what is sial

Sial is a term used in geology to refer to the composition of the upper layer of the Earths crust, which is rich in aluminum silicate minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but it is a geochemical term rather than a plate tectonic term. The uppermost layer of the crust is called the sial, consisting of silicate and aluminum (Si = silicate, Al = aluminum) . On average, the thickness of the sial is up to 25 km from the surface. The continents are composed mainly of lighter rock material formed from silicon and aluminum, so the sial is thick over the continents and very thin or absent on the ocean floor, especially the Pacific Ocean. The average density of the sial is 2.7 gm/cc. Geologists often refer to the rocks in this layer as felsic, because they contain high levels of feldspar, an aluminum silicate mineral series. The name sial was taken from the first two letters of silica and of alumina. Sial is often contrasted to the sima, the next lower layer in the Earth, which is rich in silica and magnesium and is often exposed in the ocean basins.