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A conductor whose purpose is to emit and/or receive electromagnetic waves using the bulk oscillations of electric charges within the conductor. The conduction electrons in antennas are free to move throughout the conductor. The electrons form a bulk…

The subfield of computer science concerned with understanding the nature of intelligence and constructing computer systems capable of intelligent behavior. Many activities, such as perception, learning, planning, natural language, and understanding,…

Earth's oldest desert and the most arid place outside of Antarctica's Dry Valleys. Located along the edge of the Pacific Ocean and to the west of the Andes in Chile's Antofagasta Province, the Atacama Desert (Fig. 1) has long been recognized as…

Cameras in mobile phones or smartphones that can capture images and videos.The cellphone, the world’s favorite must-have gadget, has been with us for nearly a half-century, while experiencing continuous and often phenomenal improvements. Much of the…

A branch of computer science that deals with the theory and techniques of computer-generated images. Computers produce images by analyzing a collection of dots, or pixels (picture elements). Computer graphics is used to enhance the transfer and…

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…

A radar system used to measure the relative velocity of targets and their position. The operation of these systems is based on the fact that the Doppler frequency shift in the target echo is proportional to the radial component of target velocity.…

An interdisciplinary science area that is concerned with modeling aspects of natural evolution in order to solve real-world problems of optimization. Living organisms, as well as those that are long extinct, demonstrate optimized complex behavior at…

A biological organism whose genetic material has been manipulated artificially through genetic or genomic editing techniques. In contrast to the long history of artificial selection (domestication), in which humans selectively breed organisms to…

An ordered, internally consistent, internationally recognized sequence of time intervals, each distinct in its own history and record of life on Earth, including the assignment of absolute time in years to each geologic interval. The geologic time…

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…

A phaneritic (visibly crystalline) plutonic rock composed chiefly of sodic plagioclase (oligoclase or andesine), alkali feldspar (microcline or orthoclase, usually perthitic), quartz, and subordinate dark-colored (mafic) minerals (biotite,…

A strongly cemented or hardened (indurated) dark gray sandstone that is characterized by abundant dark-colored detrital rock fragments and more than 15% clay minerals (matrix) between sand grains. The term graywacke evolved from initially informal…

The most common sulfate mineral, characterized by the chemical formula CaSO4 · 2H2O; it shows little variation from this composition.Gypsum is one of the several evaporite minerals. This mineral group includes chlorides, carbonates, borates,…

A metamorphic rock that has been subjected to heating during contact metamorphism around intrusive igneous rocks. Hornfels is typically fine-grained, although where it is subjected to high temperatures, large crystals called porphyroblasts can form.…

A naturally occurring geologic body that may contain one or more metals. The metals may be present as native elements, or, more commonly, as oxides, sulfides, sulfates, silicates, or other compounds. The term ore is often used loosely to include…

Rock formed by the accumulation of fragmental volcanic materials (also called pyroclasts) ejected during explosive eruptions. Pyroclasts can vary widely in origin, fragment size and shape, composition, texture, and structure (such as their…

The evolving internet technologies for content sharing and collaborating among communities of online users. Since the advent of the Web 2.0—the more dynamic and interactive second generation of the World Wide Web—the term social media has become…

The theory of circuits made up of ideal digital devices. Included in switching theory are the theories of circuits and networks for telephone switching, digital computing, digital control, and data processing. Computer Electrical circuit Electronic…

A management technology involving the interactions of science, an organization, and its environment as well as the information and knowledge that support each. The purpose of systems engineering is to support organizations that desire improved…

A process of making physical objects directly from digital models, usually layer by layer; also known as additive manufacturing (AM). 3D printing has been hailed by many as the technology that will revolutionize how we live. As information…

A continent that straddles the Equator, extending between 37°N and 35°S. It is the second largest continent, exceeded only by Asia (see illustration). The political geography of the continent continues to evolve as the traditional states face…

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 use of live nonhuman animals for scientific research and experimentation. Animal testing has been a cornerstone of biomedical research, having contributed greatly to the understanding of human biology, to the unraveling of the complex underlying…

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…

An oxygen-depleted region in a marine environment. The dynamic steady state between oxygen supply and consumption determines the oceanic oxygen concentration. In regions where the rate of consumption equals or exceeds the rate of supply, seawater…

The coldest, windiest, and driest continent. The lowest temperature ever measured on Earth, −89.2°C (−128.5°F), was recorded at the Russian Antarctic station of Vostok in July 1983. Katabatic (cold, gravitational) winds with velocities up to 50 km/h…

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 most common volcanic rock on Earth. Basalt is an igneous rock characterized by small grain size (less than about 5 mm) and approximately equal proportions of calcium-rich plagioclase feldspar and calcium-rich pyroxene, with less than about 20%…

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…

A discipline that combines biology, mathematics, and computer science to acquire, store, analyze, and disseminate complex biological data. The term bioinformatics was coined by Paulien Hogeweg and Ben Hesper in the early 1970s to describe the "the…

