Great Geologists | 113
Fred
Vine
Fred Vine, photographed at Princeton University in 1968.
The development of plate tectonics involved a number of individuals, and teams of scientists, each working on separate
strands of the theory. At the beginning of the 1960s, the concept of seafloor spreading was introduced by Harry Hess and
by Robert Dietz, who sought to explain observations from bathymetric and gravity surveys of the World’s oceans. Seafloor
spreading was an intriguing hypothesis, suggesting that ocean crust rose up from the Earth’s mantle along mid-ocean ridges,
and then spread away from these ridges in both directions to be subducted in ocean trenches.
But how could the notion of crust moving like a conveyor belt be proven? The answer was to come from paleomagnetics,
studies of the Earth’s ancient magnetic field as recorded in the rock record. At the forefront of this work was Cambridge
University research student, Fred Vine. In 1963 Vine, together with his supervisor Drummond (Drum) Matthews, developed a
speculative proposal tying together seafloor spreading and paleomagnetism as a means of explaining the patterns of magnetic
anomalies found parallel to mid-ocean ridges. They suggested that oceanic crust was not just a conveyor belt, but also a tape
recorder, recording the Earth’s past magnetic polarity and, in so doing, confirming seafloor spreading. It was now possible to
accept the notion of large-scale lateral crustal motion, accelerating the progress towards a complete theory of plate tectonics.
PALEOMAGNETIC THEORY
The alignment and inclination of the magnetic properties of minerals in certain rocks (e.g. basalts) is effectively fossilised at the
moment when the rock is formed. At the beginning of the 20th century, the French geophysicist Bernard Bruhes recognised
that rocks could be classified into two groups according to their magnetic properties. One group has so-called normal polarity,
characterised by the magnetic minerals in the rock having the same polarity as that of the Earth’s present magnetic field. That
is to say, the magnetic field is oriented south to north (why a compass needle points north). The other group, however, has
reversed polarity, indicated by a polarity alignment opposite to that of today’s magnetic field. That is to say, oriented north to
south (such that a compass needle would point south).
Based on the study of (mostly igneous) rocks at outcrop, it progressively became known that magnetic polarity has periodically
switched through the course of geological time. In 1929, the Japanese geophysicist Motonori Matuyama recognised periodic
magnetic polarity switching during the Pleistocene, the first step towards a magnetic polarity timescale. It would take the
progressive integration of the observations of magnetic polarity reversals in the rock record with radiometric data on their
ages for the notion of a global magnetic polarity reversal record through geological time to be accepted. When Vine began his
studies, this was far from a fully agreed upon concept amongst many geologists and geophysicists.