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John Smith Flett, Encyclopædia Britannica
METAMORPHISM (Gr. μετά, change of, and μορφή, shape), in petrology, the alteration of rocks in their structural or mineral characters by which they are transformed into new types. In the history of rock masses changes of many kinds are inevitable. Loose sands, clays and heaps of shells are gradually converted into sandstones, shales and limestones by the action of percolating water and the pressure of over-lying accumulations. All rocks exposed at the earth’s surface or traversed by waters circulating through the earth’s crust, undergo changes in their component minerals due to weathering and the chemical action of the atmosphere and of rain. ’These processes of cementation and decomposition, though not unlike those of metamorphisrn, are not regarded as essentially the same. They are considered, so to speak, normal episodes in the history of rocks to which all are subject. When rocks, however, are exposed to the heat of intrusive masses (granite, &c.) or have been compressed, folded, crushed, and more or less completely recrystallized, they assume new characters so different from their original ones that they are ascribed to a quite distinct class, namely, the metamorphic rocks.
The transformation is always gradual, so that in suitable districts every stage can be followed from an unaltered or nearly unaltered sedimentary or igneous rock to a perfectly metamorphic one. The transition may be slow or rapid, and the abundance of intermediate forms renders it impossible to lay down any hard and fast lines of distinction. A black shale with fossils may in two or three feet pass into a splintery hornfels; a sandstone or grit becomes a sheared grit, a granulitic gneiss, and a completely recrystallized gneiss sometimes within a few hundred yards; in a thoroughly metamorphic hornblende schist or chlorite-schist small kernels sometimes occur which can easily be recognized as little modified dolerites or diabases. Still, the metamorphic rocks as a class have many well-defined characteristics, and in perfectly typical development cover enormous areas of the earth’s surface and must be, in the aggregate, of vast thickness. A great number of them are recognizably of igneous origin; others are equally certainly sedimentary. Hence’ some writers have suggested that they are not entitled to rank as a separate class, but only as states or conditions of other rocks. It is generally agreed, however, that when the primitive structures and the original minerals of sedimentary or igneous rocks are so transformed as to be no longer easily recognizable the rock should be included in the metamorphic class.
Only rarely, however, does metamorphism produce much difference in the chemical composition of the rocks affected. Sandstones become quartzite’s and quartz schists, limestones are converted into marbles, granite passes into gneiss, and so on, without their bulk composition being greatly modified. From all that we know it seems established that however great the heat and pressure to which metamorphic rocks have been exposed they have very rarely been melted or reduced to the liquid state. Hence there has been no opportunity for intermixture by solution or diffusion; the changes, including the growth of crystals of new-formed minerals, have gone on in the solid rocks. The chemical molecules already present have aggregated into new combinations and have built up new minerals without travelling for more than infinitesimal distances from the places they occupied in the original rock. Exceptions to .this occur, but they are so few that they do not invalidate the general rule. Thin bands of limestone, for example, may be followed for miles in belts of mica-schist or gneiss, never losing their identity by blending with the rocks on either side of them. By tracing out zones such as these it is often possible to unravel the highly complicated stratigraphy of metamorphic regions where the rocks have been greatly folded and displaced. Another important consequence of the persistence of the chemical individuality of metamorphosed rocks is that very often an analysis indicates in the clearest possible fashion what was the original nature of the rock mass. Sandstones, limestones, ironstones, shales, granites, dolerites and serpentine may be totally changed in structure and very completely also in mineral composition, but their chemical characters are practically indelible. Confusion arises sometimes from the fact that two rocks of different origin may have much the same composition, e.g. a felspathic sandstone may closely approach a granite, or an impure dolomite may simulate a basic igneous rock. Individual specimens, consequently, cannot always be relegated with perfect certainty to sediments or igneous rocks; but in dealing with a complex containing a variety of types the geologist is rarely long in doubt as to their original nature.
Two distinct kinds of metamorphism are recognized, namely contact or thermal metamorphism, and folding or regional metamorphism. The former is associated with intrusive masses of molten igneous rock which were injected at a very high temperature and produced extensive changes in the surrounding rocks. The second occurs in districts where earth folding and the movements attendant on the formation of mountain ranges have flexured and crushed the strata, probably at the same time considerably raising their temperature. Although these processes are very different in their origin, and in the great majority of cases produce quite different effects on the rocks they involve, there are instances in which the results are closely comparable. A sandstone may be converted into quartzite and a limestone into marble by either kind of metamorphism. It is best, however, to describe them as phenomena essentially different from one another.
