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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anemometer (an-e-mom'-e-ter). Any type of instrument that measures the speed of wind.

alexandrite effect . Said of a mineral or gemstone whose color changes in hue when it is viewed under two different light sources.

Pennsylvanian (Penn-syl-va'-ni-an). The upper subsystem of the Carboniferous System of the Standard Global Chronostratigraphic Scale, above the Mississippian Subsystem and below the Permian System. Also the time during which these rocks were formed, the Pennsylvanian Subperiod, covering the time span between 320 and 295 Ma. It is named after the State of Pennsylvania, in which rocks of this age are widespread and yield much coal. It is the approximate equivalent of the Upper Carboniferous of some chronostratigraphic scales.

bathythermograph (bath-y-ther'-mo-graph). An oceanographic instrument that measuress temperature as a function of depth within a few hundred meters of the sea surface. Obsolete, except as expendable bathythermograph.

eruption magnitude . The size of an eruption, expressed as km3 of magma or as kg mass.

dryline . A boundary betweeen continental tropical air mass and maritime tropical air mass in the southeast sector of a mature extratropical cyclone; it is a likely site for squall line and severe thunderstorm development.

chute [speleo] . An inclined channel or passage in a cave.

topsoil . (a) The layer of soil moved in cultivation. Frequently designated as the Ap layer or horizon. (b) Presumably fertile soil used to topdress roadbanks, gardens, and lawns.

sandstone pipe . A clastic pipe consisting of sandstone. It may originate in various ways: gravitational foundering of sand into underlying water-saturated mud; filling of a spring vent; filling of a cavity caused by solution of underlying limestone or by volcanic explosion; or penecontemporaneous sag due to removal of support by flowage. Syn: sandstone cylinder.

rhodostannite (rho-do-stan'-nite). A metallic reddish tetragonal mineral: Cu2FeSn3S8 . It may contain appreciable Ag.

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