Cosmogenic refers to rare radioactive isotopes created when cosmic radiation interacts with an atomic nucleus. These isotopes are produced on Earth, in Earth's atmosphere, and in extraterrestrial items such as meteorites.
Some elements that have cosmogenic isotopes are:
Because cosmogenic isotopes have very long lives (anywhere from thousands to millions of years), scientists find them useful for dating extremely old geological features and activities.
Refer to the indicated sources for more detailed information.
Sources:
Purdue University Prime Lab, "Cosmogenic nuclides" http://plsv1.physics.purdue.edu/primelab/cosmo.html "Cosmogenic Exposure Dating and the Age of the Earth" http://www.geocities.com/earthhistory/tcn.htm Cosmogenic Isotope Laboratory, Quaternary Research Center and Department of Earth and Space Sciences, University of Washington (note: State of Washington, not DC) http://depts.washington.edu/cosmolab/index.html
