How do geologists know how old something is?
How old is the earth?

Relative Age Dating
 WITHOUT TALKING!
 With the students in your row
 Line up from youngest to oldest
 WITHOUT TALKING!
 You have 5 min
 WITHOUT TALKING!

Relative Age Dating
 Analyze: Determine your group ’ s accuracy. (You can talk now! Don ’ t move yet!) How accurate were you?
 Exact! (tumb up)
 Nearly. (thumb sideways)
 Yikes! Not at all.(thumb down)
 EXACT AGE DATING: 2nd attempt, talking allowed, fix the age line up.

Relative Dating Laws review
1. Law of superposition
2. Law of original horizontality
3. Law of lateral continuity
4. Principle of cross cutting relationships.
5. Principle of fossil succession/correlation
 All of these give us relative ages.
 This rock is older than that one, but younger than this other one.

Law of original horizontality
 Layers have to be formed before anything can be done to them. Any layer that has folds, tilts, or erosion can use this law to help date it.

Law of lateral continuity
 Layers continue until they reach an edge or run out.

Fossil Correlation
 Rocks containing the same index fossils are the same age.
 Fossils must be: widespread, develop and go extinct quickly, & cannot be tied to a type of rock.

Relative Play Dough

Relative dating practice problem #1
 B-superposition
 A-cross cutting
 C-superposition
 D- superposition
 K-original horizontality
 L- cross cutting
 Folding - original horizontality
 J- superposition/lateral continuity
 I- superposition
 H- superposition
 G- superposition
 F- superposition
 E- superposition

Relative dating practice #2
Tilt may have occurred before N & L or after.
 Tilt- original horizontality
 F, H, K, A, Q- (in order) superposition
 Tilt? And erosion-original horizontality
 L-cross cutting
 N-cross cutting
 M-superposition
 J-superposition
 I-superposition
 Erosion of C/B
 B-cross cutting
 E/G?-Cross cutting
 C-superposition
 D-superposition

Exact Age Dating
 Measurable! Like lining up by height.
 Radiometric Dating
 Uses radioactive isotopes and decay rates to determine exact ages of rocks and fossils.
 Half life- the amount of time it takes for one half of a radioactive isotope to decay.

Radioactive Decay
 For example, uranium decays into lead.
 A sample starts off with all uranium and no lead.
 Over time, the ratio of lead to uranium increases.

Radioactive Decay
 Different elements have different decay rates
 Must use different elements for different ages of rocks
 Limitations:
 Presence of radioactive isotopes (igneous rock most common)
 Age of rock (too young/old for the isotope present)
 Sedimentary rock is made up of sediments or pieces of other rock. (not accurate)

Elements for Radiometric
Dating

How do they do it?
 Radiometric dating process.