Bottom Configuration of the Oceans

The major features of the topography of the ocean bottom are of such large dimensions that they are readily shown on a chart with contour intervals 1000 m apart. Such a representation is given in chart I, where the contours are entered for 1000-m intervals between 3000 m and 7000 m. The areas with depths less than 3000 m represent a rather small part of the sea floor, and the complex nature of the contours for depths less than this would confuse rather than add to the value of a chart of this kind. The topography is based upon the most recent charts available, and primarily upon the bathymetric chart prepared by the International Hydrographic Bureau in 1939 (Vaughan et al, 1940). Other sources that may be consulted for details concerning the configuration of the ocean floor are listed on page 29. It will be noted that the complexity of the topography varies in different regions. This difference must be attributed, in part, at least, to the variable amount of data available, because in those regions where the soundings are widely spaced the contours will be smooth and rounded, whereas in those areas where there are numerous soundings the contours are more complex and irregular. The Atlantic Ocean, the central part of the North Pacific Ocean, the Northern Indian Ocean, and the area surrounding Antarctica are fairly well sounded, but in many other regions, such as the North Polar Sea and the Southern Indian and South Pacific Oceans, the observations are very sparse. The increase in the complexity of the known topographic features that follows the accumulation of more depth measurements can be seen by comparing recent bathymetric charts with those published in the early years of the present century. The status of bathymetric knowledge in 1937 is shown by a series of charts in Vaughan et al (1937).

As stated above, the topography of the ocean bottom is characterized by depressions and elongated ridges. Some of these features are of very

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great dimensions, as may readily be seen from chart I. Longitudinal ridges divide the three oceans into elongated troughs. This feature is most conspicuous in the Atlantic Ocean, where the Atlantic Ridge, extending from Iceland to Bouvet Island in about 55°S, separates the western and the eastern troughs. Depths exceeding 5000 m exist on both sides of the ridge, which is continuous at depths less than 3000 m over the greater part of its length and in several places extends above sea level. There is one small but significant break in the ridge, in the Romanche Furrow just north of the Equator, where the saddle depth is located between 4500 and 4800 m. The Walfisch Ridge, which extends northeast from the Atlantic Ridge in the vicinity of Tristan da Cunha (37°S) to the coast of Africa in latitude 20°S, is continuous at 3500 m and almost so at 3000 m. The Rio Grande Ridge extends westward from the Atlantic Ridge (30°–35°S) and is almost continuous at 4000 m. The presence of these two transverse ridges has a pronounced effect on the deep-water circulation in the Western and Eastern Atlantic and, hence, on the distribution of properties (chap. XV).

A longitudinal ridge, the Indian Ridge, is present in the Indian Ocean and extends from India to Antarctica, but differs from the one in the Atlantic Ocean in that it is wider and does not extend so near the surface. In the Pacific Ocean the longitudinal elevations are not so conspicuous; however, the West Pacific Ridge, which is actually composed of several shorter ridges, can be traced from Japan to Antarctica, and is continuous at depths less than 4000 m except for breaks at 11°N, 10°S, and 53°S. A second elevation extends from Central America to the south and west, reaching Antarctica in the longitude of New Zealand. This East Pacific Ridge is continuous at depths less than 4000 m and separates the central depression from the deep basins bordering Central and South America and the Pacific Antarctic Basin. The effect of these major elevations on the distribution of bottom-water temperatures is shown in fig. 211, p. 749.

Within the major depressions or troughs which are bordered by the continents and the longitudinal ridges are transverse ridges that separate to a greater or lesser degree a number of basins. Wüst (Vaughan et al, 1940) has suggested that the 4000-m contour be used as the boundary in designating basins, but this is a purely arbitrary delimitation that places undue emphasis upon the absolute depth rather than upon the relative relief, which in many instances is of greater significance. For example, the Mediterranean Sea Basin is virtually excluded from such a classification, although it is a deep, isolated basin, much of it extending more than 3000 m below the sill in the Strait of Gibraltar. In the tabulation accompanying chart I are listed the names for the major parts of the oceanic depressions which Wüst has termed basins; namely, those parts which have depths exceeding 4000 m. Certain individual basins are clearly

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defined by the presence of ridges that are continuous at 4000 m, but names have also been applied to various parts of troughs, in which case the boundaries are located at the shallowest or narrowest part of the depression. In some areas the nomenclature is incomplete, and a single name is applied to a number of more or less isolated depressions, such as the Madagascar Basins. In the Central Pacific, where the knowledge of the topography has increased rapidly in recent years and where the basin and ridge type of topography does not appear to be present, no names have been applied. The names used to designate the major features of the topography are discussed at length by Vaughan et al (1940), and the names indicated on chart I generally conform to the recommendations made in their report.

In the tabulation of the basins given on chart I are listed some of the more prominent deeps; namely, those features where the depths exceed 6000 m. Some deeps are located more or less centrally in the large basins; for example, Wharton Deep, Byrd Deep, and the numerous deeps in the central part of the North Pacific, but these rarely exceed 7000 m in depth. On the other hand, numerous deeps of elongate character are located near and parallel to continental coasts, island arcs, or submarine ridges which correspond to the furrows discussed on p. 23. These marginal deeps, to which the term trench or sometimes trough is applied, are the features within which the greatest depths are found, in nearly all cases exceeding 8000 m. Only one such trench is found in the Indian Ocean; namely, the Sunda Trench. In the Atlantic are to be found the Romanche Trench, the South Sandwich Trench, and the Puerto Rico and Cayman Troughs. The greatest number are in the western part of the Pacific Ocean, although there is a chain of such features paralleling the mountainous coast of parts of Central and South America. As stated before, the regions in which these deep trenches occur are sites of volcanic and seismic activity. The complex topography of the East Indian Archipelago, which has been described by Kuenen (1935), is shown schematically in fig. 208, p. 736.

For a detailed description of the features of the sea bottom the reader should consult Littlehales (1932). Vaughan (1938) has described the topography of the Southern Hemisphere. There is much information of value in the large report by Vaughan and others (1940), which also contains the small-scale bathymetric chart prepared by the International Hydrographic Bureau on a Mercator projection, a special chart of the North Pacific prepared by the U. S. Hydrographic Office, and an excellent, detailed chart of the Caribbean Sea region prepared by the same agency. The standard charts on the bathymetry of the oceans are those in the series known as the Carte Générale Bathymétrique des Océans, published by the International Hydrographic Bureau at Monaco. These charts comprise twenty-four sheets which are revised from time to time

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and are issued periodically as the new charts are completed. The third edition is now being issued. On these charts the depths are entered in meters. General bathymetric charts of the oceans are included in the publications of Schott (1926, 1935). Detailed charts of restricted regions and general charts of the oceans are issued by the various national agencies responsible for the publication of navigational charts. The reports of the Meteor Expedition (Deutsche Atlantische Expedition “Meteor” 1925–1957, Wissenschaftliche Ergebnisse) will contain charts of the Atlantic Ocean, to be issued in thirteen sheets. The Snellius Expedition has produced excellent bathymetric charts of the East Indian region (van Riel, 1934). The Geological Society of America has sponsored the preparation and publication of detailed topographic charts of the eastern and western coasts of the United States based upon the soundings made by the U. S. Coast and Geodetic Survey (Veatch and Smith, 1939; Shepard and Emery, 1941).