olve rupture surfaces that are either planar or gently undulatory. Such surfaces, for example, may involve pre-existing discontinuities (e.g., bedding planes, faults, or joints). In contrast, slumping occurs along rupture surfaces that are curved concavely upward. This movement typically involves fragment rotation and a greater amount of internal deformation. Concave-upward rupture surfaces usually occur within homogenous sedimentary formations (Campbell et al. 1-27).
Thirdly, subaerial debris flows typically consist of rapid and watery sediment movement. Sand flows, mud flows, or mud slides often occur in arid or semiarid environments. Intense storms or rapidly melting winter snow in these regions may impart mobility to accumulated sediment. The flows have "internal differential movements that are distributed throughout their mass (Campbell et al. 1-17). In fact, such differential movements can be intergranular; visible shear surfaces may not even form. When closely spaced shear surfaces do occur they may be transient, and often disappear without leaving any evidence. The majority of debris flows consist of "poorly sorted material with cobbles and boulders embedded in a fine muddy matrix (Blatt et al. 186)." Varnes (1978) defines a mud flow as a "very slow to very rapid, wet flow of cohesive or noncohesive earth material composed of at least 50 percent grains smaller than 2 mm (Campbell et al. 14)." However, the terms "debris flow" and "mud flow" are also commonly used to refer to flowing slurries. In this sense, the terms relate to flow and depositional characteristics rather than composition. Finally, mud slides consist of slow-moving masses of "softened clayey material" which slide along discrete basal and lateral boundary shear surfaces.
Those slope displacements which exhibit more than one type of movement are called complex. In reality, practically every landslide is complex to a certain extent. Flowin...