DEFINITION OF CREEP

The imperceptible slow downslope flowing movement

of regolith (soil plus rock fragments) under the influence

of gravity. It involves the very slow plastic deformation

of the regolith, as well as repeated microfracturing of

bedrock at nearly imperceptible rates. Creep occurs throughout

the upper parts of the regolith, and there is no single surface

along which slip has occurred. Creep rates range from a

fraction of an inch per year up to about two inches per year

(5 cm/yr) on steep slopes. Creep accounts for leaning telephone

poles, fences, and many of the cracks in sidewalks and

roads. Although creep is slow and not very spectacular, it is

one of the most important mechanisms of mass wasting, and

it accounts for the greatest total volume of material moved

downhill in any given year. One of the most common creep

mechanisms is through frost heaving. Creep through frost

heaving is extremely effective at moving rocks, soil, and

regolith downhill because when the ground freezes, ice crystals

form and grow, pushing rocks upward perpendicular to

the surface. As the ice melts in the freeze-thaw cycle, gravity

takes over and the pebble or rock moves vertically downward,

ending up a fraction of an inch downhill from where it

started. Creep can also be initiated by other mechanisms of

surface expansion and contraction, such as warming and

cooling, or the expansion and contraction of clay minerals

with changes in moisture levels.

In a related phenomenon, the freeze-thaw cycle can push

rocks upward through the soil profile, as revealed by farmers’

fields in New England and other northern climates, where the

fields seem to grow boulders. The fields are cleared of rocks,

and years later, the same fields are filled with numerous boulders

at the surface. In these cases, the freezing forms ice crystals

below the boulders that push them upward, and during

the thaw cycle, the ice around the edges of the boulder melt

first, and mud and soil seep down into the crack, and find

their way beneath the boulder. This process, repeated over

years, is able to lift boulders to the surface, keeping the

northern farmer busy.

The operation of the freeze-thaw cycle makes rates of

creep faster on steep slopes than on gentle slopes, and faster

with more water, and greater numbers of freeze-thaw cycles.

Rates of creep of up to half an inch per year are common.

See also MASS WASTING.

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