1 earth’s history. 2 planet earth is approximately 4.5 x 10 9 years old –rocks of the crust...
TRANSCRIPT
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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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Geologists have divided Earth’s history into time units based on the the fossil record
Geologic Time
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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 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 SuperpositionLaw 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
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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• 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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UnconformityUnconformity• When a new rock layer is formed
atop an eroded surface
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Upper Silurian Carbonates
Tilted Ordovician Shales and Sandstones
Taconic Unconformity
unconformity
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Practice: what happened here?
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CorrelationCorrelation
• 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?
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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
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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 FossilsIndex 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 Z
Rock layer 2 W Z
Y Z
Rock layer 3 W X X X Z
Which letter would make a good index fossil?
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Absolute Dating
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Radioactive Decay• When an unstable radioactive
element changes into a stable element
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Unstable Radioactive Isotope Stable Decay Product
Carbon 14 (C14)
Uranium 238 (U238)
Nitrogen 14 (N14)
Lead 206 (Pb206)
Parent Daughter
Example
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Radiometric DatingHalf lifeHalf 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 U-238 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.