The Age of Viscous Remanent Magnetization of Hadrian's Wall (Northern England)

Abstract

Following Néel's considerations on magnetic viscosity, an experimental approach is made to determine the age of viscous remanent magnetization (VRM) of certain rocks. The time-dependent critical internal fluctuation field is measured directly using AC demagnetization techniques. There are still some unsolved theoretical problems such as the ambiguous significance of the logarithmic time constant Q and the irreversible susceptibility χirr in the formulas developed by Néel. To prove the proposed method we have used blocks of quartz dolerite (Whin Sill) which were built into the Roman Hadrian's Wall in northern England. The historical age of the wall—constructed during the first centuries after Christ—and the age of VRM calculated from the magnetic measurements coincide fairly well. The method may therefore be applicable to certain dating problems in geology, e.g. to correlate Quaternary rock deposits, and in archaeology, if well-calibrated apparatus and statistical methods are used. Clearly the method cannot be used for ages greater than that of the last main reversal of the geomagnetic field.