Everything about Silt totally explained
Silt is
soil or
rock derived
granular material of a specific
grain size. Silt may occur as a soil or alternatively as suspended
sediment in a
water column of any surface
water body. It may also exist as deposition soil at the bottom of a water body.
Source
Silt is generated by a variety of modi capable of breaking and splitting up 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 and
haloclasty. However, it's mainly by
abrasion through transport-processes such as
fluvial comminution,
aeolian attrition and
glacial grinding most effectively operating 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 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 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 (for example material passing the #200
sieve). Silts and clays are distinguished by their
plasticity.
Environmental impacts
Silt can occur as a deposit or as material transported by a
stream or by a
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.
The main source of river siltation and storm sewer sedimentation in urban areas is disturbance of soil by construction activity. The main cause of river siltation in rural areas is
erosion from extensive plowing of farm fields, clearcut logging or
slash and burn treatment of
tropical forests. When the total ground surface is stripped of vegetation, 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 can cause extensive fish kills, hampering economic development.
Silt, deposited by annual floods along the
Nile River, created the rich and fertile soil that sustained the ancient
Egyptian civilization. This silt was depended on for this purpose. A decrease in silt deposited by the
Mississippi River throughout the
20th century due to a system of
levees has contributed to the disappearance of protective
wetlands and
barrier islands in the
delta region surrounding
New Orleans.
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