Fig. 468.—Mt. Shasta, a typical extinct cone, furrowed by erosion, but retaining its general form. (Diller, U. S. Geol. Surv.)
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.