The answer is the heaviest elements settled in the Earth's center.
The interior structure of the Earth is layered in spherical shells, like an onion. The geologic component layers are called:
CRUST --The thin, outermost layer of the earth is called the crust. It makes up only one percent of the earth's mass. This consists of the continents and ocean basins.
MANTLE -- The mantle is a dense, hot layer of semi-solid rock approximately 2,900 km thick and is composed mainly of ferro-magnesium silicates. This is where most of the internal heat of the Earth is located.
The heaviest elements settled in the Earth's center.
The Earth on the other hand starts with a core of iron and radioactive materials such as uranium and plutonium, which release energy in the form of heat, through a process known as nuclear fission. It went through a fusion stage - a "liquid-pasty" state - which allowed that, due to gravity, the denser materials sank towards the center, while the lighter ones floated towards the crust, a process called planetary differentiation.
