Relative Age Dating of Fossils

So how do we date the fossils that we find? As mentioned in an earlier article, carbon isotopes are quite good at allowing us to calculate the age of fossils. Another way that fossils are dated is through relative age dating. Fossils tend to be found in sedimentary rock because metamorphic rock and igneous rock tend to destroy the fossils, due to high pressure and heat that the rocks undergo in formation. Sedimentary rocks tend to be formed in layers with each being from a different time period. Given this, the law of superposition, an axiom of geochronology, allows us to determine that the highest layers are newer, while the lower ones are older because the layers just stack onto each other continuously. The layers are quite visible in the picture below of a stratigraphic column along the northern bank of Isfjorden, Spitsbergen, Svalbard, Norway. One could easily see the relative age dating come into use in practice.

File:IsfjordenSuperposition.jpg

If a paleontologist were to find a dinosaur fossil in a layer of strata in between two strata with ages that were known, then we could be able to infer that the dinosaur lived in between the two ages of the layers of strata. This is obviously imperfect in the sense that a more precise answer cannot always be derived, but it at least places the fossil in a small time frame that it could have roamed the earth.

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