An obelisk-shaped granitoid tower at Mt. Jizogadake in the Southern Alps of Japan: A 3-D morphological study

Abstract

Well-developed tors and associated boulder fields are present in exposed granitoid pluton, but their geomorphological origins have not been fully addressed. Although the tor-like features are commonly attributed to the orthogonal joint system along which weathering proceeds and the weathering materials are removed, the majority of previous studies lack three-dimensional (3-D) morphological observations. One of the well-known granitoid rock towers in Japan, namely, the obelisk at Mt. Jizogadake in the Japanese Southern Alps, was investigated using unmanned air vehicles (UAVs). A 3-D observation of the granitoid obelisk found that it is shaped by columnar joints with an undulation along a long axis and irregularly shaped cross-sections. Because of these features and intersection angles between the rock columns and exposure surfaces, the obelisk exhibits a peculiar morphology appearing as a heap of rock blocks when observed from the ground. The columnar jointing might have occurred in a marginal zone of the granitoid pluton due to faster cooling rates. The 3-D observations also confirmed the columnar joints formation was predated by high-angle, planar joints. The columnar joints divide the rock mass between those planar joints. This may imply that the high-angle joints would have formed probably during cooling under the presence of tectonic and thermal stresses.