Geology Reference
In-Depth Information
Figure 3.3. Wilson's 1965 sketch map of the plates [23]. Reprinted from
Nature
with permission. Copyright Macmillan Magazines Ltd.
In these four sentences Wilson defined the problem and presented its solution with
simple clarity. His sketch map (Figure 3.3) gave the world its first view of the
tectonic plates. It is only a sketch map, but the concepts are clear and precise.
Wilson was thinking as a structural geologist, and that was crucial. He envis-
aged rigid blocks bounded by three types of boundary that correspond to the three
standard fault types: strike-slip (transform fault), normal (ridge) and reverse (sub-
duction zone). Conceptually, he narrowed the old notion of mobile belts down to
sharp boundaries, and he explicitly adopted the long-standing implication of that
old term, that there is little deformation outside the mobile belts, taking it concep-
tually to the limit of proposing that there is
no
deformation. He was explicit in the
fourth sentence, quoted above, that the plates are 'rigid'. The point is explicit also
within the paper: 'These proposals owe much to the ideas of S. W. Carey, but differ
in that I suggest that the plates between the mobile belts are not readily deformed
except at their edges.'
By looking and thinking as a structural geologist, Wilson was able to see the
plates in all their simplicity. A unique and crucial feature of mantle convection,
as distinct from other forms of convection, is that the convecting material behaves
sometimes as a viscous fluid and sometimes as a brittle solid, as we will see in later
chapters. This was a major source of the confusion in earlier attempts to formulate
and relate ideas about continental drift, seafloor spreading and mantle convection.
This can be seen, for example, by contrasting Arthur Holmes' earlier concept of a
new ocean in which there is broad deformation across the sea floor, reflecting the
behaviour of a viscous fluid, with Hess' and Dietz' narrow spreading centres, and
