The metamorphic rocks are those which, have been changed subsequent to their original formation either in mineral composition or in texture, or in both, so that their primary characters have been altered, or even entirely effaced. Both chemical and physical changes can occur. Here constantly, as elsewhere in geology, gradations exist, and no definite line can be drawn on the one hand between the sedimentary rocks and their metamorphic products, or between the igneous rocks and the metamorphic ones formed from them, on the other. Thus loose chalks pass into limestones, and these into crystalline marbles, just as dolerites merge into greenstones, and so on into hornblende schist, without any sharp line of demarcation. But there comes a point in the change of each original rock, either of composition or of texture and usually of both, where its characters and relations to other rocks have become so individual that, for practical purposes, it is best regarded as a distinct kind of rock. Where this line shall be drawn must depend upon the experience and judgment of the observer; in this work only those cases are treated where the change has been so definite and pronounced as to produce typical metamorphic rocks.
The changes in metamorphic rocks are brought about by means of high temperature and pressure aided by the action of water and other chemical agents. The changes involve the formation of new minerals, the adding or subtracting of chemical constituents and a physical readjustment of the mineral particles to conform to the existing pressure. The original rock from which a metamorphic rock has been derived may be either igneous or sedimentary. As these rocks become involved in movements of the earth's crust, they are subjected to extreme pressures accompanied usually by high temperatures. The result will be frequently to transform the existing minerals into others more stable under the new conditions. The physical structure of the rock will also ordinarily be changed during the process. Because of the pressure to which the rock is subjected the mineral particles will be more or less broken and flattened and rearranged in parallel layers. This banded or laminated character given by the parallel arrangement of its minerals is the most striking peculiarity of a metamorphic rock. Because of this structure a metamorphic rock can be distinguished from an igneous rock.
Further, in the great majority of cases a metamorphic rock has a crystalline structure which distinguishes it from a sedimentary rock. There are, of course, all gradations from a typical metamorphic rock into an unaltered sedimentary rock on the one hand and into an unaltered igneous rock on the other. Rocks for the most part are composed of minerals, and minerals for the most part are definite chemical combinations, which are only, as a rule, permanent under stable conditions. If the minerals are submitted to new conditions, quite different from those under which they were formed, with new chemical and physical factors operating upon them, they will tend to change into other minerals, that is, to turn into new chemical combinations, which will be the most stable under the new conditions. A familiar example is the decay of the feldspar of igneous rocks, and its change into clay and other substances through the action of water and carbon dioxide. The change in conditions may be so slight that some rock minerals may be able to resist them nearly indefinitely, while others less stable may succumb much faster. Thus igneous rocks, formed by the cooling and crystallization of molten magmas, may remain in the depths for millions of years, and on coming to the surface through erosion and denudation, may be found entirely unchanged, or with only one or two of the constituent minerals altered. At the surface they are at once subjected to new conditions, to the combined effects of changes of temperature, to moisture, the various gases of the atmosphere, the products of organic life, etc., and they commence to break up and to form into new compounds. Then their ultimate conversion is only a question of time. The same is true of the sedimentary rocks, only in lesser degree. They are formed of mineral particles, deposited in water and, usually, cemented by pressure and deposits from solution. While they remain deeply buried and under fairly stable conditions, they are unchanged; when they are exposed to the atmosphere they also tend to change and decay, especially in those minerals that are susceptible.
All these changes which occur upon the surface are strictly to be classed as metamorphic ones, and the products, in a geologic sense, are metamorphic rocks. But for practical purposes all these materials formed by the action of weathering and by the decay of rocks on or near the surface, such as the soils, are not here included. They have been previously mentioned under the foregoing rock types, so far as seems desirable for the object of this work, and only those rocks are treated as metamorphic which, while buried at depth below the surface, have suffered, through the action of certain agencies to be presently described, changes; which have practically converted them into new kinds of rocks. The chief metamorphic agencies are mechanical movements of the earth's crust and pressure, the chemical action of liquids and gases, and the effect of heat. We may simplify these into the effects of movement, water solutions, and heat, and all three of these are required to produce complete metamorphism in rocks, though not necessarily all to the same extent, since sometimes one factor is more predominant, and sometimes another. Thus in the metamorphism which has been already described as contact metamorphism, induced by the intrusion of a body of magma, the effect of heat is the most important, that of gases and liquids less so, while the effect of movements of the crust, or pressure, is negligible. The rocks produced, however, are actually metamorphic, but for practical reasons they have been given separate consideration, and are not included among these under treatment. We will consider the different agencies separately.
