Methods of dating rock layers
A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
However, by itself a fossil has little meaning unless it is placed within some context.
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.
But this sediment doesn't typically include the necessary isotopes in measurable amounts.
Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
A fossil will always be younger than fossils in the beds beneath it and this is called the principle of superposition.
In an undisturbed sequence of rocks, such as in a cliff face, it is easy to get a rough idea of the ages of the individual strata – the oldest lies at the bottom and the youngest lies at the top.
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.
Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.