Contact Metamorphism (thermo-metamorphism).—Any kind of rock—igneous or sedimentary—which has come in contact with an igneous molten magma is likely to show alteration of this type. The extent and intensity of the changes depend principally on two factors: (1) the nature of the rock concerned, and (2) the magnitude of the igneous mass. It is to be expected that a great intrusion of granite will produce more extensive effects of this kind than a narrow dike a few inches or a few feet broad. At the edges of such dikes only a slight induration may be noticeable in the country rock, or there may be recrystallization with formation of new minerals for a few inches. Rarely does the alteration extend beyond this. Shales are baked and hardened, sandstones are rendered more compact or occasionally are partly fused, limestones may be converted into marble containing garnet, wollastonite, augite or other calc-silicates. A great granite boss, which may be ten or twenty miles broad, is often surrounded by a wide aureole of contact alteration. This may be a few hundred yards broad or a couple of miles; in rare cases the breadth of the aureole is only a few yards. These variations may have structural causes; thus when the aureole is narrow the junction of granite with country rock may be vertical; when the aureole is broad the granite may be a flat-topped mass which dips at low angles outwards on each side. When a broad aureole accompanies a vertical junction we may suppose that molten rock has owed upwards along this boundary line for a prolonged period, and has gradually raised the rocks to a very high temperature, even at some distance away from the contact. Where the alteration is slight and local there is usually something in the composition of the rocks or in their crystalline state to account for this.
No less important is the nature of the rocks involved. Where a granite intrudes into a succession of various types of sedimentary and igneous rocks the differences in their behaviour are often very marked. Sandstones alter less readily than shales or slates, and limestones, especially if they be marly or argillaceous, are often full of new minerals, when purer shales on each side of them are not visibly affected. Schists and gneisses, being already highly crystalline, are very resistant to thermal alteration, and may show it only for a few inches where they are in actual contact with the granite, or in minute fragments which have been broken off and surrounded by the invading magma. Igneous rocks, since they consist of minerals which have formed at very high temperatures, may show no change whatever. If they are decomposed, however, their secondary products, including those which fill veins and amygdaloidal cavities, are often entirely recrystallized in new combinations Instances of this will be given later.
The intensity of the alteration depends very greatly on the proximity to the intrusive rock. A typical aureole surrounding a granite boss, for example, consists of rocks in all stages of alteration, the most affected being nearest the granite, while as we travel outwards we pass over zones of successively diminishing metamorphism. Around the granites of Cornwall, the Lake District and Ireland there are tracts of altered slate which show these stages very well. The first sign of metamorphism is a slight increase in hardness and glossiness, making the slate a little brighter and more brittle. This is due to the formation of mica in small crystalline plates mostly parallel to the cleavage of the rock. Nearer the granite a faint spotting is visible on broken surfaces of the slates, and this becomes more pronounced as we enter the middle part of the aureole. These spotted slates, in Cornwall for instance, often occupy a zone a mile in breadth. They are less fissile than the unaltered slates and have rounded or elliptical spots about a quarter of an inch across. The spots are usually darker than the body of the slate, though sometimes paler. Angular, branched, lenticular and rhomboidal spots sometimes occur. Under the microscope these rocks consist mainly of brown mica, quartz and organic matters, iron oxides, &c.; the spots may be due to aggregation of biotite or of quartz, but often differ little in composition from the surrounding rock. Their dark colour is due to, abundance of iron oxides or graphite, with chlorite and biotite. Still closer to the granite a development of crystals takes place in the slates; the commonest are andalusite, chiastolite (with cross-shaped dark enclosures), cordierite, staurolite and garnet. At the same time the minerals formerly enumerated crystallize in larger individuals (biotite, quartz, iron oxides, &c.), so that the rock becomes rather more coarse-grained. At this stage the fissility and cleavage structures of the slate tend to be obliterated, and the rocks are dark, lustrous (from the abundance of mica), hard and splintery. To this type the name homfels is given. The innermost zones of the aureole consist mainly of hornfelses, and where there are slate fragments enclosed in the granite they usually show these characters in their most pronounced form.
