U.S. Geological Survey Water Science for Schools: What Is Hydrology and What Do Hydrologists Do? http://ga.water.usgs.gov/edu/hydrology.html See also: Aquifers; Atmosphere; Biodiversity; Geo- chemical cycles; Glaciation; Groundwater; Lakes; Oceans; Streams and rivers; Water pollution and water pollution control; Water rights. Hydroponics Categories: Scientific disciplines; environment, conservation, and resource management The term “hydroponics” literally means water culture and originally referred to the growth of plants in a liq- uid medium.It laterapplied toall systemsused togrow plants in nutrient solutions with or without the addi- tion of inert material (synthetic soil) for mechanical support. Background The ability to produce food and fiber for an ever- growing population is the most fundamental of all re- sources, and hydroponics has become an important method ofcrop production. Theincrease inthe num- ber of commercial greenhouse operations has re- sulted in a tremendous increase in the use of hydro- ponic systems. Greenhouses are utilized in the production of a wide array of bedding plants, flowers, trees, and shrubs for commercial as well as for home and garden use. Cash receipts from greenhouse and nursery crops total billions of dollars annually. In some aridregions, thevast majorityof vegetable crops are produced in greenhouses. Types of Hydroponic Systems The four most commonly used hydroponic systems are sand-culture systems, aggregate systems, nutrient film techniques, and floating systems. While these sys- tems are similar in their useof nutrientsolutions, they vary in both the presence and type of supporting me- dium and in the frequency of nutrient application. In sand culture, coarse sand is used in containers or spread over an entire greenhouse floor or bed on top of a recirculating drain system. A drip irrigation sys - tem is used to apply nutrient solution periodically, and a drainage system is used to collect the excess solution as it drains through the sand. In an aggre- gate open system, plants are transplanted into plastic troughs filled with an inert supporting material, and nutrient solution is supplied via drip irrigation. The aggregate and sand culture systems are open systems because the nutrient solution is not recycled. In the nutrient film technique,thereis an absenceofsupport- ing material. Seedlings are transplanted into troughs through which the nutrient solution is channeled, and the plants are in direct contact with the nutrient solution. In this closed system,the nutrient solution is channeled past the plant, collected, and reused. The floating hydroponic system involves the floating of plants over a pool of nutrient solution. While the nutrient film technique and floating hy- droponic systems are primarily used in research ap- plications, the sand culture and aggregate systems are commonly used in commercial plant production. These two systems require the use of a nutrient solu- tion and synthetic soil for mechanical support. Al- though a variety of nutrient solutions have been for- mulated, one of the earliest was developed in 1950, and this solution and slight modifications of it remain popular. Beginning in 1950, other nutrient solutions with different concentrations of chemical salts were developed, but the elemental ratios remained similar to the original solution. Materials Used for Mechanical Support A large variety of both organic and inorganic materi- als have been used to formulate the synthetic soils used for mechanical support in hydroponic systems. Commonly usedorganic materials includesphagnum moss, peat, manures, wood, and other plant residues. Sphagnum moss, the shredded, dehydrated remains of several species of moss in the genus Sphagnum,is specifically harvested for the purpose of producing synthetic soil. “Peat” is a term normally used to de- scribe partially decomposed remains of wetlands veg- etation that has been preserved under water. Moss peat is the only type of peat suitable for synthetic soil mixes. Moss peat is harvested from peat bogs, dried, compressed into bales, and sold. Animal manures are almost never used in commercial synthetic soil mix- tures because they require costly handling and steril- ization procedures. Wood residues such as tree bark, wood chips, shavings, and sawdust are generally pro - duced as by-products of the timber industry. A variety of other plant residues, including corn cobs, sugar - 588 • Hydroponics Global Resources cane stems, straw, and peanut and rice hulls, have been substituted for peat in synthetic soil mixtures in localities where there is sufficient supply of these ma- terials. Commonly used inorganic materials include ver- miculite, sand, pumice, perlite, cinders, and calcined clay. Vermiculite is a very lightweight material pro- duced by heating mica to