Lateral variations in seismic velocity (through its dependence on temperature) can easily be generated at the gravest harmonics, including degrees one and two, by the dynamic interaction between plates and convection. Models of thermal convection with a single non-subducting plate have been formulated in a cylindrical geometry. Plates of width one to four times the thickness of the convecting region strongly modulate the flow by being pushed over cold downwellings and inhibiting cooling of the fluid beneath. During rapid motion off of hot regions, a large-scale pattern of shear is developed causing small uprising limbs to be swept into the largest upwellings. Both insulation and plume-plume collisions pump energy into the lower wavenumber harmonics. ¿ American Geophysical Union 1991