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Geologic Geologic TimeTime

ChapterChapter 3434

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Dating rocksDating rocks Relative datingRelative dating

Compare events and put them in order based on their sequence of formation, oldest to youngest.

Absolute datingAbsolute dating Using radioactive decay to

determine the exact age of rocks

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The Geologic Time ScaleThe Geologic Time Scalep. 182

Cenozoic

Mesozoic

Paleozoic

Pre

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an

Phanerozoic

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Geologic Time ScaleGeologic Time Scale

Geologists have divided the earth’s Geologists have divided the earth’s history into various eras, periods history into various eras, periods and epochs using fossil evidence and epochs using fossil evidence (which life forms lived when)(which life forms lived when)

The chart has been updated many The chart has been updated many times as more fossil evidence is times as more fossil evidence is located located

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Principles of relative datingPrinciples of relative dating

Law of superpositionLaw of superpositionIn an undisturbed sequence of

sedimentary rocks the oldest rocks are on the bottom, youngest on top

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Law of superposition

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Principles of relative datingPrinciples of relative dating Principle of original horizontalityPrinciple of original horizontality

Layers of sediment are generally deposited in a horizontal, flat-lying position

Rock layers that are flat have not been disturbed

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Principles of relative datingPrinciples of relative dating

Principle of cross-cutting Principle of cross-cutting relationshipsrelationships

Younger features cut across older feature

A fault or dike that cuts across sedimentary layers is younger than the layers.

Igneous dikes cutting Igneous dikes cutting across metamorphic rockacross metamorphic rock

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Principles of relative datingPrinciples of relative dating Included FragmentsIncluded Fragments

An inclusion is a piece of rock that is enclosed within another rock (2 examples: xenolith and pebble in conglomerate)

Rock containing the inclusion is younger, the inclusion is older

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Principles of relative datingPrinciples of relative dating UnconformityUnconformity

An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock layers

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Sketch of an unconformity:Sketch of an unconformity:

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Geological events of an unconformity:Geological events of an unconformity:

1. Sedimentation / deposition1. Sedimentation / depositionand lithificationand lithification

2. Deformation / folding 2. Deformation / folding and erosionand erosion

3. Subsidence, water is covering3. Subsidence, water is covering

4. More sedimentation happened 4. More sedimentation happened once covered with wateronce covered with water

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Unconformity in rocks:Unconformity in rocks:

Angular unconformity at Siccar Point, ScotlandAngular unconformity at Siccar Point, Scotland