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].
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But this sediment doesn't typically include the necessary isotopes in measurable amounts.
Fossils can't form in the igneous rock that usually does contain the isotopes.
These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
Knowing when a dinosaur or other animal lived is important because it helps us place them on the evolutionary family tree.
During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral.