Abstract
In Yakushima, granitic and sedimentary boulders are cracked polygonally on their surface. They are called “Kikkouboulders”, which mean polygonal pattern in Japan. The size of the polygonal pattern is proportional to the size of the Kikkou-boulder, therefore bigger boulder has bigger polygonal pattern. The Kikkou-boulders show a concentric crust and core structure. The depth of the polygonal cracking reaches the boundary between the crust and core structure. The characteristics of the Kikkou-boulders indicate that they are core-stones, which are ellipsoidal blocks separated from bedrock by the cause of weathering. It is also suggested that the polygonal cracking is generated from the weathering of the rock.
