THE ATTACK OF THE WEATHER

The two contrasted processes of weathering.—It has already been pointed out that change and not stability is the order of nature. Within the earth’s outer shell and upon it rock alteration goes on continually, and from some portions of its surface the changed material is as constantly migrating to neighboring or even far distant regions. Before such transportation can begin the hard rock must first be broken down and reduced to fragments which the transporting agencies are competent to move.

To accomplish this breaking down, or degeneration, of the rock masses, either a wide range in temperature or chemical reaction is essential. In the atmosphere are found such active chemical agents as oxygen and carbon dioxide, the so-called carbonic acid gas; and these agents in the presence of water react chemically with the minerals of the rocks and form other minerals such as the hydrates and carbonates, which are lighter in weight and more soluble. This chemical attack upon the outer shell of the lithosphere is described as decomposition.

On the other hand the rock may succumb to changes which are purely mechanical and are due either to the stresses set up by differences between surface and interior temperatures, or to the prying action of the frost in the crevices. Such purely mechanical degeneration of the rocks is in contrast with decomposition and is described as disintegration. The two processes of decomposition and disintegration may, however, go on together; and the changes of volume that are caused by decomposition may result directly in considerable disintegration, as we are to see.

The rÔle of the percolating water.—In order to effect chemical change or reaction, it is essential that the substances which are to react must be brought into such intimate contact with each other as it is seldom possible to attain except by solution. The chemical reactions which go on between the gaseous atmosphere and the solid lithosphere are accomplished through solution of the gases in water. This water, derived from rain or snow, percolates into the ground or descends along the crevices in the rocks, carrying with it a certain measure of dissolved air. This air differs from that of the surrounding atmospheric envelope by containing relatively large amounts of oxygen and of the other active element carbon dioxide. It follows from the important rÔle thus performed by the percolating water that the process of decomposition will be relatively important in humid regions where the atmospheric precipitation is sufficient for the purpose.

Within hot and dry regions there is a larger measure of rock disintegration, and distinct chemical changes unlike those of humid regions take place in the higher temperatures and with the more concentrated saline solutions. The discussion of such changes will be deferred until desert conditions are treated in another chapter.

Mechanical results of decomposition—spheroidal weathering.—From an earlier chapter it has been learned that the rocks of the earth’s outermost shell are generally intersected by a system of vertical fissures which at each locality tend to divide the rock into parallel and upright rectangular prisms. It is these joints which offer relatively easy paths for the descent of the water into the rocks. In rocks of sedimentary origin there are found, in addition to the vertical joints, planes of bedding originally horizontal, and in the intrusive and volcanic rocks a somewhat similar parting, likewise parallel to the surface of the ground. The combined effect of the joints and the additional parting planes is thus to separate the rock mass into more or less perfect squared blocks (Fig. 155, upper figure) which stand in vertical columns.

The water which percolates downward upon the joints, finds its way laterally along the parting planes, and so subjects the entire surface of each block to simultaneous attack by its reagents. Though all parts of the surface of each block are alike subject to attack, it is the angles and the edges which are most vigorously acted upon. In the narrow crevices the solutions move but sluggishly, and as they are soon impoverished of their reagents in the attack upon the rock, fresh solution can reach the middle of the faces from relatively few directions. The edges are at the same time being reached from many more directions, and the corners from a still larger number.

The minerals newly formed by these chemical processes of hydration and carbonization are notably lighter, and hence more bulky than the minerals from whose constituents they have been largely formed. Strains are thus set up which tend to separate the bulkier new material from the core of unaltered rock below. As the process continues, distinct channels for the moving waters are developed favorable to action at the edges and corners of the blocks. Eventually, the squared block is by this process transformed into a spheroidal core of still unaltered rock wrapped in layers of decomposed material, like the outer wrappings of an onion. These in turn are usually imbedded in more thoroughly disintegrated material from which the shell structure has disappeared (Fig. 156).

Exfoliation or scaling.—A fact of much importance to geologists, but one far too often overlooked, is that rocks are but poor conductors for heat. It results from this that in the bright sun of a summer’s day a thin skin, as it were, upon the rock surface may be heated to a relatively high temperature, although the layer immediately below it is practically unaffected. The consequent expansion of the surface layer causes stresses that tend to scale it off from the layer below, which, uncovered in its turn, develops new strains of the same sort. This process of exfoliation acquires exceptional importance in desert regions where the rock surfaces are daily elevated to excessively high temperatures (see Chapter XV).

