Sand

Sand is a granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.[1]

The composition of sand varies, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide, or SiO2), usually in the form of quartz. The second most common type of sand is calcium carbonate, for example, aragonite, which has mostly been created, over the past half billion years, by various forms of life, like coral and shellfish. For example, it is the primary form of sand apparent in areas where reefs have dominated the ecosystem for millions of years like the Caribbean.

Sand is a non-renewable resource over human timescales, and sand suitable for making concrete is in high demand.[2] Desert sand, although plentiful, is not suitable for concrete, and 50 billion tons of beach sand and fossil sand is needed each year for construction.[3]

Libya 4608 Idehan Ubari Dunes Luca Galuzzi 2007
Sand dunes in the Idehan Ubari, Libya.
Sand from Gobi Desert
Close-up (1×1 cm) of sand from the Gobi Desert, Mongolia.

Composition

HeavyMineralsBeachSand
Heavy minerals (dark) in a quartz beach sand (Chennai, India).
CoralPinkSandDunesSand
Sand from Coral Pink Sand Dunes State Park, Utah. These are grains of quartz with a hematite coating providing the orange color.
PismoBeachSand
Sand from Pismo Beach, California. Components are primarily quartz, chert, igneous rock and shell fragments.

The exact definition of sand varies. The scientific Unified Soil Classification System used in engineering and geology corresponds to US Standard Sieves,[4] and defines sand as particles with a diameter of between 0.074 and 4.75 millimeters. By another definition, in terms of particle size as used by geologists, sand particles range in diameter from 0.0625 mm (or ​116 mm) to 2 mm. An individual particle in this range size is termed a sand grain. Sand grains are between gravel (with particles ranging from 2 mm up to 64 mm by the latter system, and from 4.75 mm up to 75 mm in the former) and silt (particles smaller than 0.0625 mm down to 0.004 mm). The size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0.02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century. The grains of sand in Archimedes Sand Reckoner written around 240 BCE, were 0.02 mm in diameter. A 1953 engineering standard published by the American Association of State Highway and Transportation Officials set the minimum sand size at 0.074 mm. A 1938 specification of the United States Department of Agriculture was 0.05 mm.[5] Sand feels gritty when rubbed between the fingers. Silt, by comparison, feels like flour.

ISO 14688 grades sands as fine, medium, and coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to 2.0 mm. In the United States, sand is commonly divided into five sub-categories based on size: very fine sand (​116 – ​18 mm diameter), fine sand (​18 mm – ​14 mm), medium sand (​14 mm – ​12 mm), coarse sand (​12 mm – 1 mm), and very coarse sand (1 mm – 2 mm). These sizes are based on the Krumbein phi scale, where size in Φ = -log2D; D being the particle size in mm. On this scale, for sand the value of Φ varies from −1 to +4, with the divisions between sub-categories at whole numbers.

Volcanic sand (Perissa, Santorini, Greece)
Close up of black volcanic sand from Perissa, Santorini, Greece

The most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide, or SiO2), usually in the form of quartz, which, because of its chemical inertness and considerable hardness, is the most common mineral resistant to weathering.

The composition of mineral sand is highly variable, depending on the local rock sources and conditions. The bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material, suggesting sand formation depends on living organisms, too.[6] The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their bright, white color. Arkose is a sand or sandstone with considerable feldspar content, derived from weathering and erosion of a (usually nearby) granitic rock outcrop. Some sands contain magnetite, chlorite, glauconite or gypsum. Sands rich in magnetite are dark to black in color, as are sands derived from volcanic basalts and obsidian. Chlorite-glauconite bearing sands are typically green in color, as are sands derived from basaltic lava with a high olivine content. Many sands, especially those found extensively in Southern Europe, have iron impurities within the quartz crystals of the sand, giving a deep yellow color. Sand deposits in some areas contain garnets and other resistant minerals, including some small gemstones.

