114 | Great Geologists
The Vine-Mathews-Morley hypothesis relating reversals of
paleomagnetic polarity with seafloor spreading.
Symmetric striped pattern of magnetic anomalies on the
Reykjanes segment of the Mid-Atlantic Ridge, south-west of
Iceland. The positive anomalies are shaded according to their
age, as indicated in the vertical column.
APPARENT POLAR WANDER PATHS
During the 1950s, studies of paleomagnetism began to present the geoscience community with some intriguing results,
especially for those with an interest in the much disputed notion of continental drift. Highly sensitive magnometers were
developed that allowed the direction and inclination of the remanent magnetism preserved in some rocks to be determined. A
growing database of this information from rocks of different ages and from different continents was developed. This suggested
that the positions of continents in relation to the poles had moved through time.
The ‘apparent polar wander’ was initially explained by movement of the poles through time, as opposed to movement of the
continents. However, it soon became evident that each continent had a distinctive apparent polar wander path. Therefore,
the continents must have moved relative to each other. This interpretation, coupled with the observation that the latitudinal
positions of continents could be different from those today (as determined by magnetic inclination), aroused the suspicion of
experts in marine geology, such as Harry Hess, that drifting continents were viable.
RECOGNISING PALEOMAGNETIC REVERSALS IN THE OCEANS
At the same time as paleomagnetic data were being gathered on land, magnetometers were developed to be towed behind
ships that enabled magnetic surveys of the ocean floor. These studies revealed regions of magnetic anomalies, alternating
strips with a magnetic intensity greater or lesser than the Earth’s mean field today. These alternating stripes lay parallel to
ocean ridges.
When visualised on a map of the seafloor, the pattern resembled that of the stripes on a zebra. However, what it meant was
uncertain. The general assumption was that it was a local phenomenon related to compositional differences in the rocks from
which the records came. Fred Vine would come to a different set of conclusions, not least because from the beginnings of his
interest in geology, he was conscious of the ‘big picture,’ and in particular continental drift, the forerunner of plate tectonics.