They are quasi-periodic, that is to say, some of their conditions as to causation rest upon a really periodic basis, as, for example, the recurrence of storms upon the periodic march of the earth, and sun and moon, etc., and the recurrence of Earthquakes upon the secular cooling of our earth; but the conditions in both are so numerous and complicated with particulars, that we cannot fully analyse them—hence, cannot reduce the phenomena to law, and so cannot predict recurrence.Yet storms and tempests—which were, along with pestilences and Earthquakes, amongst the natural phenomena which Bishop Butler deemed in his own day impossible of human prediction—have already, through the persistent and systematised efforts of meteorological observers, become to a certain extent foreseeable; and medical science assures us that it has rendered that, though to a much less degree of probability, true of pestilences.
We may, therefore, give the utilitarian some hope, that if he will help us along—who value our accessions of knowledge primarily upon a different standard to his—in our talk of discovery, our posterity, in a century or two hence, may not improbably possess the advantage of being able, in some degree, to predict their Earthquakes. I fear the inducement will go but a small way with the utilitarian generation, whose bent tends much towards asking, "What has posterity ever done for them?"
But though we cannot as yet predict the time when an Earthquake may take place in any locality, we can, on mixed statistic and dynamic grounds, in many cases state the limits of probable violence of the next that may recur. For example, the three shafts of marble columns of the Temple of Serapis, at Pozzuoli, each of about 411/2feet in height, and 4 feet 10 inches in diameter at the base, remain standing alone, since they were uncovered, in the year 1750.
Now, as we can calculate exactly what velocity of earthquake-wave motion would be required to overset these, we are certain that, during the last one hundred and twenty-two years, the site of the Temple, and wemay say Naples and the Phlegræan fields generally, have never experienced a shock as great as the very moderate one that would overset these columns. A shock whose wave particle had a horizontal velocity of only about 31/2feet (British) per second would overturn these columns; which is only about one-fourth the velocity (within the meizoseismic area) of the great shock of 1857, that produced wide-spread destruction in the Basilicatas, and not enough to throw down any reasonably well-built house of moderate height.
Naples, so far as Earthquake is concerned, whether coming from the throes of Vesuvius or elsewhere, has a pretty good chance of safety. She may possibly (though not probably) be some day smothered in ashes; but is in little danger of being shaken to the earth. During this time there have been taking place, larger eruptions of Vesuvius and earthquake shocks from other centres, together probably about the same number of times as the numbers of those years, when those columns have been more or less shaken.
We may therefore affirm that the probability (on the basis of this experienceonly) is, say 120 to 1, that the next shock, whether derived from Vesuvius, or elsewhere, that may shake Pozzuoli, will be one less in power than would be needed to overturn the shafts of the Temple of Serapis there.
Let us now turn to the second branch of our subject—viz., Vulcanology—upon which, as yet, we have secured less firm standing ground than we have seen we possessin Seismology, for which reason we took that first into consideration.
It is the part of Vulcanology to co-ordinate and explain all the phenomena of past or present times visible on our globe which are evidences of the existence and action, whether local or general, of temperatures within our globe greatly in excess of those of the surface, and which reach the fusing points of various mineral compounds as found arriving, heated or fused, at the surface.
The stratigraphic geologist sees that such heated or fused masses have come up from beneath, throughout every epoch that he can trace; but he cannot fail to discern more or less a change in the order or character of those outcomings, as he traces them from the lowest and oldest formations to those of the present day. He sees immense outpourings of granitoid or porphyrytic rocks that have welled up and overflowed the oldest strata—huge dykes filling miles of fissures that had been previously opened for the reception of the molten matter that has filled them, and often passing through those masses of previously outpoured rock; later he sees huge tables of basaltic rock poured forth over all. One grand characteristic common to all these—commonly called plutonic products—being that, whether they were poured forth over the surface or injected into cavities in other rocks, the movements of the fused material were, on the whole, hydrostatic andnot explosive.
At the present day, whatever other evidences we have of high temperature below our globe's surface, that which primarily fixes the eye of the geologist is theVolcano, whose characteristic, as we see it in activity,is explosive. But though there is this great characteristic difference between the plutonic and the volcanic actions and their products, the two, when looked at largely, are seen so to inosculate, that it is impossible not to refer them to an agency common to both, however changed the modes of its action have been between the earliest epochs of which traces are presented to us and the present day.
To us little men, who, as Herschell has well said, in referring to the methods of measuring the size of our globe, "can never see it all at once, but must creep like mites about its surface," the Volcano, in the stupendous grandeur of its effects, tends to fix itself in our minds in exaggerated proportions to its true place in the cosmic machine; and, in fact, nearly all who have sought to expound its nature and mode of origination have occupied themselves far too exclusively with describing and theorising upon the strange and varied phenomena which the volcanic cone itself and its eruptions present, and too often, in the splendour and variety of these, have very much lost sight of what ought to be the centre-point of all such studies, namely, to arrive at some sound knowledge of what is theprimum mobileof all these wonderful efforts. Nor has the distinction been very clearly seen between the main phenomena presented at and about volcanic active mouths, which can be employed to elucidate the nature of the causation at work far below, and those most varied and curious, and in other respects most pregnant and instructive phenomena, mechanical and chemical,which are called into action in and by the ejected matter of the volcanic cone after its ejection. It can help us but little or very indirectly, in getting at a true conception of the nature and source of the heat itself of the Volcano, to examine, for example, all the curious circumstances that are seen in the movements and changes in the lava that has already flowed from its mouth; but it would be of great importance if we can ascertain, by any form of observation around the cone, from what depth it has come, or at what depth the igneous origin lies.
