So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.
Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.
Geologists call this the principle of lateral continuity.
A fossil will always be younger than fossils in the beds beneath it and this is called the principle of superposition.
In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history).
Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped charge dating methods such as thermoluminescence dating of glazed ceramics.
Scientists find out the age of a dinosaur fossil by dating not only the rocks in which it lies, but those below and above it.
Sometimes, scientists already know the age of the fossil because fossils of the same species have been found elsewhere and it has been possible to establish accurately from those when the dinosaur lived.
Some critics, particularly religious fundamentalists, argue that neither fossils nor dating can be trusted, and that their interpretations are better.
Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating.
Fossils are generally found in sedimentary rock not igneous rock.
Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.
Scientists use two kinds of dating techniques to work out the age of rocks and fossils. This considers the positions of the different rocks in sequence (in relation to each other) and the different types of fossil that are found in them.