Chapter 12 — Water (Oceans)

The hydrological cycle is the circulation of water within the earth's hydrosphere in liquid, solid, and gaseous phases. It refers to the continuous exchange of water between the oceans, atmosphere, land surface, subsurface, and organisms.

About 91 per cent of the planetary water is found in the oceans. The remaining water is held as freshwater in glaciers, icecaps, groundwater, lakes, soil moisture, atmosphere, and streams.

The four major divisions are the continental shelf, the continental slope, the deep sea plain, and the oceanic deeps (trenches). Minor features such as mid-oceanic ridges, seamounts, guyots, submarine canyons, and atolls are also present.

The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean with an average gradient of 1° or less and an average width of about 80 km, ending at a steep shelf break.

The thermocline is a boundary region in the ocean, usually beginning around 100–400 m below the surface, where temperature decreases rapidly with increasing depth. It separates the warm surface layer from the cold deep ocean; about 90 per cent of ocean water lies below it.

Salinity is the total content of dissolved salts in seawater. It is calculated as the amount of salt in grams dissolved in 1,000 gm (1 kg) of seawater and is expressed as parts per thousand (o/oo or ppt).

The salinity for normal open ocean ranges between 33 o/oo and 37 o/oo. The land-locked Red Sea has salinity as high as 41 o/oo, and in hot dry regions where evaporation is very high, salinity can sometimes reach 70 o/oo.

As many as 57 deeps have been explored so far, of which 32 are in the Pacific Ocean, 19 in the Atlantic Ocean, and 6 in the Indian Ocean. Trenches are 3–5 km deeper than the surrounding ocean floor and are associated with active volcanoes and strong earthquakes.

A guyot is a flat-topped seamount that shows evidence of gradual subsidence through stages. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

The four main factors are latitude (insolation decreases poleward), unequal distribution of land and water (northern hemisphere oceans receive more heat), prevailing winds (offshore winds cause cold-water upwelling; onshore winds pile up warm water), and ocean currents (warm currents raise temperature in cold areas; cold currents lower temperature in warm areas).

The halocline is a distinct zone in the vertical profile of ocean water where salinity increases sharply with depth. High-salinity dense water generally sinks below lower-salinity water, leading to stratification by salinity.

Yes, it is free to download with no sign-up.