Earth Materials: Sand

Sand is the most continental sediment. It is important as a source of precious gems and ores, an abrasive, and a reservoir for storing valuable fluids. Sand dunes form where wind or water deposits sand. Moving dunes can overrun cropland and forests and have many other functions in the dynamics of environmental change.

Formation and Composition

Sand is any earth material that consists of loose grains of minerals or rocks larger than silt but smaller than gravel. It includes grains less than 2.12 millimeters but greater than 0.06 millimeters in diameter. Materials meeting this definition are of widely diverse origins.

Five processes lead to the formation and release of sand-sized grains. The first of these processes is weathering, including both disintegration and decomposition. Some rocks crumble by the action of the air, rain, or frost. The decomposition of rocks containing quartz crystal inclusions is probably the origin of most quartz sands. Most of the rock is converted to fine-grained, clay-sized particles, which liberate the inert, undecomposed quartz grains. Sands produced in this manner are called epiclastic sands. The second source of sand is explosive volcanism. The explosive action of volcanoes yields vast quantities of sand-sized debris: glass, crystal fragments, and lava particles. Sands produced in this manner are called pyroclastic sands. Sand-sized materials may also be produced by crushing action, which, as opposed to ordinary abrasion, produces a significant volume of sand. Tectonic movements may crush rocks but do not produce a significant deposit. Glacial crushing, in contrast, does produce a considerable body of sand-sized material as rocks transported by glaciers are ground against other rock. Sands produced in this manner are called cataclastic sands. Much sand-sized material is also produced by chemical and biochemical precipitation. They are generated within the oceanic basins and are not the products of the wastage of landmass. Biochemical and chemical precipitation form oölitic sands that may accumulate in a significant deposit. Sands produced in this manner are called endogenetic sands. The fifth process that may produce sand is pelletization. It involves the sand-sized pellets produced by organisms and the pellets of other origins, such as those that are blown into “clay dunes.” Like endogenetic sands, these sands are often found in basins.

Because quartz is the principal product of rock decomposition, it is the principal component of sand. Quartz, a form of silicon dioxide, may be classified in three ways: igneous, including volcanic quartz; metamorphic, including pressure quartz; and sedimentary, including new crystals and overgrowth. One important type of metamorphic quartz is polycrystalline quartz, which includes grains composed of one or more crystal units. Next to quartz, feldspar is the most common mineral of sand. Feldspar is released by the granular disintegration of acidic rock. Several types of feldspar are usually present in sand, although the alkali-rich feldspars seem more abundant than the calcareous feldspars. Although mica is conspicuous in some sandstones, it is never a major component because it is derived from igneous rocks. In general, abundant mica points to a metamorphic provenance for sand.

In addition to quartz and feldspar, sandstone usually contains rock particles. These rock particles, most of which are fine-grained, carry their own evidence of provenance. Most common are the shales, clays that have undergone a metamorphic change driven by heat and pressure, which tend to be molded about more resistant quartz. Because they are among the most informative of all the components of sand, any attempt to trace the origin of sand usually begins with examining these constituents.

Distribution

Sand is distributed across the world. The deep oceanic basins are practically devoid of sand because the grains are too large to be blown or washed to any great distance from the continents before settling due to gravity. Therefore, the principal environments of sand are found on the continents. Sand is produced on the continent and is shifted from higher elevations to lower ones. Only a small amount is carried to the deep sea by turbidity currents that transport sand down the continental slopes to the abyssal plains. The absence of any great quantity of sand on the sea bottom probably results more from an absence of supply than from conditions unfavorable to its accumulation.

The most obvious places to find sand are the rivers and beaches and, to a lesser extent, glacial outwash plains, dunes, and shallow shelf seas. Sand is transported to floodplains and deltas. A little sand also escapes river channels and ends up in swamps and bayous. Shoreline sand includes that which is found on beaches, in lagoons, and on tidal flats. Although windblown dunes are closely associated with beaches and major rivers, the most impressive are found in the dune fields of some desert basins. Marine sands are primarily shelf sands.

