Obtaining Lithium Lithium chloride is obtained by treating either lith- ium hydroxide or lithium carbonate with hydrochloric acid. Chemistsobtain pure metalliclithium bypassing electricity through moltenlithium chlorideor through solutions of lithium chloride in ethanol or acetone in low-carbon steel cells having graphite anodes. Uses of Lithium Lithium is used to make batteries found in electric meters, cameras, and other electronic equipment, and lithium compounds have numerous practical ap- plications. Lithium carbonate and lithium borate are used in the ceramic industry as glaze constituents, while lithium perchlorate is a powerful oxidizing agent usedin solidfuel forrockets. Lithiumhydride, a powerful reducing agent, is used in fuel cells, as a shielding material forthermalneutrons emitted from nuclear reactors, and to inflate lifeboats and air bal- loons. Lithium fluoride is usedin infrared spectrome- ters and as a flux in ceramics, brazing, and welding. Lithium chloride, the most common lithium salt, is used toincreasethe conductivityof electrolytesin low- temperature dry-cell batteries, as a dehumidifying agent in air-conditioners, and in metallurgical appli- cations. Lithium is combined with aluminum and magnesium toproduce structural alloys; lithium-mag- nesium alloys have the highest strength-to-weight ra- tio of all structural materials. In medicine, lithium amide is important in the synthesis of antihistamines, and lithium carbonateis used as a drug to treat a form of mental illness known as bipolar affective disorder (or manic-depressive disorder). Alvin K. Benson Web Sites Natural Resources Canada Canadian Minerals Yearbook, 2005: Lithium http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/content/2005/35.pdf U.S. Geological Survey Minerals Information: Lithium Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/lithium/ See also: Aluminum; Carbonate minerals; Ceramics; Fuel cells; Glass; Magnesium; Nuclear energy; Rub - ber, natural. Lithosphere Category: Geological processes and formations The usable mineral resources of the Earth are all within the lithosphere, and knowledge of its properties is particularly important inthe search forgasand oil. Definition The lithosphere (“stone sphere,” from Greek lithos) consists of the outer, brittle portions of the Earth, in- cluding the upper mantle and crust. Overview The interior of the Earth has a number of layers, or concentric spheres. At the center of the Earth is the inner core. Then, moving outward, come the outer core, the lower mantle, the upper mantle, and the Earth’s crust. Scientists subdivide the upper mantle into the asthenosphere, a partially molten zone, and, above that, the lithosphere. The lithosphere, then, is the rigid(or brittle)outer shellof theEarth,which ex- tends to a depth of between 70 and 100 kilometers and rests on the asthenoshere. It includes the Earth’s crust and part of the upper mantle. The upper mantle is approximately 700 kilometers thick. Theasthenosphere begins ata depthof approx- imately 70 to 100 kilometers and shows a rapid in- crease in density and a temperature in excess of 1,000° Celsius. The asthenosphere is partially molten ultramafic material. Because of its partially molten properties, the asthenosphere probably exhibits plas- tic flow. Above the asthenoshere, the upper brittle portion of the upper mantle that is part of the litho- sphere is a dense ultramafic material that directly un- derlies the Earth’s crust. The lithosphere comprises seven to ten major lithospheric “plates” that move slowly as they rest on the asthenosphere. Plate tecton- ics refers tothe movementof theseplates andthe land and ocean forms that are created as a result. Within the lithosphere, the boundary between the upper mantle and the crust is called the Mohorovi5i6 discontinuity, or Moho, which marks a compositional change in the rock. The earth’s crust contains two ba- sic types of crustal material, oceanic and continental, with an average density of 2.9 and 2.6, respectively. Oceanic crust ranges from 5 to 10 kilometers thick and isthinnest over seafloor-spreading areas.Oceanic crust is primarily composed of dense basaltic rock 698 • Lithosphere Global Resources with a thin veneer of silt and carbonate precipitates; however, a variety of minerals have been observed at seafloor vents. Continental crust is primarily com- posed of felsic granitic rock, which is less dense than oceanic crust; however, continental crust also includes sedimentary and metamorphic rock and even up- lifted oceanic basalt. A variety of minerals of varying economic importance occur in the continental crust. The continental crust averages 30 to 40 kilometers in thickness, butit may bemore than70 kilometers thick in some mountain areas. Oceanic crust is less dense than the parent mantle material. This is probably attributable to partial melt- ing and crystal fractionation. Felsic minerals have a lower melting