2. Outline
⢠Geologic time: perspective & a bit of history
⢠Dating geologic materials
-General: relative & absolute dating
-Relative dating:
-7 Principals & their application to a geologic history
-Fossil successions
⢠Gaps in the geologic record (unconformity)
-3 types of unconformities
-Stratigraphic correlation & the global geologic column
⢠Numerical (absolute) dating
-Radioactive decay
-Meaning of a radiometric date
-Other numerical dating methods
-Dating the geologic column, geologic time scale, & age of Earth
Chapter 12
Chapter 12
4. Geologic Time
⢠Understanding time permits assigning ages toâŚ
Chapter 12
5. Geologic Time
⢠Prior to late 1600s, geologic time was thought to =
historical time.
⢠Archbishop James Ussher, Ireland, 1654.
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6. Geologic Time
⢠Scientists began to find clues to an ancient Earth.
⢠Nicolaus Steno (1638â1686) â Danish physician.
fossil
shark
tooth
Chapter 12
7. Outline
⢠Geologic time: perspective & a bit of history
⢠Dating geologic materials
-General: relative & absolute dating
-Relative dating:
-7 Principals & their application to a geologic history
-Fossil successions
⢠Gaps in the geologic record (unconformity)
-3 types of unconformities
-Stratigraphic correlation & the global geologic column
⢠Numerical (absolute) dating
-Radioactive decay
-Meaning of a radiometric date
-Other numerical dating methods
-Dating the geologic column, geologic time scale, & age of Earth
Chapter 12
Chapter 12
8. Geologic Time
2 ways to date geological materials:
1. Relative age â
2. Numerical (absolute) age â
Chapter 12
20. Geologic History
⢠Intrusion solidified into sill
⢠Tectonic compression
Chapter 12
21. Geologic History
⢠Compression results:
⢠Folding (inference: layers had to exist to be folded).
⢠Uplift (above sea level) & erosion.
⢠Intrusion of a pluton. *baked contact/cross-cutting*
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22. Geologic History
⢠Extension -> normal faulting.
⢠Faulting cross-cuts pluton & rock layers.
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23. Geologic History
⢠Dike intrusion.
⢠Dike cross-cuts everything (even normal fault).
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24. Geologic History
⢠Erosion to present landscape.
⢠Removed volcano and cuts down the dike top.
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25. Geologic History
⢠Relative ages help to unravel a complicated history.
⢠Those rules permit one to decipher this diagram!
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29. Fossil Succession
⢠Fossil range â first to last appearance.
⢠Permit correlation of strata.
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30. Outline
⢠Geologic time: perspective & a bit of history
⢠Dating geologic materials
-General: relative & absolute dating
-Relative dating:
-7 Principals & their application to a geologic history
-Fossil successions
⢠Gaps in the geologic record (unconformity)
-3 types of unconformities
-Stratigraphic correlation & the global geologic column
⢠Numerical (absolute) dating
-Radioactive decay
-Meaning of a radiometric date
-Other numerical dating methods
-Dating the geologic column, geologic time scale, & age of Earth
Chapter 12
Chapter 12
36. Unconformities
3 Types:
3. Angular unconformity â represents a big gap in time
Horizontal rocks deposited, then deformed
Then eroded
Then sediments horizontally deposited on erosion surface
Chapter 12
37. Types of Unconformity
Types of Unconformity
This animation shows the stages in the development of
three main types of unconformity in cross-section, and
explains how an incomplete succession of strata provides a
record of Earth history. View 1 shows a disconformity, View
2 shows a nonconformity and View 3 shows an angular
unconformity. For more information, see Section 12.5
Unconformities: Gaps in the Record starting on p.423 and
Figure 12.9 in your textbook.
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38. Unconformities
⢠Earth history is in
strata.
⢠Missing strata =
missing history
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39. Stratigraphic Correlation
⢠In 1793, William âStrataâ Smith noted strata could be
matched across distances.
