Despite wide acceptance of the Hess model of convection currents, it is very difficult to see how the out-flowing magma along the mid-Atlantic ridge can contribute sufficient ‘ridge-push’ forces to drive the African and South American continents apart. The mechanical based reason for this doubt by the author is that the resistance to the westward motion of the South American plate by the rigid Nazca plate and the relatively stationary African plate would have resulted in the gross deformation and by definition the destruction of the evidence of the mirror imaged almost parallel lines of the paleo-magnetic reversal cycles either side of the ridge. A study of more recent publications 34, 13, 25, 39 suggest that plate movement is not directly controlled by ‘ridge –push’. Current thinking is that magma extrusion at the ridges is associated with up-welling or diverging convection currents and as such it exerts a negligible contribution to tectonic forces.
Despite the general consensus34, 38 et al in favour of the Hess 'Slab-Pull' model, exhaustive examination of the literature shows that there is still no general consensus of agreement on the model needed to adequately describe the source, locality and direction of the heated circulation currents that are considered necessary to move the lithosphere over the asthenosphere. Experimental data on the varying composition of the mantle obtained from igneous petrology studies 5, seismic wave propagation 45, mathematical 1 and thermal modelling 13, as well as consideration of mantle plumes or hot spots 3 has resulted in several different heat convection currents systems being described. Some of the different proposed circulation systems are summarised in Fig.3
A very puzzling feature of the convection current 'slab-pull' model is that sub-duction is associated with convergent plate boundaries. If convection currents are indeed responsible for causing denser oceanic crust to move under the lighter continental crust, then by the noted omni-directional movement of these heated currents, 'slab-pull' forces with the creation of trenches would be more random and visible on the continental crusts.
Fig. 3. Various convection current systems proposed for tectonic movement. (a) circulation pattern from the core through to the lithosphere (Holmes 19), (b) circulation pattern as above plus thermal plumes 2 (c) circulation currents confined to the upper mantle 23 and (d) a composite current pattern incorporating sub-current cells in the upper mantle 34
In contradiction to the growing acceptance that the magnitude of the forces formerly attributable to 'ridge-push' as described above are negligible, there is a high level of agreement, that the major force for plate movement is due to the 'slab-pull' forces associated with the sub-duction of the heavier oceanic slab under the lighter continental crust From an engineering point however, there appears to be an absence of similar magnitude 'action- reaction' mechanical forces with which to determine a force vector diagram to allow the 'slab pull' vector to be unambiguously represented.