At a glance
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Book DigestGeographyNCERTOceanographyCurrentsTidesSalinity
Oceans cover about 71 percent of the Earth's surface and hold about 97 percent of its water. They moderate climate, carry currents that warm or cool coasts, and provide some of the richest fishing grounds on the continental shelves.
| Feature |
Description |
| Continental shelf |
The gently sloping, shallow extension of the continent; richest fishing grounds (sunlight, plankton) and the site of most offshore oil. |
| Continental slope |
A steeper drop marking the true edge of the continent. |
| Deep-sea (abyssal) plain |
The vast flat floor of the deep ocean. |
| Mid-oceanic ridges |
Submarine mountain chains where new crust forms (sea-floor spreading), see geomorphology. |
| Ocean trenches (deeps) |
The deepest parts, at subduction zones; the Mariana Trench (Challenger Deep) in the Pacific is the deepest point on Earth (about 11 km). |
| Minor relief |
Seamounts, guyots, atolls, banks and submarine canyons. |
- Temperature falls with depth (with a sharp thermocline in between) and from the equator to the poles; the surface is warmest in the tropics.
- Salinity is the total dissolved salts, averaging about 35 parts per thousand. It is highest in enclosed warm seas with high evaporation and little freshwater inflow: the Dead Sea and the Lake Van (Turkey) are extreme; the Red Sea is among the most saline open seas. Salinity is lower where rivers add fresh water or where rainfall and melting ice dilute the sea.
Currents are large-scale movements of surface (and deep) water, driven mainly by prevailing winds, the Coriolis force, differences in temperature and salinity (density), and shaped by coastlines.
- Warm currents flow from the equator towards the poles along the eastern coasts of continents in low and middle latitudes; cold currents flow from polar regions towards the equator along the western coasts.
- Key currents for CAPF:
- Gulf Stream and the North Atlantic Drift (warm), which keep the ports of Western Europe ice-free in winter.
- Labrador Current (cold), meeting the Gulf Stream off Newfoundland to create the rich Grand Banks fishery (and fog).
- Kuroshio (warm) and Oyashio (cold) in the north Pacific.
- Humboldt (Peru) Current (cold), supporting the rich Peruvian anchovy fishery, whose collapse during El Nino events is a classic ocean-atmosphere link.
- The Benguela and Canary (cold) along Africa's west coast.
- Where warm and cold currents meet, mixing of waters and nutrients creates the world's great fishing grounds (the Grand Banks, the North Sea, the waters off Japan).
- Waves are the surface oscillation produced by wind energy; the water particles move in circles while the energy moves forward.
- Tides are the periodic rise and fall of sea level caused by the gravitational pull of the Moon and the Sun and the Earth's rotation.
- Spring tides (highest range) occur at new and full moon, when the Sun, Earth and Moon are in line and the pulls add up.
- Neap tides (lowest range) occur at the first and third quarters, when the Sun and Moon are at right angles and partly cancel out.
- Tides aid navigation (entry to ports), fishing, and tidal-energy generation; bores form in funnel-shaped estuaries.
The oceans are central to the hydrological cycle, the carbon cycle and global biodiversity. Coral reefs, mangroves and continental-shelf ecosystems are biodiversity hotspots, and warming and acidification threaten them (a link to the climate-change theme in climatology).
- The deepest ocean trench (Mariana). The current that warms Western Europe (Gulf Stream / North Atlantic Drift). Average ocean salinity (about 35 ppt).
- Spring versus neap tides (new and full moon versus quarters). Why western coasts have cold currents.
- Match the current (warm or cold) to the coast and to the fishing ground it creates.
- Spring tides occur during: (a) the first quarter of the Moon (b) new and full moon (c) the third quarter (d) a solar eclipse only. Answer: (b) new and full moon. Authored practice, not a verbatim PYQ.
- The deepest known point in the oceans lies in which trench? (Answer: the Mariana Trench, Pacific Ocean.) Authored practice, not a verbatim PYQ.