1. Geologic Time

2. Geologic Time Introduction The Geologic Time Scale Relative Dating
Fossils
Radiometric Dating

3. GEOL131: Geologic Time
Introduction

4. GEOL131: Geologic Time: Introduction
Deep Time
Current evidence indicates Earth is about 4.6 billion yrs old
Dating of Earth minerals using radioactivity
Dating of meteorites and moon rocks
Rocks and fossils form the record of geologic time

5. Deep Time Two ways to date rocks
GEOL131: Geologic Time: Introduction
Deep Time
Two ways to date rocks
Relative to other rocks
Numerically: age in years before present
Relative dating principles are among oldest in geology
Established in 1600s and 1700s
Numerical (absolute) dating is one of geology’s youngest branches
Post-World War II

6. The Geologic Time Scale
GEOL131: Geologic Time
The Geologic Time Scale

7. 570 million to 4.6 billion yrs ago
GEOL131: Geologic Time: The Time Scale, basic structure
PRESENT
Precambrian
570 million to 4.6 billion yrs ago
EARTH FORMS

8. GEOL131: Geologic Time: The time scale shown to correct scale

9. The Precambrian 4.6 billion to 570 million years ago
GEOL131: Geologic Time: The Time Scale
The Precambrian
4.6 billion to 570 million years ago
88% of Earth history
First single-celled organisms
First multi-celled organisms

10. The Precambrian Very little known compared to more recent past
GEOL131: Geologic Time: The Time Scale
The Precambrian
Very little known compared to more recent past
Older rocks have been destroyed
No organisms with hard parts, so no fossils

11. The Paleozoic Era First fish First land plants First land animals
GEOL131: Geologic Time: The Time Scale
The Paleozoic Era
First fish
First land plants
First land animals
Ended with largest mass extinction in Earth history
96% species mortality rate

12. Species existing before
GEOL131: Geologic Time: The Time Scale
“Adaptive radiation”
Post-mass extinction, ecological niches are mostly empty
Few surviving species give rise to many new species to fill these niches
younger
TIME
MASS EXTINCTION
older
Species existing before
mass extinction

13. Few extinction survivors
GEOL131: Geologic Time: The Time Scale
“Adaptive radiation”
Post-mass extinction, ecological niches are mostly empty
Few surviving species give rise to many new species to fill these niches
younger
TIME
MASS EXTINCTION
Few extinction survivors
older

14. Many new species evolve from few extinction survivors
GEOL131: Geologic Time: The Time Scale
“Adaptive radiation”
Post-mass extinction, ecological niches are mostly empty
Few surviving species give rise to many new species to fill these niches
younger
TIME
MASS EXTINCTION
Many new species evolve from few extinction survivors
older

15. The Mesozoic Era Dinosaurs dominant First flowering plants
GEOL131: Geologic Time: The Time Scale
The Mesozoic Era
Dinosaurs dominant
First flowering plants
Ended by another mass extinction
Likely caused by asteroid impact
Mass extinction & adaptive radiation

16. The Cenozoic Era Mammals, including humans Most recent Ice Age
GEOL131: Geologic Time: The Time Scale
The Cenozoic Era
Mammals, including humans
Most recent Ice Age
2 mya – 10,000 yrs ago
Mass extinction & adaptive radiation

17. GEOL131: Geologic Time
Relative Dating

18. Principles Superposition Original Horizontality
GEOL131: Geologic Time: Relative Dating
Principles
Superposition
Original Horizontality
Cross-cutting Relationships
Inclusions

19. Superposition If Layer A is above Layer B, Layer A is younger
GEOL131: Geologic Time: Relative Dating
Superposition
If Layer A is above Layer B, Layer A is younger
Assumes layers are not overturned

20. Original Horizontality
GEOL131: Geologic Time: Relative Dating
Original Horizontality
If a sedimentary rock layer is not horizontal, tectonic forces pushed it into its current position

21. Cross-cutting Relationships
GEOL131: Geologic Time: Relative Dating
Cross-cutting Relationships
If A crosscuts B, A is younger
Usually used with igneous intrusions and faults
The igneous dike is the youngest feature, since it crosscuts everything else.

22. Cross-cutting Relationships
GEOL131: Geologic Time: Relative Dating
Cross-cutting Relationships

23. Inclusions If pieces of B are included in A, A is younger
GEOL131: Geologic Time: Relative Dating
Inclusions
If pieces of B are included in A, A is younger
Sandstone contains inclusions of granite, so sandstone is younger.
Granite is younger than sandstone.

