Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.In a way this field, called geochronology, is some of the purest detective work earth scientists do.There are two basic approaches: relative age dating, and absolute age dating.
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. Highland County igneous rock intrudes sedimentary rock (Photograph by Stan Johnson) This light-colored Highland County igneous intrusion cuts through the darker sedimentary rock.Unlike people, you can’t really guess the age of a rock from looking at it.Based upon such compound cross-cutting relationships it can be seen that the fault is older than the unconformity which in turn is older than the dike.Using such rationale, the sequence of geological events can be better understood.