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How does absolute dating give rocks numerical ages?
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geochronology
The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.
Absolute dating provides numerical ages for rocks primarily through radiometric dating, a method based on the constant rate of radioactive decay.12
How Radiometric Dating Works
Radioactive isotopes (parent atoms) decay into stable daughter products at a fixed, immutable rate known as a half-life. Because this process is unaffected by external conditions like heat or pressure, it serves as a reliable "clock" for geologic time.12
- Measurement: Scientists use a mass spectrometer to measure the ratio of parent isotopes to daughter isotopes within a rock or mineral sample.23
- Calculation: By knowing the decay constant of the specific isotope, researchers can calculate the time elapsed since the mineral or rock formed.23
- Requirements: For an accurate age, the system must have remained "closed"—meaning no parent or daughter atoms escaped or were added—since the time of formation.24
While radiometric dating is the standard for geochronology, other specialized methods, such as obsidian hydration dating, can also provide absolute ages by measuring the thickness of hydration rims on specific materials.5