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The Shifty Business of True Polar Wander
To understand polar shift (known to geologists as the True Polar Wander, or TPW, hypothesis), it helps to have a clear picture of how the Earth is put together.
The Earth isn't a solid ball of rock; it consists of concentric layers, each with its own heat and density characteristics. The outermost layer, the crust, is made up of rocky, interlocking pieces. These aluminum-silicate plates float like rafts atop a molten outer mantle, which surrounds a more fluid inner mantle. Farther in, a liquid nickel-iron outer core encompasses the Earth's solid, iron inner core. Put another way, the Earth consists of a solid shell surrounding a liquid interior, which encircles a solid center.
Most of the Earth's internal heat is stored in the mantle. There, temperature differentials cause convection -- the same process observed in a pot of boiling water, except it takes place over hundreds of kilometers and involves what Dr. Evil would call "liquid hot magma." Hotter magma rises toward the crust while cooler, denser materials -- such as subducted oceanic plates -- sink toward the core [source: Sager]. Convection drives tectonic processes and also redistributes the internal mass of the Earth.
Above the mantle, the crust tilts, rocks, sinks and rebounds in response to changes in pressure and load, such as those that occur after an ice age, when glacial ice returns to the sea as meltwater. The motion is like how a boat reacts to a person exiting or climbing aboard, only much slower.
When internal and/or surface mass distributions become uneven, TPW might occur, because the centrifugal force of the Earth's spin drives mass anomalies -- whether on the crust or in the mantle -- toward the equator. Some geologists argue that this has happened in the past. One possibility occurred about 800 million years ago; another, 510-610 million years ago, might have caused climate shifts that helped bring about the Cambrian explosion -- the relatively rapid appearance of most of the major groups of animals in the fossil record [source: Maloof].
Some scientists believe that a polar shift is happening right now, at a rate of around nearly 4 inches (10 centimeters) per year [source: Tarduno]. This gradual "righting of the boat" is a physical response to the retreat of the Laurentide ice sheet at the end of the Pleistocene Epoch, 20,000-plus years ago [source: Maloof]. But don't pack the kids into John Cusack's limo just yet; this rate, although fast by plate tectonic standards, is still very, very slow. In fact, TPW takes 1 million to 100 million years to complete an adjustment -- the geological equivalent of watching fingernails grow -- and the current one will stop before making much progress [source: Tarduno; Kirschvink].
However you look at it, rapid polar shifts, like the kind portrayed in the movie "2012," simply don't happen.