Earth History, Stratigraphy, and the Fossil Record
Domain 6 tests relative and absolute dating, stratigraphy, correlation, and the fossil record.
Core rules and concepts
- Relative dating principles: superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, and faunal succession.
- An unconformity is missing time: angular unconformity, disconformity, or nonconformity.
- Radiometric dating uses parent-to-daughter decay; each half-life leaves half the parent. Closure temperature matters.
- Index fossils are widespread, abundant, and short-ranging; they correlate strata.
- The geologic time scale is ordered by eons, eras, periods, and epochs. Major boundaries often mark extinctions or tectonic changes.
- Fossils form mainly in sedimentary rock; hard parts preserve best. Trace fossils record behavior.
Worked example
A rock has 25% of its original parent isotope remaining. After one half-life, 50% remains; after two, 25%. If the half-life is 50 million years, the sample is about 100 million years old. The reasoning is 25% = (1/2)^2, so two half-lives have elapsed. A date is valid only if the system stayed closed and no daughter was lost.
Traps that cost marks
Do not date a whole metamorphic rock by an igneous intrusion that cuts it; the intrusion is younger. An unconformity represents erosion or nondeposition, not continuous deposition. None of the relative-dating principles gives a numerical age. A fossil’s presence does not always mean the same exact age everywhere; reworking can move fossils.