The influence of MgSiO3 majorite on mantle convection has been investigated via 2-D numerical simulations that incorporate the stability field of majorite. According to a recent first principles study, wadsleyite decomposes into an assemblage of majorite plus periclase with a large negative Clapeyron slope. Since the stability field of majorite is limited to be greater than similar to 2200 K in a depth range of 500-660 km for Mg2SiO4, very hot upwelling plumes are expected to be strongly influenced by the phase transitions related to majorite. These hot upwellings are occasionally observed in simulations, even though the average temperature of hot plumes is far less than the stability field of majorite. The dynamics of these upwellings are controlled by the release and the absorption of latent heat induced by majorite's phase transitions as well as by the interruption of currents due to the large negative Clapeyron slope related to majorite.