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One of the four fundamental operations of arithmetic and algebra. The symbol + of addition is thought to be a ligature for “et,” used in a German manuscript of 1456 to denote addition. Its first printed appearance is in Johann Widman's Behennede und…

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 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…

The branch of mathematics dealing with the solution of equations. These equations involve unknowns, or variables, along with fixed numbers from a specified system (Fig. 1). The origins of algebra were based on the need to develop equations that…

The study of zero sets of polynomial equations. Examples are the parabola y − x2 = 0, thought of as sitting in the (x, y)-plane, and the locus of all points (t, t2, t3, t4), which is defined by Eqs. (1), 1 in the coordinate space with coordinates w,…

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…

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…

Total variation in experimental data is partitioned into components assignable to specific sources by the analysis of variance. This statistical technique is applicable to data for which (1) effects of sources are additive, (2) uncontrolled or…

A branch of mathematics in which algebra is applied to the study of geometry. Because algebraic methods were first systematically applied to geometry in 1637 by the French philosopher-mathematician René Descartes, the subject is also called…

A framework for solving a problem. The analytic hierarchy process is a systematic procedure for representing the elements of any problem. It organizes the basic rationality by breaking down a problem into its smaller constituents and then calls for…

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…

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…

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…

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 superficial contents of a geometrical figure of two dimensions. The area of any rectangle or square is the product of two adjacent sides, one of which may be called the base and the other the altitude. In general, any line segment that partially…

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…

A branch of mathematics dealing with numbers, operations on numbers, and computation. Arithmetic is useful in solving many practical problems, such as buying, selling, budgets, sports statistics, and measurement. The usual numbers of arithmetic are…

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…

If the graph of a function y = f (x) goes out to infinity, each “end” of the graph may get closer and closer to a straight line, which is then called an asymptote. In other words, some straight line may well approximate the graph as the function…

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…