Radioactivity is the process by which we've estimated the age of the Earth. It's also crucial in understanding how the Earth works and the source of geological activity. The interior rocks of the Earth contain small concentrations of radioactive elements or compounds. The radioactive decay process releases energy in the forms of particles and radiation. This energy suffuses through the rock and melts the rock driving geological activity. In any sufficiently massive planet, radioactive decay of the interior rocks causes geological activity. Thus leads to a situation were the surface of a planet can be solid while parts of the interior are liquid. In a sense radioactivity is the engine that drives geological activity.