Creep and slump are distinct types of mass wasting, which refers to the downhill movement of rock, soil, and sediment under the direct influence of gravity. While both involve material moving down a slope, they differ significantly in speed and how the material moves.
Creep is a very slow, almost imperceptible process, whereas slump involves a more rapid movement of a coherent block of material.
Understanding Creep
Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. This process is so gradual that it is often difficult to detect visually over short periods. It typically occurs on gentle to moderate slopes.
The movement in creep is caused by the repeated expansion and contraction of soil or rock particles due to factors like freezing and thawing, wetting and drying, or daily temperature changes. Each cycle causes the particles to move slightly downhill.
Signs of Creep
Evidence of creep can be observed in various ways over time, including:
- Curved tree trunks: Trees attempting to grow vertically on a creeping hillside will have their lower trunks bent downslope. Trees with curved trunks are often signs that the hillside is slowly creeping downhill. (See Figure 2 reference).
- Tilted fences, utility poles, or gravestones.
- Broken or displaced retaining walls.
- Buckled roads or railway tracks.
Creep is a continuous process, and its effects accumulate over many years, leading to significant landscape changes.
Understanding Slump
Slump is another type of mass wasting characterized by the movement of material as a whole unit, leaving behind a crescent shaped scar. Unlike creep, slump is typically faster and involves a rotational movement of a block of earth along a curved surface.
Slumps often occur when the base of a slope is undercut, perhaps by erosion from a river or coastline, or when the slope becomes saturated with water, reducing the shear strength of the soil.
Characteristics of Slump
Key features of a slump event include:
- Coherent Block Movement: The material moves as a single, distinct mass, maintaining much of its internal structure initially.
- Rotational Slip: The movement often occurs along a concave-upward failure surface, causing the upper part of the block to tilt backward towards the slope and the lower part to bulge outward at the toe.
- Crescent-Shaped Scar: The area where the slump block detached from the hillside is marked by a distinct, curved escarpment or scar.
- Pressure Ridges/Bulges: Material accumulating at the base of the slump can form hummocky terrain or bulges.
Slumps can vary in size and speed, from small, rapid movements to larger, slower ones, but they are generally faster and more dramatic than creep.
Key Differences Summarized
| Feature | Creep | Slump |
|---|---|---|
| Speed | Imperceptibly slow, steady | Generally faster, can be sudden |
| Movement Type | Particle-by-particle, gradual downhill movement | Coherent block moving along a curved failure surface (rotational) |
| Scar | No distinct scar at the source | Leaves behind a crescent shaped scar. |
| Visual Signs | Curved trees, tilted structures | Scarp at the head, hummocky toe, tilted blocks |
| Affected Area | Entire slope surface | Localized area |
| Driving Force | Cycles of expansion/contraction | Shear failure due to factors like undercutting, saturation |
Understanding these processes is crucial in geology and civil engineering for assessing slope stability and mitigating potential hazards.
