The Jaramillo reversal was a reversal of the Earth's magnetic field that occurred approximately one million years ago. In the geological time scale it was a "short-term" positive reversal in the then-dominant Matuyama reversed magnetic chronozone; its beginning is widely dated to 990,000 years before the present (BP), and its end to 950,000 BP (though an alternative date of 1.07 million years ago to 990,000 is also found in the scientific literature).
The causes and mechanisms of short-term reversals and excursions like the Jaramillo, as well as the major field reversals like the Brunhes–Matuyama reversal — The Matuyama reversed chronozone flipped into the Brunhes positive chronozone c. 780,000 BP — are subjects of study and dispute among researchers. One theory associates the Jaramillo with an impact event that occurred at the time, as evidenced by a tektite strewnfield in the Ivory Coast — though this hypothesis has been described as both "highly speculative" and "refuted."
- Herrero-Bervera, Emilio, and S. Keith Runcorn. "Transition Fields during the Geomagnetic Reversals and Their Geodynamic Significance." Philosophical Transactions: Mathematical, Physical, and Engineering Sciences, Vol. 355 No. 1730 (September 15, 1997), pp. 1713–42.
- Courtillot, Vincent. Evolutionary Catastrophes: The Science of Mass Extinction. Cambridge, Cambridge University Press, 1999; p. 104.
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