Sand is transported primarily by water. It moves mostly along the bottom of a stream as “bed load.” The particle size of sand dictates that a very strong turbulent current is required to suspend it in a fluid medium such as water. Thus, sand does not normally move continuously; instead, sand grains travel in periodic, short jumps along the bottom by a process termed “saltation.” The alluvial transport of sand creates forms ranging from small ripples only a few centimeters high to belts many meters high and more than 50 meters thick. A wide range of dunes and bars occur in between.

Sand dispersal on beaches depends on the interaction of shoreline configuration and underwater topography with the energy supplied by waves and currents. As a deep-water wave moves toward a beach, it pushes against the bottom so that the wave tends to parallel the shore. As the water shallows, wave steepness increases, causing loose grains to move back and forth on the bed. This process is called surge transport. In response to passing waves, fluid particles follow a circular path except near the bottom, where the particles move back and forth above the bed, perpendicular to the shoreline. This action, over time, contributes to the continual erosion of larger particles into smaller particles, creating more sand in the process.

Wind transports great quantities of sand in deserts and can also carry inland the sands of beaches supplied by currents. When the wind reaches a certain velocity, it exerts sufficient force to cause dry grains of sand to begin to roll and accelerate. The round shape of the grains of desert sands can be attributed to this rolling action. Unlike silt and clay, which travel long distances suspended by the wind, sand generally travels by saltation on the ground. The grains are lifted briefly into an arcing trajectory and return to the surface on an elongated, parabolic path.

Turbidity currents are a unique type of transport system for sand. These currents are turbid because of the suspension of fine particles of sand within them and have an overall density greater than that of the body of water in which they occur. Turbidity currents often form as sediment-laden streams enter an ocean, freshwater lake, or reservoir where the turbid water passes beneath the clear, less dense water above. The current slows down as it enters the water body and is maintained as a separate layer of water with a significantly higher density than the overlying water. Under the force of gravity, the turbid layer moves along the basin's floor until it loses momentum, when the suspended material gradually settles. Oceanic turbidity currents may have enough force to extend their flow 150 kilometers or more away from the point of entry.

Diagenesis includes all the changes that sand undergoes after it has been deposited. The major direct evidence of diagenesis in sandstones is the nature of the textural relations between mineral grains and crystals. During diagenesis, original materials are preserved within a matrix that is not original. The most frequently occurring types of replacements are those that preserve faint outlines of the original. The most obvious diagenetic modification is the introduction of cementing agents. Calcite is the most common carbonate mineral cementing sandstone, although silica cementation also occurs frequently. Not all sands undergo diagenesis at the same rate. Sands that are millions of years old may be incoherent, while relatively recent sands may be already bound together.

Sand Dunes

Sand dunes form where wind or water deposits sand. Unlike many other geological processes that produce no visible change in a single human life span, the deposition of dunes occurs on a short timeline. Dunes are not solely a product of desert dynamics. They also form on the beaches of lakes, rivers, and oceans, on barrier islands and river floodplains, and underwater, and have also been observed on Mars. A ready supply of sand and an agent of transportation, such as wind or water, can lead to the formation of a dune. Dunes cover less than 12 percent of Earth’s arid lands.

Both shoreline and desert sand dunes begin with an obstacle such as a large rock, pebbles, a small sand heap, or vegetation around which sand can be deposited from eddy currents and build up. With constant wind and sand supply, dunes continue to build in size. Some Sahara dunes may reach 100 meters in height, although dunes of thirty meters are more common. Sandbanks Provincial Park in Ontario, Canada, features a sand dune some 450 feet (about 150 meters) in height.

