Final answer:
Rock strata show evidence of Earth's magnetic field instability through recorded magnetic field reversals over time.
Explanation:
Rock strata provide evidence that Earth's magnetic field is not stable through paleomagnetism, which records the remnant magnetism in rocks. By studying rock strata, geologists have observed magnetic field reversals over time, where the Earth's magnetic field changes polarity periodically. Iron-bearing minerals in rocks align with the Earth's magnetic field at the time of their formation, providing a record of past magnetic field orientations.
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