STICK a penny to the edge of a spinning top and its axis of rotation will shift. Now evidence is emerging that something similar may have happened more than once to the Earth.
The clumping of ancient continents in one hemisphere would have unbalanced the Earth and caused its axis to slowly migrate, geologists suggest. This would mean that a land mass formerly at one of the poles, for instance, would appear to have slipped across the Earth鈥檚 surface. Evidence for this 鈥渢rue polar wander鈥� is sparse because it is hard to distinguish from the normal drift of continents associated with plate tectonics.
However, Earth鈥檚 magnetic field would have tracked any changes in the planet鈥檚 axis of rotation, so Bernhard Steinberger of the Geological Survey of Norway in Trondheim and Trond Torsvik at the University of Oslo, also in Norway, studied the field鈥檚 鈥渇ingerprint鈥� in magnetic minerals formed over 320 million years.
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They found that during several periods, all the continents appeared to be moving in unison relative to the magnetic field. That鈥檚 evidence that the entire Earth was tilting markedly during those intervals, says Steinberger (Nature, ).
