THE MECHANISM OF VOLCANIC ACTION. [With 3 plates.] By H. J. JOHNSTON-LAVIS, M. D., F. G. S., etc., In a discussion of this kind it is advisable to be as concise as possible, eliminating minor details, so as to give prominence to the main outlines of any theory one holds. This communication, which the Council of the Ninth International Congress of Geography have honored me by asking me to address to you, I propose to put into the form of a "credo." To this I shall add a few fundamental facts upon which my reasoning was based, leaving minor ones for discussion at greater leisure elsewhere. For convenience, I propose to divide my theory into two sections. In the first I shall review what may be conveniently called deep volcanic action, and in the second, that group of phenomena that occur when igneous matter nearly reaches the surface or actually finds an exit thereon. Unfortunately, in the first case I am obliged to rely on hypotheses and deductions, whereas in the second section, that of superficial volcanic action, there are a number of fundamental facts and observations upon which to base speculation, and to which I propose to draw your attention. Of one fact we are certain, and that is our globe is surrounded by a solid crust, which wherever it can be examined shows unmistakable and almost universal evidence of compression, wrinkling, and dislocation. This crumpling and crushing are equally inexplicable, unless we admit that since the initial solidification of the earth's crust its lower, still cooling part or support has undergone contraction so as to crowd together the already cooled burden of the upper part that this contracting mass carries. No one has yet attempted to even suggest that the part of our globe subjacent to the solid crust has shrunk from other causes than a loss of heat. We may, therefore, look upon the idea of contraction as due to cooling to be a universally accepted fact. "Reprinted by permission from The Geological Magazine, London, new series, decade v, vol. 6, No. 10, October, 1909. Address to the International Geographical Congress. In the old theory of the earth crust crumpling over a contracting and cooling nucleus, fluid, or partially so, it always appeared to me to be inexplicable how fluid matter could be squeezed out, or why the water on the surface of the earth did not rush down to fill up the vacancy that the contracting interior tended to produce between itself and the arch of the crust. This perhaps is expressing the facts in simple commonplace terms, but is sufficient to illustrate the incompatibility of this hypothesis with the fact of some of the liquid interior of the earth rising through the fissures toward the surface and being squeezed out by the contracting crust. The hypothesis that tangential thrust did not exist, but that the earth crust was shrinking on an entirely or partially fluid nucleus, would have satisfied the vulcanologist, but is contrary to the incontrovertible evidence of tangential compression, as seen in the plications and overthrusts existing upon the entire surface of the globe, or at least that part above sea level. This hypothesis was based upon the conception that the earth's crust was acting as a single unit. To Messrs. Mellard Reade and C. Davison is due the credit of making an analytical study of the functions of different parts of the earth's crust. That work demonstrated that theoretically we can divide the cooling surface of the earth into a series of shells. The outer shells that have reached approximately the mean atmospheric temperature will, of course, have stopped contracting, whereas the shells nearest to the heated nucleus will be those losing their heat most rapidly, and therefore undergoing greatest contraction. This contraction must inevitably cause crowding, crushing, and crumbling of those shells that are nearer the surface, just as a stretched sheet of rubber coated with a layer of stiff clay would do when allowed to contract. Somewhere between the surface shells of compression and the deepest shells of greatest cooling and contraction there will be a shell in a state of equilibrium, which the authors call the zone of no contraction. This zone, which was originally quite at the surface of our globe, tends to sink lower and lower as the general refrigeration or isotherms of our planet proceed downward. Were the shells of cooling and contraction of great tensile strength, such as the experi a C. Davison: "On the Distribution of Strain in the Earth's Crust resulting from Secular Cooling, etc." (Phil. Trans. R. S., 1887, vol. 178); “Note on the Relation between the Size of a Planet and the Rate of Mountain Building on its Surface" (Phil. Mag., Nov., 1887); "On the Straining of the Earth resulting from Secular Cooling (Phil. Mag., Feb., 1896); “On Secular Straining of the Earth" (Geol. Mag., May, 1889, Dec. III, vol. 6, No. 299, p. 220). T. Mellard Reade: "The Origin of Mountain Ranges," 1886. See also H. J. Johnston-Lavis: "The Extension of the Mellard Reade and C. Davison Theory of Secular Straining of the Earth to the Explanation of the Deep Phenomena of Volcanic Action" (Geol. Mag., June, 1890, Dec. III, vol. 7, pp. 246–249). mental sheet of rubber, already referred to, we can quite understand how any fluid in the earth's interior would tend to be squeezed out, but are met by two difficulties (1) is there any fluid in the earth's interior? and (2) is the tensile strength of the contractile shells sufficient to have a squeezing power? Three classes of views have been held as to the constitution of our globe. Some hold that it is like an egg, a solid shell with a fluid interior; others maintain that by the increase of gravity as the center is approached there is a solid nucleus which is potentially fluid were it not for this gravitational condensation, so that there would be a solid nucleus, a solid crust, and a stratum of liquid rock separating them. Finally, there is a third school who holds that the highly heated nucleus, although potentially fluid, is really solid in consequence of pressure or, more correctly, gravitational condensation. No known rock that we are acquainted with gives the conception of having sufficient tensile strength to be capable of exerting any really contractile or squeezing power on fluid inclosed within it or surrounded by it. There will be a tendency as the inner shells contract to split by fissures. Such fissures would extend from within outward, and would be top-shaped in section, with the edge extending up to the neutral zone of no contraction, and their lower limit at the inner surface of the lowest shell (pl. 1, E, F.) Such a fissure might be simultaneously filled by the fluid rock paste beneath. How this filling will take place requires consideration. As there is reason to disbelieve in any considerable constricting power of the inner cooling shells, and that even