Sedimentary processes, rocks, and mineral deposits Categories: Geological processes and formations; mineral and other nonliving resources Sedimentary processes occur only at the Earth’s sur- face, because they are driven by various components of the hydrologic and biologic cycles. Sedimentary pro- cesses involvethe breakdown, movement,and ultimate deposition of broken rock fragments and chemicals in solution. These processes create many of the important resources used by humans. Background Most sediments originate from the weathering of ex- isting rocks. Weathering is the breakdown of earth material by physical and/or chemical processes. Phys- ical weathering only breaks the original material into smaller sizes. This is accomplished through mechani- cal means such as freezing and thawing, plant-root wedging, differential heating of rocks, and crystal growth in rock cracks. Chemical weathering, on the other hand, actually changes the composition of the original material into completely different compo- nents through solution, oxidation, hydration, and/or hydrolysis. The by-products of physical and chemical weathering provide the different types of sediment and ion-bearing solutions that create sedimentary rocks and minerals. Biological activity can also create sediments. Many invertebrates and some algae utilize calcium carbon- ate in seawater to make shells. Organisms, such as coral, live in warm, shallow seas and construct reefs. In addition, both macro- and microscopic shells of or- ganisms that do not live in reef communities become sediments that blanket the seafloor after the organ- isms’ deaths. Also, some algae and bacteria, as well as swamp vegetation, can become organic sediments un- der certain conditions of oxygen deficiency, creating valuable fossil fuels. Transportation Once particles of rock are loosened and broken free by weathering processes, those particles can be car- ried off by water, wind, or glaciers. Ions released through chemical activity are also free to travel in moving water, but they will never settle out of the water unless a specific chemical reaction occurs that causes the ions to precipitate as solid particles, or until shell-bearing organisms use the ions to build shells that later become sediments. The amount of energy available in a transporting medium determines which rock particles are picked up (eroded) and moved along. For example, more energy is usually required to move large (or more dense) grains thansmall (lessdense) grains.If a trans- porting medium loses energy, the sediments being carried will drop out in order of relative size, the larg- est ones first. Sediment sizes range from boulders to clay. Each has a specific size determination for the name: boulder (more than 256 millimeters), cobble (256 to 64 millimeters), pebble (64 to 4 millimeters), granules (4 to 2 millimeters), sand (2 to 0.062 milli- meter), silt (0.062 to 0.0039 millimeter), and clay (less than 0.0039 millimeter). This separation of grains ac- cording to size by the transporting medium is called sorting. A well-sorted sediment is one that contains nearly all the same size grains. The farther sediments travel, the better sorted they become. Thus, a poorly sorted sediment probably is still fairly close to its source area. Transport of material also tends to make grains more rounded in shape. Rounding occurs as particles bump into one another or into objects in the environ- ment, causing abrasion of their edges. The longerand farther a grain is moved, the smaller and rounder it becomes. Transportation tends to winnow material either by sorting or by continued weathering of the particles. Some minerals are more resistant to break- down than other minerals. Quartz is more resistant, while minerals such as feldspar, mica, pyroxene, and amphibole are less resistant. The longer and farther the sediments are transported, the more likely the weaker minerals are to disappear, which would in- crease the percentage of quartz present. Therefore, rocks composed of nearly all quartz can be inter- preted as having traveled a great distance from their source of weathering. These rocks also exhibit good sorting and are well rounded. Most sediments are carried by running water. Run- ning water begins as rainfall, flows into increasingly larger streams, and eventually runs into the oceans. Running water is present nearly everywhere on the Earth, including in arid regions (where, although un- usual, it is particularly effective as an agent of erosion because of the lack of vegetation for ground cover and because much of the rain in arid lands occurs in the form of downpours). 