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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…

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. The time necessary to transform resin…

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 (see illustration) is rare in the deep colors that characterize fine quality. Amethyst is usually colored unevenly…

A nesosilicate mineral, composition Al2SiO5, crystallizing in the orthorhombic system. Andalusite (Fig. 1) occurs commonly in large, nearly square prismatic crystals. The variety chiastolite has inclusions of dark-colored carbonaceous material…

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

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 (anhydrous calcium sulfate) occurs commonly in white and grayish granular masses, rarely in large, orthorhombic crystals (see illustration). Fracture is uneven and luster is pearly to…

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

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…

Arenite (rock composed of sand-size fragments) that contains a high proportion of feldspar in addition to quartz and other detrital minerals. Arkose is also known as feldspathic sandstone. Although there is no universal agreement, many geologists…

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…

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 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…

The primary ore used to produce aluminum metal and alumina (Al2O3). Bauxite (see illustration) 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 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 (Fig. 1) 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…

An iron-magnesium-rich layer silicate; it is also known as black mica. Biotite (see illustration) 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,…

A mineral having composition BeAl2O4 and crystallizing in the orthorhombic system. Chrysoberyl crystals (see illustration) are usually tabular parallel to the front pinacoid and frequently in pseudohexagonal twins. There is good prismatic cleavage.…

The response of corals to environmental stress in which they eject the single-cell algae that live within the transparent coral tissue, making the white coral skeleton visible. Corals (Fig. 1) are animals that live in a partnership (symbiosis) with…

Marine animal life inhabiting the portion of the oceans below the upper limit of the continental slopes. The marine animals that make up the deep-sea fauna live below the photic zone of the oceans. In general, the deep sea may be regarded as that…

A rare mineral tectosilicate found in volcanic rocks high in potassium and low in silica. Kaliophilite is one of three polymorphic forms of KAlSiO4; the others are the rare mineral kalsilite and an orthorhombic phase formed artificially at about…

A branch of meteorology that examines the effects and impacts of weather and climate on crops, rangeland, livestock, and various agricultural operations. Agricultural meteorology, or agrometeorology, addresses topics that often require an…

Natural and artificial substances in the atmosphere that affect human health or well-being, or the well-being of any other organisms in the environment. Air pollution also applies to situations in which contaminants affect structures and artifacts…

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…

System behavior that depends so sensitively on the system's precise initial conditions that it is, in effect, unpredictable and cannot be distinguished from a random process, even though it is deterministic in a mathematical sense. This article…

The long-term records of precipitation, temperature, wind, and all other aspects of the Earth's climate. Climate, like the Earth itself, has a history extending over several billion years. Climatic changes have occurred at time scales ranging from…

Construction of a mathematical model of the climate system of the Earth capable of simulating its behavior under present and altered conditions. The Earth's climate is continually changing over time scales ranging from hundreds of years to a few…

The scientific study of climate. Climate is the expected mean and variability of the weather conditions for a particular location, season, and time of day. The climate is often described in terms of the mean values of meteorological variables such…

A calcarenite or clastic limestone whose detrital particles are chiefly fossils, whole or fragmented. The term coquina is most frequently used for an aggregate of large shells more or less cemented by calcite (see illustration). If the rock consists…

Pathological conditions affecting the health of coral and causing its physiological dysfunction. Broadly defined, coral disease (Fig. 1) is any impairment to coral health resulting in physiological disturbance and breakdown. It usually involves…

A measure of the three-dimensional structure of live coral communities on reefs. Coral reef structural complexity is an intrinsic element of coral reef ecosystems and quantitates the surface area of coral in relation to its linear area. The…

An acceleration which arises as a result of motion of a particle relative to a rotating system. Only the components of motion in a plane parallel to the equatorial plane are influenced. Coriolis accelerations are important to the circulation of…

A vortex in which the sense of rotation of the wind about the local vertical is the same as that of the Earth's rotation. Thus, a cyclone rotates clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. In meteorology…

A vertical sequence of several different kinds of distinctive sedimentary rock units that is repeated upward through the stratigraphic succession. Originally defined in the rock succession of Pennsylvanian age in the Illinois Basin in the 1930s, the…