Dome structure in granite masses.—In large granite masses, such as are to be found in the ranges of the Sierra Nevada of California, a peculiar dome structure is sometimes found developed upon a large scale, and has had an important influence upon the breaking down of the rock and upon the shaping of the mountain (Fig. 157). Such a structure, made up as it is of prodigious layers, can have little in common with the veneers of weathered minerals which are the result of exfoliation, and it is quite likely that the dome structure is in some way connected with the relief of these massive rocks from their load—the rock which once rested upon them, but has been carried away by erosion since the uplift of the range.

The prying work of frost.—In all countries where winter temperatures range below the freezing point of water, a most potent agent of rock disintegration is the frost which pries at every crevice and cranny of the surface rock. Important in the temperate zones, in the polar regions it becomes almost the sole effective agent of rock weathering. There, as elsewhere, its efficiency as a disintegrating agent is directly dependent upon the nature of the crevices within the rock, so that the omnipresent joints are able to exercise a degree of control over the sculpturing of the surface features which is hardly to be looked for elsewhere (see plate 10 A).

Talus.—Wherever the earth’s surface rises in steep cliffs, the rock fragments derived from frost action, or by other processes of disintegration, as they become detached either fall or slide rapidly downward until arrested upon a flatter slope. Upon the earlier accumulations of this kind, the later ones are deposited, until their surface slopes up to the cliff face as steeply as the material will lie—the angle of repose. Such dÉbris accumulations at the base of a cliff (Fig. 158) are known as talus, and the slope is described as a talus slope, or in Scotland as a “scree.”

Soil flow in the continued presence of thaw water.—So soon as the rocks are broken down by the weathering processes, they are easily moved, usually to lower levels. In part this transportation may be accomplished by gravity slowly acting upon the disintegrated rock and causing it to creep down the slope. Yet even in such cases water is usually present in quantity sufficient to fill the spaces between the grains, and so act as a lubricant to facilitate the migration.

Upon a large scale rocks which were either originally incoherent or have been made so by weathering, after they have become saturated with water, may start into sudden motion as great landslides or avalanches, which in the space of a few moments materially change the face of the country, and by burying the bottom lands leave disaster and misery in their wake.

Within the subpolar regions, where a large part of the surface is for much of the year covered with snow, the underlying rocks are for long periods saturated with thaw water, and in alternation are repeatedly frozen and thawed. Essentially similar conditions are met with in the high, snow-capped mountains of temperate or torrid regions. For the subpolar regions particularly it is now generally recognized that somewhat special processes of soil flow, described under the name solifluction, are characteristic. The exact nature of these processes is as yet imperfectly understood, but there can be little doubt concerning the large rÔle which they have played in the transportation of surface materials. Such soil flow is clearly manifested under different aspects, and it is likely that by this comprehensive term distinct processes have been brought together.

Possibly the most striking aspect of the soil flow in subpolar regions is furnished by the remarkable “stone rivers” and “rock glaciers”; though the more generally characteristic are peculiar stripings or other markings which appear upon the surface of the ground and thus betray the movements of the underlying materials. Upon slopes it is not uncommon for the surface to be composed of angular rock fragments riven by the frost and crossed by broad parallel furrows as though a gigantic plow had gone over it (Fig. 159). The direction of the furrows is always up and down the slope, and the striping is marked in proportion as the slope is steep. Where the bottom is reached, the furrows are replaced by a sort of mosaic pavement of hexagonal repeating figures, each of which may be an area of the surface six feet or more across (Fig. 160, and Fig. 390, p. 368). The depressions which separate the “blocks” of the pavement are often filled with clay, while the inclosed surfaces are made up of coarsely chipped stone.

The splitting wedges of roots and trees.—In the mechanical breakdown of the rocks within humid regions a not unimportant part is sometimes taken by the trees, which insinuate the tenuous extremities of their rootlets into the smallest cracks and by continued growth slowly wedge even the firmer rocks apart (Fig. 161). In a similar manner the small tree trunk growing within a crevice of the rock may in time split its parts asunder (Fig. 162).

The rock mantle and its shield in the mat of vegetation.—Through the action of weathering, the rocks, as we have seen, lose their integrity within a surface layer, which, though it may be as much as a hundred feet or more in thickness, must still be accounted a mere film above the underlying bed rock. The mechanical agents of the breakdown operate only within a few feet of the surface, and the agents of rock decomposition, derived as they are from the atmosphere, become inert before they have descended to any considerable depth. The surface layer of incoherent rock is usually referred to as the rock mantle (Fig. 163). Where the rock mantle is relatively deep, as it is in the states south of the Ohio in the eastern United States, there is found, deep below the outer layer of soil, a partially decomposed and disintegrated rock, of which the unaltered minerals lie unchanged in position but separated by the new minerals which have resulted from the breakdown of their more susceptible associates. While thus in a certain sense possessing the original structure, this altered material is essentially incoherent and easily succumbs to attack by the pick and spade, so that it is only at considerably greater depths that the unaltered rock is encountered.

