Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

(1) In most cases of denudation, cooling below probably keeps pace with loss above. At any rate, many volcanoes rise from the bottoms of the oceans where no denudation takes place, and this phase of the hypothesis is not workable there.

(2) The theory of relief by faulting finds encouragement in the fact that many volcanoes occur on fault-lines. There is no evidence, however, that this is a universal or necessary relation. Computation as to the amount of lowering of the melting-point that might arise from the faulting associated with volcanoes indicates that it is necessary to suppose that the rocks were already very close to the melting-point when the faulting took place, to make the doctrine applicable. It is to be observed that in faulting the relief of pressure on one side of the fault-line is likely to be balanced by increased pressure on the other side, and that this difference in pressure may be lost by distribution at a depth of 20 or 30 miles, where, at the nearest, this delicate relation between solidity and liquidity, on which the theory is dependent, may perhaps be reached.

(3) Immediately under an anticlinal arch there may doubtless be some relief of pressure within the limits of strength of the arch, which is not great ([p. 582]). The pressure under the arch as a whole is greater than before it was bowed up by lateral thrust, and in depth this excess becomes distributed so as to obliterate the local relief under the center of the arch, and so adds the effects of folding to the average pressure of the crust. Besides, as a matter of fact, volcanoes do not appear to be especially associated with mountain folds where arching reaches its best expression.

(4) The same general considerations bear on the assignment of liquefaction to relief of pressure in connection with the more general deformation of the earth’s body. Besides, while relief of pressure might account for liquefaction, it leaves the extrusion without an obvious cause; indeed, it would seem to furnish a condition opposed to extrusion, and if pressure were subsequently added to force the liquid out, it would tend to restore the solid condition.

Hypothesis 5. Lavas assigned to melting by crushing.—Mallet[286] and others have attributed melting to the crushing of rock. Crushing, in the ordinary sense of the term, can only take place in the zone of fracture, and that is apparently too shallow to meet the requirements of the case. Below this zone, the pressure on all sides is too great to permit any separation of fragments, and a solid mass can only change its form by what is called “solid flowage.” The rock under these conditions may be compressed, and this compression must give rise to heat, but at the same time the melting-point is raised, according to all experiments. It seems improbable that melting can be produced in this way. If great pressure could be brought to bear upon a tract of rock so as to heat it by compression, and if then the pressure were relaxed before the heat generated could be distributed by conduction, and if re-expansion did not follow, possibly melting could be effected, but this makes the process complicated and apparently inapplicable. It is scarcely possible that such a sequence of events can have affected all the tracts that are now volcanic, much less all those that have been such throughout geologic history. As noted in the preceding case, relaxation would seem to be unfavorable to expulsion. Besides, volcanoes do not seem to be confined to tracts that show signs of great crumpling and crushing, as the Alps, the Appalachians, and the closely folded ranges generally. Extrusions seem rather more common with faulted ranges where crushing is less notable and where surface tension replaces compression.

Hypothesis 6. Lavas assigned to melting by depression.—It is observed that in certain regions great thicknesses of sediments have accumulated by the slow settling of the crust below, and as these sediments obstruct the outward flow of heat while the lower beds settle nearer to the interior source of heat, it is conceived that they become heated below and, being saturated with water, take on aqueo-igneous fusion and rise as lavas, well supplied with internal gas and steam from the water and volatile constituents that were entrapped and carried down with them. The question obviously arises whether such depression is sufficient to give the temperatures the lavas show, and whether volcanic action is confined to such areas of depression and deep sedimentation. At the highest credible estimates—which are none the less to be taken with reserve—the post-Archean sediments rarely reach five or six miles in thickness at any given point, and probably never exceed ten or twelve, while twenty or thirty miles is the computed depth required for the acquisition of the temperatures the lavas actually possess. If the Archean terranes be included among the sedimentaries, the thickness may be adequate, but what then of the Archean vulcanism, which much surpasses that of later times, and the other early extrusions before the sediments were thick; and what of the moon, where there are probably no sediments at all?

