Primary structures of granite and following weathering schemes define landslide behavior in granite areas.

Granite is distributed all over the world and one of the rock types that are very susceptible to various kinds of mass movements including rockfall, rock slide, debris slide and debris avalanche. For example in Japan, Hiroshima rainstorm disasters in 1999, 2014, and 2018, and southern Miyagi rainstorm disaster induced by typhoon 19 in 2019. This is because its special characteristics of formative processes and weathering behavior. The primary structures of granite have long been believed as orthogonal cooling joints since the pioneer work of Cloos (1921, 1922), but we found that a granite body has columnar joints near its roof using UAV and SfM. Whether granite has columnar joints or not leads to different mass movement types. Rock columns separated by columnar joints form high unstable rock towers or tors, which are susceptible to rockfalls. When rock columns are weathered under the ground, they form boulders surrounded by saprolite; when they are eroded to form hills they frequently fail during rainstorms and transform to debris avalanche or debris flow with high destructive potential because of large mass of boulders. Granite without columnar joints is not suitable for spheroidal weathering but is sheeted by unloading; sheeting forms dip slopes, on which rock slides occur. Some granite is micro-sheeted by unloading and micro-sheeted granite is weathered to form a loose soil layer beneath slope surfaces. Such soil layers are very prone to heavy rainfalls and frequently slide, transforming debris avalanches and debris flows.

Primary structures of granite and following weathering schemes thus define landslide behavior in granite areas.