organic soil materials . Soil materials that are saturated with water and have 174 g kg-1 or more organic carbon if the mineral fraction has 500 g kg-1 or more clay, or 116 g kg-1 organic carbon if the mineral fraction has no clay, or has proportional intermediate contents, or if never saturated with water, have 203 g kg-1 or more organic carbon.
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.
unimodal sediment (u-ni-mod'-al). A sediment whose particle-size distribution shows no secondary maxima (modes).
disorder in minerals (dis-or'-der). In a substitutional solid solution, the random occupation of one atom site in a crystal by two or more different atoms of similar size and charge, or of similar size and different charge if there is a concomitant substitution to balance charges, as in plagioclase, in which (Na and Si) in albite is substituted by (Ca and Al) as the composition approaches anorthite. Cf: order in minerals.
antapex (ant-a'-pex). (a) The posterior end of a dinoflagellate when oriented in the direction of swimming; the corresponding end of a dinoflagellate cyst. (b) The most posterior point of the hypocyst of an acritarch (Williams et al., 2000, p.9). Adj: antapical.
cartography (car-tog'-ra-phy). (a) The science and art of making maps and charts. Syn: chartology. (b) The study of maps as scientific documents and works of art.
lambda transformation . A phase transformation in which the second derivatives of Gibbs free energy with respect to temperature and pressure tend to infinity at the transformation temperature.
crossed acicular structure . A crossed structure consisting of acicular crystallites (generally aragonitic) which show two predominant dip directions; the crystallites are not aggregated into first- or second-order lamellae, but they may locally form groups of a few mutually parallel crystallites (Erben, 1971, p.56; Flajs, 1972, p.59; Bandel, 1977b, p.74; Carter, 1980b).
isomorphism [cryst] (i-so-mor'-phism). The characteristic of two or more crystalline substances to have similar chemical composition, axial ratios, and crystal forms, and to crystallize in the same crystal class. Such substances form an isomorphous series. Adj: isomorphous. Cf: isostructural. Syn: allomerism.
earthflow (earth'-flow). A mass-movement landform and process characterized by downslope translation of soil and weathered rock over a discrete basal shear surface (landslide) within well defined lateral boundaries. The basal shear surface is more or less parallel with the ground surface in the downslope portion of the flow, which terminates in lobelike forms. Overall, little or no rotation of the slide mass occurs during displacement, although, in the vicinity of the crown scarp, minor initial rotation is usually observed in a series of slump blocks. Earthflows grade into mudflows through a continuous range in morphology associated with increasing fluidity. Also spelled: earth flow.
central meridian . The line of longitude at the center of a map projection; the meridian about which the geometric properties of a map projection are symmetric and which is a straight line on the map. It is used to determine the directions of axes of plane coordinates. See also: principal meridian.
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