⢠Similar rock types in a similar order
⢠Rock layers contained same distinctive fossils
⢠He made the 1st geologic map of the UK
Chapter 12
40. Stratigraphic Correlation
⢠Stratigraphic columns depict strata in a region.
⢠Drawn to portray relative thickness
⢠Rock types depicted by fill patterns
⢠Divided into formations (mapable rock units)
⢠Formations separated by contacts
Chapter 12
41. Stratigraphic Correlation
⢠National Parks of Arizona & Utah.
⢠Formations can be traced long distances
⢠Overlap in rock type sequences
⢠Overlapping rock columns are used to build a composite
Chapter 12
42. The Geologic Column
⢠A composite global stratigraphic column exists.
⢠Constructed from incomplete sections across the globe
⢠It brackets almost all earth history
Chapter 12
43. Outline
⢠Geologic time: perspective & a bit of history
⢠Dating geologic materials
-General: relative & absolute dating
-Relative dating:
-7 Principals & their application to a geologic history
-Fossil successions
⢠Gaps in the geologic record (unconformity)
-3 types of unconformities
-Stratigraphic correlation & the global geologic column
⢠Numerical (absolute) dating
-Radioactive decay
-Meaning of a radiometric date
-Other numerical dating methods
-Dating the geologic column, geologic time scale, & age of Earth
Chapter 12
Chapter 12
44. Numerical (Absolute) Dating
⢠Based on radioactive decay of atoms in minerals.
⢠Radioactive decay proceeds at a known fixed rate
⢠Radioactive elements act as internal clocks
⢠Numerical dating is called geochronology
Chapter 12
45. Radioactive Decay
Isotopes
Atoms with same # of protons, different # of neutrons
Have similar but different mass numbers
Some are Stable â never change
Some are Unstable (radioactive) âspontaneously change to
something else (decay) at a fixed rate
Chapter 12
46. Radioactive Decay
⢠Decay process has 2 main components:
⢠Parent â isotope that decays
⢠Daughter â decay product isotope
⢠Decay process can:
⢠Have 1 step (parent>daughter)
⢠Have many steps (parent>daughter>etc)
⢠Decay product is unstable and hence also decays
⢠Eventually proceeds to a stable endpoint
Chapter 12
47. Radioactive Decay Time
⢠Half-life (t½) â time for ½ unstable parent to decay.
⢠t½ is unique for each isotope
⢠After one t½ -1/2 original parent remains
⢠After three t½ -1/8 original parent remains
⢠Parent disappears (nonlinear) daughter accumulates
Chapter 12
48. Radiometric Dating
⢠Mineral age can be determined by:
⢠Measuring parent/daughter isotope ratio
⢠Calculating time
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50. What Is a Radiometric Date?
⢠Time since a mineral began to retain all parent &
daughter isotopes.
⢠Requires cooling below âclosure (blocking)
temperature.â
⢠Daugther retained only below closure T
⢠Daughter leaks out above closure T
⢠Thus, if rock is reheated above closure T, the
radiometric clock can be reset to zero
Chapter 12
51. Other Numerical Ages
⢠Numerical ages are possible without isotopes.
⢠Growth rings â annual layers from trees or shells
⢠Rhythmic layering â annual layers in seds or ice
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52. Other Numerical Ages
⢠Magnetostratigraphy âmagnetic signatures in strata are
compared to global reference column
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53. Other Numerical Ages
⢠Decay process can cause scars (tracks) in minerals.
⢠Decay by fission (explosion) produces scar (track)
⢠Daughter isnât another isotope, itâs a damage zone
Chapter 12
54. Dating the Geologic Column
⢠Use geochronology to:
⢠Date specific strata OR
⢠Bracket those that cant be dated directly
Chapter 12
56. Age of the Earth
⢠Oldest rocks are 3.96 Ga.
⢠Zircon minerals in some sandstones are 4.1-4.2 Ga.
⢠Earth is ~4.57 Ga based on correlation withâŚ
⢠Meteorites, moon rocks.
Chapter 12