ore output was expected to be in the range of 100 to 105 million metric tons per year by 2010. A further limited increase in iron-ore production was projected to theyear2020 withouta significant expansionof the resource base. The resource base for iron ore was not considered profitable for investment because of taxa- tion issues and technological problems related to mining and processing the low-grade ores. Other Resources Russia has many world-class resources other than the ones described. The Noril’sk-Talnakh deposit in Rus- sia is not only the world’s richest nickel deposit but also one of the world’s largest platinum-group-metal and copper deposits. Global platinum-group-metal production and reserves are dominated by South Af- rica. According to Russia’s minister of natural re- sources, Russia has more than 40 percent of the world’s platinum-group-metal reserves and almost all reserves are in mixed sulfide ores at the Noril’sk com- plex. More than 50 percent of Russia’s copper metal production was produced by Noril’sk Nickel from ore mined by the company. The remainder came from a much smalleramount ofore minedin the UralMoun- tains and a large amount of secondary material. In 1999, Russia ranked sixth in the world in alu- mina production and eighth in the world in bauxite output. Russiaranked fourth inthe worldin mineout- put of antimony in 1999. All antimony reserves are in the Sakha Republic. The only sources of antimony production are goldantimony quartz vein-typedepos- its, which account for about 50 percent of the anti- mony reserves. Russia is not only the leading country for available fossil fuels such as oil, natural gas, and coal, but also a sourceof substantialunconventional energyresources, such as coal-bed methane, peat, and oil shales, which contain large amount of fuels. These resources are un- economical to exploit at present, but emerging tech- nologies are being developed to allow for economical production in the near future. Russia is among the world’s largest peat producers. Peat isa natural, renew- able, organic matterthat coversabout 4percent ofthe world’s land surface. Research shows that peat can also be converted into methane gas by bacterial diges- tion or by thermal breakdown at 400°-500° Celsius. Coal-bed methane is being increasingly developed as anew source ofnatural gas.Russia holdsthe world’s largest coal-bed methane resource. Methane gas in coal mines haslong been considered potentially risky. Methane explosions have killed tens of thousands of miners in the world. On the other hand, coal- bed methane is viewed as an undeveloped resource. Emerging technologies are being developed to sys- tematically extract methane gas from coal seams tobe used as energy sources, which would also reduce the likelihood of methane gas explosions in coal mines. Over time, coal-bed methane will become an impor- tant energy resource. Yongli Gao Further Reading Bradshaw, Michael, et al., eds. Essentials of World Re- gional Geography. Boston: McGraw-Hill, 2007. Butterman W. C., and Earle B. Amey III. Mineral Com- modity Profiles: Gold. Reston, Va.: U.S. Geological Survey, 2005. Craig, James R., David J. Vaughan, and Brian J. Skin- ner. Resources of the Earth: Origin, Use, and Environ- mental Impact. 3d ed. Upper Saddle River, N.J.: Prentice Hall, 2001. De Blij, Harm J., and Peter O. Muller. Geography: Realms, Regions, and Concepts. 13th ed. Hoboken, N.J.: Wiley, 2008. Evans, Anthony M. Ore Geology and Industrial Minerals: An Introduction. 3d ed. Boston: Blackwell Science, 1993. Levine, Richard M., and Glenn J. Wallace. “The Min- eral Industries of the Commonwealth of Indepen- dent States.” In USGS Minerals Yearbook 2005. Reston, Va.: U.S. Geological Survey, 2005. Misra, Kula C. Understanding Mineral Deposits. Boston: Kluwer Academic, 2000. Peacock, Kathy Wilson. Natural Resources and Sustain- able Development. New York: Facts On File, 2008. Web Sites Central Intelligence Agency The World Factbook: Russia https://www.cia.gov/library/publications/the- world-factbook/geos/rs.html Energy Information Administration International Energy Data and Analysis for Russia http://tonto.eia.doe.gov/country/ country_energy_data.cfm?fips=RS See also: Chernobyl nuclear accident; Coal; Dia - mond; Gold;Methane; Nickel;Oil and naturalgas dis - tribution; Oil and natural gas reservoirs. 