placer deposits formed in this manner. Among the more famous nuggets found are a 93-kilogram nugget found in Hill End, Australia, and a 153-kilogram nug- get found in Chile. The spectacular classic placer de- posits found in the Klondike, in the Yukon (Canada), and near Sacramento, California, explain the subse- quent gold bonanzas and migration of prospectors, then settlers, into the American West. Secondary de- posits have also yielded the abundant alluvial gold de- posits found near Johannesburg. By far, most gold is found in placers of sedimentary origin. In areas of re- cent erosion, gold is usually found in small, shapeless grains, in small sheets, or as flakes. When fine-grain gold is found in alluvial deposits, “panning for gold” is performed to separate the precious metal from the sand. Formed in primary deposits, crystals of elemental gold may occur as veins or as dendritic (arborescent) aggregates in association with quartz crystals. Den- dritic aggregates look as though the metallic crystal developed with a fernlike growth on large, colorful and translucent quartz crystals. Gold veins are often natural alloys of gold and silver rather than pure gold. These naturally occurring gold-silver alloys are known as electrum, in which the silver content may range from 15 percentto50percent. Other natural alloys, as of gold and palladium (porpezite) or gold and rho- dium (rhodite), are less frequently found. Gold also occurs in telluride ores, such as tetradymite, nag- yagite, and sylvanite. These ores are primarily sulfide compounds of tellurium. In addition to tellurium (Te) and sulfur (S) atoms, tetradymite contains gold and lead. Similarly, sylvanite and nagyagite (black tel- lurium) contain gold and silver, but in different ar- rangements and ratios. Elemental gold can be ex- tracted from these minerals via chemical reactions. Gold is not an essential element for life, although trace amounts are found in humans and some plants concentrate the element. Trace amounts in humans may arise from ingestion of gold from certain alco- holic beverages, from gold dental amalgams, or from exposure to gold therapy for arthritis. Because gold is minimally absorbed by the digestive system, these trace amounts pose no toxic concern. Most of the world’sgolddepositshavebeenwellex- ploited and are therefore nearly devoid of the pre- cious metal. In the South American continent, which was the least mined of all continents up to the 1980’s, the environment has begun to suffer from the hunt for gold. Past methods of obtaining gold have yielded to the more dangerous practice of using liquid mer - cury to form a mercury-gold amalgam in the panning process. To recover even the tiniest amount of gold, a large quantity of mercury must be used. In South American rivers, gold occurs in brown, iron-stained sand. Some deposits have been profitable even though only a few dollars’ worth of gold may be gath- ered per metric ton of sand panned and amalgam- ated. Whether or not gold is actually found, the left- over mercury is dumped directly into the rivers. Mercury, a neurotoxin, is lethal in high amounts. Re- portedly, the dumping of untreated mercury has re- duced populations of fish, has caused high levels of mercury in fish eaten by people, and is likely to have health impacts on children, pregnant women, and fu- ture generations. Some researchers suspect that neu- rological symptoms that suggest mercury poisoning can already be seen in some South American popula- tion areas. History The Group IB metals, or coinage metals, were the first metals used in primitive cultures. It is believed that el- emental metals were easy to find in nature because their bright lusters shone in natural light. Precious metals have been in use since at least seven thousand years ago by civilizations of the Middle East and Af- ghanistan. Wealthy members of these groups pos- sessed decorative jewels fashioned from gold. The metalworkers of these ancient societies manipulated the gold physically using hammers or other tools to carve or cut the soft metal. Exploration of the tomb of King Tutankhamen, from the fourteenth century b.c.e., revealed an entry guarded by gold funerary masks inlaid with colorful glass. A gold sarcophagus and gold panel behind the king’s throne were also found. Between 4000 and 3500 b.c.e., the Egyptians and Sumerians learned to smelt silver and gold. They were able to generate