if such constricting power did exist is would be annulled by the development of fissures within its mass, it is evident one must look to other causes. The welling up into the fissure of the fluid rock, if we admit a fluid nucleus or a stratum or shell of such fluid, might be due to the settling down by gravitation of the cooled blocks of crust limited by the fissures. If, on the contrary, we admit the immediate contact of the lowest cooling shell with a highly incandescent nucleus (P), solid by pressure, but potentially fluid when this pressure is removed, we can well see what would take place. As soon as the fissures and therefore fluid in the inner cooling shells begin to form, their location and their edges will represent a site of diminished pressure. The subjacent and neighboring but potentially fluid rock will in consequence liquefy and expand and fill the fissure. As the fissure broadens and extends so will the expansion and liquefaction increase pari passu. a Liquid rock may thus reach up to the neutral zone of no contraction, but its extension further must be a matter of chance. It is evident that if the shells of compression were in every part homogeneous and a These blocks are quite different to the blocks referred to by some recent writers on terrestrial mechanics, coherent, then no upward-pointed fissure could be formed. In practice neither of these conditions is fulfilled. It is obvious that the crowding and crushing will be most complete in the shells of compression (pl. 1, C) where these are carried on a continuous block of contraction (pl. 1, A). I mean by a block a portion bounded by fissures formed in the contracting part of the earth crust. Where the shells of contraction are fissured (E, F) there the crowding of the superincumbent masses of cooled rock will not take place. As a result, each block or island of contractile crust, with its compressed burden, will tend to tear away from the adjoining blocks or islands, so that the limiting fissures in the contractile joints will extend up into the compressional shells (G, H). This exactly fits in with what is frequently found in the distribution of volcanoes along the edges of areas of marked compression or mountain regions. It will explain also the presence of volcanoes having a linear arrangement between closely situated mountain chains or areas, as in South America. Great rifts, such as those of Central Africa and some canyon districts, are probably of such origin. In earthquakes of tectonic origin it has been pointed out" that the piers of damaged bridges have usually been found to have approached each other. This would evidently take place in the areas of positive compression (pl. 1, C, C). On the other hand, in exceptional cases the piers have been found to have been separated. This might well occur in the area of negative compression (R), or what might well be termed the areas of retraction. The much larger proportion of the former effect on the bridge piers would no doubt be in the much greater ratio of compressional areas to retractional areas on the earth's surface. May not ocean basins be in part due to blocks or islands of the contracting zones exerting that diminution of volume in a vertical more than in a horizontal direction, as we have so far been considering it to be? The peculiar abysmal ocean troughs, often at the edge of ocean basins and parallel to chains of volcanoes or inter-. rupted by them, could well be explained by the same circumstances. I do not claim that ocean basins are alone due to this cause, but to a combination of these conditions, with perhaps the slipping, shearing, and corrugating of the primitive crust over a fluid envelope, and even the tetrahedral collapse of a cooling globe. I lay down here but a general principle to which there may be many exceptions, due to the vicissitudes of cooling and the variation in the materials concerned in any particular region, not to speak of the changing position of the earth's axis, the crustal inertia of Prof. G. H. Darwin, etc. a Professor Hobbs, Ninth International Congress of Geography, 1908. Liquid rock having thus reached a considerable way to the surface, either as simple dikes (pl. 1, I, I), laccolites, or sills (pl. 1, L, L), and so forth, is now in a situation suitable for the second series of phenomena, constituting what I call surface volcanic aetion, to come into play. Surface volcanic phenomena.-Two schools of vulcanologists have held opposed views as to the origin of the volatile constituents contained in fluid igneous rock. One class of writers maintain that the gaseous contents are primordial, and have been contained in the igneous paste from the time that our globe condensed from the nebulous state. Others attribute all the volatile matter still retained in cooled igneous rock or evolved at volcanic mouths and fumaroles to the volatilization of water met by the igneous rock in its journey toward the surface Probably both are right, but I propose to bring before you a series of my observations that point incontrovertibly to the fact that by far the major part of the volatile constituents of a magma are acquired by it on its journey toward the surface. As condensation took place in our planet from a nebulous state, as each layer or shell of rock materials passed from the gaseous to the liquid state, and probably solid, it is evident that those most volatile would be the last to change their physical state. That some of the more volatile ones were entangled or held in solution by the less volatile is quite likely, but the amount must have been small. Another possible source of volatile matter in the deep-seated igneous matter may well be due to a slow osmosis or diffusion extending over vast periods of time and directed by the varying affinities of one class of matter for the other. A quarter of a century ago, as a result of a careful and detailed study of Vesuvius and other volcanoes, I was able to show that a volcano the more continuously active it was in the emission of igneous material the more tranquil was the character of emission, and that practically under such conditions lava was the only product. I showed also that the longer were the intermissions in the extrusive efforts of a volcano the more the ejecta tended to issue in a broken up and fragmentary condition, from the larger and more violent. evolution of volatile or gaseous materials. We thus had the whole gamut of products-scoria, pumiceous scoria, scoriaceous pumice, pumice, and pumice dust-bearing a distinct ratio to the time that any volcano had been in a condition of "repose." Two explanations offered themselves to my mind for this state of things. One was that the persistent evolution of volatile materials primordially stored up in the original volcanic paste escaping con a"The Geology of Monte Somma and Vesuvius:" Q. J. G. S., 1884, vol. 40, pp. 35-119. |