Earth´s inner core and outer core are both
made of an iron-nickel alloy. The state of
matter (solid, liquid or gas) of a given
material depends on its temperature and
pressure. Most materials, including iron and
nickel, change from liquid to solid at lower
temperatures and/or higher pressures. It´s easy
to understand going from a solid to a liquid at
higher temperatures because we are all familiar
with ice melting on a warm day. Why do materials
change from liquid to solid at higher pressure?
For almost all materials (except water), atoms
are packed closer together in the solid state
than in the liquid state. So when you squeeze
the atoms together with high pressure the
material changes from liquid to solid.
As you go deeper in the Earth both
temperature and pressure increases. Although the
inner core is very hot, it is solid because it
is experiencing very high pressure. The pressure
in the outer core is not high enough to make it
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