The nature of the new minerals produced depends principally, of course, on the chemical composition of the rocks affected? In pure sandstones only quartz is formed, and pure limestones merely recrystallize as marbles. Argillaceous rocks are characterized by abundance of alumina; hence, when thermally altered, they may contain corundum, or silicates of alumina such as sillimanite, kyanite, andalusite and chiastolite. Most rock masses, however, are far from pure and hence the variety of minerals which may arise in them from contact alteration is very great. Argillaceous limestones, for example, very frequently contain garnet, vesuvianite, wollastonite, diopside, tremolite, sphene, epidote and feldspar; that is to say, minerai; in which lime is resent along with silica, alumina, magnesia and other substances. Calcareous sandstones yield augite, garnet, sphene, epidote; argillaceous sandstones are characterized rather by biotite, sillimanite and spinel.
In each case the materials already present in the rock have united to form new mineral combinations. Crystallization has been stimulated by the rise of temperature, aided, no doubt, by moisture. Water vapour, even at comparatively low temperatures when the pressure is considerable, is a powerful mineralizing agent and greatly facilitates crystallization. Often the rocks acquire) ultimately a pseudo porphyritic or porphyro-blastic structure, as they contain large or conspicuous crystals scattered through a finer grained ground-mass; not only these porphyritic ingredients but the body of the rock shows increased crystallization, for contact alteration as a rule makes rocks more coarse-grained than before.
In rare instances fusion may take place, but this must be exceptional, as the finest original structures are often very perfectly preserved by rocks which have been in great measure recrystallized. Finely laminated argillaceous sandstones, for example, may pass into cordierite—or andalusite—hornfelses showing a mineral banding which corresponds exactly with the original lamination. For this reason the newly developed minerals are not frequently of good crystalline form. When weathered out of the rock they have mostly rough, imperfect faces, but exceptions to this occur in garnet, staurolite, tourmaline and a few others which often produce good crystals even in these adverse circumstances.
is only true in a general way that the rocks which are thermally altered experience no change in their chemical composition. The new minerals which are substituted for the original ones are such as are stable at high temperatures. Many of the silicates which form a large part of sedimentary rocks contain combined water; examples are chlorite, kaolin and clay. The water, or part of it, is expelled, forming silicates with little or no water, e.g. biotite, felspar, andalusite. Carbonic acid may be retained or driven out; in a siliceous limestone the silica tends to combine with the lime producing calc-silicates by replacing the carbonic acid. In a pure limestone the carbonate merely recrystallizes as marble. This loss of volatile ingredients must occasion a diminution in the bulk of the sedimentary mass involved; in cooling there will be contraction, and fissures are produced which may be filled with igneous dikes or with veins deposited by ascending hot waters. Hence contact aureoles are common sites for mineral deposits of economic value.
In some aureoles the sediments or schists have their bedding and foliation planes wedged apart by the intrusive force of the granite, and are permeated by igneous material invading them along these fissures. In this way a mélange is produced of sedimentary rock with threads and veinlets of igneous nature, and to some extent a blending of the two rocks takes place, though usually each preserves its identity however intimately mixed. In microscopic sections veins of granite not more than a tenth of an inch in width may be traced, sharply distinct from the slate or schist they penetrate. Cases, however, are described in which the rocks of the aureole have been felspathized or filled with new felspar derived from the granite; this, however, is not common. Shales are often converted, when in contact with diabase, into pale-coloured, flinty-looking rocks known as adinoles. These are exceptionally rich in albite and contain as much as 10 % of soda, an amount which is not met with in unaltered shales. It seems probable that alkalis have been transferred from the igneous rock to the sedimentary, perhaps through the medium of the vapours exhaled. The breadth of the adinole belt is as a rule only a few inches or a foot or two.
The vapours given off by intrusive igneous masses may contain substances which combine with the ingredients of the surrounding rocks and thus modify their composition. Boron, fluorine and phosphorus are the principal elements which are transferred in this way, and minerals such as tourmaline, topaz and mica are the characteristic products in quartzose or argillaceous rocks; while apatite, fluorspar, axinite, datolite and chondrodite are commonest in limestones. This is a form of pneumatolytic action (see Pneumatolysis).