Sources

Rocks erode/weather over a long period of time, mainly by water and wind, and their sediments are transported downstream. These sediments continue to break apart into smaller pieces until they become fine grains of sand. The type of rock the sediment originated from and the intensity of the environment gives different compositions of sand. The most common rock to form sand is Granite, where the Feldspar minerals dissolve faster than the Quartz, causing the rock to break apart into small pieces. In high energy environments rocks break apart much faster than in more calm settings. For example, Granite rocks this means more Feldspar minerals in the sand because it wouldn't have had time to dissolve. The term for sand formed by weathering is epiclastic.[7]

Sand from rivers are collected either from the river itself or its flood plain, and accounts for the majority of the sand used in the construction industry. Because if this, many small rivers have been depleted, causing environmental concern and economic losses to adjacent land. The rate of sand mining in such areas greatly outweighs the rate the sand can replenish, making it a non-renewable resource. [8]

Sand dunes are a consequence of dry conditions or wind deposition. The Sahara Desert is very dry because of its geographic location and is known for its vast sand dunes. They exist here because very little vegetation is able to grow and there's not a lot of water. Over time, wind blows away all the fine particles, such as clay and dead organic matter, leaving only sand and larger rocks. Only 15% of the Sahara is sand dunes, while 70% is bare rock.[9] The wind is responsible for creating these different environments and shaping the sand to be round and smooth. These properties make desert sand unusable for construction.[10]

Beach sand is also formed by erosion. Over thousands of years, rocks are eroded near the shoreline from the constant motion of waves and the sediments build up. Weathering and river deposition also accelerate the process of creating a beach, along with marine animals interacting with rocks, such as eating the algae off of them. Once there is a sufficient amount of sand, the beach acts as a barrier to keep the land from eroding any further. This sand is ideal for construction as it's angular and of various sizes.[11]

Marine sand (or ocean sand) comes from sediments transported into the ocean and the erosion of ocean rocks. The thickness of the sand layer varies, however it's common to have more sand closer to land. This type of sand is ideal for construction and is a very valuable commodity. Europe is the main miners of marine sand, which greatly hurts ecosystems and local fisheries.[8]

Study

Sand under electron microscope
An electron micrograph showing grains of sand
Pitted sand grains Western Desert
Pitted sand grains from the Western Desert, Egypt. Pitting is a consequence of wind transportation.

The study of individual grains can reveal much historical information as to the origin and kind of transport of the grain.[12] Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. It is called grus in geology or sharp sand in the building trade where it is preferred for concrete, and in gardening where it is used as a soil amendment to loosen clay soils. Sand that is transported long distances by water or wind will be rounded, with characteristic abrasion patterns on the grain surface. Desert sand is typically rounded.

People who collect sand as a hobby are known as arenophiles. Organisms that thrive in sandy environments are psammophiles.[13]

Uses

Песчинки-1. Желтый строительный песок
Sand grains of yellow building sand. Microscope Lumam P-8. EPI lighting. The photo of each grain of sand is the result of multifocal stacking..
  • Abrasion: Before sandpaper, wet sand was used as abrasive element between rotating devices with elastic surface and hard materials such as very hard stone (making of stone vases), or metal (removal of old stain before re-staining cupper cooking pots).
  • Agriculture: Sandy soils are ideal for crops such as watermelons, peaches and peanuts, and their excellent drainage characteristics make them suitable for intensive dairy farming.
  • Aquaria: Sand makes a low cost aquarium base material which some believe is better than gravel for home use. It is also a necessity for saltwater reef tanks, which emulate environments composed largely of aragonite sand broken down from coral and shellfish.
  • Artificial reefs: Geotextile bagged sand can serve as the foundation for new reefs.
  • Artificial islands in the Persian Gulf.
  • Beach nourishment: Governments move sand to beaches where tides, storms or deliberate changes to the shoreline erode the original sand.[14]
  • Brick: Manufacturing plants add sand to a mixture of clay and other materials for manufacturing bricks.[15]
  • Cob: Coarse sand makes up as much as 75% of cob.
  • Concrete: Sand is often a principal component of this critical construction material.
  • Glass: Sand rich in silica is the principal component in common glasses.
  • Hydraulic fracturing: A drilling technique for natural gas, which uses rounded silica sand as a "proppant", a material to hold open cracks that are caused by the hydraulic fracturing process.
  • Landscaping: Sand makes small hills and slopes (golf courses would be an example).
  • Mortar: Sand is mixed with masonry cement or Portland cement and lime to be used in masonry construction.
  • Paint: Mixing sand with paint produces a textured finish for walls and ceilings or non-slip floor surfaces.
  • Railroads: Engine drivers and rail transit operators use sand to improve the traction of wheels on the rails.
  • Recreation: Playing with sand is a favorite beach time activity. One of the most beloved uses of sand is to make sometimes intricate, sometimes simple structures known as sand castles. Such structures are well known for their impermanence. Sand is also used in children's play. Special play areas enclosing a significant area of sand, known as sandboxes, are common on many public playgrounds, and even at some single family homes. Sand dunes are also popular among climbers, motorcyclists and beach buggy drivers.
  • Roads: Sand improves traction (and thus traffic safety) in icy or snowy conditions.
  • Sand animation: Performance artists draw images in sand. Makers of animated films use the same term to describe their use of sand on frontlit or backlit glass.
  • Sand casting: Casters moisten or oil molding sand, also known as foundry sand and then shape it into molds into which they pour molten material. This type of sand must be able to withstand high temperatures and pressure, allow gases to escape, have a uniform, small grain size and be non-reactive with metals.
  • Sand castles: Shaping sand into castles or other miniature buildings is a popular beach activity.
  • Sandbags: These protect against floods and gunfire. The inexpensive bags are easy to transport when empty, and unskilled volunteers can quickly fill them with local sand in emergencies.
  • Sandblasting: Graded sand serves as an abrasive in cleaning, preparing, and polishing.
  • Thermal weapon: While not in widespread use anymore, sand used to be heated and poured on invading troops in the classical and medieval time periods.
  • Water filtration: Media filters use sand for filtering water.
  • Wuḍūʾ: the Islamic procedure for washing parts of the body.
  • Zoanthid "skeletons": Animals in this order of marine benthic cnidarians related to corals and sea anemones, incorporate sand into their mesoglea for structural strength, which they need because they lack a true skeleton.

