Geology Reference
In-Depth Information
event might develop. Whether Archaean greenstone belts are the product of plume
heads or overturns remains to be resolved.
The tectonic regime responsible for the earliest Archaean nuclei is inferred to
be more 'vertical' in character than plate tectonics [171, 172]. Reheating of a
pre-existing mafic crust is inferred, perhaps in repeated episodes. The agents of
such reheating might be very large plume heads or mantle overturns. A significant
point emphasised by Bedard [171] is that the persistence of thick lithospheric keels
associated with the Archaean nuclei indicates that we should think of the keel and
the crust forming in intimate association, and in that sense the keel would be a
differentiated product of the mantle just as the crust is.
There is active work on attempts to document past continental aggregations [173,
174], and one of several tools is the identification of past continental collisions. The
whole cycle of continental dispersal and re-aggregation, often called the Wilson
cycle, is likely to contribute to the episodic record. However, as noted earlier,
the episodes are more likely to be regional than global, and so do not have great
implications for the operation of mantle convection. Nor are studies of mantle
convection so likely to contribute to their understanding. Rather, the meanderings
of continents may be a second-order modulation of the plate-mantle system.
On the other hand, if the plates ceased to move for significant periods of Earth
history, that would affect the evolution of the mantle and require a deeper under-
standing of the mechanics of the plate-mantle system. If there was a hiatus in plate
activity, then the mantle heat loss would fairly quickly decay to fairly low levels
and the mantle temperature might well begin to increase. As noted earlier, so long
as the hiatus did not last longer than a few hundred million years, there might not
be a large effect on overall geological activity. However, the re-establishment of
subduction and plate tectonics would probably initiate an episode of fairly high
activity, as the higher mantle temperature accumulated in the hiatus would have
lowered mantle viscosity and would thus increase plate speeds. The higher tem-
perature would also result in greater degrees of melting. Thus a cycle of hiatus and
renewed activity could contribute significantly to the episodic nature of the geolog-
ical record. A hiatus would also help to resolve the heat source puzzle discussed in
Section 9.3.
Intermittent plate tectonics would bring to the fore the complex rheology
that undoubtedly characterises rocks in the varied pressure-temperature regime
involved in mantle convection. Much of our understanding of mantle convection
has been built on the assumption that the complications mostly do not matter very
much, but intermittency would immediately imply that the complications must be
given greater consideration, however difficult that might make modelling studies.
