Presentation on theme: "Time is not part of rock cycle, but is implied"— Presentation transcript:
1Time is not part of rock cycle, but is implied Example: a granite could be weathered and the particleseventually become part of a sandstone. The sandstonecould later be metamorphosed to become a quartzite.How fast these events happen would determinewhat ages the rocks were.
23radiometric dating – example 1 A granite is collected and analyzedContains 50% U-235 and 50% Pb-207Half-life of U-235 is 713 million yearsHow old is the granite?
24radiometric dating – example 1 Refer to Figure 18.1450% parent, 50% daughter corresponds to 1 half-lifeHalf-life of U-235 is 713 million yearsThus, granite is 713 years old
25radiometric dating – example 2 A granite is collected and analyzedContains 2 micrograms of U-235 and 6 micrograms of Pb-207Half-life of U-235 is 713 million yearsHow old is the granite?
26radiometric dating – example 2 8 micrograms of parent + daughter2/8 micrograms = 1/4 = 25% U-2356/8 micrograms = 3/4 = 75% Pb-207Refer to Figure 18.1425% parent / 75% daughter corresponds to2 half lives.Half-life of U-235 is 713 million yearsThus, granite is 2 x 713 = 1,426million years old = 1.4 billion yrs.What if U-235 = 75%, Pb-207 = 25%?