Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

The most obvious disturbance of sea-water resulting from the winds is the generation of waves. Waves are not primarily parts of the general oceanic circulation. Since they are generated in other ways than by winds, and since the gradational effects of waves are independent of their origin, the effects of wind-waves will not be considered separately.

Movements generated by attraction.—One of the movements of the sea-water which is not primarily circulatory results from the attraction of the moon and sun. The tide is really the result of the inequalities of the attraction of these bodies on different parts of the earth. The lunar tide is more important than the solar, not because the attraction of the moon is greater, for it is not, but because its differential attraction, the result of its lesser distance, is greater.

The distance of the moon from the earth is about 240,000 miles. If this be taken as the distance from the center of the moon to the center of the earth, 236,000 and 244,000 miles respectively are the distances from the center of the moon to the nearest and most distant points on the earth. The distance of the sun from the earth is about 93,000,000 miles. If this be taken as the distance between the centers of these bodies, then the distances from the center of the sun to the nearest and most distant points on the earth’s surface are 92,996,000 and 93,004,000 miles respectively. The ratio of 4000 to 236,000 or to 244,000 is much greater than the ratio of 4000 to 92,996,000 or to 93,004,000. Hence the tide-producing force of the moon is greater than that of the sun.

The tides show themselves along shores in the form of waves which, in shallow water, become translatory. They differ from the wind-waves in their periodicity, and locally in their greater height. The effects of the tidal waves on the shores of the sea, and on the bottom in shallow water, are the same as the effect of wind-waves of equal strength, and need not be separately considered in connection with the gradation of the sea-bottom. In passing through narrow straits or narrow passes of any sort, the tidal movement becomes a current which, under favorable conditions, abrades or “scours” the bottom effectively. The tidal currents in the narrow passes about New York harbor may serve as an illustration.

Aperiodic movements.—In addition to the wind-waves which are essentially constant and universal, and to the tidal waves, which are periodic, there are accidental waves which are locally and temporarily of importance. Such are earthquake-waves, which are sometimes extremely destructive. Thus an earthquake-wave on the coast of Peru in 1746 swept a frigate several miles inland and deluged Lima, seven miles from the shore. The havoc of most earthquakes affecting coasts, such as that of Lisbon in 1755, is greatly aggravated by accompanying sea-waves. Earthquake-waves differ from ordinary waves in being translatory, and so in being more effective on the bottom in deep water. Their greatest force, however, is felt in shallow water and on shores. Volcanic eruptions likewise give rise to exceptional aperiodic waves. The same is true of landslides where they affect the coast or any part of the sea-bottom. The fall of glacier ends and the capsizing of icebergs likewise generate strong waves. To the category of exceptional waves also belong those generated by the winds of exceptional storms, such as that which devastated Galveston in 1900.[149]

Summary.—From the point of view of their direct geological results in shallow water, all movements of the sea-water may be grouped into two main classes—(1) waves, with the undertow and the littoral currents they generate, and (2) ocean-currents.[150]

WAVES.

Wave-motion.[151]—The most common waves, and from the present point of view the most important, are those generated by winds. During the passage of a wave, each particle affected by it rises and falls, and moves forward and backward describing an orbit in a vertical plane. If the passing wave is a swell, the orbit of the particle is closed and is either a circle or an ellipse; but in the case of a wind-wave the orbit is not closed. In such a wave two things move forward, the undulation and the water. The velocity of the undulation is relatively rapid; that of the water, slow and rhythmic. On the crest of the wind-wave each particle of water moves forward, and in the trough it moves less rapidly backward, and the excess of the forward movement over the backward gives it a slight residual advance. This residual advance is the initiatory element of current. By virtue of it, the upper layer of water is carried forward with reference to the layer below, in the direction toward which the wind blows. The waves of any considerable or long-continued wind, therefore, generate a current tending in the same direction as the wind.

The agitation of which waves are the superficial manifestation is not restricted to the surface, but is propagated indefinitely downward. Near the surface the amount of motion diminishes rapidly with increasing depth ([Fig. 299]), but the rate of diminution itself diminishes, and there seems no theoretic reason for assigning any definite limit to the downward propagation of the oscillation.