Unlike people, you can’t really guess the age of a rock from looking at it.
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.
Often when geologists examine rock outcrops like the one illustrated above they are interested in not only the types of rocks present, but the order in which they formed.
With absolute age dating, you get a real age in actual years.Relative age dating also means paying attention to crosscutting relationships.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.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.