temperatures above 1,090° Celsius. Sand is one of the most preferred materials for formulating synthetic soils because it is both inert and inexpensive, but it is heavier than other com- monly used materials. Pumice, a natural glasslike ma- terial produced by volcanic action, provides a good inert supporting material when ground into small particles. Perlite, a porous material that will hold three to four times its weight in water, is produced by heating lava at temperatures above 760° Celsius. Cin- ders are derived from coal residues that have been thoroughly rinsed to remove harmful sulfates. Cal - cined clay is derived from the mineral montmorillo - nite baked at temperatures above 100° Celsius. Future Use of Hydroponics The use of hydroponics will increase in the future as the population continues to grow and as more and more farmland is converted to urban use. Modern greenhouses can be constructed almost anywhere— on land that is unsuitable for agriculture and wildlife and on the tops of buildings in metropolitan areas. Improved technology will result in the development of better hydroponic systems as well as an increase in the economic feasibility of greenhouse production. D. R. Gossett Further Reading Brady, Nyle C., and Ray R. Weil. The Nature and Prop- erties of Soils. 14th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. Bridgewood, Les. Hydroponics: Soilless Gardening Ex- plained. Marlborough, England: Crowood Press, 2003. Janick, Jules. Horticultural Science. 4th ed. New York: W. H. Freeman, 1986. Global Resources Hydroponics • 589 A hydroponic farmer displays a head of lettuce grown in an Illinois greenhouse. (AP/Wide World Photos) Jones, J.Benton, Jr. A Guide for the Hydroponic and Soilless Culture Grower. Portland,Oreg.: Timber Press, 1983. _______. Hydroponics: A Practical Guide for the Soilless Grower. 2d ed. Boca Raton, Fla.: CRC Press, 2005. Resh, Howard M. Hydroponic Food Production: A Defini- tive Guidebook of Soilless Food-Growing Methods, for the Professional and Commercial Grower and the Advanced Home Hydroponics Gardener. 6th ed. Santa Barbara, Calif.: Woodbridge Press, 2001. Web Site U.S. Department of Agriculture Perlite and Hydroponics: Possible Substitute for Methyl Bromide? http://www.ars.usda.gov/is/np/mba/apr99/ perlite.htm See also: Horticulture; Monoculture agriculture; Plant domestication and breeding; Soil. Hydrothermal energy. See Geothermal and hydrothermal energy Hydrothermal solutions and mineralization Categories: Geological processes and formations; mineral and other nonliving resources Hydrothermal solutions are “hot-water” solutions rich in base metals and other ions that create deposits of minerals. Most hydrothermal solutions are exhala- tions from magmas, but some hydrothermal deposits have no identifiable magma source. Hydrothermal processes are responsible for the major part of the world’s basemetals uponwhich modern society is so de- pendent. They havegiven rise tomanyof the greatmin- ing districts of the world. Background Essential conditions for the formation of hydrother- mal mineral deposits include metal-bearing mineral - izing solutions, openings in rocks through which the solutions are channeled, sites for deposition, and chemical reaction resulting in deposition. The term “ore” is used for any assemblage of minerals that can be mined for a profit. “Gangue” is the nonvaluable mineral that occurs with the ore. During the crystallization of igneous rocks, water and other volatilefluids concentrateinthe upperpart of themagma. These volatilescarrywith themvarying amounts of the ions from the melt, including high concentrations of ions that are not readily incorpo- rated into silicate rock-forming minerals. If the vapor pressure in the magma exceeds the confining pres- sure of the enclosing rocks, the fluids are expelled to migrate though surrounding country rock. These so- lutions travel along natural pathways in the rock such as faults, fissures, orbeddingplanes in stratifiedrocks. As the solutions migrate away from their source re- gion, they lose their mineral content through deposi- tion in natural openings in the host rock (forming open space-filling deposits) or by chemical reaction with the host rock (forming metasomatic replace- ment deposits). A part of these solutions may make it to the surface to form fumaroles (gas emanations) or hot springs. In addition, some hydrothermal solu- tions may be derived from water trapped in ancient sediments or by dehydration of water-bearing miner- als during metamorphism. The observed volatiles from magmas, as seen dur- ing volcanic eruptions and at fumaroles, are 80 per- cent water. Carbon dioxide, hydrogen sulfide, sulfur, and sulfur dioxide are also abundant. Nitrogen, chlo- rine, fluorine, boron, and other elements are present in smaller amounts. In addition, metal