The physician, endeavouring to ascertain the real nature of small-pox or measles, will scarcely make much progress who, however curiously or minutely, confines his attention to the pustules that he sees upon the skin.
Yet the Volcano, or rather all volcanic activity as now operative upon our globe, is, as it were, an experiment of Nature's own perpetually going on before us, the results of which, if well chosen—that is, as Bacon says, by keeping to the main and neglecting the accidents—can, when colligated and correctly reasoned upon, in relation to our planet as a whole, give us the key to the enigma of terrestrial Vulcanicity in its most general sense, and at every epoch of our world's geognostic history, and show us its true place and use in the cosmical machine. Let us glance at the history of past speculation on this subject, from which so little real knowledge is to be derived, and then at the salient facts of Vulcanology as now seen upon our earth, and finally see if we can connect these with other great cosmicalconditions, so as to arrive at a consistent explanation in harmony with all.
We gain nothing absolutely from the knowledge of the so-called "ancients" as to Volcanoes in Europe at least, where alone historic records likely to refer to them exist. The Volcanoes of Europe are few and widely scattered. The Greeks saw but little of them, and the Romans were all and at all times most singularly unobservant of natural phenomena.
Cæsar never mentions the existence in France of the Volcanoes of Auvergne, so much like those he must have seen in Italy and Sicily; and Roman writers pass in silence that great volcanic region, though inhabited by them, and their language impressed upon the places, as Volvic (volcano-vicus) seems with others to indicate; and though there is some reason to believe that one or other of the Puys was in activity within the first five hundred years of our epoch, the notices which Humboldt and others have collected as from Plato, Pausanius, Pliny, Ovid, etc., teach nothing.
Whatever of mere speculation there may have been, volcanic theory, or what has passed for such, there was none before 1700, when Léméry brought forward a trivial experiment, the acceptance of which, even for a moment, as a sufficient cause for volcanic heat (and it retarded other or truer views for years), we can now only wonder at. Breislak's origin, in the burning of subterranean petroleum or like combustibles, was scarcely less absurd than Léméry's sulphur and iron filings.
Davy, in the plenitude of his fame, and full of theintense chemical activities of the metals of the alkalies which he had just isolated, threw a new but transient verisimilitude upon the so-called chemical theory of Volcanoes, by ascribing the source of heat to the oxidation of those metals assumed to exist in vast, unproved and unindicated masses in the interior of the earth. But Davy had too clear an intellect not to see the baseless nature of his own hypothesis, which in his last work, the "Consolations in Travel," he formally recanted; and it only survived him in the long-continued though unconvincing advocacy of Dr. Daubeny. So far, the origin of the heat had been sought always, in the crude notion of some sort offuel consumed, whether that were petroleum or potassium and sodium; but as no fuel was to be found, nor any indicated by the products, so far as known, of the volcanic heat, so what has been called the mechanical theory, in a variety of shapes, took its place.
This, in whatever form, takes its lava and other heated products of the volcano ready made from a universal ocean of liquid material, which it supposes constitutes the interior or nucleus of our globe, and which is only skinned over by a thin, solid crust of cooled and consolidated rock, which was variably estimated at from fourteen to perhaps fifty miles in thickness. Here was a boundless supply of more than heat, of hot lava ready made, the existence of which at these moderate depths the then state of knowledge of hypogeal temperature, which was supposed to go on increasing with depth at the rate of about 1° Fahrenheit, for every thirty or forty feet, seemed quite to sustain.
The difficulty remained, how was this fiery ocean brought to the surface or far above it? To account for this two main notions prevailed, and, indeed, have not ceased to prevail. Some unknown elastic gases or vapour forced it up through fissures or rents pre-existent, or produced by the tension of the elastic and liquid pressure below.
The form in which this view took most consistency, and approaching most nearly to truth, finds the elastic vapour in steam generated from water passed down through fissures from the sea or from the land surface. But to this the difficulty was started, that fissures that could let down water would pass up steam. The objection, when all the conditions are adequately considered, has really no weight; and it has been completely disposed of, since within a few years it has been proved that capillary infiltration goes on in all porous rocks to enormous depths, and that the capillary passages in such media, though giving free vent to water—and the more as the water is warmer—are, when once filled with liquid, proof against the return through them of gases or vapours. So that the deeply seated walls of the ducts leading to the crater, if of such material, may be red hot and yet continue to pass water from every pore (like the walls of a well in chalk), which is flushed off into steam that cannot return by the way the water came down, and must reach the surface again, if at all, by the duct and crater, overcoming in its way whatever obstructions they may be filled with.
And this remarkable property of capillarity sufficientlyshows how the lava—fused below or even at or above the level of infiltration—may become interpenetrated throughout its mass by steam bubbles, as it usually but not invariably is found to be.
Nor is it difficult to see such a mechanism between volcanic ducts and fissures conveying down water, as large and open pipes, for a large part of their depth, as shall bring down water to foci of volcanic heat, without the power of the water flowing back except as steam and through the crater.
