GeoWord of the Day

The GeoWord of the Day is a free service of the American Geosciences Institute. All of the terms and definitions are from the Glossary of Geology, 5th Edition Revised.

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reach [coast] . (a) An arm of the sea extending up into the land; e.g. an estuary or a bay. (b) promontory.

expanding-Universe hypothesis (ex-pand'-ing-U'-ni-verse). Interpretation of the universally extragalactic red shift as a Doppler shift, suggesting that the Universe is expanding at a uniform rate.

hydric (hy'-dric). Said of a habitat that has or requires abundant moisture; also, said of an organism or group of organisms occupying such a habitat. Cf: xeric; mesic. See also: hydrarch.

stannomicrolite . A yellowish-brown cubic mineral of the pyrochlore group: (Sn,Fe,Mn)2(Ta,Nb,Sn)2(O,OH,F)7. Syn: sukulaite.

thermal resistance . Temperature difference divided by heat-flow rate measured in kelvin per watt (KW-1). Symbol: R.

bedding plane . (a) A planar or nearly planar bedding surface that visibly separates each successive layer of stratified rock (of the same or different lithology) from the preceding or following layer; a plane of deposition. It often marks a change in the circumstances of deposition, and may show a parting, a color difference, or both. (b) A term commonly applied to any bedding surface, even when conspicuously bent or deformed by folding. (c) A term commonly applied to a plane of discontinuity (usually the bedding plane) along which a rock tends to split or break readily. Syn: bedding [stratig]; bed plane; stratification plane; plane of stratification.

Wenner array (Wen'-ner). An electrode array in which the four electrodes are in line and equally spaced, and in which the outer pair is used to inject current into the ground while the inner pair is used to measure differences in potential.

plagiaplite (pla-gi-ap'-lite). An aplite composed chiefly of plagioclase (oligoclase to andesine), possibly green hornblende, and accessory quartz, biotite, and muscovite; a fine-grained diorite. Obsolete.

ground roll . A seismic surface wave, generally of low frequency and velocity. Often considered to be a modified Rayleigh wave.

defect lattice (de'-fect). A crystal lattice in which the expected systemic repetition is interrupted by an omission, an inclusion of an extra item, or the substitution of an unexpected item. Cf: Schottky defect.


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