The Thermal Demagnetization of Natural Magnetic Moments in Rocks

This paper describes a palaeomagnetic investigation of some laterites, containing hematite and maghemite, which have been baked by an overlying Tertiary lava in Northern Ireland. The laterite was magnetically unstable, but a technique of thermal demagnetization eliminated a secondary component of magnetization along the Earth's present field. This technique established that the lava and baked laterite agreed in direction, and it further allowed a determination of the strength of the Earth's field, which was apparently reversed at the time of baking, and of the temperature of baking. This determination of a geologically ancient field strength is particularly significant, since many such determinations from rocks of different ages might allow estimates of ancient latitudes for the various continents, independent of estimates derived from directions of magnetization.