B. THE ATMOSPHERE AS A CONDITIONING AGENCY.
The most obvious mechanical work of the atmosphere is effected by the wind, but mechanical results of great importance, conditioned by the atmosphere, are also effected when the air is still.
I. Temperature Effects.
When the sun shines on bare rock its surface is heated and expanded, and the expanded particles crowd one another with great force. Since rock is a poor conductor of heat its surface is heated and expanded notably more than parts beneath the surface. It follows that strains are set up between the expanded outer portion and the cooler and less expanded parts within. In the cooling of the same rock mass it is the outermost portion which cools first and fastest, and, contracting as it cools, strains are again set up between the outer part, which is cooled more, and the inner part, which is cooled less. The result may be illustrated by the effect of cold water on hot glass, or of hot water on cold glass. In either case the fracture is the result of the sudden and considerable differential expansion or contraction. Since the heating and cooling of rock are much slower than the heating and cooling of glass under the conditions mentioned, the rupturing effects are less conspicuous, but none the less real. The actual effects of temperature changes are illustrated by familiar phenomena. The surface portions of bowlders exposed to the sun are frequently seen to be shelling off ([Fig. 26]). The loosened concentric shells may be a fraction of an inch, or sometimes even several inches in thickness. This process of exfoliation affects not only bowlders, but bare rock surfaces wherever exposed to the sun (Figs. [27], [28]). It is often conspicuous on the faces of cliffs.
Fig. 26.—Exfoliation. A bowlder of weathering, the rock being granite. Wichita Mountains, Oklahoma.
Fig. 27.—A weathered summit of granite in the Wichita Mountains. Oklahoma. (Willis, U. S. Geol. Surv.)
Several conditions, some of which are connected with the atmosphere and some with the rock, determine the efficiency of this process. Since the breaking of the rock results from the expansion and contraction due to its changes of temperature, it follows that, other things being equal, the greater the change, the greater the breaking; but the suddenness of the temperature change is even more important than its amount. It follows that great daily, rather than great annual, changes of temperature[20] favor rock-breaking, though with changes of a given frequency their effectiveness is greater the greater their range. A partial exception to this generalization should be noted. If abundant moisture is present in the pores and cracks of the rock a change of temperature from 45° to 35° (Fahr.) might be far less effective in breaking the rock than a change from 35° to 25° in the same time, for in the latter case the sudden and very considerable expansion (about one-tenth) which water undergoes on freezing is brought into play. This may be called the wedge-work of ice. The daily range of temperature is influenced especially by latitude, altitude, and humidity. Other things being equal, the greatest daily ranges of temperature occur in high-temperate latitudes, though to this general statement there are local exceptions, depending on other conditions. High altitudes favor great daily ranges of temperature, so far as the rock surface is concerned (see Figs. [29], [30]), for though the rock becomes heated during the sunny day, the thinness and dryness of the atmosphere allow its heat to radiate rapidly at night. Here, too, the daily range of temperature is likely to bring the wedge-work of ice into play. Since the south side of a mountain (in the northern hemisphere) is heated more than the north, it is subject to the greater daily range of temperature, and the rock on this side suffers the greater disruption. Similarly, rock surfaces on which the sun shines daily are subject to greater disruption than those much shielded by clouds. Isolated peaks, because of their greater exposure, are subject to rather greater daily ranges of temperature than plateaus of the same elevation.
Fig. 28.—Exfoliation on a mountain slope. Mt. Starr-King (Cal.) from the north.
The daily range of temperature is also influenced by humidity. Because of the effect of water vapor in the atmosphere on insolation and radiation, a rock surface becomes hotter in the day and cooler at night beneath a dry atmosphere than beneath a moist one. Aridity therefore favors the disruption of rock by changing temperatures.
Turning from the conditions of the atmosphere which affect the disruption of rock to the conditions of the rock which influence the same process, several points are to be noted. In the first place, the disrupting effects of changes of temperature are slight or nil where the solid rock is protected by soil, clay, sand, gravel, snow, or other incoherent material. If the constituent parts of the loose material are coarse, like bowlders, their surfaces are affected like those of larger bodies of rock. The color of rock, its texture and its composition, also influence its range of daily temperature by influencing absorption and conduction. Dark-colored rocks absorb more heat than light-colored ones, and compact rocks are better conductors than porous ones. Great absorption and slow conduction favor disruption. A given range of temperature is unequally effective on rocks of different mineral composition. In general crystalline rocks (igneous and metamorphic) are more subject to disruption by this means than sedimentary rocks, partly because they are more compact, but especially because they are made up of aggregates of crystals of different minerals which, under changes of temperature, expand and contract at different rates, while the common sedimentary rocks are made up largely of numerous particles of one mineral.
Fig. 29.—Top of Notch Peak, Bighorn Mountains, Wyo. Shows the thoroughly broken character of the rock on the summit, the absence of soil, vegetation, etc. (Kümmel.)
