THE VOLCANO ASO AND ITS LARGE CALDERA 



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ORIGIN OF THE CALDERA 



There are three ways in which the old crater may be conceived 

 as having originated, namely, by being built up around a vent of 

 great magnitude, by the explosive removal of a volcanic mountain 

 mass, or by the subsidence of the area now inclosed within the wall. 



The most acceptable conclusion is that the bowl of Aso is a caldera 

 produced by the sinking in of a volcano, as the result of the escape of 



Fig. II. — Hot springs at Yunotani near the western end of the central range. 

 There is said to be a small geyser here that spouts out boiling water and red mud. 



lava from an underlying reservoir at low points of discharge and the 

 consequent undermining of the mountain iianks. The process was 

 probably one of the gradual enlargement of an original summit 

 crater. 



There are three main reasons why this supposition seems to apply. 

 In the first place, the bowl has a wide extent of almost level floor, 

 a symmetrical oval shape, and a practically continuous ring wall of 

 regular form. It is difficult to conceive of a violent explosion destroy- 



