Sand is a naturally occurring, unconsolidated sediment that is mainly composed of mineral grains with a grain size of 0.063 to 2 millimeters . On average, sand is coarser than silt (grain size predominantly 0.002 to 0.063 millimeters) and finer than gravel (grain size predominantly 2 to 63 millimeters). Sand is also one of the non-cohesive soils .
The term "sand" does not depend on the mineral composition. Most of the sands, however, consist mainly of quartz grains. Above all, this quartz sand is an important raw material for the construction industry as well as for the glass and semiconductor industries .
The first sand in the history of the earth originated from igneous and metamorphic rocks (e.g. granite or gneiss ), which disintegrated into smaller blocks due to physical weathering or, due to chemical weathering of correspondingly vulnerable rock components, disintegrated directly into individual mineral grains.
Such blocks and grains are initially transported away from their place of origin by gravity and subsequently, when the gradient decreases, mainly by water from their place of origin ( erosion ). Through continuous water transport, they are more or less strongly sorted according to size and specific weight (depending on the mineral from which they consist), in that the larger blocks are deposited after falling below a certain flow speed and remain behind and only grains the size of sand and below are transported on. Also, wind can transport sand, but due to the lower density of air generally greater screening efficiency and needs moreover higher speeds. As soon as the flow speed of the transport medium decreases enough, the grains of sand settle - the result is a sandy sediment. However, this can start moving again when the flow speed increases, i.e. it can be eroded.
Since the inner surface of sand is larger than that of an identical volume of coarser grain sizes , the weathering of sand can attack a larger area, so that some minerals, viewed in geological time, are relatively quickly chemically converted into clay minerals or completely dissolved (e.g. feldspars , mafic minerals or carbonates ) and their share in the total amount of sand decreases significantly compared to more chemically resistant minerals such as quartz .
Mechanical stress during transport changes the shape and size of the individual grains by splitting them along the crystal interfaces or by breaking out fragments during transport. Corners and edges are rounded and sanded down the longer the transport route. However, this is not a linear process: the rounder and smaller the grains, the more resistant they are to further mechanical changes. Investigations have shown that a transport route of thousands of kilometers is often necessary in order to even moderately round angular grains of sand of medium size.
When transporting along rivers, these distances can only rarely be achieved, and even the constant movements in the surf zone of a coast are in most cases insufficient to explain the good rounding of many grains of sand that can be observed today, especially not when the sand is mainly made of durable quartz. This is explained by the fact that the vast majority of the sand that occurs on earth today comes from the weathering of sandstones and has therefore already undergone several erosion and sedimentation cycles: Sand is deposited ( sedimented ), covered by other sediments and thereby compacted. The sand grains are finally cemented together during the diagenesis by a binding agent and a sandstone is created. When a sandstone returns to the earth's surface as a result of tectonic uplift and is exposed to weathering and erosion, the individual grains are exposed and rounded off a little further during the subsequent transport, ultimately deposited and another cycle follows. Even if one assumes a cycle duration of 200 million years, a well-rounded grain of quartz sand today can have passed through ten such cycles and thus almost half the history of the earth . After all, sands as loose sediments on the earth's surface can be more than a million years old.
A special case is sand that was created from the limestone skeletons of dead marine animals, for example from mussel shells or corals. In geological terms, this sand is very short-lived, since the individual grains are usually so strongly changed during diagenesis that they can no longer be removed in their original form after renewed uplift and erosion. In addition, limestone only weathers physically in arid climates , otherwise preferably chemically, i.e. that is, it is dissolved rather than broken down into small fragments.
In soil science , sandy soil is the coarsest of the four main soil types . The grain size of sand , which makes up the main part of the sediment of the same name, ranges from 0.063 to 2 mm equivalent diameter according to the preferred classification in German-speaking countries according to DIN 4022 and is thus between the grain size of silt (<0.063 mm) and gravel (> 2 mm). Sand is further divided into:
|Sand (S / Sa)||Grain size|
|Coarse sand (gS / CSa)||0.63-2 mm|
|Medium sand (mS / MSa)||0.2-0.63 mm|
|Fine sand (FS / FSa)||0.063-0.2 mm|
In practice, however, one also finds class boundaries and designations that differ to a certain extent:
- Fine sand is traditionally separated in soil science as 0.125–0.250 mm - according to DIN, this grain size would be classified as fine sand
- Coarse silt and sand have been referred to as psammites since 1953 according to the Von Engelhardt classification (in contrast to the fine-grained pelites ).
