The Laschamp excursion
Earth's magnetic field nearly collapses for a few centuries, and cosmic rays flood in.
- When
- —
- Duration
- 1 thousand years
- Known to
- ± 1 thousand years
- Era
- Late Pleistocene, Quaternary
Around 41,000 years ago the geomagnetic field weakens to perhaps 5 per cent of its normal strength and briefly reverses before recovering. The event is recorded worldwide as a spike in cosmogenic beryllium-10 and carbon-14 in ice cores and sediments. Claims that it caused extinctions or drove Neanderthals under are speculative and contested, but as a chronological marker it is invaluable, and it is a reminder that the shield we take for granted is not constant.