The monoclinic pyroxene mineral which in pure form has the formula CaMgSi2O6. Its space group is C2/c. Pure diopside melts congruently at 1391°C (2536°F) at atmospheric pressure; the effect of pressure up to 50 kilobars (5 gigapascals) on its…

Skin diving, or diving without the aid of special equipment, is a technique to investigate underwater environments and to gather shellfish or other items of commercial importance. In breath-hold diving, fins and faceplate are usually employed to…

The carbonate mineral CaMg(CO3)2. Dolomite is a very common mineral, occurring in a variety of geologic settings. It is often found in ultrabasic igneous rocks, notably in carbonatites and serpentinites, in metamorphosed carbonate sediments, where…

A type of convective transport in fluids that depends on the difference in diffusion rates of at least two density-affecting components. This phenomenon was discovered in 1960 in an oceanographic context, where the two components are heat and…

A nesosilicate mineral with composition Al7(BO)3(SiO4)3O3. Dumortierite crystallizes in the orthorhombic system but well-formed crystals are rare; the mineral usually occurs in parallel or radiating fibrous aggregates. There is one direction of poor…

A state of fluid flow in which the distribution of mass and momentum is unstable. Fluid flows are subject to a variety of instabilities which generally cause the flow to become more complex and which often lead to turbulent, chaotic flow.…

The low-density outermost layer of the Earth above the Mohorovičić discontinuity (the Moho), a global boundary that is defined as the depth in the Earth where the compressional-wave seismic velocity increases rapidly or discontinuously to a value in…

A disruption of the usual tropical Pacific Ocean and atmospheric circulation patterns that includes a few seasons of warmer-than-normal sea-surface temperatures in the central and eastern equatorial Pacific Ocean, weaker mid-Pacific trade winds, and…

The effect of changes in pressure and temperature on electrical properties of rocks. There has been increasing interest in the electrical properties of rocks at depth within the Earth and the Moon. The reason for this interest has been consideration…

The application of geology and other Earth sciences to solve engineering problems. The branches of the Earth sciences most applicable are surficial geology, petrofabrics, rock and soil mechanics, geohydrology, and geophysics, particularly…

The study of the flows of air and water, of the chemical and biological species carried by them, and of their interactions with geological, biological, social, and engineering systems in the vicinity of a planet's surface. The environment on the…

Radioactivity that originates from natural and anthropogenic sources, including radioactive materials in food, housing, and air, radioactive materials used in medicine, nuclear weapons tests in the open atmosphere, and radioactive materials used in…

A semienclosed coastal body of water which has a free connection with the sea and within which the seawater is measurably diluted by freshwater derived from land drainage. In estuaries, the seawater diluted by freshwater is called brackish water,…

Variation with time of the chemical composition and total weight of the Earth's atmosphere. The atmosphere is a most tenuous envelope; its mass is less than one-millionth that of the solid Earth; its density even at sea level is less than…

A method of dating geological and archeological specimens by counting the radiation-damage tracks produced by spontaneous fission of uranium impurities in minerals and glasses. During fission two fragments of the uranium nucleus fly apart with high…

A physiologically interactive system within which growth, adaptation, species composition, nutrient cycling, biological productivity, and energy flows among inland aquatic microbial, plant, and animal communities are integrated with the environment.…

An elongated, sloping zone in the troposphere, within which changes of temperature and wind velocity are large compared to changes outside the zone. Thus the passage of a front at a fixed location is marked by rather sudden changes in temperature…

The formation of ice on the ground, on plants, or on structures near the Earth's surface as a result of the drop in air temperature to lower than 32°F (0°C) and the presence of water vapor or liquid water in the air. The causes of the cooling, and…

Conditions in which two or more phases of fused-salt mixtures can coexist in thermodynamic equilibrium. Phase diagrams of these equilibrium conditions summarize basic knowledge about fused salts. Numerous advances in the technologies which are based…

The scientific study of the processes and impacts of ice sheets, valley glaciers, and other ice masses on the Earth's surface, both on land and in ocean basins. The processes include understanding how ice masses move, erode, transport, and deposit…

The history of glaciation on Earth, extending back in geological time to the Proterozoic and possibly the Archean. Global glaciations have occurred during every geological period except the Jurassic. This persistence of global glaciation, as well as…

