A magnetic study of the lava stone of the Chaîne des Puys was conducted by Norbert Bonhommet and Jean Babkine (1967). The goal was to determine the declination and inclination of the magnetic field recorded by certain minerals in these rocks (magnetite and ilmenite) during cooling, in order to reconstruct the changes in orientation of the field over the last 100,000 years. Most of the measures met expectations: all the lava had “direct” magnetism, with the north of the samples more or less close to the current magnetic north, in accordance with the ages of the eruptions, which all took place at the end of the “Brunhes magnetic age” covering the last 700,000 years.
There was a surprise with the lava at Puy de Laschamps, whose magnetism was “reversed”: the north was at the south, a feature shared by the lava at Olby and at Louchadière, and found throughout the world on around a dozen sites. One of them, a borehole in the Atlantic Ocean (Laj and Channel, 2007), showed that the pole had not settled in the southerly position. For the first time, a very young (40,000 years) reversal of very short duration (just a few thousand years), due to instability of the external core of the globe where the “dipole model of the Earth's magnetic field” lies, was revealed during the course of a magnetic age. To highlight these characteristics, the name “magnetic excursion” was given to this type of event, and this one in particular was called the “Laschamps event”.
Jacques Kornprobst
Honorary Director of the Observatoire de Physique du Globe de Clermont-Ferrand.
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