One of the most popular of these is known as radiometric dating.
can be In other words, something in the past caused a significant amount of helium to build up inside these zircons (such as from a rapid decay episode of uranium), yet, in spite of the fact that helium has been observed to leak out readily from these zircons, it has not done so: simply because it hasn't had enough time to do so -- suggesting that the zircons themselves are only a few thousand years old."There is evidence to show ...
Dating schemes based on rates of radioactivity have been refined and scrutinized for several decades.
With the exception of Carbon-14, radiometric dating is used to date either igneous or metamorphic rocks that contain radioactive elements such as uranium.Now when the uranium or thorium disintegrates, the alpha particles which are emitted are slowed down by the crystals in which the grains of the uranium- or thorium-bearing minerals are embedded.Some isotopes have half lives longer than the present age of the universe, but they are still subject to the same laws of quantum physics and will eventually decay, even if doing so at a time when all remaining atoms in the universe are separated by astronomical distances.Various elements are used for dating different time periods; ones with relatively short half-lives like carbon-14 (or C) are useful for dating once-living objects (since they include atmospheric carbon from when they were alive) from about ten to fifty thousand years old. Longer-lived isotopes provide dating information for much older times.Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.
It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials.
And even though various radioactive elements have been used to "date" these rocks, for the most part, the methods are basically the same.
This means that if you had some pure uranium-238 with no lead in it, 4.5 billion years later one half of it would have decayed into its stable daughter product (lead-206).
In the case of carbon dating, it is not the initial quantity that is important, but the initial ratio of C, but the same principle otherwise applies.
Recognizing this problem, scientists try to focus on rocks that do not contain the decay product originally.
The key is to measure an isotope that has had time to decay a measurable amount, but not so much as to only leave a trace remaining.