Silt

Silt is soil or rock derived granular material of a grain size between sand and clay. Silt may occur as a soil or as suspended sediment in a surface water body. It may also exist as soil deposited at the bottom of a water body.

Source

Silt is generated by a variety of physical processes capable of splitting the generally sand-sized quartz crystals of primary rocks by exploiting deficiencies in their lattice [cite journal |last= Moss |first= A J |authorlink= |coauthors= Green, P |year= 1975|month= |title= Sand and silt grains: Predetermination of their formation and properties by microfractures in quartz |journal= Australian Journal of Earth Sciences |volume= 22 |issue= 4 |pages= 485–495|doi= 10.1080/00167617508728913] . These involve chemical weathering of rock [cite journal |last= Nahon |first= D |authorlink= |coauthors= Trompette, R|year= 1982|month= |title= Origin of siltstones:glacial grinding versus weathering |journal= Sedimentology |volume= 29 |issue= |pages= 25–35|doi= 10.1111/j.1365-3091.1982.tb01706.x] and regolith, and a number of physical weathering processes such as frost shattering [cite journal |last= Lautridou |first= J P |authorlink= |coauthors= Ozouf, J C |year= 1982|month= |title= Experimental frost shattering: 15 years of research at the Centre de Geomorphologie du CNRS |journal= Progress in Physical Geography |volume= 6 |issue= |pages= 215–232|doi= 10.1177/030913338200600202] and haloclasty [cite journal |last= Goudie |first= A S |authorlink= |coauthors= Viles, H A |year= 1995 |month= |title= The nature and pattern of debris liberated by salt weathering: a laboratory study |journal= Earth Surface Processes and Landforms |volume= 9 |issue= |pages= 95–98|doi= 10.1002/esp.3290090112] . It is mainly formed by abrasion through transport-processes such as fluvial comminution, aeolian attrition and glacial grinding [cite journal |last= Wright |first= J S |authorlink= |coauthors= Smith, B J; Whalley W B|year= 1998|month= |title= Mechanisms of loess-sized quartz silt production and their relative effectiveness: laboratory simulations |journal= Geomorphology |volume= 23 |issue= |pages= 15–34|doi= 10.1016/S0169-555X(97)00084-6] . It is in semi-arid environments [cite journal |last= Haberlah |first= D |authorlink= |coauthors= |year= 2007 |month= |title= A call for Australian loess |journal= AREA |volume= 39 |issue= 2 |pages= 224–229|doi= 10.1111/j.1475-4762.2007.00730.x] that substantial quantities of silt are produced. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. Sedimentary rock composed mainly of silt is known as siltstone.

Grain size criteria

In the Udden-Wentworth scale (due to Krumbein), silt particles range between Fraction|1|256 and Fraction|1|16 mm (3.9 to 62.5 μm), larger than clay but smaller than a sand. ISO 14688 grades silts between 0.002 mm and 0.063 mm, with clay particles being smaller and sands larger. In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are approximately the same size in all dimensions; furthermore, their size ranges overlap. Clays are formed from thin plate-shaped particles held together by electrostatic forces, so present a cohesion. According to the USDA Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size. [cite web |url= http://soils.usda.gov/technical/handbook/contents/part618.html#43 |title=Particle Size (618.43) |work=National Soil Survey Handbook Part 618 (42-55) Soil Properties and Qualities |publisher=United States Department of Agriculture - Natural Resource Conservation Service|accessdate=2006-05-31] The USDA system has been adopted by the Food and Agriculture Organization (FAO). In the Unified Soil Classification System (USCS) and the AASHTO Soil Classification system, the sand-silt distinction is made at the 0.075 mm particle size (i.e. material passing the #200 sieve). Silts and clays are distinguished by their plasticity.

Environmental impacts

Silt is easily transported in water and is fine enough to be carried long distances by air as 'dust'. Thick deposits of silty material resulting from "aeolian" deposition are often called "loess" (a German term) or "limon" (French). Silt and clay contribute to turbidity in water. Silt is transported by streams or by water currents in the ocean.

Silt, deposited by annual floods along the Nile River, created the rich and fertile soil that sustained the ancient Egyptian civilization. Silt deposited by the Mississippi River throughout the 20th century has decreased due to a system of levees, contributing to the disappearance of protective wetlands and barrier islands in the delta region surrounding New Orleans. [http://biology.usgs.gov/s+t/SNT/noframe/ms137.htm]

The main source of silt in urban rivers is disturbance of soil by construction activity Fact|date=September 2008. The main cause of river siltation in rural areas is erosion from plowing of farm fields, clearcut logging or slash and burn treatment of forests Fact|date=September 2008. When the total ground surface is stripped of vegetation the upper soil is vulnerable to wind and water erosion. Entire regions of countries have been rendered unproductive by erosion. On the Madagascar high central plateau, approximately ten percent of the country's land area, virtually the entire landscape is sterile, with gully erosive furrows in excess of 50 meters deep and one kilometer wide Fact|date=September 2008.
Shifting cultivation is a farming system which incorporates the slash and burn method in some regions of the world. The resulting sediment load can cause extensive fish kills, hampering economic development Fact|date=September 2008.

ee also

*Erosion control
*Nonpoint source pollution
*Sediment control
*Silt fence

References