fires in furnaces that could achieve the extreme tempera- tures required to melt metals, to cast molten metal into molds, to forge metal, and to make alloys (by blending molten metals). The use of gold for dental fillings among wealthy Egyptians dates back to be- tween 2680 and 2160 b.c.e. In Mesopotamia, a region that is now part of Iraq, an ornate headdress of Queen Puabi, dated to 2700 b.c.e., was fashioned with gold-carved leaves to adorn her face. Trading and business deals of Mesopotamia involved the exchange of precious metals, although 528 • Gold Global Resources there was no system of standardized coins. Archaeo - logical studies have also shown that the Incan civiliza- tion of pre-Columbian South America possessed con- siderable gold-working skill and achieved mastery of soldering and welding techniques. The alchemists of the medieval period believed that gold was one of the most important keys to im- mortality. They also believed that base metals, which were abundant and cheap, could be converted into gold, which was rare and expensive. It was assumed that by simple manipulation in the presence of a spiri- tual agent—such as the Philosopher’s Stone—an elixir could be formed that possessed all the ingredients required for immortality. Because of its inert behav- ior and timeless beauty, gold was believed to impart some qualities required to achieve worldly immortal- ity. During medieval times it was widely thought that the emperors and kings who had the most gold would have the longest lives. If a king ruled for many years there could be long periods of economic stability, ac- cess to food, security of family, and safety from con- querors. Thusthepursuitofgoldwas serious business, and the king’s magician, who was usually an alche- mist, was highly regarded in the king’scourt. As a final historical note, gold amalgams, mixtures of mercury and gold, were described in the year 27 b.c.e. by a Roman architect, Vitruvius. Mercury and gold amalgamation is still in use today as a means of collecting gold from sand deposits of riverbeds. Obtaining Gold Gold is separated from rocks, minerals, and al- luvial deposits by panning or sluicing methods. The extraction of gold from telluride ores (tetra- dymite, nagyagite, and sylvanite) requires chemi- cal reactions. The use of cyanide compounds, for- mation of amalgams, or smelting gold may be necessary to extract the gold from ores. The ex- tracted gold is frequently refined by electrolysis (the use of an electric current). Electrolysis is par- ticularly useful in separating mercury-gold amal- gams back into their separate and purified metal- lic state. In telluride ores (minerals), gold is not in the free, elemental state; rather, it is in a cationic form. As a metallic cation, each atom of gold car- ries a positive charge of either +1 or +3. A chemi- cal reaction involving the addition of potassium cyanide tothecrushed rocks (coveredwithwater) makes a new compound of gold that dissolves in the water. This layer of water can be collected off the crushed rock, and through electrolysis the gold cat- ions can be converted into gold crystals. Uses of Gold As described previously, jewelry and decorative orna- ments fashioned from gold are marketed using carats to describe the quantity of gold present. Compounds of gold are used for decorating china or glass items. Gold chlorocompounds,containinggoldcationshav- ing a +3 charge, are mixed with sulfurized terpenes or resins toform a mixtureknownas“liquidgold,” which can be applied directly to glass or china. Compounds of gold with +1 cations are used in rheumatology as an anti-inflammatory agent for the treatment of active, refractory forms of juvenile and adult rheumatoid arthritis. These biologically active compounds are sodium gold thiomalate and sodium gold (or auro) thioglucose; aurothioglucose seems to be less painful when injected into a muscle near the joint. The gold therapy must be started before perma- nent changes have occurred in the afflicted joints if it is to benefit the patient. Some of the side effects of these therapies include skin, liver, and kidney changes or damage. Approximately 20 percent of patients who Global Resources Gold • 529 Jewelry & the arts 80% Electrical & electronics 8% Dental &other 12% Source: Percentages are based on data from the U.S. Geological Survey and are rounded to the nearest hundredth percent. U.S. End Uses