The earth's core is mostly made up of iron and has a radius of about 760 miles. It is surrounded by a liquid outer core composed of a nickel-iron alloy.
Unlike the mineral-rich crust and mantle, the core is made almost entirely of metal—specifically, iron and nickel. The shorthand used for the core's iron-nickel alloys is simply the elements' chemical symbols—NiFe. Elements that dissolve in iron, called siderophiles, are also found in the core.