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.
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
. These involve chemical weathering
of rock and regolith
, and a number of physical weathering processes such as frost shattering
. It is mainly formed by abrasion
through transport-processes such as fluvial comminution
, aeolian attrition
grinding . It is in semi-arid environments 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
. 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 and 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. 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
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.
The main source of silt in urban rivers is disturbance of soil by construction activity . 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 . 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 .
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 .