The Economic Aspect of Geology Part 2

No estimate is made of abundance, but obviously the total volume of these products is small as compared with the major cla.s.ses of earth materials above noted, and in large part they may be included with these major cla.s.ses.

WATER (HYDROSPHERE)

It has been estimated that all the water of the earth, including the ocean, surface waters, and underground waters, const.i.tutes about 7 per cent of the volume of the earth to a depth of 10 miles.[3]

COMPARISON OF LISTS OF MOST ABUNDANT ROCKS AND MINERALS WITH COMMERCIAL ROCKS AND MINERALS

Of the common rocks and minerals figuring as the more abundant materials of the earth's crust, only a few are prominently represented in the tables of mineral resources. Of these water and soils stand first.

Others are the common igneous and sedimentary rocks used for building and road materials. Missing from the lists of the most abundant minerals and rocks, are the greater part of the commercially important mineral resources--including such as coal, oil, gas, iron ore, copper, gold, and silver,--implying that these mineral products, notwithstanding their great absolute bulk and commercial importance, occur in relatively insignificant amounts as compared with the common rock minerals of the earth.

THE ORIGIN OF COMMON ROCKS AND MINERALS

The common rocks and minerals develop in a general sequence, starting with igneous processes, and pa.s.sing through stages of weathering, erosion, sedimentary processes, and alterations beneath the surface. The commercial minerals are incidental developments under the same processes.

IGNEOUS PROCESSES

The earliest known rocks are largely igneous. Sedimentary rocks are formed from the breaking down of igneous rocks, and the origin of rocks therefore starts with the formation of igneous rocks. Igneous rocks are formed by the cooling of molten rock material. The ultimate source of this molten material does not here concern us. It may come from deep within the earth or from comparatively few miles down. It may include preexisting rock of any kind which has been locally fused within the earth. Wherever and however formed, its tendency is to travel upward toward the surface. It may stop far below the surface and cool slowly, forming coa.r.s.ely crystallized rocks of the granite and gabbro types.

Igneous rocks so formed are called _plutonic_ intrusive rocks. Or the molten ma.s.s may come well toward the surface and crystallize more rapidly into rocks of less coa.r.s.e, and often porphyritic, textures. Such intrusive rocks are porphyries, diabases, etc. Or the molten ma.s.s may actually overflow at the surface or be thrown out from volcanoes with explosive force. It then cools quickly and forms finely crystalline rocks of the rhyolite and basalt types. These are called effusives or extrusives, or lavas or volcanics, to distinguish them from intrusives formed below the surface. The intrusive ma.s.ses may take various forms, called stocks, batholiths, laccoliths, sills, sheets and dikes, definitions and ill.u.s.trations of which are given in any geological textbook. The effusives or volcanics at the surface take the form of sheets, flows, tuffs, agglomerates, etc.

Some of the igneous rocks are themselves "mineral" products, as for instance building stones and road materials. Certain basic intrusive igneous rocks contain t.i.taniferous magnet.i.tes or iron ores as original const.i.tuents. Others carry diamonds as original const.i.tuents. Certain special varieties of igneous rocks, known as pegmat.i.tes, carry coa.r.s.ely crystallized mica and feldspar of commercial value, as well as a considerable variety of precious gems and other commercial minerals.

Pegmat.i.tes are closely related to igneous after-effects, discussed under the next heading. As a whole, the mineral products formed directly in igneous rocks const.i.tute a much less important cla.s.s than mineral products formed in other ways, as described below.

=Igneous after-effects.= The later stages in the formation of igneous rocks are frequently accompanied by the expulsion of hot waters and gases which carry with them mineral substances. These become deposited in openings in adjacent rocks, or replace them, or are deposited in previously hardened portions of the parent igneous ma.s.s itself. They form "contact-metamorphic" and certain vein deposits. Pegmat.i.tes, referred to above, are in a broad sense in this cla.s.s of "igneous after-effects," in that they are late developments in igneous intrusions and often grade into veins clearly formed by aqueous or gaseous solutions. Among the valuable minerals of the igneous after-effect cla.s.s are ores of gold, silver, copper, iron, antimony, mercury, zinc, lead, and others. While mineral products of much value have this origin, most of them have needed enrichment by weathering to give them the value they now have.

WEATHERING OF IGNEOUS ROCKS AND VEINS

No sooner do igneous rocks appear at or near the earth's surface, either by extrusion or as a result of removal by erosion of the overlying cover, than they are attacked vigorously by the gases and waters of the atmosphere and hydrosphere as well as by various organisms,--with maximum effect at the surface, but with notable effects extending as far down as these agents penetrate. The effectiveness of these agents is also governed by the climatic and topographic conditions. Under conditions of extreme cold or extreme aridity, weathering takes the form mainly of mechanical disintegration, and chemical change is less conspicuous. Under ordinary conditions, however, processes of chemical decomposition are very apparent. The result is definitely known. The rocks become softened, loose, and incoherent. Voids and openings appear.

