Gaining estimates of ages of rocks is crucial for establishing not only the history of geological events but also for determining the rates of geological processes. It is possible to establish the relative order of events in some rocks. This is called stratigraphy and is most commonly used for sedimentary layers, laid down successively on top of one another. However for most purposes we need absolute ages. These can be established using radioactive decay. The underlying principle is that the probability of an individual radioactive atom breaking down (to create a daughter atom) is constant. Different radioactive decay systems have different probabilities and these are expressed as their DECAY CONSTANT. For a given parent to daughter decay system (e.g. potassium 40 goes to argon 40) and its unique decay constant, the number of daughter atoms created depends on the amount of time and the original number of parent atoms. This can be tracked graphically.
In practice the determination of ages uses ratios between different isotopes, measured with great precision in modern mass spectrometers. The results can be interpreted graphically on something called an isochron plot. Isochron plots for the rubidium-strontium system applied to old rocks from Greenland and for chrondritic meteorites.
In practice great care is necessary in applying isotopic methods to date rocks. A key assumption is that a sample has remain a closed system so that the number of parent and daughter atoms can be fully audited. To examine these problems of diffusion, click here. Note however, these problems also work to our advantage. We can use the leaky nature of rocks and minerals to isotopic diffusion to estimate the cooling history of rocks - which is very important in tracking the passage of rocks to the surface as their overburden is eroded.