Methods for Dating the Earth

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Methods for Dating the Earth

• Relative and Absolute Dating Methodologies

Rocks of Ages
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Dating By Sedimentary Rock
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Stratification

• Sedimentary rocks form when new sediments are
  deposited on top of old layers of sediment.
  
• As the sediments accumulate they are compressed
  and hardened into sedimentary rock layers called
  beds.

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Relative Dating Methods

• Stratification
• Law of Superposition
• Principle of Horizontality

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Stratification

• The rock beds occur in layers called strata.
• These strata show the sequence of events that
  took place in the past.
  
• How Are Sedimentary Rocks Formed?

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Nicolaus Steno

• Nicolaus Steno, a Danish anatomist, geologist,
  and priest (1636 - 1686) observed the changes in
  a sequence of rock layers while working in the
  mountains of Italy.

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Law of Superposition

• Steno's observations became known as the Law of
  Superposition states that
  
• sedimentary layers are deposited in a time
  sequence, with the oldest on the bottom and the
  youngest on the top.
  

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• The Law of Superposition meant that the Coconino
  Sandstone is older than the Toroweap Formation
  and younger than the Hermit Shale as seen in the
  image above.

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Law of Superposition

• Rock layers will only tell us the relative age of
  different layers.
  
• Rock layers do not give us an exact age.
• Relative and Absolute Time

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Principle of Original Horizontality

• Scientists know that sedimentary rock generally
  forms in horizontal layers.

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Crustal Disturbances

• The principle of original horizontality is that
  sedimentary rocks left undisturbed will remain in
  horizontal layers.
  
• Therefore, sedimentary rock that is not in
  horizontal layers has been disturbed by crustal
  movements that happened after the layers formed.
  
• Give examples of crustal movements that could
  cause deformities.

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Examples of Forces That Cause Crustal Movements
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Faults and Intrusions

• A fault is a break or a crack in Earths crust
  along which rocks shift their position.
  
• An intrusion is a mass of igneous rock that forms
  when magma is injected into rock and then cools
  and solidifies.
  

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Crustal Movements

• In cases where crustal movement has disturbed the
  original position of the layers, scientists must
  look for clues to the original position of each
  layer and then apply the law of superposition.

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Law of Cross Cutting Relationships

• Described James Hutton (1726 - 1997), the Law of
  Crosscutting Relationships stated that if a fault
  or other body of rock cuts through another body
  of rock then it must be younger in age than the
  rock through which it cuts and displaces.

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Cross Cutting
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Crosscutting

• How do scientists determine relative age when
  rock layers have been disturbed by faults or
  intrusions?

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Law of Inclusions

• The Law of Inclusions was also described by James
  Hutton and stated that if a rock body (Rock B)
  contained fragments of another rock body (Rock
  A), it must be younger than the fragments of rock
  it contained. The intruding rock (Rock A) must
  have been there first to provide the fragments.

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Law of Inclusions

• Which rock is younger, A or B?

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Law of Faunal Succession

• In 1790, William Smith, a geologist, observed
  that fossils of invertebrate animals found in the
  rock layers appeared in a predictable sequence.
  
• The Law of Faunal Succession states that fossils
  occur in a definite, invariable (predictable and
  unchanging) sequence in the geologic record.

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Faunal Succession

• As you can see in this image the fossil remains
  of living things are present in the rock layers
  at definite intervals, and exist within a
  definite period of time.

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Other Relative Dating Techniques

• Cation Ratio
• Cultural Affiliation
• Fluorine Dating
• Obsidian Hydration
• Patination
• Pollen Analysis
• Rate of Accumulation
• Seriation

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Cation Ratio

• Cation RatioUsed to date modified rocks such as
  prehistoric rock carvings called _________.
  
• The method measures the relative percentage of
  calcium to potassium.
  
• This is not considered a very reliable technique.

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Cultural Affiliation

• This technique is based on using the histories of
  native peoples through historical record, myth
  and scientific evidence to date the age of a
  community.
• Again, the accuracy is questionable because
  stories often change through retelling and
  rewriting.

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Fluorine Testing

• Fluorine is found in most groundwater.
• When fluorine in the water comes in contact with
  bones and other remains it leaves deposits.
  
• Older deposits will have a higher amount of
  fluorine.
  
• Fluorine deposits do not occur at a constant rate
  so this is only a relative technique.

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Obsidian Hydration

• This relative technique can
• be used to measure the
• age of tools and weapons made with obsidian.
• Because obsidian is not truly solid crystal, it
  can absorb water.
  
• The more water absorbed, the older the artifact.

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Patination

• Patination is a technique involving the measuring
  of the patina on an artifact.
  
• The patina is the outermost surface of the
  artifact that differs in color, texture, luster
  or composition from the rest of the artifact.
  
• This difference is the result of chemical,
  physical or biological change in response to the
  surrounding soil and environmental conditions.

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Pollen Analysis

• Pollen analysis is the study of vegetation
  history using the pollen grains which are the
  male sex cells of plants.
  
• The outside of the pollen wall is made of very
  strong material.
  
• The pollen spores from 400 million years ago can
  be found today and studied for dating.

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Seriation

• Seriation is the study of pottery over time.
• Changes in pottery can tell us a relative date of
  a society.

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Absolute Age

• Relative age only tells us that one rock
  formation is younger or older than another.
  
