Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity

1. The blue planet: Earth's water and the hydrological cycle

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• The Earth is the "Blue Planet". Water is rare in the solar system, and almost three-fourths of Earth's surface is water (Class 6).
• Hydrological cycle (water cycle): water keeps moving as a liquid, a solid (ice) and a gas (vapour). It moves between the oceans, the air, the land surface, the ground and living things. It has run for billions of years. The total amount of renewable water stays the same, but demand keeps growing.
• Class 11 Table 12.1 pairs each store of water with its processes:
• Oceans: evaporation, evapotranspiration, sublimation (ice turning straight into vapour)
• Atmosphere: condensation, precipitation
• Ice and snow: snowmelt runoff
• Surface runoff: streamflow, freshwater storage, infiltration
• Groundwater: discharge, springs

• Trap: "hydration" is NOT part of the cycle.

• Distribution of the Earth's water: about 97% is in the oceans (NCERT: 91%; USGS: 96.5%). Only about 2.5% is fresh. Of this fresh water, about 69% is locked in glaciers and ice caps and about 30% is groundwater. Only about 1% is in lakes, rivers, soil, air and living things.

• Evapotranspiration: water lost from soil and water surfaces (evaporation) plus water given off by plants (transpiration). It decides how much water crops need.
• Infiltration: water soaking into the soil and refilling groundwater. It depends on the soil, slope, plant cover and how hard the rain falls. Paved ground cuts infiltration and makes floods worse.
• Surface runoff: the share of rain or snowmelt that flows over land into streams. It is high on steep, bare, paved or already soaked ground.
• Why a water planet faces a water crisis: fresh water is fixed, demand is rising and pollution shrinks the usable supply. Oceans supply most of the moisture in the air (about 86% of global evaporation, verify). NCERT's "59%" sentence is muddled, so skip it.

2. Oceans and continents: five oceans and the unequal hemispheres

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• About 71% of Earth's surface is water and 29% is land (Class 6).
• Distribution of land and water: the two halves of the globe are not equal.
• The Northern Hemisphere is the "land hemisphere". It holds about two-thirds of all land, and its centre lies near western France.
• The Southern Hemisphere is about 81% ocean. The "water hemisphere" is centred near New Zealand.

• Effects: northern oceans are warmer and have bigger temperature ranges. The open southern seas have strong, steady westerly winds (the Roaring Forties).

• Five oceans, largest to smallest: 1. Pacific – largest and deepest; about one-third of Earth's surface, bigger than all the land put together 2. Atlantic 3. Indian 4. Southern 5. Arctic – smallest

• The oceans merge into one another, so their boundaries are only conventions. Seas, bays and gulfs are parts of oceans. (Class 11 slips and says "four large oceans" right after naming five.)

• Southern Ocean: the ocean around Antarctica, south of about 60°S. It is defined by the eastward-flowing Antarctic Circumpolar Current. The IHO's 2000 draft that recognised it was never formally ratified. National Geographic recognised it in 2021.
• Indian Ocean:
• Third largest, and the only ocean named after a country.
• It is bounded by Asia (north), Africa (west), Australia (east) and the Southern Ocean (south).
• The Arabian Sea lies west of India and the Bay of Bengal east.
• Land closes it off to the north. This gives it the monsoon and currents that reverse with the seasons.

• It carries about two-thirds of the world's oil shipments and half of its container traffic, through chokepoints such as Hormuz, Bab-el-Mandeb and Malacca.

• India has more than 1,300 islands. The two main groups are the Andaman and Nicobar Islands (Bay of Bengal) and Lakshadweep (Arabian Sea).

3. Relief of the ocean floor: shelf, slope, deep-sea plain and trenches

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• Relief of the ocean floor: most of the floor lies 3–6 km deep. It is as rugged as land, with the biggest mountain ranges, the deepest trenches and the largest plains. Tectonic, volcanic and depositional (sediment-laying) processes shape it. It has four major divisions.

Division	Depth	Gradient	Key facts
Continental shelf	30–600 m (usually under 200 m)	1° or less	Average width 80 km; widest is the Siberian shelf (about 1,500 km); almost absent off Chile and west Sumatra; ends at the shelf break
Continental slope	200–3,000 m	2–5°	Marks the true edge of the continent; cut by canyons
Abyssal (deep-sea) plain	3,000–6,000 m	Nearly flat	Flattest, smoothest areas on Earth; covered in fine clay and silt
Oceanic deeps (trenches)	3–5 km below the nearby floor	Steep-sided	Deepest parts of the ocean

• Continental shelf: the drowned edge of a continent. Sediments from rivers, glaciers and wind pile up here and form fossil fuels; for example, Bombay High lies on India's wide western shelf. Shelves also hold the richest fisheries.