of Gold try gold therapy have to discontinue treatment be - cause of these adverse reactions. However, newer anti- inflammatory agents have limited the number of pa- tients who need to try gold therapy for relief. Finally, gold has been used in an abstract manner as the basis or standard of valuation for currencies and monetary systems throughout the world. The ori- gins of this ancient practice lie in Mesopotamian, As- syrian, and Lydian societies. Mary C. Fields Further Reading Bernstein, Peter L. The Power of Gold: The History of an Obsession. New ed. New York: Wiley, 2004. Boyle, Robert W. Gold: History and Genesis of Deposits. New York: Van Nostrand Reinhold, 1987. Green, Timothy. The New World of Gold: The Inside Story of the Mines, the Markets, the Politics, the Investors. New York: Walker, 1981. Greenwood, N. N., and A. Earnshaw. “Copper, Silver, and Gold.” In Chemistry of the Elements. 2d ed. Bos- ton: Butterworth-Heinemann, 1997. Macdonald, Eoin H. Handbook of Gold Exploration and Evaluation. Boca Raton, Fla.: CRC Press, 2007. Marx, Jenifer. The Magic of Gold. Garden City, N.Y.: Doubleday, 1978. Pellant, Chris. Rocks and Minerals. 2d American ed. New York: Dorling Kindersley, 2002. Schumann, Walter. Handbook of Rocks, Minerals, and Gemstones. Translated by R. Bradshaw and K. A. G. Mills. Boston: Houghton Mifflin, 1993. 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 Gold: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/gold/index.html#mcs See also: Alloys; Australia; Canada; Hydrothermal so- lutions and mineralization; Metals and metallurgy; Mexico; Mineral resource use, early history of; Native elements; Placer deposits; Russia; Sedimentary pro - cesses, rocks, and mineral deposits; South Africa; United States. Gore, Al Category: People Born: March 31, 1948; Washington, D.C. As congressman, senator, and vice president of the United States, and as author of several best-selling books, Al Gore tried to alert the American government and public to environmental problems and their dele- terious effects on world resources. Biographical Background The son of a U.S. senator, Albert Arnold Gore, Jr., went to school in Washington, D.C., during the winter and spent summers working at the family farm in Ten- nessee. He first became aware of environmental prob- lems when seeing topsoil washing into rivers during floods. A course at Harvard College taught by Roger Revelle, whose pioneering research brought the ques- tion of global warming to the attention of scientists, strongly impressed Gore. After serving in the Army during the Vietnam War, Gore attended Vanderbilt Theological Seminary and Vanderbilt Law School. In 1976, he dropped out of law school and successfully ran for a seat in the United States House of Representatives, where he served un- til being elected to the Senate in 1984. In 1992, Gore was elected vicepresidentoftheUnitedStates.He lost the 2000 presidentialcontestdespitewinninga major- ity of the popular vote. Impact on Resource Use Gore believed the environment was the most funda- mental global resource, whose degradation threat- ened the life and economy of the entire world. His major role in the environmental movement was as a publicist, bringing data on global warming and other forms of pollution to the attention of his government colleagues and the general public. While in the House of Representatives he was the subcommittee chair- man presiding over hearings examining dumping of toxic chemicalsintheNiagaraFalls Love Canal area. In the Senate, Gore called attention to the chal- lenge of global warming and traveled the world col- lecting data on environmental problems affecting global resources, which informed his first book, Earth in the Balance (1992). The book described many threatening developments—destruction of rain for - ests, indiscriminate dumping of toxic residues, and 530 • Gore, Al Global Resources the growing danger posed by greenhouse gases—and called for action to control these activities before they caused irreparable damage. Published in the spring of 1992, the book became a best seller, informing a wide audience of possible perils to the globe. As vice president Gore pushed, without great suc- cess, for government action to protect the environ- ment. He helped facilitate the Kyoto Protocol on global warming, but the United States never adopted the pact. Gore’s most effective contribution to environmen- tal