Fossils and relative dating
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.Both the principle of original horizontality and the law of superposition seem very intuitive and obvious, but they weren’t proposed until the 1600s, both by Danish anatomist, geologist, and priest Nicolaus Steno (1636–1686).In a nice, simple world, we would have a complete and undisturbed record of sedimentary rocks to help us plot out Earth’s past. All kinds of things happen to rock layers after they form.
It’s all a bit of a mess—but a mess that can be put in chronological order using relative dating principles.
In either case, weathering and erosion are left free to work their mischief and erase some evidence of the geologic past. We’ll never know what happened during those 1.2 billion years? The Grand Canyon is not the only location with a rock record.
Scientists can correlate rock layers in different locations to get a more complete geologic record.
There are two basic approaches: relative age dating, and absolute age dating.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.