Relative dating

Excavations in Hamburg: The different layers (or strata) have different colors.

Principle of superpositions: Layers which are farther below are older than those above. This means that layer 1 is older than both layers 2 and 3.

Relative dating is the kind of geochronology that determines the relative order of past events. The idea is to find the age of an object or event by comparing it to another object or event, or by using clues from the environment or circumstances where it was found. Relative dating is not about finding the absolute age. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Before the discovery of radiometric dating in the early 20th century, archaeologists and geologists used relative dating to determine ages of materials.

Radiometric dating, on the other hand, is a means of absolute dating. Relative dating can only determine the order in which a series of events occurred, not when they occurred. Even so, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. Steno's Law of Superposition states that older layers will be deeper in a site than more recent layers. This law was the 'relative dating' method of choice in geology from the 17th century to the early 20th century

Relative Dating Media

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  The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is a great example of Original Horizontality 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.

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  Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures. Explanations: A – folded rock strata cut by a thrust fault; B – large intrusion (cutting through A); C – erosional angular unconformity (cutting off A & B) on which rock strata were deposited; D – volcanic dyke (cutting through A, B & C); E – even younger rock strata (overlying C & D); F – normal fault (cutting through A, B, C & E).

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  Schematic representation of the principle of lateral continuity

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  Multiple melt inclusions in an olivine crystal. Individual inclusions are oval or round in shape and consist of clear glass, together with a small round vapor bubble and in some cases a small square spinel crystal. The black arrow points to one good example, but there are several others. The occurrence of multiple inclusions within a single crystal is relatively common