Extreme cases of the mutual interaction of the intrusive rock with the masses invaded by it are provided by the fragments enclosed in the molten magma (known as xenoliths). These are often rounded and eroded, as if softened or partly fused and dissolved. Similar changes are found in the rocks of the aureole for a few feet or yards where in actual contact with the granite. This belt of indurated hornfelses often weathers much more slowly than the igneous rock, and stands out as a prominent, sharp-edged ridge running round the granite margin.
Where sediments are dissolved in igneous rock we may expect to find modifications in the chemical composition and in the minerals produced on crystallization of the magma. Some granites, for example, which contain many rounded, partly dissolved enclosures of slate are themselves full of corundum, andalusite, cordierite and other minerals, which appear to indicate the effect of absorbed slate material. Much discussion has taken place as to the importance of such processes in modifying the facies presented by igneous rocks. Granites are alleged to have absorbed impure limestones and thus to be changed to diorites (Pyrénées). At the contact of the two rocks a narrow zone of diorite intervenes between the granite and the limestone. In this case an acid rock has become basic (or intermediate) in character; similarly, basic rocks—such as gabbros—are said to become granitic where they have melted down large quantities of felspathic quartzite. On the other side it is argued that as precisely the same modifications of the igneous rocks are known to occur where these explanations cannot possibly hold good—e.g. zones of diorite at the contact of granite with quartzite or mica-schist—they are really due to chemical segregation or differentiation in the magma and not to any admixture with foreign material.
Such modifications in the igneous rock at its contacts are often said to be endomorphic, while those which take place in the aureole or country rocks are exomorphic. The endomorphic changes are not always strictly of the nature of contact alteration. The commonest are the presence of a fine-grained, sometimes glassy, chilled edge due to rapid solidification from sudden coolin of the magma. The fine-grained marginal facies is often porphyritic, while the interior of the mass is granular or eugranitic. There is often a tendency to the development of special minerals in the edge of intrusive masses. Some of these arise probably from absorption of country rock, e.g. cordierite, andalusite, iron oxides (in granite). At the same time there may be a great abundance of angular or rounded enclosures, so that the marginal rock is brecciform. Where granite penetrates gabbro the fragments of the latter are sometimes melted down and digested in the granite till only the crystals of their augite or diallage are left (Skye). Granite margins are not always more basic than the average of the mass; they may be exceedingly rich in quartz and at the same time very coarse-grained or pegmatitic. This seems to arise from the production of fissures at the contact after the granite has to a large extent solidified. In these fissures the pegmatites are laid down by escaping vapours. Metasomatic changes are especially common also in this situation, and have often formed very valuable mineral deposits along igneous contacts. There also pneumatolytic processes often concentrate their attack; schorl-rock, greisen, topaz-rock and china-stone (or kaolinized granite) are characteristic products, and the active vapours often transform the sediments around, forming schorl-schist, calc-silicate rocks and sericite-schists.
Regional Metamorphism.—The second kind of metamorphism is known as “ regional " because it is not confined to narrow areas like contact metamorphism, but affects wide tracts of country. Metamorphic rocks of this kind often cover a large part of a continent (e.g. the centre of Africa or Scandinavia and Finland). Whatever the causes be which produced it, they must have been of widespread operation and connected either with great geophysical processes or with definite stages of the earth’s development. Where such rocks occur there is generally much evidence of earth movement accompanied by crushing and folding. They are very characteristic of the central axes of great mountain chains, especially when these have been denuded and their deeper cores exposed. Most geologists believe that this connexion is causal, holding that the contraction of the outer layers of the earth’s crust, due to shrinkage of a nearly rigid shell upon a cooling and contracting interior, has bent and folded the rocks, and at the same time has crushed and largely recrystallized them. According to this view regional metamorphism is the result of pressure and folding; hence the name dynamo-metamorphism is frequently applied to it.
