Polygonal Cracked Granitic Boulder in Yakushima, Japan: Pore Size Distribution and Genesis of the Boulder

Abstract

Polygonal cracked “Kikkou” boulders are distributed in Yakushima, southern Japan. Polygonal cracks are propagated on the surface of granitic boulders. On the section of the boulder, there is a concentric structure composed of core and crust parts. In addition, polygonal cracks are propagated from the surface to the boundary of them.

The boulders are measured by Mercury Intrusion Porosimetry（MIP）, and the followings are clarified.

1：The porosity of core part is larger than that of crust part.

2：The peak pore size of core part is larger than that of crust part.

Weathered and non-weathered granitic specimens which have been sampled from outcrops and non-polygonal cracked boulders are also measured by MIP. Pore size distributions of core and crust parts in the granitic boulders are different from the ones of weathered and non-weathered granitic rocks. It is inferred that the genesis of the polygonal cracked boulders is related to alteration in boulders, from the view point of porosity structure and microscopic observations.