Absolute and relative fossil dating

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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.

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.

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.

Layers of volcanic ash are igneous deposits, while layers of rock these deposits surround are usually sedimentary. Igneous intrusions form when magma breaks through a layer of rock from beneath, or lava flows down from above. When igneous intrusion causes newer sedimentary layers to sink into older ones, it's called subsidence.

Methods fall into one of two categories: relative or absolute.

One way to find the age of a xenolith or subsidence area surrounded by volcanic debris is to correlate its layers with the layers of wall or parent rocks.

Stratigraphy is the study of sedimentary rock layers.

Dating the ash layers above and below a sedimentary rock layer to determine its age is called bracketing.

Radiometric dating uses the decay of unstable isotopes -- atoms with specific electrical charges -- to calculate something's age.

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