[SVC47-09] Stratigraphy of Aso pre-caldera stage lavas distributed at the western edge of the caldera wall
Keywords:Aso volcano, caldera-forming eruption, pre-caldera stage, stratigraphy, whole-rock composition
The lava stratigraphy of the Shinsho district is, from the lowest order, high alumina basalt, andesite, and trachy dacite, which are covered by Aso-1 pyroclastic flow deposits. The top of the steep cliff in Shinsho district forms a flat surface that slopes gently southwest. The direction of the lava continuous to the side is almost parallel to the flat surface.
On the other hand, the lava stratigraphy in Tateno district is andesite, hornblende-andesite, high alumina basalt, and andesite from the lower to upper. We found the gravels of trachy dacite in collapse sediment distributed in the top surface of Tateno district. The lithology of Tateno is similar to that of Shinsho. At the top of Tateno, there is no flat surface confirmed in Shinsho district.
The stratigraphy in sequence of the pre-Aso volcanic rocks distributed on the western part of the Aso caldera is summarized as follows from bottom to top; andesite, hornblende andesite, high alumina basalt, andesite, and trachy dacite. Aso-1 pyroclastic flow deposit covered these lavas. High alumina basalt is effective as key layers in order to establish stratigraphic relationship between Shinsho and Tateno district. The stratigraphy suggests that the near surface lava at the top of the Shinsho district was correlated with the top of the large landslide site of the Aso bridge in Tateno district. It is considered that geological situation of Tateno district is more vulnerable than Shinsho district, and the current topography may have been formed as a result of repeated collapses