1048 • Russia Global Resources S Sagebrush Rebellion Category: Historical events and movements Date: Late 1970’s-early 1980’s The Sagebrush Rebellion was a political movement that blossomed in the late 1970’s to minimize the im- pact of federal stewardship over the public lands of the American West. After mixed success, the movement faded after the election of Republican Ronald Reagan to the presidency in 1980. Background The Sagebrush Rebellion was a political reaction to a decade of gains for the American environmental movement as well as an expression of resentment at the strong federal presence in the states of the Ameri- can West. The federal government owns a large per- centage of the land in many western states, ranging from 29 percent of Montana to 85 percent of Nevada. As former Colorado governor Richard Lamm said in his book The Angry West (1982), the three federal “superbureaus”—the Bureau of Land Management, the United States Forest Service, and the National Park Service—controlled “virtually as much of the West as theWest ownsof itself.”Because of this,Lamm declared, “[W]e cannot control our own destiny.” The short-term catalyst for the “rebellion” was the actions of President Jimmy Carter. By canceling fund- ing for eighteen western reclamation projects, Carter opened himself to charges of federal insensitivity to the needs of the West. Other westerners were ani- mated by anger over what they viewed as increasing federal restrictions over the use of public lands. In 1979, the Nevada legislature demandedthe cession of 20 million federally controlled hectares to the state. Before long, other western public-land states joined what became known as the Sagebrush Rebellion. Provisions Carrying the movement to Congress, Senator Orrin Hatch, a Utah Republican, introduced legislation to transfer 220 million federal hectares to the control of thirteen western states. The Sagebrush Rebellion be - came a major issue in the West during the 1980 presi - dential campaign between Carter and Reagan, who announced that he too could be counted “as a rebel.” During the campaign he pledged to pay careful atten- tion to the economic needs of the West. Reagan re- marked that “we can turn the Sagebrush Rebellion into the Sagebrush Solution.” When Reagan won the presidency it appeared as though the Sagebrush Re- bellion had a chance for success. However, by the early 1980’s the fires of rebellion were cooling just when the Sagebrush Rebellion ap- peared to have the greatest opportunity for success. President Reagan appointed James G. Watt as secre- tary of the interior. Watt, the former director of the conservative Mountain States Legal Foundation, fa- vored easing restrictions on private-sector exploita- tion of the West’s natural resources. Soon Watt leased northern plains coal lands at low prices to private companies. This and other actions sparked a resur- gence of environmentalist political concern. Impact on Resource Use A combination of factors brought the Sagebrush Re- bellion to anend. The election ofa conservative West- ern president placated many Sagebrush rebels. Then a series of minor scandals, coupled with Watt’s ac- tions, led to a widespread perception that the Reagan administration was giving away the public lands of the West. By 1983, Watt and several of his lieutenants had resigned from office amid varying degrees of contro- versy. Steven C. Schulte See also: Bureau of Land Management, U.S.; Carter, Jimmy; Energy politics; Environmental movement; Public lands. Salt Category: Mineral and other nonliving resources Where Found Salt (sodium chloride) is widely and abundantly dis - tributed in nature. It is present in dissolved form in seawater, salt lakes, and groundwater in various parts of the world. There are also many substantial deposits of salt in solid form, notably in the United States, Great Britain, France, Germany, Russia, China, and India. In 2008, the primary forms of salt sold or used in the United States were salt in brine (44 percent), rock salt (38 percent), vacuum pan salt (10 percent), and solar salt (8 percent). Primary Uses Salt has numerous uses, chiefly in the chemical indus- try; metallurgy; ceramics, glass, and glaze manufac- ture; agriculture; medicine; refrigeration; and foods. In addition to its importance as an industrial raw ma- terial, salt is an essential nutrient, although its ubiqui- tous use in commercial food processing has made over-intake in industrialized nations a major health concern. Technical Definition Salt is a general term for naturally occurring sodium chloride (NaCl). Synonyms include halite, common salt, and rock salt. Its average molecular weight is 58.448. Pure salt may be colorless or white; impurities may add a yellow, red, blue, or purple tint. Its hard- ness on the Mohs scale is2 to 2.5.Salt usually occursas cubic crystals. Its specific gravity is 2.17. It is readily soluble in water and is insoluble or only slightly solu- ble inmost otherliquids. Ithas amelting pointof 801° Celsius