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An approach to management of natural resources that emphasizes how little is known about the dynamics of ecosystems and that, as more is learned, management will evolve and improve. Adaptive management is a systematic strategy that seeks to improve…

Aeronomy concentrates on the regions above the tropopause or upper part of the atmosphere. The region of the atmosphere below the tropopause is the site of most of the weather phenomena that so directly affect all life on the planet; this region has…

A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often…

A model for the functioning of an agricultural system, with all inputs and outputs. An ecosystem may be as small as a set of microbial interactions that take place on the surface of roots, or as large as the globe. An agroecosystem may be at the…

A sodium-rich plagioclase feldspar mineral whose composition extends over the range Ab100An0 to Ab90An10, where Ab (= albite) is NaAlSi3O8 and An (= anorthite) is CaAl2Si2O8 (see illustration). Albite occurs in crustal igneous rocks as a major…

Light emissions in the upper atmosphere from elemental lithium, potassium, and especially sodium. These alkali metals are present in the upper atmosphere at altitudes from about 50 to 62 mi (80 to 100 km) and are very efficient in resonant…

Plant growth forms characteristic of upper reaches of forests on mountain slopes. In such an environment, trees undergo gradual changes that, though subtle at first, may become dramatic beyond the dense forest as the zone of transition leads into…

Intergrading regions on mountain slopes characterized by specific plant life forms or species composition, and determined by complex environmental gradients.Along an altitudinal transect of a mountain, there are sequential changes in the physiognomy…

A mineral of composition KAl3(SO4)2(OH)6. Alunite occurs in white to gray rhombohedral crystals or in fine-grained, compact masses. It has a hardness of 4 and a specific gravity of 2.6–2.8.Alunite is produced by sulfurous vapors on acid volcanic…

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. Resins generally are complex mixtures…

The transparent purple to violet variety of the mineral quartz. Although quartz is perhaps the commonest gem mineral known, amethyst is rare in the deep colors that characterize fine quality. Amethyst is usually colored unevenly and is often heated…

Determination of the relative or absolute age of materials or objects by measurement of the degree of racemization of the amino acids present. With the exception of glycine, the amino acids found in proteins can exist in two isomeric forms called d-…

A mineral with a framework structure in which all the aluminosilicate tetrahedral vertices are linked, thus allying it to the feldspars, feldspathoids, and zeolites. The structure is cubic, space group Ia3d, a = 1.371 mm. Its formula is…

A nesosilicate mineral, composition Al2SiO5, crystallizing in the orthorhombic system. It occurs commonly in large, nearly square prismatic crystals. The variety chiastolite has inclusions of dark-colored carbonaceous material arranged in a regular…

A plagioclase feldspar with composition Ab70An30 to Ab50An50 (Ab = NaAlSi3O8; An = CaAl2Si2O8). Andesine occurs primarily in igneous rocks, often in a glassy matrix as small, chemically zoned, lathlike crystals known as microlites. The rock types…

A typical volcanic rock erupted from a volcano associated with convergent plate boundaries. The process of subduction, which defines convergent plate boundaries, results in the sinking of oceanic lithosphere beneath either oceanic lithosphere or…

A mineral with the chemical composition PbSO4. Anglesite occurs in white or gray, orthorhombic, tabular or prismatic crystals or compact masses (see illustration). It is a common secondary mineral, usually formed by the oxidation of galena. Fracture…

A mineral with the chemical composition CaSO4. Anhydrite occurs commonly in white and grayish granular masses, rarely in large, orthorhombic crystals (see illustration). Fracture is uneven and luster is pearly to vitreous. Hardness is 3–3.5 on Mohs…

A method used by animals to conceal themselves from an enemy by appearing to be a section of the natural background. Animal camouflage is a strategy that animal organisms use to avoid or deflect the attacks of predators by misleading the latter's…

The carbonate mineral Ca(Fe,Mg)(CO3)2, also commonly containing some manganese. The mineral has hexagonal (rhombohedral) symmetry (see illustration) and has the cation-ordered structure of dolomite. The name is applied only to those species in which…

The calcium-rich plagioclase feldspar with composition Ab0An100 to Ab10An90 (Ab = NaAlSi3O8; An = CaAl2Si2O8), occurring in olivine-rich igneous rocks and rare volcanic ejecta (for example, at Mount Vesuvius, Italy, and Miyakejima, Japan). Hardness…