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 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 500 megapascals (5 kilobars) and below 400°C (750°F). Metamorphic rocks of the relatively uncommon blueschist facies contain assemblages of minerals that record…

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 2 mm, which means that 50% or more of the particles (by volume) are this…

A mineral composed of calcium carbonate (CaCO3). Calcite is one of the most common minerals found in the Earth's crust. Calcite (Fig. 1) may be found in a great variety of sedimentary, metamorphic, and igneous rocks. It is also an important…

A type of carved gemstone in which the background is cut away to leave the subject in relief. Often cameos are cut from stones in which the coloring is layered, resulting in a figure of one color and a background of another. The term cameo (see…

A mineral with the chemical composition SrSO4. Celestite occurs commonly in colorless to sky-blue, orthorhombic, tabular crystals (see illustration). Fracture is uneven and luster is vitreous. Hardness is 3–3.5 on Mohs scale and specific gravity is…

A common mineral in the upper oxidized zone of ore deposits containing galena (PbS). Cerussite (see illustration) is usually associated with anglesite, smithsonite, malachite, and other secondary minerals. Normally cerussite is close in composition…

A mineral belonging to the zeolite family of silicates. Chabazite (see illustration) commonly occurs in well-crystallized groupings of crystals with external rhombohedral symmetry (point group 3 2/m) with nearly cubic angles. This well-developed…

A mineral with the chemical composition CuSO4· 5H2O. Chalcanthite commonly occurs in blue to greenish-blue triclinic crystals or in massive fibrous veins or stalactites (see illustration). Fracture is conchoidal and luster is vitreous. Hardness is…

A fine-grained fibrous variety of quartz, silicon dioxide. The individual fibers that compose the mineral aggregate usually are visible only under the microscope. Subvarieties of chalcedony recognized on the basis of color differences (induced by…

The term “chalk” is sometimes used in a broad sense for any soft, friable, or weathered fine-grained limestone; however the term is mostly restricted to pelagic (biogenic) limestones. Chalk is a uniformly fine-grained, typically light-colored marine…

A hard, dense sedimentary rock composed of fine-grained silica (SiO2), most commonly quartz. Chert is characterized by a semivitreous to dull luster and a splintery to conchoidal fracture, and is most commonly gray, black, reddish brown, or green…

One of a group of layer silicate minerals, usually green in color, characterized by a perfect cleavage parallel to (001). Chlorite cleavage flakes are flexible but inelastic, with a luster varying from pearly or vitreous to dull and earthy. The…

A hydrous iron aluminum silicate mineral commonly found in low-grade metamorphic rocks such as slate and phyllite. Chloritoid is found as platy, black or dark green crystals, rarely more than a few millimeters in size (Fig. 1). Its density ranges…

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.…

A silicate mineral with composition essentially (Cu,Al)2H2Si2O5(OH)4·nH2O; iron, and more rarely manganese, may substitute for copper. Although acicular monoclinic crystals have been reported, the wide variation in the composition of analyzed…

A fibrous asbestos mineral with a tubular morphology for each fibril. Chrysotile (Fig. 1) is a member of the serpentine mineral group, as are antigorite and lizardite. Chrysotile aggregates make up serpentine asbestos, which is the most common type…

Fine-grained, hydrous, layer silicates that belong to the larger class of sheet silicates known as phyllosilicates. Clay minerals form on Earth in many different environments, including the weathering environment, the sedimentary environment, and…

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…

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 device that receives, processes, and presents information. The two basic types of computers are analog and digital. Although generally not regarded as such, the most prevalent computer is the simple mechanical analog computer, in which gears,…

Any device connected internally or externally to a computer and used in the transfer of data to or from a computer. A personal computer (PC) processes information. Data (unprocessed information) must get into the computer, and the processed…

Designing and writing a detailed sequence of operating instructions for a particular problem to be run on a computer; also known as programming. A computer is able to perform useful tasks only by executing computer programs. A programming language…

The process of protecting against threats to computing systems. A threat is an event that can cause harm to computers, data or programs, or computations (Fig. 1). A failure of computer security occurs because of a vulnerability or weakness in a…

The hardware components, both internal and external, of a computer used to read (retrieve) or write (save) digital data. The ability to read and write data is an essential function of a computer and of the so-called von Neumann architecture…

A program or script that is loaded and executed on a computer without the knowledge of the user. A typical computer virus is a very small program, several kilobytes or less in size, constructed in a matter of hours or days. When executed, the virus…

The technology concerned with computational understanding and use of the information present in visual images. In part, computer vision is analogous to the transformation of visual sensation into visual perception in biological vision. For this…

A loosely defined term used for a sedimentary mineral segregation that may range in size from millimeters to meters. Concretions (see illustration) are usually distinguished from the sedimentary matrix enclosing them by a difference in mineralogy,…

The consolidated equivalent of gravel. Conglomerates are aggregates of more or less rounded particles greater than 2 mm in diameter. Frequently they are divided on the basis of size of particles into pebble (fine), cobble (medium), and boulder…