The volume tends to increase, if all end products are taken into account. The original minerals, largely feldspar, ferro-magnesian minerals, and quartz, become changed to clay, mixed with quartz or sand, calcite or dolomite, and iron oxide, together with residual particles of the original feldspars and ferro-magnesian minerals which have only partly decomposed. In terms of elements or chemical composition, water, oxygen, and carbon dioxide, all common const.i.tuents of the atmosphere and hydrosphere, have been added; and certain substances such as soda, pota.s.sa, lime, magnesia, and silica have in part been carried away by circulating waters, to be redeposited elsewhere as sediments, vein fillings, and cements. Figure 1 ill.u.s.trates the actual mineral and volume changes in the weathering of a granite--one of the most common rocks. The minerals anorthite, albite, and orthoclase named in this figure are all feldspars; sylvite and halite are chlorides of potash and soda. The weathering processes tend to destroy the original minerals, textures, and chemical composition. They are collectively known as _katamorphic_ alterations, meaning destructive changes. The zone in which these changes are at a maximum is called the _zone of weathering_. This general zone is princ.i.p.ally above the surface or level of the ground-waters, but for some rocks it extends well below this level. In some regions the ground-water level may be nearly at the surface, and in others, especially where arid, it may be two thousand or more feet down. Disintegrated weathered rocks form a blanket of variable thickness, which is sometimes spoken of as the residual mantle, or "mantle rock."

[Ill.u.s.tration: FIG. 1. Graphic representation of volume change in weathering of a Georgia granite.]

Mineral products formed by weathering from common igneous rocks include soils, clay, bauxite, and certain iron, chromite, and nickel ores. Again the commercial importance of this group is not large, as compared with products formed in other ways described below.

The same weathering processes described above for igneous rocks cause considerable changes of economic significance in deposits formed as igneous after-effects. In some cases they result in removing the less valuable minerals, thus concentrating the more valuable ones, as well as in softening the rock and making it easier to work; and in other cases they tend to remove the valuable const.i.tuents, which may then be redeposited directly below or may be carried completely out of the vicinity. The _oxide zones_ of many ore bodies are formed by these processes.

SEDIMENTARY PROCESSES

Sedimentary rocks are formed by the removal and deposition of the weathered products of a land surface. Air, water, and ice, moving under the influence of gravity and other forces, all aid in this transfer. The broken or altered rock materials may be merely moved down slopes a little way and redeposited on the surface, forming one type of _terrestrial_ or _subaerial deposits_, or they may be transferred and sorted by streams. When deposited in streams or near their mouths, they are known as _river_, _alluvial_, or _delta deposits_. When carried to lakes and deposited they form _lake deposits_. Ultimately the greater part of them are likely to be carried to the ocean and deposited as _marine sediments_.

Part of the weathered substances are carried mechanically as clay and sand, which go to make up the _shale_ and _sandstone_ sediments. Part are carried in solution, as for example lime carbonate and magnesium carbonate, which go to make up _limestone_ and _dolomite_. Some of the dissolved substances are never redeposited, but remain in solution as salts in the sea, the most abundant of which is sodium chloride. Some of the dissolved substances of weathering, such as calcite, quartz, and iron oxide, are carried down and deposited in openings of the rocks, where they act as cements.

The sediments as a whole consist of three main types,--_shales_ (kaolin, quartz, etc.), _sandstones_ (quartz, feldspar, etc.), and _limestones_ or _dolomites_ (carbonates of lime and magnesia). Of these, the shale group is by far the most abundant. There are of course many sediments with composition intermediate between these types. There are also sediments made up of large undecomposed fragments of the original rocks, cemented to form _conglomerates_, or made up of small fragments of the original rocks cemented to form _arkoses_ and _graywackes_. These, however, may be regarded as simply stages in the alteration, which in repeated cycles of weathering must ultimately result in producing the three main groups,--shales, sandstones, and limestones.

Mineral products formed by sedimentary processes include sandstones, limestones, and shales, used as building stone and road materials; certain sedimentary deposits of iron, like the Clinton ores of the southeastern United States and the Brazilian ores; important phosphate deposits; most deposits of salt, gypsum, potash, nitrates, etc.; comparatively few and unimportant copper deposits; and important placer deposits of gold, tin, and other metals, and precious stones. With the aid of organic agencies, sedimentary processes also account for the primary deposition of coal and oil.

