The most commonly used radiometric dating methods are potassium-argon, uranium-lead, and rubidium-strontium. Half-life is simply the time required for half of the atoms in a pound of uranium, for example, to disintegrate into lead.The concept of how these methods work is simple: one element decays into another at a rather predictable rate. If these assumptions are correct, then the radiometric dates are correct.
These assumptions include: 1) the initial amount of the daughter isotope is known, 2) neither parent or daughter product has migrated into, or out of, the closed rock system, and 3) decay has occurred at a constant rate over time.
But what if one or some combination of these assumptions is incorrect?
Both the decaying isotope and the isotope it produces (its daughter) can be compared to an isotope of the daughter’s elemental family that does not decay.
These two ratios, when plotted on a graph for many different samples from a rock suite, should hypothetically produce a straight line under certain assumed conditions.
other isotope pairs cover intermediate time periods between the spans for carbon 14 and uranium.
Some radiometric dating methods depend upon knowing the initial amount of the isotope subject to decay.For this purpose, isochron dating was developed, a process "that solves both of these problems (accurate date, assumptions) at once" (Stasson 1992). 2) The process must occur at a relatively uniform rate.It has been established through extensive experimentation that radioactive decay occurs at a constant rate. In this case, the initial condition is the amount of daughter isotope in the rock when it was formed. Francis Crick (co-discoverer of one of the most important discoveries of 20th century biology) arrived at the theory that life could never have evolved by chance on planet earth."One need only look in virtually any reference text to quickly find that the earth is thought to be some 4.5 billion years old.As the Encyclopedia Britannica notes, methods such as measuring radioactive decay (radiometric dating) make it possible to estimate the time period when earths rocks and associated fossils were formed. All three of these decay processes have half-lives measured in billions of years.Radioactive decay has become one of the most useful methods for determining the age of formation of rocks.