Transcript Geologic Dating

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 Dating
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There are two types of
Geologic Dating
• Absolute
–Determines how many years old
something is
• Relative
–Used to determine if one thing is
younger or older than another
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Tools used to determine
Absolute Age
• Using radiometric
dating
Relative Age
• Law of superposition
• Use of index fossils
• Correlation of rock
layers
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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
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GRAND CANYON- LAW OF SUPERPOSITION
YOUNGEST ON TOP
OLDEST ON BOTTOM
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 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
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Which fossil would
make the best index fossil?
Found in only 1 layer (short lived)
Found in all samples (widespread)