Two methods of dating rocks
If a zircon crystal originally crystallizes from a magma and remains a closed system (no loss or gain of U or Pb) from the time of crystallization to the present, then the Discordant dates will not fall on the Concordia curve.
Essentially, by looking at fossils in a rock column, geologists are able to determine which fossils are geologically older than others.
Fossils are the remains of plants and animals that have been preserved in rocks.
Geologists have found fossils that are only a few thousand years old and others that are more than 600 million years old.
Radioactive decay is a process where an isotope of an element spontaneously loses particles from its nucleus to create a daughter isotope. The half-life of a radioactive isotope is the amount of time it takes for one-half of the radioactive isotopes to decay.
By comparing the ratio of parent isotopes to daughter isotopes, scientists are able to determine the age of the fossil and the rock.
Isotopes that have long half-lives are necessary to date fossils because many fossils are millions of years old.