Rocks are dated using a process that leverages the predictable decay of radioactive isotopes. This is a method to determine a rock's absolute age rather than its relative age.
Understanding Radioactive Decay
At the heart of rock dating lies radioactive decay.
- Radioactive isotopes are unstable forms of elements.
- These isotopes naturally transform into more stable forms (daughter isotopes).
- This transformation process happens at a consistent, clock-like rate.
This constant rate is what allows scientists to calculate how old a rock is. As the provided reference states, "The nuclear decay of radioactive isotopes is a process that behaves in a clock-like fashion and is thus a useful tool for determining the absolute age of rocks. Radioactive decay is the process by which a “parent” isotope changes into a “daughter” isotope."
How it Works: Radiometric Dating
Radiometric dating, therefore, is used to age rocks by measuring the amounts of parent and daughter isotopes:
- Parent Isotope: The original radioactive element within the rock.
- Daughter Isotope: The stable element the parent isotope transforms into.
- Ratio Analysis: Scientists measure the ratio of parent to daughter isotopes in a rock sample.
- Calculation: Using the known decay rate (half-life) of the radioactive isotope, they calculate the time elapsed since the rock formed.
Example of Radiometric Dating using Uranium-Lead
| Isotope Pair | Parent Isotope | Daughter Isotope | Half-life |
|---|---|---|---|
| Uranium-Lead | Uranium-238 | Lead-206 | 4.47 billion years |
Using Uranium-238 as an example:
- A fresh, molten rock contains mainly Uranium-238.
- As time passes, Uranium-238 gradually decays into Lead-206.
- By measuring the amount of Uranium-238 and Lead-206 in the rock, scientists determine its age.
- A 50/50 ratio indicates the rock is approximately one half-life old.
Key Points
- Not all rocks can be dated radiometrically: Sedimentary rocks cannot be directly dated by this method.
- Igneous rocks are ideal: Formed directly from cooled molten material.
- Accuracy: The accuracy of radiometric dating relies on the precision of isotope measurement and known decay rates.
Conclusion
The consistent and predictable decay of radioactive isotopes provides a reliable method for determining the absolute age of rocks, offering key insights into Earth's history.
