ON COSMIC-RAY EXPOSURE AGES OF TERRESTRIAL ROCKS - A SUGGESTION

An important recent development in the field of geomorphology has been the application of in-situ cosmic-ray-produced nuclides to obtain mod el erosion rates and surface exposure ages. These concepts emerged som e four decades ago in studies of cosmogenic nuclides in meteorites, bu t cannot generally be used analogously for terrestrial rocks. The diff erences in the two cases are outlined. For the case of steady-state er osional histories, the terrestrial surface exposure ages depend on the half-life of the radionuclide studied. A suggestion is made for prese nting the surface exposure ages, which allows a clear definition of th e meaning of the estimated exposure ages. In the case of a discrete ex posure history, the meaning of ''exposure age''-which should more appr opriately be called ''event age''-is however quite unambiguous.