Chapter 4 · Topic 4.1 · Prelims + Mains1 of 3
Oceanography: Ocean floor, temperature and salinity
Key points
- Shelf, slope, rise, abyssal plain, trenches, ridges, seamounts and guyots; Rann of Kutch (2023).
- Temperature layers and the thermocline; OMT and the monsoon (2020).
- Salinity: 35 ‰ average, subtropical peak, Red Sea (2024), Bay of Bengal low; effects (2017).
Prelims box: facts and traps
| Feature | What it is |
|---|---|
| Continental shelf | Gently sloping edge of the continent under shallow sea, down to about 200 m |
| Continental slope | Steep drop (about 2– 5°) from the shelf edge to about 3,000 m |
| Continental rise and deep-sea plain | Sediment apron at the foot of the slope, then flat abyssal plains at about 3,000–6,000 m |
| Oceanic deeps (trenches) | Long, narrow, very deep troughs at subduction zones |
| Mid-oceanic ridges | Underwater mountain chains where plates pull apart |
| Seamounts and guyots | Isolated undersea volcanoes; a guyot is one with a flat, eroded top. |
- Three layers form: a warm, well-mixed surface layer (up to about 500 m in the tropics); the thermocline, where temperature drops quickly (about 500 to 1,000 m).
- Oceans cover about 71% of the Earth's surface, with an average depth of about 3.7 km.
- Trap: Only 2 (option b): the 26 °C isotherm lies at about 50–100 m, not a fixed 129 m.
- Trap (UPSC Prelims 2024): all the statements are correct.
- Trap (UPSC Prelims 2020, Ocean Mean Temperature (OMT)): “OMT is measured up to a depth of 26°C isotherm which is 129 meters in the south-western Indian Ocean during January – March” is incorrect.
The ocean floor: shelf, slope, deeps and ridges
Oceans cover about 71% of the Earth’s surface, with an average depth of about 3.7 km. If you could drain them, you would see a landscape as varied as the continents: mountain ranges, plains, volcanoes and canyons. Geographers divide the floor into four major zones, going outward from the coast.
| Feature | What it is | Remember |
|---|---|---|
| Continental shelf | Gently sloping edge of the continent under shallow sea, down to about 200 m | Average width about 80 km; very wide off Siberia, almost absent off Chile. Richest fishing grounds, oil and gas (Mumbai High) |
| Continental slope | Steep drop (about 2– 5°) from the shelf edge to about 3,000 m | Cut by submarine canyons |
| Continental rise and deep-sea plain | Sediment apron at the foot of the slope, then flat abyssal plains at about 3,000–6,000 m | The flattest surfaces on Earth, blanketed by fine ooze |
| Oceanic deeps (trenches) | Long, narrow, very deep troughs at subduction zones | Mariana Trench, Challenger Deep: about 11 km, the deepest point on Earth |
| Mid-oceanic ridges | Underwater mountain chains where plates pull apart | Mid-Atlantic Ridge; Iceland is a part above water |
| Seamounts and guyots | Isolated undersea volcanoes; a guyot is one with a flat, eroded top | Many guyots carry coral caps |
Sea level has risen and fallen many times; a fall exposes the shelf, a rise drowns river valleys. Prelims 2023 asked which one of the following is the best example of repeated falls in sea level, giving rise to present-day extensive marshland: the Rann of Kutch (option d), an old arm of the sea that was raised and left as a vast salt marsh, flooded in the monsoon and dry in summer.
Which one of the following is the best example of repeated falls in sea level, giving rise to present-day extensive marshland?
- Bhitarkanika Mangroves
- Marakkanam Salt Pans
- Naupada Swamp
- Rann of Kutch
Show answer
Answer: (d) · UPSC official key
Temperature and salinity of ocean water
Ocean water is heated by the Sun at the surface, so temperature falls with depth. Three layers form: a warm, well-mixed surface layer (up to about 500 m in the tropics); the thermocline, where temperature drops quickly (about 500 to 1,000 m); and a very cold, uniform deep layer close to 0–4 °C. In high latitudes the surface is already cold, so there is hardly any thermocline.
Surface temperature falls from the equator to the poles, but the warmest water is slightly north of the equator, and the northern hemisphere oceans are warmer than the southern, because they are partly enclosed by land. Warm currents raise temperatures (the Gulf Stream warms north-west Europe) and cold currents and upwelling lower them (off Peru and Namibia).
This heat drives the monsoon. Prelims 2020 asked about Ocean Mean Temperature (OMT), the average temperature of the upper ocean down to the 26 °C isotherm. Statement 1 said OMT is measured up to a depth of 26°C isotherm which is 129 meters in the south-western Indian Ocean during January – March; statement 2 said that OMT collected in those months can be used to judge whether the monsoon will be above or below the long-term mean. Only 2 (option b): the 26 °C isotherm lies at about 50–100 m, not a fixed 129 m. OMT predicts the monsoon better than surface temperature alone.
