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.

Granite rock towers shaped by mesh‐like joint sets, which formed in the shallower portion of a granite body during cooling at depth

“…The purpose of this study is to characterize the Tengu rock, which consists of rock towers and corestones surrounded by systematic mesh‐like high‐angle joints in Hiroshima, using UAV and SfM (Figure 1). Such a combination is different from the columnar joints of granitoids with irregularly shaped polygonal cross‐sections reported by Chigira (2021, 2022), and this combination should have a special history, which is discussed in this paper.…”

“…Chigira (2021) and Chigira (2022) investigated rock towers of Miocene granite on Mt. Mizugaki and Mt.…”

“…Instead, the recently developed unmanned air vehicle (UAV) and image processing software structure from motion (SfM) have been successfully used in geomorphological and geological studies (Bemis et al, 2014; James & Robson, 2012; Kasprzak et al, 2018). Using these techniques, some granitoid rock towers have been demonstrated to be shaped by columnar joints (Chigira, 2021; Chigira, 2022) with irregularly shaped polygonal cross‐sections; these columnar joints separate rock bodies between parallel joints, which formed during an early stage of cooling according to the tectonic stress and thermal stress. Such a sequence, however, should be dependent on the stress history and cooling history of a granitoid pluton, and other cases should exist.…”