Geology Reference
In-Depth Information
6.7 Summary of the plate mode
In Chapter 5 we developed a simple theory of mantle convection and showed
that it was capable of predicting the velocities of plates to quite good accuracy,
given our initial ignorance and the simplicity of the ideas and approximations
used. In this chapter we have developed this picture by taking account of the
distinctive mechanical properties of the lithosphere, and deduced some important
results.
Mantle material changes its mechanical properties quite radically as it rises
from the deep, hot interior, where it behaves like a viscous fluid, and cools at the
surface, becoming strong and effectively brittle. Mantle convection takes a dis-
tinctive form because of this change. The lithosphere is strong, but broken into
pieces that we call plates. The plates control the spatial pattern of convection,
because upwelling and downwelling occur only (or predominantly) at plate mar-
gins. The plates thus define what I have called the
plate mode
of mantle convection,
with 'cells' that can be up to four times wider than the depth of the mantle. The
appearance of this mode of convection at the Earth's surface is unusual because
the convecting material is a brittle solid at the surface, and this results in plates
having a range of sizes, odd shapes and in some places quite angular boundaries.
This is part of the reason it was hard to recognise as convection, and why there
has been a lot of confusion about the relationship between the plates and mantle
convection.
In this picture, the plates are an integral part of mantle convection. In fact, they are
the most active component, comprising the driving thermal boundary of the plate
mode. The theory of thermal diffusion gives us a good account of the thickness of
the oceanic plates. As a direct consequence, it also gives a good account of seafloor
subsidence away from mid-ocean ridges, and of the variation of seafloor heat flux
away from ridges. The plate mode accounts for most of the heat observed to be
emerging from the Earth's interior.
This empirical success is obtained by considering only the top 100 km or so
of the mantle, as we have done in the theory of the cooling, thickening plate.
An important implication is that the underlying mantle must be relatively passive,
otherwise it would disrupt the regular subsidence of the lithosphere. There are
deviations from this regular behaviour, as we will see in Chapter 8, but they are
secondary, and we can describe much of the important behaviour of the plate mode
with this relatively simple theory.
There is more to remark about this empirical success. We noted in Chapter 2 that
the mid-ocean ridge system is the second-largest topographic feature of the planet,
after the continent-ocean dichotomy. A rather simple theory has successfully and
quantitatively accounted for this topography, which we can recognise as having a
