Fig. 337.—Volcanic bomb of unusual form, 13 foot Long. Cinder Buttes, Idaho. (Russell, U. S. Geol. Surv.)
The rougher, irregular fragments of a clinker-like nature ejected by volcanoes are known as scoriæ or cinders. They are more or less distended by gas-bubbles and are hence light and pumiceous.
The larger masses of lava ejected into the air are often caused to rotate by the unequal force of the projection, or by the unequal friction of the air, and to assume spheroidal forms, the internal gases at the same time often expanding and rendering the mass vesicular. These rounded projectiles are known as volcanic bombs (Figs. [336] and [337]). Balls of lava that have originated in rolling movements of the seething mass, or in other ways, are also styled bombs. Usage is not altogether harmonious or consistent in the application of the term.
The larger masses that are projected into the air are more or less vesicular from the expansion of included gases, as already noted, and so the fragmental products of volcanic action grade into the vesicular. The type of this class is pumice, in which the gas cavities make up by far the larger part of the volume of the whole mass, and the whole is reduced to the condition of a solidified froth or foam. So thin are the dividing films of glassy material in some cases that the whole is pure white, though the same material in solid mass would be dark. This solidified glassy froth is often lighter than water and floats freely on the sea until it becomes “water-logged” and sinks. Dredgings of the deep sea show that much pumice has accumulated there, and being far from the land has escaped burial by the sediments borne in by the rivers.
All of these fragmental rocks produced by volcanic action are known as pyroclastic (fire-fragmented) rocks, a general term of much convenience in distinguishing them from lava-flows, on the one hand, and from the fragmental rocks produced by air and water (ordinary clastics), on the other.
THE GLASSY ROCKS.
The solid glasses.—The quick cooling of lava-flows into solid glasses is chiefly dependent on their exposure at the surface. Hence it is often the case that the exterior of a lava-flow is glassy in greater or lesser degree, while the interior is more or less crystalline. Quick cooling is sometimes also due to the intrusion of the lava in thin sheets into fissures in cold rocks. When massive bodies of lavas penetrate solid rocks, the lava does not usually cool so fast as to prevent some degree of crystallization, and the crystallization may even become complete; but if the intruded lava sheet be very thin, the lava is liable to be cooled to a nearly perfect glass. The glassy condition is, therefore, subject to indefinite gradations. As a rule, the acid lavas are stiffer at the same temperature than the basic ones, and crystallize more slowly, so that acid glasses are more common than basic ones. The basic rocks usually crystallize pretty thoroughly, except on the immediate surface of the flows.
The first stages of crystallization.—The microscopic study of the volcanic glasses reveals great numbers of minute forms known as crystallites, microlites, globulites, etc., that appear to be first steps in crystallization, though many of them do not take definite geometrical shapes and some do not show the optical characters of crystals. There are minute globules (globulites), needles, and hair-like bodies (trichites) of more or less indeterminate nature, together with other forms that can be seen to be certainly the initial forms of well-known minerals.