awareness was his 2006 book An Inconvenient Truth, made into a powerful film the following year that won an Academy Award for Best Documentary. The movie dramatizes the statistics, graphs, and charts that Gore used in his lectures and in the book, stress- ing the evidence for global warming and refuting skeptics who question whether the threat to the globe is real. The Assault on Reason (2007) excoriates Presi- dent George W. Bush’s administration and others who weakened environmental security by ignoring the ra - tional arguments of scientists regarding human con - tributions to climate change. Gore’s lifelong devotion to defending the environment won him many awards, climaxed by the 2007 Nobel Peace Prize, which he won jointly with the Intergovernmental Panel on Cli- mate Change. Milton Berman See also: Climate Change and Sustainable Energy Act; Environmental degradation, resource exploita- tion and; Environmental movement; Greenhouse gases and global climate change. Granite Category: Mineral and other nonliving resources Granite is a medium- to coarse-grained igneous rock composed principally ofinterlockinggrainsofthe light- colored silicate minerals—potassium feldspar, sodium- rich plagioclase, and quartz. The overall color may Global Resources Granite • 531 Nobel Prize winner Al Gore discusses climate change at a 2009 summit in Mexico City. (AP/Wide World Photos) blend to reddish, pink, or white depending on which mineral predominates in the rock. Dark minerals may add a spotted appearance to the rock. Granite is an ig- neous rock formed at great depths intheEarth’s crust. Definition The three essential minerals in granite are quartz, which makes up 20 to 40 percent of the rock, and feld- spars in which potassium feldspar is more abundant than plagioclase. Five to 10 percent ferromagnesian minerals, usually biotite or hornblende, or muscovite are common as accessory minerals. Garnet, tourma- line, corundum, or even pyroxene may be present in some granites. Overview The continents are primarily granite, with a thin ve- neer of sedimentary rocks. Granite is found in the ex- posed core of linear mountain chains and regions of highly eroded continental shields associated with re- gional metamorphism. The Sierra Nevada mountain range consists of a composite granitic batholith that is 640 kilometers by 110 kilometers. Granites also form such notable sites as Mount Rushmore, South Dakota; Half Dome in YosemiteNationalPark, California; and Stone Mountain, Georgia. Granite is used extensively as building stone. It is strong and weather resistant. Cut and polished slabs are used for internal and external facing, and pol- ished or horned blocks are used for ornamental stones as tombstones and monuments. Large blocks areused in sea walls and jetties. Smaller blocks and crushed stone are used as rip-rap. Variations in texture and composition give rise to distinctive varieties of granite. Pegmatite is an ex- tremely coarse-grained rock of granite composition formed in the late, fluid-rich stage of magma crystalli- zation. Individual crystals may reach several centime- ters or tens of meters in length. Aplite is a fine-grained granite with a sugary texture. Graphic granite is con- spicuous by its intergrowth of quartz within ortho- clase crystals, which gives a pattern similar to cunei- form writing. Alaskite is a granite with no dark minerals. Charnockite is granite containing hyper- sthene as its chief ferromagnesian silicate. Granite magma is formed by melting continental crustal rocks and thick prisms of sediments that form along the margins of convergent plates. The melt mi - grates upward in the crust through overlying rocks by assimilation of surrounding rocks and by forcefully pushing rocks out of the way. As the magma moves up - ward, blocks of overlying rocks are incorporated into the melt. If the melt is hot enough, the included rocks may be melted. If the magma has cooled sufficiently, the blocks are preserved as xenoliths (foreign rocks) within the magma. Granite magmas cool to form large intrusive bodies known as batholiths (bathos for deep, and lithos meaning rock) and smaller intrusions such as dikes and sills. As batholiths are emplaced fairly deep in the crust, the surrounding materials are usually high-grade metamorphic rocks such as schist and gneiss. Some granites may form by extreme meta- morphism in which existing rocks are converted to