A great number of observations collected in all regions of the globe may be adduced in support of this by othesis, forming a mass of evidence so strong as to be almost overwliielming. The structural features which prove that there has been great folding in these rocks are accompanied by microscopic and lithological characters which demonstrate that extensive crushing has taken place. Through progressive stages a slate with fossils may be traced into a phyllite, which becomes a mica-schist, or, in places, a micaceous gneiss. At first the fossils are distorted or torn apart, but they disappear as crystallization advances. Limestones under great pressure flow almost like plastic masses, losing their fossils and becoming crystalline. Grits, quartzites and granites show the effects of crushing in the pulverization of their minerals and the breaking down of their original clastic or, igneous textures, fine slabby mylonites (q.v.) and granulates being produced. Moreover, the degree of metamorphism in the rock can often be shown to correspond closely to the extent to which it has been folded and crushed.
Another argument in favour of dynamo-metamorphism, which has been urged with much insistence by the extreme supporters of these theories, is the retention of original chemical characters in the metamorphic rocks. Some of them bear unmistakably the stamp of sedimentary origin, e.g. the limestones and marbles, quartzites, graphite-schists and aluminous mica-schists. Others have the normal composition of granites, diorites, gabbros and other types of plutonic igneous rocks. This leads to the inference that these were originally normal sediments and intrusives or lavas, and that their present crystalline state and foliated structure are the result of agencies which operated on them subsequently to their formation. Where the degree of metamorphism is not too high, and the folding and dislocation not too complex, the sandstones, shales and limestones may be mapped out, and igneous bosses, dikes and sills, with their contact aureoles, veins, pegmatites and segregations, convincingly delineated on the maps. This shows that a whole complex or terrane, consisting of diverse petrological types of normal sediments and igneous rocks, may be converted by metamorphism into a great series of gneisses and schists. Although recrystallization has been complete, the original rock masses still retain their identity in their new state.
The metamorphism in a rock series may be of nearly uniform intensity over a large area; the sediments, for example, may have all their clastic and organic structures effaced, and in the igneous rocks the porphyritic, ophitic, graphic and other textures may have completely disappeared. This, however, is not always the case, especially when the metamorphism is not of very intense degree. Parts of the rock may retain original structures, while others are typical crystalline schists and gneisses. Kernels, lumps or phacoids of massive rock are often found embedded in schists, and it is clear upon inspection that the phacoids represent the original state of the rock, while the schist is the effect of metamorphism. At other times a rock mass, such as an intrusive sill, is schistose at its edges and surrounded by schistose sediments, while near its centre it is almost entirely massive. The hard igneous rock has proved more rigid than the soft and plastic sediments; in folding, the latter have yielded to the stresses, and internal movement has produced foliation. The crystalline rock of the intrusive sheet has been strong enough to withstand the pressures and has folded like a rigid mass. At the junctions the effect of differential movement is shown by the presence of a belt of rock which often has a most pronounced schistosity. Some intrusive dikes show foliation especially marked along their edges; or they may be traversed by planes of movement, running obliquely or directly across them, and characterized by, the development of very marked schistosity. Exceedingly sudden transitions between normal igneous rocks and schists or gneisses have been described in sheared dikes. A normal dolerite, with ophitic structure and abundant augite, has been shown to pass in a few feet or inches into an epidiorite, where hornblende has replaced the primary augite, and lastly into a perfectly typical hornblende-schist, completely recrystallized with development of epidote, green hornblende, sphene and other minerals of metamorphic facies from the original constituents of the dolerite. These-phenomena are regarded as establishing that the rock had consolidated as a normal dolerite before the processes which caused the metamorphism began to act; that these processes resulted in internal movement in the rock mass along’certain narrow belts; and that recrystallization was set up along with the development of schistose structure. The operating cause cannot have been anything but pressure, especially as the foliated rocks occur not infrequently in lines of dislocation and shear; in other cases the foliated, types are at the margins of the dike, and the transition from massive igneous rock to metamorphic schist may take place within the space of one inch. The best examples of phenomena of this order are those described by J. J. H. Teall from Scourie in the north-west of Scotland.
