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Our transcription: Minerals which contain radioactive isotopes act like tiny clocks inside of rocks, clocks which are reset when a rock is formed or metamorphosed. Before geologists learned to measure the absolute ages of rocks using radioactive isotopes, they could only establish relative ages using the common sense logic of superposition, original horizontality, and cross-cutting relationships. They knew that one rock formed before another, but never how old the rocks actually were. The reliability and accuracy of radiometric age dating is truly impressive. The results of over half a century of radiometric age dating studies are entirely consistent with the relative age relationships developed by early geologists. So the science of geology has given us two ways to understand Earth history. First, we determine the relative age of rock formations and geologic structures and then use these ages to construct a historic sequence of events through time. Then, by applying radiometric age dating techniques, we determine the absolute timing of these events and measure the rates of geologic change as well. So not only does our study of geologic time allow us to understand Earth's past, it gives us a historic framework and the tools for predicting the planet's future.
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