Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive decay.
Radioactive decay occurs at a constant rate, specific to each radioactive isotope.
If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.
Radioactive elements were incorporated into the Earth when the Solar System formed.
All rocks and minerals contain tiny amounts of these radioactive elements.
Geologists often need to know the age of material that they find.
They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.
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Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.