Crust (geology) magma ocean and floated to the top; however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water. The Martian meteorite ALH84001 might represent primary crust of Mars; however, again, this is debated. Like Earth, Venus lacks primary crust, as the entire planet has been repeatedly resurfaced and modified. Secondary crust is formed by partial melting of silicate materials in the mantle, and so is usually basaltic in composition. This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and
Sial Sial In geology, the term 'sial' refers to the composition of the upper layer of the Earth's crust, namely rocks rich in silicates and aluminium minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but "sial" is a geochemical term rather than a plate tectonic term. As these elements are less dense than the majority of the earth's elements, they tend to be concentrated in the upper layer of the crust. Geologists often refer to the rocks in this layer as felsic, because they contain high levels of feldspar, an aluminium