This is a radiometric technique since it is based on radioactive decay.
Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide.
For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.
One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains.
a metamorphosed igneous rock may give a crystallisation age of 500 million years and a metamorphic age of 400 million years indicating the metamorphism is younger than the igneous rock, which of course this would be self evident from the relative dating observation that the igneous rock was metamorphosed (since the igneous rock could not have been metamorphosed before it formed!