Geology, Minerals, Energy and Geophysics Science Center home. The information found here reflects completed USGS work. The content of this page is static and has not been updated since the mid 's. Radioactive elements were incorporated into the Earth when the Solar System formed. All rocks and minerals radioactive isotope used for age dating tiny amounts of these radioactive elements.
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. Since the radioactive isotope used for age dating, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. Radiometric clocks are "set" when each rock forms. It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.
A commonly used radiometric dating technique relies on the breakdown of potassium 40 K to argon 40 Ar. In igneous rocks, the potassium-argon "clock" is set the moment the rock first crystallizes from magma. Precise measurements of the amount of 40 K relative to 40 Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized.
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. Carbon is a method used for young less than 50, year old sedimentary rocks. This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon by all living things. When living things die, they stop taking in carbon, and the radioactive clock is "set"!
Any dead material incorporated with sedimentary deposits is a possible candidate for carbon dating. Radiometric dating has been used to determine the ages of the Earth, Moon, meteorites, ages of fossils, including early man, timing of glaciations, ages of mineral deposits, recurrence rates of earthquakes and volcanic eruptions, the history of reversals of Earth's magnetic field, and many of other radioactive isotope used for age dating events and processes.
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