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STRATIGRAPHY
Lesson 3 - Page 6

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Absolute age dating determines how many years before the present a rock formed. It gives the age of the rock in years. Geologists use radioactive elements to do absolute age dating. Radioactive elements are unstable. Over time, they decay or change into stable elements or will decay to become either some nearby element in the periodic table or some isotope of the same element. For example, radioactive potassium-40 decays into calcium-40 (11%) and Argon-40 (89%). We know how long it takes for such a transformation, by calculating the amount of daughter material (calcium and argon) that is produced from potassium. The time it takes is called its ``half_life'' and represents the average time an atom will survive in its original state.

Radiometric clocks are "set" when each rock is created. So the moment an igneous rock solidifies from magma or a sedimentary rock layer is deposited or a rock changed by metamorphism, the clock begins. It is this process that gives us the ability to date rocks that formed at different times in earth history.


Radioactive decay of Potassium-40

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