Sand is a granular material defined by the size of its individual grains, not its composition. Geologists classify sand particles as having a diameter ranging from 0.0625 millimeters to 2 millimeters, placing them between finer silt and coarser gravel. While this size definition is universal, the composition of sand varies greatly depending on the local geology and environment. The vast majority of sand found in continental settings, such as deserts and non-tropical beaches, is composed of silica. However, not all sand consists of silica, and the exceptions reveal fascinating differences in Earth’s geology and biology.
Defining the Dominant Component: Quartz
The most common constituent of sand globally is silica, or silicon dioxide (SiO2), which is usually found in the mineral form known as quartz. Quartz dominates continental and temperate sand deposits because it is one of the most abundant minerals in the Earth’s crust, and its crystalline structure is exceptionally durable.
The Mohs scale of hardness ranks quartz at a 7 out of 10, making it harder than most other common rock-forming minerals. This hardness allows quartz to resist physical abrasion as it is transported by wind and water across long distances. Furthermore, quartz is chemically inert, meaning it resists chemical weathering and dissolution that would break down less stable minerals like feldspar.
This resilience ensures that as rocks break down over geological time, less durable components are dissolved or ground into finer sediments like clay and silt, leaving the tough quartz grains behind. Through repeated cycles of erosion, transport, and deposition, quartz becomes increasingly concentrated, resulting in the pure, often white, silica sand seen in many parts of the world.
How Sand Forms Through Geological Processes
The formation of sand requires the mechanical and chemical breakdown of larger parent rocks, such as granite, followed by prolonged transportation. Weathering is the initial process where exposed rock surfaces are fractured by physical forces, like freezing and thawing, and altered by chemical reactions. These processes liberate mineral fragments of various sizes.
Rivers and wind then act as the primary agents of transportation, carrying these rock fragments downstream or across landscapes. During this journey, the relentless movement and collision of particles—a process called abrasion—reduces the fragments to the specific size range that defines sand. The longer the distance traveled, the more rounded and uniform the sand grains become, and the more concentrated the most resistant minerals, like quartz, will be.
When the carrying power of the water or wind diminishes, the sand-sized particles are dropped, or deposited, to form beaches, riverbeds, and dunes. The resulting deposit is a mixture of grains that fall within the defined size range. This geological sorting process creates the vast, homogeneous sand bodies that define many coastal and desert landscapes.
When Sand Is Not Silica
While silica is the most common component, sand is not exclusively made of quartz, and its composition is highly dependent on the local source material. In tropical coastal environments, the dominant sand type is frequently biogenic, composed of calcium carbonate (CaCO3). This white or light-colored sand originates from the skeletal remains of marine organisms, such as fragmented coral, sea shells, and foraminifera.
Another significant exception is found in areas with active or recent volcanic activity, which results in dark-colored sand. This volcanic sand is primarily made of minerals like basalt, olivine, or fragments of volcanic glass, which are rich in iron and magnesium silicates but lack the high purity of quartz. These dark sands are common on the beaches of Hawaii and other volcanic islands.
Other less common sand compositions include deposits rich in minerals like feldspar, which can be abundant in geologically young deposits. Additionally, places like White Sands National Park in the United States feature gypsum sand, which is a form of calcium sulfate.