Outer core, internal Earth structure. Artwork showing the appearance of the Earth's outer core. This is thought to be a region of liquid iron and nickel surrounding an inner core of a solid iron-nickel alloy. The internal structure of the Earth is divided into the outer crust, the upper and lower mantle, and the outer and inner core. For examples of the material composition of each layer of the Earth, see images C018/0208 to C018/0212.