and a boiling point of 1,413° Celsius. Description, Distribution, and Forms Sodium chloride is an important and abundant inor- ganic chemical. It is as essential to life as it is to mod- ern industry. Human blood is composed of 90 per- cent water, 0.9 percent salt, and small amounts of proteins and othersubstances. As the salt isexpended it must be renewed. This fundamental need for salt has been a driving force behind exploration, com- merce, and conflict throughout human history. Salt has long been a crucial industrial material as well. It has approximately fourteen thousand different re- ported uses.Total worldproduction ofsalt in2008 was about 260 million metric tons; the United States ac- counted for about 18 percent of the total. Salt is widely distributed throughout the world and the geologic column. Salt is produced by more than one hundred nations worldwide; most of them are able to fulfill their own consumption requirements from indigenous sources. The world’s largest salt reserve is its oceans, which contain 2.5 percent dissolved salt by weight. The oceans are estimated to contain 44 × 10 15 metric tons of salt,which wouldform acube roughly 18.76million cubic kilometers in volume. Dissolved salt is also pres- ent in salt seas and lakes, such as the Dead Sea in the Middle East, the Aral Sea in central Asia, and the Great Salt Lake in Utah. Subsurface brines are an- other source of dissolved salt. These brines can be an- cient seawater that was entrapped in sediments at the time of deposition orsaline waters that formed locally by solution of rock salt beds. Extensive bedded deposits are also found in the form of rock salt. These sedimentary deposits occur interbedded with common strata and with other evaporite minerals, such as gypsum and anhydrite. The deposits were created as salts precipitated and ac- cumulated on the floor of an ancient landlocked ma- rine bodyof water. Extensive andwidespread evapora- tion led to the formation of the deposits, which can reach thicknesses of up to 900 meters. Examples of bedded depositscan be foundin Michigan,New York, Ohio, New Mexico, Canada,England, and central Eu- rope. In North America bedded salt deposits occur mostly inSilurian,Permian,and Triassic formations. When vertical or lateral stress is applied to strati- fied salt deposits, the lower-density salt flows plastic- ally through the surrounding higher-density rock to form saltdomes. Thesesalt domesare usuallycylindri- cal in shape and are often capped by anhydrite, gyp- sum, and calcite. Sulfur and hydrocarbons are fre- quently associatedwith saltdome deposits.Salt domes are found in Texas, Mississippi, Louisiana, Mexico, Germany, Poland, Romania, Russia, and the Middle East. In arid regions salt occurs along with borax, pot- ash, and other evaporite minerals as a surface deposit from desiccated salt lakes. Such playa deposits are im- portant in California, Nevada, Utah, and India. Salt occurs in nature as halite. It is often found interbedded with shale, limestone, dolostone, and rock-gypsum or rock-anhydrite in extensive beds and irregular masses. It is frequently associated with gyp- sum, anhydrite, calcite, sylvite, sand, and clay. In arid regions it can occur as a white powder, or efflores- cence, onthe soil surface. Itcan also bedissolved in the waters of salt springs, salt lakes and seas, and oceans. History Salt manufacture is one of the oldest chemical indus - tries. Its availability influenced the locations of cities, 1050 • Salt Global Resources Global Resources Salt • 1051 Source: Mineral Commodity Summaries, 2009Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009. Metric Tons 70,000,000 60,000,000 50,000,000 40,000,000 30,000,000 20,000,000 10,000,000 Australia Brazil Canada Chile China Egypt France Germany India Iran Italy Mexico Netherlands Poland Romania Russia Spain Turkey Ukraine United Kingdom United States Other countries 12,000,000 7,000,000 12,000,000 5,000,000 60,000,000 2,400,000 6,000,000 19,000,000 15,800,000 2,000,000 2,200,000 8,400,000 5,000,000 4,400,000 2,500,000 2,200,000 4,600,000 2,700,000 5,500,000 5,800,000 46,000,000 29,500,000 Salt: World Production, 2008 the migration of populations, and the establishment of trade routes. Salt’s dietary importance led toits fre- quent useas auniversal currency.Salt derives itsname from sal, the Latin word for the substance. The word “salary,” which also comes from the Latin term, re- flects the Roman practice of paying a portion of their soldiers’ wages with rations of salt. Salt production in the United States began in 1614 with colonists in Virginia, who evaporated seawater to obtain the resource. Extraction of salt from