Continental Dynamics
If the continents are not rigid what kind of material are they? Several lines of evidence suggest that the lithosphere of the continents may act like a viscous fluid over geological timescales - just as the mantle does. If the continents are fluid, however, they are not like everyday fluids. Most probably they become less viscous as they deform more rapidly, and are also very strongly dependent on temperature - like toffee, but more so. Although such fluids are complex, some fairly simple rules have emerged from studying them. It has become clear over recent years that much of the complexity of continental deformation arises from the interaction between two sets of forces. One is the compression, or extension, applied by the relative motion of the plates but the second force is due to pressure differences arising from changes in crustal thickness brought about by the deformation. (Thick crust means high pressure and vice-versa). Rules derived from theoretical arguments allow us to predict the style of deformation that we expect to find in real deforming zones. Recent advances in technology have allowed us to make such measurements in real time (Figure 2).
Figure 2. Field of velocity in Asia calculated from slip rates on Quaternary faults, compared with a theoretical velocity field calculated under the assumption that Asia is a viscous fluid, deformed by India's northward movement.