Methods of absolute fossil dating
Look closely at the Geologic Time Scale chart, and you might notice that the first three columns don't even go back 600 million years.That last, pink Precambrian column, with its sparse list of epochal names, covers the first four billion years of Earth's history, more than three quarters of Earth's existence. Paleontologists have used major appearances and disappearances of different kinds of fossils on Earth to divide Earth's history -- at least the part of it for which there are lots of fossils -- into lots of eras and periods and epochs.On Earth, we have a very powerful method of relative age dating: fossil assemblages.Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear.
The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer.There's no absolute age-dating method that works from orbit, and although scientists are working on age-dating instruments small enough to fly on a lander (I'm looking at you, Barbara Cohen), nothing has launched yet. Relative age dating has given us the names we use for the major and minor geologic time periods we use to split up the history of Earth and all the other planets.When you talk about the Precambrian, Paleozoic, Mesozoic, and Cenozoic on Earth, or the Noachian, Hesperian, and Amazonian for Mars, these are all relative ages.Age of deposition should not be confused with the date of material enclosed in deposit.Cross dating involves comparing similar artifacts from similar sites that may be of similar age.