On the granitic rocks and their collapse at Kamo District, Shimane Prefecture

Abstract

The geology, petrographic characters of the granites, and the types of collapses of the rocks caused by heavy rain fall in the Kamo District, Shimane Prefecture, are described to clarify the relation between rock types and collapsing phenomena in the granite mountain area. The granitic rocks comprise granodiorite and biotite granite, and both are weathered remarkably in most parts. From the genetic point of view, the collapses are classified into six types. 1) Collapse, closely related to the joint structure of the rocks; they occur characteristically at the foot of slopes in the region of weathered granodiorite. 2) Collapse of the weathered zone (uppermost layer covered with vegetation) of slopes, caused by difference of physical properties to the ground water between the weathered zone and the basement rocks. 3) Collapse with characters of (1) and (2). 4) Collapse closely related with dikes in granite. 5) Collapse accompanying landslide in fault breccia zones. 6) Collapse or resliding of the talus deposit, made by an older land slide of the adjoining rock masses. The majority of the collapses in this district belong to types (1), (2) and (3). Types (1) and (2) predominate in granodiorite and (3) in biotite granite.