index A

 

A

abampere (aA) The unit of electric current in the CGSeniu system, defined as that current that, if flowing through two parallel conductors of negligible cross section and infinite length, placed 1 cm apart in vacuo, would produce on each conductor a force of 1 dyne per centimeter of length. 1 abampere = 1 abcoulomb/s = r statampere (where c = speed of light in cm/s) = 10 ampere.

aberration Imperfect image formation due to geometric imperfections in the optical elements of a system

ablation 1 . The wasting of glacier ice by any process (calving, melting, evaporation, etc.). 2. The shedding of molten material from the outer sur- face of a meteorite or tektite during its flight through the atmosphere.

absolute age The age of a natural substance, of a fossil or living organism, or of an artifact, obtained by means of an absolute dating method. See absolute dating method.

absolute density Density in kg/m' or, more commonly, in g/cm\ both at STP. Cf. density, relative density

absolute magnitude (M) The apparent magnitude of a star reduced to the standard distance of 10 parsecs. See magnitude.

absolute temperature Temperature in kelvins (K), starting at the absolute zero.

absolute zero The temperature at which atomic and molecular translational motion ceases. It is equal to 273.16 K below the triple point of pure water or 273. 1 5 K below the freezing point of pure water at 1 atm

absorbance (a) The common logarithm of the re- ciprocal of transmittance: a = log 1/r where T = transmittance.

absorptance The ratio of radiant flux absorbed to the incident radiant flux. absorption The intake of matter or energy by a medium

absorption law (Bouguer's law) A law giving the flux through a substance in terms of incident flux, coefficient of absorption, and thickness of the substance. where /q = incident flux. / = flux passing through thickness x of the substance, a = absorption coefficient. Cf. attenuation, attenuation coefficient.

absorption spectrum Spectrum resulting from the absorption of specific wavelengths when light from a continuous source passes through a given substance.

abyssal hill A common low-relief feature of the deep-sea floor, where sediment cover has not obliterated bedrock topography. Abyssal hills cover 50% of the Atlantic and Indian Ocean floor and 85% of the Pacific Ocean floor.

abyssal plain The flat surface of the ocean floor covered with sediments largely contributed by tur- bidity currents.

acceleration (a) The derivative of velocity with respect to time, or the second derivative of posi- tion in space with respect to 

accretionary plate boundary The boundary be- tween two plates moving away from each other, where new lithosphere is created.

accuracy A measure of the closeness by which a measurement gr a set of measurements ap- proaches the true value.

acid (Chemistry) 1. Arrhenius acid A chemical substance that dissociates in water to give H^ (or HjO^) ions. Cf Arrhenius base. 2. Brensted acid A chemical substance capable of donating one or more protons. Cf. Brensted base. 3. Lewis acid A chemical substance capable of forming a bond by accepting an electron pair donated by a Lewis base and sharing it with it. Cf Lewis base. (Petrology) 1. Defining an igneous rock that contains more than 60% of SiOj ("silicic acid"). 2. Referring to any rock primarily composed of light-colored sili- cate minerals. Syn. silicic.

acme-zone A stratizone characterized by the maximum abundance development of a given taxon. 

acoustic basement The deepest seismic reflector below which seismic energy return is poor or absent. 

acritarch Any of the single-celled spores or similar structures found in the geological record from Precambrian to Recent.

activation analysis The identification of a stable isotope by rendering it radioactive through bombardment with neutrons, other particles, or radiation.

activation energy The energy above ground state necessary to initiate a process. 

aftershock Any of the smaller shocks following the main shock of an earthquake.

age A division of geologic time longer than sub- age but shorter than epoch, during which the rocks of a stage are formed. Cf absolute age.

agonic line The line connecting the points on the Earth's surface where magnetic declination is zero.

ahermal Defining an organism that does not par- take in the construction of a reef. 

Albedo Reflectivity of a nonluminous surface, ranging from (total absorption) to 1.0 (total re- flection). It is usually expressed as percent. Representative values: open ocean, smooth, vertical sun, 0.02-0.04; forest, 0.1; grassland, 0.2; desert, 0.2; ice, 0.7; fresh snow, 0.8; clouds, 0.5-0.8; Mercury, 0.06; Venus, 0.72; Earth, 0.39; Moon, 0.068; Mars, 0.16; Jupiter, 0.70; Saturn, 0.75; Uranus, 0.90; Neptune, 0.82; Pluto, ?. 

alidade A ruler with a simple or telescopic sight used in plane table mapping. The sight may be mounted on a vertical graduated circle to deter- mine elevation angles.

alkali metal The metals in group 1 of the Peri- odic Table of the Elements, including Li, Na, K, Rb, Cs, and 

alkaline cell A primary cell that uses an alkaline electrolyte.

alkyl acids The family of organic acids of the general formula C„H:,.,COOH (saturated). E.g. palmitic acid, CH3(CH0i4COOH (saturated); oleic acid, CH3(CH:)7CH:CH(CH2)7COOH (unsaturated). Syn. fatty acids. 

alkyl amines A family of organic compounds consisting of an alkyl radical and the -NH. group. E.g. methylamine, CH3— NH2.

alkyl halides A family of compounds consisting of an alkyl radical and a halide. E.g. methyl chloride, CH3CI.

