Origins of rock columns and corestones of granitoids and their contributions to landslides

Tors or rock columns and corestones are well known as characteristic topography of granitoids; most of them are made in the ground and subsequently exposed by the exhumation of the materials of weathering, which proceeds along joint sets. The primary joint sets of granitoids have long been believed to be orthogonal since the pioneer work of Cloos (1921, 1922). However, such studies are based on the observations from limited angles mainly from the ground surface, so 3-D morphology of joint sets has been poorly characterized. On the other hand, recently developed analytical technique using unmanned air vehicle and photogrammetric software, give us completely different configurations and find that columnar joints are common in granitoids as well as volcanic rocks like basalt. We found granitoid bodies that have columnar joints near its roof because of rapid cooling down. Exposure of rock columns separated by columnar joints form high unstable rock towers or tors, which are susceptible to rockfalls. Rock columns separated by columnar joints tend to be spheroidally weathered to form corestones surrounded by saprolite under the ground. 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.