Earth's History and the Fossil Record
This domain tests relative and absolute dating, the fossil record, geologic time, and major events in Earth history. You should be able to sequence events and calculate radiometric ages.
Core rules and concepts
- Relative dating principles: superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, and faunal succession.
- An unconformity is a gap in the rock record caused by erosion or nondeposition; it represents missing time.
- Radiometric dating uses parent-to-daughter isotope ratios and known half-lives; each half-life leaves half the parent.
- Index fossils are widespread, short-lived, and distinctive, making them useful for correlation.
- The geologic time scale is divided by major biological and geological changes; Precambrian time dominates Earth history.
Worked example
A zircon crystal retains 25% of its original U-235. U-235 has a half-life of about 704 million years. After one half-life, 50% remains; after two, 25% remains. Therefore the crystal is about 1.408 billion years old, assuming no daughter isotope loss or inheritance. If the sample contained inherited zircon, the age would be too old; if lead had leaked, it would be too young.
Traps that cost marks
- Using radiocarbon dating on dinosaur bones or rocks older than about 50,000 years.
- Forgetting that cross-cutting features are younger than the rocks they cut.
- Treating an unconformity as a continuous layer rather than missing time.
- Assuming the oldest rock is always at the bottom in a deformed sequence; overturning and faults can invert order.