ions are car- ried in this residual fluid. Especially abundant are the base metals—iron, tungsten, copper, lead, zinc, mo- lybdenum, silver, and gold. Quartz is the most com- mon nonore, or gangue, mineral deposited. Calcite, fluorite, and barite are also common as gangue min- erals. Base metalscombinedwith sulfur assulfidemin- erals, with arsenic as arsenides, or with tellurium as tellurides form the most common ore minerals. Gold often occurs as a native mineral. Nature of Open Spaces Hydrothermal solutions find ready-made escape routes through the surrounding country rock in the form of faults and fissures. Ore and gangue minerals of cavity-filling deposits are found in faults or fissures (veins), in open spaces in fault breccias, in solution openings of soluble rocks, in pore spaces between the grain of sedimentary rocks, in vesicles of buried lava 590 • Hydrothermal solutions and mineralization Global Resources flows, and along permeable bedding planes of sedi - mentary strata. The shape of the mineral deposit is controlled by the configuration of structures control- ling porosity and permeability. Fracture patterns, and therefore veins, may take on a wide variety of geomet- ric patterns, ranging from tabular to rod-shaped or blanketlike deposits. Some deposits are characterized by ore minerals that are widely disseminated in small amounts throughout a large body of rock such as an igneous stock. These igneous bodies undergo intense fractur- ing during the late stage of consolidation, and resid- ual fluids permeate the fractured rock to produce massive deposits of low-grade ores. In such deposits, the entire rock is extracted in miningoperations. The famous porphyry copper deposits of the southwest- ern United States—including those of Santa Rita, New Mexico; Morence, Arizona; andBingham, Utah— are of thistype,as are themolybdenumdepositsof Cli- max, Colorado. Metasomatic Replacement Some hydrothermal deposits are emplaced by reac- tion of the fluids with chemically susceptible rocks such as limestone or dolostone. Metasomatic replace- ment is defined as simultaneous capillary solution and deposition by which the host is replaced by ore and gangue minerals. These massive deposits orlodes take on the shape and the original textures of the host. Replacement is especially important in deep- seated deposits where open spaces are scarce. Re- placement deposits of lead-zinc are common in lime- stones surrounding the porphyry copper of Santa Rita, New Mexico, and at Pioche, Nevada. Classification by Temperature and Depth Veins are zoned, with higher-temperature minerals deposited nearthe source and lower-temperature min- erals farther away. Hypothermal or high-temperature and high-pressure mineral assemblages include the minerals cassiterite (tin), scheelite and wolframite (tungsten), millerite (nickel), and molybdenite (mo- lybdenum), associated with gangue minerals quartz, tourmaline, topaz, and other silicates. The mineral deposits of Broken Hill, Australia, the tin deposits of Cornwall, England, and Potosí, Bolivia, and the gold ofthe HomestakeMine, South Dakota, are hypo- thermal. Mesothermal, or moderate-temperature and mod - erate-pressuredeposits consistofpyrite (iron sulfide), bornite, chalocite, chalcopyrite and enargite (cop - per), galena(lead), sphalerite (zinc), and cobaltiteor smaltite (cobalt). Gangue minerals include calcite, quartz, siderite, and rhodochrosite. The zinc-lead- silver replacement deposits of Leadville, Park City, and Aspen, Colorado, and the Coeur d’Alene, Idaho, lead veins are mesothermal. Epithermal or low-temperature, near-surface de- posits are often associated with regions of recent vol- canism. The ore is characterized by stibnite (anti- mony), cinnabar (mercury), native silver and silver sulfides, gold telluride,nativegold, sphalerite, andga- lena. Gangue minerals include barite, fluorite, chal- cedony, opal, calcite, and aragonite. The extensive silver-gold mineralization of the San Juan Mountains of Colorado, including Cripple Creek, Ouray, and Creede, are epithermal deposits. Telethermal deposits are formed by hydrothermal solutions that have cooled to approximately the same temperature as the near-surface rocks. These solu- tions may originate as mobilized connate and deeply circulating meteoric waters rather than fluids expelled from magma.The principalore minerals are sphalerite and galena, with gangue minerals marcasite, fluorite, calcite, and chalcopyrite. The Mississippi Valley-type deposits of thetristate district ofMissouri,Kansas, and Oklahoma exemplify this low-temperature mineral- ization. René A. De Hon Further Reading Barnes, Hubert Lloyd. “Energetics of Hydrothermal Ore Deposition.” In Frontiers in Geochemistry: Or- ganic, Solution, and Ore Deposit Geochemistry, edited by W. G. Ernst. Columbia, Md.: Bellwether for the Geological Society of America, 2002. _______, ed. Geochemistry of Hydrothermal Ore Deposits. 