The universe is very far from being chemically uniform, which is
a result of the way the universe developed. The earliest stars,
perhaps thirteen billion years ago, condensed from a thin gas of
hydrogen. As a star condensed, the heat generated in its center
triggered the hydrogen fusion process, in which four hydrogen nuclei
coalesce to a helium nucleus with the release of a large amount
of energy. The star "switched on." In big stars with sufficient
ability to retain heat, higher temperatures in the center led to
the successive triggering of helium fusion and then to reactions
producing the heavier elements. The stars were the "crucibles" in
which the heavier elements were formed. Supernova explosions scattered
these elements through the cosmos as debris from which, in time,
the second-generation suns such as our own formed.
Our solar system was thus enriched in heavy elements from its very
beginning. As the sun coalesced at the center of a cloud of diffused
matter, so did the various planets farther out. The large planets
with enough gravitational pull to retain all of their original material,
such as Jupiter and Saturn, remained sunlike in overall composition.
The Earth and the other small inner planets had their volatile elements
driven away by the heat of the sun and by the weakness of their
own attraction for them. The only substances left were the nonvolatiles;
thus Earth became a denuded ball of rock. This is why our planet
is so rich in silicon-oxygen minerals today; these were the substances
that would not boil away.