Art. XLII.—Notes in reference to the Prime Causes of the Phenomena of Earthquakes and Volcanoes.
[Read before the Wellington Philosophical Society, 25th August, 1886.]
The recent outburst of volcanic activity in the Lake District naturally excites our curiosity in relation to the prime cause of earthquakes and volcanic phenomena; and I propose, in this paper, to call attention to some points which appear to me materially to affect the solution of this question, but which are not referred to, so far as I have been able to ascertain, in any geological works. In order, however, that the bearing of the matters to which I am about to call attention may be understood, it is necessary that I should refer, in the first place, to the speculations of astronomers and physicists respecting the original condition of our globe as a concrete mass, because, if those speculations be well founded, it is clear that the phenomena of earthquakes and volcanoes must be associated with the continued existence of fused matter at no great depth below its surface.
Herschel long ago pointed out how, under the action of gravitation, cosmical matter “so diffused as to be scarcely discernible” might be condensed into a comparatively small mass. Kant, in his “Naturgeschichte des Himmels,” (published in 1755), assumed that all the materials composing the spheres that
belong to our solar world were, in the beginning of all things, resolved into their elementary substance, and filled the whole space of the system in which these spheres now move. Laplace, who is said to have been ignorant of Kant's hypothesis, published his “Exposition du Système du Monde” in 1796, in which he referred the formation of our planetary system to a gradual cooling and contraction of the atmosphere of the sun, contending that this atmosphere previously extended, under the influence of excessive heat, beyond the orbits of the farthest planets. Mayer, in his “Celestial Dynamics,” (published in 1848), tells us that the Newtonian theory of gravitation, whilst it enables us to determine, from its present form, the earth's state of aggregation in the past, at the same time points to a source of heat powerful enough to produce such a state of aggregation, and teaches us to consider the molten state of a planet as the result of the condensation of cosmical matter, and to derive the radiant heat of the sun and the heat of the bowels of the earth from the same sources. Those who are curious as to these speculations will find a criticism of the various phases which the Nebular Hypothesis, as a cosmogenetic theory, has assumed, in Stallo's “Concepts and Theories of Modern Physics,” published as vol. xlii. of the International Scientific Series, in which the objections to each of the views propounded in relation to this hypothesis are pointed out and discussed. But the general idea that our planetary system originated from the condensation of cosmical matter has been confirmed by, or at all events receives strong support from, our recently acquired knowledge of the present condition of two of the largest of its members,—namely, Jupiter and Saturn, and of that of our own satellite. As to the latter, it is abundantly proved, that it is composed of the cooled relics of a once intensely heated mass, its whole surface giving evidence of extinct eruptive action. The absence of any appreciable atmosphere around it leaves that surface permanently unchanged, the ruggedness of the ejected material in no degree effaced, or even moderated, by the distribution of light volcanic ash, if any such substance happens to exist upon it, of which I have considerable doubt. We also now know that each of the two great outer planets, Jupiter and Saturn, is still in a condition of intense heat throughout its whole mass. “We recognize,” says Mr. Proctor, “in the appearance of Jupiter the signs of as near an approach to the condition of the earth, when as yet the greater part of her mass was vaporous, as is consistent with the vast difference between two orbs containing such unequal quantities of matter;” and the same author, speaking of the “great red spot” which has, for some years past, excited the attention and curiosity of astronomers, says: “It may well be that the movements by which a disturbed cloud-belt on Jupiter returns to its normal condition
are sluggish, compared with the fierce action by which disturbances are brought about at, or it may be below, the fiery surface of the planet itself.”
I think, indeed, that it may almost be received as a postulate, that, in whatever manner the cosmical matter of which our globe was formed became aggregated, it must, for a very long period after that aggregation had been completed, have remained in a condition of intense heat at its surface.
I have already dealt with this subject in papers on the “Cause of warmer climates which existed in high northern latitudes during former geological epochs,” published in the 10th volume of the “Transactions of the New Zealand Institute,” to which I may refer any person desirous of going more fully into it, and I do this without hesitation, because the views contained in those papers were received with approval by several scientific inquirers of high position and authority in Europe. In the first of those papers I remarked that geologists, including so eminent an authority as the late Sir Charles Lyell, have hitherto treated such speculations as those I have referred to as having only a remote bearing on geology; but I cannot help thinking, that so long as we continue to recognize the extent to which the surface conditions of the earth have been, and are still being modified by the action of forces operating at great depths below that surface, and especially by such exhibitions of those forces as earthquakes and volcanoes, we are bound to be guided in our inquiries by a regard to those speculations, before we can hope to arrive at any sound understanding of the phenomena in question.
