What kind of rock can radioactive dating be used on
In all radiometric procedures there is a specific age range for when a technique can be used.
If there is too much daughter product(in this case nitrogen-14), age is hard to determine since the half-life does not make up a significant percentage of the material's age.
For example, by using a laser, researchers can measure parent and daughter atoms in extremely small amounts of matter, making it possible to determine the age of very small samples [source: New Scientist].
As shown in the diagram above, the radioactive isotope carbon-14 originates in the Earth's atmosphere, is distributed among the living organisms on the surface, and ceases to replenish itself within an organism after that organism is dead.
Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.
This means that as the rubidium-87 decays and more strontium-87 is formed, the ratio will change.
The half-life of rubidium-87 is 48.8 billion years, meaning it can accurately measure rocks as old as the Earth itself.
While the oldest known rocks on Earth are about 3.5 billion years old, researchers have found zircon crystals that are 4.3 billion years old [source: USGS].
Based on the analysis of these samples, scientists estimate that the Earth itself is about 4.5 billion years old.
But this sediment doesn't typically include the necessary isotopes in measurable amounts.