Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Climate has both a direct and an indirect effect on erosion. Its direct influence is through precipitation, evaporation, changes of temperature, and wind; its indirect, through vegetation. Like declivity and rock structure, climate does not affect all elements of erosion equally.

The chief elements of climate are temperature, moisture, and atmospheric movements; the principal factors which influence it are latitude, altitude, distance from the sea, direction of prevailing winds, and topographic relations.

The effects of variations in temperature on rock weathering have already been discussed ([p. 43]). They are chiefly mechanical, and are seen at their best where the daily range is great.

High temperature favors chemical action, and the weathering of rock by decomposition is at its best in the presence of abundant moisture in regions where the temperature is uniformly high. Furthermore, a warm moist climate favors the growth of vegetation, the decay of which supplies the water with organic acids which greatly increase its solvent power. The climatic conditions favoring mechanical weathering are therefore different from those favoring chemical weathering. High temperature and abundant moisture and vegetation are found in many tropical regions, and here the rock is often decomposed to greater depths, on the whole, than in high latitudes. How far this is the result of rapid weathering, and how far of slow removal, due in part to the protective influence of the plants, cannot be affirmed. If the weathered material is not removed, it will presently become a mantle thick enough to retard the processes which brought it into existence.

So long as the water of the surface and that in the soil remains unfrozen, temperature affects neither corrasion nor transportation. But in middle and high latitudes the surface is frozen for some part of each year. During this time corrasion is at a minimum, for although the streams continue to flow there is relatively little water running over the surface outside the drainage channels, and that little is relatively ineffective. Under some conditions, therefore, temperature affects both corrasion and transportation.

The humidity of the atmosphere has an influence even more important than that of temperature on the rate of erosion, and its influence is exerted on each of the elements of that complex process. A moist atmosphere favors oxidation, carbonation, hydration, and the growth of vegetation, all of which promote certain phases of rock weathering. On the other hand, humidity tends to prevent sudden and considerable variations in temperature, thus checking the weathering effected by this means. Precipitation, the most important single factor in determining the rate of erosion, is dependent on atmospheric humidity. Its amount, its kind (rain or snow), and its distribution in time, are the elements which determine its effectiveness in any given place.

Other things being equal the greater the amount of precipitation the more rapid the corrasion and transportation. Much, however, depends on its distribution in time. A given amount of rainfall may be distributed equally through the year, or it may fall during a wet season only. The maximum inequality of distribution would occur if all the rainfall of a given period were concentrated in a single shower. With such concentration the volume of water flowing off over the surface immediately after the down-pour would be greater than under any other conditions of precipitation, and since velocity is increased with volume, and erosive power with velocity, it follows that the erosive power of a given amount of water would be greater under these circumstances than under any other. Furthermore, a larger proportion of the precipitation would run off over the surface under these circumstances than under any other, for less of it would sink beneath the surface and less would be evaporated. If erosive power and rate of erosion were equal terms, this would therefore be the condition for greatest erosion; but erosive power and rate of erosion do not always correspond. If the water falling in this way could get hold of all the material it could carry, extreme concentration of precipitation would be the condition favorable for most rapid erosion. But if the amount of available material for transportation is slight, a large part of the force of the water could not be utilized in erosion. It follows that if there were a large amount of disintegrated material on the surface, erosion would be greater the greater the concentration of precipitation. If, on the other hand, there were but little disintegrated material on the surface, frequent showers, with intervening periods when conditions were favorable for weathering, that is, for preparing material for transportation, might be more favorable for rapid erosion. While the total energy of running water available for erosion under these conditions would be less than before, there might in the long run be more material for transport; for weathering in the presence of moisture, and all that goes with it, might be more effective in preparing material for transportation, than weathering during the long periods of drought which would occur if the precipitation were concentrated to its maximum. Temperature favoring, the uniform distribution of moisture through the year would allow the growth of vegetation, which, although favoring some processes of weathering, retards erosion in general. While therefore it is not possible to say what distribution of rainfall favors most rapid erosion without knowing the nature of the surface on which it is to fall, enough has been said to show that the problem is by no means a simple one. Some of the most striking phases of topography developed by erosion, such as those of the Bad Lands (Figs. [75 to 78], and [108]), are developed where the rainfall is unequally distributed in time, and too slight or too infrequent to support abundant vegetation.

Fig. 108.—Bad-land topography developed under conditions of aridity and unequal distribution of rainfall. Slope of Pinal Mountains, Ariz. (Ransome, U. S. Geol. Surv.)

During its fall, and immediately after, rain is more effective than an equal amount of snow; but the snow may be accumulated through a considerable period of the year, and then melted rapidly, when it has an effect comparable to that which would be produced by the concentration of the rainfall into a limited period of the year. If the ground beneath be frozen when the snow melts (and this is often the case) the erosion accomplished by the resulting water will be diminished.