THE FRAGMENTAL PRODUCTS OF SUDDEN COOLING.
Pyroclastic rocks.—The extreme example of sudden cooling is presented when lavas are violently exploded into the air and solidify almost instantly. The resulting glassy particles or filaments, if small, constitute volcanic ash. The explosion appears to be due to steam and other gases which are held in the deeper lava under great pressure, but which, as they rise toward the surface of the lava where the pressure is relieved, expand with explosive violence. It is probably also due in part to progressive crystallization, which forces the gases out from the part that crystallizes and overcharges the rest. Sometimes the projected particles draw after themselves long filaments like the threads of spun glass, and sometimes while in the air they divide and draw apart, spinning a filament of viscid lava between them. A variety of this kind at the volcano of Kilauea in Hawaii is known as “Pele’s hair.” These light filaments drift with the wind and lodge on the lee side of the volcano, covering the surface “like mown grass” (Dana).
Fig. 336.—Volcanic bomb. About half natural size. (Photo. by Church.)
When the exploded fragments are coarser they fall about the volcanic vent and form the tuffs (tufa) of which most steep volcanic cones are chiefly built. In these larger fragments, crystals are not infrequently found, and the same is even true of the volcanic ash. These crystals are undoubtedly such as had already been formed in the lava before it exploded, and their formation, as suggested above, may have contributed to the explosion.
Fragments too large to be borne far away by the air, but still small, are known as lapilli, especially if they are somewhat rounded and gravel-like. A finer variety, of the nature of sand, much used in making Portland cement, is locally known as puzzolana.
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.