LAB: DETERMINING THE AGE
OF ROCKS AND FOSSILS
Background Information: Some very straightforward principles are used to
determine the age of fossils.
Rock Layers: Geologists in the late 18th and early 19th century studied rock
layers and the fossils in them to determine relative age. William Smith was one of
the most important scientists from this time who helped to develop knowledge of
the succession of different fossils by studying their distribution through the
sequence of sedimentary rocks in southern England.
Principle of superposition: Younger sedimentary rocks are deposited on top of
older sedimentary rocks.
Principle of cross-cutting relations: Any geologic feature is younger than anything
else that it cuts across.
Radiometric Dating:
It wasn't until well into the 20th century that enough information had
accumulated about the rate of radioactive decay that the age of rocks and fossils
in number of years could be determined through radiometric age dating.
Some elements have forms (called isotopes) with unstable atomic nuclei that have
a tendency to change, or decay. For example, U-235 is an UNSTABLE isotope of
uranium that has 92 protons and 153 neutrons in the nucleus of each atom. When
it goes through a series of changes in the nucleus, it ends up with 82 protons and
125 neutrons which make it STABLE – and then the changes stop.
A nucleus with that number of protons is called lead (chemical symbol Pb). The
protons (82) and neutrons (125) total 207. This particular form (isotope) of lead is
called Pb-207. U-235 is the PARENT isotope. Pb-207 is the DAUGHTER isotope.
Many rocks contain small amounts of unstable isotopes and the daughter isotopes
into which they decay. Where the amounts of parent and daughter isotopes can be
accurately measured, the ratio can be used to determine how old the rock is, as
shown in the following activities.
At any moment there is a small chance that each of the nuclei of U-235 will
suddenly decay. That chance of decay is very small, but it is always present and it
never changes. In other words, the nuclei do not "wear out" or get "tired"—they
just always have a chance of breaking down
Very careful measurements in laboratories, made on VERY LARGE numbers of U235 atoms, have shown that each of the atoms has a 50:50 chance of decaying
during about 704,000,000 years. What that means is, during 704 million years,
half the U-235 atoms that existed at the beginning of that time will decay to Pb207. This is known as the half life of U- 235.
Many elements have some isotopes that are unstable, essentially because they
have too many neutrons to be balanced by the number of protons in the nucleus.
Each of these unstable isotopes has its own characteristic half life. Some half
lives are several billion years long, and others are as short as a ten-thousandth of
a second. THEREFORE, ALL HALF LIVES ARE DIFFERENT DEPENDING ON THE
ISOTOPE!
PURPOSE: This activity will help students to determine relative age of a
geologically complex area using the concepts of radioactive decay and
superposition.
MATERIALS REQUIRED FOR EACH GROUP
1)
2)
3)
4)
5)
Block diagram (Figure 1).
Large cup or other container in which M & M's can be shaken.
100 M & M's
Decay table (Figure 2).
Watch or clock that keeps time to seconds. (A single watch or clock for the
entire class will do.)
6) Graph of M&M half life (Figure 3)
7) Graph of U-235 half life (Figure 4)
8) Data table of Half life of U-235 (Figure 5)
Procedure Part 1:
1. With your group, discuss together how to determine the relative age of each
of the rock units in the block diagram (Figure 1). Remember to use the
principles of superposition and cross cutting relationships.
2. List the rock layers in order at the bottom of Figure 1 on the limes provided
from most recent at the top to oldest at the bottom. Radiometric age will be
determined later.
Procedure Part 2:
3. Take the 100 M&Ms and place the printed M face down on a piece of paper.
This represents the PARENT isotope.
4. Pour the candy into a container which is large enough for them to bounce
around freely, shake them and pour them back onto the paper so they can be
spread out, not piled. This represents one half life and all the M&Ms that
have the printed M facing UP represents a change to the DAUGHTER
isotope
5. Pick up and set aside ONLY the daughter isotopes, then count the number
of pieces of candy left with the M side facing down. These are the PARENT
isotopes that did not change during the first half life.
6. Record the number of parent isotopes after the first run in your team’s
box( Your teacher will designate the team numbers). Continue to repeat the
same procedure of shaking and counting the “survivors” and filling in the
next row on the decay table. This should be done 7 or 8 more times. Each
time represents a half life.
7. The teacher will instruct each team to supply their classmates with their
data and each team is then responsible for figuring out class total and class
average for each run.
8. Using Figure 3- Graph of M&M half lives, plot the number of parent isotopes
remaining after each run and connect the points on a graph using one color.
9. On the same graph, plot the average values for the class as a whole and
connect these points with a different color.
10.
Lastly, on the same graph , plot the starting number of M&Ms (100) for
the first run and divide by 2 for each run after that.( For ex. The line begins
at 100, the next point is 100/2, and the next point is 50/2…and so on….)
Procedure Part 3:
11.
Imagine that you have 128 buttons that have U-235 written on one
side and Pb- 207 written on the other. Remember U – 235 is the parent
isotope that decays into its daughter isotope Pb- 207.
12.
Imagine that all 128 discs have the U- 235 side up. Within 2 minutes,
imagine that you flipped over ½ of the discs to show the side reading Pb-207,
This would represent one half- life.
13.
If a new 2 minute interval passes, ½ of the remaining U-235 will flip
over to Pb-207. How many are still U-235? How many are Pb-201? This
represents the passage of 2 half lives.
14.
Fill in the data table for U-235 in Figure 5 for 5 half lives.
15.
The actual half life of U-235 is 704 years, not 2 minutes. Complete the
chart using the actual half-life.
Procedure Part 4:
16.
Go back to the block diagram (Figure 1) at the beginning of this lab and
figure out how old these rocks layers are using the following information.
17.
The ratio of U-235:Pb-207 atoms in the pegmatite is 1:1, and their
ratio in the granite is 3:1. Use what you learned in part 2b to determine how
old the pegmatite and granite are. Put those ages in the lines in Figure 1.
18.
Figure 4 is a special scale called a logarithmic scale—different than a
regular graph. Look at the chart- You can find the age of shale/siltstone
layer and the basalt layer. Shale/siltstone contains 94% U-235. How old is it?
Fill in the radiometric age in the line provided in Figure 1.
19.
The ratio of U-2325 in basalt is 70%. How old is the basalt layer? Fill
in the radiometric age of basalt on the diagram in Figure 1.
Determining Age of Rocks and Fossils
NAME ___________________________
DATE ______________
ANALYSIS QUESTIONS
1) What is the rock layer that has acritarchs and bacteria? Based on the available
radiometric ages, can you determine the possible age of this rock? Why can't you
say exactly what the age of the rock is?
2) Which rock contains trilobites? Based on the available radiometric ages, can
you determine the possible age of this rock? Why can't you say exactly what the
age of the rock is?
3) What is the age of the rock that contains the Triceratops fossils? Why can you
be more precise about the age of this rock than you could about the ages of the
rock that has the trilobites and the rock that contains acritarchs and bacteria?
4) Why didn’t each group get the same results when you calculated the M&M half
life??
5) In the M&M graph, did the single group line or the line based on the class
average follow the mathematically calculated line better? Why?
6) Would students have an easier time predicting the results of the M&M half life
if there were a lot of M&Ms in the cup or few? Why?
Figure 5 – Half Life of U-235
U- 235
start
After
2 mins
(1 half
life)
128
64
0
64
parent
Pb- 207
daughter
Ratio
1:1
U: Pb
Half
lives
1
Years
704 yrs
After
4 mins
(2 half
lives)
After
6 mins
(_____
half
lives)
After
8 mins
(____
half
lives)
After
10 mins
(____
half
lives)