The first-order cause of lithospheric motion at the Earth's surface is convection within the mantle. I examine how lithospheric thickness variations affect the dynamics of the upper mantle and the impact they can have on the surface in a series of analytical and numerical experiments. Perturbations to the thickness of mantle lithosphere from horizontal shortening are considered as Rayleigh-Taylor instabilities. This deformation is considered in the context of the Sierra Nevada range in California, where lithosphere may be downwelling and the lower crust may be weak. Lithospheric instabilities are also considered in relation to intracontinental magmatism, several hundred kilometers away from active plate boundaries or rift zones. In cratonic regions, where lithosphere can be several times thicker than the global average, the motion of continental keels cause pressure gradients within the upper mantle. Constraints on upper mantle viscosity can be obtained by considering the dynamic gravity effects from these induced pressure gradients. At subduction zones the motion of subducted lithosphere within the mantle is examined along with its effects on the pressure field and dip angle evolution. Overall, lithosphere of varying thickness can have significant regional impacts on Earth's near surface dynamics, which occur against the background first-order dynamics.