salt Lake 220‰ (NCERT)
- Ocean took 91% of climate-system heating, 1971–2018 (IPCC AR6); ocean CO₂ sink 26% of emissions, 2014–2023 (GCB 2024)
- India's coastline 11,098.81 km (MoPSW, 29 April 2025); old figure 7,516.6 km
Core Concepts
- Trenches lie at the base of continental slopes and along island arcs, not beyond the deep plains
- Ocean temperature falls with depth, sharply in the thermocline; polar seas have only one layer
- Surface salinity follows the balance of evaporation against fresh-water input
- Salinity generally rises with depth, sharply in the halocline, and saltier water sinks
- Enclosed seas are warmer than open seas in low latitudes and colder in high latitudes
Confused Pairs
- Thermocline (temperature) vs halocline (salinity)
- Bay of Bengal (low salinity, rivers) vs Arabian Sea (high salinity, evaporation)
- Seamount (pointed top) vs guyot (flat top)
- Major division (oceanic deep) vs minor feature (seamount, guyot, atoll)
Data Points
- Fish production over 184 lakh tonnes, 2023-24 (Ministry of Fisheries, February 2025)
- Deep Ocean Mission: CCEA approval 16 June 2021, Rs 4,077 crore over 5 years
PYQ Pattern
- Mains GS1: salinity (2017), coral and global warming (2019), ocean resources (2014), coastline resources (2023); GS3 2023 Indian Ocean sea-level rise
- Prelims: Red Sea salinity controls (2024); Deep Ocean Mission (2026)
Sources
Sources
- NCERT, Fundamentals of Physical Geography, Class XI, Chapter 12 "Water (Oceans)", Reprint 2026-27: kegy212.pdf
- NOAA Ocean Exploration, "What is a mid-ocean ridge?": oceanexplorer.noaa.gov
- Greenaway et al. (2021), Challenger Deep depth (NOAA Institutional Repository): repository.library.noaa.gov/view/noaa/33477
- Bongiovanni, Stewart and Jamieson, Geoscience Data Journal 9(1), 2022, deepest point of each ocean: NERC Open Research Archive
- NOAA National Ocean Service, Corals Tutorial: zooxanthellae, reef formation, distribution, importance; "What is coral bleaching?": coral_bleach; "How does climate change affect coral reefs?": coralreef-climate
- V. Hoon, "Coral Reefs of India: Review of Their Extent, Condition, Research and Management Status", FAO (1997): fao.org
- USGS Pacific Coastal and Marine Science Center, "Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake" (India Plate subducting at the Sunda Trench): usgs.gov
- IPCC AR6 WGI Summary for Policymakers (2021), A.4.2: ipcc.ch
- Friedlingstein et al., Global Carbon Budget 2024, Earth System Science Data 17, 965 (2025): essd.copernicus.org
- PIB (Ministry of Earth Sciences), "Parliament Question: Coastline of the Country", 4 December 2025: pib.gov.in
- PIB (CCEA), "Cabinet approves Deep Ocean Mission", 16 June 2021: pib.gov.in
- All India Radio News, "Fish production in country doubles in last 10 years: Ministry of Fisheries", 15 February 2025: newsonair.gov.in
- Statement by India, International Seabed Authority Council, 10 August 2017: isa.org.jm
- Ministry of Earth Sciences, Draft Blue Economy Policy for India, released for comment 17 February 2021 (MoES document; no link given here because the URL could not be checked)