subsur- face brines began in the United States in 1788 in New York. In 1791, French chemist Nicolas Leblanc devel- oped a commercial process that used salt to manufac- ture soda ash. The Solvay process, in which salt was also the chief raw material, supplanted the Leblanc process in the 1860’s. In 1862, the first rock-salt mine in North America opened at Avery Island, Louisiana. In about 1882, the United States first employed solu- tion mining methods. The 1887 invention of the vac- uum pan was a significant contribution to the salt in- dustry, as applying avacuum during evaporation made water boil at a lower temperature, thereby reducing the amount of fuel needed to heat the evaporation pans. Obtaining Salt Rock salt may be extracted from deposits using conventional underground mining or solution mining methods. Solution mining involves intro- ducing pressurized and often heated fresh water into an injection well drilled into the salt deposit. The water dissolves the salt, and the resulting brine is pumped back to the surface for mineral recovery. Whether brinesare created bysolution mining or obtained from the ocean, a sea, a lake, or an- other natural source, theymust be evaporatedfor their salt contents to be harvested. Solar evapora- tion is effective in areas where the evaporation rate is high and the precipitation rate low. In many parts of the world, seawater or saline lake water is pumped into large, specially constructed ponds, where it is allowed to evaporate naturally. The brine passes through a series of these ponds during the solar evaporation process. In mechan- ical evaporation, brines are dehydrated in steam- heated vessels. This process is often augmented by applying a vacuum to make evaporation pro - ceed at a lower temperature. Desalination, the process of converting salt water into fresh water, produces salt as a by-product. Desalination methods include distillation, membrane osmosis, freezing, and ion exchange. Some salt pro- duced by desalination is used in industry. Salt obtained through evaporation is not usually pure sodium chloride. Impurities are removed by aer- ation and chemical treatment. Small amounts of other substances, such as magnesium carbonate, hy- drated calcium silicate, or tricalcium phosphate, may be added to prevent lumping. Iodized table salt usu- ally contains small amounts of potassium iodide, so- dium carbonate, and sodium thiosulfate. Uses of Salt The chiefuse ofsalt isas araw materialfor theproduc- tion of chlorine, sodium metal, and sodium hydrox- ide; it is also an ingredient in the Solvay process for manufacturing soda ash. Salt is used in making soaps, textile dyes, lacquers, cements, glass, ceramics, and glazes. It is employed in the treatment, smelting, and refining of ores and metals.While used as a refrigerat- ing agent, it is also spread in large quantities to melt ice and snow on streets and highways. In agriculture, salt is a component of livestock feed, fertilizers, soil amenders, herbicides, and insecticides. In the medi- 1052 • Salt Global Resources Chemicals 40% Deicing 39% Distributors 8% Agriculture & food 6% Other 7% Source: Mineral Commodity Summaries, 2009 Note: Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009. “Other” includes general industrial and water treatment. U.S. End Uses of Salt cal field, salt is used in pharmaceuticals and specialty cleansers. Salt is an essential part of human physiology. It is found in most body fluids, such as blood, sweat, and tears. Italso provideschlorine formaking hydrochloric acid, asmall butvital part ofhuman digestivefluid. Di- etary intake of salt replaces the mineral as it is con- sumed through normal metabolism. The average per capita consumption of salt is approximately 5.44 kilo- grams ayear. Saltis widelyusedas aseasoning forfoods, a curingagent formeats, anda preservative forfish and other foods. While salt is an essential nutrient, exces- sive amounts in the diet can lead to health complica- tions. Persons suffering from high blood pressure or heart disease often must restrict the amount of salt in their diets to avoid aggravating these conditions. Karen N. Kähler Further Reading Adshead, S. A. M. Salt and Civilization. New York: St. Martin’s Press, 1992. Gevantman, L. H., ed. Physical Properties Data for Rock Salt. Washington, D.C.: U.S. Government Printing Office, 1981. Jensen, Mead L., andAlan M. Bateman. Economic Min- eral Deposits. 