The expressive term creep is used for all slow downslope movements of regolith under the pull of gravity that are so slow as to be imperceptible except to observations with long duration (days to weeks in the case of solifluction; years to decades to centuries in the case of slower creep). It is unspectacular in comparison to the sudden, large-volume, high-speed landslides that make the news reports, and only negligibly destructive (nobody dies from creep!). Its great geological importance arises from its ubiquity.
We perceive creep to be a continuous process, as over the years we observe the slow downslope movement it engenders, but in fact it’s the sum of innumerable small and discrete movements of the slope-mantling regolith. These slow movements are brought about by a number of processes. Classic creep is brought about not by bodily movement of the surface layer above a plane of failure, as discussed at length in earlier sections, but by individual cyclic movements of the material.
The basic idea is this: particles or small masses of material at or near the surface are lifted upward perpendicular to the surface by any of a number of cyclic processes that involve lifting of material perpendicular to the surface and then lowering of that material in a much more nearly vertical direction. Here’s a list of some of the important processes that are thought to contribute to creep:
- wetting and drying
- heating and cooling
- freezing and thawing
- transfer of material to the surface by burrowing organisms, then vertically downward collapse of material overlying the resulting cavities
All such processes affect only the very near-surface layer of the regolith, generally down to depths of no more than a few meters. The result is a kind of “sawtooth”, step-by-step movement of material downslope. Figure 8-7 shows how this effect works in the case of soil particles lifted up by expansion and dropped down again by contraction. Similar figures could be drawn to show the sawtooth effect of other raising-and-lowering processes.
The sawtooth movement involves a variety of kinds of material:
- individual particles resting directly on the surface
- more extensive masses of the surface layer of regolith
- material brought up from deeper in the profile and deposited on the surface
The role of the lowly earthworm is believed to be of special importance in soil creep. (I seem to remember reading once that Darwin himself was aware of the importance of earthworms in soil movement.) The volume of soil processed by earthworms in an ordinary soil in humid temperate regions is staggering. As you must know from your own observations, earthworms work their way to the soil surface at night, leaving those telltale little piles of soil around their holes. In that way there is a net upward movement of soil material normal to the surface. Eventually the tube left behind by the burrowing earthworm collapses, and the tendency is for the collapse to be vertically downward under the pull of gravity. The net result is a sawtooth movement of soil material downslope.
Figure 8-8 shows common effects of creep. You are not likely to see all of them in one place, but all are clear manifestations of creep.
Solifluction is a special kind of creep, by which a surface layer of water- saturated regolith flows imperceptibly slowly downslope over an impermeable lower layer of some sort. The impermeability of the lower layer prevents drainage of the overlying soil, causing it to remain for long periods of time in a thickly soupy condition, which predisposes it to downslope flow. It is common in, but not restricted to, high-latitude regions of permafrost, where summer thawing affects only the surface layer, leaving frozen and impermeable material beneath. Solifluction happens also where there is an impermeable layer of clay-rich “hard pan” beneath a permeable surface layer, and even where there is an impermeable layer of bedrock just below a surface soil zone. In any case, solifluction is promoted by a high percentage of clay in the surface layer. Downslope speeds of solifluction, although still not something you can detect by standing there watching, are much higher than in other kinds of creep: up to some centimeters per day.