VOLCANO. 



VOLCANO. 



071 



Jorullo itself is the simple mult of the heaping np of cinder* and toon* 

 ejected by gaseous explosions from the principal orifice of eruption, 

 after the outbursts of Ura had ceased. The eruption took place 

 originally from an axial fissure running north and south, but there ia 

 no trace of the deration of the beds along this axis. The volcanic pres- 

 sure from beneath had only forced an exit through this fault for the 

 escape of the liquid and aeriform matters erupted. In short, tin- 

 phenomena of Jorullo show that the most powerful volcanic outbursts 

 can take place without the slightest derangement of the superficial 

 beds. (' Quart. Journ. of Geol. Sac.' vol. xvii. ; ' Translations/ Ac., 

 p. 15, 14.) 



The subject of subterranean heat and its consequences is so vast, and 

 its ramifications in science are so unlimited, that the preceding view 

 of reasonable explanations of Plutonic sad volcanic action is far from 

 having exhausted the catalogue. Not merely geologists, mineralogists 

 and chemists, but astronomers and mathematicians have attempt*. 

 the solution of the problems involved; the former directing their 

 attention chiefly to special structures, products, or phenomena, the 

 latter to such probable causes as point to hypotheses of wide generality 

 embracing the entire system of the relations between the earth as a 

 planet to the heat of its interior regions, and to that which it receives 

 from the solar radiation. 



While Dr. Daubeny was engaged in perfecting what has been called 

 the chemical theory of volcanoes, originated by Davy, other inquirers 

 gave their attention to that subject under different aspects. Sir C. 

 Lyell, as a uniformitarian in geological speculation, and the advocate, 

 not only of the sufficiency of existing causes, but of their persistence 

 without the trace of a beginning, or the prospect of an end, naturally 

 sought fur elements of chemical causation, by which a perpetual 

 circulation of cause and effect returning through effect to cause might 

 be supposed to be maintained ; and the late Professor Daniell, ol 

 King's College, London, suggested to him hydrogen, the continual 

 separation of which from water by means of oxidable bodies and its 

 re-union with oxygen effected by high temperature, as such an element. 



Two mathematicians concurred, though independently, in enun- 

 ciating a theory of Plutonic and volcanic action, dependent on that ol 

 the secular variation of the isothermal surfaces within the globe. The 

 foundation of this was the observed augmentation of temperature as 

 we descend from the surface of the earth towards its interior ; of which 

 subject weadopt from Mr. W.Hopkins the following condensed statement. 

 A considerable number of observations have been made to ascertain 

 the temperature of the earth at considerable depths beneath its surface, 

 and the law according to which that temperature increases in 

 descending. This law, in a considerable number of localities, may be 

 considered as approximately determined to be that the increase of 

 temperature above that of the mean temperature at the surface in any 

 proposed locality, is proportional to the depth beneath the surface. 

 The results of observation also lead to the conclusion that the rate of 

 increase of temperature in descending beneath the earth's surface is 

 nearly uniform in each locality, and nearly the same in different 

 localities, being equal to about 1 Fahr. for 60 feet of depth. At all 

 depths, therefore, there will be, mathematically speaking, spheroidal 

 concentric surfaces of the same temperature throughout, or itvthcrmal 

 turfact*. 



The upward migration of heat from the interior towards the surface 

 of the globe, in consequence of the deposition of fresh matter upon its 

 surface, had been indicated as a cause of geological phenomena by 

 Mr. Poulett Scrope ; but the theory of the secular variation of the 

 isothermal surfaces of the interior of the globe considered as so caused 

 was proposed by Mr. Babbage, in a paper read before the Geological 

 Society, in 1834, and by Sir F. W. Herschel, in letters commu- 

 nicated to that Society three years afterwards, and eventually 

 printed by Mr. Babbage, together with his own paper, in the appendix 

 to his work entitled ' The Ninth Bridgewater Treatise,' published also 

 in 1837. His application of the theory to volcanic phenomena, 

 properly so called, had been announced, however, in terms of extreme 

 generality, and the main object of his paper was to explain 

 by its means the pyrometric expansion of rocks as the cause of 

 elevation. From these circumstances, apparently, it happened that his 

 views remained comparatively unregarded until the subsequent 

 promulgation by Sir John Herschel of view* identical with them in 

 their leading features, but more explicitly developed in their appli- 

 cation to those phenomena. 



