Terminology of Submarine Topography
The terms applied to features of submarine topography will be classified according to the origin of the features rather than according to their size, although the latter procedure is the common one (for example, Niblack, 1928, Littlehales, 1932). The features of submarine relief may be grouped in two main categories, depending upon whether they have gained their characteristic form through diastrophic activity (crustal movements) or through erosion or deposition. The primary large-scale process involved in the development of relief must be diastrophic, but in many cases the characteristic feature is produced by erosion or deposition. No distinction will be made here between features that have been formed below the sea surface and those that may possibly owe their origin to subaerial erosion or deposition. As pointed out before, deposition in the sea tends to fill in the depressions and thus to level out the minor irregularities of the bottom, and, with the exception of those cases in which organisms play an important role (for example, in the formation of coral reefs), little or no deposition takes place on topographic highs.
There has been much discussion as to the processes that have led to the formation of the continental and insular shelves. Some authors maintain that they are wave-built (depositional); others consider that they are wave-cut (erosional), or that they are a combination of both processes (Johnson, 1919; Shepard, 1939). Geophysical studies on the two sides of the North Atlantic (Bucher, 1940) indicate that the shelves are composed of great prism-shaped accumulations of sedimentary rock that at the outer edge of the shelf bordering the eastern United States are 4000 m thick. To what extent these features resulted from the slow accumulation and sinking of the crust and to what extent violent diastrophic movements have been involved has not yet been decided. The characteristic form of the shelf and of isolated flat-topped banks and shoals, and other features of the shallow bottom indicate that wave erosion and transportation by currents have played an important part
The terms used to designate certain types of topographic features, their French and German equivalents, and their definitions, which are given below, correspond to those suggested by the International Hydrographic Bureau (Niblack, 1928). Unfortunately, there is still considerable confusion in the use of certain terms, particularly those which apply to the larger features of the topography. Sometimes several different descriptive terms have been applied to the same structure, and in other instances the same term is applied to features of vastly different size and probably of different origin. A committee of the International Association of Physical Oceanography (Vaughan et al, 1940) attempted to clarify many of the problems relating to the terminology, but much confusion still prevails. In order to designate any individual feature, the descriptive term is prefixed by a specific name. The specific names attached to large-scale features are generally geographical, but those assigned to such features as banks, shoals, seamounts, canyons, and sometimes deeps are often those of vessels or individuals associated with their discovery or mapping.
Features Resulting from Crustal Deformation
Elevations. The large-scale elevations of the ocean bottom are termed ridges, rises, or swells.
Ridge (F, Dorsale; G, Rücken). A long and narrow elevation with sides steeper than those of a rise.
Rise (F, Seuil; G, Schwelle). A long and broad elevation which rises gently from the ocean bottom.
Isolated mountain-like structures rising from the ocean bottom are known as seamounts. Where the ridges are curved, and particularly if parts of them rise above sea level, they are sometimes termed arcs. The broad top of a rise is termed a plateau. The expression sill is applied to a submerged elevation separating two basins. The sill depth is the greatest depth at which there is free, horizontal communication between the basins.
Depressions. The terms trough, trench, and basin are those most commonly applied to the large-scale depressions on the ocean bottom.
Trough (F, Dépression; G, Mulde). A long, broad depression with gently sloping sides.
Trench (F, Fossé; G, Graben). A long and narrow depression with relatively steep sides.
Basin (F, Bassin; G, Becken). A large depression of more or less circular or oval form.
The terms defined above are used rather loosely and are applied to features of a wide range in size.
For those parts of a depression which exceed 6000 m in depth, the term deep (F, Fosse; G, Tief) is used. As originally suggested by Murray, the term designated areas where the depths exceeded 3000 fathoms (5486 m), but it is now generally restricted to those depressions of greater depth (Vaughan et al, 1940). The term depth (F, Profondeur; G, Tiefe), prefixed by the name of the vessel concerned, may be used to designate the greatest sounding obtained in any given deep.
Features Resulting from Erosion, Deposition, and Biological Activity
As pointed out above, the features in this category have been produced by erosion of, or deposition upon, structures which may be primarily of diastrophic origin. The most prominent types of features in this group are the shelf and the slope.
Shelf. The zone extending from the line of permanent immersion to the depth, usually about 120 m, where there is a marked or rather steep descent toward the great depths. Continental Shelf (F, Plateau continental; G, Kontinental-Schelff) is applied to the feature bordering the continents, while Insular Shelf (F, Socle; G, Insel-schelff) is used for the feature surrounding islands.
Slope. The declivity from the outer edge of the shelf into deeper water. Continental Slope (F, Talus continental; G, Kontinental-Abfall) and Insular Slope (F, Talus insulaire; G, Inselabfall) are applied to the slopes bordering continents or islands.
The following terms are applied to the upper parts of elevations which show the effects of erosion or deposition.
Bank (F, Banc; G, Bank). A more or less flat-topped elevation over which the depth of water is relatively small, but which is sufficient for surface navigation.
Shoal (F, Haut-fond; G, Untiefe or Sandgrund). A detached elevation with such depths that it is a danger to surface navigation and which is not composed of rock or coral.
Reef (F, Récif; G, Riff). A rocky or coral elevation (generally elongate) which is dangerous to surface navigation. It may extend above the surface.
A variety of names has been applied to the steep-walled fissures that penetrate the slope and cut across the shelf. The most commonly used terms are canyon and valley, but gully, gorge, and mock-valley are also applied to these features.
In addition to the terms given above, many expressions are employed in descriptions of submarine topography with the same meanings that they have when used for land topography.