Geology Reference
In-Depth Information
pattern of variations in magnetic intensity: alternating strips of the sea floor had
stronger and weaker magnetic field strengths. The pattern was parallel to the local
fabric of seafloor topography, and later was found to be offset by fracture zones, by
about 1000 km in the case of the Mendocino fracture zone. It was presumed that the
pattern might be explained by strips of sea floor with differing magnetisations, but
its origin was obscure. In retrospect it was unfortunate that in this area the ocean
spreading centre at which the sea floor formed no longer exists, and so there was
no obvious association with mid-ocean ridges.
In subsequent surveys, elsewhere, it was found that ridge crests have a positive
magnetic anomaly (meaning merely that the field strength is greater than average),
which some people presumed to indicate 'normal' (i.e. not reversed) magnetisation.
Beyond this, on either side of the ridge crest, there was a negative (i.e. weaker
than average) anomaly. In 1963 Fred Vine, then a graduate student at Cambridge
University, was analysing the results of one such survey over the Carlsberg Ridge in
the Indian Ocean. He noticed that the seamounts near the ridge crest were reversely
magnetised. This is easier to infer for seamounts, because they are more like point
sources and produce a more distinctive three-dimensional pattern of anomalies,
whereas a long strip of sea floor produces a two-dimensional pattern that is more
ambiguous. While his supervisor Drummond Matthews, who had collected the data,
was away, Vine conceived an explanation for the magnetic stripes ([7], p. 219).
Lawrence Morley in Canada was involved in aeromagnetic surveys over Canada,
and was familiar with many aspects of geomagnetism. In his seafloor spreading
paper, Dietz had commented on how the magnetic stripes off the western USA
seemed to run under the continental slope, and had suggested that they were carried
under the continent by subduction and destroyed by subsequent heating. It is well
known that magnetisation does not survive if rocks are heated. Conversely, it is
reacquired by magnetic materials upon cooling. Morley realised that the oceanic
crust could be magnetised as it formed and cooled at a spreading ridge ([7], p. 217).
What has become known as the Vine-Matthews-Morley hypothesis combines
the hypotheses of seafloor spreading and magnetic field reversals. The idea is
that oceanic crust becomes magnetised as it forms at a spreading centre, and a
strip of sea floor accumulates that records the current magnetic field direction
(Figure 3.4(a)). If the magnetic field then reverses and the seafloor spreading
continues, a new strip will form in the middle of the old strip (Figure 3.4(b)), the
two parts of the old strip being carried away from the ridge crest on either side.
Subsequent reversals would build up a pattern of normal and reverse strips, and the
pattern would be symmetric about the ridge crest (Figure 3.4(c)).
Vine later commented that the hypothesis required three assumptions, each of
which was, at the time, highly controversial: seafloor spreading, magnetic field
reversals, and that the oceanic crust (the seismic 'second layer') was basalt and not
