Rock pillars shaped by columnar joints in granite at Mt. Mizugaki,Central Japan

Rock pillars of granite form a special type of characteristic topography in granite, but their origin is not well understood, partly because their 3D morphology has never been well characterized. Rock pillars were investigated at Mt. Mizugaki, which is underlain by Neogene granite that intruded into the Cretaceous accretionary complex in central Japan. Three rock pillars and their surrounding areas were investigated in detail using unmanned aerial vehicle (UAV) images and structure from motion (SfM). The rock pillars were up to 70 m high and were shaped by columnar joints, which has hardly been reported before in granitic rocks. Columnar joints in granite are much larger than columnar joints in volcanic rocks. The columnar joints curved vertically, and some of them showed dipyramid shapes. The large size and curved morphology of the columnar joints may be the result of the slow cooling of the granite, compared to that of volcanic rocks. Thus, the columnar joints at Mt. Mizugaki made large, high rock pillars, which are not common in volcanic rocks.     

A zone of columnar joints beneath the roof of a granitic pluton: The Okueyama granite,southwestern Japan

Igneous rocks are fractured during cooling from magma to form cooling joints, which are typically columnar joints in volcanic rocks, while orthogonal joints are considered typical for plutonic rocks. We performed a 3D study of joint systems in a granitic batholith of the Okueyama granite in western Japan, which has its roof and its internal structures from the roof to 1000 m downward exposed. We used an unmanned aerial vehicle (UAV) to observe the joints in outcrops from various angles. Based on our study, we propose a schematic model for joint systems in a granitic pluton. A granitic pluton has zones of rock columns below the roof and next to the wall. The rock column zone below the roof is as thick as 300 m, and its higher portions form steep cliffs, probably because of increased resistance to weathering. The axes of the rock columns are nearly vertical below the roof and gently plunge next to the walls, with high intersection angles with the wall. The distribution of columnar joints near only the roof and walls suggests that the granite cooled more rapidly near the roof and walls than in the core of the pluton. When the granite was jointed by parallel joints during cooling, the rock slabs between the parallel joints near the roof and the walls are subdivided into columns with polygonal cross-sections. This suggests that the granite was fractured by parallel joints at a temperature immediately below the solidus, after which the rock slabs were subdivided into rock columns during further cooling.