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[b]Possible Pole-Shift Mechanism [/b]
Pole shift is generally believed to be due to the gradual redistribution of mass in outer Earth's mantle and the crust. Our planet is most stable when its most massive parts are farthest from its spin axis; that is, on the equator. If mass redistribution were to occur somewhere on an otherwise uniform Earth, the planet would reorient itself so that the mass concentration would move to the equator. Over the past century, the north pole moved about 33 feet toward eastern Canada. Most geophysicists believe the movement is due to true polar wander (TPW), or a shift of the Earth's entire mantle and crust relative to Earth's core.
Think of Earth’s spin axis being fixed relative to the stars and the crust and mantle slowly moving like the skin and flesh of a peach over its pit. If we observe the point (pole) where the spin-axis meets Earth’s surface, the pole will appear to shift if the crust and mantle move horizontally beneath it. This is pole shift of the TPW type. The other type of pole shift, not considered here, involves the movement of the Earth's spin axis relative to fixed stars..
V. Courtillot and J. Besse wrote a provocative paper on TPW in 1987. They proposed that the emission of hot thermal currents of material in the boundary layer between Earth’s core and mantle leads to increases in mantle convection, to mass anomalies near Earth’s surface, and, ultimately, to true polar wander. I’ll assume here that the scientists’ “hot thermal currents” are equivalent to the “upheavals in the interior of the Earth”.