526 ROBERT ANDERSON 



SUMMARY 



The caldera of Aso is a great depression at the summit of a low 

 mound-shaped cone in the center of the island Kiushiu, within 22 

 to 35 miles of the sea and only 1,000 to 3,000 feet above it. It has 

 been the center of vast outpourings of lava and fragmental material 

 that have filled a depressed area in the topography of the older forma- 

 tions during Quaternary time. It is one of the largest, if not the 

 largest, of craters known on the earth. 



The caldera has been worn considerably, but the wall has not been 

 removed by erosion at any point, the single barranco being considered 

 as due chiefly to structural weakness or subsequent disruption. 

 The floor has been built up as well as worn down and probably retains 

 fairly well its original level. The caldera appears to date from about 

 middle Quaternary time. 



In the history of the volcano both effusive and explosive eruptions 

 have been characteristic, and have occurred contemporaneously, the 

 amount of material emitted as lava flows having probably been 

 greater. 



The lava is intermediate between andesite and basalt, and is of 

 comparatively easy fusibility. 



There existed formerly a volcanic cone above the site of the present 

 caldera, of which the truncated base is preserved in the outer slopes. 

 The caldera grew as a result of the discharge of great lava flows and 

 the collapse of this cone. The process was probably one of gradual 

 enlargement of a summit crater. 



A high, continuous interior range was later built up by eruptions 

 along a fissure at right angles to the long axis of the caldera; and the 

 cone building still continues, though with diminishing vigor, in a 

 modern crater centrally situated with respect to the old one. 



