The Laschamp event was a short reversal of the Earth's magnetic field. It occurred 41,400 (±2,000) years ago during the last ice age and was first recognised in the late 1960s as a geomagnetic reversal recorded in the Laschamp lava flows in the Clermont-Ferrand district of France. The magnetic excursion has since been demonstrated in geological archives from many parts of the world. The period of reversed magnetic field was approximately 440 years, with the transition from the normal field lasting approximately 250 years. The reversed field was 75% weaker whereas the strength dropped to only 5% of the current strength during the transition. This reduction in geomagnetic field strength resulted in more cosmic rays reaching the Earth, causing greater production of the cosmogenic isotopes beryllium 10 and carbon 14.
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- Bonhommet, N.; Zähringer, J. (1969). "Paleomagnetism and potassium argon age determinations of the Laschamp geomagnetic polarity event". Earth and Planetary Science Letters. 6: 43–46. doi:10.1016/0012-821x(69)90159-9.
- Helmholtz Association of German Research Centres (16 October 2012). "An extremely brief reversal of the geomagnetic field, climate variability and a super volcano". Retrieved 19 October 2012.