Earth’s History
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Earth’s History
• Planet Earth is approximately
4.5 X 109 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 Time
• Plate motions
and mountain
building events
can be placed
on the geologic
time scale
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Geologic
Time
Geologists
have divided
Earth’s history
into time units
based on the
the fossil record
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Geologic Time
• 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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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
• Sediments are laid down
underwater in horizontal layers
and form sedimentary rocks
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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
• Igneous intrusions are younger than the
rock that they cut through or flow out of
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Igneous Intrusion - Cross
Cutting
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Unconformity
• When a new rock layer is formed
atop an eroded surface
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Upper Silurian
Carbonates
Tilted
Ordovician
Shales and
Sandstones
unconformity
Taconic Unconformity
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Practice: what happened here?
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Correlation
• Matching
similar rock
layers in
different
locations to
see if they
formed at the
same time
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Which fossil might be found in Devonian
rock layers?
Volcanic Ash Falls
• Can also be used to correlate rock layers
over a large area
• Ash is a good indicator because:
– The ash from one explosion has distinct
characteristics
– ash can be deposited around the globe
– The event occurs at one, geologically brief,
time
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Fossils
• Naturally occurring preserved remains
or impressions of living things
• Generally only hard parts get
preserved
– Bones, teeth, shells
• Hard parts are replaced by naturally
occurring minerals
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Eurypterus
NY State
Fossil
Silurian
index
fossil
Fossils
• Can be a mold (impression)
–Ex: footprint
• Or a cast
–Ex: filled in footprint
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Fossils (Cont)
• Other types
– Ice
– Tar
– Carbonaceous film
– Amber
– Imprints
– Petrified
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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
• The living organism
that made the fossil
lived during the time
the rock layer was
forming
– i.e. when the sediment
was being deposited
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Fossils
• Fossils can provide
information about
ancient
environments
– Marine fossils
indicate a marine
environment, wooly
mammoths indicate
a cold environment,
etc…
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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 (strata)
–Fossils from creatures that existed for
a geologically short period of time
• Ex. less than 2.0 x 107
–Fossils from creatures that had a wide
geographic distribution
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Location A
Location B
Location C
Rock layer 1
W
W
W
Rock layer 2
W
Rock layer 3
W
Z
X
Z
Y
X
Z
X
Z
Which letter would make a good index fossil?
Absolute Dating
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Radioactive Decay
• When an unstable radioactive
element changes into a stable
element
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Example
Unstable Radioactive Isotope
Stable Decay Product
Carbon 14 (C14)
Nitrogen 14 (N14)
Uranium 238 (U238)
Lead 206 (Pb206)
Parent
Daughter
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Radiometric Dating
Half life: The time required for 1/2 of a parent
material to break down to daughter material
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• Another look
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Radiometric Dating
• The age of objects can be
determined by measuring the
relative amounts of radioisotope
and decay products
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Radiometric Dating
• The half lives of radioisotopes
vary depending upon the isotope
–Examples:
• C14 = 5,770 years
• U238 = 4,500,000,000 years
• Rb87 = 47,000,000,000 years
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• Carbon 14 is used to date
biological remains
–Carbon is incorporated into the
cells of living organisms and
begins to decay when the
organism dies
• Uranium 238 is used to date
rocks
–Larger half life
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Half Life
Practice
Problems
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• If half the K -40 of a rock has
decayed into Ar-40, how old
is the rock?
• How many years would it
take for 75 % of a sample of
K-40 to disintegrate?
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How old are:
• A sample that has ½ c-14
and ½ n-14?
• A sample that has ¼ K-40
and ¾ Ar-40?
• A sample that has ½ U238 ½ Pb-206?
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How old are:
• A sample that has 1/8 Rb-87
and 7/8 Sr-87?
• A sample that has ¼ C-14
and ¾ N-14?
• A sample that has ¾ C-14
and ¼ N-14?
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What fraction remains of:
• A 50 billion year old sample has
____ Rb-87 and ____ Sr-87.
• A 2.6 billion year old sample
has____ K-40 and ____Ar-40.
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This is so boring
Stranz should take a break ;)
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