How do we determine geologic age? Uniformitarianism = the processes that shape Earth have not changed.

Relative Age 2. Law of Superposition = rocks at bottom are oldest.

Relative Age 3. Law of Horizontality = sediments deposit in horizontal layers.

Explain This!

Folding and Faulting

Folding

Relative Age 4. Law of Crosscutting = features that cross other rock layer are younger than those layers.

Unconformities Uplift, erosion, and weathering can leave a missing piece to puzzle.

Unconformities

Determining Relative Age

How do we correlate rock layers from different locations?

If locations are close together Walk the outcrop: Compare similarities like rock type, color, and mineral composition.

If locations are far apart Volcanic Ash – unique composition, short time span, and broad distribution.

If locations are far apart 2. Asteroid Impact – unique composition, short time span, broad distribution.

K-T Boundary

If locations are far apart 3. Index Fossils – easily identifiable, short time span, and broad distribution.

Absolute Age Radioactive decay = the decay of unstable isotopes (parent) into stable elements (daughter). Isotope = element has different masses (or amount of neutrons). Ex. Uranium 238 (radioactive) decays into Lead 206 (stable).

Why use radioactivity? Decay rate is predictable. Immune to external forces: heat, pressure, chemical, and mass. Decay begins: rocks: when formed. organic: when life ends.

Half-Life time it takes for half or 50% of the parent to decay into the daughter.

Limitations Air, Water, Life: C14 / C12 ratio is used for dating air, water, and organic material, but is limited to only ~50,000 years. Rocks: radioactive isotope must be in rock. U238 date range: 10 million to 4.6 billion K40 date range: 50,000 to 4.6 billion