1. Geologic Time Relative vs. Absolute Time
Stratigraphy and Relative Time Relationships
Unconformities and Gaps in Stratigraphic Record
Stratigraphic Correlation
Geologic Time Scale

2. Figure 7.1

3. How Old is the Earth? Historical Records, written word
5000 years
Modern View, ~ 4.6 billion years old
Hutton (1795)
Uniformitarianism
“deep” time very old
Earth
Bishop Usher (1664)
9:00 AM, Oct. 23, 4004 BC
Earth is ~6000 years old
Geologic eventscatastrophism

4. How Old is the Earth Relative Dating
Geochronology is the study of time in relation to earth’s existence
Relative Dating
Determines how old a rock is in relation to its surrounding
Absolute (Numerical) Dating
Determines actual age in years

5. Geologic Time and the Rock Record
Rocks record the processes and events and help us measure geologic time.
By studying outcrops with the scientific method, we can figure out the relative order of events! Even absolute ages!
How?
Geometric relationships  Stratigraphy.
Fossils  Biostratigraphy.
Radiometric dating  Geochronology.

6. Stratification (Strata) Layering of Sedimentary Rocks
Distinct layering
Beds
Sedimentary Rock
Bed 1
Bed 2
Different thicknesses
Color, & other characteristics

7. Relative Age Inferences Original Horizontality
Sedimentary rocks are formed in layers (strata) which were originally horizontal.
If layers are inclined at an angle, then something tilted them - they didn’t form that way

8. Original horizontality

9. Relative Age Inferences Superposition
If one layer is on top of another, then it came later (it’s younger).
Note that layers can be completely upside down, and you need something like ripple marks to tell which way the layers are “facing”
Youngest
Oldest
Youngest
Oldest

10. Relative Age Inferences Cross-cutting relationships
Faults are younger than what they cut.
Crosscutting igneous rocks are younger than what they intrude.

11. Relative Age Inferences Inclusion
Units that include bits of another came later (are younger)
younger
older
older
younger

12. Relative Age Inferences
Assumptions / Geometric Principles:
Sediments deposited horizontally
Younger sediments on top of older
Units that cross-cut (e.g. faults or intrusions) came after (i.e., are younger than) those that they cut
Units that include bits of another came later (are younger)

13. Let’s practice
List events from oldest to youngest (including faulting and erosion)
Deposition of Abo Formation, Yeso Formation, Moenkopi Formation, Agua Zarco Formation
Fault (covered) offsets the four sedimentary units
Erosion (especially of Moenkopi)
Emplacement of Bandelier Rhyolite (as hot ash flow)
Erosion

14. Missing Time-Gaps Happen
Buried and tilted erosional surface

15. Conformable Contact
Layers of rock that have been deposited without any interruption.
No gaps in time.
No missing record due to erosion, non-deposition, etc.

16. Unconformity 3 types of break in the rock record.
Such surfaces represent:
A hiatus in deposition and/or…
A period of erosion.
“Missing time”
Significant events.
Popostosa Fm. Playa deposits & post-Santa Fe Group (Pl) alluvium.
N of San Lorenzo Cyn (Socorro, NM) – P. A. Scholle (1999).

17. Angular Unconformity
A sharp discontinuity in the rock record separating strata that are not parallel.
Indicates that during the break, a period of deformation occurred.

18. Disconformity
A break in the rock record across which there is little change in orientation of strata.
Often just a pause in deposition (subtle).
May also be obvious erosion surface.
River Road

19. Nonconformity
Horizontal sedimentary rocks on top of eroded crystalline rocks (metamorphic or igneous).
Requires erosion to bring crystalline rocks to the surface.

20. Missing Time: Unconformities

21. practice relative dating

22. Fossils as Timepieces
Fossils remains of ancient organisms
trilobites

23. Fossil Record
Recent
Older
Very Old

24. Relative Dating faunal succession
groups of fossil animals and plants occur in the geologic record in a definite chronological order
periods of time recognized by characteristic fossils
dinosaurs
trilobites

25. Correlation of Rock Units: Index Fossils
Common occurrence
Wide geographic distribution
Very restricted age range

26. Key Beds

27. Correlation Match rocks between different areas:
Key Beds & Index Fossils
Fossil successions.
Unique minerals.
Unique rock sequence.
Can extend relative age sequence elsewhere.

