We have developed a comprehensive physical description of the processes and pathways by which planetary bodies acquire internal energy. Our next task is to examine how this internal energy drives planetary processes. The hallmark of an active planetary body is that it has surface features, other than impact craters, that have ages that are negligible compared to the age of the Solar System. This is true for any epoch of the Solar System. For instance, the youngest features on the Moon, the immense basaltic plains that we call lunar maria, are about 3 billion years old. This means that the Moon is dead today, but it was active when the age of the Solar System was of the order of 1.5 billion years

Active planetary processes are associated to heat flow, but the causal connection is not always the same. Consider the ascent of magmas. This is a process that transfers mass and heat from the planet's interior towards its surface, and that is made possible by melting, which entails conversion of thermal energy to chemical energy. Ascent of magma and construction of volcanoes, however, are not driven by thermal energy but by gravitational energy. Magmas rise to the surface of a planet if they are buoyant, and magmas are buoyant if melting causes a decrease in density.