system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we find concentrations of minerals in the ground instead of a more homogeneous mixture?

I'm trying to rectify my understanding of stellar element genesis with what I know about planetary formation theory. Here's what I (think I) know:

1. All the metals/elements heavier than iron on the periodic table had to have been formed in supernova type events. This has to do with the hydrostatic equilibrium of stars and the energy released by different fusion reactions.

2. Most theories of planet formation seem related the material that is left over after a star is initially formed clumping together into larger masses.

If this is true, then why do we find areas of Earth's crust which are relatively more abundant in different minerals (gold, silver, copper, aluminum, etc)? This this perceived increase in material only a small fluctuation typical of a random distribution of material?

This idea is also related to how we use radioactive dating to find the age of different strata. How do we know the concentrations of specific materials at different times in Earth's history?
gpt
The layered structure of the earth isn't due to the supernova that formed our elements, nor is it due to the clumping process of accretion. The layered structure comes from the process of differentiation, that involves both gravity and alot of chemistry. 

As the earth first began cooling, the heaviest parts of the molten rock began to sink to the core. Lighter mixtures then were relatively more buoyant. As minerals cool near the surface, crystals with the highest melting points crystallize first, and based on the energy gained from crystallization and the local availability of certain elements, crystals grow and consume or exclude various elements during their growth. Once growth of certain ones subsides, and the elemental composition of the melt has changed, as more cooling occurs, a time will come when the temperature is right for another phase to begin to crystallize out of the remaining elements, further consuming or excluding some, until everything is solid. It's quite like a natural zone refining process used in industry actually.  And yes, this is an example of spontaneous organization which some like to think violates the second law, but does not.   

Coinage metals are different because, like salts in the ocean, they can be etched out by hot sea water under high pressure. Currently there's about 20 g/km^3 of sea water in the oceans. Underneath mountain ranges like the Andes for instance, water gets squeezed deep into the rocks during the tectonic processes that build the mountain. The water can become supersaturated with gold and copper. Eventually the mountain gets pushed up and the metals precipitate out in veins that were once water filled crevices.