Indeed, the facts known as to geysers, and those of half-drowned-out Volcanoes such as Stromboli—whose action is intermittent just as much as that of a geyser—show that this is not merely probable. There is, therefore, no need for the hypothesis of those who have supposed all the huge volumes of steam blown off from Volcanoes in eruption to come from vesicular water pre-existent in the minute cavities of crystalline or other rocks before their fusion into lava: a fact not proved for many classes of rock, and for none in sufficient quantity to account for the vast volume of steam required and for the irregularity of its issue.
It is rather to anticipate, but I may state at once that, so far as the admission of superficial waters to the interior, and to any depth to which fissures or dislocation can extend, I believe no valid physical or mechanical difficulties exist, taking into accountallthe conditions that may come into play together.
Another set of views has been suggested and supported by various writers, which proposes to account for the rise of lava on purely hydrostatic principles. Thesolid crust, fractured into isolated fragments by tensions due to its own contraction, is supposed to sink into the sea of lava on which it floats; and much ingenuity has been expended in imagining the mechanism by which, in places, the liquid matter is supposed to riseabovethe surface of the 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 bytensionis 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 oftangential compression.
However tenable, in other respects, may be the volcanic theory which rests upon the assumption of a verythincrust and a universal ocean of fused rock beneath, it fails wholly to explain many of the most important circumstances observable as to the distribution and movements of existing Volcanoes on our globe.
It affords no adequate explanation of the configuration of the lines of Volcanoes, nor of their occurrence in the ocean bed, nor of their existence in high latitudes, near the Poles, where, no matter how or at what rate our globe cooled from liquidity, the crust must be thickest; nor of the independence of eruptive action of closely adjacent volcanic vents; nor of the non-periodicity, the sudden awakening-up to activity, the as sudden exhaustion, the long repose, the gradual decay of actionat particular vents, and of much more that might be stated and sustained as difficulties left by that theory unexplained, or that are of a nature even opposed to it.
The researches of the last few years have, however, as it appears to me, rendered any theory that demands as its postulates avery thin crust, and a universal liquid nucleus beneath it, absolutely untenable.
Without attaching any importance to the arguments of Mr. Hopkins, based upon precession and nutation, it appears to me, on various other grounds, some of which have been urged by Sir William Thompson, that the earth's solid crust is not a thin one, at least not thin enough to render it conceivable that water can ever gain admission to a fluid nucleus, if any such still exist, situated at so great a depth; and without such access we can have no Volcano. It is not necessary to go to the extent of a crust of 800 or 1,000 miles thick: with one of half the minor thickness, I believe it may be proved, on various grounds, hydraulic amongst others, that neither water could reach the nucleus, nor the liquid matter of the nucleus reach the surface. Mr. Hopkins having proved to his own satisfaction an enormous thickness for the crust, and seeing clearly the difficulties that this involved to the generally accepted volcanic theory, and having no other to substitute for it, fell back upon that most vague and weak notion of the existence of isolated lakes of liquid rock, existing at comparatively small depths beneath the earth's surface within the solid and relatively cold crust, each supplying its own Volcano, or more than one, with ready-made lava.What is to produce these lakes of fused matter in the midst of similar solidified matter? what is perpetually to maintain their fluidity in the midst of solid matter continually cooling? what has given them their local position? why near or less near the surface? what should have arranged them in directions stretching in some cases nearly from Pole to Pole?
Surely this creation of imaginary lakes, merely because it happens to fit the vacant chink that seems needed to wedge up a falling theory, is an instance of that abuse of hypothesis against which Newton so vehemently declaims—"Hypotheses non fingo."
Hypothesis, to be a philosophic scaffolding to knowledge, must, as Whewell has said, "be close to the facts, and not merely connected with them by arbitrary and untried facts." Yet this appears accepted by Lyell (10th edition, Vol. II., p. 227, and elsewhere); by Phillips ("Vesuvius," pp. 331, 332); by Scrope, if, as I hope, I mistake him not ("Volcanoes," pp. 265, 307-8); though none of these excellent authorities seem either quite clear or quite satisfied with the notion; and in the very passage referred to, Lyellmayhave possibly a much more philosophic notion in view, where he says: "It is only necessary, in order to explain the action of Volcanoes, todiscover some cause which is capable of bringing about such a concentration of heat as may melt one after the other certain portions of the solid crust, so as to form seas, lakes or oceans of subterraneous lava." (Vol. II, pp. 226, 227). If by this is meant, that all that is needed to complete a true theory of volcanic action is to discoveran adequate cosmical cause for the heat—thatis to say, a prime mover to which all its phenomena may be traced back, which shall be at once reconcilable with the conditions of our planet as a cooling mass in space and with facts of Vulcanology as they are now seen upon it—then I entirely agree with it.
It has been my own object to endeavour to discover and develope that adequate cause in a Paper "On Volcanic Energy, an Attempt to develope its True Nature and Cosmical Relations," read (in abstract) before the Royal Society of London ("Proceedings, Royal Society," Vol. XX., May, 1872), and now (October, 1872) under consideration of Council with a view to publication.
I propose concluding this review of the progress of Vulcanology (in which I have had to limit myself to reviewing merely the chief stages of advance towards knowledge of the nature and origin of volcanic heat itself, and have had to pass without notice the vast and important mass of facts and reasonings collected by so many labourers as to its visible phenomena and products, and the still greater mass of speculation, good and bad, on every branch of the subject), by giving a necessarily very brief and imperfect sketch of my own views as in that Paper in part developed. It will first be necessary to retrace our steps a little, in order to gain such a point as shall afford us a fuller view of the whole problem before us.
