Geologic Time

1
Geologic Time
2
Geologic Time

• A major difference between geologists and most
  other scientists is their attitude about time.
  
• A "long" time may not be important unless it is
  1 million years.

3
Two ways to date to date geologic events

• relative dating (fossils, cross cutting
  relationships, structural relationships)
  
• (2) absolute dating (isotopic, tree rings, etc.)

4
Amount of Time Required for Some Geologic
Processes and Events
Fig. 9.1
5
Some geologic processes can be documented using
historical records(Brown is new land from
1887-1988)
Fig. 9.2
6
Steno's Laws

• Nicolaus Steno (1669)
• Principle of Superposition
• Principle of Original Horizontality
• Principle of Lateral Continuity

Laws apply to both sedimentary and volcanic rocks.
7
Principle of Superposition

• In a sequence of undisturbed layered rocks, the
  oldest rocks are on the bottom.

8
Principle of Superposition
Youngest rocks
Oldest rocks
Fig. 9.3b
Jim Steinberg/Photo Researchers
9
Principle of Original Horizontality

• Layered strata are deposited horizontal or nearly
  horizontal or nearly parallel to the Earths
  surface.

10
Principle of Lateral Continuity
Layered rocks are deposited in continuous contact.
11
Principle of Lateral Continuity
Map view
12
Paleontology

• The study of life in the past based on fossilized
  plants and animals.
  
• Fossil Evidence of past life
• Fossils preserved in sedimentary rocks are used
  to determine 1) Relative age 2) Environment
  of deposition

13
Using Fossils to Correlate Rocks

• Index Fossil A fossil known to be restricted to
  a specific period of geologic time
  
• Faunal Succession Groups of different fossils
  occur in a specific stratigraphic order

Fig. 9.5
Fig. 9.5
14
Unconformity

• A buried surface of erosion or nondepositon
• Types of unconformity
• Disconformity (Simple unconformity)
• Angular unconformity
• Nonconformity

15
Sedimentation of Beds A-D Beneath the Sea
Fig. 9.6
16
Uplift and Exposure of D to Erosion
Fig. 9.6
17
Continued Erosion Removes D and Exposes C to
Erosion
Fig. 9.6
18
Subsidence and Sedimentation of E over C
Unconformity a buried surface of erosion
Fig. 9.6
19
Formation of a Disconformity

• Lack of deposition of sedimentary units at a
  specific time interval

Fig. 9.6
20
South Rim of the Grand Canyon
21
South rim of the Grand Canyon
250 million years old
Paleozoic Strata
550 million years old
1.7 billion years old
Precambrian
22
South rim of the Grand Canyon
250 million years old
550 million years old
1.7 billion years old
Nonconformity
23
The Nonconformity of the Grand Canyon
Fig. 9.7
24
The Great (Angular) Unconformity of the Grand
Canyon
Geoscience Features Picture Libraryc
Fig. 9.7
25
Angular Unconformity, Grand Canyon
26
Generalized Stratigraphic Section of Rocks
Exposed in the Grand Canyon
after Beus Moral (1990)
27
Some of the Geologic Units Exposed in the Grand
Canyon
Michael Collier
28
Sedimentation of Beds A-D Beneath the Sea
Fig. 9.8
29
Deformation and Erosion During Mountain Building
Fig. 9.8
30
Erosional Surface Cuts Across Deformed Rocks
Fig. 9.8
31
Subsidence and Subsequent Deposition Buries
Erosional Surface
Angular
Unconformity
Fig. 9.8
32
(No Transcript)
33
Cross-cutting Relationships
Fig. 9.9
34
The Geologic time scale

• Divisions in the worldwide stratigraphic column
  based on variations in preserved fossils
  
• Built using a combination of stratigraphic
  relationships, cross-cutting relationships, and
  absolute (isotopic) ages

35
The Geologic Time Scale

• Phanerozoic
• Cenozoic
• Mesozoic
• Paleozoic
• ---- 540 Ma ---
• Precambrian
• 4.5 Ga (Age of the Earth)

Fig. 9.13
36
Absolute geochronology

• Add numbers to the stratigraphic column based on
  fossils.
  
• Based on the regular radioactive decay of some
  chemical elements present in minerals.

37
Isotopes

• Different forms of the same element containing
  the same number of protons, but varying numbers
  of neutrons.
  
