Geology Reference
In-Depth Information
0.15
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0.05
0.00
0
1000
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4000
TIME (Ma)
Figure 9.16. A generalised impression of the age distribution of the continental
crust. From McCulloch and Bennett [162]. Copyright Elsevier Science. Reprinted
with permission.
equi-dimensional granite-greenstone structure bears little resemblance to the elon-
gate, laterally compressed mobile belts typical of later history. There are of course
also many more subtle differences between the Archaean and younger geology
[160, 161].
There is also a decided clustering of ages of continental rocks and provinces,
illustrated in Figure 9.16. This clustering could in principle be due either to episodic
tectonic activity or to uneven preservation of continental provinces. The weight of
current opinion seems to incline to episodic tectonics [160].
Thus we know that the Earth has changed in important ways, and we know from
the preceding models that mantle dynamics has very likely changed in important
ways. While some of the modelling results are suggestive, they are far from pro-
viding clear interpretations of geological observations. On the other hand, they are
suggestive enough to deserve to be included in the multi-stranded conversation that
will be required to make sense of the sparse evidence surviving from the Earth's
remote past.
Perhaps the primary questions remaining about the Earth's geological history
are these: Has plate tectonics always operated? If it has not, what tectonic mode
or modes acted in its place? And what role have plumes played? The preceding
models have also identified other potential tectonic agents: the mantle overturn
following the breakdown of mantle layering, and the cessation and reactivation of
subduction and plate tectonics. Continental collisions have been recognised agents
independent of modelling. These agents will be discussed in turn.
The results presented in Figures 9.8 and 9.11 show that oceanic-type crust may
have been no thicker than at present, despite higher mantle temperatures in the past,
because of dynamical depletion of the upper mantle of its more fusible components.
