DEFINITION OF CREEP

Creep is the imperceptible slow downslope flowing movement

of regolith. 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

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. The ground freezes and 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, finding 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, with more

water, and greater numbers of freeze-thaw cycles. Rates of

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

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