Sandstone is a common clastic sedimentary rock, comprising roughly 20 to 25 percent of all sedimentary rocks in the Earth’s crust. Sedimentary rock forms from particles of pre-existing rocks, minerals, or organic material deposited by agents like water, wind, or ice. Its widespread presence and durability make it significant for studying Earth’s history and for human architecture. Sandstone formations preserve evidence of ancient environments and serve as important natural resources.
Defining Characteristics and Composition
Sandstone is defined by its texture, consisting of sand-sized framework grains cemented together. The specific size range for these grains is between 0.0625 millimeters and 2 millimeters in diameter. Grains exhibit varying degrees of sorting (uniformity of size) and roundness (smoothness or angularity). The framework grains are typically dominated by quartz and often include feldspar, as these minerals resist breakdown at the Earth’s surface. The spaces between grains contain a fine-grained matrix (silt or clay) and a chemical cement. This cement binds the grains, commonly precipitating from water as silica, calcium carbonate (calcite), or iron oxides, which often impart red or brown coloration.
The Geological Process of Sandstone Formation
The creation of sandstone involves a sequential geological process beginning with the breakdown of older rocks. This initial phase is weathering and erosion, where parent rocks are fractured and chemically altered into sand-sized grains. These grains are then transported by wind, water, or ice away from the source area. Transport shapes the grains, with longer distances resulting in more rounded and better-sorted particles.
When the transporting agent loses energy, the sand particles settle in a process called deposition. Over time, these layers accumulate in basins and become buried under subsequent sediment. The final step is lithification, the transformation of loose sediment into solid rock, which occurs in two main stages: compaction and cementation.
Compaction happens as the weight of overlying sediments squeezes grains closer, reducing pore space and forcing out excess water. Cementation follows as dissolved minerals precipitate from circulating water, filling the gaps and chemically bonding the grains into cohesive sandstone.
Environments of Deposition and Global Distribution
Sandstone is found wherever large volumes of sand have accumulated, been buried, and subsequently lithified. These environments are broadly categorized based on their location: continental, transitional, and marine. Continental environments include eolian settings, such as ancient desert dune fields, and fluvial environments, like vast river systems and their floodplains.
Transitional environments occur where land meets sea, such as in deltaic settings where rivers empty and deposit sediment, or along coastlines in beach and barrier bar systems. Marine settings, both shallow and deep, also serve as major accumulation sites. Deep marine sandstones, often deposited by density currents called turbidites, are found in submarine fan deposits far from the coast.
The resulting formations are distributed worldwide, with some of the most famous examples found in the American Southwest. The iconic red cliffs and arches of this region, such as those in Zion and Arches National Parks, are remnants of massive ancient desert dune and fluvial systems.
Economic and Historical Significance
The physical properties of sandstone have made it a valuable resource throughout human history. Its durability and ease of cutting established it as a premier building stone for centuries, used in countless historic monuments and modern architectural projects. Certain varieties, like brownstone, have defined the architecture of entire city districts.
Beyond construction, the rock’s naturally high porosity and permeability give it significant economic importance beneath the surface. Sandstone layers function as reservoir rocks, capable of storing and allowing the flow of fluids. This makes them major sources for the extraction of crude oil and natural gas, as well as being important aquifers that hold vast underground reserves of potable groundwater.
Historically, the hardness of the quartz grains within certain sandstones made them useful as abrasives, serving as material for grindstones to sharpen tools and mill grain. High-purity quartz arenites are also a source of silica, a raw material used in the manufacturing of glass.