TRACES of the Earth鈥檚 magnetic field frozen in rocks are yielding surprises
about the planet鈥檚 past. A re-analysis of old measurements of these fields has
forced geologists to conclude that either the migrating continents were
clustered closer to the equator than previously thought, or that the Earth鈥檚
magnetic field was not the simple pair of poles it is today.
Geologists track the history of continental motion by measuring the magnetism
of ancient rocks. As some rocks form, they retain an imprint of the Earth鈥檚
magnetic field. The field direction and the age of the rock together show the
latitude of the continent at the time the rock formed, provided that the shape
of the terrestrial magnetic field at the time can be worked out.
Today, the Earth鈥檚 magnetic field lines, which emanate from the poles and
surround the planet, have a simple and predictable distribution. Geologists have
proved that for at least five million years the field has been a dipole, like a
bar magnet with poles aligned on the planet鈥檚 axis. And they calculate ancient
latitudes assuming the field has always been a dipole, says Dennis Kent of the
Lamont-Doherty Earth Observatory in Palisades, New York.
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But now Kent and Mark Smethurst of the Geological Survey of Norway in
Trondheim have analysed palaeomagnetic data from rocks up to 3.5 billion years
old. Instead of the magnetic distribution expected from a dipole, they found an
excess of rocks from older eras with low-angle fields, as if they had formed at
lower latitudes than those predicted by standard models that assume a random
distribution of the early continents (Earth and Planetary Science
Letters, vol 160, p 391). 鈥淭he surprising result is that in the Palaeozoic
and Precambrian, the distributions differ markedly,鈥� Kent says.
One possible explanation is that the Earth鈥檚 magnetic field has not always
been a dipole. Kent calculates that if the ancient Earth contained elements of
between four and eight poles, its magnetic field lines would have met the
migrating continents at lower angles than the lines of the modern dipole field.
That would account for the distribution he and Smethurst observed, he says. Such
an arrangement might have been possible before the solid part of the
core鈥攚hich started growing as late as a billion years ago鈥攔eached
its present size.
The other possible explanation for the findings, Kent says, is that the
continents were once clustered near the equator. Such clustering could be the
result of centrifugal force tilting heavy parts of the outer layers of the Earth
away from the poles (鈥淭wist of fate鈥�, New 杏吧原创,
2 August 1997, p 15).
Gary Glatzmaier of the Los Alamos National Laboratory in New Mexico says his
unpublished simulations of the Earth鈥檚 magnetic field may be able to discover
which explanation is right. According to his models, multiple poles are
unlikely, he says. 鈥淲hen the inner core was smaller, our simulations suggest the
dipole was even stronger than today.鈥� If correct, Glatzmaier鈥檚 results would
mean that geologists have to redraw their maps of the ancient continents.