Knowledge of mantle petrology and the constitution of the deeper mantle is important in trying to understand several aspects of plate tectonics. For instance, is there whole-mantle convection or two-layer convection? What are mantle plumes? What are superplumes? Does the subducting slab penetrate into the lower mantle? What happens to the slab at depth? Is the sub-continental mantle different from the oceanic mantle? First, some basic facts.

Principal Internal Subdivisions of the Earth

Note that the crust makes up quite a small proportion of the total Earth. The main problems that have occupied geologists over the years are: What is the nature of the crust-mantle boundary (the MOHO). What is the nature of the low velocity zone? Is the lithosphere diferent in composition from the asthenosphere? What happens in the transition zone? What is the nature of the deep mantle? 

Fig. 1. The Earth in proportion. The thickness of crust is exaggerated. Note that the convecting upper mantle is quite shallow; but it is now known that at least some subducting slabs penetrate the 700 km discontinuity, and could well descend to the bottom of the lower mantle. When heated by the hot liquid core they could eventually rise as mantle plumes.