Silt

Silt is granular material of a size somewhere between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspension with water (also known as a suspended load) in a body of water such as a river. It may also exist as soil deposited at the bottom of a water body. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury feel when dry, and a slippery feel when wet. Silt can be visually observed with a hand lens.

Sources

Silt is created by a variety of physical processes capable of splitting the generally sand-sized quartz crystals of primary rocks by exploiting deficiencies in their lattice.^[1] These involve chemical weathering of rock^[2] and regolith, and a number of physical weathering processes such as frost shattering^[3] and haloclasty.^[4] The main process is abrasion through transport, including fluvial comminution, aeolian attrition and glacial grinding.^[5] It is in semi-arid environments^[6] 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 0.0039 and 0.0625 mm, larger than clay but smaller than sand particles. ISO 14688 grades silts between 0.002 mm and 0.063 mm. 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 U.S. Department of Agriculture Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size.^[7] 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 mechanically by their plasticity.

A silted lake located in Eichhorst, Germany

Environmental impacts

Silt is easily transported in water or other liquid and is fine enough to be carried long distances by air in the form of dust. Thick deposits of silty material resulting from deposition by aeolian processes are often called loess. Silt and clay contribute to turbidity in water. Silt is transported by streams or by water currents in the ocean. When silt appears as a pollutant in water the phenomenon is known as siltation.

Silt, deposited by annual floods along the Nile River, created the rich, 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.^[8]

In south east Bangladesh, in the Noakhali district, cross dams were built in the 1960s whereby silt gradually started forming new land called "chars". The district of Noakhali has gained more than 28 square miles (73 km²) of land in the past 50 years.

With Dutch funding, the Bangladeshi government began to help develop older chars in the late 1970s, and the effort has since become a multi-agency operation building roads, culverts, embankments, cyclone shelters, toilets and ponds, as well as distributing land to settlers. By fall 2010, the program will have allotted some 27,000 acres (100 km²) to 21,000 families.^[9]

A main source of silt in urban rivers is disturbance of soil by construction activity. A main source in rural rivers is erosion from plowing of farm fields, clearcutting or slash and burn treatment of forests.

Culture

The fertile black silt of the Nile river's banks is a symbol of rebirth, associated with the Egyptian god Anubis.^[10]

See also

• Erosion control
• Nonpoint source pollution
• Sediment control
• Silt fence
• Siltation
• Clay

References

1. Moss, A J; Green, P (1975). "Sand and silt grains: Predetermination of their formation and properties by microfractures in quartz". Australian Journal of Earth Sciences 22 (4): 485–495. Bibcode:1975AuJES..22..485M. doi:10.1080/00167617508728913.
2. Nahon, D; Trompette, R (1982). "Origin of siltstones:glacial grinding versus weathering". Sedimentology 29: 25–35. Bibcode:1982Sedim..29...25N. doi:10.1111/j.1365-3091.1982.tb01706.x.
3. Lautridou, J P; Ozouf, J C (1982). "Experimental frost shattering: 15 years of research at the Centre de Geomorphologie du CNRS". Progress in Physical Geography 6 (2): 215–232. doi:10.1177/030913338200600202.
4. Goudie, A S; Viles, H A (1995). "The nature and pattern of debris liberated by salt weathering: a laboratory study". Earth Surface Processes and Landforms 9: 95–98. Bibcode:1984ESPL....9...95G. doi:10.1002/esp.3290090112.
5. Wright, J S; Smith, B J; Whalley W B (1998). "Mechanisms of loess-sized quartz silt production and their relative effectiveness: laboratory simulations". Geomorphology 45: 15–34. Bibcode:1998Geomo..23...15W. doi:10.1016/S0169-555X(97)00084-6.
6. Haberlah, D (2007). "A call for Australian loess". AREA 39 (2): 224–229. doi:10.1111/j.1475-4762.2007.00730.x.
7. "Particle Size (618.43)". National Soil Survey Handbook Part 618 (42-55) Soil Properties and Qualities. United States Department of Agriculture - Natural Resource Conservation Service. Retrieved 2006-05-31.
8. "Mississippi River". USGS Biological Resources. Archived from the original on 2005-10-28. Retrieved 2006-03-08.
9. http://web.archive.org/web/20100212215115/http://www.pulitzercenter.org:80/openitem.cfm?id=1973. Archived from the original on February 12, 2010. Retrieved October 22, 2009. Missing or empty |title= (help)
10. Hart 1986, p. 22; Freeman 1997, p. 91.