Cosmogenic dating group
Cosmic rays that have sufficient energy to break the intranuclear bonds of nucleons interact with nuclei in minerals exposed within the upper few meters of the Earth's surface.Concentrations of cosmogenic nuclides produced Cl) are routinely measured in minerals or whole rock samples to determine the exposure age of a wide range of landforms, many of which could not otherwise be dated directly.
The excess relative to natural abundance of cosmogenic nuclides in a rock sample is usually measured by means of accelerator mass spectrometry.For example, beach berms, moraines, terraces, alluvial fans, landslides, lavas, fault scarps, meteorite craters, felsenmeer, and tors ranging in age from 10 terrestrial cosmogenic nuclide (TCN) dating methods.TCN methods are used to directly determine erosion rates of landforms (boulders to mountain peaks) and landscapes (entire catchments), at timescales that are intermediate between modern stream sediment flux measurements and long-term thermochronology estimates of exhumation.It is most useful for rocks which have been exposed for between 10 years and 30,000,000 years.The most common of these dating techniques is Cosmogenic radionuclide dating.The cumulative flux of cosmic rays at a particular location can be affected by several factors, including elevation, geomagnetic latitude, the varying intensity of the Earth's magnetic field, solar winds, and atmospheric shielding due to air pressure variations.
Rates of nuclide production must be estimated in order to date a rock sample.
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.
Decay rates are given by the decay constants of the nuclides.
These equations can be combined to give the total concentration of cosmogenic radionuclides in a sample as a function of age.
Using certain cosmogenic radionuclides, scientists can date how long a particular surface has been exposed, how long a certain piece of material has been buried, or how quickly a location or drainage basin is eroding.