Chapter 8 Geologic Time
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Transcript of Chapter 8 Geologic Time
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Geologic TimePhysical Geology Chapter 8Tim Horner, CSUS Geology Department
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Dating rocks: Relative dating: Puts events in order Doesnt necessarily give an exact age Absolute (Numerical) dating: Used principles of radioactive decay Assigns a numerical age to a rock Only works for certain rock types
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Figure 8.1
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Figure 8.2Step 1) Rocks are deposited in the ocean
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Figure 8.3Step 2) Sequence of rocks is deposited (oldest first):
Lutgrad FmBirkland FmTarburg FmLeet Junction Fm
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Figure 8.4Step 3) Sequence is intruded, with contact metamorphism at edges of pluton
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Figure 8.5Step 4) Sequence is tilted, top of new surface is eroded
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Figure 8.1
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Figure 8.6Step 5) Area subsided, was covered by ocean, Larsonton Fm was deposited
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Figure 8.8Step 6) Larsonton Fm was intruded by dike
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Figure 8.9Step 7) Erode material (expose dike)
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Figure 8.10Step 8) Subsidence, deposit more marine sediments (Foster City Fm) above unconformityunconformity
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Figure 8.11Step 9) Raise entire sequence above sea level, begin downcutting by modern stream
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Figure 8.1Final product:
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Figure 8.14a-dFormation of angular unconformity
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Figure 8.14eAngular unconformity
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Figure 8.14fAngular unconformity
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Figure 8.12Non-conformity
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Figure 8.15Non-conformity
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Figure 8.13Disconformity
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Figure 8.16Lithologic correlation- tracing beds
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Figure 8.17Lithologic correlation- similar rock sequences (Gondwana Sequence)
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Figure 8.18Correlation by fossils
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Table 8.2Geologic time scale
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Figure 8.19
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Table 8.3Commonly used radiogenic isotopes:
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Figure 8.22Radiogenic dating: parent and daughter isotopes
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Figure 8.21
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Figure 8.23Radiogenic dating brackets events
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Figure 8.24Geologic time scale, with absolute dates
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Figure 8.25Absolute time scale
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Figure 8.20Numeric (absolute) dating
Uses radiogenic isotopes
Decay rates are known,constant
3 decay methods:
Alpha decayBeta decayElectron capture