The work of the rain, which is both chemical and mechanical, varies greatly in accordance with climatic factors. The annual precipitation of two regions may be the same in amount, but in one the rainfall may be in very frequent, gentle showers, and in the other in less frequent, but far heavier and more violent downpours; under such different conditions the destructive work of the rain will be very different. Effects of still another kind are produced in those regions which have regularly alternating rainy and dry seasons. Temperature also modifies the work of the rain to an important degree, so that results are brought about in warm countries quite different from those observed in temperate and cold regions. Thus, each climatic zone exhibits the work of the rain with characteristic differences.
Perfectly pure water would act upon rocks with extreme slowness, but such water is not known to occur in nature. The raindrops, formed by the condensation of the watery vapour of the atmosphere, absorb certain gases which very materially increase the solvent power of the water. Of these gases the most important are oxygen (O) and carbon dioxide (C02), and all rain-water contains them.
It was formerly supposed that rain-water in percolating through the soil acquired additional destructive efficiency by absorbing certain products of vegetable decomposition called humous acids. Recent exact investigations, however, have thrown grave doubts upon the existence of these acids in the soil, and the effects which had been ascribed to them are now referred to carbon dioxide acting out of immediate contact with the atmosphere.
One of the first and simplest effects of atmospheric moisture consists in the hydration of the minerals exposed to it. Hydration, the taking up of water into chemical union, is an important agency of decay; it causes an increase of volume and thus greatly increases the pressure in the lower parts of rock masses which contain hydrating minerals. In the District of Columbia "granite rocks have been shown to have become disintegrated for a depth of many feet, with loss of but some 13.46 per cent of their chemical constituents. . . . Natural joint blocks brought up from shafts were, on casual inspection, sound and fresh. It was noted, however, that on exposure to the atmosphere, such not infrequently fell away to the condition of sand." (Merrill).
Oxidation affects especially the iron minerals and thus brings about conspicuous colour changes, for iron compounds form the principal colouring matter of the rocks and soils. Ferrous compounds give little colour, but the rocks in which they occur are apt to have a blue orgrey tint, due to other substances, both organic and inorganic. When such rocks are exposed to the action of air and water, the ferrous compounds are oxidized, producing ferric oxide and ferric hydrates, the former giving a red colour and the latter various shades of yellow and brown.
When fired in a kiln, a blue clay will yield red bricks, by the conversion of FeC03 into Fe203. In nature, rain-water effects a similar change, and the contrast between the superficial and deep-seated parts of the same rock is often as great as between blue clay and red brick. Weathered blocks stained red on the outside are often blue, grey, or nearly black on the inside, because the change has not affected the whole mass.
An especially important and wide-spread change is carbonation, due to the carbon dioxide which all natural waters contain in greater or less quantity. The silicates are attacked and decomposed in a manner that will be explained below, and, under certain circumstances, the insoluble ferric hydrates are converted into soluble ferrous carbonate.
Finally, solution plays a highly important role in the destructive work of the rain. All rocks contairf some soluble material, and when this soluble material is removed, the rock crumbles into a friable mass, which, on complete disintegration, forms soil. This may be illustrated by a block of frozen earth, which is as hard as many rocks, being cemented by the ice crystals, which bind the particles of soil together. When the ice is melted, the mass immediately becomes incoherent. So, in the rocks, the removal of even a small quantity of soluble material often causes the whole to crumble. Except vegetable moulds, all soils are derived from the decay and disintegration of rocks.
The chemical composition of the rock-forming minerals varies so much, that the processes which destroy them must vary correspondingly, differing in the case of the igneous rocks, on the one hand, from that of the sedimentary rocks, on the other.
Most igneous rocks are made up of crystals of some kind of felspar (see p. 13), associated with such minerals as augite (p. 16), hornblende (p. 16), and quartz (p. 12). In granite, for example, which is composed of orthoclase felspar, quartz, and mica or hornblende, rain-water slowly attacks the orthoclase by dissolving out the potash, probably as a carbonate, and also a considerable proportion of the combined silica. The aluminous silicate, which forms the residue, is hydrated as kaolinite (A1203, 2 Si02, 2 H20), forming clay, while the quartz is little or not at all affected. The mica, if present, is very slowly attacked, the hornblende more readily.
The decomposition of granite, then, results in the formation of clay, through which are scattered flakes of mica (if mica were originally present) and the unaltered grains of quartz. In the other igneous rocks the manner of decomposition is essentially similar; the complex silicates are broken up into simpler compounds, clay being derived from the aluminous silicates, especially the felspars, while the quartz, if present, is broken up into fragments and forms sand. The bases, potash, soda, lime, magnesia, iron, etc., are removed in solution, chiefly as carbonates, and more or less of the silica is also dissolved and carried away. Even when an igneous rock is yet firm and hard and, to the naked eye, appears to be quite unchanged, the microscope often reveals the first stages of decay.
Fig. 33. - Boulders of weathering, Eldon Mt., Arizona. (Photograph by A. E. Hackett, Flagstaff, Arizona).