So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash.
These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed.
Scientists have also made improvements to the standard radiometric measurements.
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].
But this sediment doesn't typically include the necessary isotopes in measurable amounts.The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.Nevertheless, there is substantial evidence that the Earth and the other bodies of the Solar System are 4.5-4.6 billion years old, and that the Milky Way Galaxy and the Universe are older still.The principal evidence for the antiquity of Earth and its cosmic surroundings is: Spontaneous breakdown or decay of atomic nuclei, termed radioactive decay, is the basis for all radiometric dating methods.