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basinA huge crater. Lunar basins are multiringed structures that are several hundred kilometers in diameter and were all produced during the first 750 million years of lunar history by intense meteoritic impact. The youngest basins (Orientale, Imbrium, Crisium, Nectaris, and Serenitatis) may have been excavated during a cataclysmic period around 3900 million years ago. The ejecta blankets of basins are extensive and provide the basis for highland stratigraphy. Basins are so defined by their systems of concentric ring structures, most of which are now in the form of ridges and mountain arcs. Basins on the Moon's nearside were filled with lava at some time up to 3000 million years ago to produce maria: the resulting mare has the same name as the basin (e.g. Mare Imbrium). Several unfilled basins (thassaloids) exist on the farside, the largest of which was revealed as a vast depression by the Apollo 15 laser altimeter. At least 28 basins are now known to exist. See also rille; mascons.
the accumulation of water flows with no drainage or with slow drainage in natural or artificial depressions in the earth’s surface. Basins are formed when there are en-closed hollows on the surface and when the water flowing into them exceeds the water lost by evaporation and seepage into the soil. Basins can be permanent or temporary, arising only during periods of the year which abound in water. Basins are classified as freshwater or saltwater, depending upon their chemical composition and the amount of salts dissolved in the water. The physical, chemical, and bio-logical processes in basins progress in various ways, depending upon the type of basin. Reservoirs, ponds, and canals are artificial basins.
negative (hollow) form of relief of the earth’s surface with more or less isometric or slightly elongated outlines. A basin may be closed on all sides or open in one or two directions. On the basis of morphology flat-bottom, bowl-shaped, and other basins are distinguished; by origin basins are divided into tectonic, volcanic (for example, calderas), erosion, glacial (for example, moraine-dammed basins), deflation, and karst. By drainage conditions they are divided into basins through which water flows, basins with outlets, and basins without outlets (in arid regions).
In the geomorphology of the sea floor a distinction is made between basins in the transition zone, which are occupied by geosynclinal seas, and oceanic basins, which are the largest negative forms of relief on the ocean floor.