temperature than mafic minerals, and mafic minerals are the first to crystallize out of a melt. As oceaniccrustsubducts belowcontinental crust, the subducting plate eventually melts, and its upwelling liquid fraction produces less mafic intermediates. The lithosphere is highly variable, according to re- gional studies. In parts of the middle United States and in the Gulf of Mexico region, for example, the crust has thick sedimentary layers. Oil companies were able to measure the seismic wave patterns gener- ated by many controlled explosions and discover pe- troleum and natural gas within these layers. The later discovery of oil in northern Alaska was prompted by the similarity ofthe crustthere to thecrustof these re - gions. As the study of the characteristics of the litho - sphere—including plate tectonics—continues, scien - tists will increasingly be able to usetheir knowledge to discover sites of mineral resources. Raymond U. Roberts See also: Earth’s crust; Igneous processes, rocks, and mineral deposits; Magma crystallization; Marine vents; Metamorphic processes, rocks, and mineral deposits; Plate tectonics; Plutonic rocks and mineral deposits; Seafloor spreading; Sedimentary processes, rocks, and mineral deposits; Volcanoes. Livestock and animal husbandry Category: Plant and animal resources Animal husbandry refers tothe management of domes- ticated animals such as beef or dairy cattle, sheep, goats, pigs,and chickens: livestock.Such animals con- stitute a renewable resource providing humans with food, fiber, fuel,power,implements, andother benefits. Background Effective animal husbandry requires an affinity for the animals being managed, skill in handling them, and knowledge of them and their environment. Re- spect for animals is important to good management, as is skillin handling tominimize injuries and stress to both animal and handler. Knowledge is needed of Global Resources Livestock and animal husbandry • 699 Moho Moho Ocean Upper mantle Oceanic crust Continental crust Lithosphere (70-100 kilometers deep) The Mohorovi i Discontinuity (Moho) Between the Crust and Mantleã ã their nutrition, reproduction, and behavior as well as the physical, biological, cultural, and economic con- text in which they are managed. While some inputs (such as aberrant weather and governmental regula- tions) are beyond the control of the producer, good management will ensure the most efficient productiv- ity from the available inputs. Intensive and Extensive Management Intensive and extensive management are the two main options for animal husbandry. Intensive management refers to confinement-type operations that provide animals with shelter, food, and water. It has been called “landless” because it requires very little prop- erty. Examples include beef feedlots, concentrate- based dairy farms, and confinement swine or poultry operations. In extensive systems, on the other hand, the animals are provided with an area in which they fend for themselves, finding their own food, water, and shelter. Examples are rangeland beef operations, pasture-based dairying, and free-range poultry farms. In practice, animal husbandry often includes both in- tensive and extensive management. In the early twenty-first century, the U.S. beef in- dustry generally involved extensive operations for at least the first year of life and an intensive phase just prior to market; availability and prices of feed grains may determine the extent to which intensive manage- ment is practiced. Dairy operations around the world range from intensive to extensive—from no to exclu- sive pasture, respectively. Seasonal variation of pas- ture maydictate whenitis availableand used. Because dairy cows must be milked two or three times a day, dairy operations are never as extensive as some beef operations, where the producer may have contact with the animals no more than once a year. Intensive animal management generally requires more management expertise, more capital invest- ment, andmore energy utilization.Since theanimal is totally under control of the producer, all needs of the animal must be provided. The inevitably greater con- centration of animals requires closer attention to their housing and health. The larger capital invest- ment is attributable to facilities and equipment. More energy utilization is needed to maintain temperature and ventilation as well as to operate equipment. In- tensive management also places greater emphasis on maximizing animal performance.Because morecapi - tal and energy are used, effort is made to extend ani - mal performance by genetics, nutrition, and other management tools.Intensive managmentalso requires more dependence on others for feed. While some in- tensive livestock producers raise their own feedstuff, many do not. They may depend on crop farmers within the region or half a world away. Contemporary 700 • Livestock and animal husbandry Global Resources Meat: Leading Producers, 