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**Relative Age Dating Vs. Exact Age Dating**

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

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**Relative Age Dating WITHOUT TALKING! With the students in your row**

Line up from youngest to oldest You have 5 min

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Relative Age Dating Analyze: Without moving, determine your group’s accuracy. (You can talk now!) How accurate were you? Exact! Nearly. Yikes! Not at all. EXACT AGE DATING: 2nd attempt, talking allowed, fix the age line up.

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**Relative Dating Laws review**

Law of superposition Law of original horizontality Law of lateral continuity Principle of cross cutting relationships. 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.

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**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.

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**Law of lateral continuity**

Layers continue until they reach an edge or run out.

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Relative Play Dough

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**Relative dating practice problem #1**

B-superposition A-cross cutting C-superposition D- superposition K-original horizontality L- cross cutting J- superposition/lateral continuity Folding - original horizontality I- superposition H- superposition G- superposition F- superposition E- superposition

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**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.

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**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.

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**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.

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**Radioactive Decay Different elements have different decay rates**

Must use different elements for different ages of rocks Limitations: Presence of radioactive isotopes (some types of rocks don’t have any) Age of rock (too young/old for the isotope present) Sedimentary rock is made up of sediments or pieces of other rock.

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**Elements for Radiometric Dating**

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**Radiometric dating process.**

How do they do it? Radiometric dating process. Particle accelerator piece is from 3:14 on

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