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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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Determining Geologic Time Many geologic events that Earth Scientists study occurred millions of years ago. The ages of these events can be determined.

Determining Geologic Time Many geologic events that Earth Scientists study occurred millions of years ago. The ages of these events can be determined.

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