1068 • Sedimentary processes, rocks, and mineral deposits Global Resources Waves and currents along shorelines also move large amounts of sediment, as can some currents out in deeper waters. Energy is usually greater nearer a shoreline, so, if grains accumulate, beaches form. De- pending on the energy level, the beaches can be com- posed of sand, pebbles, cobbles, or even boulders. The energy usually decreases offshore, so finer- grained silt and clay (mud) often accumulate in the deeper, quieter water. Groundwater can carry dissolved minerals. Some- times when the rock is dissolved, minerals of eco- nomic importance, such as copper, aluminum, or iron, can be left behind. In some limestone areas, groundwater can dissolve enough limestone to form caves. If conditions are right in a cave, the ground- water can redeposit some minerals in the forms of dripstone and flowstone, featuresadmired by tourists. Wind can move sediment grains as well, but nei- ther in the quantity nor in sizes as large as does water. Fine sand is usually the upper size limit that wind can transport. Grains carried by wind do not have the cushioning effect that water provides, so grains are rapidly abraded as they hit each other during trans- port. Quartz sand grains become frosted in this manner. Sediments are also transported by glaciers. Move- ment of grains in glacial ice will break the grains, mak- ing them smaller, but they remain fairly angular be- cause they do not tumble around next to each other as they would in water orwind and, thus, donot undergo much rounding. Because ice will not let particles readily settle, as water or windwill, the particles do not become sorted atall. Glacialdeposits are probably the most poorly sorted of all sediments. However, some sediments do not end their journeys under a glacier; they may move away from the ice in glacial meltwater. When this happens, the sediments are transported by running water and take on the characteristics of any sediment carried in running water. Deposition (Sedimentation) All particles eventually are carried to specific environ- ments where the transporting energy decreases to the point of no longer being able to carry the material, and the grains are deposited. If not destroyed by fur- ther erosion, they may harden into various sedimen- tary rocks. There are three main depositionalsystems: marine, transitional, and terrestrial. Marine environments are important mainly, but by no means exclusively, for deposition of chemical sedi - ments. Marine environments include shallow-marine environments (from the shore to the edge of the con- tinental shelf), reefs, and deep marineenvironments. Transitional and terrestrial depositional environments are important for deposition of mainly clastic sedi- ments. Transitional environments include beaches, deltas, barrier islands, lagoons, and tidal marshes. Terrestrial environments includerivers, lakes, alluvial fans, glacial environments, and windy areas such as deserts. Diagenesis Diagenesis is any postdepositional alteration of sedi- ments or sedimentary rocks. Diagenetic processes usually take place after sediments are buried by newer sediments. An important diagenetic process is lithification, the process by which loose sediments are hardened into rock. There are two main processes in- volved in lithifying sediments: cementation and com- paction. Cementation occurs when pore spaces be- tween the grains are large enough for mineral-rich water to seep around the grains, depositing either silica or calcite crystals that grow and eventually co- alesce and hold the grains together. Many sediments typically lithify by physical compaction. As more and more sediments collect, the weight of overlying sedi- ments compresses the lower sediments, squeezing out much of the water and pressing the grains close enough together that they become a harder mass of material. Lithification processes can occur below water; an ocean basin does not have to dry up before the sediments it contains can lithify into rock. Sedimentary Rocks and Minerals Sedimentary rocks are the most abundant type of rocks found at the surface of the Earth. All sedimen- tary rocks and minerals can be putinto one of two cat- egories: clastic, composed of rock and mineral frag- ments that were weathered from preexisting rock materials and lithified in new combinations to create a sedimentary rock, and nonclastic, made of precipi- tated chemicals or of organically derived material such as shells or plants and animals. Identification of sedimentary rocks begins with the determination of whether a rock is clastic or nonclastic. Once such tex- tural characteristics have been determined, the grain sizes and composition of the rock are used to com- plete the identification process. Clastic rocks are fairly easy to identify because indi - vidual particles in the rock can usually be seen. The Global Resources Sedimentary processes, rocks, and mineral deposits • 1069 rock is identifiedmainly on the sizes and shapes of the grains it contains. Rocks made of large particles such as boulders, cobbles, and/or pebbles in a matrix of sand are called conglomerate if the grains are mostly rounded or breccia if the grains are angular. The sand- stone family contains many varieties of rocks depend- ing on the composition of grains present. Sandstone feels like sandpaper, and thepurer variety can be white, tan, or pink and is composed almost entirely