Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

INTERNAL ALTERATIONS OF ROCKS.

Besides the extreme alterations of rocks at the surface of the earth by which they pass into solution and into residual mantle-rock, and at length by transportation and re-sedimentation become stratified rocks, as just described, those rocks which are not at the surface are subject to changes that give rise to several varieties of altered rocks. These changes are taking place constantly under ordinary conditions, though usually very slowly. Under great pressure and heat the changes are relatively rapid and intense, and lead to results not reached under other conditions. These more profound changes are termed metamorphism, and will be considered later. It is, however, important to recognize the great fact that the outer part of the earth, for perhaps 20,000 or 30,000 feet, is more or less fractured and permeated by water containing in solution various substances dissolved from the atmosphere, the soil, and the rocks through which it has already passed, and that this permeating and circulating water is now, and for long ages has been, working changes in the rocks, partly by dissolving matter out of them, partly by depositing matter in them, and partly by furnishing a medium through which new combinations of their constituents may take place. This outer fractured portion of the lithosphere has been called the zone of fracture.[201]

Oxidation and deoxidation.—At and above the surface of the underground water, where the rocks are easily reached by atmospheric waters carrying much free oxygen, and by the air itself, oxidation prevails. Through oxidation the ferrous oxides are changed to ferric oxides, a change which is usually manifested by a transition from a gray, green, or blue color, to buff, brown, yellow, or red. The partial progress of such oxidation is often shown in a fractured block or bowlder whose exterior shows the latter colors, while the interior shows the former. The sulphides, of which common pyrites (FeS₂) is the most familiar, are oxidized into sulphates, and then sometimes pass on into the higher oxides and other compounds. Thus copperas (FeSO₄) arises from pyrites (FeS₂) by direct oxidation of both Fe and S. The sulphuric acid of this compound, uniting with some base stronger than the ferrous oxide, gives rise to further oxidation and results in hematite (Fe₂O₃) and limonite (Fe₂O₃,3H₂O). In general, the mineral constituents of the rocks in this upper zone take on their maximum states of oxidation. This oxidation affects more or less profoundly the character of the rock as a whole. Deeper in the earth oxidation is less prevalent, and the action is sometimes reversed and deoxidation takes place. So also wherever organic matter is undergoing decomposition deoxidation is likely to occur.

Solution and deposition.—Solution preponderates in the upper part of the zone of fracture, but deposition is prevalent in its deeper parts. The calcium carbonate and silica dissolved near the surface are often deposited below as calcite and quartz. The sulphates and other sulphur compounds that are formed and dissolved near the surface are apt to be changed into sulphides lower down by deoxidation. The soluble oxides and other compounds formed near the surface are often likewise precipitated below. This is particularly true where the descending waters encounter decomposing organic matter, and where they mingle with waters that have followed other routes and have become charged with different solutes. On coming together, reaction between the constituents takes place, resulting sometimes in new solutions and sometimes in precipitation.

If these lower deposits of calcite, quartz, sulphides, etc., are made in the pores of the rock, they change its texture and composition. If they are made in fissures they constitute veins, and if a sufficient percentage of the vein matter consists of valuable metallic compounds, they constitute ores.

As the waters descend they suffer greater and greater pressure and some increase of temperature, and these changes modify their power to hold substances in solution. In general, the waters increase in solvent power, but the effect is different for different mineral substances, and hence as a rule the waters are taking up some substances and laying down others as they proceed. After penetrating to greater or less depths, the waters may come again to the surface, either because they are pushed up by the higher head of the waters behind, or because they become warmer and thus lighter, and are forced up by the heavier cold waters above, or else they pass up by diffusion through the descending waters. In any case, the deep, warm waters, usually rather highly charged with material dissolved in their previous courses, are apt to deposit some of their burden as they ascend to horizons of lower pressures and temperatures. They are particularly liable to make deposits where they commingle with other waters differently charged with solutes. Thus internal changes in the body of the rocks are, and for ages have been, taking place. In the upper part of the depositing zone, calcite is the dominant mineral deposited, while in the lower, quartz is more common; but much depends on local conditions and other influences, and no rigid rule holds good.

