In the mid twentieth century, the study of radioactivity led to a new method of giving rocks an absolute date. This revolutionised our understanding of palaeontology and evolution. Elements which are radioactive are very unstable and they release radioactivity steadily as they decompose over time. Different elements decay at different rates but for each one the rate is constant and can be measured. Igneous rocks, such as those formed when volcanoes erupt, often contain some radioactive elements.
Scientists can then measure the proportion of each element which has decayed to more stable forms.
Since they know the standard rate at which it should decay, scientists can calculate the amount of time since the volcanic eruption. Although such igneous rocks are relatively rarely associated with fossil remains, they have been used to scatter absolute dates throughout the fossil chronology. They therefore give us a relatively accurate portrayal of the amount of time since the fossils were formed. This has shown us just how long ago it was that the great dinosaurs lived and how long they were around for. Before radiometric dating, some estimated that the 'age of dinosaurs' had lasted nine million years and ended three million years ago.