Mass movement, also known as mass wasting, describes the downslope motion of rock, soil, and debris under the direct influence of gravity. This geological process ranges from sudden, catastrophic events to movements that are almost impossible to perceive. The slowest type of gravitational transport phenomenon is called creep, specifically soil creep, which moves at rates measured in mere millimeters per year. This subtle, pervasive process is a major factor in shaping the long-term topography of terrestrial slopes.
Classifying Mass Movement by Speed and Material
Geologists classify mass movement events based on three primary criteria: the material involved, the mechanism of movement, and the speed at which the material travels. Material is typically categorized as rock, debris (coarse, fragmented material), or earth (fine-grained soil). The mechanism of motion is described as a fall, slide, flow, or slump, each involving different internal dynamics.
Speed is the most useful factor for distinguishing between types of mass wasting, creating a spectrum from extremely rapid to exceedingly slow. Fast events like rockfalls and debris avalanches can reach speeds of meters per second. Intermediate movements, such as slumps and earthflows, move at rates from centimeters per day to meters per year. Creep occupies the slowest category, with motion so gradual it is imperceptible in real-time, often measured in less than a centimeter annually.
Defining Creep: The Slowest Form of Movement
Creep is the slow, continuous, and progressive downslope deformation of surficial material, mostly soil and weathered rock. This movement is a form of plastic flow, meaning the material deforms without the distinct, localized failure surface that characterizes a slide. Gravity is the driving force, but the mechanism involves repeated cycles of expansion and contraction within the soil layer.
These cycles are typically related to changes in moisture and temperature, such as freezing and thawing or wetting and drying. When water in the soil freezes, it expands, pushing soil particles outward and perpendicular to the slope surface. When the ice thaws or the soil dries, gravity pulls the particles straight down, resulting in a net shift downslope.
This repeated, incremental displacement ensures the soil mass slowly migrates downward. The movement is most pronounced in the shallow, upper layers of the soil, decreasing significantly with depth. Increased water content acts as a lubricant and increases the mass, accelerating the rate of creep. Steeper slopes also experience faster creep rates because the gravitational component pulling the material downward is more powerful.
Observable Indicators of Creep
Because creep is too slow to observe directly, its presence is identified by the long-term effects it has on landscape features and structures. One characteristic sign is the formation of curved or bowed tree trunks, sometimes called “pistol-butt” trees. As the soil at the base moves downslope, the tree attempts to reorient its new growth vertically toward the sun, resulting in a distinct curvature near the ground.
Vertical structures embedded in the creeping soil also show evidence of movement over time. Utility poles, fence posts, and older gravestones frequently appear tilted downhill, having been slowly pushed out of their original upright position. Retaining walls constructed to stabilize slopes often show signs of bulging, cracking, and eventual failure as the creeping soil exerts constant pressure against them.
On the soil surface, creep can create small, wave-like wrinkles known as terracettes. These miniature ridges and steps form due to the differential movement of the soil particles. In certain geological settings, even bedrock can undergo a form of very slow movement called rock creep, visible as bent or bowed layers of rock near the surface.