Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in silicates and magnesium minerals. Typically the sima when it comes to the surface is basalt, so sometimes this layer is called the 'basalt layer' of the crust. The sima layer is also called the 'basal crust' or 'basal layer' because it is the lowest layer of the crust. Because the ocean floors are mainly sima, it is also sometimes called the 'oceanic crust'. Earth cutaway from core to exosphere. ... In chemistry, a silicate is a compound consisting of silicon and oxygen (SixOy), one or more metals, and possibly hydrogen. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... Basalt Basalt is a common gray to black volcanic rock. ...

The name 'sima' was taken from the first two letters of silica and of magnesium. Comparable is the name 'Sial' which is the name for the upper layer of the Earth's crust. A petrologic name for the upper layer of continental crust that is composed of rocks rich in silica and aluminium. ...

Petrology

The sima has a higher density (2800 to 3300 kg/m3) than the sial, which is due to increased amounts of iron and magnesium, and decreased amounts of aluminium. When the denser sima comes to the surface it forms mafic rocks, or rocks with mafic minerals. The densest sima has less silica and forms ultramafic rocks. A petrologic name for the upper layer of continental crust that is composed of rocks rich in silica and aluminium. ... In geology, mafic minerals and rocks are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. ... Ultramafic (or ultrabasic) rocks are igneous rocks with very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). ...

References

• Bates, R.L., and Jackson, J.A., (1987) Glossary of geology American Geological Institute, Alexandria, Virginia.
• Dilek, Y. and Newcomb, S. (eds.) (2003) Ophiolite Concept and the Evolution of Geological Thought Geological Society of America Special Paper 373, Boulder, Colorado.