- Coarser sand is called Grand in northern Germany , a designation that is also used in the von Engelhardt classification for a grain size range that includes most of the coarse sand and fine gravel class of the DIN standard.
- Sands, which mainly consist of grains of one grain size , are called well-sorted , in a technical context also single-grain aggregates ; accordingly, poorly sorted sands are those in which a wide range of grain sizes is represented.
- Poorly sorted sands with a high fines content are more cohesive than well-sorted, fine sands are more cohesive than coarse ones : They absorb more water, but also more binding agent , regardless of the respective grain size and the type of rock .
- Low cohesion sands can be "liquefied" with a certain water content and are then known as quicksand .
- Round sands consist primarily of rounded components (such as rubble or gravel ), while angular sands consist of such grains ( broken and crushed sands ). Sharp-edged sands behave less plastically, both in sedimentation and in building materials, because the grains tilt. They are more difficult to mix, are less fluid in dispersions and lead to increased tool wear. When mixed with binding agents , they result in resistant, abrasion and pressure-resistant building materials (see also sand as a finite resource ).
- Broken sand, natural sharp-edged sands as a weathering product
- Squeeze sand is artificially produced sand with broken, sharp-edged grains, see Broken Minerals
- Flying sand is the name given to sand that is particularly easy to move by the wind due to its purity, small grain size and good sorting. If it occurs over a large area, it often appears in the form of dunes .
- River sand is fine-grained sand that was transported in a river by the current and thereby sorted and the grains of which were rounded by friction. It is washed out and therefore has a low proportion of suspended matter and water-soluble substances. It is therefore often used as a raw material in the construction industry and for concrete production. In sand pits mined material usually has to be washed, because clay and organic components are enriched.
Since only the grain size is decisive for the term sand, but not the mineral composition, sands can be composed in many ways. In the vast majority of the sands occurring on earth today, the proportion of grains made of quartz (SiO 2 ) dominates over other minerals ( quartz sands in the broader sense). The reason for this is the high proportion of quartz in the rocks of the earth's crust and its relatively high hardness (7 on the 10-step Mohs ' hardness scale ), as well as its resistance to chemical weathering. However, depending on the local geology and other conditions, sands with a completely different mineral composition can also occur.
- Carbonate sand, named after its main component calcium carbonate (CaCO 3 ), is found mainly on the beaches of islands with offshore coral reefs. These are therefore also English. back reef sands or, due to their relatively high proportion of coral fragments, colloquially known as “coral sand”. Even without an offshore coral reef, under certain conditions, e.g. B. in the absence of quartz sand, on beaches relatively pure carbonate sands, usually consisting of more or less rounded fragments of molluscs , accumulate, which are colloquially referred to as "shell sand". Mollusc fragments are also common in quartz-dominated marine beach sands.
- On islands of volcanic origin, sands arise from the physical weathering of volcanic rocks . These include the greenish olivine - and the dark basalt sands on the Hawaiian Islands. In contrast to olivine or quartz sand, basalt sand does not consist of such mineral grains, but of rock fragments the size of sand grains , due to the predominantly microscopic size of the mineral grains of basalt. Since the Hawaiian Islands are both of volcanic origin and surrounded by reefs, mixed carbonate-volcanic sands occur on their beaches.
- While olivine is one of the relatively weather- prone heavy minerals , more weather-resistant heavy minerals such as magnetite , ilmenite , monazite or garnet can accumulate locally in the form of so-called heavy mineral sands under suitable sedimentation conditions, even at a greater distance from their delivery area . Some of these enrichments are of economic importance as heavy metal deposits.
- In extremely dry regions of the world, sands can also be formed from minerals that are relatively easily soluble in water. An example of this is the fine gypsum sand, the white dunes of White Sands National Monument in the Chihuahuan Desert is in New Mexico.
In addition to the naturally occurring sands, artificially produced fine mineral granules are also referred to as "sands". This includes, but as blast furnace slag designated ground blast furnace slag .
