Journal of The School of Marine Science and Technology,Vol.6 No.2
Control factors on initial caldera geometry；
quantitative estimation by numerical simulation
Shigekazu KUSUMOTO and Keiji TAKEMURA
Abstract
In this study we demonstrate that the important factors that control initial caldera geometry are the radius and depth of the magma chamber and the elastic constants of the crust. We approximate the collapse of the magma chamber by the contraction of a small sphere (point source) in an elastic medium, and calculate the surface distribution of the rupture area using the Coulomb failure criterion under the assumption of an elastic-perfectly plastic material. As a result, it was shown that the funnel and piston calderas can be formed by the same collapse mechanism and physical processes, and the geometry of the caldera is controlled by the depth and radius of the magma chamber and the proportionality coefficient in the linear relationship between the caldera radius and the magma chamber depth.In the case that values of Poisson’s ratio,Young’s modulus,compressive strength and angle of internal friction are given in the wide range covering almost all igneous rocks, it was found that the coefficient is strongly dependent on the Poisson’s ratio.In addition,it was found that the volume change required for the caldera formation is described by a third-power polynomial expression of the depth of the magma chamber. Although the physical meaning of each coefficient is unknown at present,it was shown that the third degree of the polynomial is the most important term.