Hydration and dehydration.—Water sometimes unites directly with some of the constituents of a rock and produces hydrated minerals, i.e., minerals that have water as an element of their constitution, not simply water absorbed into their pores. A large class of minerals known as zeolites, because they swell up and undergo life-like contortions when their basic water is driven off by heat, are examples of hydrous products. A more familiar example is limonite (Fe₂O₃,3H₂O), of which yellow ocher is a variety, which on heating sufficiently gives off its water and becomes hematite (Fe₂O₃) or red ocher. The turning of yellow clay to red brick on burning is a familiar example of dehydration. The general tendency in the upper zones penetrated by water is toward hydration. In the lower zones, where the pressure is great, Van Hise holds that there is a tendency toward dehydration, if the rocks have been previously hydrated. This may be the case if rocks have once been near the surface and later deeply buried by the accumulation of sediments on them. If the principle holds, rocks subjected to intense lateral pressure may be dehydrated.

Carbonation and decarbonation.—The igneous rocks are largely silicates. The carbonic acid of the surface-waters and of the air acting upon them, converts them, in part, into carbonates. In this way has arisen most of the original supply of calcium and magnesium carbonates. Original carbonates formed in this way are precipitated and redissolved again and again. The carbonates in river-waters are much more largely solutions of previously solid carbonates than original carbonates formed from the silicates. The potassium and sodium of the silicates also form carbonates, but by preference they unite with the sulphur and chlorine, and hence appear more largely as sulphates and chlorides.

Carbonation is usually accompanied by oxidation and hydration. These several processes break up the complex and relatively unstable silicates into simpler and more stable silicates, carbonates, and oxides. This is illustrated by the following formulas illustrative of the changes undergone by augite and labradorite, two common rock-forming minerals.

The composition of augite may be represented by the formula

	CaO.(Mg,Fe)O.2SiO2
	(Mg,Fe)O.(Al,Fe)2O3.SiO2.

Assuming Mg and Fe to be equal in amount in the first half of the above formula, and Mg and Fe to be equal in the first part of the second half, and Al and Fe to be equal in the last part of the second half, doubling the whole and allowing it to be acted on by CO₂ and H₂O, we have

2CaO.2MgO.2FeO.Al₂O₃.Fe₂O₃.6SiO₂ + 6CO₂ + 2H₂O
= 2CaCO₃ + 2MgCO₃ + 2H₂O.Al₂O₃.2SiO₂ + 2FeCO₃ + Fe₂O₃ + 4SiO₂.

The hydrous silicate of the last part of the equation is kaolin.

The composition of labradorite is represented by the formula

	CaO.Al2O3.2SiO2
	Na2O.Al2O3.6SiO2.

Assuming the two molecules represented by this formula to be equally abundant, and allowing the whole to be acted on by H₂O and CO₂, we have

CaO.Na₂O.2Al₂O₃.8SiO₂ + 4H₂O + 2CO₂
= CaCO₃ + Na₂CO₃ + 2(2H₂O.Al₂O₃.2SiO₂) + 4SiO₂.

When waters charged with carbonates descend into the earth they are likely to precipitate a portion of their burden, forming calcite and other crystalline carbonates, and hence these are among the most common minerals found in veins and rock cavities. Carbonates are also deposited when carbonate-charged waters come to the surface and evaporate or lose a part of their carbon dioxide.

Decarbonation also takes place, but it is, at least at the surface, a much less common process, and its conditions are less well understood. Sufficiently high heat will drive off the carbon dioxide, as in the artificial process of burning lime, but this is rarely observed in nature. Even lava intrusions do not usually reduce limestone to caustic lime at any appreciable distance from the contact. It is believed, however, that in the deeper zones, where high pressure and heat prevail, carbonates are changed into silicates, thus in a way reversing the process that prevails at the surface, and setting free again a portion of the carbon dioxide that had become locked up in the formation of the carbonates. To this action some of the carbon dioxide of deep-seated thermal springs is assigned.

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.