Page:Collier's New Encyclopedia v. 10.djvu/287

LEFT VOLCANOES 247 VOLCANOES perlmentally proved. That hot lava, even after eruption and before solidifica- tion, still holds occluded water is also known. That enormous quantities of steam rush forth from the erupting and escape from the surface of flowing lava streams is the most conspicuous fact at- tending every eruption. The energetic action is plainly that of an elastic, high expansible agent, like steam, escaping at a very high temperature and pressure, and in enormous quantity, and cannot be due to the molten rock, which has no such elasticity, and, which, without any volatile ingredient, would be merely an inert, passive, liquid mass, incapable of producing any such dynamic effects. The vigor of the action differs widely in different volcanoes. In some the dastic or explosive action is very mild, as in the Hawaiian volcanoes. Here the eruptions are usually unattended with the extreme violence which makes them so formidable elsewhere; the destruction is limited to that caused by the quietly flowing lava streams, which are gener- ally of great magnitude and run for many months. In most volcanoes the action is far more energetic, and in many of them it is of the most terrible character. Those of the East Indies and some in Central America belong to the violent class, and the catastrophes pro- duced by their outbreaks have been the most destructive which have afflicted the world. The memorable outbreaks of Papandayang, in Java, in 1772; of Tora- boro, on the island of Gumbawa, in 1815; of Casaguina, in Nicaragua, in 1835; and of Krakatoa, in the Straits of Sunda, in 1883, may be regarded as literal explosions on a stupendous scale, in each of which a great mountain was blown up, and its fragments showered down over vast regions, burying or de- stroying everything within reach of their trajectories. The lava, instead of being poured out in a liquid stream, was blown into dust by the sudden release of its imprisoned vapors. This dust filled the air for hundreds of miles around, and was wafted by the winds to vast distances. These, however, are extreme cases. The violence of the eruption may be inferred to depend on the quantity of eruptive matter to be extravasated, the proportion of volatile ingredients contained by it, and the rapidity with which it is released from the pressure to which it is subject in the depths. Volcanoes whose vents are always open seldom cause serious trouble; and where the eruptions are frequent they are sel- dom of an extreme character. But when the intervals between the outbreaks are long they are apt to increase in severity. The piles of extravasated matter built up around the eruptive orifice assume numerous forms, depending on the vary- ing conditions of the eruptions. The simplest form is a nearly symmetric cone with a crater in the truncated summit. As the forces of the eruption exhaust themselves the escaping blast of steam carries up with it clots and pellets of viscous lava, termed scoriae and lapilli, which are blown high in the air and then descend, gradually building up a cone as they fall. This eruption may be followed by others, which de- form the original cone by adding a mass of lava and secondary cone. As succes- sive eruptions follow each other they build up a large composite pile of mingled lava, scoriae, and lapilli. When the eruptive action is well centralized in a single large vent, giving many eruptions in the course of ages, a great mountain is slowly built, having a dom^ inant central orifice with a crater in the summit, and a form which is rudely coni- cal; but in detail the structure is highly complex. When the mountain becomes very large the eruptions tend to break out on its slopes, or even at its base, forming what are termed parasitic cones. Most of the great volcanic piles have large numbers of these parasitic cones, each being a secondary vent. Volcanoes, active or extinct, are found in those portions of the earth which have been subject to extensive disturb- ances of the strata. It has been custom- ary to speak of their distribution as occurring in chains. This is but a partial truth, and, as usually stated, involves a rather strained use of the idea of linear arrangements; and it is believed that the notion is in some important re- spects a misleading one. The general law seems to be that volcanoes occur in regions which have been subject to that mysterious subterranean action which elevates the land, builds moun- tains and plateaus, and deforms the strata; and there is reason to infer that they constitute one of several classes of phenomena which ultimately originate in the same category of causation. It was suggested by Charles Darwin that volcanoes occur in regions which are undergoing elevation. So far as it has been possible to put this suggestion to test it seems to have been sustained by observation. The determining cause of volcanic action has been subject to much speculation, but no satisfactory theory of it has yet been proposed. The view which has received the most considera- tion is that surface waters penetrate the earth to depths where the tempera- ture is sufficient to melt the rocks, thus