The mechanical properties of sand are determined by the nature (shape, size, material) of the sand grains.
The angle of friction of sand cones (also called angle of repose or stable angle of slope ) depends on the internal friction of the sand, which is strongly influenced by the shape and size distribution of the sand grains. In the case of natural sands with good grading or sifted sands (small variance in grain sizes in each case ), in the case of generally good grain rounding, it is 30 °, in the case of predominantly angular grains up to 35 °. In poorly sorted sands (high variance in grain sizes) with dense grain packing (smaller grains are in the spaces between larger grains), the angle of repose can be even higher.
The density ( volume weight) of a dry sand γ t results from the mean density of the sand grains γ s and the proportion of the pore volume (grain space volume) in the total volume n according to the formula
- γ t = (1 - n ) γ s .
In Central Europe sands originate often in quartz and feldspar rich crystalline rocks such as granite and gneiss , but mostly in (quartz) sandstones . The grains of typical Central European sands thus have an average density that roughly corresponds to that of quartz (2.6 g / cm³). For a sand with a pore volume of around 40%, which is not unusual, this results in a density of around 1.6 g / cm³. In the case of water-saturated sands, the density of the water ρ H 2 O (≈ 1 g / cm³) must be taken into account. The corresponding formula is
- γ g = (1 - n ) γ s + n ρ H 2 O = γ s - n (γ s - ρ H 2 O ) .
For a typical Central European sand with a pore volume of around 40% and water saturation, this results in a density of around 2.0 g / cm³. In general, it should be noted that the grain sorting, i.e. H. the breadth of the grain size spectrum has an influence, because the greater the variance in the grain size distribution, the smaller the pore space and consequently the greater the density.
The thermal conductivity of sand depends, as u. a. also that of other loose sediments or soil types, essentially depends on three factors: the mean thermal conductivity of the grain material (e.g. quartz ≈ 8.0 W / mK) the pore volume and the water content in the pore space (water saturation). With the same mineral composition in dry sand, the thermal conductivity increases with decreasing pore volume (increasing grain packing density) due to the larger total contact surface of the grains. The thermal conductivity increases even more with the degree of water saturation, since water (0.6 W / mK) is a better heat conductor than air (0.03 W / mK). Typical values for the thermal conductivity of dry quartz sand (soil) are 0.3 W / mK (pore volume / solid volume ≈ 1) to 0.6 W / mK (pore volume / solid volume ≈ 0.5). With a water saturation level of 100%, the value of a slightly porous sand or sandy soil can be just under 3.0 W / mK.
Sand, and in this context the term mainly refers to quartz sand, occurs in more or less large concentrations all over the earth's surface. However, there are clear differences in the size of the deposits, which are due to factors such as parent rock, climate , regional and local geological conditions, relief and transport medium. Sand is a sediment and is therefore mainly found in sedimentary basins. In the high mountains , a marked area of erosion, sand is therefore only found sporadically, especially in the moraines of valley glaciers and in the deposits of rivers . In the low mountain range , but mainly in lowlands , large amounts of sand are transported and sedimented by meandering rivers. Also at the bottom of lakes there are sometimes huge sand deposits, insofar as larger rivers flow into them. Of sandbanks and flood plains fine sand can be blown and transported over long Plug ( aeolian transport ), as indeed the lack of a closed vegetation cover favors the attack of the wind. For many people, the term " desert " is associated with the image of dunes , and in fact large parts of the Sahara and the Namib as well as the western Asian deserts are characterized by sand as so-called sand deserts (although it does not always appear in the form of dunes) . In the cold climate zones, there are large expanses of sand in the vicinity of foreland glaciers and inland ice , which are known as sanders . The meltwater of the inland ice sheets of the last ice ages is responsible for the abundance of sand in northern Germany and especially in Brandenburg . There are also significant sand deposits where rivers flow into the sea to form a delta . The sand is then further distributed by currents parallel to the coast and appears on flat coasts as a beach and sandbank. During storms this sand is churned up and transported away from the coastal areas. Large amounts of sand are deposited on the continental shelves through rivers, currents and storms , from where parts of it reach the edge areas of the deep-sea plains through suspension currents .
