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Encyclopedia Britannica - Main :: ORC-PAI |
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PACIFIC OCEAN , the largest division of the hydrosphere, lying between Asia and Australia and North and South America. It is nearly landlocked to the N., communicating with the Arctic Ocean only by Bering Strait, which is 36 m. wide and of small depth. The southern boundary is generally regarded as the parallel of 4o S., but sometimes the part of the great Southern Ocean (400 to 661 S.) between the meridians passing through South Cape in Tasmania and Cape Horn is included. The north to south distance from Bering Strait to the Antarctic circle is 9300 m., and the Pacific attains its greatest breadth, 10,000 m., at the equator . The coasts of the Pacific are of varied contour. The American coasts are for the most part mountainous and unbroken, the chief indentation being the Gulf of California; but the general type is departed from in the extreme north and south, the southern coast of South America consisting of bays and fjords with scattered islands, while the coast of Alaska is similarly broken in the south and becomes low and swampy towards the north. The coast of Australia is high and unbroken; there are no inlets of considerable size, although the small openings include some of the finest harbours in the world, as Moreton Bay and Port Jackson. The Asiatic coasts are for the most part low and irregular, and a number of seas are more or less completely enclosed and cut off from communication with the open ocean. Bering Sea is bounded by the Alaskan Peninsula and the chain of the Aleutian Islands; the sea of Okhotsk is enclosed by the peninsula of Kamchatka and the Kurile Islands; the Sea of Japan is shut off by Sakhalin Island, the Japanese Islands and the peninsula of Korea; the Yellow Sea is an opening between the coast of China and Korea;the China Sea lies between the Asiatic continent and the island of Formosa, the Philippine group, Palawan and Borneo. Amongst the islands of the Malay Archipelago are a number of endosed areasthe Sulu,. Celebes, Java, Banda and Arafura seas. The Arafura Sea extends eastwards to Torres Strait, and beyond the strait is the Coral Sea, bounded by New Guinea, the islands of Melanesia and north-eastern Australia. The area and volume of the Pacific Ocean and its seas, with the mean depths calculated therefrom, are given in the article OCEAN. Extent. The Pacific Ocean has one and three-quarter times the area of the Atlanticthe next largest division of the hydrosphereand has more than double its volume of water. Its area is greater than the whole land surface of the globe, and the volume of its waters is six times that of all the land above sea-level. The total land area draining to the Pacific is estimated by Murray at 7,500,000 sq. m., or little more than one-fourth of the area draining to the Atlantic. The American rivers draining to the Pacific, except the Yukon, Columbia and Colorado, are unimportant. The chief Asiatic rivers are the Amur, the Hwang-ho and the Yangtsze-kiang: none of which enters the open Pacific directly. Hence the proportion of purely oceanic area to the total area is greater in the Pacific than in the Atlantic, the supply of detritus being smaller, and terrigenous deposits are not borne so far from land. The bed of the Pacific is not naturally divided into physical regions, but for descriptive purposes the parts of the area lying Relief oP east and west of 150 W. are conveniently dealt with Bed. separately. The eastern region is characterized by great uniformity of depth; the 2000-fathom line keeps close to the American coast except off the Isthmus of Panama, whence an ill-defined ridge
The western Pacific is in complete contrast to thb part just described. Depths of less than 2000 fathoms occur continuously on a bank extending from south-eastern Asia, on which stands the Malay Archipelago. This bank continues southwards to the Antarctic Ocean, expanding into a plateau on which Australia stands, and a branch runs eastwards and then southwards from the north-east of Australia through New Zealand. The most considerable areas over 3000 fathoms are the Aldrich deep, an irregular triangle nearly as large as Australia, situated to the east of New Zealand, in which a sounding of 5155 fathoms was obtained by H.M.S. " Penguin," near the Tonga Islands: and the Tuscarora deep, a long, narrow trough running immediately to the east of Kamchatka, the Kurile Islands and Japan. A long strip within the Tuscarora deep forms the largest continuous area with a depth greater than 4000 fathoms. All the rest of the western Pacific is a region of quite irregular contour. The average depth varies from 1500 to 2500 fathoms, and from this level innumerable volcanic ridges and peaks rise almost or quite to the surface, their summits for the most part occupied by atolls and reefs of coral formation, while interspersed with these are depressions, mostly of small area, among which the deepest soundings recorded have been obtained. The United States telegraph ship " Nero," while surveying for a cable between Hawaii