3d ed. New York: Wiley, 1979. Johnson, K. S. “Salt Resources and Production in the United States.” In Industrial Minerals and Extractive Industry Geology: Based on Papers Presented at the Com- bined 36th Forum on the Geology of Industrial Minerals and 11th Extractive Industry Geology Conference, Bath, England, 7th-12th May, 2000, edited by Peter W. Scott and Colin M. Bristow. London: Geological Society, 2002. Kogel, Jessica Elzea, et al., eds. “Salt.” In Industrial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metal- lurgy, and Exploration, 2006. Kurlansky, Mark. Salt: A World History. New York: Walker, 2002. MacGregor, Graham A., and Hugh E. de Wardener. Salt, Diet and Health—Neptune’s Poisoned Chalice: The Origins of High Blood Pressure. New York: Cambridge University Press, 1998. Multhauf, Robert P. Neptune’sGift: A History of Common Salt. Baltimore: Johns Hopkins University Press, 1978. Warren, John K. “Salt Tectonics.” In Evaporites: Sedi - ments, Resources, and Hydrocarbons. New York: Springer, 2006. Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/com-eng.htm U.S. Geological Survey Salt: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/salt See also: Evaporites; Lakes; Oceans; Salt domes; Sed- imentary processes,rocks, andmineral deposits;Soda ash. Salt domes Category: Geological processes and formations Salt domes are a major source of the world’s salt. Their caprocks are major sources of gypsum and sulfur. Up- turned sediments on the flanks of salt domes form stratigraphic traps for oil and natural gas. Definition Salt domes consist of roughly cylindrical to mush- room-shaped plugs of massive rock salt extending to- ward the Earth’s surface from depths as great as 6,000 meters. These salt pillars typically range in diameter from 1 to 3 kilometers; however, some cores reach 12 kilometers in diameter. The plug is usually topped by a limestone, gypsum, and anhydrite caprock. Overview Salt beds, ranging in thickness from a meter to a few hundred meters, are deposited in shallow, hyper- saline, marine environments such as basins of re- stricted circulation in regions where evaporation ex- ceeds precipitation. The salt is commonly pure white and is associated with gypsum, anhydrite, and shales. Some deeply buried salt beds form mobile salt col- umns thatrise toward thesurface. Saltdomes occur in the Colorado-Utah area, the Gulf Coast of the United States andMexico, Spain,France, Romania, Iran,Ara- bia, and India. Salt domes are emplaced when beds of salt deform plastically under the pressure of overlying rocks and Global Resources Salt domes • 1053 rise through overlying layered sedi - ments. The rising salt forces the over- lying rocks into domes and punches through them to leave rock layers upturned along itsflanks. The depth of the salt core beneath the surface varies widely. Deep domes may be more than 1,750 meters beneath the surface,but others may expose saltat the surface. As salt reaches near the surface, it encounters groundwater, which dissolves the rising salt. A cap- rock of less soluble minerals, mostly anhydrite, forms on top of the ris- ing plug. Often, anaerobic bacteria in groundwater break down the an- hydrite of the caprock, forming cal- cite andnativesulfur inthe process. Commercial quantities of sulfur are dispersed within the caprock of a fewdomes. Sulfuris extracted from the caprock by the Frasch process. Water heated to 150° Celsius is dis- charged into the caprock to melt the sulfur, and hot air is used to drive it to the surface. The molten sulfur is then pipedto storage,where itsolidi- fies. At shallow domes, anhydrite, gypsum, and lime- stone in the caprock may be quarried for road metal or building materials. Salt is recovered by under- ground mining techniques. Salt domes account for only 5percent of theworld’s reservesof salt, but alone they could supply the world’s demand for thirty thou- sand years. Upturned sedimentary beds around the flanks of the domes provide traps for oil and gas that migrate updip and are impounded against the imper- meable salt.The limestoneof thecaprock alsoformsa petroleum reservoir in some salt domes. Cavities may be excavated within salt domes either by standard mining techniques or by pumping fresh water into the dome to form a solution cavity. The re- sulting cavity may then be used to store oil or gas. Be- cause salt is impermeable and self-healing when frac- tured, it has been used as a storage site for nuclear waste disposal. René A. De Hon See also: Native elements; Nuclear waste and its dis - posal; Salt. Sand and gravel Category: Mineral and other nonliving resources Where Found Sands and gravels are widely distributed on the Earth’s surface;all fiftyof theUnited Stateshave producingde- posits. Sand and gravel deposits are not spatially ubiq- uitous, however. Sand and gravel are heavy or dense, high in bulk, and low in value, and they cannot be shipped economically for long