Despite the fact that the radioactive isotopes mentioned above are now effectively extinct, the record of their existence is found in their decay products and are very useful to geologists who wish to use them as geochronometers. Their usefulness derives from a few factors such as the fact that their short half-lives provide high chronological resolution and the chemical mobility of various elements can date unique geological processes such as igneous fractionation and surface weathering. There are, however, hurdles to overcome when using extinct nuclides. The need for high-precision isotope ratio measurements is paramount as the extinct radionuclides contribute such a small fraction of the daughter isotopes. Compounding this problem is the increasing contribution that high-energy cosmic rays have on already minute amounts of daughter isotopes formed from the extinct nuclides. Distinguishing the source and abundance of these effects is critical to obtaining accurate ages from extinct nuclides. Additionally, more work needs to be done in determining a more precise half-life for some of these isotopes, such as Fe and Sm.