It is not necessary to dilate, even did space allow, upon the many points which bind together Earthquakes and Volcanoes as belonging to the play of like forces. These are generally admitted; and in various ways, moreor less obscure, geologists generally have supposed some relations between these and the forces of elevation, which have raised up mountain chains, etc.
No one, however, that I am aware of, prior to myself, in the Paper just alluded to, has attempted to show, still less to prove upon an experimental basis, that all the phenomena of elevation, of volcanic action, and of Earthquakes, are explicable as parts of one simple machinery—namely, the play of forces resulting from the secular cooling of our globe. We have seen that, on the whole, both Earthquakes and Volcanoes follow along the great lines of elevation of our surface. Any true solution of the play of forces which has produced any one of those three classes of phenomena must connect itself with them all, and be adequate to account for all. And this would have earlier been seen, had geologists generally framed for themselves any correct notions of the mechanism of elevation itself, and seen its real relation with the secular cooling of our planet. But the play of forces resulting from this secular cooling has never, until very recently, been adequately or truly stated. The arbitrary assumption and neglect of several essential conditions by La Place, in his celebrated Paper "On the Cooling of the Earth," in the fifth volume of the "Mécanique Céleste," and the arbitrary and unsustainable hypothesis of Poisson upon the same subject, have tended to retard the progress of physical Geology as to the nature of elevation: the first, by leaving the geologist in doubt as to whether our globe were cooling at all; the second, by suggesting distorted notions as to the mode of its cooling and consolidation. On the other hand, neither geologistsnor mathematicians generally have framed for themselves any clear notions of the mechanism of elevation. Had a true conception been formed of the forces and interior movements brought necessarily into operation by the secular cooling of the globe, geologists could scarcely have failed to see that their notion as to the way and direction in which the forces producing elevation have actually acted could not, if arising from refrigeration, be those which they have almost universally supposed, namely—some force acting vertically upwards,i.e., radially from the centre of the sphere. Had geologists only looked at Nature with open eye, they must have seen that mountain ranges, and elevations generally (exclusive of volcanic cones), presented circumstances absolutely incompatible with their having been thrust up by any forceprimarilyacting in the direction of a radius to the spheroid.
Yet this is the erroneous notion of the mechanism of elevation which to the present hour prevails amongst geologists, so far as they in general have framed to themselves any distinct idea of such mechanism at all.
Thus, only to cite two examples from recent authors of justly high reputation. Lyell says of the probable subterranean sources, whether of upward or downward movement, when permanently uplifting a country, and in reference to the crumpling of strata on mountain flanks by lateral pressure, it would be rash to assume these able to resist a power of such stupendous energy, "if its direction, instead of being vertical, happened to be oblique or horizontal." This is somewhat vague—and I trust I do not mistake or misrepresent the illustriousauthor—yet it is the most explicit expression I can find in the "Principles of Geology" as to his notion of the primary direction of elevatory force (Edit. 10, Vol. I., p. 133). That Mr. Scrope's idea is that only of primary radial or vertical direction of such forces, is apparent on inspecting his Diagram No. 64 ("Volcanoes," p. 285), and in the use of the words, "an axial wedge of granite," which, on the next page, we find is "liquefied granite;" and if we read on to page 294, and refer also to pages 50 and 51, I believe there can be no doubt thatverticalordirect up-thrustis the author's notion of the primary direction of all forces of elevation. The true nature of these forces was, however, clearly seen and most justly stated by Constant Prevost ("Compt. Rend.," Tome XXXI., 1850, and "Bulletin de la Société Géolog. de France," Tome II., 1840) as consisting, not in forces of some unknown origin acting primarily in the vertical, but intangential pressures acting horizontally, and resolved by mutual pressures at certain points into vertical resultants. These Prevost rightly attributed to the contraction of the earth's solid crust. The same idea has been adopted by Elie de Beaumont as the true mechanism of the elevation of mountain ranges; and although De Beaumont's views as to the thinness he assigns to the solid and contracting crust, and his strange deduction as to the parallelism of contemporaneous mountain chains uplifted by its spasmodic action along certain lines, may be untenable, his notion generally as to the play of forces producing mountain elevation is much more nearly correct.
Mr. Hopkins's notion is simply that of the geologists.Anyone who reads his well-known papers on elevation and the formation of fissures, etc., must see that he views all elevatory forces as of liquids or quasi-liquids forced up and acting primarilyverticallyupon the strata above them, and that these strata are not under tangential compression, but under tension. Hence the mathematical deductions contained in those papers as to the directions in which elevatory forces act, and in which fissures are formed by them, are not in any way a setting forth of such facts as occur in Nature, and, much attention as they have attracted, can only now be viewed as exercises of mathematical skill misapplied, because based upon data not to be found in Nature. In fact, those papers do but misrepresent Nature, and, like many other mathematical investigations based on untrue or insufficient data, have tended to retard knowledge.