2006 Metric Tons World total 212,776,000 Beef and veal United States 11,981,000 Brazil 9,020,000 European Union 8,060,000 China 7,492,000 Argentina 3,100,000 India 2,375,000 Australia 2,183,000 Mexico 2,175,000 Russia 1,430,000 Canada 1,391,000 Pork China 51,972,000 European Union 21,677,000 United States 9,559,000 Brazil 2,830,000 Canada 1,898,000 Russia 1,805,000 Vietnam 1,713,000 Japan 1,247,000 Philippines 1,215,000 Mexico 1,200,000 Poultry United States 16,043,000 China 10,350,000 Brazil 9,355,000 European Union 7,803,000 Mexico 2,592,000 India 2,000,000 Japan 1,227,000 Argentina 1,200,000 Russia 1,180,000 Thailand 1,100,000 Source: U.S. Department of Agriculture, National Agricultural Statistics Service, Agricultural Statistics, 2007. swine operationsin Japanand Korea requirecornand soybeans from the U.S. Midwest. Extensive animal management demands more land and more dependence on the animals’ abilities than intensive management. The larger land requirement is a primary feature of this system. The greater depen- dence on the animals’abilities follows from lessdirect provision by the producer for their needs. Survival and growth may dependon their locating food, water, and shelter as well as avoiding danger. Reproduction may be left to natural service, easy birthing, and good mothering. Extensive management involves more tol- erance for decreased animal performance. When weather conditions do not provide sufficient food, the animals will have less than maximal growth and fertility. Neonatal losses attributed to weather, preda- tors, or terrain are tolerated. Indeed, human inter- vention may not be a realistic option when animals are widely dispersed. An important parameter is the “stocking rate,” the number of animals per land area. Too few animals will not fully use the vegetation, as many grasses are most nutritious at an early stage of development and become less nutritious and coarser if not eaten then. Too many animals will overgraze, impairing regrowth of the vegetation. Optimum “stocking rate” corresponds closely to the ecological concept “carrying capacity,” the number of animals that an area can sustain over an extended period of time. Extensive systems can demand substantial management expertise. For instance, pasture-based dairying in New Zealand requires considerable knowl- edge to optimize pasture growth and utilization. Biological and Nonbiological Parameters Any animal management system must take into ac- count numerous biological parameters pertinent to the animal under management. These include nutri- tional requirements, biological time lag (time from conception to market), reproduction (gestation length and number of newborn, newborn survival), efficiency of feed conversion, nature of weight gain, genetic selection, and susceptibility to disease. Deci- sions are madeabout using naturalservice orartificial insemination. The extent to which agricultural by- products, crop residues, and/or production enhanc- ers are useddepends on theirefficacy,availability, and price. Any animal management system also involves a number of nonbiological parameters. The available climate, water supply, and land are physical attributes that bear upon the husbandry options. Two other fac - ets of the land affecting management are its tenure, whether owned, leased, or occupied, and its use, whether restricted or not. Husbandry is also affected by the availability and skill level of labor. Another fac- tor is the infrastructure—the dependability of trans- portation providing access to markets, postfarm pro- cessing, and communication systems. Profitability, the difference between receipts and cost of inputs, as well as any subsidies, determines whether one can engage in any agricultural activity for long. Personal values, including lifestyle and risk management, also impact involvement in animal agriculture. Finally, historical and societal values, particularly those directly touch- ing on the use of animals and natural resources, influ- ence the extent and nature of animal husbandry. Issues Three issues of contemporary interest relative to live- stock and animal husbandry concern the need for an- imal agriculture, its sustainability, and its increasing corporate nature. The willingness of people to pur- chase and consume products of animal origin will al- ways determine theneed foranimal agriculture.Ifthe price people must pay for such products is too high, demand will decline. As the general affluence of a country increases, the demandfor foods of animal or- igin increases. The sustainability of contemporaryagriculture has been called into question because of its heavy depen- dence on fossil fuels for energy and its adverse effects on theenvironment. Properly managed,animals have a role to playin sustainable agriculture. Theycan help dispose of some agribusiness by-products—crop resi- dues andcrops notsuitable for humanconsumption— and generatewaste thatcanbe