ACT Style Passage
Geologic Time
Name _________________
Individual Assignment
Archaeologists use many methods to determine the age of artifacts that they discover. Two of
these methods are described below.
Method I: The Law of Superposition
This method uses the position of the layers
of rock to determine the relative age of each
layer. The basic idea is that if the layers of
rock have generally been left undisturbed,
then the deeper rock layers are older than
the layers above it. Another useful tool
archaeologists use is the law of crosscutting
relationships. This law states that if a
geologic feature cuts across a layer of rock,
then the layer of rock must be older than the
feature that cuts across it.
Figure 1
Method 2: Radioactive Carbon Dating
Radioactive carbon dating uses the half-life of Carbon-14 (C-14) to determine the absolute age of an
artifact. The basic idea is to determine the amount of C-14 in the artifact initially and then measure the
amount of C-14 in the artifact now. From this you can calculate the number of half-lives that have
occurred and then calculate the age of the artifact. The half-life of C-14 is 6000-years.
Figure 2 shows the amount of C-14 would be present in an artifact during each half-life. Table 1 gives
the same information along with the age of the artifact.
Mass of C-14 (g)
Mass of C-14 vs Half-Life
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Half-Lives
Figure 2
Table 1
Analysis
1)
Describe how the law of superposition can be used to determine the age of an object. Is this
relative or absolute age?
2)
What is the Law of Crosscutting Relationships? What geologic features in figure 1can this law be
applied to?
4)
Describe how radioactive carbon dating can be used to determine the age of a fossil.
5)
Describe the relationship between the mass of C-14 and the number of half-lives.
6)
How much C-14 is left after 3.75 half-lives?
7)
A fossil starts with 64-g of C-14 and is reduced to 2-g. How old is the fossil?
8)
If the fossil from #7 was found in layer V, what can you say about the age of a fossil found in
layer I?
Multiple Choice
1)
Which of the lists below gives the correct relative ages, oldest to youngest, of layer I, II, III and
the fault in figure 1.
a.
b.
c.
d.
2)
If an artifact began with 64-g of Carbon-14 and now has 12-g, use figure 2 to determine the
number of half-lives that have passed.
a.
b.
c.
d.
3)
b.
c.
d.
Since layer II and layer V are the same layer that was broken by the fault, the two
artifacts have the same relative age.
Since layer V is slightly lower than layer II, the arrowhead is slightly older than the piece
of metal.
Since layer V is slightly lower than layer II, the metal piece is slightly older than the
arrowhead.
There is really no way to tell which artifact is older since the two layers are broken by the
fault.
An artifact is found to be 28,000 years old. Approximately how much C-14 should be present in
this artifact if it started with 64-g?
a.
b.
c.
d.
5)
2.25
3.00
1.50
3.75
An archaeologist finds a piece of metal in layer II and an arrowhead in layer V as shown in figure
1. What conclusion is valid about the relative age of these two artifacts?
a.
4)
III, II, I, Fault
I, II, III, Fault
Fault, I, II, III
Fault, III, II, I
3.0-g
6.0-g
1.5-g
12-g
A piece of pottery was found in layer IV and a wooden spoon was found in layer VI, as shown in
figure 1. The pottery was found to have 4-g of C-14 present in it after starting with 64-g. What
can be said about the age of the wooden spoon?
a.
b.
c.
d.
The wooden spoon is more than 24000-years old.
The wooden spoon is 24000-years old.
The wooden spoon is less than 24000-years old.
The wooden spoon is greater than 36000-years old.