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Time is not part of rock cycle, but is implied Example: a granite could be weathered and the particles eventually become part of a sandstone. The sandstone could later be metamorphosed to become a quartzite. How fast these events happen would determine what ages the rocks were.

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RELATIVE DATING OF GEOLOGIC EVENTS / MATERIALS: PRINCIPLES Superposition Original Horizontality Cross-cutting relationships

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Superposition

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Original Horizontality

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original horizontality

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Erosion and creation of unconformity

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Cross-cutting relationships

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EventPrinciple Deposition of 1 – 5Superposition Folding of 1 – 5Original Horizontality ErosionSuperposition / Cross-cutting Deposition of 7 – 9Superposition time

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Layer A Layer B Layer C Layer D Layer E Layer F Layer G Layer H Figure 18.4.5

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Uranium-238 Unstable parent Rn-222 Radon gas Lead 206 Stable daughter

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Example radioactive decay pairs ParentDaughterHalf life (years) Uranium-238Lead-2064.5 billion Uranium-235Lead-207713 million Rubidium-87Strontium-8747 billion Carbon-14Nitrogen-145,370

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Figure 18.14

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radiometric dating – example 1 A granite is collected and analyzedA granite is collected and analyzed Contains 50% U-235 and 50% Pb-207Contains 50% U-235 and 50% Pb-207 Half-life of U-235 is 713 million yearsHalf-life of U-235 is 713 million years How old is the granite?How old is the granite?

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radiometric dating – example 1 Refer to Figure 18.14Refer to Figure 18.14 50% parent, 50% daughter corresponds to 1 half-life50% parent, 50% daughter corresponds to 1 half-life Half-life of U-235 is 713 million yearsHalf-life of U-235 is 713 million years Thus, granite is 713 years oldThus, granite is 713 years old

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radiometric dating – example 2 A granite is collected and analyzedA granite is collected and analyzed Contains 2 micrograms of U-235 and 6 micrograms of Pb-207Contains 2 micrograms of U-235 and 6 micrograms of Pb-207 Half-life of U-235 is 713 million yearsHalf-life of U-235 is 713 million years How old is the granite?How old is the granite?

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radiometric dating – example 2 8 micrograms of parent + daughter8 micrograms of parent + daughter –2/8 micrograms = 1/4 = 25% U-235 –6/8 micrograms = 3/4 = 75% Pb-207 Refer to Figure 18.14Refer to Figure 18.14 25% parent / 75% daughter corresponds to25% parent / 75% daughter corresponds to 2 half lives. 2 half lives. Half-life of U-235 is 713 million yearsHalf-life of U-235 is 713 million years Thus, granite is 2 x 713 = 1,426Thus, granite is 2 x 713 = 1,426 million years old = 1.4 billion yrs. million years old = 1.4 billion yrs. What if U-235 = 75%, Pb-207 = 25%?What if U-235 = 75%, Pb-207 = 25%?

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Modern humans Archaic humans incl. Neanderthals Present time 33,000 years Africa Europe Asia Evolution of humans ~3 million years ago Small Archaic Humans 13,000 years 95,000 years

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Correlation: establishing equivalence between rock layers that were once continuous

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