Page:Popular Science Monthly Volume 71.djvu/51

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45
THE GREAT JAPANESE VOLCANO ASO

from its outer rim, and of the rugged mountain bulwark in its center, on the summit of which the life of the volcano has been preserved in a far smaller inner crater. It seems inconceivable that processes alone of building-up could have resulted in such forms as those of Aso; and in attempting to outline its history one always reverts to some theory of destructive action on a very large scale.

The large crater of Aso may have been formed in either one of two ways, by the blowing off and away in some cataclysmic explosion, or series of explosions, the whole mass that must once have filled and overlain the present cavity, or by the sinking in of this same mass and its engulfment in a great void produced by the removal of the material that formerly gave support to the earth's surface at this point. A calculation, such as given below, of the mass displaced in either case affords an impressive sense of the magnitude of the task that was accomplished. The roughly-bedded strata in the walls of the big crater seem to dip away on all sides at a low angle, and their slope is probably reflected in the gently inclined surface of the outer plateau that forms the sides of the Aso cone. From the regularity of these slopes it seems likely that they represent the truncated base of an old conical mountain that continued upward with the present slope to a culminating point high above the center of what is now the crater bowl. It is probable that if such a mountain existed its upper portion rose with a gradually increasing slope into a peak, but even with a constant slope such as now exhibited in the base, its height would have been 7,000 to 7,500 feet above the sea, or about 6,000 feet higher than the present crater floor.

It is probable that during the early history of the volcano such a cone was built up by successive eruptions of lava and fragmental material that formed sheets one upon another down the sides and became roughly stratified in conformity with the slope of the mountain; and that before the close of the period of greatest activity of the volcano this cone was beheaded by some disruptive force. Not only was the summit removed, but the very heart of the volcano was opened, leaving a vast bowl on the site of the old eminence surrounded by the truncated lava flows of the outer circle of the mountain's base. Still later, the processes of building up recommencing, a new mountain was constructed, this time not over a single center as seems to have been the case before, but along the line of the short diameter of the former oval mountain, and in this way the present chain of peaks was raised. But the volcano was gradually dying down, and reconstruction on a grand scale ceased long before the new Aso had reached the dimensions of the old, or even effectually obscured, except to casual observation, the nature of its basal wreck.

The volume of the bowl of Aso, not subtracting the space that is taken up by the supposedly subsequent range, is at least nine cubic