3d ed. New York: John Wiley & Sons, 1997. Guilbert, John M., and Charles F. Park, Jr. The Geology of Ore Deposits. LongGrove,Ill.: WavelandPress,2007. Pirajno, Franco. Hydrothermal Processes and Mineral Sys- tems. London: Springer/GeologicalSurveyof West- ern Australia, 2009. Thompson, J. F. H., ed. Magmas, Fluids, and Ore De- posits. Nepean, Ont.: Mineralogical Association of Canada, 1995. See also: Magma crystallization; Open-pit mining; Pegmatites; Secondary enrichment of mineral depos - its; Underground mining. Global Resources Hydrothermal solutions and mineralization • 591 I Ickes, Harold Category: People Born: March 15, 1874; Frankstown Township, Pennsylvania Died: February 3, 1952; Washington, D.C. Ickes, U.S. secretary of the interior from 1933 to 1946, expanded theresponsibilities andpowers of the Depart- ment of the Interior in the areas of conservation and preservation of the nation’s natural resources. Biographical Background Harold L. Ickeswasa lawyer,journalist, andmunicipal reformer in Chicago before his appointmentas secre- tary of the interior. His selection was political; Presi- dent Franklin D. Roosevelt, a Democrat, was eager to gain the support of progressive Republicans and chose Ickes,who quickly becameone ofthe mostpow- erful figures in the nation. Always contentious and ready to battle for his beliefs, Ickes’s enemies and ad- mirers were legion. Impact on Resource Use As interior secretary, Ickes administered the Biological Survey, the Bureau of Fisheries, and the Grazing Divi- sion. Particularlycommitted tothe wilderness ideal,he added several parks and monuments to the National Park System and opposed their overdevelopment. He fought to have the Forest Service transferred to the Department of the Interior but lost; he also failed to obtain his ultimate dream: to turn the Department of the Interior into the Department of Conservation. In the enduring struggle within the conservation movement between preservationists and utilitarian conservationists, Ickes personified both strains but leaned toward the former. Nevertheless, as head of the Works Progress Administration (WPA), one of the New Deal agencies, he supported the building of dams and other massive public works projects that re- made the land and provided jobs during the Depres- sion. Still, like few others in American government, Ickes exemplifiedthe importance ofthe wilderness to the human spirit. Eugene Larson See also: Conservation; Department of the Interior, U.S.; National Park Service; Roosevelt, Franklin D.; Roosevelt, Theodore; Taylor Grazing Act. Igneous processes, rocks, and mineral deposits Categories: Geological processes and formations; mineral and other nonliving resources Igneous rocksand mineral deposits, created by the crys- tallization and solidification of magma, are found all over the world. Many of the world’s most economically important mineral deposits result, directly or indi- rectly, from igneous activity. Background Igneous rocks are created by the crystallization and solidification of hot, molten silicate magma. Magma consists of silicate liquid (the major component is the silica molecule SiO 4 −4 ), solid crystals, rock fragments, dissolved gases such as carbon dioxide, water, and var- ious sulfurous oxides. Familiar examples of igneous rocks are granite (an “intrusive” or “plutonic” rock that is crystallized at depth)and basalt (asin dark “ex- trusive” lava flows, such as those in Hawaii). Igneous rocks are found worldwide on all continents, on oce- anic islands, and on the ocean floors. They are partic- ularly common in mountain ranges or other areas where the Earth has undergone tectonicactivity. Oce- anic islands, suchas HawaiiandIceland, are nearly ex- clusively igneous in origin, and the world’s oceansare floored by basalt lava flows. Metallic ores produced by igneous activity may be mined directly from the igneous rocks or obtained through the injectionof hydrothermal(hot water) veins into adjacentrocks. Some of the most important com- modities obtained from igneous sources include cop- per, nickel, gold, silver, platinum, iron, titanium, tung- sten, and tin. Nonmetallic products include crushed stone, construction stones for buildings and monu - ments, and somepreciousandsemiprecious gemstones. Global Resources Igneous processes, rocks, and mineral deposits • 593 Typical Ore Minerals Associated with Igneous Rocks Rock Type Mineral Metal or Other Commodity Obtained Felsic—Intermediate Granite Feldspar Porcelain, scouring powder Native gold Gold Pegmatite Cassiterite Tin Beryl Beryllium, gemstones (emerald; aquamarine) Tourmaline Gemstone Spodumene Lithium Lepidolite Lithium Scheelite Tungsten Rutile Titanium Apatite Phosphorus Samarskite