usedto fertilize crops. Animal agriculture is increasingly conducted by corporations rather than by family-owned farms or ranches. Once farming moves away from subsistence farming and generates excess over what thefarm fam- ily needs, it becomes a business. The pressure for effi- ciency, as well as forhigh and consistent product qual- ity, is driving animal agriculture toward increasingly specialized and integrated enterprises.While this ten- dency appears to be inevitable, serious concerns arise concerning the oligopolies, if not monopolies, that may control the production of animal products and the management of domestic animals, a valued re - newable resource. James L. Robinson Global Resources Livestock and animal husbandry • 701 Further Reading Campbell, John R., M. Douglas Kenealy, and Karen L. Campbell. AnimalSciences: TheBiology, Care, andPro- duction of Domestic Animals. 4th ed. Boston: McGraw- Hill, 2003. Campbell, Karen L., and John R. Campbell. Compan- ion Animals: Their Biology, Care, Health, and Manage- ment. 2d ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2009. Cheeke, Peter R. Contemporary Issues in Animal Agricul- ture. 2d ed. Danville, Ill.: Interstate, 1999. Ensminger, M. Eugene. The Stockman’s Handbook. 7th ed. Danville, Ill.: Interstate, 1992. Field, Thomas G., and Robert E. Taylor. Scientific Farm Animal Production: An Introduction to Animal Science. 9th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. Gillespie, James R., and Frank Flanders. Modern Live- stock and Poultry Production. 8th ed. Clifton Park, N.Y.: Delmar Cengage Learning, 2009. Shapiro, Leland. Introduction to Animal Science. Upper Saddle River, N.J.: Prentice Hall, 2001. Web Site U.S. Department of Agriculture Animal Production http://www.usda.gov/wps/portal/!ut/p/_s.7_0_A/ 7_0_1OB?navid=ANIMAL_PRODUCTION&pare ntnav=AGRICULTURE&navtype=RT See also: Animal breeding; Animal domestication; Animal power; Farmland; Overgrazing; Rangeland. Logging. See Clear-cutting; Timber industry; Wood and timber Los Angeles Aqueduct Categories: Historical events and movements; obtaining and using resources Construction of the Los Angeles Aqueduct generated considerable controversy; ultimately the aqueduct en - abled Los Angeles to expand by taking water from sources in central California. Definition The LosAngeles Aqueductis a 544-kilometer-longsys- tem that transports water from the Owens Valley and Mono Basin east ofthe SierraNevada south to the Los Angeles metropolitan area. The original aqueduct was proposed in the early 1900’s as a means of supply- ing the growing Los Angeles region with an enlarged and reliable water source for the twentieth century. The original aqueduct was completed in 1913 and its extension was completed in 1941. A second aqueduct was completed in 1970. Overview Los Angeles’ Department of Water and Power, under the leadership of William Mulholland and with the help of former Los Angeles mayor Fred Eaton, ob- tained the water rights tothe Owens River by purchas- ing more than97,000hectares of land in InyoCounty. Much of the population of the prosperous Owens Valley bitterly opposed the aqueduct but could not stop the construction once the water rights had been bought by the Los Angeles Department of Water and Power. The citysold bonds worth morethan $24 millionto fund the construction of the aqueduct down the Owens Valley, across part of the Mojave Desert, and into the Los Angeles basin. Mulholland directed the construction of the mammoth project, which began in 1907 and took five years to complete. The entire 375 kilometers of the original aqueduct transports water by gravity flow and consists of more than 274 ki- lometers ofopen ditch, 19kilometers ofsteel siphons, and 142 tunnels that totaled 85 kilometers. In addi- tion, the project required the construction of more than 800 kilometers of trails and roads, 190 kilome- ters of railroad tracks, and 272 kilometers of transmis- sion lines. The project was one of the greatest engi- neering accomplishments of the early twentieth century. In 1930, Los Angeles approved another $38 mil- lion toextend the aqueductnorthward into theMono Basin in order to tap rivers and streams that feed into Mono Lake. The extension was completed in 1941, and waters were diverted into the aqueduct 544 kilo- meters north of the city. The diversion of water from Mono Lakeeventually caused thelake level todrop 14 meters and the salinity of the lake to rise. Environ- mental groups went to court to halt the diversion of water, and lengthy litigation ensued. As Los Angeles continued togrow, thecity saw theexpanded need for 702 • Los Angeles Aqueduct Global Resources more water from the eastern Sierra Nevada, and in 1963, it appropriated more money to build another aqueduct from the Owens Valley. This second aque- duct was completed in 1970 and increased the total amount of water that could be transported by about 50 percent to a total average