This section discusses the methods geologists use to determine how old a fossil or rock is. Relative age-dating methods determine when an event happened compared to another event. Absolute age-dating tells how long ago an event occurred. Using these methods, geologists have created a geologic time scale for organizing past times in earth’s history.
Relative age-dating involves comparing a rock layer or rock structure with other near-by layers or structures. Using the principles of superposition and cross-cutting relationships, and structures such as unconformities, one can determine the order of geological events. Examples are given below.
• Usually, the oldest layers of rock are found at the bottom of a sequence and the youngest layers at the top. This is called the principle of superposition.
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. The oldest layers are at the bottom. The youngest layer is at the top.
• If a geologic feature such as a fault or igneous intrusion cuts across other features, the fault or intrusion must be younger than the rock it cuts. These relationships are called cross-cutting relationships.
Highland County igneous rock intrudes sedimentary rock (Photograph by Stan Johnson)
This light-colored Highland County igneous intrusion cuts through the darker sedimentary rock. Therefore, the sedimentary rock must be older than the intrusion.