Page:Popular Science Monthly Volume 20.djvu/65

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55
VOLCANOES, THEIR ACTION AND DISTRIBUTION.

ment, and are nearly all situated along three well-marked bands and the branches proceeding from them. The volcanoes of the eastern coast of Africa, with Mauritius, Bourbon, Rodriguez, and the vents along the line of the Red Sea, may be regarded as forming a fourth and subordinate band. Nearly all of them are situated near the limits which separate the great land and water masses of the globe, either on the parts of continents not far removed from their coast-lines, or on islands in the ocean not very distant from the shores. Two conspicuous exceptions to this rule are the volcanoes of the Thian-Shan range in Asia, in the center of the largest unbroken land-mass of the globe; and the group of the Sandwich Islands, almost in the center of the largest ocean, and rising almost from the greatest depths of that ocean. Geological researches have, however, shown that the Thian-Shan Mountains in Pliocene times stood on the southern borders of a great inland sea. A regular parallelism seems to exist between volcanic bands and the great mountain-chains; and the researches of Mr. Darwin have shown that "nearly all the active volcanoes are situated upon rising areas, and that volcanic phenomena are conspicuously absent from those parts of the earth's crust which can be proved at the present day to be undergoing depression."

Inferences are sometimes hastily drawn from the fact that most volcanoes are near the ocean which the facts themselves will hardly warrant. Thus, it is frequently assumed that we may refer all the phenomena of volcanic action to the penetration of sea-water to a mass of incandescent lava in the earth's crust and to the chemical and mechanical actions which result from the meeting. This argument, however, as Mr. Scrope has shown, involves a reasoning in a circle. "It is assumed, on the one hand, that the heaving subterranean movements which give rise to the fissures by which steam and other gases escape to the surface are the result of the passage of water to the heated masses in the earth's crust. But, on the other hand, it is supposed that it is the production of these fissures which leads to the influx of waters to the heated materials. If it is the passage of water through these fissures which produces the eruptions, it may be fairly asked, What is it gives rise to the fissures? And if, on the other hand, there exist subterranean forces competent to produce the fissures, may they not also give rise to the eruptions through the openings which they have originated?"

Many of the various theories which have been proposed to account for volcanic action—depending upon the supposed presence of active forces within the earth; upon the contact of water with the hot solid or liquid matter of the interior of the globe; upon the chemical actions that may be taking place within the earth; upon the heat that may be developed by the contraction of the earth's crust; or upon the occlusion of gases by the metallic elements of which many suppose the core of the earth to be composed—have a certain probability. They