capacity of 19 cubic me- ters per second. Much of the water for the second aq- ueduct was to be groundwater pumped from the Owens Valley. However, the Los Angeles Department of Waterand Power hasbeen restricted intheirappro- priations by litigation brought by local residents and environmental groups. Jay R. Yett See also: Irrigation; Water rights; Water supply sys- tems. Global Resources Los Angeles Aqueduct • 703 M Maathai, Wangari Category: People Born: April 1, 1940; Ihithe village, Nyeri District, Kenya An environmental and social activist, Maathai estab- lished the far-reaching Green Belt movement, a grass- roots organization whose members have planted more than thirty million trees since the group’s founding in 1977. Maathai received the Nobel Peace Prize in 2004 and helped launch the Billion Tree Campaign in 2006. Biographical Background Wangari Muta Maathai was born Wangari Muta on April 1, 1940, in the village of Ihithe, Nyeri District of Kenya, the daughter of subsistence farmers. With the help of scholarships, she was able to study in the United States, where she earned bachelor’s and mas- ter’s degrees in biological science. She then returned to Kenya to study anatomy at the University of Nai- robi. According to her memoir, Unbowed (2006), in 1971, she became the first woman in east and central Africa to earn a Ph.D. However, her progressive views spurred criticism from male colleagues and govern- ment officials. These pressures strained her marriage to MwangiMathai, with whomshe hadthree children; he eventuallysued fordivorce anddemanded that she change her surname. In her memoir, Maathai ex- plains that she chose instead to insert an extra “a,” thus signifying her right to identify herself. In 2004, Maathai became the first African woman to win the Nobel Peace Prize. Impact on Resource Use Maathai’s inspiration had two sources: deforestation and poverty. Upon her return to Kenya from the United States,she realized howmuch of hercountry’s landscape had changed, as farmers were forced to cut down increasingly more trees. Maathai also was deter- mined to help her husband keep his campaign prom- ises to create jobs. She created a business called Envirocare, which hired people to raise tree seedlings in nurseries for eventual planting throughout Kenya. The program faced many obstacles, but in 1977, Maathai gained the support of Kenya’s National Council of Women and renamed the endeavor the Green Belt movement. In The Green Belt Movement: Sharing the Approach and the Experience (1988, revised 2003), Maathai states that the organization’s “one person, one tree” motto dictated its goal of planting fifteen million trees, oneforeach person in Kenya. By the early 2000’s, Maathai and other members of the movement hadplanted morethantwicethat number. Maathai simultaneously continued to build her in- fluence in the environmental movement, campaign- ing vigorously against a planned skyscraper in Nai- robi’s Uhuru Park. Although the government evicted the Green Belt movement from its offices in response to the protest, the project was ultimately stopped. Wangari Maathai, winner of the 2004 Nobel Peace Prize, at the 2009 NAACP Image Awards. (Getty Images) Maathai similarlyopposed thegovernment’s attempts to sell off valuable forestland to developers, shaming prospective financiers into withdrawing their sup- port. In retaliation, Maathai was imprisoned several times, but her growing stature in the international community made detaining her without cause in- creasingly difficult for the authorities. In 2002, Maathai won a seat in Kenya’s parliament and was appointed as the assistant minister of the En- vironment, Natural Resources, and Wildlife the fol- lowing year. After winning the Nobel Peace Prize in 2004, she helped the United Nations Environment Programme launch the Billion Tree Campaign. The group’s target was reached more quickly than ex- pected, and a new goal of planting seven billion trees by the endof2009 was established. Althoughmany in- dividuals and organizations have contributed signifi- cantly to reforestation efforts, Maathai has had a pro- found influence on this issue. Amy Sisson See also: Forests; Greenhouse gases and global cli- mate change; Nobel, Alfred; Reforestation. McCormick, Cyrus Hall Category: People Born: February 15, 1809; Rockbridge County, Virginia Died: May 13, 1884; Chicago, Illinois As inventor of the mechanical reaper, McCormick transformedagriculture in the mid-nineteenth century by streamlining the process of harvesting grain, result- ing in dramatic increases in grain production and the fueling of westward expansion. Biographical Background Cyrus Hall McCormick, the son of a prosperous Vir- ginia farmer, developed the first successful mechani- cal grain reaper in 1831 by improving upon a design conceived by his father. Sales of the reaper—which was capable of cutting, threshing, and bundling up to 5 hectares of grain per day—grew