28. Correlation
Bryce
Zion
Grand
Canyon

29. The Geologic Column and the Geologic Time Scale
In 19TH Cent., geologists began to assemble a geologic column
composite column containing, in chronological order
the succession of known strata, fitted together on the basis of their fossils or other evidence of relative age.
The corresponding column of time is the geologic time scale.

30. Geologic Column Catalog of all known strata
Not one physical locality but a chronological compilation of all localities

31. Precambrian
Geologic Time
Scale

32. Eon: largest interval into which geologic time is divided.
Hadean Eon
Some moon samples were formed during the Hadean Eon.
Archean Eon
Archean rocks, which contain primitive microscopic life forms are the oldest rocks we know of on the Earth.
Proterozoic Eon
Phanerozoic Eon

33. Relative Time Scale Worldwide changes in fossils give break points
When did dinosaurs go mostly extinct?

34. Fossils Algae Single-celled organisms range? ?
(J. William Schopf; © 1993 AAAS)

35. Fossils
Mollusks

36. Fossils
Dinosaur range?
Dinosaurs:
M.y.a.

38. Relative Time Scale Worldwide changes in fossils give break points
The relative time scale doesn’t give us numerical ages.
Where do these numbers come from?

40. Relative Dating: Correlations

42. Inclusion
Units that include bits of another came later (are younger)

44. Absolute Time Early attempts to measure absolute time Radioactivity
Radiometric Dating Methods
Absolute time and Geologic Time

45. Geologic Time Scale RECAP: Relative time relationships
Fossil assemblages

46. Relative Time Scale Worldwide changes in fossils give break points
When did dinosaurs go mostly extinct?

47. Fossils Algae Single-celled organisms range? ?
(J. William Schopf; © 1993 AAAS)

48. Fossils
Mollusks

49. Fossils
Dinosaur range?
Dinosaurs:
M.y.a.

50. Relative Time Scale Worldwide changes in fossils give break points
The relative time scale doesn’t give us numerical ages.
Where do these numbers come from?

51. Early Attempts to Measure Geologic Time Numerically
Quantity of Something
Time =
Rate Quantity changes with time
For example, Rates of sedimentation & thickness of sedimentary rocks
Problem: did not account for past erosion
differences in sedimentation rates

52. Early Attempts to Measure Geologic Time Numerically
Saltiness of Seawater (date the ocean)
Edmund Halley (1715)
John Joly (1889)
Salt
rivers
Answer: ~ 90 million years
Oceans
Incorrect!!!
Salts are added both by erosion and by submarine volcanism, but salts are also removed by solution.

53. Early Attempts to Measure Geologic Time Numerically
Lord Kelvin (1870’s), a physicist, attempted to calculate the time Earth has been a solid body.

54. Early Attempts to Measure Geologic Time Numerically
Lord Kelvin (1897’s), a physicist, attempted to calculate the time Earth has been a solid body.
Time=0
Time=Today
Cooling off by conduction
Earth
Solid
Earth
molten
No more heating
Theory of heat conduction
Experimental data (melting temp. of rocks, size of Earth)
Answer: million years
Too Young for Geologists!

56. Radioactivity: A Little History
H. Becquerel (1896)
discovers radioactivity in Uranium
Marie Curie (1900)
discovers radium & heat is given off as byproduct of radioact.
E. Rutherford (1905)
Radioactive elements transform from one chemical element to another
B. Boltwood (1907)
Radiometric dating of minerals ( million years)

57. Radioactive Atoms Atoms contain Protons, Electrons & Neutrons
Carbon: Atomic number =6 (6 protons)
Stable
Isotope
Unstable
Isotope
Isotope: atoms of the same element containing different # neutrons

58. Radioactive Decay
It turns out that some elements will spontaneously turn into other elements. This is called radioactivity

59. Half-life (T1/2)
Time needed for ½ of parent atoms to decay (rate of decay)
T1/2= 1 hour
Time
# of Parent atoms
# of daughter atoms
1000
1 hr
500
2 hr
250
750
3 hr
125
875

61. Decay rate is a non-linear process
All radioactive elements follow the same law
But, each element will have different decay rates (half-life)
1 nanosecond 49 billion years

