Geologic time

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Geologic time

• Absolute-Dating

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Geologic Time

• Homework Geologic time scale fit into something
  more familiar
• See web page for events to place
• Choose a time period for your project
• Group of 4
• Cambrian to Quaternary
• See class website or book for ideas
• Email me your first 2 choices

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Last time

• Neptunism
• Catastrophism
• Uniformitarianism
• Modern uniformitarianism

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Last time

• Relative dating relative sequence of events

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(No Transcript)
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• Relative dating relative sequence of events
• Absolute dating establishes numbers for ages

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Rock types

• Igneous
• Sedimentary
• Detrital sedimentary
• Metamorphic

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Structure of the Carbon Atom

• Nucleus
• Protons ()
• Neutrons (n)
• Electrons (-)
• protons atomic
• protons neutrons atomic mass
• Isotope element with different neutrons

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Isotopes of Hydrogen

• Hydrogen
• 1 proton, 0 neutrons
• Deuterium
• 1 proton, 1 neutron

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Radioactive decay

• Unstable nucleus spontaneously transforms into a
  nucleus of different element

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Radioactive decay

• Decay rate in terms of half-life
• Half-life time for 1/2 of all atoms to decay
  from parent element to daughter element

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Radioactive Decay

• Mineral forms with only parent atoms
• Through time, decay to daughter atom

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How to use half-life

• Half-life of potassium 40 is 1.3 billion years
• After 3.9 billion years,how many half-lives have
  occurred
• What fraction of potassium is left?
• 1/2 1/2 1/2 1/8

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Radioactive decay in rocks
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Radiometric Dating

• Knowing the decay rate
• Measuring the ratio of parent and daughter
  isotopes in a mineral
• An age can be calculated

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Isotopes used for dating
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How to date a rock

• Choose rock to date
• Separate minerals of interest

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How to date an igneous rock

• Find and separate out zircons
• Zircons contain U
• Common mineral in granites

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How to date a rock

• Extract the elements

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How to date a rock

• Analyze the isotope ratios

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Dating of a mineral

• You know
• 1,250 parent atoms
• 18,750 daughter atoms
• 1/2 life is 40 million years

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Dating of a mineral

• How many half-lives are needed to end up with
  1,250 parent atoms and 18,750 daughter atoms?
• Start with 20,000 atoms total
• Divide by two to undergo a half-live
• Keep dividing until you get to 1250 parent atoms
• 4 half lives
• 4 half lives 40 million years 160 million
  years

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Problems with dating

• Errors
• Usually 0.5 of age, as good as 0.1
• 2 billion year rock, /- 2-20 million
• Dating detrital sed rocks
• Age of sed deposit or igneous formation?
• Dating metamorphic rocks
• Did isotope ratio survive metamorphism?

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Loss of daughter isotopes

• Did isotope ratio survive metamorphism?
• Are you dating metamorphic event or igneous
  formation?

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You cant date every rock

• Expensive and may not have good minerals to date
• Sedimentary rocks are almost impossible to date
• Best rocks igneous
• Complicated rocks metamorphic

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Applying relative and absolute dating..
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Relative Dating

• Which rock is younger, the Foster City Fm or the
  Larsonton Fm? By which principle?
• Which is younger, the Larsonton Fm or the igneous
  dike? By which principle?
• Has the Tarburg Fm been deformed at all? Which
  principle did you use to answer this?
• Put the Larsonton Fm, Foster City Fm and dike in
  order from youngest to oldest