THE baffling way Earth鈥檚 magnetic poles wander and occasionally flip may be
caused by a battle for supremacy between the inner and outer core, a
geophysicist suggests.
Every 300 000 years or so, the Earth鈥檚 magnetic field spontaneously reverses
direction. Even more often, the magnetic poles move more than 45掳 away from
their original location and then return to it. During such 鈥渆xcursions鈥�, the
field strength can vary enormously over just a few thousand years. 鈥淭he magnetic
field has lost half its strength since Roman times,鈥� says David Gubbins, a
geophysicist at the University of Leeds.
Nobody knows what drives this strange behaviour, but Gubbins has now
suggested that it is caused by the interplay between the Earth鈥檚 liquid outer
core and the solid inner core. He thinks the fluid iron in the outer core is
responsible for the excursions. The magnetic field changes in response to the
flow of the liquid iron, which typically moves 10 or 20 kilometres per year.
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The inner core is resistant to change and usually makes the outer core return
to its original state, he says. Once in a while, however, the outer core鈥檚 new
magnetisation is so strong that it makes the inner core follow suit. 鈥淭he field
reverses in the inner core, and the magnetic field reverses everywhere,鈥� says
Gubbins, whose theory appears in this month鈥檚 Geophysical Journal
International (vol 137, p F1).
Gary Glatzmaier, a geophysicist from the University of California at Santa
Cruz, says this suggestion fits with the results emerging from computer
simulations of geomagnetic change. 鈥淎 lot of the time, it happens only in the
outer part of the core,鈥� he says. 鈥淚t takes time to work down to the inner core.
Often it dies out before it succeeds.鈥