WEATHERING OF SEDIMENTARY ROCKS

After sedimentary rocks are formed, and in many cases covered by later sediments, they may be brought again by earth movements and erosion to the surface, where they in turn are weathered. The weathering of sedimentary rocks proceeds along lines already indicated for the igneous rocks. Residual mantles of impure clay and sand are commonly formed.

The mineral composition of sedimentary rocks being different from that of igneous rocks to start with, the resulting products are in slightly different proportions; but the changes are the same in kind and tend merely to carry the general process of alteration farther in the same direction,--that is, toward the production of a few substances like clay, quartz, iron oxide, and calcite, which are transported and redeposited to form clay, sand, and limestone. Cycles of this kind may be repeated indefinitely.

By weathering of sedimentary rocks are produced some soils, certain commercial clays, iron ores, lead and zinc ores, and other valuable mineral products.

CONSOLIDATION, CEMENTATION, AND OTHER SUBSURFACE ALTERATIONS OF ROCKS.

=Cementation.= No sooner are residual weathered mantles formed or sedimentary rocks deposited, whether under air or water, than processes of consolidation begin. Settling, infiltration of cementing materials, and new growths, or recrystallization, of the original minerals of the rock all play a part in the process. The mud or clay becomes a shale, the sand becomes sandstone or quartzite, the marl becomes limestone or marble. All the minute openings between the grains, as well as larger openings such as fissures and joints, may thus be filled. At the same time the cementing materials may replace some of the original minerals of the rock, the new minerals either preserving or destroying the original textures. This process is sometimes called _metasomatic replacement_. Igneous rocks as a rule are compact, and hence are not so much subject to the processes of cementation as sedimentary rocks; but certain of the more porous phases of the surface lavas, as well as any joints in igneous rocks, may become cemented. All of these changes may be grouped under the general term _cementation_.

A special phase of consolidation and cementation is produced near intrusive igneous rocks through the action of the heat and pressure and the expelled substances of the igneous rock. This is called _contact metamorphism_ or _thermal metamorphism_. The processes are even more effective when acting in connection with the more intense metamorphism described under the next heading.

By cementation some of the common rocks, especially the sediments, become sufficiently compact and strong to be useful as commercial products, such as building stones and road materials.

More important as mineral products are the cementing materials themselves. These are commonly quartz, calcite, or iron oxide, of no especial value, but locally they include commercially valuable minerals containing gold, copper, silver, lead, zinc, and many other mineral products.

It is a matter of simple and direct observation, about which there is no controversy, that many minerals are deposited as cements in the openings in rocks or replacing rocks. As to the source of the solutions bringing in these minerals, on the other hand, there has been much disagreement.

In general, the common cementing materials such as quartz and calcite, as well as some of the commercial minerals, are clearly formed as by-products of weathering, and are transported and redeposited by the waters penetrating downward from the surface. The so-called _secondary enrichment_ of many valuable veins is merely one of the special phases of cementation from a superficial source. In other cases it is believed that deep circulation of ordinary ground-waters may pick up dispersed mineral substances through a considerable zone, and redeposit them in concentrated form in veins and other trunk channels. For still other cementing materials, it is suspected that the ultimate source is in igneous intrusions; in fact, deposits of this general character show all gradations from those clearly formed by surface waters, independently of igneous activity, to those of a contact-metamorphic nature and others belonging under the head of "igneous after-effects."

Hypothesis and inference play a considerable part in arriving at any conclusion as to the source of cementing materials,--with the result that there is often wide lat.i.tude for difference of opinion and of emphasis on the relative importance of the different sources of ore minerals.

=Dynamic and contact metamorphism.= Beneath the surface rocks are not only cemented, but may be deformed or mashed by dynamic movements caused by great earth stresses; the rocks may undergo rock flowage. The result is often a remarkable transformation of the character of the rocks, making it difficult to recognize their original nature. Also, igneous intrusions may crowd and mash the adjacent rocks, at the same time changing them by heat and contributions of new materials. This process may be called _contact metamorphism_, but in so far as it results in mashing of the rocks it is closely allied to _dynamic metamorphism_. The former term is also applied to less profound changes in connection with igneous intrusions, which result merely in cementation without mashing.

