Distribution Recognition of Lava Flows at the Southern Foot of Nakadake and Takadake Volcanoes in Aso Caldera, SW Japan

The Nangodani Valley is part of the caldera floor at the southern foot of Nakadake, a post-caldera volcano of Aso Volcano, SW Japan. In this valley, lava flows from the Nakadake and Takadake volcanoes are widely distributed. However, their exact distribution was not known because they were covered by fan deposits and mantle-betting tephra layers. In this study, we used various topographic maps and aerial photographs provided by the Geospatial Information Authority of Japan (GSI). Although outcrops of the lava itself are scarce, we have succeeded in estimating the distribution of lava flows by carefully observing the topography, especially the flow paths of rivers. The marginal cliffs of lavas are not easily eroded and the natural flow paths from the volcanic slopes are blocked. In fact, we found outcrops of lava were outside the previous distribution area of the lava. This fact indicates that the lava is likely to be distributed in a wider area than the previously indicated distribution area. As for the outflow area of the lava, it was confirmed from the microscope observation that the lava flowed at the point which was not considered as the outflow area of Takadake lava. The distribution of the Nakadake old-edifice lava is partly hidden by the Takadake lava, which flows around it to the vicinity of the Takamori lava. Therefore, the Nakadake and Takadake lava flowed almost at the same time, but the Takadake lava is considered to have flowed slightly after the Nakadake. There are many natural springs in Nangodani Valley, which is consistent with the distribution of the lava revealed in this study.