Fig. 30.—A detail from [Fig. 29] showing the size of the rock blocks. (Kümmel.)
Fig. 31.—Peak north of Kearsarge Pass, the Sierras. Shows the way in which serrate peaks break up into angular blocks.
The freezing of water in the pores of rock is effective in disrupting them only when the pores are essentially full at the time of freezing. Otherwise there is room for the expansion attending the freezing. If the pores of the rock are large, the expansion on freezing may force out sufficient water to balance the increase of volume, even though the rock was completely saturated. If the pores be very small the water passes out less readily, and if the rock is saturated, freezing is more likely to be attended with disruption.[21]
In view of these considerations the breaking of rock by changes of temperature should be greatest on the bare slopes of isolated elevations of crystalline rock, where the temperature conditions of temperate latitudes prevail, and where the atmosphere is relatively free from moisture. All these conditions are not often found in one place, but the disrupting effects of changing temperatures are best seen where several of them are associated (Figs. [29], [30], and [31]).
The importance of this method of rock-breaking has rarely been appreciated except by those who have worked in high and dry regions. Climbers of high mountains know that almost every high peak is covered with broken rock to such an extent as to make its ascent dangerous to the uninitiated. High serrate peaks, especially of crystalline rock, are, as a rule, literally crumbling to pieces ([Fig. 31]). The piles of talus which lie at the bases of steep mountain slopes are often hundreds of feet in height, and their materials are often in large part the result of the process here under discussion. In mountain regions where atmospheric conditions favor sudden changes of temperature, the sharp reports of the disruption of rock masses are often heard. Masses of rock, scores and even hundreds of pounds in weight, are frequently thus detached and started on their downward course.[22] Small pieces of rock are of course much more commonly broken off than large ones. The disruption of rock by changes of temperature is not usually the result of a single change of temperature, but rather of many successive expansions and contractions.
The sharp needle-like peaks which mark the summits of most high mountain ranges ([Fig. 32]) are largely developed by the process here outlined. The altitude at which the serrate topography appears varies with the latitude, being, as a rule, higher in low latitudes and lower in high. But even in the same latitude it varies notably with the isolation of the mountains and with the aridity of the climate. Thus within the United States the sharply serrate summits appear in some places, as in Washington and Oregon, at altitudes of 6000 to 10,000 feet, while in the isolated Wichita range of Oklahoma, much farther south, but in a much drier climate, the same sort of topography is developed at altitudes of 2500 to 3000 feet.
Even in low latitudes and moist climates the effects of temperature changes are often seen. Thin beds of limestone at the bottom of quarries are sometimes so expanded by the heat of the sun as to arch up and break.[23] In desert and arid regions,[24] whatever the altitude, the effects of temperature changes are often striking.
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.
Every rain-drop strikes a blow. If the drops fall on vegetation, they have little effect, but if they fall on sand or unprotected earthy matter they cause movements of the particles on one another, and this movement involves friction and wear. While the results thus effected are inconsiderable in any brief period of time, they are not so insignificant when the long periods of the earth’s history are considered.
Clayey soils contract and often crack on drying. Falling on such a soil when it is dry the rain causes it to expand, and the cracks are healed by lateral swelling. The same soils are baked under the influence of the sun, and when in this condition are softened and made more mobile by the falling of rain. Under the influence of the expansion and contraction occasioned by wetting and drying, the soils and earths on slopes creep slowly downward. When rain falls on dry sand or dust the cohesion is at once increased, and shifting by the wind is temporarily stopped.
After the water has fallen on the land its further work cannot be looked upon as a part of the work of the atmosphere; but any conception of the geological work of the atmosphere which did not recognize the fact that the waters of the land have come through the atmosphere would be inadequate. The work of the water after it has been precipitated from the atmosphere must be considered in another chapter.
III. Effects of Electricity.
Another dynamic effect conditioned by the atmosphere is that produced by lightning. In the aggregate this result is inconsequential; yet instances are known where large bodies of rock have been fractured by a stroke of lightning, and masses many tons in weight have sometimes been moved appreciable distances. Incipient fusion in very limited spots is also known to have been induced by lightning. Where it strikes sand it often fuses the sand for a short distance, and, on cooling, the partially fused material is consolidated, forming a little tube or irregular rod (a fulgurite) of partially glassy matter. Fulgurites are usually only a few inches in length, and more commonly than otherwise a fraction of an inch in diameter. Strictly speaking these results are the effect of the electricity of the atmosphere rather than of the atmosphere itself, but they are best mentioned in this connection.
Allusion has already been made to the chemical changes in the atmosphere occasioned by electric discharges.
Fig. 33.—Stratified jointed rock in process of weathering. (Cross, U. S. Geol. Surv.)
Fig. 34.—Represents a later stage of the processes illustrated by [Fig. 33]. (Darton, U. S. Geol. Surv.)