Some global features of palaeointensity in geological time

SUMMARY A global palaeointensity data base was constructed from all published data from volcanic rocks in geological time older than 0.03 Ma. The data base contains a total of 1123 flow mean data retrieved from 83 original papers. Various features of the Earth's dipole moment were examined from the data which are based on Thellier and Shaw methods. Long-term variation of the Earth's dipole moment seems to have existed in the past 300 Ma with a broad minimum at 120–180 Ma as suggested by Prevot et al. (1990). However, due to limited site distribution we cannot regard this Mesozoic dipole low as being completely established. Precambrian palaeointensity data are still insufficient to conclude any long-term variation in this time range, although the geodynamo processes of moderate magnitude definitely existed in the early time of the Earth's history. The χ2 test was applied to the distribution of the virtual dipole moment for the past 5 Ma in which the transitional data were excluded; the results indicate that the distribution of virtual dipole moment is better represented by a log-normal distribution rather than a normal distribution, and this tendency seems to be true for the past 20 Ma. The relation of mean palaeointensity versus palaeomagnetic colatitude was examined for the past 10 Ma for the data excluding transitions. The relation is concordant with a theoretical curve from a geocentric axial dipole. This reconfirms that the dipole field was dominant in the past geomagnetic field, which is the dipole hypothesis in palaeomagnetism. On the other hand, the virtual dipole moment is much smaller for transitional palaeomagnetic fields, and a virtual geomagnetic pole lower than 45° seems to be a reasonable criterion to be categorized as a transition. The mean dipole moment for the pole latitude of 30°N-50°N band is larger than that for 30°S-50°S, indicating that there might have been a persistent asymmetry of palaeointensity between normal and reversed states, or some kind of geometrical asymmetry between the Northern and Southern Hemispheres.