This paper Deep-sea sediment records of the Laschamp geomagnetic field excursion (∼41,000 calendar years before present) concludes that the narrow Laschamps excursion event, or class I excursions in general, are more closely related to normal secular variation and are not necessarily a prelude to magnetic field reversal. So what are they and what causes them? The geomagnetic field suffered for some time and so one would imagine that life did as well: "magnetic field intensities less than 10% of normal that persisted for almost 2000 years". How relevant is this event to the megafaunal extinctions for example? Wikipedia Geomagnetic Excursions Wikipedia Earth's Magnetic Field .