Absolute time is a method for determining the age of a rock or object most often using radiometric isotopes. Atoms are made of three particles, protons, electrons, and neutrons. All three of these particles are important to the study of geology: the number of protons defines a particular element, the number of electrons control how that element bonds to make compounds, and the number of neutrons changes the atomic weight of an element. Isotopes are atoms of an element that differ in the number of neutrons in their nucleus and, therefore, their atomic weight. If an element has too many or too few neutrons in its nucleus then the atom becomes unstable and breaks down over time, which is called radioactive decay. The process of radioactive decay involves the emitting of a particle from a radioactive atom, called the parent atom, which changes it to another element, called the daughter atom. We can study and measure the radioactivity of different elements in the lab and calculate the rate of decay. Though the rate of decay varies between isotopes from milliseconds to billions of years, all radiometric isotopes decay in a similar way.
Radiometric decay follows a curve that is defined by a radiometric isotope’s half-life. The half-life is defined as the amount of time it takes for half of the atoms of the radiometric parent isotope to decay to the daughter. The half-life is independent of the amount of atoms at a given time so it takes the same amount of time to go from 100% of the parent isotope remaining to 50% as it does to go from 50% of the parent isotope remaining to 25%. If we know the length of the half-life for a particular radiometric isotope and we measure the amount of parent and daughter isotope in a rock, we can then calculate the age of the rock, which is called Radiometric Dating. Given the shape of the decay curve, a material never runs out of the parent isotope, but we can only effectively measure the parent up to 10-15 half-lives.