Where rocks of any kind are traversed by powerful dislocations or thrusts they often present a schistose facies in the immediate vicinity of the planes of movement. In the Highlands of Scotland great thrusts occur, along which the rocks are displaced for distances which may be as much as ten miles; and immediately adjoining these thrust-planes very perfect foliation is induced in all kinds of rocks, sedimentary, igneous or metamorphic, which have been involved in the movements. The minute structure of these rocks is generally of the mylonitic, granulitic or finely crushed type. In the same way the serpentine of the Lizard in Cornwall passes into line talcose and tremolitic schists along narrow zones of displacement. Many other examples of this might be cited from regions where folding and crushing have taken place on a large scale. As a rule, almost without exception, the foliation thus produced is parallel to the direction of movement in the rock masses.
In the mineral transformations which accompany metamorphism the operation of pressure is no less clearly indicated. There are, for example, three minerals which consist of silicate of alumina, viz. andalusite, sillimanite and kyanite. The last of these has the highest specific gravity. In andalusite-bearing rocks which have been sheared, with production of foliation, we sometimes find pseudo morphs of kyanite after andalusite, retaining the characteristic form of the original mineral. Compression, it seems reasonable to suppose, would produce that one of the three crystalline silicates of alumina which has its molecules most closely packed, and consequently the highest specific gravity. This explains the conversion of andalusite into kyanite. The principle that substances tend to assume that mineral form which has the least molecular volume is of wide application among metamorphic rocks. It has been calculated, for example, that when olivine and anorthite felspar are replaced by garnet (a change which takes place not infrequently when basic igneous rocks are metamorphosed) the molecular volume of the mineral aggregate diminishes from 145 to 121 or about 17%. On the other hand, when garnet is fused it recrystallizes as a mixture of olivine and anorthite. This has led to the generalization that all minerals formed by the crystallization of a fused magma at high temperatures have a large molecular volume, while those which are produced in rocks at temperatures below their fusion oints and under great pressures have smaller molecular volumes. Loewinson Lessing pointed out that some minerals have a greater molecular volume than the oxides which enter into their composition; in other minerals the reverse holds good. The former group are, on the whole, characteristic of igneous rocks and products of contact alteration, both of which classes have been formed at high temperatures (e.g. wollastonite, spinel, nepheline, leucite and andalusite). The minerals of the second oup are often of common occurrence in metamorphic schists and gneisses (e.g. staurolite, kyanite, hornblende, tale, epidote and garnet). Although there are exceptions to this rule, there can be no doubt that it expresses a generalization which is of great value in the study of mineral paragenesis.
The mineral changes are usually not of so simple a kind as those above enumerated. Mutual interaction takes place between adjacent components of the rocks. Titaniferous iron oxides, for example, obtain silica and lime from such minerals as augite or lime felspar and sphene results. Felspar often breaks up into epidote, quartz and albite; the epidote obtains its iron from adjacent crystals of augite or hornblende. Equations can be written to show the transformation of one rock to another; thus, diabase (labradorite, augite, ilmenite) may be converted into amphibolite (acid plagioclase, hornblende, garnet, sphene and quartz). In this case, the molecular volumes are for diabase 671 and for amphibolite 635.6, indicating a diminution on metamorphism. Many striking illustrations of this principle have been adduced. Caution, however, is required in applying it to concrete cases; if it was always strictly correct the metamorphic rocks should have higher specific gravities than their representatives among sediments and igneous rocks. Very frequently this is not the case, and there must be some counteracting process at work. We find this antagonistic principle in the tendency for the minerals of metamorphic rocks to contain water of combination, e.g. epidote, muscovite, chlorite, hornblende, talc. This indicates that they were formed at comparatively low temperatures.
We arrive then by many inde endent lines of reasoning (stratigraphical, microscopical, chemical)and mineralogical evidence being abundantly available) at the conclusion that pressure acting on sediment and igneous rocks at temperatures below their fusion points has been able to change them into metamorphic rocks. This is the theory of dynamo-metamorphism, which has won acceptance from the majority of geologists who have made the petrology of metamorphic rocks their special study. It has still, however, many incisive critics, and in recent years dissent has on the whole gained strength.