granitic rock by recrystallization and chemical reac- tion with chemically active fluids. René A. De Hon See also: Aggregates; Earth’s crust; Feldspars; Igne- ous processes, rocks, and mineral deposits; Peg- matites; Plutonic rocks and mineral deposits; Quar- rying. Graphite Category: Mineral and other nonliving resources Where Found Natural graphite is distributed widely in the world. Major deposits are found in Sri Lanka, North and South Korea, India, Austria, Germany, Norway, Can- ada, Mexico, China, Brazil, and Madagascar. The United States imports virtually all of the natural graph- ite it needs from the latter four countries. However, most of the graphite used in the United States is syn- thesized from a wide variety of carbon-containing ma- terials—for example, anthracite coal and petroleum coke. Synthetic graphite is denser, purer, and more expensive than the natural form. Primary Uses Graphite is used in “lead” pencils. To a lesser extent, it is also used in brake linings, steelmaking, and lubri- cants. Technical Definition Graphite is composed of parallel planes of fused hex - agonal rings of carbon atoms. It exists in two forms, al - pha (also called hexagonal) and beta (also called 532 • Graphite Global Resources rhombohedral), which have apparently identical phys - ical properties but differ in their crystal structure. In the alpha form, the carbon atoms in alternate layers are directly above each other, while in the beta form, the carbons do not line up again until every fourth layer. In both forms, the distance between neighbor- ing carbon atoms within the layers is 142 picometers, which is intermediate between the length of typical single and double C—C bonds. The distance between the layers is 335 picometers. The larger distance be- tween the layers reflects the weaker forces holding the planes together compared with the forces holding neighboring atoms together within the planes. Because of graphite’s weak interplanar forces, the planes can readily slip past each other, causing graph- ite to cleave easily and preferentially parallel to its planes. This process accounts for its flaky appearance and excellent lubricating ability even when dry. The planar structure also causes several of its physical properties to be highly anisotropic (exhibiting differ- ent properties when measured in different directions). For example, its thermal conductivity is several hun- dred times larger, and its electrical conductivity is sev- eral thousand times larger, when measured parallel to the planes than perpendicular to them. The density of synthetic graphite is 2.26 grams per Global Resources Graphite • 533 Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009.Source: Mineral Commodity Summaries, 2009 130,000 5,000 13,000 30,000 2,000 3,000 2,000 8,000 6,000 Metric Tons 900,000750,000600,000450,000300,000150,000 Ukraine Norway North Korea Mexico Madagascar India Sri Lanka Turkey Other countries 76,000 28,000 800,000 3,000Czech Republic China Canada Brazil Graphite: World Mine Production, 2008 cubic centimeter, but that of natural graphite is usu - ally lower, varying from 2.23 to 1.48 grams per cubic centimeter, due to the presence of pore spaces and impurities. Description, Distribution, and Forms Graphite and diamond arethetwopredominant forms in which free carbon is found in nature. Graphite is a greasy, opaque, highly reflective black or gray solid. Although graphite can be found throughout the world, much of it is of little economic importance. Large crystals, called flake, occur in metamorphosed sedimentary silicate rocks such as quartz, schists, and gneisses and have an average crystal size of about four millimeters (ranging from fractions of a millimeter to about six millimeters). Deposits have also been found in the form of lenses up to 30 meters thick and stretch- ing several kilometers, with average carbon content of 25 percent (reaching 60 percent in Madagascar). The graphite in these cases was probably formed from the carbon in organic materials. Deposits containing microcrystalline graphite (sometimes referred to as “amorphous carbon”) can contain up to 95 percent carbon. In Mexico such amorphous carbon occurs in metamorphosed coal beds. The graphite deposit in New York occurs in a hydrothermalveinand was prob- ably formed from carbon-bearing rocks during meta- morphism intheregion. Graphite occurs occasionally as an original