Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

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.

MOVEMENTS OF THE SEA-WATER.

The movements of the sea-water fall into several categories. There is (1) a general circulation of sea-water, determined chiefly by three factors: differences in density in the sea-water, differences of level, and the general movements of the atmosphere; (2) periodic movements which are not primarily circulatory, brought about by the attraction of the sun and moon; and (3) aperiodic movements, due to occasional causes, such as earthquakes, volcanic explosions, landslides, etc., which determine local and temporary movements, often of exceptional strength.

Differences in density and their results.—Differences in density result from differences in temperature and salinity. Temperature alone considered, water would be densest where it is coldest, namely in the polar regions. Differences in salinity result from differences in evaporation and from inequalities in the supply of fresh water. Evaporation alone considered, the sea-water should be densest where evaporation is greatest; but the equatorial region, where evaporation is greatest, is also a region where precipitation is heavy, and precipitation, by freshening the water, opposes the effect of great evaporation. The greatest differences in density due to the unequal supply of fresh water are to be found near the borders of continents, where the precipitation on the land is discharged into the sea. In the polar regions, the great supply of fresh water, especially during the season when the ice is melting, opposes the effect of the low temperature, so far as the density of the water is concerned. The result of the operation of these factors affecting the density of the sea-water is to insure a general circulation, directed to the end of equalizing the densities; and since the disturbing factors are constantly in operation, equilibrium is never established, and the movements of the water are perpetual.

The pressure gradients resulting from differences of density are so slight that the resulting movements are scarcely more than a creep of the waters. In general they are far too slow to be of importance in gradational work; but the earth’s rotation deflects the creeping waters and tends to concentrate the equator-ward movement into currents on the east sides of the continents, and the pole-ward movement on the west sides. In favorable situations these currents may be competent to produce sensible mechanical results. Even where this is not the case the circulation helps to equalize the temperatures of the sea, and so of the air above and of the land about. Indirectly, therefore, the circulation of the ocean-waters affects every geological process which is sensitive to climate.

Differences in level and their results.—While the surface of the ocean is the common datum plane to which elevations and depressions are referred, it is to be remembered that the sea has “a very complicated undulating surface in consequence of the attraction which the heterogeneous and elevated portions of the lithosphere exercise on the liquid hydrosphere. In the opinion of geodesists, the geoid may in some places depart from the figure of the spheroid by 1000 feet.”[146] These variations in level would, however, not occasion circulation. The differences in level which determine circulation are much more trivial. Every stream which pours fresh water into the sea tends to raise the level of the water where it enters. The waters brought to the ocean by the Amazon, the Mississippi, and other great rivers would appreciably change the level of the sea at their debouchures, if the excess did not promptly flow away. The ready mobility of the water, however, prevents its accumulation, and the discharge of every stream generates widespread movement. This movement is strongest at the debouchure, and weakens with increasing distance from it, though in the case of great streams, such as the Amazon, the movement is traceable, by means of the sediment which the water carries, hundreds of miles out to sea.

Changes of level are also brought about by the winds, which pile up water along the shore against which they blow. The level of the water is said to have risen 24 feet at Calcutta on October 5, 1864, as the result of a severe storm. While this is exceptional, a rise of 2 feet is not rare. This piling up of the waters along shore insures a compensating movement (undertow, littoral currents, etc.) in some other direction. Unequal evaporation and precipitation likewise disturb the level of the sea and occasion movement. In the open sea the movements generated by differences of level, like those generated by differences of density, are chiefly slow, creeping movements, but movements which never cease. In bays and gulfs, on the other hand, the surface of the water may be so raised, either as the result of wind, river discharge, or heavy precipitation, as to give rise to strong outward currents. There is little doubt at the present time that the Gulf Stream owes its origin primarily to the difference of level between the Gulf of Mexico and the Atlantic.[147]

Movements generated by winds.—The circulation resulting from the tendency of the winds to change the level of the sea-water has already been mentioned, but the wind also works in other ways. Where the winds have a somewhat constant direction and are at the same time strong, they determine a general movement of the surface-waters in their own direction, the surface-water being dragged along at a rate somewhat less than that of the wind itself. The constant trades appear to be the chief generators of the equatorial ocean-currents. Once generated, these currents may be concentrated and their courses modified. The currents generated by trades are turned north and south when directed against a continent; they are modified by the configuration of the bottom if the water be shallow, and always and everywhere, except, at the equator, they are deflected by the rotation of the earth, in the northern hemisphere to the right, and in the southern to the left. The pole-ward currents generated in the equatorial region by the trades, and directed by the winds, the lands, the configuration of the bottom, and the rotation of the earth, determine compensating currents from high latitudes to low, and the same influences which control the course of the former direct the latter as well.

Since the atmospheric movements are so far constant that there is a prevailing direction of winds in all latitudes, the winds, as well as differences of density and differences of level, insure a general and continual circulation of sea-water. The geological effects of this circulation are direct and indirect; direct, by gradation of the bottom over which they flow, and indirect, by the modifications of climate they produce. Since rotation deflects the pole-ward currents to the east sides of the oceans (west sides of the continents) and the equator-ward movements to the west sides of the oceans (east sides of the continents), the east shores of the oceans are warmer than the west in corresponding latitudes, and the west sides of the continents are both warmer and moister[148] than the east sides.