Absolute Dating in the Atomic Age

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Absolute Dating in the Atomic Age

• February 6th, 2008

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Geologic Time Scale

• Divides Earth history into intervals marked by
  distinctive sets of fossils
• Boundaries of intervals are marked by abrupt
  changes in sets of fossils eras
• Paleozoic ? paleo old, zoi life
• Mesozoic ? middle life
• Cenozoic ? new life

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Periods

• Eras subdivided into periods, named for
  geographic locality in which formations were
  first or best described
• Jurassic named for the Jura Mountains of France
  and Switzerland

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Boundaries as Extreme Events

• Many of major boundaries correspond to mass
  extinctions
• These represent times when life on Earth was
  rapidly reshaped (e.g., meteors, ice ages)
• Many species die out
• Many new species appear in fossil record

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So how de we assign a real age?

• In 1896, Henri Becquerel discovers radioactivity
  in uranium
• Some elements can occur in forms which are
  unstable
• Over time, these unstable atoms will break down
  to form atoms of other elements

Uranium 235
"The Cheapest Way to Kill People Is To Use
Nuclear Weapons"
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Isotopes Why Some Atoms are Unstable

• The number of protons in an element is called the
  atomic number
• The atomic mass is the number of protons
  neutrons in an atom
• Normally, protons neutrons
• In isotopes, they arent equal
• Some combinations break down

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Ex-Russian Spy, Col. Alexander Litivinenko dies
of Polonium-210 poisoning 11/24/2006
Gives off Helium ion Highly ionizing, low
penetration
Generally emits an electron or a positron, gamma
ray Highly ionizing, low penetration
Emits high energy rays Highly ionizing (lots of
damage), low penetration
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Alpha decay

• A helium ion (He2) is spontaneously ejected
• Helium has an atomic number of 2 and mass of 4 (2
  protons and 2 neutrons)

Seaborgium
Rutherfordium Helium
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Benefits with the Badness

• The parent or original atom decays to a daughter
  atom of some new element
• This decay occurs at a constant rate
• We can define a half-life
• The time it takes for half of the parent to decay
  to daughter
• This doesnt vary with temperature, chemistry, or
  pressure

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Everyone Stand Up
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How We Apply This to Earth

• Isotopic dating only useful if you have some
  parent AND daughter left in the rock
• Isotopes that decay slowly over time like
  rubidium-87 (HL 47 billion years) are most
  useful for dating old rocks
• Can be used to date rocks from 10 million to 4.6
  billion yrs

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Other Isotopes

• Carbon-14 (HL 5730 yrs) useful for dating bone,
  wood, and other organics
• Can date organic material that has an age between
  100-70,000 yrs
• Carbon-12 is non-radioactive

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The Absolute Geologic Time Scale

• When we found absolute ages for the time periods,
  we realized that the periods/eras were of vastly
  different lengths
• The Precambrian, amounted for 9/10ths of the
  entire age of Earth
• Concept of eon
• Eon gt era gt period

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Homework

• Read Ch. 4, 84-end
• Due Monday
• For Review (pg. 95) 3, 7, 12, 14
• For Discussion (pg. 95) 2 (THINK!)