constituent of igneous rocks (for exam- ple in India), and it has been observed in meteorites. Graphite has the unusual property that it is very soft at roomtemperature(withahardness between 0.5 and 1 on the Mohs scale, which is similar to talc) but has increasing strength at high temperatures. At about 2,000° Celsius, its crushing strength is increased by 20 percent, and at about 3,000° Celsius, its tensile strength is increased by 50 to 100 percent. Other im- portant propertiesof graphite that are exploited in its many uses listed previously are its stability at high tem- peratures and in the presence of corrosive and reac- tive chemicals. History Carbon was known in prehistory in the forms of char- coal and soot, but it was not recognized as a chemical element until the second half of the eighteenth cen- tury. In 1779, graphite was shown to be carbon by Carl Wilhelm Scheele, a Swedish chemist; ten years later the name “graphite” was proposed by Abraham Gottlob Werner, a German geologist, and D. L. G. Harsten, from the Greek graphein (to write). Commer - cially, “lead” pencils were first manufactured in about 1564 in England during Queen Elizabeth’s reign, us- ing Cumberland graphite. In 1896, Edward Goodrich Acheson, an American chemist, was granted a patent for his process whereby graphite is made from coke, and within one or two years, production began on a large scale. Diamond was firstsynthesizedfrom graph- ite between 1953 and 1955. Obtaining Graphite Graphite can be made to sublime directly to carbon vapor or to melt to liquid carbon at temperatures above approximately 3,500° Celsius, depending on the pressureandotherconditions.Itcanalsobe trans- formed into diamond at extremelyhighpressures and temperatures (for example, 100,000 atmospheres and 1,000°-2,000° Celsius). The rate of conversion of dia- mond back to graphite at atmospheric pressure is not significant below temperatures of about 4,000° Cel- sius. The mining and purification process of natural graphite includes flotation followed by treatment with 534 • Graphite Global Resources Refractory 28% Steel & foundry 23% Brake linings 12% Batteries & lubricants 4% Other 33% Source: Mineral Commodity Summaries, 2009 Data from the U.S. Geological Survey, .U.S.GovernmentPrinting Office, 2009. U.S. End Uses of Natural Graphite acids and then heating in a vacuum to temperatures on the order of 1,500° Celsius. Uses of Graphite The most familiar use of graphite is in the manufac- ture of “lead” pencils, where it is mixed with clay and other materials and baked at high temperatures. The “lead” increases in softness as the ratio of graphite to clay increases. Graphite has much more extensive use in the manufacture of lubricants and oilless bearings; electrodes in batteries and industrial electrolysis; high-temperature rocket casings, chemical process equipment, furnaces, and crucibles for holding mol- ten metals; tanks for holding corrosive chemicals; and strong and lightweight composite materials that are used, for example, in airplanes and high-quality sports equipment such as tennis rackets and golf clubs. Graphite is also a component in the cores of some nuclear reactors as the moderator to slow down the neutrons, and it is the major raw material forsynthetic diamonds. Leslie J. Schwartz Further Reading Chatterjee, Kaulir Kisor. “Graphite.” In Uses of Indus- trial Minerals, Rocks, and Freshwater. New York: Nova Science, 2009. Delhaès, Pierre, ed. Graphite and Precursors.BocaRaton, Fla.: CRC Press, 2000. Greenwood, N. N., and A. Earnshaw. “Carbon.” In Chemistry of the Elements. 2d ed. Boston: Butter- worth-Heinemann, 1997. Inagaki, Michio. New Carbons: Control of Structure and Functions. New York: Elsevier Science, 2000. Kogel, Jessica Elzea, et al., eds. “Graphite.” In Indus- trial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006. Morgan, Peter. Carbon Fibers and Their Composites. Boca Raton, Fla.: Taylor & Francis, 2005. Pellant, Chris. Rocks and Minerals. 2d American ed. New York: Dorling Kindersley, 2002. Petrucci, Ralph H., William S. Harwood, Geoff E. Herring, and Jeffrey Madura. General Chemistry: Principles and Modern Applications. 9th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2007. Pierson, Hugh O. Handbook of Carbon, Graphite, Dia - mond, and Fullerenes: Properties, Processing, and Appli - cations. Park Ridge, N.J.: Noyes, 1993. 