VOLCANOES, orifices in the earth's strata from which molten rock, hot va pors, and fragmental material are ejected, and around which the solidified productions of eruptions accumulate. We know by observation only the sur face phenomena of volcanoes. What takes place in the profounder depths, where volcanic forces are generated, or what is the nature of the subterranean process, is only conjecture. We may affirm that a volcanic eruption usually consists in the escape from the depths of the earth to the surface, of molten rock, charged with intensely hot vapors, of which the most abundant is steam. The most characteristic and impressive fea tures of the action are those produced by the vapors suddenly relieved from the pressure to which they were subject within the earth, and blowing off into the atmosphere with extreme energy. Mol ten lava, prior to the eruption, is charged with these vapors in much the same way as soda water or wind is charged with carbonic acid; and when the pressure is relieved the gas is given off copiously. That molten rock may, under pressure, be made to absorb and "occlude" great quantities of water vapor has been ex perimentally 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 elastic 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 Tom 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 scori 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, scoria;, 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.