One of the principal objections is that by these processes it is possible to destroy original structures and to break down the minerals of which a rock consists, but not to induce crystallization and build up rock structures of a new type. It is pointed out that in many regions the rocks though intensely folded are not highly metamorphic; in other places immense dislocations can be proved to exist, yet the rocks are only slightly altered or are converted into fine-grained mylonites and not into typical schists and gneisses. Conversely, it is argued, there are man districts where metamorphism is very intense, yet evidence ofy folding and pressure is only slight. It seems clear that another factor must be taken into account, and in all probability that factor is the action of water in rocks at a comparatively high temperature. All rock masses contain interstitial water, andg many also consist of minerals in some of which water exists in combination. Hence all metamorphism must be regarded as taking place in presence of water. It is almost equally certain that metamorphism must be accompanied by a rise of temperature in nearly every case—in fact it is difficult to imagine such a process going on without considerable heat. Now heated water (or water vapour) is a most potent mineralizer. Crystals of quartz, for example, have been produced in glass tubes containing a little water, heated in a furnace to a temperature of about 300° C.
The heat rgguired for the more intense sta es of metamorphism may be derived from more than one source. Most regions of gneiss and schists contain igneous rocks in the form of great intrusive masses. These rocks themselves are frequently gneissose, and the possibility must not be overlooked that they were injected into the older rocks at a time when folding was going on. The metamorphism would then be partly of the contact type and partly the effect of pressure and movement, “ pressure-contact-metamorphism.” The vapours already present would be augmented by those given out from the igneous rock, and intensely crystalline, foliated masses, often containing minerals found in contact zones (andalusite, cordierite, sillimanite, staurolite, &c.), would be produced. Cases are now known where it is in every way probable that the metamorphism is the result of a combination of causes of this order. Some of the Alpine schists which surround the central granite gneisses have been referred to this group.
Heat must also have been produced by the crushing of the rock components. In many metamorphic rocks we find hard minerals possessing little cleavage (such as quartz) reduced to an exceedingly fine state of division, and it is clear that the stresses which have acted on regions of metamorphic rocks are often so powerful that all the minerals may have been completely shattered. The interstitial movement of the particles must also have generated heat. There are no experimental data to enable us to say what rise of temperature may have been produced in this way, but we cannot doubt that it was considerable. If the crushing was slow the heat enerated may have been conducted away to the surface almost as fast as it was produced. If the belt of crushing was narrow, heat would rapidly pass away into the colder rocks beyond. This may explain why in some rocks there has been much grinding down but little crystallization. The heat also may be absorbed in promoting chemical combinations of the endothermal type, but it is not likely that much was used up in this way. With rising temperature the rocks would become more plastic and fold more readily. Then if the crushing and folding ceased, a long period would follow in which the temperature gradually fell. The minerals would crystallize in larger grains after the well-known law that the larger particles tend to grow at the expense of the smaller ones, and finely granulitic aggregates would ge re laced by mosaics of coarser structure. If there has been a considerable rise of temperature we might expect analo ies in structure and constitution between the folded rocks and those which come from a contact aureole; this has in fact been noted by many geologists.
Another factor which must have been of importance is the depth below the surface at which the rocks lay at the time when they were folded. In the deeper zones the pressures must have been greater, and the escape of the heat generated must have been less rapid. The uppermost members of a complex which was undergoing folding are under the lowest pressures, are at the lowest temperatures and probably also contain most moisture. Hence minerals such as epidote, chlorite, albite, sericite and carbonates, which are often produced by weathering alone, might be expected to prevail. In the deepest zones the temperature and pressure are high from the first and are increased by folding; such minerals as biotite, augite, garnet, felspar, sillimanite, kyanite and staurolite might be produced under these conditions. The earth’s crust might in this way be divided into bathymetric zones, each of which was characterized by distinctive types of mineral paragenesis. Some geologists ascribe the greatest importance to this conception; they establish two or three types of metamorphism, each of which belongs, in their opinion, to a definite horizon. This is to some extent a resuscitation of the old idea, now discarded, that the Archean rocks are sediments of a peculiar kind formed onl in the heated waters of the primal globe; the first deposits were laid down under great heat and pressure and are typical gneisses which may resemble igneous rocks; the schists of later origin exhibit. a progressive transition to normal sediments. Without admitting that it is possible to classify metamorphic rocks according to the depth at which they were situated when metamorphosed, we may admit that there is much reason to believe that the more intense stages of alteration characterize as a rule the rock masses which were oldest or most deeply situated during the epoch of folding.
