Of a surface or layer, spanning time. The word diachronous is needed most commonly when the surface or layer did not form instantaneously. The summer surface may be diachronous, forming at different t
imes over a span of days or weeks, but it is assumed to be instantaneous. In a record of a ground-penetrating radartraverse, a marker horizon may be valuable in the determination of mass balance if it is an isochrone, but not if it is diachronous.
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surfac
e alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. The boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure, is gradational. After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization.
An optically and physically thin layer of ground-level cloud composed of small ice crystals that settle slowly. Typically diamond dust forms by the mixing of relatively moist air from aloft into a low
-level inversion layer in which the temperature is 40
A type of precipitation composed of slowly falling, very small, unbranched crystals of ice which often seem to float in the air; it may fall from a high cloud or from a cloudless sky, it usually occur
s under frosty weather conditions (under very low air temperatures).