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Tephrochronology: Within hours or days of a volcanic eruption, tephra — fragments of rock and other material hurled into the atmosphere by the event — is deposited in a single layer with a unique geochemical fingerprint.Researchers can first apply an absolute dating method to the layer.A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.Paleomagnetism: Earth’s magnetic polarity flip-flops about every 100,000 to 600,000 years.Here are some of the most common radiometric methods: Radiocarbon dating: Sometimes called carbon-14 dating, this method works on organic material.Both plants and animals exchange carbon with their environment until they die.Both methods date rock instead of organic material. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at least 500,000 years old.
Certain unstable isotopes of trace radioactive elements in both organic and inorganic materials decay into stable isotopes. By measuring the proportion of different isotopes present, researchers can figure out how old the material is.Whenever possible, researchers use one or more absolute dating methods, which provide an age for the actual fossil or artifact.Unlike observation-based relative dating, most absolute methods require some of the find to be destroyed by heat or other means.The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity.Paleomagnetism is often used as a rough check of results from another dating method.