Significant and persistent changes in the state of Earth’s climate. Earth's past climates, or paleoclimates, have alternated between ice ages and periods warmer than today over relatively long time scales (millions to tens of millions of years)…

The rise in the surface temperature of the Earth. Global warming is one of the names given to the systematic increase of globally averaged surface temperature that has been measured since the mid-nineteenth century. There is abundant evidence that…

The ability of a planetary atmosphere to inhibit heat loss from the planet's surface, thereby enhancing the surface warming that is produced by the absorption of solar radiation. The greenhouse effect acts like a thermal blanket, limiting heat loss…

The Gulf of Mexico is a subtropical semi-enclosed sea bordering the western North Atlantic Ocean. The Gulf of Mexico (Fig. 1) is the northern-most basin of the Intra-Americas Sea. The Gulf of Mexico connects to the Caribbean Sea on the south through…

Either of two large circles of light surrounding the Sun or Moon that result from the refraction of sunlight by small, hexagonal ice crystals falling slowly through the air. Light passing through the side faces of a hexagonal prism is refracted by…

A mineral belonging to the zeolite family of silicates and crystallizing in the monoclinic system. It usually occurs in crystals with prominent side pinacoid, often having a diamond shape. There is perfect side pinacoid cleavage on which the luster…

The study of the occurrence, movement, and changes in the state of water in the atmosphere. The term is also used in a more restricted sense, especially by hydrologists, to mean the study of the exchange of water between the atmosphere and…

A hot spring on the ocean floor, where heated fluids exit from cracks in the Earth's crust. Most hydrothermal vents occur along the central axes of mid-oceanic ridges, which are underwater mountain ranges that wind through all of the deep oceans.…

The application of meteorological information to industrial, business, or commercial problems. Generally, industrial meteorology is a branch of applied meteorology, which is the broad field where weather data, analyses, and forecasts are put to…

Low-frequency sound waves that cannot be sensed by the human ear. Infrasound was first detected on barometers around the world following the eruption of the volcano Krakatoa in 1883. We now know subaudible sounds are generated by a wide variety of…

Surfaces along which the entropy and potential temperature of air are constant. Potential temperature, in meteorological usage, is defined by the relationship given below,in which T is the air temperature, P is atmospheric pressure expressed in…

A curve along which pressure is constant. Leading examples of its uses are in weather forecasting and meteorology. The most common weather maps are charts of weather conditions at the Earth's surface and mean sea level, and they contain isobars as…

A line along which the speed of the wind is constant. Isotachs are customarily represented on surfaces of constant elevation or atmospheric pressure, or in vertical cross sections. The closeness of spacing of the isotachs is indicative of the…

A map showing the distribution of air temperature (or sometimes sea surface or soil temperature) over a portion of the Earth's surface, or at some level in the atmosphere. On it, isotherms are lines connecting places of equal temperature. The…

The complex, disseminated organic matter present in sedimentary rocks that remains undissolved by sequential treatment with common organic solvents (such as a benzene-methanol mixture, toluene, and methylene chloride) followed by treatment with…

The optical analog of radar. The term lidar is an acronym for light detection and ranging. Lidar systems employ intense pulses of light, typically generated by lasers, and large telescopes and sensitive optical detectors to receive the reflected…

An abrupt, high-current electric discharge that occurs in the atmospheres of the Earth and other planets and that has a path length ranging from hundreds of meters to tens of kilometers. Lightning occurs in thunderstorms because vertical air motions…

A cubic mineral and member of the spinel structure type, space group Fd3m, with composition [Fe3+]IV[Fe2+Fe3+]VIO4. It possesses the inverse spinel structure, in which half the ferric iron is tetrahedrally coordinated and the remaining half as well…

An instrument that measures the magnitude and/or direction of a magnetic field. A magnetometer can be either a scalar instrument that measures the magnitude of the total field or a vector instrument that measures one or more vector components of the…

A term applied commercially to any limestone or dolomite taking polish. Marble is extensively used for building and ornamental purposes.Limestone Stone and stone productsIn petrography the term marble is applied to metamorphic rocks composed of…

The collection and observation of living organisms in the sea, including the quantitative determination of their abundance in time and space. The biological survey of the ocean largely depends on specially equipped vessels. Sampling in intertidal…

