Russel Wallace : Alfred Russell Wallace (sic)
It is only during the present century that facts have been accumulating in various directions, bearing more or less directly on the question of the earth's internal condition. These have been partially dealt with, both by geologists and by physicists; but the problem is such a complex one, and the evidence of so varied a nature and often so difficult to interpret, that the conclusions reached have been usually doubtful and often conflicting. This seems to have been due, in part, to the fact that no properly qualified person had, till quite recently, devoted himself to a thorough study of the whole subject, taking full account of all the materials available for arriving at a definite conclusion, as well as of the various groups of phenomena which such a conclusion must harmonize and explain. But for many years past a good practical geologist, who is also an advanced mathematician--the Rev. Osmond Fisher--has made this subject his speciality, and in a most interesting volume, of which a second and carefully revised edition, with an appendix, has been recently published, he has brought together all the facts bearing on the problem, and has arrived at certain definite conclusions of the greatest interest. The object of the present article is to give a popular account of so [[p. 573]] much of his work as bears upon the question of the thickness and density of the earth's crust and the constitution of the interior.1
We will first consider the nature of the evidence in favour of the view that, below a superficial crust, there is a molten or highly heated substratum. The existence of volcanoes, geysers, and hot springs irregularly scattered over the whole surface of the globe, and continually ejecting molten rock, ashes, mud, steam or hot water, is an obvious indication of some very widespread source of heat within the earth, but of the nature or origin of that heat they give little positive information. The heat thus indicated has been supposed to be due to many causes, such as the pressure and friction caused by contraction of the cooling crust, chemical action at great depths beneath the surface, isolated lakes of molten rock due to these or to unknown causes, or to a molten interior, or at least a general substratum of molten matter between the crust and a possibly solid interior. The first two causes are now generally admitted to be inadequate, and our choice is practically limited to one of the latter.
There are also very important evidences of internal heat derived from the universal phenomenon of a fairly uniform increase of temperature in all deep wells, mines, borings, or tunnels. This increase has been usually reckoned as 1° F. for each 60 feet of descent, but a recent very careful estimate, by Professor Prestwich, derived from the whole of the available data, gives 1° F. for every 47.5 feet of descent. It is a curious indication of the universality of this increase that, even in the coldest parts of Siberia, where the soil is frozen to a depth of 620 feet, there is a steady increase in the temperature of this frozen soil from the surface downwards. Much has been made by some writers of the local differences of the rate of increase, varying from 1° in 28 feet to 1° in 95; and also of the fact that in some places the rate of increase diminishes as the depth becomes greater.2 But when we consider that springs often bring up heated water to the surface in countries far removed from any seat of volcanic action, and the extent to which water permeates the rocks at all depths reached by man, such divergences are exactly what we might expect. Now this average rate of increase, if continued downwards, would imply a temperature capable of melting rock at about twenty miles deep, or less, and we shall see presently that there are other considerations which lead to the conclusion that this is not far from the average thickness of the solid crust.