• i.e.
• 235U, 238U 87Sr, 86Sr 14C, 12C

38
Radioactive Decay of Rubidium to Strontium
87Rb ? 87Sr
Fig. 9.14
39
Half-life
The half-life of a radioactive isotope is defined
as the time required for half of the atoms of the
isotope present in a geologic material it to
decay to the daughter isotope.
40
Proportion of Parent Atoms Remaining as a
Function of Time
Fig. 9.15
41
Proportion of Parent and Daughter as a Function
of Time
42
Parent Daughter Ratios and the Number of Half
Lives
43
Isotopic dating

• Radioactive elements (parents) decay to
  nonradioactive (stable) elements (daughters).
  
• The rate at which this decay occurs is constant
  and knowable.
  
• Therefore, if we know the rate of decay and the
  amount present of parent and daughter, we can
  calculate how long this reaction has been
  proceeding.

44
Example of Radiometric Age Calculation

• Isotope X has an half life of 1 billion years, it
  decays to Isotope Y of a different element
  
• In a mineral the proportion of X to Y is 17 (D/P
  7)
  
• This equals 3 half lives (3 billion years)

45
Parent Daughter Ratios and the Number of Half
Lives
46
Geologically Useful Decay Schemes
Parent Daughter Half-life (years)
235U 207Pb 4.5 x 109 238U 206Pb 0.71 x 109 40K 4
0Ar 1.25 x 109
87Rb 87Sr 47 x 109 14C 14N 5730
47
Potassium-Argon Dating Laboratory
48
The geologic timescale and absolute ages

• Isotopic dating of intebedded volcanic rocks
  allows assignment of an absolute age for fossil
  transitions

49
The big assumption

• The half-lives of radioactive isotopes are the
  same as they were billions of years ago.

50
Test of the assumption

• Meteorites and Moon rocks (that are thought to
  have had a very simple history since they
  formed), have been dated by up to 10 independent
  isotopic systems all of which have given the same
  answer. However, scientists continue to
  critically evaluate this data.

51
Uniformitarianism
The present is the key to the past.
James Hutton

• Natural laws do not change however, rates and
  intensity of processes may.

52
1871
Fig. 9.17
53
1968
Fig. 9.17
54
Fig. 9.18
55
Many methods have been used to determine the age
of the Earth

• 1) Bible In 1664, Archbishop Usher of Dublin
  used chronology of the Book of Genesis to
  calculate that the world began on Oct. 26, 4004
  B.C.
• 2) Salt in the Ocean (ca. 1899) Assuming the
  oceans began as fresh water, the rate at which
  rivers are transporting salts to the oceans would
  lead to present salinity in 100 m.y.

56
Many methods have been used to determine the age
of the Earth

• 3) Sediment Thickness Assuming the rate of
  deposition is the same today as in the past, the
  thickest sedimentary sequences (e.g., Grand
  Canyon) would have been deposited in 100 m.y.
• 4) Kelvins Calculation (1870) Lord Kelvin
  calculated that the present geothermal gradient
  of 30C/km would result in an initially molten
  earth cooled for 30 100 m.y.

57
Flawed assumptions

• Bible does not record science observations
• Salt is precipitated in sedimentary formations
• Both erosion and non-deposition are major parts
  of the sedimentary record
  
• Radioactivity provides another heat source

58
The Heat Inside the Earth

• The discovery of radioactivity at the turn of the
  century by Bequerel, Curie, and Rutherford not
  only provided the source of the heat to override
  Kelvins calculations but provided the basis for
  all later quantitative estimates of the ages of
  rocks.

59
Oldest rocks on Earth

• Slave Province, Northern Canada
• Zircons in a metamorphosed granite dated at 3.96
  Ga by the U-Pb method
  
• Yilgarn block, Western Australia
• Detrital zircons in a sandstone dated at 4.10 Ga
  by U-Pb method.
  
• Several other regions dated at 3.8 Ga by various
  methods including Minnesota, Wyoming, Greenland,
  South Africa, and Antarctica.

60
Age of the Earth

• Although the oldest rocks found on Earth are
  3.96 Ga (or even 4.1), we believe that the age of
  the Earth is approximately 4.6 Ga. All rocks of
  the age 4.6 to 4.0 Ga have been destroyed (the
  rock cycle) or are presently covered by younger
  rocks.

61
Age of the Earth

• This is based on the age of rocks brought back
  from the Moon (4.4 Ga), and meteorites (4.6 Ga),
  that are thought to be good representatives of
  the early solar system as well as more
  complicated geochemical modeling. This data
  suggests that the present chemical composition of
  the crust must have evolved for more than 4.5 Ga.

62
Major Radioactive Elements Used in Isotopic Dating
Table 9.1
63
Ammonite Fossils
Petrified Wood
Fig. 9.4
Chip Clark
Tom Bean