Major Features of Topography
The discussion of the bottom topography of the oceans will be restricted to a brief consideration of the large-scale topographic features that are represented on small charts with large contour intervals. In regions where many soundings have been obtained, it has been found that the sea bottom may be virtually as irregular as the land surface, but such details can be shown only on large-scale charts with small contour intervals, and are not included in this volume.
Submarine geology is concerned with the topography of the sea floor, the composition and physical character of the sedimentary and igneous materials that are found on the ocean bottom, and the processes involved in the development of topographic relief. The field is a relatively new one which has received great impetus from the development of sonic sounding methods that made it possible to obtain accurate maps of the sea floor, and from the development of geophysical methods (measurement and interpretation of gravity anomalies, of the earth's magnetic field, and of the path and velocity of earthquake and artificial seismic waves) that yielded estimates of the character and thickness of the materials forming the crust of the earth. However, there is yet no agreement concerning the processes involved in the geological history of the ocean basins, and the various hypotheses will not be discussed here. General reviews of the problems will be found in Johnstone (1928), Bucher (1933), Kuenen (1935), and Gutenberg (1939). A symposium on the geophysical exploration of the sea bottom (Field et al, 1938) covers many of the developments.
The distribution of elevations and depressions on the earth's crust (fig. 2) shows that there are large portions with elevations between sea level and 1000 m, and with depths between 4000 m and 5000 m. According to Bucher (1933) the larger, lower ones are related to the character of the earth's crust, while the upper ones are the result of subaerial erosion and sedimentation. The question then arises as to the extent to which the topography of the ocean bottom with reference to a depth of about 4500 m corresponds to that of the land with reference to sea level or a slightly higher level. According to Bucher, the large-scale features are essentially similar, and elevations and depressions of comparable dimensions are found both on land and on the ocean bottom. Although the major features are comparable, the details are quite different, because erosion, which plays such an all-important role in the creation of sharp relief and in the ultimate destruction of elevations on land, is virtually absent in the sea. In the sea the most effective agents of erosion are the surface waves, and these tend to produce flat-topped features that are restricted to shallow depths, since the velocity of the water particles in such waves decreases rapidly with increasing depth (p. 528). Other processes which may contribute to erosion of the sea floor are discussed in chapter XX and in the section dealing with the origin of submarine canyons (p. 41). Deposition is the characteristic process that modifies the topography of the sea bottom. Sedimentary debris accumulates in depressions, while there is little or no accumulation on topographic highs, which are devoid of fine-grained sediment and are subject to erosion if near the surface or in localities of exceptionally strong currents.
Bucher (1933) has stated that there are essentially two types of large-scale topographic features on the land and on the ocean bottom: (1) those of approximately equidimensional lateral extent, to which he applies the names swells and basins, and (2) those of elongate form, generally with steeper sides, to which he applies the names welts and furrows. On the ocean bottom the elongate welts and furrows appear to be the more common, and there is a considerable range in the size of such structures. There is a tendency for the large welts on the sea bottom to be parallel to the continental coasts, so that the oceans are divided into elongate troughs. Transverse ridges in turn subdivide these major depressions into a series of basins that are separated from one another to a greater or lesser degree. This ridge and basin topography is clearly shown by the bottom of the Atlantic Ocean and the Indian Ocean and in the western part of the Pacific Ocean, but does not appear to be so conspicuous a feature in the main part of the Pacific.
Within the smaller welts and furrows, the steepest slopes, the highest elevations, and the greatest depths are found. The welts and furrows are commonly close together, with arced outlines, and are characteristically found near the continents. The deep furrows are generally on the convex