|Fig. 9. One of the most active volcanoes of Japan, Kirishima-yama in Southern From its summit, which is 6,000 feet high, may be seen Aso-san 70 miles away to the north.|
miles. The mass that must once have overlain it, measured as the cone formed by the upward projection of the outer slopes, was at least 28 cubic miles in volume. Thus there must have been removed no less than 37 cubic miles, or about five and a half millions of millions of cubic feet of volcanic rock, a mass equal to over two and a half mountains like Vesuvius.
Furthermore the likelihood that the cone steepened toward its summit makes it possible that the old mountain was of greater size than estimated.
If we conceive of such a vast block of the earth's surface being blown up by some terrific explosion within the volcano, it is natural to suppose that great irregular deposits of the erupted material would be in evidence round the outside of the pit. There are immense areas of volcanic debris that have settled after being blown into the air, whole hills in places, within a radius of many miles of Aso. But these deposits seem to be regularly bedded and not to exhibit the rough and tumble structure that would probably result from their being tumultuously cast up by such a great explosion, and they do not form a rim around the crater rising above the old slopes of the cone. And further the walls of the pit seem to be too regular to have been explosively broken.
More acceptable appears the theory that the Aso crater is a sunken pit. A volcano of such magnitude must certainly have been underlain at some unknown depth by a large body of molten rock, the source of the lava that built up the cone. With all the weight exerted upon it by the overlying rocks and the pressure of steam from within, this fluid or viscous, intensely-heated mass must have sought violently for