Resources and environmental concerns

Only some sands are suitable for the construction industry, for example for making concrete. Because of the growth of population and of cities and the consequent construction activity there is a huge demand for these special kinds of sand, and natural sources are running low. In 2012 French director Denis Delestrac made a documentary called "Sand Wars" about the impact of the lack of construction sand. It shows the ecological and economic effects of both legal and illegal trade in construction sand.[16][17][18]

To retrieve the sand, the method of hydraulic dredging is used. This works by pumping the top few meters of sand out of the water and filling it into a boat, which is then transported back to land for processing. Unfortunately, all marine life mixed in with the extracted sand is killed and the ecosystem can continue to suffer for years after the mining is complete. Not only does this affect marine life, but also the local fishing industries because of the loss of life, and communities living close to the water's edge. When sand is taken out of the water it increases the risk of landslides, which can lead to loss of agricultural land and/or damage to dwellings.[19]

Sand's many uses require a significant dredging industry, raising environmental concerns over fish depletion, landslides, and flooding.[20] Countries such as China, Indonesia, Malaysia and Cambodia ban sand exports, citing these issues as a major factor.[21] It is estimated that the annual consumption of sand and gravel is 40 billion tons and sand is a US$70 billion global industry.[22]

The global demand for sand in 2017 was 9.55 billion tons as part of a $99.5 billion industry.[23]

Hazards

While sand is generally non-toxic, sand-using activities such as sandblasting require precautions. Bags of silica sand used for sandblasting now carry labels warning the user to wear respiratory protection to avoid breathing the resulting fine silica dust. Safety data sheets for silica sand state that "excessive inhalation of crystalline silica is a serious health concern".[24]

In areas of high pore water pressure, sand and salt water can form quicksand, which is a colloid hydrogel that behaves like a liquid. Quicksand produces a considerable barrier to escape for creatures caught within, who often die from exposure (not from submersion) as a result.

Manufactured sand

Manufactured sand (M sand) is sand made from rock by artificial processes, usually for construction purposes in cement or concrete. It differs from river sand by being more angular, and has somewhat different properties.[25]

Case studies

In Dubai, United Arab Emirates, the use of sand has been very demanding in the construction of infrastructure and creating new islands. They completely ran out of their own reserves and now import most of their sand from Australia. There have been three projects to create artificial islands needing more than 835 million tonnes of sand, which costed more than $26 billion USD.[26]