of quartz grains. More commonly sandstone is gray, indicating that it contains particles otherthan quartz. Arkoseis a sandstone that contains fairly large, angular granules of pink feldspar. Sedimentary rocks composed of silt are called siltstone, and shale is composed of grains of clay that are so small they cannot be seen, even with a microscope. (Because the grains are so small, shale can easily be misidentified as a nonclastic rock.) The nonclastic rocks are dominated by limestone and dolostone (or dolomite). Limestone is calcium carbonate that formed in warm, shallow seas. Most limestone originated as accumulationsof shellseither in reefs or in beds on the seafloor. Some limestone can also form by chemical precipitation. Dolostone, a close relative of limestone, is thought to form diagenetically from limestone deposits when mag- nesium ions in the environment replace some of the calcium in the limestone. Microscopic shell accumu- lations from calcareous algae havecreated thick layers of chalk, while similar accumulations from siliceous plankton create diatomaceous earth and chert (flint). Plants that die in swamps and do not completely rot away can evolve into peat and eventually into coal, and microscopic algae and bacteria are the source of petroleum deposits. Although petroleum is not a rock, it is a resource that originates in a sedimentary setting. Some resource minerals form under evaporative conditions. Ions in solution travel with running water and eventually end up in the oceans or in lakes. In shallow embayments where seawater can flush into 1070 • Sedimentary processes, rocks, and mineral deposits Global Resources Sedimentary Rock Types Type Definition Subcategories Explanation Examples Clastic Rocks that consist of fragments of other rocks Conglomerates Grains are boulder-, cobble-, or gravel-sized Breccia Sandstone Grains are sand-sized Quartz sandstone, arkose, graywacke Mudstone Grains are silt- or clay- sized Mudstone, shale Precipitates Chemically precipitated or replaced; inorganic in origin Evaporites Solids have precipitated after evaporation of water in which they were dissolved Salt, gypsum, anhydrite, borax, potash Carbonates Compounds of calcium or magnesium Calcite (inorganic limestone), dolomite Siliceous Chemically precipitated silicas Chert: flint, jasper Organic (biochemical) Remains of plant or animal organisms — — Coal, organic limestone the bay and not be diluted with fresh stream water, salts can accumulate if the water evaporates. Likewise, in arid regions where streams enter landlocked lakes (playa lakes), evaporative conditions cause salts to ac- cumulate. There are a great variety of salts (gypsum, halite, magnesium sulfate, and potassium salts, to name a few), andeach oneforms in turn as the chemi- cal concentrations change as more and more of the water evaporates. Economic Importance of Selected Sedimentary Rocks and Minerals Sedimentary resources are classified into four main groups: sedimentary metallic ore deposits, sedimen- tary nonmetallic deposits, evaporites, and energy re- sources. The sedimentary ore deposits contain some of the world’s most valuable mineral resources. Many of these deposits were formed in depositional envi- ronments where large amounts of dissolved metals collected. For example, the iron ores of the famous Me- sabi Range in Minnesota originated when the Earth’s early atmosphere was poor in oxygen. This permitted an abundance of iron in its soluble (ferrous) form to be leached from large areas of the Earth’s surface and transported in solution to vast, shallow marine envi- ronments, where it oxidized to its insoluble (ferric) form and precipitated in thin layers. Although gold originates from igneous and hydro- thermal processes, once it weathers out of its original rock setting, it becomes influenced by the sedimen- tary processes of transportation by running water and ultimate deposition in streambeds. This is called a placer deposit. Placerdeposits are not limited to gold. Diamonds, tin, chromite, platinum, and magnetite can undergo similar histories. Placers can sometimes be traced upstream to find the source rock, which can then be mined. The nonmetallic deposits are also of great eco- nomic importance. Limestone is quite extensive and has a variety of uses. It is obtained by quarrying; the rock can be either cut into large blocks or blasted into fragments. The blocks, used for building stone, are taken to mills and cut to order for particular build- ings; decorative carvings can also be made. Limestone that has been blasted is usually ground into lime for either agricultural purposes or the manufacture of cement. Larger blocks may be used as riprap along shorelines or rivers to protect those areas from exces - sive erosion. Pure quartz sandstone, which usually originates from beach deposits, is quarried for use in glassmaking and fiber-optic cables. Coarser sand and gravel, which originates from glacial deposits or channeldeposits in rapidly moving rivers, is quarried for construction