Sand

Sand, tiny, loose grains of mineral material. Sand usually consists of quartz (silica), with a small proportion of mica, feldspar, magnetite, and other resistant minerals. It is the product of the chemical and mechanical disintegration of rocks under the influences of weathering and abrasion (see Erosion). When freshly formed the particles are usually angular and sharply pointed, becoming smaller and more rounded by attrition by the wind or by water. Sand deposits can turn into a sedimentary rock called sandstone.

In addition to quartz, other types of mineral materials can form sands. Coral sand is found mainly in the tropics and contains particles of limestone from broken-up coral and shells of sea creatures. It is typically white in color. Volcanic sands can form from particles of basalt and are usually black. Gypsum particles make up the sand in places such as White Sands National Monument in New Mexico.

Large deposits of sand can form on Earth’s surface along the courses of rivers and on the shores of lakes and oceans, forming beaches and sometimes sand dunes. Although sand may accumulate as a layer on the seafloor near shorelines (see Longshore Drift), sediments deeper in the ocean consist of muds. Under certain conditions, areas of sand on beaches or near the mouths of rivers can become saturated with water, creating quicksand.

In arid regions, dry conditions and wind can make soil lose its organic material (humus) as well as its fine-grained inorganic silt and clay, leaving mainly sand behind. With little water, weathered rock in such regions also does not break down chemically to form soil. Such conditions can result in sand deserts where the wind may shape the sand into large dunes. The Sahara in Africa and the Mojave Desert in North America are examples. Constant movement by the wind can make sand grains found in deserts smoother and smaller than grains found along shorelines. In some regions, strong winds can lift dust and fine sand into the air as sandstorms.

Sand is an important constituent of most soils (see Soil; Soil Management). One specific form of sand is the major ingredient in glassmaking. Other types of sand are used in foundries to make casting molds and in ceramics, plasters, and cements. Sand is used as a grinding and polishing abrasive in the form of sandpaper, which is a sheet of paper covered on one side with sand or a similar abrasive substance. Sandblasting is an important technique used for cleaning stone or for smoothing rough metal surfaces by blowing a stream of sand under air or steam pressure.