Great Geologists | 129
The main tectonic plates of the earth and their relative motions.
Mason, Vine & Matthews, Morley, and
others.
Simultaneous advances in seismic
imaging techniques along the trenches
bounding many continental margins,
together with many other geophysical
(e.g., gravimetric) and geological
observations, showed how the oceanic
crust could be subducted, providing the
mechanism to balance the extension of
the ocean basins with shortening along
its margins. The global seismic network
and the ability to locate earthquakes and
define focal mechanisms was also critical
for understanding the geometry of plates,
mid-ocean ridges, and the nature of
transform faults, all in turn of paramount
importance to the development of the
plate tectonics paradigm.
By 1967, all that was required was
a model describing translations and
rotations on a sphere to define plate
motions. This was independently
and almost simultaneously proposed
by McKenzie and Parker, and by the
American, Jason Morgan. In The North
Pacific: An Example of Tectonics on a
Sphere, McKenzie employed Euler’s
Fixed Point Theorem, in conjunction with
magnetic anomalies and earthquakes
foci to determine a precise mathematical
theory for plate tectonics. Tuzo Wilson
had suggested in 1965 that the surface
of the Earth could be divided into
rigid aseismic regions (i.e., plates). In
the previous year, Teddy Bullard had
used Euler’s theorem to describe rigid
movements on a sphere when he made
continental reconstructions. McKenzie
combined the two concepts, which
became the modern theory of plate
tectonics.
This work was published some months
after (unknown at the time to McKenzie)
similar ideas had been presented
by Jason Morgan of Princeton at an
American Geophysical Union conference.
McKenzie and Parker’s published paper