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A rock froth, formed by the extreme puffing up (vesiculation) of liquid lava by expanding gases liberated from solution in the lava prior to and during solidification. Some varieties will float in water for many weeks before becoming waterlogged.…

A large, geologically significant group of dark, rock-forming silicate minerals. Pyroxene is found in abundance in a wide variety of igneous and metamorphic rocks. Because of their structural complexity and their diversity of chemical composition…

The mathematical theory of the formation and behavior of queues or waiting lines. The name is also applied loosely to the mathematical study of a wide variety of problems connected with traffic congestion and storage systems. Uneven flow through a…

A branch of environmental geochemistry and isotope geology concerned with the occurrence of radioactive nuclides in sediment, water, air, biological tissues, and rocks. The nuclides have relatively short half-lives ranging from a few days to about…

A rigid and wave-resistant marine structure that stands above its surroundings. To navigators, a reef is any rocky structure that poses a threat to navigation. Marine biologists and geologists specify that reefs are constructed by organisms that…

The mineral form of manganese carbonate. Calcium, iron, magnesium, and zinc have all been reported to replace some of the manganese. The equilibrium replacement of manganese by calcium has been determined and found to increase with the temperature…

One of the geomorphological expressions between two tectonic plates that are opening relative to each other or sliding past each other. The term originally was used to describe the central graben structures of such classic continental rift zones as…

A relatively common aggregate of mineral grains. Some rocks consist essentially of but one mineral species (monomineralic, such as quartzite, composed of quartz); others consist of two or more minerals (polymineralic, such as granite, composed of…

Finding the age of rocks based on the presence of naturally occurring long-lived radioactive isotopes of several elements in certain minerals and rocks. Measurements of rock ages have enabled geologists to reconstruct the geologic history of the…

A secondary, planar structure of deformed rocks. A cleavage is penetrative and systematic, as opposed to fractures and shear zones which may occur alone or in widely spaced sets. It is generally better developed in fine grained rocks than in coarse…

The permanent and induced magnetism of rocks and minerals on scales ranging from the atomic to the global, including applications to magnetic field anomalies and paleomagnetism. Natural compasses, concentrations of magnetite (Fe3O4) called…

A dark to dark-brown coating on rock surfaces exposed to the atmosphere (Fig. 1). Rock varnish is probably the slowest-accumulating sedimentary deposit, growing at only a few micrometers to tens of micrometers per thousand years. Its thickness…

The red to red-violet variety of the gem mineral tourmaline. Perhaps the most sought-for of the many colors in which tourmaline occurs, it was named for its resemblance to ruby (see Illustration). The color is thought to be caused by the presence of…

Deposits of bedded sedimentary rocks composed of salts precipitated during solar evaporation of surface or near-surface brines derived from seawater or continental waters. Dominant minerals in ancient evaporite beds are anhydrite (along with varying…

An upwelling of crystalline rock salt and its aureole of deformed sediments. A salt pillow is an immature salt dome comprising a broad salt swell draped by concordant strata. A salt stock is a more mature, pluglike diapir of salt that has pierced,…

Unconsolidated granular material consisting of mineral, rock, or biological fragments between 63 micrometers and 2 millimeters in diameter. Finer material is referred to as silt and clay; coarser material is known as gravel. Sand is usually produced…

A clastic sedimentary rock comprising an aggregate of sand-sized (0.06–2.0-mm) fragments of minerals, rocks, or fossils held together by a mineral cement. Sandstone forms when sand is buried under successive layers of sediment. During burial the…

Any gem variety of the mineral corundum (Al2O3), except those called ruby because of their medium- to dark-red color. Sapphire has a hardness of 9 (Mohs scale), a specific gravity near 4.00, and refractive indices of 1.76–1.77.Although blue…

A term possessing genetic implications, originally defined as an aquatic sediment rich in organic matter that formed under reducing conditions (lack of dissolved oxygen in the water column) in a stagnant water body. This contrasts with the term…

An aluminosilicate mineral containing sodium (Na), calcium (Ca), chlorine (Cl), and carbonate (CO3). It is commonly found as light-colored, translucent tetragonal prisms (Fig. 1). Scapolite is normally white, but many other colors are known,…

A mineral consisting of calcium tungstate, CaWO4. Scheelite occurs in colorless to white, tetragonal crystals (see illustration); it may also be massive and granular. Its fracture is uneven, and its luster is vitreous to adamantine. Scheelite has a…

