The effect of a viscosity stratification on phase change dynamics have been investigated with axi-spherical convection models. As in previous studies with a constant viscosity mantle an intermittent layering appears for a Clapeyron slope from -2 MPa/K to -3 MPa/K. A viscosity increase in lower mantle requires a more negative Clapeyron slope to produce the layering. This shift is sensitive to the mechanical boundary condition. With a viscosity contrast of 30, a no-slip top condition does not lead to layering in the range of the possible values for the Clapeyron slope. With a free-slip condition, the threshold is at -4 MPa/K. Just below this threshold, a whole mantle circulation driven by a cylindrical hot plume coexists with layered mantle domains over several billion years. ¿ American Geophysical Union 1996