4. The contours of the floors of all the great oceans, now fairly well known through the soundings of exploring vessels and for submarine telegraph lines, also give confirmatory evidence that they have never been continental land. For if any part of them were a sunken continent, that part must have retained some impress of its origin. Some of the numerous mountain ranges which characterise every continent would have remained. We should find slopes of from 20° to 50° not uncommon, while valleys bordered by rocky precipices, as in Lake Lucerne and a hundred others, or isolated rock-walled mountains like Roraima, or ranges of precipices as in the Ghâts of India or the Fiords of Norway, would frequently be met with. But not a single feature of this kind has ever been found in the ocean abysses. Instead of these we have vast plains, which, if the water were removed, would appear almost exactly level, with no abrupt slopes anywhere. When we consider that deposits from the land never reach these remote ocean depths, and that there is no wave-action below a few hundred feet, these continental features once submerged would be indestructible; and their total absence is, therefore, itself a demonstration that none of the great oceans are on the sites of submerged continents.
How Ocean Depths were Produced
It is a very difficult problem to determine how the vast basins which are filled by the great oceans, especially that of the Pacific, were first produced. When the earth's surface was still in a molten state, it would necessarily take the form of a true oblate spheroid, with a compression at the poles due to its speed of rotation, which is supposed to have been very great. The crust formed by the gradual cooling of such a globe would be of the same general form, and, being thin, would easily be fractured or bent so as to accommodate itself to any unequal stresses from the interior. As the crust thickened and the whole mass slowly cooled and contracted, fissures and crumpling would occur, the former serving as outlets for volcanic activities whose results are found throughout all geological ages; the latter producing mountain chains in which the rocks are almost always curved, folded, or even thrust over each other, indicating the mighty forces due to the adjustments of a solid crust upon a shrinking fluid or semi-fluid interior.
But during this whole process there seem to be no forces at work that could lead to the production of such a feature as the Pacific, a vast depression covering nearly one-third of the whole surface of the globe. The Atlantic Ocean, being smaller and nearly opposite to the Pacific, but approximately of equal depth, may be looked upon as a complementary phenomenon which will be probably explained as a result of the same causes as the vaster cavity.
So far as I am aware, there is only one suggested cause of the formation of these great oceans that seems adequate; and as that cause is to some extent supported by quite independent astronomical evidence, and also directly bears upon the main subject of the present volume, it must be briefly considered.
A few years ago, Professor George Darwin, of Cambridge, arrived at a certain conclusion as to the origin of the moon, which is now comparatively well known by Sir Robert Ball's popular account of it in his small volume, Time and Tide. Briefly stated, it is as follows. The tides produce friction on the earth and very slowly increase the length of our day, and also cause the moon to recede further from us. The day is lengthened only by a small fraction of a second in a thousand years, and the moon is receding at an equally imperceptible rate. But as these forces are constant, and have always acted on the earth and moon, as we go back and back into the almost infinite past we come to a time when the rotation of the earth was so rapid that gravity at the equator could hardly retain its outer portion, which was spread out so that the form of the whole mass was something like a cheese with rounded edges. And about the same epoch the distance of the moon is found to have been so small that it was actually touching the earth. All this is the result of mathematical calculation from the known laws of gravitation and tidal effects; and as it is difficult to see how so large a body as the moon could have originated in any other way, it is supposed that at a still earlier period the moon and earth were one, and that the moon separated from the parent mass owing to centrifugal force generated by the earth's rapid rotation. Whether the earth was liquid or solid at this epoch, and exactly how the separation occurred, is not explained either by Professor Darwin or Sir Robert Ball; but it is a very suggestive fact that, quite recently, it has been shown, by means of the spectroscope, that double stars of short period do originate in this way from a single star, as already described in our sixth chapter; but in these cases it seems probable that the parent star is in a gaseous state.
These investigations of Professor G. Darwin have been made use of by the Rev. Osmond Fisher (in his very interesting and important work, Physics of the Earth's Crust) to account for the basins of the great oceans, the Pacific being the chasm left when the larger portion of the mass of the moon parted from the earth.
Adopting, as I do, the theory of the origin of the earth by meteoric accretion of solid matter, we must consider our planet as having been produced from one of those vast rings of meteorites which in great numbers still circulate round the sun, but which at the much earlier period now contemplated were both more numerous and much more extensive. Owing to irregularities of distribution in such a ring and through disturbance by other bodies, aggregations of various size would inevitably occur, and the largest of these would in time draw in to itself all the rest, and thus form a planet. During the early stages of this process the particles would be so small and would come together so gradually, that little heat would be produced, and there would result merely a loose aggregation of cold matter. But as the process went on and the mass of the incipient planet became considerable—perhaps half that of the earth—the rest of the ring would fall in with greater and greater velocity; and this, added to the gravitative compression of the growing mass might, when nearly its present size, have produced sufficient heat to liquefy the outer layers, while the central portion remained solid and to some extent incoherent, with probably large quantities of heavy gases in the interstices. When the amount of the meteoric accretions became so reduced as to be insufficient to keep up the heat to the melting-point, a crust would form, and might have reached about half or three-fourths of its present thickness when the moon became separated.
Let us now try to picture to ourselves what happened. We should have a globe somewhat larger than our earth is now, both because it then contained the material of the moon and also because it was hotter, revolving so rapidly as to be very greatly flattened at the poles; while the equatorial belt bulged out enormously, and would probably have separated in the form of a ring with a very slight increase of the time of rotation, which is supposed to have been about four hours. This globe would have a comparatively thin crust, beneath which there was molten rock to an unknown depth, perhaps a few hundreds, perhaps more than a thousand miles. At this time the attraction of the sun acting on the molten interior produced tides in it, causing the thin crust to rise and fall every two hours, but to so small an extent—only about a foot or so—as not necessarily to fracture it; but it is calculated that this slight rhythmic undulation coincided with the normal period of undulation due to such a large mass of heavy liquid, and so tended to increase the instability due to rapid rotation.
The bulk of the moon is about one-fiftieth part that of the earth, and an easy calculation shows us that, taking the area of the Pacific, Atlantic, and Indian Oceans combined as about two-thirds that of the globe, it would require a thickness (or depth) of about forty miles to furnish the material for the moon. We must, of course, assume that there were some inequalities in the thickness of the crust and in its comparative rigidity, so that when the critical moment came and the earth could no longer retain its equatorial protuberance against the centrifugal force due to rotation combined with the tidal undulations caused by the sun, instead of a continuous ring slowly detaching itself, the crust gave way in two or more great masses where it was weakest, and as the tidal wave passed under it and a quantity of the liquid substratum rose with it, the whole would break up and collect into a sub-globular mass a short distance from the earth, and continue revolving with it for some time at about the same rate as the surface had rotated. But as tidal action is always equal on opposite sides of a globe, there would be a similar disruption there, forming, it may be supposed, the Atlantic basin, which, as may be seen on a small globe, is almost exactly opposite a part of the Central Pacific. So soon as these two great masses had separated from the earth, the latter would gradually settle down into a state of equilibrium, and the molten matter of the interior, which would now fill the great oceanic basins up to a level of a few mile below the general surface would soon cool enough to form a thin crust. The larger portion of the nascent moon would gradually attract to itself the one or more smaller portions and form our satellite; and from that time tidal friction by both moon and sun would begin to operate and would gradually lengthen our day and, more rapidly, our month in the way explained in Sir Robert Ball's volume.