Triggered by severe local rainfall in September, 1983, a large number of landslides occurred on slopes in the southern foot of the somma of Kuttara Caldera, SW Hokkaido. On the basis of slope movement features, they were classified mainly into two types: slump and debris-slide. Almost all the slump type landslides took place on slopes within a rock alteration zone in the southwestern part of the foot; whereas, the debris-slide type landslides showed a scattered distribution in the study area. Tuff breccia, parent rock in the alteration zone, was intensely altered to clay hydrothermally.
The alteration increased with increasing depth and is characterized by considerable amounts of montmorillonite and sulfides. The slipping surfaces of landslides of this type were found in the depth of tuff breccia formation. On the other hand, the slipping surfaces of debris-slide type landslides were found in a clayey crust that was formed by weathering of the surface part of the tuff breccia formation. The clayey crust is of several tens of centimeters in thickness and is composed mainly of halloysite and/or metahalloysite. It is likely that the degree and process of rock alteration played a very important role in controlling the type and distribution of landslides in the study area.