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 relationships

Deposition of 1 – 5 Superposition Event Principle Deposition of 1 – 5 Superposition Folding of 1 – 5 Original Horizontality Erosion Superposition / Cross-cutting Deposition of 7 – 9 Superposition time

Figure 18.4 .5 Layer E Layer D Layer C Layer B Layer A Layer F Layer H Layer G Layer E Layer D Layer C Layer B Layer A Figure 18.4 .5

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

Example 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 5,370

Figure 18.14

radiometric 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 old

radiometric 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%?

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

Correlation: establishing equivalence between rock layers that were once continuous