text: Some unstable isotopes which occur naturally (such as , , and ) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as  and ), or (rarely) by such processes as geonuclear transmutation (neutron capture of uranium in the case of  and ). Other examples of common naturally occurring but non-primordial nuclides are isotopes of radon, polonium, and radium, which are all radiogenic nuclide daughters of uranium decay and are found in uranium ores. The stable argon isotope Ar is actually more common as a radiogenic nuclide than as a primordial nuclide, forming almost 1% of the Earths atmosphere, which is regenerated by the beta decay of the extremely long-lived radioactive primordial isotope K, whose half-life is on the order of a billion years and thus has been generating argon since early in the Earths existence. (Primordial argon was dominated by the alpha process nuclide Ar, which is significantly rarer than Ar on Earth.)
  A similar radiogenic series is derived from the long-lived radioactive primordial nuclide Th. These nuclides are described as geogenic, meaning that they are decay or fission products of uranium or other actinides in subsurface rocks. All such nuclides have shorter half-lives than their parent radioactive primordial nuclides. Some other geogenic nuclides do not occur in the decay chains of Th, U, or U but can still fleetingly occur naturally as products of the spontaneous fission of one of these three long-lived nuclides, such as Sn, which makes up about 10 of all natural tin. Another, Tc, has also been detected. There are five other long-lived fission products known.