black chalk . A bluish-black carbonaceous clay, shale, or slate, used as a pigment or crayon.
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.
volcano (vol-ca'-no). (a) A vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure, usually conical, that is produced by the ejected material. (b) Any eruption of material, e.g. mud, that resembles a magmatic volcano. Obsolete var: vulcano. Pl: volcanoes. Etymol: the Roman deity of fire, Vulcan.
bioerosion (bi''-o-e-ro'-sion). Removal of consolidated mineral or lithic substrate by the direct action of organisms (Neumann, 1966).
ellipsoid (el-lip'-soid). (a) A mathematical figure closely approaching the geoid in form and size. It is generally defined by its equatorial radius and by the reciprocal of the flattening, a/(a-b), where a and b are the equatorial and polar radii. A task of geodesists is the determination of more exact parameters of the ellipsoid. Cf: spheroid [geodesy]; Clarke ellipsoid of 1866; reference ellipsoid. (b) A surface whose plane sections (cross sections) are all ellipses or circles, or the solid enclosed by such a surface. In geology, ellipsoid and spheroid are used interchangeably.
moat lake . A lake of annular shape such as a caldera lake lying between the caldera wall and a resurgent dome (Heiken et al., 2000). (b) A lake in a basin of which the central part is occupied by a bog or fen (Veatch and Humphrys, 1966).
peat ring . A nonsorted circle in peat.
hamulate [palyn] (ham'-u-late). adj. Of spores, having rugulate sculpture of irregularly arranged, winding or angularly bent muri of various thicknesses, forming a mazelike pattern.
electrolysis (e-lec-trol'-y-sis). A method of breaking down a compound in its natural form or in solution by passing an electric current through it, the ions present moving to one electrode or the other where they may be released as new substances.
breunnerite (breun'-ner-ite). A variety of magnesite containing 5-30% iron carbonate.
astronomic parallel . A line or circle on the Earth's surface having the same astronomic latitude at every point. It is an irregular line, not lying in a single plane.
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