As you can see qualitatively from the graph above and quantitatively from the abundance table on the following page, the ratio of oxygen to nitrogen in the entire universe is only three to one, but on Earth the ratio is 160,000 to one! The fact that 80% of our atmosphere today is nitrogen gas is deceptive; most of the oxygen on our planet is locked up in solid compounds beneath our feet, not as gas in the atmosphere.

The final fractionation and enrichment took place within the early fluid planet. This was the stratification into core, mantle, and crust, mentioned in Chapter 6. The planet as a whole has a composition approximately that of an iron core plus FeMg(Si), or olivine. In contrast, the crust is enriched in the cations found in feldspars, Na, Al, K, and Ca, and poorer in Mg and Fe.