Oceanic crust

“ I have no space for discussing these views further than to assert that, in the existing state of our globe, and even admitting a solid crust of only 60,000 metres thick, dislocation of the crust by tension is not possible. The solid crust of our globe, as I hope we shall see further on, is not in a state of tension, and has not been so since it was extremely thin, a mere pellicle as compared with the liquid nucleus, but is, on the contrary, in a state of tangential compression. ”

“ If the earth were liquid within and merely crusted over by a shell of rock of moderate thickness, it would yield appreciably to tidal stresses, and this yielding would change the capacity of the interior so that with every distortion of the spheroid a portion of its fluid interior would be forced to the outside, and with every return to the more spheroidal form there would either be a re-flow to the interior or a shrinking of the crust. ”

“ In thinking of the formation of the globe, we must at once free ourselves from the erroneous impression that the crust of the earth is a solid, steadfast foundation. So far from being immovable, it has been constantly heaving and falling; and if we are not impressed by its oscillations, it is because they are not so regular or so evident to our senses as the rise and fall of the sea. The disturbances of the ocean, and the periodical advance and retreat of its tides, are known to our daily experience ”

“ It is in these silent depths that for uncounted and innumerable years the crust of the earth has been forming and has been growing outwards, while it has been slowly hardening inwards above the fires of its unplumbed interior.
It has been calculated that in a square mile of the ocean down to a depth of one hundred fathoms there exist more than sixteen tons of carbonate of lime in the form of the bones or shells of living animals. A continual fine "snow-storm" of dead chalky animals is therefore falling on to the bottom. ”

“ If the earth had a cool thin crust upon a hot fluid interior, and that thin crust were able to sustain itself during geologic ages so that the shrinkage should accumulate within, until finally collapse came, giving an era of uplift, it is obvious that we could account for such cycles. There is very clear evidence that the outermost layer of the earth's crust is but a thin shell like the outer shuck or exocarp of a butternut, so thin that it is not at all possible that it can sustain itself for more than a hundred miles or so, or for more than a very few years at the outside. ”

“ For the whole globe, with the exception of a thin envelope, much thinner in proportion than the shell to an egg, is a fused mass, kept fluid by heat, but constantly cooling and contracting its dimensions. The external crust does not gradually collapse and accommodate itself century after century to the shrunken nucleus, subsiding as often as there is a slight failure of support, but it is sustained throughout whole geological periods, so as to become partially separated from the nucleus, until at last it gives way suddenly, cracking and falling in along determinate lines of fracture. ”

“ Hundreds or thousands of feet of sedimentary strata might in this way be heaped up round the continents, containing a fragmentary series of organic remains belonging to those forms of comparatively shallow-water life which had hard parts capable of preservation. There can be little doubt that such has, in fact, been the history of the main mass of stratified formations in the earth’s crust. By far the largest proportion of these piles of marine strata has unquestionably been laid down in water of no great depth within the area of deposit of terrestrial sediment. ”

“ The outer crust, which is probably of no great thickness, contains an enormous proportion of oxygen and silicon combined with the materials which constitute the interior portions of our globe. It may - 324 - be, as has been suggested by astronomers, that our earth consisted at one time of a solid metallic mass surrounded by a vaporous envelope of metalloids, and that the whole of the latter, with the exception of the constituents of the atmosphere and ocean, have gradually entered into combination with the metals of the nucleus to form the existing crust of the globe. ”

“ We should thus expect to have formed on the earth's surface a crust which would never sink, or, if it sunk, only for a comparatively short distance, there to give rise to a solid crust floating on a denser heterogeneous liquid. Contractions and upheavals of this crust, not unlike in their effects what we sometimes see take place in ice, would explain all the volcanic and earthquake phenomena that need to be accounted for; while the assumption of a liquid interior accords better with the facts of petrography than any other that has been made. ”

“ The raising of mountains is a difficult bit of engineering to explain, and geologists are by no means agreed as to the causes that thrust one part of the earth's crust skyward and sink other parts into ocean depths. The most commonly accepted cause is the shrinkage of the earth's interior, by cooling, or by the loss of gases, or by condensation due to gravity. In this process the solid outer crust becomes too large for the interior and must be crushed and crumpled to adjust itself. ”

“ It is that the sediment accumulated along the shores of the ocean presented an obstacle to radiation, and consequently to cooling of the crust, while the ocean-floor, unprotected and unweighted, and constantly bathed with currents of cold water, having great power of convection of heat, would be more rapidly cooled, and so would become thicker and stronger. ”

“ For untold millions of years the rocks at and near the earth’s surface have been crumbling; streams have been incessantly sawing into the lands; the sea has been eating into continental masses; the winds have been sculpturing desert lands; and, more intermittently and locally, glaciers have plowed through mountain valleys, and even great sheets of ice have spread over considerable portions of continents. Throughout geologic time, the crust of the earth has shown marked instability. ”

“ The interior, therefore, cooling and contracting faster than the exterior crust, the latter following down the ever-shrinking nucleus, would be thrust upon itself by a lateral or tangential pressure which would be simply irresistible. If the earth crust were a hundred times more rigid than it is, it still must yield to the enormous pressure. It does yield along its weakest lines with crushing, folding, bulging, and the formation of mountain ranges. ”

