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Concept
Geomagnetic excursion
Résumé
A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth's magnetic field. Unlike reversals, an excursion is not a "permanent" re-orientation of the large-scale field, but rather represents a dramatic, typically a (geologically) short-lived change in field intensity, with a variation in pole orientation of up to 45° from the previous position.
Excursion events typically only last a few thousand to a few tens of thousands of years, and often involve declines in field strength to between 0 and 20% of normal. Unlike full reversals, excursions are generally not recorded around the entire globe. This is certainly due in part to them not registering well in the sedimentary record, but it also seems likely that excursions may not typically extend through the entire global geomagnetic field. There are significant exceptions, however.
Except for recent periods of the geologic past, it is not well known how frequently geomagnetic excursions occur. Unlike geomagnetic reversals, which are easily detected by the change in field direction, the relatively short-lived excursions can be easily overlooked in long duration, coarsely resolved, records of past geomagnetic field intensity. Present knowledge suggests that they are around ten times more abundant than reversals, with up to 12 excursions documented within the current reversal period Brunhes–Matuyama reversal.
Geomagnetic excursions for the Brunhes geomagnetic chron are relatively well described.
Geomagnetic excursions in the Matuyama, Gauss and Gilbert chrons are also reported and new possible excursions are suggested for these chrons based on analysis of the deep drilling cores from Lake Baikal and their comparison with the oceanic core (ODP) and Chinese loess records.
Scientific opinion is divided on what causes geomagnetic excursions. The dominant hypothesis is that they are an inherent instability of the dynamo processes that generates the magnetic field.
Source officielle
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