1. Geologic Time
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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
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3. Geologic Time
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4. Geologic Time
• Understanding time permits assigning ages to…
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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 (1638–1686) – Danish physician.
fossil
shark
tooth
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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
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8. Geologic Time
2 ways to date geological materials:
1. Relative age –
2. Numerical (absolute) age –
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9. Relative vs. Absolute
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10. Relative Age
• Logical tools are useful for defining relative age.
• Principles of:
1. Uniformitarianism
2. Superposition
3. Original horizontality
4. Original continuity
5. Cross-cutting relations
6. Inclusions
7. Baked contacts.
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11. Geologic Time
1. Uniformitarianism – “The present is key to the past”.
•
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12. Defining Relative Age
2. Superposition.
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13. Relative Age
3 & 4. Horizontality and continuity.
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14. Relative Age
5. Cross-cutting relations.
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15. Relative Age
6. Inclusions – a rock fragment within another.
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16. Relative Age
7. Baked contacts.
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17. Relative Age
• Determining relative ages empowers geologists to
unravel complicated geologic histories.
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18. Geologic History
• Deposition of horizontal strata below sea level in order
1-> 8 (old to young). *Horizontality & continuity*
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19. Geologic History
• Igneous intrusion of a sill. *baked contact*
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20. Geologic History
• Intrusion solidified into sill
• Tectonic compression
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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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26. Geologic History
• Test yourself at home:
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27. Fossil Succession
• Fossils (organism traces) can be preserved in
sedimentary rocks.
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28. Fossil Succession
• Species evolve, exist, and then go extinct.
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29. Fossil Succession
• Fossil range – 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
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31. Unconformity
• An unconformity is
• Causes:
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32. Unconformities
3 Types:
1. Disconformity –
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33. Disconformities
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34. Unconformities
3 Types:
2. Nonconformity –
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35. Nonconformity
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36. Unconformities
3 Types:
3. Angular unconformity – represents a big gap in time
Horizontal rocks deposited, then deformed
Then eroded
Then sediments horizontally deposited on erosion surface
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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 “Strata” 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
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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
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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
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42. The Geologic Column
• A composite global stratigraphic column exists.
• Constructed from incomplete sections across the globe
• It brackets almost all earth history
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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
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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
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45. Radioactive Decay
Isotopes
Atoms with same # of protons, different # of neutrons
Have similar but different mass numbers
Some are Stable – never change
Some are Unstable (radioactive) –spontaneously change to
something else (decay) at a fixed rate
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46. Radioactive Decay
• Decay process has 2 main components:
• Parent – isotope that decays
• Daughter – 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
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47. Radioactive Decay Time
• Half-life (t½) – 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
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48. Radiometric Dating
• Mineral age can be determined by:
• Measuring parent/daughter isotope ratio
• Calculating time
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49. Chapter 12

50. What Is a Radiometric Date?
• Time since a mineral began to retain all parent &
daughter isotopes.
• Requires cooling below “closure (blocking)
temperature.”
• 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
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51. Other Numerical Ages
• Numerical ages are possible without isotopes.
• Growth rings – annual layers from trees or shells
• Rhythmic layering – annual layers in seds or ice
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52. Other Numerical Ages
• Magnetostratigraphy –magnetic 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 isn’t another isotope, it’s a damage zone
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54. Dating the Geologic Column
• Use geochronology to:
• Date specific strata OR
• Bracket those that cant be dated directly
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55. The Geologic Time Scale
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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 with…
• Meteorites, moon rocks.
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