MAGNETIC RECORDING IN NATURE - THE MEDIUM, THE MECHANISM AND THE MESSAGE

The capacity of rocks to magnetically record Earth history has played a crucial part in transforming the way we look at our planet. It has also provided much stimulus for the study of the medium which carries the record, and the mechanisms by which the record has been implanted. The medium consists of compounds of iron (oxides and sulphides) often with high concentrations of titanium. Oxides with spinel structure, which may be highly nonstoichiometric, are abundant. Particle sizes are often too large to be ideal recording materials but seem to carry the palaeomagnetic record effectively. This has led to a concentration of effort on understanding the magnetic state and the magnetization process in particles with few domains. Several recording mechanisms operate in nature: thermoremanent magnetization may be acquired as hot rocks cool to ambient temperature; chemical remanent magnetization can be acquired as particles grow in size or change intrinsic magnetic properties due to a change in composition; and a sedimentary rock may acquire a depositional remanent magnetization as the magnetic particles settle in the forming sediment. The requirements on the medium and mechanism are somewhat stringent: the message must be accessible after exposure of the recording to an environment which may be physically and chemically hostile for times up to hundreds of millions of years. The success of palaeomagnetism testifies to the robustness of nature's magnetic recordings.