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noun Geology.
  1. an assemblage of rocks, rich in silica and magnesium, that constitutes the lower layer of the earth's crust and is found beneath the ocean floors and the sial of continents.

Origin of sima1

< German (1909), equivalent to Latin si(licium) silicium + ma(gnesium) magnesium
Related formssi·mat·ic [sahy-mat-ik] /saɪˈmæt ɪk/, adjective
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British Dictionary definitions for simatic


  1. the silicon-rich and magnesium-rich rocks of the earth's oceanic crust, the most abundant individual rock being basalt
  2. the earth's continental lower crust, probably comprised of gabbro rather than basalt
Derived Formssimatic (saɪˈmætɪk), adjective

Word Origin

C20: from si (lica) + ma (gnesia)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012