understanding earth sixth edition chapter 8: clocks in rocks timing the geologic record © 2011 by...

Understanding EarthSixth Edition

Chapter 8:CLOCKS IN ROCKS

Timing the Geologic Record

© 2011 by W. H. Freeman and Company

Grotzinger • Jordan

Chapter 8:Chapter 8: Clocks in Clocks in

Rocks:Rocks:Timing the Timing the

Geologic RecordGeologic Record

About the Clocks in RocksAbout the Clocks in Rocks

• Geologists have uncovered deep time by Geologists have uncovered deep time by studying the “clocks in rocks.”studying the “clocks in rocks.”

• These clocks can be used to measure These clocks can be used to measure the duration of geologic processes and the duration of geologic processes and cycles of the Earth system. cycles of the Earth system.

• Geologists refer to both relative age and Geologists refer to both relative age and absolute age.absolute age.

Lecture OutlineLecture Outline

1.1. Reconstructing geologic history from Reconstructing geologic history from the stratigraphic record the stratigraphic record

2. Geologic time scale: relative ages2. Geologic time scale: relative ages

3. Measuring absolute time with isotopic 3. Measuring absolute time with isotopic clocksclocks

4. Geologic time scale: absolute ages4. Geologic time scale: absolute ages

5. Advances in timing the Earth system5. Advances in timing the Earth system

1.1. Reconstructing Geologic History fromReconstructing Geologic History from the Stratigraphic Recordthe Stratigraphic Record

● ● Principles of stratigraphyPrinciples of stratigraphy

● ● original horizontalityoriginal horizontality

● ● superpositionsuperposition

●● faunal successionfaunal succession

YoungerYounger

OlderOlder

Example: Marble Canyon, ArizonaExample: Marble Canyon, Arizona

Original Horizontality and SuperpositionOriginal Horizontality and Superposition

YoungerYounger

OlderOlder

Left undisturbed, the youngest layers always remain Left undisturbed, the youngest layers always remain above the oldest.above the oldest.

Original Horizontality and SuperpositionOriginal Horizontality and Superposition

sedimentation insedimentation inlake or sealake or sea

Original Horizontality and SuperpositionOriginal Horizontality and Superposition

Sediments are deposited in horizontal Sediments are deposited in horizontal layers and slowly change into rock.layers and slowly change into rock.

Original Horizontality and SuperpositionOriginal Horizontality and Superposition

Original Horizontality and SuperpositionOriginal Horizontality and Superposition

Outcrop A Outcrop B

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Fossils as Recorders of Time: Fossils as Recorders of Time: The Principle of Faunal SuccessionThe Principle of Faunal Succession

Outcrop A Outcrop B

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Outcrop A Outcrop B

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II

III

Outcrop A Outcrop B

III

II

III

Some of the fossils found inSome of the fossils found inoutcrop A are the same as outcrop A are the same as fossils found in outcrop B, fossils found in outcrop B, some distance away.some distance away.

Outcrop A Outcrop B

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II

III

Some of the fossils found inSome of the fossils found inoutcrop A are the same as outcrop A are the same as fossils found in outcrop B, fossils found in outcrop B, some distance away.some distance away.

Layers with the same Layers with the same fossils are the same fossils are the same age.age.

Outcrop A Outcrop B

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II

III

Some of the fossils found inSome of the fossils found inoutcrop A are the same as outcrop A are the same as fossils found in outcrop B, fossils found in outcrop B, some distance away.some distance away.

Layers with the same Layers with the same fossils are the same fossils are the same age.age.

1.1. Reconstructing Geologic History fromReconstructing Geologic History from the Stratigraphic Recordthe Stratigraphic Record

● ● Unconformities – gaps in the recordUnconformities – gaps in the record

● ● disconformitydisconformity

● ● angular unconformityangular unconformity

DisconformityDisconformity

DisconformityDisconformity

DisconformityDisconformity

DisconformityDisconformity

DisconformityDisconformity

AngularAngularunconformityunconformity

AngularAngularunconformityunconformity

AngularAngularunconformityunconformity

AngularAngularunconformityunconformity

AngularAngularunconformityunconformity

AngularAngularunconformityunconformity

The ‘great unconformity’ in the The ‘great unconformity’ in the Grand Canyon, Arizona.Grand Canyon, Arizona.

