| Abstract: | The sequence of formation of the Motojuku-type cauldron (Fujita et al., 1970) is summarized as follows: (1) doming due to ascent of a magma body; (2) development of normal faults which produce a polygonal cauldron; and (3) eruption of acidic to intermediate pyroclastic material. Model experiments on a scale of 1/200,000 reveal that ascent of the magma body, which was imitated by a ball of hardened putty, produced a polygonal cauldron composed of radial and concentrically arranged short fractures on the roof of a dome. No ring fractures were formed. This type of cauldron develops near the surface. On the other hand, emission of magma which was imitated by evaporation of a ball of dry ice in brittle powdered material, caused ring faults dipping outward and a circular cauldron without up- or downwarping. This type of cauldron develops upward from the magma body. The former model is equated to the Motojuku-type cauldron, and the latter model to the other types of cauldrons with ring fractures. |