62. Decay Rates
Decay rates are unaffected by geological processes (mainly chemical)
Once radioactive atoms are created they start to act like ticking clock
Know the decay rate A A B A B
Count the daughter atoms
Count the Parent atoms A A B A
Calculate the time since the atomic clock started ticking

63. Mass spectrometer
A Minnesota Connection
Alfred Nier

64. Potassium-Argon Dating
40K-40Ar half-life = 1.3 billion years
Crystallization
40K40Ar
40K
40K40Ar
40K40Ar
K-mineral
Closed system
Magma
Clock is ticking
Open system
Rock clock is reset
Age of crystallization

65. Closed system no leakage or addition of K or Ar
K-mineral
Closed system
Closed system no leakage or addition of K or Ar
Geological processes can allow material to be added or lost date will be incorrect
Rock clock is reset
Age of crystallization
Cross-check with other radiometric systems using different minerals

66. Carbon-14 Method Atmosphere Neutron + 14-Nitrogen  14-Carbon  CO2
After Death
Time=0
14-C  decays
 14-N
T1/2= 5730 yrs
Surface
CO2
Water, Plants, &
Animals

67. Isotopic Systems Used for Radiometric Dating
Rubidium-Strontium t1/2= 47 billion yr
Uranium-lead t1/2=4.5 billion yr
Potassium-Argon t1/2=1.3 billion yr
Carbon-14 t1/2=5730 yr
Long t1/2 useful for dating old material
Short t1/2 useful for dating young material

68. Dating the Geologic Time Scale Sedimentary and Igneous Rocks
Granite is older than OLD RED SANDSTONE
Volcanic is younger than OLD RED SANDSTONE

69. Absolute Geologic Time Scale
Eon
Era
Period
Starting Age (Ma)
Phanerozoic
Cenozoic
Quaternary 65
Tertiary
Mesozoic
Cretaceous
248
Jurassic
Triassic
Paleozoic
Permian
540
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Precambrian - Proterozoic
2500
Precambrian - Archean
3800
Precambrian - Hadean
4500

70. Age of Earth Oldest dated rocks 3.94 by Oldest dated material 4.2 by
Moon Rocks & Meteorites by

71. A Year of Geologic Time 1second ≈ 200 years 0:00 AM, Jan 1
Formation of Earth
Late January
Formation of Core-Mantle-Crust
Mid February
Life Begins, Oldest Know Rocks
Late March
First Photosynthetic Organisms
Mid July
Evolution of Cells with Nucleus
Mid November
First Organisms with Shells
Late November
First Land Plants/Fish
Mid December
Dinosaurs became Dominant
Dec 26
Extinction of Dinosaurs
Evening of Dec 31
Human-like Animals
11:59:45-11:59:50
Rome Ruled the Western World
11:59:59
Modern Geology Started with Hutton

73. Clair Patterson & the Age of the Earth
In early 1950’s, Clair Patterson was a graduate student at the University of Chicago.
Wanted to use lead isotope ratios to determine the Earth’s age,
but the background level of lead contamination was too high
Lead used in gasoline, paints, plumbing, solder (cans for food) and pesticides.

74. Clair Patterson & the Age of the Earth
To accurately measure very low lead concentrations, Patterson created the modern laboratory ‘clean room’.
in 1953, published estimate of Earth’s age as 4.55 BY (previously estimated at 3.3 BY)

75. By 1960’s, Patterson began to worry about the extent of lead contamination in our environment.
Patterson discovered that modern humans had 700 to 1,200 times as much lead in their bones as pre-Columbian Incas.  
over 99% of the northern hemisphere atmospheric lead originated from human activity.
The average atmospheric lead levels
were 10 to 200 times higher than in pre-industrial times and up to 1,000 to 10,000 times higher in urban areas!

76. First recognition of the global scale and early history of lead pollution
First recognition that essentially EVERYONE in 1950’s-60’s society suffered from low-level lead poisoning.
Patterson campaigned extensively for lead removal, but was vigorously opposed by industry labs and some other scientists.
Eventually, scientific data accumulated by Patterson
and others led to the 1970 Clean Air Act
By 1991, lead levels in Greenland snow had fallen by a factor of 7.5