Geomagnetic Reversals and Apparent Excursions in Icelandic Lava Sequences

Detailed paleomagnetic results from composite stratigraphic sections containing 2600 Icelandic lava flows of 1 to 15 Ma age have been published in 1985-2004: a bibliography of these and other papers on paleomagnetism in Iceland is available from the author. Some 11 per cent of the lavas have yielded "transitional" virtual geomagnetic poles, i.e. below 40 degrees N or S latitude. If we define a polarity zone in a sampled profile as consisting of one or more (commonly 10-20) lavas with VGP latitudes above 40 deg., then only about one-third of the transitional poles are found at boundaries between polarity zones. The remaining two-thirds of the transitional-pole lavas were emplaced during apparent excursions. The deviations of the field from the axial central dipole direction are difficult to classify into two distinct categories of "secular variation" and "transitions", as they most probably had a continuous range of amplitudes. The above results are similar to those found by the present author and Ian McDougall from older surveys comprising 2400 sampled lavas (Geophys. J. Royal Astr. Soc. vols. 68, 1982 and 80, 1985). In these papers it was concluded that the geomagnetic field reversed its polarity on average about 8 times per Ma in the period 1-15 Ma ago; as few radiometric dates have been obtained from Icelandic lavas subsequently, this estimate has not been revised. Excursions and polarity zones containing a few lava flows sometimes aid in stratigraphic correlation between lava profiles separated by several km or more. However, large spatial and temporal variations occurring in the lava production tend to limit the usefulness of such features.