Dating age rule of thumb
These cosmic rays originate from high-energy supernova explosions in space.Wherever we are on Earth, when we are outside, we are constantly bombarded by these cosmic rays.When particular isotopes in rock crystals are bombarded by these energetic cosmic rays neutrons, a reaction results.Spallation reactions are those where cosmic-ray neutrons collide with particular elements in surface rocks, resulting in a reaction that is sufficiently energetic to fragment the target nucleus. Counting the numbers of these isotopes, normally as a ratio to other isotopes, means that scientists can calculate how long rocks have been exposed at the Earth’s surface.These cosmic rays do not penetrate deep into the earth’s surface.
Spallation reactions occur in minerals in the rocks upon bombardment by cosmic rays.
Different isotopes are used for different lengths of times.
This long period of applicability is an added advantage of cosmogenic nuclide dating.
However, this can be difficult, as thermal boundaries within the ice sheet may mean that it is more erosive lower down than higher up, and that cold, non-erosive ice on the tops of mountains may leave in tact older landscapes.
Cosmogenic nuclide dating can also be used in this context to understand past ice-sheet thicknesses and changes in subglacial thermal regime.