Iron has a half life of 45 days. Log in. Uranium decays over time to form lead Scientists use radioactive decay to determine the age of the rock. Earth is very old and has changed over time. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. The Earth is 4.
Absolute vs. Relative Dating
A murder weapon was found in a lake and the original weapon was reported to have contained 1, grams of iron.