I've been wondering exactly why the elements are distributed the way they are on Earth. The heavier elements have their origins in the centers of stars, or in supernovae. After the death of the stars, you end up with a dust cloud containing the heavier elements. Later, planets form out of these along with new stars.
If the heavier elements were randomly distributed in these explosions, how do we end up with special deposits of minerals on Earth? We have mines and special areas where the concentrations of different materials are higher. For example, we have iron, gold, nickel and uranium mines. Why aren't these materials uniformly distributed? Why isn't the Earth composed of a substance which is a uniform mixture of all the heavy elements? Why does it seem there is sometimes a preference for like materials to clump together?
The nickel and iron mostly make up the Earth's core, which I guess makes sense in terms of them being fairly heavy while comparatively abundant relative to heavier elements, but what about everything near the surface that we observe?