How do we determine geologic age? 1.Uniformitarianism = the processes that shape Earth have not changed.

How do we determine geologic age?

1. 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 weatheringcan 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

1. 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?

1. Decay rate is predictable.

2. Immune to external forces: heat, pressure, chemical, and mass.

3. Decay begins:rocks: when formed. organic: when life ends.

Half-Lifetime 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 billionK40 date range: 50,000 to 4.6 billion