slowly at first de- spite successful early demonstrations of its ability. Westward expansion and the resultant demand for greater grain yieldsincreased interest in themechani - cal reaper during thelate 1830’s. In1839, McCormick formed a business partnership with his brothers and began mass-producing mechanical reapers in Chi- cago, the trade hub of the Midwest and western fron- tier. With the aid of innovative marketing techniques and anincreasing availability ofrailroad linesfor ship- ping, McCormick sold large numbers of mechanical reapers, particularly in grain-producing Midwestern states and territories, during the 1840’s and 1850’s. Impact on Resource Use The McCormick reaper exerted an immediate and dramatic impact upon American agriculture, com- merce, and society during the mid-nineteenth cen- tury. The reaper greatly decreased the cost of grain farming and increased grain yields per hectare, prompting farmers to produce more grain. The in- crease in production helped meet the growing de- mand forfoodstuffsresulting from populationexpan- sion in the eastern United States and transformed the United States into a major exporter of grain. The reaper also contributed to American urbanization Global Resources McCormick, Cyrus Hall • 705 Cyrus Hall McCormick invented the crop reaper that bears his name. (Library of Congress) and industrializationbyreducing demandfor agricul - tural labor in rural areas, encouraging rural farm workers to migrate to cities, and providing a growing labor pool to meet the increased demand for indus- trial workers inurban areas. Theproduction and mar- keting of foodstuffs thus assumeda larger role inbusi- ness and industry as the number of food consumers grew and the ranks of food producers diminished. Increasing urbanization prompted a growing em- phasis upon transportation in the United States: Fewer Americans produced their own food and their proximity tofoodsources decreased,which fueledthe growth of railroads, roads, turnpikes, and trails con- necting consumers to local and regional commercial centers. By increasing demand for farmland in Mid- western states, the McCormick reaper became a driv- ing force for westward expansion, producing changes in the American social and political landscape that af- fected numerous issues surrounding resource use, in- cluding conflicts with indigenous peoples over land and resources, conflicts between livestock owners over the use of grazing lands, and the escalating de- bate over utilization of slave labor in the American South. The McCormick reaper was thefirst of anumber of agricultural machines that collectively transformed agriculture, commerce, and daily living during the late nineteenth and early twentieth centuries. The mechanization of farming influenced a number ofso- cial and economic trends in the United States and worldwide, including the development of highways, the emergenceofthe petrochemical and agribusiness industries, and mass migrationsof farmlaborers from ruralareas to cities. These trends resulted indramatic changes in the production, delivery, utilization, and allocation of resources. Michael H. Burchett See also: Agricultural products; Agriculture indus- try; Mineral resource use, early history of; Population growth; Transportation, energy use in; Wheat. Magma crystallization Category: Geological processes and formations Magma crystallization is a geologic process in which molten magma in the Earth’s interior cools and subse - quently crystallizes to form an igneous rock. The crys - tallization process produces many different types of minerals, some of which are valuable natural re- sources. Background Magma is molten rock material consisting of liquid, gas, and early-formed crystals. It is hot (900° to 1,200° Celsius), mobile, and capable of penetrating into or through the Earth’s crust from the mantle, deep in the Earth’s interior. Most magma cools in the Earth’s crust; in a process similar to ice crystallizing from water as the temperature drops below the freezing point, minerals crystallize from molten magma to form a type of rock called igneous rock. Once com- pletely crystallized, the body of igneous rock is called an intrusion. Some magma, however, works its way to the surface and is extruded as lava from volcanoes. Mineral Growth Magma that remains below the surface cools at a slow rate. Ions have time to collect and organize them- selves into orderly, crystalline structures to form min- erals. These minerals grow larger with time and, if the cooling rate is slow enough,may grow to several centi- meters in diameter or larger. Igneous rocks with min- erals of this size are said to have a phaneritic texture. Magma that reaches the surface, on the other hand, cools very rapidly and forms rocks that consist of ex- tremely fine-grained minerals or quenched glass. These rocks have an aphanitic or glassy texture. Con- sequently, it is those minerals which grow beneath