See also

References

  1. ^ Glossary of terms in soil science (PDF). Ottawa: Agriculture Canada. 1976. p. 35. ISBN 978-0662015338.
  2. ^ Constable, Harriet (3 September 2017). "How the demand for sand is killing rivers". BBC News Magazine. Retrieved 9 September 2017.
  3. ^ Albarazi, Hannah. "The Slippery Slopes of the World Sand Shortage". Retrieved 29 March 2019.
  4. ^ Unified Soil Classification System
  5. ^ Urquhart, Leonard Church, "Civil Engineering Handbook" McGraw-Hill Book Company (1959) p. 8-2
  6. ^ Seaweed also plays a role in the formation of sand. Susanscott.net (1 March 2002). Retrieved on 24 November 2011.
  7. ^ Gilman, Larry (2014). Sand. 7 (5 ed.). The Gale Encyclopedia of Science. pp. 3823–3824.
  8. ^ a b Padmalal, Maya (2014). "Sources of Sand and Conservation". Sand Mining. Springer, Dordrecht. pp. 155–160. ISBN 978-94-017-9143-4.
  9. ^ "Sahara". THE COLUMBIA ENCYCLOPEDIA (6 ed.). COLUMBIA UNIVERSITY PRESS. 2000. ISBN 9780787650155.
  10. ^ "What is the reason for not using sea and desert sand for construction?". The Hindu. 2 August 2015. ISSN 0971-751X. Retrieved 9 April 2019.
  11. ^ "How Is A Beach Formed?". WorldAtlas. Retrieved 10 April 2019.
  12. ^ Krinsley, D.H., Smalley, I.J. 1972. Sand. American Scientist 60, 286-291
  13. ^ "Psammophile". Merriam-Webster.com. Retrieved 27 January 2016.
  14. ^ "Importing Sand, Glass May Help Restore Beaches". NPR.org. 17 July 2007.
  15. ^ Yong, Syed E. Hasan, Benedetto De Vivo, Bernhard Grasemann, Kurt Stüwe, Jan Lastovicka, Syed M. Hasan, Chen (5 December 2011). ENVIRONMENTAL AND ENGINEERING GEOLOGY -Volume III. EOLSS Publications. ISBN 9781848263574.
  16. ^ See Sand Wars teaser here.
  17. ^ Simon Ings (26 April 2014). "The story of climate change gets star treatment". New Scientist: 28–9.
  18. ^ Strände in Gefahr? Arte Future, last updated 23 April 2014
  19. ^ Kim, Tae Goun (14 September 2007). "The economic costs to fisheries because of marine sand mining in Ongjin Korea: Concepts, methods, and illustrative results" (PDF). ScienceDirect – via Elsevier.
  20. ^ Torres, Aurora; et al. (8 September 2017). "The world is facing a global sand crisis". The Conversation. Retrieved 9 September 2017.
  21. ^ "The hourglass effect". The Economist. 8 October 2009. Retrieved 14 October 2009.
  22. ^ Beiser, Vince (26 March 2015). "The Deadly Global War for Sand". Wired. Retrieved 26 March 2015.
  23. ^ Doyle, Alister (11 February 2019). "As ice melts, Greenland could become big sand exporter: study". www.reuters.com. Retrieved 12 February 2019.
  24. ^ Silica sand MSDS Archived 11 March 2006 at the Wayback Machine. Simplot (13 March 2011). Retrieved on 24 November 2011.
  25. ^ Pilegis, M.; Gardner, D.; Lark, R. (2016). "An Investigation into the Use of Manufactured Sand as a 100% Replacement for Fine Aggregate in Concrete". Materials. 9 (6): 440. doi:10.3390/ma9060440. PMC 5456819. PMID 28773560.
  26. ^ PEDUZZI, Pascal (April 2014). "Sand, rarer than one thinks". Environmental Development. 11: 208–218.

External links

Media related to Sand at Wikimedia Commons

Sand Mining Side Effects
Aquifer

An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt). Groundwater can be extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer, and aquiclude (or aquifuge), which is a solid, impermeable area underlying or overlying an aquifer. If the impermeable area overlies the aquifer, pressure could cause it to become a confined aquifer.

Beach

A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles. The particles can also be biological in origin, such as mollusc shells or coralline algae.

Some beaches have man-made infrastructure, such as lifeguard posts, changing rooms, showers, shacks and bars. They may also have hospitality venues (such as resorts, camps, hotels, and restaurants) nearby. Wild beaches, also known as undeveloped or undiscovered beaches, are not developed in this manner. Wild beaches can be appreciated for their untouched beauty and preserved nature.

Beaches typically occur in areas along the coast where wave or current action deposits and reworks sediments.

Desert

A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one-third of the land surface of the world is arid or semi-arid. This includes much of the polar regions where little precipitation occurs and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.

Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter and the resulting fragments and rubble strewn over the desert floor are further eroded by the wind. This picks up particles of sand and dust and wafts them aloft in sand or dust storms. Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements and little further erosion takes place. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water in the form of springs and seepages from aquifers. Where these are found, oases can occur.

Plants and animals living in the desert need special adaptations to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles and often spines to deter herbivory. Some annual plants germinate, bloom and die in the course of a few weeks after rainfall while other long-lived plants survive for years and have deep root systems able to tap underground moisture. Animals need to keep cool and find enough food and water to survive. Many are nocturnal and stay in the shade or underground during the heat of the day. They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine. Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. They then reproduce rapidly while conditions are favorable before returning to dormancy.

People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available and oases have provided opportunities for a more settled way of life. The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Desert farming is possible with the aid of irrigation, and the Imperial Valley in California provides an example of how previously barren land can be made productive by the import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara Desert, and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves were also taken northwards across the Sahara. Some mineral extraction also takes place in deserts, and the uninterrupted sunlight gives potential for the capture of large quantities of solar energy.

Dune

In physical geography, a dune is a hill of loose sand built by aeolian processes (wind) or the flow of water. Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the stoss (upflow) side, where the sand is pushed up the dune, and have a shorter "slip face" in the lee side. The valley or trough between dunes is called a slack. A "dune field" or erg is an area covered by extensive dunes.

Dunes occur in some deserts and along some coasts. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds. Dunes can form under the action of water flow (fluvial processes), and on sand or gravel beds of rivers, estuaries and the sea-bed.

The modern word "dune" came into English from French c. 1790, which in turn came from Middle Dutch dūne.

Dust storm

A dust storm is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Fine particles are transported by saltation and suspension, a process that moves soil from one place and deposits it in another.

Drylands around North Africa and the Arabian peninsula are the main terrestrial sources of airborne dust. It has been argued that poor management of the Earth's drylands, such as neglecting the fallow system, are increasing dust storms size and frequency from desert margins and changing both the local and global climate, and also impacting local economies.The term sandstorm is used most often in the context of desert dust storms, especially in the Sahara Desert, or places where sand is a more prevalent soil type than dirt or rock, when, in addition to fine particles obscuring visibility, a considerable amount of larger sand particles are blown closer to the surface. The term dust storm is more likely to be used when finer particles are blown long distances, especially when the dust storm affects urban areas.

George Sand

Amantine Lucile Aurore Dupin (French: [amɑ̃tin lysil oʁɔʁ dypɛ̃]; 1 July 1804 – 8 June 1876), best known by her nom de plume George Sand (French: [ʒɔʁʒ sɑ̃d]), was a French novelist, memoirist, and socialist. One of the most popular writers in Europe in her lifetime, being more popular than both Victor Hugo and Honore de Balzac in England in the 1830s and 1840s, Sand is recognised as one of the most notable writers of the European Romantic era.

