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The branch of meteorology that deals with atmospheric effects on the operation of vehicles in the atmosphere, including winged aircraft, lighter-than-air devices such as dirigibles, rockets, missiles, and projectiles. The air which supports flight…

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 predominantly mechanical mixture of a variety of individual gases enveloping the terrestrial globe to form the Earth's atmosphere. In this sense air is one of the three basic components, air, water, and land (atmosphere, hydrosphere, and…

In meteorology, an extensive body of the atmosphere which is relatively homogeneous horizontally. An air mass may be followed on the weather map as an entity in its day-to-day movement in the general circulation of the atmosphere. The expressions…

The force per unit area that the air exerts on any surface in contact with it, arising from the collisions of the air molecules with the surface. It is equal and opposite to the pressure of the surface against the air, which for atmospheric air in…

The temperature of the atmosphere represents the average kinetic energy of the molecular motion in a small region, defined in terms of a standard or calibrated thermometer in thermal equilibrium with the air. Temperature Measurement Many different…

Visible, infrared, and ultraviolet emissions from the atoms and molecules in the atmosphere above 30 km (20 mi), generally in layers, and mostly between 70 and 300 km (45 and 200 mi).The airglow, together with the ionosphere, is found in 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…

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…

A lithium aluminum phosphate mineral of basic formula LiAl(PO4)(F). The structure of amblygonite consists of phosphate (PO4) groups of tetrahedra and AlO6 groups of octahedra. Each PO4 tetrahedron is connected to an AlO6 octahedron. Corner-sharing…

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…

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 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…

The science of sound in the atmosphere. Atmospheric acoustics is concerned with sound outdoors rather than indoors. The atmosphere has a structure that varies in both space and time, and these variations have significant effects on a propagating…

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…

Electrical processes that take place in the lower atmosphere. This activity is ubiquitous and is of two kinds: the intense local electrification accompanying storms, and the much weaker fair-weather electrical activity occurring over the entire…

The statistical description of atmospheric motions over the Earth, their role in transporting energy, and the transformations among different forms of energy. Through their influence on the pressure distributions that drive the winds, spatial…

Those oscillations in any or all atmospheric fields whose periods are integral fractions of either lunar or solar days. Oscillations with a period of a day are called diurnal, with a period of a half day semidiurnal, and with a period of a third of…

Horizontal wavelike oscillations in the pattern of wind flow aloft in the upper troposphere, usually with reference to the stronger portion of the westerly current in mid-latitudes. The flow is anticyclonically curved in the vicinity of a ridge line…

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…

A distribution of atmospheric pressure and mass such that the specific volume, or density, of air is a function of both pressure and temperature, but not either alone. When the field is baroclinic, solenoids are present, there is a gradient of air…

An absolute pressure gage which is specifically designed to measure atmospheric pressure. This instrument is a type of manometer with one leg at zero pressure absolute.The common meteorological barometer (see illustration) is a liquid-column gage…

A distribution of atmospheric pressure and mass such that the specific volume, or density, of air is a function solely of pressure. When the field is barotropic, there are no solenoids, air temperature is constant on a surface of constant pressure,…

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…

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 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 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 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…

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…

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 mineral composed of calcium carbonate (CaCO3); one of the most common and widespread minerals in the Earth's crust. Calcite may be found in a great variety of sedimentary, metamorphic, and igneous rocks. It is also an important rock-forming…

A large volcanic collapse depression, typically circular to slightly elongate in shape, the dimensions of which are many times greater than any included vent (Fig. 1). Calderas range from a few miles to 45 mi (75 km) in diameter. A caldera may…

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…

An interval of time in Earth history (Cambrian Period) and its rock record (Cambrian System). The Cambrian Period spanned about 60 million years and began with the first appearance of marine animals with mineralized (calcium carbonate, calcium…

A family of minerals, related to the scapolite family, characteristically occurring in basic rocks such as nepheline syenites and sodalite syenites. A typical specimen is shown in the illustration. Cancrinite is hexagonal, a = 1.28, c = 0.52…

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…

The mineral species containing the CO32− unit as an essential structural and chemical component. Members are distinguished by chemical composition (type of metal cation and in some cases other anion present) and crystal structure. More than 200…

An igneous rock in which carbonate minerals make up at least half the volume. Individual occurrences of carbonatite are not numerous (527 have been claimed) and generally are small, but they are widely distributed. Carbonatites are important…

The interval of geological time, and rocks deposited during that time, from the end of the Devonian Period to the beginning of the Permian Period. The Carboniferous Period spanned from about 359 to 299 million years ago during the Paleozoic Era…

A mineral having the composition SnO2. Cassiterite is the principal ore of tin. Its crystals are of the tetragonal rutile structure type, usually in prisms terminated by dipyramids. Twins with a characteristic notch called visor tin are common.…

A natural cavity located underground or in the side of a hill or cliff, generally of a size to admit a human. Caves occur in all types of rocks and topographic situations. They may be formed by many different erosion processes. The most important…