EarthRef.org Reference Database (ERR) -- Quidelleur et al. 1999

An excursion of the geomagnetic field characterized by strongly abnormal directions associated with a field intensity as low as 7 mu T has been identified in a lava flow sequence from La Palma, Canary Islands. The age of 602 +/- 12 ka obtained by K/Ar dating, using the Cassignol technique, for a transitional flow represents our best estimate for the age of the occurrence of this episode of excursional directions. This age is concordant with a minimum observed in the global SINT800 composite record, derived from worldwide deep-sea records of relative paleointensity. We show that a geomagnetic field model composed of a present-day-like non-dipole field, superimposed on an axial dipole reduced by a factor of 2 compared to its present value, can account for excursional directions associated with low paleointensity in some areas, while most other parts of the Globe display apparently normal polarity directions. We next generate through linear inversion a synthetic field model constrained by the actual La Palma data and a set of directions at 30 sites (distributed worldwide), based on an axial dipole field with half present-day intensity. The energy spectrum of this synthetic field model is compatible with most of the non-dipole part of the present-day field, and in that sense is 'Earth like', i.e,, represents an acceptable configuration of the past geomagnetic field at 600 ka. The model shows that very anomalous excursional directions covering 1/5 of the Earth's surface may co-exist with apparently normal directions elsewhere. The new data and modeling strongly support the hypothesis that excursions are an integral aspect of secular variation. There may be a continuum between normal SV and field reversals, primarily controlled by the ratio of axial dipole field intensity relative to the remaining (non-axial dipole) part. (C) 1999 Elsevier Science B.V. All rights reserved.