The name usually given to alkali feldspars which have a chemical composition ranging from Or40Ab60 to Or10Ab90± up to approximately 20 mole % An (Or, Ab, An = KAlSi3O8, NaAlSi3O8, CaAl2Si2O8) and which deviate in one way or another from monoclinic…

A rock composed of 90 vol % or more of plagioclase feldspar. Strictly, the rock is composed entirely of crystals discernible with the eye, but some finely crystalline examples from the Moon have been called anorthosite or anorthositic breccia. Two…

Oxygen-depleted regions in marine environments. The dynamic steady state between oxygen supply and consumption determines the oxygen concentration. In regions where the rate of consumption equals or exceeds the rate of supply, seawater becomes…

The Antarctic Circle is an imaginary line that delimits the northern boundary of Antarctica. It is a distinctive parallel of latitude at approximately 66°30′ south. Thus it is located about 4590 mi (7345 km) south of the Equator and about 1630 mi…

The Antarctic Ocean, sometimes called the Southern Ocean, is the watery belt surrounding Antarctica. It includes the great polar embayments of the Weddell Sea and Ross Sea, and the deep circumpolar belt of ocean between 50 and 60°S and the southern…

A magnesium-rich orthorhombic amphibole with perfect {210} cleavage and a color which varies from white to various shades of green and brown. It is a comparatively rare metamorphic mineral which occurs as slender prismatic needles, in fibrous…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

A fold in layered rocks in which the strata are inclined down and away from the axes. The simplest anticlines (see illustration) are symmetrical, but in more highly deformed regions they may be asymmetrical, overturned, or recumbent. Most anticlines…

Apatite is a generic name used to describe calcium phosphates with the simplified formula Ca10(PO4)6X2 (X = F, Cl, OH). Three end members form a complete solid-solution series involving the halide and hydroxyl anions, and these are the species…

A fine-grained, sugary-textured rock, generally of granitic composition; also any body composed of such rock. GraniteThe light-colored rock consists chiefly of quartz, microcline, or orthoclase perthite and sodic plagioclase, with small amounts of…

A hydrous calcium potassium silicate containing fluorine. The composition is variable but approximates to KFCa4(Si2O5)4· 8H2O. It resembles the zeolites, with which it is sometimes classified, but differs from most zeolites in having no aluminum. It…

The application of ecological principles to the solution of human problems and the maintenance of a quality life. It is assumed that humans are an integral part of ecological systems and that they depend upon healthy, well-operating, and productive…

The cultivation of freshwater and marine species for human purposes. Aquaculture, or aquatic farming, is concerned with the raising of animals and the harvesting of plants in freshwater and marine environments (Fig. 1). In other words, it is the…

A subsurface zone that yields economically important amounts of water to wells. The term is synonymous with water-bearing formation. An aquifer may be porous rock, unconsolidated gravel, fractured rock, or cavernous limestone. Economically important…

One of three naturally occurring mineral forms of calcium carbonate (CaCO3). The other forms (or polymorphs) are the abundant mineral calcite and the relatively rare mineral vaterite. Still other forms of calcium carbonate are known, but only as…

A period of geologic time from about 4000 to 2500 million years ago (MYA). During the Archean Eon (Fig. 1) a large percentage of the Earth's continental crust formed, plate tectonics began, very warm climates and oceans existed, and life appeared on…

Defined primarily by climatic rather than latitudinal criteria, arctic islands are those in the Northern Hemisphere where the mean temperature of the warmest month does not exceed 50°F (10°C) and that of the coldest is not above 32°F (0°C).…

The parallel of latitude approximately 66°30′ north of the Equator, or 23.5° from the North Pole. Named for the northern constellation Bear, the Arctic Circle has the same angular distance from the Equator as the inclination of the Earth's axis from…

The Arctic Ocean is an enclosed ocean, connected to the Pacific Ocean through Bering Strait and Bering Sea, and to the Atlantic Ocean through Fram Strait and the Barents, Greenland, and Norwegian seas (see illustration). The Arctic Ocean is the…

