Geoscience Reference
In-Depth Information
That releases a considerable amount of latent heat, which passes through the liquid out-
er core, contributing to the churning of the fluid within it. As the iron or iron-nickel alloy
crystallizes out, impurities within the melt, mostly dissolved silicates, separate out. This
material is less dense than the molten outer core, so it rises through it in a steady rain of
perhaps sand-like particles. It probably accumulates on the base of the mantle like a sort
of upside-down sedimentation, collecting in upside-down valleys and depressions. There
are seismic hints of a very low velocity layer at the base of the mantle that this upward
sedimentation could explain. The sandy sediment would trap molten iron just as ocean sed-
iment traps water. By holding iron within it, the layer provides material that can magnetic-
ally couple the magnetic field generated in the core with the solid mantle. If some of this
material rises in super-plumes to contribute to flood basalts on the surface, it could explain
the high concentrations of precious metals such as gold and platinum in such rocks.
Magnetic dynamo
From the surface, the Earth's magnetic field looks as if it could be generated by a large per-
manent bar magnet in the core. But it is not. It must be a dynamo, with the magnetic field
generated by electrical currents in the circulating molten iron of the outer core. Faraday
showed that if you have an electrical conductor, any two out of electrical current, magnetic
field, and motion will generate the third. That is the principle on which all electrical mo-
tors, generators, and dynamos work. But in the case of the Earth, there are no external elec-
trical connections. Somehow both the currents and the field are generated and sustained by
the convection currents in the core. This is what is called a self-sustaining dynamo. But it
must have needed some sort of kick-start. Perhaps that came from the Sun's magnetic field
before the Earth had one of its own.
The magnetic field on the Earth's surface is relatively simple, but the currents in the Earth's
core that generate it must be far more complex. Many models have been proposed, some
of which, such as the idea of a rotating conducting disc, are purely theoretical. A model
that best accounts for the field we see involves a series of cylindrical cells each contain-
ing spiral circulation produced by the combination of thermal convection and the Coriolis
forces generated by the Earth's rotation. One of the strangest features of the Earth's mag-
netic field, as we will see in more detail in the next chapter, is that it reverses its polarity at
irregular intervals, typically of a few hundred thousand years. At other times there can be
periods of up to 50 million years without a reversal. Evidence of the strength of the field
trapped in individual volcanic crystals suggests that the field might have been stronger than
it is today during such non-reversing periods, or superchrons. The magnetic field is not
precisely aligned with the Earth's axis of rotation. At present, it is inclined at about 11 de-
grees to the Earth's rotation axis. But it hasn't always stayed there. In 1665 it was almost
