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Public message 3285 SCIENCE Area 19:52 Wednesday 4-Sep-91 From: HENRY SHAW To: MIKE DAVEY Re: EARTH'S MAGNETIC FIELD 3 ....continued from previous Over longer periods of time (hundreds of thousands to millions of years), there is evidence that the dipole field dissapeared completely and then reappeared with the opposite polarity numerous times over geologic history. Such events are called magnetic reversals. Perhaps the most compelling evidence for the reversals comes from the ocean floor, where the record of reversals is is clearly laid out as a pattern of symmetric magnetic "stripes" flanking both sides of the mid-oceanic spreading centers. This is not meant to neglect the very large body of data from land that also contains the record of reversals. Detailed studies of the period of transition from one polarity to another suggest that the transition takes 1-2 thousand years, once initiated. There is also evidence that prior to the reversals, there are larger-than-average fluctuations in the partitioning of energy between the various harmonic components of the field. There are also instances of similar fluctuations (called excursions) that are not associated with an actual reversal. These may be instances of "failed reversals", in which the convective motions of the outer core underwent a reorganization that did not result in a change in the net polarity of the field. --- TBBS v2.1/NM * Origin: Diablo Valley PCUG-BBS, Walnut Creek, CA 510/943-6238 (1:161/55)

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