Sand is an unconsolidated granular material consisting of mineral or rock fragments between 6.3 |im and 2 mm in diameter. Finer material is referred to as silt and clay, coarser material as gravel. Sand is usually produced by the chemical or mechanical breakdown of older source rocks, but may also be formed by the direct chemical precipitation of grains or by biological processes. Accumulations of sand result from hydrodynamic sorting of sediment during transport and deposition.
Origin and Characteristics
Most sand originates from the chemical and mechanical breakdown, or weathering, of bedrock. Chemical weathering is most efficient to soils, and most sand grains originate within soils. Rocks may also be broken into sand-size fragments by mechanical processes, including diurnal temperature changes, freeze-thaw cycles, wedging by salt crystals or plant roots, and ice gouging beneath glaciers.
Transport
Sand can be transported by any medium with sufficient kinetic energy to keep the grains in movement. In nature, sand is commonly transported by rivers, ocean currents, wind, or ice. Rivers are responsible for transporting the greatest volume of sand over the greatest distances. When rivers deposit sand at the ocean edge, it is commonly remobilized by high-energy breaking waves or transported along the shore by powerful longshore currents. Sand, like coarser and finer sediments, may also be entrained by glacial ice and transported to the sea by glacial flow. Where sand is widely exposed to wind currents, such as in river bars, on beaches, and in glacial outwash plains, it may be picked up and transported by the wind. As it moves, whether subaqueously or subaeri-ally, sand commonly is organized into complex moving structures (bedforms) such as ripples and dunes.
Economic Importance
Sand and gravel production is second only to crushed-stone production among nonfuel minerals in the United States. Although sand and gravel have one of the lowest average per ton values of all mineral commodities, the vast demand makes it among the most economically important of all mineral resources. Sand and gravel are used primarily for construction purposes, mostly as concrete aggregate. Pure quartz sand is used in the production of glass, and some sand is enriched in rare commodities such as ilmenite (a source of titanium) and gold.
