Geology Reference
In-Depth Information
with the angular, segmented geometry of mid-ocean ridges. By focusing on the
motions that can be discerned at the surface, Wilson recognised the behaviour of a
brittle solid, and successfully defined plate tectonics in those terms.
There was significant supporting evidence already when Wilson published his
theory. It was known that earthquakes do not occur on the fracture zones beyond
where they connect with offset ridge crests, but only on the segment between
ridge crests. This is consistent with Wilson's description of how the two plates in
Figure 3.2 pull apart. The transform fault between the ridge segments is active.
Beyond the ridge segments, the two sides of the fracture zone are moving together,
so there is no relative motion and no earthquakes would be expected.
It took a couple of years for Wilson's idea to catch on, but when it did, other
strong evidence was soon found supporting his theory. I will note three kinds of
evidence that are particularly direct, and speak to different sections of the Earth
science community.
3.3 Evidence for motion - magnetism
In about 1960, studies of palaeomagnetism began to focus strongly on the question
of whether the Earth's magnetic field has changed through time. Specifically, has
it reversed polarity? Matuyama, in 1929, had studied the magnetisation through a
sequence of lava flows erupted by a Japanese volcano. He found that the younger
flows near the top were magnetised parallel to the present Earth's magnetic field
lines, but that the older flows near the bottom of the sequence were magnetised in
the opposite direction. During the 1950s the question of whether this was due to
reversal of the Earth's field or to a peculiar response of some rocks was vigorously
debated. It seemed that there may have been many reversals of the Earth's field, but
this was difficult to demonstrate convincingly. From 1963, two groups in particular
used a combination of magnetisation measurements and potassium-argon (K-Ar)
dating to try to resolve the question and to establish a chronology of reversals. These
groups were at the US Geological Survey in Menlo Park, California, and at the
Australian National University in Canberra. They found that the ages of normally
and reversely magnetised rocks correlated around the world, which support the
idea that the Earth's field had indeed reversed. By about 1969, the time sequence
of reversals was established with some detail to an age of about 4.5 Ma, beyond
which the K-Ar dating method did not have sufficient accuracy.
Meanwhile, Ron Mason, of Imperial College, London, and the Scripps Institute
of Oceanography in California, was trying to identify magnetic reversals in oceanic
sedimentary sequences. Because of this work, but still almost by chance, a magne-
tometer was towed behind a ship doing a detailed bathymetric survey off the west
coast of the USA. From this magnetic survey there emerged a striking and puzzling