I must not, however, in justice to Dr. Page, one of the most delightful writers on geology, omit to refer to some remarks which he makes in his “Advanced Text-book,” in relation to these speculations. In dealing with the question of the density of our globe, he points out that it cannot, if the law of gravitation be acting uniformly towards the centre, be composed throughout of materials in the same condition as those which constitute its crust, because, in that case, a depth would soon be arrived at where the density of ordinary rocks would become so great as to give a mean density much higher than that which its astronomical relations seem to warrant. He also points out that, whilst the ponderable crust, calculating from precession and nutation, cannot be of less thickness than a fourth or fifth of the radius, (being about that assigned to it by Hopkins, as I mention further on), the interior layers of that crust may consist of molten rock-matter, or even rock-matter in a state of vaporiferous incandescence. He then says that, whatever be the exact proportions and conditions of the crust and interior of the earth, we know enough of its temperature to warrant certain general conclusions—namely, that the surface temperature is
mainly derived from the sun, and may, though variable and irregular, be laid down with some degree of certainty; that the heat thus derived extends to a depth of from 60 to 90 feet, and that below this stratum the temperature increases at such a rate, that a temperature must soon be reached sufficient to keep in fusion the most refractory rock-substances; that this high internal temperature is apparently the cause of hot springs, volcanoes, earthquakes, and other igneous phenomena which make themselves known at the surface. In another passage he says: “Looking at the comparative thinness of the solid crust, one can readily conceive how much it would be affected by any commotion in the interior zones, or by any contraction or expansion of the entire mass. Hence the tremors, the undulations, the upheavals and subsidences occasioned by earthquakes and volcanic convulsions; and hence, also, the fissures and fractures which everywhere traverse the rocky crust, whether they may have arisen from the efforts of local forces, or from the operations of some unknown but general law of secular contraction.”
I do not propose to enter into a discussion of the causes which may have brought about the present figure of the Earth, because, except in so far as that figure adds strength to the view of its original fluidity from heat, it does not materially affect the question under consideration; but I propose to make some observations on this subject in the sequel, in order to show its connection with the special matter dealt with in this paper. It is curious, however, that amongst physicists who have accepted the nebular hypothesis as a sound cosmological theory, considerable differences of opinion have been expressed as to the mode in which the cooling of our globe commenced. As mentioned by Sir Charles Lyell in his “Elements of Geology,” Poisson controverted the doctrine of the present high temperature of the central nucleus, and declared his opinion that, if the globe had ever passed from a fluid to a solid condition in consequence of the loss of heat by radiation, the cooling and consolidation of the surface would have begun at the earth's centre, or, in other words, that the aggregation was so slow as to admit of the dynamical heat generated in the act being radiated into space as fast as it was generated. Other physicists treat the cooling as having commenced at and extended downward from the surface of the completely aggregated mass, and, whilst admitting that the nucleus may still be in a fluid state, have assigned a very great thickness to the solidified crust. Hopkins has fixed this at from 800 to 1,000 miles at the least, whilst others have treated the fact, that the mean density of the earth exceeds that of the rocks which compose the known portions of the crust by 2 ½ to 3, as justifying the assumption that the nucleus consists chiefly of solidified metallic substances. Dr. Page has, however, given the most conclusive reasons against
the validity of such an assumption, if we are to admit that the increase in temperature found to obtain as we penetrate the crust below the stratum of invariable temperature continues beyond the depths to which our observations have extended, for it is clear that, in such case, the temperature reached at the depth of 25 miles would be sufficient to fuse nearly all the rockmaterial with which we are acquainted, whilst at the depth of 150 miles all such material would be reduced to a state of vaporiform incandescence.
Now it is very singular that, notwithstanding the admitted connection of this internal heat with the phenomena of earthquakes, volcanic disturbances, upheavals and subsidences which affect the outer crust of the earth, some very important investigations made by Messrs. Nasmyth and Carpenter, in connection with their long-continued and exhaustive examination of the surface conditions of the moon, appear to have been entirely overlooked by geological writers, although Nasmyth and Carpenter distinctly pointed out that the results of their investigations would most probably be found to have an important bearing on the origin of the phenomena referred to, and tend to show that the thickness of the solidified, and especially of the rigid, portion of the crust, must be very much less than that which has been generally assigned to it.
I will now proceed to give some idea of the nature of those investigations, and of their suggested bearing upon the matters referred to.
Messrs. Nasmyth and Carpenter were induced, as one of the results of their long and careful observations of the surface of the moon, to inquire into the relative densities of fusible matters in the fluid and solid conditions. They found that, with few exceptions,—exceptions having no influence upon the questions at issue,—all fusible substances solid at ordinary temperatures are densest when molten. They found that solid gold, silver, iron, copper, and other metals floated upon the same substances in the molten state; that solid slag floated on melted slag, and so forth; thus accounting, in part at all events, for the greater density of the deeper portions of the globe's mass, assuming those portions to be still in a fluid condition from heat. They pointed out, what is indeed a corollary to the first proposition, that molten material, solid at ordinary temperatures, expands to and attains its minimum density in the act of solidifying, and that this expansion is followed by contraction as the solidified matter afterwards parts with its heat by radiation. Thus, if the tire of a wheel has to be formed as a casting, the fused metal must in the first place be poured into a suitable mould, in which provision has been made for the expansion of the solidifying matter. After it has cooled sufficiently to become rigid, and to
bear removal from the mould without warping, but whilst still retaining a great degree of expansion from heat, it must be taken from the mould and placed in position round the wheel to which it is to form the tire. At first it is loose, but rapidly tightens by contraction, as it gradually parts with its heat by radiation, or, as is more usual, as it is cooled by the application of douches of cold water. I am not aware to what extent the density of fusible substances can be increased when in the solid condition by the mere lowering of their temperatures, but I doubt whether the density of any such substance, whatever pressure it may be subjected to, (the heat generated by such pressure being withdrawn), can be thus increased so as to bring it up to that which it possesses in the molten state.
