Sedimentary rock age dating
Based on the analysis of these samples, scientists estimate that the Earth itself is about 4.5 billion years old.
In addition, the oldest known moon rocks are 4.5 billion years old.
To determine the ages of these specimens, scientists need an isotope with a very long half-life.
Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.
Unfortunately, these elements don't exist in dinosaur fossils themselves.
Each of them typically exists in igneous rock, or rock made from cooled magma.
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Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.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].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.