Uranium, niobium, tantalium, rare-earth elements Columbite, Tantalite Niobium, tantalium, used in electronics Thorianite Uranium, thorium Uraninite Uranium Amazonite (microcline feldspar) Gemstone Rose quartz Gemstone Topaz Gemstone Sphene (titanite) Titanium, gemstone Muscovite mica Electrical insulation Zircon Zirconium Rhyolite Chalcopyrite Cooper Molybdenite Molybdenum Mafic—Ultramafic Gabbro and Anorthosite Ilmenite Titanium Labradorite (plagioclase feldspar) Gemstone Chalcopyrite Copper Bornite Copper Pentlandite Nickel Peridotite Chromite Chromium Native platinum Platinum Sperrylite Platinum Serpentine Nickel (from weathered soils) Igneous (from the Latin word ignis, meaning fire) rocks form bythe crystallization of hot, moltenmagma produced by the heat of the Earth’s interior. Surface exposures of igneous rock bodies are widespread throughout the globe. On continents they mostly oc- cur in mountainous areas or ancient “Precambrian shield” areas where billions of years of erosion reveal the roots of old mountain ranges. In the oceans, igne- ous rocks cover the floors of ocean basins belowa thin layer of sediment. Most oceanic islands owe their very existence to ocean floor volcanic eruptions that pro- duce volcanoes of sufficient stature to project above the waves. Familiar examples are the Hawaiian chain, the Galápagos Islands, and Iceland. Types of Igneous Rocks Igneous rocks are divided into two major categories defined by their mode of emplacement in or on the Earth’s crust. If moltenmagma cools andsolidifies be- low the surface, the rocks are called “intrusive” or “plutonic.” Because these rocks generally take a long time to cool and solidify (a process called “crystalliza- tion”), their component minerals grow large enough to see with the naked eye (coarse-grained rocks). On the other hand, if magma flows out onto the Earth’s surface, it forms “extrusive” or “volcanic” rock. These rocks lose heat rapidly to air or water, and the result- ing rapidcrystallizationproduces tiny, nearly invisible crystals (fine-grained rocks). Some volcanic rocks cool so quickly that few crystals have time to form; these are glassy rocks such as obsidian. Two kinds of volca- nic rock exist: lava flows and “pyroclastic” deposits formed by explosive volcanism. Pyroclastic materials (volcanic ash) are deposited as layers of particles that have been violently ejected into the air. Igneous rocks are also classified according to chemical composition. At one extreme are the light- colored “felsic” rocks that contain high concentra- tions of silica (up to about 75 percent silicon dioxide, SiO 2 ) and relatively little iron, magnesium, and cal- cium. Examples of felsic rocks are granite, a plutonic rock, and its volcanic equivalent, rhyolite (obsidian glass is rapidly cooled rhyolite). At the other extreme are the dark “mafic” rocks with relatively low silica (as low as about 46 percent SiO 2 ) but with higher concentrations of iron, magne- sium, and calcium. Examples of mafic rocks are gab- bro (plutonic) and its volcanic equivalent, basalt. Rocks of intermediate composition also exist, for ex- ample plutonic diorite and its volcanic equivalent,an- desite. It is andesite (and a more silicic variety called “dacite”) that is expelled from the potentially explo- sive volcanoes of the Cascade range in the American Pacific Northwest (Mount St. Helens, Mount Rainier, Mount Hood, and others). Intrusive (Plutonic) Structures Intrusive igneous rock bodies come in many shapes and sizes. The term “pluton” applies to all intrusive bodies but mainly to granitic rocks (granites, diorites, and related rocks). Specific terms applied to plutons mostly describe the size of the body. “Stocks” are ex- posed over areas less than 100 square kilometers, whereas “batholiths” aregiant, commonlylens-shaped, bodies that exceed 100 square kilometers in exposed area. The Sierra Nevadarange in eastern California is a good example of a batholith. Some specialized pluton varieties are “laccoliths,” commonly mountainous areas (for example, the Henry and La Salle mountains in Utah) in which in- 594 • Igneous processes, rocks, and mineral deposits Global Resources increasing silica increasing iron and magnesium Extrusive (volcanic) Intrusive (plutonic) Felsic rhyolite granite Intermediate dacite/andesite tonalite/diorite Mafic basalt gabbro Ultramafic peridotite Simple Classification of Igneous Rocks trusive granitic magma has invaded horizontal sedi - mentary layers and has bowed them up into a broad arch. A “phacolith” is similar to a laccolith only the magma has invaded folded sedimentary rocks so that the pluton itself appears to have been folded. Minor intrusive bodies include “sills,” tabular bod- ies intruded parallel to rock layers (a laccolith can be considered a “fat sill”), and “dikes,” tabular bodies that cutacross rock