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)
The principles of Relative dating use a combination of fossil study and structural interpretation to learn about the geological history of an area. The rules of relative dating for continuous stratigraphic sequences were worked out long ago, by scientists such as Nicolas Steno (1638-86):
- The Law of superposition states that in any undisturbed geologic sequence, older beds lie below younger beds.
- The principle of original horizontality states that whether rock beds are deformed or not, they were originally deposited on a horizontal surface.
- The principle of lateral continuity states that a given rock bed will extend horizontally for some distance, until it thins out or reaches the edge of a depositional basin.
- The principle of cross-cutting relationships states that any geologic feature that off-sets or intersects a series of beds (such as a fault, dike, or sill) must be younger than the beds it cuts across.
- The principle of intrusion states that if a body of rock contains intrusions of another type of rock, it must be younger than the source rock.(Monroe and Wicander, 413-21)
- Principle of Biostratic Fossil Correlation states that if an index fossil of a known age is found in a rock formation, that formation is of the same approximate age as the index fossil. This principle represents a modern extrapolation of the older principle of floral and faunal succession, which noted increasing complexity of creatures in successively younger rock strata.
These principles are highly useful in interpreting geologic history. For instance, an angular unconformity with horizontal beds above it indicates a sequence in which:
- The original strata were deposited horizontally.
- The strata were tilted and deformed.
- Much of the strata were eroded away.
- Deposition later began anew above the unconformity.
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:
- 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)
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)
- 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