ocean

Profile of an ocean floor. The ocean trench is the deepest part of the ocean and the abyssal plains constitute most of the ocean bed.

Great mass of salt water. Geographically speaking three oceans exist – the Atlantic, Indian, and Pacific – to which the Arctic is often added. They cover approximately 70% or 363,000,000 sq km/140,000,000 sq mi of the total surface area of the Earth. According to figures released in August 2001, the total volume of the world's oceans is 1,370 million cubic km/329 million cubic mi. Water levels recorded in the world's oceans have shown an increase of 10–15 cm/4–6 in over the past 100 years.

depth (average) 3,660 m/12,000 ft, but shallow ledges (continental shelves) 180 m/600 ft run out from the continents, beyond which the continental slope reaches down to the abyssal zone, the largest area, ranging from 2,000–6,000 m/6,500–19,500 ft. Only the deep-sea trenches go deeper, the deepest recorded being 11,034 m/36,201 ft (by the Vityaz, USSR) in the Mariana Trench of the western Pacific in 1957

features deep trenches (off eastern and southeastern Asia, and western South America), volcanic belts (in the western Pacific and eastern Indian Ocean), and ocean ridges (in the mid-Atlantic, eastern Pacific, and Indian Ocean).

temperature varies on the surface with latitude (−2°C/28°F to +29°C)/84°F; decreases rapidly to 370 m/1,200 ft, then more slowly to 2,200 m/7,200 ft; and hardly at all beyond that

seawater contains about 3% dissolved salts, the most abundant being sodium chloride; salts come from the weathering of rocks on land; rainwater flowing over rocks, soils, and organic matter on land dissolves small amounts of substances, which pass into rivers to be carried to the sea. Salt concentration in the oceans remains remarkably constant as water is evaporated by the Sun and fresh water added by rivers. Positive ions present in sea water include sodium, magnesium, potassium, and calcium; negative ions include chloride, sulphate, hydrogencarbonate, and bromide

commercial extraction of minerals includes bromine, magnesium, potassium, salt (sodium chloride); those potentially recoverable include aluminium, calcium, copper, gold, manganese, silver.

pollution Oceans have always been used as a dumping area for human waste, but as the quantity of waste increases, and land areas for dumping diminish, the problem is exacerbated. Today ocean pollutants include airborne emissions from land (33% by weight of total marine pollution); oil from both shipping and land-based sources; toxins from industrial, agricultural, and domestic uses; sewage; sediments from mining, forestry, and farming; plastic litter; and radioactive isotopes. Thermal pollution by cooling water from power plants or other industry is also a problem, killing coral and other temperature-sensitive sedentary species.