Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.The overall duration and relative length of these large geologic intervals is unlikely to change much, but the precise numbers may "wiggle" a bit as a result of new data. There are two main methods determining a fossils age, relative dating and absolute dating.The majority of the time fossils are dated using relative dating techniques.Using relative dating the fossil is compared to something for which an age is already known.The principle of inclusions states that inclusions found in other rocks (or formations) must be older than the rock that contain them. Geologists call it relative dating — we know which one is older but do not know how old they are.This is actually pure logic and it can be applied not only in geology, but it is especially useful for geologists. Inclusions of foreign rocks that are found in igneous rocks are named xenoliths.
The most obvious feature of sedimentary rock is its layering.These can not be included in the diagram for practical reasons, but can be found in Harland , 1990, along with a detailed description of the history of earlier-proposed time scales and the terminology, methodology and data involved in constructing this geological time scale.Because of continual refinement, none of the values depicted in this diagram should be considered definitive, even though some have not changed significantly in a long time and are very well constrained (e.g., the Cretaceous/Tertiary boundary has been at 65 -1 Ma for decades, and has been tested innumerable times, with almost all dates somewhere between 64 and 66 million years).This feature is produced by changes in deposition over time.With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?Absolute dating entails laboratory analysis of rock samples that will take a longer time.