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[p. 426]
The conception of the development of the earth prior to the earliest stage at which its history can be read directly from the strata must depend upon the view which is entertained as to its origin. The course of its early history, according to each hypothesis of its origin, may be followed separately. These possible courses are necessarily hypothetical at present, and should not be entertained without due reserve; yet their study is important, for the great features of the earth and of the earth-shaping processes were inherited from these early stages.
The hypothetical stages of the earth’s early history, according to the Laplacian view have been stated as follows:[1]
Quite apart from any doubt as to the mode of genesis, two serious questions relative to the processes outlined in this sketch have arisen from recent investigations, the one growing out of the failure to find any great basal formation having the distinctive characteristics of an original crust; and the other from doubt as to the possibility of so prodigious an atmosphere as that postulated.
1. Relative to an Original Crust
The theory of a molten earth carries the presumption that the liquid mass arranged itself concentrically, with the heaviest matter at the center and the lightest on the outside. As the granitoids are the lightest of the large classes of igneous rocks, the granitoid magmas should have formed the outer zone of the molten earth, and should have been spread out homogeneously according to their [p. 428] specific gravity. The solid crust should have been similarly light and homogeneous, and it should have formed a universal stratum susceptible of identification. Except at the very surface, it should have been completely crystallized, for the cooling must have been very slow, a condition favorable for the growth of crystals. No very large amount of fragmental volcanic material can be assumed to have covered the original crust if we entertain the view that the primitive atmosphere contained all the water of the future hydrosphere, for that leaves no adequate explosive agency in the molten globe to produce abundant volcanic fragments. Such material cannot well be supposed to have concealed the original crust permanently, for many thousands of feet of rock have been eroded from the surface of the oldest known areas. It is equally improbable that the original crust has been concealed everywhere beneath sediments derived from itself. There should therefore be areas of the original crust exposed at the surface and they should presumably be large.
Until recently, the great granitoid areas of the Archean system (the oldest known rocks) were thought to answer these obvious characteristics of an original crust; but it has been found that most of these great granitoid masses are intrusive in rocks which had previously been formed on an older surface by (1) lava outflows, (2) volcanic explosions, and (3) sedimentation. This reduces to an unknown, and apparently to a vanishing quantity, the rocks that can be referred plausibly to a supposed original crust. If the trend of further investigation shall follow the present tendency and finally exclude all the accessible rocks from an original crust . the molten theory will have lost its observational support, at least in its original form.
2. Relative to the Primitive Atmosphere
The primitive atmosphere has usually been held heretofore to have been vast, hot, and heavy, and to have contained (1) all the water of the globe, (2) all the carbon dioxide now in carbonated rocks, (3) that portion of the oxygen which has been added to the rocks by oxidation, as well as (4) that portion of all these constituents which is now found in the atmosphere and in organic tissues.
[p. 429]
The assumption back of this seems to be that heat always promotes the expulsion of gases; if so, the separation of the gases from the rock should have been most complete in the white-hot primitive globe. This conception has been widely entertained, and the reverse conception that the cooled rocks re-absorb the atmospheric constituents is expressed in the once prevalent view that the former atmosphere and hydrosphere of the moon have been absorbed into that body, and the familiar prophecies of a similar doom for the atmosphere and hydrosphere of the earth.
Adverse physical evidence. So great an atmosphere with so much carbon dioxide and water-vapor should have given the earth a warm and equable climate. Such climates indeed seem to have prevailed at certain times during the earlier parts of the earth’s history, as also during the later; but the studies of the past two decades have shown that there was extensive glaciation on the very borders of the tropics, as early as the close of the Paleozoic, and that there was glaciation in northwestern Europe, in China in Lat. 31°, in Australia, and perhaps in South Africa, as far back as the very beginning of the Paleozoic. Less striking, but perhaps not less significant, is the occurrence of extensive beds of salt and gypsum in the early Paleozoic, in rather high latitudes. These deposits seem to imply severe and protracted aridity, not readily reconcilable with an enormous, equalizing atmosphere.
There seem to have been, in Paleozoic times, much the same alternations of very uniform with very diversified climates that marked the Mesozoic and Cenozoic eras. In other words, changes of climate seem to have been much the same in early as in late geologic time.
Adverse organic evidence. The more the early life is compared with modern life, the more nearly does it appear to imply the same atmospheric conditions, and the more insecure does any view become which postulates conditions profoundly different from those of to-day. The air-breathing animals which lived early in the Paleozoic era, and the xerophytic (indicating aridity) organs of plants that lived in the later part of that era, seem irreconcilable with a vast, hot, vaporous atmosphere, overcharged with carbon dioxide and water-vapor.
[p. 430]
The hypothesis of an enormous original atmosphere, suffering gradual depletion, finds, therefore, scant support in a critical study of either the biological or the physical history of the earth.
