A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth's magnetic field. Unlike reversals however, an excursion does not permanently change the large-scale orientation of the field, but rather represents a dramatic, typically short-lived decrease in field intensity, with a variation in pole orientation of up to 45 degrees from the previous position. These events, which typically last a few thousand to a few tens of thousands of years, often involve declines in field strength to between 0 and 20% of normal. Excursions, unlike reversals, are generally not recorded across the entire globe. This is partially due to them not being recorded well within the sedimentary record, but also because they likely do not extend through the entire geomagnetic field. One of the first excursions to be studied was the Laschamp event, dated at around 40,000 years ago. Since this event has also been seen in sites across the globe, it is suggested as one of the few examples of a truly global excursion.
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—Henry David Thoreau (1817–1862)