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.


chaos [planet] . A term established by the International Astronomical Union used to describe a distinctive area of broken terrain on planetary surfaces (USGS, 2003).

tawite (taw'-ite). A plutonic rock, similar in composition to ijolite, containing 30 to 60 percent mafic minerals, but with sodalite in place of nepheline as the dominant sodium feldspathoid. The name, given by Ramsay in 1894, is from Tawojak, Kola Peninsula, Russia. Obsolete. A preferred name would be "sodalite ijolite".

delta t (Δt) [seis] . (a) The time difference between the arrivals of the same phase of a seismic event at two locations. Cf: dip moveout; dip calculation. Delta t ordinarily does not imply normal moveout unless specifically stated. (b) Interval transit time, as used with the sonic log.

link [geomorph] . An unbroken section of stream channel between two nodes in a drainage network (Shreve, 1966). The upstream node may be either a source or fork. The downstream node may be a fork or the mouth of the network. Cf: bifurcating link; cis link; trans link; exterior link; interior link.

biostratic unit (bi-o-strat'-ic). A stratigraphic unit based on paleontologic data; an abbreviation of the term biostratigraphic unit (Henningsmoen, 1961, p.63, 68).

shear resistance . In fluid dynamics, a tangential stress caused by the fluid viscosity, taking place along a boundary of the flow in the tangential direction of local motion (Chow, 1957). Cf: pressure resistance. Syn: shear drag.

aeration . The process by which air in the soil is replaced by air from the atmosphere. In a well-aerated soil, the soil air is very similar in composition to the atmosphere above the soil (aerobic). Poorly aerated soils usually contain a much higher content of CO2 and a lower content of O2 than the atmosphere above the soil (anaerobic). The rate of aeration depends largely on the volume and continuity of air-filled pores within the soil.

current density . The current per unit area perpendicular to the direction of current flow.

holocrystalline (hol-o-crys'-tal-line). Said of the texture of an igneous rock composed entirely of crystals, i.e. having no glassy part.

drift [drill] . (a) inclination [drill] . (b) The deviation of a well from its intended direction. See also: deviation [drill].

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