Crustal structures that produce eruptions of welded tuff and formation of calderas

Abstract

Accumulation of a huge amount of welded tuff and subsequent formation of calderas are typical examples of volcanisms on a large scale. Crustal structures which are related to the phenomena may be a key for the study of volcanisms on the earth. In Japan there are some 10 calderas of low gravity anomaly type, large and small in diameter, and accordingly in volume of their ejecta, welded tuff. The low gravity anomalies are very characteristic inside the calderas and just concentric with their rims, and are due to mass deficiency caused by eruptions of a huge amount of ejecta. The author (1957), hitherto, has clarified the subterranean structures resulting from the caldera formations by a gravimetric method. In this paper, the author describes the crustal structures beneath Sikotu Caldera in Hokkaido, Japan, deduced from the gravimetric, geomagnetic, seismic and geologic surveys. And referring several examples of the subterranean structures of the other calderas in Japan, the author wishes to discuss the crustal structures that would produce the caldera formations.