<P> Three decades later , when Earth 's magnetic field was better understood , theories were advanced suggesting that the Earth 's field might have reversed in the remote past . Most paleomagnetic research in the late 1950s included an examination of the wandering of the poles and continental drift . Although it was discovered that some rocks would reverse their magnetic field while cooling , it became apparent that most magnetized volcanic rocks preserved traces of the Earth 's magnetic field at the time the rocks had cooled . In the absence of reliable methods for obtaining absolute ages for rocks , it was thought that reversals occurred approximately every million years . </P>
<P> Three decades later , when Earth 's magnetic field was better understood , theories were advanced suggesting that the Earth 's field might have reversed in the remote past . Most paleomagnetic research in the late 1950s included an examination of the wandering of the poles and continental drift . Although it was discovered that some rocks would reverse their magnetic field while cooling , it became apparent that most magnetized volcanic rocks preserved traces of the Earth 's magnetic field at the time the rocks had cooled . In the absence of reliable methods for obtaining absolute ages for rocks , it was thought that reversals occurred approximately every million years . </P>
earth 's magnetic field seems to reverse orientations about every
earth 's magnetic field seems to reverse orientations about every
million years
million years