Bands of magnetic material in the sea floor that have opposite poles, or exhibit magnetic reversal, can provide evidence for sea floor spreading.

As new oceanic crust forms at mid-ocean ridges, it records the direction of Earth’s magnetic field. Because Earth’s magnetic field has reversed many times, the sea floor contains alternating bands of normal and reversed magnetic polarity on either side of the ridge. This pattern supports the idea that the sea floor spreads outward.

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