Generally it can be said that, there are large sand deposits particularly where sandstone at the surface is present and thus can serve as a starting material. In areas where the subsoil consists mainly of limestone and where there is a humid climate , on the other hand, chemical weathering dominates : the rock is broken up rather than crushed and the terrain shows the typical treasure trove of karst forms . Lime sand only has a short service life under these conditions due to its large surface-to-volume ratio. This explains, for example, the almost complete absence of sandy beaches on the Croatian coast , because both on the coast itself and in large areas of the interior, only carbonate rocks ( Dinaric karst) bite out . Furthermore, chemical weathering also plays an important role in the continental, always humid tropics , and here too, for this reason, larger deposits of sand are rather rare.
Sand moved by the wind and other fine-grained sediments can cause corrosion (wind grinding, wind erosion) on rock formations according to the principle of the sandblasting fan and develop characteristic, sometimes bizarre forms of erosion, for example wind edges , mushroom rocks or yardangs .
Sand as a habitat
In Central Europe, as almost everywhere in the world, pure sandy soils consist for the most part of quartz grains. The lowlands of North Central Europe, characterized by sandy soils, are also known as geest . They are the result of Pleistocene sand deposits. In central and southern Germany, sandy soil landscapes mainly exist in areas where the sand, silt and clay stones of the red sandstone series bite out over large areas.
Quartz sand soils are among the least fertile types of soil , as minerals that can release or store nutrients when weathered are hardly available in such soils. Water also seeps away relatively quickly into the relatively large-pored substrate and nutrients are quickly washed out. As soil preferably develop podsols or podzolic brown soils . The sandy soil landscapes of Central Europe, however, are not comparable with the relatively bare and supposedly dead deserts of Africa or Australia. This country as sufficient water is present, open sand surfaces are relatively quickly by pioneer plants, such as the beach grass ( Ammophila the spp.), Bent grass ( Carex arenaria ), the silver grass ( Corynephorus canescens ) and the couch grass ( Agropyron spp.) Populated. Later u. a. Heather ( Erica spp.), Which are very well adapted to the relatively dry locations. Under natural conditions, beech - or oak - birch mixed forests would ultimately develop, which provide a habitat for numerous plants and animals.
In Central Europe, however, there are hardly any original landscapes left. The sandy soils are used for forestry and mostly contain pine monocultures , as in Brandenburg or in the Altmark . The open landscape, e.g. B. the Lüneburg Heath, was created by clearing formerly existing forests and thus also a cultural landscape .
Since the remaining natural communities of the sandy landscapes of Central Europe are threatened by human influence, measures are taken to put them under protection. An example of this was the biotope network Sandachse Franken .
rest of the world
Sandy landscapes serve as living space not only in the humid climate of Central Europe. Also in the rather hostile sandy deserts of Africa, Asia and Australia are numerous animal and plant species that have adapted to the extreme conditions in the course of evolution. An example of a plant is Welwitschia ( Welwitschia mirabilis ), which only occurs in the Namib Desert . Of the terrestrial vertebrates , it is above all the scaled reptiles that cope best with the extreme conditions. A particularly spectacular example is the thorn devil ( Moloch horridus ) in the Australian desert. Since the sand areas are heated by the sun to over 60 ° C during the day, numerous animals move through the sand by “swimming”, among other things. a. the apothecary kink ( Scincus scincus ) and the pouch mole ( Notoryctes spp.) in Australia or the mist drinker beetle ( Onymacris unguicularis ) in the Namib. The Palmatogecko also lives in the Namib and avoids the heat of the day through night activity . His feet are equipped with skins between the toes, through which he can distribute his body weight over a larger surface of the soles of the foot and so does not sink into the loose desert sand.
Sandy sediment occurs at the bottom of lakes, rivers and coastal regions of the sea. In terms of its function as a habitat, it is also known as sandy substrate . The community that specializes in sandy substrates is called Psammon . A distinction is made between the macropsammon and the mesopsammon . The macropsammon includes swirling and sediment-eating invertebrates as well as a few predatory invertebrates . In fresh water these are mainly mussels and snails , in the sea also crustaceans , especially the thalassinidea (mole crabs), bristle worms , sea urchins and starfish . Buildings of organisms of the macropsammon have been handed down in fossil form in sandstones (e.g. Ophiomorpha , Arenicolites or Skolithos ). Mesopsammon are eukaryotes and dwarf invertebrates that live in the pore space of the sediment. These are ciliates , urochordates , nematodes and dwarf representatives of the molluscs , echinoderms , bristle worms and crustaceans. Insect larvae are also a part of freshwater.
