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GEOLOGY 12
CHAPTER 8 GEOLOGIC TIME
ABSOLUTE TIME: RADIOMETRIC DATING ACTIVITY AND WORKSHEET
KEY
DEMO
1. Record the following: (In class)
SAMPLE # ________
Number of Parent Atoms (P) ________ (natural color)
Number of Daughter Atoms (D) ________ (pink)
Ratio of Parent to Daughter (P:D) __________ = _________ (simplest form)
Percentage of Parent Remaining:
P
x100 = _________ %
96
2. Complete the following table.
SAMPLE
Percent of
Parent Atoms (P)
Percent of
Daughter Atoms (D)
Ratio of
P to D
1
100%
0%
1:0
2
50%
50%
1:1
3
25%
75%
1:3
4
12.5%
87.5%
1:7
5
6.25%
93.75%
1:15
6
3.125%
96.875%
1:31
Fraction of
Parent Atoms
1
1
1
1
1
1
1
1
1
1
1
1
( 2)
= (1 )
2
= (1 )
2
= (1 )
2
= (1 )
2
= (1 )
2
= 1
0
1
2
3
4
5
Number of
Half-Lives
0
1
2
3
4
5
3. On the following grid, sketch a radioactive decay curve for macaronium. Your
sketch must contain:
• a title (“Radioactive Decay of Macaronium”)
• vertical and horizontal axes labels (“Percent of Parent” and “Half Lives”
respectively)
• a smooth and clearly drawn decay curve for the plotted data, in pencil.
Daughter
Isotope
Parent
Isotope
0
0.5
1.0
1.5
Age in Millions of Years (my)
2.0
2.5
3.0
FOLLOW-UP QUESTIONS
1. If the half-life of macaronium is 500,000 years, how old is Sample #4? 3HL = 1.5 my
2. Add the ages for each of the half-lives to the horizontal axis of your graph.
3. Approximately what percentage of parent remains if a sample of macaronium was
dated at 125,000 years old? ~80%
4. Use the data from “Percent Daughter Atoms” to sketch the curve of the stable
daughter isotope. Label each curve as either Parent Isotope or Daughter Isotope.
5. Describe one problem you might encounter when radiometric dating very weathered
rock.
•
Chemical changes may occur to reduce the amount of parent or daughter
isotope available for measurement.
6. Describe one problem you might encounter if the daughter product was a gas.
•
If gasses escape, such as argon, the samples would appear YOUNGER than
they actually are.
ASSIGNMENT
1.
How does an atom of a radioactive element change as it undergoes radioactive
decay?
The nucleus releases alpha & beta particles & heat (gamma rays).
2.
Examine the radioactive decay curve of Element X, shown in Figure 12.23.
a. Find its half-life. ~1 by
b. Which element has this half-life? (see p.161)
Closest to potassium-40
c. Find the time required for half of the sample to
decay again (from 50 per cent to 25 per cent).
Compare this value with the half-life you
calculated in (a) above. Is there any similarity?
Same as (a) or 1by.
d. A rock containing some Elements X is found to
contain only 40 per cent of its original amount.
How old is the rock? ~ 1.4 by
3.
Can radiometric dating be used to find the absolute age of any material? List some
characteristics that would make some materials unsuitable for this dating method.
- Materials must have a measurable amounts of parent (P) and daughter (D) material.
- Samples are unsuitable if: there is not enough of either P or D, P decays too
slowly, there is loss of gas (argon) or sample has been contaminated
Use the radioactive decay graph provided to answer the following questions.
4.
The half-life of carbon-14 is 5,700 years. If you start with 1.0 g of carbon 14, how
much will remain after 5,700 years? 1HL = 5700 = ½ x 1.0g = 0.5g
5.
How much carbon-14 remains after 11,400 years (two half-lives)?
2HL = ½ x ½ x 1.0g = 0.25g
6.
Estimate the ages of the following:
a. A bone fragment with 50% of its original carbon 14 remaining.
b. A fossilized shark's tooth with ~12% of the original carbon 14 remaining.
c. A tiny piece of paper taken from the Dead Sea Scrolls with 75% of the original
carbon 14 remaining.
a) 50% P = 1HL = 5700yr
b) 12% P = 3HL = 3 x 5700yr = 17,100yr
c) 75% P = ½ HL = ½ x 5700yr = 2850yr
7.
What percentage of the original carbon 14 would remain in a sample of wood from
a shipwreck if it was:
a. 10,400years old? 10,400 / 5700 = 2HL = (1/2)2 or ¼ or 25% left
b. 30,000 years old? 30,000 / 5700 = 5.3HL ~ (1/2)5 or 1/32 or ~3% left
8.
Carbon 14 is incorporated into living plants from the atmosphere. From there it
enters into animals. To use carbon-14 dating, the samples must have come from
organisms that were once alive. With this in mind, explain:
a. Why is it often difficult to find samples for carbon 14 dating on ancient
shipwrecks? organic material decays quickly
b. What might you expect to find on an ancient shipwreck that could be dated
using carbon 14? wooden planks or masts
9.
Suggest a reason why carbon dating is not suitable for finding the age of ancient
fossils that are millions of years old.
HL too short to have measurable amounts of P & D left after millions of years
10. Using the table on page 161, which radioactive isotopes would not be suitable for
radiometric dating when
a. testing materials that were once living?
K-40 Rb-87 Th-232 U-235 U-238
b. testing inorganic materials (matter that has never been alive)? C-14
c. dating rocks older than 1 billion years, such as meteorites? C-14 Rb-87 Th-232
d. dating materials younger than 1000 years old? All except C-14
e. dating rock samples that had been heated at some time in the past? (HINT:
This would drive off any gases trapped in the rock.)
K-40 (argon is the daughter gas)
C-14 (nitrogen is the daughter gas)
BONUS
Plot a decay curve for the radioactive isotope thorium 232, assuming its half-life is 14
billion years.
See macaronium graph. Adjust timeframe to match HL = 14by.