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.

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.