Fig. 463.—Surface of lava-flow of 1881, from Mauna Loa, as seen back of Hilo, Hawaii. (Photo. by Calvin.)
3. Unimportant coincidences.—Eruptions seem to be somewhat more liable to occur at times of high atmospheric pressure than at low, doubtless because the increased atmospheric weight on a large area of the adjacent crust aids in forcing out the lava or the volcanic gases. This can only be effective when other forces have almost accomplished the result, and would doubtless have completed it a little later had not the atmospheric wave supplied the little remaining pressure needed. Eruption seems also to be more common when the tidal strains favor it, for like reasons. In the same class are probably to be put the effects of heavy rains, whether they act by gravity or by giving rise to steam. Such agencies are to be regarded as mere incidents of no moment in the real causation of vulcanism, but of some value in determining the precise moment of action. This is not to be understood as inconsistent with the view that the periodic stresses of the body-tides of the earth are important factors in vulcanism, as elsewhere explained, but merely that the special time of surface-eruption is only incidentally connected with the water-tides.
Fig. 464.—Crater of Kilauea.
Periodicity.—Most volcanoes are periodic in their stages of action. Long dormant periods intervene between eruptive periods. Volcanoes supposed to be extinct occasionally awaken with terrific violence. Sometimes also they awaken quietly. This larger periodicity yet awaits an explanation, but it very likely means a temporary exhaustion of the supply of gas or of lava, or of both, to which the active stage is due.
Formation of Cones.
Lava-cones.—The lava usually flows away from the vent in short streams which solidify before running far. As the lava-streams flow in different directions at different times, the total effect is a low cone formed of radiating tongues surrounding the point of exit. Occasionally the streams run a dozen or a score of miles, but such cases, except in the gigantic volcanoes of Hawaii and a few others, are rare. Often the streams congeal before they reach much beyond the base of the cone, and quite often while they are yet on its slope. So far, therefore, as the volcanic cone is formed of lava, it has a radiate structure made up of a succession of congealed lava-streams. In these cases the slopes are low, because the fluidity of the lava prevents the development of high gradients. It is, however, rather the exception than the rule, that the cone is made up mainly of lava-streams, though the great Hawaiian volcanoes are of this class.
Fig. 465.—Typical cinder-cone, Clayton valley, Cal. (Turner, U. S. Geol. Surv.)
Cinder-cones.—The larger portion of the lava blown into the air by the expanding gas-bubbles falls back in the immediate vicinity of the vent and builds up a cinder-cone. From the nature of the case, this often takes on a beautiful symmetry and assumes a steep slope ([Fig. 465]). The ragged cinders lend themselves readily to the formation of an acute cone, quite different from the flatter cone formed by lavas. Sometimes the cinders are still plastic when they fall, and weld themselves together and hold their places even on very steep slopes, but usually they have already hardened before they reach the surface.