hill-slope creep In landscape studies, the term ‘creep’ covers all slow downslope movements of unconsolidated material or of soft rock that is easily deformed under pressure. The rate of downslope movement through creep is typically very small, rarely more than 1–2 cm per year, but it is the ubiquity of this tendency, affecting almost any hill slope with an appreciable gradient, that makes it a significant geomorphological process. In fact, the general process of creep can arise from several independent mechanisms. The first, continuous creep, occurs by the slow plastic flow of clay-rich material, typically unconsolidated soils, shales, and clays. The key prerequisite for continuous creep is an easily deformable substrate, an attribute that depends on clay content, moisture content, the thickness and bulk density of the regolith or rock layer, and the angle of slope. Where all these are moderate to high, overburden pressure from overlying strata (caprock) or constructions (walls, power poles, buildings, etc.) can readily induce creep. This in turn may lead to the slumping or cambering of the slope, the bulging of valley sides, and the downslope curvature of surficial strata.
Creep is often referred to as ‘soil creep’, but strictly this refers to two distinct mechanisms that primarily operate within a regolith environment, although they may also affect loose rock material (talus creep). The mechanisms are creep by expansion and contraction and creep by swelling and shrinking, but they are often referred to together as ‘heave’. Expansion and contraction generally occur through the action of freeze–thaw, where the expansion of water, present as soil moisture, upon freezing causes a consequent increase in soil volume and, in turn, in the bulging of the soil surface perpendicularly outwards from the slope. On thawing, the regolith material sinks vertically downwards under the direction of gravity. This results in a net downslope movement of individual soil particles, although cohesion between the particles usually retards a pure vertical return drop. Soil particles at the surface are affected most by this tendency; the propensity for heave and its efficacy decrease with increasing depth, reaching zero at the lower boundary of the soil layer that has been frozen. Over a period of time, because repeated freezing and thawing affect surface layers more than deeper portions of the regolith, creep rates are disproportionately greater in the near-surface zone. Expansion and contraction creep can also occur in soils containing clay minerals that are capable of swelling, such as montmorillonite. Alternating wetting and drying out of clay minerals causes a smaller net downslope movement that results from freeze–thaw because shrinking may draw a clay particle back upslope before it is moved downslope during drying. Both mechanisms are extremely widespread on the Earth's surface. Their action is commonly considered to be manifest as flights of narrow steps, called ‘terracettes’, on steep, grass-covered slopes (Fig. 1).
Because expansion and contraction depend on the direction of gravity and not its strength, creep processes are also likely to affect other planetary landscapes. Thus, on the Moon and Mars large temperature fluctuations at the surface (+130 °C to −150 °C on the Moon; 15 °C to −85 °C on Mars) give rise to alternating expansion and contraction of the surface material.
Iain S. Stewart
