Sima is a term combining the dominant elements Silicon and Magnesium (SiMa), referring to the basaltic/mafic rocks of the oceanic crust and the lower continental crust. It contrasts with Sial (Silicon + Aluminium), which refers to granitic rocks of the upper continental crust. Sima rocks are denser (~3.0 g/cm³) than Sial (~2.7 g/cm³), explaining why oceanic crust subducts beneath continental crust.