Earth's History and the Fossil Record
This domain tests how geologists order and date events: relative-dating principles, unconformities, radiometric decay, index fossils, and the structure of the geologic time scale. Items often ask you to sequence events from a described cross-section or to calculate an age from a decay fraction.
Relative dating principles
- Superposition: in undisturbed sedimentary rock, the oldest layer is at the bottom.
- Original horizontality: sediments are laid down nearly flat, so tilted layers were disturbed later.
- Lateral continuity: layers extend sideways until they pinch out or are cut off.
- Cross-cutting: a fault, dike, or intrusion is younger than the rocks it cuts.
- Inclusions and baked contacts: a fragment inside a rock is older than the rock; chilled margins show the intrusion came later.
- Unconformities: an angular unconformity shows tilted layers truncated by erosion and overlain by flat layers; a disconformity is an erosional gap between parallel beds; a nonconformity separates sedimentary rock from older igneous or metamorphic rock.
Absolute dating
Radiometric dating compares parent to daughter isotopes. The half-life is constant regardless of temperature or pressure. After n half-lives the fraction of parent remaining is (1/2)ⁿ. Common systems: U-238 → Pb-206 (4.47 Ga), K-40 → Ar-40 (1.25 Ga), Rb-87 → Sr-87 (48.8 Ga), and C-14 → N-14 (5,730 yr, organic material only, useful to roughly 50,000 years). Dates are valid only if the system stayed closed.
Fossils and the time scale
An index fossil has a short stratigraphic range and a wide geographic range, so it correlates rocks across regions. Life appeared in the Precambrian; the Cambrian explosion (~541 Ma) opened the Paleozoic; the end-Permian extinction (~252 Ma) was the largest; the K–Pg impact (~66 Ma) ended the Mesozoic and began the Cenozoic.
Worked example
A basalt flow retains 25% of its original K-40. Since 25% = (1/2)², two half-lives have elapsed: 2 × 1.25 Ga = 2.5 Ga.
Traps that cost marks
- Using C-14 on igneous rock or on a sample older than ~50,000 years.
- Treating an unconformity as continuous time, when it records erosion or nondeposition.
- Choosing a long-ranging fossil (like a modern shark tooth) as an index fossil.
- Assuming the half-life speeds up with heat.