RELATIVE AND ABSOLUTE DATING

Ashley Allen Oneonta High School Alabama Paleontological Society

Objectives

Distinguish between absolute dating and relative dating.

Review law of superposition.

Discuss the importance of half-life and radioactive decay.

Conduct activity on relative and absolute dating.

Two Ways to Skin a Cat

Relative dating of fossils is a system in which a fossil is given an age designation in terms of epoch, period, or era which can be compared to other geologic units of time as older or younger, but without the burden of assigning a specific number.

  For example, a Pennsylvanian lycopod bark impression is older than a Cretaceous oyster and younger than a Mississippian brachiopod Relative dating is best explained when covering the law of superposition and a geologic time scale Absolute dating of a fossil involves assigning a specific quantity of age with a fossil such as saying that an echinoid,

Hardouinia bassleri

, is 83 million years old.

Two Inescapable Laws

The law of superposition states that older sedimentary rocks were deposited prior to younger sedimentary rocks, therefore, as one ascends a rock face with multiple formations, the oldest rock layer is on the bottom and the rock units get progressively younger as one ascends the exposure.

The law of faunal succession states that as one views progressively older fossils, they become more and more dissimilar with modern forms of life with which we are most familiar.

Law of Superposition

Law of Faunal Succession

Note the general appearance of more familiar species the closer one gets to the present time.

Radioactive Decay

Unstable parent radioactive isotopes decay at a stable rate which cannot be altered.

Each type of radioactive parent isotope has a unique half-life.

A half-life is the amount of time that it takes for half of the given radioactive parent isotope to decay into a stable daughter product.

By comparing the amount of radioactive parent isotope in remains to the stable daughter product, an absolute age may be determined.

Carbon Dating

Web Resources

Many radiometric dating activities may be found by going to http://www.kn.pacbell.com/wired/fil/ or by typing filamentality into any search engine.

At the core of each activity is the concept of half-life.

Technology will limit the amount of detectable radioactive parent isotope or stable daughter product.

References

Law of Superposition http://www.calstatela.edu/faculty/acolvil/geotime/gran dcanyon_strat.jpg

Law of Faunal Succession http://mpschmidt.homestead.com/files/GeoTimeScale .jpg

Carbon Dating http://ttevisual.com/physics/images/file5/dkph5.09 3_212.jpg

Carbon Dating Shell http://www.fmi.uni sofia.bg/fmi/contmech/kmarkov/history/gifs/carbon.gif