Cooling joints of granite as a structural cause of the tors and boulder fields of granite

Typical cooling joints of granite have been believed to be orthogonal, and characteristic topography of granitic rocks like tors and boulder fields are interpreted in combination with the cooling joints and weathering. However, most of the previous studies were performed by the observation on the ground, and the 3D observation of cooling joints and the topographic features was not sufficient. We observed tors and boulder fields of granitic rocks using UAV and 3D modelling and found that columnar joints are typical for the granite that forms tors and that boulder fields are the accumulations of rock columns as well as boulders made by the spheroidal weathering of rock columns. Tors we observed were Mt. Kinabalu of Borneo, Mt. Mizugaki, Mt. Jizo, Mt. Gozaisho, Mt. Konze and 5 other locations in Japan. We observed that tors consist of polygonal rock columns with undulating joints, more irregularly shaped than the columnar joints of volcanic rocks. The cross-sectional areas of rock columns varied from 1 to 130 m^2, much larger than typical rock columns of volcanic rocks. The rock columns of granite are typically polygonal dipyramids, of which shapes may be dependent on the cooling history of granite. Boulder fields we observed was the Kui boulder fields in Hiroshima. We found that the boulder field is the accumulation of prismatic rock columns as well as rounded rock boulders. The prismatic rock columns had basal cross-sectional areas of 0.8 m^2 on average. The rock columns had chamfering cracks at corners, which are assumed to be made during cooling and to form preliminary outlines of core stones. Core stones had surface crusts or rindlets, which exfoliate and leave more rounded core stones.

  Rainstorm-induced landslides of weathered granite reflect weathering styles of granite: Landslides that occurred recently in Japan had three types, landslides of loosened layers of decomposed granite (or micro-sheeted granite), landslides of core-stone bearing materials, and landslides of saprolite. Landslides with core-stones were particularly destructive because of their inertia. Potential sites of such landslides could be predicted using columnar joints in fresh rocks as a clue.