Besides, it is not at all clear that the distribution of vulcanism is specially related to that of thick sediments, as it should be if this hypothesis were the true one. There are many volcanoes in the heart of the great oceans where sedimentation is now inappreciable, and probably has been in all past periods.

Hypothesis 7. Vulcanism assigned to the outflow of deep-seated heat.—If the earth grew up by slow accessions of meteoroidal or planetesimal matter, in a manner to be more fully set forth in the discussion of the origin of the earth, and if its interior heat be due chiefly to compression by its own gravity, the internal temperature would be originally distributed according to the degree of compression, and this would depend on the intensity of the internal pressure. This can be approximately computed, and is shown in the diagram on page 563, where this subject has been treated. On not improbable assumptions regarding the thermometric conductivity, the flow of heat from the deep interior to the middle zone would be greater than the loss of this zone to the superficial zone. This middle zone should, under this view, experience a rising temperature. By hypothesis, this zone is composed of various kinds of matter mixed as they happened to fall in. Hence as the temperature rises, the fusion-points of some of these constituents will be reached before those of others. More strictly, the temperatures at which some of these constituents will mutually dissolve one another will be reached, while other constituents remain undissolved, and thus a partial and distributed liquefaction will arise. The gases and volatile constituents in the mixed material would naturally enter largely into the liquefied portion. It is assumed that with a continued rise of temperature, the partial liquefactions would increase until the liquefied parts found means of uniting, and the lighter portions, embracing the gaseous contingent, were able to work their way toward the surface. As they rose by fusing or fluxing their way, the pressure upon them became less and less, and hence the temperature necessary for liquefaction gradually fell, leaving them a constantly renewed margin of temperature available for melting their way through the upper horizons. Thus it is conceived that these fusible and fluxing selections from the middle zone might thread their ways up to the zone of fracture and thence, taking advantage of fissures and fractures, reach the surface. It is conceived that such liquefaction and extrusion would carry out from the middle zone the excess of temperature received from the deeper interior, and thus regulate its temperature and forestall general liquefaction, the zone as a whole remaining always solid. The independence of volcanoes is assigned to the independence of the liquid threads that worked their way to the surface. Nothing like a reservoir or molten lake enters into the conception. The prolonged action of volcanoes is attributed to the slow feeding of the liquid threads from the locally fused middle zone. The frequent pauses in action are assigned to temporary deficiencies of supply; the renewals to the gathering of new supplies after a sufficient period of accumulation. The distribution of volcanoes in essentially all latitudes and longitudes is assigned to the general nature of the cause. The special surface distributions are assumed to be influenced, though not altogether controlled, by the favorable or unfavorable conditions for escape presented by the crustal segments of the earth. The persistence of volcanic action in time is attributed to the magnitude of the interior source, to its deep-seated location, and to the slowness of conduction of heat in the earth’s interior. The force of expulsion is found in the stress-differences in the interior, particularly the periodic tidal and other astronomic stresses (see p. 580), and in the slow pressure brought to bear on the slender threads of liquid by the creep of the adjacent rock. The violent expulsions are due to the included gases, of which steam is chief. Little efficiency is assigned to surface-waters, and that little is regarded as wholly secondary and incidental. The true volcanic gases are regarded as coming from the deep interior and as being true accessions to the atmosphere and hydrosphere. The standing of the lavas in volcanic ducts for hundreds and even thousands of years with only small outflows, as in some of the best-known volcanoes, is regarded as an exhibition of an approximate equilibrium between the hydrostatic pressure of the deep-penetrating column of lava, and the flowage-tendency of the rock-walls, the outflow being, of course, also conditioned on the slow rate of supply below, and the periodic stress-differences of the interior.

For the present these hypotheses must be left to work out their own destiny, serving in the mean time as stimulants of research. All but the last have been for some time under the consideration of geologists, and are set forth in the literature of the subject ([p. 636]).

A few special phases of the problem need further discussion, though they have been incidentally touched upon.