Relative Age Dating Vs. Exact Age Dating

How do geologists know how old something is?

How old is the earth?

Relative Age Dating

WITHOUT TALKING!With the students in your rowLine up from youngest to oldest

WITHOUT TALKING!You have 5 minWITHOUT 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 superposition2. Law of original horizontality3. Law of lateral continuity4. 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 continuityLayers

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- 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

Tilt may have occurred before N & L or after.

Exact Age DatingMeasurable! Like lining up by

height.Radiometric DatingUses 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 DecayFor 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 DecayDifferent elements have different

decay ratesMust use different elements for

different ages of rocksLimitations:

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.