Relative dating of rocks and events
There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.
Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures.
The principle of inclusions and components explains that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.
For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.
Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials.
Methods for relative dating were developed when geology first emerged as a natural science in the 18th century.Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative datin g' as observed in geology from the 17th century to the early 20th century.
The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith.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.