Before going further it will be well to consider certain objections to this conclusion, which for a long time were considered [[p. 574]] insuperable, but which have now been shown to be either altogether erroneous or quite inconclusive. In Sir Charles Lyell's Principles of Geology, Mr. Hopkins is quoted as having shown that the phenomenon of the precession of the equinoxes, due to the attraction of the sun and moon on the equatorial protuberance, requires the interior of the earth to be solid, or at least to have a crust not much less than one thousand miles thick. This view was supported by Sir William Thomson and other eminent mathematicians, and so great was the faith of geologists in these calculations that for nearly forty years the theory of the earth's internal liquidity was almost wholly abandoned. But this argument has now been shown to be erroneous by the more complete investigations of Professor George Darwin, while Sir William Thomson (now Lord Kelvin) has recently shown experimentally that a rotating liquid spheroid behaves under stresses as if it were a solid. Another difficulty arises from the phenomena of the tides. It has been argued that, if the interior of the earth is liquid tides will be formed in it which will deform the crust itself, and thus, by lifting the water up with the land, do away with any sensible tides in the ocean. But Mr. Fisher has pointed out that this conclusion rests on the assumption that the liquid interior, if it exists, is not an expansible fluid; and he shows that if this assumption is incorrect it is quite possible that little or no deformation would be caused in the crust by tides produced in the liquid interior; and he further maintains, as we shall see further on, that all the evidence goes to prove that it is expansible. Moreover, in a late paper, he claims to have proved that even the deformation of the crust itself would not obliterate the ocean tides, but would diminish them only to the extent of about one-fifth.3
There remain the geological objections founded on the behaviour of volcanoes, which is supposed to be inconsistent with a liquid interior as their effective cause. We have, for instance, the phenomenon of a lofty volcano like Etna pouring out lava from near its summit, while the much lower volcanoes of Vesuvius and Stromboli show no corresponding increase of activity; and the still more extraordinary case of Kilauea, on the lower slopes of Mauna Loa at a height of about 3,800 feet, whose lake of perennial liquid lava suffers no alteration of level or any increased activity when the parent mountain is pouring forth lava from a height of 14,000 feet. Again, it is argued that if the igneous products of volcanoes are derived from one central reservoir there ought to be a great similarity between them, especially between those of the same district. But this is not the case, an example being the Miocene lavas of Hungary and Bohemia, which are of a totally different character [[p. 575]] from each other. But although the molten interior of the globe may be the common source of the heat which causes volcanic eruptions, it by no means follows that the whole, or any large portion, of the matters ejected from volcanoes are derived from it; and it is a remarkable indication of the probable truth of Mr. Fisher's theory, that, as will be shown further on, it entirely removes the two geological difficulties here noticed. At the same time it explains other geological phenomena of a striking character which the theory of solidity altogether fails to account for, as will be now briefly indicated.
It has long been known to geologists that the series of sedimentary rocks, ancient as well as modern, afford repeated examples of great piles of strata hundreds, or even thousands, of feet thick, which throughout present indications of having been formed in shallow water, and which therefore imply that as fast as one bed was deposited it sank down, and was ready to receive another bed on the top of it. As an example we may refer to the Palæozoic rocks of the Alleghany Mountains, which are not less than 42,000 feet thick; yet the lowest of these strata, the Potsdam sandstone, was not deposited in a deep sea, but evidently in shallow water near shore, several of the beds exhibiting distinct ripple markings, and the same is the case with the highest strata found there--the carboniferous. On this point Sir Archibald Geikie remarks:--
Coming now to the other end of the geological record, we find in the deltas of existing rivers an exactly similar phenomenon. At Venice a boring of 400 feet deep was entirely in modern fluviatile mud, the bottom of which was not reached; and at four separate depths, one of them near the bottom, beds of turf or of vegetable matter were passed through, showing, as Sir Charles Lyell observes, "that a considerable area of what was once land has sunk down 400 feet in the course of ages."4 At Zagazig, on the eastern [[p. 576]] border of the Nile delta, borings have been made for the Royal Society, and have not found rock at a depth of 345 feet. In the delta of the Mississippi a well at New Orleans, 630 feet deep, passed entirely through sands and clays, with fresh-water shells of living species. Again, in the delta of the Ganges, at Calcutta, a boring 481 feet deep was entirely through beds of sand, peat, gravel, and other alluvial or fresh-water deposits. This remarkable concurrence of testimony from so many parts of the world and from different geological periods, indicates a general law of subsidence so uniformly coinciding with deposition, and so regularly keeping pace with it, that we can hardly avoid the conclusion that the two phenomena are connected; and the most reasonable explanation seems to be that the deposit of matter in a shallow sea directly causes the depression of that bottom by its weight. Such depression is quite intelligible on the theory of a thin crust resting or floating on a liquid substratum, but is quite unintelligible on the supposition of a solid globe, or of a crust several hundred miles thick. It is only reasonable to suppose that depression thus caused must be accompanied by a corresponding elevation of some other area, and as there must always be an adjacent area from which an equivalent weight of rock has been removed by denudation, we should expect the elevation to occur there; and many geologists believe that there is direct evidence of elevation wherever areas are being rapidly denuded.
