Every so often (in geologic time) Earth's magnetic field does a flip. The north and south magnetic poles gradually trade places in a phenomenon called a geomagnetic reversal. Scientists long thought this happened every ten thousand years or so. However, new evidence from deep ocean cores show that at least two ancient reversals didn't follow that script. One took about 18,000 years to flip and the other took 70,000 years. Such lengthy time lapses could have seriously affected Earth's atmospheric chemistry, climate, and evolution of life forms during the Eocene period of geologic history.
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