Cosmogenic radionuclide dating
Although erosion studies can be performed in bedrock, they are actually most commonly done on sediments. Consider a landscape that is in an erosional steady state and that is irradiated by cosmic rays. Above the erosion island is the ‘forbidden zone’ of physically impossible cosmogenic nuclide compositions.
Samples plotting in this area are likely to suffer from methodological or analytical errors.
This provides a hard upper limit of the exposure ages that can be measured with cosmogenic radionuclides. The only way to avoid making such assumptions and simultaneously determining both the erosion rate and the exposure age is to measure two nuclides with different half lives.
One example is Be in the surface rocks is known, and if the production rate is known, then the exposure age of the rock can be estimated.
So if we measure the concentration (N) in atoms per gram of, say, quartz, and if we know the production rate (P), in atoms per gram per year, then we can simply calculate the age by dividing the concentration by the production rate: Next, consider the situation of a stable nuclide and a rock surface that is in steady state erosion.
To understand this situation, it is useful to imagine one in the place of a rock particle under an eroding surface.
Thus, the cosmic ray intensity at the equator is significantly lower than at the poles, although the average energy (or ‘rigidity’) of the cosmic rays is higher The primary cosmic rays which do manage to pass through the magnetic field strongly interact with the atmosphere and form a secondary cosmic ray ‘shower’, which is mostly made of neutrons and muons.
This secondary cosmic ray shower is rapidly attenuated as it travels down into the atmosphere.