Earth's History and the Fossil Record
This domain tests relative and absolute dating, the geologic time scale, fossilization, and the evidence for evolution and major transitions. Items often present a cross-section or a parent–daughter ratio and ask you to order events or calculate an age.
Core rules and concepts
- Relative dating. Superposition (oldest at bottom), original horizontality, lateral continuity, cross-cutting relationships (a fault or intrusion is younger than what it cuts), inclusions (older than the host), and faunal succession. Unconformities are gaps: angular (tilted layers eroded), disconformity (erosion between parallel layers), nonconformity (sedimentary over igneous or metamorphic rock).
- Absolute dating. Radioactive decay is exponential; after n half-lives, the parent fraction is (1/2)ⁿ. Useful systems: U–Pb for old rocks, K–Ar for volcanic rocks, C–14 for organic remains younger than ~50,000 years. Closure temperature and contamination matter.
- Time scale. Hadean, Archean, Proterozoic, Phanerozoic (Paleozoic, Mesozoic, Cenozoic). Key events: origin of life, Great Oxidation Event, Cambrian explosion, end-Permian and end-Cretaceous mass extinctions, rise of mammals.
- Fossils. Preservation favors hard parts, rapid burial, and anoxic conditions; modes include permineralization, molds and casts, amber, and trace fossils. Index fossils are widespread, abundant, and short-lived.
- Evolution. Natural selection acts on variation; populations adapt, not individuals. Biogeography and transitional fossils support common descent.
Worked example
A volcanic ash layer contains a mineral with 25% of its original parent isotope. If the half-life is 1.25 billion years, how old is the layer? 25% remaining means n = 2 half-lives because (1/2)² = 1/4. Age = 2 × 1.25 = 2.5 billion years. Reasoning: first find the number of half-lives from the fraction remaining, then multiply by the half-life. A 1:3 parent-to-daughter ratio would also mean 25% parent.
Traps that cost marks
- Using carbon-14 to date dinosaur bones or billion-year-old rocks; its half-life is too short.
- Treating an unconformity as continuous deposition; it represents missing time.
- Applying superposition to a folded or faulted sequence without first reconstructing structure.
- Saying individual organisms evolve during their lifetime; selection changes populations across generations.