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Title: Age Dating Rocks


1
Age Dating Rocks
2
Two Types of Dating
  • Relative Dating
  • Absolute Dating

3
Relative Dating
4
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.)

5
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/

6
Which layer of rock is oldest?

A
B

C!
C
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9
Radioactive Decay
10
  • Radioactive decay is one of Earths sources for
    heat.
  • Remember
  • Accretion
  • Differentiation
  • Radioactive decay
  • Suns radiation

11
Before we get to crazy
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13
92
of Protons
Atomic Symbol
U
Name of Element
Uranium
238
of Protons Neutons
14
  • 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.

20
Parents and Daughters
  • The original element is called the parent
    isotope, and the product of the decay is called
    the daughter isotope.

21
  • The moment an igneous rock crystallizes,
    radioactive atoms start to decay to a stable
    product.

22
  • 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.

23
Half Life
24
  • 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!

25
As the of parent goes down, the of daughter
goes up!
26
Radioactive Dating
27
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.
28
  • 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

29
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!
30
Lab Radioactive Decay
31
Materials
  • Jar
  • 100 pennies

32
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.

33
Data
of Half-lives (one half-life is 1000 years) of Parents of Daughters
1
2
3
4
5
6
7
8
9
10
11
12
34
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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36
Known Half-lives
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