palaeomagnetism and polar wander Central to understanding the importance of ancient magnetisms in rocks is the concept of the palaeomagnetic pole. In early studies of this subject it was necessary to compare magnetizations of identical age from widespread localities on a continent, and simple inspection of palaeomagnetic directions was inappropriate. The earlier research workers proposed an axial geocentric model for the time-average magnetic field. If the coordinates of the sampling locality were known, directions of magnetization could then be transformed into magnetic poles (strictly termed
palaeomagnetic poles) that would be expected to give rise to the field at the locality studied. By definition, a palaeomagnetic pole reflects accurately the averaging of the geomagnetic field from a set of independent measurements. The term ‘virtual geomagnetic pole’ (VGP) was used for a representation of a short part of geomagnetic field time (e.g. the field recorded by a single lava flow or sedimentary bed), regardless of whether the field could be represented as an axial geocentric dipole at that instant in time. Compilations of thousands of VGPs from young igneous rocks worldwide have demonstrated that, to a first approximation, the axial geocentric dipole hypothesis is valid. Symmetry arguments show that the position of a palaeomagnetic pole relative to a sampled continent does not determine the palaeolongitude of that continent. However, and very importantly, the distance from the palaeomagnetic pole to the sampled area of the continent, as reflected by palaeomagnetic inclination, does provide a direct estimate of the palaeolatitude of the area. The palaeomagnetic declination gives an indication of the past orientation of the continent.
One goal of palaeomagnetic research has therefore been to provide a set of well-dated palaeomagnetic poles from rocks that have remained part of the stable continent since their formation. Understandably, certain time periods have been represented by better and more abundant palaeomagnetic data for some continents than for others. Not all palaeomagnetic poles are dated using isotopic age methods. In the mid-1950s, when the early research workers began compiling palaeomagnetic poles for the various continents, they soon realized that poles older than about mid-Cenozoic time did not coincide with the spin axis, nor was there appreciable agreement among poles of similar age from different continents. In consequence, the tracks of palaeomagnetic poles from each continent were introduced as apparent polar wander (APW) paths because they recorded the motion history of the continents, rather than significant motion of the dipole relative to the spin axis. Accurate APW paths provide a powerful base by means of which ancient motion of the continents, relative to the Earth's spin axis, can be assessed and compared. APW paths provide the only means of assessing such motion prior to the early Mesozoic break-up of Pangea. Inferences based on APW path analysis are limited, however, because the palaeomagnetic pole concept assumes an axial geocentric dipole and the palaeolongitude is undetermined. In addition, APW paths are only as robust as the database, specifically the quality of palaeomagnetic pole determinations and the accuracy and reliability of the age assigned to each pole. True polar wander implies non-axiality of the spin axis and the time-averaged dipole. Although difficult to evaluate, true polar wander has been suggested for parts of the Cretaceous and the mid-Palaeozoic.
John W M Geissman
