Geologists (scientists who study rocks) divide the time since the Earth was formed until today into chunks called periods. During the various periods, different species of animals and plants lived on the Earth. For example, the Cretaceous period, which lasted from 146 million years ago to 65 million years ago, was the final period of the dinosaurs. Some rocks can be given a relative age by identifying the fossils they contain. The date of formation of some rocks can be found by using RADIOMETRIC DATING.
Over many decades, palaeontologists have built up a database of fossils called the fossil record. This shows when different species of animals and plants lived in the history of the Earth. A fossil can be matched against the fossil record to find the age of the rock it was found in. The information in the fossil record is based on using stratigraphic layers to tell the age of a rock and the characteristic life forms within it. Here are some fossils and the periods they belong to.
Radiometric dating is a way of measuring the age of a piece of rock. Igneous rocks contain tiny amounts of radioactive chemicals. As the rocks age, these elements gradually break down into elements that are not radioactive. By knowing the rate at which the elements break down and measuring their level of radioactivity, the age of a rock sample can be calculated.
In argon-argon dating, the amounts of two forms of the element argon in a rock sample are measured using a device called a mass spectrometer. One form only is produced by radioactive decay as the rock ages, so comparing how much there is of this in relation to the other form of argon reveals the age of the rock.