The compass has pointed south
Earth's magnetic field has reversed, many times. The ocean floor is striped with the record: rock that froze while the compass pointed the other way.
New crust forms at the mid-ocean ridges, already an entry here as the longest mountain chain. As the basalt cools it locks in the direction of the magnetic field at that moment. The field reverses at irregular intervals, from tens of thousands of years to tens of millions. The next strip of crust, formed after a reversal, locks in the opposite direction. Vine and Matthews read the result in 1963: a barcode of normal and reversed magnetisation, symmetric on either side of the ridge, older as you sail away from it.
A reversal is not a day when every compass spins and falls. The field weakens, wanders, and rebuilds with the opposite polarity, over thousands of years. Life has been through this often. What the stripes settled was the motion of the plates, because a tape recorder on the ocean floor is a hard thing to argue with. The last full reversal was about 780,000 years ago. The field is restless now. Restless is not a schedule.
Caveats
- A reversal is recorded in the rocks. It is not a prediction that one is due this century. The present weakening is measured. A date for the next flip is not.
- During the transition the field does not simply vanish. It becomes weaker and more complicated. 'Pointed south' describes the stable state on either side, not a single morning.
Sources
- 1Vine F. J., Matthews D. H.. Magnetic anomalies over oceanic ridges. Nature 199, 1963.