Ask A Scientist General Science Archive

Question:  Why is the earth inner core a solid if the mantle 
and outer core are mostly liquid?
 amanda f crenshaw

Answer:
From all evidences the geologists have, we can say that the Earth core
is essentially metallic iron with additional components: 10% metallic
nickel and some 20% of either sulfur or silicon. The boundary of the
inner core is about 5100 kilometers (3200 miles). At the center of
the Earth, the pression due to the upper layers is very high
(3000-5000 kilobars). Under that pression the iron density, as demonstrated
for some measurements and calculations increases drastically, increasing
also its fusion point (that is the temperature that makes it turn to
liquid.  So the inner core can be considered, as you said, to be
substantially rigid, that is, solid. But at the boundary of the outer and
inner core there is present a rather complex fine structure and also
there must be present at least two seismic discontinuities within a
transition zone whose thickness must be of several hundreds kilometers..
The structure of that transition zone may be a consequence of chemical
inhomogeneity and phases changes during crystallization of a relati-
vely pure iron-nickel inner core from a less pure liquid that
contains larger amounts of the third component, sulfur or silicon.
 
Mabel