Water (Oceans) — Class 11 Geography

Water is a cyclic resource; about 91 per cent of the planet's water is in the oceans, which is why the earth is called the "Blue Planet". The hydrological cycle is the continuous circulation of water among the oceans, atmosphere, land and organisms through evaporation, transpiration, condensation and precipitation. The ocean floor is divided into four major divisions: the continental shelf (the shallow extended margin, average width about 80 km, a source of fossil fuels), the continental slope (which marks the end of the continents), the deep-sea plain (the flattest, smoothest region) and the oceanic deeps or trenches (the deepest parts). Minor relief features include mid-oceanic ridges, seamounts, submarine canyons, guyots (flat-topped seamounts) and atolls. Ocean temperature decreases from the equator to the poles and with depth; the thermocline is the boundary zone of rapid temperature decrease, and the temperature structure has three layers. Salinity, the total dissolved salts (expressed as parts per thousand), averages about 35‰ in the open ocean and is affected by evaporation, precipitation, river inflow, freezing and currents; it changes with depth at the halocline, where salinity increases sharply.