purposes. Clays of high purity, often formed in coal swamps or in areas of prolonged weathering, are used for both craft and industrial ceramics. Phosphatic rocks, usually marine shales and limestones that have been chemically enriched in phosphate in deep ma- rine environments, are an ingredient in agricultural fertilizers. Evaporite deposits have created vast and varied salt resources. Halite isused both astable salt and asan ice melter for road clearance. Gypsum is used in the mak- ing of plaster as well as the writing utensils used on modern “chalk” boards, contrary to popular belief that these writing implements are really chalk. Potas- sium salts can be used as table salt and in fertilizer. Epsom salts (magnesium sulfate) have health bene- fits, and borates (such as borax and boron) have uses that range from manufacturingof enamelto additives to soap and gasoline. The fossil fuels, the major energy resources in use in the twentieth century and early twenty-first cen- tury, all havebiogenic originsin depositional environ- ments. Coal forms from vegetation that grew in an- cient swamps. Coal isobtained from both stripmining and underground mining. Petroleum products are buried microscopic planktonic life-forms that lived in seas of the past. Although these organics collect on small scales, the sedimentary rocks with which they are associated permit the migration and eventual ac- cumulation of great enough volumes of oil and gas that they can be extracted for human use. Diann S. Kiesel Further Reading Boggs, Sam, Jr. Petrology of Sedimentary Rocks. 2d ed. New York: Cambridge University Press, 2009. _______. Principles of Sedimentology and Stratigraphy. 4th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006. Chernicoff, Stanley, and Donna Whitney. Geology: An Introduction to Physical Geology.4th ed.Upper Saddle River, N.J.: Pearson Prentice Hall, 2007. Davis, Richard A. Depositional Systems: An Introduction to Sedimentology and Stratigraphy. 2d ed. Englewood Cliffs, N.J.: Prentice Hall, 1992. Grotzinger, John P., et al. Understanding Earth. 5th ed. New York: W. H. Freeman, 2007. Global Resources Sedimentary processes, rocks, and mineral deposits • 1071 Nichols, Gary. Sedimentology and Stratigraphy. 2d ed. Hoboken, N.J.: Wiley-Blackwell, 2009. Pettijohn, F. J. Sedimentary Rocks. 3d ed. New York: Harper & Row, 1975. Tennissen, AnthonyC. Nature of Earth Materials.2ded. Englewood Cliffs, N.J.: Prentice-Hall, 1983. Tucker, Maurice E. Sedimentary Petrology: An Introduc- tion to the Origin of Sedimentary Rocks. 3d ed. Malden, Mass.: Blackwell Science, 2001. Web Site U.S. Geological Survey Sedimentary Rocks http://vulcan.wr.usgs.gov/LivingWith/ VolcanicPast/Notes/sedimentary_rocks.html See also: Cement and concrete; Ceramics; Coal; Evaporites; Fertilizers; Iron; Limestone; Oil and natu- ral gas formation; Shale; Weathering. Seed Savers Exchange Category: Organizations, agencies, and programs Date: Established 1975 Since its founding, the Seed Savers Exchange has helped preserve the genetic material of more than twenty-five thousand plant species. Modern agriculture practices tend to focus narrowly on an increasingly small number of crops, resulting in the endangerment or extinction of thousands of plant species. However, the Seed Savers Exchange helps maintain genetic di- versity over time, which is critical to combating the fur- ther loss of species from disease, pestilence, and other environmental factors. Background The Seed Savers Exchange was founded in 1975 in Decorah, Iowa, by then husband and wife Kent Whealy and Diane Ott Whealy. The couple had been given the seeds of two garden plants that Diane’s grandfather had brought to the United States from Bavaria in the 1870’s, a gift that made them recognize the value of preserving not only the genetic but also the cultural and historical heritage of plants. Over time, the nonprofit organization has grown to several full-time employees and occupies 360 hectares, where it maintains more than twenty-five thousand varieties of vegetable, fruit, flower, and herb seeds as well as a small number of endangered cows and poultry. Impact on Resource Use The Seed Savers Exchange is a permanent repository of thousands of seeds, including those of many plant species that have otherwise virtually disappeared. Un- like many seed banks, the Seed Savers Exchange not only stores seeds but also actively plants approxi- mately 10 percent of its inventory each year in rota- tion, allowing seeds to be distributed among members or sold. The wide variety of plants the Seed Savers Ex- change grows each year helps com- bat the existence of monocultures, or the covering of hundreds or thou- sands of hectares with a single crop. While large commercial food grow- ers routinely deal in monocultures in order to maximize profit, the prac- tice risks crop annihilation if that particular strain is attacked by a dis- ease or pest. In addition, the Seed Savers Exchange’s planting activity keeps those species in contact with the larger natural environment, and thus better equipped to survive in the future, rather than “frozen” in storage and unable to react to chang - ing environmental conditions. 