With reference to Ocean Mean Temperature (OMT), which of the following statements is/are correct ? 1. OMT is measured up to a depth of 26°C isotherm which is 129 meters in the south-western Indian Ocean during January – March. 2. OMT collected during January – March can be used in assessing whether the amount of rainfall in monsoon will be less or more than a certain long-term mean. Select the correct answer using the code given below :
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: (b) · UPSC official key
Salinity is the amount of dissolved salt, measured in parts per thousand (‰). The average is about 35 ‰. It is raised by evaporation and freezing (which leaves salt behind), and lowered by rain, river water and melting ice. So it is highest not at the equator, where it rains every afternoon, but in the subtropics (about 20–30°), where the air is dry and evaporation strong.
| Water body | Salinity | Why |
|---|---|---|
| Red Sea (north) | About 41 ‰ | Hot, dry, almost no rain and no rivers |
| Mediterranean, Persian Gulf | About 37–39 ‰ | High evaporation, little inflow |
| Arabian Sea vs Bay of Bengal | Arabian Sea higher | The Bay of Bengal gets the Ganga, Brahmaputra and other rivers |
| Baltic Sea | Very low, under 10 ‰ | Many rivers, cool, little evaporation |
| Lake Van, Dead Sea, Great Salt Lake | About 330, 238, 220 ‰ | Inland lakes with no outlet |
Prelims 2024 asked about the Red Sea: The Red Sea receives very little precipitation in any form, and No water enters the Red Sea from rivers. Both correct (option c), which is why it is among the saltiest seas. Mains 2017 asked: Account for variations in oceanic salinity and discuss its multidimensional effects. Variations: evaporation, rain, river inflow, melting ice, currents
and mixing. Effects: salt makes water denser, so it drives the deep thermohaline circulation; it lowers the freezing point; it affects marine life (corals and mangroves have limits); and it decides where salt is made (Gujarat’s coast) and where desalination is needed.
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 ?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: (c) · UPSC official key
Account for variations in oceanic salinity and discuss its multidimensional effects.
Mains box: analysis and answer points
- Surface temperature falls from the equator to the poles, but the warmest water is slightly north of the equator, and the northern hemisphere oceans are warmer than the southern, because they are partly enclosed by land.
- If you could drain them, you would see a landscape as varied as the continents: mountain ranges, plains, volcanoes and canyons.
- Likely next angle: Deepest points and ridges on the map
- Asked: Mains 2017 (GS Paper I, 15 marks): Account for variations in oceanic salinity and discuss its multidimensional effects.
Practice
Practice MCQs (8)
In the vertical distribution of ocean water, the distinct zone in which salinity increases sharply with depth is called the
Answer (b) & explanation
NCERT: salinity generally increases with depth and there is a distinct zone called the halocline, where salinity increases sharply. The lower-salinity water rests above the higher- salinity, denser water, which leads to stratification by salinity. The thermocline is the comparable zone of rapid fall in temperature.
The surface salinity of the Arabian Sea is generally higher than that of the Bay of Bengal. Which one of the following best explains this?
Answer (c) & explanation
NCERT: the average salinity of the Indian Ocean is about 35 parts per thousand; low salinity is observed in the Bay of Bengal because of the influx of river water, while the Arabian Sea shows higher salinity because of high evaporation and low fresh-water inflow. The Ganga-Brahmaputra and other large rivers, together with heavy monsoon rain, freshen the Bay.
Consider the following statements about the horizontal distribution of the temperature of ocean water:
1. The highest sea-surface temperatures are generally found slightly north of the equator rather than exactly at it.
2. On average, the oceans of the Northern Hemisphere are warmer than those of the Southern Hemisphere.
3. Enclosed seas in the high latitudes record higher temperatures than the open ocean in the same latitudes.
Which of the statements given above is/are correct?
Answer (c) & explanation
Statements 1 and 2 are correct. NCERT notes that the highest temperature is not recorded at the equator but slightly to its north, and that Northern Hemisphere oceans are warmer (about 19 °C against 16 °C on annual average) because of the unequal distribution of land and water: they are in contact with a larger extent of land. Enclosed seas follow their surroundings: in the low latitudes they are warmer than the open sea, but in the high latitudes they are colder.
Consider the following statements: Statement-I: In the open oceans, the highest surface salinity is found in the subtropical latitudes rather than at the equator. Statement-II: Under the clear skies of the subtropical high-pressure belts evaporation greatly exceeds precipitation, whereas heavy equatorial rainfall dilutes the surface water near the equator.