Great Sand Dunes National Park and Preserve

Great Sand Dunes National Park and Preserve is an American national park that conserves an area of large sand dunes up to 750 feet (229 m) tall on the eastern edge of the San Luis Valley, and an adjacent national preserve located in the Sangre de Cristo Range, in south-central Colorado, United States. The park was originally designated Great Sand Dunes National Monument on March 17, 1932 by President Herbert Hoover. The original boundaries protected an area of 35,528 acres (55.5 sq mi; 143.8 km2). A boundary change and redesignation as a national park and preserve was authorized on November 22, 2000 and then established by an act of Congress on September 24, 2004. The park encompasses 107,342 acres (167.7 sq mi; 434.4 km2) while the preserve protects an additional 41,686 acres (65.1 sq mi; 168.7 km2) for a total of 149,028 acres (232.9 sq mi; 603.1 km2). The recreational visitor total was 442,905 in 2018.The park contains the tallest sand dunes in North America. The dunes cover an area of about 30 sq mi (78 km2) and are estimated to contain over 1.2 cubic miles (5 billion cubic metres) of sand. Sediments from the surrounding mountains filled the valley over geologic time periods. After lakes within the valley receded, exposed sand was blown by the predominant southwest winds toward the Sangre de Cristos, eventually forming the dunefield over an estimated tens of thousands of years. The four primary components of the Great Sand Dunes system are the mountain watershed, the dunefield, the sand sheet, and the sabkha. Ecosystems within the mountain watershed include alpine tundra, subalpine forests, montane woodlands, and riparian zones.Evidence of human habitation in the San Luis Valley dates back about 11,000 years. The first historic peoples to inhabit the area were the Southern Ute Tribe, while Apaches and Navajo also have cultural connections in the dunes area. In the late 17th century, Don Diego de Vargas—a Spanish governor of Santa Fe de Nuevo México—became the first European on record to enter the San Luis Valley. Juan Bautista de Anza, Zebulon Pike, John C. Frémont, and John Gunnison all travelled through and explored parts of the region in the 18th and 19th centuries. The explorers were soon followed by settlers who ranched, farmed and mined in the valley starting in the late 19th century. The park was first established as a national monument in 1932 to protect it from gold mining and the potential of a concrete manufacturing business.Visitors must walk across the wide and shallow Medano Creek to reach the dunes in spring and summer months. The creek typically has a peak flow from late May to early June in most years. From July through April, the creek is usually no more than a few inches deep, if there is any water at all. Hiking is permitted throughout the dunes with the warning that the sand surface temperature may reach 150 °F (66 °C) in summer. Sandboarding and sandsledding are popular activities, both done on specially designed equipment which can be rented just outside the park entrance or in Alamosa. Visitors with street-legal four-wheel drive vehicles may continue past the end of the park's main road to Medano Pass on 22 miles (35 km) of unpaved road, crossing the stream bed of Medano Creek nine times and traversing 4 miles (6.4 km) of deep sand. Hunting is permitted in the preserve during the months of autumn, while hunting is prohibited within national park boundaries at all times. The preserve encompasses nearly all of the mountainous areas north and east of the dunefield, up to the ridgeline of the Sangre de Cristos.

Hourglass

An hourglass (or sandglass, sand timer, sand clock or egg timer) is a device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated trickle of material (historically sand) from the upper bulb to the lower one. Factors affecting the time it measured include sand quantity, sand coarseness, bulb size, and neck width. Hourglasses may be reused indefinitely by inverting the bulbs once the upper bulb is empty. Depictions of hourglasses in art survive in large numbers from antiquity to the present day, as a symbol for the passage of time. These were especially common sculpted as epitaphs on tombstones or other monuments, also in the form of the winged hourglass, a literal depiction of the well-known Latin epitaph tempus fugit ("time flies").

List of A Song of Ice and Fire characters

George R. R. Martin's A Song of Ice and Fire novels feature a sizable cast of characters. The series follows three interwoven plotlines: a dynastic war for control of Westeros by several families; the rising threat of the superhuman Others beyond Westeros' northern border; and the ambition of Daenerys Targaryen, the exiled heir of the previous ruling dynasty. The Great Houses of Westeros represent the Seven Kingdoms forged across the continent: the North, the Iron Islands, the Vale of Arryn, the Westerlands, the Stormlands, the Reach, and Dorne. A massive Wall of ice and old magic separates the Seven Kingdoms from the largely unmapped area in the most northern portion of the continent.

Each chapter is narrated in the third-person limited point of view through the eyes of a single character. Beginning with nine POV characters in A Game of Thrones (1996), a total of 31 such characters have narrated over the course of the first five volumes of the series.

Reef

A reef is a bar of rock, sand, coral or similar material, lying beneath the surface of water.

Many reefs result from natural, abiotic processes—deposition of sand, wave erosion planing down rock outcrops, etc.—but the best known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and coralline algae.

Artificial reefs (e.g. shipwrecks) sometimes have a role in enhancing the physical complexity of featureless sand bottoms, in order to attract a diverse assemblage of organisms, especially algae and fish.

Earth's largest reef system is the Great Barrier Reef in Australia, at a length of over 2,300 kilometres (1,400 miles).

Sand Creek massacre

The Sand Creek massacre (also known as the Chivington massacre, the battle of Sand Creek or the massacre of Cheyenne Indians) was a massacre of Cheyenne and Arapaho people by the U.S. Army in the American Indian Wars that occurred on November 29, 1864, when a 675-man force of Colorado U.S. Volunteer Cavalry under the command of U.S. Army Colonel John Chivington attacked and destroyed a village of Cheyenne and Arapaho people in southeastern Colorado Territory, killing and mutilating an estimated 70–500 Native Americans, about two-thirds of whom were women and children. The location has been designated the Sand Creek Massacre National Historic Site and is administered by the National Park Service. This was part of a series of events known as the Colorado War and was preceded by the Hungate massacre.