Medium- to coarse-grained, mica-bearing metamorphic rock with well-developed foliation termed schistosity. Schist is derived primarily from fine-grained, mica-bearing rocks such as shales and slates. The schistosity is formed by rotation,…

The process whereby mapping technologies are used to produce highly detailed images of the seafloor. Ever since large ships began sailing the oceans, there has been a need to know what the seafloor looks like. At first this need was simply to avoid…

A seamount is a mountain that rises from the ocean floor; a submerged flat-topped seamount is termed a guyot. By arbitrary definition, seamounts must be at least 3000 ft (about 900 m) high, but in fact there is a continuum of smaller undersea…

Rocks formed of any of the materials that accumulate at Earth's solid surface at ambient temperatures, regardless of their consistency. Together with lava flows, sedimentary rocks form a cover of stratified material, known as the stratisphere, that,…

The study of natural sediments, both lithified (sedimentary rocks) and unlithified, and of the processes by which they are formed. Branches of this discipline have been known as sedimentation or sedimentary petrology, or they have been included in…

The probability that social or economic consequences of earthquakes will equal or exceed specified values at a site, at several sites, or in an area, during a specified exposure time. Historically the term seismic risk has been used to describe an…

Determination of the nature of sedimentary rocks and the fluids they contain from analysis of seismic data. Prior to the 1970s, seismic data were used only to map the structure of the Earth. Since then, improvements in seismic data quality have…

Various devices or systems of devices for measuring movement in the Earth. Ground motion is generally the result of passing seismic waves, gravitational tides, atmospheric processes, and tectonic processes. Seismographic instrumentation typically…

The study of the shaking of the Earth's interior caused by natural or artificial sources. The revolution in understanding the Earth brought about by the paradigm of plate tectonics in the early 1960s provided a rationale that has since guided an…

The study of stratigraphic sequences, defined as stratigraphic units bounded by unconformities. L. L. Sloss was the first to formally establish, in 1963, that Phanerozoic cratonic successions in North America could be subdivided into six broad…

A common rock composed of serpentine minerals, serpentinite is usually formed through the hydration of ultramafic rocks, dunites, and peridotites in a process known as serpentinization. The result is the formation of hydrated magnesium-rich…

A class of fine-grained clastic sedimentary rocks with a mean grain size of less than 0.0625 mm (0.0025 in.), including siltstone, mudstone, and claystone. One-half to two-thirds of all sedimentary rocks are shales (Fig. 1). Sedimentary…

Silica (SiO2) occurs naturally in at least nine different varieties (polymorphs), which include tridymite (high-, middle-, and low-temperature forms), cristobalite (high- and low-temperature forms), coesite, and stishovite, in addition to high (β)…

All silicates are built of a fundamental structural unit, the so-called SiO4 tetrahedron. The crystal structure may be based on isolated SiO4 groups or, since each of the four oxygen ions can bond to either one or two silicon (Si) ions, on SiO4…

Assemblages of silicate minerals. Studies of the nature of these minerals provide information on their origin and their role in rock formation. This article discusses kinetic processes in silicate systems, oxidation-reduction equilibria in…

Fine-grained sediment and sedimentary rock dominantly composed of the microscopic remains of the unicellular, silica-secreting plankton diatoms and radiolarians. Minor constituents include extremely small shards of sponge spicules and other…

A porous silica deposit formed around hot springs. It is white to light gray and sometimes friable. Geyserite is a variety of siliceous sinter formed around geysers. The siliceous sinters are deposited as the hot subterranean waters cool after…

A nesosilicate mineral of composition Al2O[SiO4], crystallizing in the orthorhombic system, space group Pbnm, a = 0.744, b = 0.760, and c = 0.575 nanometers, with four formula units in the cell. Sillimanite commonly occurs in slender crystals or…

The third oldest period of the Paleozoic Era, spanning an interval from about 444 to 419 million years ago (MYA). The Silurian system includes all sedimentary rocks deposited and all igneous and metamorphic rocks formed in the Silurian Period (Fig.…

A broad range of rock types made up of calc-silicate minerals such as garnet, regardless of their association with ores, that originate by replacement of precursor rocks. It was a term originally coined by miners in reference to rock consisting of…

Irregular surfaces occurring in certain rocks, mostly parallel to bedding planes, in which small toothlike projections on one side of the surface fit into cavities of like shape on the other side (see illustration). A cross section of a typical…

A region in which a portion of a tectonic plate sinks beneath another plate into the Earth's interior. Subduction zones are defined by deep oceanic trenches (Fig. 1), lines of volcanoes parallel to the trenches, and zones of large earthquakes that…