In a very interesting letter to Nature (Dec. 5th, 1889.) Mr. J. Starkie Gardner states that he has actually observed the results of denudation to be of this character. He says:--
Hence he concludes that--
The physical and geological phenomena of which an outline sketch has now been given, all point unmistakably to a thin crust of various rocks resting on a molten substratum; but there are certain difficulties and objections which require a fuller discussion. In order to remove these difficulties and answer these objections, we must, with [[p. 577]] the aid of Mr. Fisher's work, go more deeply into the question, and we shall then find that, by means of some of the most refined inquiries of modern physicists, we are able to obtain so much additional information as to the peculiarities of the crust and of the substratum, that most, if not all, of the alleged difficulties will be found to disappear.
It is well known that mountains attract the plumb-line, and thus render latitudes determined by its means, or by a spirit or mercurial level, inaccurate in their vicinity. During the trigonometrical survey of India the amount of this error was carefully determined in several localities near mountains, but a discrepancy appeared. When the mass of the Himalayas was estimated and its attraction calculated, it was found to be more than the observed attraction. The same thing had occurred in the original experiment by Maskelyne at Schehallion in Scotland; and a similar deficiency in the error produced was noticed by Petit in the case of the Pyrenees. Many attempts were made to explain the discrepancy, but that which was advanced by the late Sir G. B. Airy seems best to account for all the phenomena, and is that adopted by Mr. Fisher. It is, that every mountain mass on a continent has a much larger mass projecting beneath the crust into the liquid substratum, exactly as an iceberg has a larger mass under the water than above it. Sir G. B. Airy argued that, whether the crust were ten miles or a hundred miles thick, it could not bear the weight of such a mass as the Himalayan and Tibetan plateaus without breaking from bottom to top, and receiving support by partially sinking into the liquid mass. The best experiments show that the proportionate densities of most rocks in a solid and a liquid state are approximately as ice is to water, and thus no mountain masses can be formed, whether by lateral pressure or other agency, without a corresponding protuberance forming below to keep the crust in equilibrium. It is this displacement of the denser substratum by the less dense "roots of the mountains" that leads to the total attraction of such mountains being less than they otherwise would be. In our author's words--"the roots of the mountains can be felt by means of the plumb-line."
Still more important and interesting are the revelations afforded by the pendulum, since they not only support the interpretation of the plumb-line experiments above given, but furnish additional material for estimating the varying thicknesses and densities of the earth's crust. The rate of vibration of a pendulum of constant length depends upon the force of gravity at the place, and thus variations in that force can be determined with considerable accuracy. Taking the number of vibrations in a day of a seconds [[p. 578]] pendulum at the equator and at the sea-level as 86,400, the number of vibrations at any other latitude can be calculated on the theory that the earth is a perfect spheroid of revolution; and geodetic observations show that it has such a form. At any elevated station, whether on an isolated mountain or on an extensive plateau, the pendulum will vibrate more slowly on account of its greater distance from the centre of gravity of the earth, while it would vibrate more quickly on account of the additional attraction of the elevated mass immediately beneath and around it. These effects can be calculated, and the balance of the two, applied to the normal rate for the latitude, will give the theoretical rate due to the position and altitude of the station. Experiments were made at more than twenty stations in India, varying from the sea-level to over 15,000 feet above it, and at all the higher stations there was a deficiency of the observed from the calculated number of vibrations of from one to twenty-four vibrations in the twenty-four hours. In such delicate observations there were of course some irregularities, but the fact of a greater deficiency at the higher levels came out very clearly, and could be explained only by a deficiency of subterranean density due to the roots of the mountains displacing a denser substratum, as in the case of the plumb-line experiments.
