This would be all well and good, but more recent studies have suggested that in fact the story is much more complex that it appears. The conclusions of the World Stress Map Project as reported by Zoback (1992b) are that first order midplate stress fields are the result of compressional forces applied at plate boundaries, primarily ridge push and continental collisional forces, rather than the tensile force that would be expected from a predominantly slab-pull determined stress field. Furthermore, she reports no evidence of large, lateral stress gradients which would be expected across large plates, if simple resistive or driving basal drag (parallel or antiparallel to absolute motion) controlled the intraplate stress field.

Richardson (1992) analysed the torques acting on plates, and concluded that there was a strong correlation between ridge torque poles and the motion of the N American, S American, Pacific, Cocos and Eurasian plates (although Lithgow-Bertelloni and Richards, 1998 continue to contend that slab-pull effects are more dominant). Richardson (1992) also points out that the ridge torque directions agree well with the orientations of maximum horizontal stresses in large plates, and he concludes that slab forces must be largely balanced within the subducted slab itself, and that slab-pull forces contribute relatively little to the deformation or stress distribution in the surface plates. Thus it appears that ridge push is a very important mechanism in determining intraplate stress levels, and, if Richardson's conclusions are accepted, plate kinematics. We shall see how these stresses can be effectively developed in Chapter 3.

Work on the major problem of geodynamics—determining the causes of plate motion—is still continuing. The dichotomy between models that explain plate motion in terms of forces deriving from within the plates themselves, and those that attempt to describe plate kinematics in terms of mantle flow, still has to be resolved. The major problem probably lies in effectively describing the complex rheological changes that occur as the lithosphere first cools as it forms at a ridge, and then warms up after its subduction.

In addition to the problem of the factors which determine plate motion, there are several other regions where the theory behind the plate tectonic model is patchy or even non-existent. These include (i) the cause of the apparently sudden changes in plate motion which define so many geological stage boundaries (Lithgow-Bertelloni and Richards, 1998), as exemplified by the great bend in the Hawaiian-Emperor seamount chain, (ii) the reason and mechanism for continental rifting, which for example gave rise to the break up of Gondwana, and (iii) how subduction zones are formed. In this book we shall attempt to provide solutions to these problems.

If we are truly to understand how our planet evolves, it is essential for us to be able to explain and model these processes. Obviously, much information which will be vital to our understanding of such plate tectonic processes will come from closer observation of the Earth. However, some insight into how our own planet evolves may also come from the study of other bodies in our Solar System. Of particular interest are questions such as: is plate tectonics a ubiquitous mechanism for the evolution of terrestrial planets?, and: do other planets exhibit tectonic features not seen on Earth, and if so why? In the next section we shall review some of the observations which are key to answering these questions.

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