1. Timing the Geologic Record
Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 8:
CLOCKS IN ROCKS
Timing the Geologic Record
© 2011 by W. H. Freeman and Company

2. Timing the Geologic Record
Chapter 8:
Clocks in Rocks:
Timing the Geologic Record

3. About the Clocks in Rocks
Geologists have uncovered deep time by studying the “clocks in rocks.”
These clocks can be used to measure the duration of geologic processes and cycles of the Earth system.
Geologists refer to both relative age and absolute age.

4. Lecture Outline
Reconstructing geologic history from the stratigraphic record
2. Geologic time scale: relative ages
3. Measuring absolute time with isotopic clocks
4. Geologic time scale: absolute ages
5. Advances in timing the Earth system

5. ● Principles of stratigraphy ● original horizontality ● superposition
Reconstructing Geologic History from
the Stratigraphic Record
● Principles of stratigraphy
● original horizontality
● superposition
● faunal succession

6. Original Horizontality and Superposition
Younger
Older
Example: Marble Canyon, Arizona

7. Original Horizontality and Superposition
Younger
Older
Left undisturbed, the youngest layers always remain above the oldest.

8. Original Horizontality and Superposition
sedimentation in
lake or sea

9. Original Horizontality and Superposition
Sediments are deposited in horizontal
layers and slowly change into rock.

10. Original Horizontality and Superposition

11. Fossils as Recorders of Time: The Principle of Faunal Succession
Outcrop A
Outcrop B I II II
III

12. Outcrop A
Outcrop B I II II
III

13. Outcrop A
Outcrop B I II II
III

14. Some of the fossils found in
outcrop A are the same as
fossils found in outcrop B,
some distance away.
Outcrop A
Outcrop B I II II
III

15. Some of the fossils found in
outcrop A are the same as
fossils found in outcrop B,
some distance away.
Outcrop A
Outcrop B I II II
Layers with the same fossils are the same age.
III

16. Some of the fossils found in
outcrop A are the same as
fossils found in outcrop B,
some distance away.
Outcrop A
Outcrop B I II II
Layers with the same fossils are the same age.
III

18. ● Unconformities – gaps in the record ● disconformity
Reconstructing Geologic History from
the Stratigraphic Record
● Unconformities – gaps in the record
● disconformity
● angular unconformity

19. Disconformity

20. Disconformity

21. Disconformity

22. Disconformity

23. Disconformity

24. Angular
unconformity

25. Angular
unconformity

26. Angular
unconformity

27. Angular
unconformity

28. Angular
unconformity

29. Angular unconformity The ‘great unconformity’ in the
Grand Canyon, Arizona.

30. ● Cross-cutting relationships ● faults ● intrusions
Reconstructing Geologic History from
the Stratigraphic Record
● Cross-cutting relationships
● faults
● intrusions

31. Cross-cutting
relationships

32. Cross-cutting
relationships

33. Cross-cutting
relationships

34. Cross-cutting
relationships

35. Cross-cutting
relationships

36. Relative age
relationships

37. Thought questions for this chapter
As you pass by an excavation in the street, you see a cross section showing paving at the top, soil below the paving, and bedrock at the base. You also notice that a vertical water pipe extends through a hole in the street into a sewer in the soil. What can you say about the relative ages of the various layers and the water pipe?
Why did 19th-century geologists who constructed the geologic time scale find sedimentary strata deposited in the sea more useful than strata deposited on land?

38. Thought questions for this chapter
In studying an area of tectonic compression, a geologist discovers a sequence of older, more deformed sedimentary rocks on top of a younger, less deformed sequence, separated by an angular unconformity. What tectonic process might have created the angular unconformity?
A geologist documents a distinctive chemical signature caused by Proterozoic organisms. Would you consider this chemical signature to be a fossil?

39. ● Divisions of geologic time ● eras ● periods ● epochs
2. Geologic Time Scale:
Relative Ages
● Divisions of geologic time
● eras
● periods
● epochs

41. 3. Measuring Absolute Time with Isotopic Clocks
● Isotopic dating theory
● radioactive atoms
● parent-daughter
● half-life

42. 3. Measuring Absolute Time with Isotopic Clocks
● Isotopic dating methods
● uranium-lead
● potassium-argon
● rubidium-strontium
● carbon-nitrogen

44. Example: Rubidium-Strontium
Neutrons
Protons
Electron

45. Rubidium-87 nucleus
Neutrons
Protons
Electron
A neutron decays,
ejecting an electron…

46. Rubidium-87 nucleus
Strontium-87 nucleus
Neutrons
Protons
Electron
A neutron decays,
ejecting an electron…

47. Rubidium-87 nucleus
Strontium-87 nucleus
Neutrons
Protons
Electron
A neutron decays,
ejecting an electron…
…and producing a proton,
which changes the atom.

50. Thought questions for this chapter
Is carbon-14 a suitable isotope for dating geologic events in the Pliocene Epoch?
How does determining the ages of igneous rocks help to date fossils?

51. ● Four Eons of geologic time ● Hadean ● Archean ● Proterozoic
4. Geologic Time Scale:
Absolute Ages
● Four Eons of geologic time
● Hadean
● Archean
● Proterozoic
● Phanerozoic

53. ● Sequence stratigraphy ● Chemical stratigraphy
5. Advances in Timing
the Earth System
● Sequence stratigraphy
● Chemical stratigraphy
● Paleomagnetic stratigraphy
● Clocking the climate system

58. Key terms and concepts Eon Epoch Era Geologic time scale Half-life
Absolute age
Eon
Epoch
Era
Geologic time scale
Half-life
Isotopic dating
Mass extinction
Period
Principle of faunal succession
Principle of original horizontality
Principle of superposition
Relative age
Stratigraphic succession
Unconformity