“A principle, already well known in the arts, is that many, if not all, solids may be made to flow like liquids if only adequate pressure be applied. The making of lead tubes is a well-known practical illustration of this principle, for these tubes are formed simply by forcing solid lead by the hydraulic press through a mould which imparts the desired shape.
“If then a solid can be made to behave like a liquid, even with such pressures as are within our control, how are we to suppose that the solids would behave with such pressures as those to which they are subjected in the interior of the earth? The fact is that the terms solid and liquid, at least as we understand them, appear to have no physical meaning with regard to bodies subjected to these stupendous pressures, and this must be carefully borne in mind when we are discussing the nature of the interior of the earth.”
THE VOLCANO A SAFETY VALVE
Whatever be the state of affairs in the depths of the earth’s crust, we may look upon the volcano as a sort of safety-valve, opening a passage for the pent-up forces to the surface, and thus relieving the earth from the terrible effects of the earthquake, through which these imprisoned powers so often make themselves felt. Without the volcanic vent there might be no safety for man on the earth’s unquiet face.
Professor J. C. Russell, of Michigan University, presents the following views concerning the status and action of volcanoes:—
“When reduced to its simplest terms, a volcano may be defined as a tube, or conduit, in the earth’s crust, through which the molten rock is forced to the surface. The conduit penetrates the cool and rigid rocks forming the superficial portion of the earth, and reaches its highly heated interior.
“The length of volcanic conduits can only be conjectured, but, judging from the approximately known rate of increase of heat with depth (on an average one degree Fahrenheit for each sixty feet), and the temperature at which volcanic rocks melt (from 2,300 to 2,700 degrees Fahrenheit, when not under pressure), they must seemingly have a depth of at least twenty miles. There are other factors to be considered, but in general terms it is safe to assume that the conduits of volcanoes are irregular openings, many miles in depth, which furnish passageways for molten rock (lava) from the highly-heated sub-crust portion of the earth to its surface. . . .”
ERUPTIONS OF QUIET TYPE
“During eruptions of the quiet type, the lava comes to the surface in a highly liquid condition—that is, it is thoroughly fused, and flows with almost the freedom of water. It spreads widely, even on a nearly level plain, and may form a comparatively thin sheet several hundred square miles in area, as has been observed in Iceland and Hawaii. On the Snake River plains, in Southern Idaho, there are sheets of once molten rock which were poured out in the manner just stated, some four hundred square miles in area and not over seventy-five feet in average thickness. When an eruption of highly liquid lava occurs in a mountainous region, the molten rock may cascade down deep slopes and flow through narrow valleys for fifty miles or more before becoming chilled sufficiently to arrest its progress. Instances are abundant where quiet eruptions have occurred in the midst of a plain, and built up ‘lava cones,’ or low mounds, with immensely expanded bases. Illustrations are furnished in Southern Idaho, in which the cones formed are only three hundred or four hundred feet high, but have a breadth at the base of eight or ten miles. In the class of eruption illustrated by these examples, there is an absence of fragmental material, such as explosive volcanoes hurl into the air, and a person may stand within a few yards of a rushing stream of molten rock, or examine closely the opening from which it is being poured out, without danger or serious inconvenience.
“The quiet volcanic eruptions are attended by the escape of steam or gases from the molten rock, but the lava being in a highly liquid state, the steam and gases dissolved in it escape quietly and without explosions. If, however, the molten rock is less completely fluid, or in a viscous condition, the vapors and gases contained in it find difficulty in escaping, and may be retained until, becoming concentrated in large volume, they break their way to the surface, producing violent explosions. Volcanoes in which the lava extruded is viscous, and the escape of steam and gases is retarded until the pent-up energy bursts all bounds, are of the explosive, type. One characteristic example is Vesuvius.