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.RELATIVE DATING OF GEOLOGIC EVENTS / MATERIALS: PRINCIPLES • Superposition • Original Horizontality • Cross-cutting relationshipsSuperpositionOriginal Horizontalityoriginal horizontalityErosion and creation of unconformityCross-cutting relationshipsEvent Principle Deposition of 1 – 5 Superposition Folding of 1 – 5 Original Horizontality Erosion Superposition / Cross-cutting Deposition of 7 – 9 Superposition timeFigure 18.4 .5Uranium-238 Unstable parent Rn-222 Radon gas Lead 206 Stable daughterExample radioactive decay pairs Parent Daughter Half life (years) Uranium-238 Lead-206 4.5 billion Uranium-235 Lead-207 713 million Rubidium-87 Strontium-87 47 billion Carbon-14 Nitrogen-14 5,370Figure 18.14radiometric dating – example 1 • A granite is collected and analyzed • Contains 50% U-235 and 50% Pb-207 • Half-life of U-235 is 713 million years • How old is the granite?radiometric dating – example 1 • Refer to Figure 18.14 • 50% parent, 50% daughter corresponds to 1 half-life • Half-life of U-235 is 713 million years • Thus, granite is 713 years oldradiometric dating – example 2 • A granite is collected and analyzed • Contains 2 micrograms of U-235 and 6 micrograms of Pb-207 • Half-life of U-235 is 713 million years • How old is the granite?radiometric dating – example 2 • 8 micrograms of parent + daughter – 2/8 micrograms = 1/4 = 25% U-235 – 6/8 micrograms = 3/4 = 75% Pb-207 • Refer to Figure 18.14 • 25% parent / 75% daughter corresponds to 2 half lives. • Half-life of U-235 is 713 million years • Thus, granite is 2 x 713 = 1,426 million years old = 1.4 billion yrs. • What if U-235 = 75%, Pb-207 = 25%?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 yearsCorrelation: establishing equivalence between rock layers that were once
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