• Under UNCLOS Art. 76, a country's legal shelf reaches 200 nautical miles, or up to 350 nautical miles if it proves a wider physical margin. India's 2009 claim for an extended shelf is still pending (verify current).

• Continental margin: the zone between the shore and the deep ocean. It is made of the shelf, the slope and the continental rise (a wedge of sediment at the foot of the slope).

• Abyssal plain: polymetallic nodules (lumps rich in manganese, nickel, cobalt and copper) lie on these plains. India became the first "pioneer investor" in 1987. It holds an ISA contract for about 75,000 sq km in the Central Indian Ocean Basin.
• Oceanic trench: forms where one plate sinks under another (subduction). Trenches lie at the base of continental slopes and along island arcs, with volcanoes and strong earthquakes. This makes them key evidence of plate movement.

• NCERT count: 57 deeps explored, 32 in the Pacific, 19 in the Atlantic and 6 in the Indian Ocean.

• Deepest trench in each ocean:

• Mariana Trench (western Pacific, east of the Mariana Islands): its Challenger Deep is about 10,935 m deep. Here the Pacific Plate sinks under the Philippine Sea Plate.
• Puerto Rico Trench (Atlantic, north of Puerto Rico): its Milwaukee Deep is about 8,400 m deep. It lies on the Caribbean–North American plate boundary.
• Sunda (Java) Trench (Indian Ocean, off Sumatra and Java): about 7,200 m deep. Here the Indo-Australian Plate sinks, and it was the source of the 2004 tsunami.

4. Minor relief features: ridges, seamounts, guyots, canyons, vents and coral islands

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• Mid-oceanic ridge: two chains of mountains with a large depression (a rift valley) between them. Peaks reach up to 2,500 m, and some rise above the sea, such as Iceland on the Mid-Atlantic Ridge.
• The global ridge system is about 65,000 km long, the longest mountain range on Earth.

• Indian Ocean ridges: Carlsberg, Central Indian, South-west Indian and South-east Indian Ridges. The Ninety East Ridge and the Chagos-Laccadive Ridge are aseismic (they have no earthquakes).

• Hydrothermal vent: an opening along a ridge where water heated by magma gushes out full of minerals ("black smokers"). Vents were discovered in 1977 on the Galápagos Rift.

• They support life based on chemosynthesis (making food from chemicals instead of sunlight).

• They form polymetallic sulphide deposits. India holds an ISA sulphide contract on the Indian Ocean ridges and has applied for more on the Carlsberg Ridge (verify current).

• Seamount: a volcanic underwater mountain with a pointed top, 3,000–4,500 m tall, that does not reach the surface.

• Emperor Seamount Chain: runs north-west from Hawaii. It formed as the Pacific Plate moved over the Hawaii hotspot over about 80 million years. The Hawaii-Emperor bend records a change in the plate's direction.
• Guyot: a flat-topped seamount. Waves planed its top flat, and then it sank. The Pacific alone has an estimated 10,000+ seamounts and guyots.
• Submarine canyon: a deep valley cut across the shelf and slope, often off a big river mouth. The best known is the Hudson Canyon off New York. In India, the "Swatch of No Ground" off the Ganga-Brahmaputra delta feeds the Bengal Fan, the world's largest underwater fan of sediment.
• Coral reef: a limestone ridge built by colonies of tiny polyps that live with algae.
• Corals need warm (about 20–29°C), clear, shallow, sunlit, salty seas, mostly between 30°N and 30°S.

• Reefs are rare on west coasts, where cold currents chill the water.

• Three reef types:

• Fringing reef: attached to the coast (Andamans, Gulf of Mannar)
• Barrier reef: separated from the coast by a lagoon (Great Barrier Reef)

• Atoll: a ring-shaped reef around a central lagoon. The lagoon may hold seawater, fresh, brackish or very salty water.

• Darwin (1842): reefs grow upward as a volcanic island sinks, so fringing reef → barrier reef → atoll. Drilling at Enewetak in 1952 confirmed this.

• Coral island: an island built from reef limestone. All of Lakshadweep is coral, and the Maldives are a chain of atolls. The Andamans are NOT coral islands; they only have fringing reefs.
• India's reefs: Gulf of Mannar, Gulf of Kachchh, Palk Bay, Lakshadweep, and the Andaman and Nicobar Islands (plus patches at Malvan).