Sand cat

The sand cat (Felis margarita), also known as the sand dune cat, is the only cat living chiefly in true deserts. This small cat is widely distributed in the deserts of North Africa, the Middle East and Central Asia. Starting in 2002, it was listed as near threatened on the IUCN Red List because the population was considered fragmented and small with a declining trend. It was downlisted to least concern in 2016.Owing to long hairs covering the soles of its feet, the sand cat is well adapted to the extremes of a desert environment and tolerant of extremely hot and cold temperatures. It inhabits both sandy and stony deserts, in areas far from water sources.

Sand tiger shark

The sand tiger shark (Carcharias taurus), grey nurse shark, spotted ragged-tooth shark, or blue-nurse sand tiger is a species of shark that inhabits subtropical and temperate waters worldwide. It inhabits the continental shelf, from sandy shorelines (hence the name sand tiger shark) and submerged reefs to a depth of around 191 m (627 ft). They dwell in the waters of Japan, Australia, South Africa, the Mediterranean and the east coasts of North and South America. Despite its name, it is not related to the tiger shark Galeocerdo cuvier; however, it is a cousin of the great white shark Carcharodon carcharias.

Despite its fearsome appearance and strong swimming ability, it is a relatively placid and slow-moving shark with no confirmed human fatalities. This species has a sharp, pointy head, and a bulky body. The sand tiger's length can reach 3.2 m (10.5 ft). They are grey with reddish-brown spots on their backs. Shivers (groups) have been observed to hunt large schools of fish. Their diet consists of bony fish, crustaceans, squid, skates and other sharks. Unlike other sharks, the sand tiger can gulp air from the surface, allowing it to be suspended in the water column with minimal effort. During pregnancy, the most developed embryo will feed on its siblings, a reproductive strategy known as intrauterine cannibalism i.e. "embryophagy" or, more colorfully, adelphophagy—literally "eating one's brother". The sand tiger is categorized as vulnerable on the International Union for Conservation of Nature Red List. It is the most widely kept large shark in public aquariums owing to its tolerance for captivity.

Sandpaper

Sandpaper and glasspaper are names used for a type of coated abrasive that consists of sheets of paper or cloth with abrasive material glued to one face.

Despite the use of the names neither sand nor glass are now used in the manufacture of these products as they have been replaced by other abrasives such as aluminium oxide or silicon carbide. Sandpaper is produced in a range of grit sizes and is used to remove material from surfaces, either to make them smoother (for example, in painting and wood finishing), to remove a layer of material (such as old paint), or sometimes to make the surface rougher (for example, as a preparation for gluing). It is common to use the name of the abrasive when describing the paper, e.g. "aluminium oxide paper", or "silicon carbide paper".

The grit size of sandpaper is usually stated as a number that is inversely related to the particle size. A small number such as 20 or 40 indicates a coarse grit, while a large number such as 1500 indicates a fine grit.

Sandstone

Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) mineral particles or rock fragments.

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, as seen in Bowen's reaction series. 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. Fine-grained aquifers, such as sandstones, are better able to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity.

Quartz-bearing sandstone can be changed into quartzite through metamorphism, usually related to tectonic compression within orogenic belts.

Sedimentary rock

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus (organic matter). Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts (mainly shells) of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies (marine snow). Sedimentation may also occur as dissolved minerals precipitate from water solution.

The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other structures. Sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores.

The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography, paleoclimatology and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry and structural geology. Sedimentary rocks have also been found on Mars.

Shoal

In oceanography, geomorphology, and earth sciences, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material, and rises from the bed of a body of water to near the surface. Often it refers to those submerged ridges, banks, or bars that rise near enough to the surface of a body of water as to constitute a danger to navigation. Shoals are also known as sandbanks, sandbars, or gravelbars. Two or more shoals that are either separated by shared troughs or interconnected by past or present sedimentary and hydrographic processes are referred to as a shoal complex.The term shoal is also used in a number of ways that can be either similar or quite different from how it is used in the geologic, geomorphic, and oceanographic literature. Sometimes, this terms refers to either any relatively shallow place in a stream, lake, sea, or other body of water; a rocky area on the sea floor within an area mapped for navigation purposes; a growth of vegetation on the bottom of a deep lake that occurs at any depth; and as a verb for the process of proceeding from a greater to a lesser depth of water.

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