PROCESSES IN OPERATION IN THE SEA.
Within the area of the sea, as on the area of the land, three sets of processes are at work—diastrophism, vulcanism, and gradation.
Diastrophism ([p. 2]) affects the sea-bottom as the land, but the results are notably different in certain respects. So far as the lithosphere is concerned, the sea-level may be said to be the critical level. At and above it, many processes are in operation which do not appear below, and below it, many which do not take place above. Changes of level which do not involve the submergence of areas which were land, or the emergence of areas which were under water, are relatively unimportant, compared with those which effect such changes. The rise of the bottom of the sea from a depth of 500 fathoms to a depth of 200 fathoms would not lead to important consequences, so far as the area itself is concerned, while an equal rise of the bottom beneath 200 fathoms of water, or an equal subsidence of land 500 feet high, would be attended by more striking consequences. It follows that the changes effected by diastrophism are much more obvious along coasts than in the deep seas. Emergence or submergence shifts the zones of aggradation and degradation, shifts the zone of contact of ocean and land, and changes the region concerned from one appropriate for sea life to one appropriate for terrestrial forms, or vice versa.
Over the continental shelves the water is shallow and the bottom relatively smooth. If a coastal region be elevated evenly, or if the sea-level be drawn down, the new shore-line on the smooth surface of the former submerged shelf will be relatively regular, even though the coast was notably irregular before the change. Thus in [Fig. 297] the coast-line is notably irregular. A sea-withdrawal or a land-uplift of 120 feet would change the coast-line to the position of the 20-fathom line, when it would be notably less irregular than now. If it were shifted to the 100-fathom line, few irregularities would remain. In so far as new coast-lines formed by the lowering of the sea (or rise of the crust) depart from straightness, it is usually by broad, smooth curves. Local uplifts of coastal lands, and especially uplifts along axes normal to the trend of the coast, would give rise to projections of land, and so to coastal irregularities; but such uplifts are rarely so localized as to give origin to minor projections. It follows that rising coasts, and those which have recently risen, or more likely, coasts along which the sea-level is sinking or has recently sunk, are likely to be regular so far as details of outline are concerned. Subsidence of a coast-line (or rise of the sea-level) tends to the opposite results, for in this case the sea advances on a surface which has more or less relief, and the water takes possession of every depression brought to its level. The lower parts of the valleys are converted into bays, the length and width of which depend on the slope and width of the valleys drowned. The numerous bays at the debouchures of the streams along the Atlantic coast of the United States, from Long Island Sound to Carolina, such as the Delaware, Chesapeake, ([Fig. 297]) and numerous smaller bays, are the results of recent sinking, which has allowed the sea to invade the lower ends of river valleys. The ragged coast of Maine is another example, though glaciation as well as subsidence has been operative here. From the present configuration of coast-lines, it has been inferred that the present is, on the whole, an era of continental depression.[145] River valleys, the lower ends of which are embayed, are sometimes found to be continuous with submerged valleys beyond the coast-line ([Fig. 298]). Submerged river valleys show that the surface in which they lie was once land.
Fig. 297.—Sketch of the eastern coast of the United States from Cape May to Cape Henry, showing coastal irregularities. The figures represent the depths of water in fathoms. (From charts of C. and G. Surv.)
Fig. 298.—Sketch of Carmel Bay, Cal. The contours below sea-level show a deep submerged channel. (From charts of C. and G. Surv.)
Bays may be developed by local subsidence as well as by the submerging of valleys, though decisive examples are not readily cited. Bays may also be produced by uplift of the surface on either side of an area which does not change its level. For example, uplift on either side of the Gulf of California has probably been one element, though probably not the only one, in the development of this indentation. The general outline of a great bay produced by coastal warping might be regular, though it would be likely to be marked by small irregularities where the streams enter. It is not to be understood that all, or even most, bays are due to local diastrophism.
Diastrophism, then, as it affects the ocean-bottoms and the ocean-borders, may make the water of any ocean shallower or deeper; it may cause the emergence or submergence of land; it may make coast-lines regular or irregular; it may shift the habitat of life, and through these changes may greatly influence the processes of gradation, which are especially active along the contact of sea and land.
