Despite many spacecraft missions and geophysical modelling studies, many first-order questions remain regarding the dynamics and evolution of solid planets, such as why plate tectonics occurs on Earth, why Mars has a crustal dichotomy, and why Venus is so different from Earth. Additionally, terrestrial planets around other stars (so-called super-Earths) are now being discovered, raising questions. We are engaged in systematic numerical modelling studies in 3D spherical geometry designed to answer some of these questions. Results offering explanations of many first-order observations of these planets will be presented and discussed.