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