Which one of the following is correct in respect of the above statements?
Answer (a) & explanation
Both statements are correct and Statement-II explains Statement-I. Surface salinity depends mainly on evaporation minus precipitation. In the subtropical highs, descending dry air, clear skies and steady winds drive strong evaporation, so salinity peaks there; NCERT places the Atlantic maximum (about 37 parts per thousand) between 20° N and 30° N. Near the equator the high evaporation is offset by heavy convectional rainfall, so salinity is lower.
Consider the following processes:
1. High evaporation under clear skies
2. Formation of sea ice from sea water
3. Large inflow of river water
4. Heavy convectional rainfall over the sea
How many of the above tend to increase the salinity of surface sea water?
Answer (b) & explanation
Only two, processes 1 and 2. Evaporation removes water but leaves the salt behind. When sea ice forms, most of the salt is excluded from the ice crystals and the rejected brine makes the surrounding water saltier and denser; this is an important step in the sinking of polar water. River inflow and rainfall add fresh water and lower salinity, as in the Bay of Bengal.
Consider the following pairs: Relief feature of the ocean floor — Example or description
1. Seamount — Emperor seamount chain, an extension of the Hawaiian Islands
2. Submarine canyon — Hudson Canyon
3. Mid-oceanic ridge rising above sea level — Iceland
4. Guyot — A volcanic mountain with a pointed summit that does not reach the sea surface
How many of the pairs given above are correctly matched?
Answer (c) & explanation
Three pairs are correct. NCERT gives the Emperor seamount, an extension of the Hawaiian Islands, as an example of a seamount, the Hudson Canyon as the best-known submarine canyon, and Iceland as part of the Mid-Atlantic Ridge that rises above the ocean surface. Pair 4 describes a seamount, not a guyot: a guyot is flat-topped, its summit planed off before it subsided.
Consider the following statements about the density of sea water:
1. Other things being equal, sea water becomes denser as its temperature falls and as its salinity rises.
2. Like fresh water, sea water of normal salinity is densest at about 4 °C.
3. Sea water of normal salinity, about 35 parts per thousand, freezes at about -1.9 °C.
Which of the statements given above is/are correct?
Answer (b) & explanation
Statements 1 and 3 are correct. Density of sea water rises with falling temperature and rising salinity (and with pressure). Fresh water is densest at about 4 °C, but dissolved salt lowers both the freezing point and the temperature of maximum density. Above a salinity of about 24.7 parts per thousand the temperature of maximum density falls below the freezing point, so normal sea water keeps getting denser as it cools until it freezes at about -1.9 °C. This is why cold polar water sinks.
Consider the following statements about the vertical structure of temperature in the oceans:
1. In the tropics the warm surface layer is present throughout the year, but in the middle latitudes it develops only during summer.
2. In the Arctic and Antarctic regions the change of temperature with depth is very slight, and a single layer of cold water extends from the surface to the ocean floor.
3. About 90 per cent of the total volume of ocean water lies above the thermocline.
Which of the statements given above is/are correct?
Answer (a) & explanation
Statements 1 and 2 are correct. Over low and middle latitudes NCERT describes a three-layer system: a warm surface layer, the thermocline in which temperature falls rapidly with depth, and a very cold deep layer reaching the floor. The warm top layer is permanent in the tropics but only seasonal (summer) in mid-latitudes. Near the poles the surface is already close to 0 °C, so there is essentially one cold layer. About 90 per cent of ocean water lies below the thermocline, where temperatures approach 0 °C.
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In 150 words, explain the significance of Oceanography with one recent example.
Flashcards (7)
Depth of the continental shelf edge?
About 200 m.
Deepest point on Earth?
Challenger Deep, Mariana Trench (about 11 km).
A flat-topped seamount is called?
A guyot.
Example of extensive marshland from falls
in sea level? Rann of Kutch.
Average salinity of ocean water?
About 35 ‰.
Why is salinity highest in the subtropics,
not the equator? High evaporation and little rain.
Saltier: Arabian Sea or Bay of Bengal?
Arabian Sea.
Read more: related explainers
- Ocean Currents: Causes, Warm and Cold Currents of Each Ocean, Gyres, Thermohaline Circulation and Effects on Climate
- Plate Tectonics: Continental Drift, Sea-Floor Spreading, Plate Boundaries, Driving Forces and the Indian Plate
- El Niño, La Niña and ENSO: Walker Circulation, Southern Oscillation, Indian Ocean Dipole and Impact on India
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Next topic · 4.2 →Oceanography: Currents, waves and tidesSyllabus: GS-I · Salient features of world’s physical geography.