Volcanoes: What They are and What They Teach

by John W. Judd

As the igneous activity of the district declines, the temperature of the issuing gases and waters diminishes with it, until at last the volcanic forces appear to wholly abandon that region and to be transferred to another.

Yet even after all or nearly all indications of the volcanic agencies cease to make themselves visible at the surface, occasional tremblings of the earth's crust show that perfect equilibrium has not been restored below, but that movements are taking place which result in shocks that are transmitted through the overlying and surrounding rock-masses as earthquake vibrations.

NATURE OF VOLCANIC CYCLES.

Such is the cycle of changes which appears to take place at each district of the earth's surface, as it successively becomes the scene of volcanic activity.

The invasion of any particular area of the earth's surface by the volcanic forces appears to be heralded by subterranean shocks causing earthquake vibrations. Presently the origination of fissures is indicated by the rise of saline and thermal springs, and the issuing of carbonic acid and other gases at the surface. As the subterranean activity becomes more pronounced, the temperature of the springs and emitted gases is found to increase, and at last a visible rent is formed at the surface, exposing the incandescent materials below.

From this open fissure which has thus been formed, the gas and vapours imprisoned in the incandescent rock-materials escape with such violence as to disperse the latter in scoriæ and dust, or to cause them to well out in great streams as lava-flows. Usually the action becomes concentrated at one or several points at which the ejected materials accumulate to form volcanic cones.

Sometimes the volcanic activity dies away entirely after these cones are thrown up along the line of fissure, but at others some such centre becomes for a longer or shorter time the habitual vent for the volcanic forces in the district, and by repeated ejections of lavas and fragmentary materials at longer or shorter intervals the cone increases both in height and bulk.

When the height of the cone has grown to a certain extent, it becomes more easy for the volcanic energies below to rend the sides of the cone than to raise the molten materials to its summit. In this way lateral or parasitic cones are thrown up on the flanks of the volcanic mountain, the mass being alternately elevated and strengthened by the ejections from the summit and sides respectively.

When the volcanic energies no longer suffice to raise the fluid materials to the summit, nor to rend the sides of the volcano, fissures with small cones may be formed in the plains around the great central volcano.

At last, however, this energy diminishes so far that rock materials can no longer be forced to the surface, the fissures become sealed up by consolidating lava, and the volcanic cones fall into a condition of extinction and decay.