312 The Poetry of Physics and The Physics of Poetry
than 30 kilometers below the surface cannot be magnetized any longer
because the temperature is too high. It is therefore believed, that the
geomagnetic field originates in the core rather than the mantle. It has
been suggested that the liquid metallic outer core might be able to
produce a field generating electric currents as a result of its fluid motion.
The outer liquid metal core is a conductor of electricity. It is known that
an electric conductor spinning in an applied magnetic field will begin to
conduct a current, which produces its own magnetic field. If energy is
provided to the conductor to maintain its spinning motion it will retain its
current and associated magnetic field, even if the original applied
magnetic field is removed.
This mechanism can explain the geomagnetic field if one can account
for the original field to start the process and the present source of energy
to maintain the dynamo. The triggering magnetic field could have been
produced by electric discharges produced when the Earth first formed.
The energy to drive the dynamo has been attributed to radioactivity by
some. A more attractive hypothesis is that heat is being generated by the
solidification of the inner core, which is slowly increasing its radius. The
dynamo theory is very attractive theory for the origin of the Earth’s
magnetic field because it also provides some hint regarding the origin of
polarity reversals as well. Coupled disc dynamos are known to reverse
their polarity under certain conditions in a manner not unlike terrestrial
polarity flips.
Origin of the Separation of the Core, Mantle and Crust
The origins of the basic structure of the Earth, in particular its separation
into a metal core, a rocky mantle and a crust, remains as puzzling a
mystery as the origin of the geomagnetic field. Two competing theories
for the separation of the core from the mantle seem to attract nearly equal
support. According to one theory the Earth began its existence as a
molten sphere of matter. The heavier material, such as the metal, quickly
settled to the center. According to the rival theory the Earth began its
existence as a relatively cool planet in a non-molten state. The heat
generated by the gravitational accretion of the planet is postulated to
have been radiated away initially so that the planet formed in a solid
state in which the metals and the rocks are thoroughly mixed. It is
alleged that the internal temperature of the Earth has actually increased
in certain places since its formation due to the release of energy through