The typical dune shape is distinctive: a long, low-angled windward slope rising to a peak and a steeper leeward slope. Sand grains pushed over the crest eventually settle to a constant 30- to 35-degree angle on the downwind slope. This angle of repose is the maximum stable slope of the intersection of the leeward dune side and the ground. Dunes have several wavelike properties, including crests and wavelength. Different methods of classifying sand dunes have been developed. The most commonly used system distinguishes dunes by shape, sand supply, and orientation to the wind. When sand supply is limited and the wind direction is fairly uniform, a crescent-shaped barchan dune will form. Parabolic dunes occur where vegetation partly covers the sand, often along shorelines. Here, the shape of the dune is similar to the barchan but is oriented in the opposite direction, pointing into the wind rather than away from it. Longitudinal dunes, such as the sinuous line of ridges called seifs in the Sahara Desert, lie parallel to the average direction of prevailing winds. The star dune, formed in a confined basin by variable winds blowing from radically different directions, is the most complex and the least studied of the three types.

Dunes are extremely mobile. During dune growth, as the saltating grains spill over and pile up on the leeward side, or slip face, the entire dune moves downwind. The windward slope, then, is constantly eroding while the sand grains are deposited on the leeward slope. Dunes march forward at a barely detectable three meters per year, although dunes in areas of strong, constant winds can move more than 120 meters in one year.

One of the most famous programs of dune stabilization was the United States government’s attempt to stabilize 360 kilometers of barrier island beaches off the Atlantic coast. During the 1930s, a sand fence was installed to encourage the development of exceptionally large, artificial dunes. Beach grasses, shrubs, and trees were then planted on the offshore slope. Forty years later, the human-made dunes on Hatteras Island continued to rise in height, but the original beach, 220 kilometers wide, had receded to less than thirty-three meters, and the marshes behind the towering dunes were drying up. The unnaturally high dunes had prevented sand deposition on the beach and transport of seawater to the marsh at the back of the island. In 1973, the government changed its policy of dune stabilization to allow nature to take its course.

Study of Grains

The earliest method of studying the external form of sand grains, which is still used today, involves separating the particles using a sieve. Before sifting begins, the sand must be well-washed in water and stiffly brushed to detach any mud still sticking to the grains. When dry, the coarsest particles are separated by one sieve and the smallest particles by another so that the sizes of the particles may be compared. The coarse sieve allows grains of 0.36 millimeters in diameter to pass through, and the smaller sieve keeps back all grains larger than 0.25 millimeters in diameter. After a sample of medium quality is obtained, it is mixed thoroughly and spread on a horizontal glass plate so that the rounded grains do not separate from the flat and angular grains. The characteristics of the individual grains are brought into sharp relief with the assistance of a polarizing microscope.

Before the invention of powerful microscopes, the thin-section technique was devised in the nineteenth century to examine the mineral composition of sandstone. A smooth cross section of the stone is obtained in the procedure so that the natural history of the formation may be traced in the various layers of sediment. The thin-section technique has also been applied to igneous rock.

Sedimentation tubes quickly gained popularity because they provide a quicker analysis than sieves. Sedimentation tubes operate on the principle that the sample is introduced at one end of the tube and settles to the other. This method categorizes grains according to their settling velocity rather than their size.

The question of provenance is one of the most difficult for the sedimentary petrographer to solve because sands being observed are often derived from preexisting sands and because the source areas may have changed with time. Sedimentary petrographers examine internal and external evidence as they attempt to trace the “birth” of sediment. Internal evidence is provided by examination of a single grain of sand. A tourmaline grain, for example, may show a secondary growth on a rounded core. This outgrowth may imply weathering and release of the grain, followed by transportation and abrasion, or it might suggest deposition in a new deposit of sand. A more complete analysis of provenance can be made from a sample from a single grain. If more than one sample is obtained, it is possible to map sedimentary petrologic provinces.

The external evidence relating to provenance is of two types. Regional stratigraphy will contribute to the analysis of provenance by establishing the relative ages of the strata. To study the stratigraphy of an area, investigators use a paleogeologic map, which indicates the formations exposed and subject to erosion. Paleocurrent analyses are another important approach to the problem of provenance. They are especially helpful in studying sandstones of alluvial origin because the up-current direction of these alluvial sands is in the direction of the source.

