Carbon 14 the radioactive nuclide used in dating fossils
Most rocks contain uranium, allowing uranium-lead and similar methods to date them.
Other elements used for dating, such as rubidium, occur in some minerals but not others, restricting usefulness.
Radiometric dating — through processes similar to those outlined in the example problem above — frequently reveals that rocks, fossils, etc.
are very much older than the approximately 6,000 to 10,000 years reckoned by young earth creationists.
One problem is that potassium is also highly mobile and may move into older rocks.
This depends on the decay of uranium-237 and uranium-238 to isotopes of lead.
Note that although carbon-14 dating receives a lot of attention, since it can give information about the relatively recent past, it is rarely used in geology (and almost never used to date fossils).
Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life.To date older fossils, other methods are used, such as potassium-argon or argon-argon dating.Other forms of dating based on reactive minerals like rubidium or potassium can date older finds including fossils, but have the limitation that it is easy for ions to move into rocks post-formation so that care must be taken to consider geology and other factors.We can measure directly, for example by using a radiation detector, and obtain a good estimate of by analyzing the chemical composition of the sample.