Everything about Relative Dating totally explained
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's 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.
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've 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)
Further Information
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