Pressure. Pure simple downward pressure, to the amount exerted in the upper part of the earth's outer crust, appears to have little metamorphic effect. It tends without doubt to consolidate the material of sediments by bringing the grains closer together, but many instances may be cited of sediments, buried under great thicknesses of deposits for geologic ages, which on being raised and exposed by erosion without disturbance, such as folding, are found to be practically in unchanged condition. On the other hand, as commonly supposed, through the gradual contraction of the earth, the outer crust is under compression, and this finds relief from time to time by buckling or wrinkling up of the outer shell into mountain ranges. This compressive force, thus acting with lateral thrust, is therefore spoken of as orogenic, i.e., mountain forming. By it whole masses of strata with possibly included igneous rocks intrusive, extrusive and fragmental volcanic are folded, crushed, and mashed together in the most involved and intricate manner. Not only are the rocks then subjected to vast pressure, but they are also subjected to enormous shearing stresses, which tend to produce forced differential movements among the rock particles. It is particularly this latter effect which is of great potency in producing metamorphism. Its effects may often be seen megascopically by the manner in which large crystals, included pebbles, or fossils are flattened and elongated, or broken into fragments which are drawn out into thin, lenticular masses in the direction of shear. The microscope chows that even minute crystals are broken, and their optical properties affected, as the result of the strain. It is possible indeed, for this agency working alone to produce rocks having the characteristic outward metamorphic texture, without any change in their original mineral composition, but in combination with heat and water, it is of the highest importance in inducing chemical changes, and the production of new minerals. It is indeed a noticeable fact that so long as the rocks retain their original position, they are unaltered, but as we commence to find them disturbed by orogenic forces, they begin to show signs of metamorphism, and in proportion to the degree to which they have been folded up, mashed, and sheared, they become more and more metamorphosed.
Heat. The effect of heat as a metamorphic agent is very powerful, as is so well shown in local or contact metamorphism. It increases very greatly the solvent action of solutions; it tends in many cases to break up existing chemical compounds which form minerals, and to promote new chemical arrangements. The heat needed for metamorphism may come from the interior of the earth, which increases greatly with the depth; it may be supplied in part by the transformation of energy resulting from the movements, the folding and crushing of the rock masses. In part it may also result from intrusions of molten magma, which are very liable to rise and invade the rock masses as they are uplifted and folded.
Liquids and Gases. The chief of these is of course water, which under heat and pressure becomes a powerful chemical agency. It acts as a solvent, and promotes recrystallization, and taking part in the chemical composition of some of the minerals, such for example as micas and epidote, it is a substance necessary to their formation. It is, without doubt, aided also in its action by substances it may carry in solution, such as alkalies, and by volatile emanations coming from magmatic intrusions, like boric acid, fluorine, etc., as already explained under contact metamorphism. It is this which explains the presence in metamorphic rocks of such minerals as tourmaline, chondrodite, and vesuvianite, which are characteristic of pneumatolytic contacts and of micas, hornblendes and other minerals which contain fluorine.
Effect of Depth. The outer crust of the earth has been divided by geologists into different zones, according to the various geological processes at work. In the outermost one, down to the level at which ground water stands, the rocks are full of fractures, and are exposed to atmospheric agencies moisture, carbon dioxide, oxygen, etc. In this the rocks tend to decay, to be converted into carbonates and hydroxides, and to form soils. It is called the belt of weathering, and is the one of rock destruction. Below this lies another, in which the rocks are also full of fractures and cavities filled with water. Its upper level is that of ground water; below, it reaches to the point where the pressure of the superincumbent masses and the contraction of the crust becomes so great that all fractures and openings are closed up, since the stress is so much greater than the strength of the rocks, that they crush under it, and are to be regarded as being in a relatively plastic state. In this zone the chemical action of water is most important, aided by the substances it may carry in solution. The tendency is to change the minerals to hydrates, and to a lesser amount to carbonates; thus olivine, an anhydrous silicate, becomes converted into the hydrous silicate, serpentine. Substances are taken into solution and, reinforced by those leached out from the belt above and carried down, are deposited in the pores and fissures of the rocks; hence it is called the belt of cementation, because the rock grains are thus cemented together.
Below this lies the zone where, as stated above, the pressure becomes so great that all openings are closed up and the rocks may be regarded as in a plastic condition. Its upper level is variable and depends on geological conditions; in times of quiet it may be as deep as six miles below the surface; in times of mountain making, it may rise much higher than this. Of what may be its lower level, we know nothing. In this, the chief agencies are the enormous pressure and the increasing heat of the earth; the role played by liquids and volatile substances is of less importance; the tendency is for them to be gotten rid of, to be squeezed out. The chief work done in this zone is molecular rearrangement, in which less stable mineral compounds are broken up, and new ones of higher specific gravity and smaller volume, through condensation, are formed. Carbonates are converted into silicates and the carbon dioxide expelled; hydrated minerals have their water driven out and new minerals, with less or no water, are formed. This zone of rock flowage, in contrast to the zone of fracture above it, has been called the zone of anamorphism. We may term it the zone of constructive metamorphism.
It is chiefly in the lower
part of the belt of cementation (zone of fracture), and the upper part of
the zone of rock flowage, that the greater part of the work of metamorphism,
in the production of the metamorphic rocks as we see them, is done. In the
upper zone, the results are chiefly those produced by dynamic shearing, and
the imposing upon the rocks of characteristic textures. Chemical work may be
done and new minerals produced, but it is possible for new textures to be
formed without change in mineral composition. In the lower zone, the work
done is largely chemical, new and more stable mineral combinations being
formed; and here also characteristic textures are produced.
The most common types of metamorphic rocks are briefly described
below:
Gneiss Rocks
Mica Schist and Related Schist Rocks
Quartzite Rock
Slate Or Argillite
Greenstone / Chlorite-Schist / Greenschist
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