Tuesday, August 16, 2016
Oceans Of The World Big Oceans Of The World
Oceans Of The World Big Oceans Of The World
Oceans,
Ocean and Oceanography, great body of salt water comprising all the oceans and seas that cover nearly three-fourths of the surface of the earth, and the scientific study of the physical, chemical, and biological aspects of the so-called world ocean. The major goals of oceanography are to understand the geologic and geochemical processes involved in the evolution and alteration of the ocean and its basin, to evaluate the interaction of the ocean and the atmosphere so that greater knowledge of climatic variations can be attained, and to describe how the biological productivity in the sea is controlled.
OCEAN BASIN STRUCTURE
The world ocean covers 71 percent of the earth’s surface, or about 361 million sq km (140 million sq mi). Its average depth is 5,000 m (16,000 ft), and its total volume is about 1,347,000,000 cu km (322,300,000 cu mi). The three major subdivisions of the world ocean are the Atlantic Ocean, the Pacific Ocean, and the Indian Ocean, which are conventionally bounded by the continental masses (see Continent) or by ocean ridges or currents; they merge below 40° South latitude in the Antarctic Circumpolar Current, or West Wind Drift, where they are then often referred to as the Southern Ocean. In the north polar region the nearly circular Arctic Ocean, almost landlocked except between Greenland and Europe, is considered by some part of the Atlantic and by others a fourth ocean subdivision. From the shorelines of the continents a submerged part of the continental mass, called the continental shelf, extends sea ward an average distance of 75 km (43 mi); it varies in width from nearly zero to 1,500 km (930 mi). The shelf gives way abruptly at a depth of about 200 m (660 ft) to a steeper zone known as the continental slope, which descends about 3,500 m (12,000 ft). The continental rise, a gradually sloping zone of sediment that is considered part of the ocean bottom, extends about 600 km (370 mi) from the base of the continental slope to the flat abyssal plains of the deep-ocean floor. In the central parts of the oceans are the midocean ridges, which are extensive mountain chains with inner troughs that are heavily intersected by cracks, called fracture zones. The ridges are sections of a continuous system that winds for 60,000 km (40,000 mi) through all the oceans. The Mid-Atlantic Ridge extends from the Norwegian Sea through the volcanic islands of Iceland and the Azores to the South Atlantic, where it is equidistant from the African and South American coasts. The ridge continues into the Indian Ocean, with a branch that reaches into the Gulf of Aden and the Red Sea, then passes between Australia and Antarctica and into the eastern South Pacific. The East Pacific Rise extends north to the Gulf of California; Easter Island and the Galápagos are volcanic islands that are part of this submarine mountain chain. The ridge system seems to merge into the continents in several areas, such as the Red Sea and the Gulf of California, and such areas are regions of great geologic activity, characterized by volcanoes, or earthquakes and faults (see Earthquake; Fault; Volcano).
The world ocean covers 71 percent
of the earth’s surface,
or about 361 million sq km
(140 million sq mi).
The midocean ridges play a key role in plate tectonics (movements in the earth’s crust), for it is from the inner troughs of these ridges that molten rock upwells from the earth’s mantle and spreads laterally on both sides, adding new material to the earth’s rigid crustal plates. The plates are moving apart, currently at the rate of 1 to 10 cm (0.39 to 3.9 in) a year and are being forced against adjacent plates. From the Mid-Atlantic Ridge, the continents, which rest on the plates and which once were joined, have moved away from one another. In the Pacific Ocean, plates are also moving apart from the East Pacific Rise, but the bordering plates are overlapping them and forcing them under at the edges. At these places, along almost the entire rim of the Pacific, deep trenches are formed as crust is subducted and returned to the mantle. The Pacific trenches commonly reach depths of more than 7 km (4.3 mi); the deepest known point, in the Mariana Trench east of the Philippines, lies 11 km (6.9 mi) beneath the surface. Trench areas, or subduction zones, are characterized by volcanic and seismic activity, indicative of the motions and stresses of the earth’s crustal plates (see Plate Tectonics; Seismology).
Go to link download
