Relative dating

Before the advent of absolute dating in the 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques. It estimates the order of prehistoric and geological events were determined by using basic stratigraphic rules, and by observing where fossil organisms lay in the geological record, stratified bands of rocks present throughout the world.

Though relative dating can determine the "order" in which a series of events occurred, not "when" they occurred, it is in no way inferior to radiometric dating; in fact, relative dating by biostratigraphy is the preferred method in paleontology, and is in some respects more accurate (Stanley, 167-9).

Biostratigraphy

Biostratigraphic methods are usually used in tandem with structural ones. For instance, the principle of faunal succession was probably the most important factor behind the elaboration of the geologic time scale, which was more or less complete long before an absolute time scale was available. Beds with a particular fauna can be correlated with others that share it (often globally), and also distinguished from upper and lower beds without them.

Rock units that contain a distinct assemblage of fossils are biostratigraphic units, and are based on the "range", or vertical interval in which a taxon is found. A zone, or biozone is the most basic biostratigraphic unit, one bound on its upper and lower boundaries by the ranges of given species; these can be zones where certain species coexist, or which are defined by the earliest appearance or latest disappearance of taxa in neighboring zones.

Index fossils (also guide fossils) are invaluable for biostratigraphy. The best index fossils are:
* Abundant.
* Distinct from other flora/fauna.
* Geographically widespread.
* Found in many kinds of rocks.
* Narrow in stratigraphic range.

Unfortunately, few taxa fit all these criteria (Stanley, 157-8).

Unconformity is a place in a rock column where rock is missing.

Relative dating in archaeology

Relative dating methods in archaeology are similar to some of those applied in geology. Analysing the stratification of a site was developed from principles first discovered in geology. But note that in general the stratigraphic relationships in archaeology form a partially ordered set so that the true chronological sequence cannot be reconstructed by stratigraphic reasoning alone. The principles of typology can be compared to the biostratigraphic approach described above.

Planetological use

Relative dating is used to determine the order of events on objects other than Earth; for decades, planetary scientists have used it to decipher the evolution of bodies in the Solar System, particularly in the vast majority of cases in which we have no surface samples. Many of the same principles are used; for instance, if a valley on Mars cuts across a crater, the valley must be younger than the crater.

Craters themselves are highly useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.(Hartmann, 258)

ee also

*Key bed
*Stratigraphy
*Structural geology

References

* Hartmann, William K. "Moons & Planets", 4th ed. Belmont: Wadsworth Publishing Company, 1999. ISBN 0-534-54630-7
* Monroe, James S., and Reed Wicander. "The Changing Earth: Exploring Geology and Evolution", 2nd ed. Belmont: West Publishing Company, 1997. ISBN 0-314-09577-2
* Stanley, Steven M. "Earth System History". New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6