Fig. 32.—Serrate peaks of granitic rock in Black Hills. (Darton, U. S. Geol. Surv.)
The disruption of rock by changes of temperature is one phase of weathering. It tends to the formation of a mantle of rock waste, which, were it not removed, would soon completely cover the solid rock beneath and protect it from further disruption by heating and cooling; but the loose material thus produced becomes an easy prey to running water, so that the work of the atmosphere prepares the way for that of other eroding agencies.
II. Evaporation and Precipitation.
Perhaps the most important work of the atmosphere as a dynamic agent lies in its function as the medium for the circulation and distribution of water. Atmospheric temperature is the primary factor governing evaporation, an important factor in the circulation of the vapor after it is formed, and controls its condensation and precipitation.
The average amount of annual precipitation on the land is variously estimated at from forty to sixty inches, the lesser figure being probably more nearly correct. Since much of this water falls at high altitudes, the work which it accomplishes in getting back to the sea is great. The water which falls on the land, if withdrawn wholly from the ocean, would exhaust that body of water in 10,000 to 15,000 years if none of it returned. The work of evaporation is of course not done by the atmosphere, though the atmosphere determines the effect of the solar energy which does the work.[25]
The precipitation is distributed with great inequality, and this inequality affects both the rain and the snow. Some regions have heavy precipitation and some light; some regions have much rain and little snow; others have much snow and little rain; others have rain and no snow, and still others have snow and little or no rain. The amount and distribution of rain and snow determine the size and distribution of streams and glaciers, and streams and glaciers are the most important agencies modifying the surface of the land.
It is impossible to separate sharply the geologic work of the water of the atmosphere from that of other waters; but so long as moisture is in the atmosphere (including the time of its precipitation) its effects are best considered in connection with the atmosphere.
The mechanical work of the rain.—In falling the rain washes the atmosphere, taking from it much of the dust, spores, etc., which the winds have lifted from the surface of the dry land. Not only this, but in passing through the atmosphere the water dissolves some of its gases, and perhaps particles of soluble solid matter. When therefore the falling water reaches the surface of the land it is no longer pure, and some of the gases it has taken up in its descent enable it to dissolve various mineral matters on which pure water has little effect.
As it falls on the surface of the land the rain produces various effects of a mechanical nature. In the first place, it leaves on the surface the solid matter taken from the air. The amount of material, thus added to any given region in any particular shower is trivial, but in the course of long periods of time the total amount of material washed out of the air must be very great.