Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

LAVAS.

Their nature.—In the chapter on the Origin and Descent of Rocks, the nature of lavas and of the rocks derived from them has been discussed ([Chapter VII]). In view of prevalent misconceptions, it may be repeated, for the sake of emphasis, that lavas are mutual solutions of mineral matter in mineral matter, rather than simply melted rock. Into this mutual solution there enter not only rock materials, but gases. The distinction between mutual solutions and simple molten rock cannot be rigorously made, but it is at least essential to know that the minerals do not necessarily crystallize from lavas in the order of their melting temperatures, or in any uniform order, but rather in the order in which saturation of the several mineral constituents happens to be reached in the given mutual solution. Thus quartz, which has a very high melting-point, is often one of the last minerals to crystallize. The mutual solutions are exceedingly complex, embracing a wide range of chemical substances, but the chief of them, as already stated, are silicates of aluminum, potassium, sodium, calcium, magnesium, and iron, with minor ingredients of nearly all known substances, in greater or less proportion. The old idea of lavas as simply melted rock is not, however, wholly to be abandoned. The mode of solidifying is often simply that of molten matter freezing. If lava be suddenly cooled, the congelation is essentially the solidification of a melted substance. The result is a glassy body, every part of which has essentially the same composition that the liquid had. Usually, however, even in this case, the gases escape in part. If the cooling is slower, the various substances in the mixture crystallize out into minerals in the order in which they severally reach saturation. This involves the principle that solubility is dependent on temperature, and that as the temperature sinks the degree of solubility declines, and the saturation-point for some constituents of the solution is reached earlier than for others. With sufficiently slow cooling, all the material will pass into the solid state by the crystallizing of the several minerals in succession. This does not mean that two or more minerals may not be forming at the same time, for crystals often interfere with each other’s growth. It does, however, involve the doctrine that some substances may complete their crystallization while the surrounding material is yet in the fluid condition. In most igneous rocks nearly perfect crystals of certain minerals are common, while other minerals, crystallizing later, are compelled to adapt themselves to the space left. This conception is supported by the fact that lavas, while still in the fluid condition, often contain well-formed crystals, and these crystals sometimes make up a considerable percent. of the flowing mass, just as water in certain conditions may be filled with crystals of ice. So also crystals after having been formed may be redissolved in part, doubtless because of changes in the nature of the magma due to undetermined conditions which may arise in the process of crystallization, or from the accession of gas, or from new material dissolved from the walls of the passageway.

Fig. 469.—Lobular form of lava-flow, “Pahoehoe.” (Dutton, U. S. Geol. Surv.)

Fig. 470.—Terminal portion of a rough lava-flow, “aa.” Cinder Buttes, Idaho. (Russell, U. S. Geol. Surv.)

Fig. 471.—Lava flowing over a precipice near Hilo, Hawaiian Islands.

Consanguinity and succession of lavas.—The lavas that are poured forth at different stages in the succession of eruptions of a given region are usually not the same, as might naturally be expected, but form a curious series the members of which are related to one another. Iddings has called this relation consanguinity.[283] No universal law of succession has yet been established, and perhaps none exists; but Richthofen[284] many years ago announced a definite order for the Tertiary flows of western America which seems to hold fairly well in its general aspects, though not everywhere completely realized, so far as surface observation goes. Richthofen’s order is: (1) lavas of neutral types, (2) lavas of acid types, (3) lavas of basic types, (4) lavas of more acid types, and (5) lavas of more basic types. The special varieties of rock vary, and even the general order is often apparently defective. The defects are sometimes assigned to the concealment of some of the outflows. While this may be true in some cases, it is not unlikely that in others there is a real failure of the sequence. At any rate, the sequence can only be regarded as a rough generalization. It is supposed to be due to magmatic differentiation caused by the differences of temperature to which the different parts are subjected underground, by differences of specific gravity and fluidity which result from changes of temperature, and probably by other causes.

Temperatures of lavas.—Accurate determinations of the temperatures in the center of the lava-columns, where they have been least reduced by contact with the rock-walls, have not yet been made, but it is clear from the whiteness of the lavas that their temperatures are often appreciably above the melting-point. This is also a necessary inference from the length of time they remain fluid, notwithstanding the great surface contact of the column in its miles of ascent, the conversion of contact water into steam, and the expansion and escape of the gases. In cases where determination has been practicable (and they certainly do not represent the maximum temperatures) it has been found that the melting-points of silver, about 960° C., and of copper, about 1060° C., are reached. In connection with overflows, it has been found that brass is decomposed into its component metals, the copper actually crystallizing. Silver has been sublimed, and made to redeposit itself in crystalline form. This implies much more than the bare melting temperatures. Even the fine edges of flints have been fused. It is, therefore, probably safe to assume that the original temperatures of the lavas as they rise to the surface sometimes reach considerably beyond 2000° Fahr. (1093° C.), and may perhaps even attain 3000° Fahr. or more. Even these temperatures must be somewhat below the original subterranean temperatures of the lavas, because some heat must necessarily be lost in rising, partly by contact with the walls of the colder rocks through which they pass, probably for as much as a score of miles at least, and partly from the expansion of the gases within them. If any considerable part of these gases is derived from waters which joined the lava in its upward course in the fracture zone, the energy consumed in raising the water to the high temperatures of the lavas must be subtracted from the original heat, and must be a further source of reduction of temperature. It is important to emphasize this point in view of its bearing upon the origin of the lavas. It has been suggested that lavas may be due to an aqueo-igneous fusion, a kind of fusion which may take place at comparatively moderate temperatures. It seems obvious, however, from the phenomena themselves, that temperatures as high as ordinary dry fusion, and perhaps even higher, are attained. It is clear also that the maintenance of the liquid condition in a constant state of ebullition for a long period of time implies a large surplus of heat above that necessary for liquefaction simply. This is especially true if the ebullition comes from surface-waters penetrating to and becoming absorbed in the lava-column below. This process must tend rapidly to exhaust the heat in the column of lava. If, on the other hand, the gases are derived from the deep interior, and the ebullition at the surface is due to their escape, they may bring up new supplies of heat to counteract the cooling effects of their expansion.

Depth of source.—Attempts have been made to ascertain the depth from which lavas rise, by means of the earthquake tremors that accompany eruptions. The estimates have ranged from seven or eight to thirty miles. The mode of estimate is that discussed under earthquakes, and is subject to the corrections there indicated. If these could be perfectly applied, the estimates might probably all fall within ten miles, and not improbably all within six miles of the surface. But in any case the method really tells very little as to the true point of origin of the lava. At most it probably only tells where the ascending lava begins to rupture the rock through which it passes, and rupture may not be possible below the zone of fracture, which is probably not more than six miles deep. In the zone of flowage below, where the pressure is too great to permit fracture, the lava not improbably makes its way by some boring or fluxing process, which might not, because of its nature, be capable of giving rise to seismic tremors. The behavior of the tremors perhaps forces us to locate the origin of lava movement at least as low as the bottom of the fracture zone, but it probably offers no sufficient ground for limiting the lava’s origin to this or any other specific depth.