The arenaceous rocks (arenites) include all those clastic sedimentary rocks whose particle sizes range from 2 to 0.06 mm, or if silt is included, to 0.004 mm. Some arenites are composed primarily of carbonate particles, in which case they are called…

Clastic sedimentary rocks containing silt- or clay-sized particles that are less than 0.0625 mm and/or clay minerals. The argillaceous rocks (lutites) include shales, argillites, siltstones, and mudstones. They are the most abundant sedimentary rock…

Groundwater conditions formed by water-bearing rocks (aquifers) in which the water is confined above and below by impermeable beds. These systems are named after the province of Artois in France, where artesian wells were first observed.Because the…

A group of highly fibrous, naturally occurring minerals with separable, long and thin fibers. Asbestos fibers are strong and flexible enough to be spun and woven as well as resistant to heat and chemicals. Historically, asbestos was used for fire…

The largest of the world's continents. With its peninsular extension, commonly called the continent of Europe, it is the major portion of the broad east-west extent of the Northern Hemisphere landmasses. In many ways Asia is more a cultural concept…

A layer of variable thickness within the upper mantle of the Earth that is mechanically weaker (less rigid) than both the overlying lithosphere and the underlying mesosphere. The depth of transition from lithosphere to asthenosphere can be as little…

The large body of seawater separating the continents of North and South America in the west from Europe and Africa in the east and extending south from the Arctic Ocean to the continent of Antarctica. The Atlantic is the second-largest ocean water…

A gaseous layer that envelops the Earth and most other planets in the solar system. Earth, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Titan (Saturn's largest satellite) are all known to possess substantial atmospheres that are held by the…

A scientific discipline concerned with the chemical composition of the Earth's atmosphere. Topics falling within the field include the emission, transport, and deposition of atmospheric chemical species; the rates and mechanisms of chemical…

An annular coral reef, with or without small islets, that surrounds a lagoon without projecting land area. Physiography Most atolls are isolated reefs that rise from the deep sea. They vary considerably in size and shape. Small rings, some without…

A group of monoclinic calcic pyroxenes which have the general chemical formula (Ca,Mg,Fe)(Mg,Fe)-Si2O6, in which calcium is the dominant cation in the first cation position. Monoclinic pyroxene with substantial iron or magnesium in place of calcium…

An optical manifestation of a large-scale electrical discharge process which surrounds the Earth. The discharge is powered by the so-called solar wind–magnetosphere generator. The Sun continuously blows out its upper atmosphere, the corona, with a…

An island continent situated in the Southern Hemisphere and extending from 10 to 44°S and from 113 to 153°E. Australia's total area (2,941,526 mi2 or 7,618,517 km2 for the Australian mainland and 26,383 mi2 or 68,332 km2 for the island of Tasmania)…

Minerals that are formed in sediment or a sedimentary rock. Their in-place origin distinguishes them from minerals that are formed elsewhere and transported to the site of deposition (detrital minerals). Authigenic minerals form at the Earth's…

In general, a large mass of snow, ice, rock, earth, or mud in rapid motion down a slope or over a precipice. In the English language, the term avalanche is reserved almost exclusively for snow avalanche. Minimal requirements for the occurrence of an…

A basic carbonate of copper with the chemical formula Cu3(OH)2(CO3)2. Azurite is normally associated with copper ores and often occurs with malachite. Azurite is monoclinic. It may be massive or may occur in tabular, prismatic, or equant crystals…

A semienclosed brackish sea located in a humic zone, with a positive water balance relative to the adjacent ocean (the North Sea and the North Atlantic). The Baltic is connected to the North Sea by the Great Belt (70% of the water exchange), the…

A sedimentary rock that was commonly deposited during the Precambrian. It was probably laid down as a colloidal iron-rich chemical precipitate, but in its present compacted form it consists typically of equal proportions of iron oxides (hematite or…

A mineral with chemical composition BaSO4. It is isostructural with celestite, SrSO4, and anglesite, PbSO4. Barite is orthorhombic, space group Pnma, a = 0.887 nanometer, b = 0.545 nm, c = 0.714 nm. The illustration shows typical crystals. Barite…

Elongate accumulations of sediment formed in the shallow coastal zone and separated from the mainland by some combination of coastal bays and their associated marshes and tidal flats. Barrier islands are typically several times longer than their…