1072 • Seed Savers Exchange Global Resources Seeds are stockpiled in the Svalbard Global Seed Vault on the island of Svalbard near the North Pole. The Seed Savers Exchange’s initial contribution to the vault was five hun - dred seeds. (AFP/Getty Images) The Seed Savers Exchange also promotes the sav - ing and exchange of seeds among members, thus cre- ating community, spreading the impact of its work, and promoting long-term survival of plant species. It also sells seeds via print andonline catalogs, which has helped promote the organic farming industry, be- cause commercially available seeds are far more lim- ited in variety. In addition, many commercial seeds are either hybrids that do not reproduce reliably or genetically modified, whichis notpermitted inthe or- ganic food trade. The Seed Savers Exchange considers education and outreach to be important parts of its mission. In addition to providing seed-saving guidance in its newsletters and on its Web site, the Seed Savers Ex- change houses a visitors’ center that offers guided tours to individuals and groups. The organization also participates in the global seed preservation commu- nity, most notably by contributing almost five hun- dred seeds for the opening of the Svalbard Global Seed Vault in Norway in 2008. The organization has additional global donations planned. The donations help ensure that some of the Seed Savers Exchange’s seed stock will be protected in the event of local disas- ter in Iowa. Amy Sisson Web Site Seed Savers Exchange http://www.seedsavers.org/ See also: Agricultural products; Agriculture indus- try; Agronomy; Jackson, Wes;Land Institute; Plantdo- mestication and breeding; Plants as a medical re- source; Svalbard Global Seed Vault. Seismographic technology and resource exploitation Categories: Obtaining and using resources; scientific disciplines Knowledge of the Earth’s interior has been greatly en- hanced by seismic data. Without this knowledge, it would be impossible to obtainthe oil, naturalgas, coal, metals, and other earth resourcesthat are afoundation of industrial society. Background The deepest mines and drill holes penetrate only a fraction of 1 percent of the thickness of the Earth. Al- though such samples of the interior provide impor- tant information, they are neither distributed evenly enough over the surface nor numerous enough to provide a complete picture. Therefore, information must come mostly from indirect evidence provided by instruments that probe the interior without actually going there. Modern electronics and computer tech- nology have greatly improved the quality of instru- ments now used in resource explorationand exploita- tion. The most widely used instrument to discover hidden resources in the Earth is the seismograph. Seismographs A seismograph detects and records the seismic (sound) waves traveling beneath the Earth’s surface. These waves are of two principal types: compressional waves and shear waves. In compressional waves, rock particles are vibrated parallel to the direction of wave propagation, whereas in shear waves, the motion is perpendicular to the direction of wave travel. Seismo- graphs can be manufactured to record both wave types. Generally, a seismograph consists of a sensor (seis- mometer or seismic detector), an electronic ampli- fier, filters, and a recording system. In its wide rangeof uses, from resource exploration and exploitation to earthquake studies, a seismograph may be required to measure ground movements from as small as one millionth of a meter to as large as several meters, a range of more than tenorders ofmagnitude. Theseis- mic detector (sensor) consists of a weight suspended from a frame by a delicate spring. The frame moves with the ground, but because of its inertia (mass), the weight tends to remain stationary. Attached to the weight is a coil of electrical wire. Ground motion moves the coil in a magnetic fieldcreated by a magnet attached to the frame of the seismograph. The rela- tive motion between the coil and the magnet converts the mechanical ground motion into an electrical sig- nal that passes through an amplifier. The amplified voltage controls a recording device that marks the ground motion on a moving sheet of paper. The re- corded information is called a seismogram. Sources of Seismic Waves Our knowledge of the Earth’s deep interior has been mainly conveyed by earthquakes, most of which are Global Resources Seismographic resource exploitation • 1073 caused by the sudden movement of rock masses. As these rocks grind together, energy is released that produces both compressional and shear waves. These waves spread throughoutthe Earth like the ripples made by a pebble tossed into a quiet pond of water. As the waves spread out- ward, some are reflected and some are refracted. The reflected waves travel downward to bound- aries between rock layers, where