Along with air and water , sand is the most widely used natural resource on earth. Of the 47 to 59 billion tons of ores , salts , coal , stones and earth mined annually in mines, quarries, etc. , sands make up between 68 and 85 percent. The places where sand is mined are called sand and gravel pits or mills.
In many branches of industry, sand is an important raw material or starting material for the raw materials used. It is primarily used as a building material in civil engineering , traffic routes and earthworks . Furthermore, sand is an essential aggregate ( aggregate ) in building materials such as concrete and mortar , which is used as an easily malleable mass, also for the interior and facade decoration of buildings. In construction, one pit sands, different fracture sand , crushed sand , joint sand , river sand and sea sand. Quartz-rich sand is also a raw material for cement production .
Desert sand, on the other hand, is not usable for the construction industry, as the grains of sand are ground round by the wind and can no longer get caught due to the missing edges (see also: Sand as a finite resource ).
Silica sand is also used as blasting material in blasting used ( "sandblasting"). Fine-grained corundum is increasingly being used as a substitute , since the silicate dust can cause silicosis ("dust lung "). Sand is also suitable as a grinding , scouring ( scouring sand ) and polishing agent . Quartz sand is also a raw material for glass production . Furthermore, quartz sand serves as a raw material for the extraction of pure silicon as a starting material for the production of components in semiconductor technology and semiconductor electronics .
Furthermore, sand is used as a filter medium in water and wastewater treatment as well as in drainage technology (e.g. in retention soil filters ). Sands made from different materials (quartz, corundum, zirconium ) are used as a shaping medium in the sand casting process .
Since sand has a relatively large pore volume , underground sand and sandstone deposits are important as a natural storage medium for drinking water , crude oil and natural gas . Close to the earth's surface, sand, as oil sand, can also have energy-economic and political significance.
For tourism , sand is a special attraction when there are superficial deposits of sand in the form of sandy beaches and dunes on the coast . It is also used as a design element in landscape planning, horticulture, sports and children's playgrounds ( sandpit ). Certain types of sand are suitable as building materials for sand sculptures .
Rail vehicles usually have a sanding device from which sand can be discharged onto the rails in order to increase the frictional resistance of the rail during braking or when the train starts up. Litter sand is used in winter when there is black ice on roadways as well as cycle and footpaths to restore an almost normal frictional resistance of the subsoil.
Very fine-grained, well-sorted, pure, dry quartz sand trickles through a small opening in hourglasses . A mixture of mostly fine quartz sand sold under the name of bird sand is used as litter in bird cages. It is not only used for hygiene, but also as a digestive aid for the birds. Sandbags serve as makeshift flood protection and as protection from projectiles in the military and civilian areas.
Sand as a finite resource
Only quartz sands with certain compositional and textural properties can be used in the construction industry for the production of mortar and concrete . The annual consumption of sand and gravel in 2018 is around 40 billion tons (mainly for the production of concrete). Due to the worldwide population and urban growth and the associated construction activity, there is a great demand for suitable sand, the natural occurrence of which is almost exhausted in some regions. After water, sand is considered the second most important raw material in the world by volume. The great demand has already led to international conflicts. In 2012, the French director Denis Delestrac shot the documentary Sand Wars (German version: Sand - the new environmental time bomb ) about the effects of the lack of building sand and the ecological and economic effects of legal and illegal sand mining and trade.
India is a major exporter of construction sand. There reports about the so-called sand mafia are part of the everyday news. Another hot spot for illegal sand mining is Morocco . There, the world's largest mining of coastal sands takes place after a report in the Santa Aguila Foundation in 2007, and notes of Germany radio from the year 2016, according to were "there been removed already half of the beaches illegally ." From the Lake Victoria is made possible by An unclear legal situation , large-scale sand for the large construction sites in Uganda , which is booming as a result of extensive Chinese investments, is being promoted, which leads to land losses and conflicts with the fishermen on the lake. The emirate of Dubai ( United Arab Emirates ) has a lot of sand, but desert sand is not well suited as construction sand: it is too fine-grained and the grains are too round. Instead, sand from the seabed was used for the numerous local construction projects until the beginning of the 2010s, with serious consequences for the affected benthic ecosystem. In addition, sea sand has to be laboriously cleaned of salt before it is used as construction sand, otherwise the reinforcement in the reinforced concrete will corrode . Because these deposits are now almost exhausted, Dubai is now importing building sand on a large scale by ship from Australia . In Germany there is increasing local resistance to the expansion of sand mining, which among other things will lead to uncontrolled destruction of the landscape and arable land.
