Hiroaki Komuro

Experiments on cauldron formation: A polygonal cauldron and ring fractures

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.

Numerical and experimental models on the formation mechanism of collapse basins during the Green Tuff orogenesis of Japan

AbstractThe geological model about volcanism of the Green Tuff geosyncline deduced from the field observations consists of the following processes:1)Dome-shaped uplift with a mean diameter of 30 km.2)Collapse of the central part of the domes forming basins with a mean diameter of 10 km.3)Volcanic activity inside the collapse basins. It is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust. In finite element analyses performed as plane strain problems, earths crust is assumed to be an elasto-plastic homogeneous layer and to undergo sinusoidal vertical displacement at the base of the layer due to an ascending magma reservoir. These analyses reveal that the diameter of the dome is proportional to the depth of the magma reservoir rather than to its size. The magma reservoir is estimated at 12 ∼ 24 km in depth.Scale model experiments using powdered material were performed in order to reproduce a collapse basin. These three-dimensional models are reduced to a scale of 1:200,000 th of the natural size. The results of experiments show that radial and concentric cracks are produced on top of the dome and a central part encircled by concentric cracks collapses to form a basin. The boundary of the collapsed portion forms a steep cliff with a height of about 2 mm. This is equivalent to 400 m in natural size and is nearly similar to field observations.

Paleomagnetism and fission-track ages of Oki-Dogo Island in Southwest Japan

Zircon fission-track (F.T.) ages and paleomagnetic directions were measured on the Neogene series of the Oki-Dogo Island. The results of F.T. dating on the the Tokibariyama Formation show ages of 23.7 ± 1.3 and 20.0 ± 2.3 Ma, indicating the early Early Miocene. Paleomagnetic measurements of the Tokibariyama, Kori Formations (24–17 Ma) and the Oki Trachyte-Rhyolite (6.8 Ma) were done to reveal the movement of Oki-Dogo Island during the Japan Sea opening. The Tokibariyama and Kori Formations show an average declination of 39.9°. The Oki Trachyte-Rhyolite has the same average geomagnetic direction as the present day value. These results show that a clockwise rotation of 40° ± 15° of the Oki-Dogo Island occurred between 17 and 6.8 Ma which is similar to the main land of southwest Japan.