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Page:Popular Science Monthly Volume 18.djvu/797

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ORIGIN AND STRUCTURE OF VOLCANIC CONES.

different origins, were there not existing intermediate ones which demonstrate that they are all varied modifications of one almost uniform series of physical effects.

Thus to one looking first at the vast volcanic cone of Cotopaxi, almost perfect in form, and comparing it with the ring-like cavity of Astroni in the Phlegrean field, it would be almost incomprehensible to believe that these two extremes are the result of identical forces acting much in the same manner and producing such widely different effects. But in the latter district we have not to travel far to find other vents that act as interpreters in explaining these variations of forms. In the present paper it will be my endeavor to explain the building up of what we will call a normal volcanic cone, and then afterward to point out the extreme variations to which such a mass is liable.

Given a large volume of heated vapors and liquid rock that has burst its way upward through the subjacent strata, in what way will it manifest its presence, and what traces will it leave behind? This vapor does not seem to exist separately from the molten rock or lava at any great depth, but as it approaches the surface the enormous pressure is reduced, the water and other gaseous matter expand, separate themselves into little bubbles scattered through the highly heated liquid magma. These will collect, to a certain extent, and from their lightness will float to the surface of the lava and there burst. The vapor may have commenced to form at great depths, and in its upward journey have become exceedingly bulky, so when it reaches the surface it would escape with a loud explosion. If we watch lava in the crater of a volcano in a quiescent state, such as Vesuvius, we see these great bubbles, so to speak, continually forming and bursting. As they burst, the surface of the vesicle is blown up as soft, pasty fragments, to the height of many feet. These masses appear black by day, but red-hot by night; they may cool or not, before falling; if the latter, when they strike the ground, they adapt themselves to the irregularities of the surface, and form, as it were, a cast thereof. This condition is much exaggerated at the first outbreak of an eruption; the vast column of fragments often reaches an altitude of two and three thousand feet. There the pieces ascending meet those descending, and so there is a continual grinding going on between them; the fine dust is taken by the wind and transported often many miles, forming the so-called clouds of volcanic ash. The larger fragments (or lapilli, as they are named) may again fall back into the opening or around its edge, thus building up an annular bank. This is really the foundation of the cone.

If we speculate for a moment on the formation of such a heap, we shall see that the first strata deposited will be horizontal, but somewhat thicker toward the axis of explosions. (See D, diagram.) This, however, as the action continues, will begin to arrange itself in a