It will be seen, therefore, that in proportion to the heat to which they are exposed, within the limits, in the descending scale, of the lowest degree of temperature known to us on the one hand, and the state of complete fusion on the other, metallic and earthy fusible substances undergo three well-marked changes in density—namely, they have a maximum when fused, a minimum when first solidified, and an intermediate density when their heat in the solid condition is reduced by radiation.
Now, assuming that the Earth was at one time in a molten state, it is clear that so soon as it had parted with sufficient heat to admit of the solidification of its outer surface, the material so solidified would at once expand, and in course of time would pass from the plastic to a rigid state. As radiation proceeded further, the exposed surface would cool to such a degree as to cause contraction of its substance, which would then press with great force upon the less rigid solid material between it and the still molten mass below. But that molten mass would still continue to part with its heat by conduction and radiation, and its surface would solidify; and, indeed, this process would necessarily be continuous, until the rigid crust had reached such a thickness as to oppose further solidification. Until this point had been reached, however, the consequences of the processes to which I have referred would be to create constant strains upon the contracted and still contracting outer portions of the crust, and, as a result of such strains, the production of fissures, or bulgings, or foldings, according to the degree of rigidity to which it had attained. A further effect would be to create cavernous spaces at various depths, and of greater or less extent, into which masses of molten matter would be injected by the pressure created upon the nucleus by the plastic material interposed between it and the contracted outer crust. Matter so injected would solidify with greater rapidity than that which remained in general contact with the fluid mass, and its expansion would certainly produce more violent action on the surface of the globe than would result from the more
gradual solidification of films on the surface of the diminishing nucleus.
The effect of the operations referred to upon the ultimate form of a spherical mass of fused matter, occupying the position and having the motions of the Earth, would, I think, be to produce, or, at all events, materially to assist in producing, the form which the Earth now presents. Radiation from such a mass would, in the absence of any local compensating action, be equal from every part of the surface; but from the moment that a fixed axis of rotation had been established under the paramount influence of the Sun's attraction, that radiation would proceed most rapidly at the poles, diminishing gradually towards the equator.
The result of the more rapid radiation in circumpolar regions would be to reduce the sphere to a spheroid, by the pressure of the contracting outer crust within those areas upon the molten internal mass, which, in its turn, would necessarily press outwards upon the more plastic materials in equatorial regions, until equilibrium had been established. This view is supported by the distribution of volcanoes on the surface of the Earth: for, with the exception of Hecla, in Iceland, in latitude 65° North, and Mount Erebus, on the Antarctic Land, in about the same latitude South, active volcanic action is most intense within tropical regions, and extends but little into the limits of the temperate zone. This fact appears to indicate that the loss of heat which the earth originally sustained, and is still suffering, is largely compensated within the tropics, and for some distance on each side of them, by that which it receives from the sun's radiation, and, consequently, that the molten material in the interior of the earth is exposed, within that area, to pressure less effective to prevent earthquakes and resulting volcanic phenomena, than it is subject to within the circumpolar and immediately adjacent regions.
It is, no doubt, difficult to apply the mind to the consideration of operations such as these in connection with a mass of such enormous dimensions as our globe; but it is very clear that, with the exception of the cooling of its surface to such a degree as to cause any great amount of contraction, operations of this very nature must now be going on in the great planet Jupiter. I cannot say to what extent the dynamical heat, generated by the condensation of the cosmical matter of which that planet is composed, and which is being lost by radiation into space, is compensated from outer sources. Those who are curious on this subject may consult the views propounded by Mr. Mattieu Williams, in his work on “The Fuel of the Sun,” and the very similar views as to the maintenance of the sun's heat propounded by the late Sir W. Siemens in the columns of “Nature.” It is clear, however, that in the case of our globe,
whatever retarding effect its atmosphere may have exercised upon the cooling of the outer crust, that cooling was comparatively rapid, although the straining effects which I assume to have resulted from the causes referred to were still powerful enough, in Tertiary times, to result in the elevation of nearly if not all the great mountain chains now existing upon it. Whether the forces in question are still equal to bringing about changes in the surface similar to those which are revealed to us by the investigations of geologists as having occurred since the commencement of the Eocene period, can only be determined in the far distant future, although I am inclined to doubt it.
The straining referred to has, however, certainly not ceased, and will not cease until the thickness of the earth's rigid crust has become sufficiently great to prevent further solidification of the molten interior matter. The diminution which has apparently taken place in the intensity of volcanic action since the close of the Miocene period, seems to indicate the approach of such a condition of things, and that time, when it does arrive, will certainly be the commencement of the period in which the earth will attain its ultimate surface conditions.