Analogous to “ Peak Oil ”, the term “ Peak Sand ” is used in connection with the limitation of the resource building sand . The negative effects of the mining of far larger amounts of sand than can accumulate again in the course of the rock cycle in the lowlands , on the coasts and on the shelves near the coast , are intensified by other human activities: Large reservoirs form extensive artificial sedimentation spaces in mountain regions in which the Sands transported in the upper reaches of the rivers are deposited and no longer reach the lowlands and coastal waters . This increases the natural coastal erosion .
The demand for natural building sands could be increased through fresh concrete recycling and the preferred use of recycled building rubble (so-called recycled aggregates ) and the fine-grained by-products from the production of aggregates from natural stone (fine chippings, crushed sand and rock flour, summarized under the name quarry dust ) for concrete production be reduced. According to the current status (2011), however, only a relatively small part of the natural sand can be replaced by recycled concrete rubble, since otherwise the recycled concrete does not have the desired mechanical properties.
Squeaky sand can be found on beaches on every continent of the world. The conditions are a permeable, non-cohesive subsoil , a certain type of quartz sand without lime content and a certain grain size (approx. 150–500 micrometers). The sand must lie in a layered, so-called disturbed layer (usually caused by wind), have a certain moisture content and be weathered or sanded to a smooth and rounded surface . The squeaking of the sand is caused by mechanical stress, i.e. the pressure applied when running over it and the resulting friction between the grains. The beach Squeaky Beach in southeast Australia was named after the squeaky sand.
There is no connection to the singing sand , which in rare cases is caused by the wind on sand dunes. There are hardly more than 30 known dunes of singing sand in deserts and on beaches in Africa, Asia and North America.
Around the sand
- A lightning strike in quartz sand can produce fulgurite , a natural glass.
- Sand traps in flowing waters and sewage treatment plants serve to hold back sedimentable substances with a higher density than that of water.
- Sandhose refers to a local air vortex with blown sand, large areas are called sandstorms .
- The sandman in German folk myth puts sand in the children's eyes when they fall asleep . The title character of the show Our Sandman was derived from it.
- Sand roses are natural structures made of gypsum or barite (barite), which crystallize out of aqueous solutions in the intergranular space of sand
- Kinetic sand is an indoor toy made of pure quartz sand held together by a silicone-like binder and can be shaped like wet sand.
- Sand was named Rock of the Year 2016 by the German Society for Geosciences
Regions, places and roads that are or were related to the occurrence, mining, transport or storage of sand (cf. distribution ) often refer to this by names in which the words "sand" or as a proxy "Gries" or "semolina" occur, in Germany z. B. Sandhausen in the north of the Upper Rhine Plain , Riedern am Sand in Klettgau and the nearby Grießen , in Austria z. B. Gries am Brenner , Sandgasse in Linz , the Gries district in Graz and Sandgrube 13 , the address of a winemaker in Krems an der Donau . The sandy soils that characterize large parts of the North German Plain are included in the first line of the Brandenbur link ("Märkische Heide, Märkischer Sand").
"Sand" as a symbol
- Throwing sand in someone's eyes for "deceiving someone"
- Putting something in the sand for "causing a failure"
- Sand in the gear for "a disturbed process"
- Like sand on the sea for "uncountable numbers"
- Bury your head in the sand for "not wanting to see a danger" or for "giving up early"
- To be built on sand for "having an unsafe foundation"
- Fizzled out for "an inconclusive end"
- Like sand running through your fingers for "something intangible or lost"
In figurative figures (e.g. traces in the sand ), sand is a symbol for (passing) time. Hourglasses are particularly expressive in this regard , especially since they can only be used to determine finite periods of time (“time passes”).
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