One of the chief ways scientists determine an age for the earth is by measuring the rates of decay of radioactive isotopes.
Within the Earth's crystalline rocks, daughter isotopes are found alongside their radioactive parent isotopes (including whole decay chains), scars (radiohalos) are visible due to the a-particles produced by radioactive decay, the a-particles themselves have been retained as the nuclei of helium atoms, tracks are visible from decay by fission, and the heat produced by radioactive decay is present.
Based on this evidence geologists analyse the relevant radioisotopic ratios in rocks and minerals, and then use those ratios to calculate multi-million year ages for the rocks and minerals. However, it must be stressed that these radioisotopic dating techniques are only successful because it is assumed the rates of radioactive decay have been constant at today's measured rates throughout Earth history.