The management of marine species and ecosystems to prevent their decline and extinction. As in terrestrial conservation, the goal of marine conservation is to protect biodiversity and ecosystem function through the preservation of species,…

An integrative science that studies the basic structural and functional relationships within and among living populations and their physical–chemical environments in marine ecosystems. Although an outgrowth of natural history and the life sciences,…

The harvest of animals and plants from the ocean to provide food for people and animals. Marine fisheries are places or industries engaged in the harvesting of aquatic life in seawaters. Important products in marine capture fisheries include fish…

A scientific discipline applying the principles and methods of general microbiology to research in marine biology and biogeochemistry. Marine microbiology investigates microorganisms living in saltwater environments. Dealing, by definition, with…

The branch of geography that examines human and physical activities on the Earth's surface using models and statistical analysis. The primary areas in which mathematical methods are used include the analysis of spatial patterns, the processes that…

A naturally occurring solid phase with a restricted chemical composition and a definite atomic arrangement. This is really an idealized definition insofar as some minerals (especially those that form at low temperature) have poorly defined atomic…

A distorted scene of distant objects resulting from the passage of light through a nonuniform medium. In a uniform atmosphere light would travel in straight lines and distant scenes would appear without distortion. Since the density of the air…

The smallest of the four major deserts in the North American Desert region in the continental United States and is located in the southwestern quadrant of the Basin and Range physiographic province in southeastern California and portions of Arizona,…

A group name for all clay minerals with an expanding structure, except vermiculite, and also a specific mineral name for the high alumina end member of the group. VermiculiteMontmorillonite clays have wide commercial use. The high colloidal,…

The formation of nuclides present in the universe by various nuclear reactions. Theories of the origin of the elements involve synthesis with charged and neutral elementary particles (neutrons, protons, neutrinos, photons) and other nuclear building…

An isolated fertile area, usually limited in extent and surrounded by desert. The term was initially applied to small areas in Africa and Asia typically supporting trees and cultivated crops with a water supply from springs and from seepage of water…

The general circulation of the ocean. The term is usually understood to include large-scale, nearly steady features, such as the Gulf Stream (Fig. 1), as well as current systems that change seasonally but are persistent from one year to the next,…

Propagating oscillations in the ocean that carry energy and momentum from one region to another. Most ocean waves (Fig. 1) are caused directly or indirectly by wind blowing across the sea surface. Many waves can propagate through the ocean thousands…

The science of the sea, including physical oceanography, marine chemistry, marine geology, and marine biology. The need to know more about the impact of marine pollution and possible effects of the exploitation of marine resources, together with the…

Waters of varying mineral content which are found associated with petroleum and natural gas or have been encountered in the search for oil and gas. They are also called oil-field brines, or brines. They include a variety of underground waters,…

A name given to a group of magnesium-iron silicate minerals crystallizing in the orthorhombic system. Crystals are usually of simple habit, a combination of the dipyramid with prisms and pinacoids. The luster is vitreous and the color olive green,…

The study of the abundance and composition of naturally occurring organic substances, their origins and fate, and the processes that affect their distributions on Earth and in extraterrestrial materials.Organic geochemistry was born from a curiosity…

A mineral having composition As2S3 and crystallizing in the monoclinic system. Crystals are small, tabular, and rarely distinct (see illustration); the mineral occurs more commonly in foliated or columnar masses. There is one perfect cleavage…

A geographic designation that includes thousands of mainly small coral and volcanic islands scattered across the Pacific Ocean from Palau in the west to Easter Island in the east. Island archipelagos off the coast of the Asian mainland, such as…

The largest of the world's oceans, having an area of 1.65 × 108 km2 (6.3 × 107 mi2) and a mean depth of 4282 m (14,049 ft).The Pacific Ocean covers 32% of the Earth's surface and 46% of the surface of all oceans and seas and its area is greater than…

A soil of the past, that is, a fossil soil. Paleosols are most easily recognized when they are buried by sediments. They also include surface profiles that are thought to have formed under very different conditions from those now prevailing, such as…

Any of the oriented intergrowths of potassium- and sodium-rich feldspars, (K,Na)AlSi3O8, whose proportions are determined in part by the initial composition of the alkali feldspar from which they exsolved and whose physical properties are thus…