We conduct numerical simulations of incompressible infinite Prandtl number convection in a spherical shell with a single localized high viscosity lid (HVL) on the top surface to understand the possible effects of the continental lithosphere on plume generation. The temperature under the HVL increases rapidly after the emplacement of the HVL on constant viscosity convection with internal and bottom heatings. Later, upwellings at the bottom merge into a large scale flow and a large plume emerges under the HVL. This timescale depends on the existence of phase changes. In our model, whose Rayleigh number is 106, the large plume is formed on a time-scale of 1 Gyr. A simple scaling suggests that this time-scale may be reduced by a several times, if the Rayleigh number is around 107. Despite the complicated 3-D thermal structure, it is dominated by an l=1 pattern controlled by the position of the HVL. ¿ 1999 American Geophysical Union