In geology and archaeology, the process of determining an object's or event's place within a chronological scheme. Scientists may use either relative dating, in which items are sequenced on the basis of stratigraphic clues (see stratigraphy) or a presumed evolution in form or structure, or absolute dating, in which items are assigned a date independent of context. The latter type includes potassium-argon and carbon-14 dating; both are based on the measurement of radioactive decay. The record of changes in polarity of the Earth's magnetic field has provided a timescale for seafloor spreading and long-term marine sedimentation. Dendrochronology has proved useful in archaeology and climatology. See also fission-track dating; helium dating; lead-210 dating; rubidium-strontium dating; uranium-234–uranium-238 dating; uranium-thorium-lead dating.
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