The views which I have put forward in the Paper I have referred to, read to the Royal Society, recapitulated in skeleton, so to say, are as follows. Omitting those portions which treat of our globe from the period of the first liquefaction out of a nebulous condition, and of the earliest stages of the cooling by radiation into space, when the crust was extremely thin, and of the deformation of the spheroid as one of the first effects of its contraction, and through that the general shaping out of continents and ocean beds; I have endeavoured to show that the rate of contraction of the crust, while very thin, exceeded that of the large fluid nucleus supporting it, and so gave rise totangential tensionsin the crust, and fracturing it into segments; next, that as the crust thickened, thesetensionsweregradually converted intotangential pressures, the contraction of the nucleus now beginning to exceed (for equal losses of heat) that of the crust through which it cooled. At this stage these tangential pressures gave rise to thechiefelevations of mountain chains—not by liquid matter by any process being injected from beneath vertically, but by such pressures, mutually reacting along certain lines, being resolved into the vertical, and forcing upwards more or less of the crust itself. The great outlines of the mountain ranges and the greater elevation of the land were designated and formed during the long periods that elapsed in which the continually increasing thickness of crust remained such that it was still, as a whole, flexible enough, or opposed sufficiently little resistance to crushing, to admit of this uprise of mountain chains by resolved tangential pressures. I have shown that the simple mechanism of such tangential pressures is competent to account for all the complex phenomena both of the elevations and of thedepressionsthat we now see on the earth's surface (other than continents and ocean beds), including the production of gaping fissures (in directions generally orthogonal to those of tangential pressure). And as our earth is still a cooling body, and the crust, however now thicker and more rigid, is still incapable of sustaining the tangential pressures to which it is now exposed, so I by no means infer that slow and small (relatively) movements of elevation and depression may not be still and now going on upon the earth's surface; in fact all the phenomena of elevation and depression, rending, etc., which at a much remoter epoch actedupon a much grander and more effective scale. So that, for aught my views say to the contrary, all the mountain chains in the world may be possibly increasing in stature year by year, or at times; but in any case at a rate almost infinitesimally small in its totality over the whole earth to that with which their ridges were originally upreared.
But the thickness of the earth's crust—thus constantly added to, by accretion of solidifying matter from the still liquid or pasty nucleus, as the whole mass has cooled—has now assumed such a thickness as to be able to offer a too considerable resistance to the tangential pressures, to admit of its giving way to any large extent by resolution upwards; yet the cooling of the whole mass is going on, and contraction, though unequal, both of thick crust and of hotter nucleus beneath also, whether the latter benowliquid or not. Were the contraction, lineal or cubical, for equal decrements or losses of heat, or in equal times—equal both in the material of the solidified crust and in that of the hotter nucleus—there could be no such tangential pressures as are here referred to, at any epoch of the earth's cooling. But in accordance with the facts of experimental physics, we know that the co-efficient of contraction for all bodies is greater as their actual temperature is higher, and this both in their solid and liquid states.
Hence for equal decrements of heat, or by the cooling in equal times, the hotter nucleus contracts more than does its envelope of solid matter.
The result is now, as at all periods since the signs changed of the tangential forces thus brought into play—i.e., since they became tangentialpressures—that thenucleus tends to shrink away as it were from beneath the crust, and to leave the latter, unsupported or but partially supported, as a spheroidal dome above it.
Now what happens? If the hollow spheroidal shell were strong enough to sustain, as a spheric dome, the tangential thrust of its own weight and the attraction of the nucleus, the shell would be left behind altogether by the nucleus, and the latter might be conceived as an independent globe revolving, centrally or excentrically, within a shell outside of it. This, however, is not what happens.
The question then arises, Can the solid shell support the tangential thrust to which it would be thus exposed? By the application to this problem of an elegant theorem of Lagrange, I have proved that it cannot possibly do so, no matter what may be its thickness nor what its material, even were we to assume the latter not merely of the hardest and most resistant rocks we know anything of, but even were it of tempered cast-steel, the most resistant substance (unless possibly iridio-osmium exceed it) that we know anything about. Lagrange has shown that if P be the normal pressure upon any flexible plate curved in both directions, the radii of these principal curvatures being ρ' and ρ'', and T the tangential thrust at the point of application and due to the force P, then:
P = T (1/ρ' + 1/ρ'')
P = T (1/ρ' + 1/ρ'')
When the surface is spherical, or may be viewed as such, ρ' = ρ'' and
P = 2T / ρ or, T = P × ρ/2
P = 2T / ρ or, T = P × ρ/2
In the present case P is for a unit square (taken relatively small and so assumed as plane) of the shell, suppose a square mile, equal to the effect of gravity upon that unit, ρ being the earth's radius, and if we assume the unit square be also a unit in thickness, P is then the weight of a cubic mile of its material; and if we take (roughly) the earth's radius as 4,000 miles, the tangential pressure, T, is, oneach faceof the cubic mile, equal to
(4000/2) P,
(4000/2) P,
or equal to the pressure of a column of the same material of 2,000 times its weight.
If the cubic mile that we have thus supposed cut out of the earth's crust at the surface were of the hardest known granite or porphyry, it would be exposed to a crushing tangential pressure equal to between 400 and 500 times what it could withstand, and so must crush, even though only left unsupported by the nucleus beneath, to the extent of1/400or1/500of its entire weight. And what is true here of a mile taken at the surface, is true (neglecting some minute corrections for difference in the co-efficient of gravity, etc.) if taken at any other depth within the thick crust.[F]
The crust of our earth, then, as it now is, must crush, to follow down after the shrinking nucleus—if so be that the globe be still cooling, and constituted as it is; even to the limited extent to which we know anything of its nature—it must crush unequally, both regarded superficially and as to depth; generally the crushing lines being confined to the planes or places of greatest weakness; and the crushing will not be absolutely constant and uniform anywhere, or at any time, or at any of those places of weakness to which it will be principally confined, but will be more or less irregular, quasi-periodic, or paroxysmal: as is, indeed, the way in which all known material substances (more or less rigid) give way to a slow and constantly increasing, steady pressure.
