How do elements get concentrated in the Earth's crust?

I'm not an expert but since no one else seems to have answered, here's what I understand. There are multiple mechanisms simply because there are lots if things going on on the earth. read more

Oceanic crust has a higher ratio of iron to silicon than continental crust -- it consists of basalt and gabbro (hey, spell check doesn't like "gabbro"), so it contains about 50% silica. Continental crust has a higher ratio of silica, probably because semi molten rock rising through the crust tends to melt crustal rock on the way up, and it mixes somewhat. read more

Oceanic crust, being so thin, is a very small fraction of the Earth -- about 0.1 percent -- but its life cycle serves to separate the contents of the upper mantle into a heavy residue and a lighter set of basaltic rocks. read more