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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stream line . An imaginary line connecting a series of fluid particles in a moving fluid so that, at a given instant, the velocity vector of every particle on that line is tangent to it.

accumulation area . The part of a glacier or snowfield in which, over a year's time, accumulation exceeds ablation; the region above the equilibrium line. Cf: ablation area; névé. Syn: firn field; accumulation zone; zone of accumulation [snow].

absorption [grd wat] . The entrance of surface water into the lithosphere by any method. Verb: to absorb. Cf: adsorption.

Greenwich meridian (Green'-wich). The astronomic meridian that passes through the original site of the Royal Astronomical Observatory at Greenwich, near London, England. Its adoption as the worldwide reference standard, or prime meridian, was approved almost unanimously at an International Meridian Conference in Washington, D.C., in 1884. Cf: national meridian.

bunsenite (bun'-sen-ite). A pistachio-green cubic mineral: NiO . A member of the periclase group.

frictional sliding [exp struc petrol] . A brittle-deformation mechanism involving movement parallel to a preexisting fracture.

loipon (loi'-pon). A term proposed by Shrock (1947) for a residual surficial layer produced by intense and prolonged chemical weathering and composed largely of certain original constituents of the source rock. Typical accumulations of loipon are the gossans over orebodies, bauxite deposits in Arkansas, terra rossa deposits of Europe, and duricrust of Australia. Etymol: Greek, "residue". Adj: loiponic.

mean refractive index . (a) The median index of refraction for any crystalline substance, with variation due to zoning. (b) The median index of refraction in any microcrystalline substance for which specific index values related to crystal directions are not determinable. (c) In a biaxial crystal, the beta, Ny or Nm index of refraction (in which y or m = mean). This is not the average index.

Dinantian (Di-nan'-tian). European stage: Lower Carboniferous. It includes Tournaisian and Viséan.

pseudofibrous peat (pseu-do-fi'-brous). Peat that is fibrous in texture but that is plastic and incoherent. Cf: fibrous peat; amorphous peat.