layers.Sills anddikes are common features around the margins of plutons where they contact “country rock” (older, pre-intrusion mate- rials). Another intrusive body, mostly produced by mafic (gabbroic) magmas, is the “lopolith.” Lopoliths are relatively large funnel-shaped bodies (on the order of large stocks or small batholiths) in some casescreated where magma fillsthe down-warpedpart (syncline)of a fold structure. An excellent example is the Muskox intrusion of northern Canada; another possible one (one limb is unexposed under Lake Superior) is the Duluth gabbro intrusionof northeasternMinnesota. Extrusive (Volcanic) Structures The nature of volcanoes and volcanic rock deposits in general is greatly influenced by the composition of their parent magmas. Basalt magma is a low-viscosity liquid (it is thin and flows easily) and thus produces topographically low, broad volcanic features. Typical of these are the “fissure flows” (also known as plateau basalts) in which basalt lava issues from fractures in the Earth and spreads out almost like water in all directions. Examples are the Columbia River basalt plateau in Oregon and Washington, the Deccan pla- teau in India, and the Piraná basalt plateau in Brazil. The basalt flows that floor the oceans are underwater versions of fissure flows. Basaltic volcanoes tend to have low profiles but lat- erally extensive bases typified by the “shield” volca- noes of Hawaii and other areas. These volcanoes re- semble giant ancient shields lying on the ground. Pyroclastic eruptions of basalt, powered mostly by the violent release of dissolved carbon dioxide, produce cinder-cone volcanoes, otherwise known as “Strom- bolian” volcanoes, aftertheItalian volcano Stromboli. In contrast to mafic magmas, the more silica-rich felsic and intermediatemagmas aremore viscous, and thus flow less readily. This magma tends to pile up in one place, producing towering volcanoes of moun - tainous proportions. Because felsic-intermediate mag - mas also tend to contain significant dissolved water, steam trapped during eruption may explode violently, producing thick blankets of volcanic ash near the vol- cano. The best North American example of these po- tentially violentvolcanoes, called“stratovolcanoes” or “composite” volcanoes, is the Cascade Range in the Pacific Northwest. The terms for these volcanoes re- flect their tendency to have layers of mud and lava flows (generally andesiteordacite) thatalternate with pyroclastic ash deposits. Stratovolcanoes occur world- wide, particularly at continental margins and in the oceans near continents where “lithospheric plates” (thick horizontal slabsof crustand upper mantle)col- lide, with one plate moving under the other (subduc- tion zones). Volcanism associated with subduction zones has produced the Andes of South America as well as islandssuchas Japan, thePhilippines, New Zea- land, the Aleutian islands of Alaska, and the islands of Indonesia. Another important volcanic feature is the “rhyolite complex,” or “caldera complex,” exemplified by Yel- lowstone National Park in Wyoming and the Valles Caldera (Jemez Mountains), New Mexico. When fully active, these areas produce violently explosive volca- nism and rhyolite lava flows that blanket many square kilometers. The most violent activity occurs when the roof of a large underground magma chamber col- lapses into the shallow void created by expulsion of magma during previous eruptions. The crater formed during this process is called a caldera. Roof collapse during caldera formation has the effect of ramming a large piston into the heart of the magma body, vio- lently expelling gas-charged, sticky rhyolite into the atmosphere, from which it may cascade along the sur- face as a nuée ardente (French for “glowing cloud”). These roiling infernos of hot noxious gases, bubbling lava fragments, and mineral crystals are capable of speeds in excess of 300 kilometers per hour and tem- peratures in excess of 400° Celsius. They deposit ash blankets (welded ashflow tuffs) over wide regions, as in thecaseof Yellowstone. Stratovolcanoes(described above) canalso formcalderas andashflow deposits,as exemplified at Crater Lake, Oregon. Ore Deposits of Felsic-intermediate Rock Granite and related rocks are the source of manymet- als and other products that are the foundation of an industrial society. Quartz veins intruding granite may contain gold and other precious metals, as in the “mother lode” areas of the Sierra Nevada Range in California. These veins originate as hydrothermal de - Global Resources Igneous processes, rocks, and mineral deposits • 595 posits, minerals precipitated from hot-water fluids flowing through fractures in cooling granitic bodies. Felsic and intermediate composition igneous rocks contain significant dissolved water in their magmas (called “juvenile” water), which