Numerical Experiments on the Spacing of Cauldrons During Miocene Time in Japan

Abstract

The Motojuku-type cauldrons of the Miocene, which are characterized by volcanic eruption after collapse, tend to have been produced in a cluster. The distance between adjacent cauldrons ranges from several km to 30 km and is 20 km on an average. Regularly spaced cauldrons suggest the dominant wavelength caused at the boundary between overburden and a partially melted low density buoyant layer, due to gravity instability. The relation between viscosity ratio of the buoyant layer to the overburden and the thickness of the layer was calculated using Ramberg's equation of gravity instability involving two viscous media resting on rigid substratum when the dominant wavelength equals 20 km. Various thickness ratios of overburden to buoyant layer (0.2, 0.5, 1.0, 2.0, 5.0 and 10.0) were tested. The thickness of the buoyant layer was estimated to be less than about 6 km irrespective of the thickness of the overburden.