distances. Most sand and gravel in theUnited States and Canada comefrom glacial deposits, stream terraces and channels, includ- ing alluvial fans, or from beach deposits of either cur- rent or relict shorelines. Some specialty or industrial sands are derived from bedrock when more rigid con- trol over the character of the sand is required. Primary Uses By far the greatest use of sand and gravel is in con - struction, where they maybe employed as fill material 1054 • Sand and gravel Global Resources A salt dome is situated in the upper middle section of this portion of the Zagros Moun- tains. (NASA) or as the aggregate in concrete. Industrial sands are more specialized, and their usesdemand higher qual- ity. Most industrial sand is used to make glass or as molding sand in foundries. The United States pro- duces more than 1 billion metric tons of construction sand andgravel and between25 and 30million metric tons of industrial sands. Technical Definition Sand particles are 0.05 millimeter to 4.76 millimeters in diameter. Gravel particles are larger, 4.76 to 80 or 90 millimeters in diameter. Sand fragments are com- posed almostentirely of singleminerals, chiefly quartz, with significant fractions of feldspars and smaller pro- portions of mica, chert, and heavy minerals. Gravels, on the other hand, are usually fragments of rocks that are composed of several minerals. Gravels reflect the geology of the stream basin in which they are located, because this is the source of the gravel deposit. Most gravels are resistant, but if the source stream basin is underlain largely by soft sediments, the gravels are less valuable as a resource. Impurities in sand and gravel deposits consist of silts, clays, or excessive pro- portions of micas, soft sediments, or rock fragments that have an undesirable chemistry. Description, Distribution, and Forms Sand and gravel are the most widely distributedof the construction aggregates, are the easiest to recover or mine, and require only simple beneficiation, usually washing and screening. Historically, in the United States, they have dominated the market for aggre- gates. However, for many purposes, even in rough construction, they are not as suitable as their closest competitor—crushed stone or rock—because sharp- edged broken stones interlock, unlike gravels, which are rounded by stream transport. Both the quality of a sand or gravel deposit and its location with respect to market determine the re- source valueof that deposit. Qualityconcerns include the lithologyof theparticles (theirchemical andphys- ical character), the size and shape of the particles, their resistance to abrasion and cracking, the poten- tial for chemical reactivity, and the freedom of the de- posit from organic matter, silt, and clay (in other words, the deposit’s cleanness). Fortunately, most sand deposits are dominated by quartz particles, which are both resistant and inert. Gravels can pose a greater problem because of the variety ofrocks indifferentdrainage basins. Softrocks or those that weather relatively rapidly (shales, friable sandstones, some limestones, and certain metamor- phic rocks, especially schists and slate) do not make valuable gravels. A variety of rocks react with the alka- lies in portland cement and must be avoided for that particular use. Iron impurities rust, and certain other minerals weather or decompose rapidly. These condi- tions lead to weakened construction and are avoided. Thus, all gravel deposits are not equally valuable as re- sources, even ifthey are favorably locatedwith respect to markets. Just as high-quartz sands are more valu- able, so are gravels with high proportions of resistant rocks of the proper chemical composition. Market, in the case of construction sand and gravel, is defined by population and appropriate construc- tion, such as highways. Thus, a sparsely populated re- gion serves as a significant market while interstate highway construction isunder way butbecomes asmall market when the highway is completed. The fortunes of construction sand and gravel suppliers wax and wane with the general economy; boom times of ex- panded residential oroffice buildingconstruction pro- vide an excellent market. Recessions with little con- structionactivity result in ashrinkage inproduction. The low-value, high-bulk character of sand and gravel dictates that only surface mining is economical (the exception is some higher-value industrial sands, which may be mined underground). Moreover, the mining must be close to metropolitan centers, where most new construction occurs. Inevitably, the urban centers grow and encounter the sand and gravel min- ing. Zoning may then displace the