In a recent paper
published in Science geologists from Texas
A&M University and the Scripps Institute in California have
suggested that there was a 16 to 20° shift in the earth's
rotational pole at around 84 m.y. ago. Real polar wandering!
Just when you thought that you understood the concept that "Polar Wandering" doesn't actually mean that the poles were moving; it looks as if the rules are about to change! It now appears that at certain times the poles actually have moved - and quite rapidly too.
In a recent paper
published in Science geologists from Texas
A&M University and the Scripps Institute in California have
suggested that there was a 16 to 20° shift in the earth's
rotational pole at around 84 m.y. ago.
The theory is based on magnetic data from 27 Pacific seamounts. The data show a fairly typical "apparent" polar wandering curve (related to continental drift) for the period from 125 to 39 m.y. - but there is evidence of a major jump in the pole position at around 84 m.y. The authors can't yet pin-point the time of the shift, but they suggest that it might have taken around 2 m.y. to complete - which implies a rate of change of just over 1 metre per year - roughly ten times faster than the fastest plate motion rates. They suggest that the shift resulted in a change in the earth's spin axis caused by a movement of mass either on or within the earth.
A spinning earth, like any other spinning object, is most stable when most of its mass is as far away from the spin axis as possible (ie. close to the equator). If a large mass moves towards one of the poles there will be a tendency for the object to roll to bring that mass back towards the equator. In the case of the earth, the spinning axis will still be pointing towards the same stars in space, but the pole location will be a different point on the surface.
The displacement of
mass required to cause such a shift could be related movement of
plates, or to a change in the distribution of material within the
mantle. The authors of the paper in Science suggest that the
latter is the most likely case, and they point to the end of the Cretaceous long-normal
superchron, at 83 m.y., as
evidence that there was a major reorganization of the mantle at
this time.
One possible trigger for a major change within the mantle is the disposition of subducted oceanic slabs. It is predicted that the phase change in mantle mineralogy at a depth of around 660 km could affect the buoyancy of subducted slabs, and that in some cases the slabs may get "hung up" in this region, and then break through en masse - abruptly shifting the distribution of mass within the earth.
References
Sager, W. and Koppers, A., , Late Cretaceous Polar Wander of the Pacific Plate: evidence of a rapid true Polar Wander, Science, V. 287, p. 455-459 (January 2000)
Kerr, R., Did the dinosaurs live on a topsy-turvy earth?, Science, V. 287, p. 406-407 (January 2000)
Steven Earle, 2000. Return to Earth Science News