The Kui boulder fields: Formation processes controlled by columnar joints of granodiorite

Abstract

Boulder fields of granitic rocks have long been believed to be the result of subsurface spheroidal weathering and subsequent erosion of weathering products, such as saprolite, and the exhumation of corestones. However, the morphological characterization of boulder fields has not been sufficient to examine the details of their origin. We performed a 3-D morphological investigation of boulder fields using UAV and SfM for the Kui boulder field, Hiroshima, western Japan, and found that the boulder field consists of polygonal rock columns, as well as rounded corestones. Some of these rock columns had chamfering cracks making them primary corestones. An outcrop of the weathering profile near the boulder field showed that corestones covered by weathering rindlets were embedded in saprolite surrounded by a polygon that was inherited from the columnar joints. The saprolite was locally removed by subsurface erosion to leave openings, which eventually led to the collapse of the corestone framework. Thus, the columnar joints provided basic conditions for the boulder field formation.