After reading up on it to refresh my memory, the
best answer that I can give you is that I don't
think we really know yet. The lower mantle
is denser, more viscous, and apparently more
uniform in its composition and properties than the
upper mantle, given the behavior of seismic
(earthquake) waves traveling through it.
I don't get the impression that much more than
that is known: for example, it's in contact with
the core, which is molten metal. How much of
the metal from the core gets up into the lower
mantle? Could this be responsible for some of the
lower mantle's properties, or is it purely a
function of pressure? What effect does temperature
have on all of this?
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