is finally expelled as hydrothermal fluids in the late stagesof plutonic crys- tallization. Hydrothermal veins occur in the parent granite itself or are injected into the surrounding rocks. Many important metallic ore bodies formed as hydrothermal deposits. So-called porphyry copper deposits such as those of the American southwest (Arizona, New Mexico, Colorado, and Utah) are low-grade deposits of widely scattered small grains of chalcopyrite (CuFeS 2 ) and other copper minerals in felsic plutonic and volcanic rocks, mostly residing in amultitude of extremely thin hydrothermal veins. Some porphyry copper deposits also have considerable deposits of molybdenite (in the sulfide molybdenite, used in high-temperature al- loys), especially at the Questa mine in New Mexico and at Climax, Colorado. By far the greatest concentration of valuable min- erals associated with granitic rocks comes from pegma- tite deposits. Like hydrothermal deposits, pegmatites form in the late stages of granite crystallization after most of the other rock-forming minerals have already crystallized. Another similaritytohydrothermal fluids is their high volatile content—materials that tend to melt or form gases atrelativelylow temperatures,such as water, carbon dioxide, and the halogens fluorine and chlorine. Elements with large atomic sizes (ionic radii) and valence charges also tend to concentrate in pegmatitic fluids because the majority of minerals in granites (mostly quartz and feldspars) cannot accom- modate these giant atoms in their mineral structures. Thus, pegmatite deposits may contain relatively high concentrations of uranium, thorium, lithium, beryl- lium, boron, niobium, tin, tantalum, and other rare metals. The high water content of pegmatite fluids, some of it occurring as vapor, allows minerals such as quartz, feldspar, and mica to grow to enormous sizes, the largest of which are on the order of railway boxcars. Pegmatites are generally fairly small bodies; some deposits are no larger than a small house. They may also occur as veins or dikes. Excellent North American examples containing rare and exotic min- erals are located in the Black Hills of South Dakota, Maine, New Hampshire, North Carolina, the Adiron - dacks of New York state, Pala and Ramona in Califor - nia, and Bancroft and Wilberforce, Canada. Notable international occurrences are in Brazil (Minas Ge - rais), Russia (the Urals and Siberia), Greenland, Italy, Australia, Germany (Saxony), Madagascar, and Sri Lanka. Ore Deposits in Mafic and Related Rock Owing to their low viscosity, mafic magmas produce some unique mineral deposits compared with thicker felsic magmas. In plutonic settings formed early, heavy mineral crystals can easily sink through the magma to form crystal-rich layers on the bottom of the magma chamber. These gravitationally deposited layers are called “cumulates” (from the word accumu- late) and, depending on theirmineralogical makeup, may constitute important ore bodies. Because cumu- lates are generally enriched in iron and depleted in silica compared with their mafic parent magma, they are termed“ultramafic,” thecommon rock typebeing “peridotite,” a rock rich in olivine [(Fe,Mg) 2 SiO 4 ]. Most of the world’s chromium that is used in high- temperature, corrosion-resistant alloys comes from cumulate layers of the mineral chromite (FeCr 2 O 4 ), mostly mined in South Africa. The other major com- modities recovered from cumulates are the precious metals platinum and palladium, mined in South Af- rica and Russia. Intrusive mafic magmas may also form layers of sulfide-rich mineralscalled “late-stage immiscibleseg- regations” that constitute some of the richest copper and nickel ore bodies in the world. As some mafic magmas cool and change chemically, sulfur and metal- rich fluids may separate from the silicate liquid, just as oil would from water. These “immiscible” (incapa- ble of mixing) sulfide droplets then sink through the lower density silicate magma to form thick layers of “massive sulfide” deposits on the magma chamber floor. The major minerals in massive sulfide copper- nickel mines are chalcopyrite, bornite (Cu 5 FeS 4 ), pyrrhotite(Fe 1-x S), andpentlandite [(Fe,Ni) 9 S 8 ]. Plat- inum, gold, and silver, among minerals, are com- monly recovered as by-products. Major magmatic seg- regation sulfide mines are located in South Africa (Messina and Bushveld districts, Transvaal) and Nor- way, and at Sudbury, Ontario, Canada, which has ore rich in nickel. Titanium and iron ores may also form as magmatic segregations. Massive titanium ores, mostly the oxide ilmenite (FeTiO 3 ), are mined from anorthosite rock, a plagioclase [(Ca,Na)AlSi 3 O 8 ] feldspar-rich variation of gabbro. Typicalexamples ofthese deposits occurin 596 • Igneous processes, rocks, and mineral deposits Global Resources the