Almost every article in this Cyclopaedia which relates to any subject 

 of geological science describes facts, whether structural or dynamical, 

 which involve the truth, that solid materials derived from the land are 

 perpetually being distributed over and accumulated upon the bed of 

 the sea ; this having been the process also of the formation of the 

 sedimentary strata of which, mainly, the present land consists. We 

 have seen that it is also true that the temperature of the globe below 

 the surface, and to the greatest depth with which we are acquainted, 

 increases as we descend, the heat communicated to the surface at last 

 escaping from it by radiation into space. By the continued deposition, 

 therefore, of the new sedimentary strata, which are necessarily bad 

 conductors of heat, on the bed of the ocean, the interior heat, instead of 

 being permitted to escape, will be accumulated, and the original surface 

 will acquire the temperature before possessed by some isothermal 

 surface below, at a depth equal to the thickness of the matter deposited 



upon it, the amount of the accumulation, or the increase of the tempe- 

 rature, augmenting with the increase of this thickness; and conse- 

 quently, by the necessary extension of this process, the temperature 

 of every isotherm*! surface vertically below the mass of accumulating 

 matter, to an indefinite depth, will rise in the same proportion. If tin- 

 temperature at which water boils at the surface, for example, originally 

 existed at the depth of two miles, the deposition of strata of that thick- 

 ness would cause the temperature of the original sea-bed to rise to that 

 amount ; and if the isothermal surface at a certain other depth, of MX 

 or seven miles, perhaps, had the temperature of ignition, the deposition 

 of a thickness of sediinen^ equal to that depth would cause the 01 

 sea-bed to become red-hot, and, by the continuance of deposition, it woulil 

 eventually " bttime actually melted? however refractory its material.-, 

 " and that without any Imdily tranrfcr of matttr in a liquid ttatt from 

 hetote." This process, to use a familiar illustration given by Sir J. 

 Herschel, " is precisely that by which a man's skin grows warmer in a 

 winter day by putting on an additional great coat : the flow of heat 

 outwards is obstructed, and the surface of congelation carried to a 

 distance from his person, by the accumulation thereby caused beneath 

 by the new covering." In the case of the human body, howev 

 cannot carry the illustration further : a succession of great coats would 

 not now raise the temperature of the skin, because the heat of the body 

 is limited ; whereas the succession of external coverings of the earth 

 will indefinitely exalt the temperature of the original surface of depo- 

 sition, and successively that of all the isothermal surfaces below, 

 because the heat of the interior, by the theory, is conceived to be, 

 and, so far as we know, is actually, unlimited. 



The removal of matter from above to below the sea, in the pro- 

 duction of sedimentary strata, produces a subversion of the equilibrium 

 of pressure, and, as we have seen, and which is the most important 

 effect, a subversion of the equilibrium of temperature. But the pro- 

 cess, as described, by which this is effected, miat be. cjecetiivcly rime, and 

 it will depend, " 1st, on the depth of matter deposited [as already 

 explained] ; 2ndly, on the quantity of water retained by it under the 

 great squeeze it has got ; Srdly, on the tenacity of the incumbent mass, 

 whether the influx of caloric from below, WHICH MUST TAKE i 

 acting on that water, shall either heave up the whole mass as a conti- 

 nent, or shall crock it, and [the results of the action of the heat upon 

 the sedimentary matter and the water] escape as a submarine volcano 

 [or a linear series of such volcanoes, afterwards to become BU 1 

 and insular], or shall be suppressed until the mere weight of the 

 continually accumulting mass breaks its lateral supports at or near i !,. 

 coast-lines, and opens there a chain of volcanoes." For a further account 

 of these and other consequences of the rise of the isothermal surfaces, 

 the reader must consult the original papers of the authors of the 

 induction, or of what, for the sake of convenience, we have termed the 

 '' thermotic theory of Plutonic and volcanic action." But we may 

 now refer to Sir J. Herschel's account of the facts of the local dis- 

 tribution and systematic arrangement of volcanoes, cited in a rr< 

 column (666), as evincing their entire agreement with his theoretical 

 views here given. 



Such being the position of the subject, an effort was now made to 

 deduce from the thermotic theory, in the most general, but also in tin- 

 most explicit, manner, the chemical theory of Plutonic and volcanic 

 action, and to show that the latter, as originated by Davy, adopted by 

 Gay-Lussac, and explicitly advocated by Daubeny (though Davy's 

 speculation had been rejected by one of the authors of the thermotic 

 theory), was in reality a simple and necessary consequence of the 

 theory of the secular variation of the isothermal surfaces, explained by 

 Babbage and Herschel, and applied by them to account for the same 

 phenomena. This was done by Mr. Brayley, in a Friday eveniug dis- 

 course at the Royal Institution (where, thirty years before, Sir H. Davy 

 had announced his theory), deliverd May llth, 1838, and published in 

 the ' Philosophical Magazine,' series 8, vol. xii., pp. 533-536. 



Viewing the subject in the most general approximate manner, agree- 

 ably to the amount of our actual knowledge at the time, and un 

 ably disregarding a multitude of modifying considerations which must 

 enter into the discussion of the problem, in order to obtain an exact 

 solution of it, Mr. Brayley first pointed out, on the one hand, how 

 great a thickness of deposited matter would be required for the original 

 surface to attain even a moderate temperature above that due to its 

 geographical position ; but, on the other hand, at how insignificant a 

 thickness (or depth), compared to the earth's radius, adequat. 

 peratures for Plutonic action would occur, all these inferences being 

 founded on the observed law of the increase in temperature in descend- 

 ing of one degree of temperature (1 Fahr.) for every fifty (more 

 accurately sixty) feet of depth. The depths at which the temperatures 

 of boiling water and of ignition respectively would be found, are stated 

 above. At the depth of 26 miles, less than ,^,th part of the earth's 

 radius, cast-iron would melt, or the temperature of 2786 be attained ; 

 at 50 miles depth, a temperature of 5000; and at 100 miles, only 

 Ath of the earth's radius, one of nearly 11,000; either of which, 

 from all analogy, would be more than adequate to the effects required ; 

 Tor it cannot be doubted that at such temperatures even the most 

 infusible and fixed bodies known to form the earth's crust would be 

 not only liquified but volatilised. 



Davy, after repeatedly advocating his own theory, had finally relin- 

 quished it, for reasons which it is remarkable that he should not have 



