Download Creating a Time Scale - Relative Dating Principles The Geologic

11/5/2015
Creating a Time Scale - Relative
Dating Principles
Why is the Geologic Time Scale important?
• Rocks record geologic and evolutionary
changes throughout Earth’s history
• Without a time perspective, events have little
meaning
GEOL 110:
PHYSICAL
GEOLOGY
Chapter 9: Geologic
Time Scale
Not for Distribution. For study purposes only for related class.
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The Geologic Time Scale
The geologic time scale
encompasses all of Earth
history
• Subdivides geologic
history into units
• Originally created using
relative dates
• Eon, Eras, Periods,
Epochs
– Most epochs are termed
early, middle, and late,
except for the epochs in
the Cenozoic.
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© 2014 Pearson Education, Inc.
Creating a Time Scale - Relative
Dating Principles
How does it relate to
rocks and fossils?
• Numerical dates number of
years since formation.
• Relative dates place rocks in a
sequence of formation
Numerical:
Jaime is 13 years old.
Liz is 10 years old.
Relative:
Jaime is older than Liz
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Creating a Time Scale - Relative
Dating Principles
Creating a Time Scale - Relative
Dating Principles
How do we determine the age and sequence
of geologic events?
4 primary principles and clues.
1) Principle of Superposition
• Undeformed sedimentary layers are in order
from oldest to youngest.
• This principle also applies to surface features
like lava flows and beds of ash
Youngest above, oldest below
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© 2014 Pearson Education, Inc.
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Creating a Time Scale - Relative
Dating Principles
2) Principle of Original Horizontality
• Layers of sediment are generally deposited in
a horizontal position
• Rock layers that are flat have not been
disturbed
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Lateral Continuity in the Grand Canyon
3) Principle of Lateral Continuity
• Beds originate as continuous layers that extend
in all directions
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Creating a Time Scale - Relative
Dating Principles
Creating a Time Scale - Relative
Dating Principles
4) Principle of Cross-Cutting Relationships
Clues: Inclusions
• Younger features cut across older features
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• Inclusions are fragments of one rock unit that
are enclosed within another rock unit
• The rock containing the inclusion is younger
© 2014 Pearson Education, Inc.
Creating a Time Scale - Relative
Dating Principles
Creating a Time Scale - Relative
Dating Principles
Clues: Conformable Rock layers
Unconformities
Layers of rock that have been deposited
without interruption are called conformable
layers
Are all rocks layers conformable?
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• An unconformity indicates time missing within
the rock record; a result of nondeposition and
erosion of rock units
• 3 basic types: angular unconformity,
disconformity, nonconformity
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Creating a Time Scale - Relative
Dating Principles
Creating a Time Scale - Relative
Dating Principles
1) Angular unconformity
2) Disconformity
• Tilted rocks are overlain by flat-lying rocks
• Sedimentary strata on either side of the unconformity are
parallel
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Creating a Time Scale - Relative
Dating Principles
3) Nonconformity
• Sedimentary strata overlay metamorphic or igneous rocks
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Applying Principles
Sequence of Events:
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© 2014 Pearson Education, Inc.
Creating a Time Scale - Relative
Dating Principles
All 3 types of
unconformities can
be seen in the
Grand Canyon
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Applying Principles
Sequence of Events exercise:
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Fossils: Evidence of Past Life
Fossils and geologic time
• Fossils are traces or remains
of prehistoric life preserved in
rock
• Paleontology is the study of
fossils
• Helps researchers
understand past
environmental conditions
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Fossils and Geologic
Time
•
•
•
•
•
•
•
•
Quaternary: evolution of humans
Cretaceous: first primates
Jurassic: first birds
Triassic: first dinosaurs
Carboniferous: first reptiles
Devonian: first amphibians
Cambrian: first fishes
Late proterozoic – trilobites
earliest ~521 mya
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Fossils: Evidence of Past Life
Fossils: Evidence of Past Life
Best conditions for
preservation and development
of fossils ?
5 Types of Fossils
• Rapid burial
• Possession of hard parts
• Most organisms are not preserved;
soft bodies
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1) Permineralization
• Groundwater containing
Silica flows through
porous tissue and replace
the wood molecules.
• Example: petrified wood
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Fossils: Evidence of Past Life
Fossils: Evidence of Past Life
Types of Fossils
Types of Fossils
2) Molds and casts
• A mold is created when a shell is buried and
then dissolved by underground water, leaving a
cavity.
• A cast is created when the hollow spaces of a
mold are filled with sediments.
• Generally not actual organism.
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3) Carbonization and Impressions
• Carbonization occurs when an organism is buried,
then compressed, squeezing out gases and liquids
leaving a thin film of carbon
eg: leaves and fragile animals
• Impressions remain in the rock when the carbon
film is lost
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Fossils: Evidence of Past Life
Fossils: Evidence of Past Life
Types of Fossils
Types of Fossils
4) Amber
• Amber is the hardened resin of ancient trees
Eg: preserving insects
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Dating with Radioactivity
How can we date the ages of rocks?
5) Trace fossils
• Indirect evidence of prehistoric life
Eg: tracks, burrows, coprolites,
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Dating with Radioactivity
How can we date the ages of rocks?
• Radioactivity: spontaneous decay in the structure of an
atom’s nucleus
• 238U to 226Ra to 206Pb
• Radiometric dating: uses the decay of isotopes in rocks to
calculate the age of that rock. 238U to 234U indicates 100,000
years and 1.2 mya.
Capital Reef, UT
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Dating with Radioactivity
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Dating with Radioactivity
Dating with Carbon-14
Isotopes of Hydrogen
H has 1 proton
Isotopes of H have 0, 1, 2 neutrons
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• Radiocarbon dating uses the
radioactive 14C to date recent events
• Can be used to date events as old as
70,000 years
• 14C is only useful in dating organic
matter
• Living organisms intake equal
amounts of 14C and 12C
• Dead organisms: carbon intake
stops, 14C begins to decay
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Correlation of Rock Layers
Correlating
Rock layers
• Correlation
matching rocks of
similar ages from
different regions
• Correlation
provides a more
comprehensive
view of the rock
record
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The Geologic Time Scale
Why don’t we know much about
Precambrian time?
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The Geologic Time Scale
Why don’t we know much about
Precambrian time?
• 88% of Earth’s history
• Less is known about Earth further back in geologic time.
• During the Precambrian, simple life-forms that lacked a
hard parts. Algae, bacteria, worms, fungi dominated.
• Many Precambrian rocks are highly deformed
metamorphic rocks.
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End of Chapter 9
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