The magnitude of the palaeomagnetic field in Iceland between 2 and 6 Myr ago

Summary. Sixty-eight palaeomagnetic field magnitude values have been determined from a sequence of Icelandic lavas, ranging from 2 to 6 Myr in age. The results indicate large and rapid changes in the palaeomagnetic dipole field and provide a mean value of the palaeomagnetic field magnitude in Iceland for this period.     

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.     

Some results of the investigation of geomagnetic field variation in the Urals and the Carpathians

Summary. A precision magnetic survey for the investigation of current activity in the Earth's lithosphere has been carried out in the Urals and in the Carpathians. As a result of this research three types of time variation of the total field were discovered. These are:
(1) The normal field variation reflecting the general pattern of secular variation. The difference of initial and repeat observation where only this type of variation operates, is rather small and usually does not exceed 0.2–0.3 nT. The field changes in such regions can be used only to evaluate the observation errors and to provide the regional pattern of secular variation.
(2) The slow but localized'anomalous field'change from year to year corresponding, presumably, to anomalies of a tectonomagnetic nature. The normal pattern of the secular variation field here is disturbed by sources located in the upper part of the lithosphere.
(3) Irregular time changes of the field with rather large amplitudes (up to 10–20 nT). Repeated observations of such anomalies show that the field changes significantly here even during one day. Both in the Urals and Carpathians these anomalies form extended elongated structures with widths up to 10–30 km. These anomalies usually coincide with those deep faults where the strongest recent crustal movements have been determined by means of geodetic observations. The analysis of the results of precision geomagnetic surveys in the Urals and in the Carpathians shows that geomagnetic investigations can be used for the exploration of tectonically active zones.