the surfacethat reach sizeslarge enoughto beconsidered economically feasible resources. Concentration of Valuable Elements Minerals do not crystallize from magma all at once. Instead, they follow a sequence of crystallization as the temperature decreases. In general, silicate miner- als (substances with silicon-oxygen compounds) with high contents of calcium, iron, and magnesium crys- tallize early, followed by silicate minerals with high contents of aluminum, potassium, and sodium. Ex- cess silica crystallizes last as the mineral quartz. Bonding factors such as ionic size and charge prevent some elements from incorporation into early crystal- lizing minerals. Thus they are more highly concen- trated in the residual magma and become incorpo - rated into the last minerals to crystallize, forming rocks calledgranites andpegmatites. These rocks may 706 • Magma crystallization Global Resources contain minerals such as beryl, spodumene, lepido - lite, anduraninite, which includeimportant elements such as beryllium, lithium, and uranium. Granites and pegmatites are also important sources for feld- spar and sheet mica. Diamonds and Kimberlites Perhaps the best-known magmatic minerals are dia- monds. Formed deep in the mantle at extremely high temperatures and pressures, diamonds are carried by a certain type of magma as it violently intrudes upward through the crust, sometimes reaching the surface. Upon cooling and crystallizing, this magma forms a pipe-shaped igneous rock known as kim- berlite. It is in kimberlites that most diamonds are found. Most kimberlite pipes are less than one square kilometer in horizontal area, and they are often grouped in clusters. Most of the known diamond- bearing kimberlite pipes are found in southern Af- rica, western Australia, Siberia, and Canada. Magmatic Sulfide Deposits Most major metals used in industry (copper, iron, lead, nickel, zinc, and platinum) are found in sulfide minerals, which are substances that contain metal- sulfur compounds. When magma is in the early stages of cooling and crystallizing underground, certain processes can cause droplets of liquid sulfide to form within it. These sulfide droplets attract metallic cat- ions and concentrate them by factors ranging from 100 to 100,000 over their normal levels in the host magma. The droplets eventually cool and solidify to form sulfide minerals such as pyrite (“fool’s gold”), galena (lead sulfide), and sphalerite (zinc sulfide). Sulfide minerals such as these become important tar- gets for mining because of their high concentration of metals. Layered Magmatic Intrusions Some magmas give rise to layered intrusions in which a specific sequence of minerals is repeated many times from bottom to top in a process called gravity layering (also called rhythmic layering). Dark-colored, heavier minerals such as pyroxene, olivine, and chro- mite concentrate near the base of each layer, grading to predominantly light-colored minerals such as plagioclase at the top. Each mineralsequence is a sep- arate layer, averaging several meters thick and rang- ing from less than 2 centimeters to more than 30 me- ters. It has been suggested that the origin of gravity layering involves multiple injections of fresh magma into a crystallizing magma chamber, effectively re- plenishing the magma and allowing the same miner- als to crystallize repeatedly. The Bushveld intrusion in South Africa, one of the largest layered in- trusions, contains multiple gravity layers and is more than 7,000 meters in totalthickness. Layered intrusions contain theEarth’s mainreserves for chromium and platinum. In the Bushveld intrusion, chromium oc- curs in the mineral chromite, and platinum in platinum-iron alloys, braggite, and other platinum-metal compounds. The main source for platinum minerals in the Bushveld intrusion, and the source for ap- proximately half the Earth’s supply of platinum, is the Merensky Reef, a layer of chromite and platinum min- erals 1 meter thick and more than 200 kilometers long. Also present in the Bushveld intrusion is the min- eral magnetite, which yields impor - tant elements used in steel manufac - turing such as iron and vanadium. Global Resources Magma crystallization • 707 This example ofigneous rock, theend result ofmagma crystallization, isfoundin Garrizo Mountain in Arizona. (USGS) . lithos) consists of the outer, brittle portions of the Earth, in- cluding the upper mantle and crust. Overview The interior of the Earth has a number of layers, or concentric spheres. At the center of the. areas.Oceanic crust is primarily composed of dense basaltic rock 698 • Lithosphere Global Resources with a thin veneer of silt and carbonate precipitates; however, a variety of minerals have been observed. layers. The later discovery of oil in northern Alaska was prompted by the similarity ofthe crustthere to thecrustof these re - gions. As the study of the characteristics of the litho - sphere—including