Before leaving this subject of the "roots of mountains," it will be well to refer to a remarkable corroboration of their actual existence by evidence of a quite different kind. It has already been pointed out that the rate of increase of underground temperature would, if continued downwards till the heat equalled the melting point of rock, give a mean thickness of the crust of about twenty miles. But in places where the crust is so much thicker, as it is supposed to be under mountains, the rate of increase should be much less, because the lower level of the crust in contact with the liquid substratum must always be at about the same temperature--that of melting rock. This is found to be the case; the rate of increase at the St. Gothard tunnel, where the observations were most complete, being 1° F. in eighty-eight feet, and the corresponding thickness of the crust thirty-seven miles. This is certainly a remarkable confirmation of the other observations, and of the theory of mountains being supported in approximate equilibrium by means of vast protuberances into the liquid substratum beneath.
The general result of the whole series of experiments with the pendulum shows that gravity is normal at the sea-level both over land and sea, and thus proves that the surface of the globe is in a state of equilibrium. The measures of the force of gravity over the oceans have been necessarily taken on islands, and have led to a curious discovery. The pendulum experiments on oceanic islands [[p. 579]] such as the Galapagos, Ascension, St. Helena, Bourbon, Guam, and others, all show an increase in the force of gravity, which, on the average, is very nearly accounted for by the subaqueous mass of land displacing water of less than half the density of rock. Hence it is concluded that these islands or island-mountains do not have "roots" as do those on continents; and the same thing occurs with isolated volcanoes on continents, the attraction of Fujisan in Japan being exactly that due to its own bulk unaffected by the presence of "roots" projecting into the substratum. This is explained by the fact that volcanic mountains are not produced by compression forcing the crust both downwards and upwards, as other mountain masses are supposed to have been produced, but are mere heaps of materials derived either from the crust or the substratum, and probably drawn from a considerable area. Hence they are balanced not by "roots" projecting immediately below them, but by a slight depression or sagging of the crust over a wide area, and thus having little effect on the rate of the pendulum. In the case of the Falkland Islands, however, the force of gravity is less than it ought to be, and this exception affords an interesting confirmation of the general theory. For these are not volcanic, but are true continental islands, forming the outer margin of the old continent of South America though now 350 miles from land; and thus, being surrounded by water instead of by much heavier land, the force of gravity is somewhat reduced, water having here replaced a denser mass of land.
We now come to the more special researches of Mr. Fisher, which throw so much light on the hitherto unexplained phenomena of volcanoes. By means of some recent experiments on the melting-point and specific heat of rocks, made at his suggestion, he arrives at the conclusion that the average thickness of the earth's crust on lands near the sea-level is only about 18 miles. Its density is estimated at 2.68, water being 1, and the density of the liquid substratum at 2.96.5 With these new data it appears that if the melted substratum were an inert mass it would have cooled at such a rate that the crust would have attained its present thickness in about eight million years. But geologists are almost unanimously of opinion that any such period as this is absurdly too small, and that to account for the phenomena presented by the known series of rocks and their included organic remains, the very least time that must be allowed is one hundred million years. The conclusion Mr. Fisher draws from this discrepancy is, that the substratum is not inert but energetic, that is, that it is in a state of movement or circulation, convection currents continually bringing up fresh heat from below and thus preventing the crust from solidifying so rapidly as if there were no such [[p. 580]] currents. A cause of such currents is found in the friction produced by tidal action in the liquid mass, which Professor George Darwin has shown to be very great, and to be at a maximum in the central portions.6
Gravity having approximately its normal value all over the globe at the sea-level, it is evident that there must be some denser matter under the oceans to make up for the much less density of the water, which is at least three miles deep on the average. A very refined mathematical investigation shows that this can only be brought about by the sub-oceanic crust being both thinner and denser than under the continents, the denser portion being the upper layer. This distribution of matter may, it is supposed, be due to extensive outflows of heavy basalt over the original depressions forming the ocean floors, at some early period of their history.
