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Geologic Time Chapter 21.1-21.3. Geologic Time Two frames of reference –RELATIVE AGE DATING Places events in order according to their positions in the.

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Presentation on theme: "Geologic Time Chapter 21.1-21.3. Geologic Time Two frames of reference –RELATIVE AGE DATING Places events in order according to their positions in the."— Presentation transcript:

1 Geologic Time Chapter

2 Geologic Time Two frames of reference –RELATIVE AGE DATING Places events in order according to their positions in the rock record – ABSOLUTE AGE DATING Specific ages of rock units, etc. Given as Years Before Present.

3 Geologic Time Scale RELATIVE SCALE, with ABSOLUTE AGES added Modified as new information becomes available.

4 Divided into: (p. 591) Eons Eras Periods Epochs

5 21.2 Relative Age Dating Uniformitarianism –present is the key to the past –James Hutton, 1788 Beginning of MODERN GEOLOGY –Charles Lyell –Not immediately embraced by scientists of the day!

6 Relative Age Dating Six Principles –Principle#1: Superposition Oldest layer on bottom, youngest on top

7 Marine Transgression

8 Marine Regression sea regressing

9 Relative Age Dating Principle #2: Original Horizontality –Sediments originally deposited horizontally

10 Relative Age Dating Principle #3: Lateral Continuity –Layer continues equally in all directions; eventually pinches out

11 Relative Age Dating Principle #4: Cross-Cutting Relationships –Igneous intrusion or fault is younger than the rock it cuts across. fault cuts across Sedimentary layers

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13 Igneous dike cuts across granite.

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15 Relative Age Dating Principle #5: Inclusions –Rock fragments in a layer are older than the rock itself.

16 Relative Age Dating Principle #6: Fossil Succession –Fossil assemblages succeed one another in a regular and predictable order –William Smith, English surveyor, 1815 –Index fossils (guide fossils) Geographically widespread Existed for short geologic time Easily identified Abundant

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18 Unconformities Surfaces which represent geologic time not represented in the rock record Hiatus: Interval of time not represented

19 Types of Unconformities Disconformity: –Surface of erosion or non- deposition between beds which are parallel to one another

20 Mississippian Jurassic

21 Types of Unconformities Angular Unconformity: Erosional surface on tilted or folded strata over which younger strata are deposited.

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23 Types of Unconformities Nonconformity –Erosional surface cut into igneous or metamorphic rocks, overlain by sedimentary rocks. ?

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25 Absolute Age Dating A rock layer or event can be assigned a number, years before present, with an error factor Radiometric dating –Radioactive elements spontaneously decay to a more stable element

26 Radiometric dating Unstable radioactive elements or isotopes (parents) change over time into stable products (daughters) – PARENT DAUGHTER Half-life: Time for ½ of parent to change to daughter Half-life is UNIQUE and CONSTANT Absolute Age Dating

27 Radiometric dating Types of Decay –Alpha – lose a He nucleus (2p + 2n) –Beta – lost an e- –Electron capture – gain an e- Absolute Age Dating


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