The most troublesome phases of the preceding scheme arise from the assumption that the lighter gases were excluded from the hot molten globe, and so formed a vast atmosphere. If it is possible to amend the hypothesis, it must be done, apparently, by assuming that the earth held large quantities of gaseous constituents throughout the molten stage, and discharged them later. Since lavas now bring to the surface great volumes of absorbed gases, it is not, on its face, inconsistent to suppose that a globe of molten rock might contain large quantities of the atmospheric gases. It is perhaps admissible to assume further that some notable part of the atmospheric material remained in the rock after its solidification, since igneous rocks now contain notable quantities of gas. By making these assumptions, the primitive atmosphere may be held to have been less extensive than in the preceding view, and gaseous material may be supposed to have been held in reserve in the earthbody to actuate future vulcanism and to feed the atmosphere and hydrosphere.
This modified view makes it possible to suppose that the formation of the crust may have been followed by a period of exceptional volcanic activity, during which the primitive shell was buried so deeply under volcanic matter that it has not since been exposed by erosion. It is consistent then to suppose that the oldest rocks accessible are these volcanic products, mingled with such sedimentary material as was formed contemporaneously with them. In this way the hypothesis may be made to fit the earliest rocks now known, fairly well. It is not clear, however, that the physical assumption on which it is based is sound, for experimental evidence seems to indicate that highly heated rock material discharges its gases, rather than retains them.
The modified hypothesis is only partially successful in meeting the atmospheric difficulties, for there must have been added to the [p. 431] primitive atmosphere the accessions of the great volcanic period, and these together would probably have given a gaseous envelope not altogether unlike that of the preceding view, though less excessive.
The modified hypothesis furnishes a somewhat better basis than the older view for the volcanic activity of later periods, and for a supply of gases to the atmosphere to offset the loss due to chemical combination of its constituents with the surface rocks; but it is not clear that it is adequate.
Stages under the Modified Hypothesis
The stages of evolution under this view may be summarized as follows :
The astral eon. The separation of the material of the earth from the parent nebula and its aggregation into a rotating gaseous spheroid.
The molten eon. The condensation of the rock matter of the gaseous spheroid into a molten spheroid surrounded by a hot vaporous atmosphere, the molten spheroid retaining occluded within itself some notable part of the water of the present hydrosphere, as well as much of the carbon dioxide represented by the present carbonates and carbonaceous deposits.
The lithic eon. The solidification of the molten spheroid, beginning perhaps at the center, on account of pressure.
The primitive volcanic eon. Prodigious volcanic action, closely following the solidification of the crust, during which great beds of lava were poured out on the surface, followed by great intrusions of other lavas into and through them. Contemporaneous with this volcanic action, the atmosphere and hydrosphere gave rise to some sedimentary deposits, which were interbedded with the volcanic products, but were greatly inferior to them in volume. This period would correspond to the Archeozoic eon.
The sedimentary eon. The remaining time up to the present has been characterized by the dominance of the atmospheric and the hydrospheric activities over the volcanic, and the result is recorded in the common sorts of sedimentary rocks. This eon [p. 432] began with the deposition of the proterozoic sedimentaries, and continues to the present.
It is possible to suppose that the earth grew up by accessions in some other mode than that of planetesimal evolution, but the latter furnishes the basis for the following sketch of probable stages:
(1) The nuclear stage, which started with the nebular knot and proceeded by its gradual condensation as planetesimals were gathered into it, until it became a small planet which continued to grow by the capture of more planetesimals. No specific mass is assigned to the nuclear knot save that its mass was sufficient to control its constituents.
(2) The initial atmospheric stage. There may have been a stage after the nucleus was condensed during which the earth was too small to hold the gases of an atmosphere, but if the earth then had a mass one-tenth or more of its present mass, it probably had a limited atmosphere like that of Mars; if it was much smaller than this, it probably had little or no atmosphere. Whatever the atmospheric state at the start, it is assumed that as the earth grew, atmospheric molecules were gathered to it, and sooner or later it gained the power to hold them, and accumulate an atmosphere. Gaseous molecules should have come in from without, the same as other planetesimals, and gases should have been given forth from the rock material that went to form the growing earth, particularly after volcanic action had set in. The heavier gases should have been retained at an earlier stage, while the lighter ones came under control later.
Gases or gas-producing material was no doubt contained in the original planetesimals, since gases are given forth from nearly all rocks and meteorites to-day when heated in a vacuum,[2] and as the planetesimals were aggregated and heated, a part of their gaseous material should have escaped and become a part of the atmosphere. Because the presence of these gases and gas-producing materials in rocks is so general, it is thought probable that many [p. 433] of the gases now issuing from volcanoes are a part of what was held in the- original planetesimals, and that they are now reaching the atmosphere for the first time.