Once investigators have examined the internal and external evidence, they present their conclusions as a kind of “flow sheet” or provenance diagram, of which there are two types. The first type is based solely on what can be seen in the rock itself through thin-section analysis. The second type of provenance diagram is based on a study of the thin section and a knowledge of regional geology and stratigraphy. All geologic data must be pieced together to construct this type of diagram.

Significance

Sand is economically important because of the mineral content of certain shore and river sands. As the current removes the lighter components, the heavier components become concentrated. Some of these deposits, which are called placers, yield diamonds and other gemstones, gold, platinum, uranium, tin, monazite (containing thorium and rare-earth elements), zircon (for zirconium), rutile (for titanium), and other ores. During the California Gold Rush of 1849, millions of dollars worth of gold was taken from placers. In modern times, rare metals for jet engines come from placers in Florida, India, and Australia. The greensands, found over the ocean floor, are widely sought after because their green color indicates the presence of potash-bearing material. These sands have been used for agricultural land amendment and water softening. Additionally, potash has been successfully extracted from them. The search for these sands has been refined to an art called alluvian prospecting.

Sands derived from specific minerals are indispensable to certain industries. Very pure quartzose sands are used as a source of silica in the pottery, glassmaking, and silicate industries. Similar sands are required to make the linings for the hearths of acid-steel furnaces. Sands utilized in foundries for making the molds in which metal is cast usually have a clay-type bond uniting the quartz grains. Quartz and garnet sands are also used as abrasives in sandpaper and sandblasting because of their hardness and poor cleavage. They are employed in grinding marble, plate glass, and metal. Some sands are used as soil conditioners or fertilizers. Ordinary sands find a multitude of other uses. Sand is an essential ingredient of mortar, cement, and concrete. Sand is also added to clays to reduce shrinkage and cracking in brick manufacturing and to asphalt to make “road dressing.” Additionally, it is used in filtration and as friction sand on locomotives.

Aside from their usefulness as additives, sand-based structures also serve as reservoirs for storing valuable fluids. The pore systems of sand and sandstones can contain large stores of freshwater, brines, petroleum, and natural gas. Sand strata are also conduits for artesian flow. Fluids may also be injected into the sands. Before fluids can be extracted, geologists must become familiar with the shape and porosity of the particular sand reservoir. A working knowledge of diagenesis is essential for geologists searching for petroleum deposits.

Geologists and geomorphologists are concerned with shore erosion and harbor development, as well as sand production, movement, and deposition. To solve the problems of shore engineering, some understanding of sand supply and sand deficit or removal is necessary. Geologists must also have a solid understanding of sand as they try to prevent the encroachment of sand on cultivated lands and forests or on roads and other structures. Finally, sand is involved in many aspects of river management. Globally, over fifty billion tons of sand are used each year across these uses.

Principal Terms

alluvium: sediment deposited by flowing water

barchan dune: a crescent-shaped sand dune of deserts and shorelines that lies transverse to the prevailing wind direction

cataclastic: those formative processes of sand that relate to crushing

diagenesis: all physical or chemical changes after deposition

endogenetic: those formative processes of sand that relate to chemical and biochemical precipitation

epiclastic: those formative processes of sand that relate to weathering

longitudinal dune: a long sand dune parallel to the prevailing wind

paleocurrent: the geologic system at the time of deposition

precipitate: to form a solid phase of material from dissolved components, thus separating them from the solution

provenance: all the factors relating to the production of sand

pyroclastic: those formative processes of sand that relate to volcanic action

slip face: the downwind or steep leeward front of a sand dune that continually stabilizes itself to the angle of repose of sand grains

star dune: a starfish-shaped dune with a central peak from which three or more arms radiate

stratigraphy: the internal fabric and structures, the external geometry, and the nature of the basal contact of sand bodies

turbidity current: the movement under gravity of a stream of denser, sediment-bearing fluids through another fluid

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