• In order to find a numeric age scientists use
  absolute aging methods.

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Geologic and Chemical Absolute Aging Techniques

• Rates of Erosion
• Rates of Deposition
• Varve Count
• Radiometric Dating
• Carbon Dating
• Geologic Time

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Rates of Erosion

• An example of using erosion rates is to measure
  the amount of time it takes for a stream to erode
  its bed. From this information scientists could
  then determine the age of the stream.
• Is this method practical for geological
  formations that are very old? Why or why not?

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Rates of Sediment Deposition

• By using data collected over a long period of
  time, geologists can estimate the average rates
  of deposition. In general about 30 cm. of
  sedimentary rock are deposited over a period of
  1,000 years.
• What could cause this form of dating to be
  inaccurate?

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Varve Count

• Geologists use varves the way that biologists use
  tree rings.
  
• Geologists have noted that some sedimentary
  deposits show definite annual layers called
  varves that consist of a light colored band of
  sand formed in the fall and winter and a dark
  band of clay particles formed in the spring and
  summer.
• Geologists count the varves.
• Each varve equals one year.

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Chemical Aging Techniques
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Radiometric Dating

• Rocks generally contain small amounts of
  radioactive material that can act as natural
  clocks.
  
• Recall that atoms of the same element that have
  different numbers of neutrons are called _______
  
  
• isotopes.
• Nuclear Stability

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Radioactive Isotopes

• Radioactive isotopes have nuclei that emit
  particles and energy at a constant rate
  regardless of surrounding conditions.
  
• What particles could be emitted from the nuclei
  of atoms?
  

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Alpha Decay

• Alpha decay is a type of radioactive decay in
  which an atomic nucleus emits an alpha particle
  
• Two protons and two neutrons bound together into
  a particle identical to a helium nucleus.
  

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Beta Decay

• Beta decay is a type of radioactive decay in
  which a beta particle (an electron or a positron)
  is emitted.
  
• A positron is a positively charged electron.
• Welcome to the world of anti-matter.

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Gamma Decay

• In gamma decay the nucleus does not emit
  sub-atomic particles.
  
• Instead it changes energy state by emitting a
  photon.
  
• Think of a photon as a light particle.
• This changes the energy state of the atom but
  does not change the chemical state.

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Radioactive Decay

• When these particles are emitted, large amounts
  of energy is released.
  
• Scientists use this natural breakdown of isotopes
  to measure the absolute age of rocks and minerals.

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How Radioactive Decay Works

• As an atom emits particles and energy, the atom
  changes into a different isotope of the same
  element or an isotope of a different element.
  
• What type of particle emitted from the nucleus
  would change the isotope?
  
• What type of particle emitted from the nucleus
  would change the element?

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Parent and Daughter Isotopes

• The original radioactive isotope is called the
  ______ ______.
  
• parent isotope.
• The newly formed isotopes are called the _______
  _______.
  
• daughter isotope.

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Half-Life

• Radioactive decay happens at a relatively
  constant rate that is not changed by temperature,
  pressure or other environmental conditions.
  
• Scientists have determined that the amount of
  time required for half of any amount of a
  particular isotope to decay is always the same
  and can be determined for any isotope.

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Half-Life

• Half-life is the amount of time that it takes for
  half the mass of a given isotope to decay into
  its daughter isotope.
  
• If you began with 10 g of a parent isotope, you
  would have 5 g of that parent isotope after one
  half-life and 5 g of the daughter isotope.

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Half-Life Questions.

• After a second half-life, how much of the parent
  isotope would remain?
  
• How much of the parent isotope would now be the
  daughter isotope?
  

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Honors Assignment

• Honors assignment
• Using page 193 in your textbook and the diagram
  to the right, digram the steps by which uranium
  238 decays into lead 206. What type of decay is
  occurring? What particles are emitted in each
  step? Calculate how long it takes for half of
  one atom of U-238 to decay into lead 206. What
  does this tell us about disposing of nuclear
  waste?

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Carbon Dating

• Organic materials that are less than 70,000 years
  old can be aged using Carbon-14, 14C.
  
• The daughter isotope for C-14 is Nitrogen-14,
  14N.
  
• The half life of 14C is 5,730 years.
• What material does 14C actually date?

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Carbon Dating

• Carbon-14 is not used to date the rock.
• It is used to date the organic material trapped
  within the rock like wood, bones and shells.

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Carbon-14

• The isotope Carbon-14 combines with oxygen to
  make radioactive
  
• carbon dioxide.
• Only a small amount of CO2 in the atmosphere
  contains 14C.
  

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Radioactive CO2

• Plants absorb the radioactive CO2 during
  photosynthesis.
  
• When the animals eat these plants the 14C becomes
  part of the animals body tissues.
  
• All living organisms contain both 12C and 14C.

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Ratio of 14C to 12C

• To find the age of organic material, scientists
  first determine the ratio of 14C to 12C in the
  sample.
  
• Then they compare these ratios to the ratios in a
  living organism.
  
• While organisms are alive, the ratio of 14C to
  12C remains relatively constant.

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Decay of 14C to 14N

• When a plant or animal dies, the amount of 14C
  starts decreasing.
  
• The 14C will steadily decrease and the 14N will
  steadily increase.
  
• Why does the 14N increase as the 14C decreases?
• About how much 14N will you expect to find if the
  specimen is 5,000 years old?