A solidified lava lake in an explosion crater within granitic basement, SW Japan

The Pleistocene Tawanouchi basalt of SW Japan formed as a lava lake in an explosion crater within granitic basement. Volcanic activity in the Tawanouchi area started with a phreatomagmatic eruption, based on the lithofacies present near the crater and between the granite basement and the lava. The Tawanouchi basalt lava lake forms an inverted elliptical frustum with upper dimensions of approximately 340 m × 550 m, tapering downward to a depth of more than 170 m. Thermal history was evaluated based on differences in sizes and shapes of columnar joints from the margins of the lava to its interior. The vent lay near the center of the lava mass, based on deformation and trends of columnar joints in lava associated with clastic deposits. Lava repeatedly upwelled and flowed back into the vent. Mg/(Mg + Fe) ratios of some groundmass clinopyroxenes in the basalts overlap those of phenocryst cores, suggesting that less-differentiated lava was injected into more differentiated and semi-solidified lava. Paleomagnetic data show that the basement granite was heated to 530 °C or more 90 cm - 155 cm away from the contact with the lava. Numerical simulation of the thermal effect on the basement granite from the lava indicates that the lava lake with an initial temperature of 1150°C took more than 20 years to cool below 1100 °C.