Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) because they are the most resistant minerals to weathering processes at the Earth’s surface. Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.
Rock formations that are primarily composed of sandstone usually allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Quartz-bearing sandstone can be changed into quartzite through metamorphism, usually related to tectonic compression within orogenic belts.
Sandstones are clastic in origin (as opposed to either organic, like chalk and coal, or chemical, like gypsum and jasper).The silicate sand grains from which they form are the product of physical and chemical weathering of bedrock. Weathering and erosion are most rapid in areas of high relief, such as volcanic arcs, areas of continental rifting, and orogenic belts.
Eroded sand is transported by rivers or by the wind from its source areas to depositional environments where tectonics has created accommodation space for sediments to accumulate. Forearc basins tend to accumulate sand rich in lithic grains and plagioclase. Intracontinental basins and grabens along continental margins are also common environments for deposition of sand.
As sediments continue to accumulate in the depositional environment, older sand is buried by younger sediments, and it undergoes diagenesis. This mostly consists of compaction and lithification of the sand. Early stages of diagenesis, described as eogenesis, take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in the sands, with only slight compaction. The red hematite that gives red bed sandstones their color is likely formed during eogenesis. Deeper burial is accompanied by mesogenesis, during which most of the compaction and lithification takes place.
Compaction takes place as the sand comes under increasing pressure from overlying sediments. Sediment grains move into more compact arrangements, ductile grains (such as mica grains) are deformed, and pore space is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact.
Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces.Mechanical compaction takes place primarily at depths less than 1,000 meters (3,300 ft). Chemical compaction continues to depths of 2,000 meters (6,600 ft), and most cementation takes place at depths of 2,000–5,000 meters (6,600–16,400 ft).
Unroofing of buried sandstone is accompanied by telogenesis, the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the sandstone, such as dissolution of some of the cement to produce secondary porosity.
Sandstone is classified as a sedimentary rock. It is made of some of the most common minerals found on earth, which gives sandstone a significant footprint around the world. Some interesting characteristics of sandstone relate to its composition, origin, type and use.
Sandstone is composed of very small grains of minerals or rocks, usually the size of grains of sand. Individual grains normally range in size between .00394 and .07874 inches. The most common minerals found in sandstone are feldspar and quartz. Sandstone takes on the color of the materials it contains. The most common colors are brown, yellow, red, white, gray and tan, with yellow and tan predominating.
Sandstone is of clastic, as opposed to organic or chemical, origin. Sandstone is formed in two steps. First, sand layers accumulate into piles known as grus. Water or air sorts the grains of rock or minerals in these piles, and sedimentation occurs. The pressure of deposits lying over the rock or mineral grains then compacts them, and materials such as clay, calcium carbonate or silica precipitate through the sediment layer and cement the rock or mineral grains together.
Types of Sandstone
The environment where sandstone components are deposited determines the nature of sandstone. The size of component grains of rock or minerals, how the grains are sorted and the structure of the sediment formed are some of the determinant environmental factors. Sandstone environments include lake, alluvial fan, river, glacial outwash, desert, river delta, submarine channel, storm deposit, offshore bar and sand wave and tidal delta environments. Sandstones are divided into three large groups: arkosic sandstones that are composed mostly of granite; quartzose sandstones, or beach sand, that are made up of mostly quartz; and argillaceous sandstones that have a lot of clay or silt.
Uses of Sandstone
Some sandstone is easy to work with, even though it resists weathering. For this reason, sandstone is often used as a building or paving material. Sandstone is also used to make grindstones and stones for gristmills. Sandstone formations are useful as aquifers. Since sandstone is porous, water filters through it, and large quantities of water can be stored in sandstone formations. The filtering process is useful in removing pollutants from the water.