24. GEOL131: Geologic Time
Unconformities

25. Unconformities A surface between layers that represents missing rock
GEOL131: Geologic Time: Relative Dating
Unconformities
A surface between layers that represents missing rock
Means there was an extended period of erosion

26. Unconformities: Three types
GEOL131: Geologic Time: Relative Dating
Unconformities: Three types
Disconformity
Angular unconformity
Nonconformity
Separates two horizontal layers
Separates horizontal from tilted layers
Separates sedimentary from non-sedimentary

27. Unconformities Unconformities in the Grand Canyon
GEOL131: Geologic Time: Relative Dating: Unconformities
Unconformities
Unconformities in the Grand Canyon

28. GEOL131: Geologic Time
Fossils

29. What Are Fossils? Any trace of past life preserved in rock
GEOL131: Geologic Time: Fossils
What Are Fossils?
Any trace of past life preserved in rock
Body (shells, bones, etc.)
Trace (footprints, burrows, etc.)

30. GEOL131: Geologic Time: Fossils
Fossil Ranges
Each species has an age of earliest appearance in the rock record

31. GEOL131: Geologic Time: Fossils
Fossil Ranges
Most also have an age of final disappearance (extinction)
Species don’t reappear after extinction

32. Fossils Ranges Fossil range: time between appearance and disappearance
GEOL131: Geologic Time: Fossils
Fossils Ranges
Fossil range: time between appearance and disappearance
Fossil ranges of multiple species can overlap

33. Fossils Ranges and Age Bracketing
GEOL131: Geologic Time: Fossils
Fossils Ranges and Age Bracketing
Overlapping fossil ranges can be used to “bracket” the age of a rock layer

34. GEOL131: Geologic Time
Radiometric Dating

35. Atomic Structure - review
GEOL131: Geologic Time: Radiometric Dating
Atomic Structure - review

36. Radioactivity Spontaneous changes in atomic nuclei
GEOL131: Geologic Time: Radiometric Dating
Radioactivity
Spontaneous changes in atomic nuclei
Loss or gain of protons or neutrons
Occurs at known rates
Each element’s decay rate is different

37. Radioactivity Parent: The element that changes
GEOL131: Geologic Time: Radiometric Dating
Radioactivity
Parent: The element that changes
Daughter: The product of the change
Daughter can be
A different element
A different version (isotope)of the parent element

38. Radioactivity Some parent-daughter pairs commonly used in geology:
GEOL131: Geologic Time: Radiometric Dating
Radioactivity
Some parent-daughter pairs commonly used in geology:

39. GEOL131: Geologic Time: Radiometric Dating
Radioactivity
Because rates are known and don’t change, radioactive decay can be used as a “clock” to measure geologic time

40. GEOL131: Geologic Time: Radiometric Dating
Radioactivity
Mineral crystallizes and incorporates radioactive parent atoms
Parent atoms decay into daughter atoms at known rate
Percentage of remaining parent can give age of mineral

41. GEOL131: Geologic Time: Radiometric Dating
Half-life
Time required for one-half of remaining parent atoms to decay

42. Example of radiometric dating
GEOL131: Geologic Time: Radiometric Dating
Example of radiometric dating
If a mineral contains 50% K-40 and 50% Ar-40, how old is the mineral?
How many half-lives of K-Ar have elapsed since the mineral formed?
How many years is that equivalent to?
- This is the age of the mineral

43. Example of radiometric dating
GEOL131: Geologic Time: Radiometric Dating
Example of radiometric dating
How many half-lives of K-Ar have elapsed since the mineral formed?
Mineral contains 50% parent and 50% daughter
So, one K-Ar half-life has elapsed since mineral formed
Percentage of remaining parent element

44. Example of radiometric dating
GEOL131: Geologic Time: Radiometric Dating
Example of radiometric dating
2) How many years is that?
1 half-life x 1.3 billion years = 1.3 billion years old

45. Half-life If 25% parent, 2 half-lives have passed
GEOL131: Geologic Time: Radiometric Dating
Half-life
If 25% parent, 2 half-lives have passed
If 12.5%, 3 half-lives
Etc…

46. Applicability of radiometric dating
GEOL131: Geologic Time: Radiometric Dating
Applicability of radiometric dating
Igneous rocks
Minerals in rock did not come from an older rock
So, mineral’s age is rock’s age
Metamorphic rocks
OK if mineral being analyzed formed during metamorphism

47. Applicability of radiometric dating
GEOL131: Geologic Time: Radiometric Dating
Applicability of radiometric dating
Sedimentary rocks
Very difficult
Mineral grains usually derived from older rocks
Weathering and erosion can “reset” clock by allowing daughter product to escape

48. GEOL131: Geologic Time
End of Chapter