Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

The carbonation of the silicates takes place at the expense of the carbon dioxide of the atmosphere and hydrosphere, and hence in proportion as the igneous rocks are changed into carbonates, the atmosphere and hydrosphere are depleted of carbon dioxide, new supplies being neglected. As plants are dependent on carbon dioxide for their principal food, and as animals are dependent on plants for their food, directly or indirectly, the process of carbonation has a profound bearing on the life-history of the earth, and will often invite attention in the historical chapters. It is sufficient here to note that carbonation is one of the chief processes in the alteration of igneous rocks and furnishes, directly and indirectly, a larger percentage of the mineral substances dissolved in the waters that flow from the land, than any other single process.

Molecular rearrangements.—Besides these and similar changes that involve additions and subtractions through the agency of percolating water, the molecules of some of the rock constituents rearrange themselves, or the elements enter into new chemical relations; thus, pyroxene may pass into hornblende by a change of the crystalline arrangement of the molecules. The change may sometimes be caught in progress, the outer part of the crystal being hornblende (which when thus formed is called uralite), while the heart of the crystal remains pyroxene. So aragonite may pass into calcite.

By changes of the foregoing kinds, many crystalline rocks are much altered. Some become chloritic from the development of the soft, green hydrated mineral, chlorite, derived from the pyroxene, amphibole, biotite, and perhaps other silicates of the original rock. Others become talcose from the development of talc, a very soft, unctuous, hydrous magnesian silicate developed from the magnesian minerals of the original rock. Soapstone or steatite is a rock composed essentially of such secondary material. Serpentine is a rock made up of a similar secondary mineral (serpentine) apparently derived from chrysolite (olivine) and other magnesian minerals. Epidote, a complex lime-iron-alumina silicate, often recognizable by its peculiar pistachio-green color, is derived from other silicates, and is rather common in many varieties of crystalline rocks. Melaphyre is a name applied rather loosely and variously to certain altered basic rocks of the basalt family. Diabase is essentially an altered dolerite. Nearly all the very ancient basaltic rocks show notable degrees of alteration, even though they appear to have escaped unusual dynamic conditions since their original formation, and hence their alteration seems to have resulted chiefly from the operation of unobtrusive agencies, chief among which is the circulation of water.

The Salient Features of Rock Descent.

The foregoing processes by which primitive or igneous rocks are disintegrated and their constituents converted into fragmental material may be said to constitute the descent of rocks in its fuller sense. Viewed chemically, the great features of the process are (1) the breaking down of the complex silicates, and (2) the gathering of the resultant simpler silicates (mainly aluminum silicates) into the silt and clay beds, (3) the assembling of a large part of the free acidic element (the quartz) into the sand and gravel beds, and (4) the concentration of a large part of the earthy basic element (the calcium, magnesium, and iron oxides) into the calcareous, magnesian, and iron deposits, while (5) a large part of the alkaline basic remainder (the sodium, and potassium oxides) is dissolved and held in the sea-water. Physically, the great features are (1) the disaggregation of the antecedent rock, and (2) the separation from one another of products which are physically unlike, that is, the coarser from the finer, and the heavier from the lighter, and (3) the aggregation of these diverse materials in more or less distinct beds. It is to be noted that while the rearrangement of the sediments is made on the basis of their physical characters, it results in chemical differentiation as well, for the products of rock decay, which are physically diverse, are often chemically diverse as well. The physical assortment and the stratification are to be looked upon as a step in the direction of a simpler grouping of the material. On the whole, the process is descensional in character.

THE REASCENSIONAL PROCESS.

Running hand in hand with this descensional process, there has always been a reascensional process by which the coherence, the crystallization, and in some measure the complex composition of the rocks are restored. This is partially due to external mechanical agencies, but chiefly to internal chemical and molecular forces.

Two general phases of this reconstructional work are recognized. The first, simplest and most universal, is that by which the incoherent materials produced by the descensional processes, i.e., the muds, sands, and clastic materials generally, are hardened into firm, coherent shales, sandstones, and limestones, and incidentally more or less changed in composition and molecular arrangement. The second is that by which more profound changes of induration and of composition are wrought, bringing the rock back to a state resembling its original crystalline character. This is known as metamorphism. Often, however, it is but an extension and intensification of the more common processes of the first class. Metamorphism is essentially reconstruction.

Induration under ordinary pressures and temperatures.—All kinds of loose fragmental material, whether soils, earths, clays, sands, gravels, volcanic ashes, cinders, or other forms of clastic or pyroclastic material, may become hardened into firm rock either by pressure, or by cementation, or by both. Pressure and cementation commonly act together and aid each other. The ordinary pressures arise from the weight of the overlying material, and these of course increase with depth. Extraordinary pressures arise from the shrinkage of the earth and perhaps from other sources. The fragments of the clastic material, on being pressed together for long periods, weld more or less at the points of contact. If they are irregular, angular, or elongate, they come to interlock more or less like the fragments of macadam, and this coöperates with the welding. The process is greatly aided by water-bearing solutions of lime, silica, etc. which are deposited at the points where the fragments press upon each other. It is here that the capillary spaces are most minute and deposition is most liable to take place. Sometimes a film of mineral matter is laid down over the surfaces of the fragments and serves to bind them together. This process goes on wherever the ground-waters are in a depositing condition, just as the opposite process of disintegration takes place wherever the waters are in a solvent state. At and near the surface of the land, the waters are usually in the latter condition and disintegration is in progress, as already noted, but this is not always so. At times and places, the water from within the rock-mass may come to the surface and evaporate, and in so doing leave all its dissolved material on the surface, or within the outer pores of the mass, as cementing material. The exterior thus becomes firmly bound together, “case-hardened,” as it is termed. This may be seen in the drying of a lump of mud, the exterior of which often becomes quite firm. It is seen in quarry-rock, especially sandstone, which is sometimes soft and easily worked when taken wet from the earth, but which hardens as the water—the “sap” of the quarrymen—dries out and deposits its solutes in the capillary spaces of the grains of the surface. It is obvious that it is the very last of the “sap” which contains the most concentrated solutes, and that this last remnant is held in the minute capillary spaces where the grains touch each other, and hence the last stage of drying leaves the cement at the points where it is most effective. In natural exposures of sandstone, the pores of the outer shell sometimes become almost completely filled in this way with silicious deposits, and the sandstone is changed into a quartzite.

In the sea, and in the deep water underground, the common habit of the water is to deposit more than to dissolve, though it is doing more or less of both. As a rule, therefore, loose material in these situations becomes bound more or less firmly into rock, and hence what were originally loose sand beds become sandstones; what were soft muds become shales or limestone, according to composition; what was gravel becomes conglomerate; what was chipstone becomes breccia; what were volcanic ashes, cinders, and lapilli become tuffs; and what were masses of volcanic blocks and coarse fragments become agglomerates.