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
Sima (geology) magnesia. Comparable is the name 'sial' which is the name for the upper layer of the Earth's continental crust. The sima has a higher density (2800 to 3300 kg/m) 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 most dense sima has less silica and forms ultramafic rocks. Sima (geology) In geology, sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in magnesium