On 21 April 1906, at a meeting of the Société Francaise de Physique, Bernard Brunhes (Figure 1) addressed his colleagues on an important discovery concerning the magnetization of several formations in the volcanic Massif Central. The talk was published in the Journal de Physique in November of the same year under the title “Recherches sur la direction de l'aimantation des roches volcaniques” [Brunhes, 1906].
This title only partially reflects the contents of his paper, which focused mostly on the magnetization of baked clays underlying lava flows, rather than on the flows themselves. This subject had stimulated Brunhes' interest since his appointment as professor at the Université de Clermont-Ferrand and director of the Observatoire du Puy de Dôme on 1 November 1900. His first investigations on the subject, with Pierre David, date back to 1901–1902, almost exactly 100 years ago.
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