(3) The initial volcanic stage. Before the earth grew to any large part of its present mass, the self-compression which arose from its own gravity is thought to have produced sufficient internal heat to have reached the melting points of the common kinds of rock under low pressures, and as this heat crept outward, it would reach rocks at pressures that would permit liquefaction. Recent discoveries have led to the belief that heat arising from radioactivity has also been an agency in developing high internal temperatures, and in thus promoting volcanic action. It is not known how the initial stages of the atmosphere and of vulcanism were related to one another in order of time, but later they ran parallel with one another, and volcanic action is believed to have made notable contributions of gas to the atmosphere.
(4) The initial hydrospheric stage. Water in the form of gas is light and active, and may at first have escaped; but when the earth had attained sufficient size, water vapor was held in the atmosphere, and when at length the point of saturation was reached, it took the liquid form and initiated the hydrosphere. The source of the water, according to the hypothesis, was the same as that of the atmospheric gases.
It may be added here that the hypothesis gives a simple explanation of the ocean basins and continental protuberances. It is obvious that, because of unequal growth, the surface of the earth might never have been perfectly spheroidal, so that when the accumulation of water upon its surface began, it gathered into the depressions. The planetesimal material which afterwards fell into the water was protected from weathering, while the material that fell on the protuberant areas was exposed to weathering, with its attendant lessening of specific gravity. Thus the depressed areas tended toward higher specific gravities, and hence toward still further depression when deforming stresses were brought to bear on them, while the elevated areas tended to grow relatively lighter, and to suffer relative elevation, under the stress of deformative movements. Thus the differentiation of the oceanic basins from [p. 434] the continental protuberances began almost as soon as the hydrosphere began to gather, which was long before the earth had reached its present size, and has continued to the present time.
(5) The initial life stage. Suitable conditions for life did not exist until after some notable development of the atmosphere and the hydrosphere, but as these were gathered about the earth at an early stage, it is possible that some forms of life began long before the earth was full-grown. Under the planetesimal hypothesis, therefore, the time during which life may have existed on the earth is very much longer than the time assumed under the older hypotheses.
(6) The climax of volcanic action. While volcanic action may have begun soon after the beginning of the earth’s growth, it probably had to await (1) sufficient growth to give the requisite heat by compression, and (2) sufficient time for the heat so developed to creep out to zones of less pressure, where it would suffice 'to liquefy the more fusible (soluble) parts of the rock. Vulcanism was probably hastened by radioactivity. Once begun, it is believed to have gradually increased in importance, and only reached its climax some time after the more rapid growth of the earth had ceased.
For obvious reasons, the climax of vulcanism was attended by deformations of exceptional intensity. The transfer of so much material from below to the surface required readjustment within, and the intrusion of the enormous granitic batholiths, such as are found in the early formations, was in itself a cause of deformation. Diastrophism probably had its climax with the climax of vulcanism, and both came, by hypothesis, about the time of the opening chapter of the well-recorded history of the earth.
The formations of the period when volcanic action was at its height, including some contemporaneous sedimentary deposits, are regarded as constituting the oldest accessible rocks of the earth (the Archean Complex), though probably only the later part of the great volcanic series is represented by the known Archean. It is for each student to judge whether the assigned antecedents lead felicitously or otherwise into the actual state of things which the oldest known rocks reveal. The value of a hypothesis, when its [p. 435] truth cannot be at once demonstrated, lies mainly in its working qualities.
(7) The gradational stage. To complete the survey of stages, it is necessary to note that after the growth of the earth had ceased and volcanic action had passed its climax, the surface was no longer subject to continual burial, but was exposed, age after age, to the action of air and water. The material removed by these agents from the higher parts was deposited in the basins. Throughout all this remaining period, the dominant geologic processes were therefore gradational. Vulcanism and diastrophism continued to be important, but not dominant. This stage embraces the Proterozoic and later eras.
Synoptical View of the Earth’s History
The stages of the earth’s history fall into two great groups, with a transition period between. The first group includes the stages of growth and the last the stages of maturity. In tabular form, and numbered in chronologic order, these stages appear as follows:
III. The Gradational Eon (Relative maturity) |
10. The Cenozoic era 9. The Mesozoic era 8. The Paleozoic era 7. The Proterozoic era |
The better known eras |
|
II. The Extrusive Eon (Transitional) |
6. The Archeozoic era | The partially known eras |
|
I. The Formative Eon (Birth and adolescence) |
(Under planetesimal hypothesis) 5. The initial life stage 4. The initial hydrospheric stage 3. The initial volcanic stage 2. The initial atmospheric stage 1. The nuclear or nebular stage |
(Under gaseous hypothesis) The initial life stage The initial hy drospheric stage The initial congelation stage The molten stage The gaseonebular stage |
The hypothetical eras |