New Theories in Astronomy

by William Stirling

It is very difficult to find out what geologists consider to be the nature of the interior of the earth in its details, but for our purpose no particular knowledge is required. However, it is necessary to allude to the principal features of their theories in order to note and remark how far they will agree to, or be facilitated, or the reverse, when applied to a hollow sphere. It would seem that almost all geologists are agreed that the central part is solid, and possibly extremely rigid owing to the enormous pressure of superincumbent matter; that it has a solid crust of several hundreds of miles in thickness; and that under this there is a sub-crust divided into two or more layers of different densities, partially liquid or at all events plastic, extending all over the solid interior matter; the chief purpose for which it is required being apparently to supply matter for volcanic action and surface movements.

Under the theory we are advocating, the place of greatest density of the interior is calculated to be at 817 miles from the surface, and its greatest approach to solidity will be there also; consequently, if geologists consider that it will have sufficient plasticity there to provide matter for volcanic eruptions, they will be at one with us so far. But should they consider that they require, for volcanoes, matter more liquid than is likely to be found at that depth, they will have to place their magma layers either much deeper or somewhere between that depth and the surface, in which case they will encroach on the requirements of astronomers, without liberating themselves from a difficulty in which they must find themselves involved under their present ideas. They say that these plastic layers exist under the solid crust all round the interior of the earth, so that if one of the duties they have to perform is to keep the various chains of volcanoes in communication with each other, their lateral movements must extend to some hundreds of miles in the cases of the enormous volumes of matter that are sometimes thrown out in even modern eruptions, and they have to provide the means for procuring that lateral motion. Shrinkage from cooling, or falling in of part of the solid crust, might bring about these enormous outbursts of lava, but they would be more likely to produce simple overflows than the explosive ejection of such masses as are now being recorded from time to time. We have brought into remembrance, [page 148], that water cannot penetrate into the interior of the earth to a greater depth than 9 miles, more or less, as water, and that beyond that depth it can only exist in the form of steam, or dissociated into its elements of hydrogen and oxygen. As long as it continued in the form of water it could be suddenly flashed into steam, of not far from two thousand times its volume, by relief from pressure or sudden application of heat, and thus be converted into a violent explosive almost instantaneously; but when it came to have the form of a gas, it could only be heated gradually the same as any other gas. It is clear, therefore, that water cannot be looked to for producing the force, explosive or otherwise, that is required to raise even molten matter from depths of hundreds of miles to overflow from the summits or outlets of volcanoes.

A pressure of 400 atmospheres would be required to balance a column of average rock of one mile high. A mass of water, through shrinkage of the crust, might get introduced to the vent of a volcano, or some cavity connected with it, a few miles under the surface of the earth and cause an earthquake—it might be introduced by an earthquake—or eruption or both, abundantly formidable and destructive, no doubt, but only comparatively superficial, such as those of Naples and Charleston, where the extreme depth was calculated to be only a few miles; but it seems to us to be totally inadequate to produce those outpours that last for days and weeks, covering leagues of land, and filling up bays of the sea, with floods of lavas. It may be the principal agent or ally in producing the horrors and devastation of a grand eruption that has invaded the regions of water, but it is not to be conceived as possible that it can be the prime cause. The volumes of steam, water, and mud thrown out on such occasions, only tend to distract our attention from looking deeper for the true cause of the eruption. Geologists are therefore thrown back upon their magma layers to look for the motive power for producing these grand eruptions, and they cannot get water down deep enough to do it.

Tides produced by the sun and moon cannot be appealed to, otherwise the eruptions would be more or less uniform in their periods of occurrence. Sudden evolution of gases in the magma layers could not be accounted for in any way known to us, and accumulation of gases would involve the idea of immense cavities, to serve as reservoirs to be gradually filled till the pressure was sufficient to force a way out, and would imply a formation of the interior in compartments specially adapted for particular purposes, and altogether too fanciful to be entertained. Where could such enormous masses of matter, as those thrown out, come from at only a few miles from the surface? The great eruption at the Sandwich Islands, of about a century ago, after flowing over a distance of many miles of land, on which it left enormous quantities of lava, filled up a bay of the sea twenty miles long, and ran out a promontory of three or four miles into the sea; and we cannot conceive it to be possible that such a quantity of matter could be blown out from something less than 9 miles deep by water suddenly flashed into steam.

The critical temperature of water—that temperature at which it changes into steam under any pressure however great—being 412°, its pressure in the state of steam will be somewhere about 7150 lb. per square inch, let us say 500 atmospheres; then, if 400 atmospheres are required to balance 1 mile in depth of average rock, as we have stated above, the pressure of steam just cited would balance only 1¼ miles of rock. We can, therefore, see how inadequate it would be to force a column of lava up from even the depth of 9 miles. At that depth 3600 atmospheres of pressure are required to balance a column of lava, and there are only 500 available. It has been said that the downward pressure of steam would force up the lava through the vent of a volcano, but an arrangement of that kind would require a downcast shaft as well as the upcast one of the vent like as there are in collieries; but the downcast would have to go very deep to compress the steam—a gas now—to the required number of atmospheres. Far more likely that the steam itself would put an end to any increase of water, by driving it back through the channels by which it was descending; for if they are supposed to exist under a solid crust of 800 miles thick the pressure required would be 320,000 atmospheres, and with a crust of only 100 miles thick 40,000 would be required. The only way, therefore, in which volcanic eruptions can be produced in the earth, if solid or liquid, or partially solid and partially liquid, to the centre—in other words, from magma layers—is by the shrinking of the crust squeezing out the lava. With a hollow earth and shell of more or less 2200 miles in thickness, liquid to some depth on the interior surface the difficulty becomes very much less. The communication between the vents of volcanoes would be complete and simple, without any lateral forcing of the lava through magma layers made expressly for the purpose; it would be an open and natural flow from one place to another. That there are such volcano vents connected with each other has been very generally believed, and even almost proved by observation of eruptions taking place in two or more almost simultaneously, or at the least showing signs of violent agitation, the motive forces for which would be the gases which we have concluded must be imprisoned in the hollow centre. When their pressure came to be sufficient to blow or force out the liquid, or semiliquid matter, bubbling and boiling in the vents in constant activity, there would be an eruption, during and after which the gases would escape till their pressure was greatly reduced, when the volcanoes would return to their semi-active state. The gases would naturally be those of the many kinds that are found in eruptions, by reason of their being generated in the earth, mixed with steam and water in the manner we have already shown.

