Exposure dating cosmogenic nuclides
Certain light (low atomic number) primordial nuclides (some isotopes of lithium, beryllium and boron) are thought to have arisen not only during the Big Bang, and also (and perhaps primarily) to have been made after the Big Bang, but before the condensation of the Solar System, by the process of cosmic ray spallation on interstellar gas and dust.
This explains their higher abundance in cosmic rays as compared with their ratios and abundances of certain other nuclides on Earth.
By sampling the rocks and separating certain minerals (such as quartz or pyroxene) and calculating the amount of these minerals (as a ratio to other, stable, minerals), we can work out how long the rock has been exposed on the earth’s surface.
Cosmogenic nuclides are rare nuclides that form in surface rocks because of bombardment by high-energy cosmic rays.
The palette of nuclides (He) and the different dating approaches (surface exposure dating, depth-profile dating, burial dating) allow the calculation of the times of formation of myriad bedrock and depositional landforms made up of nearly all rock types.
Nuclide concentrations are also used to determine both the rates and spatial patterns of landform construction and degradation.
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.
Cartoon illustrating cosmogenic nuclide exposure ages. A glacier transports an erratic boulder, and then recedes, exposing it to cosmic rays.
Spallation reactions occur in minerals in the rocks upon bombardment by cosmic rays.
There are both radioactive and stable cosmogenic isotopes.
Some of these radioisotopes are tritium, carbon-14 and phosphorus-32.
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