Page:EB1911 - Volume 20.djvu/469

 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 enclosed areas—the 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. The Pacific Ocean has one and three-quarter times the area of the Atlantic—the next largest division of the hydrosphere—and 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 east and west of 150° W. are conveniently dealt with 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 of less than 2000 fathoms runs south-westwards, and again off the coast of South America in about 40° S., where a similar bank runs west and unites with the former. The bank then continues south to the Antarctic Ocean, in about 120° W. Practically the whole of the north-east Pacific is therefore more than 2000 fathoms deep, and the south-east has two roughly triangular spaces, including the greater part of the area, between 2000 and 3000 fathoms. Notwithstanding this great average depth, the “deeps” or areas over 3000 fathoms are small in number and extent. Five small deeps are recognized along a line close to the coast of South America and parallel to it, in the depression enclosed by the two banks mentioned—they extend from about 12° to 30° S.—and are named, from north to south, Milne-Edwards deep, Krümmel deep, Bartholomew deep, Richards deep and Haeckel deep. In the north-east the deeps are again few and small, but they are quite irregularly distributed, and not near the land. East of 150° W. the Pacific has few islands; the oceanic islands are volcanic, and coral formations are of course scanty. The most important group is the Galapagos Islands.

The western Pacific is in complete contrast to the 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 and the Philippines, sounded in 1900 the greatest depth yet known between Midway Islands and Guam (12° 43′ N., 145° 49′ E.) in 5269 fathoms, or almost exactly 6 m.

The following table, showing the area of the floor of the Pacific (to 40° S.) and the volume of water at different levels, is due to Sir J. Murray:— 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 slopes, extending in places through the whole known range of elevation above sea-level and of depression below it—from the Cordilleras of South America to the island chains of Siberia and Australia. (See .)

The deeper parts of the bed of the Pacific arc 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 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 150° 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°–10° 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 Pacific—west of New Zealand, and along the parallel of 40° S., between 80°–98° W. and 150°–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. kilometres—about 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 the Atlantic, except in the west, where the south-west 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., 160°–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 everywhere 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. 80 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. In the trade-wind region we find the characteristic heavy rainfall on the weather sides of the islands, and a shorter rainy season at the season of highest sun on the lee side. Buchan describes the island-studded portion of the western Pacific as the most extensive region of the globe characterized by an unusually heavy rainfall. Beyond the tropical high-pressure belt, the winds of the North Pacific are under the control of an area of low pressure, which, however, attains neither the size nor the intensity of the “Iceland” depression in the north