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Chpt9pptteacher - GeologicTime Geochronology the science of...

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Geologic Time
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Putting Rocks in sequence Geochronology : the science of determining a rock's age Relative geologic dating : placing geologic events and rocks in the order in which they formed without reference to actual time. Absolute geologic dating : the actual age of rocks or events in years as determined by the decay of radioactive elements.
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Steno’s Laws Developed by Nicolaus Steno in 1669 Principle of superposition : in an undisturbed sequence of strata, the oldest strata lie at the bottom and younger at the top. Principle of original horizontality : all strata are deposited nearly horizontal. Exceptions such as the cross bedding in a sand dune or sand bar. Rock layers that are flat have not been disturbed
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Superposition is well illustrated by  the strata In the Grand Canyon Figure 9.3
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Horizontal Strata, Capitol Reef Utah  
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Figure 9.3
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Figure 9.4
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Sir Charles Lyell (1797-1875) In 1830 wrote "Principles of Geology". Synthesized the important work of his day.
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Lyell’s Principles of Geology First edition of "The Principles", 1830. (N.G. McDonald Collection).
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Two Principles by Charles Lyell Principle of cross-cutting relationships : a geologic feature that cuts across or penetrates another body of rock must be younger than the rock mass penetrated. Applies to rocks, faults, and unconformities. Principle of inclusions : fragments of rock within larger rock masses are older than the rock masses in which they are enclosed. This applies to the clasts of a sedimentary rock.
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Crosscutting Relationships
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Figure 9.5
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Figure 9.6c
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Relative dating and unconformities An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units Types of unconformities Angular unconformity – tilted rocks are overlain by flat- lying rocks Disconformity – strata above and below the unconformity are horizontal (parallel) Nonconformity – metamorphic or igneous rocks are below the sedimentary strata
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Formation of an angular  unconformity Figure 9.8
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Siccar Point,  Scotland
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Figure 9.7
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Deposition of 1 Fault 2 Intrusion 3 Erosion 4 Deposition of 5
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Principle of Inclusions Which is older the granite or sandstone in A and B?
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Xenoliths Xenoliths were part of country rock incorporated into an intrusive igneous rock body during an intrusive episode. They are older than the rock that surrounds them.
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