Changes in direction of the remanence of rocks produced by stationary alternating field demagnetization

Summary. A theoretical investigation of the way in which an isotropic rock containing single-domain particles acquires both IRM and ARM (or TRM) has indicated that stationary single-axis alternating field (af) demagnetization with the af axis at an angle to the remanence vector should produce progressive angular changes in a single-component remanence as demagnetization proceeds. Just before the remanence is completely removed it should lie at 90° to the af axis irrespective of the original orientation of the remanence (apart from 0°). Experimental observations on a rock sample support these deductions.
This analysis has been extended to investigate the way in which ARM (or TRM) and IRM are demagnetized by static three-axis demagnetization methods which are used by some workers in palaeomagnetism. Theory, in conjunction with the use of a numerical model, predicts that an ARM or TRM should not undergo significant direction changes when these methods are applied but an IRM should undergo progressive direction changes as demagnetization proceeds, usually moving until it makes an angle of cos⁻¹ (1/3) with each of the three af axes just before it is removed. Confirmation that such changes do occur have been obtained by experiments on a rock sample. The relative merits of static and tumbling af demagnetization methods are also discussed.