5. Temperature of ocean waters: factors and horizontal distribution

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• Oceans heat up and cool down more slowly than land. That is why coastal climates are mild.
• Factors affecting ocean temperature:
• Latitude: sunlight weakens towards the poles, so surface water gets colder.
• Unequal land and water: northern oceans touch more land and are warmer.
• Prevailing winds:
  • Offshore winds (blowing from land to sea) push warm surface water away. Cold water then rises from below (upwelling), as off Peru and Namibia.
  • Onshore winds (blowing from sea to land) pile warm water against the coast.
• Ocean currents: the warm Gulf Stream warms eastern North America and western Europe. The cold Labrador Current chills north-east North America.

• Enclosed seas: in low latitudes (Red Sea, Persian Gulf) they are warmer than open seas. In high latitudes (Baltic) they are colder.

• Distribution of ocean surface temperature (Class 11, Fig. 12.4):

• The average is about 27°C near the equator. It falls by about 0.5°C for every degree of latitude: about 22°C at 20°, 14°C at 40° and 0°C near the poles.
• The hottest water lies slightly north of the equator (the thermal equator).
• Northern oceans average about 19°C and southern oceans about 16°C.

• Isotherms (lines of equal temperature) bend towards the poles along warm currents and towards the equator along cold ones.

• Sea surface temperature (SST): the temperature of the ocean's top layer.

• Tropical cyclones need SST above about 26.5°C. So the warmer Bay of Bengal produces more cyclones than the Arabian Sea, although the Arabian Sea is warming and now gives more severe storms.
• SST patterns drive ENSO (El Niño and La Niña in the Pacific) and the IOD (Indian Ocean Dipole), which shape India's monsoon.
• INCOIS (Hyderabad) and Oceansat-3 (EOS-06, 2022) track SST and issue Potential Fishing Zone advisories.

6. Vertical temperature: the thermocline, three layers and density stratification

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• The sun heats the sea from above, and heat moves down slowly. NCERT says this happens "by convection"; it really happens by mixing.
• So the surface is the warmest layer. Temperature falls fast down to about 200 m, then more slowly.
• Thermocline: the layer where temperature drops sharply with depth.
• It starts about 100–400 m below the surface and is several hundred metres thick.
• About 90% of ocean water lies below it, near 0°C (in fact, deep water is about 2–4°C).

• Seawater freezes at about −1.9°C, not at 0°C.

• Three-layer temperature structure of oceans (low and middle latitudes): 1. Warm surface layer: about 500 m thick, at 20–25°C. It lasts all year in the tropics but forms only in summer in mid-latitudes. 2. Thermocline layer: 500–1,000 m thick, where temperature falls rapidly. 3. Cold deep layer: reaches down to the ocean floor.

• Polar seas have only one cold layer from the surface to the floor.

• NCERT gives two depths for the thermocline that do not match. In reality, the depth of the surface layer and the thermocline changes with latitude and season.
• Density stratification: water forms layers by density (how heavy it is for its volume).
• Saltier water is denser, and warmer water is lighter. So cold, salty water sinks below warm, fresh water.

• These layers resist mixing. The layer of sharp density change between them is the pycnocline.

• Effects of stratification:

• Little mixing starves tropical surface water of nutrients, so these seas are "blue deserts".
• Upwelling brings nutrients up and feeds plankton.
• Global warming strengthens the layering, which cuts mixing and oxygen.
• Dense water sinking near the poles drives the thermohaline circulation (covered in ocean-movements).

7. Salinity: meaning, factors and the extremes

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• Salinity: the total amount of salt dissolved in seawater.
• It is measured as grams of salt in 1,000 g (1 kg) of seawater, written as parts per thousand (‰ or ppt). Scientists now use the practical salinity scale (1978), which has no unit.
• The open ocean ranges from 33 to 37‰ and averages about 35‰.

• Salinity and temperature together control density.

• Where the salt comes from: sodium chloride makes up about 78% of the dissolved salt, followed by magnesium chloride and sulphates. The salt came from weathered rocks and volcanic gases over geological time.

• Brackish water: saltier than fresh water but less salty than the sea. NCERT takes 24.7‰ as the upper limit. It is found in estuaries, lagoons (Chilika, Pulicat), mangrove creeks and the Baltic Sea.
• Factors affecting salinity:
• Evaporation raises it; rain lowers it.
• River water lowers it along coasts.
• In polar seas, freezing raises it and melting ice lowers it.
• Winds and currents move saltier or fresher water from place to place.

• Salinity, temperature and density are linked.

• The saltiest water bodies (NCERT box): Lake Van, Turkey (330‰) > Dead Sea (238‰) > Great Salt Lake (220‰).