they reflect (bounce or echo) back to the surface, while the refracted waves follow paths that bend at each rock boundary. Eventually, these seismic waves reach the Earth’s surface, where they are de- tected by a seismograph. Earthquakes release the large amounts of en- ergy needed to probe the deep layers (mantle and core) of the Earth. Other methods can pro- duce seismic waves that can be focused on the geologic features closer to the Earth’s surface. These waves can be generated by artificialexplo- sions, as of a charge of dynamite,or by dropping a weight or pounding the ground with a sledge hammer. To eliminate the environmental risks associated with using explosives, seismologists may use a system called vibroseis (pronounced VI-broh- size). In this system, a huge vibrator mounted on a special truck repeatedly strikes the Earth to produce sound waves. Geophysics The branch of Earth science dealing with the analysis of seismographic data is geophysics, the science that applies physics to the study of the Earth and its envi- ronment. Geophysicists can use the speed of seismic waves recorded by a seismograph to determine the depth and structure of many rock formations, since the speed varies according to the physical properties of the rock through which the wave travels. Seismol- ogy is the field of geophysics that deals with the study of seismic waves produced by earthquakes or other sources, such as vibroseis. These studies have helped determine the location of many natural resources in the Earth and have led to a better understanding of earthquakes and other processes that shape the Earth. Seismic Exploration with the Seismic Reflection Method In a seismic survey, geophysicists typically arrange seismic detectors along a straight line (profile) and then generate sound waves by vibroseis or an explo- sion. A seismograph records how long it takes the sound waves to travel to a rock layer, reflect, and re- turn to the surface. The equipment is then moved a short distance along the line, and the experiment is repeated. This procedure is known as the seismic reflection profiling method. Beginning in the mid- 1980’s, the advent of high-resolution seismic detec- tors and digital engineering seismographs led to ap- plications of reflection seismology to environmental, groundwater, and engineering problems, as well as to oil, gas, and mineral exploration and exploitation. Using a variety of computer programs, researchers process recorded data (seismograms) to generate cross-sectional images the Earth to a depth of 3 kilo- meters or greater. Such cross sections present an image of the rock layers beneath the seismic line. Based on the characteristic geometries for oil and gas traps and mineral ore deposits, these images outlin- ing rock structures are usedto predict where oil,natu- ral gas, coal, and other resources, such as ground- water and mineral deposits, are most likely to exist in the subsurface. Geophysicists and geologists cannot tell whether oil or other resources will be found for certain, but using processed seismic data as a basis for deciding where to drillmakes itmuch more likely that the resource will be found. 1074 • Seismographic resource exploitation Global Resources Lithosphere 100 km (including crust 5-40 km) Asthenosphere 700 km Mantle 2,885 km Outer core 2,270 km Inner core 1,216 km Earth’s Interior By using highly sensitive seismographs, geophysi - cists can detect the changes that occur in the ampli- tude (height) of the recorded sound waves. Sound waves change in amplitude when they are reflected from rocks that contain gas and other fluids. Such changes appear as irregularities, called bright spots, on the recorded sound wave patterns, and they indi- cate the presence of fluids in underground and un- derwater rock formations. In addition, with carefully planned seismic surveys that collect both compres- sional and shear wave data, the fine details of seismic records can be used to infer the types of rocks in the subsurface. Exploitation of Oil and Other Resources Historically, most applications of seismic technology have been limited to exploration. However, seismic reflection data can be used not only to explore for new oil and gas reservoirs and other resources, but also to exploit existing reservoirs and resource depos- its by more extensively mapping these locations in or- der to yield optimum production. Many companies now use teams of geophysicists, geologists, and engi- neers to plan the acquisition of data from the best sources and analyze and integrate all the information into a consistent description of the reservoir and/or deposit. This team approach requires that each mem- ber understand the technology involved in obtaining reliable, accurate data so that the best possible infor- mation is used to estimate reservoir and/or deposit properties. The geologic detail needed to develop most hydro- carbon reservoirs substantially exceeds the detail re- quired to find them. For effective planning and drill- ing, a complete understanding