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Understanding Earth Sixth Edition Chapter 8: CLOCKS IN ROCKS Timing the Geologic Record © 2011 by W. H. Freeman and Company Grotzinger Jordan.

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Presentation on theme: "Understanding Earth Sixth Edition Chapter 8: CLOCKS IN ROCKS Timing the Geologic Record © 2011 by W. H. Freeman and Company Grotzinger Jordan."— Presentation transcript:

1 Understanding Earth Sixth Edition Chapter 8: CLOCKS IN ROCKS Timing the Geologic Record © 2011 by W. H. Freeman and Company Grotzinger Jordan

2 Chapter 8: Clocks in Rocks: Timing the Geologic Record

3 About the Clocks in Rocks Geologists have uncovered deep time by studying the clocks in rocks.Geologists have uncovered deep time by studying the clocks in rocks. These clocks can be used to measure the duration of geologic processes and cycles of the Earth system.These clocks can be used to measure the duration of geologic processes and cycles of the Earth system. Geologists refer to both relative age and absolute age.Geologists refer to both relative age and absolute age.

4 Lecture Outline 1.Reconstructing geologic history from the stratigraphic record 2. Geologic time scale: relative ages 3. Measuring absolute time with isotopic clocks 4. Geologic time scale: absolute ages 5. Advances in timing the Earth system

5 1. Reconstructing Geologic History from the Stratigraphic Record the Stratigraphic Record Principles of stratigraphy Principles of stratigraphy original horizontality original horizontality superposition superposition faunal succession

6 Younger Older Example: Marble Canyon, Arizona Original Horizontality and Superposition

7 Younger Older Left undisturbed, the youngest layers always remain above the oldest. Original Horizontality and Superposition

8 sedimentation in lake or sea Original Horizontality and Superposition

9 Sediments are deposited in horizontal layers and slowly change into rock. Original Horizontality and Superposition

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11 Outcrop AOutcrop B I II III Fossils as Recorders of Time: The Principle of Faunal Succession

12 Outcrop AOutcrop B I II III

13 Outcrop AOutcrop B I II III

14 Outcrop AOutcrop B I II III Some of the fossils found in outcrop A are the same as fossils found in outcrop B, some distance away.

15 Outcrop AOutcrop B I II III Some of the fossils found in outcrop A are the same as fossils found in outcrop B, some distance away. Layers with the same fossils are the same age.

16 Outcrop AOutcrop B I II III Some of the fossils found in outcrop A are the same as fossils found in outcrop B, some distance away. Layers with the same fossils are the same age.

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18 1. Reconstructing Geologic History from the Stratigraphic Record the Stratigraphic Record Unconformities – gaps in the record Unconformities – gaps in the record disconformity disconformity angular unconformity angular unconformity

19 Disconformity

20 Disconformity

21 Disconformity

22 Disconformity

23 Disconformity

24 Angularunconformity

25 Angularunconformity

26 Angularunconformity

27 Angularunconformity

28 Angularunconformity

29 Angularunconformity The great unconformity in the Grand Canyon, Arizona.

30 1. Reconstructing Geologic History from the Stratigraphic Record the Stratigraphic Record Cross-cutting relationships Cross-cutting relationships faults faults intrusions intrusions

31 Cross-cuttingrelationships

32 Cross-cuttingrelationships

33 Cross-cuttingrelationships

34 Cross-cuttingrelationships

35 Cross-cuttingrelationships

36 Relative age relationships

37 Thought questions for this chapter As you pass by an excavation in the street, you see a cross section showing paving at the top, soil below the paving, and bedrock at the base. You also notice that a vertical water pipe extends through a hole in the street into a sewer in the soil. What can you say about the relative ages of the various layers and the water pipe? Why did 19 th -century geologists who constructed the geologic time scale find sedimentary strata deposited in the sea more useful than strata deposited on land?

38 Thought questions for this chapter In studying an area of tectonic compression, a geologist discovers a sequence of older, more deformed sedimentary rocks on top of a younger, less deformed sequence, separated by an angular unconformity. What tectonic process might have created the angular unconformity? A geologist documents a distinctive chemical signature caused by Proterozoic organisms. Would you consider this chemical signature to be a fossil?

39 2. Geologic Time Scale: Relative Ages Divisions of geologic time Divisions of geologic time eras eras periods periods epochs

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41 3. Measuring Absolute Time with Isotopic Clocks Isotopic dating theory Isotopic dating theory radioactive atoms radioactive atoms parent-daughter parent-daughter half-life

42 3. Measuring Absolute Time with Isotopic Clocks Isotopic dating methods Isotopic dating methods uranium-lead uranium-lead potassium-argon potassium-argon rubidium-strontium carbon-nitrogen carbon-nitrogen

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44 NeutronsProtons Electron Example: Rubidium-Strontium

45 Rubidium-87 nucleus NeutronsProtons Electron A neutron decays, ejecting an electron…

46 Rubidium-87 nucleus NeutronsProtons Electron Strontium-87 nucleus A neutron decays, ejecting an electron…

47 Rubidium-87 nucleus NeutronsProtons Electron A neutron decays, ejecting an electron… Strontium-87 nucleus …and producing a proton, which changes the atom.

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50 Thought questions for this chapter Is carbon-14 a suitable isotope for dating geologic events in the Pliocene Epoch? How does determining the ages of igneous rocks help to date fossils?

51 4. Geologic Time Scale: Absolute Ages Four Eons of geologic time Four Eons of geologic time Hadean Hadean Archean Archean Proterozoic Phanerozoic Phanerozoic

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53 5. Advances in Timing the Earth System Sequence stratigraphy Sequence stratigraphy Chemical stratigraphy Chemical stratigraphy Paleomagnetic stratigraphy Paleomagnetic stratigraphy Clocking the climate system

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58 Absolute age EonEpochEra Geologic time scale Half-life Isotopic dating Mass extinction Period Principle of faunal succession Principle of original horizontality Principle of superposition Relative age Stratigraphic succession Unconformity Key terms and concepts


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