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Researchers can first apply an absolute dating method to the layer.

They then use that absolute date to establish a relative age for fossils and artifacts in relation to that layer. Anything below the Taupo tephra is earlier than 232; anything above it is later.

Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.

Layers of rock build one atop another — find a fossil or artifact in one layer, and you can reasonably assume it’s older than anything above it.

Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology.

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.

Both plants and animals exchange carbon with their environment until they die.

Afterward, the amount of the radioactive isotope carbon-14 in their remains decreases.

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The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.

Paleomagnetism is often used as a rough check of results from another dating method.

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.

Thermoluminescence: Silicate rocks, like quartz, are particularly good at trapping electrons.

Researchers who work with prehistoric tools made from flint — a hardened form of quartz — often use thermoluminescence (TL) to tell them not the age of the rock, but of the tool.