What serves as a natural "tape recording" of the history of the Earth's magnetic field reversals?
antarctic ice cap
In 1950s scientists discovered zebra-like pattern of the ocean floor magnetic polarity. It turned out that stripes of rock parallel to the mid-ocean ridge crest alternate in magnetic polarity (normal-reversed-normal, etc.), suggesting that they were formed during different epochs documenting the normal and reversal episodes of the Earth's magnetic field.
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Illustration: NASA Goddard