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Absolute dating is the process of determining an age on a specified chronology in archaeology and geology.Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy.In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young (radiocarbon dating with Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes.Particular isotopes are suitable for different applications due to the type of atoms present in the mineral or other material and its approximate age.Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.Uranium is present in many different rocks and minerals, usually in the form of uranium-238.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.Geologists call this the principle of lateral continuity.
In an undisturbed sequence of rocks, such as in a cliff face, it is easy to get a rough idea of the ages of the individual strata – the oldest lies at the bottom and the youngest lies at the top.
If a fossil is found between two layers of rock whose ages are known, the fossil's age is thought to be between those two known ages.
Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is difficult to match up rock beds that are not directly adjacent.
Strata are differentiated from each other by their different colors or compositions and are exposed in cliffs, quarries, and river banks.
These rocks normally form relatively horizontal, parallel layers, with younger layers forming on top.