Geologists often need to know the age of material that they find.They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.

To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.

Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.

This is different to relative dating, which only puts geological events in time Most absolute dates for rocks are obtained with radiometric methods.

These use radioactive minerals in rocks as geological clocks.

Carbon-14 cannot be used to date biological artifacts of organisms that did not get their carbon dioxide from the air.

This rules out carbon dating for most aquatic organisms, because they often obtain at least some of their carbon from dissolved carbonate rock.

The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating.

This restriction extends to animals that consume seafood in their diet.

Most scientists and many Christians believe that the radiometric dating methods prove that the earth is 4.5 billion years old.

The textbooks speak of the radiometric dating techniques, and the dates themselves, as factual information.

Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.