Fig. 457.—Lava-flow near the Jordan craters, Malheur Co., Oregon. Though not of the gigantic order, it illustrates the general aspect of massive lava-flows. (Russell, U. S. Geol. Surv.)
b. Volcanic eruptions.—In the types of eruption prevailing at the present time, the lavas are forced out through ducts or perhaps short fissures or sections of fissures, and build up cones about the vents, the eruptive action maintaining craters in the centers of the cones. The essential feature of a volcano is the issuance of hot rock and gas from a local vent. A mountain is the usual result, but the mountain is secondary and not usually present in the first stages; the localized eruption is the primary and necessary factor. The amount of rock matter ejected is not necessarily great. Compared to the massive extrusions of fissure eruptions, it is usually rather trivial; but the volcano makes up in demonstrativeness what it lacks in massiveness of product.
Fig. 458.—The volcano Colima, Mexico, in eruption. March 24, 1903. (José Maria Arreola, per Frederick Starr.)
c. Intermediate phenomena.—On the border-line between the intrusive and the extrusive phenomena there are special cases of interest. There appear to be certain instances in which the intrusion comes so near the surface as to develop explosive phenomena without the extrusion of lava. From the nature of the case this is an interpretation rather than a demonstration. It is certain, however, that occasional violent explosions take place where no lava comes in sight. This sometimes occurs in old volcanic formations, and sometimes in regions of undisturbed horizontal strata. In the former case the phenomena may be due to the intrusion of a fresh tongue of lava below, or it may be due to the penetration of surface-waters to hot rocks that have remained uncooled from previous volcanic action, and the development, by such contact, of a volume of confined steam sufficient to produce the explosion. A case of this doubtful kind occurred at Bandai-San in Japan in 1888, where there was a sudden and violent explosion which blew away a considerable part of the side of a volcanic mountain which had not been in eruption for at least a thousand years. The mass and violence of the exploded material was such as to fill the air with ashes and débris in a fashion altogether similar to a typical volcanic eruption. A large tract of adjacent country was devastated, and many lives lost. The whole action, however, was concentrated in the initial explosion, and within a few hours the cloud of ashes had disappeared and the phenomenon was ended. An examination of the disrupted area revealed no signs of liquid lava.
An example of the latter class is Coon Butte in Arizona.[276] This consists of a rim of fragmental material encircling a crater-like pit from which the fragments were obviously forced by violent explosion. The pit is in ordinary sedimentary strata, and the material of the rim is composed of the disrupted fragments of the sedimentary rock ejected from the pit. There are no signs of igneous material, but there was igneous action in the vicinity. Fragments of a meteorite were found on the rim and in the vicinity, but this association appears to be accidental. Computation shows that the volume of the material of the rim closely matches the size of the pit. The source of the explosion is not demonstrable, and it may be an error to connect it with an intrusion of lava below; but since intrusions rise to various degrees of nearness to the surface, and in innumerable cases reach the surface, there is every reason to entertain the conception of a class of intrusions which develop explosive phenomena by close approach to the surface, without actually reaching it.
Fig. 459.—Photograph of a portion of the moon taken at Lick Observatory.
Lunar craters.—There are grounds for thinking that the remarkable craters of the moon, assuming that they are truly volcanic,[277] may belong to this class, for they are very similar to the Coon Butte pit. The capacities of the lunar craters, so far as they can be estimated, seem to equal, if they do not in many cases exceed, the volume of matter in their rims. They do not appear usually to be great cones of accumulated material with relatively small craters, like the typical products of terrestrial volcanoes. Besides, there are no clear evidences of lava-streams. The radiating tracts once interpreted as such have been shown by increased telescopic power and the resources of photography to be at least something other than lava-streams. They are vaguely defined tracts which run over heights and depths indifferently, and are plausibly interpreted as lines of débris projected to extraordinary distances because of the absence of a lunar atmosphere, and because of the low force of the moon’s gravity. Since the moon now has no appreciable atmosphere or surface-waters, and since it is doubtful whether it ever possessed either on account of its probable inability to hold atmospheric gases or the vapor of water in the form of an envelope about it, owing to its low gravity, there is reason to suppose that the external matter of the moon derived from the explosions of the multitude of lunar volcanoes would remain in a loose, incoherent condition, from the absence of dissolving and cementing agencies. It is reasonable to suppose that lava-tongues arising from the deeper interior would have a higher specific gravity, even in their heated condition, than this porous covering of the moon, and that therefore they would almost universally become intrusions rather than extrusions, or at most they would not rise beyond the bottom of the craters they had produced by explosion. This seems to furnish at least a plausible explanation of the prevailing differences between the large lunar craters encircled by mere rims and the much smaller terrestrial craters seated in relatively large cones.