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
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.