Cutaway illustration showing the Earth's interior layers. At Earth's centre is the core, which has an inner solid region (bright yellow) and an outer liquid region (bright orange), both are composed of iron and nickel. Above it is the mantle (dark orange), a solid layer about 3000 kilometres deep, formed of silicates rich in iron and magnesium. The outer layer, the crust, is about ten kilometres thick under the oceans and about forty kilometres thick under the continents. It is composed of sedimentary rocks, granite and basalt, that have been mixed under the effect of the intense geological activity of the Earth.