Slope Stability and Mass Movements

The rugged terrain of the high volcanic islands in the South Pacific is susceptible to landslides, debris flows and other types of mass movements. Many extensive slope failures are activated during severe tropical cyclones. Slope susceptibility to failure is influenced by the predominance of clay-rich soils, normally humic latosols, overlying residual red/orange saprolite. Saprolite is completely-weathered rock and sediments in situ. The regolith (both soil1 and saprolite material) is formed by chemical weathering of the common types of volcanic rocks and volcanic-derived sediments found in the South Pacific islands, especially andesites and basalts, and associated breccias and conglomerates. The regolith often has structural weaknesses inherited from the bedrock and overlying sediments from which it is derived, such as inclined bedding planes, faults and sediment/bedrock junctions. These zones of weakness may develop into shear planes along which the regolith mass fails in a landslide. If a particular area has a history of past landslides, it means that hillslopes are formed on weak residual materials and are prone to further failures.