Geology Reference
In-Depth Information
a viscosity roughly 10 to 100 times that of the upper mantle [41]. This tends to
favour large-scale flow because then the rates of deformation involved are smaller
for a given velocity, and so the viscous resistance is reduced. The other factor is the
spherical geometry of the mantle. The mantle extends about half-way to the centre
of the Earth, which means that the deep return flow has to travel only about half as
far as a surface plate. There will therefore be less time for an instability to develop
and rise under the middle of a plate.
There is another significant distinction between convection with plates and
'normal' convection. It is that only one plate subducts, so the downwelling is
completely asymmetric (Figure 6.3, bottom left panel). In constant-viscosity con-
vection, on the other hand, the downwelling is more symmetric, drawing fluid from
both sides of the downwelling.
To summarise this section, the presence of plates has a large effect on the spatial
pattern of convection. Upwellings and downwellings occur only at plate margins,
and not, so far as we can see, in between. In this sense the plates
organise
the
pattern of the flow. As a result, plates can be much wider than the depth of the
mantle. Also as a result, subduction is asymmetric, with material from only one
side descending into the mantle.
The distinctive form of mantle convection occurs because the mantle material
changes from acting like a viscous fluid to acting like a brittle solid, and back, as it
rises to the surface, cools and then sinks back into the mantle. It is the brittleness
of the material at the surface that gives rise to plate tectonics. This is also the
reason it was so hard for people to recognise the plates as part of a convecting
system. The plates have a range of sizes, strange shapes and in many places their
boundaries have sharp corners. None of the latter features look like the top of a
'normal' convecting fluid.
6.3 Heat transport - the plates are mantle convection
The further implication of this picture is that the plates are
part of
the convect-
ing system. They comprise one of the driving thermal boundary layers of mantle
convection. They do not ride passively on a convection system that occurs mysteri-
ously under the plates for unspecified reasons. Nor is the cycle of upwelling under
a spreading centre, cooling at the surface and sinking back into the mantle to be
reheated something that is independent of a convecting system. This cycle
is
the
convecting system in action. It is the process by which mantle convection removes
heat from the deep interior of the mantle.
I have called the convection involving plates the
plate mode
of mantle convection
[1]. It deserves its own name because there is another mode of mantle convection
with a rather different form, the plume mode, that is driven by the hot thermal