An igneous rock characterized by small grain size (less than about 0.2 in. or 5 mm) and approximately equal proportions of calcium-rich plagioclase feldspar and calcium-rich pyroxene, with less than about 20% by volume of other minerals. Olivine,…

A low-lying area which is wholly or largely surrounded by higher land. An example is the Hudson Bay in northeastern Canada, which was formed by depression beneath the center of a continental ice sheet 18,000 years ago. Another example, the Qattara…

Bauxite is the primary raw ore material used to produce aluminum metal and alumina (Al2O3). It is a naturally occurring weathering product composed of one or more aluminum hydroxide minerals, such as gibbsite [Al(OH)3], boehmite [AlO(OH)], and…

A parasitic disease caused by a roundworm typically found in raccoons that is transmissible to humans, other mammals, and birds. Baylisascaris procyonis is a parasitic roundworm of raccoons (Procyon lotor) and belongs to the order Ascaridida in…

The branch of ecology that focuses on the evolutionary causes of variation in behavior among populations and species. The scientific study of behavioral ecology investigates how changes of behavior are influenced by ecological factors (Fig. 1).…

The term first applied to a particular, highly colloidal plastic clay found near Fort Benton in the Cretaceous beds of Wyoming. This clay swells to several times its original volume when placed in water and forms thixotropic gels when small amounts…

A water body north of the Pacific Ocean, 875,900 mi2 (2,268,000 km2) in area, bounded by the Siberian and Alaskan landmasses to the west and east respectively, and the Aleutian-Komandorskiye island arc to the south. The Bering Sea is very productive…

The most common beryllium mineral. Beryl, Al2[Be3Si6O18], crystallizes in the hexagonal system with space group P6/mcc. The crystal structure consists of six-membered rings of corner-sharing silicon-oxygen (SiO4) tetrahedra cross-linked by…

Minerals containing beryllium as an essential component. Over 50 beryllium minerals have been identified, even though beryllium is a scarce element in the Earth's crust. The unusual combination of low charge (+2) and small ionic radius (0.035…

The destruction of organic compounds by microorganisms. Biodegradation is a process (carried out by microorganisms) that decomposes organic material into its constituent elements or new compounds. Microorganisms, particularly bacteria (and fungi to…

The number of different plant and animal species living in a defined ecosystem or study area; a contraction of biological diversity. Biodiversity includes genetic diversity (variability in the genetic makeup among individuals in a single species),…

The biological diversity of different plant and animal species living on the island of Borneo. Borneo is the third largest island in the world. Located in Southeast Asia and straddling the Equator, it is divided among three countries: Indonesia,…

A microbial (often bacterial) community that is enveloped by an adhesive substance (the glycocalyx) at the interface of a liquid and a surface. Biofilms (see illustration) are defined as highly structured communities of microorganisms that are…

The study of the cycling of chemicals between organisms and the surface environment of the Earth. The chemicals either can be taken up by organisms and used for growth and synthesis of living matter or can be processed to obtain energy. The chemical…

A synthetic discipline that describes the distributions of living and fossil species of plants and animals across the Earth's surface as consequences of ecological and evolutionary processes. Biogeography overlaps and complements many biological…

The use of parasitoids, predators, and pathogens to reduce injurious pest insect populations and consequently the damage they cause. Viruses and bacteria are the most commonly used pathogens, but fungi, protozoa, and nematodes may also be important…

The amount and rate of production which occur in a given ecosystem over a given time period. Biological productivity may apply to a single organism, a population, or entire communities and ecosystems. Productivity can be expressed in terms of dry…

Organic materials or wastes produced by plants and animals that can be used as energy sources or fuels. Biomass is a collective term used to describe renewable materials that are derived from plants and animals. In some cases, microbial metabolic…

A major community of plants and animals having similar life forms or morphological features and existing under similar environmental conditions. A biome (see illustration) is a complex biotic community covering a large geographic area and…

A branch of meteorology and ecology that deals with the effects of weather and climate on plants, animals, and humans. Thermal equilibrium The principal problem for living organisms is maintaining an acceptable thermal equilibrium with their…

A member of a chemically diverse, structurally related group of minerals that comprise substantial fractions of both the Earth's crust and upper mantle. The term was coined by Albert Johannsen in 1911 to describe collectively the rock-forming…

