Paleomagnetic volcanic data and geometric regularity of reversals and excursions

Mostly on the basis of paleomagnetic sedimentary data, it has been suggested that maps of virtual geomagnetic poles (VGPs), corresponding to directions of the magnetic field at each site, tend to fall along American and Asian longitudes during reversals and excursions. Such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled. However, studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields. In this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 Myr. Volcanic eruptions occur sporadically, thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions, but we prefer not to use averaging methods of previous investigators, who discarded or combined directions when they appeared to be similar. We find that volcanic data give intermediate VGPs which tend to fall along American and Asian longitudes, roughly consistent with the sedimentary data. This result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites. Provided the appropriate polarity is assigned to intermediate VGPs, we find that Icelandic VGPs tend to fall along Asian longitudes. Other patterns in the data, for example, latitudinal clustering of VGPs or distinguishing longitudinal preferences of excursions from reversals, are not resolved. However, it appears that in general, transitional fields are nondipolar.