Major phase transitions occur in the normal mantle at depths near 400 and 650 km accompanied by density changes of 8–10 percent. The temperature gradients of the transitions are probably positive and in the vicinity of 20–30 bars/°C. The influence of these transitions upon the mantle engine which transports lithosphere plates is discussed. Phase transitions inhibit the occurrence of ‘Vening-Meinesz’ type mantle convection, postulated to occur in the presence of very small superadiabatic temperature gradients. It does not appear likely therefore, that lithosphere plates behave as passive objects, transported on the backs of mantle-wide convection currents of this kind.On the other hand, the sinking of a slab of lithosphere beneath an oceanic trench occurs under highly superadiabatic conditions. The levels at which major phase transitions occur in the sinking slab are displaced upwards by 30 to 100 km, compared to the depths at which they occur in normal mantle. This causes a marked increase in the average density of the slab, which sinks into the mantle because of the effect of gravitational body forces arising from this excess density. Phase transformations are thus shown to play the major role in driving the lithosphere slab into the mantle at depths greater than 300 km, and also control the stress distribution within the slab.The hypothesis that deep earthquakes are caused by phase transformations is reconsidered. In a dynamic mantle, the difficulties earlier faced by this hypothesis are greatly weakened. It now appears quite likely that deep earthquakes may be caused by failure under stresses generated directly or indirectly by phase transformations. Additional possible effects of phase transformations in a dynamic mantle are briefly considered.