Sedimentation

Sedimentation involves the response of motion of molecules or particles to gravitational, centrifugal, or electrochemical forces. Sedimentation literally means the manner in which a sediment is taken out of whatever transportation process it is dissolved or entrained in (water, wind, gravity, waves, etc.), resulting in a deposit of sediment. Thus, sedimentation can occur naturally from the [Page 2522]various fluids or the media of movement, or it can occur artificially in different human environments (e.g., inside a laboratory vessel or water and wastewater treatment plants). The net result is that material is removed from further direct interaction with its entraining agent and is generally blocked by a hard boundary against which it is then lodged. This then constitutes a new collection of sediments. Many different genetic forms of sedimentation can result, but only those of the natural world are considered in this entry.

Sediment data collection in the Little Colorado River a kilometer upstream from the Colorado River, Grand Canyon, Arizona

Source: Advisory Committee on Water Information

Sedimentation resulting from cessation of gravity-induced motion without significant entrainment by fluids includes the many forms of mass movement or landslides as well as the results of subsidence, or isolated motions of creep, talus accumulation, rock glaciers, and other such forms. Both mass movement and subsidence result in accumulations of mixed up, unsorted, unstratified clastics and organics in colluvium. Glacier ice also moves downhill under the influence of gravity, which is similarly unsorted and unstratified, although these deposits commonly have grain-surface characteristics that enable differentiation from colluvium.

The motion of air and water can entrain vast quantities of clastics and organics as well as dissolved solute loads. With both fluids, once the velocity of the transporting medium drops below a certain amount required to keep the load in motion, sedimentation occurs and deposition results, commonly sorted into different forms such as clay, silt, sand, and various sizes of gravel and boulders. Sedimentation of sand from wind results in dunes, which can be easily remobilized once the wind picks up again. On the other hand, once the sedimentation of silt and clay dust occurs, the small particles are not easily entrained [Page 2523]again, resulting in loess blankets across the landscape. Water, unlike wind, has the capacity to dissolve some rock, as well as move many different grain sizes, depending on the velocity. As with wind, the clastic load undergoes sedimentation as the velocity decreases so that bars and deltas, beaches, lakebeds, and alluvial fans all result in deposits of sorted and stratified alluvium. Where water evaporates, the solute load comes out of the solution and produces the sedimentation of salt flats or the dripping deposition of stalactites and stalagmites in caves. One unusual form of sedimentation from solution involves the precipitation of pure silica (SiO2), which when it occurs in layers commonly colored by iron forms semiprecious agates and other kinds of colorful silica compounds that are quite durable. When, however, the silica is precipitated in a very regular fashion, where the individual silica molecules are lined up very precisely in rows covered with clear silica, the new sediment can separate light like a refraction grating to produce iridescent opals.

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