Sandstone is a common and recognizable clastic sedimentary rock found across the world. It is composed of fragments, or clasts, of other minerals and rocks that have been cemented together. The primary fragments are sand-sized grains, ranging from 0.0625 to 2 millimeters in diameter, which are visible to the naked eye. Sandstone makes up a significant portion of all sedimentary rocks, forming a major component of the planet’s outer crust.
Source Materials: The Building Blocks
The primary material for sandstone is sand, which is composed of framework grains and a cementing material that binds them. These framework grains are individual particles weathered and eroded from older rocks, and they determine the final properties of the sandstone. Quartz is the most abundant framework grain due to its chemical stability and resistance to physical abrasion, allowing it to survive long periods of transport.
Feldspar is the second most common mineral found in sandstone grains, though it weathers more easily than quartz. Rock fragments from igneous, metamorphic, or sedimentary sources may also be included. The cement precipitates into the spaces between these grains after deposition, acting like a natural glue to hold the loose sand together. Common chemical cements are silica, often forming quartz overgrowths, and calcium carbonate, which is also known as calcite.
The Journey: Transportation and Deposition
The process of forming sandstone begins with the weathering and erosion of existing parent rocks, breaking them down into sand-sized particles. These sand grains are then moved away from their source areas by various agents of transport, including flowing water, wind, and ice. Rivers carry the grains downstream, sorting them by size, with finer grains traveling further than coarser ones.
Wind transports sand, especially in desert or coastal environments, often leading to the formation of dunes and highly-sorted deposits. Glaciers also grind down rock and carry the debris over long distances before depositing it when the ice melts. These transportation processes eventually lead to deposition, where the sand settles in environments such as beaches, riverbeds, deltas, or shallow marine basins once the transporting agent loses energy.
During this journey, the grains become increasingly rounded and sorted, concentrating durable minerals like quartz in the final deposit. As layers of sand accumulate in these depositional environments, the weight of the overlying sediment prepares the grains for the final transformation into rock. The resulting layers, or strata, build up, marking the end of transportation and the beginning of deep burial.
Becoming Rock: The Process of Lithification
The final stage of sandstone formation is lithification, the process that converts loose sediment into solid rock. This transformation involves two main mechanisms: compaction and cementation. Compaction occurs first, as the weight of accumulating layers of sediment above exerts pressure on the sand below.
This pressure squeezes the sand grains closer together, reducing the pore space between them and driving out interstitial water. Mechanical compaction occurs primarily at shallower depths, reducing the sediment volume and increasing its density. Following this physical compression, cementation begins, which is a chemical process where minerals dissolved in the remaining pore water crystallize.
Mineral-rich fluids, often containing dissolved silica or calcium carbonate, circulate through the remaining pore spaces. As these fluids move, the minerals precipitate out of the solution, filling the gaps between the sand grains and binding them together. Silica cement forms strong bonds, and calcite cement, derived from calcium carbonate, is also common. This binding action glues the loose sand into a coherent, durable mass.