talus [geol] (ta'-lus). Rock fragments of any size or shape (usually coarse and angular) derived from and lying at the base of a cliff or very steep, rocky slope. Also, the outward sloping and accumulated heap or mass of such loose broken rock, considered as a unit, and formed chiefly by gravitational falling, rolling, or sliding. Cf: alluvial talus; avalanche talus; rockfall talus. See also: scree. Syn: rubble. Etymol: French "talu", later "talus", "a slope", originally in the military sense of fortification for the outside of a rampart or sloping wall whose thickness diminishes with height; from Latin "talutium", a gossan zone or slope indicative of gold (probably of Iberian origin). Pl: taluses.
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.
periodicity hypothesis . The proposal, based on the analysis of the stratigraphic records of marine families and genera by David Raup and John Sepkoski Jr. of the University of Chicago, that mass extinctions have occurred approximately every 26 million years from about 250 million years ago to the present.
vergence (ver'-gence). The direction of overturning or of inclination of a fold. The term is a translation of the German "Vergenz", "overturn", coined by Stille (1930, p.379) for the direction in which a geologic structure or family of structures is facing. Cf: facing.
pneumatolysis (pneu-ma-tol'-y-sis). Alteration of a rock or crystallization of minerals by gaseous emanations derived from solidifying magma. Obsolete. Adj: pneumatolytic [petrology].
intergrowth (in'-ter-growth). Interlocking of grains of two different minerals as a result of their simultaneous crystallization.
normal polarity . (a) A natural remanent magnetization closely parallel to the present ambient geomagnetic field direction. (b) A configuration of the Earth's magnetic field with the magnetic negative pole, where field lines enter the Earth, located near the geographic south pole. Cf: reversed polarity.
deviatoric stress [exp struc geol] . A stress tensor of nine components where mean stress is subtracted from each of the three normal stress components. Incorrectly used term for differential stress.
petterdite . A pale gray to pinkish-violet orthorhombic mineral: PbCr2(CO3)2(OH)4•H2O. It is the Cr analogue of dundasite.
carbon spot . (a) A misnomer referring to any apparently black inclusion or imperfection in a diamond. Under dark-field illumination, most "carbon spots" are found to be neither black nor graphite. (b) A black flecklike or flakelike graphite inclusion in the body of a diamond crystal (Liddicoat et al., 1993).
inversion point . (a) A point representing the temperature at which one polymorphic form of a substance, in equilibrium with its vapor, reversibly changes into another under invariant conditions. (b) The temperature at which one polymorphic form of a substance inverts reversibly to another at a specific pressure. (c) More loosely, the lowest temperature at which a monotropic phase inverts at an appreciable rate into a stable phase, or at which a given phase dissociates at an appreciable rate, under given conditions. (d) A single point at which different phases are capable of existing together at equilibrium. Syn: transition point; transition temperature.
Pages
