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.