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Why can't most sedimentary rocks be dated radiometrically?

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Quick Answer

According to the Smithsonian Institute, using radiodating of sedimentary rock tells the date of formation of the original igneous rock, which, through the processes of weathering and erosion, formed the layers of the sedimentary rock. Radiodating determines the maximum age of sedimentary rock. If a layer of igneous rock forms on top of the sedimentary rock, scientists determine an age bracket for the rock sample, but not an absolute age.

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The U.S. Geological Survey states that it is possible to use Carbon-14 radiometric dating for sedimentary rock younger than 50,000 years by dating once living material from the sediment. However, the relatively short half-life of approximately 5,730 years makes it inappropriate for older samples. For dating older materials, scientists use isotopes of other elements, some of which have a half-life of 106 billion years.

According to the Utah Geological Survey, nuclear decay of radioactive elements works like a clock in allowing scientists to determine an absolute age. During radioactive decay, the parent isotope transforms into a stable daughter isotope. The rates at which radioactive decay occur are constant and measurable using the isotope's half-life. The half-life of a radioactive element is the time required for half the isotope to decay to form the stable daughter isotope.

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