While rocks near the surface which are under comparatively slight pressures yield to stress by fracturing, it is conceivable that at greater depths the minerals .would become plastic and suffer deformation without rupture. For this zone of “ flowage,” as he terms it, van Hise estimates a depth of not more than 12 kilometres, depending on many factors such as the strength of the rocks and nature of the minerals concerned, the temperature, amount of moisture and rapidity of the deformation. Between it and the zone of fracture, which lies above, a gradual transition must take place. Doelter, on the other hand, believes that the depth at which plastic flow begins must be at least 35 kilometres; it is difficult to imagine that rocks which have been so profoundly buried can now be exposed at any part of the earth’s’surface.
In the attempt to explain the existence of large masses of metamorphic rocks which are perfectly foliated, but at the same time coarsely crystalline, and show no grinding down of their components, as might be expected on the hypothesis of pure dynamo-metamorphism, F. Becke brought into prominence another principle which may prove to be widely applicable. Although known as Riecke’s law, it was advanced many years ago by Sorby. It enunciates that when minerals are subjected to unilateral pressure (acting in a definite direction and not like hydrostatic pressure, equally in all directions) they tend to be dissolved on those sides which face the pressure, while the sides which are not compressed tend to grow by additional deposit. Minerals having platy or rod-like forms will thus be produced, all having a parallel orientation, and the rock will be schistose, with foliation corresponding in direction to the extension of the mineral plates, and perpendicular to the stresses which were in action. The solvents which dissolve the mineral on one side and deposit iton the other side are the interstitial moisture and vapours present in the rock. By this means schists and gneisses will be produced, which are perfectly foliated yet have their minerals homogeneous and uncrushed. Experimenta data are at present wanting to show how far this principle is operative and what are its limits, but as a supplementary contribution to the theory of dynamo-metamorphism it may prove to be of great importance. This has been described as the development of “ schistosity by crystallization."
More interesting still are E. Weinschenk’s theories of pressure-crystallization and piezo-crystallization (pressure-contact action). He adduces evidence to show that many gneisses are igneous rocks which were foliated from the first, and a large body of observations in many European countries confirms his statement. In his opinion plutonic rocks crystallizing under certain conditions of pressure necessarily assume a banded structure, and contain minerals which are not identical with those of igneous rocks but with the components of schists and gneisses. In the surrounding rocks there is contact alteration but not of the ordinary type as the recrystallized products also have a banding or foliation owing to the pressure acting on them during metamorphism. Bonney urged the hypothesis that many gneisses are merely plutonic igneous rocks which exhibit a flow banding and an imperfectidiomorplhism of their minerals owing to their having been injected in a half-solid state; the component crystals by mutual attrition assume rounded or lenticular forms. Undoubtedly there:, is much truth in these hypotheses, yet in both cases they seem to necessitate the presence of extraordinary earth-pressures such as accompany mountain building. We know that lieat greatly increases the plasticity of rocks. Assuming that intrusions take place during an epoch of earth movement, we may be certain that as solidification goes on the pressures will force the rock forward, and the structures will be very different from those assumed by a rock which has crystallized in a condition of rest.
Lastly, there are many geologists who hold that certain kinds of gneiss are due to the infection of plutonic igneous rocks as masses of all sizes into sedimentary schists forming a mélange. The igneous rock veins the sediment in every direction; the veins are often exceedingly thin and nearly parallel or branch again and again. In this way a banding or foliation is set up, and the mixed rock has the appearance of a gneiss. In the sediment, intensely heated, new minerals are set up. The igneous rock digests or absorbs the materials which it, penetrates; and it is often impossible to say what is igneous and what is sedimentary. Acid intrusions may in this way break up and partly assimilate older basic rocks. Very good examples of this process are known, and they may be much more common than is at present suspected. Conditions which favour assimilation at great depths are the enormous pressures and the high temperature of the earth’s’ crust; the igneous rocks may also be much above their consolidation points. It ’is quite reasonable’ to believe that at deep levels absorption of sediments by igneous masses goes on extensively, while in higher, zones there is little or none of this action.
Published in 1911
© John Smith Flett, Encyclopædia Britannica