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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erosion caldera . A type of caldera that is developed by the erosion and resultant widening of a caldera or by erosion of a volcanic cone, resulting in a large, central cirquelike depression. It is considered by some not to be a true caldera type, since it is not formed by volcanic processes. Cf: explosion caldera; collapse caldera.

anal (a'-nal). adj. Pertaining to or situated near the anus of an animal; e.g. an "anal opening" representing a large thecal orifice marking the position of the anus in the CD interray of a blastoid or crinoid. n. anal plate.

colloid (col'-loid). (a) A particle-size range of less than 0.00024 mm, i.e. smaller than clay size (U.S. Naval Oceanographic Office, 1966). (b) Originally, any finely divided substance that does not occur in crystalline form; in a more modern sense, any fine-grained material in suspension, or any such material that can be easily suspended (Krauskopf, 1967).

fault valley . A linear depression produced by faulting; e.g. a small, narrow valley created within a major fault zone by relative depression of narrow slices, or a large graben situated between tilted block mountains, or a valley created by relative uplift on opposite sides of two parallel thrust faults. Cf: fault-line valley.

Warburg impedance (War'-burg). The ion-diffusion impedance that arises at the interface between an electronic conductor, such as a sulfide grain, and an electrolytic conductor (Sumner, 1976, p.255).

compound cuspate bar . A bar formed by a compound spit that unites with the shore at its distal end (Johnson, 1919, p.319).

mound [arch] . A small, human-made hill, composed either of debris accumulated during successive occupations of the site (syn: tell; teppe ) or of earth heaped up to mark a burial site.

palisade tissue . Tissue composed of long cylindrical chlorophyllous cells oriented normal to the lamina beneath the upper epidermis of angiospermous leaves. Syn: palisade mesophyll.

iridium (i-rid'-i-um). (a) An element of the platinum group, symbol Ir. (b) A metallic white cubic mineral consisting of more than 80 percent Ir, the remainder being osmium, palladium, or related elements.

granular chert . A compact, homogeneous, hard to soft chert, common in insoluble residues, composed of distinguishable and relatively uniform-sized grains, characterized by an uneven or rough fracture surface and by a dull to glimmering luster (Ireland et al., 1947, p.1486); it may appear saccharoidal. See also: granulated chert. Cf: smooth chert; chalky chert. Syn: crystalline chert.

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