Alkynes A family of unsaturated aliphatic hydro- carbons C„H2„-2, with one C=C bond. E.g. acetylene, CH=CH. Syn. acetylene series.

allele .Any of the genes belonging to the set that specifies a given physical characteristic of an organism.

allobar An occurrence of an element with isoto- pic composition, and hence mass, different from the common one.

allotropy The property of an element to crystallize in two or more different forms depending upon ambient temperature and pressure. Examples: C as graphite and diamond; S as rhombic, monoclinic, and amorphous; Fe as body-centered cubic and face-centered cubic. Cf polymorphism.

alloy A macroscopically homogeneous mixture of (usually) two or more metals. Some nonmetals partake in the formation of alloys (e.g. C in carbon steel). Alloys may be ordered (exhibiting a well-defined multiatomic crystalline unit cell) or disor- dered (solid solutions)

altitude (Astronomy) (h) the angular elevation of a celestial body above ( + ) or below ( — ) the hori- zon in the horizon coordinate system. See coordi- nate systems. (Topography) The elevation of a to- pographic or other feature above a given standard datum.

amines A family of organic compounds derived from NH3 by replacing with organic groups one H (primary amines; e.g., CH3 — NHj, methylamine), two H [secondary amines; e.g. (CH3)2 — NH, dimethylamine], or all three H [tertiary amines; e.g. (CHj)^ — N, trimethylamine].

ammonia clock A clock based on the property of the pyramidal NH3 molecule to reverse itself with a frequency of 2.387013- 10'° Hz. See atomic clock

ampere (A) The SI and MKS unit of electrical current, defined as that current that, if maintained in two parallel conductors of negligible cross sec- tion and infinite length, placed 1 m apart in vacuo, would produce on each conductor a force of 10^N/m of length (exactly). It is equal to 1 coulomb/ second

Ampere's law The law relating the line integral of the magnetic field B around a closed path to the total current / through the circumscribed area

ampere-turn (At) The SI unit of magnetomotive force, equal to the mmf developed when a current of 1 ampere is flowing around a circle. Cf. gilbert. ampere-turn/meter (At/m) Syn. ampere/meter.

amu Atomic mass unit, defined (before 1960) as 1/1 6thtopic mass of oxygen in nature (chemical definition). The chemical definition was 1.000274 larger than the physical definition. Since 1960 the defi- nition of atomic mass unit (symbol u) is l/12th of the mass of the neutral atom of '^C. See atomic mass unit,

anabolism A phase of metabolism that leads to the formation of more complex organic molecules from simpler ones. Cf catabolism, metabolis

anaerobic 1. Defining an environment that lacks free oxygen. 2. Defining an organism that does not need free oxygen for its metabolism

anaglacial A climactic phase leading from an in- terglacial to a glacial age. Cf cataglacial.

antiproton The antiparticle of the proton, having identical mass and spin but opposite charge and magnetic moment.

antiquark The antiparticle of the quark, having baryon number, electric charge, color, and strange- ness of sign opposite that of the quark.

Aouelloul glass A silica-rich glass produced by fusion of countr>' rock resulting from the impact that formed the Aouelloul crater in Mauritania (diameter = 370 m; age = 3.5 ± 0.5- 10' 

apparent day The time interval between successive local noons.

Archimedes' principle  in a gravitational field, a body floating or totally immersed in a fluid experiences an upward force equal to the weight of the fluid displaced

asphalt A naturally occurring material consisting of a mixture of heavy hydrocarbons and inorganic matter. The organic component softens at about 90°C. 

Asteroidal belt A region between the orbits of Mars and Jupiter, ranging from 2 to 4 AU, within which most of the asteroids orbit

Atmosphere. Thermal structure and zonation between and 1000 km of altitude.

Auger electron An electron emitted from an atom as a result of the Auger effect. See Auger effect.

avogadro (A) A unit of number of items (elementary particles, atoms, ions, molecules, objects, or- ganisms, etc.) equal to 6.022136- \0'\ Syn. 

azimuth The angle between the North point of an observer's horizon and the vertical projection of a celestial body on that horizon, measured clock- wise from the North direction (0° to 360°).

azimuthal projection The projection, in map construction, of the surface of the Earth from the Earth's center on a plane surface tangent to the pole or to any other specified point.

azimuthal quantum number The quantum number representing the orbital angular momentum of an atomic electron. Syn. orbital angular momentum quantum number