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USGS CMG InfoBank: Magnetic Polarity

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Comment: 22:12 - 23:03 (00:51)
Source: Annenberg/CPB Resources - Earth Revealed - 3. Earth's Interior
Keywords: "James Sadd", "magnetic polarity", magnetism, "magnetic reversal", "magnetic north pole", paleomagnetism, "magnetic south pole"

Our transcription: When a rock is magnetized, its magnetic field is aligned to the magnetic North Pole of the Earth.
This is similar to the way a compass needle points to magnetic north.
This alignment, called "magnetic polarity" is also permanent in magnetized rocks.
In the early 1900s, geophysicists studying lava flows in France made a startling discovery.
The lavas and the big soils underneath them were both permanently magnetized, and their magnetic polarity was identical in orientation.
But that orientation was exactly opposite that of the Earth; that is, the magnetic north pole of the rocks pointed toward the magnetic south pole of the Earth.
This observation was the first clear evidence that the magnetic field of the Earth completely reverses itself.

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