Absolute dating notes
Many sections of the Wasatch fault disturb or crosscut the Provo shoreline, showing that faulting occurred after the lake dropped below this shoreline which formed about 13,500 years ago.
As this example illustrates determining the age of a geologic feature or rock requires the use of both absolute and relative dating techniques.
Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.
The age of formations is marked on a geologic calendar known as the geologic time scale.
Particularly useful are index fossils, geographically widespread fossils that evolved rapidly through time.
Crosscutting Relationships: Relative ages of rocks and events may also be determined using the law of crosscutting relationships, which states that geologic features such as igneous intrusions or faults are younger than the units they cut across.
Metamorphic rocks may also be radiometrically dated.
However, radiometric dating generally yields the age of metamorphism, not the age of the original rock.
Stratigraphy is based on the law of superposition--like a layer cake, the lowest layers must have been formed first.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.In other words, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers.
Cross-dating of sites, comparing geologic strata at one site with another location and extrapolating the relative ages in that manner, is still an important dating strategy used today, primarily when sites are far too old for absolute dates to have much meaning.
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.