mining to more re- mote locations because of complaints about dust, noise, truck traffic, or the unsightliness of gravel pits. Restrictions on the use of wetlands are increasing, particularly in cases in whichendangered species may be involved. There is also an increasing concern with silica dust,whichmay affect specialtyindustrial sands. History A measure of the overall relationship between popu- lation numbers and construction may be seen in the history of sand and gravel production in the United States. During the Depression of the 1930’s annual production was about 180 million metric tons. In 1946, before widespread construction began in the postwar era, production was about 230 million metric tons. By 1960 construction had expanded significantly and production stood at 641 million metric tons. In 1970, residentialconstruction andthe interstatehigh - Global Resources Sand and gravel • 1055 way program were active; production was about 856 million metric tons. By 1994, although population had grown, highway and commercial construction had declined, and total sand and gravel production was just more than 918 million metric tons, of which about 27 million metric tons were industrial sands. By 2008, U.S.productionof sandand gravelfor construc- tion was about 1 million metric tons; for industrial use, about 30 million metric tons. Throughout the post-World War II period, metropolitan population concentrations were far more important as markets for construction sand and gravel than were rural re- gions, whichgenerally failedto generateconstruction in proportion to their population numbers. The ma- jor exception to this generalization is the interstate highway construction program, which generated tem- porary markets for sand and gravel in even the most sparsely settled portions of the country. Obtaining Sand and Gravel Nearly all gravel deposits, and most sand deposits, are found in stream sediments. Present-day stream depos- its include channels, low terraces, and active portions of alluvial fans. Under these circumstances, sands and gravels removed by dredging or open-pit mining may berenewed byrecurrent floodingor highstream- flows. Relict stream deposits are those created by gla- ciation, includingoutwash fansand valleytrain depos- its (the latter extending tothe oceans from the glacial source), as well as relatively minor sources such as eskers, kames,andmoraines depositedclose tothe ice margin. Most alluvial fans in western North America are alsorelict orinactive in thatthey wereformed dur- ing the Pleistocene era and are not renewed by cur- rent geologic processes. In either case, virtually all sand and gravel are found in surficial deposits, which are frequently wetlands. This fact has advantages in terms of mining costs, but it also results in environ- mental problems and land-use conflicts. Uses of Sand and Gravel The overwhelming use of sand and gravel is in con- struction. Use of these materials for fill, base, or subgrade of highways is the least demanding of qual- ity requirements, and sand and gravel may not even be washed or screened for these uses. Usage in con- crete, however, is far more demanding, both in terms of size-of-particle requirements (sorting, screening, or crushing may be used to produce the desired size) and in terms of quality (avoiding easily weathered or alkali-reactant rocks). Substitutes for sand and gravel in construction are crushed stone or rock and light- weight aggregates. Lightweight aggregates, largely vol- canic rocks, are increasingly employed in specialty concretes and building blocks. Crushed rock is uti- lized where more rigid specifications for concrete ex- ist or in regions where sand and gravel are scarce (this high-bulk, low-value commodity is shipped largely by truck, and rarely for distances greater than 30 me- ters). Industrial sand and gravel encompass a variety of uses, each with its own specifications as to desirable characteristics in the product and its own market— hence the resultant location of mining activities. Glassmaking and foundry or molding sands lead the list of uses bytonnage; the former requires more rigid specifications and is located where construction is ac- tive, and the latter is located where metalworking is significant. The petroleum industry uses significant quantities for hydraulic fracturingof oil and gas wells. Abrasives, especially for blast sands, also rank high. 1056 • Sand and gravel Global Resources Concrete aggregates 44% Roads 23% Construction fill 14% Asphaltic aggregates 12% Other 7% Source: Mineral Commodity Summaries, 2009 Note: Data from the U.S. Geological Survey, .U.S.GovernmentPrinting Office, 2009. “Other” includes plaster and gunite sands, blocks, bricks, pipes, filtration, golf courses, railroad ballast, roofing granules, and snow and ice mitigation. U.S. End Uses of Construction Sand and Gravel Each use or type of sand has competition from substi - tutes thatmayreduce theresource valueofdeposits or the profitability ofan industry. Glass, forexample, has largely been replaced by aluminumand plastics as the material for containers in the food and beverage in- dustry. Abrasives have come under fire for reasons of health, such as the breathing of dust by workers. Neil E. Salisbury Further Reading Bell, Fred J., and LauranceJ. Donnelly. “Gravel, Sand, and Clay Pits.” In Mining and Its Impact on the Envi- ronment. New York: Taylor & Francis, 2006. Evans, Anthony M. An Introduction to Economic Geology and Its Environmental Impact. Malden, Mass.: Black- well Science, 1997. Gyr, Albert, and Klaus Hoyer. Sediment Transport: A Geophysical Phenomenon. Dordrecht, the Nether- lands: Springer, 2006. Hamilton, W. N., and W. A. D. Edwards. “Industrial Minerals in Western Canada Sedimentary Basin.” In Industrial Minerals and Extractive Industry Geology: Based on Papers Presented at the Combined 36th Forum on the Geology of Industrial Minerals and 11th Extrac- tive Industry Geology Conference, Bath, England, 7th- 12th May, 2000, edited by Peter W. Scott and Colin M. Bristow. London: Geological Society, 2002. Harben, Peter W., and Robert L. Bates. Geology of the Nonmetallics. New York: Metals Bulletin, 1984. Kogel, Jessica Elzea, et al., eds. “Industrial Sand and Sandstone.” In Industrial Minerals and Rocks: Com- modities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006. Smith, M. R., and L. Collis, eds. Aggregates: Sand, Gravel, and Crushed Rock Aggregates for Construction Purposes. 3d ed. Revised by P. G. Fookes et al. Lon- don: Geological Society, 2001. Web Sites U.S. Geological Survey Construction Sand and Gravel: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/sand_&_gravel_construction U.S. Geological Survey Silica: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/silica See also: Abrasives; Aggregates; Cement and con - crete; Glaciation; Glass; Streams and rivers; Surface Mining Control and Reclamation Act; Wetlands. Sandstone Category: Mineral and other nonliving resources Where Found Sandstone is found throughout the world. It is proba- bly themost familiar, butnot themost abundant,of all sedimentary rock, that group of rocks composed of consolidated rock fragments of all sizes. Primary Uses Sandstone has numerous uses in the construction in- dustry.It isused to makebricks, concrete, andplaster. Technical Definition Sandstone is a rock composed of abundant rounded or angular, sand-size fragments derived by the disinte- gration of existing rock. The sand fragments are com- monly cemented together by calcium carbonate, sil- ica dioxide, or iron oxide. Description, Distribution, and Forms In chemical composition, the average sandstone is principally composedof approximately 80percent sil- ica dioxide,6 percent aluminumoxide, and 3percent each calcium oxide and carbon dioxide. Arkose is a sandstone that contains fairly large, angular granules of pink feldspar. In North America two economically significant sandstones are the Oriskany sandstone of New York State and the Saint Peter sandstone of Min- nesota. Both these sandstones are important as glass sand and natural gas reservoir rock. Sandstones of awide variety ofphysical characteris- tics and mineral compositions are known. While many sandstone classification schemes exist, a common scheme lists four typical varieties. The chief constitu- ent (90 percent or greater) of siliceous sandstone is the mineral quartz, whereas more than 25 percent of arkose iscomposed of themineral feldspar. A thirdva- riety is graywacke, a heterogeneous mixture of quartz and feldspar surrounded by fine-grained clay mate- rial. In regions of volcanic activity, accumulations of sand-size detritus ejected from active volcanoes form tuffaceous sandstone. Global Resources Sandstone • 1057 . floor of an ancient landlocked ma- rine bodyof water. Extensive andwidespread evapora- tion led to the formation of the deposits, which can reach thicknesses of up to 900 meters. Examples of bedded. and coal, but also a sourceof substantialunconventional energyresources, such as coal-bed methane, peat, and oil shales, which contain large amount of fuels. These resources are un- economical. federal “superbureaus”—the Bureau of Land Management, the United States Forest Service, and the National Park Service—controlled “virtually as much of the West as theWest ownsof itself.”Because of this,Lamm declared,