• Dead Sea: a landlocked salt lake between Jordan and Israel (and the West Bank). It is the lowest land on Earth, more than 430 m below sea level. Its level is falling because the Jordan River's water is diverted. People float easily in it.
• Caution: modern data show Lake Van is a soda lake with much lower salinity, and the Dead Sea is often given as about 340‰. Use NCERT's figures for NCERT-based questions (verify).
• In hot, dry regions, very salty lagoons can reach about 70‰.

8. Where the sea is salty: horizontal and vertical distribution of salinity

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• Horizontal distribution of salinity:
• Salinity is highest in the subtropics, where evaporation is high and rain is low.
• It is lower at the rainy equator and lowest near the poles and at river mouths.

• Isohalines are lines on a map joining places of equal salinity.

• By ocean:

• Atlantic: average 36‰, the saltiest ocean. The peak is 37‰ between 20°N–30°N and 20°W–60°W.
• Indian Ocean: average 35‰.

• Pacific: falls from 35 to 31‰ in the western North Pacific (Arctic meltwater) and to 33‰ beyond 15–20°S.

• Red Sea salinity: up to 41‰. The sea is landlocked, evaporation is intense, almost no rivers flow in and it gets little rain.

• Estuaries and the Arctic swing between 0 and 35‰ with the seasons.

Sea	Salinity	Reason
North Sea	High for its latitude	The North Atlantic Drift brings in salty water
Baltic Sea	Very low (under 10‰; 2–5‰ in the Gulf of Bothnia)	Large river inflow, low evaporation, narrow outlet through the Danish straits
Mediterranean	About 38–39‰	High evaporation; dense salty water flows out at depth through the Strait of Gibraltar while Atlantic water flows in at the surface
Black Sea	Very low (about 17–18‰)	Rivers pour in: the Danube, Dnieper and Dniester, and the Don via the Sea of Azov

• Bay of Bengal and Arabian Sea salinity:
• The Bay of Bengal is fresher. The Ganga, Brahmaputra, Irrawaddy, Mahanadi, Godavari and Krishna pour in fresh water, and rain is heavy.
• The Arabian Sea is saltier. Evaporation is high, few rivers flow in, and salty water flows in from the Red Sea and Persian Gulf.

• The Bay's fresh top layer stops mixing, which keeps its SST high and helps cyclones form.

• Vertical distribution:

• At the surface, evaporation and ice formation raise salinity; fresh water lowers it.

• At depth, salinity hardly changes, because no water is lost and no salt is added.

• Halocline: a layer where salinity rises sharply with depth. Lighter, fresher water rests above denser salty water, so the water column forms layers and mixing is limited.

9. Oceans and life, oceans under stress, and the High Seas Treaty

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• Oceans and life (Class 6):
• Oceans send rain to the continents. Without them, Earth would be a desert.
• Marine plants (phytoplankton, algae and seaweeds) make more than half the world's oxygen, so oceans are "the planet's lungs".

• Oceans gave people migration routes, trade, food and coastal cultures. The Indian Navy's motto, Sam no Varunah, calls on Varuna, the Vedic god of the waters.

• World Oceans Day is 8 June. NCERT names two threats: millions of tonnes of plastic every year, and overfishing.

• Ocean acidification: the ocean absorbs CO₂ from the air, so its water becomes more acidic.
• Surface pH has fallen from about 8.2 to about 8.1 since industrialisation, which means about 30% more acidity.
• It harms corals and shell-builders such as pteropods and oysters.

• The planetary boundary for acidification is reported to have been crossed (verify current).

• Ocean deoxygenation: the ocean is losing oxygen.

• Warmer water holds less oxygen, and fertiliser runoff feeds algal blooms that use up oxygen.

• The ocean has lost about 2% of its oxygen since 1960. The Arabian Sea oxygen minimum zone is one of the largest and is spreading.

• Marine heatwave: SST stays above the 90th percentile (warmer than 90% of past readings) for at least five days (Hobday et al., 2016).

• Heatwaves bring coral bleaching, fish deaths and stronger cyclones, and they are becoming more frequent.

• The tropical Indian Ocean is warming fastest among tropical oceans.

• Microplastics: plastic bits smaller than 5 mm, from broken waste, clothing fibres and cosmetics. They are found from the Mariana Trench to Arctic ice and enter food chains. Talks on a global plastics treaty have stalled (verify current).

• Great Pacific Garbage Patch: floating plastic between Hawaii and California, gathered by the North Pacific Gyre (a huge ring of currents).
• Atlantic Meridional Overturning Circulation (AMOC): the Atlantic part of the thermohaline circulation.
• It carries warm surface water north (Gulf Stream, North Atlantic Drift) and sends cold deep water south. This keeps north-west Europe mild.
• Greenland's meltwater may be slowing it. IPCC AR6 says it will very likely weaken this century, but a collapse before 2100 is unlikely (medium confidence). Some studies warn it could happen sooner (verify current).

