Creep

Downhill Creep

Creep of many different kinds of natural materials occurs as a result of constant gravitational attraction downhill. Consequentially, any slight displacements of small portions of material from many diverse causes tend to be always moved in a generally downhill direction. The net result is that in aggregate the whole mass has moved downhill with gravity.

Rock-mass creep, or deep creep, in bedrock occurs inside some mountains where many small fractures occur along foliation planes and between certain mineral grains so that spatially continuous deformation occurs. The volumes of rock masses involved are on the order of several thousands of cubic meters with thicknesses of many tens of meters. This process involves extremely slow and generally nonaccelerating differential movements among relatively intact rock masses. The deforming masses are not necessarily bounded by continuous yielding surfaces, but rather, such movements can be distributed along many internal shear surfaces that apparently are unconnected, so that the distributed shear results in bending, bulging, and folding. The total displacement is small relative to the large magnitude of the involved rock mass. The process roughly simulates viscous fluids in the distribution of velocities, with the result that some scientists refer to it as a kind of rock flow, albeit of a brittle-failure variety rather than the ductile kind characteristic of bedrock at a depth where it is under tremendous confining pressures and higher temperatures. In addition, this deep-seated gravitational creep of whole mountains can produce a sagging (Sackung) or settlement of the mountain so that ridge-top grabens, or trenches, can result, as well as antislope scarps, or escarpments that face uphill.

Creep of surficial sediment and soils on slopes is another of the processes characteristic of slopes worldwide, where material moves slowly downward on hillsides of low to moderate inclinations. Most of this creep is accomplished by expansion and contraction of the relatively unconsolidated materials when they freeze and thaw, or are just warmed and cooled in daily cycles with the sun, or through wetting during storms or snowmelt and later drying. In each expansion cycle, the granular soil materials move outward perpendicular to the slope, but in the return contraction part of the cycle, there is a tendency for them to move back in a slightly downhill direction because of gravity. This results in a ratcheting back and forth with a net downhill creep motion. In addition to the expansion and contraction cycles, there are also several miscellaneous additions to the creeping process. For instance, animals burrowing on slopes tend to move dirt preferentially downward with gravity, plant roots push soil preferentially downslope more than upslope as they grow, and abandoned burrows and deadand [Page 605]rotted root cavities collapse downslope. Finally, deep snows on slopes push strongly downward on trees, with the result that this force is transmitted into the ground through the tree roots, with creep resulting if the ground is not frozen solid beyond the depth of the roots.

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