Although relative geologic time provides the order in which events occur, it does not provide actual dates. The situation is similar to knowing that Rome fell before the Declaration of Independence was signed, which in turn happened before humans walked on the moon, but not knowing when these events happened or even how much time occurred between them. However, absolute geologic time provides this valuable type of information. Absolute time relies on the measurement of radioactive elements and their by-products to determine the actual age of certain geologic materials, usually minerals or rocks. The spontaneous natural process by which radioactive parent isotopes decay to form stable daughter isotopes is called radioactive decay. By combining the information gained through studies of relative and absolute geologic time and relationships between rocks around the world, geologists have created the geologic time scale, an example of which is shown below.
Atoms of the same element that have different numbers of neutrons are called isotopes. Although not all isotopes are radioactive, those that are radioactive are of particular interest to geologists when they are contained within certain minerals in certain types of rocks. All atoms of the same radioactive isotope, such as rubidium-87, decay at the same rate. And, the amount of time that it takes half of the atoms of one isotope (the parent) present in a mineral to decay to stable daughter isotopes is called a half-life. So, by knowing the rate of decay of the parent isotope (i.e., its half-life), and by using a device called a mass spectrometer to measure the ratio of parent to daughter isotopes in a mineral or rock, you can estimate its age. Below is an illustration of radioactive decay.