ABSTRACT
The evolution of surface features of the Earth, driven by mantle convection, is characterised by an arrangement of quasi-rigid lithospheric plates in relative motion at speeds of a few centimetres per year. Activities such as earthquakes and volcanic eruptions are concentrated at the boundaries between the plates, where the relative motion is concentrated. Although most boundaries, especially those where plates converge, involve deformation over tens or hundreds of kilometres, the concept of a set of plates that remain coherent and undeformed as they move, with growth or disappearance only at their boundaries, is a useful approximation. Progressive deformation within plates is commonly represented by independent micro-plates but, as the multiplicity of identifications increases, the usefulness of the plate concept decreases, and it is advantageous to restrict the perceived fragmentation to recognition of plates with clearly independent motions that relate to the underlying convective driving forces. This is the philosophy of the plate selection in Table 13.1 and Figures 13.1 and 13.2. Even so, the figures indicate a distinction between the seven major plates and smaller ones. The selection differs in minor ways from some others, most noticeably in the inclusion of Sunda as a plate that cannot be regarded as part of any of its neighbours and the exclusion of Somalia, which is not moving fast enough, relative to the rest of Africa, to be considered independent of it. Primary data are from a detailed analysis by Bird (2003), who listed a much larger number of plates, but for calculation of the areas in Table 13.1, most of the minor ones are counted as parts of those listed here. The size distribution illustrated in Figure 13.2 is not materially affected by this.
