Numerical modelling of mantle convection, with lateral variations of viscosity, and both the 670 km depth endothermic and the 400 km exothermic phase changes, reveals that downwellings are weakened upon crossing the exothermic phase change. A high temperature/low viscosity section is developed within the downgoing current, owing to both the olivine to spinel phase transition and the slowing down of the flow due to the 670 km endothermic transition. This mechanism may cause the creation of a weak zone at 400 km that results in a preferential location for horizontal or vertical slab tearing as revealed by seismic gaps in many slabs. ¿ 1998 American Geophysical Union