Dynamic and contact metamorphism may in some cases produce rocks identical in appearance with those produced by ordinary processes of cementation and recrystallization without movement. For instance, it is difficult to tell how much movement there has been in the production of a marble, because both kinds of processes seem to produce much the same result. Commonly, however, the effect of dynamic metamorphism is to produce a parallel arrangement of mineral particles and to segregate the mineral particles of like kind into bands, giving a _foliated_ or _schistose_ or _gneissic_ structure, and the rocks then become known as slates, schists, or gneisses. Commonly they possess a capacity to part along parallel surfaces, called _cleavage_. The development of the schistose or gneissic structure is accompanied by the recrystallization of the rock materials, producing new minerals of a platy or columnar type adapted to this parallel arrangement. Even the composition of the rock may be substantially changed, though this is perhaps not the most common case. Whereas by weathering the rock is loosened up and disintegrated, substances like carbon dioxide, oxygen, and water are abundantly added, and light minerals of simple composition tend to develop,--by dynamic metamorphism on the other hand, carbon dioxide, oxygen, and water are usually expelled, the minerals are combined to make heavier and more complex minerals, pore s.p.a.ce is eliminated, and altogether the rock becomes much more dense and crystalline. While segregation of materials is characteristic of the surficial products of weathering, the opposite tendency, of mixing and aggregation, is the rule under dynamic metamorphism, notwithstanding the minor segregation above noted.

Dynamic metamorphism is for the most part unfavorable to the development of mineral products. Ore bodies brought into a zone where these processes are active may be profoundly modified, but not ordinarily enriched. One of the exceptions to this general rule is the development of the cleavage of a slate, which enables it to be readily split and thereby gives it value. Contact metamorphism, on the other hand, may develop valuable mineral deposits (see pp. 20, 45-46).

THE METAMORPHIC CYCLE AS AN AID IN STUDYING MINERAL DEPOSITS

All of the chemical, mineralogical, and textural changes in rocks above described may be collectively referred to as _metamorphism_. The phase of metamorphism dealing with surficial weathering, similar changes below the surface, and the formation of sediments, is called _katamorphism_ or destructive change. The phase of metamorphism dealing with the constructive changes in rocks, due to cementation, dynamic movements, and igneous influences, is called _anamorphism_. Some geologists confine the term metamorphism to the changes involved in contact and dynamic metamorphism, and call the resulting products _metamorphic rocks_.

The zone in which katamorphism is most active, usually near the surface, is called the _zone of katamorphism_. The deeper zone in which anamorphism is preponderant is called the _zone of anamorphism_. There are no definite limits of depth to these zones. A given rock may be undergoing katamorphism while rocks on either side at the same depth are suffering anamorphism.

By katamorphism rocks break down to produce the surficial rocks, and by anamorphism the surficial rocks are again consolidated and altered to produce highly crystalline rocks, which are not dissimilar in many of their characteristics to the igneous rocks from which all rocks trace their ultimate origin. In other words, anamorphism tends toward the reproduction of igneous rocks, though it seldom fully accomplishes this result. These two main groups of changes together const.i.tute the _metamorphic cycle_. Some rocks go through all phases of the cycle, but others may pa.s.s directly from one phase to an advanced phase without going through the intermediate stages. For instance, an igneous rock may become a schist without going through the intermediate stage of sedimentation.

Rocks are not permanent in their condition, but at practically all times and places are undergoing some kind of metamorphism which tends to adapt them to their environment. The conception of rocks as representing phases or stages in a progressive series of changes called the metamorphic cycle aids greatly in correlating and holding in mind many details of rock nature and origin, and brings into some sort of perspective the conditions which have produced rocks. A schistose sediment comes to be regarded as an end product of a long series of alterations, beginning with igneous rocks and pa.s.sing through the stages of weathering, sedimentation, cementation, etc., each of which stages has been responsible for certain mineralogical, chemical, and textural features now characterizing the rock. The alternation of constructive and destructive changes of the metamorphic cycle, and the repet.i.tions of the cycle itself, periodically work over the earth materials into new forms. Usually the cycles are not complete, in the sense that they seldom bring the rock back to exactly the same condition from which it started. More sediments are formed than are changed to schists and gneisses, and more schists and gneisses are formed than are changed back to igneous rocks. Salts in the ocean continuously acc.u.mulate. The net result of the metamorphic cycle, is, therefore, the acc.u.mulation of materials of the same kinds. Incidental to these acc.u.mulations is the segregation of commercial mineral products.

The metamorphic cycle becomes a logical and convenient geologic basis for correlating, interpreting, and cla.s.sifying mineral products. Because of the great variety of materials and conditions represented in mineral deposits, prodigious efforts are required to remember them as independent ent.i.ties; but as incidents or stages in the well-known progress of the metamorphic cycle, their essential characteristics may be easily remembered and kept in some perspective.

Ores of certain metals, such as iron, occur in almost every phase of the metamorphic cycle,--as igneous after-effects, as weathered products, as sediments, and as schists. The ores of each of these several phases have group characteristics which serve to distinguish them in important particulars from ores belonging to other phases of the cycle. Having established the position of any particular ore in the metamorphic cycle, a number of safe inferences are possible as to mineralogical composition, shape, extent, and other conditions, knowledge of which is necessary for an estimate of commercial possibilities.

FOOTNOTES:

[1] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol.