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Class 11 Water (Oceans): cheat sheet
CBSE Social Science. The water cycle, the relief of the ocean floor, and how the temperature and salinity of sea water vary.
Key ideas
- The hydrological cycle moves water between oceans, air, land, the ground and living things; the oceans hold about 91% of the earth’s water.
- The ocean floor has four main divisions: continental shelf, continental slope, deep sea plain and oceanic deeps (trenches).
- Minor features include mid-oceanic ridges, seamounts, flat-topped guyots, submarine canyons and coral atolls.
- Ocean temperature falls from the equator to the poles and with depth: a warm top layer, a thermocline where it drops fast, and cold deep water.
- Salinity is the salt dissolved in sea water; evaporation raises it, while rain, rivers and melting ice lower it.
- The Arabian Sea is saltier than the Bay of Bengal, which gets much river water; the Red Sea is very salty.
Remember
| Salinity | grams of salt in 1,000 g (1 kg) of water, in parts per thousand (‰) |
| Salt in a sample | salinity × kg of water = grams of salt |
| Salinity from a sample | grams of salt ÷ kg of water |
| Brackish water | below 24.7‰ |
| Typical salinity | Indian Ocean 35‰, Atlantic 36‰, Red Sea 41‰ |
| Surface temperature (book’s rule) | 27 °C at the equator, 0.5 °C less per degree of latitude: 27 − latitude × 0.5 |
| Ocean floor | shelf slopes 1° or less (average width 80 km; widest the Siberian shelf); slope 2°–5°; deep plains 3,000–6,000 m |
Watch out
- Mixing up the thermocline (temperature) and the halocline (salinity).
- Thinking more rivers make a sea saltier: fresh water lowers salinity.
- Calling a guyot pointed: it is a flat-topped seamount.
Worked example
Using 27 °C at the equator and a fall of 0.5 °C per degree of latitude, find the surface temperature at (a) 40° (b) 30°. Why is the highest temperature found slightly north of the equator?
Working: 27 − 40 × 0.5; 27 − 30 × 0.5
Answer: (a) 7 °C (b) 12 °C; the northern hemisphere has more land, which heats its oceans more.
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