The physical constitution of the liquid matter forming the substratum is the next point to be considered, and is one of the highest importance, since it is evidently what determines both volcanic action and a large portion of the disturbances to which the crust is subject. Many geologists are of opinion that the phenomena of volcanic action can only be explained on the supposition that the molten matter forming the interior of the globe holds in solution enormous quantities of water-vapour and other gases; and there is ample evidence that melted lavas and slags do contain such gases, which they give out on becoming solid. Thus Mr. Scrope, in his great work on Volcanoes, says:--
Professor Judd, in his volume on the same subject, shows that the presence of these gases in lava is in accordance with Henry's law, that liquids are able to absorb gases to an amount proportioned to the pressure they are under, and with the fact that molten substances do actually absorb large quantities of gases. He says:--
He then goes on to show that the enormous quantity of steam and other gases given off during volcanic action and from flowing lava-streams, can only be accounted for by supposing that the molten rock from which they are derived contains these gases to an amount equal to many times their volume; and that the same fact is indicated by the liquefied gases that are found in the cavities of the crystals of volcanic products which have consolidated under great pressure, such as granites, porphyries, and other rocks of allied nature.
There can, therefore, be no doubt as to the fact of the liquid substratum containing in its substance an enormous quantity of gases, the principal being water-vapour, but how the gases came there is less certain; nor does it materially concern us. Some think that these gases have been largely derived from sea-water, which has found its way by percolation to the heated interior; but there are many difficulties in this view. Others, with whom is Mr. Fisher, think that they form an essential constituent of the primeval globe, and that, instead of being derived from the ocean, it is more probable that the ocean itself has been derived from the vapours which have been always escaping from the interior. Leaving this question as one of comparatively little importance for the present discussion, we have now to point out how the facts, that the fluid substratum is saturated with water-vapour and other gases, and is also subject to convection-currents continually bringing superheated matter up to the lower surface of the crust, enable us to explain the special difficulties alluded to in the early portion of this article.
The first of these difficulties is, that neighbouring volcanoes of very different heights act quite independently, a fact which is supposed to be inconsistent with the idea that both are in connection with the same molten interior. It seems, however, to have been assumed [[p. 582]] that a mere fissure or other aperture extending from the surface to the substratum, or from the substratum to the surface, would necessarily be followed by an outflow of lava, even though the opening terminated at the summit of a mountain many thousand feet above the sea level. But it is evident that on the theory of a molten interior, with a crust of somewhat less specific gravity resting upon it in hydrostatic equilibrium, nothing of the kind would happen. When a hole is bored through an extensive ice-field, whether on a lake or in the Arctic Ocean, the water does not spout up through the aperture, but merely rises to the same level as it would reach on the sides of a detached block of floating ice, or on the outer margin of the ice-field itself. The facts that the fluid on which the crust of the earth rests is intensely heated, and that the crust is continuous over its whole surface, can make no difference in the behaviour of the fluid and the solid, so as to cause the molten rock to rise with great violence thousands of feet above its mean level whenever an aperture is made; and this is the more certain when we take account of the fact, which may now be taken as established, that the crust floats on the fluid interior, and that it is so thin and weak, comparatively speaking, that it cannot resist a strain equal to its own weight, but must bend or fracture so as to keep every part in approximate hydrostatic equilibrium. Volcanic action, especially continuous and permanent volcanic action like that of Stromboli and Kilauea, cannot, therefore, be explained by the mere existence of a thin crust and a molten interior; but it is well explained by the presence in the molten mass of vast quantities of gases existing under enormous pressure, and ready to escape with tremendous force whenever that pressure is greatly diminished, and the molten material that contains it lowered in temperature.