1.1. Reconstructing Geologic History fromReconstructing Geologic History from the Stratigraphic Recordthe Stratigraphic Record

● ● Cross-cutting relationshipsCross-cutting relationships

● ● faultsfaults

● ● intrusionsintrusions

Cross-cuttingCross-cuttingrelationshipsrelationships

Cross-cuttingCross-cuttingrelationshipsrelationships

Cross-cuttingCross-cuttingrelationshipsrelationships

Cross-cuttingCross-cuttingrelationshipsrelationships

Cross-cuttingCross-cuttingrelationshipsrelationships

Relative ageRelative agerelationshipsrelationships

Thought questions for this chapterThought questions for this chapter

As you pass by an excavation in the street, you see a As you pass by an excavation in the street, you see a cross section showing paving at the top, soil below the cross section showing paving at the top, soil below the paving, and bedrock at the base. You also notice that a paving, and bedrock at the base. You also notice that a vertical water pipe extends through a hole in the street vertical water pipe extends through a hole in the street into a sewer in the soil. What can you say about the into a sewer in the soil. What can you say about the relative ages of the various layers and the water pipe?relative ages of the various layers and the water pipe?

Why did 19Why did 19thth-century geologists who constructed the -century geologists who constructed the geologic time scale find sedimentary strata deposited in geologic time scale find sedimentary strata deposited in the sea more useful than strata deposited on land?the sea more useful than strata deposited on land?

Thought questions for this chapterThought questions for this chapter

In studying an area of tectonic compression, a geologist In studying an area of tectonic compression, a geologist discovers a sequence of older, more deformed discovers a sequence of older, more deformed sedimentary rocks on top of a younger, less deformed sedimentary rocks on top of a younger, less deformed sequence, separated by an angular unconformity. What sequence, separated by an angular unconformity. What tectonic process might have created the angular tectonic process might have created the angular unconformity?unconformity?

A geologist documents a distinctive chemical signature A geologist documents a distinctive chemical signature caused by Proterozoic organisms. Would you consider caused by Proterozoic organisms. Would you consider this chemical signature to be a fossil?this chemical signature to be a fossil?

2. Geologic Time Scale: 2. Geologic Time Scale: Relative AgesRelative Ages

● ● Divisions of geologic timeDivisions of geologic time

● ● eraseras

● ● periodsperiods

●● epochsepochs

3. Measuring Absolute Time 3. Measuring Absolute Time with Isotopic Clockswith Isotopic Clocks

● ● Isotopic dating theoryIsotopic dating theory

● ● radioactive atomsradioactive atoms

● ● parent-daughterparent-daughter

●● half-lifehalf-life

3. Measuring Absolute Time 3. Measuring Absolute Time with Isotopic Clockswith Isotopic Clocks

● ● Isotopic dating methodsIsotopic dating methods

● ● uranium-leaduranium-lead

● ● potassium-argonpotassium-argon

●● rubidium-strontiumrubidium-strontium

● ● carbon-nitrogencarbon-nitrogen

Neutrons Protons

Electron

Example: Rubidium-StrontiumExample: Rubidium-Strontium

Rubidium-87 nucleus

Neutrons Protons

Electron

A neutron decays, A neutron decays, ejecting an electron…ejecting an electron…

Rubidium-87 nucleus

Neutrons Protons

Electron

Strontium-87 nucleus

A neutron decays, A neutron decays, ejecting an electron…ejecting an electron…

Rubidium-87 nucleus

Neutrons Protons

Electron

A neutron decays, A neutron decays, ejecting an electron…ejecting an electron…

Strontium-87 nucleus

……and producing a proton,and producing a proton,which changes the atom.which changes the atom.

Thought questions for this chapterThought questions for this chapter

Is carbon-14 a suitable isotope for dating geologic events Is carbon-14 a suitable isotope for dating geologic events in the Pliocene Epoch?in the Pliocene Epoch?

How does determining the ages of igneous rocks help to How does determining the ages of igneous rocks help to date fossils?date fossils?

4. Geologic Time Scale: 4. Geologic Time Scale: Absolute AgesAbsolute Ages

● ● Four Eons of geologic timeFour Eons of geologic time

● ● HadeanHadean

● ● ArcheanArchean

●● ProterozoicProterozoic

● ● PhanerozoicPhanerozoic

5. Advances in Timing 5. Advances in Timing the Earth Systemthe Earth System

● ● Sequence stratigraphySequence stratigraphy

● ● Chemical stratigraphyChemical stratigraphy

● ● Paleomagnetic stratigraphyPaleomagnetic stratigraphy

●● Clocking the climate systemClocking the climate system

Absolute ageAbsolute ageEonEonEpochEpochEraEraGeologic time scaleGeologic time scaleHalf-lifeHalf-lifeIsotopic datingIsotopic datingMass extinctionMass extinctionPeriodPeriodPrinciple of faunal successionPrinciple of faunal successionPrinciple of original horizontalityPrinciple of original horizontalityPrinciple of superpositionPrinciple of superpositionRelative ageRelative ageStratigraphic successionStratigraphic successionUnconformityUnconformity

Key terms and conceptsKey terms and concepts