Volcanoes
orifices in the earth's strata from which molten rock, hot vapors, and fragmental material ejected, and around which the solidified productions of eruptions accumulate.
We know by observation only the surtakes place in the profounder depths, face phenomena of volcanoes. What 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 features of the action are those produced by the vapors suddenly relieved from the within the earth, and blowing off into the atmosphere with extreme energy. Molten 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 expressure to which they were subject VOLCANOES tic, high perimentally proved. That hot lava, even after eruption and before solidification, 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 attending every eruption. The energetic action is plainly that of an elastic, high steam, escaping at a very high temperature and pressure, and in enormous quantity, and cannot be due to the molten rock, which has no , without any volatile ingredient, nt, would volatile ingredient, would be merely an mass, incapable of producing any such dynamic effects.
The vigor of the action differs widely in different volcanoes. In some the different volcanoes. In elastic or explosive action is very mild, some the 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 generally of great magnitude and run for many months. In most volcanoes the action is far more er many of them it is of the most terrible energetic, and in character.
Those of the East Indies and some in Central America belong to the violent class , and the catastrophes produced by their outbreaks have been the most destructive which have afflicted the world. The memorable outbreaks of Java, in 1772; of Tomboro, on the boro, on the island of Gumbawa, in , in Nicaragua, in 1835; and of Krakatoa, in the Straits of 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 deeverything 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, by the winds to vast distances. These, however, are extreme cases. The violence of the eruption may depend on the quantity of eruptive matter to be extravasated, the proportion of volatile ingredients contained by it, and the rapidity with 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 seldom of an extreme character. But when the intervals between the outbreaks are long they are apt to increase in severity.
VOLCANOES The piles of extravasated matter built up around the eruptive orifice assume numerous forms, depending on the varying 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 the escaping blast pellets of viscous lava, termed scoriæ of steam carries up with it clots and 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 form the original cone by adding a mass may be followed by others, which dewed by others, which deof lava and secondary cone. As succesbuild up a large composite pile of sive eruptions follow each other they the eruptive action is well centralized mingled lava, scoriæ, and lapilli. When 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 conical; but in detail the structure is highly complex. When the mountain becomes very large the eruptions tend to break forming what are termed parasitic out on its slopes, or even at its base, pes, or even at its base, cones. Most of the great volcanic piles have large numbers of these parasitic cones, each being a secondary vent. in those portions of the earth which Volcanoes, active or extinct, are found have been subject to extensive disturbances of the strata . It has been customary to speak of their distribution as occurring in chains.
This is but a partial truth, and, as usually stated, involves linear arrangements; and it is believed a rather strained use of the idea of that the notion is in some important respects a misleading one. law seems to be that volcanoes occur The general in regions which have been subject to subterranean action which elevates the land, builds mountains and plateaus , and deforms the strata; and there is reason to infer that one of several classes of phenomena which ultimately originate in the same ca was suggested by Charles Darwin that category of causation. It 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 consideration is that surface waters penetrate the earth to depths where the temperature is sufficient ture is sufficient to melt the rocks, thus VOLCANOES furnishing the elastic agent under conditions of temperature and pressure which are adequate to the result.
The principal theaters of modern volcanic action are the Chilean Andes, the Andes of Ecuador and Peru, Central America, the Dutch East Indies, and the Philippines, Iceland, and the Mediterranean. To these must be added the remarkable Hawaiian volcanoes, and perhaps also those of New Zealand. We have reason also to infer that many extensive outbreaks occur beneath the ocean, but we know very little about them. The number of active volcanoes is usually reckoned as about 350; but it is difficult in many of these cases to decide whether the volcano is active or extinct. But the number now active is small indeed in comparison with those which have become extinct in comparatively recent geological times. regions abound in volcanic piles which in the course of human history have given no sign of activity. This is especially the case in the W. part of the United States, and most especially in the N. W. States and Territories. Here may be found great regions, tens of thousands of square miles in extent which have been overflowed with lava, and the successive sheets are thousands of feet in thickness. In many portions of the West the indications point to a very recent epoch for the eruptions, and it is by no means improbable that some of them have occurred since the Spanish conquest. In all ages of the geological history of the planet, and as far back as we are able to discriminate the relative antiquity of the strata, volcanic eruptions have been abundant and of the same general character as those which now prevail. Nor is it possible to say whether they were generally more extensive or less so than at present. Two of the most remarkable of active volcanoes are Mauna Loa and Kilauea, in Hawaii; the former 13,760 feet in height and the latter, though only about 4,000 feet high, 8 miles in circumference. On the summit of Mauna Loa is a circular crater 8,000 feet in diameter, its walls nearly vertical and many hundreds of feet deep. The eruptions of this volcano have been of great volume and at an average interval of eight years; each outflow representing more lava than Vesuvius has sent out since the destruction of Pompeii. The Hot Lakes district of North Island, New Zealand, was the scene of remarkable volcanic disturbances in 1886. The effect extended over 60 square miles, burying whole villages and causing much loss of life. The eruption was accompanied by VOLGA violent earthquakes and hundreds of new geysers broke out. The fine volcanic dust that settled over the province of Auckland proved to be beneficial to vegetation. The most destructive of modern volcanic eruptions was that of Mont Pelée in the island of Martinique in May, 1902.