Mass wasting is the geological process where earth materials move down a slope under the influence of gravity, often referred to as a landslide. This phenomenon involves the destabilization of soil, rock, or debris, leading to downslope transport. Slumps and slides are two frequent and impactful types of mass wasting. While both describe slope failure, they are distinguished by fundamentally different mechanics and resulting landforms, which depend on the shape of the surface along which the failure occurs.
Mass Wasting: The Shared Context
Slumps and slides are categorized as landslides because they share a common driving force and similar triggering factors. The fundamental power behind all mass wasting is gravity, which exerts shear stress on material situated on an inclined surface. This gravitational pull is opposed by the slope material’s shear strength, which is its internal resistance to movement.
A slope remains stable until the downward stress of gravity exceeds the internal strength of the soil or rock mass. Factors that reduce shear strength or increase gravitational load can trigger both slumps and slides. For instance, the saturation of the ground by heavy rainfall or rapid snowmelt adds significant weight to the slope material while simultaneously increasing the pore water pressure, which reduces friction between particles. Steepening a slope through natural erosion or human activities like construction can also remove lateral support, increasing the likelihood of failure.
Defining Rotational Slumps
A rotational slump is a type of mass wasting characterized by the distinct rotational movement of failed material. The material moves downward and outward along a deeply curved, concave-upward rupture surface. This curved surface is often described as spoon-shaped or listric, causing the detached mass to rotate around an axis parallel to the slope.
The rotation of the soil or rock mass creates several recognizable landforms at the surface. At the top of the failure, a steep, near-vertical surface called a head scarp is exposed where the moving mass pulled away from the stable slope. The displaced block often remains relatively intact, but its upper surface tilts backward, creating a depression that may collect water. At the bottom, or toe, of the slump, the material is pushed outward and upward, forming a hummocky, irregular ground surface.
Defining Translational Slides and Their Mechanics
In contrast to the rotational motion of a slump, a translational slide involves the downslope movement of material along a relatively flat or gently undulating surface. The motion is primarily sliding, where the mass moves parallel to the slope without significant internal rotation. This movement is analogous to a rigid block sliding down a ramp.
The failure surface in a translational slide is not typically a newly formed, deep-seated curve; instead, it is often a pre-existing plane of weakness within the underlying geology. These planes can include discontinuities such as bedding planes (boundaries between sedimentary rock layers) or structural features like joints and faults. Because the moving mass is generally thin relative to its length and slides along a structural weakness, the material can travel long distances and often remains cohesive.
The Geometric Difference in Failure Surfaces
The fundamental difference between slumps and translational slides lies in the geometry of their failure surfaces, which dictates the type of movement. Slumps are defined by a curved and concave rupture surface, causing the mass to undergo rotational movement. This deep, curved failure surface is common in thick, relatively homogeneous materials like unconsolidated sediment or soft soil.
Translational slides, conversely, are defined by a planar or nearly planar failure surface. This straight geometry forces the overlying material to move in a more uniform, gliding motion. The planar surface often represents a boundary between different geological units or a zone of low shear strength, such as the interface between bedrock and overlying soil. Ultimately, the shape of the surface of rupture determines the resulting movement pattern, with a curved surface yielding rotation and a flat surface causing translation.