[H-DS10] Geohazards in humid, tectonically active countries and their precursors

[HDS10-01] Weathering styles and landslide types of granite

*Masahiro Chigira¹, Yasuto Hirata² (1.Fukada Geological Institute, 2.Central Research Institute of Electric Power Industry)

Keywords:Granite, Weatheirng, Landslide, Spheroidal weathering, Micro-sheeting

Weathered granite areas have suffered from many landslides induced by rain storms: For example, Hiroshima rainstorm disasters in 1999, 2014, and 2018, and southern Miyagi rainstorm disaster induced by typhoon 19 in 2019. The basic causes of these landslides were in two types of weathering styles: micro-sheeting and spheroidal weathering. Micro-sheeting forms loosened soil layers on slope surfaces, which fail during rainstorms; and spheroidal weathering forms corestones, which are destabilized when the intervening gruss is eroded in the ground. These two types of weathering have been known, but the reasons why were not known. Topographically, granite forms characteristic features like tors and boulder fields, but the reasons why have not been known. We investigated granite with high angle joints using UAV and SfM and found that columnar joints develop in granite body in 10 places. Many of these areas have tors, which we found were surrounded by columnar joints. We also investigated the boulder field at Kui, Hiroshima prefecture, and found that many columnar prims are included in the field as well as rounded corestones, which suggests that the boulder field is made through the collapse of rock columns weathered in the ground. Columnar joints of granite are probably made in the upper marginal part of a granite body near the surrounding rocks and not in the depths, in which granite is massive rock and later experiences sheeting and micro-sheeting. Such a history suggests that the fate of granite body after being exposed to the ground surface is already determined when it is emplaced in the depths.