Geoscience Reference
In-Depth Information
Figure 8.23.
Four
possible models for
producing the Earth's
main dipole field: (a) a
dipole at the centre of the
Earth, (b) a uniformly
magnetized core, (c) a
uniformly magnetized
core and mantle (tan
I
=
2 tan
) and (d) a current
system flowing east-west
around the core-mantle
boundary. (From Bott
(1982).)
λ
different compositions, the depression of the melting temperature in the liquid
outer core due to impurities may mean that the temperature at that boundary is
below the melting temperature of pure iron. It is essential that the high-pressure
and -temperature phase diagram for iron be well determined, since it controls
both the geochemistry and the geophysics of the core as well as the evolution of
the Earth as a whole.
8.3.2 Convection in the outer core and the Earth's
magnetic field
The first suggestion that the Earth's magnetic field is similar to that of a uniformly
magnetized sphere came from William Gilbert in 1600 (see Section 3.1.2). Carl
Friedrich Gauss (1777-1855) later formally showed that the magnetized material
or the electrical currents which produce the field are not external to the Earth but
are internal. Figure 8.23 shows four possible models for producing the Earth's
main dipole field: (a) a magnetic dipole at the centre of the Earth, (b) a uni-
formly magnetized core, (c) a uniformly magnetized Earth and (d) an east-west
electrical current flowing around the core-mantle boundary. Because the mantle
is composed of silicates (see Section 8.1.5), it is not a candidate for the origin
of the magnetic field. Permanent magnetization of the mantle or core cannot
produce the Earth's magnetic field because temperatures in the deep interior far
