1. LEARNING OBJECTIVE How do geologists interpret Earth’s history, and how does uniformitarianism help explain Earth’s

2. UNIT 6 GEOLOGIC TIME Geological time scale
Record of life forms & geological events in Earth’s history.
Scientists developed the time scale by studying rock layers & fossils world wide.
1. Relative age: is the age of compared to other rocks or events.

3. 2. Age of the Earth.
4.6 billion
3. Catastrophism-earth’s
history dominated by violent
events.
3. Uniformitarianism-geologic processes have operated uniformly from origin of Earth to present time. The present is key to the past.

4. B. Age of The Earth
1. Calculation from the Bible-(John Lightfoot, Bishop Ussher) B.C.
2. Salt in ocean and thickness of sediments gives ages of 100 million years, but rates are variable.
3. Age of Sun-the Sun was believed to be burning chemical reactions gave age of 10,000 years.

5. 1. Relative Age of rock or event as compared to other rocks or events.
a. Principle of Original Horizontality-sediments are deposited in horizontal layers that parallel to surface on which they were deposited.
Tilted or folded layers indicate that the crust has been deformed.

7. b. Principle of superposition-in series of undisturbed layers, oldest layer on bottom/each overlying layer is progressively younger/ youngest layer on top.

9. c. Evidence of Events
Igneous intrusions/extrusions, faults, joints, folds

11. d. Correlation techniques
Process of matching rocks /geologic events in one location to rocks and events in another location.
Continuity of rocks

12. Fossil evidence in rocks
Index fossils lived over large
areas for relatively short
periods of time.

14. 2. Absolute age
a. Radioactive decay sequence acts as atomic clock.
Radioactive isotopes decay (change) into daughter isotopes at known rates. Parent isotopes emits subatomic particles.
Half-life-amount of time needed for one-half of a radioactive parent to decay into doughter isotopes.

15. neutrons
protrons

16. Five Radioactive Isotope Pairs Five Radioactive Isotope Pairs
Effective
Dating Range
Minerals and
Isotopes
Half-Life
of Parent
(Years)
Rocks That Can
Parent
Daughter
(Years)
Be Dated
Uranium 238
Lead 206
4.5 billion
10 million to
Zircon
4.6 billion
Uraninite
Uranium 235
Lead 207
704 million
Muscovite
Thorium 232
Lead 208
14 billion
48.8 billion
Biotite
Potassium feldspar
Rubidium 87
Strontium 87
4.6 billion
10 million to
Whole metamorphic
4.6 billion
or igneous rock
Potassium 40
Argon 40
1.3 billion
100,000 to
Glauconite
4.6 billion
Muscovite
Biotite
Hornblende
Whole volcanic rock

17. Carbon 14 dating can be used to date fossils up to 75,000 years.
For older fossils, some isotopes can be used to date sedimentary rock layers above and below the fossil.

18. Carbon 14 Cycle

19. 3. Geologic Time Scale