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ultimately dissipated as heat in the outer core and so contributes to the heat flow
into the mantle.
The formation of the core, the evolution of the core and mantle and the mag-
netic field are thus inextricably linked. We know that the Earth has had a magnetic
field since at least the early Archaean, so the inner core must have formed early,
with crystallization proceeding at a sufficient rate to power a dynamo. How-
ever, the crystallization of the inner core has proceeded sufficiently slowly that
the inner core is still only a fraction of the core. The Rayleigh number of the
flow in the outer core must be fairly low. Since the mantle contains only low
concentrations of Ni and S, the Ni and S in the core must date from the time of
core formation. Ni and S can both alloy with Fe at low pressures, so it is probable
that Fe alloyed with the available Ni and S early during the process of core forma-
tion. This explains their low concentrations in the mantle and means that the Ni
level in the Earth matches its cosmic abundance. However, it is thought that some
Smay have been lost during the accretion of the Earth because even allowing
for the maximum amount of S in the core that the seismic data imply still leaves
the Earth as a whole depleted in S compared with cosmic abundances. Unlike Ni
and S, oxygen cannot have been an early constituent of the core because it does
not alloy with Fe at low pressures. This suggests that core composition has grad-
ually changed with time, with the oxygen concentration of the outer core slowly
increasing as the result of vigorous chemical reactions at the CMB. Numerical
geodynamo models constructed to investigate the magnetic field for a younger
Earth with a smaller core still yield a magnetic field that is primarily dipolar.
However, the pattern of heat flux across the mantle-core boundary (and hence
the pattern of convection in the mantle) may be an important factor in controlling
the complexity of the magnetic field.
Much of what has been suggested in these sections about the core is still not
certain. In our quest to understand the workings of the core we are hampered by the
very high temperatures and pressures that must be attained in experiments, by the
thick insulating mantle which prevents complete measurement and understanding
of the magnetic field, by not having any sample of core material and by not yet
being able to perform realistic numerical simulations. It has not been possible to
construct a laboratory model in which the convective flow of a fluid generates a
magnetic field. Materials available for laboratory experiments are not sufficiently
good conductors for models to be of a reasonable size. Queen Elizabeth I, whose
physician was William Gilbert, regarded Canada as Terra Meta Incognita .Inthis
century the label should perhaps be transferred to the core.
References and bibliography
Ahrens, T. J. 1982. Constraints on core composition from shock-wave data. Phil. Trans. Roy.
Soc. Lond .A, 306 , 37-47.
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