Earth History and the Fossil Record
Relative dating and correlation
Sequence reasoning rests on superposition (in undisturbed layers, younger above older), original horizontality, lateral continuity, cross-cutting relationships (a fault or intrusion is younger than what it cuts), inclusions (a fragment enclosed in a rock is older than the rock), and baked contacts. Unconformities mark missing time: angular unconformity (tilted beds truncated and overlain), disconformity (erosion surface between parallel beds), nonconformity (sedimentary rocks over igneous or metamorphic basement). Correlate units using lithology, key beds, and index fossils - species with wide geographic range and short stratigraphic range.
Absolute dating
Radiometric dating requires a closed system. Parent remaining = (1/2)^n, where n is the number of half-lives elapsed. Match the method to the material: radiocarbon only for organic material younger than about 50,000 years; potassium–argon, uranium–lead, and rubidium–strontium for ancient rocks; zircons for the oldest dates. Traps include applying radiocarbon to dinosaur bone or to inorganic limestone, and misreading half-life as though half the sample vanishes rather than half the parent atoms decaying.
Reading the fossil record
The record is biased toward hard-shelled marine organisms buried in low-energy settings; soft-bodied and upland life is rarely preserved. Trace fossils record behavior, not the body. Anchor events: Archean stromatolites (early life), the Cambrian explosion of skeletonized animals, the end-Permian extinction (largest), and the end-Cretaceous impact. Fossil succession supplies relative ages even where radiometric dates are unavailable.
Worked example
A volcanic ash bed contains 25% of its original parent isotope and the half-life is 1.25 Ga. 100% → 50% → 25% is two half-lives, so the bed is about 2.5 Ga old.