We have now to ask,How muchof this crushing is going on at present year by year? And the answer to this depends upon what amount of heat our world is losing into space year by year.
Geologists who have taken on trust the statement, that La Place has proved that the world has lost no sensible amount of heat for the last 10,000 years seem generally to suppose that to be a fact; but in reality La Place hasprovednothing of the sort, as those geological teachers who have echoed the conclusion should have known, had they deciphered the mathematical argument upon which it has been supposed to rest.
By application of Fourier's theorem (or definition) to the observed rate of increment of heat in descending from the geothermalcoucheof invariable temperature, and the co-efficients of conductivity of the rocks of ourearth's crust, as given by the long-continued observations made beneath the Observatories of Paris and of Edinburgh, it results that the annual loss of heat into space of our globe at present is equal to that which would liquefy into water, at 32° Fahr., about 777 cubic miles of ice; and this is the measuring unit for the amount of contraction of our globe now going on. The figures are not probably exact, for the data are not on a basis sufficiently full or exactly established as yet; but they are not very widely wrong, and their precise exactness is not material here. Now, how is this annual loss of heat (great or small, as we may please to view it) from the interior of our globe disposed of?
What does itdoin the interior? We have already seen that it is primarily disposed of by conversion into work; into the work of diminishing the earth's volume as a whole, and in so doing crushing portions of the solid surrounding shell.
But does the transformation of lost heat into the work of vertical descent, and of the crush as it follows down after the shrinking nucleus, end the cycle? No. A very large portion of the mechanical work thus produced, and resolved, as we have seen, into tangential crushing pressure, is retransformed into heat again in the very act of crushing the solid material of the shell. If we see a cartload of granite paving-stones shot out in the dark, we see fire and light produced by their collision; if we rub two pieces of quartz together, and crush thus their surfaces against each other, we find we heat the pieces and evolve light.
The machinery used for crushing by steam-power,hard rocks into road metal, gets so hot that the surfaces cannot be touched.
These are familiar instances of one result of what is now taking place by the crushing of the rocky masses of our cooling and descending earth's crust, every hour beneath our feet, only upon a vastly greater scale. It is in this local transformation of work into heat that I find the true origin of volcanic heat within our globe. But if we are to test this, so as in the only way possible to decide is it a true solution of this great problem, we must again ask the question,How much?and to answer this, we must determineexperimentallyhow much heat can be developed by the crushing of a given volume, say a cubic mile, of such rocky materials as we know must constitute the crust of our globe down to the bottom of the known sedimentary strata, and extending to such crystalloid rocks as we may presume underlie these. We must also obtain at least approximately what are the co-efficients oftotal contractionbetween fusion and atmospheric temperature of such melted rocks, basic and acid silicates, as may be deemed representative of that co-efficient for the range of volcanic fused products, basalts, trachytes, etc., which probably sufficiently nearly coincide with that of the whole non-metallic mass of our globe.
The first I have determined experimentally by two different methods, but principally by the direct one of theworkexpended in crushing prisms of sixteen representative classes of rock; the specific gravities and specific heats of which I have also determined.
If H be the height of a prism of rock crushed to powder by a pressure, P, applied to two opposite faces,which, when the prism has been reduced to its volume in powder, has acted through a range of H -t, then
P × (H -t) / 772
P × (H -t) / 772
is the heat corresponding to the work expended in the crushing, expressed in British units of heat. The following were the rocks experimented upon: Caen stone, Portland (both oolites), magnesian limestone, sandstones of various sorts, carboniferous limestones (marbles), the older slates (Cambrian and Silurian), basalts, various granites and porphyries, thus ranging from the newest and least resistant to the oldest and most resistant rocks. The results have been tabulated, and are given in detail in my Paper, now in possession of the Royal Society. The minimum obtained is 331 and the maximum 7,867 British units of heat developed, by transformation of the work of crushing one cubic foot of rock. If we apply the results to a thickness of solid crust of 100 miles (British), of which the upper twenty-one miles consist of neozoic, newer palæozoic, older palæozoic and azoic rocks in nearly equal proportion as to thickness, and the remaining eighty miles of crystalloid rocks (acid and basic magmas of Durocher) of physical properties which we may assume not very different from those of our known granites and porphyries—and which, in so far as they may differ, would give a stillhigherco-efficient of work transformed into heat than I have attributed to them by ranging them as only equal to the granites, etc.—then we obtain a mean co-efficient for the entire thickness of crust of 100 miles of 6,472 British units of heat, developable from each cubic foot of its material, if crushed to powder. It results from thisthat each cubic mile of the mean material of such a crust, when crushed to powder, developes sufficient heat to melt 0·876 cubic miles of ice into water at 32°, or to raise 7·600 cubic miles of water from 32° to 212° Fahr., or to boil off 1·124 cubic miles of water at 32° into steam of one atmosphere, or, taking the average melting point of rocky mixtures at 2,000° Fahr., to melt nearly three and a-half cubic miles of such rock, if of the same specific heat.