The following table, showing the area of the floor of the Pacific (to 4o S.) and the volume of water at different levels, is due to Sir J. Murray: : Fathoms. Areas. Volume. (sq. m.) (cub. m.) 0-100 3,379,700 6,128,500 loo-500 1,753,450 23,348,350 500-1000 1,707,650 28,323,700 1000-2000 6,902,550 52,628,500 2000-3000 39,621,550 32,545,400 3000-4000 2,164,150 1.357,900 over 4000 94,850 70,600 55,623,900 144,402,950 So far as our knowledge goes, the present contours of the open Pacific Ocean are almost as they. were in Palaeozoic times, and in the intervening ages changes of level and form have been slight. There is no reason to suppose that any considerable part of the vast area now covered by the waters of the Pacific has ever been exposed as dry land. Hence the Pacific basin may be regarded as a stable and homogeneous geographical unit, clearly marked off round nearly all its margin by steep sharp
elevation
The deeper parts of the bed of the Pacific are covered by deposits of red clay, which occupies an area estimated at no less than 105,672,000 sq. kilometres, or three-fifths of the Deposits. whole. Over a large part of the central Pacific, far removed from any possible land-influences or deposits of ooze, the red-clay region is characterized by the occurrence of manganese, which gives the clay a chocolate colour, and manganese nodules are found in vast numbers, along with sharks' teeth and the ear-bones and other bones of whales. Radiolarian ooze is found in the central Pacific in a region between 15 N. to 10 S. and 140 E. to 15o W., occurring in seven distinct localities, and covering an area of about 3,007,000 sq. kilometres. The " Challenger " discovered an area of radiolarian ooze between 7-12 N. and 147-152 W., and another in 2-I0 S., 152-153 W. Between these two areas, almost on the equator , a strip of globigerina ooze was found, corresponding to the zone of globigerina in the equatorial region of the Atlantic. Globigerina ooze covers considerable areas in the intermediate depths of the west and south Pacificwest of New Zealand, and along the parallel of 4o S., between 8o-98 W. and 15o-118 W.but this deposit is not known in the north-eastern part of the basin. The total area covered by it is estimated at 38,332,000 sq. kilometresabout two-thirds of that in the Atlantic. Pteropod ooze occurs only in the neighbourhood of Fiji and other islands of the western Pacific, passing up into fine coral sands and mud. Diatom ooze has been found in detached areas between the Philippine and Mariana islands, and near the Aleutian and Galapagos groups, forming an exception to the general rule of its occurrence only in high latitudes. All the enclosed seas are occupied by characteristic terrigenous deposits.Partly on account of its great extent, and partly because there is no wide opening to the Arctic regions, the normal wind circulation is on the whole less modified in the North Pacific than in Meteorothe Atlantic, except in the west, where the south-west logy. monsoon of southern Asia controls the prevailing winds, its influence extending eastwards to 145 E., near the Ladrones, and southwards to the equator. In the South Pacific the north-west monsoon of Australia affects a belt running east of New Guinea to the Solomon Islands. In the east the north-east trade-belt extends between 5 and 25 N.; the south-east trade crosses the equator, and its mean southern limit is 25 S. The trade-winds are generally weaker and less persistent in the Pacific than in the Atlantic, and the intervening belt of equatorial calms is broader. Except in the east of the Pacific, the south-east trade is only fully developed during the southern winter; at other seasons the regular trade-belt is cut across from north-west to south-east by a band twenty to thirty degrees wide, in which the trades alternate with winds from north-east and north, and with calms, the calms prevailing chiefly at the boundary of the monsoon region (5 N.-15 S., 16o-185 E.). This area, in which the south-east trade is interrupted, includes the Fiji, Navigator and Society groups, and the Paumotus. In the Marquesas group the trade-wind is constant. Within the southern monsoon region there is a gradual transition to the north-west monsoon of New Guinea in low latitudes, and in higher latitudes to the north-east wind of the Queensland coast. The great warming and abundant rainfall of the island regions of the western Pacific, and the low temperature of the surface water in the east, cause a displacement of the southern tropical maximum of pressure to the east; hence we have a permanent " South Pacific anticyclone " close to the coast of South America. The characteristic feature of the south-western Pacific is therefore the relatively low pressure and the existence of a true monsoon region in the middle of the trade-wind belt. It is to be noted that the climate of the islands of the Pacific becomes more and more healthy the farther they are from the monsoon region. The island regions of the Pacific are every-where characterized by uniform high air-temperatures; the mean annual range varies from 1 to 9 F., with extremes of 24 to 27, and the diurnal range from 9 to 16. In the monsoon region relative humidity is high, viz. 8o to 90%. The rainfall is abundant; in the western island groups there is no well-marked rainy season, but over the whole region the greater part of the rainfall takes place during the southern summer, even as far north as Hawaii
