Earth layers, computer artwork. The external layer shows the Earth's surface topography and atmosphere, including land, water and clouds. This surface layer extends downwards for around 35 kilometres as the rocky crust. The mantle (red) is a viscous layer of rocks under high pressures and temperatures, extending downwards to a depth of around 2890 kilometres. The outer core (yellow) is a liquid layer of iron and nickel, around 2260 kilometres thick. The inner core (top) is a liquid sphere of a iron-nickel alloy, with a radius of 1220 kilometres.