Of the heat annually lost by our globe and dissipated into space, represented by 777 cubic miles of ice melted, as before stated, the chief part is derived from the actual hypogeal source of a hotter though not necessarily fused nucleus, and nearly, if not wholly, is quite independent of the heat of Vulcanicity, which is developed as a consequence of its loss or dissipation. But were we to take the extreme case, and suppose it possible that all the heat the globe loses annually resulted from the transformation of the work of internal crushing of its shell, we shall find that the total volume of rock needed to be crushed in order to produce the required amount of lost heat is perfectly insignificant as compared with the volume of the globe itself, or that of its shell. For, as 1·270 cubic miles of crushed rock developes heat equivalent to that required to melt one cubic mile of ice to water at 32°, and if we assume the volume of our globe'ssolidcrust to equal one-fourth of the total volume of the entire globe, 987 cubic miles of rock crushed annually would supply the whole of the heat dissipated in that time. But that is less than theone sixty-five millionthof the volume of the crust only.
But a very small portion of the total heat annuallylost by our globe is sufficient to account for the whole of the volcanic energy of every sort, including thermal waters, manifested annually upon our earth. In the absence of complete data, we can only approximately calculate what is the annual amount of present volcanic energy of our planet. This energy shows itself to us in three ways: 1. The heating or fusing of the ejected solid matters at volcanic vents. 2. The evolution of steam and other heated elastic fluids by which these are carried. 3. The work of raising through a certain height all the materials ejected. To which we must add a large allowance for waste, or thermal mechanical and chemical energy ineffectually dissipated in and above the vents. All these are measurable into units of heat.
I have applied this method of calculation to test the adequacy of the source I have assigned for volcanic heat, in two ways, viz.: 1. To the phenomena presented during the last two thousand years by Vesuvius, the best known Volcano in the world; and 2. To the whole of the four hundred and odd volcanic cones observed so far upon our globe, of which not more than one-half have ever been known in activity.
It is impossible here to refer to the details of the method or steps of these calculations. The result however is, that making large allowances for presumably defective data,less than one-fourthof the total telluric heat annually dissipated (as already stated in amount) is sufficient to account for the annual volcanic energy at present expended by our globe.
It is thus represented by the transformation into heat of the work of crushing about 247 cubic miles of (mean)rock, a quantity so perfectly insignificant, as compared with the volume of the globe itself, as to be absolutely inappreciable in any way but by calculation; and as its mechanical result is only the vertical transposition transitorily of material within or upon our globe, the proportion of the mass of which to the whole is equally insignificant, so not likely in any way to produce changes recognisable by the astronomer.
Space here forbids my entering at all upon that branch of my investigation which is based upon the experimental results, above mentioned, of the total contraction of fused rocks: for these, the original Paper can, I hope, be hereafter referred to. I am enabled, however, to prove thus how enormously more than needful has been the store of energy dissipated since our globe was wholly a melted mass, for the production, through the contraction of its volume, of all the phenomena of elevation and of Vulcanicity which its surface presents. And how very small is the amount of that energy in a unit of time as now operative, when compared with the same at very remote epochs in our planet's history.
I have said that if we can find a true cause in Nature for the origination of volcanicheat, all the other known phenomena, at and about volcanic vents, become simple. Lavas and all other solid ejecta of Volcanoes, from all parts of the earth's surface, as well as basalts, present in chemical and physical constitution close resemblance, and may be all referred to the melting of more or less fusible mixtures of siliceous crystalloid rocks with aluminous (slates, etc.) and calcareous rocks. Their general chemical composition, and the higher or lower temperaturesof fusion resulting therefrom, together with the higher or lower temperatures to which they have been submitted at the different volcanic foci, determine their difference of flow (under like surface conditions) and of mineral character after ejection and cooling.
St. Clair de Ville and Fouqué have shown that the gaseous ejections, of which steam forms probably 99 per cent., are such as arise from water admitted to apre-existent focus of high temperature.
Whether sea or fresh water is not material, when we bear in mind that the chemical constituents found in sea water and in natural fresh waters that have penetrated the soil are, on the whole, alike in kind and only differ in proportions. But I must pass almost without notice all the varied and instructive phenomena which are presented by volcanic vents, for to treat of these at all would be to more than double the size of this sketch.
In the source that has been pointed out as that from which volcanic heat itself is derived, viz., the secular cooling of our globe, and the effects of that upon its solid shell, we are enabled to point to that which is the surest test of the truth of any theory—that it not only enables us to account for all the phenomena, near or remote, but to predict them. We see here linked together as parts of one grand play of forces, those of contraction by cooling, producing bydirectmechanical action the elevation of mountain chains, and by theirindirectaction, by transformation of mechanical work into heat, the production of Volcanoes; and both by direct and by indirect action, of Earthquakes, never previously shown to have thus the physical connection of one common cause,but merely supposed, more or less, to be connected by their distribution upon our earth's surface.
We now discern thus the physical causewhyVolcanoes are distributed, viewed largely, linearly, and follow the lines of elevation; we see equally why their action is uncertain, non-periodic, fluctuating in intensity, with longer or shorter periods of repose, shifting in position, becoming extinct here, appearing in new activity or for the first time there. We have an adequate solution of the before inexplicable fact of their propinquity, and yet want of connection. We have an adequate cause for the fusion of rock at local points without resorting to the baseless hypothesis of perennial lakes of lava, etc.