An orthorhombic magnesium aluminosilicate mineral of composition Mg2[Al4Si5O18], a = 1.710 nanometers, b = 0.973 nm, c = 0.936 nm, Z = 4, space group Cccm. The crystal structure is related to beryl; magnesium is octahedrally coordinated by oxygen…

A mineral with chemical composition Na3AlF6. Although it crystallizes in the monoclinic system, cryolite has three directions of parting at nearly 90°, giving it a pseudocubic aspect. Hardness is 2½ on Mohs scale and the specific gravity is 2.95.…

A digital equivalent of paper banknotes. Just like paper banknotes, cryptocurrencies allow users to maintain privacy in digital commercial transactions while ensuring that value is transferred from one party to the other. Cryptocurrencies achieve…

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 deposit of sediment at the mouth of a river or tidal inlet. Delta, from the Greek letter Δ, was first used by Herodotus (fifth century BCE.) for the triangular delta of the Nile. The term, delta, is also used for storm washovers of barrier islands…

The computer manipulation of visual data to solve a problem. The problems under consideration are of two primary types: (1) those that require a computer to act on the resultant images, and (2) those that provide images for human interpretation. The…

The monoclinic pyroxene mineral which in pure form has the formula CaMgSi2O6. Diopside has no known polymorphs. Its structure consists of chains of SiO4 tetrahedrons in which each silicon ion shares an oxygen with each of its two nearest silicon…

A phaneritic (visibly crystallized) plutonic rock having intermediate SiO2 content (53–66%), composed mainly of plagioclase (oligoclase or andesine) and one or more ferromagnesian minerals (hornblende, biotite, or pyroxene), and having a granular…

Programmable computers integrated with electrically powered machines that perform household chores. Domestic robots, also known as home or service robots, are essentially programmable computers integrated with electrically powered machines that…

A very dense rock composed of red-brown garnet and the grape-green pyroxene omphacite. Eclogites possess basaltic bulk chemistry, and their garnets are rich in the components pyrope (Mg3Al2Si3O12), almandine (Fe3Al2Si3O12), and grossular…

The manner in which electromagnetic waves travel forward along a path. Electromagnetic waves consist of quantized, traveling, self-sustained, electromagnetic field fluctuations. The high speed of electromagnetic waves makes them useful in…

A System that locates suitably equipped users by means of measurements made by electronic sensors. Typical measurements include distance or bearing from a vehicle to a known point, or direct determination of the present position of the vehicle in a…

The medium- to dark-green gem variety of the mineral beryl, Be3Al2Si6O18, crystallizing in the hexagonal system. A flawless emerald with good color is one of the most sought after and highly prized of all precious gems. Emerald is restricted in its…

The branch of geology that deals with the ways in which geology affects people. Examples of the effect of geology on human civilizations include (1) how fertile soils develop from rocks and how these soils can become polluted by human activities;…

The group name for a family of minerals of general composition Ca2(Fe3+, Al, Mn3+)Al2O[SiO4][Si2O]- (OH) which occur widely in metamorphic and igneous rocks (Fig. 1). Epidote [octahedral ferric iron (Fe3+) dominant] and clinozoisite [aluminum (Al)…

The result of processes that entrain and transport earth materials along coastlines, in streams, and on hillslopes. Wind and water are common agents through which forces are applied to resistant rocks, soils, or other unconsolidated materials.…

An active and intentional act or practice of putting to death, in a relatively painless way, of persons suffering severely from medical conditions that are incurable, untreatable, or irreversible. Euthanasia is an act by a person, usually a…

The process by which water bodies, such as estuaries, coastal waters, and lakes, become enriched in dissolved nutrients, such as phosphorus and nitrogen, resulting in increased growth of algae and aquatic plants as well as the deterioration of the…

The test of a hypothesis under controlled conditions. The experiment is one of the distinctive tools of the scientist. It enables the scientist to put questions to nature and receive answers. These answers lead to new problems whose solutions…

Any weather event that is unusual for a particular location or severe in its effects. The most commonly considered examples of extreme weather events include heat waves, cold snaps, heavy rainfall or snowfall, ice or hail storms, droughts,…

Any of a group of aluminosilicate minerals whose crystal structures are composed of corner-sharing [AlO4] and [SiO4] tetrahedra linked in an infinite three-dimensional array, with charge-balancing cations [primarily sodium (Na), potassium (K; Fig.…

A mineral of composition CaF2 and an important structure type with space group Fm3m, cell edge = 0.546 nm. The fluorite structure type is written RX2, where the R cations are in cubic (eightfold) coordination by the X anions. Alternatively, the X…

The plutonic equivalent of its more abundant extrusive equivalent, basalt. Because gabbro crystallized from a magma intruded deep within the crust, it has a grain size visible to the naked eye with approximately equal amounts of calcic plagioclase…

A hard, dense silicate mineral which occurs as crystals of cubic symmetry in a wide range of geologic environments. The general chemical formula of the silicate garnet group is A3B2(SiO4)3 where, in natural occurrences, the A cations are dominantly…