Czamanske, Gerald K. U.S. Geological Survey, Menlo Park, California.
Last reviewed:November 2019
- Cumulus concept
- Cumulate layering
- Mechanics of layering
- Notable mafic complexes
- Economic considerations
- Related Primary Literature
- Additional Reading
In geology, an igneous rock body of large dimensions, 5–300 mi (8–480 km) across and as much as 23,000 ft (7000 m) thick, within which distinct subhorizontal stratification is apparent and may be continuous over great distances, sometimes more than 60 mi (100 km). Although conspicuous layering may be found in other rocks of syenitic to granitic composition that are richer in silica, the great layered intrusions (complexes) of the world are, in an overall sense, of tholeiitic basaltic composition. (They may be viewed as intrusive analogs to continental flood basalts.) Indeed, their basaltic composition is of paramount significance to their origin. Only basaltic melts, originating in the mantle beneath the Earth's crust, are both voluminous enough to occupy vast magma chambers and fluid enough for mineral layering to develop readily. The relatively low viscosity of basaltic melt is a consequence of its high temperature, 2100–2200°F (1150–1200°C), derived from the mantle source region, and its silica-poor, magnesium- and iron-rich (mafic) composition. See also: Basalt; Earth; Earth crust; Magma
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