Dating rocks and fossils and geologic events

Absolute dating places events or rocks at a specific time.

If a geologist claims to be younger than his or her co-worker, that is a relative age.

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.

Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.

Fortunately, distinctive features such as index fossils can aid in matching, or correlating, rocks and formations from several incomplete areas to create a more complete geologic record for relative dating.

Metamorphic rocks may also be radiometrically dated.

However, radiometric dating generally yields the age of metamorphism, not the age of the original rock.

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.

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.

Most ancient sedimentary rocks cannot be dated radiometrically, but the laws of superposition and crosscutting relationships can be used to place absolute time limits on layers of sedimentary rocks crosscut or bounded by radiometrically dated igneous rocks.

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