Landslides and the weathering of granitic rocks

Granitic batholiths around the Pacific Ocean basin provide examples of landslide types that characterize progressive stages of weathering. The stages include (1) fresh rock, (2) corestones, (3) decomposed granitoid, and (4) saprolite. Fresh granitoid is subject to rockfalls, rockslides, and block glides. They are all controlled by factors related to jointing. Smooth surfaces of sheeted fresh granite encourage debris avalanches or debris slides in the overlying material. The corestone phase is characterized by unweathered granitic blocks or boulders within decomposed rock. Hazards at this stage are rockfall avalanches and rolling rocks. Decomposed granitoid is rock that has undergone granular disintegration. Its characteristic failures are debris flows, debris avalanches, and debris slides. Saprolite is residual granitic rock that is vulnerable to rotational slides and slumps. As a granitic rock mass progressively decomposes, the critical slope angle decreases, allowing slope failures throughout its weathering history. Failures in granitic rock are more abundant during the advanced stages of decomposition. Therefore, landslides are most common in the humid tropics, where intense chemical weathering occurs. Identification of the granitoid's weathering stage will help the engineering geologist evaluate the slope-stability hazards of an area.