1 earth’s history planet earth is approximately 4.6 billion years old –rocks of the crust...
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Earth’s History
• Planet Earth is approximately 4.6 billion years old–Rocks of the crust provide clues
to Earth’s past• By analyzing these clues we can infer events from the past
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Earth’s History• Principle of Uniformitarianism
–Major assumption in geology–Events in the past occurred the same way
that they are occurring today. Examples Include:• Weathering/erosion• Deposition• Volcanism• Plate tectonics
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Geologic DatingGeologic Dating
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There are two types of Geologic Dating
• AbsoluteAbsolute–Determines how many years old
something is
• RelativeRelative–Used to determine if one thing is
younger or older than another
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Tools used to determine
• Using radiometric dating
• Law of superposition• Use of index fossils• Correlation of rock
layers
Absolute Age Relative Age
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Relative Dating
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Law of Superposition• In a series of sedimentary rocks the
bottom layer is the oldest and the top layer is the youngest– Lower layers must be in place before
younger rocks can be deposited on top of them
– Exception: when something occurs to overturn layers
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Grand Canyon
GRAND CANYON- LAW OF SUPERPOSITION
YOUNGEST ON TOP
OLDEST ON BOTTOM
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Law of Superposition• Rock layers are older than folds found in them
– Layers were there before they were folded
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Folds/Tilted
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Law of Superposition
• Rock layers are older than faults found in them
• This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault
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Law of Superposition• Fossils are generally the same
age as the rock layers in which they are found–Animal remains are deposited
along with the sediments that will turn into sedimentary rocks
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Law of Superposition• Intrusions are younger than the rock that
they cut through or flow out of
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Fossils• Generally found in
sedimentary rock layers
Why don’t fossils exist in most igneous or metamorphic rock?
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Fossils • A study of the fossil record shows:
–A great variety of plants, animals and simpler life forms have lived on Earth in the past
–That life forms have evolved through time
–Most life forms of the geologic past have become extinct
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Index FossilsIndex Fossils• Index Fossils--used to date rock
layers–Fossils from creatures that existed for
a geologically short period of time
–Fossils from creatures that had a wide geographic distribution
Which fossil would make the best index fossil?Which fossil would make the best index fossil?
Found in only 1 layer (short lived)Found in all samples (widespread)