Numerical age dating geology

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Simply stated, each bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it.This law follows two basic assumptions: (1) the beds were originally deposited near horizontal, and (2) the beds were not overturned after their deposition.Metamorphic rocks may also be radiometrically dated.However, radiometric dating generally yields the age of metamorphism, not the age of the original rock.Half-lives of these isotopes and the parent-to-daughter ratio in a given rock sample can be measured, then a relatively simple calculation yields the absolute (radiometric) date at which the parent began to decay, i.e., the age of the rock.

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However, in order to place absolute dates on the relative time scale, other dating methods must be considered.Faunal Succession: Similar to the law of superposition is the law of faunal succession, which states that groups of fossil animals and plants occur throughout the geologic record in a distinct and identifiable order.Following this law, sedimentary rocks can be “dated” by their characteristic fossil content.If a geologist claims to be 45 years old, that is an absolute age.Superposition: The most basic concept used in relative dating is the law of superposition.Inclusions: Inclusions, which are fragments of older rock within a younger igneous rock or coarse-grained sedimentary rock, also facilitate relative dating.

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