The mechanical lithosphere

In Chapter 8 we considered convection in a fluid medium. However, the earth's mantle behaves as a fluid only in its interior, where the temperature is high. Near the surface, its viscosity is much higher, so that it is effectively rigid much of the time. This is illustrated schematically in Figure 9.1.

However, as we saw in Chapter 6, with sufficient stress the cooler mantle may yield. Close to the surface, this yielding takes the form of brittle fracture. At intermediate depths, the yielding may be more fluid-like but still result in narrow zones of deformation, which geologists call ductile shear zones. At the large scale in which we are interested here, these narrow shear zones still have the characteristics of fractures or faults, and so we may consider the lithosphere at the large scale to be a brittle solid to a first approximation. The usefulness of this approximation is illustrated, for example, by the three kinds of plate margin, which correspond to the three standard types of faults in structural geology: normal (spreading centre), reverse (subduction zone) and strike-slip (transform fault).

The implication of this ‘brittle–ductile transition’ is that our convecting medium changes from being effectively a viscous fluid at depth to being a brittle solid near the surface. The material of the mantle flows from one regime to the other, and so ultimately we must consider the mantle as a single medium that undergoes radical changes in properties as it flows around. We will approach this task in Chapters 10 and 11, and we will see that there are some important consequences of these changes of properties. First, however, there are some important aspects of each regime that can be understood separately.