For the first time, too, we discern a true physical cause for earthquake movement, where volcanic energy does not show itself. The crushing of the world's solid shell, whether thick or thin, goes onper saltumand at ever-shifting places, however steadily the tangential pressures producing it may act. Hence crushingalonemay be shown to develope amply sufficient impulse to produce the most violent Earthquakes, whether they be or be not at a given place or time connected with volcanic outburst or possible injection, or with tangential pressures, enough still, in some cases, to produce partial permanent elevation.
When subterraneous crushing takes place, and the circumstances of the site do not permit the access of water, there may be Earthquake, but can be no Volcano; where water is admitted, there may be both.
And thus we discern why there are comparatively fewsubmarine Volcanoes, the floor of the ocean being, on the whole, water-tight—"puddled," as an engineer would say, by the huge deposit of incoherent mud, etc., that covers most of it, and probably having a thicker crust beneath it than beneath the land.
We see, moreover, that the geological doctrine of absolute uniformity cannot be true as to Vulcanicity, any more than it can for any other energy in play in our world. Its development was greatest at its earliest stages, when the great masses of the mountain chains were elevated. It is even now—though as compared to men's experience, and even to all historic time, apparently uniform and always the same—a decaying energy.
The regimen of our planet as part of the Cosmos, which seems to some absolute (and presented to Playfair no trace of a beginning nor indication of an end), is not absolute, and only seems to us to be so because we see so little of it, and of its long perspective in time. This the now established doctrine of the conservation of energy renders certain.
With this source for volcanic heat, too, in our possession, we can look from our own world to others, and predict within certain limits, which must widen as our knowledge of the facts of their substance and surface becomes greater, what have been and what are the developments of Vulcanicity which have taken place or are occurring in or upon them. Looking to our own satellite, we see for the first time a sufficient physical cause for the enormous display of volcanic energy there which the telescope divulges to us; one which is not to be explained alone by the commonly made statement ofthe small density of the moon, but by the fact that as the rate of her cooling from a given temperature, as compared with that of our earth (apart from questions of the chemical nature of the two bodies, or of their specific heats, etc.), has been inversely as their respective masses, and directly as their surfaces, so has the rate of cooling of the moon been vastly greater than that of the earth, and the energy due to contraction by cooling more intense and rapidly developed in our satellite than upon our globe.
We have thus traced, in meagre and broken outline only—because space admitted no more—the progress of Science to its existing state as respects Vulcanicity, in its two branches of Vulcanology and of Seismology, and pointed out their more intimate relations and points of connection, and been at length able to refer them, on the sure basis of physical laws, to one common cause, and that one derived from no hypothesis, but simply from the postulate of our world as a terr-aqueous globe cooling in space.
What I have here advanced with reference to volcanic energy, which appertains to my own researches, I do not conceal from myself, nor from the reader, has yet to await the reception generally and the award of the true men of science of the world.
That, like every new line of thought which has attempted or succeeded in supplanting the old, it will meet with opposition, I make no doubt.
My belief, however, is that in the end it will be found to have added a fragment to the edifice of true knowledge.
The interpretation which I have given of the nature and origin of volcanic activity points at once to the function in the Cosmos which it is its destiny to fulfil. It is the instrument provided for the purpose of continually preserving the earth's solid shell in a state to follow down after the descending nucleus. It does this by an apparatus or play of mechanism whereby the material of the solid shell, locally or along certain lines, is not only crushed, but the crushed material is blown out as dust, or expelled as liquid rock from between the walls of the shell, which are thus enabled to approach each other; and thus, by relief of the tangential thrusts, to permit the shell to descend, which it is obvious that crushing alone, unless it extended to the whole mass of the shell, could not accomplish.
It is a wonderful example of Nature's mechanism thus to see how simple are the means by which this end is accomplished. The same inevitable crush that dislocates the solid shell along certain lines, produces the heat necessary to expel to the surface the material crushed.
When attempted to be made the basis for philosophic discovery, "final causes" are no doubt barren, as Bacon has said; but when we have independently and by strict methods arrived at a result, we may justly appeal, as a test of its truth, to its showing itself as plainly fulfilling a needful end, and, by a distinctly discernible mechanism, preserving that harmony and conservation which are the obvious law of the universe.
As has been said, if I mistake not by Daubeny, John Phillips, by Herschell, and by myself, the function of theEarthquake and the Volcano is not destructive but, preservative. But we now see that: that the preservative scope of this function, as respects our earth, is far wider than what has been previously attributed to it. The Volcano does not merely throw up new fertile soil, and tend, in some small degree, to restore to the dry land the waste for ever going on by rain and sea; it fulfils a far weightier and more imperative task; it—by a mechanism the power of which is exactly balanced to the variable calls demanded of it, and which working almost imperceptibly, although in a manner however terrible its surface-action may at times appear to us little men[G]—prevents at longer intervals such sudden and unlooked-for paroxysms in the mass of our subsiding earth's shell as would be attended with wide-spread destruction to all that it inhabit.
To the popular mind, Volcanoes and Earthquakes are only isolated items of curiosity amongst "the wonders of the world:" few geologists even appear to realise how great and important are the relations of Vulcanicity to their science, viewed as a whole. Yet of Vulcanicity it is not too much to say, that in proportion as its nature and doctrines come to be known and understood as parts of the Cosmos, the nearer will it be seen to lie at the basis of all Physical Geology.