Here's why:
* Radioactive Decay: Igneous rocks contain radioactive isotopes that decay at a predictable rate.
* Half-Life: Each isotope has a specific half-life, which is the time it takes for half of the radioactive atoms to decay into stable atoms.
* Measuring Isotopes: By measuring the ratio of radioactive isotopes to their stable decay products within the rock, scientists can calculate how much time has passed since the rock solidified.
Example:
* A common radiometric dating method uses the decay of uranium-238 into lead-206.
* By comparing the amounts of uranium and lead in a rock, scientists can estimate the age of the rock.
Other Methods:
While radiometric dating is the most accurate method for determining absolute age, other methods can be used in combination:
* Fossil Evidence: If the rock contains fossils, their age can help constrain the age of the rock.
* Stratigraphy: Examining the layering of rocks can help determine relative age, which can be used in conjunction with radiometric dating.