Atlantic. The result is that north-westerly winds, which in winter are exceedingly dry and cold, blow over the western or Asiatic area; westerly winds prevail in the centre, and south-westerly and southerly winds off the American coast. In the southern hemisphere there is a transition to the low-pressure belt encircling the Southern Ocean, in which westerly and north-westerly winds continue all the year round. The distribution of temperature in the waters of the Pacific Ocean has been fully investigated, so far as is possible with the existing Temperature. observations, by G. Schott. At the surface an extensive area of maximum temperature (over 20 C.) occurs over to on each side of the equator to the west of the ocean. On the eastern side temperature falls to 22 on the equator and is slightly higher to N. and S. In the North Pacific, beyond lat. 40, the surface is generally warmer on the E. than on the W., but this condition is, on the whole, reversed in corresponding southern latitudes. In the intermediate levels, down to depths not exceeding moo metres, a remarkable distribution appears. A narrow strip of cold water runs along the equator, widest to the east and narrowing westward, and separates two areas of maximum which have their greatest intensity in the western part of the ocean, and have their central portions in higher latitudes as depth increases, apparently tending constantly to a position in about latitude 30 to 35 N. and S. A comparison of this distribution with that of atmospheric pressure is of great interest
movement
The surface waters of the North Pacific are relatively fresh, the salinity being on the whole much lower than in the other great salinity. oceans. The saltest waters are found along a belt extend- ing westwards from the American coast on the Tropic of Cancer to 16o E., then turning southwards to the equator. North of this salinity diminishes steadily, especially to the north-west, the Sea of Okhotsk showing the lowest salinity observed in any part of the globe. South and east of the axis mentioned salinity becomes less to just north of the equator, where it increases again, and the saltest waters of the whole Pacific are found, as we should expect, in the south-east trade-wind region, the maximum occurring in about 18 S. and 120 W. South of the Tropic of Capricorn the isohalines run nearly east and west, salinity diminishing quickly to the Southern Ocean. The bottom waters have almost uniformly a salinity of 34.8 per mille, corresponding closely with the bottom waters of the South Atlantic, but fresher than those of the North Atlantic. The surface currents of the Pacific have not been studied in the same detail as those of the Atlantic, and their seasonal variations Ciradatlon. are little known except in the monsoon regions. Speak- ing generally, however, it may be said that they are for the most part under the direct control of the prevailing winds. The North Equatorial Current is due to the action of the north-east trades. It splits into two parts east of the Philippines, one division flowing northwards as the Kuro Siwo or Black Stream, the analogue of the Gulf Stream, to feed a drift circulation which follows the winds of the North Pacific, and finally forms the Cali- fornian Current flowing southwards along the American coast. Part of this rejoins the North Equatorial Current, and part probably forms the variable Mexican Current, which follows the coasts of Mexico and California close to the land. The Equatorial Counter- Current flowing eastwards is largely assisted during the latter half of the year by the south-west monsoon, and from July to October the south-west winds prevailing east of 150 E. further strengthen the current, but later in the year the easterly winds weaken or even destroy it. The South Equatorial Current is produced by the south- east trades, and is more vigorous than its northern counterpart. On reaching the western Pacific part of this current passes south- wards, east of New Zealand, and again east of Australia, as the East Australian Current, part northwards to join the Equatorial Counter- Current, and during the north-east monsoon part makes its way through the China Sea towards the Indian Ocean. During the south-west monsoon this last branch is reversed, and the surface waters of the China Sea probably unite with the Kuro Biwo. Between the Kuro Siwo and the Asiatic coast a band of cold water, with a slight movement
the analogue of the " Cold Wall
latitudes of the South Pacific the surface movement forms part of the west wind-drift of the Roaring Forties. On the west coast of South America the cold waters of the Humboldt or Peruvian Current corresponding to the Benguela Current of the South Atlantic, make their way northwards, ultimately joining the South Equatorial Current. The surface circulation of the Pacific is, on the whole, less active than that of the Atlantic. The centres of the rotational movement are marked by " Sargasso Seas " in the north and south basins, but they are of small extent compared with the Sargasso Sea of the North Atlantic. From the known peculiarities of the distribution of temperature, it is' probable that definite circulation of water is in the Pacific confined to levels very near the surface, except in the region of the Kuro Siwo, and possibly also in parts of the Peruvian Current. The only movement in the depths is the slow creep of ice-cold water northwards along the bottom from the Southern Ocean; but this is more marked, and apparently penetrates farther north, than in the Atlantic. See Reports of expeditions of the U.S.S. " Albatross " and " Thetis
" " " End of Article: PACIFIC OCEAN If you wish, you can link directly to this article.
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