YES. The inner core is a solid sphere of iron +10
% Nickel, plus a minor amountt of other elements.
Originally the inner core was liquid, but cooling
of the Earth over the past 4.5 billion years has
caused enough heat to be removed such that the
Temperature of the inner core has fallen below the
melting point of Iron alloy at the pressure of the
inner core, which is very, very high.
Good question! The brief answer is: part of the
core is solid, and part is molten (liquid rock).
The outer core starts at a depth of about 2900 km
(1800 miles) beneath the surface of the earth, and
is thought to be molten iron, nickel, and other
heavy elements. The inner core starts at a depth
of about 5150 km beneath the surface, and is
though to be solid. The composition of the inner
and outer core are though to be similar, but the
pressure is so high in the inner core, that the
iron and nickel are stable as a solid, and not as
a liquid! It is probably still very hot in the
inner core (just as hot as the outer core), but it
turns out that really high pressures can prevent
things from melting.
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