カルデラの構造と成因

抄録

We can not have any concrete discussions on the origin of calderas without consideration for their present structures. Hitherto, many theories about their origin have been proposed mainly from the standpoints of surface geology and petrology. In this paper, first, the results of geophysical studies of the structures of several calderas in Japan are referred; The gravimetric method proves to be effective for that purpose because calderas were formed with the movements of a huge amount of volcanic ejecta. A discussion is tried on drawing a balance-sheet of caldera ejecta and it supports the conclusion that the deposits within the calderas consist of fall-backs of lithic fragments and also volcanic ejecta. In fact, this is verified by the core samples of a 1,000 meters deep boring dug at the middle of Kuttyaro Caldera, Hokkaido, which is the deepest boring within calderas at present as far as the writer knows. Next, the author refers the major volcano-tectonic depressions in New Zealand which are similar to calderas in their subterranean structures and ejecta, and suggests that they differ only in the configurations of the vents, central or linear and the magnitudes of eruptibilities. Volcanic activities would depend on the regional geologic structures. Judging from the structures of the calderas mentioned in this paper, we have no cogent reason to assume shallow "magma reservoir" immediately beneath the calderas of low gravity anomaly type. Location of magma reservoirs has an important bearing on origin and mechanism of caldera formation.