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USGS CMG InfoBank: Polarity In Rocks of the Same Age

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Comment: 15:43 - 16:52 (01:09)
Source: Annenberg/CPB Resources - Earth Revealed - 5. The Birth of a Theory
Keywords: "Scott Bogue", paleomagnetism, "magnetic field", "magnetic reversal", USGS, polarity

Our transcription: There was much scientific debate at first because it's known that some rocks can be placed in a magnetic field pointing one way and become magnetized exactly opposite to that direction.
The fact that such rocks existed meant that it was possible that reversely magnetized rocks were simply recording normal magnetic field.
The clinching evidence came when scientists from the U. S. Geological Survey and from other places went on expeditions around the world, collected rocks of different ages, and were able to show that rocks collected from around the world of the same age always had the same polarity.
Rocks that were younger than thousand years old were normally magnetized.
Rocks older than 700 thousand years were reversely magnetized.
This global pattern of magnetic polarity recorded in the rocks is what convinced people that it was the magnetic field itself changing polarity.

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