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 Graphite: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/graphite/index.html#mcs See also: Austria; Brazil; Canada; Carbon; China; Crystals; Diamond; Germany; Gneiss; India; Meta- morphic processes, rocks, and mineral deposits; Mex- ico; Minerals, structure and physical properties of; Mohs hardness scale; Native elements; South Korea; Talc; United States. Grasslands Categories: Ecological resources; plant and animal resources Grassland ecosystems contain a great diversity of plant and animal life, and grasslands have supported hu- man habitation for tens of thousands of years. Grass- lands are crucial in the growing of crops and the graz- ing of livestock. Background One way that botanists and ecologists classify regions of the Earth is according to their vegetation. Wood- land, desert, tundra, and grassland are major classifi- cations. All have their own types of climate, physical environment, soil, plants, and animals. Grasslands are so named because the dominant plant species are low plants, most notably various grass species. Through- out the world there are differing types of grasslands— some tropical, some temperate, some with a relatively moderate amount of rainfall, some with little rainfall that are subject to harsh droughts. Grasslands have many regional names, including shrub steppe, the prairie of North America, and the pampa of South America. A savanna, or parkland, is typically a mixed zone, often considered a transitional region between grassland and forest. A grassland itself may be bor - dered by desert, parkland, or forest. Humans have Global Resources Grasslands • 535 lived in grassland environments for thousands of years, and in parts of the world it is impossible to determine which aspects of a grassland ecosystem are natural and which have been changed by countless genera- tions of human activity related to agriculture and the grazing of domesticated animals. Climate and Fire The defining characteristics of grassland climates are a marked seasonal variation between the wet and dry seasons and a dry season or overall climate that is too dry for forests to develop. A major distinction can be drawn between temperate and tropical grasslands, with tropical grasslands having higher temperatures and, generally, greater rainfall. In temperate grass- lands annual rainfall is quite low, ranging from 25 to 75 centimeters. In tropical and subtropical grass- lands, rainfall is in the range of 60 to 150 centimeters. With their distinct seasonal shifts,manygrasslandsare subject to monsoons in the rainy season and drought in the dry season. Drought periods may last from sev - eral weeks to several months. Fire is a natural and prominent part of the grass- land environment, and lightning fires are common. Fire canserveanumberofpurposes.As drought does, it can help maintain the grassland boundary, keeping forests from moving into the zone. Perhaps surpris- ingly, it also fosters the growth of grasses and grasslike plants by burning offoldplantlayerswhileleavingthe growth zones of new plants, much of which are below the soil line, generally unharmed. Soil, Plants, and Animals Grassland and prairie soils are distinct from those of forest regions. Tropical grassland soils are often leached by periods of heavy rain and therefore tend to have relatively low nutrient levels. Temperate grass- land soils retain many more of their nutrients and can be rich in humus (organic matter) as well, making them quite fertile. Therefore, they have long been used for crop production and grazing. Both the plant and animal communities of grass - lands are diverse, although grasses may compose up to 90 percent of grassland biomass. Grasses are well 536 • Grasslands Global Resources Sheep graze on a Tibetan grassland. (Xinhua/Landov) adapted to endure drought because of their root masses and because they can reproduce asexually if conditions make seed reproduction impossible. Some also go into a dormant state to survive the dry season. Perennial grasses and forbs are the most com- mon plants, but there are also small shrubs, fungi, li- chens, and mosses. In addition, some grasslands do have scattered trees, most often along stream chan- nels. Small grassland animals include birds, reptiles, insects, worms, mice, and prairie dogs. Larger ani- mals include large herbivores such as bison, elk, and wildebeest as well as the carnivores (wolves, the large cat