Vulcanism affects the sea-bottom much as it affects the land. At the volcanic centers, where the great body of extruded matter accumulates, mounds and mountains are built up. Most of the mountain peaks of the sea-bottom, whether their crests are islands, or whether they are wholly submerged, have had a volcanic origin. The rock material ejected from submarine vents is probably less widely distributed than that from vents on land, and so far forth, the volcanic cones in the oceans are steeper than those on land. Where volcanic cones are built up near the surface of the sea, they often furnish a home for shallow-water life, such as polyps. Wherever built up so as to be within the reach of waves, gradational processes are stimulated.
The processes of vulcanism do not commonly influence coasts of continents directly, for few volcanoes lie immediately on coasts. In places, however, as at various points in and about Italy, the configuration of the coast is influenced by the building of volcanoes. Indirectly, vulcanism influences the shape of coast-lines, for the resistance of igneous rock is often different from that of the rock with which it is associated, and under the influences of the forces of gradation it may come to form projecting points or reëntrants, as the case may be.
The number of active volcanoes on islands is about 200, or about two-thirds of all now known. Since the area of the sea is about three times that of the land, the known active volcanoes in the sea are rather less numerous per unit area than those on the land. The number of active vents beneath the sea is altogether unknown. A few submarine eruptions have been observed, and those observed are probably but a small percentage of those which have taken place in historic time. Slight eruptions in deep water might not manifest themselves at the surface in an unequivocal way, even were observers stationed near them. Volcanic cones which fail to reach the surface are known, and the forms of many sea-bottom mountain peaks are such as to make it probable that they are volcanic. These phenomena, as well as the numerous volcanic islands, give some indication of the importance of submarine eruptions in past time.
Ocean volcanoes, and especially submarine volcanoes, affect both the temperature and the composition of the sea-water. Both the increase of temperature and the solution of volcanic gases increase the capacity of the water for mineral matter, and both the change in temperature and composition affect the life of the adjacent waters. The destruction of life during eruptions occasions the generation of the products of organic decomposition, and these stimulate further chemical changes. The diffusion of affected waters occasions chemical changes wherever they go. The effects of oceanic volcanoes on the sea-water are, therefore, appreciable, when long periods of time are considered. The deposition of the finer parts of volcanic discharges will be considered in connection with the deposits of the deep sea.
Gradation.—The gradational processes of the land and the sea are in striking contrast. On the land, degradation predominates, and aggradation is subordinate. In the sea, aggradation predominates, and degradation is subordinate. On the land, degradation is, on the whole, greatest where the land is highest, while aggradation is of consequence only where the land is low, or where steep slopes give place to gentle ones. In the sea, degradation is virtually confined to shallow water, or to what might be called the highlands of the sea, while aggradation is nearly universal, but most considerable in shallow water, or where shallow water gives place to deep. Both the degradational and aggradational work of the sea are greatest near its shores. Opposed as the gradational work of the land and sea are, they yet tend to a common end—the leveling of the surface of the lithosphere.
The gradational processes which affect the sea-bottom may be divided into three categories: (1) Those effected by mechanical means, (2) those effected by chemical means, and (3) those effected by organic agencies.
The mechanical work of gradation in the sea is effected chiefly by the movements of the water, and, very subordinately, by the movements of the ice which the water carries. The results of these movements may be degradational wherever the water is sufficiently shallow for the motion to affect the bottom. Elsewhere it is aggradational.
The direct gradational work effected by chemical means is likewise partly degradational and partly aggradational. If at any time or place the water becomes supersaturated with any mineral substance, precipitation takes place, and the precipitate accumulates as sediment on the bottom. This sometimes happens in lagoons and other small inclosures, and perhaps in open water. On the other hand, wherever solution is effected, degradation is the result. Solution is most important where the bottom consists of relatively soluble rock, such as lime carbonate.
Organic agencies are, on the whole, aggradational. Accumulations of coral, coral débris, shells, etc., help to build up the sea-bottom, and most rapidly in shallow water where the proper forms of life are most abundant. Here also should be mentioned the accumulations of carbonaceous matter, especially in the form of plant bodies. In the aggradation effected directly by organic agencies, the sea is passive. Its only part is to support the life which gives rise to the solid matter, and incidentally to float a part of it in its currents.