Relative dating

Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.

The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is a great example of Original Horozontality and the Law of Superposition, two important ideas used in relative dating. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.

In geology, rock or superficial deposits. Fossils and lithologies can be used to correlate one stratigraphic column with another, allowing relative dating to be used to show trends over long length-absolute dating in the 20th century, archaeologists and geologists were largely limited to the use of the relative dating techniques to determine the geological events.

Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, 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, 167–69). The Law of Superposition was the summary outcome of 'relative dating' as observed in geology from the 17th century to the 19th century.

The regular order of fossils in rock layers had been observed in prehistory, and was rediscovered around 1800 by a British canal surveyor named William Smith. While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a surveyor, he found the same patterns across England. He also found that certain animals were in only certain layers and that they were in the same layers all across England. Due to that discovery, Smith was able to recognize the order that the rocks were formed. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. In 1831, Smith received recognition for his work in the form of the Wollaston Medal, the highest honor of the Geological Society of London.

Principles of relative chronology

• Uniformitarianism (Hutton) Popularized by Charles Lyell
• Law of superposition (Steno)
• Original Horizontality (Steno)
• Lateral continuity (Steno)
• Principle of faunal succession(Smith)
• Cross cutting relationships (Hutton)
• Inclusions of Igneous rocks (Hutton)'

Archaeology

Relative dating methods in archaeology are similar to some of those applied in geology. Form a partially ordered set so that the true chronological sequence cannot be reconstructed by stratigraphic means. The principles of typology can be compared to the biostratigraphic approach described above.

Planetology

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 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)

See also

• Key bed
• Stratigraphy
• Structural geology
• Unconformity

References

• "Biostratigraphy: William Smith". Understanding Evolution. 2009. University of California Museum of Paleontology. 23 January 2009 <http://evolution.berkely.edu/evolibrary/article/0_0_0/history_11>
• 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