of the lateral extent, thickness, and depth of the reservoir is absolutely es- sential. This understanding can be achieved with only detailed seismic interpretation of three-dimensional seismic surveys. In three-dimensional seismic reflec- tion surveying a common practice is to place seismic detectors at equal intervals and collect data from a grid of profiles (lines) covering the area of interest. As more wells are drilled in the area, the three- dimensional data evolve into a continuously utilized and updated management tool that affects reservoir planning and evaluation for years after the original seismic survey. Ultimately, knowledge of subsurface geology comes only from drilling the targets that have been deter - mined to be most likely to contain the resources sought. Since drilling deep drill holes is very expen - sive, applied seismic technology is a key to cost-effective exploration and exploitation of oil, natural gas, and many other natural resources. Alvin K. Benson Further Reading Burger, H. Robert, Anne F. Sheehan, and Craig H. Jones. Introduction to Applied Geophysics: Exploring the Shallow Subsurface. New York: W. W. Norton, 2006. Clay, Clarence S. Elementary Exploration Seismology.En- glewood Cliffs, N.J.: Prentice Hall, 1990. Gadallah, Mamdouh R., and Ray L. Fisher.Applied Seis- mology: A Comprehensive Guide to Seismic Theory and Application. Tulsa, Okla.: PennWell, 2005. Geldart, Lloyd P., andRobert E. Sheriff. Problemsin Ex- ploration Seismology and Their Solutions. Tulsa, Okla.: Society of Exploration Geophysicists, 2004. Kearey, Philip, Michael Brooks, and Ian Hill. An Intro- duction to Geophysical Exploration. 3d ed. Malden, Mass.: Blackwell Science, 2002. Robinson, Edwin S., and Cahit Çoruh. Basic Explora- tion Geophysics. New York: Wiley, 1988. Shearer, Peter M. Introduction to Seismology. 2d ed. New York: Cambridge University Press, 2009. Sheriff, Robert E., ed. Reservoir Geophysics. Tulsa, Okla.: Society of Exploration Geophysicists, 1992. Sheriff, Robert E., and L. P. Geldart. Exploration Seis- mology. 2d ed. New York: Cambridge University Press, 1995. Stein, Seth, and Michael Wysession. An Introduction to Seismology, Earthquakes, and Earth Structure. Malden, Mass.: Blackwell, 2003. Web Sites Enviroscan, Inc. Seismic Refraction Versus Reflection http://www.enviroscan.com/html/ seismic_refraction_versus_refl.html University of Calgary, Lithoprobe Seismic Processing Facility The Seismic Reflection Method http://www.litho.ucalgary.ca/atlas/seismic.html See also: Coal; Earthquakes; Geology; Landsat satel- lites and satellite technologies; Oil and natural gas drilling and wells; Oiland natural gasexploration; Oil and natural gas reservoirs. Global Resources Seismographic resource exploitation • 1075 Selenium Category: Mineral and other nonliving resources Where Found Selenium is widely distributed in the Earth’s crust but does not occur in ore deposits ofsufficient concentra- tion to permit direct mining. In nature, selenium is principally found with metal sulfides. Seleniferous soils—soils with high selenium concentrations—are found in Canada, the United States, Mexico, Colom- bia, and Ireland. Primary Uses Selenium’s most common industrial application is in the glass industry. It is also used as a nutritional sup- plement in domesticated animals such as poultry, cat- tle, and swine. Technical Definition Selenium (abbreviated Se), atomic number 34, be- longs to Group VI of the periodic table of the elements and resembles sulfur in its chemical and physical prop- erties. It has six naturally occurring isotopes and an av- erage molecular weight of 78.96. Pure selenium has gray, crystalline, and red forms. Its densityis 4.79 grams per cubic centimeter; ithas amelting pointof 217° Cel- sius and a boiling point of 685.4° Celsius. Description, Distribution, and Forms Selenium is a widely distributed element of volcanic origin that haschemical properties resembling sulfur. It occurs as inorganic oxides such as selenate andsele- nite, as elemental selenium, and as selenide, depend- ing on the alkalinity and aeration of theenvironment. Selenium also has a variety of soluble and volatile or- ganic forms, such as dimethyl selenide and selenome- thionine, an amino acid analog. Most selenium is com- bined with metals, as in ferroselite and challomenite, or appears as a trace contaminant in metal sulfides such as galena and pyrite, where it replaces sulfur be- cause of their similarity in size. Coal and oil deposits also have appreciable selenium contents. Soluble selenium forms are used asnutritional sup- plements for mammals, since selenium is an essential trace element. Selenium also plays a crucial role as an antioxidant in the enzyme glutathione peroxidase, and it contributes to the activity of vitamin E. High concentrations of selenium are toxic, however, and some notable instances of widespread animal death have occurred when human activity made selenium more available for plants and animals to absorb. On a commercial scale, about 1,600 metric