Let it not be supposed that the gases would require to have force enough to raise lavas from depths of over 2000 miles from the surface. According to our arguments for a hollow earth, at 817 miles from the surface the two halves—outer and inner—of the matter composing it meet and balance each other, so that all the pressure required would be what is necessary to overcome the inertia, viscosity, or cohesion of the matter in the vents. What that would be we do not pretend to be able to calculate, but we believe that it would be very much inferior to that required to balance a column of lava of even 100 miles high. We have seen that gas compressed to 4835 atmospheres would be 6¼ times more dense than water, and of equal specific gravity to the heaviest matter required in any part of the earth to make up its average density to 5·66 that of water, and we cannot assume any greater pressure than this, without diminishing that maximum. If that, or any lesser degree of compression, would supply the necessary force, then all difficulty is removed further than pointing out the means of keeping the volcano vents open or openable; and the quality of openable may be facilitated by the contraction of the interior from cooling. If a greater pressure be necessary, we need not be afraid of greatly increasing it, for the only consequence would be to diminish the maximum density of solid matter required in any part of the earth, to make up the general average to 5·66, which means less compression of the matter. If the idea of the accumulation of gases in the hollow centre, or of the hollow centre itself, is inadmissible, then scientists in general can continue as before with their magma layers—aqueo-igneous if they like—but they must abandon the notion of lavas being expelled from them by steam pressure. We repeat that steam could never get down in the form of steam to the depths they require. The temperature there would be more than sufficient to resolve it into its elements of oxygen and hydrogen, and it would behave very much like the gases we have supposed to be in the hollow; there might be accumulation, but there could be no sudden flashing into existence like steam from water.

In support of our observation—if it needs support—that water as water cannot penetrate into the earth to a greater depth than where it meets a temperature of 412° we may refer to reports on earthquakes of comparatively recent occurrence. We learn from the "London Quarterly Review" of January 1869, that in the Neapolitan earthquake of 1857, Mr. Mallett found the greatest focal depth to have been 8-1/8 geographical, or 9·35 statute miles, which agrees very well with the depth to which water could penetrate and be suddenly flashed into steam. (We say nothing, for the present at least, about how the water and the heat managed to meet so instantaneously.) The shock of the instantaneous generation of steam might be felt much lower, but it would tend to interrupt, not to produce, the eruption of lavas. In speaking of the pressure on the walls of the cavity, where the shock was produced, being 640,528 millions of tons, the reviewer says, "it may have been greater because the steam might be supposed to have acquired the temperature of the lava," and that is 2000°F.; but that could not well be. In order to meet lava of that temperature the steam would have to descend to from 20 to 25 miles deep; on the other hand, if the lava is assumed to have entered the cavity, it could only do so at a comparatively low velocity and would not reach more than a fraction of the steam at a time, and even for that reason there could be no flashing, as steam is only a gas, and cannot be heated otherwise than as a gas. Here the spirit of facilitating the meeting of the lava and the steam, is as apparent as in bringing about the meeting of the water and the lava noticed above. On the whole, therefore, we think that we were right in saying that steam or water cannot be the cause of volcanic eruptions, but that the invasion of the domains of water by the lavas may be the cause, in the main, of the explosive part of eruptions, and of the most disastrous effects of earthquakes. Moreover, the focus of the Neapolitan earthquake was 75 miles distant from Vesuvius, and therefore far removed from anything like direct connection with the vent of the volcano, so that water from it in any form could have no effect upon the magmas of scientists.

"The Scientific American" of July 16, 1887, tells us that Captain E. D. Dalton has calculated that the depth of the Charleston earthquake was 12 miles; statute miles, it is to be supposed, as nothing is said to the contrary. To reach the temperature of 412° this would give an increase of 1° in 45 metres in depth, which is a considerably greater depth than what we have estimated, but does not invalidate our reasoning, as it has always been known that the gradient of increase of heat varies considerably from one place to another. Besides, and more especially, Charleston being a seaport, and, consequently, not far from the level of the sea, it is to be supposed that, owing to the presence of water, the cooling of the earth has penetrated to a greater depth there than in the heart of Italy. The same authority states that in the formidable Yokohama earthquake of 1880, the mean depth was only 3¼ miles. The mention of mean depth here makes us notice that the 12 miles may have been the extreme depth to which the earthquake, or shock, was felt at Charleston, and that the focal depth may have been considerably higher up than that. Be that as it may, there is no proof existing that water or even steam can penetrate into the earth more than a very few miles, much less to hundreds of miles.

Having referred pretty freely to the aqueo-igneous magmas, supposed by some scientists to exist deep down in the interior of the earth, it is but fair to give our reasons for refusing to believe that there can be any such mixture in any part of it, or anywhere else. In order to do so, we shall first cite some of the bases upon which such ideas have been founded. In "Nature" of December 12, 1889, we find what follows:—