In order to and mechanical relations between caldera form and regional stress field, we approximated collapse of a magma chamber by contraction of a small sphere in the elastic medium and estimated distribution of the plastic and/or rupturing area due to the contraction of the small sphere by the Coulomb yield criterion under an assumption of the elastic-perfectly-plastic material. As a result, we found that the plastic area (caldera) showed a circle distribution on the surface when the regional stress field was not given or the isotropic regional stress field was given On the other hand, when the anisotropic regional stress field was given, we found that the caldera enlarged toward the direction of the maximum compression axis (or the minimum extension axis), so that it showed an elliptic outline on the surface.

Key words:
collapse caldera, caldera form, regional stress, magma chamber