To form mantle plumes with large heads and narrow trailing conduits requires large (>102) viscosity variations within the hot thermal boundary layer at the plume source. However, new and published data from laboratory experiments show that if a layer of fluid with a temperature-dependent viscosity is heated from below and cooled from above, convection occurs beneath a stagnant lid, and the interior fluid temperature remains close to that of the hot boundary. Consequently, the viscosity reduction across the hot thermal boundary layer is always <10 and upwellings take the form of discrete plume heads (thermals). Subduction of the stagnant layer (equivalent to recycling lithospheric plates on the Earth) will cool the interior of the mantle, thus increasing the viscosity ratio. Hence, the existence of Earth-like mantle plumes may be a consequence of plate tectonics. An absence of plate tectonics on Venus may lead to upwellings composed of thermals, the surface expression of which are coronae, as well as a smaller core heat flux than on Earth.