tons of sele- nium are produced annually (1,560 in 2007, 1,590 in 2008), but this amount is dwarfed by the selenium re- leased into soil, air, and water that occurs as a result of general industrial activity. The highest average selenium concentrations oc- cur in organic deposits such as coal (3.4 milligrams of selenium per kilogram) and oil shale (2.3 milli- grams of selenium per kilogram). However, selenium is found in virtually all materials on Earth at low con- centrations. Average concentrations of selenium in crustal material range from 0.05 to 0.14 part per mil- lion (milligrams per kilogram). Areas where selenium concentrations are low (for example, the Pacific Northwest or the northeastern United States) occur where the underlying sedimentary rock predates the Cretaceous period. Certain soils are called “seleniferous” because they formed in material with elevated selenium levels. These soils typically developed from shale that was formed in the Cretaceous period. In the United States, these soils were deposited primarily in South Dakota, Montana, Wyoming, Nebraska, Kansas, Utah, Colo- rado, and NewMexico. Seleniferous soils tend to have selenium concentrations ranging from 1 to more than 80 milligrams of selenium per kilogram of soil, and it is in areas with these soils that selenium toxicity has historically occurred. Seleniferous soils also occur in Canada (in the provinces of Alberta, Manitoba, and Saskatchewan), Mexico, Hawaii, Colombia, China, and Ireland. Wells in seleniferous soils can contain as much as 1 milligram of selenium per liter. The major available forms of inorganic selenium found in well-aerated alkaline soils are selenate, sele- nite, and elemental selenium. Selenate and selenite are water soluble and can leach out of soil. Elemental selenium is relatively insoluble. In poorly drained envi- ronments, selenium will be found as selenide, usually combined with some type of metal such as lead, cop- per, or iron. These are relatively immobile forms of se- lenium. When selenides are exposed to air, however, the selenium reoxidizes to form selenates and selen- ites, which are much more easily taken up by plants. Some plants accumulate selenium. Examples are milk vetch (Astragalus), goldenweed (Haplopappus), prince’s plume (Stanleya), and woody aster (Xylor - hiza). The milk vetch can accumulate up to 20 grams 1076 • Selenium Global Resources of selenium per kilogram of tissue. The presence of these plants in an area is sometimes an indication that the soil may contain high selenium concentra- tions. In selenium-accumulating plants, the selenium is found as a water-soluble compound such as selenium methylselenosysteine. In plants that do not accumu- late selenium, the form is usually selenomethionine. Selenium can also take the place of sulfur in sulfur- containing organic compounds. Selenium would not have an impact on other natu- ral resources, except in localized areas, were it not for human activity, such as disposing of fly ash from coal- burning power plants and irrigating arid alkaline soils. The amount of selenium released each year by industrial activity is approximately fifteen times more than that which is released naturally. Selenium is usually a minor constituent of drinking water, appearing in concentrations ranging from less than 0.1 to 100 micrograms per liter. The upper limit for the allowable selenium content of drinking water in the United States was set at 10 micrograms per liter by the 1974 Safe Drinking Water Act. Wells from seleniferous soils in Colorado and Montana, however, can contain as much as 1 milligram of selenium per liter. Wildlife is a vital natural resource that can be ad- versely affected by selenium as a direct result of hu- man activity redistributing selenium to excess in the environment. A notable example of this occurred in the Kesterson Reservoir in the San Joaquin Valley of California. The Kesterson Reservoir was a series of twelve shallow ponds that were designed to be an Global Resources Selenium • 1077 Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009.Source: Mineral Commodity Summaries, 2009 75 60 15 840 75 65 20 Withheld 120 Metric Tons of Selenium Content 900750600450300150 United States Peru Japan India Finland Chile Philippines Sweden Other countries 200 120Canada Belgium U.S. data were withheld to avoid disclosure of company proprietary data.Note: Selenium: World Refinery Production, 2008 . un- usual, it is particularly effective as an agent of erosion because of the lack of vegetation for ground cover and because much of the rain in arid lands occurs in the form of downpours). 1068. conditions of oxygen deficiency, creating valuable fossil fuels. Transportation Once particles of rock are loosened and broken free by weathering processes, those particles can be car- ried off by. coal, metals, and other earth resourcesthat are afoundation of industrial society. Background The deepest mines and drill holes penetrate only a fraction of 1 percent of the thickness of the Earth. Al- though