Volcanoes: What They are and What They Teach

by John W. Judd

Some recent writers have, it is true, endeavoured to draw a distinction between what they call 'fissure-eruptions,' and eruptions taking place from volcanic cones. But all volcanic outbursts are truly 'fissure-eruptions'—the subterranean materials finding their way to the surface through great cracks, which, in a more or less vertical position, traverse the overlying rock-masses. It is true that in many cases portions of these cracks soon get choked up, while other portions become widened, and the volcanic energy is concentrated at such spots. Thus the materials ejected from these fissures are usually emitted in greatest quantities at one or more points along the fissure, and a single great volcanic vent, or a row of smaller vents, is established upon the line at which the fissure reaches the surface.

We have seen that the amount of explosive action taking place at different volcanic vents varies according to the proportion of imprisoned water contained in the lava. In the cases where there is much explosive action, vast accumulations of scoriæ, lapilli, and dust take place, and cones of great size are built up; but in those cases where the explosive action is small the lavas flow quietly from the vent, and only small scoriæ-cones are thrown up, these being probably soon swept away by the lava-currents themselves or by denuding agencies. But both kinds of eruption have equal claims to be called 'fissure-eruptions.'

FORMATION OF VOLCANIC FISSURES.

In the expansive force of great masses of imprisoned vapour, we have a competent cause for the production of the fissures through which volcanic outbursts take place. Such fissures are found traversing the rocks lying above volcanic foci, and often extending to distances of many miles, or even hundreds of miles, from the centres of activity. Some of these cracks are found to be injected with fused materials from below, others have been more or less completely filled with various minerals that have been volatilized, or carried by superheated waters from the deeper regions of the earth's crust. That many of the cracks thus produced in the superjacent rocks, by the heaving forces of imprisoned vapour seeking to escape, never reached the surface, we have sufficient proof in many mining regions.

If we now transfer our attention from the deeper portions of the earth's crust to the surface, we can well understand how the attempts of the imprisoned vapours to force a passage for themselves through the solid rock-masses would lead to shocks and jars among the latter. Each of these shocks or jars would give rise, in the surrounding portions of the earth's crust, to those vibrations which we know as earthquakes. The close connection between most earthquakes and volcanic phenomena is a fact that does not admit of the smallest doubt; and though it would be rash to define all earthquakes as 'uncompleted efforts to establish a volcano,' yet, in the efforts of the repressed subterranean forces to find a vent by the production of fissures in the overlying rock-masses, we have a cause competent to the production of those shocks which are transmitted to such enormous distances as waves of elastic compression.

We have seen that the production of the fissure upon which the small volcano of Monte Nuovo was thrown up was preceded by a succession of earthquakes, which for a period of over two years terrified the inhabitants of the district, and might have warned them of the coming event. In the same manner, doubtless, the period before the appearance of volcanic phenomena in a new area would be marked by powerful subterranean disturbances within it, due to the efforts of the imprisoned vapours to force for themselves a channel to the surface.

NATURE OF FIRST EJECTIONS FROM FISSURES.

In the case of Monte Nuovo, we have seen that the fissure, when produced, emitted water—at first in a cold, then in a boiling condition—and, eventually, steam and scoriæ. It is probable that through the first cracks which reached the surface, during the heaving of the subterranean forces, water, charged with carbonic acid, flowed abundantly, and that these cold springs, charged with carbonic acid and carbonate of lime, would be succeeded by others which were hot and contained silica in solution. In Hungary, the Western Isles of Scotland, and many other volcanic districts, we find abundant evidence that, before the eruption of lavas in the area, great masses of travertine and siliceous sinter were formed by the action of cold and hot springs.

As the volcanic action became more intense by the more perfect opening of the fissures, the evolution of carbonic add gas would be succeeded by the appearance of sulphurous acid, sulphuretted hydrogen, boracic acid, and hydrochloric acid, which recent studies have shown to be successively emitted from volcanic vents as the temperature within them rises. At last lava or molten rock becomes visible within the fissures, and the ejection of the frothy masses—scoriæ, pumice, lapilli and dust—commences, and this is sometimes succeeded by the outflow of currents of lava.

That volcanoes originate upon lines of fissure in the earth's crust we have the most convincing proofs. Not only have such fissures been seen in actual course of formation at Vesuvius, Etna, and other active volcanoes, but a study of the volcanoes dissected by denudation affords the most convincing evidence of the same fact. The remarkable linear arrangement seen in groups of volcanoes, which is conspicuous to the most superficial observer, confirms this conclusion.