Geology Reference
In-Depth Information
6
The plate mode of convection
Mechanical properties change with temperature, from brittle plate
to yielding mantle and back. This strongly affects their dynamical
behaviour and their influence on convection. Plates organise the flow.
Internal heating versus bottom heating also affects the form of con-
vection. The plate cycle (formation, cooling, subduction, reabsorption)
is convection. The plate mode of mantle convection transports a large
fraction of Earth's heat budget. Seafloor topography and heat flow can
be quantitatively explained with remarkable success.
The convection theory developed in the previous chapter applies to many forms
of convection, and it seems to apply reasonably well to mantle convection, but
with some important qualifications. Mantle convection takes distinctive forms that
in some ways are quite unlike familiar examples of convection such as occur in
familiar kinds of fluid. The main reason for the differences is that the mechanical
behaviour of mantle rocks changes quite dramatically between the temperature at
the Earth's surface and the temperature within the mantle.
6.1 The strong lithosphere
The temperature dependence of viscosity shown in Figure 4.4 tells us that reducing
the temperature from 1300
◦
C to 1000
◦
C will increase the viscosity of mantle
rocks by as much as three orders of magnitude - a factor of 1000. However, if the
mantle rocks are much cooler than that, they cease to deform like a viscous fluid.
Through an intermediate range of temperature they develop ductile shear zones, so
that the deformation is concentrated in relatively narrow zones instead of occurring
uniformly through the fluid. At temperatures lower than a few hundred degrees,
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