“ The question of the constitution of the earth's interior has in recent years been much investigated by means of seismographic records. These appear to indicate that there are three distinct divisions. The outer crust has a thickness of from 20 to 40 miles. It possesses a high power of resistance to all kinds of stress. Beneath it is a large shell possessing a density and elasticity resembling fine steel. This shell has a high rigidity against changing forces of shorter duration, like tidal action, but in its outer parts at least yields in time to unvarying long-continued stress. ”

“ These concavities of surface are so extensive and so widely distributed over the globe that no part of the outer shell can be supposed to be capable of accumulating notable stresses unless rigidly attached to the earth-body below. In other words, so far as sphericity is concerned, the crust must ease all its stresses nearly as fast as they accumulate, if, as usually assumed, it rests on a contracting or mobile substratum. ”

“ If then by the contraction of the earth’s interior the outer crust were forced to accommodate itself to a smaller nucleus, the central softer belt would yield by crumpling, the more rigid masses to the north and south, if they gave way at all, would yield by faulting. ”

“ After millions of years of work causing the deposition of thousands of feet of strata, piled layer upon layer on the sea floor, a force of lateral pressure is brought to bear and a mountain range is literally born out of the sea. No sooner is the range well formed than the destructive processes (erosion) unceasingly set to work to destroy this marvelous work. But the sediments derived from the wear of the range are carried into the nearest ocean again to accumulate and, perchance, after long ages, to be raised into another range ”

“ In any case, the earth is hot inside as proved by deep well records and igneous phenomena in general, and it is a contracting or shrinking body as proved by the many large scale zones of wrinkling or folding of rocks. If, then, highly heated solid rocks at reasonable distances down in any part of the earth are subjected to relief of pressure by an earth movement such as upward crumpling of the crust, or by readjustment of large fault blocks, such heated solid rocks would become molten. ”

“ They show that as
the sediments grow in depth there is a rising of the geotherm of
plasticity—whatever its true temperature may be—gradually
reducing the thickness of that part
130
of the upper crust which is bearing the simultaneously increasing
compressive stresses. Below this geotherm long-continued stress
resolves itself into hydrostatic pressure; above it (there is, of
course, no sharp line of demarcation) the crust accumulates
elastic energy. The final yielding and flexure occur when the
resistant cross-section has been sufficiently diminished. ”

“ Nor is there any reason for thinking that the proportion of dry land to ocean has ever been materially different to what it is at present, however greatly the areas of sea and land may have changed as regards their place. It follows from the above, that there is no sufficient basis for the view that the crust of the earth is composed of a succession of concentric layers, like the coats of an onion, each layer representing one formation. ”

“ The constitution of the interior of the earth is a subject of great interest in the science of geography, as many of the visible forms upon the crust have been wrought by the power of the agencies within it. The discussion has been warm in the past, and doubtless will be resumed with unabated interest as we find new phenomena for the argument. The apparent lull that has followed the promulgation of the theory, three years ago, that under the crust we should find a fluid, or semi-fluid, surrounding a solid nucleus, may not be of long duration. ”

“ If the above rate of increase continues uniform the temperature of the crust would be sufficiently hot to boil water at a distance of about 8,000 feet below the surface, while at a depth of about thirty miles the temperature would be sufficiently high to melt all known substances at ordinary conditions of atmospheric pressure; that is, to melt all known substances if they were subjected to such a temperature at the level of the sea. ”

“ As the nucleus contracts, it tends to draw away from the cooled and solid crust, and the latter settles, adapting itself to the shrinking nucleus much as the skin of a withering apple wrinkles down upon the shrunken fruit. The unsupported weight of the spherical crust develops enormous tangential pressures, similar to the stresses of an arch or dome, and when these lateral thrusts accumulate beyond the power of resistance the solid rock is warped and folded and broken. ”

“ The solidified crust, when cooled to the temperature derived from the joint influence of the hot sun and the cold regions around the globe, suffers no further loss of heat; but the internal parts may still grow cooler through immense periods of time; they may thus contract more than the outer parts, and fail to sustain them; fractures follow, and the equilibrium of pressure is restored, till a long period of cooling revives the irregularity of forces, and the crust breaks again. ”

“ A less fluid type will become markedly scoriaceous, where the vapours endeavour to escape from it; the rugged crust formed on its upper cooling surface will be broken up by the continued movement of the more liquid mass below, and the blocks thus formed may become rolled over the advancing front of the flow and entombed in the portion that has not yet consolidated. ”

“ The contraction of the earth's interior by cooling and by the emission of material from below the over-crust, has caused this crust to press downward, and therefore laterally, and so to effect great bends, folds, and plications; and these, modified subsequently by surface denudation, and the piling of sediments on portions of the crust, constitute mountain chains and continental plateaus. ”

“ The radioactive theory involves an actual
rise in the temperature at any distance from the surface; so that
_the level in the crust at which the rocks are softened is nearer
to the surface in the geosynclines than it is elsewhere in the
normal crust_ (Pl. XV, p. 118) .
In this manner the rigid part of the crust is reduced in
thickness where the great sedimentary deposits have collected. A
ten-kilometre layer of sediment might result in reducing the
effective thickness of the crust by 30 per cent.; a
fourteen-kilometre layer might reduce it by nearly 50 per cent. ”

“ But there is now too much diversity in the rocks of the earth's crust, because throughout most of the world the continents have in large part been above sea level during the Tertiary and Quaternary, and erosion has been going on so that many types of rocks are exposed and particularly large areas of crystallines, and when any or all of these are brought under arid climatic conditions, grains of a great range in composition are exposed to the sorting action of wind. ”