In
geology,
sima is the name for the lower layer of the
Earth's
crust. This layer is made of rocks rich in
magnesium silicate minerals. Typically when the sima comes to the surface it 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'.