titanium mines in the Adirondacks of New York state and at Allard Lake, Quebec. Iron deposits of this type, mostly the mineral magnetite (Fe 3 O 4 ), are lo- cated at Kiruna, Sweden; the Ozarks of Missouri; Durango, Mexico; and Algarrobo, Chile. Other Important Igneous Commodities Some valuable mineral commodities are recovered from igneous rocks that do not lend themselves to simple classification.For example,diamonds occur in deposits called “kimberlites,”a type ofgeneraldeposit called “diatremes,” explosively injected mixtures of mantle (mostly serpentine) and crustal materials that in rare localities contain diamonds. The diamonds form deep in the upper mantle, where pressures are sufficiently high to produce them by the reduction (removal of oxygen)of carbondioxide. Theyarethen injected into more shallow crustal levelsupon the car- bon dioxide-powered eruption of kimberlite. Dia- monds are mostly mined in South Africa, Ghana, the Democratic Republic of the Congo, Russia, Brazil, In- dia, and theUnitedStates (Murfreesboro, Arkansas). Two other deposits with chemical affinities to kim- berlites are “nepheline syenites” and “carbonatites.” Like kimberlites,these bodiesare rare, andtheir mag- mas probably originate deep in the Earth’s mantle. Nepheline syenites contain mostly the mineral neph- eline (NaAlSiO 4 ) and are sources of apatite (phos- phate mineral) and corundum (Al 2 O 3 ), used as an abrasive. Nepheline itself is used to make ceramics. Carbonatites are unusual igneous deposits in that they are composed mostly of the carbonate mineral calcite (CaCO 3 ). They have become increasingly im- portant as sources of the rare elements niobium and tantalum, used in the electronics industry. John L. Berkley Further Reading Best, Myron G. Igneous and Metamorphic Petrology.2d ed. Malden, Mass.: Blackwell, 2003. Best, Myron G., and Eric H. Christiansen. Igneous Pe- trology. Malden, Mass.: Blackwell Science, 2001. Blatt, Harvey, Robert J. Tracy, and Brent E. Owens. Pe- trology: Igneous, Sedimentary, and Metamorphic. 3d ed. New York: W. H. Freeman, 2006. Hutchison, Charles S. Economic Deposits and Their Tec- tonic Setting. New York: J. Wiley, 1983. Jensen, Mead L., and AlanM. Bateman. Economic Min - eral Deposits. 3d ed. New York: Wiley, 1979. Philpotts, Anthony R., and Jay J. Ague. Principles of Ig - neous and Metamorphic Petrology. 2d ed. New York: Cambridge University Press, 2009. Winter,John D. An Introduction toIgneous andMetamor- phic Petrology. 2d ed.NewYork: Prentice Hall,2010. Young, Davis A. Mind over Magma: The Story of Igneous Petrology. Princeton, N.J.: Princeton University Press, 2003. Web Site U.S. Geological Survey Igneous Rocks http://vulcan.wr.usgs.gov/LivingWith/ VolcanicPast/Notes/igneous_rocks.html See also: Beryllium; Boron; Chromium; Copper; Feldspars; Geology; Gold; Granite; Lithium; Magma crystallization; Molybdenum; Nickel; Pegmatites; Plate tectonics; Plutonic rocks and mineral deposits; Pumice; Quartz; Tantalum; Tin; Titanium; Tungsten; Uranium; Volcanoes; Zirconium. Incineration of wastes Category: Pollution and waste disposal The incineration ofwastes provides a means for reduc- ing the volume of various sorts of waste by destroying the organic components of waste. Background The incineration of household and hazardous waste material can help to reduce its volume and can pro- vide the potential for electric power generation. The incineration of waste material is not a preferred strat- egy,however, becauseit doesnot stop thedepletion of natural resources, and it may cause further environ- mental problems such as air pollution. Thermal methods have been developed for deal- ing with solid, liquid, and the in-between slurry types of waste. Household trash has long been incinerated, often in backyard settings, but many governments now regulate this method except in rural areas. Some cities have built large incinerators for burning solid household waste; these are designed to reduce the waste stream as well as to provide for energy genera- tion. Several types of incinerators have also been de - veloped todeal with hazardous liquid and solid wastes in carefully regulated circumstances. Some of these Global Resources Incineration of wastes • 597 . secretary of the interior from 1933 to 1946, expanded theresponsibilities andpowers of the Depart- ment of the Interior in the areas of conservation and preservation of the nation’s natural resources. Biographical. Service transferred to the Department of the Interior but lost; he also failed to obtain his ultimate dream: to turn the Department of the Interior into the Department of Conservation. In the enduring. rocks, in vesicles of buried lava 590 • Hydrothermal solutions and mineralization Global Resources flows, and along permeable bedding planes of sedi - mentary strata. The shape of the mineral deposit