• A collapse would cool Europe and shift the tropical rain belts, possibly disturbing the monsoon.

• BBNJ Agreement ("High Seas Treaty"): adopted in June 2023 under UNCLOS, and in force since 17 January 2026. It covers the roughly two-thirds of the ocean that lies beyond any country's control. It provides for:

• marine protected areas on the high seas (needed for the 30x30 target)
• environmental impact assessments
• sharing benefits from marine genetic resources

• capacity-building for poorer countries

• India signed the treaty in 2024 (ratification: verify current).

• Related efforts: the ISA mining-code debate; the UN Decade of Ocean Science (2021–2030); SDG 14; and India's Deep Ocean Mission (2021), which includes the Samudrayaan crewed submersible Matsya-6000 (verify current).

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Exam angles

Prelims — high-yield facts and traps

• Ocean-floor divisions:
• Shelf: under about 200 m, 1° or less; widest is the Siberian shelf (1,500 km)
• Slope: 200–3,000 m, 2–5°
• Abyssal plain: 3,000–6,000 m, the flattest surface on Earth

• Trenches: the deepest parts, found at subduction zones

• Deepest trench in each ocean:

• Mariana (Pacific): Challenger Deep, about 10,935 m
• Puerto Rico (Atlantic): Milwaukee Deep, about 8,400 m

• Sunda/Java (Indian Ocean): about 7,200 m

• NCERT count of deeps: 57 = 32 Pacific + 19 Atlantic + 6 Indian.

• Minor features matched:
• Seamount: pointed and volcanic
• Guyot: flat-topped
• Atoll: ring-shaped reef with a lagoon
• Submarine canyon: Hudson Canyon
• Mid-ocean ridge: Iceland rises above the sea

• Trap: "oceanic deep is a minor feature" is FALSE; it is a major division.

• Temperature:

• Falls about 0.5°C per degree of latitude.
• The hottest water is slightly north of the equator.
• Northern oceans (19°C) are warmer than southern ones (16°C).

• About 90% of the water lies below the thermocline; polar seas have one layer.

• Salinity:

• Measured in grams per kg (‰); the open ocean is 33–37‰, averaging 35‰.
• The brackish-water limit is 24.7‰.
• Atlantic (36‰) > Indian (35‰); the subtropics are saltier than the equator.
• Red Sea up to 41‰; the Baltic and Black Seas are fresh; the Mediterranean and North Sea are salty.
• The Bay of Bengal is fresher than the Arabian Sea.

• NCERT order of the saltiest water bodies: Lake Van > Dead Sea > Great Salt Lake.

• Statement traps:

• "Oceans hold 91% of Earth's water" – FALSE now (about 97%).
• "The Southern Hemisphere is about 81% water" – TRUE.
• "The Arctic is the smallest ocean" – TRUE.
• "The Andamans are coral islands" – FALSE; Lakshadweep is coral.

• "Hydration is part of the water cycle" – FALSE.

• Current facts:

• Acidification: pH has fallen from 8.2 to 8.1.
• The BBNJ Agreement entered into force in January 2026.
• The Great Pacific Garbage Patch lies in the North Pacific Gyre.
• India was the first pioneer investor for nodules in the Central Indian Ocean Basin (1987).

Mains — GS-I themes

1. Explain the factors that control the spread of temperature and salinity in ocean water, with examples.
2. Why do the Bay of Bengal and the Arabian Sea differ in salinity? What does this mean for cyclones and the monsoon?
3. The ocean floor as evidence for plate tectonics: trenches, ridges and seamount chains such as Hawaii-Emperor.
4. Ocean warming, acidification and loss of oxygen as threats to India's fisheries and coastal livelihoods.
5. Deep-sea minerals and the High Seas Treaty: resource opportunity versus harm to nature; India's Deep Ocean Mission.
6. The hydrological cycle and India's water stress: why a "water planet" faces water crises.

Current-affairs hooks

• The BBNJ Agreement's first COP and the first high-seas marine protected areas (verify current).
• The ISA mining code, and India's applications for Indian Ocean ridges and seamounts (verify current).
• Samudrayaan, Matsya-6000 and Deep Ocean Mission milestones (verify current).
• Marine heatwaves, record SSTs, and coral bleaching in Lakshadweep and the Gulf of Mannar.
• Studies on AMOC slowdown and IPCC assessments (verify current).
• Global plastics treaty negotiations (verify current).