Résumé
The suggestion that extremely rapid field changes, called geomagnetic impulses, occur during a geomagnetic reversal is investigated. A quite simple cooling model, which nonetheless takes into account latent hea t of crystallization and variable heat capacity and thermal conductivi ty, was applied to a flow that appears to have cooled during an impuls ive change in the transitional field. Thermal and palaeomagnetic data are compatible with the hypothesis that the large changes in remanence directions observed within this flow result from a rapid field change recorded as successive partial thermal remanent magnetizations acquir ed during the initial cooling. Reasonable variants of the model sugges t that the rate of change of the field would have been approximately c onstant, with extremely rapid speeds ranging from 8 to 4 degrees per d ay for cooling intervals between 10 and 20 days, respectively.