Dating rocks wiki
Although the time at which any individual atom will decay cannot be forecast, the time in which any given percentage of a sample will decay can be calculated to varying degrees of accuracy.
The time that it takes for half of a sample to decay is known as the half life of the isotope.
where is the half-life of the element, is the time expired since the sample contained the initial number atoms of the nuclide, and is the remaining amount of the nuclide.Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures.Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock.The fragments emitted by this fission process leave trails of damage (fossil tracks or ion tracks) in the crystal structure of the mineral that contains the uranium.The process of track production is essentially the same by which swift heavy ions produce ion tracks.
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Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released.