E. Schnepp, GEOMAGNETIC PALEOINTENSITIES DERIVED FROM VOLCANIC-ROCKS OF THE QUATERNARY EAST EIFEL VOLCANIC FIELD, GERMANY, Physics of the earth and planetary interiors, 94(1-2), 1996, pp. 23-41
Geomagnetic paleointensities have been determined for 18 rock units of
basanitic, thephritic or nephelinitic composition from the Quaternary
East Eifel volcanic field (Germany). In general the rocks are well su
ited for paleointensity determination and gave consistent results of p
aleointensity. For each rock unit at least five individual values have
been measured with the Thellier method. Compared with the present day
field of 48 mu T the unweighted mean paleointensity values range from
29 to 62 mu T With relative standard errors mostly below 10%. Paleoin
tensity values are concentrated at 30 and 50 mu T with a gap at 40 mu
T. The distribution is bimodal rather than Gaussian, although all pale
opoles lie close to the geographic pole. This cannot be explained by t
he very discontinuous distribution of volcanic activity in time which
took place mainly around 200 and 400 ka. In each age group both low an
d high paleointensity values are found. However, a further temporal se
paration within each age group cannot be excluded. Furthermore, the pa
leointensity values of the West Eifel volcanic field show a similar bi
modal distribution which indicates that this case seems to be connecte
d with a geomagnetic excursion recorded in those rocks. Therefore it m
ight be considered that the East Eifel rocks recorded a characteristic
feature of the earth's magnetic field namely a quasi-cyclic variation
of paleointensity. On the other hand, the now available global data s
et of 128 virtual dipole moment values from the Brunhes chron and olde
r than 30 ka shows an almost Gaussian distribution with a mean value o
f 7.4 +/- 3.4 . 10(22) Am-2, if 14 values with transitional pole posit
ions are excluded. A quasi-cyclic variation of the dipole moment there
fore does not seem to be a persistent global characteristic of the ear
th's magnetic field.