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Chlorine-36 is produced in rocks at the earth's surface almost entirely by cosmic-ray-induced reactions.
Cosmic rays are attenuated by interaction with the earth's magnetosphere and atmosphere, so the production of 36-Cl is dependent on the latitude and elevation at which the sample was collected.
Cosmic rays are also attenuated by any material overlying the dating surface, such as windblown sand or snow, so in order for a meaningful age to be obtained, the surface either must have remained bare since initial exposure, or the cover history must be accurately known.
Therefore, cosmogenic techniques can be applied to dating of any surface that is composed of material that was not exposed to cosmic rays prior to formation of the surface, and has been exposed more-or-less continuously since.
In the case of an extrusive volcanic rock, buildup of cosmogenic nuclides begins when the rock is erupted, so measurement of the ratio of a cosmogenic isotope to a non-cosmogenic isotope can provide an estimate of eruption age (Phillips et al., 1986).
Phillips et al., 1997a and b; Phillips et al, in review, Dunbar and Phillips, 1996; Zreda et al., 1991, 1993; Zreda, 1994; Anthony and Poths, 1992, Laughlin et al., 1994).
These techniques rely on measurement of cosmogenic nuclides that begin to build up as soon as a rock is exposed to cosmic rays.
This is dominantly due to large systematic uncertainties.
The calcium spallation rate determined in this study is in excellent agreement with previous whole-rock calibration measurements at Tabernacle Hill, when these are recalculated with respect to the absolute timescale.