There is plenty of evidence to suggest that the core of
Planet Earth consists mostly of iron (around 80%) with the rest probably being
nickel, although there is some doubt about this. It is also thought that there
is an inner core that is solid, surrounded by a liquid outer core. The vast
amounts of iron at the centre of the Earth, and the convection currents in the
outer core, cause the planet to have a magnetic field. This is profoundly
important for life on Earth, because the magnetic field helps to shield the
surface from harmful solar radiation. It also accounts for why compasses point
to the Magnetic North Pole.
There is also evidence that points to the fact that
compasses – had there been any around at the time – would have pointed to the
Magnetic South Pole in past times. Samples of rock from extinct volcanoes show
clearly that there have been periods in Earth’s history when the magnetic
polarity has been the opposite of what it is now, and that there appear to have
been many such switches in the past.
The last full switch in polarity occurred around 780,000
years ago, but there was a partial switch a mere 41,000 years ago. This was not
a long-lasting arrangement, with the polarity returning to its previous state
after only a few hundred years.
These switches do not seem to happen with any regularity,
and it is impossible to predict when the next one will occur.
But why should such switches happen at all? One theory –
which cannot be confirmed given our current knowledge – was proposed in 2012 by
Peter Olson and Renaud Deguen who were working at Johns Hopkins University,
Baltimore, Maryland, USA. They termed their theory “translational instability”.
The idea is that the iron at Earth’s core is not uniformly
distributed but lopsided, with a greater amount having crystallised out on one
side than the other during the cooling period after the initial formation of
the planet.
When scientists speculate about what might have happened in
the distant past, they build models (these days using computers) that can then
suggest what the effects might be were their speculation to be correct. In this
case, a lopsided iron core would – according to Olsen and Deguen’s model –
cause the axis of any magnetic field to shift to the side where there was a
greater concentration of iron. In turn, this would cause irregularities in the
patterns of convection currents in the outer core, which would cause the
polarity to switch from time to time.
The shifting position of the axis in the inner core could
also – according to the theory – account for why Magnetic North is not the same
as True North, and why the position of Magnetic North is not constant. Indeed,
the shift in Magnetic North can be measured at around 35 miles (56 kilometres)
per year, which is extremely rapid in geological terms.
Careful monitoring of the position of Magnetic North, and
calculation of the speed of change, might be one way of determining when
another polarity change is going to occur. Would that make a huge difference to
how we live our lives? It would certainly mean that all navigation systems used
by ships and planes would have to be revised, and it might also cause temporary
chaos to communication systems, but otherwise the period of adjustment should
not be too traumatic. It could, however, make life difficult for animals and
birds that rely on Earth’s magnetic field for their migrations, but even they
should be able to adjust before too long.
© John Welford
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