Mass movement, also known as mass wasting, is a geological process involving the downslope transport of rock, soil, and debris under the direct influence of gravity. This process is responsible for shaping slopes and landscapes across the planet. It is distinct from erosion because the material is not carried away by primary agents like flowing water, wind, or glacial ice. Instead, mass movement involves the bulk transfer of material as a cohesive or semi-cohesive mass. While rapid events like landslides are often the most well-known, mass movement encompasses a wide spectrum of speeds, from instantaneous collapses to movements so slow they are imperceptible over a human lifetime.
Fundamental Driving Forces
The physics of mass movement centers on a competition between two opposing forces within the slope material. Shear stress is the gravitational force component that pulls material parallel to the slope, encouraging downslope movement. The resisting force is called shear strength, which represents the internal resistance of the material, deriving from the friction and cohesive bonds holding the material together. Movement occurs when the downslope shear stress exceeds the material’s shear strength.
Water plays a powerful role in altering this balance, often acting as the immediate trigger for failure. Water saturating the soil and rock adds significant weight, directly increasing shear stress. Simultaneously, water infiltrates pore spaces, creating pore water pressure that pushes soil particles apart. This separation reduces the effective friction and cohesion between grains, lowering the material’s overall shear strength.
Classification by Movement Type and Rate
Mass movements are classified based on the type of material involved, the primary mechanism, and the speed of the downslope motion. These classifications range from rapid free-falls to gradual forms of displacement.
Falls
Falls are the fastest type of mass movement, characterized by the free-fall of rock or debris from a vertical face, such as a cliff. The material detaches along a plane of weakness and descends through the air, resulting in the accumulation of a talus slope at the base.
Slides
Slides involve a mass of material moving along a distinct surface of rupture that separates the moving block from the stable underlying material. Translational slides move along a flat or planar surface. Rotational slides, often called slumps, move along a curved, spoon-shaped failure surface, resulting in a backward tilting of the moving block.
Flows
Flows are movements where the material behaves like a viscous fluid, typically due to saturation with water. Debris flows consist of a mix of water, mud, and coarse material like rocks and boulders, moving rapidly down channels. Mudflows are similar, highly fluid events but involve primarily fine-grained material like silt and clay. These events can be destructive because of their high speed and long runout distance.
Creep
Creep is the slowest form of mass movement, often only measurable over many years, involving the gradual, steady downslope movement of soil or rock. The movement is driven by the cyclic expansion and contraction of the soil, such as through freeze-thaw cycles or wetting and drying. Evidence of creep includes curved tree trunks, tilted fence posts, and small ripples in the soil surface.
Factors Influencing Slope Stability
The long-term stability of a slope is determined by a variety of environmental and geological conditions that affect the material’s shear strength. The steepness of the slope is a major factor, as steeper angles naturally increase the shear stress component of gravity, making the slope inherently less stable. For unconsolidated materials, the steepest angle at which a slope remains stable is known as the angle of repose.
The underlying geological structure of the rock mass is highly significant, particularly the orientation of internal planes of weakness, such as bedding planes, fractures, or metamorphic foliation. Instability is maximized when these planes are oriented parallel to the slope face, providing a smooth path for movement. Mechanical and chemical weathering processes weaken the material over time, converting strong rock into weaker soil and clay minerals, which gradually reduces the overall shear strength of the slope.
The presence and type of vegetation play a role in stability by binding the soil together with root systems, which increases the material’s cohesion. Removal of this vegetation, such as through logging or wildfire, can quickly destabilize a slope by eliminating this natural reinforcement. Human activities, including cutting into the base of a slope for road construction or adding excessive weight to the top of a slope with buildings, can fundamentally change the geometry and stress distribution, making the area prone to failure.