The premise is wrong in that (according to the most widely accepted theory) the Earth's inner core is primarily composed of iron and nickel with smaller quantities of other elements including heavy metals. read more
It isn't just made of iron. It is made from lots of heavy metals as shown in the table below. The evidence actually says all the earth's heating takes place in the mantle and crust, not the core. All models of the inner Earth depend on indirect evidence. read more
Over geologic time, theinner core grows as the whole Earth cools. At the top of the outer core, iron crystals freeze out and rain into the inner core. At the base of the outer core, the iron freezes under pressure taking much of the nickel with it. The remaining liquid iron is lighter and rises. read more
Earth's core is the deepest, hottest layer, and it's made up of two layers itself: the outer core which borders the mantle and the inner core, which is a ball-shaped layer made almost entirely of metal. The outer core is about 1,400 miles thick, and it's made mostly of a combination (called an alloy) of iron and nickel, along with small amounts of other dense elements like gold, platinum, and uranium. read more