THE INTENSITY OF THE EARTHS MAGNETIC-FIELD OVER THE PAST 160 MILLION YEARS

In contrast to our detailed knowledge of the directional behaviour of the Earth's magnetic field during geological and historical times(1,2) , data constraining the past intensity of the field remain relatively scarce. This is mainly due to the difficulty in obtaining reliable pal aeointensity measurements, a problem that is intrinsic to the geologic al materials which record the Earth's magnetic field. Although the pal aeointensity database has grown modestly over recent years(3-5), these data are restricted to a few geographical locations and more than one -third of the data record the field over only the past 5 Myr-the most recent database(5) covering the time interval from 5 to 160 Myr contai ns only about 100 palaeointensity measurements. Here we present 21 new data points from the interval 5-160 Myr obtained from submarine basal t glasses collected from locations throughout the world's oceans. Wher eas previous estimates for the average dipole moment were comparable t o that of the Earth's present field(6), the new data suggest an averag e dipole moment of (4.2 +/- 2.3) x 10(22) A m(2), or approximately hal f the present magnetic-field intensity. This lower average value shoul d provide an important constraint for future efforts to model the conv ective processes in the Earth's core which have been responsible for g enerating the magnetic field.