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]).
Fig. 156.—Spheroidal weathering of an igneous rock.
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]).
Fig. 157.—Dome structure in granite mass, Yosemite valley, California (after a photograph by Sinclair).
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.