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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.
RELATIVE DATING OF GEOLOGIC EVENTS / MATERIALS:
PRINCIPLES
• Superposition
• Original Horizontality
• Cross-cutting relationships
Superposition
Original Horizontality
original horizontality
Erosion and creation of unconformity
Cross-cutting relationshipsCross-cutting relationships
Event Principle
Deposition of 1 – 5 Superposition
Folding of 1 – 5 Original Horizontality
Erosion Superposition / Cross-cutting
Deposition of 7 – 9 Superposition
tim
e
Layer A
Layer B
Layer C
Layer D
Layer E
Layer F Layer GLayer H
Figure 18.4.5
Uranium-238Unstable parent
Rn-222Radon gas
Lead 206Stable daughter
Example radioactive Example radioactive decay pairsdecay pairs
Parent DaughterHalf 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 5,370
Figure 18.14
radiometric dating – radiometric dating – example 1example 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?
radiometric dating – radiometric dating – example 1example 1
• Refer to Figure 18.14Refer to Figure 18.14• 50% parent, 50% daughter corresponds to 1 half-50% parent, 50% daughter corresponds to 1 half-
lifelife• 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
radiometric dating – radiometric dating – example 2example 2
• A granite is collected and analyzedA granite is collected and analyzed• Contains 2 micrograms of U-235 and 6 Contains 2 micrograms of U-235 and 6
micrograms of Pb-207micrograms 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?
radiometric dating – radiometric dating – example 2example 2
• 8 micrograms of parent + daughter8 micrograms of parent + daughter– 2/8 micrograms = 1/4 = 25% U-2352/8 micrograms = 1/4 = 25% U-235– 6/8 micrograms = 3/4 = 75% Pb-2076/8 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%?
Modern humans
Archaic humansincl. Neanderthals
Present time
33,000 years
Africa
Europe
Asia
Evolution of humans~3 million years ago
Small ArchaicHumans
13,000 years
95,000 years
Correlation: establishing equivalence between rock layers that were once
continuous
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