Definition of sima

i. The basic outer shell of the Earth; under the continents it underlies the sial, but under the oceans it directly underlies the water. Originally, the sima was considered basaltic in composition with a specific gravity of about 3.0. In recent years, it has been suggested that the sima is peridotitic in composition with a specific gravity of about 3.3. First used in its present form and spelling by Suess.

Ref: AGI

ii. A petrologic name for the lower layer of the Earth's crust, composed of rocks that are rich in silica and magnesia. It is equivalent to the oceanic crust and to the lower portion of the continental crust, underlying the sial. Etymol: an acronym for silica + magnesia. Adj: simatic.

Synonym of: intermediate layer, basaltic layer

Compare with: sial, sialma

Ref: AGI