Sand is a loose accumulation of tiny pieces of rocks or minerals that are larger than silt or clay but smaller than pebbles. Scientists define sand as grains that measure from 1/400 inch (0.06 millimeter) to 1/12 inch (2.1 millimeters) in diameter.
Most grains of sand once formed parts of solid rocks that have crumbled away. Rocks break and crumble by the process of weathering. For example, waves pounding against a coastline wear away rocks. When water freezes in the cracks of rocks, it can cause them to split apart. Rocks also may break down as a result of chemical reactions with air and water.
Sand is widely distributed over the earth. It lies at the bottom of the sea and many lakes. Large amounts of sand wash up from shallow sea bottoms onto beaches. Sand also rolls along the bottom of rivers and may be deposited by the rivers over lowland areas. In desert areas, sand covers vast stretches of land. Wind may pile up the sand in low hills called dunes.
Sand may be composed of many types of material. Quartz is the most common mineral in sand. But sand also may include other minerals, such as feldspar, and fragments of rocks. Some sand beaches are made up mostly of the mineral calcite, which comes from broken shells and coral. Black sand found in Hawaii and on other Pacific Islands consists of grains of basalt and basaltic glass, which are rocks formed from the hardened lava of volcanoes.
Sand has important industrial uses. Sand and gravel are mixed with cement to make concrete. Manufacturers use a pure form of quartz sand called silica sand to make glass, abrasives, furnace linings, and scientific molds. Some river deposits of sand called placers may also contain small amounts of such precious substances as gold, platinum, and diamonds.