Because of the tendency of mantle rock to creep down upon slopes it is generally found thicker upon the crests and at the bases of hills and thinnest upon their slopes (Fig. 164).

In the transformation of the upper portion of the mantle rock into soil, additional chemical processes to those of weathering are carried through by the agency of earthworms, bacteria, and other organisms, and by the action of humus and other acids derived from the decomposition of vegetation. The bacteria particularly play a part in the formation of carbonates, as they do also in changing the nitrogen of the air into nitrates which become available as plant food. Within the humid tropical regions ants and other insects enter as a large factor in rock decomposition, as they do also in producing not unimportant surface irregularities.

How important is the cover of vegetation in retaining the rock mantle and the upper soil layer in their respective positions, as required for agricultural purposes, may be best illustrated by the disastrous consequences of allowing it to be destroyed. Wherever, by the destruction of forests, by the excessive grazing of animals, or by other causes, the mat of turf has been destroyed, the surface is opened in gullies by the first hard rain, and the fertile layer of soil is carried from the slopes and distributed with the coarser mantle upon the bottom lands. Thus the face of the country is completely transformed from fertile hills into the most desolate of deserts where no spear of grass is to be seen and no animal food to be obtained (plate 5 A). The soil once washed away is not again renewed, for the continuation of the gullying process now effectively prevents its accumulation.

Reading References to Chapter XI

Decomposition and disintegration:—

George P. Merrill. The Principles of Rock Weathering, Jour. Geol., vol. 4, 1896, pp. 704-724, 850-871. Rocks, Rock Weathering, and Soils. Macmillan, New York, 1897, Pt. iii, pp. 172-411.

Alexis A. Julien. On the Geological Action of the Humus Acids, Proc. Am. Assoc. Adv. Sci., vol. 28, 1879, pp. 311-410.

Corrosion of rocks:—

C. W. Hayes. Solution of Silica under Atmospheric Conditions, Bull. Geol. Soc. Am., vol. 8, 1897, pp. 213-220, pls. 17-19.

M. L. Fuller. Etching of Quartz in the Interior of Conglomerates, Jour. Geol., vol. 10, 1902, pp. 815-821.

C. H. Smyth, Jr. Replacement of Quartz by Pyrites and Corrosion of Quartz Pebbles, Am. Jour. Sci. (4), vol. 19, 1905, pp. 282-285.

Dome structure of granite masses:—

G. K. Gilbert. Domes and Dome Structure of the High Sierra, Bull. Geol. Soc. Am., vol. 15, 1904, pp. 29-36, pls. 1-4.

Ralph Arnold. Dome Structure in Conglomerate, ibid., vol. 18, 1907, pp. 615-616.

Soil flow:—

J. Gunnar Andersson. Solifluction, a Component of SubaËrial Denudation, Jour. Geol., vol. 14, 1906, pp. 91-112.

Otto NordenskiÖld. Die Polarwelt und ihre NachbarlÄnder, Leipzig, 1909, pp. 60-65.

Ernest Howe. Landslides in the San Juan Mountains, Colorado, etc., Prof. Pap., 67 U. S. Geol. Surv., 1909, pp. 1-58, pls. 1-20.

G. E. Mitchell. Landslides and Rock Avalanches, Nat. Geogr. Mag., vol. 21, 1910, pp. 277-287.

William H. Hobbs. Soil Stripes in Cold Humid Regions and a Kindred Phenomenon, 12th Rept. Mich. Acad. Sci., 1910, pp. 51-53, pls. 1-2.

Relation of deforestation to erosion:—

N. S. Shaler. Origin and Nature of Soils, 12th Ann. Rept. U. S. Geol. Surv., 1891, Pt. 1, pp. 268-287.

W. J. McGee. The Lafayette Formation, ibid., pp. 430-448.

F. H. King. Soils. Macmillan, New York, 1908, pp. 50-54.

Bailey Willis. Water Circulation and Its Control, Rept. Nat. Conserv. Com., 1909, vol. 2, pp. 687-710.

W. J. McGee. Soil erosion, Bull. 71, U. S. Bureau of Soils, 1911, pp. 60, pls. 33.

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