The stratigraphy, impact cratering record, and geoid of Venus indicate that a major change in surface geologic activity has occurred. Before ∼800 Ma Venus was dominated by horizontal movement and tectonic deformation of the crust. A brief period of global volcanism subsequently filled in all low-lying areas and left much of the surface at a nearly uniform elevation. Current geologic activity is dominated by large volcanoes and limited horizontal movement of the crust. This geologic history may have resulted from mantle cooling causing the lithosphere to become positively buoyant globally.