Where do the Denser Elements go in the Earth?

It is my understanding that the interior of the earth is molten, and previously the entire earth was molten during some stage of developement either during formation or most certainly during the huge cataclysm when it was struck by a large object to glob off a molten piece to form the moon. My question is: why didn't all the heavy material sink to the bottom? I'd expect the earth to be like a big gobstopper of layers in order of density of the material. Yet we find Gold and Uranium and Lead up here, when it should be at the center of the earth in the heaviest layers! Why didn't the heavier elements sink, creating a lead or uranium center instead of an iron center?
- Johnathan Sandrell (age 16)
Urbana, IL USA

This was really a great question, Johnathan. The process you're talking about is called "planetary differentiation," the process by which elements, compounds, alloys, mixtures, minerals, etc. flow in the early stages of a planet's formation. The most powerful force that causes this primordial mixture to settle into what we have today is, as you correctly expected, the differences in densities between the elements and compounds in the molten early Earth. However, elements like uranium and thorium are "lithophilic," which is a term coming from the Greek words meaning "rock-loving." This means that these elements, when molten, easily form compounds with silica, which is very abundant in the Earth's mantle. These compounds are less dense than iron and rise to the surface. Elements like silver and mercury usually form compounds with sulfurs and also silica. The compounds they form are less dense than iron, but more dense than the lithophiles, so they are still a bit more rare on the surface. Gold is "siderophilic," which means "iron-loving." This accounts for its relative rarity on the surface, because gold will form compounds with iron and sink with it to the core, leaving only trace amounts here on the surface. -Jim

(published on 01/19/2011)