The diagrams below show some conceptual models of how the mantle may be convecting, and possible relationships between the upper and lower mantle (after Allègre et al.). There is still a very intense debate on whether the lower mantle is involved in manconvection.
|Fig. 20. Box models for crust-mantle evolution. On the left continental growth occurs through igneous contributions from both the upper and lower mantle. On the right the continental crust has mainly been extracted from the upper mantle, which is therefore "depleted" relative to lower mantle.|
|Fig. 22. Cartoon showing how subducting slabs may either lay themselves out along the 650 km discontinuity (a), or penetrate the discontinuity to enter the lower mantle as in (b). The latter gives active back-arc spreading (see later lecture).|
The most recent analysis of the fate of the oceanic crust as it subducts into the mantle beneath the West Pacific island arcs (van der Hilst & Seno, 1993), suggests that whereas that subducting beneath the Izo-Bonin arc and Shikoku Basin, south of Japan may be deflected and "laid-out" along the 650 km discontinuity in the transition zone (Fig. 22(a)), that further south beneath the Mariana Arc may penetrate into the lower mantle (Fig. 22(b)):