Let us now endeavour to trace what will happen when a fissure is opened gradually from below upwards till it reaches the surface. Owing to hydrostatic pressure the fluid will rise in the fissure, and in doing so will be subject to some cooling and diminution of pressure, which, as we have seen, will lead to a liberation of some of the contained gas. The pressure of this gas will aid in extending the fissure, and the liquid will continue to rise till it reaches the level of hydrostatic equilibrium, which would be somewhere about two miles below the surface. But throughout the whole mass of the liquid in the fissure, and for some depth below the under surface of the crust, there would be a continual liberation of intensely heated gases. These would no doubt carry with them in their upward rush a portion of the liquid matter which had risen from below, but they would also, owing to their intensely heated condition, melt off some portions of the rocky walls of the fissure, and thus give to the ejected [[p. 583]] volcanic products a local character. We here see the explanation of the supposed difficulty of the individuality of neighbouring volcanoes and the diversity of their products, and also of the fact of an eruption of lava from the crater of a lofty mountain while the liquid lava of one close by, and thousands of feet lower, maintains its usual level. Kilauea we may suppose to owe its permanently molten lake to a siphon-like passage through which a constant flow of heated gases is maintained, and which suffices to keep its lava in permanent ebullition; while the lofty Mauna Loa has its vent usually blocked up, and may owe its occasional eruptions to an accumulation of gases in some deep-seated cavities which, at long intervals, become sufficiently powerful to burst away the obstacle and pour out a quantity of melted material derived from the sides of the channels through which they make their way upward.
The phenomena presented by the crater of Kilauea, where an extensive lava-lake remains in a constant state of ebullition while keeping approximately the same level, can only be explained by the upward percolation of heated gases in moderate and tolerably uniform streams, sufficient to keep up the melting temperature of the lava; while occasional more powerful outbursts throw up jets or waves of the molten matter, or sometimes break up the crust that has formed over portions of the lake. Here, evidently, there is no eruption in the ordinary sense, no fresh matter is being brought up from below, but only fresh supplies of intensely heated gases sufficient to keep the lava permanently liquid, and to produce the jets, waves, and fountains of lava, and the strange surging, swirling, and wallowing motions of the molten mass, so well described by Miss Bird, Lord George Campbell, and other competent observers.
The sketch now given of Mr. Fisher's investigations as to the nature of the molten interior of the earth and of the crust which overlays it, only covers a small portion of the ground traversed in his work. He there deals also with the more difficult questions of the stresses produced by the contraction of the cooling earth, and the various theories that have been suggested to explain the great inequalities of its surface. The origin of the great oceanic depressions and of the vast mountain masses that everywhere diversify the continental areas, and the causes that have produced the compression, upheaval, folding and crumpling of the rocks at every period of geological history, are all discussed, and some light is thrown upon these confessedly obscure and very difficult problems.
But whatever doubts may still exist as to the exact causes of these last-named phenomena do not apply to those to which the present article is mainly devoted. So many distinct but converging lines of [[p. 584]] evidence indicate the existence of a molten substratum holding in solution, in accordance with well-known physical laws, great quantities of steam and other gases, and show that the crust covering it is a very thin one--while the hypothesis of such a substratum and thin floating crust so well explains the curious phenomena of great masses of strata thousands of feet thick yet from top to bottom bearing indications of having been deposited in shallow water, and the no less singular fact of a corresponding recent subsidence in all great river-deltas, and also clears up so many difficulties in the modes of volcanic action and the diversity of volcanic products--that we can hardly doubt the correctness of the hypothesis. And though at first sight the idea of our being separated by a thickness of only eighteen miles of rock from a layer of molten lava of unknown depth may appear somewhat alarming, yet the very tenuity and fragility of the crust may itself be a source both of safety and of utility. While sufficiently thick to secure us from any injurious or even perceptible effects of internal heat, except in volcanic or earthquake areas, it yet gives us the possibility and even the promise of an inexhaustible source of heat and power at such a moderate distance that we may some day be able to utilise it. On the other hand, the thin crust so readily and constantly adjusts itself to all the alternations of strain and pressure to which it may be exposed, that we are thereby secured from the occurrence of vast cataclysms capable of endangering the existence of any considerable portion of our race. A solid earth might, possibly, not be so safe and stable as is Our Molten Globe.