However, radiometric dating generally yields the age of metamorphism, not the age of the original rock.
Most ancient sedimentary rocks cannot be dated radiometrically, but the laws of superposition and crosscutting relationships can be used to place absolute time limits on layers of sedimentary rocks crosscut or bounded by radiometrically dated igneous rocks.
This section discusses the methods geologists use to determine how old a fossil or rock is. Therefore, the sedimentary rock must be older than the intrusion.
Relative age-dating methods determine when an event happened compared to another event. Geologic time scale Relative age-dating involves comparing a rock layer or rock structure with other near-by layers or structures. Roadcut in Wise County showing the principle of superposition (Photograph by Stan Johnson) Flat-lying sedimentary layers from the Appalachian Plateaus province of southwestern Virginia illustrate the principle of superposition.
This law follows two basic assumptions: (1) the beds were originally deposited near horizontal, and (2) the beds were not overturned after their deposition.
Faunal Succession: Similar to the law of superposition is the law of faunal succession, which states that groups of fossil animals and plants occur throughout the geologic record in a distinct and identifiable order.
If a geologist claims to be younger than his or her co-worker, that is a relative age.
If a geologist claims to be 45 years old, that is an absolute age.Crosscutting Relationships: Relative ages of rocks and events may also be determined using the law of crosscutting relationships, which states that geologic features such as igneous intrusions or faults are younger than the units they cut across.Inclusions: Inclusions, which are fragments of older rock within a younger igneous rock or coarse-grained sedimentary rock, also facilitate relative dating. Highland County igneous rock intrudes sedimentary rock (Photograph by Stan Johnson) This light-colored Highland County igneous intrusion cuts through the darker sedimentary rock.Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.As this example illustrates determining the age of a geologic feature or rock requires the use of both absolute and relative dating techniques.