silt - Grain size criteria, Source, Environmental impacts

The aggregate of fine mineral particles produced by the erosion and weathering of rock, ranging in size from 0·0625–0·002 mm (0·0025–0·00008 in), a size range intermediate between that of clay and sand. Silt deposits are laid down by water, and may consolidate to form the sedimentary rock, siltstone.

Silt is soil or rock derived granular material of a specific grain size.

Grain size criteria

On the Wentworth scale, silt particles fall between ⁄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. 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. Silts and clays are distinguished by their plasticity.

Source

Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. 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 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 delta region surrounding New Orleans.

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'. 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 forests. The resulting fish kills in most of these rivers have resulted in the process of extinction of a variety of Madagascar's fish species.