A harmonious balance of human needs with resource consumption over an extended period of time. Sustainable development is a closely related term to sustainability, described in the 1987 Brundtland Report—issued by the United Nations (UN) World…

A mountain or hill, generally steep-sided, formed by accumulation of magma (molten rock with associated gas and crystals) erupted through openings or volcanic vents in the Earth's crust; the term volcano also refers to the vent itself. During the…

The Earth's water supply and its natural distribution. Freshwater is a renewable resource, and it is continually being replaced by precipitation, but it is not evenly distributed and is scarce in many areas. The uses of water worldwide are for…

Processes for formulating and disseminating information about future weather conditions based upon the collection and analysis of meteorological observations. Weather forecasts may be classified according to the space and time scale of the predicted…

A map or a series of maps that is used to depict the evolution and life cycle of atmospheric phenomena at selected times at the surface and in the free atmosphere. Weather maps are used for the analysis and display of in-place observational…

The measuring, recording, and transmitting of data of the variable elements of weather. In the United States the National Weather Service (NWS), a division of the National Oceanic and Atmospheric Administration (NOAA), has as one of its primary…

The response of geologic materials to the environment (physical, chemical, and biological) at or near the Earth's surface. This response typically results in a reduction in size of the weathering materials; some may become as tiny as ions in…

A nesosilicate mineral, composition Zn2SiO4, crystallizing in the hexagonal system. It is usually massive or granular with a vitreous luster; crystals are rare. The mineral has a basal cleavage and may be variously colored, most commonly green, red,…

The motion of air relative to the Earth's surface. The term usually refers to horizontal air motion, as distinguished from vertical motion, and to air motion averaged over a chosen period of 1–3 min. Micrometeorological circulations (air motion over…

The determination of air movement or wind, is a vector that is specified by speed and direction. Meteorological convention indicates wind direction as the direction from which the wind blows (for example, a southeast wind blows toward the…

A diagram in which statistical information concerning the direction and speed of the wind at a particular location may be conveniently summarized. In the standard wind rose a line segment is drawn in each of perhaps eight compass directions from a…

The drag or tangential force per unit area exerted on the surface of the Earth by the adjacent layer of moving air. Erosion of ground surfaces and the production of waves on water surfaces are manifestations of wind stress. Surface wind stress…

The mineral form of barium carbonate. Witherite has orthorhombic symmetry and the aragonite structure type. Crystals, often twinned, may appear hexagonal in outline (see illustration). It may be white or gray with yellow, brown, or green tints. Its…

A mineral with chemical composition (Fe,Mn)WO4, intermediate between ferberite, the iron tungstate, and huebnerite, the manganese tungstate, which form a complete solid solution series. HuebneriteWolframite occurs commonly in short,…

A mineral inosilicate with composition CaSiO3. It crystallizes in the triclinic system in tabular crystals (see illustration). More commonly it is massive, or in cleavable to fibrous aggregates. There are two good cleavages parallel to the front and…

A mineral consisting of lead molybdate, PbMoO4. Wulfenite occurs commonly in yellow, orange, red, and grayish-white crystals. They may be tetragonal, tabular (see illustration), or pyramidal, with a luster from adamantine to resinous. Wulfenite may…

A black, infusible carbonaceous substance occurring in Uinta County, Utah. It is insoluble in carbon disulfide, has a density of about 1.05, and consists of 79–80% carbon, 10.5–12.5% hydrogen, 4–6% sulfur, 1.8–2.2% nitrogen, and some…

A rock fragment enclosed in another rock, and of varying degrees of foreigness. Cognate xenoliths, for example, are pieces of rock that are genetically related to the host rock that contains them, such as pieces of a border zone in the interior of…

Any mineral belonging to the zeolite family of minerals and synthetic compounds characterized by an aluminosilicate tetrahedral framework, ion-exchangeable large cations, and loosely held water molecules permitting reversible dehydration. The…

A mineral with composition ZnO (zinc oxide). It crystallizes in the hexagonal system with a wurtzite-type structure. Thus its principal axis is polar and different forms appear at top and bottom of crystals. Such crystals are rare and the mineral is…

A mineral with the idealized composition ZrSiO4, one of the chief sources of the element zirconium. Trace amounts of uranium and of thorium are often present and the mineral may then be partly or entirely metamict. The name cyrtolite is applied to…

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…

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…

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…

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…

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…