The Earth’s crust has an average density of 2800 kg/m³ and a thickness varying from about 30 km below the continents (up to 60 km beneath some mountains) to 5 km beneath the oceans. By mapping the major rock types and averaging their composition the abundance of elements can be estimated. The crust is enriched in incompatible elements (e.g. K and Rb) as well as lithophile elements, but a few elements predominate, especially in silicate minerals, while some ore metals are rare (e.g. Cu and Sn). Because the crust was formed from material extruded from the Earth’s mantle, it is to be expected that the mantle is depleted in ‘crustal’ components. Oxygen (O) constitutes almost 50% of the Earth’s crust by weight and is the most abundant element. Other major elements include: silicon (Si), which is the second most abundant, constituting 27.72% of the crust by weight; aluminium (Al) third; sodium (Na); magnesium (Mg); calcium (Ca); and iron (Fe). Other elements, including such desired metals as gold (Au), silver (Ag), and platinum (Pt), are rare in the crust.