species) that prey on them. Grasslands and Humans Humans have lived on, and relied on the resources of, grasslands for at least tens of thousands of years. Hunter-gatherers roamed grasslands and savannas, and thefirstagriculture was almost certainly practiced in grassland regions. Throughout the world, grass- lands have extensively been converted from their nat- ural state to areas used for grazing and crop produc- tion. A huge percentage of the world’s commercial grains—notably corn, wheat, and soybeans—is grown in temperate grassland regions. Many ecologists believe that human-induced changes to grasslands can have both positive and neg- ative effects, in some cases stabilizing grassland re- gions, in others abusing them and unintentionally causing desertification. Humans have introduced non- native species of plants and animals, in some cases re- placing native species. Cattle replacedthebuffalo and elk that once roamed North America, for example. Grasslands can support a considerable amount of grazing activity, even by non-native animals, as long as overgrazing does not occur. Modern range manage- ment techniques are intended to ensure that animal numbers do not exceed asustainablelevel.Inorderto protect their investments in livestock, humans have hunted, and in many areas virtually eliminated, the large grassland carnivores (wolves, bears, cats) that prey on grazing animals. Human activities can also decrease or damage grassland habitats themselves; chief among these ac- tivities are suppressing the fires that are a natural part of grassland environment and draining prairie wetlands. Desertification is a significant problem. It is likely that some areas that have been inhabited by hu - mans for thousands of years—such as lands around the Mediterranean Sea as well as in Asia Minor, Iran, and India—that are now desertlike were once grass - lands. More recently, desertificationcausedor exacer- bated by human activity has been noted elsewhere— in central Africa, for example. On the other hand, human activity can stabilize and help maintain grasslands, and in many cases grasslands seem to adapt well to human habitation. One possible positive aspect of livestock grazing is that range managers can potentially control animal numbers and density far more than wild animals can be controlled, providing a stabilizing effect. Finally, there are ongoing efforts to preserve some of the re- maining small regions of relatively unaffected prairie in North America. McCrea Adams Further Reading Coupland, R. T., ed. Grassland Ecosystems of the World: Analysis of Grasslands and Their Uses. New York: Cambridge University Press, 1979. Cushman, Ruth Carol, and Stephen R. Jones. The Shortgrass Prairie. Boulder, Colo.: Pruett, 1988. Editors of Time-Life Books. Grasslands and Tundra.Al- exandria, Va.: Author, 1985. Fast, Dennis, and Barbara Huck. The Land Where the Sky Begins: North America’s Endangered Tall Grass Prai- rie and Aspen Parkland. Winnipeg, Man.: Heartland Associates, 2007. Gibson, David J. Grasses and Grassland Ecology. New York: Oxford University Press, 2009. Licht, Daniel S. Ecology andEconomicsoftheGreat Plains. Lincoln: University of Nebraska Press, 1997. Manning, Richard. Grassland: The History, Biology, Poli- tics, and Promise of the American Prairie. New York: Penguin, 1995. Price, Elizabeth A.C.LowlandGrasslandandHeathland Habitats. Illustrations by Jo Wright. New York: Rout- ledge, 2003. Reynolds, S. J., and J. Frame, eds. Grasslands: Develop- ments, Opportunities, Perspectives. Enfield, N.H.: Sci- ence Publishers, 2005. Woodward, Susan L. Grassland Biomes. Westport, Conn.: Greenwood Press, 2008. Web Sites Environment Canada The Prairie Ecosystem http://www.pnr-rpn.ec.gc.ca/nature/ecosystems/ da00s01.en.html Global Resources Grasslands • 537 . forest. Humans have Global Resources Grasslands • 535 lived in grassland environments for thousands of years, and in parts of the world it is impossible to determine which aspects of a grassland ecosystem. benefit the patient. Some of the side effects of these therapies include skin, liver, and kidney changes or damage. Approximately 20 percent of patients who Global Resources Gold • 529 Jewelry. Definition Graphite is composed of parallel planes of fused hex - agonal rings of carbon atoms. It exists in two forms, al - pha (also called hexagonal) and beta (also called 532 • Graphite Global Resources rhombohedral),