Raffi at 03:59, 6 April 2007

2007-04-06T03:59:32Z

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'''Silt''' is [[soil]] or [[rock (geology)|rock]] derived [[granular material]] of a specific [[grain size]].

== Grain size criteria ==
In the [[Wentworth scale]], silt particles fall between {{Fraction|1|256}} and {{Fraction|1|16}} mm (3.9 to 62.5 μm), larger than [[clay]] but smaller than a [[sand]]. In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are roughly the same size in all dimensions, and their size ranges overlap. According to the [[United States Department of Agriculture|USDA]] Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size.<ref>{{cite web |url= http://soils.usda.gov/technical/handbook/contents/part618p3.html |title=Particle Size (618.42) |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}}</ref> The USDA system is also used 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 (physics)|plasticity]].

[[Image:SiltyMustang3Jan.jpg|right|thumb|250px|silt deposits around house and car in [[New Orleans]] left by flooding from a breach in the [[London Avenue Canal]]]]

== Source ==
Silt is produced by the mechanical [[weathering]] of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by [[glacier]]s, [[eolian]] [[Abrasion (geology)|abrasion]] ([[sandblasting]] by the wind) as well as water [[erosion]] of rocks on the [[stream bed|bed]]s of rivers and streams. 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]].

Silt, deposited by annual floods along the [[Nile River]], created the rich and fertile soil that sustained the ancient [[Ancient Egypt|Egyptian]] civilization. This silt was depended on for this purpose. A decrease in silt deposited by the [[Mississippi River]] throughout the [[20th century]] has contributed to the disappearance of protective [[wetlands]] and [[barrier islands]] in the [[Mississippi River Delta|delta]] region surrounding [[New Orleans]].[http://biology.usgs.gov/s+t/SNT/noframe/ms137.htm]

== Environmental impacts ==

Silt can occur as a deposit or as material transported by a [[stream]] or by a [[current (water)|current]] in the [[ocean]]. 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.

One of the main causes of river siltation in the year 2006 is as a result of [[slash and burn]] treatment of [[tropical]] [[forest]]s. When the total ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive; for example, on the [[Madagascar]] high central [[plateau]], comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of [[vegetation]], with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide. [[Shifting cultivation]] is a farming system which sometimes incorporates the [[slash and burn]] method in some regions of the world. The resulting sediment load in rivers flowing to the west is ongoing, with most rivers a dark red brown colour. The resulting fish kills in most of these rivers have resulted in the process of extinction of a variety of [[Madagascar]]'s fish [[species]].

==See also==
*[[Sediment]]

==References==
<references/>

[[Category:Pedology]]
[[Category:Sedimentology]]
[[Category:Sediments]]

Silt - Revision history

Raffi at 03:59, 6 April 2007