MadSci Network: Earth Sciences

Re: Is polar wandering the movements of the continents?

Date: Tue Nov 28 10:26:36 2000
Posted By: Andrew Elmore, Grad student, Environmental Science, Brown University
Area of science: Earth Sciences
ID: 973205981.Es

Polar wandering is not simply the movements of the continents.  We need to 
define three different terms: apparent polar wander, magnetic polar wander, 
and true polar wander.  Apparent polar wander is what you have described (a 
case where a continent drifted over the pole).  What may be confusing, is 
that magnetic polar wandering is often USED to understand the movements of 
the continents.  I will explain how this is done, but first we must 
understand what magnetic polar wander means on Earth.

The earth'’s magnetic field is not stable. In fact, it changes in magnitude, direction, and polarity through time. Currently the magnetic poles are only about 12 degrees off from the true poles of the earth (the later described by the axis of rotation) however, 500 million years ago, the magnetic poles were much further away. Since then they have migrated to their current position in a long sweeping arc. This movement is relative to the earth’saxis of spin and is caused by changes in the convection of the earth’smantel (the part of the earth between the crust and core, which is somewhat fluid).

Part a) of the figure I have included shows this path as described by rocks found in Europe. To create this figure, rocks are found with ages ranging between 500 million years old and 10 million years old. In each rock, the magnetic information indicates what the earth’s magnetic field looked like when the rock was formed from liquid rock. When compared against the record from rocks in North America (part b) we notice that the results are not the same. In fact, to make these paths the same the position and orientation of the two continents (Europe and North America) would have to have changed through time. This is some of our strongest evidence for our understanding of plate tectonics and continental drift.

Now for the third and most amazing (to me) form of polar wander. True polar wander occurs when the axis of spin of a planet changes through time. This is very difficult to do and there are many debates about this process on Earth and Mars. There was a recent paper in the journal "Science" (v.277, p.541-545) in which the authors describe a case for True polar wander. On Mars, the case is clearer. Scientists have been arguing that there are polar deposits near the equator of Mars. Changes in the distribution of mass (due to volcanism or large impacts) would cause Mars to start spinning on a different axis. Such a change would take a long time to complete but would cause the coldest spots on Mars to move around. Check out:

Remember that scientists are still developing their theories of this type of polar wander. You should expect any science journal to be rather confusing and full of needless details. Nevertheless, there is evidence for true polar wander that is separable from the continental drift and changes in the magnetic poles.

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