The method of introducing microorganisms (native or genetically engineered) into a contaminated site to consume and break down environmental pollutants. Bioremediation is a form of biodegradation. Specifically, bioremediation is the use of…

The thin zone of living organisms and their environments at the surface of the Earth. The biosphere is the layer of life of the Earth (Fig. 1). Included in the biosphere are all environments capable of sustaining life above, on, and beneath the…

The use of telemetry methods for sending signals from a living organism over some distance to a receiver. Usually, biotelemetry is used for gathering data about the physiology, behavior, or location of the organism. Generally, the signals are…

An iron-magnesium-rich layer silicate; it is also known as black mica. Biotite is the most abundant of the mica group of minerals. The name is derived from that of the French chemist J. Biot. The formula for the ideal end member, phlogopite, is…

A term used to designate naturally occurring or pyrolytically obtained substances of dark to black color consisting almost entirely of carbon and hydrogen with very little oxygen, nitrogen, and sulfur. Bitumen may be of variable hardness and…

A semienclosed marginal sea with an area of 420,000 km2 (160,000 mi2) bounded by Turkey to the south, Georgia to the east, Russia and Ukraine to the north, and Romania and Bulgaria to the west (Fig. 1). The physical and chemical structure of the…

A distinctive, fine-grained mudrock that derives its dark color from an unusually high content of organic matter, ranging from around 2% to over 50% of the weight of the rock (typical shales contain less than 1% organic matter). Most black shales…

Regional metamorphism with the highest pressures and lowest temperatures, commonly above 5 kilobars (500 megapascals) and below 750°F (400°C). Metamorphic rocks of the relatively uncommon blueschist facies contain assemblages of minerals that record…

A borate mineral with chemical composition Mg3B7O13Cl. It occurs in Germany, England, and the United States, usually in bedded sedimentary deposits of anhydrite (CaSO4), gypsum (CaSO4 · 2H2O), and halite (NaCl), and in potash deposits of oceanic…

A large group of minerals in which boron is chemically bonded to oxygen. Boron is a fairly rare element. However, because of its chemical character, it is very susceptible to fractionation in Earth processes and can become concentrated to a degree…

A clastic rock composed of angular gravel-size fragments; the consolidated equivalent of rubble. The designation gravel-size refers to a mean particle diameter greater than 0.08 in. (2 mm), which means that 50% or more of the particles (by volume)…

Part of a clay and silt sequence that accumulated at the foot of a colossal “reef” during the Cambrian explosion, a dramatic evolutionary radiation of animals beginning about 545 million years ago. Although this explosion is most obvious from the…

Terrestrial and aquatic animals that are capable of excavating holes in the ground for protection from adverse environmental conditions, as well as for storing food. Burrowing animals are uniquely adapted species, having the capacity to dig burrows…

A member of the plagioclase feldspar solid-solution series with a composition ranging from Ab30An70 to Ab10An90 (Ab = NaAlSi3O8 and An = CaAl2Si2O8) [see illustration]. In the high-temperature form, bytownite has the high-albite structure type in…

A term that may refer to either a zinc mineral, Zn4Si2O7(OH)2·H2O, which is also known as hemimorphite, or to zinc oxide, ZnO, which is used in medicinal or pharmaceutical products and in cosmetics. Hemimorphite ZincE. Eugene Weaver S. T. Irish,…

A soil that is mineralogically an impure limestone. Such soils are also known as duricrust, kunkar, nari, kafkalla, Omdurman lime, croute, and race. Many soil profiles in semiarid climates (that is, those characterized by a rainfall of 4–20 in. or…

The name given to two species of mammals that are members of the family Camelidae in the order Artiodactyla. The two camel species are the Bactrian camel (Camelus bactrianus) and the Arabian or dromedary camel (C. dromedarius). Both species are…

The unit of weight now used for all gemstones except pearls. It is also called the metric carat (m.c.). By international agreement, the carat weight is set at 200 milligrams. Pearls are weighed in grains, a unit of weight equal to 50 mg, or ¼…

A subdiscipline of stratigraphy and geochemistry that involves correlation and dating of marine sediments and sedimentary rocks through the use of trace-element concentrations, molecular fossils, and certain isotopic ratios that can be measured on…