How Is It Possible to Estimate the Relative Age of Fossils?
It is possible to estimate the relative age of fossils, that is whether they came before or after a particular period, because most fossils are found in sedimentary rocks. These are rocks made of sediment which has been compressed or cemented together in layers.
Older rock layers, or strata, are usually at the bottom. So each layer is younger than the layer below it and older than the one above. Fossils may be present in igneous rock (hardened volcanic lava’s) and metamorphic rocks (formed by pressure and heat within the earth) but they are usually destroyed.
Telling the age of fossils in terms of years, or absolute time, is a much bigger problem. But scientists use several methods. The tree ring method, counting annual growth rings, can give a scientist a reasonably accurate date back to about 3,000 years ago.
The varve method, based on counting the annual layers of sand and clay deposited in a lake, bay or river by melting glaciers can be used for deposits less than about 15,000 years old. Similar calculations based on the rate of sedimentation, erosion, salt accumulation etc., have been successfully applied to very much older rocks.
The third method is concerned with radioactive decomposition and is based on actual changes in some of the rock elements or in the fossils themselves.
Radioactive uranium gradually changes to uranium lead, radiocarbon to nitrogen and so on. From the proportion of uranium lead to uranium in the rock we can date the oldest rocks and fossils, nearly 3,000 million years old.
Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle is the difficulty of working out fossil ages. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better.
Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to its decay products shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments.