Fossils and Dating Rocks

About this topic

Relative dating places events in order: in undisturbed layers, the oldest is at the bottom (law of superposition), and cross-cutting features are younger than what they cut. Index fossils from short-lived, widespread species help correlate layers. Absolute dating uses radioactive decay: the ratio of parent to daughter isotopes (uranium–lead, potassium–argon, carbon-14) and half-lives give ages in years.

Together they build Earth's history. Fossils form when remains are buried and mineralised.

Take radiocarbon dating as an example: living things take in carbon-14, which decays with a half-life of 5,730 years. After one half-life, half remains; after two, a quarter. If a bone has one eighth of the carbon-14 of living tissue, it has gone through three half-lives, so it is about 17,200 years old. This only works for organic material up to about 50,000 years old, so for older rocks scientists use uranium-lead (half-life 4.5 billion years) on minerals such as zircon.

Index fossils like trilobites and ammonites help match layers across continents. Fossils form in several ways: mineral replacement (petrification), moulds and casts, trace fossils such as footprints, and rare preservation in amber or ice. Dating shows evolution and mass extinctions and finds oil and coal beds. A common misconception is that carbon dating gives the age of dinosaurs or rocks; it cannot, because they are too old. Another is that the fossil record is complete; fossilisation is rare and biased towards hard parts and certain environments.