Age Dating Rocks

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Age Dating Rocks
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Two Types of Dating

• Relative Dating
• Absolute Dating

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Relative Dating
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Relative and Absolute Dates

• Relative age states whether one rock formation
  is older or younger than another formation.
• Absolute age specific number of years. (example
  like 150 million years ago.)

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How old is the Earth???

• People used to think that the earth was only a
  few thousand years old!
• Now we believe that the earth is billions of
  years old! video
• From Greatest Discoveries with Bill Nye Earth
  Science. Discovery Channel School(2005).
  Retrieved October 24, 2006, fromunitedstreaming
  http//www.unitedstreaming.com/

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Which layer of rock is oldest?

A
B

C!
C
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Radioactive Decay
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• Radioactive decay is one of Earths sources for
  heat.

• Remember
• Accretion
• Differentiation
• Radioactive decay
• Suns radiation

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Before we get to crazy
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92
of Protons
Atomic Symbol
U
Name of Element
Uranium
238
of Protons Neutons
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• Radioactive decay is the spontaneous change of
  the nucleus of an unstable isotope to a stable
  one.
• Isotope an atom with the same number of protons,
  but a different number of neutrons.
• As protons and neutrons leave atoms, energy is
  produced.

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• When protons are lost during radioactive decay,
  the atom becomes a different element.

• Example When Uranium-238 decays, it loses 32
  particles (10 protons and 22 neutrons). What
  element is it now?

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• What element is it now?

Lead (Pb)!

• What is its new atomic weight? (238-32)

206!

• So, the new product is Lead-206!
• Lead-206 is stable, so it wont decay any further.

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Parents and Daughters

• The original element is called the parent
  isotope, and the product of the decay is called
  the daughter isotope.

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• The moment an igneous rock crystallizes,
  radioactive atoms start to decay to a stable
  product.

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• Half life The amount of time it takes for half
  of the parent to decay to the daughter.

• After one Half-life, half of the atoms of a
  radioactive element will have decayed to a stable
  product, and half will remain unchanged.

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Half Life
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• A rock that has a lot of parent, but very little
  daughter is very young!
• A rock that has a lot of daughter, but very
  little parent is very old!

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As the of parent goes down, the of daughter
goes up!
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Radioactive Dating
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How we can get the age
Many minerals contain radioactive isotopes. The
age of any of these minerals can be determined
by 1)counting the number of daughter isotopes in
the mineral, and 2)using the known decay rate to
calculate the length of time required to produce
that number of daughters.
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• Parent Daughter Half-Life
• 14C 14N 5730 years
• 3H 3He 12.8 years
• 40K 40Ar 1.25 Billion Years
• 87Rb 87Sr 48.8 Billion Years
• 238U 206Pb 4.5 Billion Years

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100 g of 14C _at_ time0   _at_ 1 half life, 50 g of
14C, 50 g of 14N So 5730 years have passed   _at_
2 half lives, 25 g of 14C, 75 g of 14N So 5730
(x2) or 11460 years have passed _at_ 3 half lives,
12.5 g of 14C, 87.5 g of 14N So 5730 (x3) or
17190 years have passed Total Years Passed 17190
years!
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Lab Radioactive Decay
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Materials

• Jar
• 100 pennies

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Procedure

1. Obtain all the materials needed for the lab. The
   jar represents your rock. The pennies represent
   the minerals in the rock. Heads will represent
   parents, tails will represent daughters.
2. Place all 100 pennies in the jar. Imagine that
   they are all heads-up (all parents).
3. Dump the pennies onto the table and separate the
   parents from daughters.
4. Count how many parents and how many daughters you
   have. Record the results on a data table.
5. Place all the parents back in the jar, shake
   them, and dump them out again.
6. Repeat procedures 3-5 until all the parents have
   changed to daughters, or until you have only one
   parent left.
7. If you end up with only one parent left, put it
   back into the jar, shake it, and no matter what,
   imagine it changes to a daughter.

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Data
of Half-lives (one half-life is 1000 years) of Parents of Daughters
1
2
3
4
5
6
7
8
9
10
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12
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Conclusion

• How many half-lives did it take for all your
  parents to change to daughters?
• How old is your rock when all the parents have
  changed to daughters?
• What did you learn from this lab?

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Known Half-lives